Measured Performance of a 35 Kilowatt Roof Top Photovoltaic System.
Measured Performance of a 35 Kilowatt Roof Top
Photovoltaic System.
(1829 K)
Fanney, A. H.; Henderson, K. R.; Weise, E. R.
International Solar Energy Conference. ISEC2003. ASME
International. Proceedings. March 15-18, 2003, Hawaii,
2003.
Keywords:
roofs; building integrated photovoltaics; photovoltaic
cells; renewable energy; single-crystalline; solar
energy
Abstract:
A 35-kilowatt roof top photovoltaic system has been
installed at the National Institute of Standards and
Technology (NIST) in Gaithersburg, Maryland. The system,
located on a the roof that connects NISTs Administration
Building to its adjoining conference and cafeteria
facilities, produced NISTs 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 it was
found that the original solar radiation sensor was
influenced by reflections from the south-facing wall of
the Administration Buildings tower. NISTs 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. 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. Using the utilitys rate schedule, the
monetary savings attributable to the photovoltaic system
is determined by combining the cost of the displaced
energy with the reduction in peak demand charges
attributable to the photovoltaic system. Finally, using
utility provided data and the Environmental Protection
Agencys (EPA) Environ-mental Benefits Calculator,
estimates are made of the avoided emissions of the
photovoltaic system over its projected life span.
Building and Fire Research Laboratory
National Institute of Standards and Technology
Gaithersburg, MD 20899