Handbook of Energy Storage for Transmission or ... - W2agz.com

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Technology in the Next Ten Years
It is interesting to speculate about the future performance of electrochemical capacitors.
In the next three to five years, type II capacitors cells are predicted to achieve stable
operation at 3.0 V. This represents a significant increase in energy density over the
present products, perhaps 50% higher than is available today. With this higher operating
voltage will come increased stability, possibly increased operating temperature, and
perhaps with suitable emphasis in organic electrolyte development, creation of a nontoxic
type II electrolyte capable of high power performance.
Type III capacitors in the next several years should approach an energy density of 70
kJ/kg, which represents a 100% increase in energy density over products available today.
There could also be significant cost reductions as a result of the introduction of lower
cost designs that are described in the patent literature.
Longer term, type II capacitors will probably remain fixed at 3.0 V operation because
further increases in electrolyte and electrode purity will become cost prohibitive.
Furthermore, emphasis by small-capacitor developers on increasing cell operating voltage
will wane since the portable electronic applications will decrease to below 3.0 V. But
improved stability at the 3-V level is anticipated, particularly at elevated temperatures.
Type IV electrochemical capacitors should become commercially available, for example
the graphite/carbon system and the lithium-titanate system. Energy densities of 100
kJ/kg may become available, which is solidly placed in the range of today’s lead acid
batteries.
Which type will become the dominant capacitive energy storage technology in the future
This is impossible to predict with any certainty. However, for applications where cost is
a major issue, the dominant technology will probably have an aqueous electrolyte. This
lowers the cost of materials as well as manufacturing processes. For instance, aqueous
electrolyte products generally do not require special conditioned space like dry rooms, or
special drying systems to remove water impurities from cells before sealing like what is
needed with the non-aqueous electrolytes. Yet another related cost issue is capacitor
packaging. Aqueous electrolyte products generally are sealed in a low-cost crimped
metal or plastic package to reduce loss of water—the design need not be highly
sophisticated. In contrast, organic electrolyte products must be hermetically sealed in a
low-permeability container like metal and often incorporate a sophisticated glass-to-metal
seal for electrical feed-through. These materials and package designs add considerable
costs to a product.
Of the aqueous electrolyte capacitors on the horizon today, type III electrochemical
capacitors offer significant performance advantages including higher energy density and
voltage balance. So, this particular design is predicted to become the dominant capacitor
technology of the future for applications where cost is a driver. Since many utility and
transportation applications are cost sensitive, type III capacitors are predicted to dominate
these markets.
It is possible to estimate future cost difference between the organic and the aqueous
electrolyte products by examining the present cost differences between lithium-ion and
nickel-metal hydride batteries. The organic electrolyte battery presently costs about
twice as much as the aqueous battery. Both of these technologies are in large-volume
production. So cost differences for the capacitor types when in large-volume production
Electrochemical Capacitors 34