Free-cooling / Night-cooling

Free cooling/night cooling involves cooling a building at no cost by using cooler outside air (free cooling) or by ventilating at night (night cooling) to remove heat stored in the building's thermal mass.
In HVAC systems, this is referred to as air economizers (bypass heat exchangers, increased fresh air intake), and in chilled water systems, as water-based free cooling (dry/adiabatic cooling towers + plate heat exchangers) when the outside air is cold enough to produce cooling without a compressor.
The goal is to shift and reduce air conditioning needs, smooth out peak demand, lower summer energy consumption (kWh), and improve comfort.

Free cooling / night cooling: definition and implementation

In air-side ventilation, an air handling unit (AHU) increases the supply of fresh air when the outside temperature is cooler (and ideally drier) than the inside temperature; the building management system (BMS) opens a recovery bypass, modulates dampers/flow rates, and monitors CO₂/RH. Night cooling takes advantage of cool nights: enhanced ventilation (motorized opening, over-ventilation) is used to remove heat from the walls and pre-cool the building mass.

In water-side, free-cooling water diverts the chilled water circuit to a heat exchanger supplied by a dry/adiabatic tower: as long as the outside air allows the setpoint to be reached (e.g. 14–18 °C), the compressor remains stopped.

Keys to success: weather scenarios (temperature / humidity / dust), external solar protection (to limit daytime heat gain), internal thermal inertia (exposed slabs) and control (times, thresholds, security).

Advantages, limitations and points to consider regarding free/night cooling

Interests

• Less air conditioning: compressors are switched off for part of the season.
• Peaks capped: offices/schools more stable in the afternoon.
• Improved comfort: lower temperatures at the beginning of the day.
• Reduced OPEX: kWh and machine operating hours down.

Boundaries

• Climate dependent: insufficiently cool/dry nights = limited gain.
• Humidity: humid outdoor air can negate the effect (discomfort, risk).
• Noise/security: night openings ≠ not always possible (urban).
• Air quality: pollen/PM in season → filtration needs attention.

Points to consider

• BMS: hygro-thermal logic (temperature and humidity), dynamic thresholds, exchanger bypass, schedules, anti-conflict with heating.
• Thermal inertia: favor exposed slabs/ceilings to store the "cold".
• Sun protection: Exterior blinds/awnings to reduce daylight heat gain.
• Filtration & maintenance: ΔP filters, clean grids, airtight valves.
• Acoustics & security: nighttime flow rates compatible with noise/intrusion.
• Free-cooling water: monitor ΔT, water quality, frost (winter), anti-legionella strategy in adiabatic towers.

Anecdote — “The breeze from the Garonne brought relief to Blagnac”

In Blagnac (Toulouse), an office building was overheating in the late afternoon. The team activated a weather-controlled night cooling system (bypass opening + over-ventilation as soon as the outside temperature dropped below the inside temperature) and added free water cooling via a dry tower for the shoulder seasons. With programmed exterior blinds, the temperature peaks dropped, and the cooling unit ran for 30% fewer hours from June to September. Users noted a "new coolness" in the morning—without the compressors running.