This study highlights an attempt of comparing the performance of several energy storage (ES) devices like battery ES, flywheel ES, capacitive ES, superconducting magnetic ES, ultra-capacitors and redox flow battery (RFB) in automatic generation control under bilateral deregulated scenario. The considered system comprises gas and thermal generations wherein a geothermal power plant (GTPP) is also incorporated. Gas and thermal systems are provided with appropriate generation rate constraints. A new fractional order (FO) cascade controller named as FO proportional-integral–FO proportional-derivative (FOPI–FOPD) is proposed as secondary controller and its performance is compared with commonly used classical controllers. A novel stochastic algorithm, sine cosine algorithm, has been used to optimise controller gains and other parameters. Analyses of dynamic responses reveal the superiority of FOPI–FOPD over others in terms of settling time, peak deviation and magnitude of oscillation. The effect due to GTPP introduction has been examined and the responses disclose that the integration of GTPP leads to better dynamics. Performances of various ES devices in the presence of FOPI–FOPD controller are also compared and dynamic responses of RFB found superior to others. For a more realistic scenario, analysis is done considering time delay and some practical plants of Nevada, USA. © The Institution of Engineering and Technology 2018