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a u g u s t / s e p t e m b e r 2 0 1 5 www.resourceworld.com 57 during the day when solar energy is plen- tiful. Without a home battery, excess solar energy is often sold to the power company and bought back in the evening. This mis- match adds demand on power plants and increases carbon emissions. Powerwall comes in 10 kWh weekly cycle and 7 kWh daily cycle models. Both are guaranteed for 10 years and are suf- ficient to power most homes during peak evening hours. Multiple batteries may be installed together for homes with greater energy needs, up to 90 kWh total for the 10 kWh battery and 63 kWh total for the 7 kWh battery. Home solar installations comprise a solar panel installed in an array on your roof, an electrical inverter, and now a home battery to store surplus solar energy for later use. The inverter converts direct current elec- tricity from solar panels or a home battery into the alternating current used by lights, appliances and other devices. Contained within Powerwall's out- door-rated enclosure is a rechargeable lithium-ion battery, a liquid thermal man- agement system, a battery management system and a smart DC-AC converter for controlling power flow. Until now, lithium-ion (li-ion) technology has staked a claim to the gold standard for energy storage in performance relative to cost. Redox flow batteries (redox is electron transfer, and "flow" describes how flow batteries work) are becoming an alterna- tive to li-ion batteries. Redox flow batteries consist of two tanks of liquid, which remain dormant until needed. When pumped into a reactor, the two solutions flow next to each other and generate a charge. Earlier redox battery versions were costly, inefficient affairs, but in recent years flow battery technology has improved. The main driver has been the US Energy Department's Pacific Northwest National Laboratory (PNNL). PNNL's breakthrough was to introduce hydrochloric acid into the electrolyte solution. Conventional flow batteries use sulfuric acid, and PNNL found an increase of about 70% in stor- age capacity when both acids are used. In addition, two acids enabled the battery to function efficiently at a far greater range of temperatures. As for lifecycle costs, one advantage of vanadium flow batteries is the 100% recyclability of the vanadium. The PNNL flow batteries are marketed by UniEnergy Technologies, LLC (UET) located in Mukilteo, Washington. UET, a private equity company, produces turn- key, large-scale energy storage systems for utility, micro-grid, commercial and indus- trial, and other applications. The UET flow batteries use vanadium (vanadium is a sil- very gray transition metal, also known as a micronutrient). Redox flow batteries can also help utili- ties during times of peak demand on the grid, providing additional power when needed. Successful commercialization of a US Department of Energy-sponsored tech- nology development was delivered earlier this year, and their announcement con- firms that the battery is now fully owned by Avista Corp. [AVA-NYSE], an energy company located in Spokane, Washington. HaRvesting moRe eneRgy fRom sunligHt Low-cost, solar energy could be the end product of research carried out by stu- dents at Brigham Young University (BYU) in Provo, Utah. Students are building materials designed to absorb different wavelengths of light, which common sili- con solar cells miss. Their lab experiments suggest that solar cells based on nano- crystals of titanium, iron, cobalt and manganese could achieve up to 38% solar energy conversion. Solar panels provide reliable and clean energy, but they don't do it efficiently. Most solar cells rely on silicon-based semicon- ductors, which harvest less than 29% of the available energy from sunlight. That's because silicon-based cells only convert a portion of the light spectrum into electric- ity. The light from wavelengths that are too long or too short is mostly unusable. BYU chemistry professor Richard Watt says, "We started off with one particle that could only absorb blue light and all the other light was wasted. The thought was if we could put another particle that could absorb red wavelengths and another that would absorb yellow and another green, we would be able to harvest most, if not all, of the energy from light." That hope became possible when Watt joined forces with physics professor John Colton. In his lab, they can finely tune these nanocrystals to capture very specific wavelengths of light. This lets them make combinations that work in a coordinated fashion. "What we're looking to do is use the pro- tein ferritin, which is a 12 nanometre-wide hollow sphere that your body and most ani- mals use for iron storage, but through some fancy chemistry you can take out that iron and replace it with a wide variety of dif- ferent minerals that all behave differently," Erickson said. "I've been looking at how these minerals within the ferritin protein absorb light for solar energy applications." The biggest benefit of using the nano- crystals is that researchers could control the wavelengths of light it absorbs, letting them divide up the solar spectrum and increase efficiency in the solar cells. Another advan- tage is that the materials are Earth-abundant and can be synthesized at room tempera- ture. In contrast, Erickson said one of the main expenses for creating silicon solar cells is heating furnaces to produce the neces- sary crystals. He believes eliminating the need for heating could open up the possibil- ity for low-cost solar energy. n