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Heating Your Swimming Pool With Solar Energy

Summary: This section discusses options for reducing the costs for operating and heating your pool, including solar pool heaters. Swimming pools provide a great way to exercise and beat the summer heat. Building and maintaining a pool, however, also means relatively high costs added to your household's budget. There are several ways that you can reduce operating and maintenance costs, lower water consumption, and conserve heat if you heat your pool. Many people heat their pools to extend the swimming season and/or to keep it at a temperature that they are personally comfortable with. This lets them enjoy the full value from their pool. Solar pool heaters are an option to heat the pool with "clean" energy from the sun, and can reduce heating costs.

Optimizing Water Filtration and Circulation Pumping: A study by the Center for Energy Conservation at Florida Atlantic University shows that pool owners can save energy and maintain a comfortably heated pool by using smaller and higher efficiency pumps, and operating pumps less each day. In this study of 120 pools, some pool owners saved as much as 75 percent of their original pumping bill when they used both conservation measures (see Figure 119).

Figure 119: Savings from Pump Conservation Measures

Condition

Energy Use (kWh/year)

Cost of Energy ($/year)

Energy Savings

Original

3000

240

----

Pump replacement (downsizing)

1800

140

40%

Reduced time (60%)

1200

100

60%

Combination of above

720

60

75%

Table courtesy of Home Energy magazine. These savings represent a typical pool in Florida. The average pool pump energy bill is probably higher in Florida than in many other areas of the country because of the long swimming season. While the absolute savings here will be greater there than elsewhere, the percentage savings should apply nationwide. Note that the savings for the combination of measures are not simply the sum of savings for the individual measures. When both are implemented, the energy use is 60 percent of 40 percent of the original use- percent savings.

Sizing the Pump Right: When a pump wears out or can't be repaired, a pool owner typically installs a larger one, thinking that "bigger must be better." Instead, a larger pump may increase the costs of pumping and maintenance. To choose the right size pump, consult design charts that match the hydraulic characteristics of the pump to both the piping and the pool's flow characteristics. A local pool supply dealer should have access to these charts.

The Florida study shows that a 0.75 horsepower or smaller pump is generally sufficient for residential pools. Smaller pumps, which cost less, can be used if you decrease the pool circulation system's hydraulic resistance. This can be done through one or more of the following ways: substituting a large filter (rated to at least 50 percent higher than the pool's design flow rate), increasing the diameter or decreasing the length of the pipes, or replacing abrupt 90-degree elbows with 45-degree elbows or flexible pipe. These types of changes can slash up to 40 percent off the pump's use of electricity (see table above).

Circulating the Water: Another way to save energy is to reduce the pump's operating time. Pool pumps often run much longer than necessary. Pool owners need to understand the reasons behind circulating the pool's water.

Circulating water keeps your pool's chemicals mixed. However, as long as the water circulates while chemicals are added, they should remain evenly mixed with minimal daily circulation. Secondly, circulating the water keeps the pool free of debris by drawing water out and through the filter. It is not necessary to recirculate the water completely every day to remove debris and clean the water. One complete circulation usually takes between 6 and 12 hours per day. But this may be longer than necessary since most debris either floats or sinks, and can be removed with a skimmer or vacuum. After about an hour, most of the pumping power is wasted by circulating clear water and does little to improve the water's quality.

Furthermore, longer circulation does not necessarily reduce the growth of algae. Instead, using chemicals in the water and scrubbing the walls are the best methods. In the Florida study, most people who reduced pumping to less than 3 hours per day were still happy with the water's quality. On average, this saved them 60 percent of their electricity bill for pumping. Combined with using a smaller size pump, pool owners saved up to 75 percent of their original electricity bill for pumping water (see table above).

One last simple measure for saving money is to use an accurate timer to control the pump's cycling. Use a clock that can activate the pump for many short periods each day if debris is a problem. Running the pump continuously for, say, 3 hours leaves the other 21 hours a day for the pool to collect debris. Several short cycles keep the pool cleaner all day.

  • Summary of Recommendations: Pool owners can save a great deal of money by:
  • Choosing the smallest pump and the largest filter suitable for their pool

  • Decreasing the hydraulic resistance of the pool's circulation system wherever possible

  • Circulating the water with the pump for the shortest time possible (often fewer than 3 hours)

  • Installing a timer.

Keep in mind, however, that all pools are different. Circumstances such as special cleaning and heating needs, climate, pool size, and usage all affect a pool's circulation, as well as the potential for saving money.

Reducing Water and Heat Loss: Almost all of a pool's heat loss-about 95 percent-occurs at the surface, mostly through evaporation to the air and radiation to the sky. A pool cover is an effective means to keep heat (and water) in a pool by reducing evaporation of water from the pool when it is not in use, and reduces radiant heat losses. A pool cover can reduce water loss by 30 to 50 percent. Each gallon of 80-degree water that evaporates removes around 8,000 Btu from the pool. Reducing water loss also reduces the amount of chemical water treatment required.

Outdoor pools can gain a significant amount of heat from the sun, absorbing 75 to 85 percent of the solar energy striking the pool surface. A bubble cover (sometimes called a solar cover) is one of the least expensive covers made specifically for swimming pools. It's similar to bubble-packing material except it has a thicker grade of plastic and ultraviolet (UV) inhibitors. Vinyl covers are made of a heavier material, which extends their use. You can also get vinyl covers with a thin layer of flexible insulation sandwiched between two layers of vinyl.

A transparent bubble cover may reduce solar energy absorption by 5 to 15 percent, and an opaque cover may reduce it by 20 to 40 percent. However, the decrease in solar gain can be balanced or more than offset by the cover's retention of the pool's heat, which depends on the air temperature and humidity. Generally, the drier and cooler the air, the greater the heating benefit from covering the pool during the daytime. Of course, a cover should always be used at night to prevent losses when there is no solar gain.

A cover also helps you keep the pool clean and extend the life of the chemicals in your pool. At a cost of 20 to 60 cents per square foot, a pool cover may pay for itself in 1 year. Be aware, however, that UV radiation deteriorates the cover, requiring that you replace it every 3 to 5 years. Before you buy one, make sure the cover comes with at least a 2-year warranty. Also find out how easily you can place the cover over the pool, how to remove it, and how to store it. Systems are available that move the cover off and on the pool with a motor or hand crank.

Covers should always be installed and used according to the manufacturers guidelines. Always consider that people could use the pool unauthorized or unsupervised.

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