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Energy storage is an essential element of future power systems to integrate high level of variable renewable energy resources. Earlier studies have found that energy storage can compensate for the stochastic nature of intermittent energy sources by absorbing the excessive energy when generation exceeds predicted levels and providing it back to the grid when generation levels fall short. However, earlier economic studies have shown that battery energy storage and flywheel energy storage is not economically competitive comparing to traditional generation units. An optimal control algorithm has been developed to coordinate the slow unit (having respond time greater than 1 minute) and fast energy storage unit (having response time less than 1 minute) to maximize the revenue (or minimize the total cost) of the hybrid energy storage system. The fast energy storage unit, (which can be a flywheel or battery bank) is tuned to pick up the fluctuations of regulation signal while the slow unit, (which can be a traditional generation unit or slow energy storage system) is adjusted less than once per hour to provide regulation service. Simulation models of hydro, combined cycle, and flywheel unit have been developed and implemented in MATLAB. Extensive simulations demonstrate the effectiveness of the control algorithm. The value of the algorithm has been shown from power plant wear and tear aspect and reducing system balancing reserve aspect.