Measured Impact of a Rooftop Photovoltaic System

A 35-kW rooftop photovoltaic (PV) system has been installed at the National Institute of Standards and Technology (NIST) in Gaithersburg, Maryland. The system, located on the flat roof that connects NIST’s Administration Building to its adjoining conference and cafeteria facilities, produced NIST’s first site-generated renewable energy on September 14, 2001. In addition to providing electrical energy and reducing monthly peak electrical loads, the rear surface of each module is laminated to 51 mm of extruded polystyrene enhancing the thermal performance of the roof. A unique ballast system secures the photovoltaic system, eliminating the need for roof penetrations. An instrumentation and data acquisition package was installed to record the ambient temperature, wind speed, solar radiation, and the electrical energy delivered to the grid. Additional solar radiation instruments were installed after determining that the original solar radiation sensor was influenced by reflections from the south-facing wall of the Administration Building’s tower. NIST’s electric utility billing schedule includes energy and peak demand charges. The generation charges vary significantly depending upon the time interval—off-peak, intermediate, and on-peak—during which the energy is consumed. The schedule is divided into summer billing months (June–October) and winter billing months (November–May). During the winter billing months, the distribution, transmission, and generation peak demand charges are based on the greatest power demand imposed by the site on the grid. During the summer billing months, an additional demand charge is imposed to capture electrical demand during the on-peak time interval. This paper summarizes the monthly and annual measured performance of the photovoltaic system. The monthly energy produced by the system is tabulated. The system has provided 35676 kWh of electrical energy during its first year of operation. Conversion efficiencies—computed using solar radiation measurements from a single photovoltaic cell radiation sensor, four thermopile-based radiation sensors located around the perimeter of the photovoltaic array, and a remotely located thermopile-based radiation sensor—are presented. Annual conversion efficiencies of 10.8%, 8.8%, and 7.4% were achieved using cell, module, and foot print areas, respectively. Using the electric utility’s rate schedule, the monetary savings credited to the photovoltaic system is determined by combining the cost of t