The intermittence and uncertainty of the renewable source have complicated the integration of the renewable sources in a microgrid scenario, therefore the inclusion of ESS is required in order to improve the performance of the microgrid, to avoid the utility grid overload, to reduce operating cost and more.
I suggest you to read topic related to microgrid, distributed generation and energy management.
F. Katiraei, R. Iravani, N. Hatziargyriou, and A. Dimeas, “Microgrids management,” IEEE Power Energy Mag., vol. 6, no. 3, pp. 54–65, 2008.
H. Jiayi, J. Chuanwen, and X. Rong, “A review on distributed energy resources and MicroGrid,” Renew. Sustain. Energy Rev., vol. 12, no. 9, pp. 2465–2476, 2008.
R. H. Lasseter, “MicroGrids,” in IEEE Power Engineering Society Winter Meeting, 2002, vol. 1, pp. 305–308.
N. Hatziargyriou, H. Asano, R. Iravani, and C. Marnay, “Microgrids,” IEEE Power and Energy Magazine, vol. 5, no. 4, pp. 78–94, 2007.