Science and Technology Solar power The third way

　　A new method of making electricity from sunlight has just been tested

　　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&aposs 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 therore propose, in a paper in Nature Materials, an alternative. They suggest that a phenomenon called the thermoelectric fect might be used instead—and they have built a prototype 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, such as bismuth telluride, 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. (200oC 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 ficient way of producing electricity. If the heat concentration could be done without all the paraphernalia of mirrors, though, thermoelectricity&aposs inficiency would be offset by the cheapness of the kit. And that is the direction in which Dr Chen and Dr Ren hope they are heading.

　　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 rlected. The second is to choose a thermoelectric material which conducts heat badly (so that different parts remain at different temperatures) but electricity well. The third is to be certain that the temperature gradient which that badly conducting material creates is not frittered away by poor design.

　　The two researchers overcame these challenges through clever engineering. The first they dealt with by coating the top of the device with oxides of hafnium, molybdenum and titanium, in layers about 100 nanometres thick. These layers acted like the anti-rlective coatings on spectacle lenses and caused almost all the sunlight falling on the device to be absorbed.

　　The second desideratum, of low thermal and high electrical conductivity, was achieved by dividing the bismuth telluride into pellets a few nanometres across. That does not affect their electrical conductivity, but nanoscale particles like this are known to scatter and obstruct the passage of heat through imperfectly understood quantum-mechanical processes.

　　The third objective, ficient design, involved sandwiching the nanostructured bismuth telluride between two copper plates and then enclosing the upper plate (the one coated with the light-absorbing oxides) and the bismuth telluride in a vacuum. The copper plates conducted heat rapidly to and from the bismuth telluride, thus maintaining the temperature difference. The vacuum stopped the apparatus losing heat by convection.

　　The upshot was a device that converts 4.6% of incident sunlight into electricity. That is not great compared with the 20% and more achieved by a silicon-based solar cell, the 40% managed by a solar-thermal turbine, or even the 18-20% of one of the new generation of cheap and cheerful thin-film solar cells. But it is enough, Dr Chen reckons, for the process to be worth considering for mass production.

　　He sees it, in particular, as something that could be built into the solar water-heaters that adorn the roofs of an increasing number of houses. If such heaters were covered with thermoelectric generators the sun&aposs rays could be put to sequential use. First, electric power would be extracted from them. Then, the exhaust heat from the bottom plate of the thermoelectric device would be used in the traditional way to warm water up. Two-for-one has always been an attractive proposition for the consumer. This kind of combined heat and power might enable more people to declare independence from the grid.

　　【中文对照翻译】

　　科技 太阳能 利用太阳能发电的第三种方法

　　一种新的利用太阳能发电的方法刚刚得到测试

　　目前，利用阳光发电的可靠方法有两种。 你可以使用一块太阳能电池板从硅等半导体材料中释放电子来直接制造电流。 也可以用镜子集中太阳光线，利用它们烧开水，利用蒸汽驱动发电机。

　　这两种方式都能进行也都很昂贵。 为此，麻省理工学院的陈刚和波士顿大学的任志峰在《自然-材料》杂志上刊登的一篇论文中提出了另一种方式。 他们提出可以利用一种名为热电效应的现象——而且还建立了一个模型来证明这个想法的可行性。

　　热电器件并不是什么新鲜事。 比如它们被用来捕捉从汽车引擎排出的废热。 它们之所以能起作用是因为某些材料，比如碲化铋，如果其中一部分比另一部分热，内部就会产生电位差。 通过外部电路就可以利用这一点来导通电流。

　　为什么在过去热电材料没能成功地应用到太阳能上呢，这是因为如果要获得有价值的电流必须有巨大的温度差。 (200摄氏度被视为合适的起点。) 汽车引擎里很容易达到这个温度差。 但是对于阳光来说，这意味着通过某种方式集中热量， 而如果你费尽力气用一堆镜子达到这个温度差，你很可能走回蒸汽发动的老套路上了，那是一种效率更高的发电方式。 倘若能集中热量而不需要使用镜子的复杂步骤，虽然热电效率不高，但设备的廉价却可以弥补这点。 而陈博士和任博士希望他们可以朝这个方向努力。

　　他们认为创造一种可行的太阳能热电设备需要具备三个条件。 第一是确保大多数射入该设备的阳光被吸收而不是被反射回去了。 第二是选择的热电材料的导热性差(这样不同部分就能保持不同的温度)但是导电性良好。 第三是确保那种导热性差的材料产生的温度变化率不因为设计缺陷而白白浪费。

　　两位研究者经由巧妙的工程技术克服了上述挑战。 他们在设备顶上盖上了大概100纳米厚的二氧化铪、氧化钼和氧化钛的混合物。 它们的作用类似玻璃眼镜上面防反射的覆盖层，使所有落到设备上的阳光都被吸收，这样第一个问题就解决了。

　　低导热性和高导电性则通过把碲化铋分成几纳米的粒状物来实现。 它们的导电性不会因此受到影响，但是人们知道像这样的纳米级颗粒会分散开来并通过人们还尚未完全理解的量子力学过程阻碍热量通道。

　　第三个目标是高效的设计，它涉及到把纳米级的碲化铋夹在两片铜薄片之间然后把位于上方的薄片(这个薄片被覆盖上了吸收光线的氧化物)和碲化铋封入一个真空内。 铜片可以把热量迅速地传递到碲化铋上或从碲化铋上导出，这样就能保持气温差。 容器防止该设备通过对流失去热量。

　　结果就是这样一个可以把射入阳光的4.6%转化为电能的设备。 以硅晶为基础的太阳能电池的转化率为20%甚至以上，太阳能热力涡轮的为40%，就连一种新一代价廉物美的薄膜太阳能电池的转化率也能达到18%-20%，与它们相比，4.6%并不可观。 但是陈博士认为这已经足够了，值得考虑对该设备进行大规模生产。

　　他特别指出该设备可以安装到越来越多的房屋顶上装有的太阳能热水器上去。 如果这样的热水器配上热电发动机，那么太阳光就可以被连续使用。 首先，从它们身上可以获取电能。 其次，从热电设备中位于底部的薄片中出来的排气可以用于传统方式来加热水。 对消费者来说，二合一总是很有吸引力的建议。 这种结合热力和电力的方式可以让更多人摆脱输电网。

【双语阅读】利用太阳能发电的第三种方法 中文翻译部分

　　Science and Technology Solar power The third way

　　A new method of making electricity from sunlight has just been tested

　　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&aposs 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 therore propose, in a paper in Nature Materials, an alternative. They suggest that a phenomenon called the thermoelectric fect might be used instead—and they have built a prototype 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, such as bismuth telluride, 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. (200oC 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 ficient way of producing electricity. If the heat concentration could be done without all the paraphernalia of mirrors, though, thermoelectricity&aposs inficiency would be offset by the cheapness of the kit. And that is the direction in which Dr Chen and Dr Ren hope they are heading.

　　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 rlected. The second is to choose a thermoelectric material which conducts heat badly (so that different parts remain at different temperatures) but electricity well. The third is to be certain that the temperature gradient which that badly conducting material creates is not frittered away by poor design.

　　The two researchers overcame these challenges through clever engineering. The first they dealt with by coating the top of the device with oxides of hafnium, molybdenum and titanium, in layers about 100 nanometres thick. These layers acted like the anti-rlective coatings on spectacle lenses and caused almost all the sunlight falling on the device to be absorbed.

　　The second desideratum, of low thermal and high electrical conductivity, was achieved by dividing the bismuth telluride into pellets a few nanometres across. That does not affect their electrical conductivity, but nanoscale particles like this are known to scatter and obstruct the passage of heat through imperfectly understood quantum-mechanical processes.

　　The third objective, ficient design, involved sandwiching the nanostructured bismuth telluride between two copper plates and then enclosing the upper plate (the one coated with the light-absorbing oxides) and the bismuth telluride in a vacuum. The copper plates conducted heat rapidly to and from the bismuth telluride, thus maintaining the temperature difference. The vacuum stopped the apparatus losing heat by convection.

　　The upshot was a device that converts 4.6% of incident sunlight into electricity. That is not great compared with the 20% and more achieved by a silicon-based solar cell, the 40% managed by a solar-thermal turbine, or even the 18-20% of one of the new generation of cheap and cheerful thin-film solar cells. But it is enough, Dr Chen reckons, for the process to be worth considering for mass production.

　　He sees it, in particular, as something that could be built into the solar water-heaters that adorn the roofs of an increasing number of houses. If such heaters were covered with thermoelectric generators the sun&aposs rays could be put to sequential use. First, electric power would be extracted from them. Then, the exhaust heat from the bottom plate of the thermoelectric device would be used in the traditional way to warm water up. Two-for-one has always been an attractive proposition for the consumer. This kind of combined heat and power might enable more people to declare independence from the grid.

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