Hybrid PV-T Solar Systems for Domestic Hot Water and Electricity Production

Energy Conversion and Management 47 (2006) 3368–3382

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Hybrid PV/T solar systems for domestic hotwater and electricity production

S.A. Kalogirou a,*, Y. Tripanagnostopoulos b

a Higher Technical Institute, P.O. Box 20423, Nicosia 2152, Cyprusb Physics Department, University of Patras, Patra 26500, Greece

Received 20 January 2005; received in revised form 26 July 2005; accepted 30 January 2006Available online 22 March 2006

Abstract

Hybrid photovoltaic/thermal (PV/T) solar systems can simultaneously provide electricity and heat, achieving a higherconversion rate of the absorbed solar radiation than standard PV modules. When properly designed, PV/T systems canextract heat from PV modules, heating water or air to reduce the operating temperature of the PV modules and keepthe electrical efficiency at a sufficient level. In this paper, we present TRNSYS simulation results for hybrid PV/T solarsystems for domestic hot water applications both passive (thermosyphonic) and active. Prototype models made from poly-crystalline silicon (pc-Si) and amorphous silicon (a-Si) PV module types combined with water heat extraction units weretested with respect to their electrical and thermal efficiencies, and their performance characteristics were evaluated. TheTRNSYS simulation results are based on these PV/T systems and were performed for three locations at different latitudes,Nicosia (35�), Athens (38�) and Madison (43�). In this study, we considered a domestic thermosyphonic system and a lar-ger active system suitable for a block of flats or for small office buildings. The results show that a considerable amount ofthermal and electrical energy is produced by the PV/T systems, and the economic viability of the systems is improved.Thus, the PVs have better chances of success especially when both electricity and hot water is required as in domesticapplications.� 2006 Elsevier Ltd. All rights reserved.

Keywords: Solar energy; Photovoltaics; Thermal collectors; Hybrid photovoltaic/thermal system; Water heating

1. Introduction

The temperature of PV modules is increased by the absorbed solar radiation that is not converted into elec-tricity, causing a decrease in their efficiency. For monocrystalline (c-Si) and polycrystalline (pc-Si) silicon solarcells, the efficiency decreases by about 0.45% for every degree rise in temperature. For amorphous silicon (a-Si)cells, the effect is less, with a decrease of about 0.25% per degree rise in temperature depending on the moduledesign. This undesirable effect can be partially avoided by a proper heat extraction with a fluid circulation. In

0196-8904/$ - see front matter � 2006 Elsevier Ltd. All rights reserved.

doi:10.1016/j.enconman.2006.01.012

* Corresponding author. Tel.: +357 22 406466; fax: +357 22 406480.E-mail address: [email protected] (S.A. Kalogirou).

mailto:[email protected]

Nomenclature

Aa collector area (m2)a-Si amorphous siliconC investment cost (€)Cp specific heat (kJ/kg �C)c-Si monocrystalline siliconCaux cost of auxiliary energy (€)CFA cost rate of auxiliary energy (€/kJ)CFL cost rate of conventional fuel (€/kJ)Cload cost of fuel to cover load (€)d market discount rate (%)DT temperature difference (�C)FYFS first year fuel savings (€)G total global solar radiation (W/m2)i interest rate (%)Im current of PV module operating at maximum power (A)LCS life cycle savings (€)_m mass flow rate (kg/s)N number of yearsgel electrical efficiencygth thermal efficiencyp-Si polycrystalline siliconPV photovoltaicPV/T hybrid photovoltaic/thermal collectorPW present worth (€)Qu useful energy extracted (kJ)Qaux auxiliary energy (kJ)Qload energy required to cover load (kJ)Ta ambient temperature (�C)Ti inlet temperature to collector (�C)To outlet temperature from collector (�C)TPV PV module temperature (�C)TPV(eff) effective PV module temperature (�C)TPV/T operating temperature of PV/T module (�C)Vm voltage of PV module operating at maximum power (V)

S.A. Kalogirou, Y. Tripanagnostopoulos / Energy Conversion and Management 47 (2006) 3368–3382 3369

hybrid photovoltaic/thermal (PV/T) solar systems, the reduction of PV module temperature can be combinedwith useful fluid heating. Therefore, hybrid PV/T systems can simultaneously provide electrical and thermalenergy, achieving a higher energy conversion rate of the absorbed solar radiation. These systems consist of PVmodules coupled to heat extraction devices in which air or water of lower temperature than that of the PVmodules is heated, while at the same time, the PV module temperature is reduced. In PV/T system applica-tions, the production of electricity is the main priority, and therefore, it is necessary to operate the PV modulesat low temperature in order to keep the PV cell electrical efficiency at a sufficient level. Natural or forced aircirculation are simple and low cost methods to remove heat from PV modules, but they are less effective if theambient air temperature is over 20 �C. To overcome this effect, the heat can be extracted by circulating waterthrough a heat exchanger that is mounted at the rear surface of the PV module. PV/T systems provide a higherenergy output than standard PV modules and could be cost effective if the additional cost of the thermal unit islow. Water type PV/T systems can be practical devices for water heating (mainly domestic hot water), but theyare not improved enough yet for commercial applications. Therefore, the objective of this work is to evaluate

3370 S.A. Kalogirou, Y. Tripanagnostopoulos / Energy Conversion and Management 47 (2006) 3368–3382

the performance and financial improvement of PV/T systems compared to standard PV systems, which couldprove beneficial to the greater diffusion of PV units. Also, for countries with good penetration of solar waterheaters (Cyprus = 93% and Greece = 25%), where it is a habit to produce hot water with solar energy, itwould be difficult to convince potential customers to install a PV system, whereas a hybrid system producingboth electricity and hot water has better chances of success.

The main concepts of hybrid PV/T systems have been presented in several publications from 1978 [1–5].Other interesting applications are a low cost PV/T system with transparent type a-Si cells [6] and building inte-grated PV/T systems [7]. Following these initial studies, the design aspects of a water type PV/T system [8] anda detailed analysis of liquid type PV/T systems [9] were presented. More recently, results from integrated PV/Tsystems with hot water storage [10,11] and PV/T collectors with polymer absorber [12] were given. Severalother models of water cooled PV/T systems, like models for PV/T prototypes with water heat extractionmodes [13,14], modeling results [15] and a study on domestic PV/T systems [16] have been presented. Finally,PV/T thermosyphonic solar water heaters were studied regarding their performance [17,18], and a commercialthermosyphonic system [19] was introduced in the market. Most of the above works give experimental andmodeling results regarding the performance of PV/T systems with forced or natural flow of the heat removalfluid, but only few of them [16,19] include information on cost and energy aspects for practical applications.

The work of the authors in this area includes the design and performance improvements of hybrid PV/Tsystems with water or air as heat removal fluid [20]. The investigated models include a number of modifica-tions that contribute to the increase of thermal efficiency, to the decrease of PV module temperature and tothe improvement of the total energy output of the PV/T system. Design concepts, prototype constructionand test results for water and air cooled PV/T systems were presented for PV/T systems with and withoutan additional glass cover [20]. The dual type PV/T system, operating either with water or air heat extraction,extends their practical use [21], and a life cycle analysis for water cooled PV/T systems compared with stan-dard PV modules demonstrated the environmental impact of these systems [22]. In addition, economic aspectsand performance results for water cooled PV/T systems that could be applied in houses, multi-flat residentialbuildings, hotels, etc., show the advantages of applying PV/T systems [23].

The work of the authors includes also extensive studies of solar energy applications by using advanced sim-ulation tools. A part of this work refers to hybrid PV/T water heaters and is based on the use of the TRNSYSsimulation program where a hybrid PV/T system was modeled and simulated for the environmental conditionsof Nicosia, Cyprus [24]. The system can satisfy part of the thermal and electrical needs of a family of fourpersons. From the results presented, it was shown that the mean annual electrical efficiency of the PV wasincreased considerably, and the system can satisfy 50% of the thermal needs of the family.

In this paper, we present basic design considerations for hybrid PV/T systems that were investigated as pro-totypes that can be applied in houses, multi-flat residential buildings, hotels, etc., aiming to provide both elec-tricity and hot water. The electrical and thermal efficiency of designed, constructed and tested prototype PV/Tpanels of polycrystalline silicon (pc-Si) and amorphous silicon (a-Si) PV modules are presented and TRNSYSsimulation results are given for Nicosia, Athens and Madison for a small thermosyphon unit (4 m2) suitablefor a single house and an active system for large scale hot water and electricity production (40 m2). The objec-tive is to prove the potential benefits of PV/T systems compared to typical PV modules. This work providesenergy and cost results regarding system application and could be considered useful to estimate the cost effec-tiveness of these new solar energy systems in practice.

2. Experimental PV/T systems

A PV/T system consists of a thermal unit for extraction of the heat by water, which circulates through pipesin contact with a flat sheet placed in thermal contact with the rear surface of the PV module (see details inSection 3). Practical considerations in PV/T system design include evaluation of the thermal and electrical effi-ciency improvement with respect to system cost. It should be noted that the cost of the thermal unit remainsthe same irrespective of whether the PV module is made from c-Si, pc-Si or a-Si cells, but the ratio of the addi-tional cost of the thermal unit per PV module cost is almost double in the case where a-Si modules are usedrather than the c-Si or pc-Si PV ones. In addition, a-Si PV modules present lower electrical efficiency, althoughthe total energy output (electrical plus thermal) is almost equal to that of c-Si or pc-Si PV modules.


Hybrid PV/T systems consisting of PV modules without thermal protection of their illuminated surfacefrom the ambient have high top thermal losses, and therefore, the operating temperature is not high. Toincrease the system operating temperature, an additional transparent cover is necessary (like the glazing oftypical solar thermal collectors), but this has the result of decreasing the PV module electrical output becauseof the additional absorption and reflection of solar radiation. In addition, these systems use thermal insulationto avoid thermal losses from the non-illuminated system surfaces. PV/T systems can be installed on a horizon-tal or inclined roof or on the facade of a building. The horizontal and inclined roof installations are of moreinterest for low latitude countries, while the building facade installation is more effective for medium and highlatitude applications because of the low sun altitude angles. Smaller size PV and PV/T systems using a PVaperture surface area of about 3–5 m2 and a water storage tank of 150–300 l can be installed in one familyhouses. Larger size systems of about 30–50 m2 and 1000–3000 l water storage are more suitable for multi-flatresidential buildings, hotels, hospitals, industries, etc. In the case of small size PV/T systems, a thermosy-phonic operation (without pump for circulation of the water) can also be used, aiming to replace the well-known flat plate thermosyphonic solar water heaters.

Two types of PV/T experimental models were constructed and tested, one with pc-Si and another with a-SiPV modules. The studied PV/T systems consist of PV modules in combination with water heat extraction unitsmade from copper sheet and pipes and thermally protected with 5 cm of polyurethane thermal insulation. Allmodels had glazing covers of 4 mm thickness to achieve satisfactory thermal output.

The experimental models of the hybrid PV/T water systems were constructed and tested outdoors for deter-mination of the steady state thermal efficiency gth and the electrical efficiency gel [20]. The thermal efficiency ofthe experimental PV/T models is determined as a function of the global solar radiation (G), the input fluidtemperature (Tin) and the ambient temperature (Ta). The electrical efficiency of the PV/T systems is deter-mined for all the PV module types as a function of the operating temperature TPV/T. During the testing fordetermination of the system thermal efficiency, the PV modules were connected with a load to simulate realsystem operation and to avoid PV module overheating by the solar radiation that is converted into heatinstead of electricity.

The steady state efficiency is calculated by:

gth ¼ _mCpðT o � T iÞ=GAa; ð1Þ

where _m is the fluid mass flow rate, Cp the fluid specific heat, Ti and To the input and output fluid tempera-tures, respectively, and Aa the aperture area of the PV/T model. The thermal efficiency gth of PV/T systems iscalculated as a function of the ratio DT/G where DT = Ti � Ta, with Ta being the ambient temperature. Theelectrical efficiency gel depends mainly on the incoming solar radiation and the PV module temperature (TPV)and is calculated by:

gel ¼ ImVm=GAa ð2Þ

where Im and Vm are the current and the voltage of the PV module operating at maximum power. The elec-trical efficiency of the PV cells depends on the incoming solar radiation and their operating temperature. Theformula that can be used for calculation of the PV module temperature is a function of the ambient temper-ature Ta and the incoming solar radiation G and is given by Lasnier and Ang [25] as:

T PV ¼ 30þ 0:0175ðG� 300Þ þ 1:14ðT a � 25Þ ð3Þ

This relation is used for standard pc-Si PV modules. In PV/T systems, the PV temperature depends also on thesystem operating conditions, such as the heat extraction fluid mean temperature. In PV/T systems, the PVelectrical efficiency gel can be a function of the parameter (TPV)eff, which corresponds to the PV temperaturefor the operating conditions of the PV/T systems. Thus, for the studied PV/T systems, we used the effectivevalue (TPV)eff calculated by the formula [23]:

ðT PVÞeff ¼ T PV þ ðT PV=T � T aÞ ð4Þ

The operating temperature TPV/T of the PV/T system pertains to the PV module and to the thermal unit tem-peratures and can be determined approximately by the mean fluid temperature.


For the a-Si PV modules, their lower electrical efficiency results in slightly higher PV module temperature ascompared to pc-Si PV modules. For this purpose, the following formula, which was validated by experiments,was used:

T PV ¼ 30þ 0:0175ðG� 150Þ þ 1:14ðT a � 25Þ ð5Þ
The thermal and electrical efficiencies of all the studied models are the following:
pc� Si : gth ¼ 0:71� 09:04ðDT =GÞ gel ¼ 0:1457� 0:00094ðT PVÞeff ð6Þa� Si : gth ¼ 0:75� 08:83ðDT=GÞ gel ¼ 0:0485� 0:00011ðT PVÞeff ð7Þ

For the pc-Si type PV/T systems, the (TPV)eff is calculated from Eqs. (3) and (4). For the calculation of (TPV)eff

of a-Si type PV/T systems, Eqs. (4) and (5) are used. It should be noted that inverters, regulators and otherauxiliary equipment that constitute the Balance of System (BoS) result in reduction of the final energy outputof all systems by about 15% [22] due to electrical and thermal losses.

3. Systems considered

In this paper, two types of PV/T systems are considered as follows:

(a) Small size PV/T solar water heating system of thermosyphonic type.(b) Large size system with PV/T modules in parallel rows placed on a horizontal building roof with the

water storage tank located inside the building and a pump for the water circulation.

Thermosyphon systems heat potable water or a heat transfer fluid. These systems need no pumps and con-trols to transfer the water heated by solar energy, as they use natural convection to transport it from the col-lector to storage. The water in the collector expands, becoming less dense as the sun heats it, and rises throughthe collector into the top of the storage tank [26]. There, it is replaced by the cooler water that has sunk to thebottom of the tank, from which it flows down the collector, and circulation continues as long as there is sun-shine. In thermosyphon systems, the collector is connected with a water storage tank which is always at ahigher position so as to avoid reverse operation during the night.

Active systems on the other hand use a pump to circulate the water from the collector to storage. A typicalactive system is shown in Fig. 1. The pump is operated by means of a differential thermostat. The storage tankcan be located at any place, like behind the collectors, indoors in a plant room or any other suitable location,and thus, there is an overall improvement in the aesthetics of the system. To avoid water freezing in the tubesof the collector, a heat exchanger is used in the storage tank, and the heat removal fluid is water with antifreezeliquid.

The specifications of the small solar system are shown in Table 1, whereas the specifications of the largesystem are shown in Table 2.

Hot watersupplyRelief

valveMixingdevice

Collectorarray

Auxiliary heater

Storage tank

BurnerDifferential thermostat

Solar pump Make-up water

Fig. 1. Active system schematic.

Table 1Specification of the small solar system considered

Parameter Specification

Collector area 4 m2

Collector slope Latitude + 5�Storage capacity 160 lAuxiliary capacity 3 kWHot water demand 120 l (4 persons)

Table 2Specification of the large solar system considered

Parameter Specification

Collector area 40 m2

Collector slope Latitude + 5�Storage capacity 1500 lAuxiliary capacity 10 kWHot water demand 1200 l (40 persons)


The usual type of collector employed in both thermosyphon and active units is the flat plate. A typical flatplate solar collector is shown in the detail of Fig. 2. The underside of the absorber plate and the side of thecasing are well insulated to reduce conduction losses. For the present application, PV cells are installed on topof the absorber, as shown in Fig. 2, so the absorber is the PV panel and the copper absorber acts as a heatremoval medium.

The transparent cover is used to reduce convection losses from the absorber plate through the restraint ofthe stagnant air layer between the absorber plate and the glass. It also reduces radiation losses from the col-lector. Although the transparent cover reduces the amount of electrical energy produced by the PV panel, it isretained for the present applications so as to increase the thermal performance of the collectors.

Natural or forced air circulation is a simple and low cost method to remove heat from PV modules, but it isless effective at low latitudes where ambient air temperatures are over 20 �C for many months of the year.Regarding heat extraction, the water circulates through pipes in contact with a flat sheet (heat exchanger)placed in thermal contact with the PV module rear surface as shown in Fig. 2. The additional thermal protec-tion increases the thermal efficiency of the system, but the lower thermal losses keep the PV temperature at ahigher level, therefore operating with reduced electrical efficiency. To increase the system operating tempera-ture, an additional glazing is used, but this results in a decrease of the PV module electrical output because anamount of solar radiation is absorbed by the glazing and another is reflected away, depending on the angle ofincidence.

The characteristics of the solar collector considered in this study are shown in Table 3.In PV/T systems, the collector needs to be connected electrically to the mains (for grid connected systems)

and hydraulically to a hot water storage tank. Two types of PV cells have been considered in this work poly-crystalline silicon (pc-Si) and amorphous silicon (a-Si).

Fig. 2. Hybrid and conventional flat plate collector details.

Table 3Characteristics of the solar collector considered

Parameter Characteristics

Riser pipe diameter 15 mmHeader pipe diameter 28 mmAbsorber plate thickness 0.5 mmInsulation material and thickness Fiber wool, 40 mmFixing of risers on the absorber plate WeldedGlazing Low-iron glass


4. System model

The system is modeled with the well-known TRNSYS program [27]. The program consists of many subrou-tines that model subsystem components. Once all the components of the system have been identified and amathematical description of each component is available, it is necessary to construct an information flow dia-gram for the system. The purpose of the information flow diagram is to facilitate identification of the compo-nents and the flow of information between them. From the flow diagram, a deck file has to be constructedcontaining information on all system components, weather data file and the output format.

In a previous work in which an active PV/T system was modeled, it was found that the optimum water flowrate value was 4.9 l/h m2 [24]. This low value of flow rate suggests that the system could be used in a thermosy-phon mode. For simulation of the thermosyphon system, the TRNSYS Type 45 model is used. The thermalperformance of the system is analyzed by dividing the thermosyphon loop into a number of segments normalto the flow direction and applying Bernoulli’s equation for incompressible flow to each segment. The flow rateis obtained by numerical solution of the resulting set of equations. Flow in the loop is assumed to be steadystate.

For the active system, the units are modeled by combining a number of components as shown in Fig. 3,which shows the flow diagram of the large hot water application.

In all the deck files required to run the TRNSYS models for the various applications, all equations shown inSection 2 are incorporated, and whenever possible, outputs from ready made modules were used directly. Forexample, the mass flow rate (for the thermosyphon system) and collector inlet and outlet temperatures areobtained in this way and used in the appropriate equations.

All systems are simulated on an annual basis at three different locations at different latitudes, Nicosia,Cyprus (35�); Athens, Greece (38�) and Madison, Wisconsin (43�). The first two locations represent locations

TYPE 16 SOLAR RADIATION PROCESSOR

TYPE 1SOLAR COLLECTOR

TYPE 9DATAREADER

TMYfile

Tout EQUATIONS(for the TPV-T)

TinAmbient temperature

Solarradiation TYPE 31

PIPINGTYPE 31 PIPING

EQUATIONS (for the PVsystem)

TYPE 2COLLECTORCONTROL

LEGEND: Information flow

Control signal flow

Raw data input

TYPE 3COLLECTORPUMP

TYPE 14 Mains water temperature TYPE 4

HOTWATERSTORAGE

TYPE 6AUXIL IARY HEATER

TYPE 11 DIVERTER

TYPE 11 TEE PIECE

TYPE 15 RELIEFVALVE

TYPE 3LOADPUMP

TYPE 14Hot water load

Fig. 3. Information flow diagram for the large hot water system application.

10

2

4

6

8

10

12

14

0 2 4 6 8 0 12 14 16 18 20 22 24Hours

Hot

wat

er c

onsu

mpt

ion

(l)

Fig. 4. Hot water daily consumption profile.


with hot summer weather and mild winters, whereas the latter represents a location with mild summer andsevere winter and was considered to find the difference in system performance for comparison purposes.For each of these three locations, a typical meteorological year (TMY) file, which is required in the simula-tions, is available.

With respect to the water consumption and although the hot water demand is subject to a high degree ofvariation from day to day and from consumer to consumer, it is impractical to use anything but a repetitiveload profile. This is not quite correct during the summer period where the consumption pattern is somewhathigher. However, during this period, the temperature requirement for hot water is not as high as during thewinter. Consequently, the total thermal energy requirement is reasonably constant throughout the year. Forthe present simulation, the hot water consumption profile illustrated in Fig. 4 is used, which assumes a dailyhot water consumption of 120 l at 50 �C for a family of four (30 l/person). For the large hot water application,the consumption considered is 10-fold of that shown in Fig. 4 (10 families of four persons each).

5. Results

The annual results obtained for the thermosyphon unit are shown in Table 4. As can be seen, the PV/Tsystems achieve an increase of the total energy output because hybrid systems utilise the thermal energy,whereas in a standard PV system, this is lost to the ambient. However, the electrical energy output of a hybridsystem is lower than that of standard PV modules (maximum 38%) due to operation of the PV modules athigher temperatures. The reduced electrical performance is also due to the additional glazing, which increasesthe thermal output and the optical losses. Generally, depending on the location, this system can give 222–532 kW h electrical energy, and the solar contribution varies from 29% to 72%. The solar contribution deter-mines the percentage of the hot water load covered by solar energy.

The pc-Si PV modules give higher total energy output compared to a-Si PV modules. However, the a-Sigives more thermal useful energy and, thus, a higher solar contribution in water heating. In cold climates

Table 4Annual performance of the hybrid PV/T thermosyphon system

Location Cell type Qu (MJ) Qaux (MJ) Solarfraction

PV/T electricalenergy (kW h)

PV electricalenergy (kW h)

Electricalenergy % difference

Nicosia pc-Si 5741 1736 68.6 532.1 843.2 63.1a-Si 6083 1516 72.6 257.6 353.6 72.9

Athens pc-Si 5047 2410 56.4 515.1 827.1 62.3a-Si 5370 2208 60.1 249.1 343.4 72.5

Madison pc-Si 3807 3910 29.3 499.0 774.4 64.4a-Si 4151 3643 34.1 222.7 305.2 72.9

Notes: (1) Qu = useful thermal energy. (2) Qaux = auxiliary thermal energy required to cover hot water load. (3) Electrical energy isestimated by considering an 85% efficiency for BoS.


and although the overall performance of the hybrid system is reduced due to the excessive cloudiness, thecomparative performance of the PV is better because of the operation at a lower environmentaltemperature.

The annual results for the thermosyphon PV/T system are shown in Table 4 and the monthly performancein Fig. 5. As can be seen, on a monthly basis, pc-Si cells produce more electrical energy (Pel) than the corre-sponding a-Si cells. This is due to the higher efficiency of the pc-Si cells. The a-Si cells produce more thermaluseful energy (Qu) in all three locations considered. For Nicosia and Athens, both types of cells cover all ther-mal energy required for hot water production in the summer months as represented by the zero or near zeroauxiliary energy required (Qaux). For Madison, where the temperatures and available solar radiation arelower, a substantial amount of thermal energy is covered in the summer, but some thermal auxiliary is stillrequired. All systems represent a substantial thermal energy collection and a good electrical performancethroughout the year.

It should be noted that although the model used for the thermosyphon unit cannot work with a heatexchanger, in practice, a mantle-type tank and a water with antifreeze liquid needs to be used in locationswhere freezing is a possibility, like in Madison, with a small reduction (about 15%) in the thermal performanceof the unit.

The annual results for the large active system are shown in Table 5 and the monthly performance in Fig. 6.Much higher values of electrical energy are obtained for the large systems, in the order of 10-fold that of the

smaller systems. The solar contribution of the PV/T systems varies, depending on the location, between 60%and 87%. Also, the percentage difference between the electrical energy produced from the standard and thehybrid units is smaller, in the order of 38%. This has been estimated by considering also the energy required

Qu-pc Qaux-pc Pel-pc Qu-a Qaux-a Pel-a

0

100

200

300

400

500

600

700

1 2 3 4 5 6 7 8 9 10 11 12

Months

Ene

rgy

(MJ)

Nicosia

0

100

200

300

400

500

600

700

1 2 3 4 5 6 7 8 9 10 11 12

Months

En

erg

y (M

J)

Athens

0

100

200

300

400

500

600

700

1 2 3 4 5 6 7 8 9 10 11 12

Months

En

erg

y (M

J)

Madison

Fig. 5. Monthly performance of small (thermosyphon) hybrid PV/T system.

Table 5Annual performance of the large hybrid PV/T active system

Location Cell type Qu (MJ) Qaux (MJ) f (%) PV/T electricalenergy (kW h)

PV electricalenergy (kW h)

Pumpenergy (kW h)

Electrical energy% difference

Nicosia pc-Si 52,310 8785 84.1 5640.1 8431.5 379.8 62.4a-Si 54,540 7163 87.1 2611.4 3539.3 384.5 62.9

Athens pc-Si 46,480 14,470 73.8 5502.3 8271.0 369.8 62.1a-Si 48,970 13,020 76.5 2531.1 3435.4 376.8 62.7

Madison pc-Si 37,810 21,760 60.7 5350.3 7744.4 284.8 65.4a-Si 40,780 19,640 64.5 2265.5 3048.2 294.0 64.7

Notes: (1) Qu = useful thermal energy. (2) Qaux = auxiliary thermal energy required to cover hot water load. (3) f = solar fraction, denotespercentage of hot water load covered by solar. (4) Electrical energy is estimated by considering an 85% efficiency for BoS and pump energy.

Qu-pc Qaux-pc Pel-pc Qu-a Qaux-a Pel-a

0

1000

2000

3000

4000

5000

6000

1 2 3 4 5 6 7 8 9 10 11 12Months

En

erg

y (M

J)

Nicosia

0

1000

2000

3000

4000

5000

6000

1 2 3 4 5 6 7 8 9 10 11 12Months

En

erg

y (M

J)

Athens

0

1000

2000

3000

4000

5000

6000

1 2 3 4 5 6 7 8 9 10 11 12Months

En

erg

y (M

J)

Madison

Fig. 6. Monthly performance of large hybrid PV/T active system.


by the solar pump. As the other comments about the comparison between the various systems are similar tothe ones made for the smaller system, they will not be repeated again.

A general comment on the monthly performance of the systems indicated in Fig. 6 is that the useful thermalenergy (Qu) of the amorphous cells is slightly higher than the corresponding value of the polycrystalline cellsby an equal amount in each month, which is also reflected in the auxiliary thermal energy required to cover thehot water load. However, the electrical energy produced by the polycrystalline cells is much higher than that ofthe amorphous ones due to their higher electrical efficiency.


6. Economic analysis

The viability of all the above systems depends on the initial cost and the amount of energy, electrical orelectrical plus thermal, depending on the type of system they replace. A life cycle analysis is performed in orderto obtain the total cost (or life cycle cost) and the life cycle savings (LCS) of the systems. The economic sce-nario used in this project is that 100% of the initial cost of the solar system is paid at the beginning, i.e., there isno mortgage payment. The period of economic analysis is taken as 20 years (life of the system). Although earlyPV installations showed that the life of the PVs is more than 30 years, the 20 years period applied in solarthermal systems is retained. The economic analysis can be performed either within the TRNSYS environmentor in a spreadsheet program. For the present work, the spreadsheet application is used. A detailed descriptionof the method of economic analysis of solar systems using spreadsheets is given in Ref. [28].

In general, the present worth (or discounted cost) of an investment or cost C at the end of year N at a dis-count rate of d and interest rate of i is obtained by:

PWN ¼Cð1þ iÞN�1

ð1þ dÞNð8Þ

In the case of this project, the various costs and savings are estimated annually. From the addition of electric-ity and fuel savings incurred because of the use of the system and the tax savings, the maintenance and par-asitic costs are subtracted, and thus, the annual solar savings of the system are estimated, which are convertedinto present worth values of the system. These are added to obtain the life cycle savings according to theequation:

PWLCS ¼XN

N¼1

Solar Savings

ð1þ dÞNð9Þ

For the thermal part of the system, the fuel savings are obtained by subtracting the annual cost of the con-ventional fuel used for the auxiliary energy from the fuel needs of a fuel only system. The integrated costof the auxiliary energy use for the first year, i.e., solar back up, is given by the formula:

Caux ¼Z t

0

CFAQaux dt ð10Þ

The integrated cost of the total load for the first year, i.e., cost of conventional fuel without solar, is:

Cload ¼Z t

0

CFLQload dt ð11Þ

where CFA and CFL are the cost rates for auxiliary energy and conventional fuel, respectively. In case the samefuel is used for both, CFA = CFL.

The investment cost of the solar systems is estimated by considering the current costs of the various parts ofthe systems (PV module, heat extraction unit, inverter, pipes, pump, cables, etc.). These are tabulated in Table6 together with explanations on how the costs of the various systems are estimated. As can be seen, the specificcosts for the larger systems are slightly lower due to economy of scale. For example, for the larger system, oneslightly larger inverter is used, which is slightly more expensive than the one used in the smaller systems. Itshould be noted that the cost of the hybrid PV/T systems includes also the costs for modification of thePV systems into the hybrid ones and all other equipment such as piping, pump, differential thermostat andinsulation required to complete the system. It does not include the cost of the storage tank, which is presentin an installation irrespective of whether this is solar or not. Thus, in the economic analysis, a comparison ismade of the extra equipment required by the solar system against the money saved because of the amount ofelectricity and fuel replaced by solar energy. As subsidization schemes for PV systems vary from country tocountry and as the economic analysis is performed mainly in order to compare the standard and hybrid sys-tems, no subsidies are considered in the present analysis.

Table 6Specific costs of the systems considered

System description PV panel Cost (€) Explanation of costs

Thermosyphon system (4 m2) – 900 1200€ for complete system �300€ for storage tankThermal system (per m2) – 250 Includes collectors, piping, insulation and supportsSmall PV system (4 m2) pc-Si 3200 Current market price which includes cost for BoSSmall PV system (4 m2) a-Si 2000 Current market price which includes cost for BoSLarge PV system (40 m2) pc-Si 28,000 Current market price which includes cost for BoSLarge PV system (40 m2) a-Si 16,000 Current market price which includes cost for BoSThermosyphon PV/T system (4 m2) pc-Si 4100 4 · 800€ for PV + 900€ for solar thermal systemThermosyphon PV/T system (4 m2) a-Si 2900 4 · 500€ for PV + 900€ for solar thermal systemLarge PV/T system (40 m2) pc-Si 38,000 40 · 700€ for PV + 40 · 250€ for solar thermal systemLarge PV/T system (40 m2) a-Si 26,000 40 · 400€ for PV + 40 · 250€ for solar thermal system

Note: Storage tank is assumed to be present in a conventional hot water system, thus it is not considered as cost in the present analysis, i.e.,only the cost of extra equipment is considered.


For the operating cost, maintenance and parasitic costs are considered. The former are estimated to be 1%of the initial investment and are assumed to increase at a rate of 1% per year of the system operation. Thelatter accounts for the energy required (electricity) to drive the solar pump. Thus, the total annual cost is givenby the addition of the system and operation costs. The cost of electricity is considered as 0.1€/kW h, and thecost of Diesel is 0.62€/l. The market discount rate and the general inflation rate considered in this study areequal to 6.5% and 5.2%, respectively. Finally, the inflation rate for the fuel used is equal to 10% to reflect thelatest dramatic increases in oil price.

The results of the economic analysis are shown in Table 7 for the small system and Table 8 for the largesystem. What is of interest here is mainly the comparison between the savings in electricity and thermal energyand the LCS of the various types of systems.

As can be seen in all the cases of the PV systems, the LCS obtained are negative, meaning that the paybacktime is greater than 20 years, which is considered as the life of the systems. It should be noted that no subsidiesare considered in this study and the negative amounts of money represent the money that the owner will looseby installing the PV system instead of buying the electricity from the mains. These negative figures show theneed for subsidies in order to convince people to install such systems. It should be noted that the LCS stronglydepends on the first year fuel savings (FYFS) and the solar system cost. The electrical FYFS (FYFS-e) dependon the electrical energy produced by the PVs. In hybrid systems, the thermal FYFS (FYFS-t) depend on thethermal load in each case and the auxiliary energy required. Better figures are obtained in the case of hybridsystems, as the LCS are smaller negative values, and in some cases, positive values are obtained. All cases thatgive positive LCS refer to the use of a-Si cells, and these are the applications for Nicosia. Generally, for loca-tions with higher available solar radiation, like Nicosia and Athens, the economics give better figures. Also,although amorphous silicon panels are much less efficient than the polycrystalline ones, they give better figures

Table 7Results of the economic analysis of the small system

System type (Location) Celltype

Standard PV cells Hybrid PV/T cells

FYFS-e electricity LCS FYFS-eelectricity

FYFS-tfuel

LCS FYFS-telectricity

LCS

Thermosyphon (Nicosia) pc-Si 84.3 �1070.0 53.2 71.3 �1105.0 105.3 �223.3a-Si 35.3 �1118.3 25.8 75.5 �513.3 111.5 421.7

Thermosyphon (Athens) pc-Si 82.7 �1111.7 51.5 58.7 �1476.7 86.7 �750.0a-Si 34.3 �1145.0 25.0 62.3 �876.7 92.3 �96.7

Thermosyphon (Madison) pc-Si 77.5 �1248.3 49.8 30.3 �2260.0 45.0 �1880.0a-Si 30.5 �1245.0 22.3 35.5 �1643.3 52.5 �1201.7

Notes: (1) All values in Euros. (2) FYFS = first year fuel savings. (3) LCS = life cycle savings. (4) FYFS-e = electricity replaced by solarduring first year. (5) FYFS-t = thermal energy replaced by solar during first year when backup is fuel (Diesel) or electricity.

Table 8Results of the economic analysis of the large system

Location Cell type Standard PV cells Hybrid PV/T cells

FYFS-e electricity LCS FYFS-e electricity FYFS-t fuel LCS

Nicosia pc-Si 843.2 �6623.3 526.0 874.0 �2346.7a-Si 354.0 �7106.7 222.7 904.5 2791.7

Athens pc-Si 827.2 �7041.7 513.3 767.3 �5450.0a-Si 343.5 �7376.7 215.5 794.5 �253.3

Madison pc-Si 774.5 �8408.3 506.5 630.2 �9186.7a-Si 304.8 �8383.3 197.2 670.2 �3958.3

Notes: (1) All values in Euros. (2) FYFS = first year fuel savings. (3) LCS = life cycle savings. (4) FYFS-e = electricity replaced by solarduring first year. (5) FYFS-t = thermal energy replaced by solar in first year when backup is fuel (Diesel). (6) All systems are active.


due to their lower initial cost, i.e., they have better cost/benefit ratios, as has also been observed by otherauthors [16].

For the case of the small system, both Diesel and electricity are used as a backup auxiliary for the thermalenergy. This is to account for houses that produce hot water with Diesel through the central heating systemand those that use electric immersion heaters. As the unit cost of electricity is higher that that of fuel, in thecase where an electric immersion heater is used, the solar energy replaces a more expensive fuel, and thus, ahigher LCS is obtained.

The modeled energy output results of the considered PV/T systems are generally in agreement with theresults that are presented by other authors. In the literature on PV/T solar systems however, no monthlyor annual results for these systems are given. A recent publication considers 4 m2 c-Si and a-Si PV/T systemswith glazing and 300 l water storage tank. The systems are studied with TRNSYS [16], but only a few annualresults are given, showing that the effective application of the two cell types depends on the electrical to ther-mal energy value ratio. Regarding the benefits of commercial PV/T solar water heaters [19], it is estimatedthat, in the case of cost reduction by mass production and considering that the produced heat replaces elec-tricity, the system becomes cost effective for practical applications.

Taking into account these aspects, an example is given by considering a subsidy of 55% on the initial cost ofthe PV system, which is presently applied in the case of Cyprus as an incentive to promote PV systems instal-lation. The LCS for the polycrystalline silicon panels increase to 723€ for the standard PV system, to 852€ forthe hybrid PV/T thermosyphon system with Diesel backup of the thermal energy and to 1733€ for the hybridPV/T thermosyphon system with electricity backup. It should be noted that in the case of the hybrid systems,the subsidy is considered only on the PV part of the system. In Cyprus, another incentive is given, namely thatfor grid connected systems, the electrical energy is bought by the Electricity Authority at double the normalselling rate. If this is considered, then the LCS increases to 2913€ for the standard polycrystalline silicon pan-els, to 2233€ for the hybrid PV/T thermosyphon system with Diesel backup of the thermal energy and to 3117€

0

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Nic-p Nic-a Ath-p Ath-a Mad-p Mad-a Nic-p Nic-a Ath-p Ath-a Mad-p Mad-a

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Fig. 7. Payback times of plain PV and thermosyphon PV/T systems with polycrystalline (p) and amorphous (a) silicon cells for the threelocations (electricity backup).

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Fig. 8. Payback times of plain PV and active PV/T systems with polycrystalline (p) and amorphous (a) silicon cells for the three locations(Diesel backup).


for the hybrid PV/T thermosyphon system with electricity backup. Therefore, it can be concluded that subsi-dies are a must for the introduction of both standard PV and hybrid PV/T systems.

Finally, in Figs. 7 and 8, the payback times in years of the systems studied are shown. In these figures, theadvantage of the hybrid PV/T systems is shown, as much shorter times are indicated. Also, by comparing thepolycrystalline and the amorphous silicon cells, the latter are slightly better for the hybrid type as they havebetter cost/benefit ratios, as outlined above.

7. Conclusion

Hybrid photovoltaic/thermal (PV/T) systems consisting of pc-Si and a-Si PV modules combined with waterheat extraction units, which were constructed and tested at the University of Patras are modeled and simulatedwith the TRNSYS program. The work includes the study of two PV/T systems, a small scale unit of 4 m2 aper-ture area and 160 l water storage tank and a large scale system of 40 m2 aperture area and 1500 l storage tank.

The results show that the electrical production of the system employing polycrystalline solar cells is morethan that employing the amorphous ones, but the solar thermal contribution is slightly lower. A non-hybridPV system produces about 38% more electrical energy, but the present system covers also, depending on thelocation, a large percentage of the hot water needs of the buildings considered. The derived TRNSYS resultsgive an account of the energy and cost benefits of the studied PV/T systems with thermosyphon and forcedwater flow.

As a general conclusion, it can be said that as the overall energy production of the units is increased, thehybrid units have better chances of success. This is also strengthened by the improvement of the economicviability of the systems, especially in applications where low temperature water, like hot water productionfor domestic use, is also required.

Additionally, the economics of the systems considered show that for locations with higher available solarradiation, like Nicosia and Athens, the economics give better figures. Also, although amorphous silicon panelsare much less efficient than the polycrystalline ones, they give better figures due to their lower initial cost, i.e.,they have better cost/benefit ratios. Considering the case of Cyprus, a considerable increase in LCS can beobtained when subsidies are considered, indicating the need of state subsides in order to promote the instal-lation of these systems. The same order of subsidies needs to be given for the hybrid systems as well.

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Hybrid PV-T Solar Systems for Domestic Hot Water and Electricity Production

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