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. The device can reach temperatures -12 ℃, representing a critical milestone for expanding the green solid-state elastocaloric refrigeration technology into the global freezing industry. This breakthrough has been published in Nature, offering a promising solution to combat climate change and accelerate low-carbon transformation of the global freezing market.
As global warming intensifies, the global 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 cooling industry and to mitigate global emissions and climate change. However, the existing elastocaloric devices have been limited to air conditioning scenario for room temperature applications, it is important to expand the technology into the freezing sectors which has the same market size as the air-conditioning sector.
The team’s breakthrough lies in a synergistic combination of materials, heat transfer fluid and refrigeration structures: (1) The researchers employed a binary low-transition-temperature nickel-titanium (NiTi) alloy with a high nickel content (51.2 at%), lowering its austenite finish temperature (Af) to -20.8 ℃. This alloy maintains excellent superelasticity 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) A freezing-resistant heat-transfer fluid of 30 wt% aqueous calcium chloride solution was used as the working fluid. Its low freezing point ensures flowability during sub-zero operation, while its good wettability on the NiTi surface enhances heat exchange efficiency. (3) Cascaded tubular architecture of the regenerator operates on a compression-based active Brayton cycle. The regenerator comprises 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 900 MPa without buckling, as confirmed by X-ray computed tomography.
Operating at 1 Hz, 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 1.0×0.5×0.5 m3 package and tested outdoors (20–25 ℃). It successfully cooled an insulated chamber down to a stable -4 ℃ air temperature within 60 minutes and froze 20 ml of distilled water into ice within 2 hours, validating its real-world freezing capability. The device demonstrated a specific cooling power of up to 1.43 W g-1 under zero-temperature-lift conditions. The system coefficient of performance can reach 3.4 under the ideal work-recovery assumption, indicating its potential energy efficiency.
The work has significant Impact on global decarbonization to battle the climate change. According to published data, global 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.
The research team leader Prof. SUN Qingping, Chair Professor of the Department of Mechanical and Aerospace Engineering at HKUST and member of Otto Poon Center for Climate Resilience and Sustainability, said, “This achievement demonstrates the potential for large-scale application of elastocaloric freezing technology. We are working with industry in parallel to drive its commercialization. As global regulations on hydrofluorocarbons (HFCs) tighten, this zero-emission, energy-efficient freezing technology is poised to reshape the freezing sector of 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 Otto Poon Center for Climate Resilience and Sustainability, HKUST, expressed, “This sub-zero elastocaloric ice-making technology is one of the flagship outcomes of the Otto Poon Center’s ‘Deep Decarbonization’ research theme. By integrating materials science, thermal engineering and industrial policy, the Center has rapidly translated fundamental discoveries on shape-memory alloys into a commercial-ready green-cold-chain device, positioning Hong Kong and the Greater Bay Area at the forefront of the global race to decarbonize refrigeration”.
This research was supported by the Strategic Topics Grant (STG) and General Research Fund (GRF) of the Hong Kong Research Grants Council, Innovation and Technology Commission (ITC) projects, Shenzhen-Hong Kong collaborative research project and the HKUST Otto Poon Center for Climate Resilience and Sustainability.