Opaque cover. For large pools, an opaque blanket with a thin layer (roughly 1/8-inch thick) of polyethylene insulation is sometimes used. Uninsulated, opaque covers are also commonly used with certain automated pool reels or as a safety cover to prevent children or animals from falling into the pool. Opaque covers minimize evaporation—but they also reduce or eliminate passive solar gain. This may be preferable in some climates, such as Phoenix, Arizona, in the summer, but detrimental in others. More robust insulated covers are used with spas and hot tubs due to the high temperature difference between the water and the outside air. Uninsulated vinyl covers cost from $0.20 to $0.40 per square foot, while insulated polyethylene covers may cost $2 to $4 per square foot.
Solar pool cover. A solar pool cover minimizes evaporation without fully sacrificing passive solar gain. These bubble-type covers may reduce the passive solar gain by 5% to 15%, but they also limit the heat lost from the pool’s surface due to evaporation and radiation. When the pool is in use, the solar pool cover is stored on a manual or automatic reel. The polyethylene used for solar pool covers is treated to protect it from UV degradation. Solar pool covers vary in price from $0.20 to $2 per square foot. Since the durability of the cover can vary significantly by manufacturer, it is important to consider the warranty duration.
Solar rings. For those who are less inclined to handle a single large pool cover between uses or with irregularly shaped pools that don’t lend themselves to a standard pool cover, several smaller pool rings can be used. The 5-foot-diameter discs are made from UV-resistant vinyl and contain magnets along the perimeter to link the individual rings together. The rings contain two layers of vinyl and are slightly inflated to provide additional insulation. Since they are circular, these rings are able to cover up to 75% of the pool surface, thus reducing a majority of the evaporation. Each sun ring costs approximately $30 to $40.
Liquid pool covers. There are a number of alcohol-based liquid pool covers that are applied to a pool through apparatuses that slowly release the liquid. For residential pools, these devices may be placed directly into the pool or in the filter basket. The liquid is biodegradable and specific brands have been deemed nontoxic for pool occupants. Liquid pool covers disperse across the pool’s surface, creating a thin film and inhibiting evaporation of the water below. A liquid pool cover costs about $10 to $15 per month for a 20- by 40-foot residential pool.
Residential pools require mechanical systems to maintain water quality. A pump sends pool water through a filter before returning the water to the pool. The pump is a significant energy user, and can be as large as 2 horsepower and may run for 10 or more hours per day—possibly requiring 20 kWh or more of electricity per day. At $0.15 per kWh, that adds up to more than $3 per day.
Though it may seem counterintuitive, it is best to use a pump that has to run longer to filter the contents of the pool. Let’s look at a 20- by 40-foot pool that holds 24,000 gallons of water. The pool’s mechanical system should be set up to filter the entire contents of a pool each day. If we wanted to run the pump for only four hours, it would require a flow rate of 100 gpm and a 2 hp pump. A relatively expensive filter and larger-diameter piping are required to accommodate this high flow. A 0.5 hp pump could be used instead that would provide a flow rate of roughly 50 gpm. Though this would double the required pump run time (to eight hours), it would cut the total electricity consumption in half. In addition, the pump would cost about $200 less and the filter would cost about $300 less.
Some states, like Arizona, California, and Florida, actively promote energy-efficient pool pumps. In Florida, a pool pump with a capacity greater than 1 hp must be multi- or variable-speed. The lower speed(s) are used for standard filtration, and the higher speed(s) can be used if an SPH system requires higher lift to the collector array. In addition, the code stipulates that the pump must operate at a speed that requires six or more hours to filter the volume of the pool.
A variable-speed pump integrates well with an SPH system, and pump speed can be preprogrammed. During the day, when solar energy is available, the flow can be increased. In the evening, when the pump is needed only for filtration, the flow can be decreased. Compared to standard single-speed pumps, these pumps can reduce electrical consumption by up to 90%. Unfortunately, variable-drive motors have not yet found their way into the aboveground pool pumps market.
A multiple-speed pump has a manual switch for selecting between two or more pump settings, but is less efficient than a variable-speed pump. The lowest speed uses the least energy, which allows extended filtration time to improve filtering and reduce total energy use. Two basic strategies can be used in conjunction with multi- and variable-speed pumps.
Appropriately size the filter and keep it clean. A filter that is undersized or excessively dirty will restrict the pump’s flow, which increases run time needed to adequately filter the water. The filter manufacturer can provide pressure loss and flow rate specifications for sizing. The pool owner can determine whether the filter needs cleaning by monitoring the pressure gauge on the filter.
Properly size the piping. PVC pipe is used in pool systems. The larger the pipe diameter, the less the flow restriction. Typically, 1.5- to 2-inch PVC is used. The farther away the pump and filter are from the pool, the larger the pipe that is needed. This is not usually a consideration with unheated pools but can be with systems that require long runs to an SPH.
Jeff Simons, manager at Sandollar Spa & Pool, a swimming pool and sauna company in Brewer, Maine, says that the efficiency measure most pool owners overlook is a pump timer. “Most pools with conventional pumps can be maintained by running the pump for 10 to 12 hours a day,” says Simons. “However, without a timer, the owner may forget to turn the pump on or off, either of which quickly increases the operational cost and energy use.”
Vaughan Woodruff is a contractor, engineer, and educator. He owns Insource Renewables, a design/build and consulting firm. His pool energy strategy is to relax at his generous neighbors’ pool in his hometown of Pittsfield, Maine.
Conserving Energy and Heating Your Swimming Pool with Solar Energy • NREL • bit.ly/NRELpool
Pump efficiency info • bit.ly/PoolPumpEff