Principles and efficient implementation of charge replacement in hybrid electrical energy storage systems

Hybrid electrical energy storage systems (HEES) are comprised of multiple banks of inhomogeneous EES elements with difference characteristics. They have been proposed to achieve desired performance metrics of an ideal energy storage device, i.e., high energy capacity, high output power level, low self-discharge, low cost, and long service life. Implementation of appropriate charge management policies enables efficient storage and retrieval of the electrical energy, attaining performance metrics that are close to the respective best values across the constituent EES elements in the HEES system. This paper addresses a global charge replacement problem in HEES systems, namely, how to dynamically select a subset of the EES banks and discharge them to meet the load power demand in a prespecified amount of time. Precisely, the global charge replacement optimization problem is formulated as a mixed-integer nonlinear programming problem, and a hierarchical algorithm is presented to efficiently solve this problem. At the top level the proposed algorithm considers the complete discharging process and allocates the load demand among all available EES banks, whereas at the bottom level it employs convex optimization methods to solve for discharging currents and the voltage level of a shared charger transfer interconnect. A prototype HEES system has been built to demonstrate the energy benefits brought by HEES systems and the efficacy of the proposed charge replacement algorithm.