Heat Pump driven by a Small-Scale Oil-Free Turbocompressor – System Design and Simulation

Small-scale oil-free turbocompressors driven by high-speed electric motors and supported on refrigerant vapor bearings represent a promising technology to increase the performance of domestic heat pumps. Gas bearings enable high rotational speeds with low mechanical losses. Furthermore, turbocompressors can reach high compression efficiencies and offer a compact and lightweight design. This paper evaluates the performance of a heat pump using a small-scale radial turbocompressor rotating on gas bearings. The turbocompressor has been optimized for R134a and achieves pressure ratios ion the order of 1.5 to 3.5 at rotational speeds of 160 to 280 krpm with isentropic efficiencies of up to 75%. The impeller diameter is 15.2 mm. A system model of the heat pump has been programmed in the Engineering Equation Solver (EES) software. The model uses effectiveness-NTU models for the heat exchangers and polynomial fit approximations for the non-dimensional compressor map data. The heat pump produces 4.0 kW of 30°C water from 10°C ground heat with a predicted COP of 8.1 and 53.4% 2nd law efficiency. Simulation results, the design of the turbocompressor impeller, as well as the layout of the experimental setup are presented. First experimental measurement results will be expected in the beginning of 2017. The system serves as a proof of concept before stepping forward to a two-stage heat pump cycle with two turbocompressors reaching heat sink temperatures of 55 to 65°C.