Free Cooling / Night Ventilation Instead of Air Conditioning

Using cool outside air at night or in shoulder seasons to cool the building; the air conditioning runs only in extreme heat.

How does free cooling with night ventilation work?

Even in hot summers, the outdoor temperature in Austria frequently drops below 18 °C at night. This cool air is deliberately routed through the building, either via automated window openings and ventilation dampers or via the existing ventilation system running in fan-only mode. Massive building elements such as concrete slabs and internal walls release the heat stored during the day and cool down. The next day they absorb the heat loads from people, equipment and solar gains again, so the room temperature rises much more slowly. The active chiller then only runs on extreme heat days. In systems with a chilled water network, free cooling in the narrower sense is also possible: a plate heat exchanger transfers the coolness of the outdoor air directly to the chilled water without the compressor starting. In the shoulder seasons and in data centres, this replaces a large share of mechanical cooling.

Which buildings benefit from night ventilation?

Thermal mass is decisive. Buildings of solid construction with exposed concrete ceilings are ideal; pure lightweight buildings without thermal mass are unsuitable. Typical properties are office buildings and schools that are unoccupied at night, as well as data centres with chilled water systems, where free cooling exploits hours of low outdoor temperature all year round. Window-based night ventilation requires a security and acoustic concept, such as burglar-resistant ventilation sashes and rain sensors. Suspended ceilings and carpeted floors reduce the effect because they decouple the thermal mass from the room.

What does night ventilation actually deliver?

Cooling electricity demand falls by 30 to 60 %. The remaining cooling demand with free cooling is typically 30 to 50 % of the original value. The auxiliary electricity for the fans must be deducted from the avoided mechanical cooling. An office building with 120,000 kWh/a cooling demand that halves this through night ventilation saves a net 8,300 kWh/a of electricity at a SEER of 4.5. A free cooling module for the chiller costs €3,000 to €15,000; automated ventilation openings depend on the architecture. With a service life of 20 years, payback is 5 to 12 years.

Energy carrier

Strom → Strom

Savings potential

30-60 % Kühlstrom

Worked example

An office building of solid construction has an annual cooling demand of 120,000 kWh, so far covered exclusively by the chiller with a SEER of 4.5. The ventilation system is programmed to draw cool outdoor air through the building on summer nights and discharge the concrete ceilings. As a result, the remaining cooling demand that the chiller still has to cover drops to 60,000 kWh per year. The auxiliary electricity for the night ventilation fans is 5,000 kWh per year. The calculation is: E_saved = (120,000 − 60,000) / 4.5 − 5,000 = 13,333 − 5,000 = 8,333 kWh/a. Net, the building saves around 8,300 kWh of electricity per year. The result can be read directly from the 24-hour profile of outdoor temperature, indoor temperature and cooling energy.

Investment & payback

Free-Cooling Modul Kälteanlage: 3.000-15.000 €; automatisierte Lüftungsöffnungen Architektur-abhängig · Service life: 20 a · Payback: 5-12 a

Applicability

Requirements

Exclusion criteria

Typical buildings

Frequently asked questions

How much electricity does night ventilation save compared to air conditioning?

Night ventilation and free cooling reduce cooling electricity demand by 30 to 60 %. The cooling demand the chiller still has to cover typically drops to 30 to 50 % of the initial value. The fan electricity for night ventilation has to be deducted. An office building with 120,000 kWh/a cooling demand saves a net 8,300 kWh of electricity per year.

Does night ventilation work in lightweight buildings?

No, night ventilation needs thermal mass. Massive concrete ceilings and internal walls absorb heat during the day and release it to the cool outdoor air at night. Pure lightweight buildings without thermal mass do cool down at night but heat up just as quickly the next day. Suspended ceilings reduce the effect because they shield the concrete mass from the room.

What is the difference between night ventilation and free cooling?

Night ventilation routes cool outdoor air directly through the rooms via windows, dampers or the ventilation system. Free cooling in the narrower sense uses a plate heat exchanger between outdoor air and the chilled water network, so the chiller provides chilled water without running its compressor. Free cooling is particularly suited to data centres and buildings with chilled water systems, night ventilation to offices and schools of solid construction.

How much does it cost to retrofit free cooling?

A free cooling module for an existing chiller costs €3,000 to €15,000. Automated ventilation openings for night ventilation depend heavily on the architecture, for example the number of motorised window sashes and the required burglary and rain protection. With a service life of 20 years, the measure pays back in 5 to 12 years.

Which standards apply to night ventilation and cooling load calculation in Austria?

ÖNORM EN 16798 defines the requirements for indoor climate and the design of ventilation systems, including the assessment of summer comfort. VDI 2078 describes the calculation of cooling load and room temperatures taking thermal mass into account. Window-based night ventilation additionally requires a security and acoustic concept.

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Standards & sources

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