In applications where the cooling demand lasts all year but the temperature required is not very low, in winter the outdoors is already colder than the medium to be cooled. Free cooling uses this difference: water or glycol is passed through a finned coil cooled by the outside air; the compressor stops or its load is reduced. The only energy spent is that of the fan and pump; it is of the order of a tenth to a twentieth of mechanical cooling. Because the running hours of the compressors fall, maintenance costs fall too.
In chillers the free cooling coil is placed in front of the condenser. When the outside air falls 1–2 K below the temperature of the water returning from the system, partial free cooling begins; the coil pre-cools the water and the compressor completes the rest. When the outside air falls 8–10 K below the required water temperature the compressors stop completely. In a process that wants water at 15/20 °C this means most of the year; at 7/12 °C the hours are fewer. The changeover is automatic; a three-way valve or a separate pump directs the water to the coil.
The most suitable fields are plastic injection moulding and machine cooling, data centres, air handling units that run all year, and processing areas kept at 10–15 °C. In storage a similar method is the ventilation of products such as onions, potatoes and garlic with cold outside air at night: when the outside air is 2–3 K colder than the product temperature and its humidity is suitable, the dampers open and the fans run, and mechanical cooling comes in only in the hot period. If the outside air sensors are not put in the right place, warm air is taken in at the wrong hour.
What determines the gain is the temperature required and the climate of the location; regions with a continental climate, long winters and cool nights are favourable. The calculation is done with hourly climate data. Because there is a risk of freezing in the outdoor coil, glycol is used in the circuit or a heat exchanger is placed in between. The coil creates extra air resistance for the condenser while the compressor is running. In frozen storage and in rooms at around 0 °C the share of free cooling is very small. The investment decision is taken by looking at the annual total of free cooling hours.