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 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.
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