Abstract This study reviews the current state of the art and the current research activities of high temperature heat pumps (HTHPs) with heat sink temperatures in the range of 90 to 160 °C. The focus is on the analysis of the heat pump cycles and the suitable refrigerants. More than 20 HTHPs from 13 manufacturers have been identified on the market that are able to provide heat sink temperatures of at least 90 °C. Large application potentials have been recognized particularly in the food, paper, metal and chemical industries. The heating capacities range from about 20 kW to 20 MW. Most cycles are single-stage and differ primarily in the refrigerant (e.g. R245fa, R717, R744, R134a or R1234ze(E)) and compressor type used. The COPs range from 2.4 to 5.8 at a temperature lift of 95 to 40 K. Several research projects push the limits of the achievable COPs and heat sink temperatures to higher levels. COPs of about 5.7 to 6.5 (at 30 K lift) and 2.2 and 2.8 (70 K) are achieved at a sink temperature of 120 °C. The refrigerants investigated are mainly R1336mzz(Z), R718, R245fa, R1234ze(Z), R600, and R601. R1336mzz(Z) enables to achieve exceptionally high heat sink temperatures of up to 160 °C.

High Temperature Heat Pumps: Market Overview, Stateof the Art, Research Status, Refrigerants, and Application Potentials

E-fuels promise to replace fossil fuels with renewable electricity without the demand-side transformations required for a direct electrification. However, e-fuels’ versatility is counterbalanced by their fragile climate effectiveness, high costs and uncertain availability. E-fuel mitigation costs are €800–1,200 per tCO2. Large-scale deployment could reduce costs to €20–270 per tCO2 until 2050, yet it is unlikely that e-fuels will become cheap and abundant early enough. Neglecting demand-side transformations threatens to lock in a fossil-fuel dependency if e-fuels fall short of expectations. Sensible climate policy supports e-fuel deployment while hedging against the risk of their unavailability at large scale. Policies should be guided by a ‘merit order of end uses’ that prioritizes hydrogen and e-fuels for sectors that are inaccessible to direct electrification. E-fuels—hydrocarbon fuels synthesized from green hydrogen—can replace fossil fuels. This Perspective highlights the opportunities and risks of e-fuels, and concludes that hydrogen and e-fuels should be prioritized for sectors inaccessible to direct electrification.

Potential and risks of hydrogen-based e-fuels in climate change mitigation

Direct air capture (DAC) can provide an impactful, engineered approach to combat climate change by removing carbon dioxide (CO2) from the air. However, to meet climate goals, DAC needs to be scaled at a rapid rate. Current DAC approaches use engineered contactors filled with chemicals to repeatedly capture CO2 from the air and release high purity CO2 that can be stored or otherwise used. This review article focuses on two distinctive, commercial DAC processes to bind with CO2: solid sorbents and liquid solvents. We discuss the properties of solvents and sorbents, including mass transfer, heat transfer and chemical kinetics, as well as how these properties influence the design and cost of the DAC process. Further, we provide a novel overview of the considerations for deploying these DAC technologies, including concepts for learning-by-doing that may drive down costs and material requirements for scaling up DAC technologies.

A review of direct air capture (DAC): scaling up commercial technologies and innovating for the future

Addressing the growing concerns of climate change necessitates the decarbonisation of energy sectors globally. Heating is the largest energy end-use, accounting for almost half of total energy consumption in most countries. This paper presents an extensive review of previous works on several aspects of heat pumps, including their role in the decarbonisation of the heating sector. In addition, themes related to recent technological advances of heat pumps, as well as, their roles in terms of adding flexibility to renewable-rich systems and carbon abatement are examined. Challenges and barriers facing large-scale deployment of heat pumps are identified. Generally, as the share of renewables in the energy mix increases, heat pumps can play a role in addressing a multitude of problems induced by climate change. The potential of heat pumps to abate emissions, however, is highly dependent on the type of technology, location and electricity mix. Heat pumps can be a source of flexibility in the power system and can upgrade waste heat to provide low-cost heating in district heating networks. They are environment friendly and provide a viable pathway for decarbonising the heating sector. However, economic, regulatory, structural and infrastructural barriers exist, which may hinder heat pump integration rate.

Heat pumps and our low-carbon future: A comprehensive review

Experimental investigation of modern ORC working fluids R1224yd(Z) and R1233zd(E) as replacements for R245fa

https://docs.lib.purdue.edu/cgi/viewcontent.cgi?article=2875&context=iracc

High temperature heat pumps: Market overview, state of the art, research status, refrigerants, and application potentials

Standardized efficiency and capacity measurements of residential heat pumps deviate from real time performance due to several factors. While standardized measurements are performed at steady state operation, heat pumps in the field are usually capacity controlled using on-off cycling. In order to determine the difference between these operating modes, laboratory and field measurements were conducted to verify transient models of air-source and geothermal heat pumps. Heat pump performance was then simulated for several operating conditions typical for central European installations. It was shown that air-source heat pumps experience performance losses of only 1–2% for short cycling times. In the case of geothermal heat pumps, the recovery of the ground probe during the off time can lead to efficiency gains of up to 5%. Results of this study help to provide improved control strategies for heat pumps using on-off cycling as capacity control. They also support the conclusion that standardized measurements according to EN14511 accurately represent the efficiency of residential heat pumps both in continuous and in part load operation.

Theoretical and experimental investigation of startup and shutdown behavior of residential heat pumps

High temperature heat pumps (HTHP) with heat sink temperatures of 100 to 160 °C will increasingly become commercialized in the coming years, especially for industrial drying, sterilization and evaporation processes. In particular, the HFO R1336mzz(Z) and the HCFOs R1233zd(E) & R1224yd(Z) are low GWP (< 10) replacement refrigerants for R245fa and R365mfc. This study examines the experimental performance of R1336mzz(Z) and R1233zd(E) in a laboratory HTHP with 5 to 10 kW heating capacity. The developed heat pump is single-stage, operates with a variable speed piston compressor, and contains a continuously adjustable internal heat exchanger (IHX) for superheating control. The performance data were measured at 30, 50, and 70 K temperature lift (40 to 80°C heat source, 80 to 150°C heat sink). At W60/W110 the experimental results with R1233zd(E) showed a COP of 2.8 (basic cycle) and 3.1 with IHX integration (+15%). For R1336mzz(Z) a COP of 2.4 and 3.0 was reached (+24%). At higher temperatures tested, the deviations between the measured COP with R1336mzz(Z) and R1233zd(E) were within the measurement uncertainty. Moreover, R1336mzz(Z) achieves potentially higher condensing temperatures due to its higher critical temperature. By increasing the heat sink temperature glide from 5 to 30 K, a further 15% COP increase was achieved.

High temperature heat pump using HFO and HCFO refrigerants – system design and experimental results

Aim of this project is the development of a new heat pump system with economizing that is able to improve the heating performance using two or more heat sources at different temperature levels. These heat sources are incorporated into the system with minimal loss of exergy by adding the heat at different pressure levels. Practical applications for such systems are for example buildings with a heat pump and a solar thermal collector or buildings with waste heat recovery. While solar thermal systems can be used for heating and domestic hot water in summer, they fail to produce sufficient temperatures for direct use in fall, spring and winter. By connecting the solar collectors to the here described heat pump system, they are able to improve the heat pump efficiency significantly. Furthermore, the lower temperatures of the collectors help to improve their effectiveness and raise the usable solar gains. While existing systems using two heat sources are either very costly, inefficient or need large amounts of waste heat, the proposed cycle can use varying amounts of waste heat and increases the heat pump efficiency by up to 30% over a wide range of operating conditions. The paper presents an introduction to the topic and various heat pump system designs using two heat sources. All of them incorporate the OptiRef. Simulation results and experimental analysis show that depending on the amount of waste heat and the temperature level of the heat pump cycle, efficiency and heating capacity improvements of 20-30% could be shown in a laboratory prototype. Oil management and control of the system are the main challenges when implementing it in the laboratory or field. Altogether, the functionality of the system could be proven, but there is still room for improvement with respect to the control concept and in order to reduce cost.

/pdf/heat-pump-with-two-heat-sources-on-different-temperature-3b5jrvj9yf.pdf

Michael Uhlmann

Papers

High temperature heat pumps: Market overview, state of the art, research status, refrigerants, and application potentials

High Temperature Heat Pumps: Market Overview, Stateof the Art, Research Status, Refrigerants, and Application Potentials

Theoretical and experimental investigation of startup and shutdown behavior of residential heat pumps

High temperature heat pump using HFO and HCFO refrigerants – system design and experimental results

Heat Pump with Two Heat Sources on Different Temperature Levels