Free Cooling & Air-Side Economisers
A practical guide to free cooling — using cool outdoor air or water to cool a building without running the chiller — including air-side economisers, water-side free cooling, and where each is worthwhile in the Gulf.
When the outdoor air or available water is cooler than the air being returned from the spaces, a building can be cooled wholly or partly without running the compressor — "free cooling". By substituting fan or pump energy for compressor energy, free cooling can sharply cut the electricity a cooling plant uses, whenever conditions allow.
In a hot climate like the UAE, the opportunities are narrower than in temperate regions, but they are real: cool winter nights, shoulder seasons, and spaces with high internal heat gains that need cooling even when it is mild outside (such as data and electrical rooms). Designing systems to exploit these hours is a recognised efficiency measure.
How it works
The basic idea. Cooling is normally produced by a compressor moving heat. Free cooling instead uses a naturally cool source — outdoor air or cooled water — to absorb the building’s heat directly, so the compressor can be switched off or run less. The trick is to detect when the free source is genuinely cooler (and, for air, dry enough) than what the spaces need.
Air-side economiser. An air handling unit is fitted with three sets of dampers: outdoor air, return air and relief/exhaust. In normal hot conditions it recirculates mostly return air with minimum fresh air. When outdoor air is cool and dry enough, the controls open the outdoor-air damper wide, close the return damper, and relieve excess air outside — cooling the space with fresh air while the chilled-water coil idles. This is the most common form of free cooling.
Enthalpy (total heat) control. Simply comparing temperatures can be misleading in humid climates, because cool but very humid outdoor air carries a heavy latent load. Better economisers compare the total heat (enthalpy) of outdoor and return air, only choosing outdoor air when its combined temperature-and-humidity content is genuinely lower. This avoids dragging in moisture that the coil would then have to remove.
Water-side free cooling. Instead of, or alongside, air-side methods, a building can use the cooling tower (or a dry cooler) to produce cool water directly when ambient conditions allow, bypassing the chiller via a heat exchanger. This is valuable for loads that run year-round, such as data centres, where even modest cool periods yield large savings over a year.
Controls and limits. Free cooling is sequenced by the building management system, which switches between free-cooling and mechanical-cooling modes based on sensed conditions and load. In the Gulf, careful control logic ensures the system only uses free cooling when it truly helps — and reverts smoothly to the chiller as soon as outdoor conditions become unfavourable.
Main types
In the UAE
- In the UAE’s hot climate, free-cooling hours are limited to cool nights and seasons, but it remains valuable for spaces with high year-round internal loads, such as data and electrical rooms.
- Enthalpy-based economiser control is preferred over simple temperature control in the humid Gulf, to avoid bringing in moist outdoor air that increases the latent load.
- Free-cooling strategies support Estidama Pearl Rating energy goals, and any increased fresh-air intake must still respect indoor air quality, filtration and fire/smoke-control requirements.
How GPR applies this
GPR evaluates and implements free-cooling strategies where they genuinely pay off in the UAE climate, fitting air handling units with properly sequenced economiser dampers and using enthalpy-based control to avoid importing humidity. For year-round loads such as data and electrical rooms we assess water-side free cooling, and we configure the building management system to switch cleanly between free and mechanical cooling so the chiller only runs when it must — all while maintaining indoor air quality and code compliance.
Frequently asked questions
What is free cooling?
Cooling a building using naturally cool outdoor air or water instead of running the compressor, substituting fan or pump energy for compressor energy when conditions allow.
What is an air-side economiser?
An arrangement of dampers in an air handling unit that brings in cool outdoor air to cool the space directly when it is cooler and drier than the return air, idling the cooling coil.
Why use enthalpy control instead of just temperature?
Because cool but humid outdoor air carries a large latent load; comparing total heat (enthalpy) avoids dragging in moisture the coil would then have to remove.
Does free cooling work in the UAE?
Its hours are limited by the hot climate, but it is genuinely useful on cool nights and seasons and for high internal-load spaces like data centres that need cooling year-round.
What is water-side free cooling?
Using the cooling tower and a heat exchanger to produce cool water directly when ambient conditions allow, bypassing the chiller and saving compressor energy.