Exergetic and energetic comparison of reverse Rankine, Brayton, and Stirling heat pump cycles for district heating applications
Integrating heat pumps into district heating systems is a key strategy for increasing the share of renewable energies in the heating sector, especially in urban areas. Yet, district heating networks are diverse, often posing significant challenges on heat pumps. This study provides a potential analysis of three fundamental heat pump cycles – vapor compression, reverse Brayton, and reverse Stirling – with a focus on district heating applications by conducting a comprehensive exergetic and energetic comparison. For this purpose, both ideal and non-ideal process models are applied to capture thermodynamic potentials and the impact of key loss factors under typical district heating conditions. The coefficient of performance (COP) serves as an indicator of energetic efficiency, while exergy loss quantifies the integration quality of the processes into district heating systems. The results demonstrate that the reverse Brayton heat pump constitutes the theoretical optimum under idealized assumptions, achieving the highest COPs and lowest exergy losses. However, once non-ideal effects are introduced, this advantage diminishes substantially. Under realistic conditions, the vapor compression heat pump delivers the highest COPs and proves most robust with respect to loss factors such as pressure drops and isentropic efficiency. The reverse Brayton heat pump shows strong dependence on very high isentropic efficiencies in compression and expansion stages, which poses a major challenge for practical application. The reverse Stirling heat pump consistently occupies an intermediate position in context of efficiency, exergy losses and sensitivity to loss factors. By abstracting from specific technologies, this study delivers a comprehensive evaluation of the three cycles in district heating, highlighting their thermodynamical capability.