Environmental Engineering Reference
In-Depth Information
More than half of the United States receives five kilowatt-hours or more of solar radiation per
square meter per day. Alaska receives less solar radiation during a year than Arizona, yet solar
hot water heating units are common in northern climes such as Maine and Montana. Although
it is intermittent, solar energy can be easily supplemented by thermal energy storage or another
energy source, such as natural gas, geothermal heat pumps, wind power, or hydropower, to provide
a reliable mix of energy supplies.
Solar energy is utilized in three ways: thermal collectors, solar power plants, and photovoltaic
collectors. The first type of technology makes direct use of heat from the sun and the other two
involve converting solar energy to electricity.
Thermal energy (heat) from the sun can be collected to heat air or water for use in homes, buildings,
industrial processes, and swimming pools, or to directly heat spaces inside homes, greenhouses,
and other buildings. Primitive collectors may involve locating loops of a dark-colored garden hose
on a roof and allowing gravity to draw heated water down for use in washing dishes, laundry, or
people. More sophisticated collectors involve the use of metal cabinets containing surfaces that
absorb heat and plumbing that circulates it into a building, often using water or antifreeze as a
heat-transfer medium, the flow of which is controlled by computer-assisted valves and storage
devices.
Solar space heating systems are often classified as passive or active. In a passive solar build-
ing, windows, walls, and floors are designed to collect, store, and distribute solar energy in the
form of heat in the winter and reject solar heat in the summer. This is called passive solar design
because, unlike active solar heating systems, it does not involve the use of mechanical and electri-
cal devices. Passive solar design can convert into useful heat 65 to 70 percent of the solar energy
that strikes a surface. The key to designing a passive solar building is to best take advantage of
local climate. Elements to be considered include window placement and glazing type, thermal
insulation, thermal mass, and shading. This can be done most easily when designing a new home.
However, existing buildings can be adapted or retrofitted to passively collect and store solar heat
(Mazria 1979).
Active heating systems require a collector to absorb solar radiation. Fans or pumps are used
to circulate heated air or heat-absorbing fluid. Active systems often include some type of energy
storage system such as a tank of water or rocks. Active solar thermal collectors use nonconcentrat-
ing or concentrating equipment. Flat-plate collectors, the most common type of nonconcentrat-
ing collector, are used when temperatures below about 200°F are required. In nonconcentrating
collectors, a surface absorbs solar radiation, which is then moved to an area requiring heat. Such
collectors are often used for heating buildings and water for washing.
There are many flat-plate collector designs but generally all consist of the following
components:
s!mATPLATEABSORBERTHATINTERCEPTSANDABSORBSSOLARENERGY
s!TRANSPARENTCOVERSTHATALLOWSSOLARENERGYTOPASSTHROUGHBUTREDUCESHEATLOSSFROM
energy-absorbing surfaces
s!HEATTRANSPORTMEDIUMAIRORWATERmOWINGTHROUGHTUBESTOREMOVEHEATFROMTHEAB-
sorber
s!HEATINSULATINGBACKING53%)!F
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