Experimental Exergy Analysis of a High-Temperature Brayton Heat Pump

Kurzfassung

High-temperature heat pumps (HTHP) are a promising technology to provide emission free process heat at high temperatures. HTHPs available on the market can provide process heat up to 150°C. However, many industrial processes, especially in the food industry, require higher temperatures, so the industrial sector remains one of the largest emitters of greenhouse gases due to the burning of fossil fuels such as coal, natural gas and oil. To advance the decarbonization of industry towards climate neutrality and drive forward the development of HTHPs for the provision of industrial process heat, challenges such as the temperature level to be achieved while maintaining high efficiency must be addressed to ensure applicability for the end user. Despite the knowledge of the relevance of heat pumps for the decarbonization of industry, there are only a few experimental heat pumps that work with high temperature lifts at sink temperatures above 150°C. This paper shows first experimental results of a HTHP demonstrator based on the reversed Brayton cycle with air as working medium for the simultaneous provision of process heat above 150°C and process cooling below 0°C. The design of the heat pump and the experimental test procedure are explained, followed by the execution of an exergy analysis to evaluate the efficiency of the system and the components, taking into account exergy losses in order to identify optimization potential. The HTHP demonstrator called "CoBra" at the Institute of Low-Carbon Industrial Processes of the German Aerospace Center (DLR) in Cottbus consists of two radial compressors, one turbine and three shell and tube heat exchangers. One of the heat exchangers is used as a recuperator. Dry ambient air is used as the working medium in the primary and secondary cycles. The tests are performed without and with recuperation reaching heat sink temperatures of 158°C and 168°C and heat source temperatures of 11°C and -3°C. A temperature lift of 130 K and 134 K is achieved and COPs of 1.37 and 1.4, respectively. The exergy analysis indicates the compressor to be the component with the highest optimization potential with a relative irreversibility of around 50%. The high-temperature heat exchanger has the highest exergy efficiency of all components with around 75%. The advantages of using a recuperator are confirmed by the increase in the overall heat pump performance. The present study provides new insights into the actual behavior of a heat pump that simultaneously provides process heat above 150 °C and process cooling below 0 °C, while also demonstrating the feasibility of such a system. This work contributes to the benefits and feasibility of Brayton heat pumps for industrial applications.