Free Cooling: what is it and is it right for me? [BUSINESS GUIDE]

With so much energy being consumed by data centre equipment, heat quickly builds up and cooling it all has become an expensive necessity in order to protect systems from failure.  This effectively doubles the electricity bills you already pay to produce the excess heat in the first place!

Free cooling is the perfect short-cut to lower energy bills

So how about taking advantage of a cooling approach that promises benefits for ‘free’?  This article explores how free cooling works and what kinds of data centre it is most suitable for.  

What is free cooling?
Why opening the window to your data centre is a bad idea
Access to cold air or cold water is essential to free cooling
Free cooling is never going to be completely free
You can still get free cooling benefits if your data centre is not in the artic
Water and sensitive IT equipment don’t mix
Free cooling can transalte back into free heating
The different kinds of free cooling solutions available
Glossary of terms

What is free cooling

The first thing to know about free cooling is that it harnesses something nobody ever pays for – outside air.  The most direct way to use this air to cool your datacentre is to fit vents or windows that you can open on a cold winter’s day.  That’s how the principle of free cooling was first born, but today’s free cooling solutions are significantly more sophisticated and energy efficient.

Free cooling works on the principle of heat exchange, where a medium (such as air) is used to remove heat from an enclosed space.  

Air itself does this inefficiently because it is difficult to introduce and remove from targeted areas, so in free cooling solutions it is commonly used instead to cool a water supply that is passed close to IT equipment (i.e. into the equipment cabinets).  This in turn performs the heat exchange process that makes the data centre environment colder and the water itself warmer.

Some free cooling solutions take their water supply directly from a suitably cold source such as a deep water lagoon and do not utilise outside air to perform the water chilling process.

Why opening the window to your data centre is a bad idea

Before we go on, it’s worth quickly pointing out why free cooling systems are more complicated than simply enabling cold air into warm data centre environments.

It won’t bring cooling to where you need it

Letting cold air in and warm air out is a sound enough principle, but a rudimentary ventilation system is never going to bring cooling to the parts of the data centre that need it most.  Using computational fluid dynamics or some other process of mapping hotspots in your data centre will reveal the IT components that generate the greatest amounts of heat and the areas of the data centre that require the most direct forms of cooling application.  Like opening a car window on a hot summer’s day, it makes your arm and head cooler, but doesn’t prevent your back and legs sweating through the seat!

It may do more harm than good

Letting the outside into your data centre presents a series of environmental and security risks that could have catastrophic consequences.  A vent could let cold air, but also insects or birds.  This is among the reasons why water is piped into data centre environments as the heat exchange medium for free cooling systems.  

That being said, ‘fresh air’ / ‘air-side’ economiser units are frequently deployed as free cooling solutions in datacentres around the world, as are so-called ‘water-side’ economisers.  Both reduce the requirement to operate a mechanical refrigeration system.

Access to cold air or cold water is essential to free cooling

The cost reduction and efficiency opportunities around free cooling are so great that many data centres are now situated where cold weather can be assured for most of the year.  Instead of powering chillers to make warm data centres colder, entire cooling systems now benefit from the natural injection of on-demand coldness taken directly from the surrounding environment.

This explains why the Arctic regions of North America and northern Europe are increasingly popular locations to situate massive data centre facilities.  Not only is a place like Sweden’s ‘Node Pole’ business park somewhere to enjoy cold outside temperatures that can be pumped inside the data centre, but these conditions can also be relied upon to persist for most if not all of the year round.  

In Iceland, data centres have become a new industry on the back of two unique geographical characteristics.  The first is the rich availability of cool air.  The second is the oversupply of very cheap, renewable energy from geothermal and hydroelectric sources.  This works to minimise the amount of energy required to maintain a safe operational temperature within the datacentre, and any that is needed is very cheap, reliable and environmentally friendly.

Places like Sweden, Iceland and Canada are seen as prime data centre locations because of free cooling

For most data centre operators, such resources are not easily or reliably obtained.  In these cases, cooling systems are able to derive partial benefit from the outside environment and still perform as effective ‘free cooling’ solutions.

Free cooling is never going to be completely free

It’s important not to get too carried away with the ‘free’ aspect of free cooling because any such solution is going to cost something.  For instance, even in a highly efficient system with plentiful quantities of low temperature air/water on demand, the system still needs to be designed, built, tested, pumped and maintained.

The other key principle with free cooling is that the ‘free’ elements rarely manage to accomplish the whole objective.  Rather like the electric fuel cell in a hybrid vehicle, which doesn’t exist to prevent the combustion engine from being used but to help it along.  In a free cooling system, a conventional chiller will still need to be employed to reduce the water temperature to the required level, but the cold air/water that has been introduced will have provided a ‘head-start’ to the process, thereby reducing the required electricity draw.

You can still get free cooling benefits if your data centre is not in the Arctic

Many free cooling systems in deployment today are situated in temperate climates where cold outside temperatures cannot be guaranteed for more than a few months per year.  This makes them less viable than the ‘Arctic’ based examples above, but still a sound economic and environmental investment.  

The feasibility for free cooling rests upon the ambient temperature being sufficiently low to deliver additional, incremental benefit to the conventional cooling operation.  One might wrongly assume that a data centre in the Sahara desert would not benefit from free cooling because of the extremely hot daytime temperatures all year round.  But consider the fact that night-time temperatures frequently drop below the level desired within the datacentre environment.  Here, free cooling could be feasible during these times.  At other times, the data centre would be environmentally sealed and cooled in the conventional way.

Calculating the future ROI of a free cooling investment would depend, in part, upon historic weather data for the immediate locality of the data centre.

Water and sensitive IT equipment don’t mix

It should go without saying that there are many significant dangers associated with introducing water into highly controlled data centre environment with sensitive IT equipment powered by large quantities of electricity.  This explains the reasoning behind a professionally designed, manufactured and installed free cooling solution, delivered and maintained using industry best practices.  

The two main risk areas are:

Water escape from the system

Water is circulated within a free cooling system, with cold water entering the environment from a temperature controlled chiller through pipework and being returned as warm water.  The presence of warm(er) water indicates that the heat exchange process has been successful.  Clearly it is critical to establish the correct temperature settings for water as it progresses through the system.  This relies upon accurate and automated measurement and monitoring processes for temperature, pressure and other metrics.  Should water escape from the system, it may result in serious equipment damage, fire and/or injury to any persons present.   

Condensation/humidity inside the data centre

The physical effect of placing a cold pipe into a hot environment is for any moisture in the air to condense onto the pipe surface as water.  Ordinarily, to mitigate this would require a necessary level of insulation to ensure that no condensation occurs.  However, the net effect of insulation is to allow the pipe to maintain its temperature, thereby counteracting its ability to cool the air around it.  Free cooling solutions therefore focus on operating environments where air moisture is closely monitored and controlled, and where differences in temperature between the system and the environment it cools is kept within a safe range (i.e. not too great).

    Free cooling can translate back into free heating

    By circulating water inside the datacentre environment, organisations can create additional energy efficiencies in water-based heating supply.  The effect is increased during periods of greatest heating demand by introducing a steady flow of newly warmed water from the datacentre into existing boiler plant.  

    Some data centres use the heat exchange principle to pass on the warmth that free cooling attracts

    Much like in the cooling system, the heat exchange process does not achieve 100% of the objective i.e. it does not make radiators hot to the touch.  Rather, as per the hybrid car example above, it reduces the need to use other forms of energy (in this case, gas) to heat water to the required temperature – thereby saving money and reducing the organisation’s environmental impact.

    The different kinds of free cooling solutions available

    Air-side Economiser Coils

    Applicable for dry, cool ambient air conditions, these filter and dehumidify outside air and pass it directly into the datacentre’s existing cooling systems.  Ideal for datacentres with limited space and can be implemented with minimal disruption.

    Cooling Tower

    Uses cold air to reduce the temperature of condenser water so that this can be used to circulate inside a heat exchange pipe system. These can be configured to supplement or replace the work of a mechanical chiller, depending on the ambient air conditions.  Can be retrofitted into existing datacentres or designed for new builds.

    Water-side Economiser Coils

    These use ‘free water’ to either cool the air inside datacentres or feed directly into the heat exchange pipe system.  A large, dependable supply of cool water is therefore essential, as is a filtration system to prevent pollutants and blockages.

    Adiabatic Coolers

    Adiabatic coolers operate on the principle of evaporation heat exchange and are proven to increase cooling efficiency in hot-air climates.  The process involves injecting water as a fine mist over air-side economiser coils.  As the mist evaporates, it takes heat with it, cooling the water (or refrigerant) inside.  These systems must be well maintained to prevent bacterial build-up.

    Glossary of Terms

    Adiabatic Cooling

    A form of evaporative technology employed in datacentres using air and water from the outside environment to maximise energy-efficient cooling.

    Air-side Economiser

    A form of free cooling that uses air is its primary input.


    The mechanical refrigeration component of a conventional cooling system associated with cooling water or refrigerants.

    Free Cooling

    The concept of cooling a datacentre environment using freely accessible air and/or water instead of, or in addition to, conventional chillers.

    Heat Exchange

    A core principle of thermodynamics that allows a solid and/or fluid to transfer heat to another.

    Water-side Economiser

    A form of free cooling that uses water as its primary input.

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