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Strategy & Planning

Solar Boost

Utilities are testing options for adding solar capacity to existing steam power plants. Concentrated solar thermal boosters increase plant efficiency and reduce emissions, while helping utilities to cost-effectively meet renewable mandates.

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Figure 1. A 26-MW photovoltaic array is being added to Enel’s Stillwater geothermal plant in Nevada. Source: ENEL
FPL has been ramping up output at it the Martin plant’s 75-MW solar booster. The facility recently achieved 85 MW of capacity. Source: FPL
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FPL Leads U.S. Industry
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The first utility-scale solar hybrid in the United States was the 75-MW solar baseload addition to Florida Power & Light’s 3,705-MW Martin combined-cycle generating facility near Indiantown, Fla. The lead contractor was Rioglass America LLC. The solar unit, completed in 2010, includes 190,000 reflecting mirrors spread over 500 acres to concentrate light with basic parabolic, or trough concentrating technology, which is the most common type of concentrating solar installed in the United States.

The $398 million hybrid addition was approved in advance under a 2008 state law approving the construction of 110 MW of renewable energy in the state, according to Buck Martinez, a senior director of development at the utility. FPL covered the costs with balance-sheet funding, buoyed by an $850 million 30-year bond issue last year.

The Martin solar system has been ramping up production levels steadily. “Last month we hit 85 MW nominal, our highest production to date, and our goal is to get to 155,000 MWh in the near future,” says Martinez.

The plant hasn’t been without its startup problems; last year, a burst valve resulted in the loss of 46,000 gallons of heat transfer fluid, requiring the removal of some 1,000 truckloads of affected soil.–CWT

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Tucson's Sundt Boost
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Tucson Electric Power (TEP) recently opted to add a 5-MW solar peaker to its gas-fired 156-MW Unit IV at the Sundt Generating Station. When it comes on line during first quarter of 2013, the solar facility will provide a 4-percent boost to the unit’s output. Areva Solar will provide linear Fresnel concentrating solar technology, which uses reflectors to heat a linear tube filled with water.

Sundt Unit IV can operate either on coal or gas, and was burning coal until recently. “We were considering a retrofit at Sundt, but found there would be some complication on the coal side,” says Joseph Barrios, a TEP spokesman. “So we decided to try out the solar hybrid as a demonstration for something we might want to adopt in other facilities.”

At the announcement of the project, TEP CEO Paul Bonavia said, “Areva Solar’s innovative solar boost technology gives us a cost-effective, environmentally responsible way to expand the output of our largest local power plant without increasing emissions.”

Footprint was a consideration. “TEP also looked at a PV booster, and wanted to go larger, but 5-MW thermal was their limit because of land limitations on the metro outskirts of Tucson,” says John Robbins of Areva. The hybrid also will help TEP reach its goal of installing 200 MW of solar generation by 2014, which will allow it to meet Arizona’s 15-percent-by-2025 renewable energy standard. 

To pay for the development of the plant, TEP turned to its renewable energy surcharge collections from customers, which adds 0.7182 cents per metered kWh. But once the plant is operating in early 2013, the utility will seek rate base compensation from the Arizona Corporation Commission, Barrios says.

The cost per kW of the Fresnel-type plant is relatively low, compared to other solar thermal technology. “The cost for this type of plant typically would be in the $1.50 to $2 per installed watt range, compared with a stand alone cost of $3 to $3.50,” Robbins says.–CWT

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Xcel's Solar Cameo
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Xcel Energy in 2010 successfully tested a 2-MW, $4.5 million solar hybrid solution from Abengoa Solar, to boost efficiency by about 5 percent at its Cameo coal-fired plant near Grand Junction, Colo. The Cameo hybrid addition was approved in 2009 by the Colorado Public Utilities Commission, under an Innovative Clean Technology program.

The first of its kind in the United States, the hybrid test was designed largely to help reduce emissions, according to Xcel spokesman Gabriel Romero. Overall, more than 500,000 pounds of coal were saved during the year-long test, and 2,000 tons of carbon dioxide emissions were avoided.

The measured goal of the installation was simple. “If solar provides 20 percent of the fuel for steam production, then you reduce coal consumption by 20 percent and you reduce dioxide emissions by 20 percent,” says Bruce Kelly, a technical specialist at Abengoa.

Temperatures of the food-grade oil heating fluid, heated in the 6.4 acre parabolic solar array, reached 576 degrees F. The heated fluid was used to preheat the feed water in the coal boiler, eliminating the need to divert heat from the steam turbine. “During daylight operation, solar replaces coal for much of the energy supply,” Kelly says. “As the radiation decreases during the late afternoon hours, the coal contribution replaces the solar contribution, allowing the boiler to operate at full load at all times. In the early morning hours, the process is done in reverse, gradually replacing the coal energy contribution to solar, while maintaining the technical minimum to optimize the operation of the coal boiler.”

While the Cameo test equipment has since been dismantled for use in a solar storage project elsewhere, the utility is considering other solar hybrids. At the time of the hybrid announcement, David Wilks, president of energy supply for Xcel Energy, said, “If this demonstration works, we may be able to implement this type of technological advance in other coal-fired power plants to help further reduce carbon dioxide emissions in Colorado and possibly other areas of our service territory.”

Toward that end, the vendor is currently promoting the technology. “Abengoa is now pursuing the concept where you put in solar-heated steam at the same conditions as the existing coal-heated steam plant already operates,” says Kelly.–CWT

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Sun and Earth Combine
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Thermal systems aren’t the only solar option for generators seeking to hybridize their power plants. Enel Green Power North America is building a 26-MW solar booster to complement its existing 33-MW binary-cycle, medium-enthalpy geothermal plant at Stillwater, Nev. The geothermal plant uses large-scale electric submersible pumps in the wells to pressurize and extract geothermal water. The pumps’ energy usage is partially offset by the 81,000 fixed polycrystalline-silicon PV panels, and excess generation will be sold to Las Vegas utility NV Energy under an 18-year power purchase agreement. 

Enel also is testing a solar-geothermal hybrid, in which a concentrating solar system would directly warm the heat transfer fluid that feeds the power-generation turbines.–CWT

Author Bio: 

Charles W. Thurston is a Fortnightly contributor based in Sonoma County, Calif.

Hybridizing fossil plants with solar thermal technology.

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