 At the moment, there are two reliable ways to make electricity from sunlight. You can use a panel of solar cells to create the current directly, by liberating electrons from a semiconducting material such as silicon. Or you can concentrate the sun’s rays using mirrors, boil water with them, and employ the steam to drive a generator. Both work. But both are expensive. Gang Chen of the Massachusetts Institute of Technology and Zhifeng Ren of Boston College therefore propose an alternative. They suggest that a phenomenon called the thermoelectric（热电）effect might be used instead—and they have built a prototype（原型）to show that the idea is practical. Thermoelectric devices are not new. They are used, for example, to capture waste heat from car engines. They work because certain materials generate an electrical potential difference within themselves if one part is hotter than another. That can be used to drive a current through an external circuit. The reason thermoelectric materials have not, in the past, been applied successfully to the question of solar power is that to get a worthwhile current you have to have a significant temperature difference. (200℃ is considered a good starting point.) In a car engine, that is easy. For sunlight, however, it means concentrating the heat in some way. And if you are going to the trouble of building mirrors to do that, you might as well go down the steam-generation route, which is a much more efficient way of producing electricity. If the heat concentration could be done without all the equipment of mirrors, though, thermoelectricity’s inefficiency would be balanced by the cheapness of the equipment. In their view, three things are needed to create a workable solar-thermoelectric device. The first is to make sure that most of the sunlight which falls on it is absorbed, rather than being reflected. The second is to choose a thermoelectric material which conducts heat badly but electricity well. The third is to be certain that the temperature gradient（梯度）which that badly conducting material creates is not wasted by poor design. 小题1:The following methods can be adopted to make electricity from sunlight EXCEPT .
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