Evaluation of a Cold Thermal Energy Storage System Using Alternative Refrigerants

Abstract

This study provides a detailed comparison study between clathrate hydrate of R134a and R404a refrigerants in a direct contact thermal energy storage system. Numerous closed loop cycles using hydrate of each refrigerant have been evaluated to compare the clathrate characteristics formation and the overall performance of the direct contact thermal storage closed loop system. The input parameters for conducting the comparison include the compressor speed and the mass flow rate of the refrigerant used to form refrigerant clathrate. Results of this investigation show that using R134a is better than R404a in forming the cold storage refrigerant clathrate. For R134a, the overall system coefficient of performance based on the first law of thermodynamics is evaluated under different operating conditions and found to be varying between 4.10 and 5.77. The exergy analysis shows that the exergy recovered varies between 50% and 66%. For R134a clathrate, a high system coefficient of performance of 5.77 (COP) and a high exergy recovered of 66% are obtained at the lowest tested compressor speed of 2300 rpm and a high refrigerant mass flow rate of 0.96 kg/min. At the compressor speed of 2300 rpm with refrigerant R134a, the system exergy recovered are 62%, 64% and 66% of the exergy input for mass flow rates of 0.48, 0.72, and 0.96 kg/min, respectively. On the other hand, the overall system coefficient of performance for R404a clathrate shows lower values when compared to R134a formation. For R404a, the system coefficient of performance varies between 2.9 and 3.48 while the exergetic efficiency varies between 38% and 58%. At the operating compressor speed of 2300 rpm and refrigerant R404a, the system exergy recovery are about 52%, 55% and 58% for mass flow rates of 0.48, 0.72 and 0.96 kg/min, respectively. Current results show that the thermal system operates more efficiently with R134a refrigerant than with R404a refrigerant. It also shows that the best system performance is achieved at the lowest compressor speed while the effect of refrigerant mass flow rate is insignificant. These results are in agreement with an earlier study by Kiatsiriroat et al. using refrigerant R12 and R22 [1] and [2]. Finally, the current research results conclude to use of R134a refrigerant to form the clathrate hydrate in thermal storage systems.