Materials Challenges Facing Electrical Energy Storage
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terials Challenges Facing Electrical Energy Storage
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M. Stanley Whittingham (Binghamton University, USA)
Flywheels
Abstract During the past two decades, the demand for the storage of electrical energy has mushroomed both for portable applications and for static applications. As storage and power demands have increased predominantly in the form of batteries, the system has evolved. However, the present electrochemical systems are too costly to penetrate major new markets, still higher performance is required, and environmentally acceptable materials are preferred. These limitations can be overcome only by major advances in new materials whose constituent elements must be available in large quantities in nature; nanomaterials appear to have a key role to play. New cathode materials with higher storage capacity are needed, as well as safer and lower cost anodes and stable electrolyte systems. Flywheels and pumped hydropower also have niche roles to play.
MRS BULLETIN
System Demand/MW
Why store energy, and in particular, why store electrical energy? There is a great need for electrical energy storage, not only for mobile electronic devices, such as cell phones, computers, and iPods, but also for transportation and load-leveling and for the effective commercialization of renewable resources such as solar and wind power. The storage sizes needed range from milliwatts for smart-card devices to multiple-megawatts for large load-leveling stations; unlike for electronics, there is no simple applicable Moore’s law (i.e., no general trend of exponentially increasing storage capacity). Much attention is being given to hybrid electric vehicles (HEVs), in which batteries and/or capacitors are used to capture the energy evolved in braking. A related application is the capture of the energy normally wasted when a dock crane is lowering a crate; capturing this energy through capacitors saves around 40% of total energy utilization. Similarly, subway trains, with many stops and starts, can capture the braking energy entering the station for use in accelerating out of the station. The next generation of electric vehicles might be plug-in hybrids, in which larger batteries are used and the vehicle can be recharged by plugging into the electrical power line. An effective unlimited range can be attained by using a small internal engine as a battery charger; the waste heat from the internal combustion engine can provide the heating and defogging energy. The all-electric vehicle might find a few niche markets; these include city buses and postal delivery and utility repair vehicles with much stopping and starting and limited daily ranges, high-cost hot-rod sports cars, and small commuter cars. In all of these transportation applications, low cost and long life are essential for commercial success. Neither can be attained with the present chemical storage battery and capacitor charge storage systems. An application demanding even lower costs and higher reliability is in load-leveling and central back-up. A typical daily
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