This summer’s extreme weather conditions, with high temperatures and prolonged dry spells leading to widespread droughts across the UK and Europe, has brought into sharp focus the issues we face as a country, and a planet, as a result of climate change.

Climate change, industrialisation and digitisation are conflating megatrends that demand some form of balance if we are to continue with the development of our species without destroying our environments. Fortunately, we are starting to see decisive action from many industries that are taking steps to reduce their carbon footprint. This collective movement is really gathering pace, nowhere more so than within the data centre industry where operators, suppliers and designers work tirelessly to improve the efficiency and the sustainability of their sites.

It is no secret that the rapid growth in the data centre industry has brought with it some environmental challenges, particularly water scarcity and extreme temperatures. With almost half of Europe under official drought warning conditions this summer (2022), data centre cooling systems are feeling the heat, both literally and figuratively. System providers are pondering three huge challenges:

• How do we cope with increasing peak ambient conditions?
• How do we reduce both energy and water use?
• How do we maintain 24/7/365 resilience in an industry demanding we do more with less?

In this article we look at the journey data centre cooling has been on and where it should go next, in order to keep the internet cool, without warming the planet.

Air-side optimisation

Around 10 years ago, the optimisation of air temperatures was introduced as the latest way for data centres to increase efficiencies. At the time, many data centres ran at 20°C to 22°C. However, as server technology advanced, data centres were able to run at higher temperatures which reduced the cooling requirement and provided more opportunity to utilise free-cooling.

Adiabatic cooling systems

In recent years, air-side optimisation has been built on with the introduction of adiabatic cooling systems. This technique incorporates both evaporation and air cooling into a single system. The evaporation of water, usually in the form of a mist or spray, is used to pre-cool the ambient air to within a few degrees of the wet bulb, allowing cooler and more efficient operation.

Whilst the use of spray or mist means water use is significantly lower than with more traditional evaporative systems, a conservative water usage estimate for a modern data centre employing an adiabatic cooling system would still be 500,000 litres per 1MW per annum. As data centres grow larger, this becomes a real concern, particularly in regions where water shortages have been identified as a threat.  

This water has to be stored and treated, which increases capital costs and, as with any mechanical equipment exposed to continuous water contact, the cooling plant has been seen to suffer from increased degradation, putting strain on OPEX costs too.

Water-side optimisation

Having recognised the need for cooling systems that provide something close to the efficiencies that can be achieved with adiabatic cooling, but with a more sensitive approach to water conservation, Airedale developed an approach to data centre cooling, that takes the philosophy behind air side optimisation and evolves it further. Airedale calls this water-side optimisation.

The philosophy of water-side optimisation is based on taking an optimised air environment and looking at what other variables can be adjusted in order to deliver more free-cooling. Assuming that the air within the data centre white space stays at the same temperature, the next step was to reduce the approach temperature whilst opening the difference between water supply and water return. Implementing innovations within the plant equipment means the supply and return air remain as before, but supply and return water temperatures are higher, thus the approach temperature is reduced. We design in a fixed temperature difference of 12°C on the air side with the fluid side being opened out to 10°C and the approach temperature closing from 6°C to 4°C.

The system features that deliver these variables are:

• Higher water temperatures, meaning less mechanical cooling
• Pressure sensors at aisle level, with fans controlled to a fixed pressure output
• Deep-row chilled water coils and simplified air paths in CRAHs
• Ducted hot air return
• Free cooling chiller with large free cooling coil
• Holistic controls system (not a BMS) delivering constant dynamic supervision of the whole system.

Free-cooling chillers are matched to large surface area chilled water coils in either indoor CRAC units or fan walls. The air path is simplified using hot aisle containment, creating a pressure differential that draws cool air through the servers and out of the white space via ducts and back to the air conditioning plant via a common plenum. The air is introduced directly to the space via side wall diffusion, minimising air side pressure drops.

The benefits of this are:

• Less mechanical cooling meaning more efficient chiller operation
• Lower fan speeds meaning more efficient indoor unit operation
• Lower pump power meaning more efficient water transfer
• Large coil surface leads to increased cooling for less footprint (more cooling capacity per metre).

This is all managed with an intelligent controls platform that monitors fluctuating demand within the white space and dynamically operates the system at its most efficient operating point.

Based on average temperatures for London, an extra 2°C creates many more hours of free cooling. 14% more free cooling (59% in total) can be achieved with water-side optimisation, with all but 1% of the rest of the year being covered by concurrent cooling (a combination of free cooling and mechanical), giving huge benefits in terms of chiller efficiency. This system could provide free cooling for over 50% of the year in all of Europe’s major data centre hubs (London, Frankfurt, Amsterdam, Paris, Dublin).

Climate change

As data centre chillers are now being designed for much higher ambient temperatures, companies have to future-proof their designs for Europe’s major growth regions, whilst advancing system designs and free cooling capabilities. One of the issues with high peak temperatures is the power draw on the chiller when it initiates mechanical cooling, so cooling solution providers must find ways to mitigate this. At Airedale for example, we are researching free cooling coil innovations that will reduce air side pressure drop and assist our peak operating ambient condition, delivering greater airflow across the condenser coils.

Cooling solution providers must be committed to providing sustainable cooling solutions for a data centre industry that is prioritising its environmental responsibilities and it is crucial that water usage is not overlooked, in the race for energy efficiency.