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EPRI Proprietary Licensed Material asymmetric capacitor is that it can reliably operate above 1.2 V (the breakdown voltage of water) without gas evolution, even when employing an aqueous electrolyte. Operation above 1.2 V is possible because reaction kinetics for gas evolution are slow. Therefore available asymmetric capacitors products can operate at 1.4 to 1.6 Vdc for the same reason lead-acid batteries can operate at 2.05 V per cell with an aqueous electrolyte. Pulse Ragone Plots For many applications, it is useful to determine the energy delivered by a capacitor during a given discharge time. This relationship, for instance, can express the energy delivered by a capacitor during one 60 Hz cycle. In this case, the effective energy density of the capacitor has to be measured at the pulse width of one cycle. Figure 3 is pulse discharge data for several large electrochemical capacitors. The discharge is from rated voltage to 90% of rated voltage, and the pulse lengths are from very long, 100 s, down to 1 ms. This plot shows the energy per mass that can be delivered by the capacitor for different length pulses. Specific Energy (J/g) 10 1 0.1 0.01 0.001 Voltage window of Vr - 0.9 Vr Maxwell 2500F Elit 290V NESS 5000F Panasonic 2000F SAFT 3200F 0.0001 0.001 0.01 0.1 1 10 100 PulseLe ngth (s) Figure 3 Pulsed Ragone plot for several large electrochemical capacitors As shown in this figure, several of the capacitors have an effective specific energy of approximately 3 J/g for long pulse lengths. At shorter pulse lengths, for example at 1s, the effective energy drops by a factor of three or more per decade for the majority of these capacitors. The effective energy density continues to drop as the pulse length becomes shorter. This behavior is characteristic of a multiple-time-constant circuit as exists with electrochemical capacitors. The shape of the curve depends on the capacitor design. It is possible to design the capacitors for either higher long pulse length or higher short pulse length performance. It is not possible to predict the energy delivered by a capacitor at short discharge times based on total specific energy alone. Temperature Performance Electrochemical capacitors provide good operating performance over a wide range of temperatures. Upper temperature limits are generally below 85 C, depending on the product. Lower temperature limits are as low as -55 C in some products. Capacitor properties, in particular leakage current, are affected by temperature. Property changes Electrochemical Capacitors 16
EPRI Proprietary Licensed Material observed with increased temperature are fully reversible if the temperature is not excessive. Self-discharge rates increase dramatically with temperature and often establish a practical upper operating temperature limit. Correspondingly, product life decreases at high temperatures since mechanisms responsible for the leakage current are often chemical side-reactions. Such undesired chemistry in type II capacitors results from electrochemically active impurities that were originally present in the package (water, for instance), and new impurities that are created during capacitor operation due to electrolyte decomposition or arise from permeation into the package through seals. One common method to counteract the elevated leakage current levels and thus increase operating life of type I and II cells is to reduce the average voltage applied to a cell. This reduces the effective energy density of the capacitor but can substantially increase operating life. Exceptional low-temperature performance can usually be expected in all electrochemical capacitors. This is possible because, unlike batteries, reaction kinetics do not limit the charge or discharge rate of an electrochemical capacitor. Instead, the limit is usually established by the electrolyte conductivity. Thus, capacitors can operate with good performance at very low temperatures. Generally, but not always, aqueous electrolyte electrochemical capacitors (types I and III) have the least change in performance at low temperatures compared with room-temperature values. Combining Cells into Modules Unlike conventional electrostatic and electrolytic capacitors, electrochemical capacitors are inherently low voltage devices. The maximum voltage of a single cell in a commercial product is 2.7 V. Thus, to meet the 600- to 800-V requirements of a utility application, hundreds of cells are series-connected and a dc-to-dc boost converter may also be employed. Failure of just one cell in a series string can lead to failure of the entire storage system. A cell can fail as an open circuit or as a short circuit. The most common failure is an open circuit. Of course, if the failure is an open circuit, the entire system will stop working. On the other hand, if a single cell short circuits, then other cells in the string will experience higher voltage, which may stress them. This stress could lead to accelerated aging of those remaining and premature failure of another cell, and so on. Thus, one cell failure in this scenario could start a cascade situation where the entire string of cells would rapidly become a short circuit. For long life, each cell in a series-string must remain below its maximum voltage rating under all conditions, which includes charge/discharge as well as float operation. The three key parameters affecting the cell voltage are variability in capacitance, internal resistance, and leakage current. Each of these parameters can lead to voltage imbalance among cells in a string. Thus, the construction of the cell, and its normal variability, will affect the reliability of a high-voltage string. Cell Over Voltage in a Series String Preventing cell over voltage is particularly critical for type I and II symmetrical capacitors. When gas is generated due to over voltage in a symmetric electrochemical capacitor there is no means for recombination and pressure rises inside the package. Some small capacitors have crimp seals for pressure relief that can vent small amounts of Electrochemical Capacitors 17
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