Aspects and Improvements of Hybrid Photovoltaic Thermal Solar Energy Systems

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Solar Energy 81 (2007) 1117–1131

Aspects and improvements of hybrid photovoltaic/thermalsolar energy systems

Y. Tripanagnostopoulos *

Physics Department, University of Patras, Patra 26500, Greece

Received 14 August 2006; received in revised form 9 April 2007; accepted 9 April 2007Available online 11 May 2007

Communicated by: Associate Editor Jean-Louis Scartezzini

Abstract

Hybrid photovoltaic/thermal (PV/T or PVT) solar systems consist of PV modules coupled to water or air heat extraction devices,which convert the absorbed solar radiation into electricity and heat. At the University of Patras, an extended research on PV/T systemshas been performed aiming at the study of several modifications for system performance improvement. In this paper a new type of PV/Tcollector with dual heat extraction operation, either with water or with air circulation is presented. This system is simple and suitable forbuilding integration, providing hot water or air depending on the season and the thermal needs of the building. Experiments with dualtype PV/T models of alternative arrangement of the water and the air heat exchanging elements were performed. The most effectivedesign was further studied, applying to it low cost modifications for the air heat extraction improvement. These modifications includea thin metallic sheet placed in the middle of the air channel, the mounting of fins on the opposite wall to PV rear surface of the air channeland the placement of the sheet combined with small ribs on the opposite air channel wall. The modified dual PV/T collectors were com-bined with booster diffuse reflectors, achieving a significant increase in system thermal and electrical energy output. The improved PV/Tsystems have aesthetic and energy advantages and could be used instead of separate installation of plain PV modules and thermal col-lectors, mainly if the available building surface is limited and the thermal needs are associated with low temperature water or air heating.� 2007 Elsevier Ltd. All rights reserved.

1. Introduction

Photovoltaics (PV) convert, depending on cell type, 5–15% of the incoming solar radiation into electricity, withthe greater percentage converted into heat. The solar radi-ation increases the temperature of PV modules, resulting ina drop of their electrical efficiency. For system installationin parallel rows on horizontal roofs of buildings, the expo-sure of both PV module surfaces to the ambient permitstheir natural cooling, but in facade or inclined roof instal-lation the thermal losses are reduced and PV modules oper-ate at higher temperatures. This undesirable effect can bepartially avoided by applying a suitable heat extractionmode with a fluid circulation, keeping the electrical effi-

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doi:10.1016/j.solener.2007.04.002

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ciency at a satisfactory level. The PV modules that are com-bined with thermal units, where circulating air or water oflower temperature than that of PV module is heated, con-stitute the hybrid photovoltaic/thermal (PV/T or PVT) sys-tems and provide electrical and thermal energy, increasingtherefore the total energy output from PV modules.

The PV/T solar systems can be effectively used in thedomestic and in the industrial sector, mainly for preheatingwater or air. The water-cooled PV modules (PVT/watersystems) consist of a water heat exchanger in thermalcontact with the PV rear side and are suitable for waterheating, space heating and other applications (Fig. 1, left).Air-cooled PV modules (PVT/air systems) can be inte-grated on building roofs and facades and apart of theelectrical load they can cover building heating and airventilation needs (Fig. 1, right). PV/T solar collectors inte-grated on building roofs and facades can replace separate

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Fig. 1. Cross section of the main PV/T geometries, PVT/water (left) andPVT/air (right), regarding unglazed (up) and glazed (down) types.

1118 Y. Tripanagnostopoulos / Solar Energy 81 (2007) 1117–1131

installation of thermal collectors and photovoltaics, result-ing to cost effective and aesthetic application of solarenergy systems. An additional glazing for thermal lossreduction (Fig. 1, down) increases thermal output butdecreases the electricity production due to the additionaloptical losses.

In PV/T system applications the production of electric-ity is the main priority, therefore it is necessary to operatethe PV modules at low temperature in order to keep PV cellelectrical efficiency at a sufficient level. This requirementlimits the effective operation range of the PV/T unit forlow temperatures, thus, the extracted heat can be usedmainly for low temperature applications such as spaceheating, water or air preheating and natural ventilationin buildings. Water-cooled PV/T systems are practical sys-tems for water heating in domestic buildings but theirapplication is limited up to now. Air-cooled PV/T systemshave already been applied in buildings, integrated usuallyon their inclined roofs or facades. These systems keep theelectrical output at sufficient level, covering building spaceheating needs during winter and ventilation needs duringsummer, avoiding also building overheating.

In this paper design aspects for PV/T collectors and testresults from PVT/air prototypes with improved perfor-mance regarding a new system design based on dual heatextraction operation, are presented. This work followssome other studies on hybrid PV/T systems (Tripanagnos-topoulos et al., 2000, 2001a, 2002a) and the new system canheat water or air. Considering water as heat removal fluidthe system can be used for preheating water in residentialbuildings, hotels, hospitals, etc and regarding air heatextraction, for space heating, natural ventilation, etc. Thisconcept has been introduced by Tripanagnostopoulos et al.(2001b) and can be applied to small and large PV/T instal-lations on buildings. The use of air or water heat extractiondepends on building thermal needs, the system operationregarding temperature level and heat removal fluid typeand also considering the weather conditions. Aiming tothe improvement of electrical and thermal output of thePV/T systems, new modifications in the air-duct for theair circulation have been investigated in order to improvethe heat extraction. All experimental PV/T models weretested outdoors and also combined with booster diffusereflector for effective operation on horizontal building roofinstallation, mainly from spring to fall. The presented new

PV/T designs are suggested to increase system total energyoutput by using low cost additional system elements.

2. Literature on PV/T systems

Theoretical and experimental studies are referred tohybrid PV/T systems with air and/or water heat extractionfrom PV modules. Kern and Russel (1978) present thedesign and performance of water and air cooled PV/T sys-tems, while Hendrie (1979) and Florschuetz (1979) includePV/T modeling in their works. Numerical methods predict-ing PV/T system performance are developed by Raghur-aman (1981), computer simulations are studied by Coxand Raghuraman (1985), a low cost PV/T system withtransparent type a-Si cells is proposed by Lalovic et al.(1986–1987) and results from an applied air type PV/T sys-tem are given by Loferski et al. (1988). Bhargava et al.(1991), Prakash (1994), Garg and Agarwal (1995) presentthe same aspects of a water type PV/T system and Sopianet al. (1996) and Garg and Adhikari (1997) present a vari-ety of results regarding the effect of design and operationparameters on the performance of air type PV/T systems.

Because of their easier construction and operation,hybrid PV/T systems with air heat extraction are moreextensively studied, mainly as an alternative and cost effec-tive solution to building integrated PV systems (BIPV).Following the above referred studies, test results fromPV/T systems with improved air heat extraction are givenby Ricaud and Roubeau (1994) and from roof integratedair-cooled PV modules by Yang et al. (1994). Regardingbuilding integrated PV/T systems, Posnansky et al.(1994), Ossenbrink et al. (1994) and Moshfegh et al.(1995) include in their works considerations and resultson these systems. Later, Brinkworth et al. (1997), Mos-hfegh and Sandberg (1998), Schroer et al. (1998), Brink-worth (2000), and also Brinkworth et al. (2000) presentdesign and performance studies regarding air type buildingintegrated hybrid PV/T systems. In addition, the work ofEicker et al. (2000), which gives monitoring results froma BIPV PV/T system that operates during winter for spaceheating and during summer for active cooling and of Bazil-ian et al. (2001), which evaluates the practical use of severalPV/T systems with air heat extraction in the built environ-ment, can be referred.

The building integrated photovoltaics is going to be asector of a wider PV module application and the worksof Hegazy (2000), Lee et al. (2001), Chow et al. (2003)and Ito and Miura (2003) give interesting modeling resultson air cooled PV modules. Last years, the works on build-ing integrated air-cooled photovoltaics include the studieson the multi-operational ventilated PVs with solar air col-lectors (Cartmell et al., 2004), the ventilated building PVfacades (Infield et al., 2004; Guiavarch and Peuportier,2006; Charron and Athienitis, 2006) and the design proce-dure for cooling air ducts to minimize efficiency loss(Brinkworth and Sandberg, 2006). Despite these improve-ments, commercial application of PVT/air collectors is still

Y. Tripanagnostopoulos / Solar Energy 81 (2007) 1117–1131 1119

marginal, but it is expected to be wider in the future wheremany building facades and inclined roofs will be coveredwith photovoltaics.

PVT/water systems are more expensive than PVT/airsystems due to additional cost of the thermal unit withthe pipes for the water circulation. On the other hand waterfrom mains does not often exceeds 20 �C and ambient air isusually higher during summer in low latitude countries andwater heat extraction is of more practical value at theselocations as it can be used during all seasons. The liquidtype hybrid PV/T systems are less studied than air type sys-tems and the works that follow the first period of PV/T sys-tem development are the study of Bergene and Lovvik(1995) for a detailed analysis on liquid type PV/T systems,of Elazari (1998) for the design, performance and economicaspects of a commercial type PV/T water heater, of Haus-ler and Rogash (2000) for a latent heat storage PV/T sys-tem and of Kalogirou (2001) with TRNSYS results forwater type PV/T systems. Later, Huang et al. (2001) pres-ent a PV/T system with hot water storage and Sandnessand Rekstad (2002) give results for PV/T collectors withpolymer absorber.

The combination of solar radiation concentrationdevices with PV modules is up to now the most viablemethod to reduce system cost, replacing the expensive cellswith a cheaper solar radiation concentrating system.Besides, concentrating photovoltaics present higher effi-ciency than the typical ones, but this can be achieved inan effective way by keeping PV module temperature aslow as possible. The concentrating solar systems use reflec-tive and refractive optical devices and are characterized bytheir concentration ratio (CR). Concentrating systems withCR > 2.5· must use a system to track the sun, while forsystems with CR < 2.5·, stationary concentrating devicescan be used (Winston, 1974). The distribution of the solarradiation on the absorber surface (PV module) and thetemperature rise of it are two problems that affect the elec-trical output. The uniform distribution of the concentratedsolar radiation on the PV surface and the suitable coolingmode contribute to an effective system operation and theachievement of high electrical output. PV/T absorberscan be combined with low, medium or high concentrationdevices, but low CR PV/T systems have been mainly devel-oped so far. Reflectors of low concentration, either of flattype as presented by Sharan et al. (1985), Al Baali (1986),and Garg et al. (1991a) or of CPC type as proposed byGarg and Adhikari (1999), Brogren et al. (2000), Karlssonet al. (2001), Brogren et al. (2002) and Othman et al.(2005), have been suggested to increase the thermal andelectrical output of PV/T systems. Regarding medium con-centration, PV/T systems based on linear parabolic reflec-tors (Coventy, 2005) or linear Fresnel reflectors (Rosellet al., 2005) have been investigated. Although concentra-tors of low or medium CR are interesting devices to becombined with photovoltaics, 3D Fresnel lens or reflectortype concentrators have been recently developed, aimingat the market of concentrating photovoltaics.

Dynamic 3D and steady state 3D, 2D and 1D modelsfor PV/T prototypes with water heat extraction have beenstudied by Zondag et al. (2002), systems with water circu-lation in channels attached to PV modules have been sug-gested, also by Zondag et al. (2003). Regarding recentworks, modelling results (Chow, 2003; Jie et al., 2003),the study on domestic PV/T systems (Coventry and Love-grove, 2003), the performance and cost results of a roof-sized PV/T system (Bakker et al., 2005) and the theoreticalapproach for domestic heating and cooling with PV/T col-lectors (Vokas et al., 2006) and the performance evaluationresults (Tiwari and Sodha, 2006), can be referred. PVT/water collectors can replace thermal collectors for waterheating in the domestic and industrial sectors, but theyare not yet cost effective and this is the main reason fortheir niche market penetration.

Economic aspects on PV/T systems are given by Leend-ers et al. (2000) and the environmental impact of PV mod-ules by using the life cycle assessment (LCA) methodologyhas been extensively used at University of Rome ‘‘La Sapi-enza’’. Frankl et al. (2000) presented LCA results on thecomparison of PV/T systems with standard PV and ther-mal systems, thus confirming the environmental advantageof PV/T system compared to plain PV modules (shorterenergy payback time).

Design and performance improvements of hybrid PV/Tsystems with water or air as heat removal fluid have beencarried out at the University of Patras including modifica-tions that contribute to the decrease of PV module temper-ature and to improve the total energy output (electrical andthermal) of the PV/T systems. Design concepts, prototypesand test results for water and air-cooled PV/T systems withand without additional glass cover are extensively pre-sented in Tripanagnostopoulos et al., 2002a. Also, PV/Tsolar water heaters of ICS (Tripanagnostopoulos et al.,1998) and of thermosiphonic (Tselepis and Tripanagnosto-poulos, 2002) type have been studied. The diffuse reflectoris suggested to increase both electrical and thermal outputof PV/T systems (Tripanagnostopoulos et al., 2002a) andLCA results for PVT/water (Tripanagnostopoulos et al.,2005) and PVT/air (Tripanagnostopoulos et al., 2006) sys-tems, compared with standard PV modules, give an ideaabout the positive environmental impact of the suggestedsystems. The concept of combined linear Fresnel lenseswith PV/T absorbers has been also investigated, proposedfor lighting and temperature control of internal spaces,providing also electrical and thermal output for later use(Tripanagnostopoulos et al., 2007). In addition, the appli-cation of PV/T systems in the industry is suggested as a via-ble solution for a wider use of solar energy systems (Battistiand Tripanagnostopoulos, 2005) and TRNSYS results forPVT/water collectors, calculated for three different lati-tudes (Kalogirou and Tripanagnostopoulos, 2006) and astudy on PVT/air collectors with air heat extraction (Tonuiand Tripanagnostopoulos, 2007a,b) give recently a figureof the investigated system application and their perfor-mance improvements respectively. A detailed description


of hybrid PV/T solar systems is included in a recently pub-lished Roadmap (Zontag et al., 2005; Affolter et al., 2006),where many aspects regarding technology, present statusand future perspectives of these solar energy conversionsystems are presented.

3. Application aspects for PV/T systems

3.1. General aspects

PVT/water systems are practical devices for water heat-ing, but they are not yet improved enough for cost effectivecommercial applications. There are several suggestedmodes for the water circulation and the heat extraction,but more practical is considered to circulate water throughpipes in contact with a flat sheet, placed in thermal contactwith the PV module rear surface. Regarding air type PV/Tsystems, an air channel is usually mounted at the back ofthe PV modules. Air of lower temperature than that ofPV modules, usually ambient air, is circulating in the chan-nel and thus both PV cooling and thermal energy collectioncan be achieved. In PV/T systems the cost of the thermalunit is the same irrespective if the PV module is constructedwith crystalline-silicon (c-Si), poly-crystalline silicon (pc-Si)or amorphous-silicon (a-Si) type of cells. Thus the ratio ofthe additional cost of the mounted thermal unit per PVmodule area cost is different and is almost double in caseof using a-Si compared to c-Si or pc-Si PV modules. Theelectrical and thermal output, although are of differentvalue, it is usual to be added in order to give a figure ofthe hybrid system total (electrical and thermal) energy out-put and new devices are in development towards cost effec-tive and of low environmental impact solar energyconversion systems.

The PVT/water collectors have more limitations in sys-tem design and operation than the PVT/air collectors. Thisis due to the necessary heat exchanger element, whichshould have good thermal contact with PV rear surface,while in PVT/air systems the air is heated directly fromthe front or/and the back surface of PV modules. But onthe other hand the air heat extraction is less efficient thanthe water one, due to the low density of air and improve-ments are necessary to make PVT/air system efficient andattractive for real applications. In PV/T collectors theabsorber element is less efficient compared to that of typi-cal thermal collectors as it is of lower conductivity (glass orpolymer substrate in PV modules) and also, there are lim-itations for PV module surface treatment to become selec-tive in infrared emittance (low e) and reducing heatradiation to operate effectively at higher temperatures.

In most PVT/air systems the air circulates through achannel formed between the rear PV surface and the systemthermal insulation, and in some other systems throughchannels on both PV module sides, in series or in parallelflow. The usual heat extraction mode is the direct air heat-ing from PV module rear surface by natural on forced con-vection and the thermal efficiency depends on channel

depth, air flow mode and air flow rate. Small channel depthand high flow rate increase heat extraction, but increasealso pressure drop, which reduces the system net electricaloutput in case of forced air flow, because of the increasedpower for the fan. In applications with natural air circula-tion, the small channel depth reduces air flow and thisresults to an increase of PV module temperature. In thesesystems large depth of air channel (minimum 0.1 m) is nec-essary (Bhargava et al., 1991).

3.2. Air heat extraction improvements

The design of PVT/air solar systems has some similari-ties with the solar air collectors, but the use of the PV mod-ule instead of the black absorber sheet makes PVT/airsystems of lower efficiency, for the same above mentionedreasons regarding PVT/water systems. Several publicationsare referred to investigations on air heating solar collectors.The simpler modification that is suitable for application inthe air channel of the PVT/air systems is the roughenedopposite air channel wall surface (Prasad and Saini, 1991;Bhavnani and Bergles, 1990), by which up to about 30%heat extraction increase can be achieved. Better results givethe addition of several type ribs in the air channel (Han andPark, 1988; Gupta et al., 1993). More efficient is consideredthe mounting of vortices (Turk and Junkhan, 1986; Biswasand Chattopadhyay, 1992; Zhu et al., 1995; Tiggelbecket al., 1993; Brockmeier et al., 1993 and Fiebig, 1997),which contribute to about four times better performancein heat transfer. Other modifications that have been sug-gested for the improvement of heat extraction in the airchannel are the use of pins, matrices, porous materialsand perforated plates, but most of them are not of practicalinterest for PVT/air collectors. Fins on the absorber backsurface, on the opposite air channel wall or on both sur-faces (Garg and Datta, 1989), as well as joining these twosurfaces (Garg et al., 1991b) are interesting and practicalmodifications to enhance the heat transfer in the air chan-nel. Some other finned absorber geometries (Pottler et al.,1999 and Naphon, 2005) give satisfactory results, makingpromising this type of air channel modification.

Our research is mainly concerned with the reduction ofPV module temperature, the improvement of air heatextraction by the circulating air and the avoidance of heattransfer through the thermal insulation on system backside. By applying the new design concepts the electricaland thermal efficiencies are at satisfactory levels. Severalideas were experimentally tested and the results showedthat low cost PVT/air system improvements can beachieved (Tripanagnostopoulos et al., 2000, 2001a; Tonuiand Tripanagnostopoulos, 2007a,b).

3.3. PV/T application to buildings

In BIPV applications and regarding PV installation atthe facade and the inclined roof, the rear surface of PVmodules is thermally protected from the back thermal


losses and the cell temperature rise becomes a considerablereason for the electrical efficiency reduction. In addition,heat from PV modules is transmitted to the building,mainly during summer, the building temperature rises overthe acceptable comfort level and more electrical energy isneeded to cover the increased load of the air conditioningsystem to reject this undesirable heat out to the ambientand to cool the building.

The facade and tilted roof integrated PV/T systems aremore effectively insulated on their rear surface, comparedto the ones installed on horizontal roof, as they areattached on the facade wall or the tilted roof. The addi-tional thermal protection increases the thermal efficiencyof the system, but the lower thermal losses keep PV temper-ature at a higher level, therefore they are operating withreduced electrical efficiency. Smaller size PV and PV/T sys-tems, using aperture surface area of about 3–5 m2 andwater storage tank of 150–200 l, can be installed on one-family houses (Elazari, 1998). Larger size systems of about30–50 m2 and 1500–2000 l water storage are more suitablefor multi-flat residential buildings, hotels, hospitals, indus-tries, etc. Two recent works give a figure of domestic appli-cations of PV/T systems by applying TRNSYSmethodology to water heating (Kalogirou and Tripanag-nostopoulos, 2006) and using F-chart techniques for spaceheating and cooling (Vokas et al., 2006).

An interesting building application of solar energy sys-tems is to use linear Fresnel lenses as transparent materialof atria, sunspaces, etc, to control lighting and temperatureof these spaces, providing also electricity and heat andcover building energy needs. In buildings, shading devices(Tsangrassoulis et al., 1996) and double-glazed windowswith motorised reflective blinds (Athienitis and Tzempeli-kos, 2002) aim to reduce the absorbed solar energy andto keep the average temperature of the interior space atthe comfort level. Flat or curved (CPC) reflectors are sug-gested as lightguides to provide sunlight to the buildinginterior spaces (Molteni et al., 2001; Scartezzini and Cour-ret, 2002). Fresnel lenses are optical devices of practicalinterest for solar radiation concentration, because of theirlow volume and weight and also of smaller focal lengthand lower cost compared to thick ordinary lenses. Theadvantage of linear Fresnel lenses to separate the directfrom the diffuse solar radiation makes them suitable forillumination control in the building interior space, provid-ing light of suitable intensity level without sharp contrastsand achieving shading absorbing a great part of the incom-ing solar radiation.

The concentration of the direct part of the incident solarradiation on a thermal absorber of small width located atthe focal position has been suggested by Jirka et al.(1998) to achieve lower illumination level, to avoid spaceoverheating and to contribute to the thermal needs of thebuilding. An effective combination of Fresnel lenses canbe the use of hybrid PV/T small width absorbers to extractthe concentrated solar radiation in the form of electricityand heat (Tripanagnostopoulos et al., 2007). This com-

pound system can be also used to achieve illumination con-trol of buildings during day, storing the surplus energy forspace heating during night. This system can contribute inthe ventilation needs during day and also to cover otherbuilding electrical loads. In low intensity solar radiation,the absorbers can be out of focus, leaving the light to comein the interior space and to keep the illumination at anacceptable level. Laboratory scale experimental results givean idea about the application of this new optical system forlighting (reduction by about 60–80%) and cooling control(reduction by 3–10 �C) of building interior spaces (Tripan-agnostopoulos et al., 2007), estimating that the system ispromising for building application and effectively com-bined with PV/T type absorbers.

4. PV/T design improvements

4.1. Design concepts for modified PV/T systems

From the parametric study on PVT/air systems (Tripan-agnostopoulos et al., 2000, 2001a and 2002b) it wasobserved that the efficiency is increased for smaller channeldepth, but the pressure drop must be considered due to theadditional electrical power input for the operation of thefan. The heat extraction by natural airflow depends onthe temperature difference between the inserting air in thechannel and the PV module. The operation of PV/T systemwith high rate of forced airflow gives satisfactory resultsregarding heat extraction. In natural airflow the flow rateis not usually so much high as in forced airflow applica-tions. The smaller channel depth increases heat extraction,but decreases also the air velocity. The heat extraction canbe increased using larger heat exchanging surface area inthe air channel to promote the convection heat transferto the circulating air. In order to increase radiation heattransfer, the PV rear surface as well as the opposite channelwall surface should be of high emissivity to transform theinfrared radiation to convection heat transfer mechanismsand to heat efficiently the circulating air. The last effectcan be further improved if larger heat exchanging surfacesare used in the air channel.

Elements of several geometry (Fig. 2) can be placedbetween PV module and opposite channel wall, or alsoon the wall, by which a more efficient air heat extractionis achieved (Tripanagnostopoulos et al., 2000). Roughen-ing the opposite channel wall with ribs or/and using wallsurface of high emissivity, a considerable and low cost airheating improvement is also adapted (Fig. 2a). In addition,corrugated sheet inside the air channel along the air flowcan be attached on PV rear surface and opposite channelwall surface (Fig. 2b). An alternative modification is toput light weight pipes along the air flow in the air channel,with slight elasticity to achieve satisfactory thermal contactwith PV rear surface and channel wall (Fig. 2c). Thesepipes are heated by conduction, convection and radiationfrom PV rear surface and can contribute to air heat extrac-tion, avoiding also the undesirable increase of opposite

Fig. 2. Improvement of the heat extraction in the air channel of the PVT/air system, with (a) roughened with ribs the opposite air channel wallmodification, (b) interposition of a corrugated sheet and (c) placement oftubes inside air channel.


channel wall surface temperature (Tripanagnostopouloset al., 2000).

Although the above heat transfer improvements resultto efficient air heating, two other low cost modificationswere investigated that can achieve satisfactory air heating,reduced PV module temperature and low increase of theopposite channel wall temperature (Tripanagnostopouloset al., 2000). The first is to place a thin flat metallic sheet(TMS type modification) inside the air channel and alongthe air flow (Fig. 3a). This TMS element doubles the heatexchanging surface area in the air channel and reducesthe heat transmittance to the back air channel wall of thePV/T system. The second modification is to mount finson the opposite air channel wall and along air flow (FINtype modification) and facing the PV rear surface(Fig. 3b). By using fins, the heat exchange surface can beincreased two or more times depending on the fin densityand dimensions (Garg and Datta, 1989). Fins can be alsoattached at PV rear surface but although they can contrib-ute to the achievement of higher heat extraction they

Fig. 3. Air heat extraction improvement by using (a) a thin metallic sheetinside air channel (TMS modification) and (b) fins on the opposite airchannel wall (FIN modification).

increase enough system cost because they should be lami-nated to PV modules and the higher module weightincreases the transportation cost. The mounting of fins atthe opposite to PV module channel wall can be done sepa-rately on the building tilted roof or the facade and is esti-mated of practical interest regarding flexibility and cost.The typical as well as the modified PVT/AIR systems canbe applied for space heating of building during winterand for space cooling during summer with natural ventila-tion mode and by the creation of a strong upward airstream (solar chimney effect).

4.2. Booster diffuse reflectors

Considering PV/T solar systems installed on horizontalbuilding roof, the parallel rows keep a distance from oneto the other in order to avoid PV module shading. Station-ary flat diffuse reflectors, which can be placed between theparallel rows of PV modules have been investigated (Tri-panagnostopoulos et al., 2002a). This installation increasessolar input on PV modules (Fig. 4) almost throughout theyear, resulting to an increase of electrical and thermal out-put of PV/T systems. Although diffuse reflectors result toless radiation on system absorber than the specular reflec-tors, they do not cause PV electrical efficiency drop as theyprovide an almost uniform distribution of the reflected solarradiation on the PV module surface. The diffuse reflectorscan be effectively applied in the residential and the industrialsector, overcoming some practical limitations of the usualPV/T systems as of the low operating temperature of thethermal unit in typical PV/T collectors and the reductionof electricity in the case of using additional glazing.

4.3. The PVT/dual system concept

The PVT/water collectors can effectively operate all sea-sons, mainly for application at locations in low latitudeswhere favorable weather conditions regarding the efficientoperation of the thermal collectors usually exist, or margin-ally in medium latitudes to avoid freezing. On the otherhand, the PVT/air collectors can effectively operate mainlyat locations of medium and high latitudes without freezingproblems, but for low latitude applications the summer

Fig. 4. Combined PV/T-diffuse reflector system and indication of diffusedreflected radiation on PV surface.


period with the high ambient temperatures PV cooling bythe circulating air is less effective. In addition, the hot airis not useful to the buildings during summer, except ifthe system is used to enhance natural ventilation by thesolar chimney effect, but in this case the heated air is usu-ally rejected to the ambient.

A combination of both heat extraction modes in onedevice is interesting and could possibly overcome the limita-tions of the two PV/T type collectors. Based on this princi-ple, a new type of PV/T collector with dual heat extractionoperation (PVT/dual) either to heat water or to heat airdepending on the weather conditions and building needs,was investigated (Tripanagnostopoulos et al., 2001b). Thewater heat extraction part could operate mainly duringthe periods of higher ambient temperatures, as water frommains is not usually over 20 �C and the air heat extractionpart to operate mainly when the ambient temperature islow. It should be taken care to drain the water from thepipes when ambient air drops under zero and to operatethe system only with the air circulation (except if anti-freez-ing liquid is used), while under mild weather conditions it ispossible to operate both heat extraction modes, if it is con-sidered useful for the application.

5. PVT/dual experimental study

5.1. Alternative PVT/dual system designs

In the dual PV/T collector both water and air heatexchangers (WHE and AHE correspondingly) are togetherin the same device and there are three main modes ofarrangement (Fig. 5). In the first mode the WHE is placedin thermal contact with the back surface of PV module andthe AHE exactly after it, forming also the thermal insula-tion envelope (MODE A). In the second mode the AHEis placed directly on the PV module back and the WHEinside the formed air channel (MODE B). In the thirdmode the AHE is mounted directly on the back of PV mod-

Fig. 5. Alternative PVT/dual design modes, used to determine theoptimum arrangement of the water and the air heat exchangers.

ule and the WHE is attached at the opposite air channelwall (MODE C).

MODE A has advantage in water heat extraction as theWHE is in thermal contact with PV rear surface, but airheat extraction is through the WHE back side. MODE Bhas advantage in air heat extraction as the additional ele-ment of the WHE plays the role of the TMS in the air chan-nel (mentioned in the previous section). MODE C hasadvantage from the practical point of view, as the WHEis simply placed on the opposite air channel wall. This sys-tem is obviously easier in construction than the other twoarrangements, as the mounting the WHE at PV modulerear surface with a good thermal contact and the hangingof it in the middle of the air channel. Another point ofthe suggested combination regarding WHE and AHEsub-parts is that the air heat extraction is improved com-pared to the typical PVT/air collectors because the pipesof the WHE increase the heat-exchanging surface insideair channel in all above cases.

In order to determine the difference in water and airheating performance of the three design modes of thePVT/dual collector a prototype was constructed and exper-imentally tested outdoors with steady state procedure. Theexperimental model was consisted of a pc-Si PV module of46 Wp with aperture area Aa = 0.4 m2 and an air channel atPV rear surface of 10 cm depth (AHE), formed by sheets ofthe thermal insulation 5 cm thickness. A heat exchanger forwater circulation (WHE) from copper sheet with copperpipes was constructed. The PVT/dual experimental modelwas properly designed to have flexibility in changing theplace of the WHE element inside the formed by the AHEair channel, placed on PV rear surface (MODE A), in themiddle of air channel (MODE B), or on air channel oppo-site wall (MODE C).

5.2. Experimental study of PVT/dual system

The system was tested outdoors at Physics Departmentexperimental site for the above three heat extraction modesof water and air circulation, regarding the electrical andthermal performance of it. For the measurements of thetemperatures at several positions of the device, Cu–CuNithermocouples were used. These temperatures were the fol-lowing: the input (Ti) and the output (To) fluid tempera-ture, the ambient air temperature (Ta), the water heatexchanger temperature (TWHE), the air channel oppositewall temperature (Tw), the air temperature in the air chan-nel (Tair) and the PV module temperature (TPV). Theincoming solar radiation on PV module plane (G) was mea-sured by a Kipp and Zonen pyranometer and for the con-trol of air flow a Lutron AM-4204 sensor was used. Forsystem operation at stagnation conditions (without fluidflow) the stagnation temperature (Tst) for the water wasconsidered by measuring the temperature on the WHE(Tst = TWHE). For the air heat extraction the mean valueof the PV module temperature and the opposite channelwall temperature were used (Tst = (TPV + Tw)/2).

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Fig. 6. Test results of the alternative designs of PVT/dual systems.


The steady state tests were performed during noon(±2 h), with systems oriented to the sun in order to ensureconstant value of the incoming solar radiation with almostsame diffuse solar radiation (20–25% of total). It was esti-mated that for these experiments solar radiation intensityvariation of the ±20 W m�2 and ambient air temperaturevariation of ±1 K were considered approximately constantfor the calculations and system evaluation, while all datawere collected by a CR10X Data Logger. The mass flowrate for the two operation modes of the dual type PV/Tcollector (for both water and air) was _m ¼ 0:02 kg s�1.The testing procedure for the determination of the effectof the alternative positions of the heat exchanging elementin the PVT/dual system performance, includes experimentsfor system operation with fluid inserting system input atseveral temperatures. During tests the PV module was con-nected to a load, simulating real system operation and inorder to avoid additional water or air heating. The opera-tion of the PVT/dual collector can be done either by work-ing independently the two heat extraction modes orworking them together and simultaneously. Although sometests were performed operating the WHE and AHE ther-mal sub-units simultaneously, apart of the complicatedtesting procedure it was observed that it was of less practi-cal interest and therefore the independent operation of thePVT/dual system regarding water or air heat extractionwas finally more extensively studied.

Considering the incoming solar radiation G (W) onaperture system surface of area Aa (m2), the fluid mass flowrate _m (kg s�1) and the heat removal fluid specific heat Cp

(J kg�1 K�1), the thermal efficiency gth of solar thermal col-lectors is calculated by the relation: gth ¼ _mCpðT o � T iÞ=AaG. In thermal collectors gth is determined as functionof the ratio DT/G (K W�1 m2), where DT = Ti � Ta isdetermined for fluid flow operation and DT = Tst � Ta,for operation at stagnation. The function gth = f(DT/G),is also used in PV/T systems as their thermal unit corre-sponds to a thermal collector. For the calculation of theelectrical output the load was disconnected for a shorttime and the current I (A) and the voltage V (V) of PVmodule were measured to determine the I–V curve of itunder system operating conditions. From these data thevalues of the current Im and voltage Vm at maximum powerpoint of PV module operation were determined. The valuesof Im and Vm and the incoming solar radiation G are usedto calculate the PV module electrical efficiency gel for thesystem aperture area Aa, using the relation: gel = ImVm/AaG. The electrical measurements were also used to deter-mine the electrical efficiency as function of PV moduleoperating temperature.

In Fig. 6 the test results of the PVT/dual experimentalcollector regarding the three modes of water heat exchan-ger placement inside the air channel, are shown. Theseresults are referred to the thermal efficiency gt of thePVT/dual system for the water and the air heat extraction,regarding the ratio DT/G. The thermal efficiency of PV/water sub-collector is extended in negative DT/G axis, as

some experiments were performed for ambient temperaturebeing higher than the water temperature at system input(Ta > Ti). From the presented results of Fig. 6 one cansee that the first PVT/dual design mode (MODE A) withthe water heat exchanger on PV rear surface, presents thehigher system thermal performance regarding the waterheating, while regarding air heating it is considered satis-factory. The second design mode (MODE B) presents thehigher thermal performance for the air heating and moder-ate for the water heating. The last design mode (MODE C)presents moderate performance in air heating, but the per-formance in water heating is low as the WHE is far fromPV rear surface.

Comparing the above results, MODE A is observed themost effective combination of the WHA and AHE elementsfor the heat extraction operation among the three testedsystem design modes. The other two design modes presentlower thermal performance in water heat extraction, whichis very critical for the efficient operation of the PVT/dualsystem in both thermal and electrical outputs. This is dueto the less efficient water heating and also to the less effec-tive PV cooling, resulting therefore to the reduction of boththermal and electrical outputs. The water heat extractionperformance improvement is limited as the main modifica-tion that can be applied is to use more pipes on the heatexchanger sheet, which could give a slightly higher effi-ciency, but on the contrary the additional cost is muchmore higher. In the constructed model the distance between


the pipes was considered sufficient (8 cm) regarding theheat extraction by conduction and the corresponding ele-ment cost. On the other hand, it is easier and cost effectiveto improve the heat extraction in the air circulation byapplying some of the investigations that were analyzed inthe previous section and MODE A of the PVT/dual systemwas used to be applied.

5.3. Modified PVT/dual systems

Based on the above, the MODE A combination ofWHE and AHE elements was used and the collector wasfurther modified, applying some of the above mentionedinvestigated improvements in the air channel. The firstmodification was the interposition of the TMS element inthe air channel (PVT/dual-TMS model). The second mod-ification was the mounting of the FIN element (PVT/dual-FIN model) on the opposite air channel wall and in thethird modification the TMS element was combined withroughened opposite channel wall by small ribs (PVT/dual-TMS/RIB model). These three modified PVT/dualcollectors are shown in cross section in Fig. 7.

In PVT/dual-TMS model a thin flat metallic sheet (alu-minum) was placed in the middle of the air channel andparallel to air flow (Fig. 7a). The front side of TMS elementthat faces the PV rear surface was painted mat black tohave high emittance (e � 0.9), while the back side ofTMS was left unpainted and as metallic surface it hadlow emittance (e � 0.1). By these surface properties(together with the high e back surface of WHE elementand the high e opposite air channel wall surface) a higherradiation transmission is obtained to the front side of theTMS element. The absorbed heat rises the TMS tempera-ture and results to the effective heating of the circulating

Fig. 7. Cross section of the studied PVT/dual solar systems, with (a) thethin metallic sheet (TMS) modification, (b) the fins on opposite air channelwall (FIN) modification and (c) the combination of TMS with ribs onopposite air channel wall (TMS/RIB).

air by convection, contributing to the achievement ofhigher thermal efficiency (according also to the results men-tioned in 4.1). Besides, the TMS element operates as ashield, protecting the opposite air channel wall from theheat flow from PV rear surface to it, thus it is estimatedsuitable to avoid the undesirable building overheating.

In the second modified model, the PVT/dual-FIN, finsof profile were used to form the fin plate element withtheir flat vertical surfaces being along to the air streamand to increase the heat exchanger surface of FIN element(Fig. 7b). It was considered preferable a cheap metallicmaterial to be used, as it is the galvanized iron sheet, toconstruct metallic fins of 4 cm in size, in order to formthe fin type element. The surface of the fins can remainmetallic to have low e and reduce the heat transmissionthrough the thermal insulation of the PV/T collector. Inthis case the temperature rise of FIN is not high and theeffect on the air heat extraction is lower. More efficient inair heating is the case that the surface of the fins is paintedblack (high e) and as the heat flow from the PV rear surfaceto it is higher, the achieved thermal efficiency of the collec-tor is increased.

The third modified model, the PVT/dual-TMS/RIB, issimilar to the first model but ribs of about 5 mm wereformed on the opposite air channel wall (Fig. 7c). By thismodel it was aimed to combine the advantages of TMSand FIN modifications, where the formation of ribs simu-lates small fins. The ribs were painted black to increase theheat transmittance by radiation from TMS back surface toair channel wall and overcome the lower heat transfer fromwall to circulating air due to the small size of the ribs com-pared to the fins, but to keep the low heat transmissionthrough the wall to collector back surface.

The thermal efficiency of air heat extraction modedepends strongly on air mass flow rate and the increaseof it results to a corresponding increase of electrical inputfor the operation of the necessary fan. The electrical con-sumption of fan is also increased in case of using smallair channel width or/and increased heat exchanging surfacefor air heat extraction, aiming to thermal efficiencyimprovement. In PV/T systems the electrical efficiency isof priority and the additional electrical input to overcomethe corresponding additional pressure drop must be takeninto consideration. The used air flow rate was_m ¼ 0:02 kg s�1 and the additional heat exchanging surfacefor the air heat extraction was almost two times the airchannel internal surface area without the modification.The pressure drop in air channel from entrance to exit issmall and is difficult to be accurately measured becauseof the short length of air channel (1 m). In a recent studythe values of the pressure drop and the correspondingpower to overcome it were calculated for TMS and FINtype modifications (Tonui and Tripanagnostopoulos,2007b).

The modified PV/T systems with the dual heat extrac-tion operation were also tested with additional incidentsolar radiation, which was achieved by combining properly

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Fig. 9. Thermal efficiency steady state results of PVT/dual-FIN typecollector regarding water and air heat extraction, in typical form as well ascombined with diffuse reflector.


the PVT/dual system with a flat diffuse reflector (Fig. 4)tested outdoors for the system performance determination.In the experiments with the concentrating system, a matthin aluminum sheet was attached on a rigid thin woodensheet and adjusted to the sun in order the additional solarradiation input on the surface of the PV module of thePVT/dual collector to be about 35% to observe the effectof a higher value of the additional solar input by a station-ary diffuse reflector-solar system combination. The applica-tion of diffuse reflectors is mainly concerned for horizontalbuilding roof installations of solar thermal collectors, pho-tovoltaic panels and PV/T systems, as they are usuallyplaced in parallel rows and the space between them is avail-able to put the reflectors. The additional solar input on PV/T system is not very high and the effect of it to the PV tem-perature increase is rather small, but this type of installa-tion is very simple and cheap and can be considered acost effective performance improvement for horizontallyinstalled solar energy systems.

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Fig. 10. Thermal efficiency steady state results of PVT/dual-TMS/RIBtype collector regarding water and air heat extraction, in typical form aswell as combined with diffuse reflector.

5.4. Experimental results from modified PVT/dual systems

Thermal efficiency (gth) steady state test results of themodified PV/T model regarding water and air heat extrac-tion are presented in Figs. 8–10. The obtained linear effi-ciency equations for all collectors are presented in Table1, including also for comparison the corresponding equa-tions from PVT/DUAL collector of MODE A (Fig. 5) asreference. From the results of diagrams and Table 1 andfor the water heat extraction mode, the obtained thermalefficiency is about 55% for all tested models and for oper-ation at ambient conditions (DT/G = 0 K W�1 m2). Forthe air heat extraction and for DT/G = 0 K W�1 m2, it is39% for PVT/dual-TMS type, 42% for PVT/dual-FIN typeand close to 44% for PVT/dual-TMS/RIB type. Theseresults are estimated satisfactory considering the achievedthermal efficiency of PV/T systems with air or with water

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Fig. 8. Thermal efficiency steady state results of PVT/dual-TMS typecollector regarding water and air heat extraction, in typical form as well ascombined with diffuse reflector.

heat extraction that is presented in the works of otherauthors. The thermal efficiency is reduced for lower air flowrate and increased for higher air flow rate, but the increasein pressure drop should be considered as it affects the elec-trical input for the additional needed fan power, althoughthe increase of electrical efficiency from the decrease ofPV temperature due to higher air flow can balance in somecases the additional fan power (Tonui and Tripanagnosto-poulos, 2007b).

Considering the obtained results from the reference sys-tem (MODE A PV/T system) there is a significant increasein thermal efficiency for the air heat extraction, which ishigher by about 23%, 33% and 36% for the PVT/dual solarcollectors with TMS, FIN and TMS/RIB type modifica-tion, respectively. Regarding the water heat extraction,the modifications in the air channel have a smaller effect,which is in a reverse sequence to the air heat extraction per-formance and the observed differences are about 17%, 16%

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Fig. 11. The PV module electrical efficiency as function of its operatingtemperature for the typical and the combined with diffuse reflector mode.

Table 1Efficiency equations of the studied PVT/dual collectors

System Thermal efficiency

Water Air

PVT/dual-TMS gth = 0.556 � 12.824DT/G gth = 0.391 � 8.484DT/GPVT/dual-FIN gth = 0.553 � 12.981DT/G gth = 0.423 � 8.488DT/GPVT/dual-TMS/RIB gth = 0.545 � 12.771DT/G gth = 0.434 � 8.933DT/GPVT/dual-TMS + REF gth = 0.662 � 11.969DT/G gth = 0.548 � 10.222DT/GPVT/dual-FIN + REF gth = 0.651 � 11.731DT/G gth = 0.595 � 10.286DT/GPVT/dual-TMS/RIB + REF gth = 0.651 � 11.870DT/G gth = 0.623 � 11.133DT/G

PVT/dual (MODE A) gth = 0.475 � 11.671DT/G gth = 0.319 � 7.471DT/G

Electrical efficiencyPV module gel = 0.166 � 0.001 TPV

PV module + REF gel = 0.185 � 0.001 TPV


and 14% correspondingly, for the above modifications. Theexperiments were performed operating the PVT/dual sys-tems only with water or with air heat extraction mode, todetermine independently the effect of the two heat extrac-tion modes to the system performance. Some experimentswere performed using simultaneously water and air heatextraction system operation and the results showed thatthe two modes of heat extraction complement each other,if the WHE and AHE thermal parts are operating at thesame temperature. If system operation is at different waterand air temperatures, the one thermal unit affects thermallythe other and the thermal outputs depend on the operatingtemperature of each fluid, but this operation mode is com-plicated and not so important for most of the PVT/dualsystem applications.

The PVT/dual systems were properly combined with aflat diffuse reflector, which was adjusted to achieve an addi-tional solar input on the surface of the used PV module. InFigs. 8–10 the corresponding performed thermal efficiencyfor the three PVT/dual system types with the diffuse reflec-tor regarding water heat extraction (water + REF) and airheat extraction (air + REF) are included. The calculationof thermal efficiency for the combined systems is basedon the net solar radiation on PV module surface in orderto distinguish the effect of booster diffuse reflector fromthe typical system in the efficiency diagrams. The addi-tional solar radiation from the reflector to the PV modulesurface was measured to perform experiments of same con-centration and surface distribution of solar radiation, butin the calculations the efficiency was derived consideringthe one-sun radiation only. Although the increase in energyoutput is more essential, the calculated efficiency is lower ifthe additional solar radiation input is considered, becauseof the optical losses from the diffuse reflector and the PVmodule protective glazing. The given results in the dia-grams and the Table show a considerable increase of ther-mal output for both heat extraction modes, achieving forthe water heat extraction about 18%–19% and for the airheat extraction about 40–43%, at ambient conditions ofoperation (DT/G = 0 K W�1 m2). The results are moreimpressive for higher system operating temperatures wherethe performance can be almost doubled for operation at

DT/G = 0.030 K W�1 m2. These results show that the addi-tional solar input on PV module has a positive effect inPVT/dual collectors and mainly in the air heat extraction,as these improvements contribute to overcome the problemof lower efficiency due to the low density of air.

In Fig. 11 the electrical efficiency (gel) of the used PVmodule that was used in the tested PV/T systems as func-tion of the operating temperature (TPV) is shown, togetherwith the application of the diffuse reflector (PV + REF). Inthe tests with the booster diffuse reflector the calculation ofthe electrical efficiency was based on the net solar radiationon PV module surface for a better comparison of theobtained electrical performance, following the same con-cept described above for the thermal part of the PV/T col-lector. The result from the applied diffuse reflector is anincrease of the electrical performance by about 12% forthe low (about 0 �C) operating temperatures, withgel = 16.5% and 18.5%, for the typical PV and thePV + REF cases respectively. The increase in electrical per-formance percentage is more significant if the operating PVmodule temperature is higher, as it happens usually in PV/T collectors, thus the electrical performance can be higherby about 18% for operation of PV module at about 55 �C,as gel = 13% for the PV + REF case, compared togel = 11% for the typical case.


5.5. Discussion

The PV/T type solar collectors have a conflict betweentheir electrical and thermal performance, as the photovol-taic part (PV modules) presents higher electrical outputfor lower operating temperatures, but the thermal part(water or air heat extraction units) should provide heatremoval fluid at higher temperatures to adapt effectivelyseveral solar thermal applications. This problem makesthese systems to be less often applied than the separateunits of the photovoltaics and the thermal collectors,although PV/T collectors have lower cost per producedelectricity and heat regarding the total used surface areafor their installation. The investigated performanceimprovements aim to overcome some limitations in systemefficient operation and to make these new solar energydevices more attractive for applications.

The suggested PV/T collector with the dual heat extrac-tion operation is an investigated PV/T system, which com-bines satisfactory electrical performance with flexibility inheat extraction and heat removal fluid. This device has aslightly higher cost than that of a typical PVT/water sys-tem, but the new system is suitable for multiple uses, pro-viding also hot air for space heating, space cooling bynatural ventilation, etc. In case that the heat extraction isnot effective for domestic water heating and should bedrained to avoid freezing in cold climates, the system canbe used to heat air for space heating of the building. Onthe other hand PVT/dual collectors are a little more expen-sive than PVT/air collectors because of the additionalWHE element, but their PV modules can operate at lowertemperatures during summer as the water-cooling mode ismore effective, achieving therefore higher electrical output.

The applied modifications for the improvement of theair heat extraction (TMS, FIN and TMS/RIB) are costeffective as the additional elements are cheap from thematerial point of view and can be easily mounted on theair channel or on the wall of it, independently to the typeof the used PV module. In the studied PVT/dual collectorsthe increase in thermal output is significant and can resultto a reduction of the total system cost pay back time. If oneconsiders the sum of thermal and electrical outputs fromthe PVT/dual systems, total values of about 70% and55% are obtained, for the water and the air heat extractionmodes respectively and for operation at about 20 �C inboth heat extraction modes. The corresponding valuesfor the same systems but combined with diffuse reflectorare about 80% and 75% for the water and for the air heatextraction.

These results show the advantage of using the suggestedcombination of the diffuse reflectors for the increase of PVmodule electrical and thermal output. The effect of the dif-fuse reflector is not same all year round, as the additionalsolar input to the PV module from the diffuse reflectordepends on the season and the ratio diffuse/total of theincident solar radiation. The horizontal building roofslocated at low latitudes are mainly suitable for the applica-

tion of the diffuse reflectors. Considering solar energy sys-tems installed on horizontal building roof they are usuallyplaced in parallel rows, keeping a proper distance from onerow to the other in order to avoid PV module shading. Thestationary flat diffuse reflectors can be placed between thecollector rows from the higher part of PV modules of onerow to the lower part of PV modules of next row.

In all other cases, the simple type of PVT/dual collectorscan be applied, providing electricity and heat in warmwater or air the buildings and covering therefore a partof their energy demand. The investigated configurationsfor the PVT/dual air heat extraction improvement are con-sidered satisfactory, with the TMS modification more suit-able for the building protection from overheating, the FINmodification for the thermal output and the TMS/RIBmodification for both. Regarding the applications ofPVT/dual collectors, the warm water can be used in build-ings all year, but for the air it is not effective in case ofambient air temperature over 20 �C, except if air is usedfor natural ventilation of the building. It is also interestingthe application of the studied systems to provide heat inindustrial and agricultural processes. Considering the typeof the PV modules, the used pc-Si type is estimated satisfac-tory regarding cost and electrical output, but c-Si and a-Sitype modules can be also used depending on other require-ments. The PV modules of c-Si type are more expensivethan pc-Si type, but are more stable and of slightly higherelectrical efficiency The PV modules of a-Si type arecheaper than both above, but of lower efficiency and stabil-ity. On the other hand, the use of a-Si PV modules in PV/Tcollectors can result to cost effective devices (Tselepis andTripanagnostopoulos, 2002), mainly in case of large avail-able installation surface area, because the lower electricaloutput is balanced by the higher thermal output.

The complete PV/T systems include the additional com-ponents as of the balance of system (BOS) for the electric-ity and also for the heat. Because of the BOS the finalenergy output is reduced by about 15% due to the electricaland thermal losses from one part to the other. The cost payback time (CPBT) of standard photovoltaics is more than25 years in case of no subsidies. PV/T systems presentlower values of the CPBT (10–15 years) if they are operat-ing at low temperatures, but CPBT is higher for higher sys-tem operating temperatures because the electrical andthermal efficiencies are reduced (Tripanagnostopouloset al., 2005). Considering the environmental impact ofPV/T systems, they present lower (by 20–30%) energyand CO2 payback times in all cases compared to the stan-dard PV modules and even lower values for lower operat-ing temperatures (Tripanagnostopoulos et al., 2005,2006). Finally, one more advantage of PV/T systems rela-tive to standard PV modules and solar thermal collectorsis that they can cover both electricity and thermal needswith module types of same appearance instead of usingtwo systems of different appearance, especially if the avail-able external building surfaces for the installation of solarenergy systems is limited. In case of large available building


surfaces, the PV/T collectors can operate efficiently at lowtemperatures to preheat the circulating fluid and keep PVtemperature relatively low, with the typical solar thermalcollectors to operate at higher temperatures (Tripanagnos-topoulos, 2006).

6. Conclusions

A brief overview of the literature and the aspects for thestudies, technologies and improvements of hybrid photo-voltaic/thermal solar systems show the perspectives andlimitations of these new solar energy devices. Investigationsthat contribute to PV/T system improvements at the Uni-versity of Patras were briefly analyzed and the dual PV/Ttype collector with water and air heat extraction operationwas suggested. Some experimental models consisting of pc-Si PV module, an air channel and water heat exchangermade by copper sheet and copper pipes were designedand tested. The systems aim to use either water or air forthe heat extraction from the PV module, depending onthe thermal needs of the application and the efficient oper-ation of system regarding the weather conditions and theused heat removal fluid.

Three alternative modes of placing the water heatexchanger inside the air channel were tested, with the waterheat exchanger at PV rear surface giving the best results forthe combined water and air heat extraction. For theimprovement of air heat extraction, three low cost modifi-cations that increase the heat exchange surface in the airchannel were tested to determine system performance.Steady state test results of the modified PV/T systemsshowed satisfactory thermal efficiency for both water andair heat extraction. Aiming to achieve higher thermal andelectrical output of the modified PVT/dual collectors theywere combined with diffuse reflector and the results con-firmed the advantage of using low concentration and cheapdiffuse reflector making, this combination cost effective forhorizontal building roof installations.

PV/T systems with the dual heat extraction operationcan be effectively used in houses, residential and officebuildings, in hotels, hospitals and also in the industrialand agricultural sectors providing electricity and heat withflexibility in the heat removal fluid. They are cost effectivesolar energy devices, mainly for operation in low tempera-tures and are promising systems for a wider application ofphotovoltaics.

References

Affolter, P., Eisenmann, W., Fechner, H., Rommel, M., Schaap, A.,Sorensen, H., Tripanagnostopoulos, Y., Zontag, H., 2006. PVTROADMAP. Edition of ECN, Netherlands, www.pvtforum.org.

Al Baali, A.A., 1986. Improving the power of a solar panel by coolingand light concentrating. Solar and Wind Technology 3, 241–245.

Athienitis, A.K., Tzempelikos, A., 2002. A methodology for simulation ofdaylight room illuminance distribution and light dimming for a roomwith a controlled shading device. Solar Energy 72, 271–281.

Bakker, M., Zondag, H.A., Elswijk, M.J., Strootman, K.J., Jong, M.J.M.,2005. Performance and costs of a roof-sized PV/thermal arraycombined with a ground coupled heat pump. Solar Energy 78, 331–339.

Battisti, R., Tripanagnostopoulos, Y., 2005. PV/Thermal systems forapplication in industry. In: Proceedings of (CD-ROM) of 20thEuropean Photovoltaic Solar Energy Conference, 6–10 June 2005,Barcelona, Spain, paper 6CV.2.3.

Bazilian, M., Leeders, F., van der Ree, B.G.C., Prasad, D., 2001.Photovoltaic cogeneration in the built environment. Solar Energy 71,57–69.

Bergene, T., Lovvik, O.M., 1995. Model calculations on a flat-plate solarheat collector with integrated solar cells. Solar Energy 55, 453–462.

Bhargava, A.K., Garg, H.P., Agarwal, R.K., 1991. Study of a hybrid solarsystem – Solar air heater combined with solar cells. Energy Conversionand Management 31, 471–479.

Bhavnani, S.H., Bergles, A.E., 1990. Effect of surface geometry andorientation on laminar natural convection heat transfer from a verticalflat plate with transverse roughness elements. International Journal ofHeat and Mass Transfer 33, 965–981.

Biswas, G., Chattopadhyay, H., 1992. Heat transfer in a channel withbuilt-in wing-type vortex generators. International Journal of Heatand Mass Transfer 35, 803–814.

Brinkworth, B.J., 2000. Estimation of flow and heat transfer for the designof PV cooling ducts. Solar Energy 69, 320–413.

Brinkworth, B.J., Sandberg, M., 2006. Design procedure for cooling ductsto minimise efficiency loss due to temperature rise in PV arrays. SolarEnergy 80, 89–103.

Brinkworth, B.J., Cross, B.M., Marshall, R.H., Hongxing, Yang, 1997.Thermal regulation of photovoltaic cladding. Solar Energy 61, 169–179.

Brinkworth, B.J., Marshall, R.H., Ibarahim, Z., 2000. A validad model ofnaturally ventilated PV cladding. Solar Energy 69, 67–81.

Brockmeier, U., Guentermann, Th., Fiebig, M., 1993. Performanceevaluation of a vortex generator heat transfer surface and comparisonwith different high performance surfaces. International Journal of Heatand Mass Transfer 36, 2575–2587.

Brogren, M., Nostell, P., Karlsson, B., 2000. Optical efficiency of a PV-thermal hybrid CPC module for high latitudes. Solar Energy 69, 173–185.

Brogren, M., Karlsson, B., Werner, A., Roos, A., 2002. Design andevaluation of low-concentrating, stationary, parabolic reflectors forwall-integration of water-cooled photovoltaic-thermal hybrid modules.In: Proceedings of International Conference PV in Europe 7–11October, Rome, pp, 551–555.

Cartmell, B.P., Shankland, N.J., Fiala, D., Hanby, V., 2004. A multi-operational ventilated photovoltaic and solar air collector: Applica-tion, simulation and initial monitoring feedback. Solar Energy 76, 45–53.

Charron, R., Athienitis, A.K., 2006. Optimization of the performance ofdouble-facades with integrated photovoltaic panels and motorizedblinds. Solar Energy 80, 482–491.

Chow, T.T., 2003. Performance analysis of photovoltaic-thermal collectorby explicit dynamic model. Solar Energy 75, 143–152.

Chow, T.T., Hand, J.W., Strachan, P.A., 2003. Building-integratedphotovoltaic and thermal applications in a subtropical hotel building.Applied Thermal Engineering 23, 2035–2049.

Coventry, J.S., Lovegrove, K., 2003. Development of an approach tocompare the ‘value’ of electric and thermal output from a domesticPV/thermal system. Solar Energy 75, 63–72.

Coventy, J., 2005. Performance of a concentrating photovoltaic/thermalsolar collector. Solar Energy 78, 211–222.

Cox III, C.H., Raghuraman, P., 1985. Design considerations for flat-plate-photovoltaic/thermal collectors. Solar Energy 35, 227–241.

Eicker, U., Fux, V., Infield, D., Mei, Li, 2000. Heating and cooling ofcombined PV-solar air collectors facades. In: Proceedings of Interna-tional Conference of 16th European PV solar energy. 1–5 MayGlasgow, UK, pp. 1836–1839.

http://www.pvtforum.org


Elazari, A., 1998. Multi Solar System – Solar multimodule for electricaland hot water supply for residentially building. In: Proceedings of 2ndWorld Conference On Photovoltaic Solar Energy Conversion, 6–10July, Vienna, Austria, pp. 2430–2433.

Fiebig, M., 1997. Vortices, generators and heat transfer. In: Proceedings of5th UK National Conference on Heat Transfer ICHEME, pp. 108–123.

Florschuetz, L.W., 1979. Extention of the Hottel–Whillier model to theanalysis of combined photovoltaic/thermal flat plate collectors. SolarEnergy 22, 361–366.

Frankl, P., Gamberale, M., Battisti, R., 2000. Life Cycle Assessment of aPV Cogenerative System: Comparison with a Solar Thermal and a PVSystem. In: Proceedings of 16th European PV Solar Energy Confer-ence, 1–5 May, Glasgow, UK, pp. 1910–1913.

Garg, H.P., Adhikari, R.S., 1997. Conventional hybrid photovoltaic/thermal (PV/T) air heating collectors: Steady-state simulation. Renew-able Energy 11, 363–385.

Garg, H.P., Adhikari, R.S., 1999. Performance analysis of a hybridphotovoltaic/thermal (PV/T) collector with integrated CPC troughs.International Journal of Energy Research 23, 1295–1304.

Garg, H.P., Agarwal, P.K., 1995. Some aspects of a PV/T collector/forcedcirculation flat plate solar water heater with solar cells. EnergyConversion and Management 36 (87–99), 1995.

Garg, H.P., Datta, G., 1989. Performance studies on a finned-air heater.Energy 14, 87–92.

Garg, H.P., Agarwal, P.K., Bhargava, A.K., 1991a. The effect of planebooster reflectors on the performance of a solar air heater with solarcells suitable for a solar dryer. Energy Conversion and Management32, 543–554.

Garg, H.P., Jha, R., Choudhury, Datta G., 1991b. Theoretical analysis ona finned type solar air heater. Energy 16, 1231–1238.

Guiavarch, A., Peuportier, B., 2006. Photovoltaic collectors efficiencyaccording to their integration in buildings. Solar Energy 80, 65–77.

Gupta, D., Solanki, S.C., Saini, J.S., 1993. Heat and fluid flow inrectangular solar air heater ducts having transverse rib roughness onabsorber plates. Solar Energy 51, 31–37.

Han, J.C., Park, J.S., 1988. Developing heat transfer in rectangularchannels with rib turbulators. International Journal of Heat and MassTransfer 31, 183–195.

Hausler, T., Rogash, H., 2000. Latent heat storage of photovoltaics. In:Proceedings of 16th Europ. PV Solar Energy Conference, 1–5 May,Glasgow, UK, vol III, pp. 265–267.

Hegazy, A.A., 2000. Comparative study of the performances of fourphotovoltaic/thermal solar air collectors. Energy Conversion andManagement 41, 861–881.

Hendrie, S.D., 1979. Evaluation of combined Photovoltaic/Thermalcollectors. In: Proceedings of International Conference ISES, Atlanta,Georgia, USA, May 28 – June 1, vol. 3, pp. 1865–1869.

Huang, B.J., Lin, T.H., Hung, W.C., Sun, F.S., 2001. Performanceevaluation of solar photovoltaic/thermal systems. Solar Energy 70,443–448.

Infield, D., Mei, Li, Eicker, U., 2004. Thermal performance estimation forventilated PV facades. Solar Energy 76, 93–98.

Ito, S., Miura, N., 2003. Usage of a DC fan together with photovoltaicmodules in a solar air heating system. In: Proceedings of (CD-ROM)ISES World Congress Goteborg, Sweden, 14–19 June.

Jie, Ji, Tin-Tai, Chow, Wei, He, 2003. Dynamic performance of hybridphotovoltaic/thermal collector wall in Hong Kong. Building andEnvironment 38, 1324–1327.

Jirka, V., Kuceravy, V., Maly, M., Pokorny, J., Rehor, E., 1998. Thearchitectural use of glass raster lenses. In: Proceedings of WorldRenewable Energy Congress V, Part III, pp. 1595–1598.

Kalogirou, S.A., 2001. Use of TRNSYS for modelling and simulation of ahybrid PV-Thermal solar system for Cyprus. Renewable Energy 23,247–260.

Kalogirou, S.A., Tripanagnostopoulos, Y., 2006. Hybrid PV/T solarsystems for domestic hot water and electricity production. EnergyConversion and Management 47, 3368–3382.

Karlsson, B., Brogren, M., Larsson, S., Svensson, L., Hellstrom, B., Sarif,Y., 2001. A large bifacial photovoltaic-thermal low-concentratingmodule. In: Proceedings of 17th PV Solar Energy Conference 22–26October Munich, Germany, pp. 808–811.

Kern Jr., E.C., Russel, M.C., 1978. Combined photovoltaic and thermalhybrid collector systems. In: Proceedings of 13th IEEE PhotovoltaicSpecialists, Washington DC, USA, pp. 1153–1157.

Lalovic, B., 1986–1987. A hybrid amorphous silicon photovoltaic andthermal solar collector. Solar Cells 19, 131–138.

Lee W., M., Infield D.,G., Gottschalg R. 2001. Thermal modeling ofbuilding integrated PV systems In: Proceedings of 17th PV SolarEnergy Conference. Munich, 22–26 October, pp. 2754–2757.

Leenders, F., Shaap, A.B., van der Helden, B.G.C., 2000. Technologyreview on PV/Thermal concepts. In: Proceedings of 16th European PVSolar Energy Conference, 1–5 May, Glasgow, UK, pp. 1976–1980.

Loferski, J.J., Ahmad, J.M., Pandey, A., 1988. Performance of photovol-taic cells incorporated into unique hybrid photovoltaic/thermal panelsof a 2.8 KW residential solar energy conversion system. In: Proceed-ings of 1988 Annual Meeting, American Solar Energy Society,Cambridge, Massachusetts, USA, pp. 427–432.

Molteni, S.C., Gourret, G., Paule, B., Michel, L., Scartezzini, J.L., 2001.Design of anidolic zenithal lightguides for daylighting of undergroundspaces. Solar Energy 69, 117–129.

Moshfegh, B., Sandberg, M., 1998. Flow and heat transfer in the air gapbehind photovoltaic panels. Renewable and Sustainable EnergyReviews 2, 287–301.

Moshfegh, B., Sandberg, M., Bloem, J.J., Ossenbrink, H., 1995. Analysisof fluid flow and heat transfer within the photovoltaic facade on theELSA building. In: Proceedings of JRC ISPRA 13th European PVSolar Energy Conference Nice, October 23–27, p. 2215.

Naphon, P., 2005. On the performance and entropy generation of thedouble-pass solar air heater with longitudinal fins. Renewable Energy30, 1345–1357.

Ossenbrink, H.A., Rigolini, Chehab O., Olaf van der Vebbe. 1994.Building integration of an amorphous silicon photovoltaic facade. In:Proceedings of IEEE 1st World Conference on PV energy conversion,December 5–9, Hawaii, pp. 770–773.

Othman, M.Y.H., Yatim, B., Sopian, K., Bakar, M.N.A., 2005.Performance analysis of a double-pass photovoltaic/thermal (PV/T)solar collector with CPC and fins. Renewable Energy 30, 2005–2017.

Posnansky, M., Gnos, S., Coonen, S., 1994. The importance of hybrid PV-Building integration. In: Proceedings of first world conference onPhotovoltaic energy conversion, Hawaii 5–9 December, pp. 998–1003.

Pottler, K., Sippel, C.M., Beck, A., Fricke, J., 1999. Optimized finnedabsorber geometries for solar air heating collectors. Solar Energy 67,35–52.

Prakash, J., 1994. Transient analysis of a photovoltaic-thermal solarcollector for co-generation of electricity and hot air/water. EnergyConversion and Management 35, 972–976.

Prasad, B.N., Saini, J.S., 1991. Optimal thermohydraulic performance ofartificially roughened solar air heaters. Solar Energy 47, 91–96.

Raghuraman, P., 1981. Analytical Predictions of liquid and air photovol-taic/thermal, flat-plate collector performance. Journal of Solar EnergyEngineering 103, 291–298.

Ricaud, A., Roubeau, P., 1994. Capthel, a 66% efficient hybrid solarmodule and the Ecothel co-generation solar system. In: Proceedings ofIEEE First World Conference on Photovoltaic Energy Conversion,Waikoloa, Hawaii, pp. 1012–1015.

Rosell, J.I., Vallverdu, X., Lechon, M.A., Ibanez, M., 2005. Design andsimulation of a low concentrating photovoltaic/thermal system.Energy Conversion and Management 46, 3034–3046.

Sandness, B., Rekstad, J., 2002. A photovoltaic/thermal (PV/T) collectorwith a polymer absorber plate-experimental study and analyticalmodel. Solar Energy 72, 63–73.

Scartezzini, J.L., Courret, G., 2002. Anidolic daylighting systems. SolarEnergy 73, 123–135.


Schroer et al., 1998. Hybrid thermal insulating PV facade elements. In:Proceedings of 2nd World Conference On Photovoltaic Solar EnergyConversion , 6–10 July, Vienna, Austria, pp. 2591–2593.

Sharan, S.N., Mathur, S.S., Kandpal, T.C., 1985. Economic evaluation ofconcentrator-photovoltaic systems. Solar and Wind Technology 2 (3/4), 195–200.

Sopian, K., Liu, H.T., Yigit, K.S., Kakac, S., Veziroglu, T.N., 1996.Perfromance analysis of photovoltaic thermal air heaters. EnergyConversion and Management 37 (11), 1657–1670.

Tiggelbeck, St., Mitra, N.K., Fiebig, M., 1993. Experimental investiga-tions of heat transfer enhancement and flow losses in a channel withdouble rows of longitudinal vortex generators. International Journalof Heat and Mass Transfer 36, 2327–2337.

Tiwari, A., Sodha, M.S., 2006. Performance evaluation of solar PV/Tsystem: An experimental validation. Solar Energy 89, 751–759.

Tonui, J.K., Tripanagnostopoulos, Y., 2007a. Improved PV/T solarcollectors with heat extraction by forced or natural air circulation.Renewable Energy 32, 623–637.

Tonui, J.K., Tripanagnostopoulos, Y., 2007b. Air-cooled PV/T solarcollectors with low cost performance improvements. Solar Energy 81,498–511.

Tripanagnostopoulos, Y., 2006. Cost effective designs of buildingintegrated PV/T solar systems. In: Proceedings of (CD-ROM) 21stEuropean PV Solar Energy Conference, 4–8 September 2006, Dresden,Germany.

Tripanagnostopoulos, Y., Nousia, Th., Souliotis, M., 1998. Hybrid PV-ICS systems. In: Proceedings of International Conference WREC V20–25 September, Florence, Italy, pp. 1788–1791.

Tripanagnostopoulos, Y., Nousia, Th., Souliotis, M., 2000. Low costimprovements to building integrated air cooled hybrid PV-Thermalsystems. In: Proceedings of 16th European PV Solar Energy Confer-ence, Glasgow, UK, 1–5 May, vol. II, pp. 1874 – 1899.

Tripanagnostopoulos, Y., Nousia, Th., Souliotis, M., 2001a. Test resultsof air cooled modified PV modules. In: Proceedings 17th PV SolarEnergy Conference Munich, Germany, 22–26 October, pp. 2519–2522.

Tripanagnostopoulos, Y., Tzavellas, D., Zoulia, I., Chortatou, M., 2001b.Hybrid PV/T systems with dual heat extraction operation. In:Proceedings of 17th PV Solar Energy Conference, Munich, 22–26October, pp. 2515–2518.

Tripanagnostopoulos, Y., Nousia, Th., Souliotis, M., Yianoulis, P., 2002a.Hybrid photovoltaic/thermal solar systems. Solar Energy 72, 217–234.

Tripanagnostopoulos, Y., Bazilian, M., Zoulia, I., Battisti, R., 2002b.Hybrid PV/T system with improved air heat extraction modification.In: Proceedings of International Conference on PV in Europe 7–11October, Rome, Italy, pp. 718–721.

Tripanagnostopoulos, Y., Souliotis, M., Battisti, R., Corrado, A.,2005. Energy, cost and LCA results of PV and hybrid PV/T solarsystems. Progress in Photovoltaics: Research and Applications 13,235–250.

Tripanagnostopoulos, Y., Souliotis, M., Battisti, R., Corrado, A., 2006.Performance, cost and Life-cycle assessment study of hybrid PVT/AIRsolar systems. Progress in Photovoltaics: Research and Applications14, 65–76.

Tripanagnostopoulos, Y., Siabekou, Ch., Tonui, J.K., 2007. The Fresnellens concept for solar control of buildings. Solar Energy 81, 661–675.

Tsangrassoulis, A., Santamouris, M., Asimakopoulos, D., 1996. Theoret-ical and experimental analysis of daylight performance for variousshading systems. Energy and Buildings 24, 223–230.

Tselepis, S., Tripanagnostopoulos, Y., 2002. Economic analysis of hybridphotovoltaic/thermal solar systems and comparison with standard PVmodules. In: Proceedings of International Conference on PV in Europe7–11 October, Rome, Italy, pp. 856–859.

Turk, A.Y., Junkhan, G.H., 1986. Heat transfer enhancement down-stream of vortex generators on a flat plate. In: Proceedings ofInternational Heat Transfer Conference, vol 6, pp. 2903–2908.

Vokas, G., Christandonis, N., Skittides, F., 2006. Hybrid photovoltaic-thermal systems for domestic heating and cooling – A theoreticalapproach. Solar Energy 80, 607–615.

Winston, R., 1974. Principles of solar concentrators of a novel design.Solar Energy 16, 89–95.

Yang, H.X, Marshall, G.H., Brinkworth, B.J., 1994. An experimentalstudy of the thermal regulation of a PV-clad building roof. In:Proceedings of 12th European Photovoltaic solar energy conferenceAmsterdam, The Netherlands 11–15 April, pp. 1115–1118.

Zhu, J.X., Fiebig, M., Mitra, N.K., 1995. Numerical investigation ofturbulent flows and heat transfer in a rib-roughened channel withlongitudinal vortex generators. International Journal of Heat andMass Transfer 38, 495–501.

Zondag, H.A., De Vries, D.W., Van Helden, W.G.J., Van Zolingen,R.J.C., Van Steenhoven, A.A., 2002. The thermal and electrical yieldof a PV-Thermal collector. Solar Energy 72, 113–128.

Zondag, H.A., De Vries, D.W., Van Helden, W.G.J., Van Zolingen,R.J.C., Van Steenhoven, A.A., 2003. The yield of different combinedPV-thermal collector designs. Solar Energy 74, 253–269.

Zontag, H., Bakker, M., van Helden, W.G.J., Affolter, P., Eisenmann, W.,Fechner, H., Rommel, M., Schaap, A., Sorensen, H., Tripanagnosto-poulos, Y., 2005. PVT Roadmap: A European guide for the develop-ment and market introduction of PVT technology. In: Proceedings of(CD) 20th European Photovoltaic Solar Energy Conference, 6–10June, Barcelona, Spain.

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Aspects and Improvements of Hybrid Photovoltaic Thermal Solar Energy Systems