This article discusses the thermodynamic approach to use water as a refrigerant in high-temperature heat pumps (HTHPs) to provide thermal energy at about 200°C, which is essential for decarbonizing the industrial sector through electrification. Water, also known as R718, is a good option due to its unique properties, such as high critical temperature, large heat of vaporization, and low cost. However, during the compression process, the discharge temperature of dry steam can reach a very high temperature of about 280-548°C for 15.6 bar discharge pressure, making it difficult to desuperheat the steam while keeping the same discharge pressure. Therefore, the researchers propose liquid injection as an effective desuperheating method to bring the steam to the saturated state at compressor discharge.
The compression process with liquid injection above and below the saturated vapor line, temperature/pressure build-ups, compression work, and coefficient of performance of HTHP equipped with a rotary vane compressor is examined. The concept remains the same for other compressor types. The results of the study also shows that the rotary vane compressors can compress two-phase liquid-gas mixture, allowing the superheated steam to be prevented by compressing wet steam instead of dry steam. The compression process continues until the vapor reaches the desired discharge pressure.
Furthermore, the article presents a more realistic thermodynamic approach to the compression process with liquid injection as previously presented models have several oversimplifying assumptions, such as constant pressure and/or temperature during injection, which might not be valid. The paper discusses the desuperheating process in detail and the assumptions that should be made to achieve it effectively. The compressor work for wet compression with injection requires 344.2 kW, and for dry compression with injection, it requires a higher value. Finally, the study concludes that the proposed thermodynamic approach can provide useful insights for designers of HTHP with liquid injection.
Seyed Mojtaba Hosseinnia, Sébastien Poncet, Hakim Nesreddine and Dominique Monney, Canada
This text has been shortened by the HPC team
