What To Expect Of Your System
Solar heating for swimming pools is a proven technology, it
will provide years of low cost heating for your pool, but you
should be aware of its advantages and limitations.
The biggest advantage of solar heating is of course its cost
effectiveness. The energy that the panels receive is really free
in the truest sense of the word. On a sunny day the surface of
the panel receives a continuous stream of energy from the sun.
Most systems will use the filter pump to send the water to the
panels. In most cases good results can be obtained by running
the pump about 6 hours per day. (There are exceptions: please
see the section “Maximizing Your Heating”.) This is
not an extra electrical expense since most pools need to be filtered
about that long during warm weather anyway. In that scenario there
is no ongoing expense for the heating. Of course the original
purchase price of the system is an expense, but since well-constructed
systems will last 15 to 20 years, the annual heating expense is
quite small. Ongoing maintenance expenses are negligible, since
the panels, the most expensive part of your system, are covered
by a long warranty and require no maintenance. Another advantage
is that solar energy is a “clean” energy. Unlike fossil
fuels, it is non-polluting, good for the environment.
You should be aware of some inherent limitations of solar heating.
A solar system is not like a gas heater. The heat per hour supplied
by most large swimming pool heaters (measured in “ BTUs”}
far exceeds the output of a solar system. Because of this the
solar system will take a few days, even a week or so, to reach
the maximum temperature it can reach. This maximum will depend
on a number of factors:
- Angle of sun to collector. This is often not understood by
consumers.
The closer that angle is to 90 degrees the more intense the
heat.
An easy way to understand this is to think about using a magnifying
glass to burn a hole in a piece of paper. If you turn it so
that the light is diffused over a larger area of the paper it
will not be as hot. Solar panels work the best when 1 square
foot of sunlight falls on 1 square foot of panel, not diffused
over a larger area. This is why, in the northern hemisphere
we try to put collectors on a south-facing roof. The sun for
most of the swim season is somewhat south of straight overhead
(except around the solstice-June 21), so a panel angled that
way will have more days close to that 90-degree angle. Systems
facing other compass headings can work if sized properly (See:
A Word About Sizing).Early and late in the year the sun is quite
low in the south and panel performance is decreased.
- Daytime temperature. When the ambient air temperature around
the panel is cool, the panel will not perform quite as well.
Why not? Because some of the solar heat being gathered is being
transferred to the air around the panel, rather than into the
cool water running through it. If the air is cooler than the
pool itself, the pool too, is giving some of its heat back to
the air around it. Interestingly, this is why a west facing
system is much better than an equivalent east facing system.
Yes they both receive the same angle of sunlight, but at different
times of day. The west system receives its best sunlight in
the afternoon when the ambient air temperature is much warmer.
And in very hot weather both the pool surface as well as the
collectors can transfer some ambient heat to the water in addition
to the solar energy being obtained.
- Nighttime temperature. Although no water is running through
the panels at night, the pool will always give back some of
its heat gain overnight. Again, in the fall and early spring
this will be greater, so the next day’s starting temperature
is lower. And unfortunately, the warmer the pool, the more it
gives back overnight due to the greater contrast in temperatures.
And this is why there is a maximum obtainable temperature. The
pool will reach a balance point where the heat obtained during
the day will match the loss overnight. During the swim season
this maximum can be quite high, usually higher than the low
to mid eighty degree water most swimmers prefer.
- Wind. When the air is cooler than the panels, wind will affect
performance, similar to how humans experience wind chill. The
pool surface also is cooled by wind. Brisk winds, even when
the air temperature is warm, will cool the pool because of evaporative
cooling. Cool wind even more so.
- Shade. Obviously the solar panels cannot be in full shade
all day long. If they are someone has made a mistake. Some shading
may be OK, as long as we get a 5 hr or greater period where
good direct sun is on the big majority of panel surface. Shaded
pools will naturally be cooler than pools in full sun. For these
pools solar is really needed, but maximum temperature expectations
may be lower.
So all these factors considered- what should you expect?
A well-designed and properly sized system should make your pool
warmer by an appreciable amount. A broad rule of thumb would
be about a 6-12 degree increase over what your pool would be without
solar heating, from about late April to early October.
Typically, in Northern California, late fall and winter produce
such cold overnight temperatures that solar heating cannot, by
itself, maintain comfortable heating. In some years with an “Indian
Summer” and a cover we have had customers swim even in November
but this is the exception rather than the rule. Please bear in
mind that every pool is different, and time of year and weather
affect performance. We often get calls comparing a solar pool
to another pool, with or without solar. Some pools are shallower,
less shaded, more protected from the wind. All we can do is make
your pool warmer, by an amount that makes your investment worthwhile.
Is an increase of 6-12 degrees worthwhile? The difference between
a 76-degree swim and an 82-degree swim is huge. If owners of non-solar
pools were to count up the days when they wish their pool was
warmer, either to get into the water, or to stay in longer, there
would be a lot of days.
One
other note: Please make sure you are using an accurate thermometer.
We have had multiple instances of inexpensive thermometers that
are reading as much as 10 degrees off. Some of these have the
degree scale glued-on next to the mercury column, and when the
scale gets loose it can slide up or down, giving a completely
erroneous reading.
