Solar Site Evaluation

Tools & Techniques to Quantify & Optimize Production

Improve system performance and energy harvest projections with a thorough site evaluation that includes shading analysis and insolation quantification.

There are multiple factors to consider when evaluating a site for a photovoltaic or solar thermal installation, and each may impact optimal energy production. In addition to latitude and longitude, which determine the sun path characteristics, panel or collector orientation - tilt and azimuth - defines the field of view that an array has of the sun. Shading from trees, hillsides, buildings or other obstructions can cause significant degradation in energy production. Additionally, local and regional weather patterns result in site-specific seasonal and daily fluctuations in solar insolation.

These factors combine and interact to determine the solar energy incident on an array and therefore impact both financial returns and customer satisfaction. Tools and techniques used in site evaluation emphasize shade analysis and optimizing solar access.

THE IMPORTANCE OF SITE EVALUATIONS

An early and thorough site evaluation can lead to better system designs that will result in the following benefits: increased energy production by selecting the best location for the solar array; improved accuracy in energy production estimates due to better quantification of shading and other site-specific issues; optimized financial incentives, such as state-specific rebates that adjust for panel orientation and shading; improved system installation and materials cost estimates; and increased customer satisfaction and confidence, which in turn can lead to repeat or referred business.

Leading solar system designers and installers invest significant efforts into on-site data collection and evaluation, especially during customer qualification, initial design and proposal preparation. The site information gathered includes:

  • Measurement of location parameters, including available area for the array, roof pitch or site grade, and azimuth.
  • Measurement of solar access and impact of shade- causing obstructions, as well as evaluation of shade- reduction strategies, such as tree trimming or removal.
  • Identification of issues that could jeopardize the viability of a project or result in increased design and installation complexity and implementation cost, such as conductor and trench routing; proximity of array to inverter; roofing material integrity; rafter and beam spacing for engineering calculations; and safety concerns and access issues.
  • Direct contact with the client to discuss additional issues, including possible aesthetic concerns and financing plan options.

SUN PATHS

Solar access will depend on the sun’s location, defined by elevation angle and azimuth direction, as it varies through each day and throughout the year. This path can be plotted for a given latitude and longitude. An example sun path chart is shown in both rectilinear and polar formats in Graphs 1a and 1b. Typically, sun charts are centered around south (180° azimuth) for sun path diagrams in the Northern Hemisphere, and around north for sun path diagrams in the Southern Hemisphere. Examples shown in this article are for the Northern Hemisphere with references to summer and winter from a Northern Hemisphere view.

The sun path is a function of latitude and longitude, and it shifts with changes in location. This effect is illustrated for two different locations in Graphs 2a and 2b. Moving north toward higher latitudes, the annual sun path chart shifts, indicating that the sun is at lower elevations. Moving south, the chart shifts, indicating higher sun elevations. Moving west toward greater longitude, the sun’s path remains the same, but the time for each sun location is shifted toward later in the day.

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