Free cooling or compressor cooling?

Free cooling means fulfilling cooling requirements without having to pay for the chill factor! In winter this is obvious: place perishable foods outdoors in the cold and we can use nature’s free cooling for chilling instead of a fridge or freezer. Free cooling can naturally also be used as an aid to lowering the temperature of a refrigerant without having to achieve cooling by other means, which thereby saves energy.

Cold outdoor air provides free cooling

Free cooling can be used for both waterborne and airborne systems. In its simplest form, outdoor air is used in  airborne systems where a heat exchanger to treat outdoor air is installed. For optimum operation, the outdoor temperature must not exceed 16 °C.

If a chiller is used for a waterborne system, the heat exchanger for the outdoor air is built into the chiller between the refrigerant and the cooling medium circuits. Under the effects of a given outdoor temperature, all the water is then chilled by the outdoor air (= free cooling) instead of by the chiller. The switch from chiller to free cooling typically occurs at around 10 °C.

District cooling more common

Cooling can also be distributed in the form of district cooling supplied by large-scale utilities to office complexes, shopping centres and other large properties. This cooling may derive from free cooling, e.g. a cold lake, or the energy in district heating is used locally to produce cold water, which then in the comfort unit produces the right temperature for the ventilation air in the system.

Compressor cooling – the classic means of achieving cooling

For a long time now, a compressor has been the usual means of achieving cooling, and is what is usually meant when referring to machine cooling. Compressor-powered chillers are very flexible for cooling a building. The cooling can either be supplied to the cooling coil in an air handling unit or to chilled beams in each room.

All chillers are based on a closed circuit filled with refrigerant, e.g. CFC/freons or ammonia-based fluids. When a compressor raises the pressure of the refrigerant medium, it heats and evaporates into a gas. When the medium then passes a heat exchanger/condenser, it cools and condenses and reverts to its liquid state. Next, when the fluid passes an expansion valve, the pressure falls rapidly, which then cools the refrigerant. The fluid then passes over yet another heat exchanger, called an evaporator. Inside this, the cold in the refrigerant is transferred to a cooling medium (water or air) which then finally directs the cooling to where it is needed.

The heat pump is a reverse cooling compressor

The condenser in the system can be installed outdoors and will then be cooled by outdoor air, and can be sited indoors, where it will be cooled by a hydronic circuit, which also has outdoor cooling. In other words, electrical energy supplied to the compressor transfers heat from an interior to an exterior, e.g. outdoors. With a heat pump, the opposite happens: heat is transferred from the outdoor air into the home, and the outdoor air temperature goes down. Here, the heating medium is equivalent to the cooling medium/refrigerant.