Numerical and experimental study of subcritical and transcritical high-temperature heat pump systems for industrial applications with low-GWP refrigerants

Abstract

The industrial sector is a major contributor to global greenhouse gas (GHG) emissions, necessitating prompt actions to reduce its environmental footprint. An effective approach involves substituting fossil-fuel gas boilers with high-temperature heat pump (HTHP) technology. This transition aligns with the wider trend towards sustainable energy conversion methods while significantly reducing GHG emissions. Consequently, the process of decarbonizing the industrial sector through the substitution of fossil-fuel gas boilers with electric technology in industrial processes would make a substantial contribution to ecologically safeguarding the planet. Nonetheless, the current utilisation of this technology is constrained by specific operational limitations. To facilitate its broader adoption within the industrial sector, it is imperative to address technical challenges and optimize system performance. It is worth emphasising that the selection of low global warming potential (GWP) refrigerants that adhere to F-gas regulations is fundamental to the operation and future evolution of HTHPs used in industrial processes and waste heat recovery (WHR). The present study embarks on a comprehensive evaluation of both subcritical HTHP (SC-HTHP) and transcritical HTHP (TC-HTHP) cycle techniques, exploring various system configurations and assessing a range of low-GWP refrigerants for their suitability in replacing conventional refrigerants such as HFC-245fa and HFC-365mfc. By employing a steady-state thermodynamic model, the research conducts simulations to analyse the energetic and exergetic balances within the system, as well as to assess the performance characteristics of diverse subcritical and transcritical cycle configurations.

The SC-HTHP system is designed with specific temperature settings for the heat source and heat sink, leading to temperature lifts of 30 and 70 K. Theoretical analysis focuses on energetic and exergetic efficiencies, as well as optimizing performance through superheat regulation. HCFO-1233zd(E) and HFO-1336mzz(Z) are identified as promising replacements for certain refrigerants, and the minimum superheat mapping shows optimal performance conditions. Different system configurations are compared, and empirical data aligns with simulated coefficient of performance (COP) values. This research provides a systematic approach to optimize refrigerant selection in high-temperature heat pumps. The study also explores TC-HTHP technologies, aiming to achieve a 200 °C heat sink temperature using low-GWP refrigerants. Three TC-HTHP cycle configurations are assessed, with the basic cycle and dual-IHX configuration proving the most efficient. HFO-514A and HFO-1234ze(Z) are promising refrigerant choices. A pinch-point analysis emphasizes the importance of ix optimizing the gas cooler. Environmental impact assessment indicates significant reductions in Total Equivalent Warming Impact (TEWI) compared to HFC-245fa, paving the way for eco-friendly refrigerant options in TC-HTHPs.

In experimental investigations of the SC-HTHP system, a temperature range is examined, and the importance of effective control of the primary circuit and managing the secondary sink loop is highlighted. The use of off-the-shelf components and their certifications imposes limitations. The study identifies the pivotal role of accurately mapping the minimum superheat and the primary sources of exergy destruction within the system. According to the findings from the test-bed, it was evident that accurately mapping the minimum superheat is a pivotal factor in selecting the appropriate IHX component to maintain the discharge state within a manageable range. Notably, at heat source and sink temperatures of 60 °C and 120 °C, respectively, the experimental COP was measured at 2.52. Furthermore, the analysis revealed that the compressor was the primary source of exergy destruction, with the expansion valve following as the next significant contributor to exergy losses.

Date of Award	May 2024
Original language	English
Sponsors	Department for the Economy
Supervisor	Neil Hewitt (Supervisor) & Ming Jun Huang (Supervisor)