Performance Analysis of a Solar Refrigeration System

Abstract

Because of limited sources of fossil fuels and environmental issues like global warming, researchers are focusing on renewable energy sources and mainly solar energy as it is clean, inexhaustible and abundantly and universally available source of renewable energy without any environmental pollution. Along with use of fossil fuels which are the major contributors to the climate change and global warming, the demand of refrigeration and air conditioning has increased drastically. The need of using eco-friendly refrigerants and refrigeration system using low grade energy has increased since long, which eventually increases the need of solar energy for refrigeration system. In present work, thermodynamic analysis of 3 TR ammonia-water absorption refrigeration system with and without ejector was carried out to obtain maximum performance of the system. Thermodynamic analysis of absorption refrigeration system with ejector was carried out for various combinations of condenser temperatures of 45°C and 35°C and evaporator temperatures of 15°C and 5°C and results were compared with conventional absorption refrigeration system. For combined ejector-absorption refrigeration system, effect of the generator temperature on coefficient of performance (COP) was studied for the different value of entrainment ratio in the range of 0.04 to 0.14. COP of combined cycle is improved by 14.66% to 30.1% compared to conventional cycle. Results indicate that combined ejector absorption refrigeration system requires generator pressure of 45 bar for condenser and evaporator temperatures of 45°C and 5°C respectively at the entrainment ratio of 0.14. Even for low entrainment ratio, ejector inlet pressure is twice as compared to conventional absorption refrigeration system for higher condenser and lower evaporator temperatures which causes practical limitations of solution pump. Due to the limitation further development was carried with conventional absorption refrigeration system. First and second law analysis of 3 TR absorption refrigeration system with refrigerant and solution heat exchangers was carried out to obtain maximum performance of the system at optimum generator temperature. Major components of absorption refrigeration system are generator, rectifier, condenser, refrigerant heat exchanger (RHE), expansion device, evaporator, absorber, pump, solution heat exchanger (SHE), and pressure reducing valve. The effect of generator temperature and effectiveness of SHE and RHE on COP, total entropy generation and exergetic efficiency for the different values of evaporator and condenser temperatures was studied. Results show that higher value of evaporator temperature, lower value of condenser temperature and higher values of effectiveness of SHE and RHE gives better performance of the system. The results show that for Indian weather condition, considering condenser temperature of 50ºC and evaporator temperature of 10ºC for the application of summer air conditioning, maximum COP obtained is 0.525 at optimum generator temperature of 170ºC. Optimization of the parameters of ammonia-water absorption refrigeration system using Taguchi design was also carried out. Multi linear Regression analysis was also carried out to obtain best equations of COP and exergetic efficiency of 3 TR absorption refrigeration system. COP and exergetic efficiency were considered as objective functions and condenser, evaporator, and generator temperatures as well as solution heat exchanger and refrigerant heat exchanger effectiveness were considered as independent parameters. L16 orthogonal array was selected for Taguchi method of design of experiment. Maximum COP is obtained at lower condenser and higher evaporator temperature and maximum exergetic efficiency is obtained at lower condenser and lower evaporator temperature for the selected Taguchi design. Optimum COP was 0.65 and optimum exergetic efficiency was 23.94% for the selected Taguchi design. From signal-to-noise ratio analysis optimum COP calculated was 0.6612 at Tc = 35ºC, Te = 10ºC, Tg = 150ºC, ɛRHE = 0.6 and ɛSHE = 0.75 and optimum exergetic efficiency calculated was 24.74% at Tc = 35ºC, Te = -5ºC, Tg = 120ºC, ɛRHE = 0.6 and ɛSHE = 0.75. An evacuated glass tube based parabolic trough collector was conceived to supply heat to the absorption refrigeration system. To understand the effect of solar radiation on efficiency and required mass flow rate for desired air outlet temperature in the range of 150 to 190ºC, simulation was carried out for solar radiation between 500 and 1100 W/m2, assuming air inlet temperature of 35ºC. Simulation results of solar powered absorption refrigeration system are also presented. Effect of generator temperature on solar COP for the different values of concentration ratio (CR) in the range of 2 to 4 of evacuated glass tube based parabolic trough solar collector was studied. Results show that solar COP increases with increase in CR. Solar COP is maximum between 140º and 150ºC generator temperature for the CR in the range of 2 to 4. In present work concentration ratio 3 is selected as it was intended to adjust the tilt of the collector once in a day. With higher CR, acceptance angle decreases which increases incidence losses and solar radiation is effectively utilized only around solar noon if the collector is not tracked continuously. From the simulation results, it was observed that for the concentration ratio of 3, maximum Solar COP is 0.3164 at optimum generator temperature of 150ºC. To validate the simulation results and to evaluate the benefit of using turbulator, two identical collectors, one with turbulator and the other without turbulator were constructed and tested under outdoor field conditions at Rajkot, Gujarat, India (latitude 22.3039ºN, longitude 70.8022ºE). It was intended to achieve air temperature of more than 170ºC for six hours. Experiments were carried out for the mass flow rate from 4.32 kg/h to 13.68 kg/h. For mass flow rates of 4.32 kg/h and 6.12 kg/h, efficiency of collector with turbulator is about 24% higher compared to that without turbulator, while for mass flow rates of 10.8 kg/h and 13.68 kg/h, efficiency of collector with turbulator is 4.73% and 3.57% higher compared to without turbulator. For mass flow rates of 6.12 kg/h and lower, the collector with turbulator is capable of delivering heat at temperature higher than 170°C for more than 6 h per day. Pressure drop was 2.5 to 4 times higher for collector with turbulator compared to that without turbulator for the mass flow range. It is concluded that evacuated glass tube based parabolic trough collector is technically feasible option to supply heat energy requirement of widely used conventional ammonia-water absorption refrigeration system. This will not only reduce the fast depleting fossil fuel resources but also address the critical environmental issue of global warming.