Selection of working fluids and heat pump cycles at high temperatures: Creating a concise technology portfolio

As global efforts to address climate change intensify, industrial processes are of the highest interest due to their substantial contributions to greenhouse gas emissions and use of fossil energy. The deployment of high-temperature heat pumps (HTHPs) presents a significant potential for mitigating emissions and decreasing primary energy consumption in industrial applications. However, for widespread adoption, further technological development is crucial to enhance the technology's availability at higher supply temperatures. This study is dedicated to optimising HTHPs by exploring the intricate interplay between working fluids and heat pump cycle design, with the goal of establishing a technology portfolio that guarantees high performance across a wide range of high-temperature conditions. The research evaluates 16 working fluids in 10 HTHP configurations across a broad spectrum of high-temperature conditions, with source temperatures from 0 °C to 100 °C and temperature lifts from 20 K to 150 K. A polynomial estimating the Lorenz efficiency was proposed based on these configurations and temperature conditions. A portfolio of heat pumps using either isobutane, isopentane, or hexane, in a one-stage cycle layout with a suction line heat exchanger, a two-stage cycle layout with an open intercooler using R-718, and cascade configurations using these working fluids, consistently demonstrated the highest Lorenz efficiency for 95% of the evaluated temperature conditions. Including ammonia and carbon dioxide elevated this to 98% of temperature conditions, improving particularly at large sink temperature glides. While underscoring the necessity for an approach considering both COP and production costs, the study highlights the COP as the most important aspect. The marginal improvement observed by including HFOs highlights the potential of a natural working-fluid-based portfolio to achieve high performance, instilling optimism for the future of sustainable high-temperature heat pump applications.