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TOPIC 1: OVERVIEW OF POWER TECHNOLOGIES 1 Three presentations were made on this topic, by George Blomgren of Blomgren Consulting Services, Ltd., John Miller of IME, Inc., and Daniel Steingart, a Ph.D. student at the University of California, Berkeley. Their papers are summarized below. THE INFLUENCE OF NANO MATERIALS ON ENERGY STORAGE DEVICES George Blomgren focused on the near- to medium-term opportunities to improve high-energy rechargeable batteries such as lithium-ion (Li-ion) and nickel metal hydride (NiMH), though he briefly acldressed the long-term potential of biofuel cells at the end of his talk. NiMH batteries are currently the standard battery used in hybrid electric vehicles; they offer a specific energy capacity (energy capacity per unit weight) of about 50 Wh/kg and specific power (power per unit weight) up to 1,100 W/kg. The technology is relatively mature, though modest near- to medium-term increases in discharge rate capability are likely, mainly through improvements to engineering design rather than new materials or nanostructures. Li-ion batteries, already widely used in electronics, have specific energies of around 160 Wh/kg and are expected to show more substantial near- and mid-term increases in capacity, rate capability, and stability. Discontinuous improvements in performance are expected to result from the introduction of new materials, including negative electrode materials (especially metals that alloy with Li), positive electrode materials (including materials that are more chemically stable than Co+4 materials with higher capacity), and new electrolyte salts. Chemical stability is critical not only for safety but also for reliability and service life as one proceeds to smaller and smaller particle sizes. Li-ion microbatteries are already well advanced in the lab, and printed batteries using vapor- deposited materials are becoming of interest for small or flexible cells. In the long term, nanoscale Li-ion batteries could be integrated with devices using templated materialsetched substrates and various deposition methods or cells utilizing colloidal-scale self-organization of components. Also in the long term, biofue! cells powered! by sugar solutions for example, using one enzyme- coatec] electrode to oxidize glucose and another enzyme-coated electrode to reduce oxygencould have a revolutionary effect on small, integrated power sources. 3 .
1 4 Summary of the Power Systems Workshop ELECTROCHEMICAL CAPACITOR TECHNOLOGY John Miller (liscussecl electrochemical capacitors (ECs), often caller] "supercapacitors" or "ultracapacitors." These have the highest energy density of all capacitor types, approaching 20 Wh/kg. So far, memory backup has been the most common application, but new applications include communications, transportation (they are ideal for regenerative braking in electric vehicles), and power quality. They rely on clouble-layer charge storage at the electrode/electrolyte interface. The most promising ECs are "asymmetric" ECs that feature one battery electrode and one charge storage electrode. This asymmetric arrangement results in approximately a twofold increase in capacitance and higher cell voltages. For many engineering applications, the best solution may be a combination of a battery and an EC; this enables one to exploit the high energy capacity of a battery and the high power density of an EC. ECs use porous electrodes that maximize charge storage, such as activated carbon. This gives them a different frequency response from other types of capacitors. Multiwall carbon nanotubes (MWNT) show great promise as an electrode material, and preliminary experiments indicate that MWNT electrodes have the fastest frequency response of any material yet tested. Among other benefits, use of MWNT allows enhanced charge storage, and the electrode pore volume and surface area can be decoupled (as is not the case for activated carbon), which enables greater control over performance characteristics. In the near term (<3 years), asymmetric ECs such as the PbO2/H2SO4/C system should] be able to achieve 20 Wh/kg with significant cost reductions. In the long term (>3 years), improved design as well as new materials such as MWNT should enable energy densities of 28 Wh/kg and optimization for many different power levels/profiles. MICRO POWER SYSTEMS OVERVIEW Dan Steingart noted that current wireless two-way sensors/transceivers have dimensions on the ogler of cubic centimeters and that batteries take up 90 percent of this volume. Primary batteries are not practical given the application area of most sensor systems. Steingart reviewed a variety of energy reservoir options (batteries, fuel cells, capacitors, etc.) ant! energy-scavenging options (solar, temperature gradients, human power, vibrations, etc.) for supplying power to sensor networks. The most appropriate power source clepends upon the nature of the task anti the area of deployment. There are two main paradigms for wireless architectures: modular and monolithic. In modular systems, off-the-shelf technology components are fabricated together on one small printed circuit board; this provides software flexibility at the cost of higher energy consumption. An example of where a modular design would be most appropriate is a sensor net intended to gather audio and/or visual information with 10-100 nodes, some fixed and some moving, with variable assignments that may change during the time of interest. In this case, the power source might be a micro fuel cell or micro heat engine, since audiovisual applications require much energy and a high-bandwidth transmitter. Monolithic systems feature an integrated design baser! only on the specific functionality of the crevice; this results in less flexibility but lower energy consumption. An example of where a monolithic design wouIcl be most appropriate is a sensor network intended for low-frequency measurement of simple quantities (light levels, temperature, etc.) over long durations with thousands of fixed and piggybacked mobile nobles. In this case, the extra design time for a monolithic architecture is worth the extra durability in the field, ant} the power source might be an energy scavenger coupled with an ultracapacitor or m~crobattery. The scavenging system can be chosen based on the relevant environments, and the energy storage system can be matched to the amount of energy scavenging available.
Topic 1: Overview of Power Technologies 5 1 TOPIC 1 DISCUSSION The first topic of discussion was the relationship between the structure and properties of carbon nanotubes used for electrodes. Only the external surface of the nanotubes is available for exploitation; the internal surface is not in contact with the electrolyte ant] is therefore unreactive. A MWNT structure containing about five concentric walls appears to offer optimum properties because one can etch the surface to get two, three, and four-wall edges that yield a high external surface area with access to stored energy at very short times. For MWNT, a capacitance of approximately 15 ,uF/cm2 can be achieved. Capacitance results for single-wall carbon nanotubes (SWNT) have been publisher} recently. The discussion then turned to the question of what is likely to come next after the Li-ion battery system. It was noted that there are no lighter metals than lithium, and the eventual fruit of work on Mg systems is hard to predict. The Li-FeS2 system has been studied for 40 years and is not yet viable; one must achieve reasonable power and energy densities simultaneously. The NRC has a parallel committee working on soldier power, where developments in Zn-air and Mn-air are being examined (potentially useful in soldier applications because air is free). Indeed the next battery may be a fuel cell. It was noted that some failed battery chemistries might turn out to be viable in hybrid capacitor systems. One pane] member commented that the energy density of batteries is one or two orders of magnitude less than that of liquid fuels such as kerosene. The reason for this is that most batteries inclucle the weight of metal casings and electrolyte; also, batteries can be made reversible, a feature not offered by liquid fuels. A more global response is that this is not a valid comparison, because if the goal is to produce electricity, as batteries do, one must look at the entire system. Conversion of liquid fuels to electricity involves several inefficient steps that would have to be accounted for in any valid comparison. The discussion then turned to the commercialization of battery technology. The commercial marketplace drives most investment in new battery technologies; the government invests a comparatively small amount. Most new battery ideas break down in the implementation stages of engineering and manufacturing- areas that are not typically funded by the government. In the case of capacitors, major government development funding was provided by the National Institute of Standards anti Technology (NIST), though much of the manufacturing has moved offshore to China and Mexico. ~ industry, applications always drive the R&D. If one did not have to manufacture a battery system for industry but was willing to invest significant government money in a specialty application because of the benefits to the nation, one could be more creative. To the question of where the biggest improvement might come from in battery technologies, it was replied that no rechargeable battery currently uses more than about 25 percent of its volume as active materials. In theory, one coup] build a battery in which between 50 and 75 percent of the theoretical maximum could be utilized if one could simultaneously achieve high surface area ant} high space-fi~ling. It was believed that achieving a two- or threefold improvement in performance did not depend so much on the amount of R&D money as it did on the formation of interdisciplinary teams of researchers. 1