Entry Level Solar Hot Water- Real People Can Do This Right Now

I blew this diagram up so you can make out the details; it's from a Williams College site describing a system they put in a graduate dorm. The essentials are this: the panels receive cool water from tank dedicated to solar, warm it up in the passage through the panel, return it to a heat exchanger in the solar tank that warms the domestic water. When hot water is demanded, the solar "pre-heated" water passes into a tank warmed by a boiler which increases its temperature to a setpoint for use in the building.
It's called pre-heating, and that's the little point I want to make in this post. When you aim a low-temperature all-weather solar setup at a low temperature need, it performs beautifully. Asking a solar hot water system to finish your water off to 125 F or more is asking too much. Only during sunny summer weather will the system carry that burden.
In one pass through two panels on a sunny winter's day, you might raise the fluid temperature by only three degrees F. You might run that system all day until sundown and have warmed an 80 gallon tank only to 85 degrees (from 45 to 50 degrees incoming temp, depending upon whether you have city supply or well water). But 40 degrees rise for 80 gallons comes to almost 13 thousand btu that you didn't have to pay for. And it means your finishing, or backup source will run that much less to get the water hot enough to use. On a sunny day you might raise the same tank of water all the way to terminal temperature (app. 130 degrees). That's 26,000 btu from the sun. You might want to run your finishing source a little, but it only has to raise the temp a few degrees. Energy is being saved on a grander scale by allowing the solar panels to operate at a lower temperature, where the sun and the heat exchanger are able to deliver energy more efficiently.
I have clients with solar systems sharing a tank with electric finishing elements. They only get solar benefit when the panels are as hot as the terminal temperature setting of the domestic water. There just aren't that many days in CT when the panels get hot enough to finish off the water in one pass.
And the cost? Fewer panels and smaller tanks do more work at lower temps. A tank only has to be a about as large as your daily water demand to deliver its full potential as a preheater. It needs to be much larger to store heated water against cloudy days and night time losses. So while you're waiting for the cost of photovoltaics to come down, and wondering what you can do to join the green movement, solar hot water in a pre-heating configuration is the most cost effective entry level investment. Most systems can be installed for less than ten thousand dollars US, and they attach to your hot water piping just ahead of whatever your water has been heated by in the past: electric tank, boiler coil, external heat exchanger or woodstove. With photovoltaic systems starting at about 30,000 d0llars, and paying back rather slowly, this solar hot water option is appealing at several levels. You can get free energy from the sun, with not much red tape, and get the federal and state tax credits that reduce the cost of the system by as much as 50% depending upon your location and the system cost. That's a game we normal people can think about jumping into, and nothing feels as good as a nearly free shower.