Transcritical CO₂ Heat Pump Space Heating

After a long-term development of solving the performance deterioration caused by high water inlet temperature, the subcooler-based transcritical CO₂ heat pump system was finally proposed as an ideal solution under space heating conditions. Employing the subcooler’s cooling capacity to decrease the CO₂ temperature before throttling point and its heating capacity to assist the heating supply, the performance of the baseline transcritical CO₂ system could be remarkably enhanced with an acceptable power consumption. Based on plentiful experimental and theoretical studies, the subcooler-based system was found more suitable to be adopted in running conditions with higher ambient temperature, lower water inlet temperature and higher temperature difference between water inlet and outlet compared with a same-scale cascade system. Besides, unlike the cascade system that has only one quantity to be optimized, there are two optimal terms, named the optimal discharge pressure and optimal medium temperature in the subcooler-based system. Overall, this chapter gives a brief perspective on the theoretical gist and experimental validation of the subcooler-based system for referring the further development and applications in the future. In recent years, the consumption of fossil fuels has become the most significant cause of environmental pollution, and around 32% of energy consumption can be attributed to buildings’ requirements in the forms of water heating, space cooling and space heating, with more than 30% CO₂ emission occurring. Based on reports, space heating and water heating, which consumed 42% and 18% of energy, occupied the first and third places of energy end-use in the U.S. However, a new notion was proposed to alter fossil fuel consumption by using the heat pump technologies that could transport the heat from ambient to users. Specifically, the transcritical CO₂ heat pump systems have found worldwide appeal because of their environmental friendliness and other thermodynamic advantages, such as the capability for hot water supply with very high temperatures. However, the inefficiency of the CO₂ cycle has been the main issue blocking its popularization, especially in space heating applications with very high water feed temperatures.