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  3. High in the Mojave desert, 130 miles northeast of Los Angeles, lies a vast f

High in the Mojave desert, 130 miles northeast of Los Angeles, lies a vast f

游客2024-08-09  9
问题     High in the Mojave desert, 130 miles northeast of Los Angeles, lies a vast field of mirrors. Crisscrossing rows of glass and metal, glinting in the sunlight, cover a full square mile of dirt. It is a fully operational array of power plants churning out an average of 180 megawatts of electricity.

    Most people think of solar power as a flat panel on every rooftop. But photovoltaic panels have limitations. They work fine when the sun is strong, but when the clouds roll in you’ d better have batteries to run the TV and dishwasher. And even on the sunniest days the panels are not very well suited for cities, where roof space is limited.
    For decades, engineers have been working on ways to catch the sun over a broad area concentrating it and using it to produce electricity on the same scale as centralized coal, hydro or nuclear power plants—hundreds of megawatts at a time. Several pilot plants have been operating in California, some for decades, but so far they have not had enough volume to force costs down to competitive levels.
    Some countries are expected to go with much larger plants capable of generating more than 100 megawatts each. If some of the projects are completed, costs could come down from the current 15 cents a kilowatt-hour for the Mojave plant to 8 cents per kwh in the next 8 to 10 years. That would go a long way toward closing the gap with gas and oil, which now cost as little as 4 cents per kwh.
    The Mojave plant is one of the world’s first commercial solar power plants, with five solar Electric Generating Systems (SEGS) supplying electricity to southern California. The basic component of a SEGS plant is a row of mirrors that reflect sunlight onto a pipe filled with oil. The oil heats up and is used to produce steam, which turns an electrical turbine. Assemble a few dozen rows of these trough-mirrors, and you have got capacity to generate 30 megawatts of power, enough for half a small town. The problem with trough technology is that the oil loses its heat too quickly. When the sun goes down, so does the power.
    A Solar Tres design uses molten salt instead of oil. Since salt holds more heat longer than oil, it can drive turbine through the night. Concentric rings of mirrors direct sunlight to a tank of molten salt. When the stuff is hot enough, some goes straight to a generator to produce steam, while the rest is stored for use at night. The 15-megawatt Solar Tres plant would be the first long-term commercial power production project. Since the electricity is expected to be costly close to 20 cents per kwh, the Spanish government plans to subsidize the plant.
    The next big thing—Dish systems—is already in the works. The building block of such a plant is a parabolic mirror, shaped like a satellite dish, that reflects sunlight onto a small generator suspended in front. The heat drives a turbine. Theoretically a dish configuration would produce more energy per acre than other solar concentrating plants, that is, if engineers could figure out a good way of linking many dishes together.  [br] Why couldn’t we widely use solar power as flat-panel ones in cities?
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答案 We don’t have enough roof space.
解析 文章第二段提到，即使在阳光最充足的日子，这种面板也不太适合屋顶空间有限的城市。说明局限性体现在屋顶空间的不足。由此可知本题答案。
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