Solar water heaters give radiant heat without the normal energy that would be required to heat the water, and without the accompanying need to burn fossil fuels or otherwise put contaminants into the air, water, or land. The setup is fairly straightforward and easy to understand, too.
There are two main kinds of solar water heaters; active and passive.
Active solar water heaters are generally used with solar panels that drive energy into heating the water; much like electricity and gas is used to do the same thing for conventional water heaters. Active heaters allow for the accumulation of hot water in a storage tank, which can be used in the same ways that water from a conventional water heater tank can be; bathing, washing clothes or dishes, and the like.
Active heaters do have a downside. In areas of the country that have periods of time when there are storms and other severe weather that prevent the operation of the solar panels, there must be an alternate means of heating the water. This is normally supplied by electricity. In northern climes, this can be a bother. Still, in a 12-month period, the costs of heating the water will still be low enough to make a big difference, and the pollutants will be more limited.
Passive solar water heaters are usually a lot cheaper to set up or build, however their main purpose is the production of radiant heat. This can still cause a huge reduction in energy costs and resultant byproducts of the heating of the water.
A great example of a passive heater is a use in a greenhouse, though it is just one example.
In the summer, it isn't difficult to maintain a steady temperature inside the greenhouse. All that is required is sunlight and venting, when the interior gets too warm. However, it is another matter in the fall and winter when there is little or no direct sunlight.
Conventionally, this is dealt with by either shutting down the greenhouse until warm weather and sunshine returns, or by placing heaters in the greenhouse. The latter can become very expensive quickly, offsetting any benefits of having the greenhouse to begin with.
This is much less a problem with passive solar water heaters. In a good-sized greenhouse, four 55-gallon drums, painted black and filled with water, can provide the heating needs for most of the year.
During even overcast skies, the black paint draws heat from sunlight. Since water is such a tremendous conductor of heat, it takes little time for the water to warm up. When it is warm, it radiates the heat outward, and since it takes some time to dissipate all the heat energy, it will still be emitting heat through the night and into morning.
We aren't talking about enormous amounts of heat, mind you. It doesn't mean that your 20' by 10' greenhouse will produce tomatoes in late December in Minnesota, or bananas in Montana in January. However, even in northern states, it isn't uncommon for this sort of solar water heater to cause the interior of the greenhouse to remain at 50 or 60 degrees F., even when the outside temperatures are dropping below single digits. This is warm enough to grow lettuce, peas, or other cold weather crops, long after everything else has died for the winter.
While many people can no doubt think of the benefits of lowered energy costs and attendant pollution for the energy, the above example points out another benefit of solar water heaters. With population continuing to grow, food and land capable of growing food is becoming more important than ever. Passive solar water heaters allow the means to grow more, in greater areas, using less man produced energy.
There is little doubt that as time goes by, methods of using both passive and active solar water heaters will become more efficient, and as they do, they will become more and more important to the welfare of everyone on the globe. The benefits of solar water heaters are many while the problems with them are few.