Trade-off between COP and heat production temperature in high-temperature Brayton heat pumps

High-temperature heat pumps support the integration of renewable energy into industrial processes and drive the decarbonisation of industrial thermal production. Among available options, Brayton heat pumps are gaining research interest because they can produce heat at higher temperatures than vapour-compression systems, which are commercially limited to about 200 °C. Brayton heat pumps, by contrast, can achieve peak production temperatures exceeding 350 °C. This paper examines the trade-off between COP and the average heat production temperature (TAve) for different configurations of Brayton heat pumps, accounting for the practical constraints on component operation, such as pressures, temperatures, and heat exchangers’ effectiveness. Moreover, a parameter called split is introduced to account for how the total pressure ratio is divided between the two compressors. The results show that the value of this parameter can be chosen to favour solutions with high COP or solutions with high values of average heat production temperature. Eventually, to boost both the COP and TAve, solutions with a low-pressure ratio are to be preferred, while the maximum and the minimum temperature of the cycle can be chosen in order to promote COP (by keeping these temperatures low) or TAve (by keeping these temperatures high). In the framework analysed in the paper a COP up to 1.67, or a TAve up to 330 °C, depending on the aim, maximizing the COP or maximizing the temperature at which the heat is delivered to the user.