Environmentally friendly lubricants for high temperature heat pumps with low GWP refrigerants
This project aims to provide expert knowledge, disseminate findings, and support the development of new POE lubricants in collaboration with leading manufacturers. Given that lubrication plays a critical role in compressor durability, energy efficiency, reliability, and life-cycle cost, attention must be directed towards selecting lubricants compatible with emerging low-GWP synthetic refrigerants.
This project is funded within the 5th funding call of Termo program ”Heating and cooling for future energy systems”, (contributing to a Sustainable Energy System through Heating and Cooling) by the Swedish Energy Agency.
Funding Agency: Swedish Energy Agency
Project Number: P51502-1
Time period: 2021-01-01 – 2025-04-31
Project partners: KTH, Perstorp AB
Background
High-temperature heat pumps (HTHPs) can play a significant role in reducing the internal waste heat and replace fossil fuel and electrically driven boilers in different industrial heat treatment and drying processes. The choice of low Global Warming Potential (GWP), zero Ozone Depletion Potential (ODP) and low trifluoroacetic acid yield (TFA) refrigerant for high temperature applications is of highest importance. The introduction of new high-temperature refrigerants also carries certain environmental, safety, compatibility, and economic risks. Historically, compressor failures due to inadequate lubrication have been observed during the transition from CFCs and HCFCs to HFCs and even to propane-based systems. These issues were eventually mitigated through appropriate lubricant selection, crankcase heating strategies, and proper superheat control. In today's refrigeration and heat pump industry, synthetic lubricants such as polyol esters (POEs), polyalkylene glycols (PAGs), and polyvinyl ethers (PVEs) are increasingly used, especially in Europe, as safer alternatives to mineral oils. POEs and PAGs are commonly applied not only with HFOs and HCFOs, but also with CO₂, hydrocarbons, and HFCs. The customizability of synthetic lubricants to achieve specific thermal and chemical properties makes them particularly suitable for use with low-GWP refrigerants.
The refrigeration and heat pump sector, together with lubricant manufacturers, must remain at the forefront of technological advancements to ensure informed choices of lubricants in collaboration with producers and installers. As many older systems are now being retrofitted or replaced to comply with new environmental regulations, the industry is entering a new transition phase involving 4th generation refrigerants.
Aim and Objectives
This project aims to provide expert knowledge, disseminate findings, and support the development of new POE lubricants in collaboration with leading manufacturers. Given that lubrication plays a critical role in compressor durability, energy efficiency, reliability, and life-cycle cost, attention must be directed towards selecting lubricants compatible with emerging low-GWP synthetic refrigerants. Lubricants for HTHP applications must exhibit specific properties, including high thermal stability, appropriate volatility, chemical compatibility with system materials, sufficient lubricity, long-term stability throughout the compressor life, and environmental biodegradability. While there is extensive experience with POE lubricants in low- and medium-temperature applications, their performance at higher temperatures remains insufficiently explored. The main objectives of this project are:
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Investigate the performance of commercial POE lubricants in high-temperature heat pump (HTHP) systems.
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Develop an environmentally friendly lubricant prototype suitable for high-temperature applications.
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Evaluate the thermophysical properties and chemical stability of the developed lubricant prototypes.
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Analyze the solubility and viscosity behavior of lubricant–refrigerant mixtures.
Methodology
The core objective of this project was to design and construct a small-scale high-temperature heat pump (HTHP) system capable of operating with various synthetic high-temperature HFO and HCFO refrigerants and delivering up to 120 ⁰C condensation temperatures. By July 2024, the construction of the new test rig was completed at the Division of Applied Thermodynamics and Refrigeration, KTH. The prototype system can reach condensation temperatures of up to 140 °C and can work with both synthetic and natural refrigerants.
The compatibility and chemical stability of refrigerants and lubricants were evaluated using the HTHP test rig with in-line viscometer and specially designed stainless steel gas reactors. These tests followed the updated ASHRAE Guideline 38-2023 (Guideline for Using Metal Pressure Vessels to Test Materials Used in Refrigeration Systems).
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