Build a Solar Water Heater: An Intregal Passive Solar Water Heater
(Page 2 of 5)
January/February 1984
by David Bainbridge
The Evolution of Passive Solar Water Heaters
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Let's look now at the history and evolution of the IPSWH, and at five of the many designs that have proved effective and popular with home-based handypersons.
The principles used in modern IPSWH's are undoubtedly the same as those that were first applied to solar water heating. For example, Butch Cassidy's roost in Utah reportedly still bears remnants of a passive solar water warmer: a black can filled with water and placed in the sun to heat. Similar primitive IPSWH's are being used today in locations where people live without piped-in water and conventional energy sources such as gas and electricity.
The first commercial solar water heater, patented in 1891 by Clarence Kemp, was dubbed the Climax solar heater. Kemp's Climax used four cylindrical water tanks housed in a pine box lined with felt paper and covered with single-pane glass. This system typically was mounted on a roof, with cold water from a reservoir entering the first tank and passing through the other two in series, with gravity carrying the heated liquid down to the tap. By the year 1900, more than 1,600 Climax units were in use in America, primarily in the sunny regions of California and Florida.
In 1898, Frank Walker of Pasadena, California applied for a patent on an improved IPSWH. Walker's model was recessed into the roof, instead of being exposed as were the Climax heaters. And more important, the Walker unit incorporated backup connections to a "wetback" woodstove. Thus, the Walker model was the direct forerunner of today's most usual IPSWH application, a polar preheater feeding into a standard gas or electric water heater located inside the louse.
But successful as these early IPSWH's proved to be, they were destined to gradually disappear as enticingly cheap natural gas became available and was aggressively promoted.
A New Wave of Solar Hot Water
Fortunately for us, the popularity of natural gas didn't bring solar water-heating research and development to a total standstill. In one valuable experiment conducted at the University of California at Berkeley in 1936, a researcher named F. A. Brooks tested several IPSWH designs and demonstrated that tank-type solar heaters were capable of producing water warmed to more than 120°F. He found, too, that upright tanks placed on an incline delivered hotter water than horizontally situated units. Brooks also concluded that IPSWH's could produce hot water at a cost consistently below that of flat-plate systems. The only shortcoming of the IPSWH was lower early morning temperatures as a result of nighttime heat loss.
Contemporary investigators have reduced nighttime cool-down in several ways, the most effective involving the use of manual or automatic lids on the heater boxes. These hinged lids are raised during the day, their reflective undersides catching and directing additional solar Btu onto the tanks. At night, the insulated lids are closed to retain the stored heat. A second method of heat-saving relies on an ultramodern, specially coated metal foil, tagged "selective surface tape." This expensive — $75 to $80 for a 4' X 8' sheet! — but effective material offers a combination of high absorptance (on the average, about the same as flat black paint, approximately 95%) and low emissivity (thereby cutting heat loss).
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