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While large rooftop solar projects have their share of system integration challenges, getting a racking configuration wind tunnel-certified to 120mph (193kph) is usually not one of them. But that was part of the engineering process as construction preparations began on a now-generating 1.795MW (DC) PV installation atop IKEA’s huge distribution center in Tejon Ranch, CA. Facilities manager Eric Ranney recalled how a January day made it clear that a wind-tolerant design was necessary, as gusts topped 89mph (143kph) and nearly two-dozen skylights and a handful of modules staged for installation blew off the roof.
REC Solar’s Ben Collinwood said that the original design called for a ballasted system, but that idea had to be scrapped because of the roof’s seismic capacity and other considerations. A custom racking solution with a relatively small amount of penetrations (also known as “stand-off connections”) was drawn up, with the number of holes shrinking by two-thirds or so, from several thousand down to just a couple thousand for the 7,980-panel, 12-degree tilted system, according to the national account director.
The site benefits from between 314 and 321 days of sunshine per year, according to Ranney, but seasonal temperatures can dip to an icy 20°F and soar to a blistering 135°F (-6.7°C to 57°C)on the slightly contoured white-membrane roof. As a result, the racking design also has to accommodate sizeable thermal swings over the course of the year at the location, with expansion joints positioned along the racks to allow for the heat- and cold-induced contractions.
In addition to the dozens of south-facing rows of black-framed REC Group PE Series 225W multicrystalline-silicon modules (the two RECs are not directly related, btw), the system employs a trio of Advanced Energy 500KW (AC) Solaron central inverters, which are located at ground level on the frontside of the structure. Through AE’s Remote PV-Tie wiring approach, which uses smaller-gauge copper wire to help facilitate voltage conversion when long cabling runs are necessary to connect arrays with inverters, Collinwood explained that string losses (and Tejon has 570 strings of 14 panels each) will be minimized.
Projects of this size generate a lot of construction and packing materials trash—usually around 800 cubic yards’ worth of cardboard, wooden spools, plastic, aluminum, copper, and the like, according to Ranney —but a concerted effort by IKEA and REC Solar resulted in the reuse and recycling of 98% of the refuse. In one altruistic example, the wooden frames erected to protect the numerous rooftop skylights during construction were salvaged and donated to the Bakersfield chapter of Habitat for Humanity, which will be able to use them on the construction of two houses.
A more mundane aspect of the power plant concerns its upkeep. Since the 1.77 million sq ft building is situated where the “Grapevine” section of Interstate 5 spills out of the Tehachapi mountains into the San Joaquin Valley, dirt and other particulates from nearby agricultural activities tend to collect in the bowl-shaped topography, making the cleaning of the system a bit more demanding.
REC Solar has contracted to provide operations and maintenance support for IKEA for five years, and will be modeling the system’s output and performance to determine when to wash off the soil from the glass surfaces of the modules, noted Collinwood. “We can watch that degradation as the dirt builds up, and say now’s the time to clean the panels because it will be more beneficial to have that system operating at a better capacity.”
Said to be the second-largest single-rooftop commercial PV system in California (and one of several installed or soon to be on IKEA properties in the Golden State and elsewhere), the Tejon array occupies about 370,000 sq ft (~4.85 installed watts per sq ft) and has been operational and hooked up to the PG&E grid since the latter part of April. It is expected to provide 2.88 million kilowatt-hours annually (~7.8 KWh per sq ft)—equivalent to between 68% and 74% of the site’s total energy requirements, according to Ranney.
In addition to the HVAC, lighting, and cranes used in the massive warehouse (which has reduced its power consumption >50% since 2005), a fleet of 180 high frequency-charged electric forklifts will benefit from the photovoltaic power source. He’s already seeing around a 7000KWh daily drop in grid-consumed electricity since the system was activated.
Collinwood explained that “for commercial customers, if you target the system size to be about two-thirds of their original utility bill, that’s usually the economic sweetspot for paybacks and financial rates of returns and things like that for solar investments.” In this case, Ranney said the expected payback time should be less than 6 years.
Although IKEA is reticent to talk about the cost of the project (and the Swedish firm does own the system—no power purchase or lease-type deal here), Collinwood did share the range of pricing he’s been seeing on the market of late. He told me that the “raw costs” (preincentive) of large commercial installations (>1MW) have been running between $3.50 and $4.25 per installed watt, which would put the guesstimated price tag of the Tejon rooftop at somewhere between $6.3 million and $7.6 million.
If we split the difference and call it $7 million, that represents about $1 for every 12.8 pounds of carbon dioxide that will be offset by the solar power plant over the next 25 years—a total of 45,000 tons. Throw in the site’s rapid payback time, and that’s what I call a clean and green investment.
(A time-lapse video of the construction of the IKEA distribution center project has been posted on the REC Solar YouTube channel.)
