Should you be using power or water to cool your data centre?

I'd make a bet you've used your phone today. Maybe it was messaging someone. It could be doing some online banking. Likely, you scrolled on social media. No matter what you did, a data centre was definitely involved. These buildings are so important to daily life that the UK deemed them mission-critical infrastructure so that they are better supported in case of an outage. To keep these buildings operational requires large quantities of energy and water. For data centre operators who are also trying to reach sustainability targets, their design is vital.

One key aspect of data centre design is the cooling system. The equipment in data centres runs hot, so efficient cooling of the equipment is a primary aspect of operation. It is up to mechanical engineers to select the system that will best suit the data centre. They are typically power-based or water-based, and each comes with advantages and setbacks.

Data centres are in every country, and local climates affect facility’s design in different ways. Some countries have a scarcity of water, whereas others could rely on high carbon power grids. When bringing reference designs to different countries, choosing between water or power systems is rarely a one-size-fits-all solution.

Water usage

In the past decade, power usage efficiency (PUE) has been the primary metric for measuring a data centre's sustainability impact. Driving this figure down is the goal, and so many operators use water-based systems. These systems also have two categories, those being direct-air or indirect-air.

In a direct-air system, air from outside the building is brought into the data centre through an evaporative cooling humidifier. The water evaporates as air passes over the humidifier, cooling the incoming air. This system also uses air-side economisers to circulate warm air, which increases efficiency.

The indirect-air system operates similarly. However, in this, the air stream in the data hall is passed over a heat exchanger, and the external air stream is rejected by the atmosphere. This means that the external air does not enter the data hall. To improve the performance of the heat exchanger, water is run over it. Doing this means you don't need to run chiller compressors in the system unless it is in a particularly hot climate. This minimises power consumption and reduces PUE whilst maximising power for the IT systems.

However, the water use of data centres has been a focal point for many when discussing the sustainable impact of data centres. This water could instead be used for residential or agricultural purposes. Additionally, the growth of AI has necessitated liquid cooling to be used in data centres due to the higher power of the data racks. For liquid cooling, a hydronic system is used. This passes liquid through the racks to a series of chillers. This provides a greater cooling density but increases the power consumption of the data centre, in turn raising its PUE. That being said, when water is scarce or clean electricity isn't available, it can be a suitable solution for non-AI data centres.

Location matters

 From one country to another, conditions vary greatly. In California, water is scarce, and a drought can heavily impact data centres. Water-based cooling is oftentimes not an option. However, they do have access to relatively clean and reliable power so that they can use power-based cooling instead. On the other hand, a region like Poland operates on the converse. Their power grid relies on fossil fuels, so using them would increase their carbon footprint. The most sustainable cooling solutions rely on the conditions in which the data centre will operate.

Waste heat recovery or using river or sea water are relatively new alternatives for cooling. Waste heat recovery involves capturing the heat generated by the servers, where it is redirected to facilities like district heating networks, making what was once waste a valuable resource. We worked on a hyperscale data centre in Denmark that used heat pumps providing around 165,000MWh of heat per year when the facility was fully built out.

Current sustainability metrics are inaccurate.

PUE or WUE – water usage effectiveness – have been traditionally used to measure the sustainability of data centres. PUE measures the total energy consumed ratios against the energy used by the IT equipment. The trouble is that this doesn't reflect the true environmental impact of the data centre. For instance, when water plays a larger role in cooling systems. Meanwhile, WUE measures the amount of water consumed against the IT power. Again, though, WUE can vary depending on the cooling system. Also, it doesn't consider the water used when generating power. This means that it's tough to compare data centre to data centre.

Recently, carbon usage effectiveness (CUE) has been put forward as a better solution. This metric can be best to find what the best cooling solution is for the data centre. This is because it can measure the benefits of reducing power and water against the potential carbon output. To make this most accurate, though, the carbon intensity of the power and water production needs to be included in the calculation.

Consider your location

The number and scale of data centres being built is massive. To speed up their delivery, operators use reference designs using a data centre template and bring it to different regions. This means designers can edit the design rather than start from a blank page. Whilst this does mean that not every data centre needs to be designed from scratch, it doesn't reduce the need for engineering and design teams. When bringing a reference design to a new location, the climate and local regulations must be brought into the design process. These will heavily impact the best cooling solution. Whilst there isn't a simple solution to power vs water-based cooling, understanding how the data centre fits into the wider ecosystem holistically will ensure the correct choice.