At APS’s Saguaro Generating Station, six rows of 15-foot-high mirrors generate electricity by heating oil to drive steam turbines. (Photo by Arizona Public Service Company)

Anyone who wants to see the solar comeback story of the year need only look in the mirror—one of the mirrors covering 100,000 square feet of desert between Phoenix and Tucson.

On Earth Day, Arizona Public Service Company dedicated the 1-MW Saguaro Solar Trough Generating Station, the first such plant to be built in the United States since 1990. "It's also the only powerplant we know of in the world that marries concentrating solar technology with a process usually associated with geothermal powerplants," said APS Technology Development Manager Peter Johnston.
         
Instead of generating power through photovoltaics, concentrating solar technology uses mirrors to heat mineral oil. The hot oil then boils water, producing steam that drives a turbine. At APS's facility, six rows of 15-foot-tall, trough-shaped mirrors concentrate sunlight to heat mineral oil, but the oil is used to boil an organic liquid instead of water.

The process, called the Organic Rankine Cycle, generates more power at lower temperatures than a steam-driven system and is common in geothermal and biomass applications. "The Saguaro Station generates electricity with the same technology," Johnston explained, "except that the heat comes from the sun instead of the earth." 

Innovation needed to build solar portfolio

The investor-owned utility is no stranger to solar power, or to innovation. Starting with its first large-scale PV system in 1997, APS has developed more than 5 MW of solar power. Projects include installations at Prescott and Tempe that track the sun to extend generation hours. A highly concentrated photovoltaic plant at the Glendale Airport employs special lenses to concentrate the sun’s rays 250 times onto each solar cell.

The state renewable portfolio standard approved by the Arizona Corporation Commission in 2002 was the impetus behind the utility's latest project. It required APS to generate 1.1 percent of its energy through renewable sources — 60 percent of that portion with solar — by 2007. "The RPS was a major catalyst for the solar trough project," Johnston acknowledged. "We realized that we needed something on a large scale if we were going to meet the goals."

The utility issued a challenge to developers to come up with a solar system that could compete cost-wise with photovoltaics. Parabolic trough technology seemed to offer an answer, but only at a minimum size of 50 MW. "We had something along the lines of one MW in mind," said Johnston. "That's when we became interested in combining the technology with the Organic Rankine Cycle turbine."

Hank Price, the parabolic trough project manager at the National Renewable Energy Laboratory, observed that the ORC could be easily adapted for smaller solar projects. "It opens up a whole new size range that is still utility scale."

NREL provided technical expertise to APS and Solargenix Energy and Ormat International, the development team that won the contract to build the facility. Solargenix provided the parabolic troughs and integrated the system with the power conversion unit Ormat designed and installed. Ormat is the company behind four waste-heat powerplants in the Midwest that use ORC turbines.

Technology lends itself to storage

APS and NREL may collaborate on an addition to the solar trough plant that will build on the ORC innovation.

Because parabolic technology uses heated fluid, it offers the potential for thermal energy storage, which could increase solar power's dispatchability. Adding a TES system to the powerplant, as NREL proposes, could allow APS to tap the powerplant hours after the sun goes down or on cloudy days.

"If PV electricity isn't used when it is generated, it has to be stored in a battery or possibly used to make hydrogen, which adds to the cost of the system," said Johnston. "Heated fluid is stored energy. It can be used when it is needed."

Peak solar generation occurs in the middle of the day, while APS's load peaks around 6 PM on summer days. "When people get home from work, they turn up the air conditioning," said Johnston. "If we could pull off that time shift, it would be a huge step toward integrating solar power into our mix."

TES would do much to increase the already-growing acceptance of concentrating solar technology. According to Price, the Saguaro Station has opened the door for other parabolic powerplants. A 64-MW project is under construction in Nevada and the California Public Utilities Commission approved a contract for a 500-MW facility last fall, with expansion options up to 850 MW.

The technology is not suited to residential applications or to northern climates that don't have high, direct normal solar resources. For southwestern states, however, the solar trough could offer a cost-effective alternative to photovoltaics at utility scale. California is home to all nine of the nation's parabolic powerplants, built in the 1980s before dropping energy prices discouraged the technology.

Opportunity, interest converge

Rising energy prices, incentives and the RPS are playing a key role in the return of concentrated solar systems.

The RPS was the driver for this project, however, since it was completed before the 30-percent investment tax incentive in the 2005 Energy Bill went into effect. “The ITC won’t be available to utilities like APS,” Price added, “but the incentive will be necessary to make larger CSP plants competitive with conventional powerplants. The incentives need to be longer, particularly for this technology."

Installing solar panels is relatively quick and easy compared to building a parabolic facility, he explained. "Larger plants take around three years to site and build, so the only projects that benefit from incentives are the ones that are already in the pipeline."

APS customers fund all APS renewable energy projects through an environmental portfolio surcharge the Corporation Commission implemented four years ago. The Commission's strong commitment to renewable energy—it recently gave preliminary approval to boost the RPS to 15 percent renewable resources by 2025—is improving the picture for third-party developers, too. "It's getting easier for them to get the long-term power purchase agreements they need to finance private developments," Johnston said. 

The solar industry may have the public to thank for that administrative boost. A recent poll showed that 75 percent of Arizona customers are willing to pay a little more to ensure energy independence.

The public response to the Saguaro Station indicates strong support for renewable energy. More than 400 people showed up for the plant dedication. "We get a lot of industry calls, and people love to have their picture taken here," Johnston said. "It's a very photogenic plant."

That's a good thing, since it looks like solar trough technology is going to get another day in the sun.