Operation of a thermal energy storage system

A method of operating a thermal energy storage system having:

• a gas as a working fluid;
• a first thermal reservoir having a first end and a second end, the first end of the first thermal reservoir having a temperature higher than the second end of the first thermal reservoir;
• the first thermal reservoir containing a first thermal energy storage medium for exchanging thermal energy with the gas during flow of the gas through the first thermal reservoir;
• a second thermal reservoir having a first end and a second end, the first end of the second thermal reservoir having a temperature higher than the second end of the second thermal reservoir, and the first end of the second thermal reservoir having a temperature lower than the first end of the first reservoir;
• the second thermal reservoir containing a second thermal energy storage medium for exchanging thermal energy with the gas during flow of the gas through the second thermal reservoir;
• turbomachinery connected to the first and second ends of the first and second thermal reservoirs and configured for two-way conversion between electrical and thermal energy by compression and expansion of the gas circulating through the first and second thermal reservoirs;
• and
• a drive unit and a motor driving a compressor of the turbomachinery during charging of the thermal energy storage system and a generator driven by an expander turbine of the turbomachinery during discharging of the thermal energy storage system;
• the method comprising:
• during charging, raising the temperature of the gas by motor-driven compression when the gas is flowing from the first end of the second thermal reservoir to the first end of the first thermal reservoir;
• during discharging, reducing the temperature of the gas by expansion when the gas is flowing from the first end of the first thermal reservoir to the first end of the second reservoir;
• and
• one or more of
• (i) predetermining a first target temperature for the first end of the first thermal reservoir for a specific charging cycle;
• during the specific charging cycle, repeatedly measuring the temperature of the gas after compression but before transfer of thermal energy from the gas to the first thermal energy storage medium;
• and varying a compression ratio of the turbomachinery in real time based on the temperature measurement during charging of the energy storage system for controlling the temperature of the first end of the first thermal reservoir and reaching the first target temperature;
• and
• (ii) predetermining a second target temperature for the first end of the second thermal reservoir for a specific discharging cycle;
• during the specific discharging cycle, repeatedly measuring the temperature of the gas after expansion but before transfer of thermal energy from the gas to the second thermal energy storage medium;
• and varying the compression ratio of the turbomachinery in real time based on the temperature measurement during discharging of the energy storage system for controlling the temperature of the first end of the second thermal reservoir and reaching the second target temperature.