Combat Hybrid Power System Component Technologies Technical Challenges and Research Priorities (2002) / Chapter Skim
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6 Capacitor Technology
6 Capacitor Technology
Pages 47-56
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From page 47... ...
capacitors. There is now something of a continuum between a film capacitor, which truly stores energy in an electrostatic field and can absorb and release energy extremely rapidly, and a true battery (e.g., lead acid)
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From page 48... ...
Advances in materials processing technology have focused on smaller ceramic grain sizes and reduced porosity in the dielectric, which has increased the operating fields to 10 kV/mm. Effective dielectric constants in the range of 1000 with low loss can be achieved in 48
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From page 49... ...
Other technologies approaching this energy density include soggy foil capacitors with a high dielectric constant polymer coating applied to the nonmetallized side of the foil. Thus, the present state of the art in high-voltage film capacitors appears to be in the range of J/cm3.
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From page 50... ...
POTENTIAL FOR HIGHER ENERGY DENSITIES High Dielectric Constant Polymer Film Capacitors As noted above, self-clearing is essential for high energy density polymer film capacitors, as it allows operation very near the breakdown field of the film. Self-clearing characteristics are a strong function of interracial pressure within the winding, which is much more uniform in a round winding than in an oval winding.
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From page 51... ...
However, if the relative dielectric constant of capacitor film can be increased to 8, in the range that can be achieved through asymmetric fluorine substitution into appropriate polymers, the required operating stress for an energy density of 2.5 J/cm at 50 percent packing efficiency would be about 375 kV/mm, which is very reasonable for a metallized film capacitor. Dielectric constants in the range of ~ are probably feasible, and if such a film couict be developed with the dielectric strength of BOPP, a packaged energy density of about 3.5 J/cm shouici be possible.
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From page 52... ...
However, as in the case of polymer film capacitors, substantial progress has been macle over the last clecade. Ceramic capacitors tend to have very high dielectric constant but relatively low (lielectric strength as a result of substantial porosity, which is typical of sistered materials.
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From page 53... ...
Understanding of high-temperature, high-stress failure mechanisms would clearly benefit, or perhaps should be a prerequisite to the development of, new materials. Double Layer Capacitors Double layer capacitors are energy storage devices that convert chemical into electrical energy and are particularly suited to providing the energy to power electrical devices.
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From page 54... ...
based on improved polypropylene clielectric. From the preceding discussion, and acknowledging that technology forecasting is extremely risky, the limits to microsecond polymer film capacitor technology appear to be in the range of 5 J/cm based on the use of yet-to-be-cleveloped high dielectric constant films at operating Relets typical of BOPP.
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From page 55... ...
TABLE 6-T Technical Challenges, Performance Metrics, and Research Priorities Associated with High Energy Density Capacitors Component/System Polymer film capacitors Technical Challenges Films with improved dielectric properties Performance Metrics Dielectric constant Dielectric withstand R&D Priorities New polymer films with increased dielectric constant and dielectric withstand similar to biaxially oriented polypropylene Filled polymer films: either inorganic filler to improve dielectric strength, high dielectric constant filler to increase dielectric constant, or high dielectric polymer filler to reduce volume within the film, resulting in a combination of increased operating field and increased dielectric constant Ceramic capacitors Double layer capacitors Lack of understanding of aging/failure mechanisms Dielectrics with improved properties Improved operating electric field Lack of understanding of aging and degradation processes at high temperature Improvement of properties of electrolytes, increase in cell voltage, and reduction of equivalent series resistance Predictability of performance over time Research on aging/failure mechanisms under hightemperature, high-field conditions Dielectric constant Dielectric withstand Operating field Cell voltage equivalent series resistance Stability of properties Research to improve high energy density, hightemperature ceramic dielectrics Ceramic-polymer composites or other technologies that reduce the free volume within the ceramic Investigate role of impurities in the carbon electrodes and interactions among the electrodes, electrolyte, and separator Materials and processes that achieve reproducible cell characteristics that are stable s5
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From page 56... ...
over Umc, or age uniquely over time Reduction of current densities Effective electrode surface area Research into materials and manubc~dug processes 1ba1 increase 1be effective surface area of electrodes 56
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