Designing a Large-Scale Commercial System
People often ask: “What’s the difference between designing a larger commercial array versus a residential system? It’s just more panels, right?” In truth it’s a little more complicated than that. It’s an interesting exercise in system design, forcing engineers to think differently (and larger) than we normally do when working on residential systems. It’s also very satisfying to finally see the system commissioned and running. I was able to experience this recently, with the commissioning of a 275kW DC (256kW AC) 480VAC 3 Phase system out at Armco Park, in Lebanon, Ohio. This project was 8 years in the making; I worked on it during the final 3 years with the primary driver of the project, Jim Yockey, CEO of Rocknoll Energy, Inc. I was happy to be invited for the ribbon cutting ceremony for the project, and I wanted to take a moment to discuss the interesting challenges involved in designing such a system.
The Drawing Board
When the project was first broached, the goals were big but pretty simple – the array would produce enough power to light Armco Park’s fields and courts, spread across 311 acres, with enough left over to eventually handle additional features. Ideally, the array’s workload would include charging the 18 hole golf course’s golf cart batteries, and powering EV charging station(s) for park visitors.
One of the primary requirements of the project was that as many of the parts in the system as possible be of US manufacture. To that end, we used Ironridge’s ground mount system, 4 Solectria PVI 60kW transformerless string inverters, 1 PVI 36TL transformerless string inverter, and were going to use Solarworld 350W 72 cell modules, but due to logistical issues and timing, ended up going with 745 Heliene 370W 72 cell panels.
One of the first challenges faced in any large scale installation is determining the best racking solution for the application. Depending on the type of soil, different racking solutions work better. If there are a lot of rocks, debris, a ledge, or if there are significant elevation changes across the area, working with concrete and pipe can make for additional complications during installation. Armco Park has a 110-acre lake, which means a high water table at the array site. This made something like a C channel pounded into the ground more difficult, as the channel would need to be quite long to hit solid earth for good anchoring. We chose the Ironridge ground mount, and the concrete provided the stability needed to anchor the array into the earth.
Installation began in January of 2019. Rocknoll employees worked during the cold, wet and rain to put racking in:
As the snow melted, the temperatures rose and the water evaporated. The array could be brought up to its full height of 5 modules in landscape:
With the panels in position, the next challenge was to string them for the inverter. One advantage of a 480VAC 3 Phase system, is you can wire up to higher voltages. Many commercial inverter manufacturers are allowing for 1000V and 1500V DC inputs, allowing for fewer parallel strings and higher voltage coming in to the inverter. This makes stringing much easier, requiring fewer home runs from the array back to the inverter. All of this reduces installation time and cost, while keeping voltage drop losses to a minimum. This particular inverter used 1000V strings, so we used strings of 17 72-cell panels in series to keep the number of strings even, and to ensure the voltage stayed below 1000VDC during the colder periods of the year. Counter-intuitively solar panels produce higher voltages when they are colder – watch our video on this here.
Solectria’s PVI60TL inverter is a great platform, as it is a NEMA (National Electrical Manufacturers Association) 4X unit which can be mounted right at the array, again reducing the amount of trenching and conduit needed for the installation:
Positioning the DC wiring box directly under the inverter allows for easier maintenance and reduced downtime. In this configuration, you can leave the DC wiring box alone while swapping out the inverter, reducing the amount of rewiring that needs to be done.
This was a very satisfying project to work on. Armco Park got an excellent source of sustainable power to reduce the overall cost of the park. The local wildlife got a 1.5-acre butterfly pollinator station right next to the array, providing a much needed stop for the butterflies on their annual migration (not to mention providing an excellent area to see the butterflies as they make their journey!). With the solar array producing no pollution, the butterflies have cleaner air to fly through on their way. While working with the challenges of larger racking requirements, higher voltage systems, and unique stringing setups, I learned a lot about designing these systems. I look forward to designing more efficient and cost-effective large-scale systems going forward.
As of this blog post, the system has already produced over 150 megawatt hours!