HKUST Develops Sub-Zero Elastocaloric Cooling Device
Researchers in Hong Kong have developed the world’s first sub-zero Celsius elastocaloric freezing device, potentially offering an eco-friendly alternative to traditional cooling systems.
A team at the Hong Kong University of Science and Technology (HKUST) School of Engineering has achieved a milestone by developing an elastocaloric device capable of reaching temperatures as low as -12℃. Solid-state cooling technology, based on the elastocaloric effect of shape memory alloys (SMAs), is considered a scope alternative to vapour compression technology, due to its zero greenhouse gas emissions and high energy efficiency scope.
The technology harnesses the latent heat from the cyclic phase transition of shape memory alloys to provide cooling without refrigerants. 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℃. The researchers tested their desktop elastocaloric device outdoors at temperatures between 20℃ and 25℃. They report that 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.
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 ideal work-recovery assumptions. The device employs a binary low-transition-temperature nickel-titanium (NiTi) alloy with a high nickel content, lowering its austenite finish temperature to -20.8℃. This alloy is said to maintain 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℃. The heat transfer fluid is a 30wt% aqueous calcium chloride solution with a low freezing point that ensures it remains fluid in sub-zero operation. 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.
“This achievement demonstrates the scope for large-scale application of elastocaloric freezing technology. We are collaborating with industry to drive its commercialisation,” said the research team leader Prof Sun Qingping.
Researchers are also claiming to have developed an elastocaloric cooling device capable of achieving a record-breaking temperature lift of 75K, according to a related report.
Looking ahead, Prof Sun Qingping said his team will focus on optimising system efficiency, power density, and cost-effectiveness through advances in shape memory alloy materials, manufacturing, heat exchange design, and system integration and optimisation to achieve larger cooling power and high energy efficiency.
