Solar Water heaters were big then in areas with NO access to Natural Gas heating (i.e. Southern Florida). Most were home made. That is someone had copper tubing bent into coils under enclosed glass. The Glass turned the light into infrared light, which heat up the Copper panels which ran down to the hot water tank ready for use. The water is pushed to the Panels by Water pressure in the water line itself. This is sometime referred to as an "open" System, for water is always being added and subjected (i.e. used) through the pipes.
In areas where temperatures drop below freezing, the water in the pipes are replaced with glycine which runs to the water tank and heats the water in the tank. The Glycine is a "Closed" System, the Glycine NEVER exits the system, it is just a conduit to heat the water in the Water Tank. You need a pump in such a system to pump the glycine between the panels and the water tank, but you do NOT need to worry about bust water pipes if the water in the pipes would Freeze.
Sellers of Solar Water Panels:
http://www.solarroofs.com/http://www.solardirect.com/swh/swh.htmhttp://www.advancepower.net/http://www.gaiam.com/realgoods/http://www.apricus.com/index.htmInstallation of Solar Water heaters is more expensive then installing a Solar Electric Panel, do to the need to run water pipes up to and down from the panels. With Solar Electric Panels all you need is to hook the panel up with your electric Grid.
On the other hand the Solar Water Panels are more efficient AND cheaper if you plan to use Solar power to heat your water.
You can even use the Solar water System to run a Air Conditioner system. There are two different ways to run an Air Conditioner. The first and most Common is Electric compressor system. This method is used on most Refrigerators (and all the Refrigerators I know of made in the last 50 years). The Electric Compression system is simple, the Compressor compresses the Freon (or other refrigerate in the System). As things are compressed, they release heat. The now liquid refrigerate is than run through the back of the Refrigerator (or A/C Unit). Upon reaching the inside of the Refrigerator the refrigerate is permitted to expand, which absorbs heat from the surrounding area. The Freon then returns to the Compressor to be compressed and lose this heat (and the system keeps on going).
In the 1930s and 1940s in addition to the above electric Compression System, you had the the Natural Gas/Absorption system. Such systems are cheaper to run then Electric/Compression systems but are more complex and more expensive to produce. My mother had such a Refrigerator in her home in the late 1940s, but had to give it up when she moved to an area without Natural Gas service. The company that made the Refrigerator had switched to making Huge A/C units in the 1950s and closed in Refrigerator business (people kept buying the cheaper to buy Electric/ Compression system).
For how the Absorption system works see:
http://en.wikipedia.org/wiki/Absorption_refrigerationThrough if you read the paper remember, the Ammonia, Hydrogen and Water are all in a "closed" system. There is NO need to add any of them to the system once made (Unless a leak occurs). Absorption works through the interaction of these three Elements/Compounds.
I bring this up, for the absorption system needs a source of heat. In most cases that was Natural Gas, but you can replace the Heat from Natural Gas by the heat from Hot Water. When I did my research in the 1970s, some people had done so in Florida and other places to keep their electric bills down AND still have A/C.