Cold outdoor air provides free cooling
Free cooling may be used for both waterborne and airborne systems.
In its simplest form, outdoor air is used in airborne systems where a heat exchanger is fitted to treat the outdoor air before it is supplied to the building.
For this solution to be efficient, the air taken in must be cooler than the temperature aimed for indoors. It is optimal when the outdoor temperature is not exceeding 14-16 °C. The airborne free cooling method works very well in the Nordics since the outdoor temperature is below mentioned for about 90% of the year.
If a chiller using free cooling is provided to a waterborne system, the heat exchanger for the outdoor air is built into the chiller and placed between the refrigerant and the cooling medium circuits. When the outside air temperature is low enough to cover the required cooling, it is possible to operate without the power of compressors, hence almost cost free. This is called total free cooling.
For the above to work as efficiently as possible a three way valve is recommended to be used. It will manage the change from compressor cooling to free cooling, typically at an outdoor temperature around 10 °C.
Heat pump and chiller refrigerant cycle
When cooling, electrical energy is being supplied to a compressor which then works to transfer heat away from one point to another, normally to the outdoors. In a heat pump, the opposite happens. Heat is transferred as energy from the outdoor air into the room. The outdoor air temperature then decreases and the cooling refrigerant is becoming the heating medium.
District cooling is more common
Cooling may also be distributed in the form of district cooling, supplied in large-scale to office complexes, shopping centres and other larger properties. This cooling may derive from free cooling, e.g. a cold lake, or from the energy in district heating that is being used locally to produce cold water. Comfort units placed in a room or space uses the water to produce the right temperature for the supply air in the system.
The above can be achieved, fully or in part, in a well-integrated system and reach a high TER, Total Efficiency Ratio, for both a chiller and heater respectively.