a review on photovoltaic-thermal (pv-t) air and water collectors

ISSN 0003�701X, Applied Solar Energy, 2011, Vol. 47, No. 3, pp. 169–183. © Allerton Press, Inc., 2011.

169

1 INTRODUCTION

The idea and various concepts of combined PV–Tcollector has been discussed and significant amount oftheoretical an experimental research carried out overthe last 30 years. The main aim always was increasingof the overall energy efficiency [1, 2]. The electricityconversion�efficiency of a solar cell for commercialapplication is about 6–15%. More than 85% of theincoming solar energy is either reflected or absorbed asheat energy. Consequently, the working temperature ofthe solar cells increases considerably after prolongedoperations and the cells efficiency drops significantly.The hybrid PV–T collector technology using water asthe coolant has been seen as a solution for improvingthe energy performance [3]. Hybrid of PV–T solarsystems can simultaneously provide electricity andheat, achieving a higher conversion rate of theabsorbed solar radiation than standard PV modules.When properly designed, PV–T systems can extractheat from PV modules, heating water or air to reducethe operating temperature of the PV modules and keepthe electrical efficiency at a sufficient level [4]. ThePV–T collectors mainly categorized according to thekind of working (heat transfer) fluid as PV–T air andPV–T water systems as shown in Fig. 1.

According to conclusions by various researchersPV–T water systems are more efficient than PV–T airsystems, due to high heat conductivity and hence highheat capacity, high density�resulting in a high volumetransfer. But use of water requires more extensivemodifications to enable water�tight and corrosion�free construction. Hence, natural or forced air circu�lation through an air channel on the PV rear or top orboth surface, is the simplest mode to extract heat fromPV modules [1, 2]. The rapid development and salesvolume of photovoltaic PV modules has created a

1 The article is published in the original.

promising business environment in the foreseeablefuture. However, the current electricity cost from PV isstill several times higher than from the conventionalpower generation. One way to shorten the paybackperiod is to bring in the hybrid PV–T technology,which multiplies the energy outputs from the samecollector surface area [5].

PV–T AIR COLLECTORS

There are different types of PV–T air collectorsdesigned, evaluated theoretically and experimentallyby various researches, which are mainly distinguishedaccording to the air flow pattern, as well. These are dif�ferentiated with respect to the flow of air above theabsorber, below the absorber, on both sides of theabsorber, in single and in double pass.

The works of H.P. Garg and R.S. Adhikari [6, 7]devoted to the performance analysis of a conventionalhybrid PV–T air collector by developing of computersimulation model. A simulation model is developedand various performance parameters are calculated forsingle�glass and double�glass configurations and alsovarious types of absorbers have been derived. In Fig. 2there is presented schematic configuration of a con�ventional PV–T air collector with (a) single�glass and(b) double�glass covers.

Results show, that utilization of single� and double�glass covers in a PV–T air heating system depends onthe range of temperatures for which the system isdesigned. Increased transmission losses due to theaddition of an extra cover do not justify the heat lossreduction and beyond some critical point the single�glass cover collects more heat than double�glass. Para�metric studies show that the system efficiencyincreases with increase in collector length, mass flowrate and cell density, and decreases with increase induct depth for both configurations. It has also been

A Review on Photovoltaic�Thermal (PV–T) Air and Water Collectors1

R. R. Avezov, J. S. Akhatov, and N. R. AvezovaPhysical�Technical Institute, SPA “Physics�Sun” Uzbekistan Academy of Sciences

Received January 17, 2011

Abstract—This paper presents the state�of�the�art on photovoltaic�thermal PV–T collectors. There are pre�sented two main classification groups: �Air and �Water PV�Thermal collectors, design and performance evalu�ation, comparison of the findings obtained by various researchers. The review also covers the description of dif�ferent designs of air and water PV–T collectors, the results of theoretical and experimental works, focused tooptimization of the technical and economical performances in terms of electrical as well as thermal outputs.

DOI: 10.3103/S0003701X11030042

SOLAR POWER PLANTS AND THEIR APPLICATION

170

APPLIED SOLAR ENERGY Vol. 47 No. 3 2011

AVEZOV et al.

observed that for larger values of duct depth the per�centage decrease in performance of the double�glassconfiguration is smaller than for the single�glass con�figuration. As material cost increases by increasing thenumber of glass covers, collector length, cell density,duct depth and mass flow rate, final selection of designparameters of a PV/T system must be based on thecost�effectiveness of the system by minimizing the lifecycle cost (LCC) of the system.

An extensive investigation on overall performancesanalysis of flat plate PV–T air collectors has been doneby Adel A. Hegazy [8]. Four popular designs are con�sidered with the air flowing either over the absorber(Model I) or under it (Model II) and on both sides ofthe absorber in a single pass (Model III) or in a doublepass fashion (Model IV), that are described in Fig. 3.

The various designs of PV–T air collectors areshown in Fig. 3. Each collector is covered with a 3 mmthick glass plate and has an effective absorber area oflength L = 9 m and width W = 1 m. There is only a sin�gle rectangular channel inside Models I and II, whilethe other models exhibit two identical channels aboveand under the absorber, separated in Model III butinterconnected by a close return bend in Model IV.

The results of numerical simulations show, thatunder similar operational conditions, the Model I col�

lector has the lowest performance, while the othermodels exhibit comparable thermal and electrical out�put gains. Nevertheless, the Model III collectordemands the least fan power, followed by Models IVand II. It is also shown that selective properties areinappropriate for these PV–T collectors due to theresultant reduction in the generated PV energy, espe�cially at low flow rates.

On the base of the comparisons made the followingconclusions:—for a particular model, the thermalefficiency is enhanced with the increase of air specificmass flow rate;—for each model exists a critical rate ofmass flow;—the flow channel (D/L) ratio is animportant design parameter also influencing the per�formance of PV–T air collectors;—performancecomparisons indicate that the Model III PV–T collec�tor is the most suitable candidate design for convertingsolar energy into low quality heat and high qualityelectrical energy. On the other hand, it is simple to bebuilt by local craftsmen in the rural areas of developingcountries.

The objective of the work of A. Tiwari et al. [9] wasto evaluate the performance of the photovoltaic PVmodule integrated with air duct for composite climateof India. Thermal model of PV–T collectors has beendeveloped. Created experimental setup, where the

PV�THERMAL (PV�T)

Ungla� Round and

COLLECTORS

AIR COLLECTORS

WATERCOLLECTORS

Glazed Single Double Channel passpass flow

Ungla� Glazedtube

absorber

Square/rectan�

tubeabsorber

Sheet Channelflow

Fig. 1. Classification of PV–Thermal collector types.

Transparent

Insulation

Solar cellAbsorber plate

Rear plateTo

Transparent cover�1

Transparent cover�1Solar cellAbsorber plate

Rear plate

Insulation

To

TgTsTp

Ti

Tb

hp–g

hp–f

G

hs–ghs–p

hb–f

hg–a

Tg1

TsTp

Ti

Tb

Tg2

hb–f

hs–phs–g2

hp–f

hp–g2

hg2–g1

hg1–a

G

(a) (b)

Fig. 2. Schematic configuration of a conventional PV–T air collector along with the associated energy transfer mechanism.

zed zed gulartube

Ub

cover

Ub


A REVIEW ON PHOTOVOLTAIC�THERMAL (PV–T) AIR AND WATER COLLECTORS 171

Fig. 4a shows the front view of PV–T system with airas the fluid flowing below the tedlar and Fig. 4b showsthe thermal resistance circuit diagram of current PV–Tsystem.

The experimental validation of thermal modelshows, that there is a fair agreement between theoreti�cal and experimental data. Further it is concluded thatan overall thermal efficiency of PV–T system is signif�icantly increased due to thermal energy utilization inPV module. Results show, that there is an increase in

an overall efficiency of PV–T system by 18% due tothermal energy available in addition to electricalenergy.

In another work of A. Tiwari and M.S. Sodha [10]an attempt has been made to evaluate the overall per�formance of hybrid PV–T air collector. The differentconfigurations of hybrid air collectors which are con�sidered as unglazed and glazed PV–T air heaters, withand without tedlar. Numerical computations havebeen carried out and the results for different configu�

Fig. 3. Schematics of the various PV–T air collector models along with heat transfer coefficients.

Glass cover

Back plate

Air in Air out

Pottant

Insulation Absorber platePhotovoltaic cell

Model I

I Ts

Sg

D

tc

hw

tin

Ta

Ub

hrgs

hrpg

Ta

Sp

hw

hphg To

g

p

G, Ti

g

g

g

p

p

p

b

b

b

Glass cover

Glass cover

Glass cover

Air out

Air out

Air out

Air in

Air in

Air in

Air in

Air out

Pottant

Pottant

Pottant

Absorber plate

Absorber plate

Absorber plate

Photovoltaic cell

Photovoltaic cell

Photovoltaic cell

Back plate

Back plate

Back plate

Insulation

Insulation

Insulation

Model II

Model III

Model IV

tc

tc

tc

D

tin

s

D2

tin

D1

D2

tin

D1

Ts

TsI

Ta

Ta

hrgs

hrpg

hrpb

hw

hphb

hpgSp

Sg

I

ToG, Ti

hw TaUb

Sg

Sp

hb

hw Ta Ub

hrpb

hrpg

hrgs

To1

To2

hg

hw

hp1

hp2

G1, Ti

G2, Ti

TsI

TaSg

G, TiSp

hg

hw hrgs

hrpg

hrpb

UbTahw

hbhp2

hp1

io2

io1

G, To2

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AVEZOV et al.

rations have been compared. In Figs. 5a, 5b, 5c and 5dthere are shown the cross�sectional view of unglazedand glazed PV–T air collector with and without tedlar,respectively.

The thermal model for unglazed PV–T air heatingsystem has also been validated experimentally forsummer climatic conditions. It is observed that glazedhybrid PV–T without tedlar gives the best perfor�mance. On the basis of the present study, one can inferthat the glazed hybrid PV–T system without tedlar,Model IV, is the best system among others with signif�icant increase in an overall efficiency which can be

used for various applications namely space heating,water heating, drying, greenhouse, illumination andlighting, etc. Other conclusions are as follows:—thereis no difference in the solar cell temperature ofunglazed PV–T module, with, Model I, and withouttedlar, Model II;—however, there is marginal increasein outlet air temperature in Model IV due to absenceof tedlar;—an overall efficiency of hybrid system andsolar cell efficiency increases with increase of massflow rate of air through the duct;—an overall effi�ciency of the hybrid system decreases with an increaseof the length of the module due to more losses from thesystem;—there is significant increase in an overall

Conductive resistance

Convective resistance

Radiative resistanceGlass

Solar cell

Tedlar

Air in Air out

(a) (b)

Tg

Tc

Tbs

Ti

Ta

Ts Ta

Insulating

and EVA

structure

Fig. 4. (a) The front view of PV/T system. (b) Thermal resistance circuit diagram for PV/T system.

Solar cell and EVA

TedlarAir in Air out

Insulating Material

Insulating material

Insulating material

Insulating material

Glass Glass

GlassGlass

Solar cell Solar cell

Tedlar

Solar cell and EVA

and EVAand EVAAir in

Air in

Air in

Air outAir outAir in

GlazingGlazing

I(t)

I(t)

I(t)

I(t)Ta

TaTa

Ta

(a)

(c)

(b)

(d)

Fig. 5. (A) Cross�sectional view of unglazed PV–T air heater (a) with tedlar (Model I); (b) without tedlar (Model II). (B) Cross�sectional view of glazed PV–T air heater (c) with tedlar (Model III); (d) without tedlar (Model IV).



efficiency of the PV–T system if more small modulesare connected in series for a given length of the system.

In the work of J.K. Tonui and Y. Tripanagnosto�poulos [11] investigated the performance of two lowcost heat extraction modifications in the channel of aPV–T air system to achieve higher thermal output andPV cooling so as to keep the electrical efficiency atacceptable level. The use of thin flat metal sheet sus�pended at the middle or finned back wall of an airchannel in the PV–T air configuration are the sug�gested methods. In Fig. 6 there is presented cross�sec�tional view of PV–T air collector models.

A theoretical model is developed and validatedagainst experimental data, where good agreementbetween the predicted results and measured data wereachieved. The validated model was then used to studythe effect of the channel depth, channel length andmass flow rate on electrical and thermal efficiency, PVcooling and pressure drop for both improved and typ�ical PV–T air systems and their results were com�pared. Both experimental and theoretical results showthat the suggested modifications improve the perfor�

mance of the PV–T air system. A total efficiency of80% is achieved. The experimental results approxi�mately similar to the theoretical ones, show that thethermal performances of the new hybrid collector areimproved compared to the classic hybrid collectors.

There is a direct�coupled PV–T air collector,which is designed, built, and tested at a geographiclocation of Kerman, Iran, presented by A. Shahsavar[12]. In this system, a thin aluminum sheet suspendedat the middle of air channel is used to increase the heatexchange surface and consequently improve heatextraction from PV panels. This PV–T system is testedin natural convection and forced convection modesand its unsteady results are presented in with and with�out glass cover cases. Figure 7a shows the cross�sec�tional view of studied PV–T air collector and Fig. 7bshows the thermal scheme of the studied PV–T air sys�tem with heat transfer coefficients.

A theoretical model is developed and validatedagainst experimental data, where good agreementbetween the measured values and those calculated bythe simulation model were achieved. In this device, PV

PV module

TMS sheet

PVT/AIR – FIN + UNGLPVT/AIR – FIN + GL

Back wall

PVT/AIR – REF + UNGL

Upper channelLower channel

PVT/AIR – TMS + UNGL

Fins

Glass cover

PVT/AIR – REF + GL

PVT/AIR – TMS + GL

Fins

PV moduleBack wall

Upper channelLower channelTMS sheet

Glass cover

Glass cover

Fig. 6. Cross�sectional view of PV–T air collector models.

Glass coverTg

PV array Upper channel

TMS sheet Lower channel

Outlet air

Outlet air

Intel air

Intel air

Tpv

Tb

TTMS

hw

hc

hc

Ub

hr,pv�TMS

h'c

hr,g�s

Ur

hr,pv�g

hr,TMS�b

(a) (b)

Fig. 7. (a) Cross�sectional view of studied PV–T air collector. (b) Thermal scheme of the studied PV–T air system with heat trans�fer coefficients.

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AVEZOV et al.

panels are directly used to generate required power tooperate the DC fans. During a day, mass flow ratechanges with variation of solar radiation and ambienttemperature and therefore air flow is unsteady. Thereis good agreement between experimental and theoret�ical results for air mass flow rate, outlet air tempera�ture and PV panel temperature with correlation coef�ficient of 0.95–0.99 and standard percentage devia�tion error of 0.78–7.4%. In the case of forcedconvection, air mass flow rate decreases by settingglass cover on PV panels. On the other hand, in freeconvection mode, setting glass cover leads to air massflow rate increases. Thermal efficiency increases withincreasing the air mass flow rate due to increased heattransfer coefficient. There is an optimum number offans for achieving maximum electrical efficiency. Set�ting glass cover on PV panels leads to increase in ther�mal efficiency and decrease in electrical efficiency ofthe system.

In the work of F. Sarhadi [13] a detailed energy andexergy analysis is carried out. A computer simulationprogram is developed to calculate the thermal andelectrical parameters, exergy components and exergyefficiency of a typical PV–T air collector. The simula�tion results have been validated with the experimentalresults of Joshi et al. [14]. Figures 8a and 8b show thecross�sectional view and thermal resistance circuitdiagram for current PV–T air collector, respectively.

The results of numerical simulation are in goodagreement with the experimental measurements. It isalso found that the thermal efficiency, electrical effi�ciency, overall energy efficiency and exergy efficiencyof PV–T air collector is about 17.18, 10.01, 45 and10.75% respectively for a sample climatic, operatingand design parameters. It’s concluded that the behav�

ior of modified exergy efficiency with respect to thevariations of climatic, operating and design parame�ters is so similar to the electrical efficiency of PV–T aircollector. The agent fluid has a great effect on themodified exergy efficiency and it can be increased ifwater is used in PV–T collector system. The modifiedexergy efficiency has a slight change with respect toinlet air temperature or duct length. Increasing inletair velocity or solar radiation intensity, the modifiedexergy efficiency increases initially and then decreasesafter attaining inlet air velocity or solar radiationintensity of about a maximum point. While increasingwind speed, the modified exergy efficiency increases.

In the work of R. Kumar [14] detailed analysis of adouble�pass solar PV–T air heater with fins is per�formed. The fins are arranged perpendicular to thedirection of air flow to enhance the heat transfer rateand efficiency. Figure 9 shows the cross�sectional viewof double�pass PV–T solar collector with (a) andwithout (b) fins.

Air enters the upper channel of the air heater andsubsequently flows to the lower channel in the oppo�site direction. The results of calculations show, that thepresence of fins in the lower air channel on theabsorber surface increases the heat transfer area to airand improves the thermal and electrical efficiencies.The extended fin area also reduces the cell tempera�ture considerably. The electrical efficiency is signifi�cantly affected by the cell temperature, which dependson solar irradiance, inlet air temperature, air flow rateand packing factor. The depth of the air heater is sig�nificant in both channels, but the depth of the lowerchannel plays a more prominent role in the heat trans�fer to air. The determined influenced of packing factoron the thermal, electrical and total equivalent thermal

Glass

Insulation material

Convective resistance

Air inlet

Solar cell and EVA

Tedlar

Glass

Air inlet Air outlet

Air outlet

Direction of air flow

Conductive resistance

Radiative resistance

Solar cell and EVA

L

WTf,in

x dx x + dx

m

Tf Tf + dTf

Tf,outm

Tamb

Ti

Tbs

Tcell

Tg

Insulationmaterial

TambTsky

G

(a)

(b)

(c)

Fig. 8. (a) The cross�sectional view and (b) thermal resistance circuit diagram for a PV–T air collector.

..

Tedlar



efficiencies indicate that a higher packing factor isuseful for producing more electrical output per unitcollector area and also in controlling the cell temper�ature, but marginally reduces thermal output.

A new design of a double�pass PV–T air collectorwith compound parabolic concentrator and fins wasstudied by M.Y. Othman et al. [15]. The collectordesign concept is shown in Fig. 10a. Air enters throughthe upper channel formed by the glass cover and thephotovoltaic panel and is heated directly by the sun.Next it enters the lower channel formed by the backplate and the PV panel. The compound parabolic con�centrators concentrate solar radiation onto the PVcells. The fins, on the back of the PV panel increase theheat transfer to the air and enhances the efficiency ofthe system.

The collector dimensions are 0.85 m × 1.22 m (W L).The height of the upper channel is 16.5 cm. The heightof the lower channel can be varied from 30 to 120 mm.The total area covered by solar cells is 0.38 m2. CPCwith concentration ratio of 1.86 is used as a reflectorand located parallel to the air flow.

The collector was tested at steady state operationunder indoor conditions to determine their electricaland thermal efficiency for various operating tempera�

tures. Figure 10b shows the thermal schematic modelof the double�pass PV–T with CPC and fins. Boththermal and electrical performances of the collectorevaluated for the various nodes of system. The devel�oped steady state model predicts the thermal and elec�trical performance of a PV–T collector with CPC andfins. The prediction results agreed with experimentalresults. In general, results show that electricity pro�duction in a PV/T hybrid module decreases withincreasing temperature of the air flow. This impliesthat the air temperature should be kept as low as pos�sible. On the other hand, the system should deliver hotair for other purposes. The simultaneous use of hybridPV–T, CPC and fins have a potential to significantlyincrease in power production and reduce the cost ofphotovoltaic electricity.

PV–T WATER COLLECTORS

PV–T water collectors are distinguished accordingto water flow pattern, which are differentiated to sheetand tube, channel free flow and different absorbertypes.

S. Dubey and G.N. Tiwari [16] presented (glass–glass) type PV–T solar water heater with capacity200L. Proposed PV–T collector has been investigated

Solar Radiation

Air outlet Solar cells

Insulation

Air Inlet Upper air channel

Upper glass cover

Solar Radiation

Air Inlet Upper air channelSuperstate of

Air outletPV module

Fin

L

Tf1

Solar cells

Superstate of PV module

Upper glass cover

AbsorberAbsorber

Back plate of flow channel Back plate of flow channel

Solar PV Module Solar PV Module

Insulation

Lower air channel Lower air channel

H1H1

H2 H2

Tf1

Tf2 Tf2

L L1

(a) (b)

Fig. 9. Cross�sectional view of double�pass PV–T solar air collector with (a) and without (b) fins.

Insulator

Inlet air

Outlet air

Solar cell

FinsInsulatorSolar cells

Air out

Air in

Fin

CPC

Glass cover

Tg

Tp

Tbp

Tf2

Tf1

hrgs

hrpg

hrpbp

hcgw

hcgf1hcpf1

hcbpf2

UbCPC

(a) (b)

Fig. 10. (a) The schematic model of a double�pass photovoltaic thermal solar collector with CPC and fins. (b) The thermal sche�matic model of the double�pass PV/T with CPC and fins.

1

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AVEZOV et al.

theoretically and experimentally. Two flat plate collec�tors connected in series with each having an effectivearea of 2.16 m2 are considered. Design of an absorberis shown in Fig. 11a. The whole absorber and glasscover is encased in an aluminum metallic box with0.1 m glass wool insulation below the absorber toreduce bottom losses. A glass to glass PV module withan effective area of 0.66 m2 is integrated at the bottomof one of the collector as shown in Fig. 11b.

The solar radiation is transmitted through non�packing area of PV module and finally absorbed by theblackened absorber. Further, the thermal energy asso�ciated with PV module is transferred to absorber byconvection for further heating of absorber. Waterbelow absorber gets heated and moves in the upwarddirection. The outlet of water at the end of absorberwhich is covered with PV module becomes inlet toglass�absorber combination. The outlet of PV–Twater collector is further connected to the inlet of con�ventional flat plate collector for higher operation tem�perature. Both collectors are connected to an insu�lated storage tank of 200 l capacity. There is a provisionof a DC water pump (18 V, 60 W, 2800 rpm) connected

to PV module to circulate the water between collectorsand storage tank in a forced mode. Thermal model ofPV–T solar water heating system shows good agree�ment with the experimental results. It is observed thatthe PV–T flat plate collector partially covered with PVmodule gives better thermal and average cell efficiencywhich is in accordance with the results reported byearlier researchers. The results also indicate that thereis a significant increase in the instantaneous efficiencyfrom 33 to 64% due to increase in glazing area. Thepresented combined system of PV–T solar waterheater is a self sustainable system and it can beinstalled at remote areas for fulfillment of hot waterrequirements and the electrical energy saved by thissystem can be utilized for other purposes.

In the work of Jie Ji et al. [17] there are presented adesigned and constructed flat�box aluminum�alloyPV–T water heating systems for natural circulation.The hybrid PV–T collector was an integration of sin�gle�crystalline silicon cells into a solar thermal collec�tor. In Fig. 12 there is presented the physical configu�ration of the PV–T collector, where (a) front view and(b) the constituent layers and Fig. 13 shows the electri�

Fig. 11. (a) Cut sectional side view of a PV integrated flat plate collector. (b) Cross sectional view of a combined PV–T solar waterheating system.

(a) (b)

(1) front glazing; (2) TPT; (3) EVA;

(1)

(2)(3)(4)

(5)(6)(7)

(8)

(2)(3)

(4) PV module; (5) silicon gel;(6) absorber; (7) thermal insulation;(8) back cover

Fig. 12. Physical configuration of the PV–T collector:(a) front view and (b) the constituent layers.

Pyranometer

Computer

Con�

Accumulator battery

V

LoadData logger

Ta

A

Tout

Tin12V100AHX4

Fig. 13. Electrical system of the PV–T test rig.

vector

Solar Cel

Cut section

Metallic Frame

Frame of PV PanelGlazing Surface

Thermal

Outlet

Thermal Insulation

Absorber

of tube

Tfi

Intel

Air Gap

Tfol

Tfo2Hot Water

Insulated Pipe

Supporting

Storage Tank

Cold Water

Glazing PV Module

Flat PlateConnected to

Outlet

Inlet

DC Motor

Tfi

Tfol

Tfo3

(b)(a)

Collector

Stand

Surface

Tfo2

Tfo2



cal subsystem of the PV–T test rig. The PV–T waterheating system was designed with natural circulationand experiments were conducted with different watermasses and different initial water temperatures in anoutdoor environment.

As the hot�water load per unit heat�collecting areaexceeded 80 kg/m2, the daily electrical efficiency wasabout 10.15%, the characteristic daily thermal effi�ciency exceeded 45%, the characteristic daily totalefficiency was above 52% and the characteristic dailyprimary�energy saving was up to 65%, for this systemwith a PV cell covering factor of 0.63 and front�glazingtransmissivity of 0.83. A dynamic system model wasdeveloped and the results well validated with experi�mental data. The simulation results based on the vali�dated dynamic model indicated that the higher thecovering factor and tire glazing transmissivity, the bet�ter the overall performance. Proposed flat�boxAl�alloy solar PV–T collector has many merits, ascompared to the sheet�and�tube metallic or flat�boxplastic/rubber collectors, such as the large contactarea to facilitate heat exchange between the absorberplate and the fluid, the uniform transverse temperaturedistribution across the collector width, and the flatmetallic surface as a high�quality lamination betweenthe PV cells and the absorber plate.

In the work of T.T. Chow et al. [18] there are pre�sented the results of investigations on experimentalset�up of two flat�box type PV–T collectors (Figs. 14aand 14b), one with glazing and the other without glaz�ing. Each collector had an aperture area of 1.34 m2 andcarried a thermally�insulated 155�litre water storagetank. Poly crystalline silicon solar cells of conversionefficiency 0.13 at standard testing conditions (STC)were used. The PV encapsulation was adhered to theupper portion of the aluminum alloy thermal absorber.The cell area was 0.81 m2 and the packing factor wastherefore close to 0.60.

During the winter test period, the collectors wereset facing south at a tilt angle of 30°. The absorptivityand emissivity of the thermal absorber were 0.9 and

0.8, respectively. The transmissivity and emissivity ofthe glass cover were 0.83 and 0.88, and the depth of airgap between glazing and collector plate was 0.025 m.Based on numerical models validated by experimentaldata, the use of glass cover at a thermosyphon�typePV–T collector system has been evaluated from thethermodynamic point of view. The energetic efficiencyof the glazed collector was found always better than theunglazed collector. This is for ail cases examined forthe six operating parameters, namely, cell efficiency,packing factor, ratio of water mass to collector area,solar radiation, ambient temperature, and wind veloc�ity. Hence if the system design is targeted at acquiringeither higher proportion of thermal energy or moreoverall energy output in “quantity”, the glazed systemcan be the better choice.

In the work of Wei He et al. [3] presented an alumi�num�alloy flat�box type hybrid solar collector func�tioned as a thermosyphon system. As illustrated byFig. 15a the hybrid collector carried a thermally�insu�lated 100�1 water�storage tank. The PV module wasattached to the upper portion of the aluminum alloyflat�box absorber. Poly crystal line silicon solar�cells of14.5% conversion efficiency at standard conditionswere used. The constituent layers of this hybrid collec�tor are shown in Fig. 15b a single glass�cover was pro�vided and separated from the PV encapsulation by anair gap. The solar cells were inserted within the encap�sulated materials, which included the transparent TPT(tedlarpolyester�tedlar) and the EVA (ethylene�vinylacetate) layers on the top, and the EVA and opaqueTPT layers underneath. The entire flat�box absorberwas built from a multiple of extruded aluminum alloybox�structure modules.

The test results on the energy performance of water�type hybrid collector with polycrystalline PV module ona flat�box type aluminum�alloy thermal absorber werevery encouraging. The daily thermal efficiency wasfound around 40%, which is about 0.8 of that for a con�ventional solar thermosyphon collector system. Theenergy saving efficiency was found above that of theconventional system. A high final water�temperature in

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(a) (b)

Water channels at Front glazing (optional)

PV cells on collector plate

Air gap

Thermal insulation

thermal absorber

Glazed Unglazed

Fig. 14. (a) Cross�section view of PV–T collector with flat�box absorber and multi�water channel design. (b) PV–T collectorswith and without glass cover.

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the storage tank can be achievable after a one�day expo�sure. This makes the product design a good potential forserving the domestic market.

The major purpose of the investigations ofB.J. Huang et al. [19] was to understand the perfor�mance of an integrated photovoltaic and thermal solarsystem (IPVTS) as compared to a conventional solarwater heater and to demonstrate the idea of an IPVTSdesign. A commercial polycrystalline PV module isused for making a PV–T collector. The PV–T collec�tor is used to build an IPVTS. The test results showthat the solar PV–T collector made from a corrugatedpolycarbonate panel can obtain a good thermal effi�ciency.

The primary�energy saving efficiency of thepresent IPVTS exceeds 0.60. This is higher than for apure solar hot water heater or a pure PV system. Thecharacteristic daily efficiency h ⋅ reaches 0.38 which isabout 76% of the value for a conventional solar hotwater heater using glazed collectors (h × 50.5). Theperformance of as PV–T collector can be improved ifthe heat�collecting plate, the PV cells and the glasscover are directly packed together to form a glazed col�lector. The manufacturing cost of the PV–T collectorand the system cost of the IPVTS can also be reduced.

The work of T.T. Chow [20] devoted to study ofPV–T collectors theoretically by developing dynamicsimulation model. The operation of a PV–T collectoris inherently dynamic. A steady�state model is notsuitable for predicting working temperatures of the PVmodule and the heat�removal fluid during periods offluctuating irradiance or intermittent fluid flow. Theproposed model, which is suitable for dynamic systemsimulation allows detailed analysis of the transientenergy flow across various collector components andcaptures the instantaneous energy outputs. In thestudy of transient solar system performance thatinvolves fluctuating irradiance and/or an imposed sys�tem control scheme, dynamic analysis has to be per�

formed. An explicit dynamic model of a water heatingPV–T collector suitable for dynamic system simula�tion has been presented.

The seven�node model derived from the control�volume finite�difference formulation and incorpo�rated with a transport delay subprogram. With anextension of the nodal scheme to include multidimen�sional thermal conduction on PV and absorber plates,the model is able to perform complete energy analysison the hybrid collector. The collector represents apanel with an aperture area 2 m long by 1 m wide, andis inclined at 45°. The glass cover is of non�iron typewith A = 4 and εg = 0.88. The PV plate is of εp = 0.88and αp = 0.9. Both the absorber plate and tubes are incopper. Ta, Te and Tw0 are at 30°C. It is 800 W/m2.ua is 1.5 m/s. Obtained from steady�state simulationwith the above data, under the given condition themaximum combined efficiency of a perfect collectorcan be over 70%, this may decrease to less than 60% fora low�quality collector.

In the work of T.T. Chow et al. [21] a photovoltaic�thermosyphon collector with rectangular flow chan�nels is presented and the energy performance is dis�cussed. The fin performance of the thermal absorber isknown to be one crucial factor in achieving a highoverall energy yield of the collector. Accordingly, analuminum�alloy flat�box type PVT collector was con�structed, with its fin efficiency approaching unity. Itsdesign is primarily for natural circulation and fordomestic water heating purpose. In Fig. 18 there arepresented the constituent layers of the PVT collector.

The test results showed that a high final hot watertemperature in the collector system can be achievedafter a one�day exposure. A numerical thermal modelof the proposed PV–T collector is developed based onthe finite�difference control volume method. Themodel validation test through the use of measured datafrom an experimental rig showed that the numericalmodel has been able to give accurate prediction of the

(a) (b)

Glass cove

Transparent

Air gap

Solar cell

EVA

Silicon gel

Opaque

Flat�box absorber

TPT

TPT

Water storage tank c/wthermal insulation

Water supply pipefrom tank

Water return pipe

PV module

Thermal absorberwith a black�colour

surface

Pyranometer

Fig. 15. (a) Experimental set�up of the hybrid PVT collector. (b) Constituent layers of the hybrid PVT collector.



daily thermal performance. The model has been thenused to evaluate the steady�state collector perfor�mance as well as the typical daily operation under thehot summer and cold winter climate zone of China.The energy performance of two alternative positions ofthe PV module has been compared. The overall per�formance of the hybrid collector system is foundpromising in providing an alternative means of energysource for the domestic sector of China.

The work of S.A. Kalogirou and Y. Tripanagnosto�poulos [4] presents TRNSYS simulation results forhybrid PV–T solar systems for domestic hot waterapplications both passive and active. The experimentalinvestigations carried out on prototype, where poly�crystalline silicon (pc�Si) and amorphous silicon(a�Si) PV modules combined with water heating sys�tem. Experiments were conducted on a small scaleunit with 4 m2 aperture areas collector field and

PV/T

CONTROLLER

Hot

plate

PV MODULEHEAT

COLLECTOR

COLLECTING PLATE

HOT WSUPPL

COLDWATER

PV module

Cold

Heat�collecting

water

water

Insulation

STORAGETANKPUMP

AC110V60 Hz

(b)(a)

Fig. 16. (a) Schematic diagram of integrated PV–T system. (b) Schematic diagram of PV–T collector.

Fig. 17. PV–T collector with single glazing: (a) front view and (b) section Z–Z (across one water tube).

Glazing

Silicon gel

Back cover

Transparent TPTEVA

Opaque TPT

Solar cellFlat�box absorber

Insulation material absorber

Fig. 18. Constituent layers of the PV–T collector.

PV plate above

Collector casing with

Water out

Glass cover

Absorber

Water tube welded PV plateAir gap

Metallic bond

Insulation layer

absorber plate

to the bottom side ofabsorber at spacing W

glass cover at front and thermal insulation attached to side and back surfaces

plate

Water in

and tube

X

Y

W

g

(W – D0)/2p

b

i

D0/2t

w1

Z Z

(b)(a)

180


AVEZOV et al.

1601 water storage tank and a large scale system with40 m2 aperture areas of collector field and 15001 stor�age tank. Figures 19a and 19b show scheme of activesystem and collector types, respectively.

The results show that the electrical production ofthe system employing polycrystalline solar cells ismore than that employing the amorphous ones, butthe solar thermal contribution is slightly lower. A non�hybrid PV system produces about 38% more electricalenergy, but the present system covers also, dependingon the location, a large percentage of the hot waterneeds of the buildings considered. As a general con�clusion, it can be said that as the overall energy pro�duction of the units is increased, the hybrid units havebetter chances of success. This is also strengthened bythe improvement of the economic viability of the sys�tems, especially in applications where low tempera�ture water, like hot water production for domestic use,is also required.

There is a relatively new concept, to compare pre�vious works, in improving the overall energy perfor�mance of PV installations in buildings, which calledfacade�integrated photovoltaic�thermal (BiPV–T)technology. With the use of wall�mounted water�typePV–T collectors, the system not only generates elec�tricity and hot water simultaneously, but also improves

the thermal insulation of the building envelope [22].In the work of Jie Ji et al. [22] there are illustrated thethermal and electrical behavior of a wall�mountedsolar PV–T collector system through a numericalmodel. A numerical model of this hybrid system wasdeveloped by modifying the Hottel–Whillier model,which was originally for the thermal analysis of flat�plate solar thermal collectors. Computer simulationwas performed to analyze the system performance.The influences of the mass flow rate and the packingfactor on the thermal and electrical performance ofthe water�type PV–T collector were analyzed. InFigs. 20 and 21 there are presented features of hybridPV–T collector and schematic diagram of flow circuitof the PV–T collector system, respectively.

The simulation results showed that the increase ofworking fluid mass flow rate is beneficial for PV cool�ing. However, by the increasing working fluid massflow rate the thermal efficiency decreases. Achievingof system operation at the optimum mass flow rate canimprove the thermal performance of the system, andalso can meet the PV cooling requirement so that a bet�ter electrical performance. The utilization of a PV–Twater�heating collector walls in buildings has manyadvantages, such as reducing the energy consumptionin buildings and providing electrical and thermal

Flat�plate collector detail

Riser

Glazing

Header

Casing

Insulation

Absorber

pipe

Collector

device

Solar pump

Relief

Hot

Mixing

Storage

thermostat

Auxiliary

Bumer

Make�up water

tankDifferential

array

PV/T flat�plate collector detail

water supply

valve

(b)(a)

Fig. 19. (a) Active system schematic. (b) Hybrid and conventional flat plate collector.

(a)

Glass cover

silicon gel

TPT

EVA

TPTsolar cell

flat�box absorber

Glass cover Air gap PV laminate

Insulation layer Absorber channel

(b)

Fig. 20. Features of hybrid PV–T collector: (a) collectorassembly and (b) cross�section view.

Fig. 21. Schematic diagram of flow circuit of the PV–Tcollector system.

heater

Water storage tank

Circulation pump

PV/T collector

Connecting pipe

array (6 nos.)



energy for domestic usages. This innovative design hasvery good application prospects in modern cities.

In the work of T.T. Chow et al. [5] the performanceevaluation of a new water�type PV–T collector systemis presented. The thermal collection making use of thethermosyphon principle eliminates the expense ofpumping power. Experimental rigs were successfullybuilt. Figures 22a and 22b show the cross�sectionviews of three adjacent water tubing in a sheet and�tube PV–T collector and several integrated flat�boxabsorber modules of PV–T collector, respectively.

A dynamic simulation model of the proposed PV–T collector system was developed and validated by theexperimental measurements. The introduced PV–Tsystem is able to generate higher energy output percollector surface area than the side�by�side collectorsystem. Through the numerical models developed forthe plain PV module, the solar hot water collector, andthe PVT collector system respectively, and calcula�tions are showed that the PVT system carries mucheconomical advantages over the conventional PVmodule. Benefited by the financial contribution of thesolar water heating system, the payback period can beshortened from about 52 years to 12 years. This highly

improves the business operation opportunities. It isalso a cost�effective alternative for off�grid ruralhouseholds with modest electricity needs.

S. Dubey and G.N. Tiwari [23] proposed the flatplate collectors partially and fully covered by PV mod�ule. A detailed analysis of energy, exergy and electricalenergy are presented and it is concluded that the par�tially covered collectors (case A) are beneficial interms of annualized uniform cost if the primaryrequirement of user is thermal energy yield and fullycovered collectors (case D) are beneficial when theprimary requirement is electrical energy yield(Fig. 23).

The detailed analysis of annual yield obtained interms of thermal energy, exergy and electrical energyfor five different cities of India (New Delhi, Banga�lore, Mumbai, Srinagar, and Jodhpur) shows that theJodhpur is the best place for installing such types ofwater collectors. PV–T solar water heater is also help�ful for CO2 mitigation and earning the carbon credits.

In the work of M.N. Tursunov et al. [24] the effi�ciency of PV�Thermal collector for electricity andthermal energy generation has been evaluated, takinginto account the daily variation of solar radiation and

Front glazing

insulation

PV encapsulation

Air gap

Thermal Water tube bonded to thermal absorber

Metallic frame cover

Thermal absorber modules

Metallic frame

insulationThermal

Air gap

PV encapsulation Front glazing

(a) (b)

Fig. 22. (a) Cross�section view showing three adjacent water tubing in a sheet and�tube PVT collector. (b) Cross�section view ofthe PV–T collector showing several integrated flat�box absorber modules.

Solar Cell

Inlet

Glazing Surface

Thermal Absorber

Air Gap

Exdest

Tfol

Exthermal = ηcarbotQu

1st

Inlet

Inlet

Inlet

Inlet

Outlet

Outlet

Outlet

Outlet

Outlet

Cut section

Metallic FrameThermal Insulation

of a tube

Tfo2Tfi

Exin = �I(t)

2rd 3rd1st 2rd 3rd

1st 2rd 3rd

1st 2rd 3rd

1st 2rd 3rd

4th 5th 6th

Tfo1 Tfo2 Tfo3 Tfo, N

Tfo

Tfo, N

Nth

Tfo1 Tfo2 Tfo3

Nth

Tfo

(a)

(b)

(c)

(d)

(e)

Fig. 23. (a) Cross sectional side view of a flat plate collector partially covered by PV; (b) Collectors partially covered by PV con�nected in series; (c) Collectors fully covered by PV module and fully covered by glass cover are connected in series; (d) Collectorsfully covered by PV connected in series; (e) Series and parallel combination of collectors fully covered by PV.

.

.

.

Tfi

Tfi

Tfi

Tfi

182


AVEZOV et al.

ambient temperature during the day. The results ofinvestigations show, that the temperature of solar cellsmay vary significantly within a day, however for theadopted dependency of efficiency to temperature, it’saverage daily efficiency, even in the absence of coolingis higher enough, only 14% less than the nominal effi�ciency even in summer. In another work of M.N. Tur�sunov et al. [25] presented the experimental resultsobtained on PV�Thermal collector, where siliconbased PV modules used as a solar radiation absorbingsurface of flat plate water heating collectors. Thus, it isexperimentally shown, that the temperature changesat the outlet of water heating collector during thesunny hours of a day, in a natural circulation mode ofworking fluid at different ambient temperatures arealmost identical. In this case the water temperature atthe outlet is quite different.

In the work of R. Santbergen [26] the electrical andthermal yield of solar domestic hot water systems withone�cover sheet�and�tube PV–T collectors were con�sidered. Objectives of the work were to understand themechanisms determining these yields, to investigatemeasures to improve these yields and to investigate theyield consequences if various solar cell technologiesare being used. The work was carried out using numer�ical simulations. A detailed quantitative understand�ing of all loss mechanisms was obtained, especially ofthose being inherent to the use of PV–T collectorsinstead of PV modules and conventional thermal col�lectors. In Fig. 24 there are presented (a) schematicoverview of the PV–T system for domestic hot water,where besides the PV–T collector, the electrical sub�system consists of an inverter, the thermal sub�systemconsists of a pump, heat exchanger, heat storage andauxiliary heater and (b) described cross�section of thestretched out PV–T collector with the energy flowsand temperature nodes in the thermal model.

The results show that both the annual electrical andthermal efficiency of systems with covered sheet�and�tube PV–T collectors are about 15%, lower comparedto separate conventional PV and conventional thermalcollector systems. The loss of electrical efficiency ismainly caused by the relatively high fluid temperature.This temperature is influenced strongly by the system

sizing. The loss of thermal efficiency is caused both bythe high emissivity of the PV laminate and the with�drawal of electrical energy. To obtain a high electricalefficiency, not only high cell efficiency at StandardTest Conditions, but also low�temperature coeffi�cients is required. The thermal efficiency can beimproved by the application of a low�e coating, how�ever, at the cost of a reduced electrical efficiency.

CONCLUSIONS

The possibility of generating electricity and thermalenergy by using of PV–T solar collector with eitherforced or natural flow, using air or water as a working(heat transfer) fluid demonstrated by various research�ers. This paper has been presented with the comprehen�sive review on the description on design configurationsof PV–T systems and also convoluted the principleclassifications of PV–T air and water collector systems.This classification and grouping systematically, accord�ing on the type of working fluid such as water or air helpsto clarify the design of PV–T collector system, purposesof their using and choosing of appropriate type, takinginto account of local meteorological factors. Accordingto a thorough review on latest module aspects of the var�ious techniques that had been attempted in improvingthe overall performance of the PV–T system, the choiceof technique depends on the location and its applicationwhich dictates the usage of appropriate design consider�ations. There is significant amount of theoretical anexperimental research carried out over the last 30 years,which are presents various concepts of combined PV–Tcollector. However, based on the overview of previousworks it is apparent that there is still more problems tobe undertaken in terms of design aspects before PV–Tsystems can be successfully implemented and inte�grated into domestic and commercial applications.

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PVT c

olle

ctor

s

aux.

Radiation

Conduction

4: PV cellsElectricity

Inventor

Conv.

to grid

pump

Solar irradiance

Convection

Temperature

1: cover glass (top)

2: cover glass (bottom)

3: PV glass (top)

5: sheet

6: tube

7: water

modes

Radiation Convection

Electricity

heater

heat

heatexchanger

Conducation

from mains

(T=10°C)

DHW

(T=60°C)

Conducation

Conducation

Heat

storage

for

DHW

(200I)

(a) (b)

Fig. 24. (a) Schematic overview of the PV–T system for domestic hot water; (b) Cross�section of the stretched out PV–T collector.



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a review on photovoltaic-thermal (pv-t) air and water collectors

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