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NIST and the Sun |
| NIST researchers have been working to make solar energy-based technologies, including water heaters, as efficient as possible. | A novel solar water heating system developed
by the Commerce Department's National
Institute of Standards and Technology is now hard at work in Great
Smoky Mountains National Park (Tennessee/North Carolina). The device,
which uses photovoltaic cells in combination with computer technology
to maximize capture of the sun's energy, was installed at the park's
Sugarlands Visitor Center near Gatlinburg, Tenn., in a joint project
involving NIST's Building and Fire
Research Laboratory, the U.S. National Park Service, the Tennessee
Valley Authority's Technology Advancements Division, and the Sevier
County (Tenn.) Electric System.
Participants hope that the test of the photovoltaic solar water heater will considerably reduce the visitor center's water heating energy consumption while providing valuable "on-the-job" field data to NIST researchers looking to improve the models used to predict system performance. Two ongoing tests of the solar water heater--at NIST's Gaithersburg, Md., headquarters and at the Florida Solar Energy Center in Cocoa, Fla.--led to refinements in earlier versions of the system that resulted in the model installed in Tennessee. Photovoltaic cells are semiconductors that convert sunlight into electrical energy. The NIST technology, patented in 1994, includes an array of PV cells, a water storage tank incorporating multiple electrical resistive elements and a controller that maximizes the conversion of solar energy into heated water. In the system, a microprocessor continuously monitors solar conditions and kicks in the appropriate electrical resistive elements for maximum energy conversion from sunrise to sunset. The PV design avoids many of the problems inherent in traditional thermal solar water heating systems. Such systems operate when sunlight is transmitted through a transparent cover, is absorbed by a metallic plate, and the energy then heats a fluid flowing under, through, or over the plate. Unlike thermal devices, the PV solar water heater operates without pipes through the roof, fluids which can freeze or be toxic (if antifreeze is used), or costly pumps that may fail. There also is no need for storage batteries to provide energy during the night or poor solar conditions. Electrical current produced by the PV cells is utilized by the resistive elements in the storage tank to directly heat water within the tank. The heated water then serves as the means of storing energy for poor weather or nighttime conditions. The current high cost of PV cells--equal to $5 per peak watt--is
expected to decline to $1.50 per peak watt within the next decade.
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