ENERGY TIMES
SOLAR POOL HEATING by David Lankheet of Sundu Solar

It's spring and everyone's heart turns toward … opening their pool, if they have one. All the promises, and good memories, of owning a pool come back to them: family fun, better looking body through exercise and sunbathing, keeping that teenage daughter at home and under their watchful eye. There is a tendency to remember the good times when the pool was warm and fun but some hardheaded realists can't forget the times that their pool was too cold to enjoy (goose bumps) or even get in at all. Some of these pool owners have heard of solar pool heating but don't exactly know what is involved or how to go about it.

After a homeowner finds themselves with a swimming pool in their backyard, they soon notice that it needs to be warm enough to use. Children seem to be able to tolerate temperatures as low as 72-76 degrees Fahrenheit, teenagers 77-79 degrees, and most adults prefer 80-85 degrees. These age/temperature relationships are generalizations not truths. Everyone is different but age, metabolic rate and activity level are the main factors that determine what a person considers to be a comfortable swimming temperature.

A swimming pool is a solar collector in its own right and during the day the sun warms the pool. A pool loses heat through conduction to ground or air and evaporation. The amount of heat taken from the pool to evaporate water can amount to 80% of its heat losses. Relative humidity, wind speed and ambient temperature are the main variables that affect the rate of loss. On its own, a pool reaches equilibrium with its environment. Solar blankets (aka "bubble covers") greatly reduce evaporation and have a positive effect on a pools average temperature. Since they actually block a percentage of the sunshine, I prefer to call them evaporation covers. An uncovered pool gains 4-5 degrees per sunny day, loses 4 to evaporation and averages 70-72 degrees. A covered pool gains 3 degrees, loses 1 and averages 76-78 degrees. With periods of consecutive sunny days a covered pool can work its way over 80 degrees fairly often. A cloudy, rainy day will drop it down to the mid 70's again and the "3 steps forward 1 step back" process starts again. During an average summer a covered, aboveground pool will get above 80 degrees about 30 days of a 90 day season. If there is any tree shading of the pool during the day the average temperature will be correspondingly lower. A colder summer is worse and a warmer than average one is better. These facts mean that without supplemental heat, an average adult will feel comfortable in the pool 1/3 third of the season (or less). Since the adults are spending the money and maintenance time on the pool it makes sense that they should be able to use it more often.

There are three types of solar heating panels commonly used for residential applications: 1) Solar Air Panels - used for space heat 2) medium temperature Liquid Solar Panels - used for domestic hot water, radiant floor heating, and indoor pools. 3) low temperature Liquid Solar Panels - used in the summertime in northern climates or year round in southern climates for swimming pool heating.

A company called Fafco Solar in California developed the modern day solar pool-heating panel in 1976. It is a set of 200 tubes ¼" diameter extruded in a connected group 48" wide and made from a UV stabilized, black plastic called polyolefin. Most panels developed later, by competitors, use UV stabilized polyethylene because it is cheaper and easier to work with. The sets of tubes are cut in 8', 10' and 12 foot lengths and welded to 1 ½" or 2" plastic manifolds at each end. The manifolds of each panel in a multiple panel array connect to each other with rubber couplings and stainless steel band clamps.

Solar pool heating panels are typically mounted on a roof near the pool pump. The summer sun rises in the northeast, sets in the northwest and is a bit south of straight up at midday. Therefore a south facing roof with a 4/12 pitch (like a ranch house) is preferred, but a similar pitched roof facing west or east can be used with 25% upsizing. A simple ground rack can be constructed out of treated lumber if there is no appropriate roof nearby. All solar pool panel manufacturers that wish to market in Florida (and they all do) have to pass the "Dade county hurricane wind load test". Any solar pool system that is mounted according to manufacturers recommendations does not blow away in a Midwest storm.

Another type of solar pool heating panel, which has been on the market for the past 5-7 years, is a 4' x 20' version developed primarily for aboveground pools and sold in many pool supply stores. These panels are designed to roll out on the ground near the pool pump and must be stored indoors before the first frost. These panels work fine as a solar heater but it usually takes more than one to provide what the pool owner wants and they are a little bit inconvenient as well as in the way of lawn mowing etcetera. The 4' x 20' rollouts are still a much better option than a bunch of coils of black plastic pipe. It takes 800' of 1" polyethylene pipe to equal one 4' x 10' solar pool heating panel. Manifolding, mounting and draining of polyethylene pipe is a real challenge.

Solar pool heating systems use the existing pool pump to circulate the pool water through the solar panels. The water flow is interrupted after the filter and diverted to the solar system before going back to the pool. The most common type of control is a "three valve bypass" and a pump timer. The valves are set so that the pool water always goes through the collectors and the timer runs the pump from 9 or 10 a.m. to 6 or 7 p.m. Another type of control system uses an automatic flow-diverting valve.

Sizing a solar pool heating system is done through simple "rules of thumb" which are based on a couple of formulas. The manufacturers like to say 50-75% of the pool surface area in solar collector sq. ft. This method tends to generate oversized systems and is definitely larger than necessary for someone who uses a bubble cover. I prefer to figure one sq. ft. of solar collector for every 100-125 gallons of water. On a sunny day, between May and August, 2400 BTUs (British Thermal Units) of heat strike a square foot. The solar collector is operating in the 80-85 degree range, which makes it about 80% efficient. That means we can collect around 2000 BTUs per sunny day per sq. ft of solar collector. A BTU is the amount of heat required to raise the temperature of one pound of water one degree Fahrenheit. A gallon of water weighs close to 8 lbs. A swimming pool with a volume of 25,000 gallons would contain about 200,000 lbs of water. A 200 sq. ft. solar heating system would collect 400,000 BTUs and raise the temperature of this pool 2 degrees F. The increase is in addition to the 3 degrees that a pool with a cover gains on its own. The total of a 5 degree gain enables the solar heated pool to go from 75 degrees to 80 degrees in one sunny day instead of 2 and ½ sunny days in a row (remember 3 steps forward 1 step back?).

A pool with a faster temperature recovery rate and a higher average temperature enjoys much more use. Swimming pool owners with a solar heating system and an evaporation cover can't imagine why anyone would want to do it any other way.