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Waste heat can be produced by any process that uses energy. In buildings, this might include::
- Heating, ventilation and air-conditioning systems (HVAC).
- Machinery, equipment and industrial processes.
Where this waste heat is at a low-temperature, it may have limited useful capacity for work and so it may be rejected to the environment. However, if it is suitable for use in another process, a portion of heat that would otherwise be wasted might be reused. This is known as heat recovery.
Heat rejection methods include, air cooing, evaporative cooling, and ground coupling.
Typically in heating, ventilation and air-conditioning systems, air cooling rejects heat to the outside air by circulating 'outside' air over coils containing 'hot' fluid returning from the building. Heat is transferred from the coil to the air which is then rejected to the outside. See cooling for more information.
When water evaporates, it absorbs significant amounts of heat energy (latent heat), which produces a cooling effect in its surroundings.
Direct evaporative coolers (sometimes referred to as sump coolers, swamp coolers, or desert coolers) draw hot, dry air through a continually dampened pad and supply cool, humid air.
Cooling towers reject heat through the evaporation of water in a moving air stream within the cooling tower. The temperature and humidity of the air stream increases through contact with the warm water, and this air is then discharged. The cooled water is collected at the bottom of the tower. This process can achieve lower temperatures than air-cooled heat rejection systems. See Cooling towers for more information.
 Ground coupling
Earth-to-air heat exchangers draw air through buried ducts or tubes (sometimes referred to as earth tubes). As the temperature of the ground below 3m is practically constant, it can be used to substantially reduce air temperatures. See Earth-to-air heat exchanger for more information.
Heat recovery is the process of collecting and re-using heat that would otherwise be lost. This can help reduce the energy consumption of the process or the heat can be used elsewhere, reducing running costs and carbon emissions. See Heat recovery for more information.
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