TEI Patra: 3-18 July 2006Intensive program: ICT tools in PV-systems Engineering HYBRID PV/T SOLAR WATER HEATERS Soteris Kalogirou Higher Technical Institute

TEI Patra: 3-18 July 2006 Intensive program: ICT tools in PV-systems Engineering

HYBRID PV/T SOLAR WATER HEATERS

Soteris Kalogirou

Higher Technical Institute

Nicosia-Cyprus


Contents

• Solar energy in Cyprus– Solar water heating

• PV applications

• Hybrid PV/T systems

• PV/T system modeling

• Conclusions


Island of Cyprus• Cyprus is the third largest island in the

Mediterranean at 35° north latitude.

• Area = 9,251 km2.

• Population: about 750,000.

• Member EU as from 1 May 2004.


Cyprus energy scene

• Cyprus has no natural oil resources and relies entirely on imported fuel for its energy demands.

• The only natural energy resource available is solar energy.

• The climatic conditions of Cyprus are predominantly very sunny. – Daily average solar radiation of about 5.4

kWh/m2 on a horizontal surface.


SOLAR WATER HEATING

• At present solar energy covers about 4.5% of the total annual energy requirements.

• Furthermore, it contributes to a reduction in the atmospheric pollution by approximately 260,000 tons of CO2 per year.


Solar water heating in Cyprus


Typical solar water heater


Typical solar water heaters in Cyprus

• These are of the thermosyphon type and consists of: – Two flat-plate solar collectors having an absorber

area between 3 to 4 m2, – A storage tank with capacity between 150 to 180

litres,– A cold water storage tank; and– An auxiliary 3 kW electric immersion heater.

• In buildings which are equipped with oil fired central heating systems the boiler is used as auxiliary through a heat exchanger fitted in the storage tank of the unit.


Current situation

• 93% of houses• 50% of hotels• The estimated park of solar collectors in working

order is 560,000 m2

• Total number of units = 190,000• Corresponds to 3.7 inhabitants per unit World Record

– Second: Greece– Third: Israel

Equipped with solar water heaters


Systems installed

• 96% of the collector area installed up today (540,000 m2) are installed in houses and flats.

• In the tourist industry, it is estimated that: –44% of the existing hotels; and –80% of the existing hotel apartments

are equipped with solar-assisted water heating systems.

The contribution of solar energy to the total energy consumption in the hotel industry is 2%.


Installed solar collector area per inhabitant, 1994

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994


Collector performance prediction

• Simulation studies of this kind of system were conducted with TRNSYS and the typical meteorological year (TMY) values for Nicosia-Cyprus.

• Results:– 80% of the hot water needs are covered with solar

energy (solar contribution).– No auxiliary needs for four months.– Life Cycle Savings = £427 (725 Euro).


Monthly and yearly solar contribution of thermosyphon

solar water heaters

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Predicted monthly auxiliary energy needed by the system

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PV Applications

• PV can be applied in any environment– Snow– Sea– Desert– Space

• Some of the most typical are shown in the next slides


PV in snow


PV in Alaska


PV in sea


PV in desert #1


PV in desert #2


PV in space


PV on Mars


PV Applications

• Typical applications include:– Solar roofs– Transmission stations– Emergency lighting– Shading devices– Water pumping

• And many other…..


Solar roof


Water pumping


PV Shading-1


PV shading-2


PV shading-daylight


Roof system-daylight


Street lighting


Stand alone systems


Solar race


Solar car


Portable unit


PV transmission station


Car park shading


PV tracking


Concentrating PV


PV in Cyprus

• Despite the high penetration of SWH no other application has grown in Cyprus.

• Very few applications of PV are available– Powering transmission station in mountain

peaks (no grid available)– House applications– Water pumping– Telephone booths lighting

• One factory recently start operation


Telephone booth powered by PV


Highway advertisement lighting


PV-Shading. EAC head offices


Hybrid PV/T Systems

PV characteristics

Methods of extracting thermal energy

Air and water systems


PV Characteristics

PV panel

Solar radiation 100%

Waste85%Heatmostly

Electricity 15%

If waste heat is not removed PV can reach temperatures of 40°C above ambient


Why should a solar device produce both electricity and heat

• Because user needs both• Easy to cell electricity• Hot water is also required-save

conventional energy• Use one system instead of two

– Different systems:• One system for electricity (PV)• One system for solar thermal heat


Why we should have a combined system?

• There is not enough roof space• PV/T is cheaper• PV/T is nicer (one system of same appearance)• PV/T system is more efficient• It would be difficult to introduce PVs in countries

with good penetration of SWHs

A system producing electricity and hot water has better chances of success.


Temperature increase of PV modules

• The temperature of PV modules increases by the absorbed solar radiation that is not converted into electricity causing a decrease in their efficiency.

• For monocrystalline silicon solar cells efficiency decreases by about 0.45% for every degree rise in temperature.

• For amorphous silicon cells the effect is less pronounced, with a decrease of about 0.25% per degree rise in temperature.

• This undesirable effect can be partially avoided by a proper heat extraction with a fluid circulation.

• In hybrid Photovoltaic/Thermal (PV/T) solar systems the reduction of PV module temperature can be combined with a useful fluid heating.


Cell efficiency as a function of temperature

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Cell temperature (°C)

Cel

l eff

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Monocrystalline silicon

Thin film amorphous silicon


Sample of manufacturer catalogue


PV Cooling

• PV cooling is considered necessary to keep electrical efficiency at a satisfactory level.

• Natural or forced air circulation are simple and low cost methods to remove heat from PV modules. – they are less effective if ambient air temperature is

over 20°C.

• To overcome this effect the heat can be extracted by circulating water through a heat exchanger that is mounted at the rear surface of the PV module.


Hybrid Systems

• Hybrid PV/T systems can simultaneously provide electrical and thermal energy, achieving a higher energy conversion rate of the absorbed solar radiation.

• These systems consist of PV modules coupled to heat extraction devices. – air or water of lower temperature than that of

PV modules is heated whilst at the same time the PV module temperature is reduced.


Cooling of PV with Air

Stand alone panels BIPV

Air flow by natural means

PV panel

Wall

Air flow

PV panel

Air flow is used on purpose to lower temperature of PV and wall


Stand alone system


Building integrated PV (BIPV)


Cooling of PV with Water

PV/T UNGLAZED

PV/T GLAZED

To increase the system operating temperature, an additional transparent cover is necessary (like the glazing of the typical solar thermal collectors), but this has as a result the decrease of the PV electrical output from the additional absorption and reflection of the solar radiation.


Benefits of hybrid systems

• PV/T systems provide a higher energy output than standard PV modules and could be cost effective if the additional cost of the thermal unit is low.

• Air type PV/T systems are applied in buildings, usually integrated on their inclined roof or façade.

• By using these systems the electrical output of the PV is increased, while avoiding building overheating during summer and covering part of the building space heating needs during winter.

• The water type PV/T systems can be practical devices for water heating (mainly domestic hot water).


Air hybrid systems

• Air systems use ducts and fans to draw air from PV to heat a space.

• Air is drawn from ambient which is at lower temperature compared to that of PV.

• Electricity is required for the operation of the fan which can be taken either from mains or from PV.– PV operation is preferred as system operates when

there is sunshine and PV produces power.


Air hybrid systemsAdvantages

• Easily combined with existing a/c systems.

• Control is straight forward.

• When building temperature set point is reached, excess air is damped to atmosphere.


Air hybrid systemsDisadvantages

• Air leakage is difficult to detect.

• Ducts are bulky.

• During summer in hot countries it may be difficult to use ambient air which is at about 40°C to cool the PV system.


Water hybrid systems

• Water can circulate through pipes in contact with a flat sheet, placed in thermal contact with the rear surface of the PV module.

• The PV/T system consists of the thermal unit for water heat extraction, the necessary pump and the external pipes for fluid circulation to extract the heat from PV module and transfer it to the final use.

• Electricity is required for the circulation pumps which can be provided either from mains or from PV. – PV operation is preferred as system operates when there is

sunshine and PV produces power.


Water hybrid systemsAdvantages

• Water from mains usually remains under 20°C throughout the year in low latitude countries, which have high air temperatures during summer. – Therefore, water heating is useful during all seasons

for most applications.

• PV/T systems could be cost effective if the additional cost of the thermal unit is low and the extracted heat is effectively used.

• System is compact.


Water hybrid systemsDisadvantages

• Suitable for low temperature applications like domestic water heating.

• Water heat extraction is more expensive than air (need for heat exchanger).

• Water leakages are more problematic.

• Some form of storage is usually required.


Other considerations

• The increase of the electrical output of PV/T systems is of priority, as the cost of PV modules is much higher than that of the thermal unit.

• It is required to have as low temperature for the PV and as high for the thermal unit.

• Stabilising the temperature of the PV modules at a lower level is highly desirable and offers two additional advantages: – the increase of the effective life of the PV modules – the stabilisation of the current-voltage characteristic curve of the

solar cells. • Also the solar cells act as good heat collectors and are

fairly good selective absorbers (usually are of black or deep blue colour).


Test results of the PV/T models• Hybrid PV/T water systems testing includes

outdoor tests for the determination of the steady state thermal efficiency ηth and the electrical efficiency ηel.

• The thermal efficiency of the PV/T systems is

determined as a function of the global solar

radiation (G), the input fluid temperature (Ti) and

the ambient temperature (Ta).

• The electrical efficiency of the PV/T systems is

determined as a function of the operating

temperature TPV/T.


Steady state thermal efficiency

• Qu=AaFR[(τα)G-UL(Ti-Ta)]=mCp(To-Ti)

• The thermal efficiency can be obtained by dividing Qu by

GAa

• ηth =FR[(τα)-UL(Ti-Ta)/G]=mCp(To-Ti)/GAa

– m is the mass flow rate– Cp is the fluid specific heat– FR is the heat removal factor, – UL is the heat loss coefficient– τα is the transmittance absorptance produce, – To the output fluid ambient temperature; and – Aa the aperture area of the PV/T model.

• The thermal efficiency ηth of PV/T systems is calculated as a function of the ratio ΔT/G where ΔT=Ti-Ta.


Thermal efficiency graph

• Slope =-FRUL

• Intercept = FR(τα)nth

ΔT/G

Intercept

Slope


Electrical efficiency

• The electrical efficiency ηel depends mainly on the incoming solar radiation and the PV module temperature (TPV) and is calculated by:

• ηel=ImVm/GAa

• where Im and Vm the current and the voltage of PV module operating at maximum power.


Operating temperature

• The electrical efficiency of the PV cells depends on the incoming solar radiation and their operating temperature.

• The PV module temperature can be calculated as a function of the ambient temperature Ta and the incoming solar radiation G and is given by:

• TPV=30+0.0175(G-300)+1.14(Ta-25)• This relation is used for standard pc-Si PV

modules for horizontal roof installation.


Efficiency relationsHorizontal roof installation

Unglazed• pc-Si/T:

– ηth = 0.55 - 11.99 (ΔT/G)– ηel = 0.1659 - 0.00094 (TPV)eff

• a-Si/T:– ηth = 0.60 - 12.02 (ΔT/G)– ηel = 0.0601 - 0.00011 (TPV)eff

Glazed• pc-Si/T:

– ηth = 0.71- 9.04 (ΔT/G)– ηel = 0.1457 - 0.00094 (TPV)eff

• a-Si/T:– ηth = 0.75 – 8.83 (ΔT/G)– ηel = 0.0485 - 0.00011 (TPV)eff

(TPV)eff=TPV+(TPV/T-Ta)


Horizontal Roof InstallationThermal Efficiency

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Horizontal Roof InstallationElectrical Efficiency

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TRNSYS Modeling of Complete System

Program description

Collector model

Daily and monthly results


TRNSYS MODELLING OF A COMPLETE SYSTEM

• TRNSYS is an acronym for a “TRaNsient SYstem Simulation program”.

• This program was developed by the University of Wisconsin by the members of the Solar Energy Laboratory.

• The program consists of many subroutines that model subsystem components.


Components in TRNSYS libraries

• Collectors– Flat plate – CPC

• Storage tanks• Pump, Fan• Auxiliary heater• Thermostat• Heat Exchanger• PV, battery, inverter• PV/T collector

• Data reader• Weather data generator• Solar radiation processor• Quantity integrator• Thermal load• Economic analysis• Printer• Plotter• …etc


Hybrid system construction details

Pump

HotWater

CylinderHybrid PV/T panels

Hot water supply

Mains water make-up

Insulation

Heat exchanger

PV module

Glass cover

Casing

PV#1 PV#2 PV#3 PV#4

a. Hybrid PV/T panel c. Collector Cross section

b. complete system diagrammatic


Hybrid PV/T system schematic

Regulator/Inverter (part of TYPE 49)

Battery Bank (part of TYPE 49)

Utility Back-up

Electrical Load TRNSYS TYPE 14

Hot water Cylinder TRNSYS TYPE 38

Differential Thermostat TRNSYS TYPE 2

Pump TRNSYS TYPE 3

Hybrid PV/T Collector TRNSYS-TYPE 49

Hot Water Load TRNSYS TYPE 14

Mains Water Supply TRNSYS TYPE 14

LEGEND: Electrical line Water line Control line


Type of data required

• Parameters– Constant data required for a particular type

such as area, flow rate, etc.

• Inputs– Input data either from other modules or

constants.

• Outputs– Data that can be printed or used as inputs to

other modules


Consumption profiles

• The present model considers that the system is applied to a house of four persons.

• For this application, both electricity and hot water consumption (load) profiles are required.

• Both of these loads are subject to a high degree of variation from day to day and from consumer to consumer, however, it is impractical to use anything but a repetitive load profile.


Consumption profile-Hot water (120 lt/day)

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wat

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(l)


Consumption profile-Electricity (25700 kJ/day)

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1. Optimisation of flow rate

• All simulations with TRNSYS were performed with the weather condition of Nicosia, Cyprus.

• The electrical energy output from the PV panel increases as the flow rate increases. – This is due to the fact that the panel is

working at a lower temperature. • The thermal energy output increases and

then drops.


Hybrid PV/T system model output for various flow rates

Water flowrate

Electrical energy output

(Qe)

Useful thermal energy output

(Qu)

Electrical energy required from utility to

cover the load (Qutil)

Thermal auxiliary energy required to cover hot

water load (Qaux)

[lt/hr] [GJ] [GJ] [GJ] [GJ]

0 0.977 0.000 8.589 8.296

20 2.584 6.721 7.397 3.634

25 2.646 8.293 7.142 3.742

50 2.737 4.308 7.061 4.936

100 2.763 2.769 7.037 6.355

150 2.770 1.007 7.034 6.987

Note: Total annual incident solar energy =34.47 GJ

Qutil and Qaux indicate the extra energy required from utility to cover the electrical and thermal loads respectively, which cannot be covered by the solar heat. It should be noted that thermal energy is also electrical energy as electricity (immersion electrical heater) is assumed to be used for hot water production.


Optimum system flow rate• The optimum value of flow rate can be found by

adding the total system output energy – (Qoutput= electrical + thermal)

• and the required extra energy – (Qrequired=electricity from utility + auxiliary thermal

energy) • and plotting the values against water flow rate.• The optimum value corresponds to 25 lt/hr. • This low value of flow rate suggests that the

system can be used in a thermosyphon mode, i.e., without a pump and differential thermostat which will enhance the economic viability of the system.


Optimum system flow rate

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Water flow rate (kg/hr)

En

erg

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Qoutput

Qrequired

Qoutput= electrical + thermal

Qrequired=electricity from utility + auxiliary thermal energy


System Efficiency

• The system mean annual efficiency is defined as the total electrical energy output from the cell over the total incident energy from the sun, over a period of one year.

• For the hybrid system, operated at the optimum water flow rate of 25 kg/hr, this is equal to 7.7% whereas the maximum value reached is 8% corresponding to higher values of flow rate.

• It is important to note that the corresponding efficiency value of a standard PV system (without cooling) is only 2.8%.

• The hybrid system mean annual efficiency increases to 31.7% when the thermal output is also considered. – This value corresponds again to the optimum flow rate of

25 kg/hr.


Cell efficiency and total system efficiency against water flow rate

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Water flow rate (kg/hr)

Effic

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Cell Efficiency

System Efficiency


2. Hourly performance of the system

• First figure shows both the electrical and thermal output (Qe and Qu) and input (Qutil and Qaux) of the system on an hourly basis.

• Second figure shows the solar energy input (Qins) and output of the system (Qout), as well as its total efficiency on an hourly basis.


Hybrid system performance working with optimum flow rate (April 21st)

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Time (hours)

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Electrical output (Qe)

Energy from utility (Qutil)

Useful thermal energy (Qu)

Thermal energy from auxiliary (Qaux)

The curves of the hourly variation of energy supplied from utility (electrical) and from auxiliary (thermal - immersion heater) show the energy required to cover the electrical and thermal loads imposed by the consumption profiles.


Variation of system input and output

energy and efficiency (April 21st)

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%)

Incident solar radiation (Qins)

Qout (=Qe+Qu)

Efficiency

It is interesting to note that the system efficiency reaches a value of 51.6% at 9.00 whereas the corresponding value for the non-hybrid system is 8.2%.

Both Qe and Qu curves follow, as expected, the pattern of solar radiation


3. Monthly performance• The variation of the monthly electrical efficiency

of the PV panel for flow and no flow conditions is shown in next Figure.

• The advantage of the hybrid system (with flow) is obvious.

• Additionally as it can be seen from this figure the efficiency drops during the summer months because of the higher ambient temperature and solar radiation available, which results in higher panel temperatures, thus relatively lower performance. – This is more pronounced for the case of the standard

PV panel.


Variation of monthly electrical efficiency of PV panel with and

without flow

0123456789

JAN FEB MAR APR MAY JUN JUL AUG SEP OCT NOV DECMonths

Effic

ienc

y (%

)

With f low

No flow


Monthly performance of the system

• The monthly variation of the various energy flows of the system is shown in next Figure.

• These include: – the total electrical output from the system (Qe), – the energy required from utility to cover the electricity

consumption (Qutil), – the useful thermal energy supplied to the tank (Qu), – the hot water energy requirements (HWLoad), and – the thermal auxiliary energy demand (Qaux).


Monthly performance of the system

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Qe Qutil Qu HWLoad Qaux

The maximum value of the useful thermal energy (Qu), occurs in the month of June (8.6 GJ). The electrical energy supplied from the collector (Qe) is almost constant throughout the year and is maximised in the month of August (2.6 GJ).


Monthly and yearly solar thermal contribution of the PV/T system

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Economic Analysis

• The economic analysis of the system was performed by considering the extra cost required to modify the PV modules and the other equipment required for the construction of the hybrid system against the extra energy benefit obtained by the modified system,

• i.e., extra electrical energy above the one obtained from non-hybrid PV module and the thermal energy output.


Economic factors considered

• It is estimated that the extra cost required to modify the system is 760 Euro.– 425 Euro to modify the panels and 335 Euro for the

cost of the pump, the differential thermostat, the electrical connections and the extra piping required.

• Electricity is assumed to be used for the auxiliary thermal energy required.

• The saving in electrical energy per year that results from the system modification (extra electrical plus the thermal energy output) is equal to 235 Euro.


Method used

• The economic viability study of the system is based on the life cycle analysis (LCA) method and takes into account the fuel inflation rate, the market discount rate, the annual maintenance cost and the power required by the solar pump.

• The economic scenario used assumes that all the cost of the system is paid at the beginning (i.e., no credit payments are assumed).

• The period of economic analysis is taken as 20 years.


Parameters used in the economic analysis

Parameter Value Collector area Period of economic analysis % Down payment Nominal market discount rate Extra maintenance in year 1 General inflation rate Resale value Conventional fuel inflation rate

5.1 m2 20 years 100% 9% 1% or original investment 5% 10% 3%


Results of economic analysis

• The results of the economic analysis show life cycle savings equal to 1345 Euro.

• The pay back time is found to be equal to 4.6 years which is very satisfactory.


CONCLUSIONS-1

• Hybrid photovoltaic-thermal (PV/T) collector systems may be applied in order to achieve maximum energy output by simultaneous electricity and heat generation.

• In this way the energy efficiency of the systems is increased considerably and the cost of the total energy output is expected to be lower than that of plain photovoltaic modules.


CONCLUSIONS-2

• Photovoltaic modules need to be cooled to retain their high efficiency. This can be done with water or air. The heated fluid can either be disposed or used for the heating needs of a building.

• Hybrid PV/T systems when properly designed provide benefits. These offer a possibility to increase the effectiveness of PV systems by providing both electricity and heat.


We eventually have to chooseThis or This

Thank you for your attention,

any questions please….

Documents

TEI Patra: 3-18 July 2006Intensive program: ICT tools in PV-systems Engineering HYBRID PV/T SOLAR WATER HEATERS Soteris Kalogirou Higher Technical Institute