Concerning the sustainability of geothermal energy

单项选择题

Geothermal(地热的)energy has been used for thousands of years in some countries for cooking and heating. It is simply power derived from the Earth’s internal heat. This thermal energy is contained in the rock and fluids beneath Earth’s crust. It can be found from shallow ground to several miles below the surface, and even farther down to the extremely hot molten rock called magma(岩浆). A geothermal heat pump system can take advantage of the constant temperature of the upper ten feet(three meters)of the Earth’s surface to heat a home in the winter while extracting heat from the building and transferring it back to the relatively cooler ground in the summer. Geothermal water from deeper in the Earth can be used directly for heating homes and offices, or for growing plants in greenhouses. Some US cities pipe geothermal hot water under roads and sidewalks to melt snow. To produce geothermal-generated electricity, wells, sometimes a mile(1.6 kilometers)deep or more, are drilled into underground reservoirs to tap steam and very hot water that drive turbines linked to electricity generators. The first geothermally generated electricity was produced in Larderello, Italy, in 1904. There are three types of geothermal power plants: dry steam, flash, and binary(二元的). Dry steam, the oldest geothermal technology, takes steam out of fractures in the ground and uses it to directly drive a turbine. Flash plants pull deep, high-pressure hot water into cooler, low-pressure water. The steam that results from this process is used to drive the turbine. In binary plants, the hot water is passed by a secondary fluid with a much lower boiling point than water. This causes the secondary fluid to turn to vapor, which then drives a turbine. Most geothermal power plants in the future will be binary plants. There are many advantages of geothermal energy. It can be extracted without burning a fossil fuel such as coal, gas, or oil. Geothermal fields produce only about one-sixth of the carbon dioxide that a relatively clean natural-gas-fueled power plant produces. Unlike solar and wind energy, geothermal energy is always available, 365 days a year. It’s also relatively inexpensive; savings from direct use can be as much as 80 percent over fossil fuels. But it has some environmental problems. The main concern is the release of hydrogen sulfide(硫化氢), a gas that smells like rotten eggs at low concentrations. Another concern is the disposal of some geothermal fluids, which may contain low levels of toxic materials. Although geothermal sites are capable of providing heat for many decades, eventually specific locations may cool down.

Concerning the sustainability of geothermal energy, people believe that______.

A．geothermal energy can be exploited for an indefinite period of time in one place
B．geothermal energy has an advantage over solar and wind energy
C．only geothermal energy exploited by binary power plants can last long
D．in certain places geothermal energy may dry up finally after decades of exploitation