Method of operating thermal energy storage system

A method of operating a thermal energy storage system comprising cold storage, hot storage, a heating system, and a heat transfer fluid, the method comprising:

• operating a pump to circulate the heat transfer fluid from the cold storage through the heating system to the hot storage;
• supplying electric power from an electric power grid external to the thermal energy storage system to power an electric heater in the heating system that heats the heat transfer fluid as it circulates through the heating system to the hot storage;
• regulating a flow rate of the heat transfer fluid through the heating system so that the heat transfer fluid enters the hot storage at a specified temperature;
• and
• regulating the supplying of electric power from the electric power grid to the electric heater to balance supply of and demand for power on the electric power grid and maintain a frequency, a voltage, or a frequency and a voltage of electric power on the electric power grid at specified values.