As the HVACR industry faces growing pressure to reduce carbon emissions and increase energy efficiency, new cooling innovations are emerging that promise to revolutionize the way climate control is approached. From the development of solid-state cooling technologies to using air as a refrigerant, these new technologies could pave the way for more sustainable air conditioning and refrigeration systems. 

SOLID-STATE COOLING 

One recent innovation comes from Irish technology firm Exergyn, which is developing a unique solid-state cooling technology based on shape memory alloys (SMAs). SMA solid-state cores are zero GWP, nontoxic, nonflammable, cannot leak, and open up new possibilities for increased energy efficiency and clean cooling. The company states that its cores are so durable, they can outlast the equipment in which they are used and be reused in new equipment. 

"We are now producing SMA material with performance factors beyond anything that has been achieved before in the SMA domain," said Dr. Kevin O'Toole, co-founder and CEO of Exergyn. 

According to the company, a typical coefficient of performance (COP) for SMAs, which measures useful heat output relative to mechanical work input, usually falls within the high single digits in steady state operation. However, Exergyn has created a material with a COP of 20-25 (with the potential to exceed 30 in specific conditions), all while maintaining material stability across a temperature range as high as 60°C for a single blend. Exergyn said this major advancement brings the company closer to offering SMA as a viable alternative to replace HFC refrigerants, which are considered to be potent greenhouse gases. 

Exergyn's latest prototypes are currently undergoing testing in the lab as its team continues to iterate next-generation advances in the HVACR market.  

"Exergyn's solid state refrigerant technology is the kind of breakthrough the Kigali Amendment was intended to create," said O'Toole. "We can help mitigate the causes of climate change by replacing HFCs and many less-than-ideal substitutes, and we can help protect people from the ravages of a rapidly warming world." 

ELASTOCALORIC REFRIGERATION 

Researchers at the School of Engineering of the Hong Kong University of Science and Technology (HKUST) have also developed an eco-friendly refrigeration device that relies on SMAs to provide an environmentally friendly alternative to high-GWP refrigerants. With a boost in efficiency of over 48%, the researchers say the new elastocaloric cooling technology opens a promising avenue for addressing the environmental challenges associated with traditional cooling systems. 

While traditional vapor compression refrigeration technology relies on refrigerants with high GWPs, solid-state elastocaloric refrigeration is based on latent heat in the cyclic phase transition of SMAs, which are greenhouse gas-free, 100% recyclable, and energy-efficient, said the researchers. But the relatively small temperature lift between 20 and 50 Kelvin (K), which is a critical performance indicator of the cooling device’s ability to transfer heat from a low-temperature source to a high-temperature sink, has hindered the commercialization of this emerging technology. 

To overcome the challenge, the research team led by Professors Sun Qingping and Yao Shuhuai from the Department of Mechanical and Aerospace Engineering, developed a multi-material, cascading, elastocaloric cooling device made of nickel-titanium (NiTi) SMAs. They selected three NiTi alloys with different phase transition temperatures to operate at the cold end, intermediate end, and hot end, respectively. 

By matching the working temperatures of each unit with the corresponding phase transition temperatures, researchers say the overall device’s superelastic temperature window was expanded to over 100 K, and each NiTi unit operated within its optimal temperature range, significantly enhancing the cooling efficiency. The built multi-material cascading elastocaloric cooling device achieved a temperature lift of 75 K on the water side, surpassing the previous world record of 50.6 K. 

“In the future, with the continuous advancement of materials science and mechanical engineering, we are confident that elastocaloric refrigeration can provide next-generation green and energy-efficient cooling and heating solutions to feed the huge worldwide refrigeration market, addressing the urgent task of decarbonization and global warming mitigation,” said Professor Sun. 

AIR AS REFRIGERANT 

The Korea Institute of Energy Research (KIER) has developed a new concept of refrigeration and freezing technology that uses air as a refrigerant. The research team successfully developed an integrated ultra-high-speed “compander” system, which combines a compressor and an expander, in order to achieve a temperature environment of -60°C within just one hour. 

The KIER research team focused on implementing a cooling system based on the reverse-Brayton cycle, which uses air as the refrigerant. Unlike the traditional method that involves evaporating a liquid, this system compresses a gas and then goes through heat exchange and expansion to produce a low-temperature gas, enabling cooling without the need for liquid refrigerants. 

In a reverse-Brayton cycle, KIER explained that air is compressed to a high temperature and pressure. Next, the compressed air passes through a heat exchanger, where it is cooled down to a low temperature while maintaining high pressure. The cooled, high-pressure air is then expanded in an expander, reducing it to a low temperature and low pressure. Finally, the chilled air is delivered to the required area for cooling. This cycle is continuously repeated to provide a steady supply of cooled air for the desired cooling applications. 

To implement the reverse-Brayton cycle system, KIER’s compander system connects the compressor, expander, and motor on a single shaft. Although the compressor and expander are connected to a single shaft, each device must operate at its own maximum efficiency. Additionally, the shaft system design ensures stable operation even at ultra-high rotational speeds, further enhancing the reliability and performance of the system. 

According to KIER, the complexity of designing and building such a system has been a significant challenge, preventing its application in refrigeration systems until now. The compander must be designed with extreme precision due to the ultra-high-speed rotation during the cooling process. For instance, the gaps between components and shaft displacement require a tolerance within 0.1 millimeters.  

“We are currently working on improving the system's performance to enable the production of cold temperatures below -100°C,” said lead researcher, Dr. Beom Joon Lee. “We anticipate that this technology will be applied in fields that require ultra-low temperatures, such as semiconductor processes, pharmaceuticals, and biotechnology.”