Freezing items may seem straightforward, but doing so without the century-old technology that has been the backbone of refrigeration presents significant challenges.
Current freezers utilize chemical refrigerants that cycle through evaporation and condensation. Many of these substances, particularly hydrofluorocarbons (HFCs), are potent greenhouse gases. When released, they can trap heat much more effectively than carbon dioxide.
By 2025, global emissions from HFCs are anticipated to surpass 1.2 gigatons of CO₂ equivalent annually, with a substantial portion--approximately 330 million tons--originating from freezing systems.
Is it feasible to eliminate freezers altogether? Doing so could trigger a food crisis, as freezing technology is essential for households, dairy farms, food supply chains, and various industries.
So, what's the alternative? Researchers at the Hong Kong University of Science and Technology (HKUST) have made strides in developing a solution that was once elusive.
They have engineered a solid-state cooling system capable of achieving temperatures as low as -12 °C, utilizing specialized metals, mechanical force, and a saline solution, completely avoiding the use of refrigerant gases. In essence, their innovation accomplishes freezing without any harmful emissions.
A New Approach to Cooling
This innovative device operates on the elastocaloric effect, a phenomenon observed in certain metals known as shape memory alloys. These materials change their atomic structure when subjected to pressure, releasing heat during compression and absorbing it when the pressure is released, resulting in cooling.
Imagine a metal that heats up when compressed and cools down when released. For years, scientists have leveraged this effect for cooling applications near room temperature. However, as temperatures approached freezing, the efficiency significantly decreased.
The HKUST team tackled this challenge by modifying the alloy composition. They employed a nickel-titanium alloy with 51.2% nickel, lowering the critical transformation temperature to -20.8 °C.
"We developed a compression-based, regenerative elastocaloric cooling device that utilizes a cascade of eight low-Af-temperature tubular NiTi units. The high nickel content reduces its Af temperature to −20.8 °C," the researchers noted.
This adjustment allows the metal to remain highly elastic while producing a substantial temperature variation even at sub-zero conditions.
At 0 °C, the alloy can generate a temperature change of 16.3 °C and function effectively across a temperature range of 48.5 °C, ensuring reliable heating and cooling even in freezing scenarios.
Engineering the Temperature Shift
Material design alone was not sufficient; the researchers also reconfigured the alloy into thin-walled tubes with intricate internal channels. This design maximizes the surface area relative to volume, enhancing heat transfer between the metal and the surrounding fluid.
Despite their slender design, these tubes are mechanically resilient, capable of withstanding compressive stresses up to 900 megapascals--approximately 4,000 times the pressure found in a typical car tire.
The cooling system comprises eight connected regenerator units, each containing three nickel-titanium tubes. A linear actuator compresses and releases these tubes once per second, sustaining a continuous cycle.
During compression, the alloy heats up as its internal structure shifts. A circulating liquid absorbs this heat and disperses it into the environment. When the pressure is released, the alloy cools rapidly, chilling the liquid flowing in the opposite direction.
The working fluid is a 30% calcium chloride solution, which remains liquid at sub-zero temperatures and enhances heat exchange by effectively spreading across the metal surface.
In laboratory tests, the device achieved -12 °C at its cold end within 15 minutes, while the hot end registered 24 °C, resulting in a temperature lift of 36 °C--marking the first confirmed sub-zero performance for elastocaloric cooling.
"Our device achieved a 36 °C temperature lift at an applied stress of 900 MPa and an operating frequency of 1 Hz. This advancement propels elastocaloric technology further into the sub-zero freezing range," the researchers added.
Real-World Freezing Applications
To assess the system's functionality outside of laboratory conditions, the team installed it in an insulated chamber measuring 1.0 × 0.5 × 0.5 meters and conducted tests outdoors at ambient temperatures of 20 to 25 °C.
After one hour, the internal temperature of the chamber stabilized at -4 °C. Within two hours, 20 milliliters of distilled water had solidified into ice.
Under optimal conditions, the system demonstrated a specific cooling power of up to 1.43 watts per gram of alloy, with a coefficient of performance reaching 3.4, suggesting potential energy efficiency once further optimized.
The freezing market is comparable in size to the air conditioning sector. If elastocaloric systems can replace even a fraction of traditional sub-zero units, the potential reduction in greenhouse gas emissions could be substantial--potentially hundreds of millions of tons of CO₂ equivalent annually.
Challenges Ahead for Commercialization
However, the path to commercialization is fraught with challenges. The actuator currently consumes significant energy, impacting overall efficiency compared to established vapor-compression systems. Additionally, manufacturing the precision thin-walled tubes is expensive.
"We are developing new actuation technologies as part of our system integration and optimization efforts," stated Qingping Sun, a co-author of the study and professor at HKUST.
The researchers are also focused on refining system integration and exploring cost-effective fabrication techniques. Moreover, they are investigating alternative alloys that could enhance cooling performance down to -100 °C.
While still a prototype, this device represents a significant technological milestone, proving that solid-state cooling can operate in the sub-zero realm without relying on greenhouse gas-emitting refrigerants.
The findings are detailed in a study published in the journal Nature.