An international research team has reviewed emerging solid-state and gas-cycle heat pumps capable of reaching temperatures of up to 1,600 K, assessing their challenges, applications, scalability, and technology readiness levels. The researchers say these systems could outperform conventional high-temperature heating technologies by achieving coefficients of performance above 1.
An international research group led by Slovenia’s University of Ljubljana has conducted a review of emerging ultra-high-temperature heat-pump technologies based on solids or gases.
All of those technologies, which have the potential to reach temperatures up to 1,600 K (1,326.85 C), were assessed in their current state. Diving into their challenges, the team described potential solutions, applications, scalability, and technology readiness level (TRL), and further presented a roadmap for future development.
“The main advantage of those technologies is their capabilities to operate at high temperatures, which not a lot of heat pumps can do,” said Katja Klinar, who co-authored the article, to pv magazine. “At around 1,200 K, conventional heat pump solutions are typically replaced by direct fuel combustion or electrical resistance heating, both of which effectively have a coefficient of performance (COP) of 1. In contrast, new technologies can achieve a COP greater than 1, making them inherently more energy-efficient. This has been demonstrated through numerical simulations.”
According to Klinar, “currently, there are no high-temperature solid-state and gas-cycle heat pumps in the market. Although room temperature solid-state and gas-cycle heat pumps are not on the market yet, they are at the prototype stage.” She highlighted that any of these new technologies that require electrical energy could get it from renewable sources, such as PV and wind.
“Our team was the first one to claim that solid-state electrocaloric heat pumps can be used for high temperatures – nobody described this idea before in scientific papers,” Klinar added. “We did some simulations, and now we will start planning the prototype. We are estimating the technology readiness level (TRL) of electrocaloric heat pumps at 2.5.”
The TRL measures the maturity of technology components for a system and is based on a scale from one to nine, with nine representing mature technologies for full commercial application.
Electrocaloric heat pumps are a subset of caloric heat pumps. These systems transfer heat by exploiting reversible thermal effects in solid materials when an external field is applied and removed. Depending on the field type, they are classified as electrocaloric (electric field), magnetocaloric (magnetic field), or mechanocaloric (mechanical stress). Thermoelectric heat pumps also belong to the solid-state category, using electrical current to pump heat across semiconductor junctions.
Unlike conventional heat pumps, which rely on vapor-compression cycles, solid-state systems use entropy changes in solid materials. “High-temperature solid-state heat pumping offers advantages because of its use of solid refrigerants, which eliminate leakage risks and have recycling potential,” the research team said. “Technologies such as magnetocaloric, electrocaloric, and thermoelectric (Peltier) heat pumps can operate without moving parts.”
The researchers also assessed gas-cycle heat pumps for high-temperature applications. These systems use a gas as the working fluid. The technologies evaluated include thermoacoustic heat pumps, which use sound waves to compress and expand gas and thereby generate hot and cold regions; mechanical Stirling heat pumps, which use pistons to compress and expand gas in a Stirling cycle; and reverse Brayton heat pumps, which use compressors to circulate gas and transfer heat.
Based on a qualitative assessment of device performance and technology maturity, the authors assigned technology readiness levels (TRLs) to each concept. The mechanocaloric heat pump received a TRL 2 rating, defined under EU guidelines as “technology concept formulated.” Magnetocaloric and electrocaloric systems were each rated at TRL 2.5, between TRL 2 and TRL 3. Thermoelectric and thermoacoustic systems achieved TRL 4, meaning “technology validated in a laboratory.” Mechanical Stirling and reverse Brayton heat pumps were assigned TRL 6, or “technology demonstrated in a relevant environment.”
The researchers also proposed a development roadmap through 2040. If implemented, they expect power density per unit mass to reach 15–20 W/kg for magnetocaloric systems, 15–20 W/kg for mechanocaloric, 100–150 W/kg for electrocaloric, 400–500 W/kg for thermoelectric, 200 W/kg for thermoacoustic, 300 W/kg for mechanical Stirling, and 150 W/kg for reverse Brayton heat pumps. These projections compare with current power densities of 3 W/kg, 1.5 W/kg, 30 W/kg, 300 W/kg, 75 W/kg, 60–100 W/kg, and 45 W/kg, respectively.
Under the same roadmap, projected second-law efficiencies by 2040 are 60% for magnetocaloric, mechanocaloric, and electrocaloric systems; 20% for thermoelectric; above 60% for thermoacoustic and mechanical Stirling; and above 40% for reverse Brayton heat pumps. Current efficiencies stand at 30%, 30%, 55%, 5–20%, 55%, 55%, and 33%, respectively.
In parallel, maximum device-level power output is expected to increase significantly. Present values of 15 kW (magnetocaloric), 1.5 kW (mechanocaloric), 0.01 kW (electrocaloric), 10 kW (thermoelectric), 500–1,000 kW (thermoacoustic), 500–1,000 kW (mechanical Stirling), and approximately 200 kW (reverse Brayton) are projected to rise to application ranges of 0.5–50 kW, 1–50 kW, 0.1–10 kW, 0.1–100 kW, 50–1,000 kW, 50–1,000 kW, and 100–1,000 kW, respectively, by 2040.
The research’s findings were presented in “Emerging opportunities for high-temperature solid-state and gas-cycle heat pumps,” published in Nature Energy. Researchers from Slovenia’s University of Ljubljana, China’s Chinese Academy of Sciences, Spain’s National Research Council (CSIC), the Netherlands’ University of Twente, Croatia’s University of Zagreb, and the United Kingdom’s University of Cambridge have participated in the study.
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