In this paper, a variable and smart energy storage system, consisting on a two-level voltage source converter with LCL filter and Microbial Electrosynthesis (MES) stack, is presented. This energy storage system is able to contribute to damp the variability of renewables and provide grid services, by controlling the active and reactive power exchanged with the grid and can be considered as another alternative in the market of storage normally centered in batteries. The proposed system converts electrical energy into methane, taking waste water as another input. The converted energy inside the MES presents a new degree of freedom, which can be exploited to provide ancillary services such as energy management, frequency regulation, voltage profile improvement and oscillation damping. The big challenge for controlling this system lays on the fact that the dc bus voltage of the converter has to be changed in order to regulate the exchanged active power with the grid. This paper presents a cascaded approach to control such system by means of combining extern control loops with fast inner loops. The outer power loop, with a PI controller with special limitation values and anti-windup capability, is used to generate dc bus voltage reference. An intermediate loop is used for dc bus voltage regulation and current reference generation. A new proportional resonant controller is used to track the current reference. The proposed scheme is validated through real-time simulation in OPAL OP4510 hardware in the loop system.