Manipulating the sun

If you've ever held a magnifying glass over a piece of paper on a bright summer day, you can appreciate the potential of concentrated solar energy.

By SAM SER
January 11, 2007 11:43
solar panel 88 298

solar panel 88 298. (photo credit: Courtesy: Jacob Blaustein Institute for Desert Res)

 
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If you've ever held a magnifying glass over a piece of paper on a bright summer day, you can appreciate the potential of concentrated solar energy. What scientists have had to do to realize some of that potential, though, has turned that simple phenomenon into a complicated operation that now entails p-n junction diodes, depletion regions and the Czochralski process. Different approaches to capturing and transforming solar energy have led to a number of amazing technologies. The simplest, called a hot box, goes back 240 years - when a Swiss scientist who took notice of the greenhouse effect, in which a glass enclosure traps heat from the sun, started building contraptions to do just that. By the end of the 19th century, crude systems for heating water had been developed; today their descendants, not much more complex than the originals, can be found on rooftops all over the country (and the world). The same concept can be used to make low-tech devices offering a cheap, safe cooking method for the world's poorest people. That the sun can produce heat seems obvious. That it can be used to produce electricity is another matter, and requires a whole lot more engineering. There are essentially two ways to go about it. One way exploits the heat of the sun's rays, while the other exploits its light. The Israeli company Luz developed one of the world's most stunning examples of solar thermal power by utilizing parabolic troughs. These curved mirrors reflect and amplify the sun's heat, directing it onto a pipe carrying a liquid that is capable of reaching and maintaining very high temperatures. The liquid flows to a facility where its heat turns a turbine, and the turbine generates electricity. With rows upon rows of troughs in a large solar field - and with improvements to the technology from Beit Shemesh-based Solel significantly increasing its efficiency - the method can produce enough electricity to power hundreds of thousands of homes. "So far," notes Prof. David Faiman of the National Solar Energy Center in Sde Boker, "this approach has proved the most economically viable." A similar idea is to use flat mirrors arrayed in a circle or a semi-circle to reflect sunlight onto a central tower, where the heat can be used to create electricity. This is the approach used by the Weizmann Institute of Science in Rehovot. An alternative to solar thermal energy is using the sun's light to manipulate a metal's molecules. As their name implies, photovoltaic (PV) systems turn light into electricity. Photons in sunlight come zooming through the atmosphere and smacking into an absorbent material, knocking electrons loose and setting off a reaction that gets a direct current (DC) flowing. An inverter turns this into alternating current (AC), which can then be directed into your home to power your appliances. PV technology is highly adaptable. Since a series of breakthroughs in the mid-1950s that allowed satellites to use solar panels to power themselves in space, PV systems have been used for a wide variety of applications - from solar-powered wristwatches to solar-powered factories and villages. In Israel, PV panels power remote Beduin encampments, small schools and cash-strapped clinics, street lights and even irrigation systems. The traffic probe readers that monitor the Trans-Israel Highway and its automated billing system are powered by PV panels; the system is the first of its kind in the world. A major downside of PV technology is its reliance on silicon. Although the wafers of semiconductive material used in solar panels are incredibly thin, they still amount to a huge expense because the material is scarce and very expensive to create. Competition with the computer industry for access to silicon has been fierce; both fields are growing at a tremendous rate, and production of silicon is limited. Researchers around the world, including numerous teams here, are developing alternative materials that can be made into multi-layered, thin-film composites and used instead of silicon. Other avenues include light-absorbing dyes and even more complicated technologies such as photoelectrochemical cells, polymer solar cells and nanocrystal solar cells. None of these has proven yet that it can replace silicon. And in the meantime, silicon systems are being designed with greater efficiency to reduce the amount of the material needed. Taking that idea a giant step further, the contraption that Faiman and his colleagues set up in Sde Boker uses only one silicon solar cell. Like the solar thermal systems, it uses mirrors to concentrate sunlight - for its light in this case, not for its heat. No matter which of these approaches is used, though, solar electricity is hampered by one obvious drawback: the sun only shines for half the day. In theory, this is not as big of a problem as it seems. Peak energy demand time is during the day; there is much less demand for electricity at night. Since a conventional power plant cannot just be shut down after sundown, because it needs to be kept firing constantly - a huge waste of resources, alternative energy proponents note - most of the electricity that a conventional power plant creates at night just goes to waste. That's why electricity costs much less at night than it does during the day. In that sense, a solar plant is much more efficient. However, there is no denying that at least some power needs to be generated at night. To overcome their inability to function after dark, solar power systems can be outfitted with supplementary power generators fueled by coal or natural gas, if need be. The combination of the two would provide the efficiency of solar power with the stability and on-demand production of conventional power. (Another solution would be to add batteries to a solar power plant to store excess energy from the day for use at night. At present, such storage solutions are impractical, but a commercial-level model may be ready within a few years.) For now, it seems that solar power is destined to augment, rather than replace, conventional electricity. Even for those involved in solar power's development, like Faiman, that's not a bad scenario. "I think that, for now, one doesn't want to replace conventional electricity," he says. "First of all, there is a tremendous amount of money invested in the infrastructure, and to simply junk that would be a major perturbation to any country's economy. "Secondly, major international companies rely on these power plants to keep them in business, and if you were to threaten to put them out of business, you would generate a backlash and they would probably destroy you. What is necessary is for them to perceive solar power not as a threat, but as something that they themselves could eventually offer." - S.S.

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