Yesterday we spoke about different options for mounting a solar array. Today we are going to dive deeper into ground mount systems and talk specifically about some of the considerations one would need to take when siting a ground mount system in order to get the most optimum solar exposure.
The nice thing about ground-mounted systems is they can be
optimally-aligned with respect to both their southern orientation and their
tilt angle. I use a tool called PVWatts in order to look at the different options for azimuth and what kind of impact that will have over the course of the year. In my area, a more Southwesterly orientation is better than due south when looking at overall system production. Where I live, if I were grid ties, I would have my panels facing about 220 degrees, or 40 degrees off due south.
However since I am more concerned with when I get my energy than squeezing every watt our of daily production, we aren’t pointing at 220 degrees. When I get up in the morning I work many times from my upstairs office or my front porch. I need energy in order to run my office machinery to include my routers as well as a fan to keep me cool during the summer, where mornings are pleasant but still hot and muggy. If I were maximizing the daily output, it would be 11 AM before I was getting any real production from my panels. Lets talk for a inut about reducing the amount of shade on the panels. Lets say I have an array of three solar panels. And each of those panels has 72 cells…thats 216 total cells Now if a few of those cells are shaded, they reduce the amount of current that can pass through the system by the proportion of shading on those cells times the total number of cells. So lets say one cell is 50% shaded, now I have the equivalent of 1.5 panels worth of energy coming from my three panels. In other words by shading one out of 216 cells, I have erased the generation of 108 of those cells. That’s not a good utilization of my investment at all. The reason for this is that every cell in the string has to operate at the current set by the shaded cell.
One strategy for mitigating this issue is to place panels that may recieve shading on a parallel string, so that only the string the shaded cells are in is affected and all the parallel strings are not. This is how our system is set up in that the eastern most string which receives early shade, and the westernmost string which receives evening shade, wont impact the other two respective strings.
An interesting emergiung tech is Module Level Power Electronics which are essentially microinverters that operate on the individual module level and therefore limit shading losses to a single panel. These are initially more expensive but in the right environments can pay for themselves over time. I plan to do a stand alone show on microinverters in the future. Some panel manufacturers are also looking into in panel electronics that may act as microinverter for each cell, limiting the shading losses to the individual cells that are shaded and not impacting any of the other cells on that panel.
As with any solar project, ground-mounted PV systems must be designed with care to meet your individual 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 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. As a rule of thumb, If your latitude is below 25°, use the latitude times 0.87. If your latitude is between 25° and 50°, use the latitude, times 0.76, plus 3.1 degrees. if you above 50 degrees latitude, there are many more factors that must be taken into account so a simple formula doesn’t apply. Remember that we are taking the whole day into consideration here, so we aren’t pointing the panels directly at the noon sun in most instances.
I want to take a minute to remind everyone that right now, there is a 30% tax credit for solar systems, pv or thermal, constructed through 2019, then the credit drops to 26% in 2020, 22% in 2021, and after 2021 there is a permanent 10% credit.
Ok back to business, 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. One strategy for optimizing space might be to raise the height of the northern array so that the shade in the winter falls under the bottom panel. ANother thing to consider is the height of trees near your area. I love using the sun seeker app on my iphone to look into where the sun will be at different points of the year and I can see which trees will be a problem, and which will be a problem in the future. Just like panel spacing, a 2.5 x height ration can be a good rule of thumb for determining setback from a treeline to a solar array.
ANother great tool for this is google sketchup, you can actually import a map of your property, draw the array in with the proper azimuth and angle, and then see during different times of year what the shading is going to look like. You can additionally draw in a wall to act as a treeline and do the same thing by pulling a line out from the wall to the edge of the shading to find the minimum setback for your array.
Ok in closing, if you are going to place your panels near a property boundary be sure to look into creating a solar easement.