A 2.3 MW single-axis tracking solar system helps an Arizona town cut electricity costs and enables process changes that will save even more


The Neely Wastewater Reclamation Plant in Gilbert, Ariz., now gets 44 percent of its electricity from photovoltaic solar arrays it didn't have to pay for, doesn't have to operate, and doesn't maintain. Over the term of the deal, it's expected to save the town of 215,000 people about $2 million.

Wastewater Division manager Mark Horn says the deal came about because SPG Solar (SPG), of Novato, Calif., was marketing solar opportunities for municipal facilities in Arizona. "They approached me, and over the next month or so I had a lot of conversations with them about the concept," says Horn. "I was looking for a reason why we shouldn't pursue it, and I couldn't find one. It seemed to be a no-brainer."

Sealing the deal was a renewable energy incentive from the local utility, Arizona Public Service (APS), who purchases the renewable attributes of the power generated from the approximately $10 million, 2.3 MW-DC project. "Instantly, that made this project very attractive in everyone's eyes," says Horn. "It made the whole project come together."

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With production expected to be around 4.8 million kWh per year, the solar installation will make enough electricity to power about 440 homes for a year while preventing the emission of 86 million pounds of carbon dioxide over the next 20 years.

Space considerations

The 11 mgd (design) Neely plant's Class A+ reclaimed water is used in parks, golf courses and homeowner association neighborhoods for irrigation, and about 65 percent is put back into the aquifer through groundwater recharge basins.

That offered plenty of land for the 8,000 solar panels. "Gilbert has three separate recharge facilities," says Horn. "The Neely Recharge Facility where the solar arrays were constructed has 11 basins that are each 3 to 4 acres in size. The recharge facilities also serve as a wildlife habitat. So they really serve multiple purposes."

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The Neely Recharge Facility is right next to the wastewater plant, making it a prime location for the SPG Solar SunSeeker single-axis tracking array.

The area gets plenty of fuel for the solar panels, averaging 310 days of sunshine a year. By shading the recharge basins, the solar panels will also reduce evaporation.

The solar plant is owned by Gilbert Solar Facility I LLC, a wholly owned subsidiary of Integrys Energy Services Inc. Gilbert Solar Facility I contracted with SPG for the design, construction and operation of the facility, choosing a single-axis tracking system that follows the sun as it travels across the sky. "Single-axis panels are 20 percent more efficient than a fixed array that doesn't follow the sun," says Horn. Dual-axis systems that also adjust for the height of sun from the horizon provide more power, but they are more expensive to build and require much more maintenance.

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"We looked at the past 12 months of the plant's power demands because we wanted to make sure the system was sized accordingly," Horn says. "We wanted it to produce just enough power for us during our peak demand. We didn't want to oversize the solar system."

While it is possible to earn credit for excess electricity put back onto the power company's grid, Horn says it was more important to make sure the solar project was sized right to fit the financial picture.

Finding more savings

A local, inexpensive supply of electricity has allowed the treatment plant, one of two in the town, to make operational changes to save money. Since the Neely plant has no solids handling facility, it pumps all waste activated sludge to a regional treatment plant in nearby Phoenix. "It used to be cheaper to do that at night when we had reduced time-of-use rates from the power company," Horn says. It is now done during the day, powered by the less-expensive electricity from the solar plant.

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There are also three backup generators at the plant that used to be tested during the day for an hour once a week. During testing, the generators provide the plant's electricity. "We don't do that during the day anymore because that's when our solar panels are producing power," Horn says.

Horn says such tweaking will continue as the plant staff learns more about optimizing its operations to match up with the new source of power.

Due diligence

It took about 15 months to go from the initial discussions to full solar power operation. SPG approached Gilbert in June 2010, the renewable energy incentive was approved by APS in September, and the town council accepted the long-term power purchase agreement (PPA) with SPG in November. Project design started in December, and construction began in February 2011. The solar plant began operating in September 2011.

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The Town of Gilbert provides only the land for the solar panels and buys the electricity at a set rate. "That's where the savings come from," says Horn. "Eventually, the amount we pay for utility power is going to be much higher than what we pay for the solar power."

Gilbert pays just under the current utility rate for the electricity, and the PPA includes a fixed price escalator that's lower than the utility's past rate increases. "Looking at the last ten years, the power company had raised rates an average of 5.5 percent per year," says Horn. "We used a projection of 4 percent per year over the term of the agreement."

Most of the savings will come in the later years of the PPA. Horn says the annual savings for the treatment plant will initially be about $15,000, but that will increase to about $200,000 annually toward the end of the term.

Town manager Patrick Banger called it "ingenuity at its best" during a tour for elected officials from across the country in November 2011. He added, "This facility also clearly demonstrates all of the possibilities for the town in the clean and renewable industry."


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