Performance Improvement of a Transcritical Carbon Dioxide Compression Refrigeration System by Means of an Absorption After-cooler

Introduction: Two-stage Transcritical Carbon dioxide Compression (TCC) refrigeration system is a useful tool to provide low-temperature refrigeration but with a low coefficient of performance (COP). In this research, in order to improve this COP, a heat exchanger, called After-cooler (AFC), is embedded into the system configuration. The AFC cools down the temperature of the refrigerant leaving the gas-cooler to the lowest possible temperature. The cooling load of the AFC is provided by an absorption refrigeration system and the heating power required to run the absorption chiller is supplied by the waste heat, recovered from the hot refrigerant, which leaves the compressors. When the recovered heat is not enough to provide the maximum cooling load, an auxiliary boiler is used in order to provide the redundant heat, required by the generator of the absorption chiller. Single-effect LiBr/H2O and Ammonia/Water absorption refrigeration systems are two options for the AFC part. The proposed configurations are modeled in EES software and analyzed from energy and exergy viewpoints. A parametric study is also performed, in order to evaluate the performance of the two proposed systems in different operating conditions. Finally, an economic analysis reveals lower annual life-cycle cost (ALCC) of the system.
 
Materials and methods: In order to simulate the proposed combined systems, mass, energy and exergy balance equations are applied on all components. These equations are solved for each component, so that the energy consumption and exergy destruction of that component are calculated. The results of the simulations of the TCC and absorption systems are validated by the presented literature. Then, a parametric study is performed in order to point out the optimal operating conditions. Finally, the proposed systems are studied for the optimal operating condition from an economic viewpoint.
 
Result: Results show that in mutual operating conditions, using an ammonia/water absorption chiller results in a higher COP for TCC system, say 22% higher. But compared to the LiBr absorption system, the ammonia absorption system reduces the Energy Utilization Factor (EUF) and the exergy efficiency of the whole system, 25% and 29% respectively. Results of the parametric study state that the accurate design of the operating conditions of a TCC system results in an increase in system performance. Therefore, the COP and EUF of the system with LiBr AFC could respectively increase up to 4.7% and 59.4%, compared with the NH3-AFC counterpart. On the other hand, economic analysis illustrates that with current energy carriers’ prices in Iran, the annual cost of the system which has combined the TCC and the LiBr-AFC is 3720$ less than that of a single TCC, and it is 4590$ less than that of the combined TCC and the NH3-AFC.
 
Discussion and Conclusion: Energy and exergy analyses show that by use of a LiBr absorption chiller as the after-cooler, COP and EUF of the TCC system can be increased up to 48.73% and 73.4% respectively. Also, it can be concluded that a LiBr After-cooler can reduce the ALCC of the combined system up to 5%, compared with a single TCC system.