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EPRI Proprietary Licensed Material 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
EPRI Proprietary Licensed Material may mimic this behavior, i.e., organic electrolyte capacitors will continue to cost more than aqueous electrolyte capacitors, perhaps two-times higher. Applications Until recently, most applications for electrochemical capacitors have been in low-voltage circuits such as computer memory backup. Larger capacitor modules with advanced power electronics have expanded the possible applications of energy storage capacitors into electric power systems. Capacitors can now store enough energy to compete with batteries in many short-term energy storage applications. The best fit for capacitors is in applications requiring relatively high cycle life, high round-trip efficiency, wide operating temperature range, maintenance-free operation, quick charge, and high power. Often these short-term operations compliment other power system components such as weak feeders, small-distributed generators, fluctuating or high-inrush loads, etc. Consequently, effective application of the electrochemical capacitor is expected to help make a wide range of energy system applications more practical. In other words, there are short-term storage applications that were not technically or economically viable in the past that should now be reconsidered because of this new technology. The applications of interest use energy storage to supplement normal power delivery. One dimension of a power system application is how the delivery is supported or enhanced by using energy storage. Important electrical parameters include voltage (V), current (A), real power (W), reactive power (VA), and energy (Wh). Baseline for Applying Energy Storage The cost and performance of lead-acid batteries are well known and provide a practical baseline for comparison with other energy storage technologies. Electrochemical capacitors store less energy per unit mass (or volume) than batteries, but can deliver higher power per unit mass (or volume). This is shown graphically by the hypothetical Ragone curves shown in Figure 14. In this figure, the battery delivers more energy, per unit mass, than the capacitor for discharges longer than 15 seconds, while the capacitor delivers more energy than the battery for discharges less than 15 seconds. Electrochemical Capacitors 35
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