Performance of contact and non-contact type hybrid photovoltaic-thermal (PV-T) collectors

Performance of contact and non-contact type hybrid photovoltaic-thermal (PV-T) collectors S. Khandelwal, K. S. Reddy and S. Srinivasa Murthy (corresponding author) Department of Mechanical Engineering Indian Institute of Technology Madras, Chennai-600 036, India E-mail: ssmurthy@iitm.ac.in Abstract The conceptualized non-contact Photovoltaic-Thermal (PV-T) collector consists of a PV panel separated by a conventional sheet and tube solar thermal collector. Simulation of both non-contact type and contact type collectors is carried out and correlations are proposed for both thermal and electrical efficiencies in terms of irradiation, inlet water temperature, ambient temperature and PV transmissivity. At high values of transmissivity (t > 0.75), the thermal efficiency of the non-contact type system exceeds that of the contact type collector at higher values of inlet temperatures. The PV-T system yields thermal and electrical efficiencies in the order of 30 35% and 8 9% respectively. Experiments are carried out to validate the simulation results and to study the influence of performance parameters. It is found that non-contact type collectors can perform better than the contact type collectors at high water inlet temperatures and high PV panel transmissivity values. Keywords hybrid solar photovoltaic-thermal collector; contact type; non-contact type; modeling; experiments Nomenclature A ap Exposed surface Area (m 2 ) A c Collector surface Area (m 2 ) C Specific heat (J/kg K) D Gap between the panel and plate (m) E g Band gap (ev) F Shape factor F 1 Collector efficiency factor F R Collector heat removal factor G Solar irradiance (W/m 2 ) h Heat transfer co-efficient (W/m 2 K) h w Convective heat loss coefficient due to wind (W/m 2 K) I Current (Ampere) K Thermal conductivity (W/m K) l Characteristic length of collector (m) M Mass flow rate (kg/m 2 s) Nu Nusselt Number P Power (W) Pr Prandtl Number

360 S. Khandelwal et al. Q Heat lost or gained (J) R Thermal resistance Ra Rayleigh Number Re Reynolds Number t Thickness (m) T Temperature (K) T Temperature difference (K) U Heat loss co-efficient (W/m 2 K) U B Bottom heat loss coefficient (W/m 2 K) U L Overall heat loss coefficient (W/m 2 K) U S Side heat loss coefficient (W/m 2 K) U T Top heat loss coefficient (W/m 2 K) V Voltage (Volts) v Wind speed (m/s) W Collector width (m) a Absorptivity t Transmissivity s Stefan Boltzmann Constant (W/m 2 K 4 ) e Emissivity b Collector tilt angle (degree) b! Volumetric coefficient of expansion (1/K) u Kinematic viscosity (m 2 /s) Subscripts a ambient ab absorber plate Bin bottom insulation c collector e electrical f working fluid fi fluid inlet fm mean fluid g glass i inlet ins insulation o outlet pv photovoltaic r radiative sin side insulation th thermal

Contact and non-contact type hybrid PV-T collectors 361 Introduction A hybrid PV-T collector produces both electricity and hot water. The common configuration consists of a flexible PV panel pasted over an absorber plate using a conductive adhesive. A glass cover may be put over it to reduce heat losses. The hybrid PV-Thermal panel (also called combi-panel) is superior to separate PV and thermal panels because it produces more total (electrical and thermal) energy per unit area than the two units kept separately. This would prove advantageous when installation space is limited and there is need for both hot water and electricity. For low water temperature applications, the efficiency of the PV panel would be higher than the efficiency of the standalone PV panel due to the cooling effect of tube water. Moreover, the life of the PV panel would be enhanced. Theoretical and experimental studies on PV-T collectors have been carried out by Kern and Russell [1]. Florschuetz [2] extended the Hottel-Whillier model for flat plate collectors to PV-T systems. Use of a transparent amorphous silicon cell for hybrid systems was proposed by Lalovic et al. [3]. Bergene and Lovvik [4] proposed a theoretical model for PV-T systems. To improve the overall performance of the PV-T system, an experimental investigation with booster reflectors was carried out [5]. The yield of different PV-T collector designs was studied by Zondag et al. [6]. Sorensen [7] reviewed the temperature effects on the efficiency of various types of solar cells. He compared Mono-Silicon, Poly-Silicon, Ga-As, Cd-S, a-si and organic dyes sensitized solar cells over a temperature range of 0 C to 90 C. Radziemska [8] studied the effect of temperature on current, voltage, power and efficiency variations of mono-crystalline solar panels for a temperature range of 20 C to 80 C. He found that as the temperature of the solar panel increases, the power generated by solar panel decreases (at about 0.65% per K). The current as a function of temperature remained constant at lower voltages. However, as the voltage increased, the output current and the efficiency also decreased with increase in temperature. Cox and Raghuraman [9] presented numerical methods of predicting the performance of both air and liquid PV-T collectors. Their studies suggested that PV-T air collectors were thermally less efficient than PV-T collectors using water as the heat transfer fluid due to the lower absorber-to-air heat transfer coefficient. Garg and Agarwal [10] presented a simulation model for a PV-T collector with air as the heat transfer fluid and an algorithm for making quantitative predictions of the performance of the system. Thermal efficiency curves for the solar PV-T hybrid collectors corresponding to various types of absorbers were derived. Hegazy [11] carried on extensive investigations on the thermal, electrical, hydraulic and overall performance of PV-T air collectors. Four designs were considered with the air flowing over the absorber or under it, and on both sides of the absorber in a single pass or in double pass mode. Heat balance equations were written for each model and were numerically solved incorporating measured climate data. Huang et al. [12] carried out experiments to compare the performance of a water-cooled PV-T system with a conventional solar heater. A flat plate collector with forced circulation and a

362 S. Khandelwal et al. storage tank to allow recirculation were used in the experiment. The photovoltaic panels were of poly-crystalline silicon. It was found that efficiency of the PV-T system was higher than that of the conventional solar thermal water heating system by as much as 76%. It was also found that the thermal and electrical efficiencies of the system reduced with the increase in temperature of the hot water. Saitoh et al. [13] reported experiments on a brine-circulated hybrid PV-T system with a storage tank. Experiments yielded photovoltaic efficiency of 10 13% while the thermal efficiency varied from 40 50% at 20 C and 20% at 40 C. The PV-T system was compared with a separate solar collector and a photovoltaic panel. It was found that the hybrid PV-T system had an advantage over the photovoltaic panel in terms of exergy efficiency, but there was some reduction in the collector thermal efficiency. The common feature of all the above referenced works is that the PV is pasted on the absorber plate. Due to the direct contact between PV and the absorber, heat is extracted from the PV panel mainly by conduction, increasing PV efficiency only in case of low temperature applications. However, when the need exists for a higher water temperature, the PV panel temperature can reach high values, which would be detrimental to both the efficiency and life of the PV panel. Recently, the PV panels having high transparency are available in the market, e.g., those used for integration with building facades (BIPV) in this paper, a non-contact design of PV-T collector is proposed. The performance of both contact and non-contact PV-T collectors are comparatively studied. Modeling of PV-T systems Mathematical models based on the classical energy balance approach are proposed for both non-contact type and single-cover contact type PV-T collectors. When the PV panel is pasted on the thermal collector, it is referred to as a contact type PV-T system (Fig. 1a). The non-contact PV-T collector consists of a PV panel separated by a sheet and tube-type absorber plate by a small distance. It has been named as a Non-Contact system here because the panel and absorber are not in contact (Fig. 1b). A Building Integrated Photovoltaic panel (BIPV) is well-suited to a non-contact system because of the high transmittivity value. Commercial BIPV panels are available with a broad range of transmittivity values, which makes transmittivity an important parameter for study. Analysis done in this paper may help in deciding between the two types of design depending on application, climatic conditions and PV panel properties. The PV-T system under investigation along with associated thermal network is shown in Fig. 2. The governing equations for the PV-T system are as follows: The energy balance for a PV panel under a steady state condition can be given as: Apvαpv G = Q1+ Q2 + ηegapv (1) where Q 1, Q 2 are heat loss terms involved in total heat loss from absorber to ambient. The heat transfer from the absorber to the PV panel is expressed as

Contact and non-contact type hybrid PV-T collectors 363 Figure 1. Solar Photovoltaic-Thermal (PV-T) systems: (a) Contact type (b) Non-contact type. Figure 2. Schematic of PV-T collector with thermal network. [ ] (2) Q1= ( hc1+ hr1) Tab Tpv The energy balance for absorber can be written as: Acτα ( G)= AU c L( Tab Ta)+ Qw (3) The Hottel-Whiller model [14] is adopted here. The actual useful energy gain by the water is equal to the collector heat removal factor times maximum possible useful energy gain. Therefore the heat collected by the water can be rewritten as Qw = Ac[ FR( τα ) G FRUL( Tab Ta) ] (4) The overall heat transfer coefficient (U L ) values may be computed by using the concept of thermal network (Fig. 2) and is given as

364 S. Khandelwal et al. UL = UT + US + UB (5) The top heat loss coefficient U T = where R1 = h 1 R + R 1 2 1 + h c1 r1 1 R2 = hc2 + hr2 Convective heat transfer coefficient from absorber to PV panel is NuKa hc1 = (9) d where Nu is the Nusselt number; Hollands et al. [15] have given a relation for the Nusselt number in terms of Rayleigh number and collector tilt angle as follows: ( ) Nu = 1+ 1 44 1 1708 1 8 16. sin. β 1708. 1 + 1/3 Ra cosβ Ra cosβ R a cosβ 1 (10) 5830 where the meaning of the + exponent is that only positive values of the terms in the bracket are to be used. The Rayleigh number is given by 3 gβ TL Ra = 1 (11) υα The thermal conductivity of air, K a is given by 15. 0. 002528Tm Ka = (12) Tm + 200 Radiative heat transfer coefficient from absorber to PV panel is 4 4 σ ( Tab Tpv ) hr1 = (13) 1 1 + 1 Tab Tpv ( ) ε ε c a The heat transfer from the PV panel to the ambient is given by Q2 = hwapv( Tpv Ta)+ hr2apv Tpv Ta Convective heat transfer coefficient due to wind is expressed as [16] + + (6) (7) (8) ( ) (14) h w = 57. + 38. v (15) Radiative heat transfer coefficient between PV panel and ambient is

Contact and non-contact type hybrid PV-T collectors 365 ( ) 4 4 σε pv Tpv Ts hr 2 = (16) Tpv Ta The side and bottom heat loss coefficients are given [17 19] by 1 4lhKins US = + lkins + t Kins Ac 216. 06. sin t (17) sin 1 AK c ins UB = + lkins + tbinkins Ac 216. 06. tbin (18) The performance of the collector can be evaluated based on the overall heat transfer coefficient and internal fluid heat transfer coefficient. The heat transfer coefficients are to some degree functions of temperature. The mean fluid temperature in the collector can be given as Qw FR Ac 1 F Tfm = Tfi + (19) ULFR The mean absorber temperature is given by Qw Ac Tab = Tfi + ( ) ( 1 FR ) (20) FU R L Thermal efficiency of the system is given by ( ) FU R L Tfi Ta ηth = FR ( τα) (21) G Where F R is the heat removal factor, defined as the ratio of heat extracted by the collector to heat extracted when the whole collector is at fluid inlet temperature. For a fixed mass flow rate, F R and U L are almost constant quantities for a thermal collector. Therefore, if thermal efficiency (h th ) is plotted against (T fi T a )/G, a straight line is obtained. The thermal efficiency (h th ) of PV-T system for fixed mass flow rate and (ta) value can be given as ( ) η th = a1+ a2 T fi T a G (22) Where a 1 & a 2 are constants. If U 1 and U 2 represent the heat loss coefficient from the absorber to the PV panel and from the PV panel to ambient, the energy balance of the PV panel can also be written as: U1( Tab Tpv )+ αpvg = η eg+ U2( Tpv Ta ) (23) G( αpv ηe)+ U2Ta + U1Tab = Tpv ( U1+ U2 ) (24) Substituting equations (19) (21) in equation (24) yields,

366 S. Khandelwal et al. G( αpv ηe)+ U2Ta U1[ Tfi + Gηth ( 1 ( ULFR ) ( 1 FR ))]= Tpv ( U1+ U2) (25) Rearranging the above equation one gets, U1 K1 G ( αpv ηe)+ U Ta U Tfi Tpv U U a + 2 1 = ( 1+ 2 ) (26) 2 Where K 1 = h th (1 F R ) The temperature of the photovoltaic panel may be written as: T = ( X X η ) G+ X T + X T (27) pv 1 2 e 3 a 4 fi α pv + ( UK 1 1) a2 1 U2 U1 where X1 =, X2 =, X3 =, X4 = U1+ U2 U1+ U2 U1+ U2 U1+ U2 The expressions for X 1, X 2, X 3, and X 4 suggest that these should be constant because the variations of quantities such as U 1, U 2, etc are so small that it can be neglected. It can be noted that a linear relationship exists between the temperature of the PV panel (T pv ) and electrical efficiency (h e ). For constant value of solar radiation (G), the electrical efficiency of the system can be expressed as: η e = Y1 T a + Y2 T fi + Y3 (28) If Y 1, Y 2 and Y 3 are constant values, then the variation of electrical efficiency (h e ) of the system with T a and T fi should give a plane. For constant solar radiation, G = 800 W/m 2, the effect of ambient and fluid inlet temperatures on electrical efficiency for t = 0.5, 0.6 and 0.7 is shown in Fig 3. It is observed that Y 1, Y 2, Y 3 are constant values. Through regression, values of Y 1, Y 2 and Y 3 have been determined and it has been found that maximum error in electrical efficiency (h e ) is in the order of 10 5, which is negligible. From the figure, it is clear that it is possible to develop linear relations between h e, T a and T fi for various t values. Since the value of Y 1, Y 2 and Y 3 depends on t, one can develop a correlation for these in terms of t. From Fig. 3, it can be said that a straight-line equation can be used to express the relationship between Y i (i = 1,2,3) and t. For t varying from 0.5 to 0.8, correlations have been developed, as follows: ( τ ) (29) 3 Y 1 = 0. 200 1. 4 10 ( τ ) (30) 3 Y 2 = 0. 2058 0. 5060 10 ( τ ) (31) 3 Y 2 = 0. 2058 0. 5060 10 Combining equations (27) (31) a relation between h e and fundamental quantities like T fi, T a and t can be obtained, for constant value of G = 800 W/m 2. On similar lines, an expression for h th in terms of T fi, T a, G and t has been found to be: ηth = ( 0. 7524τ 3. 8655) ( Tfi Ta) G+ ( 0. 5207τ + 0. 2567) (32) Simulation of one cover contact type was done based on a model proposed by Bergene and Lovvik [4]. Since the bond between PV and absorber plate is highly

Contact and non-contact type hybrid PV-T collectors 367 Figure 3. Effect of ambient (T a ) and inlet fl uid (T fi ) on electrical effi ciency for various transmissivity of a PV panel. conductive, it is logical to assume the temperature of PV and absorber plate will be the same. Both h e and h th follow a linear relationship with the quantity (T fi T a )/G. Hence, total efficiency can be expressed in terms of basic parameters, as it is for non-contact type. Thus, one can derive correlations for h e and h th for both types of collectors. This would facilitate evaluating and comparing the performances of these collectors. Results and discussion The performances of contact and non-contact type systems are evaluated. The thermal efficiency (h th ) for various transmissivity values is shown in Fig. 4. The electrical efficiency (h e ) variation for both non-contact and contact type collectors is shown in Fig. 5. For t = 0.7, h th is lower for the non-contact collector for all values of (T fi T a )/G or in other wards T fi. As t is increased to 0.75, h th for a non-contact system may exceed that of a contact system at higher values of inlet temperatures. For even higher t values, say for t = 0.8, h th for a non-contact type is higher than that for a contact type for the range of inlet temperatures studied. With variation in transmissivity, the thermal performance of a contact type system decreases by a small value. But in the case of a non-contact type, the thermal efficiency increases significantly because of an increase in the ta value of the system. Thus, for noncontact type, h th has a strong positive correlation with t, whereas for contact type,

368 S. Khandelwal et al. Figure 4. Effect of transmissivity on thermal performance for contact and non-contact systems. Figure 5. Effect of transmissivity on electrical effi ciency for contact and non-contact systems. h th remains almost the same. Therefore, the thermal efficiency of a non-contact type PV-T system is higher for higher t values. The contact type system involved more optical losses due to additional glass cover. The reflection and absorption from the glass cover (which is not there in the non-contact type) reduces efficiency. The electrical efficiency (h e ) of the non-contact type is far greater than that of the contact

Contact and non-contact type hybrid PV-T collectors 369 Figure 6. Schematic of solar PV-Thermal system with forced circulation mode. type system at high values of t. The reason is that a PV panel with high t value gets heated up, instead of cooling the absorber plate. In the case of non-contact PV-T systems, the heat transfers from the thermal collector to the PV and vice versa by only convection and radiation with minimum thermal resistance, while in the case of contact PV-T systems there is conduction thermal resistance, which enhances the heat losses. It was also seen that the change in h e of the contact type with t is small. The schematic of the PV-T system with accessories for experiment is shown in Fig. 6. The PV-T system consists of a conventional solar water heater with 100-litre tank capacity and 0.75 m 2 (1.38 m 0.54 m) flat plate collector area. A pressure relief valve and vacuum breaker are provided at the top of the tank. The flow rate of water was measured by electronic turbine flow meter located in the downcomer of the tank. The flat plate collector is covered with a transparent photovoltaic panel (BIPV) of the same size. The electrical output from the BIPV panels was used to charge the 12V, 150 Ah@20 hrs batteries. A 12V, 50A charge regulator is used to protect the batteries against over charging and load discharging. The temperature measurements were taken at various nodal points using K-type (Copper-Constantan) thermocouples. The experimental setup was made to operate in dual mode i.e. natural circulation and forced circulation conditions. The solar radiation data was measured using a Kipp and Zonnen pyranometer. The data for thermal, fluid and electrical

370 S. Khandelwal et al. Figure 7. Experimental set up of an integrated PV-T system. Figure 8. Characteristic curves of BIPV at different temperatures and radiation. parameters was obtained continuously by a 40-channel online data acquisition system. The photograph of the PV-T system is shown in Fig. 7. The photovoltaic panels (12V, 75W) were tested for their output under load and they were matched so that a comparison could be made between single and hybrid panels. During testing of the solar panels it was found that both solar panels have the same performance characteristic curves and hence could be used for the comparative study. The characteristic curves of the BIPV under different operating conditions of temperatures and solar radiation are shown in Fig. 8. Fig. 9 shows the variation of power for the

Contact and non-contact type hybrid PV-T collectors 371 Figure 9. Variation of power output with voltage of BIPV for various insolation conditions. Figure 10. Diurnal variation of solar radiation on a sunny day. same operating conditions. At each value of temperature and solar radiation, the BIPV exhibits a peak as seen in the figure and all experiments were conducted at this peak point. The proposed model was validated with the experimental data. The solar radiation data at Chennai (80.18 E, 13.05 N), India on a typical sunny day is used for this validation and is shown in Fig. 10. The heat is collected in the form of hot storage water. The variation in storage water temperature with time is shown in Fig. 11. It

372 S. Khandelwal et al. Figure 11. Variation of water temperature with time. Figure 12. Variation of thermal effi ciency with time. is observed that the temperature of water increased by about 20 C. The variation of thermal efficiency of the system is shown in Figs. 12 and 13. The experimental values are found to be in good agreement with predicted values. As the day progresses the thermal efficiency decreases because the absorber plate temperature increases resulting in higher heat loss to the ambient. The thermal efficiency of the

Contact and non-contact type hybrid PV-T collectors 373 Figure 13. Thermal performance of the PV-T system. Figure 14. Electrical performance of the PV-T system. system is found in the order of 30 to 35%. The electrical efficiency of the PV-T system shown in Fig. 14 reveals a difference of 7 to 12% between the predicted and experimental values. The simulation model predicts the performance of system at mean water and cell temperatures whereas the experimental data is based on mean values of discrete locations of collector and stratified storage water temperatures.

374 S. Khandelwal et al. Apart from this, some inaccuracy was involved in accounting top, bottom, and side experimental heat loss data, which might result in deviation of experiment from simulation. Therefore, the simulation thermal and electrical efficiencies were always reported higher values than the experimental. Conclusions The concept of a non-contact type solar PV-T collector, using a PV panel of high transmissivity is studied. The proposed analytical model shows good agreement with experimental results. At high values of PV panel transmissivity, and also at high heat collection temperatures, the non-contact type PV-T collector yields significantly better thermal and electrical efficiencies compared to the conventional contact type PV-T collector. References [1] E. C. Kern and M. C. Russell, Combined photovoltaic and thermal hybrid collector systems, Proc. Thirteenth IEEE Photovoltaic Specialists Conference, (1978), 1153 1157. [2] L. W. Florschuetz, Extension of the Hottel-Whiller Model to the analysis of combined Photovoltaic-Thermal flat plate collectors, Solar Energy, 22 (1979), 361 366. [3] B. Lalovic, Z. Kiss and H. Weakliem, Hybrid amorphous silicon photovoltaic and thermal solar collector, Solar Cells, 19 (1986), 131 138. [4] T. Bergene and M. O. Lovvik, Model calculations on a flat-plate solar heat collector with integrated solar cells, Solar Energy, 55 (1995), 453 462. [5] Y. Tripanagnostopoulos, Th. Nousia, M. Souliotis and P. Yianoulis, Hybrid photovoltaic/thermal solar systems, Solar Energy, 72 (2002), 217 234. [6] H. A. Zondag, D. W. De Vries, W. G. J. Van Helden, R. J. C. Van Zolingen and A. A. Van Steenhoven, The Thermal and Electrical yield of a PV-Thermal Collector, Solar Energy, 72 (2002), 113 128. [7] B. Sorensen, PV Power and Heat Production: An added value, Renewable Energy, Academic Press, London (2000). [8] E. Radziemska, Thermal Performance of Si and Ga As based Solar Cells and Modules, Progress in Energy and Combustion Science, 29 (2003), 407 424. [9] C. H. Cox and P. Raghuraman, Design consideration for flat-plate photovoltaic/thermal collectors, Solar Energy, 36 (1985), 227 241. [10] H. P. Garg and R. K. Agarwal, Some Aspects of a PV/T Collector Forced Circulation Flat Plate Solar Water Heater with solar cells, Energy Conversion and Management, 36 (1995), 87 89. [11] A. A. Hegazy, Comparative study of the performances of four photovoltaic/thermal solar air collectors, Energy Conversion and Management, 41 (2000), 861 881. [12] B. J. Huang, T. H. Lin, W. C. Hung and F. S. Sun, Performance evaluation of solar photovoltaic/ thermal systems, Solar Energy, 70 (2000), 443 448. [13] H. Saitoh, Y. Hamada, H. Kubota, M. Nakamura, K. Ochifuji, S. Yokoyama and K. Nagano, Field Experiments and Analyses on a Hybrid Solar Collector, Applied Thermal Energy, 23 (2003), 2089 2105. [14] J. A. Duffie and W. A. Beckman, Solar Engineering of Thermal Processes. Wiley Interscience, New York, 2 nd Edition, (1991). [15] K. G. T. Hollands, K. N. Marshall and R. K. Wedel, An approximate equation for predicting the solar transmittance of transmittance of transparent honeycombs. Solar Energy, 21 (1978), 231.

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