CO2 thermal network prototype: Identifying control parameters for optimal operation in transcritical mode

Abstract

An innovative concept has been developed and patented recently called CO2 Thermal Network (CO2TN) to improve the performance of heating and cooling systems in buildings. It is a decentralized system circulating two-phase CO₂ at 20 ± 5 °C inside a building, instead of water, as a heat carrier fluid (HCF) in a single pipe. Heat pumps connected to this pipe provide heating and cooling inside a building using CO₂ at their source side. Thus, the heat pumps operate with enhanced and consistent performance. The pipe also facilitates heat recovery between the connected units. Additionally, leveraging the latent heat of two-phase CO₂, the CO2TN operates with a reduced mass flow rate in the loop, requiring a lower circulation energy consumption than conventional hydronic systems.

This study introduces the concept and the first prototype of a CO2TN system. Then, the paper experimentally investigates the influential parameters that control the system’s operation and optimize its performance in transcritical mode. The results showcase that a CO2TN’s performance depends on its gas cooler (GC) temperature and pressure, the number of working heat pumps, and their mode of operation. An optimal GC outlet temperature exists for each GC pressure, which can improve system performance by up to 30%. Moreover, the GC pressure depends on the temperature range of the thermal energy sources used to balance the CO2TN. Additionally, the system shows significant performance improvement—up to 63%—when multiple heat pumps operate simultaneously in different modes.