Real-world
application of the sub-zero Celsius elastocaloric cooling device. Credit: Nature (2026). DOI: 10.1038/s41586-025-09946-4
Researchers
at the School of Engineering of The Hong Kong University of Science and
Technology (HKUST) have developed the world's first sub-zero Celsius
elastocaloric freezing device, capable of reaching temperatures as low as -12℃. This represents a significant milestone in expanding
green solid-state elastocaloric refrigeration technology into the global
freezing industry, offering a promising solution to combat climate change and
accelerate low-carbon transformation of the global freezing market.
The findings have recently been published in Nature,
under the title "Sub-zero Celsius Elastocaloric Cooling via
Low-transition-temperature Alloys."
As global warming intensifies, the
demand for freezing has been growing rapidly and accounts for a significant
portion of global electricity consumption. Mainstream freezing based on vapor
compression cooling technology relies on refrigerants with high global warming
potential (GWP).
As an eco-friendly alternative,
solid-state cooling technology based on the elastocaloric effect of shape
memory alloys (SMAs) has drawn substantial attention from both academia and
industry due to its zero greenhouse gas emissions and high energy efficiency
potential. The technology harnesses the latent heat from cyclic phase
transition of shape memory alloys to provide cooling without greenhouse gas
refrigerants, offering a promising path to decarbonize the freezing sector of
the cooling industry and to mitigate global emissions and climate change.
Challenges and recent breakthrough in elastocaloric cooling
However, the existing elastocaloric
devices have been limited to air conditioning scenarios for room temperature
applications. It is important to expand the technology into the freezing
sector, which has the same market size as the air-conditioning sector.
A research team led by Prof. Sun
Qingping, Chair Professor from the Department of Mechanical and Aerospace
Engineering at HKUST, has achieved a breakthrough in Sub-zero Celsius
elastocaloric cooling. This advancement results from a synergistic combination
of materials, heat transfer fluid and refrigeration structures. The features include:
1.
Super-elastic alloy: employing a binary low-transition-temperature nickel-titanium (NiTi) alloy with a high nickel
content (51.2 at %) and lowering its austenite finish temperature (Af) to -20.8℃. This alloy maintains excellent super-elasticity and
a substantial latent heat even at -20℃, with a peak adiabatic temperature change of 16.3℃ at 0℃ and a functional temperature window of 48.5℃.
2.
Freezing-resistant
heat transfer fluid: using a 30wt% aqueous calcium chloride solution as the working fluid. Its
low freezing point ensures that it remains fluid in sub-zero operation, while
its good wettability on the NiTi surface enhances heat exchange efficiency.
3.
Cascaded tubular architecture: the regenerator operates on a
compression-based active Brayton cycle and consists of eight cascaded units,
each containing three thin-walled NiTi tubes. This design offers a high surface
area-to-volume ratio (8.68 mm-1) and withstands a compressive stress of 900MPa
without buckling, as verified by X-ray computed tomography.
Real-world freezing application of our
elastocaloric device. Credit: Nature (2026). DOI:
10.1038/s41586-025-09946-4
Operating at 1Hz, the desktop-scale
device achieved a cold-source temperature of -12 ℃ from a room-temperature heat sink (24℃), establishing a temperature lift of 36 ℃. This is the first reported sub-zero Celsius
performance in elastocaloric cooling.
In a real-world demonstration, the
system was integrated into a package measuring 1.0×0.5×0.5 m3 and tested outdoors at temperatures between 20 and 25℃. It successfully cooled an insulated chamber down to
a stable -4℃ air temperature within 60 minutes
and froze 20ml of distilled water into ice within 2 hours, validating its
real-world freezing capability.
The device demonstrated a specific cooling power of up to 1.43W g-1 under zero-temperature-lift
conditions. In addition, the system's coefficient of performance can reach 3.4
under the ideal work-recovery assumption, highlighting its potential energy
efficiency.
Potential impact on global emissions
The work has a significant impact
on global decarbonization to battle climate change. According to published
data, global Hydrofluorocarbon (HFC) emissions are projected to exceed 1.2
gigatons of CO2 equivalent annually by 2025, with roughly 27%
originating from sub-zero freezing applications. This translates to
approximately 330 million tons of CO2 equivalent each year.
The successful demonstration of
sub-zero elastocaloric cooling provides a viable, emission-free alternative for
these applications. Widespread adoption of this technology could, therefore,
potentially mitigate around 330 million tons of CO2 equivalent emissions annually, contributing substantially to global
climate goals.
Future directions and expert perspectives
The research team leader, Prof. Sun
Qingping stated, "This achievement demonstrates the potential for
large-scale application of elastocaloric freezing technology. We are
collaborating with industry to drive its commercialization. As global regulations
on HFCs tighten, this zero-emission, energy-efficient freezing technology is
poised to reshape the freezing sector of the refrigeration industry and provide
a key technical solution for carbon neutrality.
"Looking ahead, we will focus
on optimizing system efficiency, power density, and cost-effectiveness through
advances in shape memory alloy materials, manufacturing, heat exchange design,
and system integration and optimization to achieve larger cooling power and
high energy efficiency."
Prof. Lu Mengqian, Director of the HKUST Otto Poon Center for Climate Resilience and Sustainability, where Prof. Sun and Prof. Zhou are affiliated members, said, "This groundbreaking advancement in elastocaloric freezing technology by our center's members represents a significant step forward in our fight against climate change. By offering a zero-emission alternative for sub-zero applications, we are addressing the urgent need for sustainable freezing solutions. This achievement has been phenomenal since the center's establishment in July 2025. The work accelerates the center's mission to deliver impactful strategies for climate resilience and sustainable development worldwide."
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