A National Institute of Standards and Technology-built photovoltaic residential hot water heater that uses computer technology to maximize the conversion of sunlight into electricity has begun a year-long trial at the Florida Solar Energy Center in Cocoa, Fla. The transfer of the system from NIST's Gaithersburg, Md., laboratories to the Sunshine State's solar experimental station for field tests represents the latest advance for a system with the potential to replace more conventional solar thermal water heating systems. Future field tests of the technology are expected to be held later this year at the Kadena Air Force Base housing in Okinawa, Japan, and at a site selected by the Tennessee Valley Authority.
The Florida researchers will evaluate the NIST system's ability to serve the hot water needs for a representative "family of four." They will collect their data from the system, which includes an array of photovoltaic cells, a storage tank incorporating multiple electrical resistive elements and a controller that maximizes the conversion of solar energy into heated water. In the new system, a microprocessor reacts to light intensity changes and kicks in the appropriate electrical resistive elements for maximum energy conversion from sunrise to sunset.
The PV design avoids many of the problems that inhibit greater market penetration of current thermal solar water heating systems. Such systems operate when sunlight is transmitted through a transparent cover and absorbed by a metallic plate, with the energy then heating a fluid flowing under, through or over the plate.
According to A. Hunter Fanney, NIST mechanical engineer who invented the new system with his colleague Brian Dougherty, "Photovoltaic cells heat water without the need for pipes through the roof, fluids which can freeze or offer toxic risks if antifreeze is used, and costly pumps that can fail."
He said the new system also enjoys two distinct advantages over previously proposed uses of photovoltaic cells for residential homes. "During the night and during other poor solar conditions, previous systems have required the use of storage batteries to provide energy. They also have required the use of an expensive electronic inverter to convert the direct current produced by the photovoltaic cells into alternating current required by household appliances. In this invention, the resistive elements within the storage tank use direct current provided by the photovoltaic panels and, in turn, use the water within the storage tank as a means of storing energy for poor weather and night time conditions," he said.
The current high cost of PV cells, equal to $5 per watt, is expected to decline to $1.50 per peak watt within the next decade. At that cost, a PV water heater could be installed for approximately $4,000, about the same cost as a solar thermal heater.
The payoff to the nation could be great if price and convenience can be combined in tomorrow's PV water heater. According to Fanney, a photovoltaic solar water heating system sized to provide 60 percent of the water heating needs of a typical family using an electric water heater would result in yearly savings of $300. If 50 percent of the households within the United States utilized a photovoltaic solar water heating system the annual savings to the country would be approximately $5 billion. This reduction would decrease the balance of trade with oil-producing countries and reduce the emissions to the atmosphere created when non-renewable energy resources such as coal, oil and gas are used to produce energy required for water heating.
NIST holds a patent on the photovoltaic hot water system. Industry partners for the project are welcome. Interested parties should contact A. Hunter Fanney, B222 Building Research Building, NIST, Gaithersburg, Md. 20899-0001, (301) 975-5864, e-mail: hunter [at] micf.nist.gov (hunter[at]micf[dot]nist[dot]gov).
An agency of the Commerce Department's Technology Administration, NIST promotes economic growth by working with industry to develop and apply technology, measurements and standards.
NOTE TO EDITORS: Photographs and artwork of the PV solar water heater are available by faxing a request to (301) 926-1630.