Centrifugal compressor design for high-temperature heat pumps

Abstract

Heat pumps are identified as crucial technology for decarbonising the heat production. Especially for large-scale, high-temperature heat pumps the use of centrifugal compressors has been seen as an attractive technological option. However, there are very few detailed studies available discussing the compressor designs with different refrigerants for high-temperature heat pumps. This study combines the analyses of a 500 kW two-stage heat pump cycle and centrifugal compressor designs. The study qualitatively assesses the feasibility of compressor designs and provides information on how the dimensions, efficiency, and rotational speed are influenced by cycle conditions and the refrigerant. Temperature lift and evaporation temperature were varied with eight working fluids, including hydrocarbons and hydrofluoro-olefins. The estimated COPs were 2.0–2.55 with the highest values observed with pentane and isopentane. The compressor rotational speeds were between 20 – 50 krpm with hydrocarbons requiring higher values than fluids containing fluorine. The predicted compressor efficiencies were between 77% to 84%, with low pressure compressor reaching higher efficiencies than the high-pressure compressor. However, high compressor Mach numbers were observed, and the Mach number seems to be the main obstacle for the increase of the heat pump temperature lift when centrifugal compressors are used.