Load leveling using EDLCs under PLL control

The power demand has been increasing year after year in Japan and a steep projection appears during daily peak-load periods particularly in high summer due to growing demand for space cooling, resulting in the problem of a great day versus night demand gap. Such a demand gap appearing daily and seasonally is a priority for power utilities to ensure reduced service costs and dependable power supply. The study was intended to demonstrate the feasibility of an energy storage system for load leveling based on the electric double-layer capacitor (EDLC), which is safer, has a longer service life and needs far less maintenance service than the secondary cell. The system as a whole works to store surplus energy from a commercial AC line in an EDLC bank during the nighttime and release such energy for use during the daytime peak-load period through a novel interface circuit. This paper particularly reports on the working principle and experimental results of the interface circuit, which comprises a voltage control oscillator (VCO), a bi-directional DC/DC converter, a bi-directional inverter, a coupling inductor, the whole circuit being subjected to PLL control, so that automatic transfer of energy between DC from an EDLC bank and AC from a commercial power line is allowed to take place in a simple, reliable and less costly manner on the basis of DC voltage as if electric charging and discharging take place in a full DC system.