Experimental investigation of a two-stage oil-free domestic Air/Water heat pump prototype powered by an oil-free high-speed twin-stage radial compressor rotating on gas bearings

Domestic heat pumps have been identified as a key-technology to decrease the energy consumption of the domestic sector, which represents 29% of the world final energy consumption. Space & water heating represent more than two third of the domestic sector energy consumption in many countries. Next generation domestic heat pumps powered by oil-free high-speed radial compressors are expected to (a) be more efficient at rating operating points and in partial loads, (b) to match better the energy demand characteristics, (c) to be more compact and lighter, (d) more silent, and (e) to need lower refrigerant charges. As a first step towards the development of such advanced heat pumps, an oil-free twin-stage Air/Water domestic heat pump prototype powered by an oil-free high-speed twin-stage radial compressor has been developed and tested. The heat pump layout corresponds to a two-stage heat pump cycle with an open economizer. The two main functions of the economizer are (a) to separate the liquid from the gas before the second-stage compressor inlet and offer a proper mixing rocess, and (b) to store the liquid refrigerant not located in the heat exchangers at a given time. This key-component has been coupled with the compression unit and improved through incremental experimental steps. The heat pump driven by an oil-free twin-stage radial compressor has been successfully tested at the rating operating point A-7/W35. The performance reached with the prototype is considered very promising and constitutes a breakthrough in the domain. This article describes the experimental setup, the Operating Point (OP) A-7/W35, the methodology applied for data analysis, followed by the results. The issues limiting the performance of the prototype are also identified and briefly documented.