IEEE TRANSACTIONS ON POWER ELECTRONICS, VOL. 26, NO. 3, MARCH 2011 923 Composite Energy Storage System Involving Battery and Ultracapacitor With Dynamic Energy Management in Microgrid Applications Haihua Zhou, Student Member, IEEE, Tanmoy Bhattacharya, Duong Tran, Tuck Sing Terence Siew, and Ashwin M. Khambadkone, Senior Member, IEEE Abstract—Renewable-energy-based microgrids are a better way of utilizing renewable power and reduce the usage of fossil fuels. Usage of energy storage becomes mandatory when such microgrids are used to supply quality power to the loads. Microgrids have two modes of operation, namely, grid-connected and islanding modes. During islanding mode, the main responsibility of the storage is to perform energy balance. During grid-connected mode, the goal is to prevent propagation of the renewable source intermittency and load fluctuations to the grid. Energy storage of a single type cannot perform all these jobs efficiently in a renewable powered microgrid. The intermittent nature of renewable energy sources like photovoltaic (PV) demands usage of storage with high en- ergy density. At the same time, quick fluctuation of load demands storage with high power density. This paper proposes a composite energy storage system (CESS) that contains both high energy den- sity storage battery and high power density storage ultracapacitor to meet the aforementioned requirements. The proposed power converter configuration and the energy management scheme can actively distribute the power demand among the different energy storages. Results are presented to show the feasibility of the pro- posed scheme. Index Terms—Bidirectional converter, energy management, energy storage, interleaved modulation, modular design and microgrid. I. INTRODUCTION D UE to the intermittent nature of renewable energy sources and the continuous variations of the load, storage (e.g., battery, ultracapacitor, flywheel etc.) is usually needed in a re- newable powered microgrid. The renewable output power pro- file and the load profile are two important factors in deciding the capacity and type of the energy storage components. The varia- tion in output power from a utility-scale PV system is presented Manuscript received July 5, 2010; revised November 3, 2010; accepted November 8, 2010. Date of current version May 13, 2011. The work was supported by the Science And Engineering Research Council (SERC) under MODERN project Fund R-263-000-507-305. Recommended for publication by Associate Editor J. M. Guerrero. H. Zhou, T. Bhattacharya, D. Tran, and T. S. T. Siew are with the De- partment of Electrical and Computer Engineering, National University of Singapore, Engineering drive-3, Singapore 117576 (e-mail: g0500090@nus. edu.sg; eletb@nus.edu.sg; g0700253@nus.edu.sg; terence_siew@nus.edu.sg). A. M. Khambadkone is with the Department of Electrical and Computer En- gineering, National University of Singapore, Engineering drive-3, Singapore 117576, and also with the Experimental Power Grid Center, Agency for Science, Technology and Research (A*STAR), Singapore 138668 (e-mail: ashwinmk@ices.a-star.edu.sg). Digital Object Identifier 10.1109/TPEL.2010.2095040 Fig. 1. Typical 24-h (a) PV output power and (b) residential load profile. in [1], which is shown as curve (a) in Fig. 1, whereas curve (b) of Fig. 1 depicts a typical load profile. Both the PV power and load power profiles are normalized such that they have equal average power. The PV power output profile and the load pro- file shows low-frequency as well as high-frequency fluctuations, which are mutually independent in nature. Hourly average vari- ations can be considered as low-frequency variation, whereas power transients, which sustain for minutes, seconds, or mil- liseconds come under the high-frequency segment. To buffer out the low-frequency oscillations and to compensate for the intermittency of the renewable energy sources, energy storage with high energy density is required. To provide high-frequency component of power and also to supply or absorb the high-power transients, energy storage with high power density is required. Fig. 2 shows the energy density and power density profiles of different energy storages, whereas a general theory of Ragone plot is provided in [2]. It is to be noted that, the load profile and renewable source profile strictly decides the desired location of the optimum energy storage on the Ragone chart and this lo- cation will be different for different microgrids. If we use only battery as storage, then it has to be oversized to take care of the peak load demand. If we use only ultracapacitor, then it has to be oversized for storing large amount of energy to take care of the intermittency of the renewable sources and loads. Hence, use of a Composite energy storage system (CESS) comprising both high power density and high energy density storage units is practically unavoidable. Now, the selection of the type of storage is also crucial. For energy storage in the high power range for standard power systems, the most suitable ones would be pumped hydro storage, compressed air storage, etc. 0885-8993/$26.00 © 2010 IEEE