Evaporative air-cooling uses less energy, is healthier and has lower running costs than standard methods. Seeley's Steve Kirkwood, left, discusses the benefits and ponders why it is not more widely used
HEAT exists in two forms, sensible and latent. Sensible is the heat you can sense. It is a natural property that can be detected with a thermometer. A pavement heated by the sun, a stove burner, or a hair dryer are examples of sensible heat.
Latent heat is hidden heat that cannot be detected with a thermometer. The heat used to evaporate water into vapour is called latent heat of evaporation. For example, it is the heat from the hot pavement that is given up to evaporate the water after a summer rainstorm.
As the liquid water changes its state into vapour, it absorbs heat from its surroundings; the temperature does not change, but the amount or energy it absorbs is contained in the molecular structure of the vapour.
Evaporative air conditioning (EAC) is only possible because of this natural phenomenon of latent heat.
Total heat is the sum of latent heat and sensible heat. It is the total amount of heat in a room, made up of heat you can feel and heat you can not. Total heat is measured in kilojoules (kJ). The complete evaporation of one litre of water absorbs about 2,000kJ of heat energy, and that occurs within the process without any external energy input. In this way, evaporative air conditioners use a small amount of electrical power to operate.
Efficient cooling
Until recently, companies have discounted the use of EAC through lack of knowledge. Instead, they have either had to put up with no cooling at all or have installed extremely expensive refrigerative air conditioning systems. This is unfortunate both for the user and the environment.
Evaporative systems use less energy because, unlike refrigerative systems, they do not have to use a compressor to compress vapour and condense it back into liquid to repeat the cooling cycle. Instead, evaporative coolers just use fresh water.
According to a World Bank report, four million EAC units in operation in the US provide estimated annual energy savings equivalent to 12 million barrels of oil, in addition to an annual reduction of 2.4Mtonnes of CO2 emissions.
In industrial applications, EAC may be used to moderate the effects of an additional internal heat source that does not depend on the climate or the time of the year. For example, temperatures may rise inside warehouses or buildings because of the operations of ovens, machines or the presence of people or livestock.
These heat sources exacerbate already high ambient temperatures and may need cooling year-round. In many of these cases, EAC is the only practical solution, as air conditioning cannot function or compete effectively because of high installation and operating costs.
Historically, in hotels, restaurants and offices, air conditioning has been the first choice. Now, because of cost savings, the added advantages of air washing (to avoid sick building syndrome) and forced ventilation, many commercial establishments are finding EAC the most cost-effective solution.
Evaporation is an efficient natural heat exchange process because there is no third element in the exchanger. As the air is in direct contact with the refrigerant, in this case water, the heat is transferred extremely efficiently.
In compressor systems, the exchangers involve a third element - the metal coil in the evaporator, which interferes with the efficiency of the heat transfer. With compressor systems there are two heat exchangers, the evaporator and the condenser, so there is a double negative effect.
The only power-consuming components of a direct evaporative cooler are the fan and small water pump. The energy savings of EAC systems vary with humidity levels and temperatures. However, typically in the UK, EAC systems will yield 50-70% energy savings compared with conventional air conditioning. For example, in a typical hourly cycle, a 3.5tonne refrigeration unit consumes 8,698W of electricity, compared with an equivalent evaporative cooler, which consumes 1,360W.
Best solution
Evaporative air conditioning uses between 25% and 70% less energy than air conditioning, brings in 100% outside air, and utilises water for cooling instead of CFCs, HFCs and HCFCs. In addition, maintenance is minimal requiring just an annual pad clean, or change, compared with air-conditioning, which requires a bi-annual filter change.
Last, but not least, there are no CFC pollutant emissions from evaporative air conditioning, and water consumption is moderate. That all adds up to environmentally responsible and energy-saving behaviour.
Conventional air-conditioning impacts significantly on a building's operating costs as it can use in excess of 25% of the building's energy consumption. It therefore makes sense to create a pleasant environment for workers to maximise productivity while at the same time minimising energy costs.
Evaporative cooling enhances the well-being of individuals in areas where ambient temperatures are high, providing comfort cooling in buildings such as schools, libraries and offices. It is also the best, and often the only solution for commercial and industrial applications such as greenhouses, laundries, warehouses, factories and commercial kitchens.
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