How do you space a ground-mounted array?
Here are some pointers for a well designed ground-mounted array. Jump to another CivicSolar article for some practical and free shading tools.
Ground-mounted PV systems are increasingly prevalent in the solar industry with the trend towards larger systems and systems built as investments. In addition, the decreasing cost of PV systems has created demand even by customers who lack rooftops amenable to solar installations. Rooftop installations are in any case limited by the size of the roof; whereas ground- mounted systems can be constructed on a much larger scale, offering investors projects with greater financial rewards. Furthermore, ground-mounted systems can be optimally-aligned with respect to both their southern orientation and their tilt angle. In markets where there has been extensive penetration of larger systems such as in Germany and Spain, there has been widespread deployment of ground-mounted systems.
As with any solar project, ground-mounted PV systems must be designed with care to meet project goals and address practical issues. Siting issues for ground-mounted systems are different than for roof-mounted systems and include such things as property boundaries, terrain, soil properties, the location of electrical interconnections and potential permitting issues. But in addition to these general siting concerns, energy generation optimization in a ground-mounted system (or, for that matter, on most flat-roof installations) must address the unique issue of how to space the rows of solar panels to maximize energy harvest while preventing the panels from shading one another. This issue can of course be avoided by simply keeping the rows of panels sufficiently far apart, but generally one needs to minimize this inter-row spacing to most efficiently utilize the available site.
Ground-mounted arrays are arranged in rows of panels in an east-west alignment that allows the panels to have an ideal south-facing orientation. One can then utilize the site’s latitude to determine the optimal tilt angle for the panels. However, there is a tradeoff between using a tilt angle as high as the latitude and how close one can place the rows in the array. The size and configuration of the site may place constraints on this inter-row spacing. Therefore, it is standard practice to perform optimization studies to determine tilt angle and row spacing for the array.
For small systems, it may be sufficient to simply use a rule of thumb, such as the inter-row separation should be about 2.5 times the row height (reasonably good for 40 degree latitude) or that that latitude minus 10 degrees is a good compromise tilt angle in a multiple-row fixed-tilt ground-mounted system. Historically, simple calculations based on geometry were used. A standard formula is "d = h + tanø" where d is the minimum distance between rows, h is the height differential between the top of one row and the bottom of the row to the north, and ø is the solar altitude angle.
However, there are successively more sophisticated ways of calculating the inter-row spacing that take into account seasonal variations of the sun’s path at the site.
Shade studies can be performed on-site using a shade analysis tool, on paper using math and geometry, or on a computer using 3-D CAD programs or specialized PV software such as PVSyst or PV*SOL Expert. Ultimately, most systems are designed to permit some self-shading loss between rows in the heart of the winter, typically resulting in a loss of no more than a couple of percent of annual production. This is because it is often not practical to utilize the additional space required to completely eliminate such shading. Ultimately, the goal is to get the most production of the system given the constraints imposed by its site.
Finally, for the visually oriented, try Google Sketchup - it's a free drawing tool with incredible shading functions. Build your site on an actual map and then lay it out with PV for predicted spacing - then watch the sun for the whole year to see what will happen. Jump to the article on CivicSolar HERE.
“Calculating Inter-Row Spacing,”
December/January 2009, SolarPro Magazine