STUDY OF ELECTRICAL AND THERMAL PERFORMANCE OF A HYBRID PVT COLLECTOR

Transcription

1 International Journal of Electrical and Electronics Engineering Research (IJEEER) ISSN X Vol. 3, Issue 4, Oct 2013, TJPRC Pvt. Ltd. STUDY OF ELECTRICAL AND THERMAL PERFORMANCE OF A HYBRID PVT COLLECTOR H. BEN CHEIKH EL HOCINE & M. MARIR-BENABBAS Department of Electronic, Laboratory Modeling Renewable Energy Devices and Nanometric, University Constantine 1, Algeria ABSTRACT The combination of renewable energy sources to optimize the maximum power generation systems, both from technical and economic point of view. The hybrid photovoltaic / thermal (PV/T) collector converts solar energy into heat and electricity. We present in this article, a study of the electrical and thermal performance of a hybrid collector through the development of a heat balance that involves heat exchange between the different components of hybrid collector. The results suggest that this type of collector is a good alternative to photovoltaic modules and thermal collectors installed separately. KEYWORDS: Solar Collector, Photovoltaic, Thermal, Hybrid, Electrical Efficiency, Thermal Efficiency INTRODUCTION A typical photovoltaic module (PV) crystalline silicon technology converts between 12 and 18% of the incident solar energy. To achieve this, a PV module absorbs between 80 and 90% of the incident radiation, most of which is dissipated as heat, increasing the temperature inside the module. This increase in temperature substantially reduces the electrical performance of the module. Noting that the efficiency of the photovoltaic panel decreases with increasing temperature, and that the function of a solar collector is to transfer the heat absorbed by the surface of a heat transfer fluid, the idea was born to combine these two technologies and make a hybrid prototype PVTh. Research on solar panels began in the 70s and has been intensified in the 80s. The hybrid collectors using air and water absorber were evaluated experimentally [1-3], analytically [4-6] and economically. Work has been conducted for three years at the Massachusetts Institute of Technology. The most important conclusion of this work stated that the viability of the hybrid PV / T collector will be decided by the system's ability to meet the thermal and electrical loads required. Kern et al [7], gives the main concepts of these systems through the use of water or air as a coolant. Hendrie, 1979 [8] presents a theoretical model of the PV / T using the techniques of conventional thermal plane collector. Bhargava et al [9] and Prakash, 1994 [10] present the results of their work on the effect of flow and air duct. Works on the performance of hybrid collectors have the effect of flow and air duct. Work on the performance of hybrid collectors have been studied by Sopian et al., 1995 and [11] In the above work, the thermal efficiency of the PV / T was in the range of 45% to 65%. Can be considered for hybrid collectors, total conversion efficiency is the sum of the thermal performance and energy efficiency. In 2005, Zondag [12] offers a state of the art on the solar PV / T hybrid, based on the report of the European project PV-catapult. [13] Among the first studies identified by Zondag [12], some emphasize modeling methods.

2 96 H. Ben Cheikh El Hocine & M. Marir-Benabbas 2007, Tripanagnostopoulos [14] has study PVT hybrid solar collectors which the coolant is either air or water, and can be integrated to the frame. The objective of this work was to reduce the operating temperature of the PV modules, to increase the production of preheated air and reduce heat loss through the insulation on the underside of the component. A mathematical model of air temperature collector PVT with double passage with fins proposed by Ebrahim.M and al (2009) [15]. Chow et al [16] present the modeling and comparative performance of a solar PV / T water. Kribus et al. (2006) discussed the design of a PVT system using a parabolic concentrator small size; system design could provide heating at high temperature. In this paper, the concept of hybrid photovoltaic-thermal collector is presented. The objective of this work is to study the electrical and thermal performance of a hybrid PV / T water. CONCEPT OF HYBRID PHOTOVOLTAIC-THERMAL COLLECTOR The concept of hybrid photovoltaic-thermal collector consists of superimposing both electrical and thermal energy functions. It is characterized by a combination sandwich between air and water. The lower face is isolated and does not absorber. Figure 1 presents a description of a PV-T collector using water as coolant. Schematic of Heat Transfer Figure 1: Descriptive Scheme for a Solar Photovoltaic Thermal (PV/T) Water Collector figure: The heat exchange between the different layers of the collector in the prototype can be presented by the following Figure 2: Schematic of Heat Transfer in the PV / T

3 Study of Electrical and Thermal Performance of a Hybrid PVT Collector 97 MODELING OF SYSTEM PVTH The traditional electrical analogy greatly simplifies the problems of heat. It is the nodal method that allows modeling electrical analogy systems. Diagram of the Electrical Analogy PVT Figure 3: Electrical Equivalent Heat Transfer of Hybrid PV / T Collector Equations Characterizing the Heat Transfer it. The PVTh is a complex system that involves coupling of heat transfer between the various elements constituting Balance equations show the parameters that describe the geometry of the system, the nature of the water flow and the ambient air, losses by convection and radiation. The accuracy of the model is highly dependent on these parameters. Heat Balances of the Various Components External Glass Cover-Ambient Air Q r,ge-a : The radiative heat flux exchanged between the external cover and ambient. with (1) Q c,ge-a : The convective heat flux exchanged between the external cover and ambient. It primarily depends on the wind speed and can be calculated by [19]: (2) (3) (4) Solar flux absorbed by the glass cover outside; calculated by: (5)

4 98 H. Ben Cheikh El Hocine & M. Marir-Benabbas Internal Glass -External Glass Q s-gi : Solar flux absorbed by the internal glass cover; calculated by: (6) Q r,ge-gi : Flux exchanged by radiation between the external and internal glass, it can be calculated by considering the outer glass cover and the inside glass as two infinite parallel surfaces: with (7) (8) Losses from glass internal to cover external is through expressed as the sum of the convective losses due to the radiation and natural convection. (9) Natural convection heat transfer coefficient between glass external and glass internal is given by the expression: (10) Nu is the Nusselt number calculated by means of a flow characteristic correlation between two flat plates defining an enclosed volume [18]. (11) (12) Otherwise (13) G r is the Grashof number defined by (14) β : is being the thermal dilatation, for the air β T -1 w air : duct depth of air. ΔT: Temperature difference between the two plans. Internal Glass- Photovoltaic Cell The total energy absorbed by the photovoltaic module: (15) E ce produced by the PV cell is expressed by the following equation:

5 Study of Electrical and Thermal Performance of a Hybrid PVT Collector 99 Q r,gi-cell : The radiative heat transfer coefficient term between the internal glass and photovoltaic module (16) with (17) (18) Between the glass/panel and water [7], the convective heat transfer coefficient hf is calculated according to the flow regime and the Nusselt number, in our work, it was assumed that its value is the same that for the one air channel Nu2=5.385 [20]. Q 1 : Heat flux transferred by convection to the working water from the internal glass is defined as Q 2 : The heat flux transferred by convection to the working water from the photovoltaic module. Calculate the convective heat flux Q1 exchanged between the inside glass to T vi and the water flow of at temperature T one hand and the heat flux Q2 exchanged between the PV module to T cell and water flow at temperature T on the other. The sum of these two flows gives the amount of useful energy recovered by the fluid. The heat loss from the bottom of the collector down to write: The heat balance at the nodes can be written [21]: (19) (20) (21) (22) (23) Solving this system of equations we can reach value of different temperature: T ge, T gi, T fo and T cell, this resolution allows us to characterize the prototype power. A Matlab program was created, it has solved the system of equations by a numerical method is the Newton-Raphson method. STUDY OF ELECTRICAL AND THERMAL PERFORMANCES Depending on the temperature of the panel, the electrical efficiency η e decreases when the temperature rises above a reference temperature T ref [22]. η o nominal electrical efficiency under standard condition [23] : (24)

6 100 H. Ben Cheikh El Hocine & M. Marir-Benabbas V max and I max are voltage and maximum current. The electrical performance is linked to the performance of η cell by the ratio between surface of the cell and the entire surface (which is known by the filling factor). (25) Useful thermal energy: (26) (27) Dependent on the mass flow and the heat input and output of the exchanger, respectively T fe and T fs, the thermal efficiency is expressed in terms of the total area based on the total surface receiving the radiation S and collector surface Ac [24]. RESULTS Table 1: Ambient Conditions Used in Simulation Solar PV/T Collector Parameters Value Width of PV/T collector, b 0.45(m) Length of duct, L 1.2 (m) Water flow, 76 (m 3 /h) Collector angle, 36 The duct depth, air, w air 0.01(m) The duct depth,water, w eau 0.01(m) The wind speed, V w 1 (m/s) Transmission-absorption factor of the PVT-collector [21], τα 0.62 transmission-absorption coefficient of the water, τα eau 0.16 Emissivity of glass, ε g 0.88 The thickness of glass cover, e g (m) Conductivity of glass, λ g 1 (W/m K) The absorptivity of glass, α g Transmittivity of glass, τ g 0.95 The thickness of silicon solar cell, e si (m) The conductivity of silicon solar cell, λ si (W/m K) The transmittivity of cell, module, τ pv 0.87 The emissivity of cell, module, ε cell 0.95 The absorptivity of cell, module, α cell 0.85 The thickness of back insulation, e i 0.05 (m) The conductivity of back insulation, λ i (W/m K) The packing factor of solar cell, β c 0.83 temperature coefficient, β ( C -1 ) Solving the system of equations governing the heat transfer in the prototype allows estimate the thermal and electrical performance of the prototype. Eqs (20)-(23) of the thermal energy balance have been computed by the Matlab program for the external temperature of cover (T ge ), internal temperature of glass (T gi ), outlet water temperature (T fo ), solar cell temperature (T cell ) in the PV/T collector. (28)

7 Temperature (K) Temperature (K) Study of Electrical and Thermal Performance of a Hybrid PVT Collector 101 Typical results of the simulation program under the following conditions: T fin =T amb =298K, V=1m/s, G=1000 W/m². Table 2: Typical Results of the Simulation Program Paramètres de Collecteur Hybride PV/T à eau Value Thermal efficiency, η th 60.38% Electrical efficiency, η el % Outlet water temperature, T fout K Solar cell temperature, T cell K The average temperature, T av K Heat capacity of water, C p 4180 J kg -1 C -1 mass flow rate (kg/s), ṁ 0.02 kg/s transmission- absorption coefficient, (τα) eff The temperature in different nodes is linearly proportional to the irradiation and ambient temperature, figure 4 shows the influence of radiation G on different temperatures of the hybrid collector. The value of radiation varies from 200W / m² to 1000 W / m², Figure 5 shows the influence of temperature on different temperatures of the hybrid collector for irradiation equal to 1000W / m², keeping the other values constant speed wind is maintained at 1m/s. These curves show a linear temperature changes depending on the solar radiation and ambient temperature Tge Tgi Tfo Tcell Irradiation (W.m-²) Figure 4: Variation of Different Temperatures Depending on the Illumination Tge Tgi Tfo Tcell Ambient temperature (K) Figure 5: Variation of Different Temperatures as a Function of Temperature A parametric study was carried out on the fluid temperature by varying the mass flow rate and radiation G. The fluid temperature at the outlet decreases with the mass flow and its value increases with increasing radiation G (Figure 6 and 7).

8 Cell temperature (K) Outlet temperature Tfo ( C) Outlet temperature Tfo ( C) 102 H. Ben Cheikh El Hocine & M. Marir-Benabbas kg/s 0.1 kg/s 0.15 kg/s 0.2 kg/s Irradiation (W.m-²) Figure 6: Effect of Solar Radiation in the Outlet Temperature Tfs for Different Values of Mass Flow W/m² 600W/m² 1000W/m² mass flow rate (kg/s) Figure 7: Variation of the Outlet Temperature Tfs a Function of Mass Flow for Different Values of the Illumination The temperature of solar cell is linearly proportional to the solar radiation, as shown in Figure 8. It value is higher in the case of a PV module than in the case of a PVT hybrid collector cools the PV modules. And therefore we find that the electrical efficiency in the case of a PVT hybrid collector is higher than in the case of a PV module as shown in Figure PVT PV solar radiation (W.m-²) Figure 8: Temperature of the Solar Cell a Function of the Illumination

9 Thermal efficiency Electrical efficiency Study of Electrical and Thermal Performance of a Hybrid PVT Collector PVT PV solar radiation (W.m-²) Figure 9: The Variation of Electrical Efficiency as a Function of Illumination Figure 10 shows the influence of radiation G on the thermal efficiency of the hybrid collector, it varies linearly with the illumination, it reaches a value of 60.38% for an illumination of 1000W / m² and for a inlet temperature of the fluid equal to ambient temperature, holding the other constant values solar radiation G (W/m²) Figure 10: Thermal Efficiency Depending on the Illumination CONCLUSIONS An interesting alternative to ordinary photovoltaic modules using photovoltaic generators combined with thermal collector to form the hybrid collector, the collector thus produced can simultaneously produce electricity and heat. This work has enabled the study tied the hybrid PVT water, determining its thermal and electrical performance, such as cell temperature and the outlet temperature of fluid, electrical and thermal efficiency for different values of mass flow rate and different values solar radiation. The results suggest that it is a good alternative to conventional photovoltaic generators and thermal collector installed separately. The extracted heat could be used to heat water or be transformed into another energy (mechanical or electrical), and could help to avoid the problem of'' hot spot'' in the PV generator, and the increase is electrical output of the collector and thermal energy is collected.

10 104 H. Ben Cheikh El Hocine & M. Marir-Benabbas NOMENCLATURE Table 3 G Solar radiation intensity [W/m²] h Heat transfer coefficient [W/m² K] e thickness [m] λ Conductivity [W/mK] ε émissivity σ Stefan-Boltzmann Constant [W/m².K 4 ] α absorptivity, τ transmittivity βc The packing factor of solar cell T Temperature [K] w air Width of the air channel [m] w eau Width of the water channel [m] R thermal resistance [m² K/W] Vw wind speed [m/s] Nu Nusselt number Collector angle [ ] g Acceleration of gravity [m/s²] ν Kinematic viscosity [m²/s] Indice ge External glass gi Internal glass cel solar cell amb ambient g glass si silicium i insulation REFERENCES 1. R. Tscharner, H. Curtins, J.P. Häring, R. Schwarz and A.V. Shah, Low Temperature Liquid PV/T Collector, Proceedings of the 5th E.C. Photovoltaic Solar Energy Conference, CEC,Athen, pp , October B. Lalovic, T. Pavlovic, Z. Kiss and J. Van Dine, The Application of Hybrid a-si:h PV and Thermal Collectors for Different Usages, Proceedings of the 8th E.C. Photovoltaic Solar Energy Conference, (CEC), pp , S.V. Sudhakar and M. Sharon, Fabrication and Performance Evaluation of a Photovoltaic/Thermal Hybrid System, SESI Journal, Vol. 4, N 1, pp. 1-7, L.W. Florschuetz, Extension of the Hottel-Whillier Model to the Analysis of Combined Photovoltaic/Thermal Flat Plate Collectors, Solar Energy, Vol. 22, N 4, pp , T. Takashima, New Proposal for Photovoltaic/Thermal Solar Energy Utilization Method, Solar Energy, Vol. 52, N 3, pp , T. Bergene and O.M. Lovvik, Model Calculations on a Flat-Plate Solar Heat Collector with Integrated Solar Cells, Solar Energy, Vol. 55, N 6, pp , J.A. Duffie and W.A. Beckman, Solar Energy Thermal Process, Wiley-Interscience, NewYork, 1974.

11 Study of Electrical and Thermal Performance of a Hybrid PVT Collector ASHRAE Standard 93-86, Methods of Testing to Determine the Thermal Performance of Solar Collectors, American Society of Heating, Refrigeration, and Air Conditioning Engineers, Atlanta, USA, SRCC Document RM-1, Methodology for Determining the Thermal Performance Rating for Solar Collectors, Solar Rating and Certification Corporation, Florida, K.G.T Hollands, T.E. Unny, G.D. Raithby and L. Konicek, Free Convection Heat Transfer Across Inclined Air Layers, Transactions of ASME, Series C, Journal of Heat Transfer, Vol. 98, pp , Y. Yiqin, K.G.T. Hollands and A.P. Brunger, Measured Top Heat Loss Coefficients for Flat Plate Collectors with Inner Teflon Covers, Proceedings of the Biennial Congress of the International Solar Energy Society, Denver, Colorado, USA, August 19-23, pp , Zondag H.A Flat plate PV-thermal collector s au systems Areview. Renewable and Sustainable Energy Reviews, Zondag H.A, BAKKER M, HELDEN W.G.J.Eds, PV/T Roadmap-a European guide for the development and warket introduction of PV-Thermal technology. Rapport Eu-projet PV-Catapult,2005,87p. 14. Tripagnostopoulos Y. Aspects and improvements of hybrid photovoltaic /thermal solar energy systems solar energy, 2007, vol 81.n 9,pp Ebrahim M. Ali Alfegi, Kamaruzzaman Sopian, Mohd Yusof Hj Othman and Baharudin Bin Yatim, Mathematical Model of Double Pass Photovoltaic Thermal Air Collector with Fins American Journal of Environmental Sciences 5 (5): , 2009, ISSN X. 16. Chow T.T, HE W, JI J, et al. Performance evaluation of photovoltaic therosyphon system for subtropical climate application. Solar Eergy, 2007, vol.81, pp Rapport de Stage, Pré-étude d un système couplé Photovoltaïque/Thermique, Universite Joseph Fourier, Jacques BERNARDS, «énergie solaire Calculs et optimisation», Août 2004 France. 19. G.N. Tiwari, Solar Energy: Fundamentals, Design, Modeling and Applications, Narosa Publishing House, New Delhi (2002). 20. H.A. Zondaga, D.W. de Vriesa, W.G.J. van Heldenb, R.J.C. van Zolingenc, a A.A. van Steenhoven, T he yield of different combined PV-thermal collector Designs, Solar Energy 74 (2003) Yves JANNOT, THERMIQUE SOLAIRE, Florschuetz LW, On heat rejection from terrestrial solar cell arrays with sunlight concentration, IEEE Photovolt Spec Conf Rec Mater 1975: H.G. Teo, P.S. Lee, M.N.A. Hawlader An active cooling system for photovoltaic modules, Applied Energy 90 (2012) Simon BODDAERT, Rodolphe MORLOT, Christophe MENEZO, Daniel QUENARD, Jeau BRAU ; Etude et optimisation du potentiel d un capteur thermique photovoltaïque de faible épaisseur intégrable au bâti, 12èmes Journées Internationales de Thermique, Tanger, Maroc du 15 au 17 Novembre 2005.

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