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Solar Power Projects

NCAT Hooks Up to Solar Power

Photovoltaics fill some of the power needs at the headquarters of the National Center for Appropriate Technology in Butte. The PV system installed on NCAT's roof in May by Solar Plexus of Missoula consists of 60 Siemens SP75 modules. Each module is rated at 75 watts and is flush-mounted on the south-facing, third-floor roof; total module area is about 480 square feet. Peak electrical output of the system is 4.5 kilowatts.

Thirty modules make up each of the east and west arrays. Each array is electrically connected to produce direct current (DC) at 48 volts. The output from each array can be disconnected for comparison purposes. The system's DC electrical output is connected to a Trace SW4048UPV utility intertie inverter.

The inverter, connected to the utility grid, senses the utility's voltage and frequency and produces alternating electricity at the utility's voltage and frequency. The inverter is designed to operate only when utility electricity is available. Whenever the utility goes down, the inverter shuts down and will only turn back on when the utility line comes back on.

The system will provide approximately 13 percent of the NCAT building's electrical consumption. It is assumed that the annual kWh use of the building will be 50,416 and the PV system will provide 6,750 kWh. Helena solar data was used for the performance calculation.

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