About Solar Heating For Swimming Pools
Introduction
Solar swimming pool heating is the most cost-effective application of solar energy. Unglazed collectors (solar panels), which are the simplest and least expensive of all collectors, are used almost universally for heating outdoor pools and spas. The heating system itself is normally already required for pools, regardless of whether a solar heating system is added.
Why Unglazed Collectors are used for Outdoor Pool Heating
Since many solar collectors have expensive glass covers and insulation, many people need considerable convincing that an unglazed collector will work at all. Nevertheless, it not only works, but is highly cost effective for outdoor pool heating.
Most of the heat loss from a pool is directly from the surface of the water through direct conduction and convection to the air, and through evaporation. If the temperature of the water is much higher than the temperature of the surrounding air (called the "ambient" temperature) surface losses will become very large. Thus, it is not practical by any means of heating to maintain the pool temperature more than 15 or 20 degrees above ambient temperature. Even if it were practical, few persons would want to swim in 70 degree water when the air temperature is 40 degrees. If the air temperature is comfortable, the water temperature cannot be comfortable at a much higher temperature. For these reasons, it is not generally desirable to heat a pool more than a few degrees below or above ambient air temperature.
Since the temperature difference will always be maintained at about 20 degrees or less, the efficiency of unglazed collectors will always be near that of the glazed collector, or higher.
The reasons for the higher efficiency at lower temperature differences are as follows: First, the radiation losses from the collector are so small they are negligible at these small temperature differences, regardless of whether the collector is glazed or not. Conduction and convection losses are greater for the unglazed collector, but they are smaller than the transmission losses caused by the glazing on the glazed collector, unless there is a high wind to drive forced convection. Even in a 15 MPH wind, where the efficiency of the unglazed collector may be less than that of a glazed collector, it is not enough less to justify the added cost of the glazing, so the unglazed collector is less expensive for the same heat output.
During a realistic outdoor swimming season, while solar collectors are operating, the air temperature is frequently higher than the pool temperature, so the efficiency becomes higher for unglazed collectors than for glazed collectors.
Please note, that although the conclusions drawn above are correct, this is not intended to be an accurate engineering analysis of pool heating. Also, remember that this applies to outdoor pools only.
Typical Pool Heating Installation
The solar pool heating schematic diagram, illustrates in a simplified form a typical solar pool heating installation using motorized 3-way valve control.
The system consists of the following basic subsystems:
A. Collector subsystem
B. Circulation subsystem
C. Control subsystem
D. Optional backup heater
The collector subsystem consists of the necessary number of collectors to replace the heat losses from the surface of the pool during sunny days for the design swimming period, and perhaps to supplement the supply of losses during an additional portion of the year.
The circulation subsystem includes plumbing, motorized 3-way valve and pump. In the typical pool heating schematic, note the check valve used to prevent direct drain down of the collectors back through the filter. Most pool systems have this valve in the main line, so the valve may not be needed in the location shown. However, you must provide a bypass around the heater for drainage to prevent freeze damage. The vacuum relief valve is needed at the highest point in the circulation system to insure that air can enter to replace the water and permit the collectors to drain completely when not circulating, for protection against freezing.
The control subsystem consists of a differential thermostat control for the circulation subsystem. A temperature sensor on a the roof detects the temperature available at the collector and transmits this information to the control unit to be compared with the temperature of the pool water. When the collector temperature is higher than the temperature of the water by a predetermined amount, the control unit turns on the collector pump or actuates the control valve(s). This causes the water to circulate through the warm collector and to be delivered to the pool. If the temperature of the collector falls below the required difference, the control unit causes the collectors to be bypassed to prevent the water from being cooled. If a heat exchanger is not used, the collectors automatically drain back into the pool (from elevated units with positive drainage) when circulation to them is interrupted.
Note that, if the collectors do not naturally drain when the solar circulation stops, some form of freeze protection must be provided.
For the most economical heating system with high reliability, a fired backup heater should be used. The backup heater automatically maintains the pool temperature during periods of low insolation, and also provides quick recovery capability. The backup heater also improves the average efficiency of the solar system, since the solar system will provide pre-heated water to the fired heater, which would not otherwise be usable in periods of low insulation.
As in all solar applications, the designer should never design a pool solar heating system to provide 100% of the heating requirements. A good figure for southern California is 80 to 90% during the normal swimming season only. The remaining 10 to 20% of heating is much more economically supplied by conventional gas heaters.
To meet this optimum percentage of heating for pools in the southwest, the "average" pool installation requires a minimum area of unglazed collectors equal to 50% of the pool surface area. A suitable pool cover or pool blanket can be used to increase further the length of the swimming season. Dealers and pool contractors are cautioned to be sure they know the performance and durability of the pool blanket or cover to avoid disappointing their customers.
The manifolds on most pool collectors are designed to handle a maximum of about 40 gallons of water per minute. Since each collector is operated at 4 to 6 gallons per minute, no more than 10 collectors may be connected directly together. When more than 10 collectors are required, divide the collectors into banks of equal numbers of collectors and plumb each bank with a maximum of 1 1/2" diameter copper tubing or PVC pipe. Be sure to have all branch lines that feed the same number of collectors the same size to avoid causing unequal flow through the collectors, or use balancing valves.
Collector Mounting
Unglazed collectors are capable of collecting a large proportion of the diffused, or sky radiation from the sun, therefore they may be mounted close to horizontal with added area to make up for loss of output. However, output will be increased substantially if the collectors are tilted to an angle of approximately the same as local latitude as measured from the horizontal. If the collectors are tilted, they must face near south (in the northern hemisphere).
Some metal pool collectors are self-supporting, and may be mounted on posts or directly on the ground or decking, as long as they are in a location with full sun exposure.
The designer should be very careful to observe obstructions to full sunlight, such as trees and adjacent buildings. In particular, watch for young trees that may later grow to a height that will cause shading of the collectors. It is also easy to overlook possible shading when the site is visited during the season when trees have shed their leaves. When looking for possible obstructions, make a visual sweep of the entire horizon from about 20 degrees to the east of south around to due west.
Note: For large commercial pools, detailed heat load calculations may be needed which are beyond the scope of this page of information.
Contact AMECO to evaluate your home for a solar swimming pool heating system installation
