Field Performance of Photovoltaic Solar Water Heating Systems

Energy consumed for water heating accounts for approximately 17.9 EJ of the energy consumed by residential and commercial buildings. Although there are over 90 million water heaters currently in use within the United States (Zogg and Barbour, 1996), durability and installation issues as well as initial cost have limited the sales of solar water heaters to less than 1 million units. Durability issues have included freeze and fluid leakage problems, failure of pumps and their associated controllers, the loss of heat transfer fluids under stagnation conditions, and heat exchanger fouling. The installation of solar water heating systems has often proved difficult, requiring roof penetrations for the piping that transports fluid to and from the solar collectors. Fanney and Dougherty have recently proposed and patented a solar water heating system that eliminates the durability and installation problems associated with current solar water heating systems. The system employs photovoltaic modules to generate electrical energy which is dissipated in multiple electric heating elements. A microprocessor controller is used to match the electrical resistance of the load to the operating characteristics of the photovoltaic modules. Although currently more expensive than existing solar hot water systems, photovoltaic solar water heaters offer the promise of being less expensive than solar thermal systems within the next decade. To date, photovoltaic solar water heating systems have been installed at the National Institute of Standards and Technology in Gaithersburg, MD and the Florida Solar Energy Center in Cocoa, FL. This paper will review the technology employed, describe the two photovoltaic solar water heating systems, and present measured performance data.