You’ve left a garden hose in the sun, and nearly scalded yourself when you turn it on? Well done, you’ve discovered solar water heating! Well, actually, you didn’t discover it, as solar water heaters have been used for at least a hundred years. It’s an appropriate use of the sun’s power, making use of it for our needs.
The modern solar water heater is little different from the types first used. The modern solar collector is often made up of a series of pipes fixed to a backing plate, painted in a dark color, and covered with glass which may have a special heat-trapping coating. This is called a flat plate collector, and it can go on the roof, like a photovoltaic, or electric, solar panel, although it is a bit thicker and may be heavier when full of water.
As with conventional solar power, before you install a solar water heater you may want to check on reducing the requirements, by installing water saving faucets, etc., to keep the size down and the performance up.
Solar water heating can be installed with a few variations on the theme. One version involves using an additional pre-heating tank to be heated by water circulated through the collector, and this tank supplies the incoming water for your regular water heater. The normal water heater will then, by the usual means of gas or electricity, additionally heat the water as necessary to the required temperature as set on the thermostat. Another means is to have a water heater with an internal pipe coil through which the solar heated water flows, heating the stored water indirectly. Most methods involve using an electric water pump, although in some circumstances you can arrange for natural circulation (that is, hot water rises) to circulate the water. Generally this doesn’t work, as the hot water is on the roof, and thus needs to go down, not up.
There are a couple of things to watch for when planning a system, although it really is basically as simple as described above. It is possible that the water in the collector can get too hot, and make the stored water hot enough to scald you. You will need to have a method of avoiding this. Often this is done by mixing the too hot water with the incoming mains cold water to produce the correct temperature before delivery. The other potential problem is the water freezing in the collector. This can be overcome by what is called a “draindown” system, where the collector is drained in cold weather, or by using antifreeze with the indirectly heating system.