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Better Solar Energy Systems: More Heat, More LightSolar photovoltaic thermal energy systems, or PVTs, generate both heat and electricity, but until now they havent been very good at the heat-generating part compared to a stand-alone solar thermal collector. Thats because they operate at low temperatures to cool crystalline silicon solar cells, which lets the silicon generate more electricity but isnt a very efficient way to gather heat.Thats a problem of economics. Good solar hot-water systems can harvest much more energy than a solar-electric system at a substantially lower cost. And it，s also a space problem：photovoltaic cells can take up all the space on the roof, leaving little room for thermal applications.In a pair of studies, Joshua Pearce, an associate professor of materials science and engineering, has devised a solution in the form of a better PVT made with a different kind of silicon. His research collaborators are Kunal Girotra from ThinSilicon in California and Michael Pathak and Stephen Harrison from Queens University, Canada.Most solar panels are made with crystalline silicon，but you can also make solar cells out of amorphous silicon, commonly known as thin-film silicon. They dont create as much electricity, but they are lighter, flexible, and cheaper. And, because they require much less silicon, they have a greener footprint. Unfortunately，thin-film silicon solar cells are vulnerable to some bad-news physics in the form of the Staebler-Wronski effect.“That means that their efficiency drops when you expose them to light pretty much the worst possible effect for a solar cell,” Pearce explains，which is one of the reasons thin- film solar panels make up only a small fraction of the market.However, Pearce and his team found a way to engineer around the Staebler-Wronski effect by incorporating thin-film silicon in a new type of PVT. You dont have to cool down thin-film silicon to make it work. In fact，Pearces group discovered that by heating it to solar-thermal operating temperatures，near the boiling point of water, they could make thicker cells that largely overcame the Staebler-Wronski effect. When they applied the thin-film silicon directly to a solar thermal energy collector , they also found that by baking the cell once a day，they boosted the solar cells electrical efficiency by over 10 percent.“Liquefaction” Key to Much of Japanese Earthquake DamageThe massive subduction zone1 earthquake in Japan caused a significant level of soilliquefaction2 that has surprised researchers with its widespread severity, a new analysis shows.Weve seen localized3 examples of soil liquefaction as extreme as this before, but the distance and extentof damage in Japan were unusually severe, said Scott Ashford, a professor of geotechnical engineering4 at Oregon State University5. Entire structures were tilted and sinking into the sediments, Ashford said. The shifts in soil destroyed water, drain and gas pipelines6, crippling the utilities and infrastructure these communities need to function. We saw some places that sank as much as four feet.Some degree of soil liquefaction7 is common in almost any major earthquake. Its a phenomenon in which soils soaked with water, particularly recent sediments or sand, can lose much of their strength and flow during an earthquake. This can allow structures to shift or sink or collapse. But most earthquakes are much shorter than the recent event in Japan, Ashford said. The length of the Japanese earthquake, as much as five minutes, may force researchers to reconsider the extent of liquefaction damage possibly occurring in situations such as this8.With such a long-lasting earthquake, we saw how structures that might have been okay after 30 seconds just continued to sink and tilt as the shaking continued for several more minutes, he said. And it was clear that younger sediments, and especially areas built on recently filled ground, are much more vulnerable.The data provided by analyzing the Japanese earthquake, researchers said, should make it possible to improve the understanding of this soil phenomenon and better prepare for it in the future. Ashford said it was critical for the team to collect the information quickly,beforedamage was removed in the recovery efforts9.Theres no doubt that well learn things from what happened in Japan10 that11 will help us to reduce risks in other similar events, Ashford said. Future construction in some places may make more use of techniques known to reduce liquefaction, such as better compaction to make soils dense, or use of reinforcing stone columns.Ashford pointed out that northern California have younger soils vulnerable to liquefaction -on the coast, near river deposits or in areas with filled ground. The young sediments, in geologic terms, may be thosedepositedwithin the past 10,000 years or more. In Oregon, for instance, that describes much of downtown Portland, the Portland International Airport and other cities.Anything near a river and old flood plains is a suspect12, and the Oregon Department of Transportation has already concluded that 1,100 bridges in the state are at risk from an earthquake. Fewer than 15 percent of them have been reinforced toprevent collapse. Japan has suffered tremendous losses in the March 11 earthquake, but Japanese constructionstandardshelped prevent many buildings from collapse -even as they tilted and sank into the ground.
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