Build a Solar Water Heater: An Intregal Passive Solar Water Heater
(Page 3 of 5)
January/February 1984
by David Bainbridge
Five Types of Solar Water Heating Systems
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Let's take a look at five of the many types of integral passive solar water heaters in use today.
The single-tank IPSWH is often dubbed a "breadbox" heater, because it looks a lot like an oversized breadbox (see the illustration in the Image Gallery at right). Overall, it's probably the most economical and least complicated of the permanent IPSWH's to build, requiring only easily scrounged materials and basic construction skills.
For the "econo-model" breadbox unit, a standard electric water heater tank is painted flat black and housed in an insulated plywood box. Glass or another transparent glazing material covers the top and the south-facing side of the box. (Glass is generally the most suitable glazing for owner-built heaters.) And, while not always absolutely necessary, an additional hinged, insulated lid will reduce nighttime cool-down and increase the collector's efficiency.
Incoming water enters the breadbox near the bottom of the tank, with the sun-warmed water being drawn off from near the top and routed on to the backup unit inside the house. A horizontally oriented IPSWH provides the lowest heat gain, but is the easiest to build and is less visible than a vertical one. By using tilted tanks — which can be attained by vertically flat-mounting the unit on a pitched roof — the stratification of the water is increased and the solar angle is improved, resulting in higher temperatures.
The vertical three-tank IPSWH has as simple a design as the breadbox, but it's constructed on a larger scale. For the triple-tanker, three glass-lined electric water heater cores are painted flat black (or are covered with selective surface tape) and enclosed in a large, well-insulated box. Glass or other glazing material covers the south-facing side and the top of the box, and shuttering in the form of a lid or interior insulated "drapes" can be installed to improve heat retention. The three tanks are plumbed in series, with the protected central tank serving as the final stage before the heated water is discharged (see the photo).
Naturally—because of the greater collection surface area and improved thermal layering—the tilted position of the tanks and the series hookup provide better heating than is offered by the smaller, horizontal, single-tank breadbox. To test the efficiency of the three-tank IPSWH, I installed and monitored one for a full year at my home in north-central California . . . and found that it supplied an impressive 70% of my family's hot water!
A greenhouse IPSWH installation is the ticket for best performance and freeze protection in colder climates or during the winter months. Of course, you can increase the cold-weather efficiency of your solar water heater by locating it in virtually any protected, warm, sunlit enclosure . . . but an attached solar greenhouse is usually the best choice. Almost any IPSWH configuration can be used inside the plant room, with size and weight being the primary limiting factors. Usually, the heater is tucked up under the peak of the greenhouse roof to take advantage of the warm, rising air. And as with all IPSWH's, the collector tank should be painted flat black or coated with selective surface tape. For maximum efficiency and cold-proofing, an insulated box with top and south-facing glazing, and a hinged, insulated, after-hours lid is still recommended.
The inverted IPSWH offers another good method of reducing heat loss in colder climates and seasons, and is especially effective protection against after-dark cool-down. Visualize, if you will, a standard IPSWH with a black tank enclosed in an insulated box with south-facing and top glazing . . . and the whole works flipped on its lid! Now, the insulated portions of the heater box are looking at the sky, thereby significantly reducing nighttime heat loss. Sunlight gets to the collector tank(s) by way of one or a series of reflectors. At first, these upended heaters seem almost absurd. But they've performed quite well at sites throughout the country.
A low-cost IPSWH, which can be built for just a couple of bucks, was developed a few years ago by the Minimum Cost Housing Group at McGill University in Canada. This unit offers more Btu per dollar than any solar water heater designed to date, and should be useful to summer campers and owners of weekend homes with no conventional water-heating facilities. This budget water warmer can be built with a minimum of time, effort and cash by anyone in need of a little hot water for washing purposes, and requires only a plastic garbage bag, some PVC pipe and fittings, a few hand tools and a lighted cigarette for "welding" the plastic joints.
The heart of this heater is the common plastic garbage bag of 1.6-mil-thick, dark green polyethylene in a 26" X 35" size. The drain outlet is made of PVC pipe, washers and nuts, and is fastened to the bag mechanically (that is, by tightening the fittings over a hole punched in the plastic). Tests at McGill University demonstrated that a plastic bag of this type and size is strong enough to hold water to a depth of three inches, thus offering a capacity of about a dozen gallons. Temperatures of about 104°F were reached by just setting the bag of water out in the sun, and when the bag was placed inside an insulated box with a glass cover, the water temperature rose to a respectable 132°F!
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