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Heat loss from the water can occur in several different ways, such as conduction through the walls, but most is lost by evaporation. When compared to evaporation, all other losses are comparitively small. Evaporation uses up huge amounts of energy, so to restore water temperatures, that energy has to be put back in to the water. According to information from the United States Department of Energy, it only takes 1 BTU of energy to raise 0.22 gallons of water by 1°F. However, each 0.22 gallons of 80 °F water that evaporates takes a very large 1048 BTU's of heat out of the pool. The metric equivalent means that it takes 0.001163 kilo watt hour of energy to raise one litre of water 1°C and each litre of water at 27°C that evaporates wastes 1.218 kWh of energy. Testing on outdoor pools clearly show how evaporation from the surface area is key in heat loss from the water. Typical Heat Loss: 20% radiation into the air, 10% conduction to surrounds, 70% evaporation. Evaporation rates are determined by four factors: The temperature of the water. The temperature of the air. The humidity of the air. The wind speed at the surface of the water. The higher the pool temperature and wind speed and the lower the humidity air temperature, the greater the evaporation rate. How can this energy problem be addressed? The simple answer is use a pool cover. To gain the maximum benefit from your cover it should be placed over the water surface area at all times when the pool is not in use, and only taken off just before you would like to swim, and then replaced afterwards. Covers are manufactured in a wide range of materials, offering different weights and colours. Any type of pool cover, whatever the fabric, will achieve: Reduced energy consumption by up to 50%. Lower chemical usage by up to 60%. Eliminate water evaporation by 98%. Potentially increase water temperature up to 8°C. Reduce growth of algae. Absorb the heat of the sun. Possibly reduce the pump running time by half. Reduce debris contamination and subsequent cleaning costs. Types of Covers. Solar covers offer the advantage of solar gain, raising the water temperature, in addition to the evaporation reduction of sealing the surface of the water. These covers are usually used on domestic outdoor pools. A solar cover will usually last for between 5 and 10 years. The initial cost of buying these covers is therefore more than recouped during the life of the cover. Heat Retention blankets are produced from 5mm thick foam fabrics. These are usually very robust and excellent at reducing evaporation, but they offer no solar gain.
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