Evaluation of a CO₂ Network System for District Heating and Cooling Using Energy Storage Tanks

District heating and cooling (DHC) systems have attracted interest in reducing CO2 emissions, and a 5th-generation DHC (5GDHC) is being proposed in Europe, which supplies a heat-transport medium at ambient temperature and utilizes various unused heat sources. 5GDHC systems have the characteristics of being nonlinear, bi-directional and decentralized. The CO2 network system has similar concepts in these aspects, but the system adopts CO2 refrigerant as heat-transport medium and uses the latent heat of vaporization of CO2 refrigerant to transport and exchange unused heat to specific areas via decentralized heat pumps (DHPs). Previous studies have confirmed that the CO2 network system can reduce the total annual energy consumption by up to 80% compared to the conventional 1st-generation DHC (1GDHC) system using fossil fuels as heat source in Japan. In this study, evaluation of the CO2 network system, properly storing surplus variable renewable energy (VRE) into storage tanks (STs), is clarified by a flexible grid operation. In addition, it is confirmed that the “zero CO2 emission” can be achieved by the CO2 network with STs in optimal operation. The effectiveness of the CO2 network system with STs is quantified compared to the 1GDHC system.