By Kirk Stelsel
Custom precast concrete pads are the perfect solution for solar power inverter stations.
A precast concrete company with an experienced staff, an in-house design team, metal fabricators and a history of creating custom products is perfectly suited for unique projects. That’s certainly the case at Lindsay Precast’s plant in Canal Fulton, Ohio, as well as those in Florida, Colorado, and North and South Carolina.
Lindsay Precast employees have designed, manufactured and delivered products like wine vaults, luxury vehicle storage units and even the vault that holds the Charters of Freedom at the National Archives. Now, Lindsay Precast counts solar power companies among its customers.
A bright idea
The idea to create a precast concrete pad for solar power inverter stations was a joint concept between Lindsay Precast and the customer. The precast pads are more durable and cost effective than steel and eliminate the need for cast-in-place foundations on some sites. But this product is no ordinary piece of precast concrete. The project required quite a bit of custom design work and precise engineering due to the complexity of the equipment it supports.
Solar power collected by the field array is linked to a photovoltaic inverter station. That’s where the precast pads come into play. Each pad is 31 feet 3 inches long, 10 feet 6 inches wide and 10 inches thick. Features cast into the product include conduit for 120/240 volt and data connections, a 4/0 copper ground loop with stainless steel landing plates for field grounding connections and stainless weld plates. The weld plates anchor the control station and the power panel components as well as a site-installed canopy. The slab also has cast-in cable tray grooves for running the large alternating current, or AC, power cables with inserts for attaching and sealing the aluminum cable tray covers.
Lindsay Fabrication, Lindsay Precast’s in-house steel fabrication division, designed the mold with the intent to pour, flip, strip and stage product every day. To get everything right, the company enlisted the assistance of Concrete Engineering Solutions for structural design and A.L. Patterson for lifting design.
“Since the most efficient way to make this was pouring upside down, the first challenge was determining the best way to flip the full mold the next day for stripping the product without stressing it and causing cracking,” said Mike Hoffman, vice president at Lindsay Precast. “Another challenge was building the tapered cable grooves since they were cast in and running many different directions to properly connect the components. The tolerances were critical so the finished product would strip free without chipping or spalling.”
Each pour is 8 1/2 yards of 6,500-psi concrete. Employees flip the product by rolling the mold over with a spreader beam and crane. It’s important to avoid torsional stress during this process. Once it’s stripped, the pad is set on level blocking prior to integrating the inverters and other components. The finished product is then moved using four cast-in lifting anchors per side and roller blocks to avoid overloading any one anchor.
The devil is in the details
In the field, the pads help facilitate a complex process that converts the sun’s rays into usable energy for the power grid. Broken down simply, wires combine the direct current, or DC, electricity from the solar panels and send it to the hardware fixed to the precast pad. They convert it to AC electricity. The AC electricity runs through nine, 1 1/4-inch diameter cables connected to the low voltage side of the transformer using the grooves in the pad. Lastly, the medium voltage side of the transformer transfers the electricity to the grid for use.
The complexity of the process translated into plenty of intricate details for Lindsay Precast. But Hoffman and the rest of the team thrive on complexity and look forward to these types of projects.
“Custom work continues to grow, and has always been one of our specialties at Lindsay Precast,” Hoffman said. “With our own structural steel/fabrication division, we can offer better design solutions and can get projects done with much shorter lead times. We love the challenge and are always up for creating potential new product lines.”
Lindsay Precast shipped the first pads in late 2015, which are now being used successfully in the field. With the production process in place, the team can now efficiently manufacture the pads as needed while the engineering and fabrication teams move on to determine the company’s next product.
Kirk Stelsel is NPCA’s director of Communication and Marketing.