wp3

ASPECTS OF DIFFERENT DISTRIBUTED GENERATION TECHNOLOGIES –

CODGUNET WP 3

Saga Häggmark, Viktoria Neimane,

Urban Axelsson, Per Holmberg, Gerth Karlsson – Vattenfall Utveckling AB

Kimmo Kauhaniemi - Technobothnia Margareta Olsson - SwedPower AB

Christer Liljegren - CLEPS

2003-03-14

Aspects of different distributed generation technologies – CODGUNet WP 3

From Date Serial No.

Vattenfall Utveckling AB 2003-03-14 U 03:04

Author/s Access Project No.

Saga Häggmark, Viktoria Neimane, Urban Axelsson, Per Holmberg, Gerth Karlsson –Vattenfall Utveckling AB

Kimmo Kauhaniemi - Technobothnia

Margareta Olsson - SwedPower AB

Christer Liljegren - CLEPS

Confidential 11645-03

Customer Reviewed by

Anders Holm

Issuing authorized by

Ulf Arvidsson

Elforsk AB

S-101 53 Stockholm

Stefan Melin

Key Word No of pages Appending pages

Distributed generation, Wind power, Photovoltaic, Micro-turbines, Fuel cells, Reciprocating engines, Stirling engines, Steam cycle.

48 5

Summary

This is the report of WP 3 in the project CODGUNet. The main scope of WP3 was to provide information to WP 4 and 5 about the characteristics of different Distributed Generation (DG) –technologies.

The most common and prospective technologies, which are used for distributed genera-tion, are presented in this document. The document summarises the main characteris-tics of different DG technologies, describes technical solutions for their network con-nection and the possible impact from DG on network operation is also discussed. The document also contains a brief discussion concerning environmental issues for each DG technology and economic aspects regarding e.g. investment costs. Finally the barriers and opportunities associated with utilisation of each technology are high-lighted and also an overview over existing energy storage technologies.

Distribution list

Company Department Name Number of

Elforsk AB Ulf Arvidsson

Table of Contents Page

1 BASIC INFORMATION 1

1.1 Definition 1

1.2 Purpose 1

1.3 Background 1

1.4 Scope 2

2 GENERAL DESCRIPTION AND CHARACTERISTICS OF DIFFERENT DG TECHNOLOGIES 3

2.1 Wind power 3

2.2 Photovoltaic 4

2.3 Micro-turbines 5

2.4 Fuel cells 7

2.5 Reciprocating engines 9

2.6 Stirling engines 10

2.7 Steam cycle 12

3 TECHNICAL SOLUTIONS FOR GRID CONNECTION OF DIFFERENT DG TECHNOLOGIES 13

3.1 Typical arrangements 13

3.2 Start-up and shut-down 19

3.3 Control in normal operation 21

3.4 Fault response 23

3.5 Protection 24

4 IMPACT ON POWER SYSTEM OPERATION 26

4.1 General impact 26

4.2 Impact on power availability 26

4.3 Impact on distribution network planning 28

4.4 Impact on power losses 28

5 ENVIRONMENTAL ASPECTS 29

6 OBSTACLES, BARRIERS AND OPPORTUNITIES 31

6.1 Technically related 31

6.2 Economically related 33

6.3 Fuel related 36

7 COMPARISON OF DIFFERENT FORMS OF DG 37

7.1 Technical aspects 37

7.2 Economic aspects 38

7.3 Overview 38

8 ENERGY STORAGE 39

8.1 Benefits of energy storage 39

8.2 Energy storage technologies 41

9 REFERENCES 45

Appendices Number of Pages

APPENDIX 1 Small-scale energy production in Nordic countries 5

Vattenfall Utveckling AB U 03:04

1 Basic information

1.1 Definition

A large number of terms are used in relation to distributed generation. The following three examples define distributed generation from the point of view of size, location and interconnection point, respectively:

• Any on-site generator with less than “X” kW or MW of capacity. “X” ranges everywhere from 10 kW to 50 MW;

• Facilities located at or near load centre;

• Any generation interconnected with distribution facilities.

In [1] the following definition is given:

“Distributed generation is an electric power source connected directly to the distri-bution network or on the customer side of the meter.”

In the CODGUNet project the definition of “Distributed Generation” is:

“Under 20 MW electrical power production and 24 kV and below voltage levels”.

CODGUNet is an acronym for “Connection of Distributed Energy Generation Units in the Distribution Network and Grid”.

1.2 Purpose

The purpose with the project CODGUNet is to identify and specify the issues (prob-lems and possibilities) related to more common use of distributed energy generation (DG) in the network.

The CODGUNet project is a co-operation project between the Nordic countries. The project consists of 8 different work packages (WP). In the 3rd WP specific charac-teristics of different generation technologies are to be handled. This document is a part of WP3.

1.3 Background

Different kinds of DG technologies are developed world wide with large R&D bud-gets. Typical for these technologies are a great number of production units, which are small in size (from 10 kW up to about 10-20 MW) and located near the consumption of energy. These units often produce electricity for a certain end-user. One main idea of these units is that while the consumption of the end-user increases, additional elec-tricity is taken from the distribution network and when the consumption decreases, the generation unit may provide electricity to the network.

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Connection of lots of small DG units to the distribution network will have conse-quences related both to technological and legal matters. Network or electricity com-panies do not own typical DG units. When DG units become more common and the unit sizes increases, connection and disconnection effects on the net increase. The network and grid companies must pay certain attention to the variation of electricity generation causing problems in network balance and possible need for reserve capa-city. Effects on power quality from different kinds of generators are also of interest for the network and grid companies.

The quality of electricity has become critical by many customers, especially when it comes to changes of voltage (peaks, dips). Quality indexes and standards for elec-tricity products are being developed. Thus it is of great interest to study the effects of DG on the quality of electricity.

1.4 Scope

The most common and prospective technologies, which are used for distributed genera-tion, are presented in this document. The document summarises the main characteris-tics of different DG technologies and describes technical solutions for their network connection. Possible impact from DG on network operation is also discussed. The document contains a brief discussion concerning environmental issues for each DG technology and economic aspects regarding e.g. investment costs. Barriers and opportunities associated with utilisation of each technology are highlighted. Finally an overview is presented over existing energy storage technologies.

An overview on small-scale energy production in the Nordic countries is done in Appendix 1. This is done in order to make it easier to relate DG to the market for small-scale energy production.

The project CODGUNet has several work packages (WP). Items that are handled within other WP’s will not be treated deeply in this document. For example:

• In WP1 a general overview on issues related to the connection of DG in the network has been done. The work in WP1 also includes what kind of barriers network companies see with DG [2]

• In WP2 the present status of DG in the Nordic countries regarding existing national and company recommendations is described [3]

• In WP4 various technical aspects related to the network connection are analyzed by applying simulations [4]

• In WP5 influence of DG on power system is discussed [5].

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2 General description and characteristics of different DG technologies

2.1 Wind power

In a wind turbine the kinetic energy of the streaming air is converted to electric power. Normally power is produced in the wind speed range 4 to 25 m/s. The wind aero dynamical power is proportional to the cube of the wind speed and to the circle area swept by the wings. The efficiency of this conversion is theoretically close to 0.6 but in practice lower (0.4 to 0.55).

Wind turbines have developed rapidly from unit sizes below 20 kW (fixed speed, stall control) in the seventies to the present sizes of up to 4 MW. In order to withstand the mechanical stress, most wind turbines above 1.0 MW are equipped with a variable speed system incorporating power electronics in combination with pitch control. If advanced enough these systems are capable of decoupled active and reactive power control on the grid side and of decoupled torque and generator excitation control on the generator side.

Single units can normally be connected to the distribution grid 10, 20 kV. The present trend though is that wind power is being located off shore in larger parks that are con-nected to higher voltage levels (even to the transmission system). For such large in-stallations the transmission system owners in several European nations recently have written new connection regulations that imply e.g. that the wind turbines shall survive prolonged periods with low grid voltage and in some cases even frequency support is a requirement. This is a challenge that requires new concepts to be designed by the wind turbine manufacturers and will definitely rule out the uncontrolled systems. An un-controlled wind turbine may however be used if a DC connection system is used where the rectifier is capable of controlling the generator speed and thereby allowing generator speed to vary and thus controlling the mechanical stress of the turbine. In Denmark such comparisons have been made and in the Tjaereborg installation the HVDC Light rectifier can control the frequency to the wind turbines.

2.1.1 Characteristics

Asynchronous generators are used in most system designs. If synchronous generators are used they are connected to the grid by power electronics.

The power quality depends on system design. Direct connected asynchronous genera-tors may contribute to increased flicker levels and relatively large active power varia-tions. Converter connected systems may contribute to higher harmonic levels.

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Today the manufacturers believe that wind turbines for off shore applications will manage operation with a maintenance interval of at least 1 year.

The yearly production time depends on system technology, hub height and site. Front end 4 MW wind turbines at off shore sites are believed to produce power during 3500- 5000 hours/year while on land sited 600 kW turbines may produce during only 2000-3000 hours/year.

The yearly aero dynamical energy content is estimated to around 4000 kWh/m2 [6] for Swedish mean wind conditions at 100 m height. For groups of wind turbines the reduction in energy production is estimated to 2 – 8 %, depending on the distance between the wind turbines.

2.2 Photovoltaic

Photovoltaic (PV) systems convert the sunlight directly to electricity. PV technology is well established and widely used for power supplies to sites remote from the distri-bution network [7].

Photovoltaic systems are commonly known as solar panels. PV solar panels are made up of discrete cells that convert light radiation into electricity [8] connected together in series or parallel.


There are two basic types of PV equipment, stand-alone and grid connected. In the project CODGUNet, the latter is handled. PV systems can be single phase or three-phase, with or without transformer.

The output of PV modules is specified in standard conditions with power density 1000 W/m2 and cell temperature of 25 °C [7]. In practice the latitude which determines the path length of the solar radiation trough the atmosphere affects the power density and thus the output. Large variations in output are also due to the varying cloud conditions.

The maximum power output of a PV module is obtained near the knee of its charac-teristics (Figure 1). Since the voltage depends on the cell temperature a maximum power point tracking (MPPT) stage is needed in the converter to always get the maxi-mum power output [7].

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Figure 1 Typical characteristic of a PV module [7]

Production and performance of PV equipment is characterised by a number of parameters, e.g.

• Efficiency (inverter, array, overall plant)

• Final PV system yield Yf : the portion of the daily energy of the entire PV plant, which is delivered to the load per kilowatt peak of installed PV array (Euse,PV,d/P0). Typical annual values: Germany and Netherlands: 700 h/y, Israel: 1600 h/y

• reference yield Yr : the solar energy theoretically available per kilowatt peak of installed PV per day

• performance ratio PR: Yf /Yr, typically 0.72

• performance ratio vs. nominal power

• contribution to and from utility grid (ETU, EFU)[9].

Current units have efficiencies of 24% in laboratory conditions and 10% in actual use. The maximum theoretical efficiency that can be attained by a PV cell is 30% [8].

PV units are inverter connected and potentially cause harmonics. On the other hand, they may be sensitive to harmonics. Existing standards seem adequate, but the effect of multiple inverters should be investigated. Can inverters of the same type ‘synchro-nise’ and amplify the same individual harmonics [10]?

The inverters of PV system could operate, in the future, as active filters to reduce low order harmonics in the distribution system [10].

2.3 Micro-turbines

Distributed generation with micro-turbines is a new and fast growing business. The market is worldwide. If there is a demand of power there is a possible market for distributed generation with micro-turbines.

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In the Nordic countries distributed generation with micro-turbines are expected to be operated in combined heat and power mode. The reason for this is that the cost of power is close to the cost of heat. For each produced kilowatt-hour of electricity the micro-turbines will produce two kilowatt-hour of heat. With inexpensive fuels (if there are any in the Nordic countries) the power could be produced without taking care of the heat. Example of inexpensive fuel is landfill gas. The micro-turbines could also be used for peak shaving stand-by power, capacity addition, stand-alone generation and others. In the case of capacity addition the short time from decision and order to operation will be a heavy argument for DG with micro-turbines in the future.

The leading Micro-turbine Company is Capstone in the USA [11]. In USA we also find IR PowerWorks a subsidiary of Ingersoll&Rand [13] and Elliot [15]. In Europe we have Bowman in UK [12] and Turbec in Sweden [14].

Table 1 Micro-turbine companies and size of gas turbines

Company Size of micro-turbine kWe

Bowman [12] 80

Capstone [11] 28 and 60

Elliot [15] 75

Ingersoll&Rand [13] 70

Turbec [14] 100


The technology varies between the different manufactures.

The main components are: • Gas turbine and recuperator • Permanent magnet generator • Electrical system • Exhaust gas heat exchanger • Supervision and control system • Gas compressor

The most micro-turbines uses a turbine mounted on the same shaft as the compressor and a high-speed generator rotor. The rotating components can be mounted on a single shaft that spins up to 96 000 rpm.

The micro-turbine could be equipped with oil-lubricated bearings or air bearings. The micro-turbine manufacture Capstone uses the technology with air bearings and Turbec use oil-lubricated bearings.

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Figure 2 Principle components of micro-turbine (www.turbec.com)

Table 2 Micro-turbines Efficiency and Weight

Capacity kWel Efficiency Weight

Capstone 28 23% 500 kg

Capstone 60 25% 760 kg

IngerSoll&Rand 70 29% 1 860 kg

Elliot 80 28% 860 kg

Bowman 80 26% 1 900 kg

Turbec 100 30% 2 000 kg

2.4 Fuel cells

Fuel cells generate power through the electrochemical reaction between hydrogen and oxygen. The conversion is highly efficient and leaves only water and heat as by-products, which is the main motivation for the increasing interest in the technology.

The fuel cell is not a new technology, the principle has been known for two hundred years and development has been done for forty years. Fuel cells are very versatile and can potentially be used for every application needing power from cell phones to multi MW power plants.

For DG purpose fuel cells from 0,5 kW and upward are developed. Proposed appli-cations include continuous generation, cogeneration or power only, remote power, backup power etc. As hydrogen usually not is directly available most DG fuel cells are fuelled with hydrogen, which is processed from available hydrocarbons, e.g. natural gas. Normally the fuel processing is an integral part of the fuel cell system.

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http://www.turbec.com/


As of today there is only one fuel cell system that can be seen as at least semi-com-mercial, UTC Fuel Cell’s PC25. This is a 200 kW phosphoric acid fuel cell (PAFC) of which some 200 units have been sold worldwide. On the other hand there are numer-ous activities on-going with the objective of putting fuel cell systems on the market in the 2005-2010 time frame. While the development covers the entire range up to 1 MW (with loose plans of even larger systems) an emphasis can be seen on so called resi-dential fuel cells. Residential fuel cells are systems in the 0,5-10 kW range that are targeted against single or multi family houses, small enterprises etc and planned to be produced in very large numbers.

The large number of companies involved in developing fuel cells covers the whole range from small, specialised companies to large multinational companies. Among the leaders are companies such as Ballard, Plug Power, Siemens Westinghouse, Sulzer Hexis and Fuel Cell Technology, to name a few.


There are four major fuel cell technologies with somewhat different characteristics. The main apparent difference is the electrolyte, which also have far reaching effects on the design and operating characteristics of the fuel cell. In Table 3 these four tech-nologies are listed with some key characteristics.

Table 3 Major fuel cell technologies

PEMFC (PEFC)

PAFC

MCFC

SOFC

Electrolyte Protone Exchange Membrane

Phosphoric Acid

Molten Carbonate Solid Oxide

Operating tem-perature (°C)

80 200 650 800 – 1000

Electric effi-ciency based on natural gas* (%)

30 – 35 35 - 40 45 - 55 45 – 55

* With hydrogen as fuel the electric efficiency is the same or even higher for low temperature fuel cells as this is not a Carnot process. The reason for the higher efficiency with higher tem-perature for natural gas (or any reformed fuel) lies primarily in that fuel processing can be thermally integrated with the fuel cell and to a lesser extent to lower internal electric resis-tances.

All fuel cells generate a direct current, the voltage depending on cell voltage and the number of cells in series. Furthermore the voltage varies with the load and also to some extent with time as the fuel cell stack ages.

To obtain AC current the equipment has power-conditioning equipment handling DC to AC conversion and current, voltage and frequency control. Apart from supplying power to the external point of supply the fuel cell also has to cover some internal power needs, e.g. pumps, fans and control system.

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As fuel cells are in a development stage it is difficult to make general statements about operating characteristics as for example operating procedures tends to be on the cau-tionary side. Tentatively the following characteristics can be listed:

• Start-up time depends on type of fuel cell and type of fuel processing system. A low temperature fuel cell (PEMFC) with partial oxidation could probably be started in a couple of minutes while a high temperature fuel cell takes 3-4 hours due to the need of avoiding thermal stresses during warm up. Generally speaking, high temperature fuel cells are not suited for start-stop operation.

• A fuel cell in it self can facilitate nearly instant load changes. However, a fuel cell system has a limiting factor in the fuel processing system which has a certain time lag (varying depending on type) and a truly load following system would need a buffer, e.g. batteries or hydrogen storage capacity. A typical turndown ratio of a fuel system is about 1:5 and high efficiencies are kept to at least 50% load.

• Fuel cells have a potential for high reliability as the number of moving parts is low and consists of auxiliary equipment such as fans and pumps but this remains to be proven. The target for life length of fuel cells is usually given as 40000 h for the stack and at least twice the number of hours for the system. This target has been reached for a small number of fuel cells but in general still remains to be proven.

2.5 Reciprocating engines

Reciprocating engines, developed more than 100 years ago, were the first among DG technologies. Both Otto (spark ignition, SI) and Diesel cycle (compression ignition, CI) engines have gained widespread acceptance in almost every sector of the eco-nomy. They are used on many scales, ranging from small units of 1 kVA to large seve-ral tens of MW power plants. Smaller engines are primarily designed for transpor-tation and can usually be converted to power generation with little modification. Larger engines are most frequently designed for power generation, mechanical drive, or marine propulsion [8].

Reciprocating engines are usually fuelled by diesel or natural gas, with varying emis-sion outputs. Almost all engines used for power generation are four-stroke and operate in four cycles (intake, compression, combustion, and exhaust). The process begins with fuel and air being mixed. In turbo-charged applications, the air is compressed before mixing with fuel. The fuel/air mixture is introduced into the combustion cylin-der and ignited with a spark. For diesel units, the air and fuel are introduced separately with fuel being injected after the air is compressed. Reciprocating engines are cur-rently available from many manufacturers in all size ranges. They are typically used for either continuous power or backup emergency power. Co-generation configura-tions are available with heat recovery from the gaseous exhaust. Heat is also recovered from the cooling water and the lubrication oil [7],[8].

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Typically, synchronous generators are applied with internal combustion engines although some examples can be found where induction generators are applied.

The output of power quality from engine driven synchronous generators is good. In addition, from the viewpoint of reliability and availability the diesel power plants are relatively good.

Reciprocating engines are maintenance intensive, but they can provide high levels of availability [17]. Availability factor is usually between 90 and 97 % depending on the size of engine and the fuel applied [16],[17].

Typically, medium (275 – 1000 rpm) or high-speed (1000-3600) engines are applied for distributed power generation (engine speed classification from [17]). The output of medium-speed engines ranges up to 20 MW while high-speed engines are applied up to 5 MW level.

Figure 3 Classification of reciprocating engines [17]

The electric efficiency is 30 – 50 % for diesel engines and 24-45 % for natural gas engines [16],[18]. In co-generation application, a total efficiency of 80-85 % is achieved.

2.6 Stirling engines

On September 27, 1816, Robert Stirling applied for a patent for his Economiser at the Chancery in Edinburgh, Scotland. By trade, Robert Stirling was actually a minister in the Church of Scotland and he continued to give services until he was eighty-six years

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old. But, in his spare time, he built heat engines in his home workshop. Lord Kelvin used one of the working models during some of his university classes.

In 1850 Professor McQuorne Rankine first explained the simple and elegant dynamics of the engine. Approximately one hundred years later, Rolf Meijer coined the term ”Stirling engine” in order to describe all types of closed cycle regenerative gas engines [19].

Stirling engines are powered by the expansion of a gas when heated followed by the compression of the gas when cooled. The Stirling engine contains a fixed amount of gas, which is transferred back and forth between a "cold" and a "hot" end. The "dis-placer piston" moves the gas between the two ends and the "power piston " changes the internal volume as the gas expands and contracts.

The gasses used inside a Stirling engine never leave the engine. There are no exhaust valves that vent high-pressure gasses, as in a gasoline or diesel engine, and there are no explosions taking place. Because of this, Stirling engines are very quiet. The Stir-ling cycle uses an external heat source, which could be anything from gasoline to solar energy to the heat produced by mouldering plants.

The best working gas in a Stirling engine is hydrogen. Helium is working nearly as well as the hydrogen, but it is much more expensive. The cheapest alternative is air, but it has properties much inferior to the other two gases.

In spite of its long history, Stirling engines are still mostly used in some very special-ised applications, as in submarines or auxiliary power generators, where quiet opera-tion is important. Stirling engines are unique heat engines because their theoretical efficiency is nearly equal to their theoretical maximum efficiency, known as the Carnot Cycle efficiency.

The example of the co-generation system with a Stirling engine is shown in Figure 4. The heat source for the Stirling engine is situated outside the engine. The fuel is burned in the combustion chamber in presence of air. The hot gases from the combus-tion process warm up the Stirling engine. At the same time water or the surrounding air is cooling it down. Evidently, water is preferable because in this case the engine can be used also for heating purposes. The exhaust gases on their way out first warm up the air coming into the combustion chamber and then water for the heating system.

Advantages of Stirling engines are relatively high efficiency, good operation possi-bilities with partial load, good modularity and quiet operation.

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Stirling engine G

Air

CombustionFuel

Heatingsystem

Exhaust

Cooling

Figure 4 Co-generation system based on Stirling engine


Electrical efficiency of the engine is about 25%. The total efficiency in co-generation application is as high as 90%. If water is condensed from the exhaust gases the total efficiency can approach 100%.

Since the combustion is external almost any fuel is suitable for utilisation in Stirling engines. The most typical fuel is natural gas, but also bio-fuel and even solar energy can be used if future development is successful.

2.7 Steam cycle

Another possibility for co-generation consists of the system with boiler and steam turbine. Principal scheme is depicted in Figure 5. In the boiler the incoming water is transformed into dry steam under high pressure. The steam is transmitted to the tur-bine where it expands and as a result the electricity is produced. The wet steam leaves the turbine and passes through a heat condenser where it exchanges heat with water in the heating system. The water obtained as a result of steam condensation is transferred to the water tank. The task of the pump is to force the water to the boiler under the proper pressure.

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G Steam turbine

Water tank

Boiler Heat

condenser

PumpWater heater

Figure 5 Co-generation system with steam turbine

There are different types of steam boilers and almost any fuel can be used. Thus, there are boilers based on natural gas, fuel oil and bio-fuel.


The average electrical efficiency is about 20 to 30 %, but the total efficiency can reach 80-85% depending on the efficiency of the boiler, other losses and size of the unit. If an exhaust condenser is used and the boiler is fuelled with biofuel, the total efficiency can reach 110% [6].

Environmental properties of the steam turbine units depend on the fuel, which is used in the boiler and cleansing technology of the exhaust gases. Typically for steamcycle a 10 MWel biofuel plant has emissions NOx: 50 mg/MJfuel and dust: 20 mg/MJfuel [6].

3 Technical solutions for grid connection of different DG technologies

3.1 Typical arrangements

DG units can be grid independent or grid parallel as well as a combination of the both. In the latter case a grid failure means that the DG unit disconnects from the grid and works as grid independent and thus creates an “island” (islanding).

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3.1.1 General interface

The general interface for the typical arrangement for the DG connection to the medium voltage network is depicted in Figure 6. Connection and disconnection of the generator is made by the circuit breaker at the generator side (main breaker). Depend-ing on the size of the plant the disconnect switch on the grid side of the main trans-former may be replaced by circuit breaker. The scheme presented in Figure 6 is general and illustrates connection of synchronous (and asynchronous) generator tech-nologies.

Figure 6 One-line diagram of the typical arrangement [20]

There are a number of considerations, which are specific for different technologies. These considerations are presented below.

3.1.2 Wind power

Uncontrolled wind turbine systems

The dominating wind turbine technology for power below 1 MW is still the stall con-trolled fixed speed system. The stall control means that the wings are firmly attached to the hub and the wind attack angle of the wings is fixed. Thereby at higher wind speeds the turbulent flow will increase which reduces the transfer from aerodynamic to mechanical power. Fixed generator speed means that the rotor speed is stuck to the grid frequency and cannot be changed. A stall regulated fixed speed system normally is equipped with gearbox, capacitor banks and soft starter.

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Controlled wind turbine systems

The electrical power can be controlled in two ways, aerodynamical power control (active stall, pitch) and generator speed control.

Aerodynamical power control: The mechanical power can be controlled by intro-ducing a system with flexible coupling of the wings that can change the wind attack angle of the wings hydraulically or electrically and thereby maintaining a laminar flow around the wings also at high wind speed. In a pitch control system this angle can be changed in a large domain while in the active stall system the variation is limited. There are several benefits with a pitch system besides the obvious advantage of con-stant power at higher wind speeds e.g. reduction of thrust force at high wind speed and assisted start and emergency stop. Due to a higher impact on the dynamic voltage variations pitch regulation is normally not combined with a fixed speed system.

Generator speed control: The time constant of the pitch control is too large compared with wind variations from gusts and tower shadow effect and mechanical oscillations in the drive train. With a fixed generator speed system these variations will transfer to the electrical side giving unwanted variations in active and reactive power and thereby also in the grid voltage.

To avoid these disturbances a variable speed system can be used, incorporating power electronics to make possible that the generator speed differs from the synchronous speed, so that variations in the mechanical power are absorbed by changes of the rotor speed and thereby maintaining a relatively constant electrical power. By doing so also the reactive power will be constant which will reduce the voltage variations.

The speed control can be divided in narrow speed control and broad speed control [21]. A broad speed range increases the power production (before losses) and reduces the noise more than a narrow speed range.

The narrow speed control has two main candidate systems and the generator is of asynchronous type.

The first is a system where it is possible to vary the rotor resistance with power elec-tronics. This gives a slip (the relation between grid frequency and rotor speed) range from 1-10% at nominal load. Here only active power is directly controlled within this speed range but the reactive power will follow.

A slip and pitch controlled system is normally also equipped with gearbox, capacitor banks and soft starter.

The second system is referred to as a DFIG (Double Fed Induction Generator). In this system the stator side is directly connected to the grid while the rotor windings are fed with a 25 % of nominal power frequency converter (rectifier and inverter). The inverter side of the converter is connected to a common transformer for both stator and rotor side. Thereby the voltage imposed to the rotor can be vector controlled giving possibility to decoupling of mechanical and electrical behaviour of the generator and

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the grid side inverter can be controlled to give decoupled control of active (control the DC-link voltage) and reactive power.

A pitch controlled DFIG system is normally equipped with gearbox but there is no need for capacitor banks or soft starter. The speed variation range is typically –10% to +30 %.

The broad speed control system may use either synchronous or asynchronous genera-tors. A 100% of nominal power frequency converter connects the generator stator to the grid. No need for capacitor banks or soft starter. The speed variation range is typi-cally –30% to +30 % of synchronous speed. This implies that the broad speed control system can start at reduced cut-in wind speed and thereby increase the energy produc-tion compared with narrow speed control systems. The net energy production depends though on the overall losses and control principle and to our knowledge no such energy comparison has been made between the two systems.

• Synchronous generator: Low speed multi-pole large diameter synchronous generators allow gearbox free solution. This reduces noise and maintenance requirements. The excitation may be either electrically or by permanent mag-nets. The rectifier is free of choice and a diode rectifier may be a good choice.

• Asynchronous generator: Only a force-commutated rectifier can be used as the asynchronous machine need reactive power for excitation.

For both the DFIG system and the broad speed control system the grid connecting inverter is today of PWM-regulated forced commutated type (IGBT-transistors) that, compared with line commutated thyristor inverters, have several advantages e.g. reduced harmonics content and much better voltage control characteristics. Price and losses may though be higher than in the case of thyristor inverters.

It is believed that the generator voltage level will increase from the present normal value of 690 V, even for units up to 3 MW, to a future voltage level of several kV to reduce the winding currents and magnetic saturation during faults and starts. The con-verter voltage level preferably shall be designed to have the same voltage level but if that is not the optimal solution then adjustment has to be done electronically or with a transformer.

As mentioned in the introduction possible enlarged utilisation of the uncontrolled system may be realised together with e.g. a HVDC Light transmission system. This could be compared with a broad speed system including an asynchronous generator.

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3.1.3 Photovoltaics

A schematic picture of a small grid connected PV inverter is shown in the following figure.

Figure 7 Schematic diagram of a small PV inverter for grid-connected operation [7]

The DC/AC conversion is usually accomplished by a self-commutated PWM inverter that is current controlled. The amount of current depends on the DC power available from the PV array.

In order to avoid DC injection to the grid an isolation transformer is used. Alterna-tively a shunt or DC-current sensor can be used, that initiates inverter shutdown when the DC component of the output current exceeds the specified threshold [22]. It is also possible to make the transformer considerably smaller by applying a configuration as given in Figure 8 where it carries a high frequency AC current.

Figure 8 Diagram of a PV inverter applying high frequency transformer [23]

Considering the whole PV system there are alternative ways to achieve suitable modularity (see Figure 9). The central configuration is the most classical for especially residential PV system. There are several PV modules connected to supply one inverter. For reducing the amount DC wiring needed other solutions were adopted mid-nineties. String inverters are connected to one string of PV modules so that the junction box for parallel connection can be omitted [24].

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Figure 9 Different photovoltaic system configurations [24]

An AC Module is defined as an integrated combination of single solar module and inverter. The inverter converts the DC energy from the module into AC energy and feeds this energy into the AC network. The main advantage of AC Modules is modu-larity. A “Plug and play” type if grid connection is desirable. It is expected that AC Modules will be used worldwide within a few years [25].

The most important unresolved question is how to connect small residential AC Modules to the network. Is a separate feeder necessary or should connection to a regular outlet be allowed? Some countries have strict regulations due to safety reasons - some have more liberal guidelines [25].

3.1.4 Micro turbines

An example on electrical system of the micro-turbine is presented in [14]. Before the generated AC reaches the grid it needs to be rectified to DC and than converted to a three-phase AC. An inductor stabilises the AC output. An EMC filter protects the operation to prevent generated interference. The electrical system is entirely controlled and automatically operated by the power controller. The electrical system is used in reverse when it works as the electric starter to the gas turbine.

G ~= ~

===

Generator Rectifier/Start converter DC bus Converter Line filter EMC filter

Main circuitbreaker

Figure 10 Electrical system of micro-turbine [14]

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3.1.5 Fuel cells

A typical layout of a fuel cell system can be seen in Figure 11.

Figure 11 Typical power conditioning system of a residential fuel cell

In this system the DC/DC converter creates and maintains a constant DC voltage, here called DC Link, from the varying output from the fuel cell. A constant DC input to the inverter means that the latter can be efficiently designed and, furthermore, is “of-the-shelf product” from many manufacturers.

The grid connection is principally the same as for other DG technologies especially for large fuel cells. For residential fuel cells there is a need to keep costs down due to the small allowable unit cost. As development is ongoing it is not yet clear to which extent the need for cheap interconnections will be consistent with the demands of the grid operators [26].

At a loss of load the fuel cell cannot be stopped immediately. This will lead to that the voltage from the fuel cells races until the process is stopped or reduced by the internal controller if no remedy is introduced. In the PC 25 this is handled by dummy loads, which are used during the shut down [26]. In small fuel cells, as an alternative, bat-teries or super capacitors may be used to absorb the excess energy.

3.2 Start-up and shut-down

In work package 4 in the CODGUNet project [4], voltage behavior at DG start-up and shut-down is thoroughly handled, especially concerning wind- and microturbines.

3.2.1 Inverter based technologies

Inverters are typically equipped with automatic control that enables smooth start-up and shutdown. Inverters must include automatic synchronisation to the grid. To reduce transients resistive dummy loads can be used at start-up or shutdown.

The start-up and shutdown sequences may be illustrated by a micro-turbine example. During the start-up sequence the grid power is used to motor the micro-turbine. Power flow through the power controller is reversed. When power is available from the turbo

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generator, the power controller converts the generator output to 3-phase power syn-chronised with the utility grid.

Shut down procedures differs for normal and emergency stop. During a normal shut-down the following sequence of events occurs:

• Fuel flow to the turbo generator stops

• Rotation speed of the turbine decreases

• Output power flow ceases

• The power electronics switches off.

When an emergency shutdown is initiated by one or more protective relay functions, the following events occur:

• Output power flow ceases

• The power electronics switches off

• Fuel flow to the turbo generator stops.

3.2.2 Synchronous generators

The grid connection interface of a synchronous generator is equipped with automatic synchronisation, which ensures that no extra voltage or current transients occurs when the power plant is connected to the grid. In normal operation the power output can be driven close to zero before making the disconnection to avoid transients.

3.2.3 Wind turbine systems

CODGUNet WP4 [4] simulates and reports in more detail the transient and dynamical behaviour of the uncontrolled wind turbine system during start, shutdown and fault events. Here and in the next paragraph only an attempt is made to compare responses between the uncontrolled and the controlled systems for these kinds of events. In the CODGUNet WP5 report [5] the possibilities the controlled systems gives are discussed.

The shutdown will occur either at cut-in wind speed (4 m/s) or at cutout wind speed (25 m/s). At low wind speeds the disconnection will be smooth as both active and reactive flows are low. At high wind speeds the grid impact will be much higher.

Uncontrolled system:

During the start-up sequence the speed of the turbine is increased until the generator speed is close to the synchronous speed. The generator needs to be connected quickly why the soft starter operates for a short period. This in combination with the sub-sequent capacitor bank connection gives a high current peak followed by current oscillations caused by mechanical oscillations. The voltage dip will be rather large,

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depends on the SCR (short circuit ratio), due to the need for reactive power from the grid at the start.

Re-start at high wind speeds (below the cut-out level): The wing tip tilt system (see below) is not used. Therefore the turbine acceleration is much faster and the connec-tion procedure will give even worse grid impact.

The shutdown sequence at low wind (below synchronous generator speed): The gen-erator breaker will disconnect the generator and at the same time the capacitor banks (The main breaker is though still closed). The generator is then freewheeling awaiting higher wind (above the cut-in speed) that gives possibility for a new start attempt. The negative power quality (flicker) contribution is in this case only from the step in re-active power but as the banks are divided in several small units it could well be close to zero.

The shutdown at high wind speed (above the cut-out value) is started with tilting the wing tips to reduce the mechanical power. It's a regulation demand to have two inde-pendent brake systems. The following procedure is the same as at low wind to reduce the speed and active power as much as possible before opening of the generator breaker.

Emergency shutdown at line disconnection: In this case there is no time for tilting the wing tips. Full mechanical braking is needed, which gives a very high mechanical stress to the wind turbine. The worst case is at nominal power as the step in active power will be the nominal value at the line disconnection. The step in reactive power depends on the local compensation degree.

Controlled systems:

The use of pitch control as well as speed control and no need for capacitor banks (except slip controlled system) gives much smoother performance during start-up and shut-down. It also gives much less impact on the power quality.

At emergency shutdown due to line disconnection the pitch control will be used with-out need for the mechanical brake. The mechanical brake is then used as a parking brake. Manually there is a possibility to order both pitch controlled braking together with mechanical braking in case of large danger.

In normal case the rotor converter is not short-circuited.

3.3 Control in normal operation

In work package 4 In the CODGUNet project [4], voltage and current harmonics caused by converter based DG units is handled.

3.3.1 Wind turbines

The control principle of variable speed generators is normally confidential and varies for different manufacturers and system designs. Some possible combinations of prin-

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ciples are mentioned above (3.1.2) e.g. torque control, speed control, pitch control and voltage control.

The impact of various designs and control strategies on power quality differs rather much. Disconnections will always influence voltage and, if large enough, also the frequency. Very large disconnections due to domino effects or lack of redundancy in the grid design may even endanger the system stability.

The uncontrolled systems have several disadvantages as flicker contribution from the tower shadow effect, large 20% active power variations as function of wind gusts that even may activate tap changers, increased voltage dips during faults due to decreased capacitor compensation and thereby larger risk for protection activation and discon-nection.

In the controlled systems the converters are normally sensitive to disturbances. These have to be designed so that blocking and disconnections are avoided as much as pos-sible. With controlled systems it is though possible to smoothen the active power variations and assist in grid voltage control. Possible differences regarding this topic between different controlled systems are not well covered so far in published articles. Depending on switching methodology some broad speed systems may, except har-monics, produce inter-harmonics. According to the Swedish recommendation AMP [27] inter-harmonics shall be avoided.

3.3.2 Inverter connected technologies

As with harmonic distortion, the older line-commutated inverters generated current waveforms with poor power factor, but modern PWM inverters can generate power at unity power factor, i.e. the output current is exactly in phase with the utility voltage. Inverter designs that generate other than unity power factor and therefore can be used for power factor correction, are possible. However, these units must necessarily store energy through part of each cycle and are thus generally more expensive and less effi-cient than unity power factor inverters [25].

IEEE standard 929-2000 says that the PV system should operate at a power factor > 0.85 (lagging or leading) when output is > 10% of rating [22]. Utility-interconnected PV systems do not regulate voltage; they inject current into the utility. Therefore, the voltage operating range for PV inverters is selected as a protection function that responds to abnormal utility conditions, not as a voltage regulation function [22].

Micro-turbines are controlled and supervised with automatic control systems. In case of critical distortion the system automatically shuts down and records the fault. The power output depends on the type of the unit and selected mode of operation. The most interesting application for the Nordic countries is combined heat and power (CHP) mode, where power output follows the heat production. However, micro-tur-bines can be operated in the electric load following mode. In typical application the power factor is set to one. But there is a possibility to adjust power factor between for example 0.8 leading and 0.8 lagging [14].

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The load controls power on the DC-side as fuel cell acts similarly to a battery (with high internal resistance). In a grid-connected mode the fuel cell feeds in power against the full load of the grid, which means that there are special demands on the inverter in order to control the load.

3.3.3 Reciprocating engines

By adjusting the torque produced by the engine, the active power output of gensets is controlled. This means in practice the change of air/fuel ratio of the mixture to be burned in the engine [28]. Another control is needed for maintaining the desired termi-nal voltage at the generator. By adjusting the magnetising current of the synchronous generator, the reactive power output can also be controlled. Depending on the applica-tion several alternative modes for these main controls are available.

The voltage controller typically keeps the generator terminal voltage at a given value. An outer loop can also be added to voltage controller that takes care of reactive power balance.

The control for active power output is based on either the generator speed or the out-put power. The latter means that the power plant produces the given constant MW amount continuously. Plain speed control is appropriate only in island operation while in the case of grid with other generators a control with output MW based droop can be applied. This means that for each generator a share of the required output power change, during grid frequency variation, is defined by a droop factor. Special arrangements such as the one presented in [29] are needed when optimal operation of more than one different type of DG units is required.

The power controller also includes a down-kick module that shuts down the fuel in-jection in the case of sudden loss of load.

In practice modern engines also include emission control system [28] that is closely linked to the power controller.

3.4 Fault response

During faults, the grid voltage is reduced and the uncontrolled system generator, such as wind power plants with asynchronous generators, will reduce its torque and speed up. The sub-transient short circuit current, including the DC-component, from these systems may increase to values several times the nominal current. The connecting switchyard has to be dimensioned for this high current. As the generator speeds up there is also concern whether the kip torque, very much reduced at low grid voltage, is passed and for excessive consumption of reactive power. If the generator is not dis-connected before the fault is cleared the active power will increase, when decelerating, to values much higher than before the fault during typically half a second. The grid protection settings have to be co-ordinated with this.

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During faults the uncontrolled system will physically assist in grid frequency control by its inertia. In some grids with less frequency control strength (or speed) this may be rather valuable and there will be a balance between voltage and frequency protection settings.

After faults in some cases the uncontrolled systems will not recover meaning that the local grid voltage will remain low. They have to be disconnected. The system design including capacitor banks contributes to this possible negative behavior.

The controlled systems on the other hand probably control the wind turbine response rather differently and it is very much depending on whether tripping is allowed or not. A combination of converter and pitch control is essential.

The narrow speed systems may reduce the short circuit current significantly by in-creasing the rotor resistance. This also contributes to reduce active and reactive power flows after the fault is cleared. The voltage control is anyhow much better in the DFIG system than in the system with slip control due to lack of capacitor banks.

In the broad speed system the short circuit current may be completely controlled as the active power is entirely controlled.

3.5 Protection

In work package 5 In the CODGUNet project [5], protection issues are deeply handled. The focus is on protection of the generating units and protection of the net-work.

Standard protection for DG technologies includes protection against grid disturbances, over-under voltage, etc. As there is additional risk for islanding for DG equipment connected through inverters there are possibly a need for more sophisticated protec-tion for safety reasons. Thus, there usually is a requirement for an external switch to ensure personal safety for grid maintenance personnel.

The relay protection for DG typically include the following functions:

Under/Over Voltage (typically line voltage ±10%)

• Under/Over Frequency (typically line frequency ±1 Hz).

Any deviation outside these limits will cause the unit to shut down and disconnect from the utility line within a few cycles.

Protective functions recommended by the IEEE Power Systems Relaying Committee Working Group C-5 include [25], [30]:

• Transient over-voltage suppressors

• AC & DC under-voltage/over-voltage trips

• Current overload and short-circuit protection

• Under-frequency and over-frequency trips

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• Abnormal current flow in grounding conductor

• Loss and return of utility line voltage (re-closing)

• Over-temperature.

There is also an IEEE standard 929-2000 [22] that specifies the following normal operation ranges:

• Voltage: 88 % - 110 %

• Frequency: 59.3 - 60.5 Hz (at 60 Hz system)

According to this standard the DC current injected to the network must be limited to 0.5 % of the rated inverter output current [22].

Perhaps the most critical protection issue is “islanding” that can be defined as follows [10]:

Islanding is the continued operation of a grid-coupled inverter (or generator in general) in cases where the utility grid has been switched off, cut off or the distri-bution lines have been damaged so that no electric energy is delivered from the utility side.

In such a situation, the safety of persons and/or the safety of equipment might no longer be guaranteed.

In work package 5 In the CODGUNet project [5], controlled island operation is thoroughly handled. The focus is on different types of islands in power systems and the circumstances when island operation becomes advantageous are described. Diffi-culties as well as possible solutions are also handled.

Many anti-islanding methods have been identified in the literature and have been tested in practice. They can be divided into 2 groups [10]:

• Passive methods: a detection circuit monitors grid parameters (e.g. voltage, fre-quency, voltage phase jumps, and voltage harmonics); these methods do not have any influence on grid quality

• Active methods: a detection circuit deliberately introduces disturbances (e.g. active or reactive power variation, frequency shift) and deduces from the reaction to these disturbances if the grid is still present. The grid quality is somehow affected; however, ordinary devices like TV sets have a much bigger (negative) effect.

IEEE standard 929-2000 defines a test procedure for “non-islanding PV inverters”, which verifies whether an inverter will cease to energize the utility line under certain conditions [22].

There is also a German standard DIN VDE 0126 [31] that covers one-phase grid con-nected PV inverters. It is a prerequisite that anti-islanding protection is to be based on the monitoring of grid impedance and the disconnection is made by an independent switch.

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4 Impact on power system operation In work package 5 in the CODGUNet project [5], the effects on the power system when connecting DG are thoroughly handled.

4.1 General impact

Distribution systems are initially designed to transmit power in one direction – from the primary substation to the consumers. Generation within the distribution system may impact network operation issues such as Voltage profile along the feeder, Power losses, short circuit currents, Power quality and reliability indices, Voltage transients.

The impact on network operation may be both positive and negative. This depends upon the type and amount of DG, its location, as well as the power interfaces and the control schemes to connect the DG units to the grid.

4.2 Impact on power availability

One of the common arguments for utilisation of DG is additional power availability and improved reliability indices. There are a number of studies, which analyse this influence and make some conclusions [32],[33].

• Island operation – DG cannot support the system during a momentary interrup-tion, but there is a possibility to use DG to restore at least part of the load during a sustained interruption. Thus the intentional island can be created and parts of the load supplied by DG. Intentional islanding is presently not allowed in most of the cases of customer-owned DG.

• Impact on utility re-closing – DG on the feeder typically forces the utility to change their re-closer settings. The DG must disconnect during the fault in order to let the fault clear. Because of the DG the fault current increases and flows for a longer period of time. There is some risk that more equipment or conductor dam-age will occur, and that temporary fault could become a permanent fault, which could result in SAIFI (System Average Interruption Frequency Index) increasing. The typical dead time for the instantaneous re-close is 0.2 to 0.5 seconds. The recommended re-close interval on feeders with DG is 1 to 5 seconds. This may reduce power quality to the sensitive groups of customers.

• Out-of-phase re-closing – normally with anti-islanding protection the DG will trip when grid is disconnected. However, the possibility of DG run-on always exists. If the DG continues to feed an island with motor loads and capacitive com-ponents a situation may arise wherein the re-closer closes onto an island when the grid and DG voltage are not in phase. The out-of-phase re-closing will cause over voltage and large inrush currents, which may damage equipment in the system.

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G

DG

FaultA

B

Figure 12 Nuisance trip of breaker B caused by fault current contribution from relatively

large DG

• False tripping – relatively large DG near the substation can cause false tripping of the breaker. Consider a fault on a parallel feeder as shown in Figure 12. If breaker B does not have directional relays the fault can cause nuisance tripping of breaker B in addition to proper tripping of breaker A. Possible solutions to this problem are directional over-current relays or increased time delay for breaker B.

• Prevention of correct tripping – consider the fault near the line protection device that has the least available fault current at its location (Figure 13). The substation breaker will typically not be required to sense faults beyond this device. Fault cur-rent contribution from DG reduces fault contribution from the substation. In this case the protective device at substation takes longer to trip the breaker A or does not trip until the DG trips.

DG

Fault

Firstsectionalizingdevice

Systemfault current

DG fault currentA

Figure 13 Prevention of correct tripping of breaker A

• DG tripping during the network fault – the response from the DG protection to the fault in the network (or voltage sags caused by other events) may be their trip-ping. This may considerably worsen the situation in the network and lead to a domino effect.

If the DG unit is properly installed and operated the reliability of supply of the local grid improves radically, if controlled islanding is allowed. However, if there are only few DG in the network, reliability improvement for separate customers will hardly be noticeable on overall system reliability indices.

There can be impact on the utility indices in these cases:

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• Positive: DG can reduce the number or the duration of interruptions if automated

switches are available to restore power in DG supplied islands.

• Negative: There is a risk that while availability of DG owners increases, the over-all availability of the network will degrade.

4.3 Impact on distribution network planning

Some DG installations may allow the network utility to save money by deferring up-grades of certain facilities. In order for DG to provide this benefit the following crite-ria must be fulfilled:

• The DG must be installed on a circuit that is reaching its capacity

• Load on this circuit cannot be growing too fast, or the growth will overwhelm the capacity of the DG

• The DG technology must be capable of providing power consistently at times when the circuit is strained

• The network utility must have the right to call the unit on when needed.

• Several DG units connected to the distribution network can provide the possibility to omit the upgrades on the higher voltage levels.

An important case of very large benefit of DG to the network utility is pointed in [54]. The risk of planning for uncertain “block” loads can be hedged relying on the cus-tomer DG. These loads represent a significant quantum increase in feeder load in a single year, such as commercial or industrial facility. If the load is delayed or fails to appear, any investment the utility may have made in wire upgrades to accommodate the load will become a negative financial impact. In the Nordic countries, most com-mon DG are CHP and wind power.

4.4 Impact on power losses

In many cases the DG unit can reduce the feeder losses. It is possible to site the DG units on optimal locations for loss reduction. The principle would be the same as finding the optimal location for the capacitor banks. On feeders where losses are high a small amount of strategically placed DG with an output of 10-20% of the feeder demand can have a significant loss reduction benefit for the system. However, most DG units are not situated based on the most beneficial location for the utility, instead it is the DG owners decision.

One example on the impact on power losses is that wind power on the island Gotland increases the feeder losses locally, but decreases the losses for the whole system.

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5 Environmental aspects In work package 1 In the CODGUNet project [2], environmental barriers to wider DG implementation is handled.

5.1.1 Wind power

Wind power is an environmentally clean production source. The governmental per-mission procedures for approval of new wind power erections are nevertheless rather lengthy (two years is not unusual). Questions that are addressed are e.g. noise, bird and sea biology interferences, electromagnetic AC and DC fields and related travelling fish disturbances and in many cases maybe most important the visual impact and sun set disturbances. Power quality disturbances though are normally not discussed in the handling of the governmental permission. This is a subject for the grid owner.

Construction permission is always necessary for a wind power plant if at least one of the following conditions is present:

• The diameter of the wind turbine is larger than 2 meters

• The power plant is situated at a shorter distance from the real estate boarder than the height of the power plant

• The power plant shall be erected on a building.

The local authority can demand that a local plan shall be developed, at the developer’s expense, if a planning permission for a group of wind power plants is to be handled. This can take 6-9 months.

Wind power plants are regarded as activity, which influences environment, mainly due to the noise. An environmental permission is always required when a wind power plant shall be built. This is a fact independent of if the power plant is to be built on land or at sea.

If a wind power plant is to be built at sea, a concession is required for the cable con-necting the power plant with land. A report regarding the consequences on the envi-ronment shall also be done.

The policy in Sweden for outdoor noise levels is maximum 40 dBA during night in new living areas. For recreation areas the figure is 35 dBA.

In Finland the policy for outdoor noise levels is maximum 55 dBA during daytime and 50 dBA during night. In new living areas the outdoor figures are 5 dBA lower. For indoor the figures are 35 dBA during daytime and 30 dBA during night.

In Denmark the noise from wind power plants must not exceed 40 dBA outdoors in living areas. The plants shall be placed in groups in an easily comprehended pattern. If groups of wind power plants are intended to be placed closer than 2½ km, the affect on the landscape must be treated in a special report in the proposal. The distance

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between the plant and close living areas must be at least four times the height of the wind power plant. If plants are planned closer than 500 meters to a living area, pos-sible disturbances must be handled separately in the proposal.

5.1.2 PV

If the PV-cells are incorporated in the roofing from the beginning, they are a part in the construction permission for the entire building. If the roofing is to be changed in order to connect PV-cells, new construction permission is needed.

An environmental permission is not needed for PV-cells since they do not generate noise, pollution, vibration, light or other things that might be of disturbance for the environment.

5.1.3 Micro turbines

Construction permission is necessary before:

• Construction of a building

• Additional building extension

• Equip a building for an essentially different use than the original permission allows

Micro turbines can be regarded as activities of environmental impact, mainly due to the noise and pollution that can interfere with the environment. Therefore it is prob-able that an environmental permission is needed, even if there are some exceptions.

For an indoor installation noise levels in the range 70-80 dB is not expected to be a problem. But for outdoor placement the requirement for a quiet unit may be less than 45 dB.

Table 5 Noise from micro-turbines

Bowman 77 dB (A) at 1 meter in free field. Capstone 28 kW model 65 dB (A) on a distance of 10 meters. Capstone 60 kW model 70 dB (A) on a distance of 10 meters. Elliot 65 dB (A) on a distance of 10 meters. Ingersoll&Rand 73 dB (A) at 1 meter Turbec 70 dB (A) at 1 meter

The emissions vary little between the companies and also depend on the fuel. NOx< 15 ppm/v and CO<15 ppm/v are expected.

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5.1.4 Fuel cells

Fuel cells are virtually emission free. With hydrocarbons as fuels there are, apart from the inevitable carbon dioxide, trace amounts of NOx and CO from the fuel processing system. For governments fuel cells represent one important component in reducing green house gases due to its high electric efficiency and large number of possible applications.

5.1.5 Reciprocating engines

From the environmental point of view, it might be considered the fact that the engines have a relatively high sound level and therefore, e.g., embedded applications where generators are in houses are not so welcomed.

The exhaust gases of reciprocating engines include varying NOx and CO levels depending on the fuel and engine type. With suitable control systems and filters, the content of harmful constituents is kept below the limits currently stated by authorities, but still the level is relatively high for diesel gensets. However, the engines operating with natural gas have very low level of NOx output.

5.1.6 Stirling engines

From the environmental point of view Stirling engines are relatively good. According to [34] the 10 kWe Stirling engine fuelled by natural gas exhausts in average 15 ppm NOx at λ=1.4. This corresponds to about 10 mg/MJ NO2.

5.1.7 Steam cycle

Application of bio-fuel and especially waste requires installation of advanced external filters in order to reduce dust and NOx content in exhaust gases.

6 Obstacles, barriers and opportunities In work package 1 In the CODGUNet project [2], problems and barriers in association with DG is thoroughly handled. In the study also drivers and benefits of DG is briefly discussed.

6.1 Technically related

6.1.1 Requirements

Experiences from the wind power production in Denmark, and from the Swedish island Gotland, are that it is important to follow the technical requirements for keeping

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the stability in the network. The grid operator must be sure that the technical require-ments are followed and be sure of the quality of the equipment.

6.1.2 Power quality

Reverse power flow may cause over-voltage problems. The voltage at the customer’s terminals may exceed statutory limits. Selection of a proper power factor of the DG unit is one countermeasure. It is necessary to conduct further studies on the effect on distribution line voltage variation [10].

Short-circuit current from a DG unit may cause malfunction of over-current relays and fuses. It may be necessary to develop a new fault detection system [10].

One obstacle may be the need of ensuring safety of DG units: Should there be regular safety inspections of the equipment?

The possibility to operate as active filters reducing distribution system harmonics could make DG units with electronic interface more attractive from the point of qual-ity service [10].

6.1.3 Islanding

Islanding, see definition in chapter 3.5, seems to be the most controversial topic with grid-coupled DG units. However, theoretical studies show that islanding can only occur under very special and unlikely circumstances if basic safety methods are implemented. These methods are:

• Monitoring of grid voltage

• Monitoring of grid frequency [10].

In work package 5 In the CODGUNet project [5], difficulties and possible solutions concerning island operation is thoroughly handled.

Common international guidelines addressing the problem of islanding do not yet exist. Dangerous situations are very unlikely, but the consequences could be grave. One of the main questions is: “Which measures lead to an acceptable degree of security?” To reach an international consensus the following aspects have to be clarified:

• Definition of voltage and frequency limits for the operation of DG units

• Definition of the allowable duration of islanding

• Definition of standard test methods for islanding-prevention devices.

6.1.4 Location

Some DG technologies can be placed both indoors and outdoors. In the case of indoor solution it might be a problem to find space inside in buildings for example in the boiler room. In the Nordic countries, for example the micro-turbines can be placed

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outdoors or in a container. Low temperature can cause freezing problem during stop in wintertime for outdoor units and even for container solutions.

Regarding photovoltaic systems it should be noted that in the north the annual yield is far less than in countries closer to the equator. It is possible to integrate PV into resi-dential and commercial buildings.

6.1.5 Availability

In the Nordic countries we have a very high availability of power from the grid. This is a barrier for many DG installations. A DG-unit might decrease the power avail-ability for the DG-owner if he is dependent on one single unit.

A clear benefit of fuel combustion technologies is that the power is available practi-cally always in opposite of e.g. wind power or PV, which are available only in windy and sunny weather. From operational point of view the engine genset is superior in many ways. It can be started very fast and the power output can be controlled to give the required output at any time. Due to this diesel gensets are in many cases used for back-up or peak-shaving purposes. Thus, an opportunity might be to apply for exam-ple a diesel genset combined with e.g. a wind power plant. These kinds of hybrid applications can be found in the literature.

6.2 Economically related

In work package 6 In the CODGUNet project economic issues will be handled. The focus will be on how large scale DG introduction affects network business.

6.2.1 Grid connection

In Sweden, The Company, which has the concession for electricity distribution, has an obligation to connect all types of customers (both consumers and producers) in the area. The distribution company has the right to take a connecting fee corresponding to the cost of the specific connection.

Small-scale producers with plants smaller than 1500 kW and wind power units with generators smaller than 1500 kW, have today favoural special conditions for connec-tion and power transmission on the grid. If connection of the small-scale generators and wind power parks leads to reduction of the operation cost for the distribution system e.g. by reducing losses, the producers can receive compensation. On the other hand, if the connection increases the operation cost, the distribution company have no right to demand compensation from the producers.

A proposal has been put forward on “green certificates”, which also include a proposal to remove the special rules for production sources with a nominal power less than 1500 kW from 2003.

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When the producer starts planning the new DG unit he has to be sure that it is possible to connect the unit for a reasonable cost before spending too much money on the envi-ronment permission. The network company gives an offering for the specific connec-tion. Getting permission may take several years. During this time both the situation in the network and the legislation can change. This may be a problem for the network company to handle and give the relevant information. The investment is big and the situation is uncertain. Furthermore, the technology for the plant may be changed during the process of getting permission, which also is a problem for the network operator.

At the same time the costs for connection differs according to the grid situation. The first plant may have to pay for new lines and transformers etc, while the next project only pays for additional costs that can be several times less for the same maximum power. It could be difficult to co-ordinate the permission for two projects in order to optimise the investment and solution and get an altogether lower connecting cost.

In Denmark there are similar technical rules for connecting wind power to the net-work. The difference is where the responsibility is divided. All investment costs in the grid and all transmission costs are included into the price for the customers. It means that the grid operator has to pay these costs in the first place. Hereafter the costs are equally distributed among all customers. The economical problem related to the net-work connection does not concern the producers. With respect to the current preferen-tial status of DG, this status is expected to change.

In Germany regulations for grid connection are sometimes a barrier. One example is that an external disconnection switch is required, but this is often very expensive com-pared to the total system cost [9]. This is referring to small plants and probably this will be a fact even in the Nordic countries.

Rates and metering are barriers, before there is proper standardization of these issues.

6.2.2 Heat load demand

In the Nordic countries the fuel combustion technologies have their main market for combined heat and power production. The unit needs to be connected both to the elec-tric grid and to a heat distribution system. Only electricity may be produced, but in this case the heat created in the generation unit has to be stored for later use or to be cooled away. The heat from the unit can as an alternative be used for cooling system (air conditioning) but the cooling period is (normally) short in the Nordic countries. There are however projects for cooling in connection with micro-turbines in Norway and in Finland.

One interesting opportunity is Virtual Power Plant, which means that many DG units together can operate as a larger power plant and have a very high availability. This will give an excellent opportunity to use several small heat demands. One opportunity is to operate them in combination with heat storage that can store the heat produced

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for later use. The hot water in the storage will take the peaks in the heat load. An obstacle for this may be the cost of heat storage.

Another aspect with gensets is that units can be made so compact that they can be moved easily to desired location, e.g., in containers.

6.2.3 Development

There is a possible huge market for micro-turbines especially on the European conti-nent. But to reach the market the manufactures have to produce cheap and cost effi-cient standard engines. The new micro-turbines should have low maintenance and low investment costs. They must meet environmental demands. The units should not require special permits. A small unit cannot carry the cost for an expensive handling with engineering, permit and other costly questions. There are expectations that micro-turbines shall have low maintenance cost. But this has to be shown. To have costly engineering work will be an obstacle for the technology.

The short time from decision until a micro-turbine or other DG technology can be in operation is one other important opportunity. A traditional power station need years until it produces power and heat. In the future it is expected that new power from CHP DG can be in operation a couple of weeks from decision. The capacity can be in-creased in many small steps.

As a highly efficient, versatile and environmentally compatible new technology there is an abundance of opportunities for fuel cells. The potential market for stationary fuel cells in DG applications has been estimated to billions of dollar in various studies, which of course is a major attraction for the involved companies.

Obvious obstacles or rather uncertainties are the economic and technical challenges that lie ahead for fuel cells. If these are solved there are also questions regarding in-stallation, technical support and business model that needs to be addressed. The first two items is about building up a new service structure and the last is how to handle the at least initial technology risk.

For residential fuel cells there is a need for standards and codes with regard to the installation in buildings.

Today there are developed Stirling engines available for commercial application. However the interest for Stirling engines for co-generation is still low. For a wider commercial application of Stirling engines more documented experience from the installed units is required. Right now this experience is still insufficient in order to get serious attention to the technology. If Stirling engines are considered to be interesting for future applications, it is important to continue research concerning application of Stirling engine for co-generation and to perform more tests on real installed units.Price of electricity produced by for example PV systems is an obvious barrier. Many countries have programs to promote residential PV installations, and new programs

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are being launched [9]. Many government-subsidised projects are going on in various countries.

Steam boiler is a mature, well-tested technology with more than 100 years operation and development experience.

6.3 Fuel related

At most sites the micro-turbine or engine needs to be connected to a chimney and sometimes also to the gas grid. This can be costly and time consuming. Installation of a new chimney can be costly so connection to an old is recommended or to have an out door unit without chimney as for example Capstone.

If the micro-turbine is connected to low pressure (and a gas compressor increase the pressure to the demanded level of the combustion chamber) the barrier with permits may be reduced. To have a gas grid with natural gas is an opportunity but not neces-sary for micro-turbines. Denmark has an opportunity with a large gas grid but also Sweden and Finland have natural gas grid. The use of different fuel is an opportunity for the micro-turbine. Natural gas, town gas, biogas from landfill or sewage, oil, methanol are some of the fuel that can be used.

The low emissions are an opportunity and it is possible to find cases when the exhaust gas from the micro-turbine or engine is used in greenhouses. The carbon dioxide in the exhaust gas is used as fertiliser for i.e. growth of cucumber in greenhouses.

A barrier for fuel cells is the availability of fuel, primarily natural gas although other fuels as propane, biogas and diesel are feasible.

An obvious barrier for gas engines is the availability of the gas. For making the plant operation economical a gas pipe is required. For engines using diesel or other fuels the availability of fuel is generally good.

For Stirling engines an important research field is related to the fuel. One of the promising alternatives is bio-fuel. Thus in Denmark there is a test installation where the combustion chamber is replaced by the bio-fuel boiler. The engine is 35 kWe and has the electrical efficiency 19% and total efficiency 87% [35].

Almost any fuel can be used in steam boiler; for example in Sweden bio-fuel and waste are used more often. However, combustion of waste is very demanding. The fuel must be prepared for combustion – turned into a homogeneous mass, otherwise the fuel properties, particularly moisture content, will differ considerably, which will influence the thermal value of fuel.

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7 Comparison of different forms of DG

7.1 Technical aspects

There are different ways to classify the different DG technologies. The most important are the following criteria:

• Location: energy based or customer based

• Controllability and availability

• Energy conversion system: rotating machines or static inverters.

Energy based sites with hydro, wind, solar, biomass, geothermal, waves and other energy sources, as a rule, are situated far from the load centres. Therefore, the prob-lems related to the network connection are not an exception.

The CHP technologies such as micro-turbines, reciprocating engines, Stirling engines as well as fuel cells and photovoltaic can be placed at the customer site. The important condition is availability of gas infrastructure for the gas fuelled DG units.

Another important feature of various DG technologies is their availability and as a result the possibility to control power output. Thus, wind and solar energy are non-controllable and uncertain. Furthermore, the experience shows that days without wind and sun often correlate with peak load demand [36]. Therefore, it can be concluded that these technologies have no or little installed capacity value, since the reserve power units are needed in order to provide power supply when there is no wind or sun.

Most of the new DG technologies are connected to the network via an electronic inter-face. This class of rotating machines is represented by reciprocating engines, gas tur-bines and some wind turbines. There are several substantial differences in application and influence on the network from the two different systems. For example, short cir-cuit current for the induction and synchronous generator can reach 500-1000% of rated power current, while converter connected generator at the same site will produce only about 100-400% current [37]. Most generators are operated with a power factor between 0.85 lagging and 1. However, some inverter technologies can provide re-active compensation and thus perform the voltage control.

Distributed generators may introduce harmonics. In case of inverters there has been particular concern over the possible harmonic current contributions they may cause. Fortunately, these concerns are in part due to the older line commutated inverters. Most new inverter designs are self-commutated and use pulse width modulation to generate the injected sinusoidal wave. These newer inverters are capable of generating a very clean output, furthermore, they are able to perform the function of harmonic filter. Synchronous generators can be another source of harmonics. Depending on the design of the generator windings (pitch of the coils), core non-linearity, grounding and other factors, there can be significant harmonics.

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7.2 Economic aspects

The investment cost for wind power is decreasing with size and time, but increases when going to off shore sites. Rough estimates for present factory delivery costs are in the range 650 – 900 €/kW. Transportation, building and grid connection costs in the range 300 – 500 €/kW and the yearly operation and maintenance cost is around 1,7 % of the total investment cost. For off shore sites the cost may increase with 50% for the building and grid connection part. The total production cost is believed to decrease 15 – 25 % until 2010.

Cost targets for fuel cells are commonly given as 1000-2000 $/kW with O&M costs in region of 0,005 - 0,010 $/kWh. Today’s costs are in order of magnitude larger but is on the other hand for small, more or less hand built, series.

The values given for the installed costs of reciprocating engines varies greatly depending on the source; the overall range is from 250 to 1500 $/kW. For the opera-tion and maintenance costs values from 0,005 to 0,015 $/kWh are given [16],[18].

As an example, the investment cost for a complete micro-CHP from SOLO-Klein-motoren is about 1800 €/kWe. Annual operation and maintenance costs are estimated at about 6% of the total investments cost.

Particular investment costs for the boiler with steam turbine depends very much on the utilised fuel. Thus, the turbine unit with boiler based on natural gas or fuel oil costs around 1000 €/kWe. Units based on bio-fuel combustion are more expensive, for example investment cost for the unit with electrical capacity 10 MW can lie around 2000 €/kWe[34]. Smaller units are usually more expensive.

7.3 Overview

Table 6 summarises the main features of the principal DG technologies used today or having the potential application in the nearest future. The table also gives rough mar-gins for the installed and operation and maintenance costs for each technology [8],[38]. Fuel cells are a very promising technology, but they are presently very expen-sive. Micro-turbines presently are more expensive than the traditional combustion engines, but they are smaller, lighter, and operate with no vibration and less noise.

Table 6 Overview of different DG technologies Wind

generators Landbased

Wind generatorsOffshore

PV Micro-turbines

Fuel cells Stirling engines

Recipro-cating

engines

Steam cycle

Size, kW 10-3000 3000-6000

<1-100 25-500 5-3000 2-500 50-25000+

10000

Installed costs, €/kWe

950-1500 1100-1650

6000-10000

1000-1800

1000-2000

~1800 250-1500 1000-2000

Operation and mainte-

0.008 0.01 Little 0.008-0.015

0.005-0.01

0.018 0.005-0.015

0.005

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nance costs, €/kWh Emissions No No No Low Almost

no Low Fairly

low Fairly low

Availability on demand

Low* Low* Low* High High High High High

Location Energy-based

Energy-based

Energy / customer-

based

Customer-based

Customer-based

Customer-based

Customer-based

Customer-based

Commer-cial status

Available well

estab-lished

Available well

estab-lished

Available Availablecoming

into commer-

cial applica-

tion

2005 Available newly

introdu-ced

Available well

estab-lished

Available well

estab-lished

Application Green power

Remote locations

Green power

Remote locations

Green power

Base load

Co-gene-ration

Back-upPeak

reduction

Power quality

Base load

Co-gene-ration

Back-up Peak

reduction

Back-up Co-gene-

ration Peak

reduction

Co-gene-ration

Fuel - - - Natural gas

Natural gas

Any heat source

Natural gas,

diesel, biofuel

Natural gas,

diesel, biofuel

*Low availability of these technologies can be explained by uncertainty in energy source

8 Energy storage Although electricity cannot be directly stored, it can be easily stored in other forms and converted back to electricity when necessary. The peak value of the electricity may then cover the cost of storing the power.

In this document techniques regarding storage of electrical energy is treated. One rele-vant purpose to store energy is to compensate for fluctuations in production and demand. An important condition for energy storage to be interesting is that the effi-ciency rate is very high and also that the investment costs are not too high.

In [39] economical calculations regarding energy storage has been made. The high cost for energy storage has in [39] been seen as an obstacle for wide implementation.

8.1 Benefits of energy storage

The demand for electricity is seldom constant over time. Excess generating capacity available during periods of low demand can be used to charge an energy storage device. The stored energy can then be used to provide electricity during periods of high demand, helping to reduce power system loads during these times.

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The share of renewable electricity production is expected to increase by extension of intermittent generation. The extension of wind power plants will accentuate the need of electric energy storage. Power production variations in hours, days and even longer periods require some kind of adjustment. Use of electric energy storage can also reduce the need of investments in reinforcements in the network and grid [40].

From the network owners perspective there are several positive aspects with distri-buted generation in combination with energy storage. Increased network utilisation ratio, availability, transfer capacity and power quality are some examples.

Energy storage can improve efficiency and reliability of the electric utility system by:

• Reducing the requirements for spinning reserves to meet peak power demands

• Making better use of efficient base-load generation

• Allowing greater use of renewable energy technologies / distributed generation

Many renewable resources, wind and solar power for example, are intermittent. They are not available all of the time. Storing energy from the renewable source allows supply to more closely match demand. For example, a storage system attached to a wind turbine could store energy captured around the clock, whenever the wind blows, and then dispatch that energy into the higher priced midday market. And energy stor-age allows solar electricity to be used at night.

By reducing peak demands for power generation and offering greater flexibility among power supply options (including renewable), energy storage systems not only can help utilities by improving their cost-effectiveness, reliability, power quality and efficiency, they also may reduce the environmental impact of electricity generation, transmission, and distribution.

Other applications / motives that could be of interest for energy storage [41]:

• Possibility to postpone reinforcements in the grid

• Spinning reserve

• Peak power, load shedding

• Network stability

• Frequency control and power quality

• DSM (Demand Side Management)

• Energy efficiency

The discussion of the benefits that can be gained with the introduction of energy stor-age in the network has been going on for a long time. There are examples of true commercial introduction, as pumped storage for load levelling on the large scale and battery storage for peak shaving. There are also installations of SMES for power qual-ity purposes and flywheel storage for power quality and Ride-through capacity. Most of these applications are however special cases where the economic benefits are higher

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than what is the normal situation in the network. Many times the introduction of energy storage is technically well motivated but the rules governing the responsibili-ties for system and energy delivery makes it hard to find the economic motivation for such an installation.

8.2 Energy storage technologies

There are mainly six various energy storage technologies in use or under consideration by electric utilities [42],[43]. These are:

• Batteries

• Pumped hydro

• CAES (Compressed air energy storage)

• Flywheels

• Super capacitors

• SMES (Super-conducting magnetic energy storage)

Below is a short description of these six energy storage technologies.

Beside the specific energy storage technologies, the production of hydrogen from electricity is interesting, but not handled here. The hydrogen can be used as fuel in for example fuel cells.

8.2.1 Batteries

Batteries are the most common devices used for storing electrical energy. Prototype utility-scale battery energy-storage installations are currently being field tested, and manufacturing facilities for advanced batteries specifically designed for energy stor-age applications on power systems are being built in North America, Europe, and Asia.

• Lead-Acid Batteries: The traditional lead-acid battery is made up of plates, lead, and lead oxide immersed in a solution consisting of 35% sulphuric acid and 65% water. This solution is called “electrolyte,” and causes a chemical reaction that produces electrons.

Lead-acid batteries are the most common type of battery in both utility and non-utility applications.

A couple of variations on the traditional design have emerged:

Valve-regulated lead-acid (VRLA) batteries— are sealed and need no topping off with water, and so require less maintenance than regular lead-acid batte-ries. VRLAs are popular in distributed power applications.

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Gel-type lead-acid batteries — are filled with a gel instead of liquid, making

them much less likely to spill. These are becoming popular in Europe.

• Flow Batteries: A central battery unit provides power, but total energy is furnished by reservoirs of rechargeable anolyte and catholyte that can be as large as needed, and situated where convenient.

Flow batteries can be scaled to large-scale applications.

Three types of flow batteries are currently undergoing development and demon-stration for distributed power applications:

Zinc-bromine batteries

Vanadium redox batteries

Sodium-bromide batteries

A special technique that we refer to this category is the regenerative fuel cell that is represented by the commercial application “Regenesys”

The biggest advantage with the flow cell technique is the scalability. If you need to store more energy you only increase the volume of tanks containing the elec-trolyte fluids and if you want to increase power you increase the number or size of the conversion cells. One other advantage is that there are no time dependent losses, only conversion losses.

• Advanced Batteries: Advanced battery technologies currently under development include:

Lithium-ion

Lithium polymer

Nickel metal hydride

Sodium sulphur

Rechargeable lithium batteries are a very promising new energy storage tech-nology. Lithium-ion batteries in laptop computers, for example, can provide twice as much operating time as conventional batteries. Researchers have been explor-ing ways to develop this technology by studying how thin films of vanadium oxide improve the charge capacity and durability of these batteries.

8.2.2 Pumped Hydro

Pumped hydro facilities use off-peak electricity to pump water from a lower reservoir into one at a higher elevation. When the water stored in the upper reservoir is released, it is passed through hydraulic turbines to generate electricity.

The off-peak electrical energy used to pump the water up hill can be stored indefi-nitely as gravitational energy in the upper reservoir. Thus, two reservoirs in combina-

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tion can be used to store electrical energy for a long period of time, and in large quan-tities.

In the 1970s and 1980s many pumped hydro facilities were built in the USA. In the Nordic countries, Sweden had one that is no longer in use. Switzerland imports elec-tricity from France to use in Pumped Hydro Plants.

8.2.3 Compressed Air Energy Storage (CAES)

Off-peak electricity is used to power a motor/generator that drives compressors to force air into an underground storage reservoir. To produce electricity the compressed air is returned to the surface and used in gas turbines thus eliminating compressor work.

Utilities can use off-peak electricity to compress air and store it in airtight under-ground caverns. When the air is released from storage, it expands through a combus-tion turbine to create electricity.

The concept of compressed-air energy storage to help generate electricity is more than 30 years old. Two plants currently exist — an 11-year-old plant in McIntosh, Alabama, and a 23-year-old plant in Germany, both in caverns created by salt deposits.

8.2.4 Flywheels

A flywheel is a cylinder that spins at very high speeds, storing kinetic (movement) energy.

A flywheel can be used to store energy by combining it with a device that operates either as an electric motor that accelerates the flywheel to store energy or as a genera-tor that produces electricity from the energy stored in the flywheel.

The faster the flywheel spins, the more energy it retains. Energy can be drawn off as needed by slowing the flywheel.

Modern flywheels use composite rotors made with carbon-fibre materials. The rotors have a very high strength-to-density ratio, and rotate in a vacuum chamber to mini-mize aerodynamic losses. The use of super-conducting electromagnetic bearings can virtually eliminate energy losses through friction.

No large-scale applications of the technology have been made. Applications for power quality are interesting.

8.2.5 Super-capacitors

Capacitors are electronic devices. Conventional capacitors have enormous power but store only tiny amounts of energy. Batteries can store lots of energy but have low

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power — they take a long time to be charged or discharged. Super-capacitors offer a unique combination of high power and high energy.

Batteries are charged when they undergo an internal chemical reaction. They deliver the absorbed energy, or discharge, when they reverse the chemical reaction. In con-trast, when a super-capacitor is charged, there is no chemical reaction. Instead, the energy is stored as a charge or concentration of electrons on the surface of a material.

Super-capacitors found their first application in military projects. Common applica-tions today are within vehicle technology and for power quality.

8.2.6 Super-conducting Magnetic Energy Storage (SMES)

SMES systems store energy in the magnetic field created by the flow of direct current in a coil of super-conducting material that has been cryogenically cooled.

A super-conducting material enhances storage capacity. In low-temperature super-conducting materials, electric currents encounter almost no resistance. The challenge is to maintain that characteristic without having to keep the systems quite so cold.

Several 1-MW units are used for power quality control in installations around the world.

8.2.7 Data for energy storage

In table 7 some data are shown for alternatives of energy storage that in a short-term perspective are of big interest for integration with the grid [39].

Table 7 Prospective data for energy storage technologies

Size MWh Size MW Storage time Efficiency Investment cost €/kW

Batteries 40 10 Minutes – 24hours

100 ±30

Pumped hydro

<1 - >1000 10sec - season

appr. 75% 1000 - 2000

Capacitor msec - sec

Flywheel 100 – 1000 10 – 100sec

SMES 0.0001 - 10000

0.1 – 10000 0.01sec – 10hours

Depending on size

Thermal storage

>10 hours – 24hours

Hydrogen 10sec - season

High

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8.2.8 Grid connection

Electrical energy storage techniques are in general based on technologies that either provide the electrical energy as DC current or as an AC current with a frequency other than the nominal frequency of the network. Due to this the connection between the storage and the network is provided by converter technology. This means that grid connection and controllability of these technologies are similar to the connection of converter based DG as given in previous chapters. The exceptions from this are pumped storage and CAES, these produce their electrical energy in generators “directly” connected to the grid.

9 References [1] T. Ackermann, G. Andersson, L.Söder, “Distributed Generation: A Definition”,

Proceedings of the First International Symposium on Distributed Generation and Market Aspects, June 11-13, 2001, Royal Institute of Technology, Sweden.

[2] CODGUNet WP1, “General overview on issues related to the connection of DG in the network”, November 2002.

[3] CODGUNet WP2, “Present status of DG in the Nordic countries, existing national and company recommendations”, August 2002.

[4] CODGUNet WP4, ”Technical analysis of network connection of different types of generation units”, draft version December 2002.

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[6] Elforsk rapport 00:01, “El från nya anläggningar”, 2000.

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[39] Elforsk rapport 97:22, ”Elsystem med distribuerad elproduktion”, 1997.

[40] H. Bernhoff, “Review of electric energy storage”, UPTEC 2002006 R, 26th April 2002, Division for Electricity and Lightning Research, Uppsala University.

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Vattenfall Utveckling AB Appendix 1 U 03:04

Appendix 1

Small-scale energy production in Nordic countries Denmark

Denmark has long been an international front-runner in the development and imple-mentation of sustainable alternative energy solutions. As a result, Denmark is the country with extensive wind energy production. At the end of 2001, almost 6,500 wind turbines were installed in Denmark with a total capacity of approx. 2,500 MW - an excess of 1000 MW compared to the goal of 1500 MW by 2005 set in the Danish energy plan Energy 21. Therefore only few wind turbines sites are still available in the regional plans made by the local authorities. Furthermore, a new Danish payment system still needs to be introduced and the continued uncertainty of future payment conditions makes it difficult for potential wind turbines investors to finance new wind turbines.

In the future, Danish wind turbine installations are mainly expected to take place off-shore and by replacing small, old turbines with big, modern ones.

With only 80% of the wind energy content of an average wind year, 2001 was an un-usually poor wind year in Denmark. The Danish wind turbines produced 4.3 TWh (4.3 billion kWh) electricity in 2001 - equal to approx. 13% of Danish electricity con-sumption or the electricity consumption of one million Danish households. If the energy content in the wind had been average, wind power would have produced more than 16%. In 2001, 86% of the wind turbine electricity came from privately owned turbines and 14% from utility owned turbines. This picture will change somewhat in 2002 and 2003 with the construction of the two 160 MW utility owned offshore wind farms Horns Rev and Rodsand. The two large offshore wind farms will boost the share of wind power in the Danish electricity consumption. In 2002, wind power is expected to cover approx. 18-19% of the Danish electricity consumption and approx. 21% in 2003.

Denmark has a very high solar energy penetration per inhabitants compared to most other countries. The reason for this is a strong tradition for exploitation of renewable energy. Most important for the development has been a long and stable market development for small collectors until 1997 combined with the installation of a few very large solar heating systems for district heating. Today there are a total of about 35000 installed solar heating systems in Denmark. It is characteristic that Denmark has a very well established subsidy and quality scheme, and that manufacturers and installers have been on the market for many years. Sweden, Norway and Finland are one step behind, but also have solar heating systems, the number of which is increas-ing every year.

There is also some experience with application of photovoltaic (PV) units in Denmark. The first project (called SOL 30) consisted of 30 family houses in the town of

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Brædstrup. The next project (called SOL 300) consisted of 300 installations, where different customer categories where represented. The latest project (called SOL 1000) has been approved and will result in 1000 new PV installations.

Bio energy is available in the form of biogas, forest chips, fuel wood, wood waste, wood pellets, and furthermore it is produced to a very limited extent from willow crops in short rotation forestry. Today energy production based on biomass (and waste) is an important pillar for Denmark to live up to its obligations for a more sus-tainable energy sector and reduction of CO2 emissions. In Denmark biomass has become an increasingly important energy source over the last twenty years and bio-mass has thereby made a significant contribution to reduce Danish CO2 emissions.

The bio energy in Denmark is produced on around:

• Number of wood fired stoves: approx. 300.000-500.000

• Number of small boilers: approx. 75.000

• Number of district heating plants: approx. 120

• Number of combined heat and power plants: approx. 16

• Number of centralised biogas plants: approx. 20

• Number of farm biogas plants: approx. 20

In 1999 5 % of total Danish energy consumption was covered by biomass energy pro-duction.

The Danish hydrogen and fuel cells programs aim at early commercialisation of PEM fuel cells to use in the transportation sector as well as for decentralised stationary uses in buildings. The large share of wind power provides an incentive for this develop-ment.

Sweden

Sweden has very good natural conditions for both small- and large-scale hydro power plants. Today there is about 1200 small hydropower plants in Sweden, which annually produce about 1.5 TWh energy.

There are about 600 wind power units with total capacity over 300 MW. Most of the units are situated along the southwest cost of Sweden and on the island of Gotland (where wind power covers 12 % of the consumption). The total wind energy produc-tion was 482 GWh in 2001, which is about 0.34 % of the total energy production. There are no accepted official goals for development of the wind power production, but in different scenarios the theoretical potential for wind power in Sweden is esti-mated to be between 10 and 25 TWh, which corresponds to 7-18 % of the present consumption.

In Sweden, as well as in Norway and Finland, PV is often employed to power remote summer cottages. There are now more than 20 000 such PV-powered cottages in Sweden, and the market is increasing rapidly with more than 5,000 new installations

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every year. In addition, several hundred lighthouses along remote stretches of the Swedish coast are now powered with PV. Several larger units supply the energy to the bigger buildings and are connected to the power network, for example IKEA in Älmhult and several apartment buildings in Kristianstad. Total energy production from these network-connected units is almost negligible: there are about 20 units with overall capacity 127 kW.

Bio fuels today meet 19% of Sweden’s total energy use, but only about 2.4 % elec-trical energy. The major source of bio fuels in Sweden is from forest products, in the form of thinning and trimmings residues from trees, or secondary products from industrial processes, such as chips bark and black liquor. Utilisation of bio fuels has increased by about 50 TWh during the last years. It is forecasted that the increase will continue and bio energy production will reach 150 TWh in 2020.

Fuel cells have the big potential in the future due to effective conversion of fuel to electrical and thermal energy with low emissions. The major obstacle to commercial application of this technology is high price. During the last three decades the price decreased considerably and there is potential for farther reduction. However it can still take some time before fuel cells can compete economically with other established energy production technologies. In Sweden a number of demonstration installations, mainly for the research and development purposes have been performed. In 2002 there will be installed 2 fuel cells of type SOFC for both power production and research purposes in Hammarby Sjöstad.

Finland

By the end of 2001 there was 39 MW wind power installed in Finland. These 63 units produced 0.1 % of the total country’s electricity consumption. The Ministry of Trade and Industry has set the goal to reach 500 MW installed wind capacity in 2010. In order to reach this goal the state subsidies the investments into the wind power pro-jects. These subsidies have gradually decreased from 40% to the present 15-25%. In addition there is a tax relief of about 0.7 eurocents for each kW produced by the wind power units.

Finland's share of renewable energy sources in energy consumption is among the highest in Europe. As a user of bio energy Finland is number one. With a 20 per cent share wood is the main renewable source. Wood accounts for 10 per cent of electricity production, which is produced mainly in connection with heat production. By 2010, Finland aims to produce more than a third of electricity from renewable energy sources. Major part of this increase is expected to consist of bio energy.

The photovoltaic market in Finland is dominated by the small solar home systems for summer cottages, typically 50-100 W in size. The estimated potential is some 120 000 units. Navigational aids (typically 50-150 W) along the coastline form another sig-nificant market segment. Examples of larger applications in remote areas are tele-communication base stations. Also the Finnish coast Guard operates some 20 larger stand-alone hybrid systems with a PV capacity of 500-1000 W.

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The ambitious goal set by the Action Plan for Renewable Energy until 2010 is to in-crease the production by 50% when compared with the year 1995. A further goal is to double the use of renewable energy sources by the year 2025. This increase is to a large degree foreseen to rely on bio energy and hydropower, but ambitious goals have been set also for photovoltaic. The objective for photovoltaic electricity generation in 2010 is 40 MWe, meaning a 20-fold increase when compared with the 1998 situation. The prognosis for 2025 is 500 MWe. Thus the main emphasis in the coming decade is in creating the needed infrastructure (awareness, information dissemination, export, industrial activities) whereas volume effects are sought later. Then impact of photo-voltaic on the total environmental effects of the Action Plan is assessed to be less than 1% in 2010.

Norway

In Norway the wind power production is not extensive, but the potential is very big. In 2001 there were 17 MW installed capacity. The total wind power production was 20 GWh, which is less than 0.02% of the total production. In September 2002 20 new windmills with 40 MW power and expected yearly production of 120 GWh were in-stalled. Concession is given for another 204 MW of wind power. The Norwegian Energy policy sets the goal that in 2010 about 3 TWh will be produced using wind power. To reach this goal about 1000 MW installed wind power is required.

Siemens Westinghouse Power Corporation announced the first demonstration of a unique solid oxide fuel cell (SOFC) power generation technology fuelled by natural gas. A 250 kW plant will be installed in Norway and operated by Norske Shell. Shell has also signed an agreement with Aker Kvaerner and Statkraft to launch the project in Norway to explore possible large-scale applications of new zero-emission solid oxide fuel cell technology driven by natural gas. A project team from the three companies will perform a pilot study to explore the technology’s potential on a technical and commercial basis more closely. Shell and its partners aim to complete the project by 2010 and intend to become the first in the world to develop and commercialise large-scale, multi-megawatt fuel cells of this type.

The contribution of bio energy to the total energy production is about 6%. There is a goal to produce about 30 TWh bio energy by 2020. This corresponds to 10% of present energy consumption.

About 99.3% of the energy in Norway is produced by hydro power plants. One part of them can be related to the small-scale production. In January 2002 there were:

• 252 hydro power plants in power range 1-10 MW, having the total installed capacity of 915 MW and producing 4 321 GWh/year

• 98 hydro power plants in power range 0.1-1 MW, having the total installed capacity of 14 MW and producing 74 GWh/year

• 74 hydro power plants with power range 0-0.1 MW, having the total installed capacity of 3 MW and producing 18 GWh/year

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There are more than 80 000 PV units installed with capacity about 3.5 MW. These are mainly small units for summer cottages, which provide alternative to the power supply by diesel generators. The potential for solar energy in Norway is estimated between 5 and 25 TWh power production in 2030. The big gap can be explained by uncertainty when it comes to the future prices for the conventional energy, technological development and application of competing technologies.

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