Same Sun of Vermont: Same Sun Solar Society
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Same Sun of Vermont’s inaugural Solar Society community solar project brought together a shared vision, over a decade of solar experience, an open-minded landowner and more than a dozen Vermonters in the market for off-site solar. Various community solar models are gaining traction across Vermont and nationwide, and Same Sun had a growing list of potential customers who were unable to install solar on their residences due to excessive shading, slate roofs or rental agreements.
A Solar Society is distinguishable from other community solar models in the region. Rather than subjecting customers to a leasing model that offers a small percentage discount on the monthly bill, a Solar Society allows customers to buy and own outright a specific percentage of the array based on their energy requirements. They can completely zero out their monthly electric bill and still enjoy all benefits of true ownership, including the federal tax credit. They also have the option to retire or sell their renewable energy certificates.
A dedicated Same Sun team spent a year in research and due diligence to address successfully the legal, land use, financial and permitting challenges associated with the solar society model. With a land lease in place and some members (off-takers or subscribers) signed up, the team still faced challenges such as soft soils and the site’s rural single-phase service.
Geotechnical analysis and multiple pull tests revealed that the site’s soil was almost entirely moist clay. The first racking company on-site was unable to yield passing pull-test results, even with an 8-foot embedment depth. RBI Solar’s longer 20-foot piles just passed muster at an impressive 14-foot embedment depth. The longer piles were crucial to avoid a costly change to a concrete or helical pier foundation.
The second major challenge was the site’s single-phase service. During the initial project planning stage, Same Sun’s design target was a 150 kWac system interconnected to a 3-phase service. However, the nearest 3-phase distribution for the actual site was too far away to justify the cost of a line extension. Given the specific line location and proximity to a substation, Green Mountain Power allowed for a maximum plant capacity of 100 kWac, which was somewhat larger than anticipated considering the site’s single-phase service.
To help mitigate the increased costs associated with the single-phase system’s long, high-current conductor runs, Same Sun aggregated the 12 SMA Sunny Boy 7700TL-US inverters behind the array closest to the point of interconnection. This reduced the ac run by approximately 200 feet. The system’s source-circuit layout mitigates early morning and late afternoon shading on row ends. To optimize component costs, the electrical design bumped the dc:ac power ratio up to just under 125%. In addition, the design specified parallel sets of aluminum homerun conductors to minimize costs while keeping ac voltage drop at an acceptable level.
“In my 15 years working in the solar industry, this project was undoubtedly one of the most rewarding. Like all solar projects, it had its unique challenges—most notably developing a progressive community solar model and orchestrating an off-site generation project with so many stakeholders. Connecting the dots between a parcel of fallow land and Vermonters who couldn’t otherwise go solar was priceless.”
—Khanti Munro, Same Sun of Vermont
DESIGNER & PROJECT MANAGER: Khanti Munro, director of development and technical design, Same Sun of Vermont, samesunvt.com
LEAD INSTALLER: Tyler Crow, Same Sun of Vermont
DATE COMMISSIONED: August 2015
INSTALLATION TIME FRAME: 20 days
LOCATION: Poultney, VT, 43.5°N
SOLAR RESOURCE: 4.21 kWh/m2/day
ASHRAE DESIGN TEMPERATURES: 84.2°F 2% average high, -14.8°F extreme minimum
ARRAY CAPACITY: 113.4 kWdc
ANNUAL AC PRODUCTION: 144,800 kWh
MODULES: 360 SolarWorld Sunmodule Pro-Series XL SW 315, 315 W STC, +5/-0 W, 8.71 Imp, 36.5 Vmp, 9.35 Isc, 45.6 Voc
INVERTERS: Single-phase 120/240 Vac 600 A service, 12 SMA Sunny Boy 7700TL-US, 7,680 W rated output, 600 Vdc maximum input, 270–480 Vdc rated MPPT range, 125–500 Vdc operating MPPT range
ARRAY: 10 modules per source circuit (3,150 W, 8.71 Imp, 365 Vmp, 9.35 Isc, 456 Voc), three source circuits per inverter (9,450 W, 26.13 Imp, 365 Vmp, 28.05 Isc, 456 Voc), 113.4 kWdc array total
ARRAY INSTALLATION: Driven-pile ground mount, RBI Solar GM-I racking, 180° azimuth, 25° tilt
INVERTER AC COMBINERS: Two 12-circuit two-pole ac panels, 40 A breakers
SYSTEM MONITORING: Solar-Log 1000 revenue-grade meter with CTs and cellular modem, Solar-Log WEB Commercial Edition monitoring package