Spray cooling for servers, a good option

Researchers at Singapore’s Nanyang Technical University (NTU) are proposing a more sustainable method to cool servers in datacenters, reducing energy costs and carbon footprint by up to 26%.

In Singapore, in particular, datacenters account for 7% of total electricity consumption, according to the researchers. With the growing demand for computing resources, it is crucial to find a solution that the energy consumed and the environmental impacts of these environments.

The component that gets warmest in datacenters servers is the CPU, which requires a dedicated, air-cooled heat sink. In addition, the datacenter space as a whole also needs to be cooled by air conditioners. However, conventional air-cooling schemes are thermally inefficient and energy-intensive as they require a large temperature difference between the heat source and the cooling medium. As a result, between 30% and 55% of the energy consumed by datacenters goes to cooling systems.

More importantly, the increasing miniaturisation of electronic chips will inevitably lead to increased heat flux, while the air cooling scheme is limited to a relatively low 37 W/cm2 plateau, according to the NUT researchers. Therefore, they draw attention to the urgent need for more effective cooling strategies.

Taking all these points into account and considering that, due to better thermal properties, liquid cooling has been seen as a promising alternative to the air-based method, the solution developed by NTU scientists adopted a special spray of non-conductive fluids to cool the CPU directly.

The indirect liquid cooling scheme, which uses fluid circulation systems, was disregarded because of concerns about coolant leakage. While immersion cooling, which according to studies is energy efficient and superior to air cooling in the tropics, can be challenging and expensive to implement in large datacenters.

That’s when the idea of spray cooling, which sprays dielectric fluid directly onto CPUs and uses a combination of highly efficient heat removal mechanisms such as evaporation and boiling, came about. Excess gases and fluids are collected in a closed system, condensed into liquid at room temperature in the tropics (around 30 degrees Celsius) and circulated back to be reused by the system.

“The inspiration for our solution is simple. If we want to put out a burning wood, we are taught to aim the extinguisher at the base of the fire instead of trying to eliminate the flames or around the fire. Thinking in a similar way, we are wasting a lot of energy trying to cool the air around the heat source, when we should be cooling it directly,” explains Wong Teck Neng, NTU professor leading the research project.

The professor comments that the targeted approach is smarter, especially in tropical environments where high humidity and heat can put pressure on traditional air cooling systems. For example, a conventional datacenter has to operate at around 18 degrees Celsius. In comparison, environments using spray cooling can maintain their optimum temperature at around 55 degrees Celsius, without the need to use energy-intensive air conditioning systems. The Power Usage Effectiveness (PUE) – or the ratio of total energy used by the datacenter versus the actual energy delivered to the servers – of the new prototype can reach values close to 1.08 (the closer to 1, the better). Air-cooled datacenters, in general, have PUE values around 1.8.

Based on the power consumed by servers in a rack, the waste heat is around 7kW per cubic metre in conventional air-cooled models. In comparison, the spray-cooled prototype used in the research has been shown to be able to dissipate significantly more heat, coping with densities of up to 23kW per cubic metre.

In addition, with the new spray cooling solution, it is estimated that servers with greater computing power can be brought together in a smaller space than seen in current datacenters. The researchers estimate that this can translate into around 30 per cent space savings over conventional datacenters using air-cooled systems, a significant advantage in the face of the land scarcity we are likely to see ahead in some regions.

According to the researchers, spray cooling will also allow CPUs to run at higher speeds than current CPUs, which are limited by air cooling (higher speeds lead to higher temperatures as well). Another advantage is that the new approach proposed by NTU scientists is highly scalable and easily adaptable to modern datacenters.

In terms of environmental impact, studies conducted by the research team showed that, considering an IT load of 1 megawatt in the datacenter, the spray-cooled system can reduce up to 1,550 tonnes of CO2 emissions annually compared to conventional air-cooled systems. In addition, energy-efficient operations resulting from spray cooling can lead to 26 percent savings in annual energy costs.

During the research, a full-scale spray-cooled rack system housed in an enclosure capable of accommodating up to 12 servers was used. Sprays with multiple nozzles spray the fluid onto each CPU. There is also a collection system for the vaporized liquid and a condenser that works at room temperature to convert the gases into liquid. No cooling system is required.

Patents for the technology have already been applied for; one has already been granted in the United States in July 2022.