A method of operating a thermal energy storage device comprising a body of heat transfer fluid, said body of heat transfer fluid comprising an upper temperature region comprising heat transfer fluid having a temperature above a upper threshold temperature, a lower temperature region comprising heat transfer fluid having a temperature below a lower threshold temperature and a thermocline region separating the upper and lower temperature regions and comprising heat transfer fluid having a temperature above a lower threshold temperature and below an upper threshold temperature, wherein during charging of the thermal energy storage device, heat transfer fluid is removed from the thermocline region of the body of heat transfer fluid and when the temperature of the heat transfer fluid being removed from the thermocline region of the body of heat transfer fluid rises above a maximum temperature, said heat transfer fluid being removed is brought to a temperature equal to or below said maximum temperature by combining the heat transfer fluid being removed and having a temperature above a maximum temperature with heat transfer fluid having a temperature below the lower threshold temperature, which heat transfer fluid having a temperature below the lower threshold temperature is sourced from the lower temperature region of the body of heat transfer fluid, wherein the maximum temperature is above the lower threshold temperature and/or wherein during discharging of the thermal energy storage device, heat transfer fluid is removed from the thermocline region of the body of heat transfer fluid and when the temperature of the heat transfer fluid being removed from the thermocline region of the body of heat transfer fluid falls below a minimum temperature, said heat transfer fluid being removed is brought to a temperature equal to or above said minimum temperature by combining the heat transfer fluid being removed and preferably having a temperature below a minimum temperature with heat transfer fluid having a temperature above the upper threshold temperature, which heat transfer fluid is sourced from the upper temperature region of the body of heat transfer fluid, wherein said minimum temperature is below the upper threshold temperature. wherein during charging of the thermal energy storage device, heat transfer fluid is removed from the thermocline region of the body of heat transfer fluid and when the temperature of the heat transfer fluid being removed from the thermocline region of the body of heat transfer fluid rises above a maximum temperature, said heat transfer fluid being removed is brought to a temperature equal to or below said maximum temperature by combining the heat transfer fluid being removed and having a temperature above a maximum temperature with heat transfer fluid having a temperature below the lower threshold temperature, which heat transfer fluid having a temperature below the lower threshold temperature is sourced from the lower temperature region of the body of heat transfer fluid, wherein the maximum temperature is above the lower threshold temperature and/or wherein during discharging of the thermal energy storage device, heat transfer fluid is removed from the thermocline region of the body of heat transfer fluid and when the temperature of the heat transfer fluid being removed from the thermocline region of the body of heat transfer fluid falls below a minimum temperature, said heat transfer fluid being removed is brought to a temperature equal to or above said minimum temperature by combining the heat transfer fluid being removed and preferably having a temperature below a minimum temperature with heat transfer fluid having a temperature above the upper threshold temperature, which heat transfer fluid is sourced from the upper temperature region of the body of heat transfer fluid, wherein said minimum temperature is below the upper threshold temperature. <EOS>