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Theoretical investigation of high-temperature heat pump cycles for steam generation.

Number: No 062

Author(s) : BLESS F., ARPAGAUS C., BERTSCH S.

Summary

This paper presents a thermodynamic analysis of different heat pump cycles that are capable of delivering 150 °C (4.8 bar) saturated steam from 80 °C recovered waste heat. A detailed comparison of the advantages and disadvantages of the following cycles is presented:
1. Transcritical cycle using butane (R600),
2. Classical closed-cycle using low GWP hydrofluoroolefin (HFO) refrigerant (e.g. R1233zd),
3. Reversed Brayton cycle using carbon dioxide (R744) or argon (R740),
4. Closed-cycle using R600 or R1234ze(Z) with flash tank and additional steam compressors for
mechanical vapor recompression, and 
5. Open-cycle using water (R718) as working fluid and multiple steam compression stages.
Performance parameters such as COP, compressor efficiency, pressure ratio, steam generation rate, cycle controllability, safety, running and investment costs, and component availability are evaluated and discussed. Under the assumed operating conditions (e.g. temperature lift of approx. 70 K), the theoretically calculated COPs of these cycles range between 1.3 and 3.1. In the transcritical R600 cycle, only a small part of the heat can be used to generate steam, limiting the COP to about 1.3. However, the efficiency can be increased by parallel hot water generation in a second gas cooler. Decisive for a system with a classical closed-cycle heat pump (COP of 1.8) are the use of an environmentally friendly refrigerant (e.g. HFO) and a temperatureresistant compressor with cooling function and stable lubrication. The Reversed Brayton cycle (COP of 2.3) requires a compressor with very high isentropic efficiency (> 0.9) in order to achieve reasonable efficiencies. Depending on the application, a temperature glide on the heat source can be advantageous to transfer more heat. The availability of oil-free compressors is crucial for open steam compression cycles (COP of 3.1). Their achievable pressure ratio determines the number of compression stages. Overall, the study contributes to a better understanding of steam generating heat pump cycles for industrial heating processes and it highlights the current research and technological gaps.

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  • Original title: Theoretical investigation of high-temperature heat pump cycles for steam generation.
  • Record ID : 30029958
  • Languages: English
  • Subject: Technology
  • Source: 13th IEA Heat Pump Conference 2021: Heat Pumps – Mission for the Green World. Conference proceedings [full papers]
  • Publication date: 2021/08/31

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