Electric or propane pool heaters extend the pool season for our enjoyment. They are, however, expensive to run. Solar pool heaters can provide the same hot water while only using the sun's energy.
A solar pool water heater can be as simple as a dark tarp over the pool. That system has been used for years to increase the temperature during the day and lower the heat loss at night. It works well, but the efficiency is low when the sun is not at zenith.
An economical heating system can use black hoses with water running through on continuous bases taking water from the pool through the hose before returning it to the pool. Depending on your climate and how efficient you want the system to be, it can be either a closed system with the hose enclosed in an insulated case or an open system with the hose exposed.
You can also use black irons, aluminum or even copper pipes to build the system. While the prices of those components are usually higher than black hose, they will handle the heat better and transfer heat more efficiently, and they do not degrade with time. If solder is used, be sure to use solder with a high melting point as the temperature inside the case in the summer can reach over 300 degrees F.
If you choose to build a hose-based system, buy hoses rated for the temperature the water will reach as well for the amount of UV it will be exposed to; plastic degrades when exposed to UV. When using the pool pump to circulate the water, be sure to plug the hose or pipes on the outtake port and not the intake. Debris could plug the hose/pipe and destroy the pump if it runs dry. For economical reasons, pumps shipped in pool kits are usually only capable of running with the length of hose provided in the cleaning system. When using long hoses/pipes, be sure the pump is powerful enough to handle the load, and if it is not, buy a new, more powerful pump. For a closed system, remember to use material that will not catch fire because of the high heat, usually over 200 degree F inside the system.
Water coming out of a water heating system can be of scalding temperature when water is allowed to stand in it. Do not turn those systems on when people are in the pool. When in the pool, be sure the system is either off or the running system provides only warm water. A temperature gauge on the outtake is strongly recommended to avoid accidents.