Here's an easy to build passive solar pool heater that really gets the job done. My father built one of these and installed it all in one day. You can make it as large as you want but remember that it will be installed next to your pool so you don't want to get too carried away. You can install it laying flat or upright and you can design your own configurations. All you need for it to work is full day exposure to the sun.
You'll want to have the passive collector placed on a hard surface which is painted flat black to get maximum value from the sun. A simple wood platform about 12 inches off the ground works great, or a concrete pad can make the project look especially professional. In areas where there is a high concentration of swimming pools an industrious person can even keep pretty busy building and installing these passive heaters for client pool owners. Additionally, similar solar collectors are being used to preheat water for in home use but that requires special engineering and a consultation with local building inspectors.
Build your solar collector using standard black PVC
pipe of a diameter the same as or slightly larger than your pools own water lines. If you can't get black PVC pipe in your area, you can use the lighter colored types by lightly sanding them and then applying a coat of flat black spray paint. Your collector must be
installed on the discharge side of your filter because sending heated water into your filter will compromise the filters own operating temperature as well as create an increased potential for breeding dangerous bacteria. Let the glue joints of your newly built solar collector cure for 24 hours before putting the pool filter system back into service. You may also want to consider using flexible, hose clamp connections at the intake and outflow ends for easy draining of your solar collector at the end of the swimming season.