Design recommendations for R-718 heat pumps in high temperature applications.

The transition towards carbon neutral industry requires sustainable and climate friendly heat supply. Heat pumps can meet these requirements, but currently achievable temperatures for supply of process heat are limited to around 100 °C. Using water (R-718) as refrigerant shows a good thermodynamic and environmental performance at higher temperatures, while a sophisticated design is required to compensate increased investment cost due to relatively large volume flows. This work analyses the design of two-stage R-718 heat pumps with turbo compressors with a focus on desuperheating and suggests design recommendations to ensure a competitive thermodynamic and economic performance. The results have shown that the economic performance becomes especially competitive at evaporation temperatures above 100 °C with COPs of up to 4.3 and specific investment cost below 250 €/kW of supplied heat. The suggested approach to realize the desuperheating was found as the optimal solution with respect to pressure drop and space requirements.