Charge migration efficiency optimization in hybrid electrical energy storage (HEES) systems

Electrical energy is high-quality form of energy, and thus it is beneficial to store the excessive electric energy in the electrical energy storage (EES) rather than converting into a different type of energy. Like memory devices, no single type of EES element can fulfill all the desirable requirements. Despite active research on the new EES technologies, it is not likely to have an ultimate high-efficiency, high-power/energy capacity, low-cost, and long-cycle life EES element in the near future. We propose an HEES system that consists of two or more heterogeneous EES elements, thereby realizing the advantages of each EES element while hiding their weaknesses. The HEES management problems can be broken into charge allocation into different banks of EES elements, charge replacement (i.e., discharge) from different banks of EES elements, and charge migration from one bank to another bank of EES elements. In spite of the optimal charge allocation and replacement, charge migration is mandatory to leverage the EES system efficiency. This paper is the first paper that formally describes the charge migration efficiency and its optimization. We first define the charge migration architecture and the corresponding charge migration problem. We provide a systematic solution for a single source and single destination charge migration considering the efficiency of the charger and power converter, the rate capacity effect of the storage element, the terminal voltage variation of the storage element as a function of the state of charge (SoC), and so on. Experimental results for an HEES system comprising of banks of batteries and supercapacitors demonstrate a migration efficiency improvement up to 51.3%, for supercapacitor to battery and supercapacitor to supercapacitor charge migration.