INDIANTOWN — Florida Power & Light Co.'s newest solar-energy plant will have enough mirrors to cover 80 football fields. But those mirrors will focus sunlight onto surfaces that add up to slightly less than the area of a single football field.That concentration of solar power will generate temperatures of more than 700 degrees — hot enough to make electricity for 11,000 homes.
The Martin Next Generation Solar Energy Center here will rank as the world's second-largest solar plant when it begins pumping out as many as 75 megawatts of electricity late this year. It will also be the only system of its kind in the world.
Conventional wisdom holds that solar plants using mirrors — which generate heat that produces steam that, in turn, spins an electrical generator — aren't worth the effort in Florida because of the regularity of afternoon rain clouds much of the year. So far, all of the solar plants built in the state convert sunlight directly into electricity using photovoltaic panels, which produce a charge, if only a reduced one, even on cloudy days,
But FPL is building its "thermal" solar plant on a campus near Lake Okeechobee that already has 13 generators fueled by oil and natural gas. Steam from the solar plant will be combined with steam produced with the heat exhaust from four natural-gas plants to spin an existing generator — an approach not taken before. FPL thinks that makes thermal more feasible, because the utility won't have to spend millions of dollars building a generator for the solar plant.
The project costs about $420 million, which will add about 16 cents a month to the average FPL residential customer's bill.
FPL also owns the world's biggest solar plant, a thermal unit in California's Mojave Desert that is four times the size of the Martin County project. The Florida plant is based largely on the technology of the 30-year-old Mojave system, though it has been given far stronger pylons, frames and mirrors to withstand hurricane winds of up to 130 mph.
John Gnecco, FPL project development director, said dropping one of the California plant's glass mirrors could lead to much bad luck, because it would shatter. But the Martin County solar mirrors, though also made of glass, bounce unscathed when they hit the ground.
To demonstrate, Gnecco laid one of the curved mirrors on a gravel parking lot recently and jumped on it repeatedly, causing it to flex trampoline-like. The special mirrors were made in Spain, one of the few countries where FPL could find a suitable manufacturer with kilns large enough to temper the 56-by-67-inch pieces of glass.
The thermal unit's mirrors are also highly reflective — much more than a typical bathroom mirror — and there are a lot of them: more than 190,000.
Workers are installing the mirrors in aluminum frames to create long, linear dishes. The more than 6,800 frames each contain 28 mirrors and will be arranged in parallel rows that are linked together for a total length of about 50 miles.
Each frame also holds a tube a few feet in front of the mirrors. The tube contains a synthetic, oil-like fluid that costs $15 a gallon and is designed for heating to 740 degrees. The hot fluid flows through a separate component that acts something like a boiler to create steam.
The tubes are made of stainless steel and painted black but encased in the airless vacuum of a glass tube. Birds can land on the glass tubing and not be roasted, Gnecco said.
Nearly 150 miles of pipe and related plumbing, some as much as 30 inches in diameter, will hold 1.2 million gallons of the synthetic fluid.
The relatively costly project is likely to fuel the debate among state lawmakers about the risks and rewards of government incentives and mandates for solar-power development. FPL and other power companies — including Central Florida's two other major utilities, Progress Energy and Orlando Utilities Commission — have been experimenting with solar in response to growing government concern about climate change triggered, in part, by relatively cheap power plants that burn coal and natural gas.
One thing utility engineers hope to solve once the Martin County plant is operating is the problem posed by partly cloudy days, when some of the plant's mirrors will be reflecting full sunlight but others will be shaded. Plant engineers don't want alternating pulses of cooler and hotter steam arriving at an electrical generator that runs most efficiently, and with the least wear and tear, when operating conditions are kept constant.
But they'll have plenty of time to figure that out: Gnecco said the Martin Next Generation Solar Energy Center is likely to be operating for the next 50 years.
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