Energy and exergy efficiencies of a hybrid photovoltaic–thermal (PV/T) air collector

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Energy and exergy efficiencies of a hybrid photovoltaic–thermal (PV/T) air collector

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  Renewable Energy 32 (2007) 2223–2241 Energy and exergy efficiencies of a hybridphotovoltaic–thermal (PV/T) air collector Anand S. Joshi  , Arvind Tiwari Centre for Energy Studies, Indian Institute of Technology Delhi, Hauz Khas, New Delhi 110016, India Received 22 January 2006; accepted 21 November 2006Available online 1 February 2007 Abstract In this communication, an attempt has been made to evaluate exergy analysis of a hybridphotovoltaic–thermal (PV/T) parallel plate air collector for cold climatic condition of India(Srinagar). The climatic data of Srinagar for the period of four years (1998–2001) has been obtainedfrom Indian Metrological Department (IMD), Pune, India. Based on the data four climaticconditions have been defined. The performance of a hybrid PV/T parallel plate air collector has beenstudied for four climatic conditions and then exergy efficiencies have been carried out. It is observedthat an instantaneous energy and exergy efficiency of PV/T air heater varies between 55–65 and12–15%, respectively. These results are very close to the results predicted by Bosanac et al.[Photovoltaic/thermal solar collectors and their potential in Denmark. Final Report, EFP Project,2003, 1713/00-0014, www.solenergi.dk/rapporter/pvtpotentialindenmark.pdf ]. r 2006 Published by Elsevier Ltd. Keywords:  Exergy; Hybrid photovoltaic–thermal; Solar energy 1. Introduction To evaluate the performance of a photovoltaic–thermal (PV/T) collector, the value of electricity versus the heat from the collector is important. The performance of a PV/Tcollector can be evaluated in terms of energy efficiency and exergy efficiency. Bosanac et al.defined energy efficiency as the total energy yield for a year [1]. The results are calculatedfrom 1st law of thermodynamics and exergy efficiency as the total exergy yield per year, ARTICLE IN PRESS www.elsevier.com/locate/renene0960-1481/$-see front matter r 2006 Published by Elsevier Ltd.doi:10.1016/j.renene.2006.11.013  Corresponding author. Tel.: +919810934365. E-mail address:  anandsj75@rediffmail.com (A.S. Joshi).  ARTICLE IN PRESS Nomenclature A  area of module (m 2 ) b  breadth (m) C  a  specific heat of air (kJ/kgK)d x  elemental length (m)EVA ethyl vinyl acrelate h T  heat transfer coefficient from back surface to air through tedlar (W/m 2 K) h p1  penalty factor due to presence of solar cell material, tedlar and EVA h p2  penalty factor due to presence of interface between tedlar and working fluidthrough absorber plate I  ( t ) incident solar intensity (a function of time) on the inclined module surface(W/m 2 ) I  HB  terrestrial beam solar radiation on a horizontal surface at ground level(W/m 2 ) I  HD  diffuse solar radiation on horizontal surface at ground level (W/m 2 ) I  ON  normal extra terrestrial solar radiation (W/m 2 ) I  N   normal terrestrial solar radiation at the ground level (W/m 2 ) K  1  perturbation factor (dimensionless) K  2  background diffuse radiation (W/m 2 ) m  air mass (dimensionless) _ m a  rate of flow of air mass (kg/s) n  day of the year starting from January 1 L  length of the PV module (m)PV photovoltaicPV/T photovoltaic–thermal _ Q u  monthly thermal energy (W/m 2 ) _ q u  rate of useful energy transfer (W/m 2 ) _ q exergy  rate of exergy (W/m 2 ) T   number of sunshine hours (h) T  a  ambient temperature ( 1 C) T  air  temperature of flowing air ( 1 C) T  airin  inlet air temperature ( 1 C) T  airout  outlet air temperature ( 1 C) T  bs  back surface temperature of tedlar ( 1 C) T  c  temperature of solar cell ( 1 C) T  R  cloudiness/haziness factor (dimensionless) T  0  reference ambient temperature (K) D T   difference between the ambient temperature and collector outlet temperature U  b  overall heat transfer coefficient from bottom to ambient (W/m 2 K) U  L  overall heat transfer coefficient from solar cell to ambient through top andback surface of insulation (W/m 2 K) U  t  overall heat transfer coefficient from solar cell to ambient through glasscover (W/m 2 K) U  T  conductive heat transfer coefficient from solar cell to air through tedlar(W/m 2 K) A.S. Joshi, A. Tiwari / Renewable Energy 32 (2007) 2223–2241 2224  which is the part of energy that could theoretically be converted to work in an initialCarnot process. The results are calculated from the 2nd law of thermodynamics known asthe exergy efficiency. According to Coventry, exergy (sometimes called availability) isdefined as the maximum theoretical useful work obtainable from a system as it returns toequilibrium with the environment [2].Jones and Underwood have studied the temperature profile of a photovoltaic (PV)module in non-steady-state conditions [3]. They conducted experiments for cloudy as wellclear day condition for energy analysis. The carrier of thermal energy associated with thePV module can either be air or water. The integrated arrangement for utilizing thermalenergy as well as electrical energy, with a PV module is referred to as the hybrid PV/Tsystem.The hybrid PV/T system can be used for:(i) Air heating [4 – 12] and (ii) Water heating [6,13 – 19]. Performance of a PV/T system can be carried out either in terms of energy orexergy. Chow has analyzed the PV/T water collector with a single glazing in transientconditions, consisted of tubes, in thermal contact with the flat plate on account of metallic bond [16]. It has been observed that the electrical efficiency (the ratioof maximum power to the incident solar radiation) is increased by 2% at a mass flowrate of 0.01kg/s due to decrease in temperature of solar cell of PV module and a plate tobond heat transfer coefficient 10 4 W/m 2 K with an addition thermal energy efficiencyof 60%. Huang et al. have experimentally analyzed unglazed integrated photovol-taic–thermal solar system (IPVTS) for water heating under natural mode of operation [20].They observed that the primary energy-saving efficiency of the IPVTS exceeds 0.60,which is higher than that for a conventional solar water heater or pure PV system.Kalogirou has observed the monthly performance of a glazed hybrid PV/T system underforced mode of operation for the climatic condition of Cyprus [14]. He observed an ARTICLE IN PRESS U  tT  overall heat transfer coefficient from glass to tedlar through solar cell(W/m 2 K) U  tair  overall heat transfer coefficient from glass to ambient (W/m 2 K) Greek Letters a  atmospheric transmittance a c  absorptivity of solar cell a T  absorptivity of tedlar b c  packing factor of solar cell y Z  zenith angle ( 1 ) Z c  efficiency of solar cell Z exergyov  overall exergy efficiency Z o  electrical efficiency under standard test condition Z ov  overall thermal efficiency t  transmitivity of glass cover A.S. Joshi, A. Tiwari / Renewable Energy 32 (2007) 2223–2241  2225  increase of the mean annual efficiency of the PV solar system from 2.8% to 7.7%with an associated thermal energy efficiency of 49%. A similar study has alsobeen made by Zondag et al. [13]. They have referred to a hybrid PV/T as a combi-panel, which converts solar energy into both electrical and thermal energy. The efficiencyof the combi-panel was reported as 6.7% (electrical) and 33% (thermal energy),respectively.Sandnes and Rekstad have studied the performance of a combined PV/T collector,which was constructed by pasting single crystal silicon cells onto a black plastic solarheat absorber (unglazed PV/T system) [18]. They recommended that the combinedPV/T concept must be used for low-temperature thermal applications and for increasingthe electrical efficiency of the PV system. Zakharchenko et al. have also studied unglazedhybrid PV/T systems with a suitable thermal contact between the panel and thecollector [17]. To operate a PV module at low temperature, PV module shouldcover the low temperature part of the collector (e.g. at cold water inlet portion). Further,an unglazed hybrid PV/T system with a booster diffuse reflector was integratedwith the horizontal roof of a building by Tripanagnostopoulos et al. [6]. They concludedthat a PV/T system with a reflector yields distinctly clearly higher electrical andthermal energy output. They have studied the performance characteristic of a PV/T-wateras well as a PV/T air system. An overall heat loss coefficient ( U  ) and the thermalenergy gain factor ( g ) for a combination of a ventilated vertical PV module anda double glazed window (PV facades) have derived by Infield et al. [5]. It is observedthat the ventilated facades lead to enhancement of the electrical efficiency of aPV module on account of corresponding low temperature (generally below 45 1 C). Hagazyand Sopian et al. investigated a glazed PV/T air system with a single or double pass airheater for space heating and drying [4,21]. They have developed thermal models for each system. The thermal energy, obtained from the glazed PV/T system gets increasedalong with lower electrical efficiency (due to high operating temperature). Further,Coventry has made an attempt to study the performance of a concentrating PV/T solarcollector and concluded that overall thermal and electrical efficiencies of a PV/Tconcentrating system are 58% and 11%, respectively [22]. This gives a total energyefficiency of the system as 69%.Little work has been carried out for exergy analysis of PV/T air/water heaters. RecentlyBosanac et al. [1] have briefly carried out exergy analysis of PV/T system and reported thatmaximum exergy efficiency of PV/T system is about 12% against an overall maximumenergy efficiency of 60%. Coventry and Lovehrove have also studied the exergy analysis of PV/T water system and reported that energy to exergy ratio of electrical and thermal are 1and 16.8, respectively [2].In this paper, an attempt has been made to study the exergy efficiency of unglazed PV/Tair heating module for the cold and cloudy condition of Srinagar. 1.1. Hybrid PV/T air collector Fig. 1a shows the schematic diagram of a PV/T air collector. Air has been considered asthe medium for transport of thermal energy. Two PV modules with an effective area of 0.61m 2 each are connected in series. The module has been mounted on a wooden structurewith the air duct below the module known as tedlar. In other words tedlar is an insulatingand non-corrosive material used beneath the solar cell for better support to PV module. ARTICLE IN PRESS A.S. Joshi, A. Tiwari / Renewable Energy 32 (2007) 2223–2241 2226  ARTICLE IN PRESS abc Fig. 1. (a) Schematic diagram of integrated PV/T system (IPVTS) when air is used as the medium to collectthermal energy. (b) Cross-sectional view (rotated at 90 1  from horizontal) of hybrid PV/T air collector withtemperature profile. (c) An elemental length ‘d x ’ shows flow pattern of air below tedlar. A.S. Joshi, A. Tiwari / Renewable Energy 32 (2007) 2223–2241  2227
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