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EPRI Proprietary Licensed Material gas, eventually leading to dry-out and failure. Hermetically sealed packages may swell as the pressure rises and the package can eventually rupture causing a catastrophic loss of electrolyte and failure of the cell, usually as an open circuit. Before total failure these conditions may cause additional voltage stress on the remaining cells and lead to unusual performance of the series string. Voltage de-rating, decreasing the average cell voltage in the string, is often applied as an effective way to avoid cell overvoltages. That is, the average voltage, V ave , on each cell in the string must be below its maximum allowable value. This means the number of cells connected in series need to operate at voltage V must be greater than V/V ave . As described, this will prevent a single cell in the string from reaching the maximum voltage and causing problems. The resultant effect is lower power (more cells in series means higher series resistance), less energy storage (more cells means less capacitance), and higher cost. The type III asymmetric are more tolerant to overvoltages. In this case the recombinant mechanism seen in some aqueous batteries 2 helps to maintain voltage balance in a series string. When the string is charged with a controlled current, cells that first reach overvoltage conditions start to evolve oxygen. They do not rise in voltage while the lower charge cells “catch up.” For healthy cells this condition continues until all of the cells reach full voltage. Provided the rate of oxygen generation is not too high compared to the rate of gas recombination, there is practically no loss of electrolyte. Therefore, type III electrochemical cells have a valuable self-leveling characteristic. Like recombinant batteries, these devices can operate at a slight overpressure and normally release no gas. Nevertheless, commercial products have a pressure release safety valve similar to that used on batteries. At higher over voltage conditions, there can be gas releases with consequential loss of electrolyte, but without damage to the cells. Because of the valve there is generally no swelling of the cells and no deterioration in performance. If overvoltage conditions continue and lead to excessive consumption of electrolyte, then the cell will fail as an open circuit due to electrolyte loss. Type III capacitors differ from type II devices with respect to cell voltage de-rating. In fact, it is undesirable to de-rate the voltage of an asymmetric aqueous capacitor. It is best to operate series-strings of such cells with average voltage equal to their rated value, which helps maintain cell voltage uniformity. Restated, in contrast with type I and II electrochemical capacitors, type III capacitors should not be de-rated when series connected to form high-voltage series strings. Cell Balancing in a Series String Series connecting a number of electrochemical capacitor cells usually requires an active or passive voltage leveling system. For example attaching a parallel string of precision resistors to help pin the voltage of each cell. Typically, resistance values are selected so that the current flowing through the resistor string is approximately ten times the current flowing through the capacitor string. With this ratio, and during static operation, the 2 There is a fundamental difference between aqueous batteries and symmetric electrochemical capacitors. Such rechargeable batteries can be subjected to conditions that might lead to over voltage, but they do not actually rise in voltage. Instead, the high voltage causes the evolution of oxygen gas at the positive electrode of the cell. The gas travels to the negative electrode and recombines to form water. This mechanism is used in recombinant lead-acid batteries as well as in sealed nickel cadmium and sealed nickel metal hydride batteries. Electrochemical Capacitors 18
EPRI Proprietary Licensed Material resistor string establishes the individual cell voltages. A disadvantage of this passive approach is high self-discharge rate, since the string of resistors will discharge the capacitor. A related approach to provide cell balance but without the self-discharge problem is to use a parallel string of Zener diodes. Such devices appear as open circuit below a specified voltage, and a short circuit above that voltage. These methods add cost and complexity to the system. There are also active approaches for balancing cell voltages where each cell is monitored. This information can be used to report over-voltage problems that may occur in series strings, or it may be used to actually control the voltage on each cell by charging or discharging individual cells in the string. Active balancing has been used with batteries and some electrochemical capacitors in the past. It is often used at the cell level, but sometimes this balancing is only needed between modules in a multi module system. Note that balancing is normally at a low level, that is a few hundred milliamps during float conditions. If dynamic cell balancing is needed for a particular application a much higher rated leveling circuit will be required for the higher currents. This may add substantial cost to the system. Nevertheless, such an approach can be effective for raising the voltage of capacitors in a string to an average value that is closer to the maximum possible value, increasing energy density and perhaps offsetting the additional cost. Temperature Variations in a Series String Even with highly uniform cells there are still potential problems when cells are connected in series that have temperature non-uniformities. If a large module is warmer at the center due to cycling or warmer at the perimeter because of environmental factors, a temperature gradient will exist and could create cell voltage imbalance. This situation is true for all electrochemical capacitor designs. The solution to this problem is to engineer the system so that every cell within the system is held to within some specified temperature tolerance. Without this consideration, cells that are from a theoretically perfect manufacturing line (no variability) still may have cell voltage balance problems when operated within a series string. Power Electronics Requirements A unique characteristic of a capacitive energy storage system is that the state of charge of the system is always known–it is determined by the voltage. This is very different from most battery storage systems. It is usual to exploit this feature when charging and discharging a capacitor. The sloping discharge of a capacitor, however, does present problems in applications that demand a constant voltage. In this case, power electronics are needed to boost the voltage of the discharging capacitor to a higher, constant value. Generally, a capacitor storage system will have very large capacitance, small inductance, and small resistance. Thus, it can act as its own filter during charge. The single limitation is that self-heating from its charging source must not create over-temperature conditions in the cells. Heat dissipation depends on the value of the ripple current, the value of the charging current, and the cell equivalent series resistance. Thus, low-cost charging sources can be employed, ones that are typically unsuited for battery charging. A practical difference between the power source used for charging a capacitor and that used for charging a battery is the power level. Charging can be much faster for a Electrochemical Capacitors 19
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