Analysis of Innovative Vapor Compression and Thermal Energy Storage Systems Using Alternative Refrigerants

Abstract

This research presents detailed analyses of innovative single and two stages vapor compression refrigeration cycles (VCRCs) as well as a cold thermal energy system (CTES) for residential and commercial cooling applications. Different subcooling systems, such as dedicated mechanical subcooling (DMSC) and liquid-vapor heat exchangers (LVHX) using various refrigerants are investigated and analyzed. Results indicate that subcooling systems have great impacts on increasing the overall energetic and exergetic COPs of VCRC. The energetic and the exergetic COPs of the single stage VCRC with LVHX using R134a are 5.50 and 0.178, respectively. Whereas, for the innovative two stages VCRC with DMSC systems and R134a, energetic end exegetic COPs are found to be 5.83 and 0.189, respectively. A direct contact heat transfer CTES system using R134a-clathrate as a storage medium is investigated experimentally on a single-stage VCRC at different operating conditions. Some of the investigated operating conditions used to evaluate the storage medium behavior (clathrate) and the overall system performance are compressor speed, refrigerant mass flow rate, and different water to refrigerant mass ratios. Using R134a, it is found that at lower refrigerant mass flow rate between 0.5 to 0.8 kg/min, the clathrate is formed at longer durations, and COPs of operating cycles are found to be low compared to VCRCs. At a higher mass flow rate of 1.2 kg/min, the COPs of the CTES systems are found to be between 11.81 and 11.85, which is higher than the corresponding single stage VCRCs with LVHX by 42.5 to 44.5% and higher that the VCRC with DMSC by 41.3% to 43%. Current results show that at lower compressor speed of 2100 rpm and constant R-134a mass flow rate of 1.2 kg/sec, the COP is improved by 17%; whereas, at higher compressor speed of 2900 rpm, the COP is decreased by 23%. At higher water/R134a mass ratios in the crystallizer (storage tank), COPs are found to be decreasing due to the increase in charging times needed to form the clathrate compared to all other cases. Finally, the experimental and simulation results indicate that the CTES systems have higher COPs as compared to innovative VCRCs.