A solar pool heating system is an economical way to extend your swimming season and improve comfort in your outdoor pool.
By the end of 2010, the United States had 15.3 billion kilowatts of installed solar heating capacity—and roughly 85% of this capacity was used to heat swimming pools. In fact, there are more solar pool heating (SPH) systems installed in the United States than any other type of solar energy system. The widespread adoption of SPH can be attributed to the popularity of residential swimming pools in Florida, California, Arizona, and Texas. In these climates, outdoor pools can be used almost year-round, and SPH provides the most economical solution for actively heating these pools during stretches of cool weather.
Auxiliary pool heating is an option for pool owners who want to extend the swimming season and increase comfort by providing greater control over pool temperatures. For pools that are significantly shaded, are used as therapy pools, or are used so much that a cover may be impractical, active heating is typically required.
In heated pools, once water from the pool is pumped through the filter, it is sent to a solar pool heating collector array or a thermostat-regulated auxiliary heater—or both. If the water is at or above the set temperature, the auxiliary heater will not activate.
A common auxiliary heater is typically fueled by propane or natural gas, or an air-to-water heat pump. Gas heaters are the most responsive—an appropriately sized heater can reasonably raise the pool temperature 5°F in four hours. A heat-pump pool heater may take six to eight hours to raise the same pool water 5°F. A properly sized SPH system may require several days. The other trade-offs between these technologies are cost and environmental impact.
As shown by the “Pool Heating Fuels & Costs” table, the most economical method for pool heating is a SPH system. The initial costs of a SPH system are comparable with heat pump and liquid fuel pool heaters, while the operating costs are far less.
The lower cost of SPH systems is due to their simplicity and efficiency. For most SPH systems used for outdoor pools, the only moving part is a motorized valve activated by a differential controller. The controller uses two sensors—one near the solar pool collectors, where it is exposed to the same solar radiation as the collectors, and one that measures the water temperature from the pool. When there is enough solar energy available to heat the pool water, the valve is opened. (A common temperature differential is 4°F, although this temperature can be field-set on some controllers.) The pool water is pumped through the collectors before being returned to the pool. When the pool reaches the controller’s high temperature setting or there is not sufficient solar radiation to maintain a collector temperature that is 1°F to 2°F more than the pool water, the motorized valve closes and the mechanical system functions as it would in a pool without a SPH system.
The difference in temperature between the fluid entering the collector and the ambient air temperature has a significant impact on a system’s efficiency. If you are trying to heat your outdoor pool to 80°F on an 85°F day, unglazed pool collectors are the best tool for the job.
When the pool water entering the collector is equal to the ambient outdoor temperature, unglazed collectors can reach efficiencies near 90%, meaning that 90% of the solar radiation that strikes the collector is converted to heat.
If your priority is to heat an indoor swimming pool to 80°F during a 30°F winter day, then glazed flat-plate collectors will be much more effective. Since the glazing decreases the amount of sun hitting the absorber, glazed flat-plate collectors have a maximum efficiency of about 80%; the maximum efficiency of certain evacuated-tube collectors is roughly 50%. When you consider that medium-temperature collectors typically retail for four to five times the cost of unglazed pool collectors, using them does not make sense for most pool applications.
Given the popularity of pools and solar pool heating, there are a number of products on the market that are readily available in department stores and home centers. Some do-it-yourselfers and frugal types have also been known to construct their own devices that behave similarly to the manufactured collectors. When selecting a pool collector, consider these factors: