Single-Source, Single-Destination Charge Migration in Hybrid Electrical Energy Storage Systems

In spite of extensive research it is still quite expensive to store electrical energy without converting it to a different form of energy. As of today, no single type of electrical energy storage (EES) element can fulfill all the desirable features of an ideal storage device, e.g., high-efficiency, high-power/energy capacity, low-cost, and long-cycle life. A hybrid EES system (HEES) consists of two or more heterogeneous EES elements, realizing the advantages of each EES element while hiding their weaknesses. HEES systems exhibit superior performance compared with homogeneous EES systems when appropriate charge allocation and replacement policies are developed and used. In addition, charge migration is mandatory because the optimal EES banks for charge allocation and replacement are in general different, and each EES bank has limited storage capacity. This paper formally describes the notion of charge migration efficiency and its optimization. We first define the charge migration architecture and the corresponding charge migration optimization problem. We provide a systematic solution for the single-source, single-destination charge migration problem considering the efficiency variation of the converters, the rate capacity and internal power loss of the storage element, the terminal voltage variation of the storage elements as a function of their state of charge, and so on. We also introduce the optimal solutions for both the time-constrained and unconstrained versions of the charge migration problem formulations. Experimental results demonstrate significant charge migration efficiency improvement of up to 83.4%.