THERMAL AND ELECTRICAL PERFORMANCES OF A NEW KIND AIR COOLED PHOTOVOLTAIC THERMAL SYSTEM FOR BUILDING APPLICATION R.S. Adhikari, F. Butera, P. Caputo, P. Oliaro and N. Aste Department of Building Environment Science and Technology (BEST), Politecnico di Milano Via Bonardi 3, 2133 Milan, Italy Phone: +39-2-2399-7257, Fax:+39-2-2399-7255, e-mail:rajendra.adhikari@polimi.it Abstract In this paper, a simulation model has been presented for the modeling of thermal and electrical performances of an air cooled photovoltaic/thermal (PV/T) system. Based on the detailed energy balances of various components of the system, a new type (TYPE25) has been developed in FORTRAN for its integration with the TRNSYS software. The usefulness of developed model has been shown by carrying out numerical calculations for the annual performance of the system corresponding to the typical climate of Milan in Italy (45 27 N). Further, a simple economical analysis has been carried out to calculate the cost of unit energy production ( /kwh) for PV/T systems. The potential use of thermal energy produced from PV/T system is discussed and finally a sensitivity analysis on the cost of unit energy production has also been shown. 1. INTRODUCTION It is well established that the efficiency of PV cells drops as the temperature of cell increases. Various studies have shown that the PV temperature can be lowered by heat extraction with a proper natural or forced fluid circulation. An interesting alternative to conventional PV module is to use hybrid PV/T system, which consists of PV modules coupled to a heat extraction devices for the generation of heat and electricity simultaneously. In the literature, several theoretical and experimental studies on PV/T systems have been carried out. Most of these system configurations were based on the conventional flat-plate collectors with pasted solar cells on the absorber. Recently, Tripangnostopoulos et al. (22) have presented a summary on the work conducted by several researchers in the past in respect of PV/T systems. The concept of hybrid PV/T can be realized by providing a simple well insulated air duct beneath the PV modules for cooling of the PV cells and the thermal energy produced by the system can be used for low and intermediate temperature building applications viz. space and water heating etc. Applications of photovoltaic in built environment and its financial viability has been discussed by Bazilian et al. (21). At Politecnico di Milano, presently, research work is being carried out on the development of solar energy systems regarding their integration with building envelope. In this respect, a proto-type of PV/T system was studied experimentally (Aste et al., 22) and it was observed that the PV/T systems have great potential in terms of higher system efficiency and their integration with building envelop. In the present study, a mathematical model has been developed for the simulation of thermal and electrical performances of an air-cooled PV/T system. Based on this model, a new type (TYPE25) was written for its integration with TRNSYS simulation software. For the appreciation of the developed model, numerical simulation was carried out in TRNSYS to predict the annual thermal and electrical performances of the PV/T plants corresponding to a typical climate of Milan (45 27 N) in Italy. Further, a simple economic analysis has been made to calculate the unit cost of unit energy production ( /kwh) for PV/T systems. The analysis is based on the life cycle costing (LCC), and the annual cost of the system is calculated based on initial capital costs, maintenance costs, operating costs, salvage value, useful life and interest rate. A sensitivity analysis regarding the effect of capita cost and useful life on the cost of unit energy production is also discussed. 2. MODEL FORMULATION A conceptual design of the air cooled PV/T system is shown in Fig. 1. The system consists of a PV laminate, a duct for air flow, absorber and well insulated plate. Air-In (T a ) PV Cells (T c ) Insulated Back Plate (T b ) Glass Cover (T g ) Absorber Plate (T p ) Air-Out (T o ) Fig. 1. Schematics of an air cooled PV/T system
Energy balance equations for PV sandwich covered with cells and without cells, absorber plate, back plate and air inside the duct are respectively written as follows: P f I c τ g α c η c = (h ca + h ca ) (T c -T a ) + h cf (T c -T f ) + h cp(t c -T p ) (1) (1-P f )I c α g = (h ga + h ga) (T g -T a ) + h gf (T g -T f ) + h gp(t g -T p ) (2) (1-P f )I c τ g α p + h cp (T c -T p ) + h gp (T g -T p ) = h pf (T p -T f ) + h pb (T p -T b ) (3) h pb (T p -T b )=h ba(t b -T a ) (4) mc f (T o -T i ) = h cf (T c -T f )+ h gf (T g -T f ) + h pf(t p -T f ) (5) where packing factor, P f is the ratio of area covered by cells to the total PV laminate area. The average air tempearture inside the duct is considered as the mean of the inlet and outlet air temperatures. The effective PV cell efficiency (η c ) as a function of its temperature, is calculated by expression given by Florschuetz (1975). In the present study, the forced convective heat transfer coefficients h cf, h gf, and h pf ; conductive and radiative heat transfer coefficients h pb and h pb; radiative heat transfer coefficients h cp, and h gp are calculated by the well established expressions described by Garg and Adhikari (1998). The radiative heat trasfer coefficients between PV laminate and sky, h ca and h ga are calculated based on view factors between surface and sky (CNR, 1985). Eqs. (1-5) can be solved analytically for obtaining the temperatures of various components of the system. The total thermal (Q t ) and electrical (Q e ) energy produced by the PV/T system can be calculated by following expressions: Q t = m C f (T o -T i ) A c (6) Q e = P f A c I c η c η bos κ dust (7) For the numerical calculations, a PV/T system with typical specifications has been considered, These specifications represent the type, configuration of PV modules, air duct characterstics and the components material. The same are described in Table 1. Based on the PV/T system specifications, simulations have been carried out to predict the annual thermal and electrical performances of the system for the climate of Milan (45 27 N) in Italy. PV Module Module area PV area No. of cells Nominal Power Packing Factor Table 1. PV/T System Specifications 1.254 m2 (1.587 m x.79 m) 1.158 m2 72 15 Wp.89 PV cells Type Multicrystalline silicon Size 12.5 x 12.5 cm Nominal efficiency 15% Air Duct Duct spacing.1 m TRNSYS simulations were run using the new TYPE25 corresponding to different PV/T plant capacities (kw p ). The total PV/T collector area required corresponding to different plant capacity was calculated based on the specifications given in Table 1. The weather data used in simulation were generated using Meteonorm software (1998). The system simulations have been performed corresponding to air flow rate of 6 kg/ h m 2 inside the duct. The results of the simulations are presented and discussed subsequently. 25 2 15 1 5 5 1 2 3 4 5 6 7 8 9 1 PV Plant capacity (kwp) Thermal Electrical (PV/T) Electrical (PV) Fig. 2. energy performance a PV/T system Fig. 2 shows the annual thermal and electrical energy production (kwh) of a PV/T system for different PV plant capacities. For the comparision purpose, the electrical energy produced by a conventional PV plant is also shown in the figure. It is clear from the results that a PV/T system performs better than a conventional PV system in terms of electrical energy production and in addition, also provide the thermal energy.
To explore the possible application of thermal energy generated by a PV/T system, statistical analyses were made and the frequency distribution curves (annual and winter period) were generated for ambient and outlet temperatures of air obtained from a PV/T system as shown in Figs. 3 and 4. attractive applications for space heating (fresh air preheating) in winter and water heating, drying etc. all the year around.. Frequency Distribution (Thermal Energy) 2 18 16 14 12 1 8 6 4 2 Frequency Distribution () Frequency (hrs) 3 25 2 15 1 5 2517 111 95 255 144 34 9 1-1 1-2 2-3 3-4 Energy Range (W/m2) Temperature Range ( C) Fig. 5. Frequency distribution of thermal energy produced for annual and winter Periods Ambient Outlet air Fig. 3. statistics of ambient and outlet air temperatures 2 18 16 14 12 1 8 6 Frequency Distribution () Frequency (hrs) Frequency Distribution (Thermal Efficiency) 2 18 16 14 12 1 8 6 4 2 61 27 1777 743 148 547 257-1 1-2 2-3 3-4 Efficiency Range Range (W/m2) 83 4 2 Temperature Range ( C) Ambient Outlet air Fig. 3. statistics of ambient and outlet air temperatures The results of the statistics show that the increment in air temperature across the PV/T system is low and in the range of 5-15 C. Such low temperatures, however, have Fig. 6. Frequency distribution of thermal efficiency for annual and winter periods Figs. 5 and 6 show the frequency distribution of produced thermal energy and corresponding efficiency from a PV/T system. It is clear from the figure the maximum obtainable thermal efficiency is in the range of 3-4%. 4. ECONOMICS OF PV/T SYSTEMS Considering the initial capital cost (C s ) maintenance (M s ) and pumping (C f ) costs, salvage value (S s ), thermal
revenues (C th ), the annual cost of a PV/T system can be expressed by following expression: C T = C s f s + M s C s S s F s C s + C p + C f - C th (8) where f s and F s represent the capital recovery factor and sinking fund factor, respectively, and can be evaluated using the standard expressions. The cost of unit electricity production (C unit ) from a PV/T system can be evaluated by dividing total annual cost with the total electricity production (Q e ) and can be expressed as follows: C unit = C T / Q e (9) Various cost parameters used in the present study are based on the prices prevailing in the European market. The PV/T system cost is considered as 9 /kw p. The annual maintenance cost and salvage values are 2% and 5 % of initial capital investments. The useful life of the system and interest rate are considered as 2 years and 6% respectively. The cost of pump (C p ) is a function of the supply airflow rates and which, in turns depends on the plant capacity (kw p ). The pumping cost (C f ) is evaluated on the basis of pressure drops in the system. The cost of electricity used for the calculations of pumping cost is.2 /kwh. Cost of thermal energy (C th ) generated by the system was estimated on the basis of equivalent cost of gas savings. The cost of unit electricity production (C unit ) for various plant capacities (kw p ) were made for both the cases when the thermal energy produced by the PV/T system is used throughout the year and during winter period only and the values of C unit are obtained as.77 and.8 /kwh respectively. The obtained values of C unit are quite higher (4-5 times) as compared to the cost of electricity (.2 /kwh) purchased by the consumer from the utility. However, it is to be noted that the electricity cost from the utility include subsidy benefits from the Government and does not include the environment cost regarding the pollution generated by conventional energy production systems. In view of the fact that presently all over the world, substantial financial incentives have been provided for the promotion of PV technology. For example, in Italy, the Government bears 75% of capital costs of the system. In this case, the economics of the PV/T systems becomes very interesting as it competes fairly with the conventional electricity price. The effect of capital cost on the cost of electricity production from the PV/T system is shown in Fig. 7. The calculations are also carried out to see the effect of useful life on the cost (Fig. 9). It has been observed from the Figs. 7 and 8 that these parameters have a significant effect on the cost of energy production. Fig. 7. Effect of capital cost of the system on the cost of unit electricity production Cost of Energy ( /kwh) 1.2 1..8.6.4 1.2 1..8.6.4 Fig. 9. Effect of useful life of the system on the cost of unit electricity production It is foreseen that as the PV technology grows advanced and becomes more efficient, the electricity generation from PV/T systems are going to be competitive in the near future. REFERENCES 7 8 9 1 11 Capital cost ( /kwp) 1 15 2 25 3 Useful Life (Years) Aste N., Beccali, M. And Chiesa G. (22). Experimental evaluation of the performance of a proto-type hybrid solar photovoltaic-thermal (PV/T) air collector for the integration in sloped roof. Proc. EPIC 22 AIVC Conference., October 22, Lyon, France, pp. 339-344.
Bazilian M. D., Leenders F., Van Der Ree B. G. C. And Prasda D. (21). Photovoltaic cogeneration in the built environment. Solar Energy 71, 57-69. η τ κ efficiency transmittance coefficient Consiglio Nazionale Delle Ricerche (1985). Guida al controllo energetico della Progettazione. Final Report of the project, Rome, Italy. Florschuetz, L.W. (1975). On heat rejection from terrestrial solar cell arrays with sunlight concentration. Proc. IEEE Photovoltaic Specialists Conference. Records, May 1975, pp. 318-326. Garg H.P. And Adhikari R. S. (1998). Transient simulation of conventional hybrid photovoltaic/thermal (PV/T) air heating collectors. Int. J. Energy Res. 22, 547-562. METEONORM-Global Meteorological Database for Solar Energy and Applied Climatology, Handbook, Meteotest, Fabrikstrasse 14, CH-312 Bern, Switzerland. Subscripts a b c f g i o p bos Acknowledgments ambient back plate PV cells air transparent cover inlet air outlet air absorber plate balance of system One of the authors (R.S. Adhikari) undertook this work with the support of the "ICTP Programme for Training and Research in Italian Laboratories, Trieste, Italy. Tripanagnostopoulos Y., Nousia Th., Souliotis M. And Yianoulis P. (22). Hybrid photovoltaic/thermal solar systems. Solar Energy 72, 217-234. TRNSYS Reference Manual (1996), Solar Energy Laboratory, University of Wisconsin, Madison, USA, 1996. Nomenclature A c collector area, m 2 C f specific heat of air, J/ Kg o C C s capital cost of PV/T system, /kw p C unit cost of unit electricity production, /kwh C P blower cost, C b pumping cost, /kwh C th cost of produced thermal energy, /kwh h conductive heat transfer coefficient, h convective heat transfer coefficient, h radiative heat transfer coefficient, I c solar irradiance on inclined plane, W/ m 2 M s maintenance cost m mass flow rate, Kg/ h m 2 P f packing factor, ratio of area covered by cells to the total PV laminate area Q t produced thermal energy, kwh Q e produced electrical energy, kwh T temperature, o C Greek letters α absorptivity