Commercial Grid-Direct Photovoltaic System: Denver Museum of Nature & Science

When it was commissioned, the PV array at the Denver Museum of Nature & Science was the largest building sited PV system in Colorado. The system achieved even greater prominence when President Obama and Vice President Biden toured the site before signing the American Recovery and Reinvestment Act on February 17, 2009.

The project size was dictated by Xcel Energy’s rebate rules. This, along with the pricing structure for the PPA, favored a system as large as possible without exceeding 100 kW STC. Using SMA Sunny Towers allowed for significant design flexibility and a final array capacity of 99.98 kW.

The inverter and interconnection locations presented major implementation issues. The rooftop penthouse is large enough to house multiple inverters but not structurally strong enough for a single central inverter. Using Sunny Towers spreads out the inverter point loading. The mechanical subpanel in the penthouse is a 3-wire, 277/480 Vac panel without a neutral conductor. An isolation transformer is used to generate a neutral reference, and the Sunny Towers are configured for 277 Vac operation. This allows the system to interconnect on the rooftop, minimizing both the ac and dc wire runs and reducing the voltage drop and installation expenses.

"The Denver Museum of Nature & Science is the perfect place to showcase solar technologies. By using the PV array as a teaching tool, not just a power source, the Museum sets an example for other community organizations and visitors to follow, encouraging them to do their part for the environment."

Ray Tuomey, Namaste Solar

Overview

DESIGNER: Jason Sharpe, COO, & Stephen Irvin, CFO, Namaste Solar, namastesolar.com
PROJECT MANAGER: Ray Tuomey, co-founder, & Sam Mason, lead installer, Namaste Solar
DATE COMMISSIONED: June 11, 2008
INSTALLATION TIMEFRAME: 30 days
LOCATION: Denver, CO, 39.5°N
SOLAR RESOURCE: 5.5 kWh/m2/day
RECORD LOW/AVERAGE HIGH TEMPERATURE: -31°F / 88°F
ARRAY CAPACITY: 99.98 kW
AVERAGE ANNUAL AC PRODUCTION: 135 MWh

Equipment Specifications

MODULES: 465 SunPower SPR-215-WHT, 215 W STC, +5%/-5%, 5.4 Imp, 39.8 Vmp, 5.8 Isc, 48.3 Voc
INVERTERS: 3-phase, 277/480 Vac system, 3 SMA Sunny Towers, 36 kW each, 6 SB6000US per tower, 600 Vdc max. input, 250–480 Vdc MPPT range
ARRAY: Eleven 27 module subarrays with nine modules per string (1,935 W, 5.4 Imp, 358.2 Vmp, 5.8 Isc, 434.7 Voc) and three circuits per inverter (5,805 W, 16.2 Imp, 358.2 Vmp, 17.4 Isc, 434.7 Voc); and seven 24 module subarrays with eight modules per string (1,720 W, 5.4 Imp, 318.4 Vmp, 5.8 Isc, 386.4 Voc) and three circuits per inverter (5,160 W, 16.2 Imp, 318.4 Vmp, 17.4 Isc, 386.4 Voc)
ARRAY INSTALLATION: Flat roof mounted, SunPower T-10 racking, 180° azimuth, 13° tilt
ARRAY COMBINER: Inverter integrated, 15 A fuses
SYSTEM MONITORING: Fat Spaniel Technologies, inverter direct with weather station; museum designed and installed mobile, wireless flash display for educational purposes

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