Turbocompressors for Domestic Heat Pumps – A Critical Review

Small-scale, oil-free radial compressors on gas lubricated bearings represent a promising alternative to state-of-the-art compressor technology for driving domestic sized heat pumps. The inherent characteristics of turbocompressors manage to fit the heat pump load while the oil-freeness allows the implementation of advanced heat exchanger technology and the deployment of advanced multi-stage cycles, both validated means to significantly enhance performance and efficiency. The paper presents experimental investigation and results of both electrically and thermally driven 20mm 2kW radial compressor rotating at rotor speeds of up to 210krpm. The compressor stage has been tested in R134a, reaching isentropic efficiencies above 70%. Dynamic, gas lubricated bearings are supporting the high-speed rotors that are lubricated with vapor phase working fluid, thus offering an oil-free and hermetic solution. Insights into the experimental investigation of a 6kW two-stage radial compressor for driving retrofit heat pumps with high temperature lifts are introduced as well. Besides comparing experimental data, the paper also sheds light into critical design aspects related to reduced scale machines and compares them with regards to their impact on efficiency. Identified key issues are (1) aerodynamic challenges related to the severe downscaling of radial compressors such as tip clearance, surface roughness and non-adiabatic operation (2) stable gas bearing technology and (3) challenges related to high power density designs.