24 April 2026
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Data centres are under intensifying pressure to reduce energy consumption. Rising electricity costs, power grid limitations, net-zero commitments and tightening regulatory scrutiny have all placed cooling performance firmly in the spotlight. Tim Mitchell, Sales Director at Klima-Therm, explains the important role free cooling - using ambient air or water to reject heat rather than relying solely on mechanical refrigeration – can have in keeping data centre energy usage in check. He warns that not all free-cooling solutions are created equal, however.
Free cooling has been around for some time, but is experiencing increased popularity in data centres for two main reasons:
- Air-cooled systems: Hot aisle containment raises cooling coil air-on temperatures, which means chilled water temperatures can also be raised whilst still delivering the required cooling duty and increased efficiency.
- Liquid-cooled systems: The efficiency of direct to chip cooling also permits elevated chilled water temperatures.
In theory, the case for free cooling is compelling. In practice, many free cooling installations fail to deliver anything like their promised performance due to a lack of adaptability. Despite the fact that many chillers claim to prioritise free cooling, with mixed mode operation where possible, the reality is that competing component requirements create a far more binary affair.
The mixed mode problem
Most modern free cooling chillers claim to support ‘mixed mode’ or ‘partial’ free cooling, where mechanical refrigeration and free cooling operate simultaneously, allowing a smooth transition from full free cooling at low ambient temperatures to full mechanical cooling at high ambient temperatures, maximising free cooling hours across the year.
In most cases, however, free cooling is either fully on, or it is effectively off. The reason lies in a fundamental control conflict.
To maximise free cooling, condenser fans need to run fast, increasing airflow across the free cooling coils. When compressors are operating, refrigerant controls typically require much lower fan speeds - particularly in cold weather - to prevent over condensing and over cooling of the refrigerant. If fans are allowed to run too fast while compressors are active, the refrigerant system can be driven outside safe operating limits, risking serious compressor damage.
To protect the compressors, control systems would prioritise refrigerant stability over free cooling potential. Fan speeds are reduced to suit the vapour compression cycle, but in doing so, the available free cooling capacity collapses. The result is a system that technically supports mixed mode operation but spends very little time exploiting it.
Paying twice for poor control
The consequences of ineffective mixed mode operation are significant. When free cooling is not used to its maximum potential, compressors are forced to run far more often than they should. Energy consumption rises, operating costs increase, and the efficiencies and carbon savings that justified the free cooling investment fail to materialise.
As a result, operators end up paying twice: first for free cooling technology that is never fully utilised, and second for compressor energy that should not be required for large portions of the year. In some installations, thousands of potential free cooling hours are lost annually simply because the system cannot manage the transition between cooling modes effectively.
Designing free cooling as a primary function
One of the most common mistakes in free cooling design is treating it as an add on rather than a core operating mode. Many chillers are fundamentally vapour compression machines with free cooling bolted on, rather than systems designed from the outset to prioritise free cooling whenever conditions allow. By contrast, in systems where free cooling is integral to the design this state is achieved for as many hours as possible, not just during periods of very low ambient temperature.
Adaptive controls: The real differentiator
The single most important factor in effective mixed mode free cooling is adaptive control. Rather than forcing all components to operate according to a single, rigid strategy, adaptive systems actively manage and isolate parts of the refrigerant circuit to suit prevailing conditions.
By using isolating valves within condenser coil banks, redundant sections of the refrigerant system can be shut off when free cooling potential is high. This allows fan speeds to increase to maximise free cooling without compromising the vapour compression cycle. The result is genuine partial free cooling: maximum heat rejection from ambient air, with minimum compressor run time.
True adaptive control avoids the ‘all or nothing’ behaviour seen in many mixed mode systems and restores the smooth operating curve that free cooling promises in theory.
Addressing the glycol penalty
Another often overlooked factor in free cooling performance is the use of glycol. While glycol is commonly added to chilled water systems to provide freeze protection, it comes with significant penalties: increased pumping energy, reduced heat transfer efficiency and higher environmental risk in the event of leaks.
Integrated glycol free cooling systems eliminate these drawbacks entirely, reducing parasitic losses and avoiding the long term costs associated with maintaining large volumes of glycol over the life of the plant. For many data centres, particularly those with elevated chilled water temperatures, glycol free designs represent a straightforward route to improved sustainability and efficiency.
What the numbers really show
The impact of effective mixed mode free cooling becomes clear when operating hours are examined. In a typical London data centre scenario, a system designed to maximise free cooling can operate in partial mode for more than 99% of the year, with full free cooling available for a significant proportion of those hours.
Systems with less sophisticated controls may technically qualify for free cooling during similar ambient conditions, but in practice achieve only a fraction of the full free cooling hours due to over condensing constraints and conservative fan control strategies. The resulting difference in annual energy consumption can be substantial, even though both systems are marketed as ‘free cooling’ solutions.
Making free cooling work in practice
For engineers and operators looking to implement free cooling effectively, several lessons stand out:
- Design for reality, not extremes. Systems should be optimised for the conditions they will experience most of the time, not the rarest design day.
- Prioritise adaptive control. Without intelligent isolation and modulation, mixed mode free cooling will always underperform.
- Select components that support flexibility. Compressors, coils and fans must be capable of stable operation across a wide range of conditions.
- Minimise parasitic losses. Eliminating unnecessary glycol and reducing pumping energy can unlock further efficiency gains.
Free cooling can deliver exceptional energy savings in data centres - but only when it is treated as a primary operating mode, supported by the right controls and components. When mixed mode operation is engineered properly, free cooling becomes a reliable, everyday contributor to lower energy use, reduced carbon emissions and improved operational resilience.
Best in class free cooling
QUANTUM Air is Engie Refrigeration's highly efficient air-cooled chiller series, developed for maximum energy efficiency and sustainability. Using state-of-theart compressor technology and an optimised free- cooling system, QUANTUM Air provides reliable and environmentally friendly cooling for data centres. What sets this product apart from other similar products is that free-cooling is integral to its operation; it has been designed with no compromises, specifically to deliver free-cooling for as many hours as possible, not just when ambient temperatures are very low.
Compared with its competitors, in some instances QUANTUM Air provides thousands more hours of free cooling at points where comparable technology might still be using compressors to keep data centre temperatures stable.
