Evaporation is a function of the temperature difference between the water and the air, as well as the relative humidity and wind conditions. Evaporation is far greater in hot, dry, windy climates where the air has a greater capacity to absorb water than in cooler, wet climates where the air is closer to its saturation point.
When a pool is uncovered, evaporation will occur. For example, with a pool temperature of 82°F, an air temperature of 86°F, and a relative humidity of 50%, an uncovered 20- by 40-foot pool will lose 4.5 gallons of water per hour. If the pool temperature is increased to 86°F, the pool will lose 6 gallons per hour to evaporation. In areas where rain is prevalent, this evaporation can be replenished easily. (At a rate of 6 gallons of water per hour, it would take more than 80 hours of uncovered pool time for the water level to drop 1 inch.)
In arid regions, evaporation will be much more severe, and a lack of precipitation may require the pool owner to add water periodically. Pool chemicals also evaporate, which increases the costs and impacts of pool maintenance.
But the loss of water and pool chemicals from a pool pales in comparison to the heat loss that occurs when water is lost. When a gallon of water evaporates from a pool, it removes the equivalent of almost 8,700 Btu of heat. As a result, evaporation is the most significant source of heat loss from a pool.
Pool covers are extremely effective at minimizing evaporative losses. In the next issue of Home Power, we will explore this and other efficiency measures that can be used to minimize a pool’s heating, chemical, and electrical demands.
| A pool cover can reduce heat loss due to evaporation and radiation, making a solar pool heating system even more effective. |
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