Energy Reports (Nov 2022)
Exergy destruction characteristics of a transcritical carbon dioxide two-stage compression/ejector refrigeration system for low-temperature cold storage
Abstract
Cold storage is an irreplaceable part of food cold chain. In the present study, a novel transcritical carbon dioxide two-stage compression/ejector refrigeration system for low-temperature cold storage is proposed to freeze and refrigerate food. Also, a combination of conventional and advanced exergy methods is adopted to evaluate the exergy destruction characteristics inside the system. According to the advanced exergy method, 69.88% of the exergy destruction is endogenous, which suggesting that exergy destruction of the system is primarily contributed to the irreversibility of components themselves, meanwhile, the interactions between components are not very close. Among all components, the high-pressure compressor possesses the most tremendous avoidable endogenous exergy destruction, indicating that it has the maximum potential for optimization. And the rest of the improved order is the ejector, low-pressure compressor, evaporator, gas cooler and intercooler, while the high-pressure and low-pressure expansion valves have the lowest priority for optimization. Remarkably, there is a significant discrepancy between the conclusions obtained from these two exergy methods. Furthermore, the system was also found to have a COP of 0.42 under the actual operating condition and 0.62 under the unavoidable operating condition. The impacts of the ejector and compressor efficiencies, as well as discharge and intermediate pressure upon theoretical exergetic performance of the system are considered. The system exergy efficiency is respectively increased by 5.78%, 36.2% and 50% as the ejector, low-pressure and high-pressure compressor efficiencies ranging from 0.5 to 0.9. In addition, there exists an optimal intermediate pressure of 4.25 MPa and an optimal discharge pressure of 8.75 MPa from the perspective of system exergy efficiency.
