Modeling and Experimental Investigation of an Oil-Free Microcompressor-Turbine Unit for an Organic Rankine Cycle Driven Heat Pump

Domestic heating and cooling will more and more have to rely on heat pumps (HPs) in order to support a more rational use of primary energy consumption. The HP market is mainly dominated by electrically driven vapor compression cycles and by thermally driven sorption processes. The drawback of electrically driven vapor compression cycle is their dependence on an electrical grid and the fact that they increase the winter or summer electricity peak demands. Hence, a thermally driven vapor compression cycle would offer substantial advantages and flexibility to the end user for heating and cooling applications. This paper presents the investigation of an oil-free compressor-turbine unit (CTU) used for a thermally driven HP (TDHP) based on the combination of a HP compression cycle and an organic Rankine cycle (ORC). The CTU consists of a radial inflow turbine and a centrifugal compressor of the order of 2 kW each, directly coupled through a shaft supported on gas lubricated bearings. The CTU has been tested at rotor speeds in excess of 200 krpm, reaching compressor and turbine pressure ratios up to 2.8 and 4.4, respectively, and isentropic efficiencies around 70%. Comparisons between the experimental data and predictions of models, that are briefly described here, have been carried out. A sensitivity analysis based on the experimentally validated models shows that tip clearance, for both compressor and turbine, and surface roughness of the compressor are key parameters for further improving performance.