Pool Water Evaporation

There is a direct link between the energy consumption of ventilation systems and evaporation of water vapour from the pool. The amount of heat energy in the pool which is lost to evaporation depends on the air conditions immediately above the pool. This energy, together with a small amount of heat loss through conduction and radiation, represents a major part of the energy exchange from the pool to the pool hall air. Controlling this is therefore key to saving energy.

The rate of evaporation from the pool depends on the difference between the moisture content of the air at the water surface, the moisture in the air just above the surface and also the air velocity.

If the air above the surface can be as stationary as possible, it will become saturated and evaporation will be minimised. If the air moves, however, drier air will enter the space immediately above the pool, the moisture difference will increase and evaporation will increase. Thus the key to reducing evaporation is to minimise the air movement immediately above the pool. This will inevitably conflict with bathing activity in the area, so some evaporation will always take place when the pool is occupied. This is why the pool hall air must be continually heated even after it has been warmed up, to maintain bather comfort.

There must be some movement of air from the water surface for this is needed to remove the potentially harmful chloramines and trihalomethanes that are a natural consequence of pool water disinfection.

The key is to encourage a natural flow of air from the relatively wet air at the water surface to a drier area of heated air someway, perhaps 3m above the water surface.

Evaporation is also increased if the water is sprayed or agitated by water features such as wave machines or flumes, because the surface area is increased. To prevent wastage, limit the use of water features, rather than operating them continuously. As well as reducing evaporation, this will also reduce pumping energy and provide a more varied swimming environment. When the pool is unoccupied it may still be necessary to run the pool ventilation system to maintain environmental conditions within the pool hall to prevent condensation.

Pool covers

The use of pool covers out of hours reduces the evaporation rate from the pool surface and can save significant amounts of energy in swimming pools. 

The lower evaporation rate reduces the energy needed to keep the poolwater heated to the required temperature and the resulting reduction in energy use is significant.

The reduced evaporation lessens the need for high ventilation rates and air heating in the pool hall can be reduced or shut off. This allows the air circulation system to be shut off, or turned down to a very low rate, when the covers are on.

In practice, some water may get on to the top of the pool cover and there may be some evaporation at the sides where the cover does not quite fit. There may also be some residual moisture in the air after the cover has been deployed so it may still be necessary to run the ventilation system at a low setting. Nevertheless, an effective pool cover will reduce the need for dehumidification out of hours and therefore substantially reduce energy consumption. Even where covers do not match the shape of a free form pool, they are still worthwhile: evaporation is still reduced in proportion to the area covered and hence yields energy and cost savings.

A wide variety of covers is available, including motorised models which can be withdrawn to the side or bottom of the pool to minimise the labour input at changeover. When selecting a suitable cover, consider the following factors:

Pool covers also provide additional benefits in terms of building maintenance. A reduction in pool hall humidity and condensation can help to prolong the life of the structure and reduce building maintenance.

It is important to train staff to use covers properly – and regularly – in order to achieve maximum energy savings. Ensure staff and lifeguards have adequate safety training to ensure that they know what to do should anybody become trapped on top of or underneath a cover.

Insulated pool covers are one of the most cost effective investments available to swimming pool managers. Typical savings can be 10–30% of total pool energy use with a payback period of 18 months to three years.