An empty plot of land that a developer might have once considered for tract housing or a parking lot may these days be a more likely candidate for a solar power plant. Increasingly, a plant chosen for such a location is ground mounted.

Ground-mounted PV projects are becoming more relevant in the solar industry as project sizes get larger and the nature of projects changes from an asset held by a building owner to a pure investment. Unlike rooftop systems, which are limited by the size of roofs, ground-mounted systems can be constructed on a much larger scale, offering investors financially attractive projects. In addition, the growing market penetration of low-cost thin-film PV technologies, which require a larger area than traditional crystalline silicon PV, drives growth in ground-mounted systems. More mature solar markets like Germany and Spain have a good history of large ground-mounted systems. Most industry analysts predict tremendous growth in the US for large ground-mounted systems over the next several years. Chris Edgett, director at StrateGen Consulting, for example, believes that growth in ground-mounted systems will be significant. “We are expecting to see massive growth in the deployment of utility-scale ground-mounted systems, representing more than 50% growth over 2008 in many regions,” he says.

In this article, I focus on fixed-tilt ground-mounted PV systems, but much of the information is relevant to fixed-tilt solar thermal and tracking PV systems as well. For my purposes, small systems have less than 2,000 square feet of collector surface area, which equates to about 26 kW of PV at 14% module efficiency; medium systems have in the range of 2,000–20,000 square feet of collector area, or 26–260 kW at 14% efficiency; large systems are any that have more than 20,000 square feet of collector area.

If you design solar system mechanical assemblies, consider transitioning to the use of collector area rather than power capacity ratings for design purposes. PV technology has a wide range of efficiencies. When people refer to system capacity in terms of watts or kilowatts, they are generally referring to a product with a module efficiency of 13%–14%. In fact, the range of efficiencies is anywhere from 6%–19%. The collector area for a 100 kW PV array using 6% efficient Signet Solar SI-344 modules, for example, is approximately 17,850 square feet. If 18.7% efficient SunPower SPR-305-WHT modules are used, the collector area decreases to approximately 5,830 square feet. While both array capacities are the same, these are very different mechanical assemblies.

As with any solar project, if you are considering a ground-mounted PV system you should evaluate several factors. First, carefully describe the goals of the project, as these strongly influence the project design. Next, optimize the design to meet the goals within the context of broader siting concerns, including property boundaries, any restrictions on the property size, the location of electrical interconnections and potential permitting issues. More detail on the property, including geotechnical data on the soil and other relevant site classifications—such as wind zone, snow zone, seismic zone, weather conditions and soil properties—should be the next concern. With geotechnical and civil issues settled, you can then begin to select a foundation and racking system. The next logical steps involve the electrical aspects of the design, including grounding and other Code compliance issues. Finally, consider constructability.

MEETING PROJECT GOALS

The typical PV system’s purpose is to produce the most favorable return on investment, given the constraints of the site. The goals for a specific project, however, should be carefully defined with the owner. Does the owner want to maximize ROI according to time-of-delivery factors per the utility rate schedule? Is there a maximum capacity that is dictated by the utility or other regulatory restrictions that may limit the total size of the project? Are there transmission capacity issues? Energy production estimates are important for any PV system and should be delivered to the owner in the proposal stage of the project to aide in this process. In addition, will aesthetic considerations drive the design of the project? How will other parts of the property or adjacent properties be affected? These considerations are all part of the goal setting process for successful projects.

INITIAL SITING ISSUES AND OPTIMIZATION

If you are siting a ground-mounted system, you need to consider several factors, starting with those most important to the project’s success.