An attractive way to cool the world

Staying cool in the sweltering heat accounts for a significant fraction of our energy consumption. In Singapore, for example, more than half of all electricity used is consumed by cooling-related systems.

Cooling is particularly important in sectors such as transport, food storage and buildings, as well as for industrial applications in data centres, consumer electronics and power engines. Conventional gas compressor-based cooling is energy-intensive and relies on hazardous chemicals known to damage the atmosphere’s ozone layer.

An alternative available right now—heat pipe-based two-phase cooling—typically uses water or ammonia for cooling and is limited by low maximum operating temperatures and toxicity, respectively. Hence, a new technology compatible with high power and wireless remote control over a broad range of temperatures is urgently needed.

Magnets to the rescue

To achieve the goal of a “Green Singapore”, my research team has developed an industrially relevant liquid magnet-based cooling system, with funding from Singapore’s National Research Foundation’s Campus for Research Excellence and Technological Enterprise programme.

Compared to existing cooling systems, magnetic cooling has significant advantages including higher energy efficiency, zero emission of greenhouse gases and decreased operational noise and vibration. Magnetic cooling can be used in stand-alone cooling operations or integrated with existing cooling technologies to enhance their performance.

Currently, industry giants, such as BASF, GE, Haier, Camfridge and CoolTech, are competing to develop magnetic cooling technology. However, since the magnetic cooling materials used by these companies typically contain either expensive rare earth metals or toxic materials, commercialisation efforts have stalled.

As an alternative, we have developed a variety of low-cost, iron-based magnetic nanoparticles with high cooling power. Suspending these nanoparticles in a carrier fluid, such as water or silicone oil, results in a magnetic fluid that behaves like a liquid magnet. When exposed to a magnetic field and a temperature gradient, the fluid flows due to thermomagnetic convection—an effect similar to atmospheric circulation and one that does not require external energy.

An optimised performance

In the course of our research, we have optimised the properties of the nanoparticles as well as of the carrier fluid. When placed within suitable tubes, magnetic fluids can be used to efficiently transfer heat from a high temperature heat load to a low temperature heat sink (Figure 1).

Figure 1: The thermomagnetic cooling system can be driven by waste heat generated by various processes or equipment such as laptops or industrial cooling towers. Credit: Raju Ramanujan/Varma Vijaykumar.

Interestingly, our device transfers heat faster at higher heat load temperatures. Our autonomous and self-regulating cooling systems are currently capable of handling heat load powers in the kilowatt range and heat load temperatures of about 400℃. The system performance depends strongly on heat load characteristics, magnetic field strength, volume fraction of nanoparticles, fluid density and viscosity.

Our liquid magnet-cooling technology has drawn considerable interest from industry and shown potential for a variety of applications, including cooling of power electronics, batteries, solar panels, servers in data centres, and air conditioning inverter systems. In addition, we are developing methods to harvest energy from the magnetic fluid flow to make magnetic cooling even more attractive in the future.

Author: Raju Ramanujan
Assoc Prof Raju V. Ramanujan is Assistant Chair (Graduate Studies) in NTU’s School of Materials Science and Engineering.
More details of the research presented in this article can be found in a review paper published in Progress in Materials Science (2019), DOI: 10.1016/j.pmatsci.2018.09.005. You can also watch a video describing the concepts of the patented technology at bit.ly/2OI6dC1.

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