This paper highlights an attempt of comparing the performance of several energy storage (ES) devices such as battery ES, flywheel ES, capacitive ES, superconducting magnetic ES, ultra-capacitors, and redox flow batteries (RFBs) in automatic generation control of an interconnected system. The considered system comprises conventional thermal, hydro, wind, and solar photovoltaic generations wherein a geothermal power plant (GTPP) is also incorporated. The thermal and hydro systems are provided with appropriate generation rate constraints. A new fractional order (FO) cascade controller named as the FO proportional-integral-FO proportional-integral-derivative (FOPI-FOPID) is proposed as a secondary controller, and its performance is compared with the commonly used classical controllers. A powerful stochastic algorithm called the Sine Cosine Algorithm has been used to optimize the controller gains and other parameters. Analyses of the dynamic responses reveal the superiority of FOPI-FOPID over the others in terms of settling time, peak deviation, and magnitude of oscillation. The effect due to introduction of GTPP has been examined, and the responses disclose that integration of GTPP leads to better dynamics. The comparison of performances of various ES devices in the presence of the FOPI-FOPID controller highlights the predominance of RFB over others. © 2018 Author(s).