Researchers at the School of Engineering at the **Hong Kong University of Science and Technology (HKUST)** have introduced the world’s first **Sub-Zero Celsius elastocaloric freezing device**, capable of achieving temperatures as low as **-12°C**. This groundbreaking innovation marks a significant advancement in the realm of green refrigeration technology and offers a sustainable alternative to traditional freezing methods that contribute to climate change.
The findings of this research were published in the prestigious journal **Nature** under the title “Sub-zero Celsius Elastocaloric Cooling via Low-transition-temperature Alloys.” As global temperatures rise, the demand for effective freezing solutions has surged, accounting for a substantial portion of global electricity consumption. Traditional freezing methods predominantly utilize vapor compression cooling technologies, which depend on refrigerants with high global warming potential, exacerbating environmental issues.
In contrast to these conventional methods, the newly developed elastocaloric technology harnesses the unique properties of **shape memory alloys (SMAs)**. This solid-state cooling approach operates without greenhouse gas emissions, presenting a viable pathway for decarbonizing the freezing sector and addressing global emissions challenges.
Innovative Features and Performance
The research team, led by **Prof. SUN Qingping**, Chair Professor in the Department of Mechanical and Aerospace Engineering at HKUST, achieved a breakthrough by integrating advanced materials and innovative designs. The device includes several notable features:
1. **Super-elastic alloy**: It employs a binary low-transition-temperature **nickel-titanium (NiTi)** alloy containing **51.2%** nickel. The alloy’s austenite finish temperature is lowered to **-20.8°C**, ensuring excellent super-elasticity and significant latent heat retention even at low temperatures.
2. **Freezing-resistant heat transfer fluid**: A **30wt% aqueous calcium chloride solution** serves as the working fluid, maintaining its liquid state in sub-zero conditions. Its effective wettability on the NiTi surface enhances heat exchange efficiency.
3. **Cascaded tubular architecture**: The device operates on a compression-based active Brayton cycle, featuring eight cascaded units with three thin-walled NiTi tubes each. This configuration maximizes the surface area-to-volume ratio, enabling the device to withstand compressive stress of **900 MPa** without structural failure.
The prototype demonstrated impressive performance by achieving a cold-source temperature of **-12°C** from a standard room temperature heat sink of **24°C**, establishing a temperature lift of **36°C**.
In practical tests, the unit was integrated into a compact package measuring **1.0×0.5×0.5 m³** and effectively cooled an insulated chamber to **-4°C** within **60 minutes**. It also froze **20 ml** of distilled water into ice in just **two hours**.
Environmental Impact and Future Directions
The significance of this advancement extends beyond technological innovation. Current projections indicate that global emissions of **hydrofluorocarbons (HFCs)** could surpass **1.2 gigatons of CO2 equivalent** annually by **2025**, with approximately **27%** of these emissions originating from sub-zero freezing applications. The adoption of this new elastocaloric cooling technology could potentially mitigate around **330 million tons** of CO2 equivalent emissions each year, contributing significantly to worldwide climate goals.
Prof. Sun emphasized the potential for large-scale application of this technology, stating, “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.”
**Prof. LU Mengqian**, Director of the HKUST **Otto Poon Center for Climate Resilience and Sustainability**, echoed these sentiments, highlighting the importance of sustainable freezing solutions in the ongoing fight against climate change. He noted, “This groundbreaking advancement in elastocaloric freezing technology represents a significant step forward in our efforts.”
The research received support from the **Strategic Topics Grant** and the **General Research Fund** of the **Hong Kong Research Grants Council**, as well as the **Innovation and Technology Commission**. This marks the second time within a year that the research team has had their work published in Nature, with a previous article focusing on achieving kilowatt-scale elastocaloric cooling through a multi-cell architecture.
Looking ahead, the research team plans to enhance system efficiency, power density, and cost-effectiveness through continued advancements in shape memory alloy materials and system integration. This work not only showcases the potential of elastocaloric technology but also aligns with global initiatives aimed at achieving carbon neutrality and sustainable development.
