【雙語閱讀】利用太陽能發(fā)電的第三種方法.

　　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.

　　【中文對照翻譯】

　　科技 太陽能 利用太陽能發(fā)電的第三種方法

　　一種新的利用太陽能發(fā)電的方法剛剛得到測試

　　目前，利用陽光發(fā)電的可靠方法有兩種。 你可以使用一塊太陽能電池板從硅等半導(dǎo)體材料中釋放電子來直接制造電流。 也可以用鏡子集中太陽光線，利用它們燒開水，利用蒸汽驅(qū)動發(fā)電機。

　　這兩種方式都能進行也都很昂貴。 為此，麻省理工學(xué)院的陳剛和波士頓大學(xué)的任志峰在《自然-材料》雜志上刊登的一篇論文中提出了另一種方式。 他們提出可以利用一種名為熱電效應(yīng)的現(xiàn)象——而且還建立了一個模型來證明這個想法的可行性。

　　熱電器件并不是什么新鮮事。 比如它們被用來捕捉從汽車引擎排出的廢熱。 它們之所以能起作用是因為某些材料，比如碲化鉍，如果其中一部分比另一部分熱，內(nèi)部就會產(chǎn)生電位差。 通過外部電路就可以利用這一點來導(dǎo)通電流。

　　為什么在過去熱電材料沒能成功地應(yīng)用到太陽能上呢，這是因為如果要獲得有價值的電流必須有巨大的溫度差。 (200攝氏度被視為合適的起點。) 汽車引擎里很容易達到這個溫度差。 但是對于陽光來說，這意味著通過某種方式集中熱量， 而如果你費盡力氣用一堆鏡子達到這個溫度差，你很可能走回蒸汽發(fā)動的老套路上了，那是一種效率更高的發(fā)電方式。 倘若能集中熱量而不需要使用鏡子的復(fù)雜步驟，雖然熱電效率不高，但設(shè)備的廉價卻可以彌補這點。 而陳博士和任博士希望他們可以朝這個方向努力。

　　他們認為創(chuàng)造一種可行的太陽能熱電設(shè)備需要具備三個條件。 第一是確保大多數(shù)射入該設(shè)備的陽光被吸收而不是被反射回去了。 第二是選擇的熱電材料的導(dǎo)熱性差(這樣不同部分就能保持不同的溫度)但是導(dǎo)電性良好。 第三是確保那種導(dǎo)熱性差的材料產(chǎn)生的溫度變化率不因為設(shè)計缺陷而白白浪費。

　　兩位研究者經(jīng)由巧妙的工程技術(shù)克服了上述挑戰(zhàn)。 他們在設(shè)備頂上蓋上了大概100納米厚的二氧化鉿、氧化鉬和氧化鈦的混合物。 它們的作用類似玻璃眼鏡上面防反射的覆蓋層，使所有落到設(shè)備上的陽光都被吸收，這樣第一個問題就解決了。

　　低導(dǎo)熱性和高導(dǎo)電性則通過把碲化鉍分成幾納米的粒狀物來實現(xiàn)。 它們的導(dǎo)電性不會因此受到影響，但是人們知道像這樣的納米級顆粒會分散開來并通過人們還尚未完全理解的量子力學(xué)過程阻礙熱量通道。

　　第三個目標是高效的設(shè)計，它涉及到把納米級的碲化鉍夾在兩片銅薄片之間然后把位于上方的薄片(這個薄片被覆蓋上了吸收光線的氧化物)和碲化鉍封入一個真空內(nèi)。 銅片可以把熱量迅速地傳遞到碲化鉍上或從碲化鉍上導(dǎo)出，這樣就能保持氣溫差。 容器防止該設(shè)備通過對流失去熱量。

　　結(jié)果就是這樣一個可以把射入陽光的4.6%轉(zhuǎn)化為電能的設(shè)備。 以硅晶為基礎(chǔ)的太陽能電池的轉(zhuǎn)化率為20%甚至以上，太陽能熱力渦輪的為40%，就連一種新一代價廉物美的薄膜太陽能電池的轉(zhuǎn)化率也能達到18%-20%，與它們相比，4.6%并不可觀。 但是陳博士認為這已經(jīng)足夠了，值得考慮對該設(shè)備進行大規(guī)模生產(chǎn)。

　　他特別指出該設(shè)備可以安裝到越來越多的房屋頂上裝有的太陽能熱水器上去。 如果這樣的熱水器配上熱電發(fā)動機，那么太陽光就可以被連續(xù)使用。 首先，從它們身上可以獲取電能。 其次，從熱電設(shè)備中位于底部的薄片中出來的排氣可以用于傳統(tǒng)方式來加熱水。 對消費者來說，二合一總是很有吸引力的建議。 這種結(jié)合熱力和電力的方式可以讓更多人擺脫輸電網(wǎng)。

　　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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