Natural Gas Working with Renewables


Published In: EnergyBiz Magazine September/October 2011


IN RECENT MONTHS, the choice for new power generation is either renewables or natural gas. In some limited circumstances the answer could be evolving into: Why not both?

It seems like a match made in energy heaven. Take decidedly clean technologies like wind or solar and the in-vogue, cheap and plentiful energy choice of the present, natural gas, and combine them to create a seamlessly operating power plant.

The concept has been around for decades, but recent developments worldwide have shown this as an idea whose time has come. Some existing concentrating solar plants use natural gas as a backup, but the new breed of plants is using concentrating solar power as a supplemental fuel source.

A few demonstration plants have been either proposed or built worldwide, and one operating hybrid plant in Florida is approaching its first anniversary. And General Electric has broadened its game by expanding a combined cycle plant that integrates solar and wind generation.

Florida Power & Light last year started operating its first hybrid energy plant. The Martin Next Generation Solar Energy Center is a natural gas-solar power hybrid. The utility took an existing 3,705-megawatt combined-cycle plant, the largest fossil fuel power plant in the United States, and installed a $476 million 75-megawatt solar plant to supply heat.

"It's just a way to augment what we already have in a heat recovery steam generator," said John Gnecco, director of project development at FP&L. "Combustion turbines are going to run at full capacity most of the time, and when we get the sun, for free, we'll actually generate more electricity as a unit than we normally would have done."

And they'll do it without the emissions.

The concentrating solar power system of parabolic troughs uses 190,000 solar thermal tracking mirrors to create heat. And with existing transmission lines, the company was able to leverage infrastructure to keep costs down.

"Obviously, it makes this more cost-competitive than if we built a stand-alone 75-megawatt solar thermal plant," Gnecco said.

But don't look for too many plants to copy this exact model. For starters, the solar plant required 500 acres of adjacent land. Ideally, the parabolic troughs would have ringed the turbines instead of being primarily concentrated on one side of the plant.

But Martin had enough "head room," or excess capacity in the combined-cycle plant to efficiently allow a supplemental heat source, in this case, solar.

The plant started making solar steam in September 2010 as the first of its four phases was connected, and total operation was achieved last December. FP&L had projected a 23 percent capacity factor or 155,000 megawatt-hours a year on average.

We're tracking that number pretty well, but we really won't know until we get a full year or two of operations," Gnecco said.

The marriage of fossil fuels with solar has gotten worldwide attention. A hybrid plant combining 30 megawatts of concentrating solar power with 470 megawatts of natural gas generation in Morocco went online last November. A 150-megawatt natural gas hybrid with 20 megawatts of CSP capacity was commissioned south of Cairo late last year.

A recent development in the combined power plant design, with an American twist, has been proposed in Turkey and could be available in the United States in a year or two. The 530-megawatt plant would include 22 megawatts of GE wind turbines and 50 megawatts of CSP technology.

GE this spring invested in CSP technology company eSolar, which employs a "power tower" alignment with a steam boiler surrounded by concentric circles of reflective mirrors concentrating sunlight on it.

The combination is made possible, GE says, with its new combined cycle technology, which is 61 percent efficient.

"This is all about putting together high efficiency, lower fuel costs and the operating flexibility that goes with it," said Guy DeLeonardo, product manager at GE Energy.  "As renewable penetration increases, you need to back that with stable generation that drives the need for operational flexibility."

Intermittent resources like wind and solar ramp up or down during the course of a day, and system integrators often need to balance load with fossil generation. One reason natural gas combined-cycle plants have gained favor is their ability to start quickly or turn down as more renewable energy is integrated into the grid.

"Customers are going that way, that's the way the market is going, so you have to put the two together," DeLeonardo said.

Unlike the FP&L natural gas generator described earlier, this base plant is not expected to run constantly. It is designed for a high number of starts and stops, perhaps 250 to 300 starts per year, operating for perhaps 4,000 hours a year.

"It's the idea that when the wind's blowing and the sun's shining, they will always be `on' but this type of gas plant would be the first in numeric order to start when the renewable resources tail off," DeLeonardo added.

For now, the plant is being marketed in Europe and places that accommodate a 50-hertz line frequency. For the American market, a 60-hertz capability is expected to be available in 2012.

With the shale gas boom in the United States and increasing demand for renewable energy, technologies are providing the opportunity to meld intermittent solar and wind power with natural gas.








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