New zeotropic CO2-based refrigerant mixtures for cascade high-temperature heat pump to reach heat sink temperature up to 180 °C

Abstract

There is a great demand for clean, high-temperature heating in industries. The present work demonstrates the modeling and analysis of a high-temperature heat pump (HTHP) using a two-stage cascade refrigeration system with newly developed refrigerant mixtures. Environment-friendly CO2/butane and CO2/pentane mixtures were introduced in the Low-stage (LS) and High-stage (HS) cycles, respectively. The model was built in MATLAB and explored the possibilities of high-temperature heating. A heating source temperature of 10 to 50 °C was considered as the system's input. The HTHP was designed to deliver hot water at a temperature above 115 °C. The performance of the system was investigated in terms of the heat sink and hot water temperatures, temperature lift, heating capacity, LS, HS, and total system’s coefficient of performance (COP), volumetric and isentropic efficiencies, and discussed. The analysis indicated that the system could deliver high-temperature water with a temperature lift of 70 °C. Mixtures of 5% CO2/ 95% butane and 5% CO2/ 95% pentane exhibit excellent thermo-physical properties making them suitable for high-temperature heating applications with high COP. For instance, such mixtures obtain the system's COP and heating capacity of 3.6 and 201.5 kW, respectively for the heat sink temperature of 116.7 °C. It shows an improvement of 20% compared to pure fluids. Also, the heat sink temperature could reach up to 181 °C when the CO2 composition in the refrigerant mixtures is 20%. The volumetric (0.72–0.87) and isentropic (0.61–0.71) efficiencies of the compressor were studied and displayed.

Keywords

• Cascade system
• High-temperature heat pump
• Natural refrigerants
• Zeotropic mixtures
• MATLAB