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Solar collectors reduce the cost of hot water heating at home
The luxurious practice of bathing and showering in hot water for health and wellbeing can be traced back thousands of years across cultures from the Far East to ancient Rome. Only within the last century, however, have Americans come to rely on hot water available in every bathroom with the twist of knob or push of a button. Not only do we expect it on demand for bathing, but also for washing our clothes and getting our dishes squeaky clean. Barring the presence of geothermal springs in the earth below, this means raising the temperature of water. Throughout time, man has been heating hot water by burning wood, oil, gas, coal, or compost; and in many parts of the world, natural sunlight is used to heat water on rooftops. Now, solar hot water systems are taking their rightful place in history. In fact, heating hot water is among solar energy's most efficient usages.
Laurel Wilkinson likes knowing that most of the energy which heats the water in her home is coming directly from the sun. Photo by Martha Shaw
It's easy to guesstimate the amount of energy coming from the sun. Ten square feet of surface on the planet exposed to direct sunlight receives about 1.4 kilowatts of sun power. The power level of one kilowatt for one second is an energy measurement called a BTU (British Thermal Unit). Think of a BTU as about the same amount of energy that comes from lighting one kitchen match. A BTU is also defined as the amount of energy needed to raise one pound of water one degree Fahrenheit. In general, it's fair to assume that a fixed solar collector on an unobstructed roof on a sunny day receives 1 BTU per 10 square feet. A typical solar collector panel is 32 square feet so it can receive about three BTU's per second. That's roughly 10,000 BTU's per hour for a collector about the size of a sheet of plywood. Five hours of bright sunshine in a day can make about 50,000 BTU of solar heat from one collector panel.
In the solar hot water system, this heat energy is then used to raise the water temperature of the well or town water. For instance, if well water is 50 degrees, and you want to raise its temperature 70 more degrees to reach 120 degrees, you can calculate how many gallons can be heated. If you divide the 50,000 BTU from the collector panel, by the 70 degree temperature rise needed, you can conclude that 714 pounds of hot water, or about 79 gallons, could be obtained per day from a single solar collector, minus the losses in transferring heat and storing the hot water.
Sarah Kuh, right, pictured here with daughter Sofia DeGeofroy, center, and Maisie Jarrell, finds that solar collectors reduce her hot water heating bill, especially when she uses energy efficiency, like washing clothes in colder water or only on sunny days. Photo by Brian Jolley
Back in the 1970s, during the energy crisis, solar collectors rose in popularity, but the movement lost its momentum as soon as the tax benefits and rebates disappeared.
South Mountain Company Inc. chose Laurel Wilkinson's house in Island Co-Housing to demonstrate and monitor a new Enerworks solar hot water system that cost about $7,500 total. Laurel, who works at the eco-friendly firm, prefers the soft hum of the system's pump to the sound of the oil furnace firing up. She says, "As a child, I was raised not to use what I don't need, and that includes both water and energy. In my family, we turned down the thermostat at night and wore an extra layer to stay warm. College years are the age when people look at their own values and what's important to them. For me this was during an energy crisis. I began to study various environmentally conscious topics around architecture. Being here at South Mountain I am fortunate to have the opportunity to integrate my values with my work."
This article is sponsored by the Vineyard Energy Project through a grant from the US Department of Energy. The Vineyard Energy Project promotes sustainable energy choices through education, outreach, and renewable energy projects. The author, Martha Shaw, is a member of the Vineyard Energy Project's advisory board. The Times publishes these columns as a service to its readers.