Advanced Absorber Design for Photovoltaic Thermal (PV/T) Collectors Kamaruzzaman Sopian, Goh Li Jin, Mohd. Yusof Othman, Saleem H. Zaidi, Mohd Hafidz Ruslan, Solar Energy Research Institute, University Kebangsaan Malaysia, 43600 UKM Bangi, Selangor Darul Ehsan, Malaysia k_sopian@yahoo.com Abstract:- The application of solar energy can be broadly classified into two categories; thermal energy systems which converts solar energy into thermal energy and photovoltaics energy system which converts solar energy into electrical energy. The vital component in solar energy system is the solar collections systems. Two solar energy collection systems commonly used are the flat plate collectors and photovoltaic cells. Normally, these two collection systems are used separately. It has been shown that these two systems can be combined together in a hybrid photovoltaic thermal (PVT) energy system. The term PVT refers to solar thermal collectors that use PV cells as an integral part of the absorber plate. The system generates both thermal and electrical energy simultaneously. The number of the photovoltaic cells in the system can be adjusted according to the local load demands. In conventional solar thermal system, external electrical energy is required to circulate the working fluid through the system. The need for an external electrical source can be eliminated by using this hybrid system. With a suitable design, one can produce a selfsufficient solar collector system that requires no external electrical energy to run the system. The different options in the development in PVT systems have been categorized by the heat transfer fluid used i.e. air, water, refrigerant. The choice of the heat transfer fluid is fundamental to the design of PVT systems. The absorber design of the PVT is very important since it will be the basis for better heat transfer and higher efficiency systems. Absorbers attached to the surface with more coverage area on PV cell can increase its thermal, electrical and combined efficiencies. Other than increasing solar irradiance, reducing fluid flow input temperature can also be another option for increasing the thermal performance. Best performance of PV/T collector s thermal efficiency in this study can be as high as 51.4 %. The Split flow PV/T design had shows better performance compared to 2 other convention PV/Ts which are Direct flow and Parallel flow. Key words: Photovoltaic thermal, PV/T, absorber, thermal efficiency, simulation 1 Introduction One of the most common of renewable energy generating is by photovoltaic technology. Photovoltaic can be use in anywhere since it depends solely on the sunlight or solar irradiance. Receiving more photon from light can increase the electrical generation as well as thermal energy within photovoltaic cell. It was proven that electrical efficiency of photovoltaic cell reduces with the increase of cell s temperature. The efficiency of the system will lose about 0.3% with ISBN: 978-1-61804-052-7 77
cells temperature increased by 1 C (Kemmoku et al., 2004). A special type of solar collectors was design to collect electric energy and thermal energy simultaneously known as Photovoltaic-Thermal (PV/T) solar collector. Solar hybrid PV/T system can generate more energy per unit area compared to system of solar panel and thermal collector separately side by side (Charalambous et al., 2007). The purpose of using hybrid solar photovoltaic-thermal system is to prevent the electrical efficiency to drop and to collect thermal energy in water or air as heat carrier. Kern and Russel are the first to propose the concept of photovoltaic thermal collector s design by using water or air as working fluid (Kern & Russell et al., 1978). Cox and Raghuraman simulated the performance on some various design of photovoltaic thermal collectors (Cox & Raghuraman et al., 1985). Zondag had conduct an experiment study on four different design of PV/T which use both water and air as heat carrier medium (Zondag et al., 2003). His best collector design can achieve 58% of thermal efficiency and electrical efficiency of 9.7 % of different PV/T collectors. His experiment suggested that adding glazing to collector will increase the thermal efficiency but reduce the electrical efficiency. A glass-glass photovoltaic thermal (PV/T) collector had been fabricate and tested in door and under New Delhi climate condition (Dubey et al., 2008). The PV/T collector above uses water and air as medium to collect heat energy. 2 Design and Methodology 2.1 PV/T Collector Designs In this paper, 3 PV/T collectors was design and compared the thermal performance before fabricating into prototype. PV/T collector in this study used water as medium to collect heat from PV panel. 3 PV/T collectors designed include Direct flow PV/T, Parallel flow PV/T and Split flow PV/T as shown in Fig. 1 to Fig 5. Material use in the collector will affect factors like energy gain, heat transfer and heat loss that occurs inside and outside the PV/T collector. The different of these designs lay on the absorber place under PV panel that using material of copper tube. Copper was chosen as absorber in PV/T because of the high thermal conductivity of 401 Wm -1 K -1 at temperature of 300 K. Design of absorber is crucial to the thermal performance and temperature output of water produce by the PV/T collector. Fig. 1: Schematic Direct flow PV/T diagram Fig. 1 shows the parameter diagram of Direct PV/T collector. It is the most common, conventional and simple design use in solar water collector or photovoltaic thermal collector. An experiment performance study done on PV/T using Direct flow PV/T by Jie Ji (Ji J. et al., 2009) Fig. 2: 3D view of Parallel flow PV/T ISBN: 978-1-61804-052-7 78
Fig. 3: Schematic Parallel flow PV/T diagram Fig. 2 and Fig. 3 shows the 3D diagram and parameter diagram of Parallel flow PV/T collector design. This design is being used in most product of current solar water collector without PV panel. But this Parallel flow PV/T s design is slightly different than the existing design produce in industry. The input and output of the collector is bend to 90 o considering effective heat transfer using conduction, so the absorber can fit nicely under the PV panel. Fig. 5: Schematic Split flow PV/T diagram 2.2 Theoretical analysis 2.2.1 PV/T collector performance Theoretical analyses of a PV/T collector start with collecting all the characteristic parameter that will affect the performance. After determine the parameter sizing of the PV/T collector designed, choosing material used in the collector will affect the performance of the collector greatly. Results to be obtained in this study include energy gain and thermal efficiency. Energy can be calculates from the equation below: (1) Efficiency of thermal energy of the PV/T collector can be calculated by using equation below: (2) Fig. 4: 3D view of Split flow PV/T Fig. 4 and Fig. 5 is the new design of PV/T collector shown in 3D and schematic diagrams. This design is to be tested and compare to other collectors design in this simulation. Where F R, (τα) PV, and U L are the parameter to be found and calculated. All the parameter above various depending on the design, material used and parameter sizing of the PV/T collector. Vokas 2006 and Ibrahim 2009 use the thermal efficiency above in the simulation study of their research on various designs of PV/T collectors (Vokas et. Al., 2006; Ibrahim et. Al., 2009). ISBN: 978-1-61804-052-7 79
(τα) PV is the effective transmitivity and absorbtivity of the product or collector. This can be found by using equations below: (3) (4) (5) (6) The equation above use to calculate transmitivity and absorbtivity are suggested by Tiwari (Tiwari et. Al., 2006). Value of U T, U t, α sc, α p, η sc, β sc and τ g are suggested by Tiwari (Tiwari et. Al., 2006) and can be found in Table 1. Calculation of getting U T and U t can be found in Appendix. Heat loss, U L is essential to the collector s performance which can be avoided to minimum during the fabrication of the collector. As proven by equation 2, decrease U L will improve overall thermal efficiency of the collector. Below is equation for U L : Table 1: Parameters Values U T 66 Wm -2 K -1 U t 9.24 Wm -2 K -1 U tt 8.1028 Wm -2 K -1 K T 0.033 Wm -1 K -1 K abs 204 Wm -1 K -1 L G L T h cα h fi 0.003 m 0.0005 m 45 Wm -1 K -1 300 Wm -1 K -1 h T 500 Wm -1 K -1 α sc 0.70 0.85 α p 0.8 η sc 0.09 β sc 0.90 τ g 0.95 (7) efficiency factor. The heat removal factor is common equation used in simulation of PV/T collector. Heat removal factor is calculated with equation 8 shown below: (8) To calculate heat removal factor, collector efficiency, F is needed to be found first. F can be calculated by considering heat transfer and heat loss of all dimensions and medium of the collector. F can show the heat transfer from outside the collector to the medium inside through different medium and materials. F can be found with equation 9 suggested by Vokas (2006). (9) h cα and h fi can also be found in Table 1. Fin efficiency factor, F can be calculated using the equation below: Where (10) (11) To determine the temperature output of fluid from various module no. of PV/T collectors connected in series, a methodology method was developed by previous researcher on PV/T s simulation (Dubey et al., 2009). Where k k,m is (12) (13) Equation of Duffie and Beckman (Duffie and Beckman et. al., 1991) and Vries (Vries et. Al., 1998) are used to determine heat removal ISBN: 978-1-61804-052-7 80
3 Results and Observations The above methodology determines the characteristic parameter of every PV/T collector design s performance. The results show in Table 2. Table 2: Parameter DF PF SF Unit F R U L 6.361 6.342 6.434 W/m 2 C F R (τα) PV 0.514 0.513 0.520 - Where DF, PF and SF means PV/T collector design of Direct flow, Parallel flow and Split flow designs. Thermal efficiency of a PV/T collector calculated by collector parameter can be achieved using Equation (2). From Equation (2), it is important to keep F R (τα) PV high since it s the ability to absorb heat energy. Reducing heat loss of F R U L is essential to assure the performance and achieve high thermal efficiency. Thermal Efficiency, η th 0.516 0.514 0.512 0.510 0.508 0.506 0.504 0.502 0.500 0.498 0.001 0.0012 0.0014 0.0016 0.0018 0.002 0.0022 (Ti -Ta)/ G Split flow Parallel flow Direct Flow Fig. 7: Thermal efficiency when water input is 26 o C, ambient 25 o C under various solar irradiance (500-1000 W/m 2 ) Fig. 7 and Fig. 8 shows the simulation result using methodology equation above and by determine some parameter. Ambient temperature in Fig. 7 and Fig. 8 is set to same at 25 o C and the variables are solar irradiance which run from 500-1000 W/m 2. Water input temperature in Fig. 7 is 26 o C which is different from Fig. 8 that use 31 o C. The result show lower temperature input of water gives higher thermal efficiency of PV/T collector. Thermal Efficiency, η th 0.485 0.480 0.475 0.470 0.465 0.460 0.455 0.450 0.445 0.440 0.435 0.430 0.005 0.006 0.007 0.008 0.009 0.01 0.011 0.012 0.013 (Ti -Ta)/G Split flow Parallel flow Direct flow Fig. 8: Thermal efficiency when water input is 31 o C, ambient 25 o C under various solar irradiance (500-1000 W/m 2 ) Fluid Temperature output, To 54.00 52.00 50.00 48.00 46.00 44.00 42.00 40.00 0 2 4 6 8 10 12 14 Panels No. 0.02 kg/s 0.03 kg/s 0.04 kg/s Fig. 9: Fluid temperature output from PV/T collectors for various modules no. under solar irradiance of 500 W/m 2 and ambient temperature of 25 o C Using Split flow design PV/T collector as the following simulation to determine fluid temperature output of PV/T collector since Split flow was the best out of all 3 designs. Fluid (water) temperature flowing out from PV/T collectors of Split flow design was simulated using methodology of equation 12 and 13 and the result was as shown in Fig. 9. 4 Conclusions Conclusions can be made that Split flow design of PV/T collector has better performance compared to Direct flow and Parallel flow. Some other specifications observe from the results include: 1. The gap between tube absorber, W can affect the performance since reducing W can increase the thermal efficiency. ISBN: 978-1-61804-052-7 81
2. Minimize heat loss, U L from inside to outer surface of collector is important. 3. From equation (2), smaller water temperature input, T i or higher ambient temperature, T a can increase the overall thermal efficiency. 4. Fin efficiency factor, F and collector efficiency factor, F are important parameters when designing a PV/T collector since they include the heat transfer coefficiency and heat loss of overall collector. Recommendations: The solar collector can be change to double pass collector by adding air medium to compare its performance with single pass system. Adding concentrator to collector can increase both the electrical and thermal efficiency since PV cell temperature is not to be worried. Nomenclature A m C p Area of PV module Specific heat D f Tube diameter Fluid F Fin efficiency factor F Collector efficiency factor F R Flow rate factor G / I(t) Solar irradiance h cα Heat transfer coeffienct between cell and absorber h fi Heat transfer coefficient inside tube h o Heat transfer coefficient from glass to ambient h T Heat transfer coefficient of Tedlar K abs Thermal conductivity of absorber K T Thermal conductivity of Tedlar L T m Tedlar thickness module PF 1 penalty factor due to the glass cover of PV module PF 2 penalty factor due to the absorber below PV module PV/T Photovoltaic thermal collector T a Ambient temperature T i Fluid input temperature U T conductive heat transfer coefficient from solar cell to water through tedlar overall heat transfer coefficient U t from solar cell to ambient through the glass cover U tt overall heat transfer coefficient from glass to tedlar through solar cell W Gap between tube α sc Absorbtivity of solar cell α p Absorbtivity of Plate η sc Solar cell efficiency β sc Solar cell packing factor τ g Glass transmittivity τα Effective absorbtivity and transmittivity Appendix ACKNOWLEDGMENT (12) (13) The researcher is grateful to have financial support from Solar Energy Research Institute (SERI), National University of Malaysia (UKM). References: Charalambous, P.G., G.G. Maidment, S.A. Kalogirou and K. Yiakoumetti, 2007. Photovoltaic Thermal (PV/T) collectors: A review. Applied Therm. Eng., Vol. 27, pp. 275-286. Kemmoku, Y., Egami. T., Hiramatsu. M., Miyazaki. Y., and Araki. K.. 2004. Modelling of module temperature of a concentrator PV system. http://www.physics.usyd.edu.au/app/ solar/research/syracuse/pdf/19theupvsec_5bv- 2-40.pdf Kern, E.C., and Russell. M.C. 1978. Combined photovoltaic and thermal hybrid collector system. Proc. of 13th IEEE Photovoltaic Specialist: 1153-1157 Ji, J., He. H., Chow. T., Pei. G., He. W., and Liu. K. 2009. Distributed dynamic modeling and experimental study of PV evaporative in a PV/T ISBN: 978-1-61804-052-7 82
solar-assisted heat pump. International Journal of Heat and Mass Transfer, Vol. 52, pp. 1365-1373. Vokas. G., Christandonis. N., Skittides. F. 2006. Hybrid photovoltaic-thermal systems for domestic heating and cooling- A theoretical approach. Solar Energy, Vol. 80: 607-615. Ibrahim. A., Othman. M.Y., Ruslan. M.H., Alghoul. M.A., Yahya. M., Zaharim. A., Sopian. K. 2009. Wseas Transactions on Environment and Development, issue 3, Vol. 5, 321-330. Tiwari. A., and Sodha. M.S. 2006. Performance evaluation of solar PV/T system: An experimental validation. Solar Energy, Vol. 80: 751-759. Duffie, J.A., Beckman, W.A., 1991. Solar Engineering of Thermal Processes, second ed. John Willey and Sons Inc., New York, pp. 777, 250 326. Wiebe de Vries, D., 1998. Design a photovoltaic/thermal combi-panel. Eindhoven University Press, Eindhoven, Vol. 58, pp. 111 116. Zondag. H.A., Vries. D.W., Helden. W.G.J., Zolingen. R.J.C., and Steenhoven. A.A. 2003. The yield of different combined PV-thermal collector designs. Solar Energy, Vol. 7, pp. 253-269. Dubey. S., and Tiwari. G.N. 2008. Thermal modeling of a combined system of photovoltaic thermal (PV/T) solar water heater. Solar Energy, Vol. 82, pp. 602-612. Dubey. S., and Tiwari. G.N. 2009. Analysis of PV/T flat plate water collectors connected in series. Solar Energy, Vol. 83, pp. 1485-1498. ISBN: 978-1-61804-052-7 83