Same Sun of Vermont: Shrewsbury Mountain School
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Same Sun of Vermont won the Shrewsbury Mountain School project via competitive bid. The PV system is designed to provide at least half of the small elementary school’s annual electrical usage. Given the school’s location along the spine of Vermont’s Green Mountains, the project posed some unique design challenges, including 70 psf snow loading, the need to make the system kid-proof, NEC 2014 Section 690.12 rapid-shutdown compliance and a 1-week spring break installation window.
When Same Sun inspected the roof in early spring of 2015, the crew immediately recognized the significance of the rooftop snow and ice loading. The standing seams along the two valleys were noticeably deformed downhill, and the glacier-like drift from the lower valleys to the ground was high enough to negate the need for ladder access.
To help mitigate the long-term effects of the snow, the array design keeps the modules well out of the roofs’ valleys. In addition, the installers fabricated custom snow guards from short pieces of PV rail that they mounted perpendicular to the ends of each interior module row. The guards help deflect the downward flow of snow and ice. To protect the two rooftop conduit runs, Same Sun installers let the module rails run slightly long and installed aluminum rigid conduit on top of the rails. Running the conduit in this manner not only simplified conditions-of-use conductor sizing, but also keeps the conduit a few inches above the primary flow of the snow and ice. Lastly, the installation team attached racking clamps to every seam, rather than every other or every third seam as is typical in less-challenging environments.
Same Sun specified microinverters for the Shrewsbury Mountain School project for several reasons. Module-level optimization mitigates the impact of morning shading from a tree line to the east of the array. It also reduces the impact of some very obvious variations in the amount of snow buildup and shedding on different areas of the roof. The building was short on appropriate wall space for string inverter mounting. The only available locations were far from the point of interconnection and difficult to make inaccessible to young children. Specifying micros also facilitated adherence to the relatively new 690.12 rapid-shutdown requirements. Lastly, module-level monitoring is a real boon for the teachers at the school, who are excited to incorporate information from the new solar array into their curriculum.
Physical array layout and the necessity to keep modules as far away from the roof valleys as practical primarily dictated the characteristics of each inverter output circuit (IOC). The longer 12- and 13-module IOCs have a center-fed configuration to minimize ac voltage drop. While it was not a specific requirement of the RFP, an ambitious project goal was to complete the installation during the school’s 1-week spring break in early April. The tight timeline would minimize liability and the disruption of school activities. With the added labor associated with the microinverter and trunk cable installation, unique conduit run and custom snow guards, and a bonus half-day lost to snow removal, the installers completed the system with just under a half day to spare.
“Working with schools inevitably provides a unique set of challenges, but always a very gratifying result. Shrewsbury was no exception. A very cooperative school administration, a dedicated installation crew and a 70-hour work week yielded a robust system with exceptional educational potential.”
—Khanti Munro, Same Sun of Vermont
DESIGNER & PROJECT MANAGER: Khanti Munro, director of development and technical design, Same Sun of Vermont, samesunvt.com
INSTALLERS: Tyler Crow, Jon Klos, Jake Boulier, Same Sun of Vermont
DATE COMMISSIONED: April 2015
INSTALLATION TIME FRAME: 8 days
LOCATION: Shrewsbury, VT, 43.5°N
SOLAR RESOURCE: 4.3 kWh/m2/day
ASHRAE DESIGN TEMPERATURES: 84°F 2% avg. high, −20°F extreme min.
ARRAY CAPACITY: 25.2 kWdc
ANNUAL AC PRODUCTION: 30,440 kWh
MODULES: 90 SolarWorld Sunmodule Plus SW 280 Mono, 280 W STC, +5/-0 W, 9.07 Imp, 31.2 Vmp, 9.71 Isc, 39.5 Voc
MICROINVERTERS: 120/240 single-phase service, 90 Enphase M250, 250 W peak output power, 48 Vdc maximum input, 16–48 Vdc operating range, 27–39 Vdc MPPT range, 16 microinverters per single-phase 20 A branch circuit maximum
ARRAY: One microinverter per module, 9–13 micros per inverter output circuit (IOC), 9 A–13 A at 240 Vac per IOC, eight IOCs total combined at new dedicated ac inverter combiner panel via two-pole 20 A breakers; 25.2 kWdc array total, 90 Aac at 240 Vac
ARRAY INSTALLATION: Flush mount, standing seam roofing, IronRidge XRS-1000 rails with grounding mid-clamps, EcoFasten ASGU-2 clamps installed on every seam (approximately 20-inch spacing) to handle snow loading, 140° azimuth, 18° tilt
SYSTEM MONITORING: Enphase Envoy communications gateway hardwired to school network, Enphase Enlighten web-based monitoring