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EPRI Proprietary Licensed Material solvent in large capacitors. It offers higher power operation, but at the expense of using a toxic and flammable material. One feature common to all electrochemical capacitors is the requirement that some pressure be applied to the cell so that its electrodes remain in contact with the separator, that the electrodes are in contact with the current collectors, and that everything is wetted with electrolyte. The amount of pressure required depends on the design and electrode form. Winding pressure is typically used for type II products. External pressure plates are usually used for type I and III products. Performance Features and Limitations Power-Energy Relationships (Ragone Plots) A convenient way to compare various energy storage technologies is to use so called Ragone plots. These plots show energy available (work performed) as a function of power level. Relative units for energy and power are used such as specific energy in Wh/kg or power density in kW/ liter. At low power levels essentially all of the energy is available to perform work. Less energy is available as the power level increases, until a maximum power value is reached. This behavior is typical for all sources of energy and is therefore useful for comparison purposes. For example, a horse that is walking (at low power output) can likely perform more work until fatigue than one that is running (at high power output). This behavior is true for capacitors. More energy is released at slow discharge rates than at faster rates. Losses increase and efficiency drops off significantly at high rates thus reducing the amount of energy that can be delivered in any particular application. The Ragone plots are particularly useful for matching application requirement with various energy storage technologies. When using Ragone plots it is important to keep in mind that attributes other than power and energy such as cycle life, self-discharge rate, operational life and safety are not considered. These other factors may also be key to selection of the best technology for a particular application. Ragone Plots for Available Capacitors Electrochemical Capacitors 14
EPRI Proprietary Licensed Material There are many ways to compare capacitor products. One way is to examine their powerenergy behavior. Figure 2 shows Ragone plots of several large electrochemical capacitors available as commercial products or as fully packaged prototype products. Most of the devices were tested as single cells. However, the ELIT was tested as a multicell module rated at 290Vdc. The operating voltage window was from rated voltage Vr to one-half rated voltage, which represents 75% of the stored energy in an ideal capacitor. 100.00 Specific Energy (kJ/kg) 10.00 1.00 Maxwell 2500F NESS 5000F 0.10 Panasonic 2000F SAFT 3200F Elit 290V (sym) ESMA 130000F Montena 2600F 0.01 0.01 0.10 1.00 10.00 Specific Power (kW/kg) Notes: 1. Calculated using equivalent circuit models and voltage window of V r to 0.5 V r 2. Module and cell voltages vary, Elit 290 V, ESMA 1.6V, others are rated at 2.3 - 2.7 V 3. Montena has since been acquired by Maxwell Figure 2 Energy and power relationships for several large electrochemical capacitors (i.e., Ragone plots) As shown in Figure 2, at low power levels different capacitors types tend to group, depending on their design. For instance, energy performance at low power of the type II capacitors are all approximately 10 kJ/kg (3 – 4 Wh/kg). The type I (Elit) capacitor is at a lower energy value of ~1 kJ/kg (~0.3 Wh/kg). The type III (ESMA) capacitor is at a higher level ~35 kJ/kg (~10 Wh/kg). This type III is a “traction” type capacitor, which has been optimized for, high-energy density applications. Note that the type I capacitor is rated at 290V and is comprised of hundreds of cells connected in series. This product uses bipolar construction as opposed to individual cell construction. The voltages of individual cells in the series stack have been de-rated to allow for the unique high voltage module. On the other hand, capacitor power performance is not well grouped, but widely spread. For the type II capacitors, this suggests that these commercial devices have different types of carbons with different electrode thickness. The electrolyte for all of these type II capacitors is believed to be acetonitrile based. Even with the larger mass and volume required to achieve the higher voltage rating, the type I capacitor shows good specific power and power density, albeit at lower energy density and specific energy. The asymmetric capacitor design can offer energy density advantages over symmetric designs, as explained under Operating Principles below. Another advantage of an Electrochemical Capacitors 15
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