Project acronym: EE-VERT Project ... - trimis.ec.europa.eu · SC Beespeed Automatizari SRL BEE RO . EE-VERT Deliverable D2.1.1 23 December 2009 Document history Revisions: Version

EE-VERT

© 2009 The EE-VERT consortium

Project acronym: EE-VERT

Project title: Energy Efficient VEhicles for Road Transport – EE-VERT

Grant Agreement Number: 218598

Programme: Seventh Framework programme, Theme 1.1, Greening

Contract type: Collaborative project

Start date of project: 1 January 2009

Duration: 36 months

Deliverable D2.1.1:

POWER GENERATION REPORT

Authors: Organisation Name

MIRA David Ward

MIRA Bob Simpkin

CRF Carlo D‘Ambrosio

BOSCH Marcus Abele

LEAR Antoni Ferré

FH-J Manuela Midl

FH-J Raul Estrada Vazquez

UPT Ion Boldea

BEE Sever Scridon

VTEC John Simonsson

Reviewers: Organisation Name

VTEC John Simonsson

FH-J Raul Estrada Vazquez

FH-J Hubert Berger

CRF Carlo D‘Ambrosio

Dissemination level: Public

Deliverable type: Report

Work Task Number: WT2.1

Version: 1.0

Due date: 31.12.2009

Actual submission date: 23 December 2009

Date of this Version 23 December 2009

EE-VERT

© 2009 The EE-VERT consortium

Consortium Members

Organisation Abbreviation Country

MIRA Limited MIRA GB

Volvo Technology AB VTEC SE

Centro Ricerche Fiat Società Consortile per Azioni CRF IT

Robert Bosch GmbH Bosch DE

LEAR Corporation Holding Spain SLU Lear ES

Engineering Center Steyr GmbH & Co KG /

MAGNA Powertrain

ECS AT

FH-JOANNEUM Gesellschaft mbH FH-J AT

Universitatea ―Politehnica‖ din Timisoara UPT RO

SC Beespeed Automatizari SRL BEE RO

EE-VERT Deliverable D2.1.1 23 December 2009

Document history

Revisions:

Version Description Planned

date

Actual

date

0.0 Draft document outline – sent to contributors 28/09/2009

0.1 First internal version of deliverable with all contributions 10/12/2009

0.2 Incorporating revisions suggested by John Simonsson 15/09/2009

0.3 Incorporating revisions suggested by Raul Estrada Vazquez 18/12/2009

1.0 First version of deliverable 23/12/2009

A brief summary

Despite improvements in modern vehicles, a considerable amount of energy is still wasted due to the

lack of an overall on-board energy management strategy. Further electrification of auxiliary systems

promises energy and efficiency gains but there is an additional need for a coordinated approach to the

generation, distribution and use of energy.

The objective of work task 2.1 is the development of high efficiency power generation concepts,

including their operational strategies, within EE-VERT‘s system approach. Also the integration of

innovative systems will be studied, such as solar panels or thermo-electric generators to recover

energy from exhaust gases. Energy recovery from waste energy e.g. thermo-electric generators to

recover energy from exhaust gases can make a significant contribution to energy efficiency and CO2

reduction. This is particularly important for the overall system when different energy sources are

combined.

This deliverable reports on the power generation concepts investigated by EE-VERT and needed for

the operation of an advanced electrical power net aimed at generating and reusing energy at a very

high level of efficiency.


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Contents

A BRIEF SUMMARY ..................................................................................................................... 1

1 INTRODUCTION ................................................................................................................... 4

1.1 BACKGROUND .................................................................................................................... 4

1.2 PURPOSE ............................................................................................................................ 5

1.3 SCOPE ................................................................................................................................ 5

1.4 PROCEDURE OF WORK ........................................................................................................ 5

2 REQUIREMENTS .................................................................................................................. 7

2.1 BASIC FUNCTIONAL REQUIREMENTS ................................................................................... 7

2.2 REQUIREMENTS FROM AN ARCHITECTURE POINT OF VIEW................................................... 8

2.3 REQUIREMENTS FROM THE STORAGE SELECTION ................................................................ 9

2.4 SUMMARY - REQUIREMENTS FOR POWER GENERATION COMPONENTS ............................... 11

3 POWER GENERATION CONCEPTS ................................................................................. 14

3.1 GENERATOR ..................................................................................................................... 14

3.1.1 State of the art .............................................................................................................. 14

3.1.2 Two selected solutions for the future ............................................................................. 21

3.1.3 Characterization of the proposed solutions ................................................................... 22

3.1.4 Decision ....................................................................................................................... 25

3.1.5 Selected Electric Alternator: IPM- Lundell configuration ............................................. 25

3.2 SOLAR PANELS ................................................................................................................. 28

3.2.1 Current Vehicle Applications for PV Panels ................................................................. 28

3.2.2 Solar Panel Performance ............................................................................................. 29

3.2.3 Types of solar cell ........................................................................................................ 35

3.2.4 Panel Electrical Characteristics ................................................................................... 36

3.2.5 Possible Sizes of PV Panel on the Alfa Romeo 159 Reference Car ................................ 40

3.2.6 Possible use of PV panels within EE-VERT .................................................................. 41

3.2.7 Energy Saving Calculation ........................................................................................... 42

3.3 WASTE HEAT RECOVERY .................................................................................................. 44

3.3.1 Technology overview .................................................................................................... 44

3.3.2 On vehicle heat recovery management .......................................................................... 48

3.3.3 Current vehicle applications for TEG ........................................................................... 49

3.3.4 TEG Performance ........................................................................................................ 51

3.3.5 DC/DC requirements for TEG management .................................................................. 53

3.3.6 Possible use of TEG within EE-VERT ........................................................................... 54

3.3.7 TEG energy saving calculation ..................................................................................... 54

3.4 DC/DC CONVERTER ......................................................................................................... 59

3.4.1 Basic architectures ....................................................................................................... 59

3.4.2 Baseline ....................................................................................................................... 62

3.4.3 MIPEC (Multi-input Power Electronic Converter) ........................................................ 66

3.4.4 MIPEC working operation............................................................................................ 70

3.5 CONCLUSIONS .................................................................................................................. 74

4 SYSTEM INTEGRATION AND MANAGEMENT ............................................................. 76

4.1 SYSTEM CONCEPT............................................................................................................. 76

4.1.1 Basic EE-VERT approach ............................................................................................ 76


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4.1.2 EE-VERT architecture with the power generation components ..................................... 77

4.2 OPERATION STRATEGY ..................................................................................................... 78

4.3 LINK TO HYBRID VEHICLES ............................................................................................... 78

4.4 IMPACT ON SAFETY RELEVANT APPLICATIONS .................................................................. 79

4.5 IMPACT ON COMMERCIAL VEHICLES ................................................................................. 81

5 CONCLUSIONS AND OUTLOOK ...................................................................................... 86

REFERENCES .............................................................................................................................. 87


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1 INTRODUCTION

1.1 Background

The electrical system in conventional vehicles consists of a single electrical power bus, a generator

mechanically linked to the engine, an energy storage device (usually a 12 V lead acid battery) and

many different loads. In present-day vehicles, even in those regarded as the state-of-the-art electrical

power is generated with little knowledge of the actual loads. In general, the energy required for

auxiliary systems (e.g. power steering, water pump, oil pump) is generated and consumed

continuously, regardless of demand. Similarly, the energy generation for the vehicle‘s electrical

system operates continuously.

Currently the generator is responsible for the conversion of mechanical energy from the vehicle‘s

engine to electrical energy for the power distribution network. Due to the current operating conditions

the generator can only be optimised selectively for special operating points. Today it has to fulfil its

task over the whole range of engine speed with only a low average efficiency. With a new machine

concept and enhanced control and regulation methods a significant improvement of efficiency is

possible. These enhanced control and regulation methods have to be integrated in the vehicle overall

system approach.

Auxiliaries 5%

Rolling friction 10%

Aerodynamic resistance 7%

Drive train 2%

Brake energy 8%

Fu

el

de

ma

nd

10

0%

Exhaust gases

Cooling

system,

thermal

radiation

22%

40%

Idling (6%)

Drive

(94%)

Idling losses 5%

Thermal

energy

(62%)

Driving

(25%)Driving resistance (17%)

Auxiliaries during Idling 1%

Mechanical energy

(32%)

Fig. 1.1.1 Energy flow diagram of a conventional vehicle with diesel engine [4]

Fig. 1.1.1 shows the energy flow diagram with average values for a medium-class passenger car with

a diesel engine on the NEDC. The combustion engine converts less than one third of the chemical

energy supplied by the fuel into mechanical energy which can be used to propel the vehicle and

overcome frictional losses and aerodynamic drag as well as drive the auxiliaries such as pumps and

the generator. The predominant part, more than two-thirds of the chemical energy, is converted into

heat and will be released to the environment via the engine cooling system and the exhaust system.

Only about 17 % of the chemical energy is used to overcome the actual driving resistances.

Despite improvements in modern vehicles, a considerable amount of energy is still wasted due to the

lack of an overall on-board energy management strategy. Further electrification of auxiliary systems

promises energy and efficiency gains but there is an additional need for a coordinated approach to the

generation, distribution and use of energy.


Version 1.0 Page 5

1.2 Purpose

The objective of work task 2.1 is the development of high efficiency power generation concepts,

including their operational strategies, within EE-VERT‘s system approach. Also the integration of

innovative systems will be studied, such as solar panels or thermo-electric generators to recover

energy from exhaust gases. Energy recovery from waste energy e.g. thermo-electric generators to

recover energy from exhaust gases can make a significant contribution to energy efficiency and CO2

reduction. Nevertheless this is strongly dependent on energy management and the mission profile.

Additionally, the link to hybrid vehicles will be studied, since power generation technologies are also

important for hybrid vehicles. Since hybrid vehicles use still combustion engines, the technology of

energy recovery from waste energy is also applicable to hybrid vehicles. Hence, the application of

thermo-electric generators or other EE-VERT power generation concepts for hybrid vehicles will also

be investigated within WT2.1. A special focus will also be given to the possible application of EE-

VERT‘s smart power electronic sub-systems for hybrid vehicles. They could lead to cost advantages

for hybrid vehicles.

Also the link to safety relevant applications will be studied. For safety-relevant applications predictive

diagnosis algorithms will be considered along with reaching a high quality and certainty of service for

power systems.

Finally, the link to commercial vehicle applications will be studied.

1.3 Scope

The central EE-VERT approach is the electrification of auxiliary systems, and supplying their energy

through high efficiency electrical power generation and reuse of waste energy. The EE-VERT concept

considers the combination of several different approaches to energy saving within an overall energy

management strategy (thermal and electrical). The approaches include:

Greater efficiency in energy generation with a new concept for the electrical generator and an

optimised operation strategy;

Energy recovery from waste energy such as waste heat recovery or an optimised braking

energy recuperation with a temporarily increased generator output power with up to 8kW at a

higher voltage level;

Energy scavenging from unused and new sources of energy, for example the use of solar

cells.

The increased electrification of auxiliary systems with optimised operation promises efficiency gains.

But this can only be accomplished if the energy generation and distribution is optimised and adapted

to the current driving conditions and the power demands.

1.4 Procedure of work

Fig. 1.4.1 shows the procedure of work. To support the work a list of components (LoC) has been

generated which was used to identify the partner contributions to each power generation component.

The intention was to have a list with all of the components and their development status. On the basis


Version 1.0 Page 6

of this spreadsheet the working groups and the responsible partners have been determined.

Furthermore, the activities for every component within the consortium have been determined.

Collection of

partner

contributions

Definition of

working

groups

Definition of

responsible

partners

Selection of

components

for EE-VERT

Fig. 1.4.1 Procedure of work

EE-VERT develops a system that will optimise the energy performance and efficiency of

conventional vehicles in general. But special attention will be given to validate the results on a

reference car, an Alfa Romeo 159 1.9JTD. Hence, the LoC contains information on whether a

component is available in the reference car and how it is realised (mechanical or electrical or if there

are any special implementations we have to consider). Furthermore, the partner contributions are

listed and what activities can be undertaken within the project consortium (Fig. 1.4.2).

Power generation Power generation Power generation Voltage converter System control

Optimised generator with

recuperation capabilitySolar panels Reuse of thermal energy DC/DC converter / power interface ECU or dSpace

Reference car

Available in the

reference car

Y= Yes

N= NOY N N N

State of the art in

the reference car

Fixed voltage regulation (14 V).Fixed

LRC time (3s). LRC time cut off =

3000 RPM.

Activities Stage

Concept study C x x x x x

Simulation S x x x x x

Development D x - - x x

Prototyping P x x - x x

Test bench T x x - x x

Vehicle V x - - x x

Partner

contributionsPM

MIRA 4 C C C C

CRF 30 C,P,V - C C,P,V

BOSCH 49 C,S,D,P,T,V - - C,S C, T, V

LEAR 14 - C,S,P,T C, S C,S,D,P,T,V

UPT 49 C,S,D,P - - C,S,D,P

Bee 13 C,S,D,P C,S,D,P

Working groups

Responsibel

partnerBosch Mira CRF Lear CRF, Lear

Working group Mira, CRF, Bosch, UPT, Bee Mira, Lear Mira, CRF, Lear Mira, CRF, Bosch, Lear, UPT, Bee CRF, Bosch, Lear

Working groups and responsible partners

WT2.1 Power generation

Role of beneficiaries and contributions to activities

Classification

Picture

Component

What we are able to do within the project consortium

Fig. 1.4.2 List of components with the partner contributions to each power generation component


Version 1.0 Page 7

2 REQUIREMENTS

2.1 Basic functional requirements

In conventional vehicles a 14 V low voltage power net distributes the supply voltage for the auxiliary

systems and loads via direct connections. The generator contains a voltage regulator to obtain a

constant voltage for charging the vehicle battery, so the full potential of this equipment is not utilised

for most of the time. Also, some systems e.g. ECUs require higher voltage stability to function

properly and regulate this internally in the ECU. Furthermore, there is no coordination between these

systems with respect to current consumption.

ECU #1AlternatorR Battery

Load #1BMS

Voltage

Stabilizer

ECU #N

Voltage

Stabilizer

Load #NStarter

CommunicationCommunication NetworkNetwork

14 V 14 V PowernetPowernet

R Alternator regulator

BMS Battery Monitoring System

Fig. 2.1.1 Diagram of a standard power net architecture [3]

Fig. 2.1.1 shows the diagram of a standard power net architecture [3]. As discussed in D1.3.1 [3], the

EE-VERT concept is focused on introducing new architectures for conventional vehicles with better

energy efficiency both at generation and at consumption stages. Functional requirements for this

improved electric architecture within a conventional vehicle, i.e. maintaining the low voltage power

net, are the following:

assure the same degree of functionality and availability of the standard power net;

provide a stable 14 V power net, protected against load dump and similar glitches

(including cranking) to allow the hardware requirements and design of ECUs to be

simplified and lead to an ECU cost reduction;

improve the efficiency of electrical power generation;

add brake energy recovery up through the generator to reduce the generator mechanical

power request to the engine; currently there is some rotation of the generator during

motor braking but with only a small amount of benefit;

include energy recuperation from other sources;

improve electrical energy storage (charge/discharge) capability (both in size and peak

demand);

convert mechanically driven auxiliaries to electrically driven for better fuel economy, by

using recovered energy and by improved control of them;


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introduce an overall electrical energy management approach for power net management

with the potential for temporarily switching off systems that are not necessary at that

specific time;

allow on the easy addition of a start and stop system.

2.2 Requirements from an architecture point of view

The architectures overview in D1.3.1 [3] has shown some of the possible solutions and has

highlighted critical issues to be considered. Particular attention must be paid to:

the energy flow for engine cranking;

the stability of the 14 V power net voltage in every vehicle condition;

the efficiency of energy generation and energy recovery;

the availability of suitable energy storage for energy recovery.

As proposed in §1.1 of D1.3.1 [3] the EE-VERT concept for power generation is based on two main

assumptions:

Possibility of using multiple power sources: To improve the overall efficiency of the

vehicle, new and improved generation sources may be introduced. In all cars, alternators with

braking recuperation capabilities will be incorporated. Also, other sources may be (optionally)

introduced including waste heat recovery, solar cells and grid connection. The characteristics

of these sources are highly variable: some of them may be available at any time, but with

varying efficiency depending on operating conditions, others are only available under certain

conditions. For instance, if a grid connection is installed in the vehicle it will be available

only when the car is parked in the vicinity of a charging point.

Generation is decoupled from the conventional power net for consumers. Instead, an

energy converter device is introduced. This energy converter device is a multiple-input power

electronics converter (MIPEC). In this way, each generation device can be used in an efficient

manner because each source may be conditioned for an optimized power output. Also, MIPEC

gives more freedom to implement management strategies for saving energy during the vehicle

operation by avoiding current consumption by systems that are not in active use at the time.

It is also necessary to have a storage element with high power / high energy capabilities in order to

store the energy produced when it can be generated with low energy losses / high efficiency. The

stored energy should be used when generation is not recommended due to low efficiency. Due to the

unbalancing of power generation capabilities, it is recommended that the storage element is connected

directly to the output of the generator, as discussed in D1.3.1.

In order to store this energy, two possibilities have been considered: Lithium-Ion battery and

ultracapacitor. If a battery is used, it could be more beneficial to regulate the generator output current

(externally controlled current level) to charge the battery more optimally, within certain fixed limits,

of course, for the voltage level. Furthermore, it is possible to provide a high quality supply voltage for

the more sensitive systems by means of a switch between the low voltage node and the starter and the

starter battery. The switch is opened during cranking. This gives the possibility to save some energy


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and expensive protection components on each single device. The resulting general architecture is

shown in Fig. 2.1.1. A detailed explanation is given in section 4.1 of this report.

Fig. 2.2.1 EE-VERT architecture

2.3 Requirements from the storage selection

Storage device

The recommended general EE-VERT architecture is shown in Fig. 2.2.1. One important question is

which high voltage storage device type to use. With an ultracapacitor the system has to start charging

on a low voltage level and increase the voltage during charging of the ultracapacitor. At the beginning

of a braking phase the car has a high velocity and therefore a high kinetic energy but the generator has

to start on a low voltage level. So the power from the generator is limited due to the low voltage. At

the end of the braking phase the ultracapacitor is already charged and works on a higher voltage level

but the kinetic energy of the car is lower and restricted.

If a battery is used instead of an ultracapacitor the system can have a higher voltage level during the

whole braking phase and therefore is able to receive more energy from the generator. Furthermore, a

battery allows the supply of electrified auxiliaries when the vehicle is stationary enabling the engine

to be turned off. Hence, a battery on the higher voltage level is presumably the better choice. But a Li-

Ion battery has a higher weight and with an aimed recuperation power of 8 kW it has to be able to

tolerate a charging current of up to 200 A or even more (dependent on the system voltage level).

Hence, current Li-Ion battery technology was reviewed to see if it is able to fulfil the given

requirements. Specific questions included:

Which system voltage level on the generator side is adequate?

Can the Li-Ion technology provide a lifetime of up to 5 years under the boundary conditions?

Is the Li-Ion battery able to handle a charging current of up to 200 A (8 kW charging power)?

What is the weight and required space for the necessary Li-Ion battery?

To clarify these questions it was agreed during the plenary meeting in Graz in June 2009 to convene a

task force to investigate specific aspects of the architecture, especially the storage device to be used

(Li-Ion battery or ultracapacitor) and the operational voltage range of the higher-voltage bus. The task

force comprised MIRA, VTEC, Lear, CRF and Bosch.

Storage

Alternative

energy

sources

Load # 1

Volt . stab .

Load # X + 1

Load # X

Volt . stab .

Load # N S

Lead acid

Battery High

Power

Loads G

Low voltage power net

DC/DC

converter

High voltage power net


Version 1.0 Page 10

Key constraints on the architecture included:

The ability to permit the estimated maximum recuperation of up to 8 kW on each recuperation

event in NEDC (though the effect on drivability must be considered);

The storage device must last a comparable time to the life of the vehicle (typically 8–10 years,

but at least 5 years);

The voltage level in the ―power‖ bus must be within ―safe‖ limits for human working (e.g.

Low Voltage Directive < 50 V AC, < 75 V DC; previous ―42 V‖ specifications < 60 V DC;

and [19]).

Main recommendation

The main recommendations from the task force activities are to use a Lithium-Ion battery operating at

40 V DC (nominal) with a capacity of 64 Ah for the main solution. The main reasons were:

Most flexible solution: 40 V 64 Ah Li-Ion ―power‖ battery; 40 V is a good compromise

between power capability and safety (<60 V);

Charge current capability: 64 Ah are necessary with one currently available Li-Ion technology

to charge a current of 200 A;

Permits electrified auxiliary operation during stop phase, for instance electrified air-

conditioning;

Possible downsizing of 12 V ―energy‖ battery, if a voltage modified starter is connected to the

high voltage level and not the 14 V power net;

Present-day mass and size of Li-Ion pack is an acknowledged issue, but this is expected to

improve due to the impetus of current of research programmes into battery technology.

Maximum recuperation power and voltage level

With an aimed recuperation power of maximum 8 kW the storage device has to be able to tolerate a

charging current of up to 200 A. The proposed 40 V Li-Ion battery is based on the manufacturer

LiFeBatt 8 Ah lithium iron phosphate cells with 12 cells in series giving a nominal voltage of 39.6 V

(3.3 V/cell) and 8 cells in parallel to accept a maximum charge rate of 200 A (25 A/cell). This results

in a maximum charge power of 8.8 kW and a 64 Ah capacity. The maximum charging voltage is 3.65

V/cell, giving a maximum battery charging voltage of 43.8 V. The battery includes cell balancing

electronics that operate during charging. Warning signals are available to the control system: High

voltage 3.85 V and low voltage 2.4 V as well as temperature warnings. The high and low warning

signals can be used to provide an outer loop control. When the high voltage signal is triggered the

charging current should be limited to 1 A by the system management and the generator regulator.

Lifetime of the battery

The ―high‖ capacity leads to an acceptable lifetime for the battery. With charging and discharging

calculations it has been estimated that the lifetime of the battery in EE-VERT‘s recuperation approach

is about 8.5 years for 15000 kilometres travelled per year and 6.4 years for 20000 kilometres travelled

per year respectively. More information on this is reported in the quarterly report for quarter 3 of

2009. Since the intended lifetime is at least 5 years – which comes from the comparable lifetime

requirements of lead acid batteries in conventional passenger cars – the 40 V DC 64 Ah battery is

sufficient and offers an additional optimisation potential for downsizing the 12 V battery.

Weight and required space


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The physical size is approximately 600x400x210 mm and a weight of around 30 kg. With CRF it has

been clarified that it is possible to integrate a battery with this dimensions into the demonstrator car.

Baseline solution

The main recommendation for EE-VERT is to use a Li-Ion battery due to the reasons above.

Nevertheless an ultracapacitor could be used in a baseline architecture solution, if only brake energy

recuperation is required. In this case it is not possible to store energy from different sources over a

longer period due to the low energy level of an ultracapacitor. Note that the 12 V lead-acid battery

which is still in the system could not be used to store this energy because the operational strategy of

this battery is to keep the state of charge always near to 100 %. With the low energy level it is also not

possible to supply electrified auxiliaries during stand-still phases. Hence, an ultracapacitor can only

fulfil the requirements for a baseline braking recuperation solution.

Subchapter 2.3 provides only a summary of the task force results. More information is reported in the

EE-VERT report for quarter 3 of 2009. As a consequence of the task force results, some important

component requirements have been defined. They are summarised in the following section.

2.4 Summary - Requirements for power generation components

This subchapter summarises the requirements identified for the power generation components. They

have been derived from sections 2.1 – 2.3. These requirements are necessary to develop adequate

power generation solutions. The main further activities in EE-VERT and the following basic

requirements are based on the architecture in Fig. 2.2.1.

Boundary requirements

power generation is decoupled from the conventional power net;

inputs from multiple power sources (braking energy recuperation, solar cells, waste heat

recovery);

voltage level below 60 V direct current in order to avoid additional safety means for personal

safety [19];

deployment of a high power storage device – recommendation is a Li-Ion battery;

high braking energy recuperation power generation with a maximum of between 4 and 8 kW;

Li-Ion battery

The recommended Li-Ion battery has the following characteristics:

voltage level of 40 V (nominal voltage of 39.6 V – due to the number of cells);

charging power up to 8 kW that corresponds to 200 A at 40 V;

connected directly to the output of the generator;

lifetime should be higher than 5 years;

dimensions have to allow the integration in the vehicle;

warning signals: low cell voltage level = 2.4 V; high voltage level = 3.85 V;

DC/DC converter

decoupling power generation from conventional power net;

provide a stable 14 V voltage for the conventional consumers;

output power on 14 V must be the same as the current generator output power on 14 V,

because the 14 V loads will not be modified in EE-VERT;


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multi input port to link a generator, a solar panel and a thermal generator to the power supply;

the efficiency should as high as possible in the main operation points;

lifetime = vehicle lifetime;

The main challenge is to design a unit that is able to link several different voltage levels of the power

net and accommodate inputs from a generator, a solar panel and a thermal generator, whilst delivering

high efficiency. Hence, a multi-input power converter (MIPEC) is proposed.

Generator

Since a Lithium-Ion battery is recommended in the EE-VERT architecture a well regulated voltage

and current respectively is required for charging the battery. Key points for the generator:

at 40 V the maximum efficiency should be as high as possible but greater than 70 % at 3000-

4000 rpm (larger than a conventional generator);

the maximum output power should be 8 kW at 8000 rpm (using a 3:1 drive ratio);

at 35 V the maximum efficiency should be as high as possible but greater than 75 % with a

maximum output of 7.0 kW (in case the battery has a low state of charge level);

at 47 V the maximum output power should be 9 kW;

voltage range has to be between 28.8 V (lowest cell voltage 2.4 V and 12 cells in series, for

instance after a long stand-still period) and 43.8 V (highest cell voltage 3.65 V and 12 cells in

series);

interface to send control signals to and from the generator;

dimensions that allow integration into a vehicle;

lifetime = vehicle lifetime;

If an ultracapacitor is used, the generator would not have to provide a constant voltage. Instead the

generator would have to provide a voltage with a range from 0 V to the maximum charging voltage.

Solar panels

Solar panels, consisting of interlinked solar cells, are able to generate electricity from sunlight. To

maximise the power output high efficiency cells should be selected and the largest surface area of

panel used. The output will fall with temperature of the panel and if any of the cells are shaded or

covered by dirt.

the roof is the most suitable area where approximately 1m2 is available;

the typical maximum power output that can be expected is 150 W at a voltage of 20 V and a

current of 10 A for instance; but this is quite flexible and can be decided by how many cells

are series and parallel coupled; the voltage level is selectable and should be chosen to be less

than 60 V;

Waste heat recovery

Thermo electric devices (TEGs) are able to generate electricity from heat. Also other solutions like

steam turbines are possible to convert heat energy to electrical energy.


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DEVICE VOLTAGE CURRENT (PEAK) POWER (PEAK)

Generator (for Li-Ion battery architecture) 28 – 44 V 200 A 4 – 8 kW

Generator (for ultracapacitor architecture) 0 – <60 V 200 A 4 – 8 kW

DC/DC converter

Input: see

generator

Output: see

lead-acid

battery

See generator See generator

Waste heat recovery <60 V

(e.g. 0 – 20 V)

Power / Voltage

(e.g. 35 A) 750 W

Solar Cell <60 V

(e.g. 0 – 20 V)

Power / Voltage

(e.g. 10 A) 200 W

Table 2.4.1: Characteristics of power sources and storage elements for near-future conventional

electrical vehicle systems

Table 2.4.1 gives a summary of the characteristics identified for power sources and storage elements

of near-future conventional electrical vehicle systems. More information is to be found in chapter 3.

The next chapter describes the development of adequate power generation concepts for the different

energy sources and the development of the DC/DC converter.


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3 POWER GENERATION CONCEPTS

3.1 Generator

3.1.1 State of the art

Motivation

The present feasibility study is prompted by:

the larger electrical power demand over the entire speed range, on automobiles;

the low efficiency of electrical energy production on automobiles is no longer acceptable as

the power level increases;

the need to reduce energy (fuel) consumption and pollution from automobiles by intelligent

strategies; for instance to store and reuse vehicle braking energy.

Scope

The scope of the feasibility study is to investigate competitive electric alternators (with their control)

capable of higher power at higher efficiency for moderate cost and weight additions.

Introduction

The new electric accessories on automobiles - such as electric drives for air conditioning power

assisted steering, electrically heated catalytic converters, various ventilators, fuel pump, electric

brakes, electric throttle - require increased average (and peak) electrical power from the on board

alternator Fig 3.1.1 [7]. There is also a demand for the introduction of innovative power electronics

for all these electric loads to secure intelligent energy use. The emerging 40 V / 14 V bus is one way

to reduce power electronics costs for the growing electric power loads and enable more energy

retrieval from regenerative braking.

Fig. 3.1.1 Average automobile electrical power;

Fig. 3.1.1 shows the average automobile electrical power since 1970. But note that the average

electrical power consumption of a typical car on the NEDC is between 200 W and 350 W because

most of the electrical loads are turned OFF. Fig. 3.1.2 shows the typical (existing) Lundell alternator.

Fig. 3.1.3 gives the performance requirements and limitations of the Lundell alternator at 14 V.


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Fig. 3.1.2 Typical (existing) Lundell alternator

Fig. 3.1.3 Performance requirements and limitations of the Lundell alternator at 14 V

For the 40 V the Lundell generator with switched mode rectifier (SMR) – boost DC-DC converter

with high MOSFET or 3 MOSFET SMR within the existing power diode rectifier – is shown to

almost double the power produced at high speeds and 14 V, with 6-7 % more efficiency [9, 10], at 40

V, but also, at idle engine speed about 15 % more power is produced with the same losses

(efficiency), [9, 10]. However, the additional costs of power electronics are likely to more than double

the costs of the existing alternator to meet the scope, but with twice the power (at high speeds).

Fig. 3.1.4 The system structure/Lundell alternator with 3-MOSFET SMR


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Fig. 3.1.4 – 3.1.8 show the performance improvement of Lundell alternator with 3 MOSFET SMR

and independent control of MOSFETs:

Fig 3.1.4: The system structure;

Fig 3.1.5: The principle of 3 MOSFET SMR operation;

Fig 3.1.6: Modulation patterns for the MOSFETs;

Fig 3.1.7: Power enhancement versus speed;

Fig 3.1.8: Power enhancement and current increase at idle speed.

Fig. 3.1.5 The principle of 3 MOSFET SMR operation (Vag-phase to ground voltage)

Fig. 3.1.6 Modulation patterns for the MOSFETs

Fig. 3.1.7 Power enhancement versus speed/Lundell alternator


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Fig. 3.1.8 Power enhancement and current increase at idle speed for Lundell alternator

To summarize the 3 MOSFET SMR solution Fig. 3.1.4 – 3.1.8 [9] illustrate key results on a small

power alternator with 37sR m , 120sL H , ( ) 10,716sV RMS V , at 1 180f Hz , field

current 3.6Fi A , operating at 14 VDC power bus.

Though the power is doubled at high speeds, the increase in power at idle speed, within the

thermal limit, is still small, 10-12 %.

The same solution (SMR with 3 MOSFETs) has been applied to the permanent magnet –

reluctance synchronous machine (PM-RSM) and a 66 % increase of power at idle speed was claimed

[12] from idle engine speed. A summary of the results is shown in Fig. 3.1.9 – 3.1.12. The Figures

show the PM-RSM with 3 MOSFET SMR plus diode rectifier control as an automotive alternator:

Fig 3.1.9: The rotor topology and emf waveform;

Fig 3.1.10: Power enhancement with engine speed (3:1 transmission ratio);

Fig 3.1.11: Efficiency vs. speed;

Fig 3.1.12: Power versus id, iq currents at 1800 rpm alternator speed (600 rpm, idle engine

speed) – F is the best operation point [12]: SMR is here the single MOSFET device after the

diode rectifier.

Fig. 3.1.9 The rotor topology and emf waveform for PM-RSM


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Fig. 3.1.10 Power enhancement with engine speed (3:1 transmission ratio)/PM-RSM

Fig. 3.1.11 Efficiency vs. speed for PM-RSM

Fig. 3.1.12 Power versus id, iq currents at 1800 rpm for PM-RSM


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The results seem to suggest that the PM-RSM produces the required 66 % power enhancement from

the idle speed in contrast to the 3 MOSFET SMR Lundell alternator. However, we should mention

that the PM-RSM, even if designed with 150 % emf (at maximum speed), still remains uncontrollable

at high speeds, when emf > Vdc, when the DC-DC converter is faulty (off). So, in fact, a single switch

DC-DC converter (1 switch SMR) is required for load control (and dumping) at high speeds.

If we were to use only the single switch SMR, the maximum power at idle speed will be of 2.6 kW

(point C in Fig. 3.1.12) and not 4.5 kW as for the 3 MOSFET SMR (points G or F in Fig. 3.1.12).

Consequently the power increase at idle engine speed with single switch SMR is again only 10-15 %

as for the Lundell machine with 3 MOSFET SMR.

So in fact, to retain the large (66 %) power increase at idle speed, with IPMSM, we need the 3

MOSFET SMR, operational up to 66 % of maximum speed; above that speed load control has to be

achieved through a one switch SMR.

Fig. 3.1.13 Proposed full speed range controllable alternator system with PM-RSM

Unfortunately 4 MOSFETs are required and at high speeds most part of the short-circuit current losses

remain for zero load. The 4 MOSFET switches use simplified control circuitry and sensorless

simplified control hardware/software, but still their cost is substantial.

Other existing improvements on Lundell generator

Use thin sheet copper wire in the field coil with an on-rotor DC converter to boost the field

current to the level needed by the smaller number of turns (sheets) per coil. As the space

filling factor increases from 0.6 to 0.74, for the same rotor temperature, 10-15 % more air gap

flux density is obtainable; consequently 10-15 % more power at idle speed is expected. [11],

provided magnetic saturation is not too heavy.

Also the improvement of filling factor in the stator slots will result in lower copper losses and

better efficiency.

An initial proposal to place PMs on the ―claw poles‖ was not followed through, despite of the

potential advantages evident at low speeds mainly due to the reduction of stator inductance.

Other proposed alternator solutions:

Heteropolar hybrid (DC excited plus PMs) rotor synchronous generator [14, 15]; all

heteropolar DC. excited solutions, with PMs along same (d) axis imply a few times larger

rotor copper losses which reduce the efficiency notably though the machine reactance is


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decreased; as the Lundell machine uses 2p=12, 14, 16, 18 poles, its size is notably smaller

than all smaller number of poles heteropolar rotor synchronous generator (SG) solutions.

Increasing the number of heteropolar rotor DC excited poles leads to further rotor copper

losses per Nm of torque. So 4-6 poles should be a limit.

Induction generators with inverter-controlled parallel capacitors (or full inverter) for machine

excitation have been proposed but shown to be less practical than the Lundell machine system

in costs, with an only incremental increase in efficiency [16].

Switched reluctance generators have also been proposed for the scope but they are totally

power electronics dependent; they have been found notably costlier than the Lundell

generator system, while offering only an incremental increase in efficiency [17].

Unless the alternator is used also as a starter and torque assistant for the ICE, when a full

power PWM converter is required, we are stuck with DC excited or interclaw permanent

magnet (IPM) rotor synchronous machine (PM-RSM) with diode rectifier and switched mode

rectifier to control the alternator power output at reasonable costs.

Improving further on Lundell alternator is a first choice in this case as the technology is there

and at low cost and volume/power, burdened still by the somewhat reduced (but still up to 70

%) efficiency up to 6000 rpm alternator speed (3:1 belt transmission ratio ) in up to date

versions.

Up to date Lundell alternator performance

Fig. 3.1.14 shows an exemplary state-of-the-art Lundell alternator (Bosch H8 Li-X Series, 2.5 kW,

115/180 V at 1800/6000 rpm generator speed, 3/1 transmission, (maximum generator speed 18.000

rpm) 2p=16, air gap=0.3 mm, stack length: 0.037 m, Dis=106 mm, Dos=143.2 mm, weight: 7.1 kg)

performance with speed.

Fig. 3.1.14 Lundell alternator performance (Bosch H8-Li-x Series)

It should be noticed that the efficiency decreases drastically above alternator speed 6000 rpm and up

to 18000 rpm (6000 rpm-maximum engine speed), but it is about 70 % (max) below 6000 rpm. This is

caused mainly by large core losses due to flux density space harmonics and large stator current along

axis d to keep the unity power factor condition as imposed by the diode rectifier, and, perhaps mainly,

due to inevitably increased skin effect in the stator coils.


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3.1.2 Two selected solutions for the future

To try to meet the higher power levels required by the future automobiles, with both a 14 V and a 40

V power bus, with lower losses and reasonable initial costs (Euro/Watt) the conclusion of the

thorough investigation of the existing (Lundell) and proposed car alternator systems, that two

solutions stand out as the most competitive (and practical).

1. Hybrid (PM) claw pole (Lundell) alternator

2. PM-RSM alternator with 4 MOSFET SMR control

In view of the above, the two proposed solutions are illustrated in Fig. 3.1.15.

a) b)

Fig. 3.1.15 Proposed alternator solutions: a) Hybrid (surface PM) claw pole alternator; b) PM-RSM

(Ld/Lq>3)

16 pole hybrid (PM) Lundell alternator rotor

q=1 slot/pole/phase (48 slots), air gap: 0.3 mm

Improved fill factor stator winding

Lower core loss higher flux-density stator laminations

Increased stator slot area

PM-RSM 2p=8 poles (48 slots); air gap=0.4 mm

q=2, y/τ=5/6 which is the stator coil chording factor

2 rotor segmental skewing

Increased air gap flux density

Lower inductance;

Fig. 3.1.16 Proposed control system for hybrid (SPM) Lundell alternator


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Fig. 3.1.17 Proposed control system for PM-RSM alternator

Fig. 3.1.16 and Fig. 3.1.17 show the proposed control systems for the hybrid (PM) Lundell alternator

and for the PM-RSM alternator.

3.1.3 Characterization of the proposed solutions

The hybrid (PM) claw pole alternator

It keeps the present geometry of claw pole rotor alternator but the rotor pole surface is

machined such that the total magnetic air gap is more than doubled. The PMs placed on the

rotor poles (Fig. 3.1.15) are 0.4-0.5 mm thick (the present air gap is 0.35 mm).

It is inherent that, because the magnetic saturation influence in p. u. is reduced, the PMs could

produce up to half the maximum air gap flux density with an equivalent 50 % inductance; that

is, with about the same (or 50 %more) DC field current and mmf the same total air gap flux

density or more is produced.

For emf control up and down, the field current should be now ±, so a four quadrant chopper is

required for fast field current control. This way, when the speed increases the field current

will become smaller positive and even negative if the load decreases.

For load dumping the PM field is drastically reduced by negative field current, until the emf

becomes small enough so that the diodes in the rectifier do not conduct anymore.

With PMs on the claw poles the machine saliency decreases, which is good in terms of

magnetization requirements and stator copper losses, for almost unity power factor operation

imposed by the diode rectifier.

The PMs, glued to the rotor surface (against centrifugal and attraction forces), will put the

claw poles at a larger distance from stator (radially) and thus the claw pole eddy current

losses should be smaller.

To further decrease the copper losses in the stator, the stator winding will be made with a

larger filling factor, the slots will be larger in area by using a larger saturation (low core loss,

flux density lamination core/0.18 mm thick, Hiperco 50), and by increasing the slot area and

thus of stator outer diameter (by 12-14 mm).

The reduction of the machine inductance Ld≈Lq to about 60 % of its initial value should

allow for more DC current in the battery at idle engine speed (1800 rpm for generator speed).

A 40 % increase in current would imply a notable increase in stator copper losses unless the

stator resistance is not reduced by at least 30-40 % by methods indicated above. But at least


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40 % more power can be produced at higher efficiency and perhaps less than 25 % more

stator weight.

However, the placing of PMs on the rotor claws, the presence of a 4 quadrant DC/DC

converter to control the field current, the over-voltage protection at high speed and the PM

eddy current losses are the main disadvantages of this solution.

The PM-RSM alternator with SMR

The PM-RSM will be mostly likely a 4 (8) pole 3 flux barrier/pole, bonded NdFeB PM rotor

machine with q=2 slots/pole/phase in the stator.

To control power:

o At low speeds a 3 MOSFET SMR which acts as a voltage booster on positive current

polarity will basically provide one phase active at a time and some overlapping of

phases. The system works as a constant voltage source.

o At high speeds when the 3 MOSFETs (S1, S2, S3) SMR is idle, the single switch (S4)

SMR has to be activated to control power as a constant current source.

Though there are 4 MOSFETs, the power source for their control is unique; however

their control is independent.

o There is no need for a position sensor to control the 3 MOSFET SMR as only the zero

crossing of stator currents is required which corresponds to observing phase to

ground phase voltage polarity change for positive value on the off-state MOSFET.

Fig. 3.1.18 Control angles of 3 MOSFET SMR (phase a);Vag-phase to

ground voltage

o The angle δ is crucial for controlling the DC current; for maximum power at idle

speed δ ≈45˚, ε =5˚;

o Also, the levels of Vag (during ε) and during Φ angle (Vov) are allowing for degrees

of freedom in control. For stator Vov+Vbase ≈V and Vag≈1/2V, or so. Given the

complexity of phenomena a trial and error method will lead to a practical solution for

a given IPMA;

o For large speed the single MOSFET (S4) SMR becomes active;


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Fig. 3.1.19 Single switch (S4) SMR operation at high speeds

Fig. 3.1.20 Single switch (S4) SMR operation

Typical DC input and DC output current relationships with single switch SMR are shown in Fig.

3.1.21 for various duty cycles and speeds.

Fig. 3.1.21 DC input and output currents of single switch SMR dependency on speed and duty

cycle of S4

o The single switch (S4) SMR retains control of output current at all speeds down to zero

by 100 % duty cycle, that is short circuiting the machine. This however implies notable

losses at zero output current in contrast to the Lundell generator.

So the combined usage of 3 MOSFET (for low speed) and single MOSFET SMR (for high speeds)

seems to be required to control power along all speed range, up and down.


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3.1.4 Decision

Comparing the two proposed competitive solutions based on additional cost in power electronics (4

MOSFETs) on the part of PM-RSM alternator system and its rotor mechanical fragility above 6000

rpm, we hereby decide to go on only with the hybrid (PM) claw pole (Lundell) alternator. Even here,

after thorough modelling and design efforts we decided to drop the surface PM Lundell alternator

configuration because it needs a 4 quadrant (instead of today‘s two quadrant) chopper in the field

current control at more excitation power. We proceeded to investigate thoroughly, then the interclaw

PM rotor (IPM) Lundell alternator which needs only a 1 quadrant DC/DC converter to control the

field current, albeit at higher power(200 W, level) and which does not require special over voltage

protection at very high speed with zero field current.

3.1.5 Selected Electric Alternator: IPM- Lundell configuration

Preliminary analytical design models and dynamics and control models for the IPM Lundell

alternators have been performed. In detail this is described in D2.1.2 [20]. In this report D2.1.1 only

the results are given.

Permanent magnets are placed only between rotor claws (Fig 3.1.22c) in a, say, si-luminum enclosure,

the air-gap is increased to 0.8 mm (from 0.3 mm), the voltage is 40 V, number of turns per coil is 8.

Moreover the stator interior diameter is brought back to 106 mm (unchanged rotor of the existing 2.5

kW, 14 V alternator). Permanent magnet dimensions and characteristics:

30 mm x 10 mm x 9.7 mm

Br_20=1.13 T (remanent flux density at 20 C)

Hc_20=860000 A/m (field strength at 20 C)

Fig. 3.1.22 Permanent magnets placement options: on shaft, a); on claw poles (SPM), b); between the

rotor claws (IPM).


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The following figures show some of the characteristics.

Fig. 3.1.23 Output power of IPM Lundell alternator at 40 V (g6ipm)

Fig 3.1.23 shows the output power at 40 V over the alternator speed range.

Fig. 3.1.24 Efficiency of IPM Lundell alternator at 40 V (g6ipm)

Fig 3.1.24 shows the efficiency characteristic at 40 V over the alternator speed range.


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Fig. 3.1.25 Efficiency of IPM Lundell alternator at 40 V (g6ipm)

Fig 3.1.25 shows the efficiency characteristic at 40 V over the alternator speed range.

Out of the simulation results and investigations at this point EE-VERT favours the solution ―g6ipm‖

(project-internal identifier for IPM Lundell alternator) as it offers the multiple advantages presented

above. The machine g6ipm though with existing rotor (Dis=107 mm) but with interpole magnets and

Dos=137+14=151 mm stator outer diameter seems capable of more than 7.5 kW of power at 40 V.

Also the power at 2000 rpm is above 3 kW. The maximum efficiency is around 80 - 85 %. No over-

voltage protection for zero current excitation is necessary as the PM flux in the air-gap is very small

(see above Fig. 3.1.22 flux at zero field current). Finally, the one quadrant field circuit chopper is

maintained though at higher power. The solution g6ipm with surface PMs (SPM Lundell alternator)

also produces 8.5 kW at 42 V but with Dis=106+10=116 mm and Dos=137+14+10=161 mm, but it

requires a 4 quadrant chopper for field current bi-directional control and a dedicated over voltage

protection (at high speeds and zero field current).

The proposed alternator (IPM Lundell alternator (g6ipm)) fulfils the requirements for high efficiency

and high output power during braking phases (30% of power can be attributed to the presence of

IPMs). However additional effort will be made to further improve it during the design optimization

stage of the project .


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3.2 Solar Panels

It is proposed that solar panels, also known as photovoltaic (PV) panels that comprise a series of

linked solar cells should be used as an additional source of power for vehicles. Whilst the power

output from these panels is relatively low, typically 50-100 W for the surface areas available on the

roof of passenger cars, they provide continuous power, provided the sun is out, even when the vehicle

is turned off. This means that the panel can provide a new source of energy when the vehicle is

stationary. This power could be used to recharge the battery or combat quiescent drain over longer

periods. Additionally, the available solar power can be used to provide cabin pre-conditioning whilst

the vehicle is parked in the sun, with a subsequent reduction in AC load when the vehicle is started. It

has been calculated that the reference vehicle requires about 350 W of power for the electrical

devices. A power output of around 100 W from the PV panel is a significant proportion of this load.

A brief description of how solar panels work can be found in section 2.1.4 of deliverable D1.1.1 [1].

3.2.1 Current Vehicle Applications for PV Panels

The use of such panels in automotive applications is not new, having been offered by Mazda in the

past and by the VW group companies as an optional fit on their high-end vehicles. For instance, a

solar panel is available on the Audi A8 as an option feature at ~€1350. The main use of such panels is

to power the Heating and Ventilation (HVAC) blower whilst the vehicle is parked up in the sun. This

function operates all year round (whilst the sun is out), so in the summer cooler ambient air is used to

displace hotter cabin air, and in the winter the airflow, albeit much reduced, can help alleviate misting

issues.

Normally the PV panel is integrated into a sunroof (Fig

3.2.1). In this way the panel is mounted onto an existing

accessory feature (no additional packaging in the roof panel

keeps implementation costs low) and the sunroof retains tilt

and slide capability. However, as the panels are normally

opaque, no transmitted light is possible through the sunroof,

into the cabin, a feature that may be off-putting for some

customers. Webasto Solar are one of the leading European

suppliers (www.webasto-solar.de/en/).

Fig. 3.2.1

Audi include the following in their glossary explanation of their solar powered sunroof;-

Even in very low sunlight, light-sensitive elements under the glass sunroof panel produce electricity to

power the ventilator inside the vehicle. Even when the ignition is switched off, the interior will be

supplied with a continuous flow of fresh air and temperature levels inside the vehicle can be reduced

by as much as 20°C with the outside air that is cleaned as is passes through the dust and pollen filter.

This kind of ventilation does not put any additional demands on the car‘s battery. This preliminary

cooling lets the air conditioner cool the interior to the desired temperature with little energy and use of

the ventilator.

http://www.webasto-solar.de/en


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More recently, Toyota has announced that the Prius hybrid will also be available with a Kyocera solar

panel roof option. These panels are not part of a sunroof but part of the main roof section.

3.2.2 Solar Panel Performance

The ability of a given panel to generate electricity is a combination of many factors but principally:-

Size of panel

Amount of solar irradiance in the physical location

Topography of terrain

Type of solar cells

System losses

Panel Size

The size of panel is dependant on a number of factors that include:-

Type (size) of cell and number of cells required (total peak power output expected)

Cost of panel vs. price customer is expected to pay

Size of roof space available

Topography of roof (although some curvature of the thin silicon cells can be accommodated)

Solar Irradiance within Europe

Broadly the power available from the sun is related to the latitude, with higher latitudes having less

ability to generate photovoltaic electricity than lower latitudes. The European Community covers a

broad range of latitudes, from Sweden and Norway in the north to Spain and Greece in the south. Fig.

3.2.2 shows a map of potential electricity generation from a 1 kW(peak) panel. It assumes a horizontal

panel and a performance ratio of 0.75. Performance ratio is a quantity, defined namely by the

European Communities (JRC/Ispra), which represents the ratio of the effective energy produced

compared with the energy which would be produced by a "perfect" system. The performance ratio

includes the array losses such as shading, PV conversion, mismatch, wiring, and the system losses

including inverter efficiency and storage/battery.


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PVGIS © European Communities, 2001-2008

Fig. 3.2.2 Map of potential solar power output

The data is taken from the PVGIS1 website (http://re.jrc.ec.europa.eu/pvgis/index.htm). The

background database represents the period 1981-1990, and has been computed by interpolation and

modelling of 580 meteorological measurements over Europe. In order to understand the year round

application of PV panels, the PVGIS data has been examined in more detail for the four European

cities shown in Table 3.2.1:-

Latitude Longitude (E) Hy, horizontal (kWh)

Göteborg, SE 57°41‘48‖ 11°59‘12‖ 917

Graz, AT 47°04‘06‖ 15°26‘30‖ 1154

Tarragona, SP 41°07‘06‖ 1°14‘42‖ 1493

Athens, GR 37°58‘48‖ 23°43‘00‖ 1600

Table 3.2.1 Irradiation data for 4 European cities

It can be seen that for the yearly Irradiance totals, there is ~75 % improvement for Athens compared

with Göteborg. How these yearly totals break down into monthly solar irradiance figures are shown in

Fig. 3.2.3 below. As expected, the most southerly city, Athens, has the highest peak value although

the winter months are very similar to Tarragona. Graz and Göteborg have similar summer peaks that

are around 75 % the Athens value. However, the minimum available solar energy in winter falls to

1 Photovoltaic Geographical Information System

http://re.jrc.ec.europa.eu/pvgis/index.htm


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20% of the Athens value. This is probably due to the differences in daylight between the two cities. In

summer, the sun is less intense in the northern latitudes, but the length of the solar day is

approximately 2 hours longer in Göteborg than it is in Graz – in winter the solar days are very short in

northern latitudes.

Also shown in the graph are the mean maximum monthly temperatures (for 2008) which, when

combined with the level of solar activity, are suggestive of overall cooling demand.

0

10

20

30

40

50

60

70

80

90

100

0

50

100

150

200

250

Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec

Ave

rage

Te

mp

era

ture

-°C

Sola

r Ir

rad

ian

ce -

kWh

/Mo

nth

Monthly Solar Irradiance and Average Monthly Maximum Temperatures

Goeteborg, Hy

Graz, Hy

Athens, Hy

Tarragona - Hy

Goeteborg, °C

Graz, °C

Tarragona, °C

Athens, °C

Fig. 3.2.3 Monthly solar irradiance and average monthly maximum temperatures

Local topography – The effect of shading

Fig. 3.2.4 The effect of shading on solar panels

Clearly the geographic location of the panel has a large effect on performance but this is also affected

by the local topography. For instance, adjacent mountains will have a shadowing effect when the sun

is at low inclination. Additionally, using the panel in built-up areas, especially larger cities, will have

a negative affect, again due to the shadowing influence of near-by buildings.


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PV modules are very sensitive to shading. Unlike a solar thermal panel which can tolerate some

shading, many brands of PV modules cannot even be shaded by the branch of a leafless tree.

Fig. 3.2.5 The effect of shading – reduction of PV module power

Shading obstructions can be defined as soft or hard sources. If a tree branch, roof vent, chimney or

other item is shading from a distance, the shadow is diffuse or dispersed. These soft sources

significantly reduce the amount of light reaching the cell(s) of a module. Hard sources are defined as

those that stop light from reaching the cell(s), such as a blanket, tree branch, bird dropping, or the like,

sitting directly on top of the glass. If even one full cell is hard shaded the current of that module will

drop to half of its unshaded value. If one cell in a series connection is shaded, the output current of the

panel will drop significantly. The weakest cell defines the panel‘s output current. The current, which

is generated by the other not shaded cells, is dissipated in the weak cell – cell gets hot! The output

voltage is affected only by temperature. To get rid of the pure series connection, bypass diodes are

used. If enough cells are hard shaded, the module will not convert any energy and will, in fact,

become a tiny drain of energy on the entire system.

Partial-shading even one cell of a 36-cell module for example, will significantly reduce its power

output. Because all cells are connected in a series string, the weakest cell will bring the others down to

its reduced power level. Therefore, whether ½ of one cell is shaded, or ½ a row of cells is shaded as

shown above (Fig. 3.2.5), the power decrease will be the same and in this case 50 %.

When a full cell is shaded, it can act as a consumer of energy produced by the remainder of the cells,

and the module is protected through bypassing diodes. The module will route the power around that

series string. If even one full cell in a series string is shaded (Fig. 3.2.5) it will likely cause the module

to reduce its power level to ½ of its full available value. If a row of cells at the bottom of a module is

fully shaded the power output may drop to zero. The best way to avoid a drop in output power is to

avoid shading whenever possible though, in practice, bypass diodes should be implemented.

PV panel system losses

Although a solar conversion efficiency of ~30 % is theoretically possible for a silicon based solar cell,

the actual efficiency in cells found on the market is around 15-23.5 %.


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The graph below (Fig. 3.2.6) shows the expected monthly energy output (Ey, horizontal kWh) for a 100

W2 panel operating at 14 % efficiency for the four European cities cited in Fig 3.2.6. The large

difference between the energy that is incident in these locations compared with what can be

realistically converted into electricity using PV panel is clear.

0

25

50

75

100

125

150

175

200

225

Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec

kWh

/Mo

nth

Solar Irradiance Available vs Estimated 100W PV Panel Output

Goeteborg, Hy

Goeteborg, Ey

Graz, Hy

Graz, Ey

Athens, Hy

Athens, Ey

Tarragona - Hy

Tarragona, Ey

Fig. 3.2.6 There are a number of factors that impact panel efficiency:-

Inclination angle

Ideally a PV panel would be mounted at an inclination angle that maximises the panels surface

exposure to the sun. This is a geometric effect and will vary with latitude. For instance, the optimum

angle in Göeteborg is 38°, whilst in Athens it is 30°C. Clearly, for automotive applications, the panel

has to be horizontally mounted, ideally on the roof to minimise shading effects.

Typically roof mounting accounts for around a 14% decrease in performance compared with

mounting at the optimum angle.

Thermal effects

From section 3.2 it can be seen that there is more solar energy at lower latitudes. At lower latitudes

the temperatures are also warmer and thus the demand for cooling is greater. Unfortunately, PV

panels become less efficient as they get hotter (panels generate electricity from light, not heat). But

when a vehicle is moving the wind helps cooling the panel. Using local ambient data, the graph below

shows how the efficiency of a PV panels differs for various European locations. Thermal effects differ

from 6% loss to 9% loss between the latitude extremes.

Reflectance effects

2 A 100 W panel, at ~1m

2 is the maximum size that could be expected on a vehicle


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Not all the light energy incident on a solar cell is absorbed, some is reflected away. Whilst anti-

reflective coatings can minimise these losses, there is a geometric factor that increases or decreases

reflective losses, these being slightly worse at higher latitudes. The graph below (Fig. 3.2.7) shows,

for a given anti-reflective coating, how the panel losses are effected by latitude. Older anti-reflective

coatings worked for only one specific wavelength. More recent developments work for a range of

wavelengths, greatly minimising these angular losses. Note that some of the most efficient anti-

reflective coatings may not be acceptable from a customer aesthetics view point.

0

10

20

30

40

50

60

70

0

1

2

3

4

5

6

7

8

9

10

Goeteborg Nuneaton Graz Turin Tarragona Athens

Lati

tud

e -

°

Loss

-%

Town

PV Panel Losses as a Function of Latitude

Loss due to Temp

Loss due to Ang Reflect

Latitude

Fig. 3.2.7 PV panel losses as a function of latitude

Other losses

Other losses between the panel and the load are primarily due to the voltage converter that is often

needed (see section 4) and cabling. These losses can account for additional losses in the order of 10 %

in power output. The output of the PV panel is proportional to the solar irradiance, so cloud cover will

reduce the system output. Typically, the output of any solar module is reduced to 5 to 20 % of its full

sun output when it operates under cloudy conditions.

Total system losses

Taking the various losses above into account, the overall system losses of a typical PV panel under

full sunlight are shown in the graph below. It shows that actual losses tend to be fairly flat, primarily

as the increase in thermal losses with decreasing latitude is counteracted by the lower angular

reflectance losses.


Version 1.0 Page 35

0

10

20

30

40

50

60

70

0

10

20

30

40

50

60

70

Goeteborg Nuneaton Graz Turin Tarragona Athens

Lati

tud

e -

°

Loss

-%

Town

Overall PV Panel Losses as a Function of Latitude

Overall System Losses

Latitude

Fig. 3.2.8 Overall PV panel losses as a function of latitude

Conditions where little or no current is generated

There are several environmental conditions under which the PV panel would not generate a

meaningful amount of current.

Night time

Excessive cloud

Day time, northern latitudes in winter

Snow/ice build-up

Parked in garage, covered car park etc

Heavily shaded areas (e.g. adjacent to large buildings)

3.2.3 Types of solar cell

Table 3.2.2, taken from Section 2.1.4 of [1], summarises the main types of solar panels available.

Table 3.2.2 Types of solar panels

Thin film solar cells can be made transparent, overcoming some of the obscuration issues associated

with crystalline silicon used in sun roof applications. Unfortunately the efficiencies are quite low at


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less than 10 %. Because of the limited surface on vehicles high cell efficiencies are essential.

Consequently monocrystalline or poly-crystalline cells are the best choice for passenger car

applications.

Typical cell sizes are as follows;-

MCSi 100x100 mm 1.5 Wp ~16 % efficient

MCSi 125x125 mm 3.24 Wp ~23.5 % efficient

PCSi 156x156 mm ~3.75 Wp ~14-16.5 % efficient

MCSI 156x156 mm ~3.89 Wp ~15-17 % efficient

3.2.4 Panel Electrical Characteristics

Typical characteristics of a crystalline silicon solar panel are shown in Fig. 3.2.9. The graph

demonstrates how the DC characteristics change as a function of solar irradiance and with

temperature. Increases in cell temperature increase current slightly, but drastically decrease voltage.

Maximum power is derived at the knee of the curve.

Fig. 3.2.9 PV panel I/V characteristic for different temperatures and irradiances

Battery Interaction

Solar panels are often used to provide trickle charging for batteries, normally at 12 V or 24 V. For

these applications, additional precautions are required. For instance, for a 12 V lead-acid battery the

following is required

DC/DC conversion: voltage step down from >17 V to 14 V

Regulation: This is required to ensure efficient and safe battery charging

Reverse current protection: If a solar panel is connected directly to a battery it will charge

whilst the sun is out but once the sun goes in the battery can then discharge back through the panel. A blocking diode could be used to prevent this.

Load control: to prevent battery overcharging

Temperature compensation: the charging voltage is adjusted according to the temperature


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A typical charging algorithm for instance for a 12 V lead-acid battery would be similar to the

following Fig. 3.2.10.

Fig. 3.2.10 Typical charging algorithm

Clearly at night or under cloudy conditions no charging is available. The first stage is the Bulk

Charge, where current is sent at a maximum safe rate until the battery has been nearly fully charged

(about 80 % - 90 % of maximum). The charger then switches to the Absorption Charge, where the

voltage is constant and the current reduces gradually according to the resistance of the battery as it

become charged. Although the current drops, the charger produces maximum voltage during

absorption charging, at around 14 V.

The third and final stage in the charging process is the Float Charge, where the charging voltage is

reduced to around 12.8 to 13.2 volts, often called the trickle charge. At this voltage the emission of

hydrogen gas is maintained at a minimum for safety reasons. The purpose of this stage is to maintain

the battery at full charge until it is needed for use.

Some regulators use Pulse Width Modulation as a more efficient charging method so as to maintain

the battery at its maximum state of charge whilst minimising adverse sulfation by pulsing the battery

charging voltage at high frequency.

Maximum Power Point Tracking (MPPT)

MPPT is, when by certain means the load to the panel is continuously adjusted to match its internal

impedance (which changes with irradiance), in order to always achieve proper impedance matching.

This impedance transformation can be achieved by integrating a converter, with special control for

MPPT.

It is clear from the Volts/Amps characteristics shown in Fig 3.2.11 that a number of factors affect the

output from the panel. Having a ‗standard, DC/DC converter with a device with such a characteristic

means that some typical output has to be chosen and deviation away from this will lead to losses in

efficiency.


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Cu

rre

nt

Voltage10V 15V ~17V

BatteryVoltageRange

MaximumPowerPoint

Fig. 3.2.11 Volts/Amps characteristics and MPPT

The diagram shows how MPPT can overcome this. The battery typically operates at 10-15 V worst

case, Vmp is of the order 17 V for Pmax in a typical solar panel configuration. The MPP changes, as the

cell/panel output current reduces with irradiance, and the cell/panel voltage reduces with temperature.

MPPT assures by means of impedance matching, that maximum available power is transferred to the

load. At the same time, the voltage/current provided to the battery can be adjusted to some extent.

Keeping the voltage of the panel at a FIXED voltage, reduces the freedom for MPPT – a properly

MPP tracked PV panel will behave as a power source, with voltage limit. Peak efficiencies of ~97 %

are achievable when using such tracking algorithms.

To date, a number of MPPT algorithms have been proposed in the literature [21], including perturb-

and observe method, open- and short-circuit method, incremental conductance algorithm, fuzzy logic

and artificial neural network. Among these, three are being considered to be implemented in EE-

VERT: perturb-and observe method, open- and short-circuit method and incremental conductance

algorithm. The other controls need complex algorithms that lead to high cost of implementation.

Perturb-and-observe (P&O) method is dominantly used in practical PV systems for the MPPT

control due to its simple implementation, high reliability, and tracking efficiency. The perturbation in

the PV output power is accomplished by periodically changing (either increasing or decreasing) the

reference current, with a small amount. Fig. 3.2.12 illustrates the flow chart of the MPPT algorithm.


Version 1.0 Page 39

Fig. 3.2.12 Flow chart of the perturb-and-observe (P&O) MPPT algorithm [22]

From the plot of PPV versus VPV, two possible operating regions, A and B, can be defined (Fig.

3.2.13). The current operating point location can be determined by a perturbation in the PV output

power. For instance, if the PV controller increases the reference for the converter output power by a

small amount, and then detects the actual output power. If the output power is indeed increased, it will

increase again until the output power starts to decrease, at which the controller decreases the reference

as shown in Fig. 3.2.13.

.

Fig. 3.2.13 PV output power curve with respect to the PV output voltage [22]


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The open- and short-circuit current method for MPPT control is based on measured terminal

voltage and current of PV arrays. By measuring the open-circuit voltage or short circuit current in

real-time, the maximum power point of the PV array can be estimated with the predefined PV current-

voltage curves. This method features a relatively fast response, and do not cause oscillations in steady

state. However, this method cannot always produce the maximum power available from PV arrays

due to the use of the predefined PV curves that often cannot effectively reflect the real-time situation

due to PV non-linear characteristics and weather conditions. Also, the online measurement of open-

circuit voltage or short-circuit current causes a reduction in solar panel output power.

The main task of the incremental conductance algorithm is to find the derivative of PV output

power with respect to its output voltage that is dP/dV. The maximum PV output power can be

achieved when its dP/dV approaches zero. The controller calculates dP/dV based on measured PV

incremental output power and voltage. If dP/dV is not close to zero, the controller will adjust the PV

voltage step by step until dP/dV approaches zero, at which the PV array reaches its maximum output.

The main advantage of this algorithm over the P&O method is its fast power tracking process.

However, it has the disadvantage of possible output instability due to the use of a derivative

algorithm. Also, when insulation is weak, the differentiation process is noisy and the algorithm has a

poor performance.

3.2.5 Possible Sizes of PV Panel on the Alfa Romeo 159 Reference Car

The available roof space on the Alfa Romeo 159 is shown as the shaded area in the picture Fig 3.2.14.

This equates to a size approximately 1277x907 mm (or 1.16 m2).

Fig. 3.2.14 Available roof space on the Alfa Romeo 159

Given the possible cell sizes listed in Section 4, the options in Table 3.2.3 are available with this open

area, assuming that the full roof curvature can be accommodated by any particular panel:-

Cell

Type

Cell size

(mm x mm)

Number of

cells (Length)

Number of

cells (Width)

Total

Cells

Potential

Peak Power

(Wp)

~Cell Cost

(€)*

MCSI 100 x100 12 8 96 144 310

MCSI 125 x 125 9 7 63 204 440

PCSi 156 x 156 8 5 40 148 295

MCSI 156 x 156 8 5 40 156 340

Table 3.2.3 Options with given cell sizes

* Approximate cost for the cells (not ‗made up‘ panel)


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It can be seen from a power production point of view that the high efficiency mono-crystalline cell

offers the best prospect, with some 200 W(peak) being theoretically available. However, from the

discussions on system losses covered in Section 4, ~150 W(peak) delivered to the point of need is more

realistic. Note, at 14.0 V, this equates to 10.7 A.

3.2.6 Possible use of PV panels within EE-VERT

There are several benefits that a solar cell would offer to an overall vehicle energy management

strategy.

Vehicle Off - Battery: Low State of Charge

If the vehicle is switched off after heavy use of the battery the battery will be in a state of low charge.

Combined with quiescent drain, after a period of time parked up, this may cause subsequent starting

issues for the customer. Clearly use of a solar panel, given the right environmental conditions, could

go some, or all, the way in overcoming this issue. Consequently, it would be prudent for any PV panel

implementation strategy to first examine the battery state of charge, and if low, direct all power to

recharging prior to any other activity, such as cabin cooling.

Vehicle Off - Battery: Quiescent Drain

Even with the vehicle switched off, there are sufficient electrical loads to draw a small, continuous

current, from the battery. This is of the order 0 - 10 mA but can vary from vehicle to vehicle. Over a

long period of time (e.g. parking at an airport whilst on holiday) can lead to a discharged battery.

Consequently, it would be prudent for any PV panel implementation strategy to include some small

part of the available current to be directed to the battery to overcome quiescent drain issues.

Vehicle On – Lower Alternator Load

With the vehicle switched on, ~13.7 A is available to the electrical circuit that would otherwise have

to be generated by the engine via the alternator.

Vehicle Off – Cabin-Preconditioning

Most OEMs offering PV panels as an option use the current generated when the vehicle is powered

off to ventilate the interior cabin using the HVAC blower. This offers a number of benefits. Even for

brief periods parked up in the sun, vehicle interiors can become uncomfortably hot. With a high solar

load, the panel will also be generating a high current and thus interior cooling will be maximised, with

cooler exterior air drawn in and vented back out. The added benefit is that as the existing ducting is

used, the very hot air that would otherwise accumulate in the ducting whilst the vehicle is parked is

also expelled. Thus when the occupant turns on the HVAC upon entering, cooler air is immediately

available, both cabin ambient and that expelled from the ducting, rather than the slug of very hot air

normally experienced.

If sufficient pre-cooling is available, then the A/C system will have to do less work as the cabin air

and interior structure has already been pre-cooled. This could lead to a reduction in the maximum

specified load for the A/C system and the possibility of down-sizing of the compressor.

Alfa Romeo 159 – Expected Airflow

The airflow versus current graph for the reference vehicle is shown in Fig. 3.2.15.


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0

20

40

60

80

100

120

140

0 5 10 15 20 25 30

Air

Flo

w -

l/s

Current - A

Alfa 159 HeVAC Performance - Vent Flow vs Blower Current

Flow (Vent)

+ Blower Setting 1

+ Blower Setting 2

+ Blower Setting 3

+ Blower Setting 4

+ Blower Setting 5

+ Blower Setting 6

Fig. 3.2.15 Reference car HVAC performance

Also indicated are the approximate blower settings on the control panel. From the possible peak

output from a full roof panel described in Section 7, it can be seen that if 150 Wpeak can be generated,

then 10.7 A corresponds roughly to blower setting between 3 and 4 and ~70 l/s airflow (peak). It is

expected that ambient outside are drawn into the vehicle at this rate would have a significant impact

on cabin pre-cooling and hence the maximum load on the A/C system that occurs at start-up.

Impact on HVAC Strategy

The introduction of a cabin cooling approach will have an impact on how HVAC strategy is

implemented on the vehicle in the following ways:

on key-off, outside air must be selected (cabin cooling will be meaningless if re-

circulating air is selected!);

on key off, the HVAC sets face vent mode;

the HVAC blower bearings will have to be upgraded to take into account continuous

running ( unfortunately this will incur additional cost);

ideally a blower with a brushless motor would be used as this is more efficient than a

brushed one (again at additional cost).

3.2.7 Energy Saving Calculation

Reduced Alternator Loading

With the vehicle running, power delivered from the PV panel is power that is not generated by the

engine via the alternator. As an approximation, it has been assumed that for all vehicles, 0.1 litre of

fuel is saved for every 100 W the alternator is saved from generating.


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159 3.2 V6

Gasoline

159 1.9l

Diesel

Fuel consumption (l/100 km) 12.2 5.9

Peak solar power (W) 150 150

Fuel saving, (l/100 W) 0.1 0.1

Fuel saving (l) 0.150 0.150

~Fuel saving, % decrease (peak) 1.2 2.5

Table 3.2.4 Potential fuel saving possible with a PV panel

Table 3.2.4 shows that a solar panel could make a significant contribution to reducing fuel

consumption of up to 2.5 % for the reference vehicle during normal driving.

Reduced AC Demand

The AC system is the largest ancillary load on the engine which is seen as an increase in fuel

consumption for the customer. For a typical EU vehicle, fuel consumption increases by 10 % when

AC is used [5] and represents some 3.2 % of total fuel used by the European automotive fleet. It was

also shown that reducing AC power load by 30 % could save some 2.5 billion litres per annum within

Europe.

The effect of cabin pre-cooling on reduced energy demand has been studied in depth by a number of

organisations, particularly the National Renewable Energy Laboratory (NREL) in the US. They have

investigated panel ventilation as part of a wider AC load reduction strategy. It was shown [6] that

running the blower in a Jeep Grand Cherokee with 81 watts of energy reduced the interior temperature

by 6.9°C (by comparison, just having the sunroof open 6cm reduced the temperature by only 2°C. A

similar study conducted on a Cadillac STS with just a 17 W panel was able to reduce interior

temperatures by 5.6°C and seat temperatures by 5-6°C. As part of a wider load reduction strategy

(solar reflective glazing and IR reflective paints) they were able to show that A/C load could be

reduced by 30 %, representing ~26 % improvement in fuel used for the A/C.


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3.3 Waste heat recovery

3.3.1 Technology overview

It is proposed that a heat recovery device should be used as an additional source of power for vehicles.

Around 30 % of the fuel potential energy is transformed into available mechanical energy in a

conventional engine. Most of the potential energy is transformed into thermal energy (Fig.

1.1.1).Different technologies are available to recover the wasted thermal energy. The thermal energy

may be converted to mechanical, thermal or to electrical energy. The focus of EE-VERT is mainly on

thermoelectric conversion and the application of available technology solutions to evaluate the

effective fuel saving.

Thermal to thermal

Engine warm-up can be sped up adding a heat exchanger between exhaust gas and engine water

temperature, transferring heat from the gas to the water. Obviously the engine friction is mainly

related to the oil temperature; so the heat from exhaust gas must reach the engine oil through water.

When the engine is hot, the heat exchanger is bypassed and exhaust gas reaches the muffle via the

traditional direct pipe.

Improved engine warm-up on NEDC cycle means improved engine combustion and less emission

during the cycle. The difference between cold and hot NEDC cycle in terms of fuel consumption is

about 10 %. A fuel reduction may be expected if during the initial part of cold homologation NEDC

(600 s), engine warm-up is sped up.

This kind of device may also speed up cabin heating at cold start if heated water is also routed to the

cabin heater. This also gives a relevant contribution to the vehicle comfort.

Heat transfer may vary according to:

o exhaust gas temperature

o heat exchanger efficiency.

Typically a high exhaust gas temperature is required (500-600 °C), so the heat exchanger must be

placed just after the catalyst devices with a proper exhaust pipe layout. If temperature is too low, the

heat exchanger efficiency is reduced.

Moreover the heat exchanger must not be a source of excessive pressure drop in the exhaust pipe;

otherwise engine performance and efficiency may be greatly affected. Greater pressure drop means

higher engine pumping losses.

It must be noted that such kind of device for fuel saving just works when the engine is cold. As soon

as the engine is hot, the bypass circuit is activated. The device can not be exploited during the

remaining part of the mission.


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Fig. 3.3.1 Fast warm up architecture

Thermal to mechanical

A significant reduction of the fuel consumption may be reached through the recuperation of the

engine waste heat by means of a Rankine bottom cycle. The system produces mechanical power and

is based on the principle of the steam engine: a fluid is heated by the exhaust gas to form steam which

is conducted into an expansion device producing mechanical power that can be used to assist the main

engine or generate electrical power for hybrid vehicle or to drive auxiliaries.

The following scheme in Fig 3.3.2 shows the layout of a typical Rankine cycle applied to the engine

exhaust gas. A system based on a Rankine cycle is made of four elements: a pump, a steam generator,

an expander (in this case a turbogenerator with electric generator), a condenser.

Water 1 bar @ 85°C

Steam

Engine

Steam

generator

Exhaust gas

Water cooled

Condenser

Steam 100 bar @ 450°C

Steam 1 bar @ 100°C

Water 100 bar @ 100°C

V1

V2

Fig. 3.3.2 Typical Rankine cycle applied to engine exhaust gas


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A thermodynamic analysis must be conducted for each component of the Rankine bottom cycle to

find out the optimum thermodynamic working points, i.e. to find out the best values for the minimum

pressure, the maximum pressure and the superheating temperature of the cycle.

The ideal thermodynamic efficiency of the cycle depends on the maximum pressure and temperature

of the cycle. The following graph Fig 3.3.3 shows the thermodynamic efficiency of a superheated

steam cycle, without extractions or reheating, as a function of the maximum pressure; for the

calculation, a minimum pressure of 1 bar and a superheating temperature of 450°C have been

assumed.

-

0,05

0,10

0,15

0,20

0,25

0,30

0,35

0,40

0 20 40 60 80 100 120 140

Pmax [bar]

id

Pmin=1bar, Tmax=450°C

Fig 3.3.3 Isentropic efficiency of steam Rankine cycle as function of cycle maximum pressure

As is well known, efficiency increases when maximum pressure increases, as the ideal work also

increases.

Another way to recover thermal energy to electric energy via first mechanical energy is the

application of a turbogenerator integrated in the gas exhaust pipe. For example such a device may

exploit a water cooled switched reluctance generator for instance coupled to an exhaust gas driven

turbine. It is capable of operating in exhaust temperatures, at high speeds, delivering a typical shaft

power of 6kW. Such kind of system is in development among vehicle suppliers: the TIGERS system

is shown in Fig. 3.3.4.


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Fig. 3.3.4 Turbogenerator with switched reluctance integrated generator

Thermal to electric (thermoelectric)

Thermoelectric conversion allows direct energy conversion from thermal to electric, being a suitable

heat recovery technology for the EE-VERT concept. Converted electrical energy may be managed

within the EE-VERT powernet concept: energy may be stored or used for low voltage or high voltage

loads through the DC/DC converter.

Thermoelectric technology is based on the well-known Seebeck effect. The basic element is the

thermoelectric cell (Fig 3.3.5). Typically efficiency ranges from 4 % to 13 %. Low efficiency is not a

big weakness since the wasted heat energy is in the order of several kW and a low efficiency would

therefore still generate considerable electric power. Cell types are developed for low temperature or

high temperature operation.

A typical cell performance (WATRONIX source) is shown in Table 3.3.1. Depending on temperature

difference between the sides, a certain cell voltage and available current is produced. To produce a

meaningful electric power several cells must be used. An output power target may be about 150 W. In

such case, taking into account an average cell efficiency of 10%, 1.5 kW thermal power has to be

available.

Only part of the thermal power is converted into electric power (150 W); the remainder must be

dissipated (about 1.35 kW). In such a device the main problem is to assure proper heat dissipation,

proper cooling of cold cell side and proper temperature difference between cell sides. Otherwise

efficiency can fall drastically. If auxiliaries are to be introduced to manage the TEG (Thermo Electric

Generator) (like water circuit, water pumps, …), their contribution must be taken into account for the

overall electric energy balance.

Fig. 3.3.5 Low temperature cell


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COLD HOT dT U (V) I (A) Q (W)

30 60 30 0.77 0.44 0.34

30 80 50 1.29 0.75 0.97

30 100 70 1.8 1.01 1.82

30 120 90 2.31 1.24 2.86

30 140 110 2.82 1.46 4.12

30 160 130 3.31 1.66 5.49

30 180 150 3.8 1.84 6.99

30 200 170 4.27 2 8.54

Table 3.3.1 TEG typical performance

Conclusion

Among the described technologies to recover waste heat energy, TEG is recommended be considered

in the new EE-VERT approach to the electric power net, since being a source of electric energy

always available during the whole vehicle mission. A motivation to this conclusion is given below.

Thermal to thermal conversion has no impact on the electric powernet and has a limited impact on

vehicle mission (cold start phase); nevertheless it must be noted that it has a potential in fuel saving.

Thermal to mechanical conversion has no impact on electric powernet. A further conversion stage

must be installed (electric generator) to produce electric energy, lowering overall efficiency. The

development of a Rankine cycle has already been addressed in other EU projects. The integration of

such a source of energy with an electric generator, has to be considered in the overall electric energy

management. A turbogenerator with integrated electric generator is a promising solution, though

impact on engine management must be verified.

It must be noted that all the above mentioned systems affect the pressure drop along the exhaust pipe.

A solution in the design process must be found which limits the pressure drop and typically must

include an electronically controlled full flow by-pass on the exhaust pipe that ensures the desired

proportion of exhaust gas is delivered to the heat recovery device as determined by the control system.

Development of new devices for thermal energy recovery is out of scope of EE-VERT. The scope of

EE-VERT is the integration and management on the vehicle powernet of new existing promising

electric generation solutions to check their contribution during NEDC and real driving conditions.

3.3.2 On vehicle heat recovery management

This paragraph describes the best way to manage heat recovery, regardless of the kind of technology

used for the conversion.

On a vehicle there are two main sources of thermal energy: engine water and engine exhaust gas. The

overall amount of energy and the allocation between them depend on engine type (gasoline, diesel,

methane,…) and engine calibration.

During engine warm-up it is better to avoid to taking heat from the engine water. Engine warm-up

must be as fast as possible to reach best thermal engine working condition regarding fuel consumption

and emissions. This condition is reached when the engine water temperature is above approximately

80°C. When the engine has reached its best working conditions, heat can be taken from the water with


Version 1.0 Page 49

no relevant effects on fuel consumption and emissions performance. Obviously not so much heat so

that the engine water temperature is lowered too much. State of the art engine thermal management

does not need engine fan ventilation during the NEDC. Engine thermal management is optimized to

get the engine warm as fast as possible and to not produce so much heat that engine fan engagement is

needed. In short, heat can be taken from water in order to avoid engine fan engagement.

So during the NEDC cycle a thermoelectric device placed on the engine water circuit will recover

heat for a really small part of the cycle.

Moreover the engine water circuit heat is used for cabin heating during the cold season. Cabin

comfort may be affected if during cold start heat is not available as soon as possible to the cabin.

Exhaust gas heat energy is indeed really wasted energy and can be used as soon as it is available. The

only recommendation is to avoid taking heat from the exhaust pipe before the precatalyst and catalyst

converter: lowering the exhaust gas temperature there may lead to lower catalyst efficiency. Heat

must be taken before the muffle.

In case of thermal recovery from water, heat is available at about 90°C. In case of thermal recovery

from the exhaust gas, heat is available at 300-600 °C. It is clear that having available heat at higher

temperature is a more suitable source for energy conversion.

3.3.3 Current vehicle applications for TEG

Fig. 3.3.6 and Fig. 3.3.7 show vehicle applications in development. Systems under development claim

interesting figures of electric power production (up to 600 W) but just in well defined vehicle

conditions. The contribution on NEDC cycle and in real use must be assessed.

BMW Thermoelectric exhaust gases recovery system Hi-Z 1kW generator for class 8 trucks

Amerigon CCSTM Vehicle Seat ApplicationGM design for TE heat recovery

Fig. 3.3.6 Ongoing development


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Fig. 3.3.7 BMW and DLR (Deutsches Zentrum für Luft- und Raumfahrt) thermal-electric prototype

BSST has started to develop a high efficiency thermoelectric waste energy recovery system for

passenger vehicle applications in November 2004 under a contract awarded by the U.S. Department

of Energy Freedom Car Office. The goal of the effort is to reduce fuel consumption by converting

exhaust gases into electricity using a Thermoelectric Generator Module (TGM). This project has

presented prototype simulation results, see Fig. 3.3.8. Depending on engine load, different amounts of

recovered electrical energy are available. 500 W are available at 130 km/h. NEDC speed is mainly

around 50 km/h with an output power of 120 W; such output is still interesting compared to the

electrical load request of the vehicle (about 240 W), allowing some fuel saving. A fuel economy of 1

% is claimed over the NEDC cycle by BSST. The system has been installed on a BMW vehicle as

shown in Fig. 3.3.6 for testing.

Fig. 3.3.8 BSST 500 Watt TEG module simulation


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On trucks the waste heat amount is larger than on a passenger car and recuperation can reach high

values. In Fig. 3.3.9 a MACK application is shown with a claimed nominal electric generation of 1

kW.

Fig. 3.3.9 1 kW TEG for trucks

Thermoelectric devices are also used to exploit the reverse principle: producing cold or warm air

supplying electric energy to the cell, as shown in the Amerigon seat application (Fig. 3.3.6). Such

type of application may be greatly exploited in hybrid and full electric vehicles, where no heat is

available from an internal combustion engine.

3.3.4 TEG Performance

The ability of a given TEG to generate electricity is a combination of many factors but principally:

Number of cells

Output voltage regulation

Amount of available heat

Cell side temperature control and heat dissipation

Type of cell

Number of cells

Typically TEG cells are placed around the external side of the exhaust pipe in a ring configuration

(Fig. 3.3.10), assuring the same temperature for all the cells. The number of cells depends on the

required power output. State of the art shows a sustainable trade-off of performance versus cost with a

power output of at least 200 W. Such power output requires about 30-50 cells. A suitable cell package

must be designed.


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Cells

insulator

Heatsink

Fig. 3.3.10 Cell package layout

Output voltage regulation

The number of cells in series determines the output voltage of the TEG. A DC/DC or active

electronic device is always needed for connection of the TEG to the vehicle powernet. The voltage

regulation is similar to the regulation for solar panels with maximum power point tracking (MPPT),

see section 3.2.4 for more information about MPPT.

Amount of available heat

There is typically lots of heat power available on the vehicle exhaust pipe. The amount may vary from

3 kW (engine idle) up to 100 kW at full load on a medium passenger car. On a truck it may be as

much as 450 kW. It is costly to design a device for optimised operation over the whole of such a wide

power range. Therefore, the TEG must be designed to control the heat flow through the section of the

pipe fitted with cells. A suitable by-pass for the exhaust gas must be available to control heat flow and

assure proper operation in the desired range (typical 5-30 kW), as shown in a prototype installation in

Fig. 3.3.11.

Fig. 3.3.11 Installation of a prototype TEG

Cell side temperature control and heat dissipation

A cell can be irreparably damaged from over temperature on the hot side. A Waltronic module

(ibnC1_127.08hts) has an optimal working temperature on the hot side of 200°C and is destroyed at a


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temperature of 232°C. Just a few °C deviations from the optimal working temperature cause cell

damage.

Exhaust gas control flow is also relevant in order to control the hot side temperature.

Typically also a thermal insulator is placed between the pipe and the hot side of the cell to lower the

contact temperature and assure reliability.

A temperature sensor must be present on the hot side to take recovery actions in case of over

temperature or in a more complex way to control the temperature within a desired optimal range to

assure a high efficiency.

To assure energy conversion, the cell cold side must be cooled. A heatsink is used in case of air

cooling. To reach better cooling performance, water cooling is used on vehicles with a more complex

device package. Water cooling has an impact on fuel consumption. If the engine water cooling circuit

is used, impact on pipes, pressure drops and engine water request must be taken into account. The

engine water circuit is typically not suitable for such cooling due to high water temperature (> 80°C).

A new cooling circuit may be required with a lower water temperature (40°C) which will have

significant impact on pipes, radiator and the introduction of a new electric water pump.

Ongoing research projects claims TEG output performances without taking into account related

factors with an impact on fuel consumption (additional cooling request to engine load, weight…).

Within the EE-VERT approach all these factors will be modelled and evaluated to get the TEG net

power contribution to the vehicle.

DC/DC

heat source 400°C

Thermal insulator To lower the contact temperature to 200 ° C

Heatsink

Cell

Temperature sensor

Fig. 3.3.12 TEG layout with control device and thermal management

3.3.5 DC/DC requirements for TEG management

The connection from cells to DC/DC converter is quite challenging. To work at the optimal working

point of each cell, each cell should be connected directly to a DC/DC input stage. Design optimization

requires putting some cells together in series in order to reduce harness complexity and DC/DC cost.

Putting cells together in series connection has the risk to force some cells to work in a non-optimal

working point. Moreover additional electronic components must be introduced to avoid that one

damaged cell will lead to cell strip open circuit; devices like diodes must be introduced in parallel to

each cell in order to allow by-pass of broken cells, i.e. similar as described for solar cells. The DC/DC

converter should also monitor the power generation in order to check for cell damage and warn the

overall energy management system.


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3.3.6 Possible use of TEG within EE-VERT

TEG is recommenced to be integrated in the EE-VERT power net concept. TEG development is out

of scope of the project, but EE-VERT can benefit from the results of other on going projects fully

devoted to TEG development (e.g. HEATRECAR). Furthermore if a TEG device will be available for

proper vehicle installation from suppliers, it will be tested on a test bench and on a test vehicle, to

assess the benefit during the NEDC cycle and in real use.

TEG device will be considered in the simulation activities. Reasonable hypothesis will be made about

real energy recovery, based on information available on state of the art and a TEG simulator block

will be integrated.

A TEG device is able to continuously produce electric power; the amount depending on the engine

load. During the cold start, the TEG is not able to work at full performance. As soon as the exhaust

gas reaches the nominal working point, TEG produces nominal output power.

From an energy management point of view, a TEG device seems to be a very good complement to

brake energy recovery. The combination of TEG and brake energy recovery allows a source of

continuous ―free‖ energy, in most vehicle conditions.

During engine cut-off, there is no engine load and consequently there is no heat production. TEG

generation drops, but energy level is restored by brake energy recuperation. Engine cut-off is read as

an intention of the driver to slightly slow down, so application of a slight brake force (controlled by

the new alternator concept) is feasible and has negligible impact on vehicle driveability. During

vehicle off condition there is no contribution from the TEG to the electric power net. Although the

exhaust pipe temperature may still be high just after an engine shut down, no exhaust gas flow is

available and no efficient heat exchange is possible.

3.3.7 TEG energy saving calculation

Experimental measurements in the exhaust gas pipe temperature

The reference vehicle (Alfa Romeo 159 1.9 jtdm) has been equipped with temperature sensors along

the exhaust pipe (Figure 1). Sensors are placed just before and after the pre-catalyst device. Another

couple of temperature sensors are placed before and after the DPF (Diesel Particulate Filter). The last

sensor is placed before the muffle.


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ENGINE

CAT

DPF MUFFLE(e)(e)(i) (e)(i)(i)

(i)

(i)

32

22

37

121187

(i)

6

5

Figure 1. Temperature sensor layout on test vehicle; (i) internal to pipe; (e) on pipe surface

5 Catalyst gas inlet

6 Catalyst gas outlet

7 DPF gas inlet

8 DPF gas outlet

11 Exhaust gas at middle DPF-muffle

12 Muffle gas inlet

37 External pipe at DPF outlet

32 External pipe at middle DPF-muffle

22 External pipe at muffle inlet

Table 1. Temperature sensor list

The scope of the temperature sensors setup is to monitor the gas temperature to check if it is suitable

for a TEG device.

The results are shown in Fig. 3.3.13 for a cold NEDC cycle. The exhaust gas temperature at the

engine outlet is not too high: it reaches about 400°C at high vehicle speed. This behaviour is typical of

efficient diesel engines where the heat losses are lower compared to gasoline engines, i.e. in a

gasoline engine a higher exhaust gas temperature may be expected.

The pre-catalyst device produces a reduction of about 20°C and a filtering of gas temperature bursts.

There is a further reduction in temperature along the pipe before it reaches the DPF. The DPF

produces the largest reduction in temperature of about 170°C during an ECE cycle and 260°C during

an EUDC cycle.

An acquisition was also done on a part of the EE-VERT cycle (Fig. ). The temperature trend is similar

to NEDC, but another issue has been highlighted: the DPF regeneration phase. The DPF regeneration

process is based on combustion on particulate, so exhaust gas temperature increases a lot. This

process may happen during the real use of the vehicle. In the acquisition it happened at the beginning

of the EE-VERT mission. The exhaust gas temperature may reach up to 600°C. It is clear that an

exhaust pipe by-pass device is really needed in that case to avoid to damage the TEG. Furthermore a


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coordination with engine control unit is recommended to coordinate the DPF regeneration process

with the TEG management for improved reliability.

No devices should be inserted before the pre-catalyst and the DPF in order to avoid lowering the pre-

catalyst and the DPF efficiencies.

The TEG may be inserted before the muffle. The available gas temperature is however low (<200°C)

and thermoelectric conversion may be less efficient. It must also be considered that thermoelectric

cells are mounted around the exhaust pipe: so further temperature reduction must be considered when

passing from exhaust gas to pipe surface.

Fig. 3.3.13 Exhaust gas temperature along the exhaust pipe.


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Fig. 3.3.14 EE-VERT cycle acquisition

It could be useful to add thermal insulation to the exhaust pipe to avoid heat dissipation. This is a

typical trick used in cogeneration (generation of electrical and hot thermal energy from an engine)

application. In actual production cars, heat is considered as a disturbance to system performance and it

must be dissipated as much as possible to allow vehicle full performance and availability. Therefore

also passive dissipation is desired in actual production cars.

Since heat is now considered as a source of energy, heat must not be dissipated. Heat must be trapped

and released in the thermal conversion device. Therefore, insulation must now be considered.

Reduced alternator load

The contribution of the TEG device leads to reduction of the alternator load and consequently of fuel

consumption. From the previous description and facts related to a real application, a contribution to

the vehicle electrical supply may be expected, but not full coverage to overall vehicle electric demand,

due to:

Heat available at low temperature (~170°C);

Heat available after 600 sec in cold NEDC cycle;

TEG cooling auxiliaries request still not considered in state of the art TEG declared

performance. Power claims are gross values. Net values must be considered, subtracting TEG

auxiliaries power request.

Simulations have been performed with the following assumptions:


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nominal TEG performance as shown in Fig. 3.3.8;

flat road: vehicle speed is assumed to be related to the engine load in such a case;

reduced efficiency during engine warm up;

efficiency of the DC/DC converter: 90 %;

50 W reduced output power due to TEG auxiliaries: additional water pump.

Fig. 3.3.15 Simulation of TEG electric generation during NEDC cycle

The final simulation result is shown in Fig. 3.3.15. The TEG contribution is able to support just a part

of the vehicle electric request. The average power supplied is 50 W. Considering 240 W as the vehicle

electric request, about 20 % of the alternator load may be replaced.

Basic requirements for a TEG application

Requirements have been defined for TEG application on a vehicle for an effective fuel saving:

Exhaust gas pipe insulation;

TEG placed after the DPF;

Added exhaust gas pressure drop <200 mbar.


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3.4 DC/DC converter

3.4.1 Basic architectures

As previously explained, a first option to augment the efficiency of the system is to have generation

(i.e. the alternator) decoupled from the conventional power net for consumers. In this way, the

generation device could be used in an efficient manner. In particular, it may be conditioned for

optimized power output (i.e. selecting an output voltage not limited by the power net voltage).

This option defines, as explained in D1.3.1, a first basic scenario (baseline) for optimizing the electric

architecture of a conventional car (Fig. 3.4.1).

ECU #1AlternatorR Battery

Load #1BMS

Voltage

Stabilizer

ECU #N

Voltage

Stabilizer

Load #NStarter

CommunicationCommunication NetworkNetwork

14 V 14 V PowernetPowernet

R Alternator regulator

BMS Battery Monitoring System

Fig. 3.4.1 Currently power net architecture of a conventional passenger car

Firstly, the alternator may be optimized by using a higher voltage as described in section 3.1. As

consequence, an energy converter (DC/DC converter) should be used to transfer the energy from the

generator to the consumer power net. Depending on system requirements, the DC/DC converter has

different characteristics: unidirectional / bi-directional and different topologies such as buck / boost /

buck-boost. In this section we will discuss different DC/DC concepts depending on the system (i.e.

depending on electric architecture).

As explained in section 2.2., the electronic architecture also requires having a storage element with

high power / high energy capabilities in order to store the energy produced when it can be generated

with low energy losses / high efficiency. The energy stored should be used when generation is not

recommended due to low efficiency.

In order to store this energy, two possibilities have been addressed: Lithium-Ion battery and

ultracapacitor. If an ultracapacitor is used, the generator does not need a well regulated constant

voltage. If a Lithium-Ion battery is used a well regulated constant voltage is needed. The resulting

general architecture is shown in Fig. 3.4.2.


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Gene - rator

DC / DC

Storage

Load # 1

Volt . stab .

Load # X + 1

Load # X

Volt . stab .

Load # N Starter

Lead acid battery

LV HV

Fig. 3.4.2 EE-VERT architecture - baseline

The DC/DC converter characteristics are determined by the voltage and current characteristics of the

generator (and therefore of the storage) element chosen. For the baseline version, the most suitable

DC/DC converters are listed in Table 3.4.1.

ARCHITECTURE INPUT RANGE TYPE DIRECTIONALITY

Generator with

Li-Ion battery 28 – 47 V Buck

Unidirectional

(although some topologies are

intrinsically bi-directional)

Generator with

ultracapacitor 0 – <60 V Buck-boost

Unidirectional

(although some topologies are

intrinsically bi-directional)

Table 3.4.1 Characteristics of DC/DC converter for baseline architecture

However, this solution represents only a limited enhancement of current systems. The capability of

energy recovery is higher than for systems with one unique power net but most of the energy is still

wasted (as shown in Figure 1.1.1) or, in other words, is left unused.

Therefore, to improve the overall efficiency of the vehicle, it is necessary to recuperate / harvest

energy from other sources that may be available. For instance, sources such as waste heat recovery,

solar cells and possibly also AC grid connection. Some of these sources are available at any time, but

with varying efficiency, depending on operating conditions, while others are only available under

certain conditions. For instance, grid connection is available only when car is parked in the proximity

of a charging point.

In order to integrate these power sources on the architecture, different options are possible. Since

auxiliary sources (i.e. all mentioned except braking energy recovery) will be considered as optional

elements in the vehicle and they generally provide a limited power to the system, a straightforward

solution is to have an independent DC/DC converter for each of them and connect it to one of the

power nets (Fig. 3.4.3).


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Fig. 3.4.3 EE-VERT architecture – Baseline with add-on power sources

If the external DC/DC converter is connected to the consumer power net, it is recommend having a

main DC/DC converter that is bi-directional in order to allow recharging of both batteries.

However, this solution is not completely satisfactory since, if an external DC/DC is plugged to the

system, the common operation point of the vehicle may not be optimal for the different subsystems.

Therefore, it is interesting to develop a more compact solution able to support all these energy sources

while providing reliability, flexibility and a unique control. A possible solution is to use a multi-input

power electronics converter (MIPEC), directly interfacing the available energy sources.

Fig. 3.4.4 EE-VERT architecture – MIPEC with alternative power sources

The different sources —the alternator, the solar cells, thermoelectric devices or grid connections —

have different voltage and current characteristics. In general, one source (the alternator) will be

preferred before others but, in some situations, a simultaneous combination of sources is appropriate

for optimal energy/economic use. Therefore, multiple-input power converters (MIPECs) are required

to enable multiple-source technology. An ideal MIPEC could accommodate a variety of sources and

combine their advantages automatically, such that some of the inputs are interchangeable. Such a

converter could also take advantage of the local environment, e.g., in some areas solar power would

be readily accommodated, in others, grid power may be especially inexpensive.


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3.4.2 Baseline

In the baseline solution, a mono-input DC/DC converter is selected for current control. Two options

are possible, depending on the alternator and the storage:

Buck (from HV to LV)

Buck-Boost (from 0-HV to LV)

Buck Converter

For energy transfer from HV (28 – 47 V) to 14 V, the classical buck converter topology is the straight

forward solution. However, the efficiency of classic buck converter is low. To improve the efficiency

several buck converters can be paralleled and the driving signals shifted. This kind of converters is

called interleaving DC/DC converter (also called multiphase synchronous DC/DC converter).

The general structure of interleaving DC/DC converters is shown in Figure 3.4.5. They consists of n

converters in parallel (each with its own inductor) sharing the output filter capacitor.

Fig. 3.4.5 Interleaving DC/DC converter with n phases

For a step-down conversion, i.e., for an interleaving Buck DC/DC converter the resulting circuit is

shown in Figure 3.4.6. The diodes which are normally used to re-circulate the current through the

inductors L have been replaced in the figure below by power MOSFETs [23]. This is called

synchronous rectification. The advantage of the synchronous rectification is the lower voltage drop on

the transistors when they are conducting.

Fig. 3.4.6 Interleaving buck DC/DC converter with n phases [23]


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This converter has bi-directional capability and high efficiency (generally >95%) can be achieved. No

isolation is required between the batteries and therefore, topologies with galvanic insulation are

unnecessary. The output current is the sum of the phase currents.

There are several advantages of using the interleaving buck converters. One of these is the reduction

of the total current ripple. This is shown in Fig. 3.4.7 which presents the maximum current ripple as a

function of the number of phases for an interleaved buck converter. The current ripple is filtered by

the output capacitor which can be much smaller than for one phase. This also gives a possibility to

shift the technology of the output capacitor. Instead of bulky electrolytic capacitors some ceramic

capacitors can be used. This fact has the consequence of reducing the power loss in the output

capacitors because the ceramic capacitors have much lower values of the equivalent series resistance.

Fig. 3.4.7 Interleaving buck DC/DC converter with n phases

The interleaving converter can be implemented using off-the-shelf inductors but there are other

possibilities to realize the inductors. One possibility is to use coupled inductors as shown in Fig. 3.4.8.

In coupled inductors the current slope of one inductor is affected by the voltage across the other

inductors. The main advantage in using coupled inductors is that the current ripple is reduced in

comparison with the case of the uncoupled inductors. If the current ripple is lower, the frequency of

the MOSFETs control can be reduced thus lowering the switching losses in power transistors.

Fig. 3.4.8 Interleaving buck DC/DC converter with coupled inductors

Buck-boost Converter

The buck–boost converter is a type of DC-DC converter that has an output voltage magnitude that is

either greater than or less than the input voltage magnitude. The output voltage is adjustable based on

the duty cycle of the switching transistors. The most usual non-inverting synchronous topology


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consists of a buck (step-down) converter followed by a boost (step-up) converter. Such a non-

inverting buck-boost converter may use a single inductor that is used as both the buck inductor and

the boost inductor.

Fig. 3.4.9 Buck-boost DC/DC converter

This converter can be used as a buck, boost or non-inverting buck-boost converter by selecting the

operating mode and using appropriate control circuitry.

1) Standard control operates all four MOSFETs during each switching cycle see (Fig. 3.4.10 c). This

type of operation generates the classical buck-boost waveforms (i.e. the converter works in buck-

boost mode). In this mode, the RMS current through the inductor and MOSFETs is significantly

higher than that of a standard buck or boost converter. This increases both the conduction and

switching losses in the classical buck-boost. Operating all four switches simultaneously also increases

gate-drive losses, which can significantly lower efficiency at lower output currents.

The physical size of the inductor must also be larger to accommodate the extra current without

saturating. Furthermore, as the output capacitor must carry the full output current during the PWM on-

time (D), and the charge current during the PWM off-time, the output capacitor must have low

equivalent series resistance (ESR).

2) The second buck-boost control scheme reduces losses by only operating two MOSFETs per switch

cycle. Referring to Fig. 3.4.10, this control scheme operates in three distinct modes. When Vin is

greater than Vout, the converter opens SW4 and closes SW3. It then controls SW1 and SW2 as a

classical buck converter. When Vin is below Vout, the control circuitry opens SW2 and closes SW1.

It then controls SW3 and SW4 as a classical boost converter. This control mode has several

operational and control problems around the transition region between the buck and boost modes. The

solution is to operate as a classical buck-boost mode during the transition region. In this operating

mode, as explained, all four switches are operational. Therefore, we have a transition region with

significant efficiency drop due to the increased switching losses and increased RMS currents.


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SW1

SW2

SW3

SW4

(a) Buck mode (Vin >> Vout)

(b) Boost mode (Vin << Vout)

(c) Buck-Boost mode (Vin > Vout)

Fig. 3.4.10 Buck-boost DC/DC converter operating modes


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3.4.3 MIPEC (Multi-input Power Electronic Converter)

For conventional vehicles, the need to reduce fuel consumption and emissions has boosted the search

of ways to optimize the energy used and wasted in the car. To achieve this goal, new and improved

generation sources should be introduced, such as braking recuperation, waste heat recovery, solar

cells and grid (AC power) connection.

In particular, some of these sources have very promising features. For instance, the TEG device is

able to continuously produce electric power; the amount depending on engine load. During the cold

start, the TEG is not able to work at full performance but, as soon as the exhaust gas reaches the

nominal working point, the TEG produces nominal output power. From an energy management point

of view, the TEG device can be integrated very well. The combination of TEG and brake energy

recovery allows a source of continuous ―free‖ energy, in most vehicle conditions.

However, to use these sources, a device able to combine the available sources is needed. Multiple-

input power electronic converters (MIPEC) have recently been developed to interface more than one

power source with a load, especially in the field of renewable sources. By using these devices it is

possible to diversify the energy sources so that the power system availability can be increased. In

automotive, MIPECs have not been used until the advent of Hybrid and Electric vehicles and the use

of high-voltage batteries for supporting traction [24]. In conventional vehicles, this strategy has not

been yet used.

High-level design

In EE-VERT architecture, the MIPEC should fulfil the following requirements:

power sources may have different power and voltage / current variation range;

power sources can deliver power to the load individually or simultaneously;

current flowing from any source / storage element to any load / storage element is possible

and controllable; set up the appropriated energy flow path and transformation strategy to

combine the multiple sources in one single power net line;

select the appropriate sources / storages supply configuration based on vehicle conditions for

energy efficiency;

maximize efficiency conversion on ―non-free‖ energy (i.e. energy obtained from fuel-

consumption);

Maximum Power Point Tracking (MPPT) for PV and TEG arrays;

design should be as compact as possible while maintaining flexibility on input types.

The general circuit topology for a MIPEC fulfilling these requirements is shown in Fig. 3.4.11. It is

based on two principal stages: a Smart Source connection/selection Bays (SSB) and the multi-input

DC/DC converter itself. The aim of SSB is to connect-disconnect at appropriate time and system

conditions, based on Energy Management strategy, the different sources/storage devices into the

power converter. In some cases, depending on the actual sources and storages devices connected and /

or the topology of the MIPEC, the SSB may be eliminated since its function is not needed and / or is

already done by the switches in the MIPEC.


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Fig. 3.4.11: Multi-input power electronic converter with smart sources connection bays

This flexible connection by means of SSB, allows the MIPEC to select among different generation

sources and storage devices to set up the more convenient power structure to satisfy the vehicle

electrical net condition, while maintaining the targets originally assigned to it. The Energy

Management algorithm (developed later in the EE-VERT project) is in charge of setting up those

targets.

Smart Sources Bays (SSBs)

The aim of SSB is to connect-disconnect at appropriated time and system conditions, based on Energy

Management strategy, the different sources/storage devices into the power converter. The SSB main

characteristics are listed below:

SSBs must be generally bi-directional blocking controlled.

SSBs work in long time ‗ON‘ conditions at high current ratings.

SSBs work at relatively low frequency.

SSB can be implemented as IGBT or series MOSFET and diode pair depending of voltage

and power of the circuit.

The low level driver acts over the SSB and the converter in order to set up the appropriated

configuration for the conversion mode and vehicle condition. The SSB technology depends on

power/dynamics of application (IGBT, MOSFET, SCR, etc.) and directionality of the power / energy

source (an alternator is unidirectional while a battery is intrinsically bi-directional). Fig. 3.4.12: shows

an example of a bi-directional SSB based on MOSFET technology as proposed in [25].


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Fig. 3.4.12: Bi-directional SSB based on MOSFET technology [25]

DC/DC Converter

As mentioned before, two kinds of qualitatively different inputs are present in a vehicular

environment: for sources such as thermoelectric generator or solar cells, only unidirectional power

flow is needed. Storage elements such as batteries or ultracapacitors require bi-directional power flow.

For the realization of the converter, several options are available, depending on the topology.

Combination strategies include sharing the output filter capacitor, sharing some switches and energy

transfer inductor and capacitor, and sharing a magnetic core [24]-[27]. These input combination

methods are shown in Fig. 3.4.13.

.

(a) (b) (c)

Fig. 3.4.13: MIPEC topologies. (a) sharing output filter capacitor

(b) sharing inductor, switch and/or capacitor, (c) sharing magnetic core [26]

Variant 1: Output filter capacitor sharing

This implementation is based on interleaving topology, i.e., the one used in mono-input converters.

This topology is typically used in hybrid / electric vehicle applications since they are easily matched

with requirements for energy flow and operation modes.

In hybrid / electric vehicles with two storage elements (battery and ultracapacitor), these topologies

are generally used to supply the inverter for traction load. In this case, a bi-directional buck-boost

converter is usually implemented such that the converter acts as step up converter (boost converter)

for one mode of operation and as step down converter (buck converter) for other mode of operation.

Each power source is connected to the DC-link by means of this bi-directional converter. Step up

mode of operation is used in order to transfer energy from each power source to the DC-link, where as


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step down operation is used to charge both ultracapacitor and battery storage system and to recover

the braking energy. Main features are:

1. Interleaved single cells sharing output filter

2. Cells based on simplest buck-boost topology

3. Variant suitable for:

Non isolated requirements

Bi and mono directional power flow

Non-overlapped input voltage sources ranges

4. Drawbacks:

No isolation

Low rate of simplification/integration (high cost)

Low-medium power rates

Output O-ring needed

Variant 2: Inductor, switch and/or capacitor sharing

This implementation is typically used in combined (wind and solar) renewable energy installations in

the form of a unidirectional buck-boost converter. Main features are:

1. Common power cells sharing power plant components

2. Cells based on simplest buck-boost topology


Non isolated requirements

Bi & mono directional power flow

Overlapped input voltage sources ranges

4. Drawbacks:

No isolation

Low-medium power rates

SSBs and its control play a key role assuring operating conditions at the input.

Variant 3: Magnetic core sharing

This implementation is preferred for developments such as battery chargers, i.e., incorporating a

connection to the grid for recharging the battery. Main features are:

1. Common Secondary cells sharing power plant components.

2. Cells based on simplest isolated buck-boost topology


Isolated requirements

Bi and mono directional power flow

High voltage applications

Un-overlapped input voltage sources ranges

Symmetrical topology

4. Drawbacks:

Cost due to number of components

Transformer characteristics are key to achieve high efficiency


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Fig. 3.4.14: MIPEC structure based on Magnetic core sharing

3.4.4 MIPEC working operation

In order to understand the system operation with the MIPEC, let us consider a version with four

power sources: generator with regenerative braking, thermoelectric generator, photovoltaic generator

and grid connection and one storage element (an Li-ion battery). The generator and the battery are

directly connected and working at voltage higher than the power net voltage (in order to improve

efficiency of generation and recuperation).

The different working operation modes of the MIPEC are directly linked to vehicle status. In

conventional electrical systems, the power supply necessities during driving phases were covered by

the alternator as main provider. The alternator charge regulator was not directly associated to specific

driving phases like traction or coasting/breaking mode. Therefore, the electrical energy was converted

independently of the state of the machine. In new alternator designs, as the proposed, the possibility to

partly regenerate the brake energy of the vehicle, known as recuperation, will be possible.

In systems like BMW microhybrid [28], a system named BER implements this function. In this mode,

the moment of inertia drives the engine with engaged gear. Therefore, the rotational speed of the

alternator is not powered by fuel consumption, but by the vehicle inertia moment, i.e. the already

‗paid‘ energy. The BER system controls the rotator exciting current to an alternator output of 14.8 V

on-board voltage. This implicates over-covering of the electrical consumers resulting in recuperative

charging of the battery.

However, two issues should be considered: firstly, the conversion of mechanical to electrical energy

strongly depends on the charge acceptance of the battery at the moment of the voltage increase. As the

charge acceptance is inversely proportional to the state-of-charge (SOC), the state-of-charge has to be

regulated to a certain level below 100 % SOC while driving. Secondly, the efficiency of the

conversion of energy depends on engine regime. By being aware of these situations, it is possible to

find appropriate time zones / vehicle state where it is more efficient to use energy from a different

source.


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Depending on status of primary inputs, the alternator, the thermoelectric generator and grid are

connected as inputs to the common input MIPEC. Due to different conduction cases of switches, the

primary converter can be operated in buck, boost and buck-boost modes for different values of input

voltages.

Figures 3.4.15 and 3.4.16 shows an example of using the MIPEC to efficiently control the different

sources to provide demanded current to the loads, by choosing the most suitable combination of

power sources at any driving situation (acceleration, braking, and steady-state) for a NEDC drive.

In PARKING MODE (engine OFF), the vehicle loads generate almost no consumption. In this

scenario, battery is recharged from available sources: energy from a solar cell (if available) or energy

from grid plug-in may be used to fill completely the battery and provide energy to the loads (like car

access system, alarm or fans for controlling vehicle interior temperature). Also, it may be recharged

from LV powernet if needed or the other way around as well, i.e. the Li-ion could be used to charge

the 14V battery.

LV powernet

CAN-bus

Grounding

Supervisor

HV source /storage

Thermoelectric source

Photovoltaic source

Electric Grid

LV powernetThermoelectric source

Photovoltaic source

Electric Grid

CAN-bus

Grounding

Supervisor

HV source /storage

(a) (b)

LV powernet

CAN-bus

Grounding

Supervisor

Thermoelectric source

Photovoltaic source

Electric Grid

HV source /storage

(c)

Fig. 3.4.15: MIPEC in PARKING MODE (a) recharge from grid. (b) recharge from solar cell. (c)

recharge from LV power net (using LV battery, external charger or external solar cell)


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In DRIVE MODE (engine ON), energy for supplying loads is initially provided by the battery. It is

assumed that vehicle starts with battery at high SOC (since has been recharged when is parking by

means of a solar cell or grid connection). Therefore, at start and low speeds (low efficiency regime for

the alternator) the vehicle electrical system is supplied with electrical energy from the battery. If an

extra power source is available, this energy may be used to support the current demand by combining

them through the primary converter, therefore reducing somewhat the battery current. However, just

during the cold start, the TEG is not able to work at full performance. As soon as exhaust gas reaches

the nominal working point, TEG produces nominal output power.

While driving, the alternator may be used in braking to recuperate energy using a regenerative braking

system to recharge the battery. Thus, the alternator control system may accomplish approximately

zero discharging / charging of the battery while driving, in order to keep it at partial state-of-charge

allowing the system to recuperate braking energy. In this way, the alternator generates electrical

power with decreased power demand on the internal combustion engine.

At specific regimes, where alternator / TEG are highly efficient, it may be used to provide current lo

loads and, depending on the SOC of the battery, to sustain the battery SOC or, if needed, to recharge

it. In general, however, direct operation of the alternator should be kept at a minimum to reduce fuel

consumption. In this regime, thermal TEG is more efficient, so this energy may be used to support the

current demand by combining them through the primary converter, therefore reducing somewhat the

alternator current or helping for a fast recharge of the battery. Or, if the load is low, the system may

use the remaining energy to recharge the HV battery.


Photovoltaic source

Electric Grid

CAN-bus

Grounding

Supervisor

HV source /storage


Photovoltaic source

Electric Grid

CAN-bus

Grounding

Supervisor

HV source /storage

(a) (b)

Fig. 3.4.16: MIPEC in DRIVING MODE: (a) Direct mode conversion (b) Direct mode conversion

plus recharge from harvesting sources.

Using this strategy, system variables depending on an exemplary driving pattern in NEDC is shown in

Fig. 3.4.17. With this strategy, the power demand of the alternator related to fuel consumption is

reduced. The total amount of this reduction depends on several factors such as the statistics of the

driving phases, the actual efficiency of power electronic modules (including the alternator and the

MIPEC) and, especially, on the charge acceptance of the storage systems. A more detailed evaluation

of this fuel reduction is addressed later in EE-VERT project.


Version 1.0 Page 73

Fig. 3.4.17: System variables depending on an exemplary driving pattern with MIPEC


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3.5 Conclusions

The objective of this part of the EE-VERT project was the development of adequate power generation

concepts for the EE-VERT approach.

In 3.1 a new generator concept was introduced and explained. In 3.2 and 3.3 innovative systems like

solar panels and thermo-electric generators have been studied. In 3.4 especially the integration and

combination of the different power generation technologies have been investigated with an adequate

DC/DC converter topology.

The generator is currently responsible for the conversion of mechanical energy from the vehicle‘s

engine to electrical energy for the power distribution network, and is therefore a crucial component

for the overall energy efficiency. But the average efficiency is very low. In addition, the capability of

the current regenerative braking is limited due to the voltage level of 14 V and the characteristics of

the lead-acid battery.

To overcome the current limitations a new generator concept was identified and selected. The

introduction of the new generator concept with permanent magnets, a higher voltage level and an

adequate storage technology greatly improves the regenerative braking capabilities. Brake energy

recuperation is now possible with up to 8 kW.

Due to the EE-VERT architecture with decoupled power generation and consumption the generator

can now be operated with more flexibility and has a higher efficiency especially during standard

operation.

The alternator concept is so promising that the EE-VERT consortium has started to develop a

prototype generator. It keeps the present geometry of the claw pole generator but with integrated

permanent magnets to improve the characteristics.

Regarding the photovoltaic study several benefits that a solar panel would offer to an overall vehicle

energy management strategy were identified. Hence, it is proposed that solar panels should be used as

an additional source of power for vehicles since they are able to deliver around half of the basic

power net demand for passenger cars which is a significant proportion (!). Also for cabin-

preconditioning PV panels offer a number of benefits. Especially since power delivered from PV

panels is power that is not generated by the engine.

Since two thirds of the chemical energy supplied by the fuel is converted into heat energy it is

proposed that a heat recovery device should be used as an additional source of power for vehicles.

Different technologies are available to recover thermal wasted energy but the study has shown that the

focus of EE-VERT should be on thermo-electric conversion. Among the described technologies

clearly TEG must be considered in the new EE-VERT approach, being a source of electric energy

always available during the whole vehicle mission. EE-VERT will benefit here from other ongoing

projects, like HEATRECAR for instance.

For the combination of all the power generation concepts a new DC-DC converter design with high

efficiency has been developed and proposed with multiple inputs with different voltage levels.


Version 1.0 Page 75

This chapter has shown that there are additional concepts for power generation that could be of

benefit for the EE-VERT approach. All the investigated concepts will be a part of the EE-VERT

system. In this stage of the EE-VERT project it is planned that the new generator concept, solar panels

and the DC/DC converter concept will be integrated into the demonstrator car while the thermo-

electric device will be at least investigated via simulations in the system context.


Version 1.0 Page 76

4 SYSTEM INTEGRATION AND MANAGEMENT

The following chapter describes how the energy generation sources are integrated in the EE-VERT

approach, the system concept and the operation strategy. Furthermore, the link to hybrids, the impact

on safety relevant applications and the impact on commercial vehicles will be considered.

4.1 System concept

4.1.1 Basic EE-VERT approach

The central EE-VERT concept is to combine several different approaches to energy saving within an

overall energy management strategy (thermal and electrical). The approaches include:

Greater efficiency in energy generation with a new concept for the electrical generator and an

optimised operation strategy;

Improved efficiency in energy consumption, through electrification and demand-oriented

operation of auxiliary systems and the use of more efficient electrical machines such as

brushless DC motors;

Energy recovery from wasted energy such as waste heat recovery or an optimised braking

energy recuperation with a temporarily increased generator output power with up to 8 kW at a

higher voltage level;

Energy scavenging from unused and new sources of energy, for example the use of solar cells

on the roof of the vehicle.

The increased electrification of auxiliary systems with an optimised operation promises efficiency

gains. But this can only be accomplished if the energy generation and distribution is optimised and

adapted to the current driving conditions and the power demands.

Solar

panels

Waste

heat

Electric

energy

Generator

Brake energy

recuperation

Engine

lubrication

system

Engine

cooling

system

Air

conditioning

system

Oil

pu

mp

Wa

ter

pu

mp

Va

cu

um

pu

mp

Ste

eri

ng

pu

mp

…

A/C

co

mp

res

so

r

Po

wer

ge

nera

tio

nC

om

po

nen

tsS

ys

tem

Electrified auxiliaries

Fig. 4.1.1 The basic EE-VERT approach [4]


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Fig. 4.1.1 shows the basic EE-VERT approach. Electric energy is recovered from regenerative

braking, waste heat and solar cells. The (engine) auxiliaries are mainly driven by the recovered

electric energy. This strategy leads to the following benefits:

Auxiliary systems can operate on a demand-oriented basis and fulfil their tasks in an

optimised way;

Less mechanical power demand on the engine;

Less engine drag torque, because the generator and the electrified auxiliaries runs only on

demand and not continuously, leading to a higher capability for braking energy recuperation.

The optimised operation of auxiliary systems allows the thermal engine management and the air

conditioning system to be enhanced. This leads to additional benefits for the fuel demand and for the

convenience of vehicle users.

4.1.2 EE-VERT architecture with the power generation components

In order to achieve the necessary flexibility in power generation and consumption the system concept

requires a change in the electrical architecture to permit the integration of multiple generation,

actuation and storage devices with different optimal operating voltages and usage profiles.

Consequently, it is necessary to divide the electrical distribution system into two parts: one electrical

distribution system for the existing standard loads (14 V) and an electrical distribution system on a

higher voltage level (40 V) for efficient energy recovery and for high power loads (e.g. electric power

steering). The subsystems are connected by a DC/DC converter and incorporate different energy

storage elements. The EE-VERT architecture is shown in Fig 4.1.2.

Fig. 4.1.2 EE-VERT architecture with different new power sources

The EE-VERT architecture will deploy a distributed network of smart components, whose

characteristics are co-ordinated to optimise their interaction and their efficiency. In addition to this, it

is crucial to manage the use of different types of energy, such as electrical, mechanical or thermal

energy, which means that an overall vehicle optimisation and management concept needs to be

developed. This is the task of work package 3. Generally the power generation and the power

consumption are decoupled. If a bi-directional or unidirectional DC/DC converter is necessary

G

DC / DC

Storage

Waste heat recovery

Solar cells

Load # 1

Volt . stab . Load # X + 1

Load # X

Volt . stab . Load # N Starter Lead acid

battery

Low voltage power net

AC power 110/220V

High

Power

Loads

High voltage power net


Version 1.0 Page 78

depends on the kind of high power loads on the higher voltage level and on the configuration of

power from other solar panels and waste heat recovery as well and how this is configured.

4.2 Operation strategy

The decoupling of the power generation and the power consumption which is one of the basic EE-

VERT concepts leads to a new flexibility in the system management. Furthermore, due to that there

are several different energy sources for the electrical network – discussed and investigated in chapter

3 - it is possible to control the power generation in a way that the efficiency is maximised in every

operation point. EE-VERT will combine the contributions from different energy sources in the best

way.

The operation strategy will deal with optimising the fuel consumption especially during NEDC but

also in real-life missions. For that, it is considered to switch on the alternator only when the cost of

electrical power generation, in terms of fuel, is low, i.e. during braking recuperation and stop phases

as much as possible. The optimization performances for the operation strategy will have as a main

constraint the feasible operative area of the storage components. These components will provide the

electrical power whether the alternator is switched off and their operative area can be established as a

function of the state-of-charge.

The calculations and simulations made in function of the storage requirements and the designed

generator have shown that on NEDC there will be enough energy to switch on the alternator only at

the low cost generation phases.

Further developments in the operations strategy will be made in WT3.2 ―Optimisation Strategies‖ and

in WT3.5 ―Systems Integration‖. This work tasks will continue to use the results and work of WT2.1.

4.3 Link to hybrid vehicles

Transferability of the solutions

A hybrid vehicle has a relatively high potential to reduce CO2 emissions but it presently requires cost-

intensive and drastic technical modifications. In the past hybrid vehicles have penetrated the

automotive market relatively slowly in Europe. Hence, hybrid vehicles can provide only a slowly

increasing effect for the reduction of CO2 emissions in the short and medium term. This is one of the

reasons why EE-VERT is focusing on marketable solutions (in respect of vehicle integration, costs

and reliability) for conventional vehicles. Additional functions and benefits especially for

convenience, and taking into account the transferability of the solutions (e.g. for hybrid or electric

vehicles) will guarantee a high market acceptance of the solutions to be developed.

Auxiliary units and power management

Hybrid vehicles typically require auxiliary units that must be modified to operate with electric power

as the ICE is switched off during EV driving:

Electrically powered steering (EPS or EHPS)

Vacuum pump for braking assistance (passenger car)

Air compressor for brake system (commercial vehicle)


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Water pump for cooling motor, inverter and DC/DC converter

Water pump and possibly AC compressor for cooling the battery pack

Actuator to operate clutch and automatic transmission - depending on hybrid topology (e.g.

electrical oil pump)

During pure electric vehicle mode, brake energy recuperation by electric generator and stop/start

operation the listed auxiliaries are required to be fully functional. The designer can decide whether the

larger loads are placed on the high voltage system, otherwise these systems will draw electrical power

from the low voltage system.

Additionally the comfort and infotainment systems may also be activated and draw electrical power

from the low voltage system. An intelligent power management can decide or control what equipment

is available during different operation modes.

It is evident that hybrid vehicles will benefit from the availability of more efficient, electrically driven

auxiliary units.

EE-VERT’s power generation components

The link to hybrids is bi-directional. As described in subchapter 3.1 the EE-VERT generator is

optimised to deliver a high electrical power especially during braking phases. This is an operation

strategy that is used already in hybrid vehicles. Here EE-VERT benefits from the experiences in

hybrid vehicles. But due to the different boundary and system conditions the generator for EE-VERT

is based on the standard claw pole generator. It is only generating energy while an electrical machine

for hybrid vehicles has to be a generator and an electrical motor. Therefore, the EE-VERT generator

is not directly transferable to a hybrid car. The EE-VERT generator is optimised especially for the

recuperation phases and to deliver a high efficiency for power generation in standard operation mode

in conventional vehicles.

EE-VERT solutions which are transferable to hybrid vehicles are the waste heat recovery and the

usage of solar panels. Both technologies are adaptable to hybrid architectures. Especially the

developed DC/DC converter operation strategy (see subchapter 3.4) is usable for hybrid vehicles. The

DC/DC converter combines several energy sources. This is highly interesting for hybrid vehicles as

well. Different is however the voltage level. The voltage level has to be adapted to hybrid

architectures since the voltages in hybrids are usually higher.

4.4 Impact on safety relevant applications

A brief overview of these issues is given here since the subject of safety-relevant applications will be

studied in detail in WT3.3 which starts in October 2010.

Top-level safety requirements

The following top-level requirements were identified in D1.3.1 [3] as applying to power net

architectures and energy management strategies:

Energy management strategies shall consider the requirements for availability of power supply to

safety-related functions.

Energy transfer shall take place only from non-critical to critical power supply busses.


Version 1.0 Page 80

Any safety-related power net or energy management function shall have a safety level, such as

SIL or ASIL, associated with it. The SIL or ASIL associated with power net and energy

management functions shall consider the SIL or ASIL of all safety-related vehicle functions that

depend on those functions.

Power net hardware shall be analysed to determine which faults could lead to a dangerous failure,

and which proportion of those failures are covered by a diagnostic capability.

The power net shall have sufficient fault tolerance and redundancy with the safety requirements of

the functions it is supplying.

Energy management strategies shall be able to provide ―state of health‖ information on the power

supplies to safety-related functions that require this information, and shall be able to provide (for

example, through prognostic functions), ―early warning‖ of impending disruption to the power

supply to safety-related functions.

Furthermore one of the key findings of the previous EASIS project was to require redundant power

supplies to certain safety-related functions.

EE-VERT is a key enabling technology

The power generation strategy and electrical architecture proposed in EE-VERT is a key enabling

technology to achieve such a redundant power supply strategy since it incorporates a number of levels

of redundancy and dissimilarity including:

Two separate power nets with differing voltage levels;

Redundant storage devices (Li-Ion battery and/or supercap and conventional battery);

Energy generation and recovery from multiple sources;

Multi-input DC/DC converter.

In respect of the requirements of safety-relevant functions, further developments in the EE-VERT

project need to consider:

The design of the multi-input DC/DC converter should support energy management strategies that

permit prioritising of available energy to critical functions.

The design of the multi-input DC/DC converter should also consider a modular approach such

that an additional output bus can be added for critical supplies if needed (see for example

Architecture 7 of [3]). This could either be an additional output block within a single DC/DC

converter (provided there is adequate independence) or a separate additional converter for the

critical power bus.

The design of the multi-input DC/DC converter should ensure that a failure in a non-critical bus

cannot affect a critical bus.

Provision for health monitoring and diagnostics should be provided in the DC/DC converter and

in control and management aspects of the power supply.

Conclusion

With two energy storages, several power generation sources and two decoupled sub nets a first simple

consideration offers that at least two failures are necessary to disconnect a safety relevant application

from the power supply.


Version 1.0 Page 81

Fig. 4.4.1 Fault tolerant power supply from several connection points

Fig 4.4.1 shows that it is possible with the EE-VERT architecture to supply a safety relevant load

from different connection points. Therefore, the safety level of the EE-VERT power net is much

higher than the safety level of the current conventional 14 V power net. A more detailed safety study

including the system management and the operation strategy will be done in WT3.3.

4.5 Impact on commercial vehicles

Buses are the commercial vehicles that are in focus in the EE-VERT project. In [2] a Volvo 7700 city

bus was chosen as reference vehicle for the commercial vehicles. Buses are the focus of this

paragraph as well although most of what is written here is applicable also for trucks.

Much of what is said in previous paragraphs about conventional vehicles is applicable also for

commercial vehicles. A few important differences in the prerequisites for commercial vehicles

compared to conventional vehicles which influence the components and power net architecture are

listed below:

● The mass is bigger for commercial vehicles. This leads to more available kinetic energy to

recuperate when braking since the available kinetic energy is proportional to vehicle mass.

● The power load sum of mechanical and electrical auxiliaries are higher in commercial

vehicles. However, the total power load of auxiliaries generally does not increase as much as

the kinetic energy available to recuperate.

Note: The most power consuming auxiliaries, which today normally are mechanically driven,

are air compressor, power steering, oil pump, water pump, motor cooling fan and A/C

compressor.

● Commercial vehicles generally obtain significantly more mileage and / or operational hours

per year.

Volvo Bus Corporation currently produces and sells a full hybrid 7700 city bus, see [18]. In this bus

some of the auxiliaries are electrified such as the air compressor, the A/C compressor and the steering

are electro-hydraulic. The kinetic energy recuperated as electric power in this hybrid bus covers the

need, with margin, of all the currently electrified auxiliaries for a typical city driving cycle. It is most

likely that there will be a surplus of electric energy even with all auxiliaries electrified. In a full hybrid

this surplus of electric energy can be used for propulsion. For a long-distance coach it will probably

40 Vdc

subnet G DC/DC

coupling

14 Vdc

subnet

Additional

power sources

Safety relevant system


Version 1.0 Page 82

be quite different though, i.e. the recuperated electric energy will not be sufficient to drive all

auxiliaries if electrified. The balance between recuperated electric energy and the auxiliaries load

might be better for an inter-city like route. More intercity bus like routes will therefore also be

investigated for the chosen reference bus. The same argument of energy balance should be applicable

also for a truck application with an optimum somewhere between the two extremes of a refuse truck

and a long-haul truck.

Power net architecture

Despite the differences listed above between commercial and conventional vehicles the power net

architecture presented in section 4.1.2 with small modifications, is considered to be the most

promising also for commercial vehicles. The major change proposed is an increase in voltage level,

see discussion about voltage level below.

Storage

No decision on storage type, battery or supercapacitor, has been taken at the moment for commercial

vehicles. Cost, especially to predict future cost in larger quantities, is difficult. Both battery and

supercapacitor solutions seem quite expensive at the moment.

A solution with a combination of storage types is possible. In this scenario the supercapacitor

interfaces the generator and a DC/DC converter is inserted between supercapacitor and battery. Here

the high power loads are connected to the battery where the voltage level is more stable. This solution

is however discarded at the moment due to the extra complexity and hardware that would be needed

and the therefore inevitable cost increase.

It will be investigated further in the project which storage type to use. Below a few advantages for the

different types are listed.

Battery Supercapacitor

Very beneficial if start-stop functionality is

included. A battery makes it easy to run

electrified auxiliaries that are needed during the

stop phase

Lower voltage swing less stringent

requirements needed for high power auxiliaries

and the DC/DC converter

Lower power losses in storage device no extra

cooling should be needed

Longer life-time is expected. This is especially

good considering the higher mileage compared to

for a conventional vehicle

Table 4.5.1 Comparison of advantages with a battery or a supercapacitor storage


Version 1.0 Page 83

Voltage level

The baseline, based on the information in the table below, is to choose 600 V in the high voltage node

of the power net architecture.

40V 600V

<60VDC according to [19]

o No galvanic isolation is necessary in

DC/DC between high voltage and low

voltage node

o Safety restrictions less severe, i.e. less

isolation of cables is necessary etc

Slightly cheaper Battery Management Unit

(BMU). This since there will be fewer cells in

series which means less voltage nodes for the

BMU to handle

Slightly higher efficiency of DC/DC is expected

It is expected that at the end of the EE-VERT

project many electrified auxiliaries will be

available at this voltage level in the future. They

will be optimised for conventional vehicles

though.

Lower currents at higher voltage

o ―Reasonable‖ cable dimensions:

Generator to battery and inside

generator.

Note: With 40V option and assumed

peak recuperation of 40-120kW the peak

currents would be in the order of 1-3kA

o Weight and cost is lower for power

cables to high power electrified

auxiliaries

o Cable power losses will be lower

o Generator with a little bit higher

efficiency

o Auxiliaries with a little bit higher

efficiency are expected

Advantage for Volvo: A few electrified

auxiliaries are already available from the hybrid

bus project at this voltage level. Standardisation

at one voltage level leads to lower prices

Table 4.5.2 Comparison of advantages with a high voltage node voltage of 40 V or 600 V

Generator

The efficiency will be a little higher for commercial vehicles due to the choice of a high voltage level

of 600V. The impact of the significantly higher power levels in commercial vehicles on for instance

generator mass and cooling arrangements need to be investigated.

Solar panels

The potential for solar generation on commercial vehicles seems to be significant. The highest

potential exists for buses due to the big area available on top of the roof. The costs for solar cells /

panels are currently quite high but the cost might drop in the future so this is planned to be

investigated further in the project.

Typical truck

For a ―typical truck‖ an area in the order of 2.5m2 should be available for solar cells placement on top

of the cab. Given the possible cell sizes listed in Section 3.2.3, the following options are available

with this open area, assuming that the full roof curvature can be accommodated by any particular

panel.


Version 1.0 Page 84

Cell

Type

Cell size

(mm x

mm)

Number of

cells (Length)

Number of

cells

(Width)

Total

Cells

Potential

Peak Power

(Wp)

~Cell Cost

(€)*

MCSI 100 x100 12 20 240 360 775

MCSI 125 x 125 9 16 144 466 1005

PCSi 156 x 156 7 13 91 336 670

MCSI 156 x 156 7 13 91 355 775

Table 4.5.3 The potential result with solar panels covering 2.5m2 of the roof of a vehicle

* Approximate cost for the cells (not ‗made up‘ panel)

It can be seen that the high efficiency mono-crystalline cell offers the best prospect, with some 460

W(peak) being theoretically available. However, from the discussions on system losses covered in

Section 3.2.2, ~345 W(peak) delivered to the point of need is more realistic.

It should be noted, however, that many cab roofs have a skylight for driver comfort (both visual and

airflow) and many trucks have aerodynamic fairings added to boost fuel efficiency. Clearly such

additions would make the use of solar panels impractical in these cases.

Typical bus

Due to their large size, buses offer the best possibility of panel coverage. However, like for the truck,

the roof space is sometimes taken up with additional features, particularly skylights for coaches and

air conditioning packs for many buses and coaches. Again this would limit coverage.

The roof of a typical bus, such as the Volvo 7700 (12 m version), covers an area of approximately

12m x 2.2m = 26.4m2. In the calculation below it is assumed that around 80 % of this area, ~21m

2, is

available for solar panels. Given the possible cell sizes listed in Section 3.2.3, the following options

are available with this area.

Cell

Type

Cell size

(mm x

mm)

Number of

cells (Length)

Number of

cells

(Width)

Total

Cells

Potential

Peak Power

(Wp)

~Cell Cost

(€)*

MCSI 100 x100 93 21 1953 2930 6310

MCSI 125 x 125 75 17 1275 4131 8900

PCSi 156 x 156 60 14 840 3150 6190

MCSI 156 x 156 60 14 840 3267 7140

Table 4.5.4 The potential result with solar panels covering 21m2 of the roof of a vehicle

Again it can be seen the high efficiency mono-crystalline cell offers the best prospect, with some 4.1

kW(peak) being theoretically available. However, from the discussions on system losses covered in

Section 3.2.2, ~3.1 kW(peak) delivered to the point of need is more realistic.

Waste Heat Recovery

Both thermoelectric generation and thermal to mechanical and then from mechanical to electrical

generation with a generator look promising also for commercial applications. Details about these

technologies can be found in section 3.3.


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DC/DC converter

A DC/DC fitting into the architecture described in section 4.1.2 is foreseen also for commercial

vehicles. If it is to be unidirectional or bi-directional are left unanswered at the moment. Decision on

some implementation details such as converter topology will be left to suppliers.


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5 CONCLUSIONS AND OUTLOOK

The project ―EE-VERT‖ is concerned with improving the energy efficiency of conventional vehicles.

The central concept is the electrification of auxiliary systems, and supplying their energy by recovered

energy from new sources or wasted energy such as recuperation of braking energy, waste heat

recovery or solar cells.

This report has described the EE-VERT concept to reduce the fuel consumption in conventional

vehicles especially by generating electrical energy from different sources. Therefore, this report has

presented the power generation concepts investigated by EE-VERT and needed for the operation of an

advanced electrical power net aimed at generating and reusing energy with a very high efficiency.

The objective of work task 2.1 was the development of adequate power generation concepts with high

efficiency including their integration and operation strategies within EE-VERT‘s system approach.

Also the integration of innovative systems has been studied, such as solar panels and thermo-electric

generators to recover energy from exhaust gases. Thereby, EE-VERT is focusing on marketable

technologies for conventional road vehicles with the potential for a fast market launch and market

penetration. The link to hybrid vehicles and to commercial vehicles was considered and furthermore a

brief overview of the impact on safety relevant applications was given.

Finally, the work and results of work task 2.1 has contributed to deliver adequate power generation

concepts with high efficiency for the EE-VERT concept.


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REFERENCES

[1] EE-VERT Deliverable 1.1.1, State of the art and standards, 31/03/2009.

[2] EE-VERT Deliverable 1.2.1, Mission profiles, 31/03/2009.

[3] EE-VERT Deliverable 1.3.1, Requirements report, 30/04/2009.

[4] Abele, Marcus and the EE-VERT consortium: ―Reduction of fuel consumption in

conventional vehicles by electrification of auxiliaries‖; AVL conference engine and

environment, Graz (Austria), September 2009.

[5] Rugh, J et al, Significant Fuel Savings and Emissions Reductions by Improving Vehicle Air

Conditioning. 15th Annual Earth Technologies Forum and Mobile Air Conditioning Summit

April 15, 2004.

[6] Rugh, J. and Farrington R., NREL/TP-540-42454, January 2008.

[7] D.W. Whaley, W.L. Soong, M. Ertugrul, ―Extracting more power from the Lundell Car

Alternator‖ Proceedings of Australasian University Power Engineering Conference (AUPEC

2004), 26-29 September 2004, Brisbane, Australia;

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Documents

Project acronym: EE-VERT Project ... - trimis.ec.europa.eu · SC Beespeed Automatizari SRL BEE RO . EE-VERT Deliverable D2.1.1 23 December 2009 Document history Revisions: Version