Analysis of double-layer capacitors supplying constant power loads

Double-layer capacitors are a recent technology based on the well-known electrochemical phenomenon of extremely high capacitance/unit area in an electrode-electrolyte interface and the high surface area achievable in activated carbon fibers. Capacitances are available in the range of a few farads to a few hundred farads. The energy stored in a bank of double-layer capacitors is related to the capacitance and the square of the voltage. The voltage of the capacitor bank decreases as energy is drawn by an external load. Seventy-five percent of the initial energy stored in the capacitor bank can be utilized if the voltage is allowed to decrease to one-half of its initial value. Because very few loads can tolerate a significant deviation in terminal voltage, a DC-DC converter can be connected between the capacitor bank and load to maintain a constant load voltage as the capacitor bank voltage decreases. A numerical procedure for computing the amount of time that a capacitor bank can supply a constant power load through a DC-DC converter is presented. The effective specific energy, which is defined as the ratio of the energy delivered to the load divided by the volume of the capacitor bank, can then be calculated and utilized to compare different bank configurations for a particular load.