Solar power might seem strange or futuristic, but it's already quitecommonplace. You might have a solar-powered quartz watch on your wrist or asolar-powered pocket calculator. Many people have solar-powered lightsin their garden. Spaceships and satellitesusually have solar panels on them too. The Am Contact online >>
Solar power might seem strange or futuristic, but it''s already quitecommonplace. You might have a solar-powered quartz watch on your wrist or asolar-powered pocket calculator. Many people have solar-powered lightsin their garden. Spaceships and satellitesusually have solar panels on them too. The American space agency NASA has even developed a solar-poweredplane! As global warmingcontinues to threaten our environment, there seems little doubt that solar powerwill become an even more important form of renewable energy in future. But how exactly does it work?
Photo: NASA''s solar-powered Pathfinder airplane.The upper wing surface is covered with lightweight solar panels that power the plane''s propellers.Picture courtesy of NASA Armstrong Flight Research Center.
Solar power is amazing. On average, every square meter of Earth''ssurface receives 163 watts of solar energy (a figure we''ll explain in more detail in a moment). [1] In other words, you could stand a really powerful (150 watt) table lamp on every square meter ofEarth''s surface and light up the whole planet with the Sun''s energy! Or, to putit another way, if we covered just one percent of the Sahara desert with solarpanels, we could generate enough electricityto power the whole world. [2] That''s the good thing about solar power:there''s an awful lot of it—much more than we could ever use.
Photo: The amount of energy we can capture from sunlight is at a minimum at sunrise and sunset and a maximum at midday, when the Sun is directly overhead.
But there''s a downside too. The energy the Sun sends out arrives onEarth as a mixture of light and heat. Both of these are incrediblyimportant—the light makes plants grow, providing us with food, whilethe heat keeps us warm enough to survive—but we can''t use either theSun''s light or heat directly to run a television or a car. We have tofind some way of converting solar energy into other forms of energy wecan use more easily, such as electricity. And that''s exactly what solarcells do.
A solar cell is an electronic device that catches sunlight andturns it directly into electricity. It''s about the size of an adult''s palm, octagonal in shape, and colored bluish black. Solar cells are often bundled together to make larger units called solar modules, themselvescoupled into even bigger units known as solar panels (the black- orblue-tinted slabs you see on people''s homes—typically with severalhundred individual solar cells per roof) or chopped into chips (toprovide power for small gadgets like pocket calculators and digitalwatches).
Photo: The roof of this house is covered with 16 solar panels, each made up of a grid of 10×6 = 60 small solar cells. On a good day, it probably generates about 4 kilowatts of electricity.
Just like the cells in a battery, the cells ina solar panel are designed to generate electricity; but where a battery''scells make electricity from chemicals, a solar panel''s cells generatepower by capturing sunlight instead. They are sometimes called photovoltaic (PV)cells because they use sunlight ("photo" comes from the Greek word for light) to make electricity (theword "voltaic" is a reference to Italian electricity pioneerAlessandro Volta,1745–1827).
Photo: A single solar cell. Picture courtesy of NASA and Wikimedia Commons.
Silicon is the stuff from which the transistors(tiny switches) in microchips are made—and solar cells work in a similar way.Silicon is a type of material called a semiconductor.Some materials, notably metals, allow electricity to flow through themvery easily; they are called conductors. Other materials, such asplastics and wood, don''t reallylet electricity flow through them atall; they are called insulators. Semiconductors like silicon areneither conductors nor insulators: they don''t normally conductelectricity, but under certain circumstances we can make them do so.
A solar cell is a sandwich of two different layers of silicon thathave been specially treated or doped so theywill let electricity flow through them in a particular way. The lower layer isdoped so it has slightly too few electrons. It''s called p-type or positive-type silicon (because electronsare negatively charged and this layer has too few of them). The upperlayer is doped the opposite way to give it slightly too many electrons. It''scalled n-type or negative-type silicon. (Youcan read more about semiconductors and doping in our articles on transistors andintegrated circuits.)
This is what we mean by photovoltaic—light making voltage—and it''s one kind of whatscientists call the photoelectric effect.
Artwork: How a simple, single-junction solar cell works.
A solar cell is a sandwich of n-type silicon (blue) and p-type silicon(red). It generates electricity by using sunlight to make electrons hopacross the junction between the different flavors of silicon:
That''s a basic introduction to solar cells—and if that''s all you wanted, you can stop here.The rest of this article goes into more detail about different types of solar cells, howpeople are putting solar power to practical use, and why solar energy is taking such a long time tocatch on.
Chart: Efficiencies of solar cells compared: The very first solar cell scraped in at a mere 6 percent efficiency; the most efficient one that''s been produced to date managed 47.1 percent in laboratory conditions. Most cells are first-generation types that can manage about 15 percent in theory and probably 8 percent in practice.
Real-world domestic solar panels might achieve an efficiency of about 15 percent, givea percentage point here or there, and that''s unlikely to get much better rst-generation, single-junction solar cells aren''t going to approachthe 30 percent efficiency of the Shockley-Queisser limit, never mindthe lab record of 47.1 percent. All kinds of pesky real-world factors will eat into the nominal efficiency,including the construction of the panels, how they are positioned andangled, whether they''re ever in shadow, how clean you keep them, howhot they get (increasing temperatures tend to lower their efficiency),and whether they''re ventilated (allowing air to circulate underneath)to keep them cool.
Most of the solar cells you''ll see on people''s roofs today areessentially just silicon sandwiches, specially treated ("doped")to make them better electrical conductors. Scientists refer to theseclassic solar cells as first-generation, largely to differentiatethem from two different, more modern technologies known as second-and third-generation. So what''s the difference?
Photo: A colorful collection of first-generation solar cells.Picture courtesy of NASA Glenn Research Center (NASA-GRC) and Internet Archive.
Over 90 percent of the world''s solar cells are made from wafersof crystalline silicon (abbreviated c-Si), sliced from large ingots,which are grown in super-clean laboratories in a process that cantake up to a month to complete.[3] The ingots either take the form ofsingle crystals (monocrystalline or mono-Si) or contain multiple crystals (polycrystalline,multi-Si or poly c-Si).
Photo: A thin-film, second-generation solar "panel." The power-generating film is made from amorphous silicon, fastened to a thin, flexible, and relatively inexpensive plastic backing (the "substrate").Photo by Warren Gretz courtesy of NREL(image id #6321083).
Classic solar cells are relatively thin wafers—usually afraction of a millimeter deep (about 200 micrometers, 200μm, or so).But they''re absolute slabs compared to second-generationcells, popularly known as thin-film solar cells (TPSC) orthin-film photovoltaics (TFPV), which are about 100 timesthinner again (several micrometers or millionths of a meter deep).Although most are still made from silicon (a different form known asamorphous silicon, a-Si, in which atoms are arranged randomly insteadof precisely ordered in a regular crystalline structure), some aremade from other materials, notably cadmium-telluride (Cd-Te) andcopper indium gallium diselenide (CIGS).[4]
Because they''re extremely thin, light, and flexible, second-generation solar cells can belaminated onto windows, skylights, roof tiles, and all kinds of"substrates" (backing materials) including metals, glass, and polymers (plastics). What second-generation cells gain in flexibility, they sacrifice inefficiency: classic, first-generation solar cells still outperformthem. So while a top-notch first-generation cell might achieve anefficiency of 15–20 percent, amorphous silicon struggles to get above7 percent, the best thin-film Cd-Te cells only manage about 11percent, and CIGS cells do no better than 7–12 percent. [5] That''s one reason why, despite their practical advantages, second-generationcells have so far made relatively little impact on the solar market.
Photo: Third-generation plastic solar cells produced by researchers at the National Renewable Energy Laboratory.Photo by Jack Dempsey courtesy of NREL(image id #6322357).
The latest technologies combine the best features of first andsecond generation cells. Like first-generation cells, they promiserelatively high efficiencies (30 percent or more). Likesecond-generation cells, they''re more likely to be made frommaterials other than "simple" silicon, such as amorphous silicon,organic polymers (making organic photovoltaics, OPVs), perovskite crystals,and feature multiple junctions (made from multiple layers of different semiconductingmaterials). Ideally, that would make them cheaper, more efficient,and more practical than either first- or second-generation cells.[6]Currently, the world record efficiency for third-generation solaris 28 percent, achieved by a perovskite-silicon tandem solar cell in December 2018.
"The total solar energy that reaches the Earth''ssurface could meet existing global energy needs 10,000 times over."
European Photovoltaic Industry Association & Greenpeace, 2011.
In theory, a huge amount. Let''s forget solar cells for the momentand just consider pure sunlight. Up to 1000 watts of raw solar power hits each square meter of Earth pointing directly at the Sun (that''sthe theoretical power of direct midday sunlight on acloudless day—with the solar rays firing perpendicular to Earth''ssurface and giving maximum illumination or insolation, as it''stechnically known).
In practice, after we''ve corrected for the tiltof the planet and the time of day, the best we''re likely to get ismaybe 100–250 watts per square meter in typical northern latitudes(even on a cloudless day). That translates into about 2–6 kWh per day(depending on whether you''re in a northern region like Canada orScotland or somewhere more obliging such as Arizona or Mexico).[11]Multiplying up for a whole year''s production gives us somewherebetween 700 and 2500 kWh per square meter (700–2500 units ofelectricity). Hotter regions clearly have much greater solarpotential: the Middle East, for example, receives around 50–100percent more useful solar energy each year than Europe.
Unfortunately, typical solar cells are only about 15 percentefficient, so we can only capture a fraction of this theoreticalenergy: perhaps 4–10 watts per square meter.[7]That''s why solar panels need to be so big: the amount ofpower you can make is obviously directly related to how much area youcan afford to cover with cells. A single solar cell (roughly the sizeof a compact disc) can generate about 3–4.5 watts; a typical solarmodule made from an array of about 40 cells (5 rows of 8cells) could make about 100–300 watts; several solar panels, eachmade from about 3–4 modules, could therefore generate an absolutemaximum of several kilowatts (probably just enough to meet a home''speak power needs).
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