Where a building or process has a net heat gain, there is a resulting requirement to remove that heat in the most economic way possible. Mechanical cooling systems such as Air-Conditioning or Chilled Water systems will perform that function but it comes at a cost. The primary energy used to drive those systems must first be purchased. The driver for using mechanical cooling is usually the fact that the temperature of the heat source (the building or process) is less than or almost equal to the temperature of the sink (the atmosphere, river or sea to which the heat energy will be discharged). Since heat energy can only be transferred with a negative temperature gradient, the energy cannot be dissipated naturally. Hence, a mechanical cooling system is used to generate a temperature gradient in the opposite direction.
The simplest example of free cooling would be the use of openable windows in a building. When the temperature of the air inside the building is higher than the external ambient, the volume of air in the building can be exchanged for cooler external air simply by opening the windows. The heat transfer is driven only by the temperature difference and has no other input cost. Of course, the presence of a light breeze will aid the process but it is not actually necessary for this form of free cooling.
Where a temperature difference is available but it is impractical to open windows (for example, due to security concerns or if the rooms are landlocked in the middle of a building with no access to outside walls) then a mechanical ventilation system can be employed. The exchange of air still occurs between the building and the external ambient but instead of being driven naturally it must be achieved by mechanical means. This represents a cost, so therefore the cooling function is not totally free but the costs are relatively small. If moving air in and out of a building is either impractical or not desirable for some reason then the heat can be transferred from the air in the building to a water based system and transferred out of the building in that way. when the heat reaches its destination (usually a Cooling Tower or Dry Air Cooler) it is transferred once more to the atmosphere (or river, sea, etc). As with the mechanical ventilation solution, this option also comes at a cost but slightly higher than the ventilation-only option. However, both of these systems are referred to as free-cooling since there is no use of refrigeration plant.
Where the temperature gradient is in the wrong direction for natural cooling then a form of mechanical cooling is used. Invariably, there will be a refrigerant-based cooling device at the core of this type of system which requires an electrical (in the case of compression cycle refrigeration plant) or heat (in the case of adsorption cycle refrigeration plant) input, both of which must be purchased. This type of system would not qualify as free cooling.
Callidus Design offer a number of services for Free Cooling systems including design and specification of entire heat transfer systems, system modifications and extensions, condition surveys, refurbishment and troubleshooting.