Selection of lubricant oils for high temperature heat pumps: A review and selection methodology guidelines

• Lubrication is essential for ensuring HTHP system performance and reliability. • The oil must meet specific viscosity traits, varying with temperature and pressure. • Viscosity, miscibility, and solubility of the medium are key in oil selection for HTHPs. • Working medium dissolved in oil lowers viscosity; Daniel Plots used to determine working viscosity. • Tribology tests replicate compressor conditions, focusing on temperatures and pressures. • The methodology: chemistry analysis, tribological and lubricity tests in HTHP systems. Lubrication is key to the performance and reliability of high-temperature heat pump systems. The primary function of the oil is to lubricate and protect the compressor. In addition, it also provides cooling of the parts heated by friction, a seal against working medium gas leakage, removes impurities, and reduces the noise produced by the moving parts. To fulfil these tasks the lubricant oil must have the required properties, in particular viscosity, which is temperature and pressure dependent and is impacted by the solubility and miscibility between oil and working medium. There is limited literature dedicated to a concrete selection methodology of appropriate lubricant oils for high temperature heat pumps operating with different working media. The objective of this work is to fill this gap and provide a guiding methodology for the selection of appropriate lubricant oils for these applications. A systematic selection methodology is proposed which involves tribology and chemistry studies, along with wear tests to determine the lubricity properties of oils and their mixtures with working media. In addition, based on an extensive literature review, an overview is given of the most suitable lubricant oils for a given working medium.