Swimming Pool Heating
When using solar energy to heat swimming pool water, you have several choices
from the types of systems available, but which system is right for your needs?
The next question is usually; What type of panels should I use?
Generally speaking there are two types of panels that are used for pool water heating, plastic or copper. Either of these
types can be exposed or encased in an insulated box with glazing.
Plastic, poly or other composite material panels can be as simple as a coil of tubing in the sun to one piece molded panels
as well as multiple tubes with a headers.  Some even have absorber fins molded with the tubes.
Copper panels consist of copper tubing either mechanically or physically bonded to absorber plates that are made from
either copper or aluminum. These absorber plates are then painted or anodized with a dark color to enhance absorbtion.
The first question is usually; How many panels do I need?
Some say you should determine the amount of panels by using the area of the pool as a factor in your calculation, but the
answer can vary greatly depending upon your needs, the depth of the pool, the amount of time exposed to the sun, pool
location (orientation and climate), type of pool, type of panel, surrounding air temperature (day and night), target
temperatures, return water distribution, insulation and on and on......
 So as you can see, a system that is designed to give a certain performance is subjected to many variables that can either
hinder or enhance performance. Unfortunately on the days you want the heat, the heat is usually not available which is
why the other factors that hinder performance should be addressed.
When using plastic panels there are several concerns;

What type of plastic is used to construct the panel?
Different plastics or polymers have different properties, some may be brittle, some may have higher UV tolerence,
some can handle freezing weather better, some may react with water conditions and conductive properties may
differ. Depending on your location you may have to sacrifice heat gain for durability.

Where is it being installed ?(climate).
If installed where freezing weather can occur, precautions should be taken when closing the pool for the season to
ensure the panels are fully drained to prevent damage from expanding, freezing water. Ice and snow can often cause
damage during winter months, especially when installed "loosely" on a roof. If installed in hot climates, expansion,
warping and softening of the plastic can cause damage, especially in drain back systems, when the panels are
empty.
Copper too is subjected to these environmental conditions and can be phsically damaged if not properly
maintained but damage is not usually associated with material properties.  Proper pool water PH should always be
maintained.
Then; What are the advantages/disadvantages of these types of panels?
Most important, cost?
Copper panels can run from $400-800+ depending on size and type.
Conventional p
lastic panels can run from $150 -500+  depending on size and special connections needed.
Installation costs can vary for both systems depending on the installation location, with factors such as gound or roof mount,
number of panels to install, distances from pump or pool and materials needed for installation and of course labor rate.
What acutal absorber area does the panel have?
If the plastic panels do not have absorber plates between the tubing, then a panel that is 4' x 8' may only have an exposed area that
is half that size. This area is the exposed tubing, and since the tubing is round, convex, some radiation actually bounces away from
the tubing.
Copper panels are usually made using a flat absorber plate, the area of the panel is equal to the absorber area so a 4' x 8 ' panel will
actually give you 32 ft ^2 of absorber area.

How hot can the pool water get?
 
The water in a pool can only get as hot as it's surroundings. So if it is 80 degrees F outside, your pool has the potential to reach
this temperature if heat is added. Most pools without heat only reach around 70 degrees F due to the surrouding  ground
temperature to which the heat is lost. Even indoor pools with large heaters are usually the same temperature as their surroundings.

How hot can the water get in a solar hot water panel?
 Most conventional plastic panels will only reach a temperature of around 100 F or so if left stagnant, especially if the panel is
not encased or insulated, once again dependent upon the material and it's conductivity
and panel design. If operated with a high
volume pump, minimal heat can be absorbed by the water passing through, due to the time the water is in the panel and the rate of
heat transfer between the panel material and the water. High roof temperatures may add to the heating process but when panel
temperatures are higher than surrounding temperatures, the heat gain reduces and can even be reversed.
Copper has a high conductivity, which is why it is used in electical wiring, this means it can absorb more radiation more
efficiently. This higher rate of conductivity allows faster heat transfer to the water. This in turn will reduce the overall panel area
needed for a system when compared to plastic.  This transfer of heat to the water with respect to time, and flow rate of water
from the pump, will determine the overall temperature gain of the pool water.  If the panel is encased, glazed and insulated, a
copper panel left stagnant will actually bring water to boiling temperatures.

How long will the panels last?
Plastic panels will last dependent upon the environment to which they are exposed, and of course to the care given to the panels.
Most plastic panels  have connections, from the header to the tubing, that often loosen and leak over time. These connections can
expand and sometimes hold small amounts of water that can freeze, causing damage. So an estimate from one to twenty years or
more can be stated, but repairs are usually a yearly task.
Copper panels can last well over twenty to thirty years and can be easily repaired if damage should occur, without hunting down
manufacturers of systems that have gone out of business, and additional panels can be added easily using conventional plumbing.
When the pool season is over, your copper panel system can be used to heat domestic hot water thus getting the all important
"payback" associated with renewable systems.
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Water Heating Systems
Consider this....
One of the most overlooked items when considering heating a pool is insulation. Most pool people only recommend using a cover
or solar cover to prevent heat loss. This heat loss is mostly from the energy that is drawn from the water in order for the water to
evaporate (evaporative cooling), just like if you blow on your skin, the added energy from the wind causes the moisture on your
skin to evaporate thus making your skin cooler. Using a cover reduces this evaporation, thus reduces heat loss. But heat loss can
happen whenever there is a temperature difference between two surfaces. So if the ground is cooler than the pool water then the
water will lose heat to the ground, same goes for the exposed walls and bottoms of above ground pools. So, if you are considering
heating a pool, you should consider using some insulation, installation can be easy with the many types of insulation that
available today.