Double layer capacitor/DC-DC converter system applied to constant power loads

Abstract

Ultracapacitors or double layer capacitors are a 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. Capacitors have been tested with a rated capacitance value of 470 F and a rated voltage of 2.3 V. Test voltages as high as 3 V (30% above rated) have been used without any short term effect on measured capacitance. At 3 V the total energy storage capacity of one capacitor is 2100 Joules. With a total volume of 245 cm/sup 3/, the specific energy of this capacitor is 8.5 J/cm/sup 3/. To tap this entire energy store would require running the capacitor to zero voltage. Of course, few loads to which a capacitor bank might be connected can tolerate any drop in input voltage. To remedy this problem a DC/DC converter between the capacitor bank and load is proposed. This paper describes optimization of capacitor bank configurations when supplying a constant power load through a DC/DC converter.