2E (Energy and Exergy) Analysis of ET-CPC Solar Collector Integrated with Different Con guration of Thermal Storage System

6 The intermittency of solar thermal energy warrants the integration/utilization of thermal 7 energy storage system for efficient operation. Effective utilization of solar water heating (SWH) 8 system can reduce nearly 70 90 % of the energy cost incurred for water heating applications. In 9 this study, a compound parabolic concentrator (CPC) solar collector is paired with thermal energy 10 storage (TES) system for the improvement of thermal performance of the collector through 11 enhanced heat transfer rate and minimizing the heat losses. Effects of varying mass flow rate and 12 different arrangement of phase change materials (PCMs) on the performance of the CPC solar 13 collector are investigated. A study of the influence of PCMs configurations in TES systems viz 14 three PCMs 15 (Case 1) and five PCMs (Case 2) on the energy efficiency, exergy efficiency and overall loss 16 coefficient of the solar collector and TES system is made and compared with sensible TES system. 17 The results show the attainment of maximum thermal efficiency of 70 % for ‘Case 2’. Comparison 18 with ‘Case 1’, ‘Case 2’ exhibited a reduction heat loss of 4 % from the TES system. Results of 19 exergy study reveal a superior performance in Case 2 over other configurations. 20 Keyword: compound parabolic concentrator, thermal energy storage, phase change materials, 21 exergy efficiency and overall loss coefficient of the solar collector 22


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Availability of enormous solar energy is perceived as the most effective source of clean 24 energy than other resources (Panwar et al., 2011). Mitigation of global warming in a sustainable 25 manner can be achieved through recent developments seen in solar thermal technology. 26 Dependence of solar harnessing mainly on a kernel solar collector that transfers the solar radiation 27 into heat energy is well known (Son et al., 2014). Among the available solar collectors, a flat plate 28 collector and the evacuated tube are being used for several applications due to ease in installation 29 in buildings along with relatively low operating and maintenance costs. Nevertheless, a large  (Sobhansarbandi and Atikol, 2015). In most of the domestic applications, water is used 38 as heat transfer fluid (HTF) owing to its desirable thermal transport properties, compatibility with 39 the collector material and availability. However, considerable reduction is seen in thermal 40 efficiency of the collector with respect to rise in temperature of the HTF at the inlet resulting in 41 temperature lift at all sections of the collector (Kürklü et al., 2002). Attempts have been made to 42 reduce temperature of the circulating HTF through storage of the required quantity of thermal 43 energy in a thermal storage system thereby simultaneously reduces the mismatch between demand 44 and supply and also the temperature of HTF is brought to the possible low temperature condition.  single and three PCMs configurations. The average energy and exergy collection efficiency of storage system, under different seasonal conditions. Results showed a 1.5 % improvement in 74 thermal efficiency of the solar collector with the use LHTES system compared to sensible heat 75 storage system. Anbazoglu et al (Canbazoǧlu et al., 2005) carried out a similar study on the 76 performance of the solar collector integrated with LHTES system using sodium thiosulfate 77 pentahydrate as the PCM. The outlet HTF temperature in the solar collector was seen as 3.5 times 78 higher using LHTES system than the sensible TES system.

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Exergy analysis is considered as an established tool for evaluating the overall thermal 80 performance of the system (Lior et al., 2006). Kalogirou (Kalogirou, 2004) used an exergy analysis 81 for isothermal and non-isothermal solar collectors for determination of the optimum temperature of 82 the absorber tube for a reduced entropy generation. The optimized absorber tube temperature was 83 the geometric mean of the stagnation temperature of the HTF and the ambient temperature. Farahat

Experimental test facility
The experimental facility was placed at the roof top in Thermal Sciences Block, Anna 124 University, Chennai, India. Water, the selected HTF, was circulated from the makeup water tank 125 through the copper tube in the receiver using a centrifugal pump. The mass flow rates of HTF were 126 continuously monitored by a Coriolis mass flow meterl. The solar radiation was effectively where m is the HTF mass flow rate (kg s −1 ), c is the specific heat of HTF (kJ kg −1 K −1 ), ΔT is the 150 difference in temperature of the HTF (K) and is the time interval (s). The total energy stored in the TES system is evaluated by summing of energy stored in 159 each zone.

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The heat losses from the LHTES system to the surrounding is calculated from where T avg -average temperature of the zone (K), T a -surrounding temperature (K), r 1 -inner 166 radius of the LHTES (m), r 2 -outer radius of the LHTES system without insulation (m) and r 3 -167 outer radius of the LHTES system (m) with insulation (0.023 W m -1 K -1 ).
The convective heat transfer coefficient (h o ) of air and TES system is predicted using 169 the correlation (VDI-Gesellschaft, 2010) as given in the following Eqn (3.8). Grashof number is given by The convective heat transfer coefficient of HTF in inside area of TES evaluated from where, The utilization ratio characterizes the amount of energy retrieved (Q d ) versus the stored 181 energy (Q s ) during the discharging process: where, A a is the aperture area of solar collector (m 2 ) and G t is the incident solar radiation on the 186 surface of solar collector (W m −2 ).

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The useful energy gained with 'F R ' and 'U L ' terms is expressed as where, 'F R ' is the heat removal factor, is the receiver-absorber area (m 2 ) and T p is the mean plate The absorbed radiation 'S' is obtained from Eqn where, and are transmissivity of inner and outer glass; , , , , , , and are 194 absorption coefficient, receiver reflectivity, reflection number, absorber reflectivity, glass 195 reflectivity, area of glass evacuated glass (m 2 ), radius of glass (m) and radius of an observer (m) 196 respectively.

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The heat losses 'Q L ' associated with the solar collector is determined from Eqn (3.17)

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Radiative thermal resistance (R abs-g2 ) between absorber and glass tube is calculated

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Initially, the TES system is supplied with HTF at 85 °C at 240 kg h -1 and the HTF 326 circulation has been taking place until the steady-state condition is achieved.