Since about 1800, near the beginning of the industrial revolution, coal, petroleum and gas have been used at a rapidly increasing rate．The supply of these【B1】is limited and electric power is very【B2】in many places．Some scientists believe solar energy is the only【B3】that can meet the world's enormous【B4】for power．Great progress had been made harnessing the sun．Not only is the sun now used in cooking,【B5】it also supplies power for【B6】things as beacon lights for ships and airplanes．It【B7】telephone lines, portable radios, electric clocks, hearing , and even communication satellites．Some homes and office buildings in USA are now being【B8】with solar energy．The three most【B9】types of solar equipment so far are the furnace, the still(蒸馏器) and the cell．Solar furnaces heat water or air,【B10】is then circulated through a building to make it comfortable．Solar stills ate【B11】important because they can provide【B12】water at a relative cheap rate．Salt can easily be【B13】from sea water with solar still．The most highly developed type of solar【B14】equipment is the solar cell．Some cells are so【B15】that they can turn 16% of the energy they receive from the sun【B16】electric energy．One of the cell's biggest advantages is【B17】it can be made either small enough to carry【B18】large enough to produce current that can run an automobile．Besides, it has a very long life．It is still too expensive for the【B19】consumer but when ways are【B20】to produce it more cheaply, we can expect its use to be widely-spread.
【B1】