While the 26-module fixed system yielded the largest annual output, the tracked system had the greatest summer yield of all three options and wasn’t far behind in estimated annual production (8,947 kWh vs. 8,929 kWh). Even though the tracker’s cost adds $1 per W, it yielded the best payoff of the compared systems—it will provide about double the income of the 20-module fixed array and 23% more than the larger 26-module fixed array.
The installed cost of Washington-made components is a premium—and in the case of the tracker, this changed the outcome considerably. Using less-expensive, out-of-state modules and inverters results in the tracked system and larger array’s costs being about even, in which case there would be no advantage to using the tracker since there will be more maintenance issues over the system’s life.
Because of the premium cost of Washington-made modules and microinverters, the larger fixed array had a greater up-front cost—about $4,000 more than the tracked system. In comparison, the cost difference by specifying non-Washington-made modules was much less (see “SAM Modeling Output Summary” table on the previous page). The larger array would have required more space on the roof and the tracked system fit perfectly in one row in the width of the roof. (Ground mounting was not feasible—the cold storage building sits in the middle of a maintenance yard in which heavy equipment is frequently being moved around.)
Compared to a fixed array, the tracked system will likely have maintenance costs, although these were not factored into the analysis. Those could negate the slight financial advantage that the tracker has compared to the fixed array.
Mounting the tracker on a roof required additional attention to attachments, due to higher wind loads of an array that was tilted from the roof plane. It also required engineering to ensure that the additional wind loads and the added weight of the tracker could be carried by the existing structure.
The tracker base is Unistrut placed on 48-inch centers, providing several connection points through the corrugated steel roof into the 2-by-12 trusses, which are on 24-inch centers and oriented north/south. They also provided the additional height needed to mount two rows of Unistrut cross rails above the corrugated roofing profile. Six or more 5/16-inch, 5-inch-long structural construction screws were used in each 8-foot Unistrut base. All penetrations were sealed with GeoGreen 4500 roof sealant. To resist wind forces on the back and sides of the array, 2 feet of Unistrut extend beyond the tracker base toward the top of the roof and an extra Unistrut was placed on 24-inch centers at each end.
DH Solar’s single-axis 10-bay tracker has upright sections every 8 feet with one pivot rod in each bay, which holds two modules each. The lightweight system has a lot of bracing that required additional assembly time compared to a fixed rail system. An arm extends the length of the tracker and moves the pivot rod for each bay using one electric screw actuator.
Two modules were attached per pivot rod with standard 1/4-inch stainless steel bolts. It was critical that the module wiring was long enough and had enough slack to handle the changing position of the modules as they are moved to track the sun. The pivot rod was then bonded to the equipment-grounding conductor using a ground pipe connector.