Thermodynamic modeling of an improved transcritical carbon dioxide cycle with ejector: Aiming low-temperature refrigeration

Abstract

Low-temperature refrigeration (aka, deep-freezing) have an essential role in the food and pharmaceutical industries. Considering environmental and economic concerns, Carbon Dioxide (R744) has presented itself as a competent refrigerant. Even though many researchers performed extensive investigations on the performance of the low-temperature refrigeration cycles, the innovations in this field still exist, and refinement and examination of new layouts remain a hot topic, among which coupling an ejector with the cycle is a popular method that has shown promising results. This article proposes a new layout for low-temperature refrigeration together with the thermodynamic studies on the effects of changing pressures before and after the ejector by introducing an additional compressor, gas cooler, and turboexpander to the conventional layout of transcritical CO2 ejector cycle. The coefficient of performance (COP) around 1.4 was obtained for evaporation at −45 °C. The first law analysis of the cycle was conducted, and optimal values for pressures before and after the ejector were identified. It was found that using a compressor and a gas cooler before the secondary entrance of the ejector is beneficial to COP, and the expansion process right after the ejector will affect the COP. It was discovered that the instant expansion after ejector is unnecessary at optimum conditions, and the phase condition of the ejector’s discharge has a huge impact on the performance. Optimization and parametric analysis of the cycle was conducted, and the effects of efficiencies of the cycle’s components on COP were investigated. A simple and comprehensive second Law analysis of the proposed system is included, and the performance of the setup was briefly compared with other cycles in low-temperature refrigeration. It was revealed that this single-refrigerant proposed cycle not only can reach a reasonable performance for deep-freezing applications, but also it has 10% less compression ratio than its R744 counterparts.