Mitigating high return water temperatures in CO₂ heat pumps for legacy district heating networks

Transcritical CO₂ heat pumps are a promising, environmentally friendly technology for decarbonizing space and water heating. However, their performance deteriorates at high central heating return water temperatures, which are common in existing building stock and legacy district-heating networks and can limit retrofit viability. This deterioration is primarily due to higher throttling losses, which increase in proportion to the square of the temperature lift. This study presents and evaluates a novel large-scale transcritical CO₂ heat pump system that combines parallel compression with an indirect auxiliary heat-recovery cycle, in which the auxiliary heat pump recovers heat from the return water stream before it enters the CO₂ gas cooler. In a case study, the system was developed for a district heating network and delivers a total heating capacity of 600 kW and aims to enhance both energy efficiency and cost effectiveness under varying return water temperature conditions. The parallel compression CO₂ system with auxiliary heat recovery achieved a 26% increase in COP over the conventional parallel compression system at a return water temperature of 50 °C. Moreover, the improvement increased further at elevated return water temperatures, underscoring the benefits of auxiliary heat recovery under such operating conditions. The analysis shows that the proposed system significantly reduces throttling losses, resulting in enhanced energy efficiency, particularly at high return water temperatures (e.g., 55 °C), and offers operational flexibility at lower return temperatures. Exergy analysis showed that the total exergy destruction decreases from 259 kW in the conventional parallel compression system to 159.9 kW with auxiliary heat recovery, primarily due to reduced expansion-valve and compressor losses. The results offer practical insights for designing district heating networks with large-scale CO 2 heat pumps and contribute to both advanced thermodynamic modeling and site-specific feasibility planning for next-generation low-carbon heating systems.