Impact of local heat pump characteristics on the performance of a CO2-based thermal network
CO2-based thermal networks (CO2TN) connected to local heat pumps (LHPs) offer a promising approach for thermal energy distribution in buildings. These systems circulate two-phase CO2 at near-indoor (room) temperature through a single pipe (the CO2 loop), which serves as the source side for LHPs connected in series. This configuration enables heat recovery between zones and as it operates at an almost constant temperature near indoor temperature, it reduces both heat losses through pipes and insulation requirements. Previous studies have shown that LHP’s performance has a significant impact on the overall system performance. Standard heat pumps, typically designed for higher temperature difference between source and sink (temperature lift), operate quite well for conventional outdoor temperature ranges. However, they may not operate optimally when the source side is near room temperature and the required temperature lift is relatively small. This study evaluates how local heat pump performance characteristics influence the system-wide efficiency of a representative CO2TN. The network includes a CO2 circulation system, LHPs, and an outdoor unit that exchanges heat between the CO2TN and the environment. A detailed simulation model is developed, incorporating all major system components: the circulation compressor, internal heat exchanger, outdoor-side heat exchangers (gas cooler, condenser, and evaporator), and local heat pumps. The base case is modeled using a commercially available LHP unit that has been modified at CanmetENERGY (Varennes, Canada) to integrate with the CO2 loop. To explore potential system improvements, additional simulations are conducted using idealized performance curves that approximate thermodynamically optimized cycles. Based on the presented simulation results, avenues to improve the efficiency of the system by better selection of the heat pump units are identified.