Seasonal solar water heaters

Posted Wednesday, March 28, 2012 in Sustainable Maine

Seasonal solar water heaters

Image from SolarNet.org

by Paul Kando

Last week we discussed how water heating is a year-round user of energy, any reduction of which is worth considering if the water is being heated by a fossil fuel or fossil-generated electricity. This is especially true when the space-heating system’s oil-fired boiler is used to heat water, requiring the boiler to operate — quite inefficiently — even outside the heating season. Examples are a heat exchanger located inside a boiler, heating water on demand, and boiler-mates: tanks of hot water hooked to a boiler like another heating zone. All tanked water heaters use energy even when no hot water is drawn because the tanks naturally lose heat to their cooler surroundings. Therefore, to conserve energy and money, fossil-fueled and electric tanked water heaters should be turned off whenever the home’s occupants plan to be away for more than a couple of days. Heating water using solar energy can be a no-brainer because, while the equipment costs money — professionally installed year-round systems cost $5,000 and up, depending on their size — the energy is free. 

Let us now consider less expensive alternatives, suitable for do-it-yourself assembly and installation. One of these is the batch or “bread-box” water heater. It consists of a steel tank, painted flat black and enclosed in a well sealed and insulated box covered with a sheet of glass or other greenhouse-type glazing. The whole assembly is installed with the transparent cover perpendicular to the incoming sunshine: In Maine, 30 to 45 degrees from the horizontal, facing due south, is a good orientation. Recycled water-heater tanks, shorn of their outer jacket and insulation, are good candidates for such water heaters.

Or a batch water-heater kit may be purchased online for around $500. Construction of the insulated box, plumbing and assembly are extra. Building such a “bread-box heater” can be an ideal weekend project, resulting in an effective seasonal solar water heater for under $800. Depending on the size of the tank, batch heaters can provide for a family’s hot water needs on sunny days, especially if people shower in the evening. Like all solar water heaters, batch heaters should be plumbed in series with an electric- or gas-fired water heater, so they pre-heat the incoming water supply. On sunny days they will keep the conventional heater off, except when the solar-heated water is not warm enough for use.

Another way to passively heat water is by a thermosyphon system. Water is circulated through a collector installed below the bottom of a well-insulated water tank. Water drawn from the bottom of the tank is piped to the bottom of the collector. The heated water rises through natural convection and enters the top of the tank, drawing more colder water to the collector. This process continues without the need of pumps as long as the temperature at the collector is higher than in the bottom of the tank. Caution: The top of the collector must be about 12 inches below the bottom of the water tank or reverse syphoning will occur, cooling the tankful of water whenever no solar energy is available.

Thermosyphon and the batch heaters may also be combined: Paint the thermosyphon system’s tank flat black and install it similarly to a batch heater.

Remember: Batch and thermosyphon water heaters are not frost-protected. They are seasonal devices that must be drained down for the frosty season. Still, in Maine, where most visitors come during the summer, such heaters can pay for themselves in short order, even though they operate only part of the year. Importantly, they are fun to build and use, not to mention the bragging rights you'll have as a special bonus.

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