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• HFO-1234ze(E) and R-515B are evaluated as low-GWP alternatives to HFC-134a. • An internal heat exchanger singularly benefits heating applications. • Heating production temperatures up to 90 °C are considered. • The alternative low-GWP refrigerants significantly reduces carbon emissions. • R-515B becomes a promising non-flammable alternative (A1) for heat pumps. This article provides a theoretical energy performance and carbon footprint analysis of the hydrofluoroolefin HFO-1234ze(E) and the mixture R-515B as low global warming potential (GWP) refrigerants to replace the hydrofluorocarbon HFC-134a in different heat pump applications. The reference configuration is a single-stage cycle with an internal heat exchanger (IHX). In this way, the impact of the IHX on medium and moderately high temperature heating applications has been investigated. HFO-1234ze(E) and R-515B result in approx. 25% lower heating capacity than HFC-134a due to a diminution of the latent heat of vaporisation and suction density. The heating capacity difference between HFO-1234ze(E) and R-515B remains below 2%. The heating coefficient of performance (COP) of the alternative low GWP refrigerants is comparable to that of the HFC-134a at the proposed conditions. The environmental analysis illustrates that HFO-1234ze(E) and R-515B can reduce down to 18% and 15%, respectively, the equivalent CO 2 emissions compared to HFC-134a in low-temperature space heating applications. In addition, this reduction arrive at 78% when compared to a natural gas boiler, considered as the conventional heating technology in moderately high temperature applications (domestic hot water, industrial processes, and radiators). Although HFO-1234ze(E) and R-515B present comparable energy and environmental performance, the latter refrigerant has an advantage in regard to installation safety requirements as a non-flammable refrigerant (A1). [ABSTRACT FROM AUTHOR]

• Ejector and parallel compression configurations are proposed for high temperature heat pumps. • Internal heat exchanger analysis is included in the proposed configurations. • Alternative sustainable refrigerant assessment is performed to replace HFC-245fa. • Multi-objective analysis evaluates the compromise between volumetric heating capacity and COP. • Advanced high temperature heat pump configurations are promoted. Climate change evolution urges us to take action to reduce greenhouse gas emissions. As one of the main contributors, the industrial sector requires cleaner methods of heat production, such as high temperature heat pumps (HTHP) with the highest energy efficiency. Facing this challenge, this paper provides a performance comparison of the ejector and economizer with parallel compression configurations in HTHPs for low-grade waste heat recovery. A single-stage cycle has been used as a reference configuration to compare the proposed options. The internal heat exchanger (IHX) is included in all the configurations to extend the analysis and illustrate the influence of this component in high temperature applications. Moreover, alternative low-GWP refrigerants have been analyzed in order to replace the reference working fluid HFC-245fa. Ejector and economizer with parallel compression configurations with IHX provide a coefficient of performance (COP) improvement of up to 36% and 72.5%, respectively, using HFC-245fa for a heating temperature production of 140 °C. Moreover, the volumetric heating capacity (VHC) increases around 36% for the ejector cycle and 80% for the economizer with parallel compression cycle with IHX. HC-601, HFO-1336mzz(Z), and R-514 have the most significant COP improvement compared to HFC-245fa, along with the highest VHC drop. Multi-objective evaluation illustrates that single-stage and ejector configurations with IHX have similar results, whereas the economizer with parallel compression cycle with IHX presents a significant COP and VHC improvement. HCFO-1233zd(E) and HCFO-1224yd(Z) presents the proper trade-off between COP and VHC. [ABSTRACT FROM AUTHOR]

• Existing regulations do not limit GWP of refrigerants used for ULT applications. • Very low operating temperatures limit the possible low GWP candidates. • Cascade and auto-cascade refrigeration systems seem promising with ejector. • Compared to other hydrocarbons, flammability of R170 and R1150 is not widely studied. • R1132a is a promising candidate but requires further study. Several environmental protection policies have been enforced restricting working fluids with high global warming potential (GWP) values used in many types of refrigeration and heat pump systems. However, ultralow-temperature (ULT) refrigeration has not been included, which commonly uses refrigerants with very high GWP values (such as R23 and R508B). Therefore, publicly available research programs seeking low GWP alternative refrigerants do not cover this application and the transition to more environmentally friendly fluids is slowed down. This work presents a comprehensive review that summarizes and discusses the available studies about ULT refrigeration systems. The current status of the technology, system architectures and refrigerants are analyzed. Moreover, the transition towards low GWP refrigerants is proposed, presenting the most promising low GWP alternatives. The most commonly used architectures for ULT refrigeration are the two-stage cascade and auto-cascade, in which the use of ejector has recently been considered in research papers. R170 and R1150 are the available natural refrigerants suitable for ULT, but they have not yet been included in many flammability and risk assessment studies. The A2 hydrofluoroolefin R1132a has been recently proposed as a blend component to avoid problems of stability. However, more information is still necessary to start with simulation and experimental studies. R41 could be an alternative due to its low GWP and suitable normal boiling point, but it has not been thoroughly investigated yet. Overall, there is a gap in the literature in terms of developing alternative refrigerants for ULT refrigeration. This study aims at shedding light on this gap to direct future research in this field towards reliable, environmentally friendly and marketable alternative refrigerants. [ABSTRACT FROM AUTHOR]

Abstract High-temperature heat pumps (HTHPs) based on vapour compression can be used for industrial low-grade waste heat valorisation, which can aid in mitigating climate change. Currently, the performance of HTHPs operating at high-temperatures lifts is limited; therefore, advanced configurations become an opportunity for their utilisation. This paper presents an HTHP cascade with configurations of internal heat exchangers (IHXs) that uses low GWP refrigerants in both high-stage (HS) (HCFO-1233zd(E), HFO-1336mzz(Z), HCFO-1224yd(Z), and pentane) and low-stage (LS) (HFO-1234yf, HFO-1234ze(E), butane, isobutane, and propane) cycles. Prior to the analysis and presentation of results, an optimisation of the operating conditions is performed based on intermediate temperature and IHX effectiveness in both stage cycles. Results indicate that butane and isobutane appear to be the most convenient working LS fluids from the point of view of coefficient of performance (COP). The highest system performance is obtained using pentane and HFO-1336mzz(Z) in the HS cycle. Compared to third-generation refrigerants (HFC-245fa/HFC-134a), a slight COP improvement is obtained using HCFO-1233zd(E), and HCFO-1224yd(Z). A comparable or even lower volumetric flow rate at the HS compression suction is also observed. The use of pentane/butane achieved maximum COP (3.15), which is a 13% improvement compared to COP obtained when HFC-245fa/HFC-134a is employed. Highlights • A high-temperature heat pump cascade with the IHXs is proposed and optimised. • Synthetic and natural refrigerants with low GWPs are proposed for both stages. • The higher effectiveness of both IHXs in similar proportions increases total COP. • Pentane/butane presents maximum COP and minimum HCFO-1224yd(Z)/HFO-1234yf. • A few refrigerant pairs are comparable with HFC-245fa/HFC-134a in the stages of V ˙ s u c . [ABSTRACT FROM AUTHOR]