Last edited 01 Jul 2016

Evaporative cooling

Phase change (or phase transition) is the transition of a system from one state of matter to another by heat transfer. For example, from a solid to a liquid (melting) or from a liquid to a gas (evaporation).

When systems change phase, they absorb or release significant amounts of heat energy (latent heat, expressed in J/kg). The systems themselves do not change temperature, as the energy is consumed or generated by the physical process of changing the state of the system. For example, when water evaporates, it absorbs heat, producing a cooling effect. So when water evaporates from the surface of a building, or when sweat evaporates from the skin, this has a cooling effect. Conversely, when water condenses it releases heat.

The process of evaporative cooling can be used to cool buildings. This can be as simple as including a water feature, such as a fountain in a courtyard, or a pond near a building or on a building roof, or might involve spraying water over a building roof.

Packaged evaporative cooling units can be used in building services systems as an alternative to absorption refrigeration or compression refrigeration.

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. The cooling effect is achieved by the process of evaporation from the pad, taking sensible heat from the hot air and converting it into latent heat. This requires little energy, with up to 90% savings reported compared to conventional chillers, and on hot days in the UK can deliver up to 15°C of cooling. Ref EcoCooling, How Evaporative Cooling Works.

Direct evaporative cooling is generally best suited to hot dry climates, where conditions encourage evaporation and the resulting humidification of the cool air supplied to the building can be beneficial. Conventional chillers tend to have a dehumidifying effect on air as cool air is less able to ‘hold’ moisture than warm air.

Direct evaporative cooling can also be achieved by misting fans and humidifiers.

Indirect evaporative cooling can be achieved by using a heat exchanger to cool the supply air. This is less efficient than direct cooling, but does not humidify the supply air. Alternatively, cooling towers can provide indirect evaporative cooling, or indirect evaporative cooling can be achieved by spraying water over the cooling coils of a conventional chiller.

Evaporative cooling tends to be simple, and relatively inexpensive compared to conventional chillers, however, the resulting humidity can cause problems such as; corrosion, condensation, and occupant discomfort. It also requires a continuous supply of water consumption and there is the potential for bacteria (such as legionella) to develop or for mineral deposits to accumulate in poorly maintained systems

Evaporative cooling is also important in conventional refrigeration, where refrigerant gases absorb heat from the cooling medium (typically water) as they evaporate, and release heat when they condense, which is rejected to the outside (or recovered). The exact opposite of this process is used to generate heat in heat pumps.

The efficiency of conventional chillers can be improved by spraying water over the heat exchange coils to increase heat loss due to the evaporative cooling effect.

See refrigerants and heat pumps for more information.

A newly-emerging application of evaporation is in the use of phase change materials (PCM). These are generally materials with a large specific latent heat capacity. They can be used in construction to reduce internal temperature changes by storing latent heat in the solid-liquid or liquid-gas phase change of a material. Heat is absorbed and released almost isothermally and is used to reduce the energy consumed by conventional heating and cooling systems by reducing peak loads.

See phase change materials for more information.

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