Glimpses from the Recent R&D Works in the Cooling Industry

Human civilization is always dynamic. One after the other challenges come and people work hard to overcome those. That is exactly happening in our HVAC-R industry too. There are many reasons behind global warming, so each group of scientists at different parts of the world has begun to work on removing or mitigating one challenge at a time, which is related to its field of activities.

The purpose of this article is to highlight some of those recent breakthrough scientific discoveries. It is important to note here that since our problems are multifaceted and involving multiple fields of activities, it is not possible to plan a complete solution for any one of the research organisations. With the aggregate of the small and effective solutions offered by each such group of researchers, we will be able to address the vast challenge of global warming. Thus, in the next few paragraphs, I will present information on two recent high-potential research outcomes. As mentioned earlier, although all these works target to attain the same goal, apparently, they may not be co-related.

Managing thermal shocking

With the growing deployment of Electric Vehicles (EVs), the need for fast charging is increasing, thus the DC-HPC technology is being applied, especially for megawatt-level charging currents (≥1000 A). But the superhigh heat density generated by the ultra-high charging current is a big challenge – as it creates the problem of instantaneous thermal shocks. Conventional cooling methods that separate current transmission and heat transfer struggle to achieve both flexibility and high-efficiency cooling.

Recently, a novel approach to address the challenges related to High-Power Direct Current Fast Charging (DC-HPC) in Electric Vehicles (EVs) has been proposed by a team from China Agricultural University. By developing a gallium-based Liquid Metal Flexible Charging Connector (LMFCC), the researchers have addressed the need for synergetic cooling and charging. Their proposed method can efficiently dissipate ultra-high heat flux while carrying superhigh current.

The researchers have optimized a compact induction electromagnet-driven method. By adjusting the current and magnetic flux distribution, they have enhanced the Liquid Metal (LM) flow rate and active cooling capacity of the LMFCC system. This method also helps suppress end effects. A three-dimensional multi-physics numerical model and a synergetic cooling and transmission test platform have been established to comprehensively evaluate the performance of the LMFCC under different conditions.

The experimental results are promising. The LMFCC demonstrated good electrical stability under torsional and bending conditions. Regarding cooling performance, at a charging current of 1000 A, the temperature difference between the maximum temperature and the external environment remained at 54.3°C, showing its excellent heat extraction and dissipation capabilities. The system’s cooling performance can be further improved by adjusting parameters such as the length, diameter of the charging cable, and the flow rate of the liquid metal.

This new synergetic cooling and charging strategy represents a significant step forward in ultra-high heat flux thermal management. It has the potential to enable the development of simple, reliable, and lightweight charging systems with high charging power. Although it is still in the research stage, it offers new possibilities for the future of the electric vehicle industry, potentially accelerating the widespread adoption of electric vehicles.

World’s first kilowatt-scale elastocaloric green cooling device

Of late the rising global temperature has been increasing the need for more and more cooling devices. However, the cooling sector already consumes 20% of global electricity. Also, it is well-known that there are challenges related to the use of refrigerants. Still our dependence is growing on vapour-compression-based cooling machines. Thus, the need of the hour is to develop eco-friendly alternative machines for cooling.

Addressing the situation, Researchers at the Hong Kong University of Science and Technology (HKUST) have developed the world’s first kilowatt-scale elastocaloric cooling device. The device can stabilize indoor temperatures at a comfortable 21-22°C in just 15 minutes, even when outdoor temperatures reach between 30-31°C, marking a significant breakthrough toward the commercial application of elastocaloric solid-state cooling technology.

As an eco-friendly alternative, solid-state cooling technology based on the elastocaloric effect of Shape Memory Alloys (SMAs) has drawn substantial focus from both academia and industry due to its zero greenhouse gas emissions and high energy efficiency potential.

However, the maximum cooling power of previous elastocaloric cooling devices was around 260 watts, which could not meet the kilowatt-scale requirement for commercial air conditioning. The HKUST research team, led by Prof. SUN Qingping and Prof. YAO Shuhuai, both Professors from the Department of Mechanical and Aerospace Engineering (MAE), identified that this bottleneck stems from two core issues: (1) the difficulty in balancing the Specific Cooling Power (SCP) of the refrigerant with the total active mass; and (2) insufficient heat transfer efficiency during high-frequency operation.

To overcome those limitations, the research team proposed an ‘SMAs in series – fluid in parallel’ multi-cell architecture design (Figure 2a). This architecture serially connects 10 elastocaloric cooling units along the direction of force application, with each unit containing four thin-walled nickel-titanium alloy tubes, totaling a mass of only 104.4 grams. The nickel-titanium tubes feature a high surface area-to-volume ratio of 7.51 mm-1 that significantly improves heat exchange efficiency. Meanwhile, the parallel fluid channel design keeps system pressure below 1.5 bar, ensuring stable high-frequency operation.

Another key innovation is replacing traditional distilled water with graphene nanofluid, a cutting-edge heat transfer medium with exceptional thermal conductivity. Experiments showed that graphene nanofluid, at just 2 grams per liter concentration, conducts heat 50% more efficiently than distilled water (Figure 2d). The diameter of its nanoparticles (0.8 micrometers) is much smaller than the width of the fluid channels (150-500 micrometers), avoiding blockage risks. X-ray tomography (Figure 2b) confirmed that the nickel-titanium tubes maintained uniform compressive deformation under a stress of 950 megapascals without buckling failure.

At a high frequency of 3.5 Hz, the device achieved a specific cooling power of 12.3 W/g and a total cooling power of 1,284 watts (under zero temperature lift conditions), demonstrating its practical viability in real-world conditions.

In practical application tests, the device successfully cooled a 2.7 m³ model house (Figure 4) in a summer outdoor environment with temperatures between 30-31 ⁰C, stabilizing the indoor temperature at a comfortable 21-22 ⁰C in 15 minutes.

Compared to the existing solid-state cooling technologies (Figure 5), this pioneering device leads in terms of cooling power and temperature lift performance. Its SCP value (12.3 W/g) nearly triples the previous record of liquid heat transfer elastocaloric devices (4.4 W/g), and it has for the first time broken through the kilowatt-scale cooling threshold.

Commenting on the development, Prof. Sun Qingping said, “This achievement demonstrates the potential for large-scale application of elastocaloric cooling technology. We are working with the industry to drive its commercialization. As global regulations on hydrofluorocarbons (HFCs) tighten, this zero-emission, energy-efficient cooling technology is poised to reshape the air conditioning industry and provide a key technical solution for carbon neutrality. Consumers will also benefit from lower energy bills while the technological advancements enable more compact cooling devices that save valuable indoor space.”

Focusing on the potential of their work, Prof. YAO Shuhuai said, “In the future, the system’s cooling performance can be further improved by developing new elastocaloric materials and optimizing the rotary drive architecture. These improvements can help achieve larger cooling powers, meaning indoor environments can be cooled down in significantly less time.”

By P. K. Chatterjee (PK)