This article was originally published in the May/June 1996 issue of Home Energy Magazine. Some formatting inconsistencies may be evident in older archive content.
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Home Energy Magazine Online May/June 1996
Swimming Pools Soak Up the Sun
By Dan Cuoghi, Paul Hesse, and Thomas Schiller
Dan Cuoghi, Paul Hesse, and Thomas Schiller are staff members of the Energy Efficiency and Renewable Energy Clearinghouse (EREC). EREC is operated by NCI Information Systems Incorporated, for DOE's National Renewable Energy Laboratory (NREL).
Solar pool heaters survived the boom and bust solar years of the 1970s and 1980s. Today they are even popular and cost-effective in parts of the country where many people think solar is impractical.
Solar pool heaters may cost more to install than conventional gas or electric pool heaters, but the higher purchase price is offset by low maintenance and no fuel costs. By circulating the water through the collectors at night, solar systems can even cool the pool during hot summers and thereby reduce evaporative water loss.
Solar pool heaters are the simplest and least expensive solar water heating systems. The solar collectors raise the temperature of the water approximately 5oF-10oF between the inlet and the outlet of the collector. Because they do not have to achieve high temperatures, the collectors do not need to be glazed, or encased in a glass-covered, insulated box, like solar domestic hot water systems. In fact, on calm, hot days solar pool heaters operate so close to the ambient air temperature that they do not lose much heat to the environment. The collector transfers most of the solar energy that strikes it to the water, often operating at efficiencies of 80% and higher. A properly operating solar pool collector feels cool to the touch.
Solar-heated pool water heats up slowly as the water circulates continuously through the collectors. Over successive sunny days, the pool temperature will increase by 10oF-20oF. The system keeps heating the pool as long as the collector temperature is higher than the pool water temperature. During the middle of the summer it may be necessary either to bypass the collectors during the day to keep the pool from getting too hot, or to operate the system at night so that the collectors cool the pool.
Solar pool-heating systems (see Figure 1) work in conjunction with the pool pump, filter, and chlorinator (if used). The pump pulls water from the pool and forces it through the filter. The filtered water passes through a pressure relief valve and then a check valve. The check valve prevents backflow of water in the collectors from reentering the filter. From there the water flow is controlled by a bypass valve. When the valve is closed, water flows to the collectors and then continues its normal path back to the pool. When the valve is open, water bypasses the collector loop and flows to the conventional heater (if used) and chlorinator and then on to the pool. The bypass valve can be either a manual valve, or more conveniently, a valve operated by an automatic differential controller.
A controller monitors the water temperature at the check valve (or at any point between the pump and the bypass piping) and the temperature of the collector. As long as the pool temperature is lower than the collector temperature and the preset desired temperature, and the pump is operating, water will circulate in the collector loop. When the collector temperature drops (at night or during cold weather), the bypass valve opens. The collectors then automatically drain out the return.
There are also two manual isolation valves in the collector loop, one leading to the collectors and the other located on the return. These make it possible to isolate the collectors during servicing, while allowing the pool filter to continue to operate. A vacuum break or air vent at the highest point of the collectors allows air to enter or exit when the collectors fill and drain.
There are four types of unglazed solar pool heater collector (see Figure 2):
Each system has its advantages and disadvantages. Ultimately, the choice of a system comes down to site-specific factors, such as the amount of space available for the collectors, and the individual user's preferences.
Metal tube-on-sheet collectors conduct more heat per ft2 of collector area, so they take up less space but are more susceptible to freezing. Copper collectors require careful monitoring of the pool pH level. If it falls below 7.2, becoming relatively acidic, the chlorine in the pool water may react with the copper, causing it to lose ions to the water. High concentrations of these ions may cause a dark film to form on the pool walls. This film can only be removed by draining the pool and cleaning the walls, or even repainting them. Be sure to follow the operating and maintenance instructions for both the solar system and the pool itself. The solar collectors should be bypassed when adding pool chemicals and when chemically shocking the pool to kill bacteria. Wait until the pH returns to normal levels (between 7.4 and 7.8) before putting the collectors back on line.
Even though they are less efficient than metal collectors, plastic-panel and rubber-mat collectors are more commonly used. Plastic and rubber have thermal conductivities as much as 1,000 times lower than those of metals, so they require a larger collector surface area. However, since they are less expensive per ft2 than metal collectors, the overall capital cost is about the same, and they are not susceptible to corrosion from pH variations.
The life expectancy of collectors varies depending on the material they are made of. Plastic and rubber collectors should have UV inhibitors added to retard degradation from sunlight. They will typically last 10 to 15 years. Collectors that do not have any UV inhibitors may last only one year. Metal collectors will last decades if they are properly maintained.
Simple, low-cost, do-it-yourself systems can be made by simply laying out black plastic pipe in the sun. However, they may perform poorly compared to commercially available systems. Commercially available pool collectors must undergo stringent testing before they can be certified by the Solar Rating Certification Corporation (SRCC).
The pool itself is a solar collector. Shading during the day will increase the collector area needed to heat the pool. A pool cover will reduce the collector area required (see Putting a Cover on Heat Loss).
On average, the collector area must equal about 75% of the pool area to provide all of the pool heating. A key factor is how warm the users want the pool to be during the coolest month that they will use it. A higher temperature requires more collector area, which is more expensive.
In many cases, pool owners who already have a gas or electric heater will choose to use solar for only part of the pool heating load. This choice may be based on first-cost and space considerations. Collectors cost $150 to $200 each, and a site may have room to accommodate only three or four collectors.
Proper pump sizing is important for overall pool energy consumption and solar heater efficiency. It is possible to use an existing pool filter pump when installing a solar pool heater on an existing pool. However, this increases the amount of work the pump must do. The pressure in the filter is usually 10 to 15 pounds per square inch (psi). Depending on the type of solar pool collector, its distance from and above the pool, the diameter of the piping, and the number and type of elbows, the collector loop could add another 4-5 psi. If this pressure exceeds the filter capacity, a new pump, a booster pump, or a larger filter may be necessary. The diameter of the pipe to and from the collectors is typically 1 1/2 inches. Increasing the diameter to 2 inches, using 45o instead of 90o elbows, and using flexible hoses will reduce pressure losses in the system (see Pool Bills Take a Dive, EA&R Mar/Apr '86, p. 35).Collector Siting and Mounting Proper collector siting and mounting is important for maximum heating efficiency. The collectors should have complete exposure to the sun between 8 AM and 4 PM. Although true south orientation is best, orientation 15o east or west of true south will have little effect on the overall heating of the pool. Adding extra collector area can help to compensate for situations where the collectors cannot be sited within 15o of true south.
The collectors can be mounted on a roof, on a rack mounted on the ground near the pool, or on any smooth surface that has unobstructed solar exposure. Rubber mat collectors are usually glued to the roof.
The collectors also can be positioned at a fixed tilt to maximize their exposure to the sun during a particular season. The optimum tilt depends on when solar heat is needed most and on the site latitude. However, since setting the collectors at this tilt will typically increase performance by less than 10%, most systems are installed flat against sloped roofs. The expense of mounting collectors at a better tilt on flat roofs in northern latitudes (at least $100 for materials for a small application) has to be gauged against the benefits of improved collector performance.
Local building codes, covenants, and restrictions may affect how and where solar collectors can be sited.
Installing a conventional electric, propane, or natural gas heater for a 450 ft2 pool costs around $1,800. The cost of a solar pool heating system varies, depending upon location, seasonal use, and desired water temperature, which determine the amount of collector area needed. Solar collectors cost about $8-$12 per ft2 on average.
We used DOE's pool energy software to analyze typical pools in Michigan and Florida. We input a pool size of 450 ft2 and used average fuel costs for the two areas. We assumed use of a pool cover and that for 14 hours per week the pool was uncovered and used.
Installing a solar heating system instead of a conventional system on a new outdoor pool in Michigan came out very attractive economically. To maintain a minimum average temperature of 80oF from May 1 through September 30 in Detroit would require about 200 ft2 of solar panels. These would cost approximately $1,800 to install-about the same as a conventional system. Because a solar pool heating system has no fuel costs, it pays for itself the moment it is installed.
On the other hand, if the pool in Michigan is currently heated conventionally, retrofitting to solar is probably cost-effective only if the current system is electric. Propane could keep a 450 ft2 pool at 80oF from May 1 through September 30 for about $130 per season. Using natural gas would cost approximately $25 per season, while heating the pool with electricity would cost about $230 per season. The solar heating system's return on investment would be 8 years to replace the electric system, and 20 or 70 years to replace the propane or natural gas systems, respectively. However, this assumes the diligent use of the pool cover. A less effective cover, or a pool that is used all day long, could shift the results dramatically.
The economics of solar pool heating systems improve in Florida, where pools can be used all year. Maintaining a minimum temperature of 82oF in a 450 ft2 outdoor pool in Miami (again with an effective cover) for an entire year with propane, natural gas, and electricity costs about $280, $220, and $550 respectively.
For a new pool, a solar pool heating system in Florida came out cost-effective when compared to all conventional heating sources. Maintaining a year-round average pool temperature of 82oF in Miami in a 450 ft2 pool requires 360 ft2 of solar collectors. The installed cost is $3,240-almost twice that of a conventional heating system, but the payback from fuel savings is only 5 years compared to propane, 6.5 years compared to natural gas, and 2.5 years compared to electricity. Replacing existing conventional systems with a solar pool heater would pay back in 11, 15, and 6 years respectively.Resources The following organizations have information on solar pool heating:
Florida Solar Energy Center
Solar Energy Industries Association and Solar Rating Certification Corporation
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