Producing solar cell-grade silicon from rice hulls

Production of high purity solar grade silicon can be made from common rice hulls. A unique process for material purification and reduction includes leaching the rice hulls in acid followed by treatment with high purity water, coking the acid-cleaned hulls in a non-oxidizing ambient, compensating the carbon or silica content of the coked hulls to obtain a desired carbon to silica ratio and reducing the silica to produce high purity silicon. The size of the rice hulls is decreased by grinding or milling, the rice hull is leached in with aqueous hydrochloric acid and rinsing in distilled water to reduce the impurity level in the rice hulls to below about 400 ppm., the leached rice hull is coked by pyrolyzing the rice hulls at a temperature of about 920.degree C in a non-oxidizing atmosphere comprising a gaseous mixture of an inert gas comprising about 1% anhydrous hydrogen chloride and at least one of the group of anhydrous acids consisting of HCl, HBR, and HI to produce a composite of carbon and silica by adjusting the carbon to silica ratio of the coked rice hulls to less than about 2:1 and thermally reducing the adjusted carbon and silica mixture to produce elemental silicon.

Silicon carbide from rice husk

Silicon carbide crystals can be manufactured from rice husk raw material. Rice husk is treated with an accelerator selected from the group consisting of boron compounds and lanthanum compounds prior to heating in the furnace. It is pretreating with an acid solution (e.g., 5N to 6N H2 SO4, HCl or HNO3) upto 10 percent and 40 percent of the weight of said rice husk raw material for a period of at least one and half hour prior to being heated in a furnace of non-oxidizing atmosphere. The acid treated raw material is arranged on a gas-permeable, heat-resistant support in a manner allowing the passage of a gas through said raw material. It is then placed in an air tight furnace at 400 to 1300 deg.C. for at least one hour to remove impurities. Then silicon carbide whisker-containing material can be removed from the furnace.

Liquid Battery for Solar Energy Storage

One of the biggest challenges currently facing large-scale solar energy technology is finding an effective way to store the energy, which is essential for using the electricity at night or on cloudy days. Recently, researchers from MIT have designed a new kind of battery that can quickly absorb large amounts of electricity, as required for solar energy storage. Unlike conventional batteries it is made of all-liquid active materials. The battery consists of three layers of liquids: two electrode liquids on the top and bottom and an electrolyte liquid in the middle forming the three distinct layers. In the first prototype, the electrodes were molten metals - magnesium on the top and antimony on the bottom - while the electrolyte was a molten salt such as sodium sulfide. When charging, the solid container holding the liquids collects electrons from exterior solar panels or another power supply, and later, for discharging, the container carries the electrons away to the electrical grid to be used as electricity. As electrons flow into the battery, magnesium ions in the electrolyte gain electrons and form magnesium metal, rising to form the upper molten magnesium electrode. At the same time, antimony ions in the electrolyte lose electrons and sink to form the lower molten antimony electrode. At this point, the battery is fully charged, since the battery has thick electrode layers and a small layer of electrolyte. To discharge the electrical current, the process is reversed, and the metal atoms become ions again. The batteries are expected to be inexpensive and simple to manufacture but likely to have the inherent problems associated with handling of liquids.