High-performance heat pump systems: Enhancing performance and range of heat pump systems for industry and district heating

Abstract

The United Nations aim for the goal of limiting the global warming to well below 2 K by 2050 compared to pre-industrial levels, which requires considerable reductions in greenhouse gas emissions. Shifting to renewable energy sources and increasing the energy efficiency is inevitable for the industry and utility sectors. Recovery of excess heat is considered as a promising approach for increasing the energy efficiency, although it is often available at too low temperatures. Heat pumps are an efficient technology for upgrading the heat to higher temperatures, suitable for covering various heat demands. This makes heat pumps an attractive technology for substituting fossil fuel based combustion processes. State of the art heat pumps operate however at a fraction of their potential maximum efficiencies and are limited to maximum supply temperatures of between 100 °C and 150 °C. This thesis analyzed approaches for enhancing the performance of heat pumps for industry and district heating and for increasing the possible maximum supply temperatures. In the first part, it was studied how the performance could be improved by selecting a favorable working fluid. Zeotropic mixtures have the potential to increase the performance of heat pumps considerably, especially in applications in which the heat source and heat sink experience temperature glides. The performance improvement potentials are case specific and exploiting these potentials requires a sophisticated procedure for selecting the working fluid and designing the heat pump cycle. This thesis analyzed the criteria and boundary conditions for comparing working fluids and consequently derived a procedure for selecting the best fluids. The interdependencies between the working fluid, the components and the system were studied for enabling a system design for fully exploiting the peculiarities of zeotropic mixtures. Exergy analysis revealed that the majority of the performance improvement resulted from matching the temperature profiles of the working fluid and the secondary fluids. Without major adjustments of the cycle, increases in the coefficient of performance (COP) of more than 30 % were obtained for a beneficial fluid choice. Reducing the required superheating inside the evaporator was found to yield further improvements. Economic analyses revealed that the additional investment for the increased heat exchanger area could be compensated by the increases in COP in most cases. In the second part, different technologies were studied with respect to their techno-economic feasibility for the supply of process heat at temperatures above 150 °C. A reversed Brayton cycle using R-744 and a cascade system with a multi-stage cycle using R-718 showed high potentials in applications with large temperature glides. The reversed Brayton cycle was cost-effective and less complex, while the cascade system had a higher flexibility with respect to the process integration. Both systems were found to be promising for extending the possible supply temperatures to 300 °C or higher, while the economic potentials were highest when combined with own renewable electricity generation. The thesis developed solutions for enhancing the performance of heat pumps and outlined options for enlarging their range of applications. Further actions were derived for exploiting the potential of heat pumps as key components in sustainable energy systems.