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How high is high: what temperatures can we achieve with high temperature heat pumps?

Number: 2339

Author(s) : HEWITT N. J.

Summary

COP28 pledged to triple the world's renewable energy capacity by 2030 and to double global energy-saving efforts over the same period. For energy efficiency, examples include an over 30 times increase in electric and hybrid light vehicle use, a doubling of building renovation, a nearly 10 times increase in smart meters, and a nearly 10 times increase in the number of heat pump installations. Domestic and commercial installation processes are well established and yet would always still benefit from system and component improvements, including new and or natural working fluids to address the per- and polyfluoroalkyl substances (known as PFAS) challenges. It is with PFAS that a concern is raised. The International Energy Agency (IEA) Heat Pump Technologies Annex 58, addressing High Temperature Heat Pumps details the significant research and commercial effort to deliver heat pumps in that heat from 100⁰C to 200⁰C and above. There are significant proportions of these systems, both existing and emerging, that use PFAS refrigerants. Therefore, what working fluids can we utilize to match the operating temperatures of IEA HPT Annex 58, while maintaining or indeed increasing efficiency and while increasing the operating temperature range. A natural working fluid with a high critical temperature is of course water (R718). Its large vapour volume is of course one of its disadvantages but there appears to be routes to its successful deployment. In investigating multistage compression for a heat source e.g., from waste heat from industry at for example 90⁰C to attaining 300⁰C, a single stage lift will have a very poor coefficient of performance (COP). Multistage compression can deliver much higher COPs e.g., approaching a COP of 4.0, if single compressor can operate with up to 7 injection ports along its compression path and initiate for example, 30K temperature lifts during each stage. Experience has shown both flash tank heat pumps and injection port positioning increase COP with a slight loss in capacity as less refrigerant reaches the evaporator. Furthermore, the system increases in complexity with additional flash tanks, control valves, injection ports and no doubt a management system. So, while multistage compression may be theoretically very efficient, in practice, achieving 300⁰C may need some support. The support may appear from the adsorption pair of a heat transformer utilising water and carbon. A thermal transformer requires pressure and instead of the 55 Bar to attain 300⁰C in a vapour compression heat pump (single or multistage), a compressor delivering 17 bar R718 vapor to a thermal transformer, could realise a reasonable system COP of about 2.7, without the complexity of multi-stage compression that may be challenging to implement. Therefore, this paper will illustrate the modelling and design aspects to achieve the higher COP multi-stage system, while also presenting a lower complexity hybrid heat pump system with a lower COP but with a potentially lower capital cost. Theoretically, it is possible to achieve higher temperatures with heat pumps. Water based systems can deliver 300⁰C with either thermal transformer systems and/or multi-stages (possibly with liquid injection). However, water and transcritical systems may reach that upper temperature limit due to material and PFAS constraints. Reversed Brayton Cycles may offer higher temperatures again but a review of materials (and compression lubrication if necessary) reveals a range of results that require further investigation.

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  • Original title: How high is high: what temperatures can we achieve with high temperature heat pumps?
  • Record ID : 30033173
  • Languages: English
  • Subject: Technology
  • Source: 2024 Purdue Conferences. 19th International Refrigeration and Air-Conditioning Conference at Purdue.
  • Publication date: 2024/07/17

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