Researchers at the Commerce Department's National Institute of Standards and Technology have invented a novel solar water heating system that is believed to be the first to use photovoltaic cells in combination with computer technology to capture the sun's energy.
"We think this system is going to be a real competitor for thermal solar hot water heaters, which have had many problems over the years, and some day even traditional electric or gas water heaters," says Hunter Fanney, an engineer in NIST's Building and Fire Research Laboratory and the system's inventor.
Photovoltaic cells are semiconductor devices that convert the energy in sunlight into electrical energy. The novel NIST system, which recently received a patent, uses an array of PV cells on the roof of a house to deliver solar power directly to multiple heating elements in a hot water tank.
A computerized controller measures the energy coming in and controls which element or combination of elements receives the energy. "Using multiple heating elements connected to computer technology enables the system to take maximum advantage of the varying intensity of solar energy that strikes the cells throughout the day," says Fanney.
Solar thermal water heaters were first widely used in Florida homes starting in the 1920s. Water is pumped through pipes up to a rooftop solar collector where it is heated and returned to the storage tank. But over the years, these systems have had numerous problems, including freezing or leaking fluid; failure of temperature sensors, controllers and pumps; and significant heat losses through piping. In addition, pipe installation often is complicated.
A big advantage of the NIST system, says Fanney, is that it directly heats the water in the tank, eliminating the need for piping and its associated problems.
"This system offers the potential for a lower-cost means of supplying a significant portion of the energy consumed for domestic water heating," says Fanney. Currently, approximately 90 million residential water heaters are used in the United States; only a million or so use solar. According to the Department of Energy, an electric hot water heater for a typical household of two adults and two children uses almost as much energy a year as an automobile driven 12,000 miles averaging 24 miles per gallon.
In the Washington, D.C., area, Fanney estimates that a solar water heater with a PV array of about 4 meters by 4 meters could supply up to 60 percent of the hot water needed for that same four-member household.
While the PV water heater has many advantages, it does have one disadvantage--cost of solar photovoltaics. According to the Commerce Department's U.S. Industrial Outlook 1994, the average cost per peak watt of solar photovoltaics is expected to be approximately $4 by the end of 1994. But, that is a far cry from the early 1980s when the average cost was more than $100.
With the introduction of new, more efficient PV technologies and an increased worldwide demand, PV prices are expected to decline to $1.50 per peak watt within the next decade. At this cost, a PV water heater could be installed for approximately $4,000, about the same cost as a solar thermal water heater. If the U.S. Department of Energy's National Photovoltaic Program reaches its long-term goal of $0.50 per peak watt, a PV solar water heating system could cost about $1,875 and pay for itself in reduced energy costs in about six years. Photovoltaic panels can last more than 20 years.
Fanney and his colleague Brian Dougherty, a mechanical engineer, are building a full-scale PV hot water system at NIST's laboratories in Gaithersburg, Md. They expect to have it operational in January 1995 and will monitor the system's performance for a year.
Fanney welcomes inquiries from industries interested in "partnering" with NIST on this project. He can be reached by telephone at (301) 975-5864 or by e-mail at hunter [at] micf.nist.gov (via Internet).
NIST has been involved in solar energy research since the 1970s. For example, Fanney helped develop a test method to characterize the performance of solar thermal water heating systems, which was adopted as an industry standard by the American Society of Heating, Refrigerating and Air Conditioning Engineers in 1988.
A non-regulatory agency of the Commerce Department's Technology Administration, NIST promotes U.S. economic growth by working with industry to develop and apply technology, measurements and standards.