There is currently a big thrust for integrating renewable resources to the electric grid. With increasing variable generation the need for energy storage devices has escalated. Traditional storage devices have bulky 60 Hz transformer to provide the electrical isolation from the grid. But, with the advent of advanced magnetic materials, power electronic topologies with high frequency link transformers are being researched. These systems have high power density and can be quickly dispatched for remote installations. This paper presents the design of the energy storage system consisting of the three phase rectifier and bi-directional dual active bride converter. It presents a methodology to optimize the switching frequency of the dual active bridge converter by minimizing the volume of the transformer and the total losses in the system. Frequency dependent and independent terms are aggregated and minimized over the range of switching frequency.