Introduction  In the context of "Dual Carbon", the demands for ancillary services of the electric power system are increasing. However, traditional thermal power units have many problems in AGC control. As a new energy storage mode, the battery energy storage has the great potential for applying in ancillary service market because of its advantages of fast response and high precision.

  Method   In this paper, the battery energy storage participating in AGC ancillary service of a coal-fired unit in Guangdong Province was taken as an example. The cost-benefit model was established to study the economic benefits of the AGC frequency control, and three aspects including the investment payback period, the net profit of life cycle and the net present value of project investment were investigated with respect to different energy storage capacity.

  Result  The net profit and the net present value of project investment increase with higher energy capacity. The increase rate of the net profit and the net present value of project investment slows down as the energy capacity changes from 17.5 MWh to 30 MWh. The investment payback period is the shortest as the energy capacity is 17.5 MWh. The investment payback period has a small increase as the energy capacity changes from 17.5 MWh to 30 MWh.

  Conclusion  For thermal power units with energy storage, when the capacity of thermal power units is configured with 3%~5% energy storage batteries, the net profit and the investment payback period of the Project can achieve a good effect, and the best effect can be achieved when 3% energy storage batteries are configured.