How close are fridges, heat pumps and air conditioners that avoid high-global-warming-potential refrigerant gases?

The road so far

4 of 7 steps done, 1 in progress.

1. Done · 2016Phase-down of HFCs agreed

   The Kigali Amendment to the Montreal Protocol was adopted on 15 October 2016 and entered into force on 1 January 2019. It covers HFCs. UNEP says developed countries will start reducing HFCs as early as 2019, developing countries follow a later schedule, and all countries will cut HFCs by more than 80 per cent over the next 30 years. [1]

2. Done · 4 Dec 2024Micro-scale thermoelectric coolers with high temperature difference

   A research highlight in National Science Review describes micro-scale thermoelectric coolers from work by Zhuang et al. It reports a peak temperature difference of up to about 89.3 K and a COP of about 6.6, and says these surpass conventional Bi2Te3 devices, without giving a numerical baseline. The highlight does not give cooling power or the operating conditions for the COP. [4]

3. Done · 2018Passive radiative cooling below ambient in outdoor tests

   A 2018 Nature Communications study reported a directional passive radiative cooler with a minimum temperature 6 °C below ambient and a maximum cooling power of 45 W/m², measured around solar noon with a polished aluminium disk and an infrared-transparent cover. It is a test device, not a refrigerator or air conditioner. [5]

4. In progress · Oct 2025Magnetocaloric prototype near room temperature

   A 2025 Case Studies in Thermal Engineering paper describes a reciprocating magnetic refrigerator with a linearly moving Halbach magnet array and gadolinium. It reached a temperature span of 2.197 K with no load, and a maximum cooling power of 7.90 W when the span was close to 0 K. Its best COP was 1.32 at moderate cooling. The useful span and cooling power have not been shown together in the sources reviewed. [3]

5. Done · Mar 2025Kilowatt-scale elastocaloric device

   Zhou et al. report a multi-cell elastocaloric device made of tubular nickel-titanium alloy, with a specific cooling power of 12.3 W/g at 3.5 Hz and a fluid-side cooling power of 1,284 W at zero temperature lift during the first 500,000 cycles. The authors say existing devices are limited to about 300 W or less. The 1,284 W figure is a fluid-side value at zero lift, not a full-system rating. [2]

6. Not startedFull-system efficiency and lifetime under appliance conditions

   The sources reviewed give COP values only at specific test conditions. They do not report full-system COP across a range of operating conditions, nor fatigue or lifetime testing of a complete appliance-scale elastocaloric or magnetocaloric unit. [2][3]

7. Not startedCommercial solid-state appliance on sale

   No commercial fridge, heat pump or air conditioner using a solid-state method with verified cooling figures was found in the sources opened for this page. Any company claim would need to be labelled as a claim and checked against independent test data.

Why it matters

Cooling equipment that uses HFCs adds to warming, and the Kigali Amendment sets a schedule for cutting those gases. UNEP says the phase-down is expected to avoid up to 0.5 °C of global warming by the end of the century. Solid-state or passive cooling could remove the need for these gases, but only if it reaches the efficiency, cost and reliability that buyers expect.

What stands in the way

• Fatigue of shape-memory alloys under repeated loading has not been shown over an appliance lifetime in the sources reviewed.
• The magnetocaloric prototype reached a span of 2.197 K with no load and 7.90 W of cooling power at a span close to 0 K. A useful span and useful cooling power have not been reported together.
• Thermoelectric results so far are at micro scale. Coefficient of performance at appliance scale is not reported in the sources reviewed.
• Passive radiative cooling depends on the sky and the surface, and the 2018 result was measured around solar noon under set conditions.
• Elastocaloric cooling powers are reported at zero temperature lift, so a full-system rating for a building or appliance is still needed.

Signals to watch

• A full-system COP and lifetime test for an elastocaloric or magnetocaloric appliance, published with its operating conditions.
• A magnetocaloric prototype that reports a useful temperature span and cooling power in the same test.
• A peer-reviewed thermoelectric result at larger scale, with stated COP and temperature span.
• Independent test data for any commercial solid-state product, not only company claims.
• Changes to the HFC phase-down schedule published by the UNEP Ozone Secretariat.

Sources

1. government report
   Kigali Amendment to the Montreal Protocol
   UNEP Ozone Secretariat, 2016.
2. peer reviewed paper
   Achieving kilowatt-scale elastocaloric cooling by a multi-cell architecture
   Guoan Zhou, Lingyun Zhang, Zexi Li et al.. Nature, Mar 2025.
3. peer reviewed paper
   Experimental investigation and performance analysis of a reciprocating magnetic refrigerator
   H. Aras, A. Zaim. Case Studies in Thermal Engineering, Oct 2025.
4. other
   Cooler, stronger, smaller: improving thermoelectric cooling
   Nagendra Singh Chauhan, Takao Mori. National Science Review, 4 Dec 2024.
5. peer reviewed paper
   Passive Directional Sub-Ambient Daytime Radiative Cooling
   Bikramjit S. Bhatia, Arny Leroy, Yichen Shen et al.. Nature Communications, 27 Nov 2018.

Last reviewed 22 Oct 2026.