The ideal place to house string inverters in commercial solar projects on the roof is indoors in an air-conditioned, locked room – but this is not always possible. When mounting options near the floor are tight, installers often place these fragile power electronics on the roof next to the array.
A project to mount inverters on the roof of Third Sun Solar.
Solar installers began moving inverters to rooftop level when the 2014 National Electric Code introduced requirements for quick shutdown. The easiest, most affordable way to ensure quick shutdowns on commercial roofs was to install the inverters within 10 feet of the solar array. Installers mounted inverters on angled steel sleds made by the inverter manufacturer or third-party suppliers like Shoals or Bentek within 10 feet of the array.
When the 2017 NEC update went into effect, which requires a quick shutdown at the module level, the 10-foot rule no longer met the code. Since solar modules now have to take shutdown measures in countries where the NEC was introduced in 2017, the inverter can be placed anywhere. The motivation for mounting string inverters on the roof is no longer primarily code-controlled, but many installers prefer the roof for placing inverters.
Considerations for installing the inverter
According to Geoff Greenfield, President of Third Sun Solar in Athens, Ohio, inverters should be kept at a stable temperature and out of direct sunlight so they can operate most efficiently without degradation. Inverters also generate heat while they are working. It is therefore important that the inverter's storage rooms indoors are air-conditioned and large enough.
A project to mount inverters on the roof of Standard Solar.
“Most [inverters] are designed for outdoor use. They have pretty wide temperature windows, but I still think the electronics perform better at modified, simpler temperatures, ”said Greenfield.
Inverters also need to be protected from potential vandalism. Therefore, fencing is a must if they are installed outdoors near the ground, especially when children are around.
Maryland-based Standard Solar primarily finances and installs ground-mounted projects, but does some commercial roofing. When installing on the roof, the company almost always installs inverters on the roof for several reasons. For one thing, many AHJs in their service area still adhere to the NEC 2014 and its 10-foot rule, according to C.J. Colavito, VP of Engineering. Standard Solar has also chosen to mount inverters on the roof for several school district installations for safety reasons.
"You don't want vandalism, you don't want people to touch it, you don't want anyone to be hurt. Putting it on the roof could be a relatively safe place," said Colavito.
Another advantage of a roof mount is a more stable cell modem connection for data acquisition and monitoring.
"Now, if I have a cell modem, I can't put it in the basement or in an electrical room because I may not get very good service. So it can be beneficial to put it on the roof," Colavito said.
Ideal roof mounting practices
A project to mount inverters on the roof of Standard Solar.
When installers put inverters on the roof, the next step is to find the best possible mounting point. If the 10 foot rule does not need to be followed, Standard Solar will prioritize a shaded, north-facing point on a vertical wall or parapet. The company prefers to place the inverters in one place for ease of maintenance. If a vertical wall is not available, the team builds a shelving system using steel braces.
"Our maintenance technicians generally like the inverters, which are vertically mounted on a wall at working height rather than on the top of a rack," said Colavito.
Even if manufacturers say their product won't fall off in the sun, it's best to install them in the shade whenever possible.
"You want to make sure that your inverter is protected from overheating. So we usually avoid exposing it to direct sunlight unless we have a shade for it, as this can lead to an early derating," said Colavito.
The CPS inverter mounting solution
Chint Power Systems designed inverter accessories specifically for rooftop applications, including a shade cover and mounting brackets that the company calls "legs". Peter Hsiung, director of product management, said that most of Chint's customers prefer to mount inverters near-flat on the roof for aesthetics and ease of installation. Chint inverters can be installed with an incline of 15 ° or higher. Although the shading and mounting accessories add cost, they are cheaper than third-party suppliers, according to Hsiung.
Water is the enemy of power electronics, so according to Colavito, mounting inverters horizontally is not ideal, especially in snowy climates. When the snow melts, all that water puts the delicate equipment at risk. He said he prefers to mount inverters on the roof with at least some slope to protect the units from condensation.
Colavito also suggested mounting the inverter as close as possible to the array to limit the amount of cable required and to place it in a location that maintenance technicians can access with relative ease.
When installing for the first time, pulling the inverters up to the roof is usually not an additional stress factor. With a little extra planning, they can be sick or lifted at the same time as the other equipment.
"It's not that big of a difference lifting six inverters on a pallet onto the roof while we are lifting pallets with solar energy and pallets with ballast blocks," said Greenfield.
The real problems arise when inverters need to be replaced on the roof. Although string inverters are getting lighter on a commercial scale, they are usually still too heavy for a person to carry.
O&M perspective
Third Sun Solar installs more than half of its workstations on commercial roofs. When installing inverters, easy access is a top priority for O&M. The company's president, Geoff Greenfield, said installing inverters on the ground is preferable to avoid additional operating and maintenance costs.
"We have jobs where we couldn't replace an inverter for service without renting a scissor lift," said Greenfield.
PVCA repaired these heat damaged inverters on the roof.
Luke Pollinger, president of California solar O&M company PVCA, said he understands that it is often easier for installers to mount inverters on the roof as the voltage on the devices has increased and their weight decreased over the years . However, he believes that an air-conditioned room is always the best option for housing inverters.
“The two biggest elements that we at O&M have to fight against in order to ensure reliable [and] long-term operation of the equipment without problems are moisture and heat. So it is generally not good practice to place inverters on the roof, ”said Pollinger. "The goal is to keep your devices reliable and to limit the harsh conditions in which they must operate."
In Pollinger's experience, inverters last much longer indoors than inverters that are mounted directly in the sun.
In this project, PVCA had to repair thermal failures on two units and replace screens on 12 units after only two and a half years of operation.
"If you can limit the operating temperatures, you are only extending the life of these devices," said Pollinger.
When installers have no choice but the roof, he recommends mounting inverters on a north-facing wall that is completely shaded and easily accessible, and at least 3 m above the roof surface to avoid excessive heat. He said installers should provide some sort of shade for inverters on the roof, even if the manufacturer's manual doesn't say it is necessary.
Even under perfect conditions, inverters are the hardest working part of a solar system and will inevitably fail. When units need to be replaced, PVCA usually uses a jib crane or a jib. Although inverters typically come with a 10 year manufacturer's warranty, project owners still have to pay for work and rental equipment required to replace units on the roof, so the operating and maintenance costs are higher than maintenance on the ground.
Chint Power Systems has found a unique solution to the O&M problem of the inverters on the roof. The company aims to avoid the need to rent expensive transportation by making a smaller, lighter commercial inverter.
Chint's string inverters consist of two parts – the wire box and the powerhead. According to Hsiung, the drive head is the part of the inverter that normally fails and needs to be replaced. The company designed it so that the drive head can slide out of the device when it is replaced, eliminating the need to disassemble the wire box.
"We wanted to make sure customers weren't renting these scissor lifts," said Hsiung. “The 25 kW unit has a power head that is less than 50 pounds so you can actually carry it in a backpack and climb the ladder if necessary
An example of the easily separable inverter from CPS America.
to swap that. "
Regardless of which brand of inverter is used in a rooftop installation, developers must factor operating and maintenance costs into the long-term project estimate, said Eric Cotney, VP and general manager of Axium Solar in Texas. Axium evaluates the installation location of the inverter on a case-by-case basis and selects the best location for each customer.
“I assume that we will continue to do projects where we install on the roof. We don't foresee this going away, but now that the module is being shut down quickly and the shutdown devices are in use, we have the flexibility to install virtually anywhere, and that's a great thing, ”said Cotney.
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