Sub-Zero Green Freezer Reaches −12°C Without Emissions in Global First

Researchers at the School of Engineering of The Hong Kong University of Science and Technology have developed the world’s first elastocaloric freezing device capable of operating below zero degrees Celsius, reaching temperatures as low as −12°C without using greenhouse gas refrigerants.

The achievement marks a major advance for solid-state cooling technologies and extends elastocaloric refrigeration beyond room-temperature air conditioning into the global freezing market. The findings were published in Nature under the title “Sub-zero Celsius Elastocaloric Cooling via Low-transition-temperature Alloys.”

 

Why Freezing Matters for Climate Targets

 

Demand for freezing and cold storage has risen sharply alongside global warming, population growth, and food supply chain expansion. Conventional freezing systems rely on vapor compression technology that uses hydrofluorocarbon refrigerants with high global warming potential. These systems account for a significant share of global electricity use and emissions.

Elastocaloric cooling offers an alternative pathway. Instead of circulating refrigerant gases, it relies on the latent heat released during stress-induced phase transitions in shape memory alloys. This approach eliminates direct greenhouse gas emissions while offering the potential for high energy efficiency.

Until now, elastocaloric systems had been limited to temperatures suitable for air conditioning, preventing their use in freezing applications where emissions are most difficult to abate.

 

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Engineering Innovations Enable Sub-Zero Performance

 

The HKUST research team, led by Professor Sun Qingping from the Department of Mechanical and Aerospace Engineering, achieved sub-zero performance by combining advances in materials science, heat transfer, and system architecture.

At the core of the device is a low-transition-temperature nickel-titanium alloy engineered to remain super-elastic at temperatures well below freezing. The alloy maintains strong latent heat performance even at −20°C, enabling large temperature changes during operation.

The system also uses a freezing-resistant heat transfer fluid based on an aqueous calcium chloride solution, ensuring reliable heat exchange during sub-zero operation. A cascaded tubular regenerator design, operating on a compression-based active Brayton cycle, allows the system to withstand extreme mechanical stress while maximizing surface area for efficient heat transfer.

Together, these elements enable a functional temperature lift of 36°C, cooling from a room-temperature heat sink of 24°C down to −12°C. This is the first documented demonstration of elastocaloric cooling below zero degrees Celsius.

 

Real-World Freezing Demonstration

 

Beyond laboratory testing, the researchers validated the technology under outdoor conditions. The elastocaloric system was integrated into a compact enclosure and tested in ambient temperatures between 20°C and 25°C.

In this setup, the device cooled an insulated chamber to a stable air temperature of −4°C within one hour and successfully froze 20 milliliters of water into ice within two hours. These results confirm that the system can perform real-world freezing tasks rather than only controlled laboratory cooling.

Performance metrics further underline its potential. The device achieved a specific cooling power of up to 1.43 watts per gram under zero temperature lift conditions. Under ideal work-recovery assumptions, its coefficient of performance can reach 3.4, suggesting competitive efficiency compared with conventional freezing technologies.

 

Implications for Global Emissions Reduction

 

The climate implications are significant. Global hydrofluorocarbon emissions are projected to exceed 1.2 gigatons of CO2 equivalent annually by the middle of this decade. Roughly 27 percent of those emissions originate from sub-zero freezing applications, equivalent to approximately 330 million tons of CO2 per year.

By eliminating refrigerants altogether, elastocaloric freezing could offer a direct, zero-emission substitute for these applications. If deployed at scale, the technology could materially reduce emissions from cold storage, food logistics, medical supply chains, and industrial freezing.

 

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Path Toward Commercialization

 

Professor Sun said the results demonstrate the feasibility of elastocaloric freezing for large-scale applications and confirmed that the team is already working with industry partners to accelerate commercialization. As global regulations tighten around hydrofluorocarbons, interest in alternative freezing technologies is expected to grow rapidly.

Future research will focus on improving system efficiency, increasing power density, and reducing costs. Areas of development include advanced shape memory alloy manufacturing, improved heat exchanger design, and system-level optimization.

 

Strategic Importance for Climate Resilience

 

Professor Lu Mengqian, Director of the HKUST Otto Poon Center for Climate Resilience and Sustainability, described the achievement as a major step toward sustainable cooling solutions. He noted that sub-zero applications have long lacked viable low-emission alternatives and that this work directly addresses one of the most challenging segments of the cooling industry.

As demand for freezing continues to rise globally, the ability to decarbonize this sector without sacrificing performance could play a meaningful role in achieving long-term climate and energy goals.