Solar Methodology

Data Sources

Existing Roof-mounted Solar PV Sites: Existing solar PV sites data was collected from the Vermont Clean Energy Development Fund, the Vermont Public Service Board, and other sources, and geocoded.  Users can click on each icon to learn more about specific sites.

Existing Ground-mounted Solar PV Sites: Existing solar PV sites data was collected from the Vermont Clean Energy Development Fund, the Vermont Public Service Board, and other sources, and geocoded.  Users can click on each icon to learn more about specific sites.

Existing Solar Hot Water (Thermal) Sites: Existing solar hot water sites data was collected from the Vermont Clean Energy Development Fund and geocoded.  Users can click on each icon to learn more about specific sites.

Solar PV and Thermal Installers / Consultants / Manufacturers: All solar industry sites were collected from Renewable Energy Vermont and geocoded. Users can click on each icon to learn more about specific sites.

Roof-mounted Solar Photovoltaic Potential

Solar photovoltaic potential was calculated with the ESRI ArcGIS Solar Analyst tool, using the best available statewide Digital Elevation Model.  Unlike the Solar Boston website, building elevations are currently unavailable statewide in Vermont. Consequently, shading from other buildings is not represented in the modeling results. This analysis was conducted on all of the building points in the latest Emergency 9-1-1 database and assumes a “flat” roof mounted photovoltaic system at a height above ground of approx. 23’ (7m) facing due South (aspect of 180 degrees).  This analysis does not account for ground-mounted solar trackers (e.g., AllEarth Renewables AllSun Solar Trackers).

While other tools such as the National Renewable Energy Laboratory’s PVWATTS program are more robust than the ESRI ArcGIS Solar Analyst tool, the Solar Analyst can be fed any custom elevation and input point data. Using an input Digital Elevation Model (DEM) allowed the analysis to capture the effects of topographic shading on modeled solar potential.

The ESRI Point Solar Radiation tool (PSR) calculates Plane-of-Array solar radiation in units of Watt hours/square meter (Wh/m2) for each E9-1-1 building point based on local level elevation, slope, and aspect derived from the VTHYDRODEM digital elevation model. The energy values calculated for each E9-1-1 building point analysis account for the effects of topographic shading, but vegetation shading (i.e., the big tree in your front yard) can only be factored in by end users. Users can further refine estimates with Tilt and Azimuth “derate” settings. The mathematical algorithms behind this tool account for variations in topography using hemispherical model “sky maps” (think of lying on the grass and looking straight up) and atmospheric weather data via the radiation parameters. Total global radiation is calculated as the sum of both the direct (sun striking the panel) and diffuse radiation (e.g., water molecules in the air) for all directions on the topographic surface at each location.

The radiation parameters, controlled by diffuse proportion (cloudiness), and transmittivity (atmospheric effect) were derived from historical recorded data at five National Solar Radiation Database stations in Vermont. The PSR tool does not account for reflected radiation in the analysis (e.g., the effects of snow cover), but it was considered to be negligible based on an NREL report that states, “Comparing surface-orientation factor contour plots based on these assumptions indicates only a small effect due to snow cover”. Effects of snow cover obscuring a solar array are also not accounted for and are considered to be minimal as these instances occur at the time of year where solar radiation is lowest.

A default array size with a DC rating of 4kW (at Standard Test Conditions) is assumed, and the PSR modeled “raw” values are “derated” by a “DC-to-AC” derate factor of 0.8 to yield the Net Power Production of the array in Kilowatt Hours. The .8 “DC-to-AC derate” factor is a slightly less conservative value than that used by the National Renewable Energy Laboratory (.77), based on feedback from a number of Vermont Solar professionals. Users can change the array size and tilt and azimuth derate factors, but the “DC-to-AC” derate factor is fixed.

Ground-mounted Solar Photovoltaic Potential

The ground-mounted solar “area” analysis uses the same ESRI ArcGIS Solar Analyst tool as the roof-mounted PV analysis to create the solar area base layer. A roof-mounted system implies easy site access due to the presence of an existing structure, as well as road or driveway access. In contrast, a ground-mounted system requires a different approach since it is subject to physiographic parameters such as slope, soils, and site access.

The feasibility of ground-mounted PV arrays can be limited by site and landing zone access (e.g., physically delivering the hardware to the site and maneuvering equipment around the site), and estimating site energy potential is complex and subject to site specifics that vary by project size. Some site features, such as panel setback from tree lines to avoid shading and a combination of both aspect and slope are fixed. Other site features can scale marginally by project size (e.g., landing zone, inverter shed(s), parking lot and access road(s)). For north facing slopes, panels must be spaced further apart to avoid shading. Conversely, south facing slopes require less spacing relative to slope. As the system size decreases these fixed features take up an increasing percentage of the total site area, making it difficult to use a single conversion value to estimate potential.

As a result, potential solar area sites are stated only in categories of percent slope and not incoming solar potential (with units of kWh/m2). This analysis was conducted statewide using the USGS 1:24k scale National Elevation Dataset Digital Elevation Model as a base for constraining the analysis to a targeted range of aspects and slopes while also excepting “masked” areas as unsuitable resources. The statewide solar area potential layer was constrained by the following 1:24k DEM based aspect and slope values: 1) Only slopes less than or equal to 10% were considered for “South facing” aspect from 90 degree (East) clockwise thru 270 degrees (West), and for slopes less than or equal to 5% from greater than 270 degrees (West) clockwise through 0 degrees (North) to less than 90 degrees (East).

The following considerations further constrained the potential solar area site: frequently flooded areas, conserved lands, transportation infrastructure, E 9-1-1 buildings, surface waters, wetlands, deer wintering areas and rare, threatened and endangered species, and natural community features. In order to model the constraint of wind loading on ground mounted panels, areas with a wind data class greater than “3”, from the AWS TrueWind “Residential Scale” 30m hub height dataset, were used as a surrogate to mask out these areas.

In order for users, planners, and decision makers to understand what portion of the solar area resources are also highly valuable for other uses, the final solar area data layer can be intersected with prime agricultural soils. This enables the Analysis Results panel to include that percentage of the total acreage meeting solar area requirements that happen to contain prime soils that may be of a higher societal value for growing food. The Soil SURvey GeOgraphic Database (SSURGO) Soils Data, created by the National Resource Conservation Service (NRCS), was used in concert with two other NRCS resources (Farmland Classification Systems for Vermont Soils, which defines Prime, Statewide & Local significant soils, and VT State Data Table TOP20) to aggregate all Federal, State and Local “Prime” soils data into a single “Prime” agricultural soils layer.

Similar to the roof-mounted PV analysis, shading from buildings or vegetation is not represented in the modeling results. This analysis does not specifically account for, nor preclude the viability of solar trackers (e.g., AllEarth Renewables AllSun Solar Trackers). For more information on the selection criteria leading to the use of the ESRI Solar Analyst tool see the “PV Solar Methodology”.