Listen and read 32: Photovoltaics - Thấm Tâm Vy

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1. LISTEN AND READ 32 Photovoltaics For gridscale electricity produced in standard solar farms this arrangement is likely to SOLAR'S NEW POWER continue. But many people think solar energy has wider potential than that. Some want Cells are getting better at converting sunshine into electricity to redesign solar farms in radical ways. Others see it as having small-scale applications SOLAR ENERGY has had a good crisis. In many parts of the world skies clear of that do not require connection to a grid. Both of these approaches will require pollution have helped photovoltaic power stations, which convert light into electricity, efficiencies that standard silicon has never managed to achieve. But both will permit become more productive and reliable. Declining demand, meanwhile, has seen coal-and high prices for cells that do so. gas-fired stations taken offline. In Britain, on April 20th, solar generation peaked at 9.7 Layer cake gigawatts. At the moment this happened that represented almost 30% of the country’s One way to boost a cell’s efficiency is to add layers tuned to different parts of the solar electricity supply—ten times the usual proportion. In Germany the proportion of solar in spectrum. This means reaching beyond silicon to other materials. So-called III-V the mix reached 23% for an entire week in April, compared with an average of about semiconductors, made of elements from group III of the periodic table (aluminium, 8% during 2019. gallium and indium) and group V (phosphorus and arsenic) are one approach. Indeed, Though temporary, such figures are impressive. Solar power, they suggest, has come gallium arsenide is already used in applications like satellites. John Geisz and his of age. In some ways, colleagues at the National Renewable Energy Laboratory, in Colorado, have produced a however, despite solar’s six-junction cell containing various III-V mixtures, each with different light-absorbing new and shiny image, this properties. This cell has an efficiency of 47.1% in laboratory conditions—a new record, is the victory of an old which the researchers reported in Nature Energy in April. With further work, they technology. reckon, an efficiency of more than 50% should be possible. The first practical solar Intriguingly, the efficiency of Dr Geisz’s cell rises as more light is concentrated on it. cell was made in the 1950s Laid out in standard solar farms it would manage a bit under 40%. The 47% figure at Bell Labs in New Jersey. comes when it is bathed in illumination equivalent to 143 suns. Roughly speaking, then, It had an efficiency of 6% a six-junction cell with a suitable arrangement of mirrors concentrating the sun’s light and was horrendously onto it could turn out the same amount of electricity as a standard silicon cell that had expensive. It did, though, 400 times the area. Those are the sorts of numbers that disruptive technologies are made prove to have a killer of. application in powering the Another promising group of materials for making new types of solar cells are satellites of the perovskites. The original substance of this name is a mineral, calcium titanium oxide, superpowers in the discovered in the Ural mountains in 1839 and called after Count Lev Perovski, a forthcoming space race. Russian mineralogist. As is often the way with minerals, though, the basic crystal lattice That kept interest alive. involved can be created from many sorts of atoms. “Perovskite” has thus now become a Gradually, costs came generic term for any of these variants. down, efficiencies tripled Not all perovskites are semiconductors. But a group based on a metal, such as tin, and to 17-20% and applications a halogen, such as chlorine, bromine or iodine, do have that property. The ingredients of these metal-halide perovskites are, moreover, abundant and inexpensive. One of the widened, until the point, now arrived at, where grid leaders in the field of making cells out of them is Oxford PV, a British firm founded in managers faced with 2010 to exploit work done on perovskites by Henry Snaith of Oxford University. The surplus capacity are preferring solar to fossil-fuel generation. For all that they have got firm’s design is a hybrid structure, known as a tandem cell, that coats a silicon layer better in detail, though, solar cells have stayed the same in principle. Two layers of with perovskite. ultrapure (99.9999%) silicon, each doped with an additive to make it semiconducting, This brings two advantages. One is that, like a multilayered III-V cell, a absorb light and use the energy from this to move electrons across the junction between perovskitesilicon tandem cell divides up the job of capturing sunlight. The upper, them, thus generating an electric current. perovskite, layer is tweaked to absorb light from the blue end of the spectrum. The lower, silicon, layer mops up the remaining wavelengths towards the red end. This makes for high efficiency. In a test in 2018 such a tandem cell set a new record for its Thẩm Tâm Vy LISTEN AND READ 31 MAY 23RD, 2020
2. type with an efficiency of 28%. Eventually, the firm’s engineers think, they can push this into the “mid-30s”. The second advantage of piggybacking the perovskite on silicon is that the cells are fairly easy to make into solar panels using standard industrial processes. That helps keep them competitive with conventional solar panels. A new factory that will do just this is currently under construction in Germany. The hope is that—the pandemic provided— the first panels made in this plant will go on sale next year. Whether an efficiency in the mid-30s will be enough to displace silicon cells from part of their existing market remains to be seen. Perovskites may, however, have applications doing jobs that silicon cannot manage. For instance, they work well in low light intensities. This has permitted a group led by Thomas Brown of Tor Vergata University of Rome and John Fahlteich of the Fraunhofer Institute’s campus in Dresden, Germany, to develop versions which operate at the levels of illumination found inside buildings. The amount of energy in artificial lighting is vastly less than that in sunshine. Nevertheless, Dr Brown and Dr Fahlteich have found, according to a paper they published this month in Cell Reports Physical Science, that their cells can achieve a conversion efficiency of up to 22.6%, thereby producing enough juice to run small, low-power devices like wireless sensors and remote-control units, which would otherwise require batteries. Though it may seem odd to turn artificial, indoor lighting into electricity, given that it has been created from electricity in the first place, the truth is that all such light which does not end up entering a human eye is wasted. This approach simply reduces the level of waste. With the growth of the so-called Internet of things, which relies on many different types of sensors, wireless control systems and other bits of electronic kit, such an approach could have wide application. If it works, the label “batteries not included” will go from being a warning to a recommendation. [The Economist 23.05.2020 US] Notes: - come of age = reach a state of development - horrendously = horrifically; horrifyingly - perovskites: a mineral that contains various types of metals - metal-halide: hợp chất được hình thành khi kết hợp các phần tử kim loại và halogen, bao gồm những thứ như natri clorua (muối) và uranium hexafluoride (nhiên liệu sử dụng trong các lò phản ứng năng lượng hạt nhân; chế tạo đèn tạo ra ánh sáng bằng cách cho dòng điện đi qua hỗn hợp giữa khí mê-tan và khí metal halide - tandem = two-rider bicycle (xe đạp đôi) - mops up: to remove liquid with something soft = hút ẩm bằng cách thấm khô - piggybacking = cõng trên lưng Thẩm Tâm Vy LISTEN AND READ 31 MAY 23RD, 2020