A true solar farm
A new 4.25-MW solar project near Charlotte, North Carolina, is different on a number of fronts, including the use of sheep as vegetation control on the 20-acre site
By Vicky Boyd
The new 4.25-megawatt solar facility near Rockwell, North Carolina, is quite literally a solar farm, complete with sheep to help control vegetation under the photovoltaic panels.
This is the ninth such facility where O2energies of Cornelius, N.C., has contracted with Sun-Raised Farms of Bunn, N.C., for sheep and their management.
Not only has O2energies employed a unique form of vegetation control, but the company also used a decentralized architecture on the Rockwell facility that maximizes energy output and simplifies construction and maintenance.
Much like earlier North Carolina solar projects that O2energies developed, the Rockwell farm took a "hyper-local" approach, whether it was hiring local labor or sourcing from local companies, said O2energies President Joel Olsen.
"We selected a local EPC (Engineering, Procurement and Construction) contractor that is headquartered less than 30 miles from the project," Olsen said. "We held a job fair for contractors and labor in Rowan County, and we encouraged our EPC to source all supplies and rental equipment locally. From the neighbors to the local schools and businesses, we engage and involve as many people as possible."
The Rockwell Solar Farm-one of 13 developed so far by O2energies-was commissioned in December 2014. Moving forward, solar projects will be developed under the firm's new venture, O2 emc LLC.
Work on the Rockwell project, about 45 miles northeast of Charlotte, began in 2011 when an area landowner offered up the property after hearing how O2energies had developed other projects maximizing local contractors and labor. The owner also was attracted by the way agriculture was integrated into the long-term operations of the firm's solar farms.
The 20 acres of land neighboring a rock quarry were already zoned for agriculture, which also allowed solar farms and public utilities. O2energies secured the necessary building and electrical permits as well as a soil erosion control permit.
The project has a 15-year avoided-cost power purchase agreement with Duke Energy for all of the power produced, Olsen said. Duke also will purchase the renewable energy credits generated from the project.
Local governmental leaders were engaged early and often to help bolster project support. An initial project kick-off meeting in April 2014 attracted more than 50 business leaders and provided an overview of the project and construction, he said.
Midway through the project on Nov. 21, 2014, more than 200 local community members visited the site during the peak of construction.
O2energies also worked with nearby middle and high schools as well as the science, technology, engineering, and math coordinators for Rowan County's kindergarten through 12th grade public schools.
In addition, the solar development firm invited leaders from Catawba College School of the Environment and Rowan Cabarrus Community College to visit so they could plan to use the facility in their curriculum.
North Carolina is becoming a hotbed of solar development, boasting more than 120 solar farms, according to the North Carolina Sustainable Energy Association. During 2014 alone, North Carolina added 335 MW of capacity, the third best in the country, according to the Solar Energy Industries Association (SEIA). All told, the state has 592 MW of solar capacity, ranking fourth among U.S. states. And North Carolina is second only to California with the number of projects under construction, according to a report from SNL Energy.
All this bodes well for solar farms, such as Rockwell, that need an experienced, local work force, Olsen said.
Part of the solar growth may be spurred by a North Carolina state investment tax credit, which the Rockwell project took advantage of. Not only is the tax credit attractive to project developers, it also benefits the state's residents in the long run.
"In turn, the project will generate state and local tax revenues for the next 30 years, which will pay back almost $2 for every dollar of state tax credit," Olsen said.
The Rockwell site sits adjacent to a granite quarry, which is indicative of the region's predominant geology. That meant pre-drilling holes for anchoring the racking piers, Olsen said. Precision was paramount to prevent the drilling of holes that were too small or too large.
Getting materials to the site-on the top of a mountain-was fairly standard. After roads were built, a laydown site was created.
Construction of the project itself started with trenching to house the wiring conduits.
Charlotte-based NARENCO act-ed as general contractor, while WB Moore Electrical Contractors, also of Charlotte, was the electrical subcontractor.
The solar project was divided into three sections for installation of a pile-driven racking system from GameChange Racking, of Manhattan, N.Y. The first section employed ground screws for anchors, while the other two thirds used driven piers.
"There was different geology, but there were also some supply constraints, so we had to make a change in the materials of the project," Olsen said.
The project involved 16,629 Hanwha 60 S-type cell solar modules chosen for their thinner frame, 1,000-volt UL certification, and PID-free module design, Olsen said. This was important because of Rockwell Solar's string inverter system.
In a conventional PV system, voltage differs between the frames and the solar cells of a module. Especially during wet weather or high temperatures, this can lead to undesirable current leakage that can decrease cell performance-often referred to as potential inducted degradation or PID. Some have pegged the performance loss at 20 percent or more, even during the first few months of a project's operations.
|O2energies, which developed the Rockwell solar farm, took a "hyper-local" approach with the project, whether it was hiring local labor or sourcing from local companies. A job fair was held for contractors and labor, and O2energies encouraged its EPC to source all supplies and rental equipment locally.|
Rockwell Solar was engineered with decentralized architecture using one Sunny Tripower 20000TL-US inverter, 144 Sunny Tripower 24000TL-US three-phase transformerless inverters, and two SMA Cluster Controllers. All were from Rocklin, California-based SMA America.
Although decentralized architecture is popular in Europe, it is just taking hold in the United States, said Brett Henning, manager of application engineering for SMA America.
"The adoption of the decentralized design approach here in the U.S. has really gathered momentum in the last couple of years," he said.
The Rockwell Solar project lent itself to a decentralized design and string inverters because of the rolling landscape.
A centralized system involves collecting power from numerous arrays, then funneling it through a single point of interconnection to the grid. In that collection process, the DC electricity is converted to AC by a small number of large inverters.
A decentralized system involves hundreds of much smaller inverters, each one handling just a few arrays and located much closer to them. It offers several benefits, including harvesting the maximum amount of power from the array at any one moment, Henning said.
"From a design and customer standpoint, a decentralized approach can give you a more modular, more repeatable design," he said. "It also simplifies construction and site logistics, especially with projects such as Rockwell, which was located atop a mountain."
With a centralized design, large cranes and other equipment are needed to install the inverters, Henning said. Roadways also would need to be engineered to handle oversized trucks to move the equipment. The smaller Sunny Tripower TL-US inverters can be transported using standard-sized trucks.
Using string inverters also reduces the amount of engineering required, both on the planning side and on site.
That same simplicity is seen with repairs. If a Sunny Tripower TL-US inverter should go down, operators see only a minimal drop-off in energy yield.
"The other key advantage of a decentralized system is it reduces the risk of a single equipment failure causing a large outage to the solar plant," he said.
Instead of needing a specially trained technician to make repairs, a local licensed electrician can typically replace the failed inverter, Henning said, comparing it to servicing a household appliance.
|O2energies is all about working with the communities where it operates solar farms. With the Rockwell solar farm, it offered tours to nearby middle and high schools and invited leaders from Catawba College School of the Environment and Rowan Cabarrus Community College to visit so they could plan to use the facility in their curriculum.|
Additionally, the Sunny Tripower TL-US is transformerless, bringing with it lower cost, lower weight, and higher efficiency.
Remotely monitoring the output of each of the 145 inverters would be daunting. That's where the SMA Cluster Controller comes in, providing a single interface for up to 75 inverters. The Rockwell Solar project uses two.
"You can control those 75 inverters as if they were one plant or one large centralized inverter," Henning said. "You have one point of communication and control. At the same time, you still have the opportunity to dial it in and look at a single individual inverter to identify if it is underperforming."
The Rockwell Solar Farm also marked the ninth North Carolina PV project where O2energies enlisted Sun-Raised Farms for vegetation management.
"Sun-Raised Farms has worked on that for several years to really refine a system that works for the solar farm and works for the sheep farmer," Olsen said.
O2energies had tried to use sheep on its own for vegetation control several years ago but failed, he said.
"It didn't work because the local farmers really need a network, they need training, they need insurance, they need access to herd genetics, they need a market for the sheep, and also a lot of protection for their sheep," Olsen said. "And you had to build [the solar project] in a way for it to work with the sheep."
As part of the arrangement, O2energies consults with Sun-Raised Farms to ensure the right grass species-planted for ground cover-that are well suited to the area and will provide good livestock forage.
"We often see solar farm designers 'rubber stamping' the same grass seed specifications on all farms independent of farm geography or soil types," said Shawn Hatley of Sun-Raised Farms. "Doing so creates limited forage establishment and promotes weed seeds and invasive plant species.
"Sun-Raised Farms works very locally, providing farm-specific information to design teams and recommending best practices. Good forage means good vegetative cover, which is good for livestock and the solar farm owner's management budgets."
Sun-Raised Farms is responsible for working with local producers near the solar farm to supply the sheep. As part of the arrangement, Sun-Raised Farms trains producers on grazing practices that won't interfere with electrical generation.
But its services aren't limited to livestock, Hatley said. "Most solar farms are not local and although farm sites are typically monitored remotely, it is hard to replicate the value of local 'farm boots on the ground.' Sun-Raised Farms' boots are the local men and women of the farm community who are active on the sites weekly and monthly checking on farms and on farm animals. If something is out of order on a farm site, Sun-Raised Farms can be the first to report activity to solar farm owners."
The deal has also allowed Sun-Raised Farms to develop a premium market for locally produced grass-fed lambs. "In today's local food environment, Sun-Raised Farms network farmers are well positioned to be the leading suppliers of premium lamb in the southeastern U.S.," Hatley said.
For O2energies, the arrangement with Sun-Raised Farms has meant supporting local economic development while spending about the same-roughly $20,000-$30,000 annually-on vegetation management, as it would using more conventional techniques.
"From predator protection to water to extra site security, it doesn't cost much more to have the site prepared for sheep when we incorporate Sun-Raised Farms during construction planning," Olsen said.