Fields northwest of Tucson that once yielded cotton by the ton are now producing clean electrical power, at the biggest solar power plant to be connected to Tucson Electric Power Co.'s power grid to date.

The Avra Valley Solar Generating Station, a 34-megawatt (DC - direct current) photovoltaic plant built by California-based NRG Solar a few miles west of Marana, was dedicated Thursday on a 300-acre expanse of reclaimed cotton fields.

"In my mind, we're replacing one type of farming with another," Randy Hickok, senior vice president of NRG, said during a dedication ceremony. "When it was cotton, farmers were turning sunlight into the clothes on our backs, and now we're using the same property to use sunlight to essentially sustain civilization - it's helping keep the lights on, without the pollution."

TEP will buy power from the solar farm, which can generate enough power to meet the annual energy needs of about 5,000 homes, under a 20-year power purchase agreement with NRG, which owns and operates the plant. The thin-film photovoltaic system, which creates electricity, was made by Tempe-based First Solar.

Against a backdrop of solar panels that seemed to stretch out as far as the eye can see, TEP Chairman and CEO Paul Bonavia said the solar plant represents the future of energy.

"This is what a power plant looks like in the 21st century, and that's a huge change in an industry that I've been a part of for a long time," he said.

Bonavia, who also heads TEP parent UNS Energy Corp., noted that the Avra Valley plant is just one of several utility scale plants serving TEP that are slated to go online in the near future.

TEP will buy power from a 25-megawatt photovoltaic plant being built by SunEdison just a few miles to the east in the Avra Valley area.

The plant is expected to be turned on commercially by mid-January, a SunEdison spokesman said. Both Avra Valley solar plants are built on land leased from Tucson Water.

It's all part of TEP's plan to meet a state mandate that regulated utilities generate 15 percent of their power from renewable sources like solar and wind by 2025. Ratepayers are helping to fund renewable-energy projects through small monthly billing surcharges.

While TEP's renewable-energy mix includes grid-connected residential and commercial rooftop installations, utility-scale projects offer less expensive power, Bonavia said.

Even so, solar energy is still much more expensive than fossil fuels like coal, which TEP depends on for most of its power generation.

While the price TEP is paying NRG for power from the Avra Valley plant is kept confidential because of competitive considerations, TEP paid about 12 cents per kilowatt-hour of power from renewable-energy power purchase agreements this year.

That compares to a generation cost of about 5.5 cents per kwh from TEP's coal and natural gas generating sources.

But the growth of the solar industry and competition have helped reduce the cost of utility-scale solar projects like the NRG plant, Bonavia said.

"We want it to be cost effective for our customers, because ultimately, consumers, citizens, pay the cost of any power plant, this or any other," Bonavia said, calling the price of power from NRG's plant "very competitive."

NRG Solar, a subsidiary of New Jersey-based NRG Energy, is a heavy hitter in the industry, with more than 2,000 MW of photovoltaic and solar thermal projects under development or in construction across the southwestern U.S. The company also is building the nation's biggest solar farm, the 300-megawatt Agua Caliente plant east of Yuma, for two California utilities.

The Avra Valley solar farm features more than 400,000 of First Solar's thin-film cadmium telluride solar modules, mounted on a single-axis tracking system that pivots east to west to maximize light-gathering capacity.

NRG and First Solar rate the plant at a capacity of 25 megawatts of AC (alternating-current) power, the kind of power it will deliver to the grid after conversion from the DC (direct current) produced by solar modules. The plant's DC rating - the measure TEP generally uses - is about 34 megawatts.

The plant's panels were visibly dusty during the dedication ceremony, but Hickok said the dust doesn't significantly affect the output of thin-film photovoltaics. The plant will use very little water, he said, adding that the panels will be washed only a couple of times a year aside from any rain.

The plant will avoid the release of some 51,500 metric tons of carbon dioxide into the atmosphere, the equivalent of taking more than 10,700 cars off the road, Hickok said.

Coming online

Other new utility-scale solar projects expected to be completed on TEP's system by the end of the year, or early 2013, are (all capacity ratings in DC):

• A 6-megawatt photovoltaic array at the University of Arizona Science and Technology Park's Solar Zone, which is expected to become operational within a week. The Gatos Montes, originally developed by AstroSol Inc., was purchased by Duke Energy, North Carolina-based Duke announced Thursday.

• The TEP-owned Prairie Fire Solar Project, a 5MW, fixed-mount photovoltaic plant being built by Tucson-based Solon Corp. about a mile north of the UA Tech Park.

• Foresight/FSP 2, a 6MW single-axis tracking photovoltaic system being built by German energy firm E.On SE, under a power purchase agreement.

Contact Assistant Business Editor David Wichner at or 573-4181.

Senior reporter covering business and technology for the Arizona Daily Star/