Design and control of a cryogenic multi-stage compression refrigeration process

Energy and exergy analysis of a modified three-stage auto-cascade refrigeration cycle using low-GWP refrigerants for sustainable development

This study proposed a modified three-stage auto-cascade refrigeration cycle (MTARC) operating with environmentally benign zeotropic mixture of R1234yf/R170/R14 at the refrigeration temperature level of - 80 °C. Compared with the conventional three-stage auto-cascade refrigeration cycle (CTARC), MTARC incorporates an additional pressure regulator between the condenser and separator to realize phase separation at a lower pressure and temperature. A comprehensive evaluation of energy and exergy performance of the two cycles was conducted theoretically. Under a typical working condition, the cooling capacity, COP and exergy efficiency of the MTARC are improved by 15.85%, 11.69% and 7.65% in comparison with the CTARC, respectively. In addition, a lower evaporating temperature was also obtained by the MTARC under the same operating condition. When the intermediate pressure drops from 2 to 1 MPa, the cooling capacity, COP and exergy efficiency are improved by 35.43%, 25.25% and 16.74%, respectively, for the MTARC, meanwhile the compressor outlet temperature increases 19.93 °C from 92.27 to 112.20 °C. Therefore, the selection of the intermediate pressure should be comprehensively considered to ensure a desirable cycle performance and a proper working condition for the compressor. The proposed modified cycle offers new pathways for designing innovative cryogenic refrigeration systems, thereby potentially improving the energy economy in a myriad of modern energy applications for sustainability concerns.

Advanced two-stage cascade configurations for energy-efficient -80 °C refrigeration

In response to the COVID-19 pandemic, some vaccines have been developed requiring ultralow-temperature refrigeration, and the number of these freezers has been increased worldwide. Ultralow-temperature refrigeration operates with a significant temperature lift and, hence, a massive decrease in energy performance. Therefore, cascade cycles based on two vapor compression single-stage cycles are traditionally used for these temperatures. This paper proposes the combination of six different cycles (single-stage with and without internal heat exchanger, vapor injection, liquid injection, and parallel compression with and without economizer) in two-stage cascades to analyze the operational and energy performance in ultralow-temperature freezers. All this leads to 42 different configurations in which the intermediate cascade temperature is optimized to maximize the coefficient of performance. Ultra-low global warming potential natural refrigerants such as R-290 (propane) and R-170 (ethane) for the cascade high- and low-temperature stage have been considered. From the thermodynamic analysis, it can be concluded that liquid and vapor injection cascade configurations are the most energy-efficient. More specifically, those containing a vapor injection in the low-temperature stage (0.89 coefficient of performance, 40 % higher than traditional configurations). Then, using an internal heat exchanger for such low temperatures is unnecessary in terms of energy performance. The optimum intermediate cascade temperature varies significantly among cycles, from -37 °C to 2 °C, substantially impacting energy performance. Parallel compression configuration improves energy performance over single-stage cycles, but not as much as multi-stage (between 20 % and 30 % lower coefficient of performance). For most of low-temperature cycles, the high-temperature stage can be based on a single-stage cycle while keeping the maximum coefficient of performance.