A combined thermo-mechanical refrigeration system with isobaric expander-compressor unit powered by low grade heat – Design and analysis

Abstract

Refrigeration and air conditioning systems consume about 17% of the world-wide electricity and their conventional refrigerants cause ozone depletion and global warming. In this study a novel thermo-mechanical refrigeration (TMR) system is developed and analyzed that is powered, instead of electricity, by thermal energy from waste heat or renewable sources in the ultra-low temperature range of 60–100 °C. A novel isobaric expander-compressor unit (ECU) is designed and combined with vapor compression refrigeration cycle to constitute the TMR system. The technological solutions (mainly towards simplification of the design) are crucial components of the study novelty. The suitable refrigerants for the system are systematically investigated, analyzed and selected from a list of 43 refrigerants. Nine fluids for the power loop (the isobaric expansion cycle) and nine fluids for the cooling loop (the thermal refrigeration cycle) were selected and compared based on their mode of operation (subcritical and supercritical), environmental effects and safety class. It is found that the HFO refrigerants such as R1234yf and R1234ze have acceptable performance with no ODP and very low GWP. Natural refrigerants R717 (ammonia) has the best performance in subcritical mode with toxicity as the main drawback. At heat source temperatures less than 85 °C, the system operation in subcritical mode is more efficient and more compact than in the supercritical mode. Thorough analysis and recommendations are made for the size of the ECU in terms of the diameters of the expander and the compressor.