Exergy analysis of a flat plate solar collector with latent heat storage

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A B S T R A C T An exergy analysis has been performed to determine the potential for useful work in a latent heat storage system with phase change material (PCM) for a flat-plate solar collector. Commercial paraffin wax is used as PCM to store and release energy in the solid-liquid transformation; this material is located in metal containers under the absorber plate on the bottom insulation of the collector. The exergy analysis is performed in outdoor conditions for days of low, medium and high radiation taken from October 2016 to March 2017 at Barranquilla city (latitude: 10º 59' 16" N, longitude: 74º 47' 20" O, Colombia). The system is evaluated throughout charge and discharge periods. The energy and exergy balance equations based on the first and second law of thermodynamics is formulated and solved for each element of the collector system as well as for the PCM. Results obtained show the energy distribution and energetic destruction for each system component and its variation as a time function. It was observed that the average energy and energetic efficiency are 28.7 %, 13.2 % for of low radiation days. 26.9%, 20.56% for of medium radiation days, and 23.2%, 18.6% for of high radiation days, respectively. Results of the analysis are shown in detail in the present paper.

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Exergy analysis of a flat plate solar collector with latent heat storage by phase change material for water heating applications at low temperature *Angie Rincon Ortega1, Dr.Mauricio Carmona2 1, 2Mechanical Engineering Department, Universidad del Norte, Barranquilla, Colombia 1 E mail:anggier@uninorte.edu.co , 2 E mail: mycarmona@uninorte.edu.co A B S T R A C T An exergy analysis has been performed to determine the potential for useful work in a latent heat storage system with phase change material (PCM) for a flat-plate solar collector. Commercial paraffin wax is used as PCM to store and release energy in the solid-liquid transformation; this material is located in metal containers under the absorber plate on the bottom insulation of the collector. The exergy analysis is performed in outdoor conditions for days of low, medium and high radiation taken from October 2016 to March 2017 at Barranquilla city (latitude: 10º 59' 16" N, longitude: 74º 47' 20" O, Colombia). The system is evaluated throughout charge and discharge periods. The energy and exergy balance equations based on the first and second law of thermodynamics is formulated and solved for each element of the collector system as well as for the PCM. Results obtained show the energy distribution and energetic destruction for each system component and its variation as a time function. It was observed that the average energy and energetic efficiency are 28.7 %, 13.2 % for of low radiation days. 26.9%, 20.56% for of medium radiation days, and 23.2%, 18.6% for of high radiation days, respectively. Results of the analysis are shown in detail in the present paper. CONTEMPORARY URBAN AFFAIRS (2017) 1(3), 43-48. https://doi.org/10.25034/ijcua.2018.3678 www.ijcua.com Copyright © 2017 Contemporary Urban Affairs. All rights reserved 1. Introduction In recent years there is a grown interest in the use of renewable energy due to the scarcity of fossil energy reserves and the environmental impact caused by their management. Among renewable energy sources solar energy has been great attention due to the ease of obtaining and high potentiality in the generation of electricity and heat, the use of solar heating systems has increased on the basis of reasonable initial costs and structure relatively simple (Jafarkazemi,and Ahmadifard, 2013). For any application with solar thermal systems solar collectors constitute an important component, their operation is based on the capture of the radiation coming from the sun, converting it to heat and the transfer of this heat to a circulating fluid through the collector . The collected energy is carried by the fluid directly to a process requiring heat or to a thermal energy storage system and subsequently withdrawn for use (Kalogirou, 2004). There are different forms of storing thermal energy, among them are the storage by sensible heat, by the thermochemical reaction and by latent heat (Oliver et al., 2010). The most common forms used are by sensible heat and latent heat. Energy stored in the form of sensible heat requires a large volume of the material used to store, while latent heat storage such as phase change materials (PCM) provides higher storage density and little temperature variation during phase change. Techniques have been investigated to improve the thermal energy storage performance with the use of phase change materials, in addition to improving the heat transfer with the application of fins and to improve the thermal conductivity with the PCM encapsulation (Nkwetta, and Haghighat, 2014). Thermal performance can be assessed based on the first thermodynamics law of (Energy Conservation) or the second thermodynamics law of (Principle exergy). The work done by (Li, 2015) shows the review of several techniques to improve energy and exergetic performance. In addition, it concludes that the evaluation of the energetic efficiency is not enough to evaluate the thermal behavior of storage. The weakness of thermodynamics first law analysis is it does not take into account the quality of energy degradation when the energy is converted from one form to another or is exchanged between materials and the currents along of heat transfer processes (Kalogirou et al., 2016). In this context, the second thermodynamics law evaluates the quality energy, and the first law focuses on amount energy. Meanwhile, the exergetic analysis uses the mass conservation and energy principles with the second thermodynamics law, for to design and to analyze energetic systems (Kocca et al., 2007). In a solar system, energy inlet is solar energy, (Petela, 1964) in his works exposed the formulas for to calculate the exergy of thermal radiation. And presented a discussion of the dependence of exergy of fluid and the radiation on the temperature. On the other hand, the investigations presented by (Kalogirou et al., 2016) are based on basic principles of exergy analysis and show a review of the thermal analysis of solar collectors and the processes to involving collectors as an energy source. (Jafarkazemi,and Ahmadifard, 2013). They performed a theoretical model and experimental validation for the energetic and exergetic analysis for flat-plate solar collectors, finding the effect of different design parameters in the efficiencies for to determine the optimum working condition. The main purpose of this study is to perform an exergetic analysis for a flat-plate solar collector with thermal storage system with phase change material. It was determined the exergetic efficiency and exergy destruction for each system component and its variation as a time function for days of low, medium and high radiation. 2. Method and materials In this study, experimental setup consists of a latent heat storage system with phase change material (PCM) for a flat-plate solar collector as shown in Figure 1 wherein its components are listed. Commercial paraffin wax is used as PCM; this material is located in metal containers under the absorber plate on the bottom insulation of the collector. Measurement of collector system temperatures was with thermocouples, located in glass cover, absorber plate and inlet and outlet water in the copper pipe. The energy and exergy performance of the system was analyzed during days of low, medium and high radiation taken from October 2016 to March 2017 in Barranquilla Atlántico. Initially, for to do energy and exergy analysis, inlet and outlet parameters to heat transfer of each component of the system was specified. Figure 1 shows heat transfer termsinvolved in the solar collector. Figure 1.Heat transfer terms involved in solar collector. The notations and diagrams of the references (Asbik et al., 2016)and (Faramarz Sarhaddi, 2016), was used. In Figure 2. shown the exergy of heat associated with the radiation heat transfer, convection, and conduction in each component of the device.

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