“You’ve come a long way baby” is an old advertising slogan that can be applied to solar water heating. In their simplest incarnations, solar water heating systems are boxes with tubes running through them, painted black to absorb the heat of the sun. Water flows through the tubes, absorbs the heat from the box and moves into a storage tank, pushing colder water back into the collector. You can build a system that works without any moving parts using just convection (look on http://www.instructables.com for some examples) but that is not very efficient.
Today’s commercially available systems come in a variety of configurations. Flat plate systems are quite common. Then there are evacuated tube models, my personal favorites. They cost more when I priced them but are adaptable to more situations and climates. Parabolic systems are also used but tend to be found in commercial applications. Let’s take a gander at these different types.
Flat plate collectors place an array of tubing on a roof or other area with proper solar exposure. These tubes are contained inside carefully engineered assemblies of glass, thermal coatings, insulation, and structural components. I had this type on my home when I lived in Florida. The collectors were attached on a south-facing portion of my roof and the piping ran to and from an electric water heater in my garage. The water produced by that system was HOT, thank you Sunshine State! Why the electric water heater? Backup. Even Florida has cloudy spells and with two young children, running out of hot water during those times was a real possibility. My particular system did not have a pump but one is frequently part of these systems.
Evacuated tube systems are sleek and sophisticated, at least to my eye. A thin tube holding the heat exchange medium is contained within a larger glass vacuum tube. Reflective coatings on the back of the vacuum tube concentrate the heat onto the smaller tube. In one system from Germany I researched, the liquid turned to steam and rose to the top of the small tube. At the top, the tube assembly connected to a header where the household water was pumped past. This water absorbed the heat from the tubes and the steam reverted to a liquid, the process repeating itself continuously. These systems can be mounted on vertical surfaces, work well in colder climates, can produce hot water on overcast days, and if a tube leaks, it can be taken out and replaced, something more difficult with a leak in a flat plate collector.
Parabolic systems use reflective materials or curved (parabolic) mirrors to focus the sunlight onto a tube or tubes containing the water. This concentration of heat speeds up the water heating process and can produce steam. In the most spectacular example I have read about to date, more than 600 large mirrors concentrate the sun onto a tower containing water pipes. This intense focus produces steam to drive an electric generator. Talk about green energy! This is the Solar Platform at Seville, Spain.
The sun is a great way to produce hot water but like all “renewable” sources of heat and power, it is intermittent. This means that you need a backup source of hot water whether it is an insulated holding tank containing more than a day’s worth of hot water or some other means of heating including electric, gas, wood, or propane. Even in Florida, my system only provided 70% of my hot water needs. The systems work and can provide a great return on your energy investment, but whether they will work for you will require some careful research on your part.