Theoretical investigation on a liquid-state pressurized carbon dioxide ejector heat pump cycle driven by waste heat

Heat pumps serve as energy-saving technologies for upgrading low-temperature heat in industrial and district heating applications. They can be driven by electricity or high-temperature heat, with the latter being widely available as waste heat in industrial processes. This study investigates a waste heat-driven system known as the liquid-state pressurized carbon dioxide ejector heat pump. Theoretical models of the cycle and the ejector were established to evaluate its performance. Using a flow of hot gas at 300 °C as the driving heat source, the system upgraded heat from 35 °C to over 100 °C. The results showed that the cycle can supply hot water at temperatures up to 107.5 °C with a COP of 1.14, while a maximum COP of 1.71 was achieved with an output temperature of 35.7 °C. The exergy efficiency increased with cooling pressure, and among all components, the vapor–liquid separator consumed the largest exergy rate while the ejector destroyed the largest exergy rate. It is helpful to increase evaporating temperature to achieve higher COP and entrainment ratio, though the final temperature of hot water shows a declining trend.