During off-peak operation of a power plant operating on a thermodynamic cycle wherein heat is rejected to an ambient fluid, heat is removed from a cold temperature storage medium. The signal supplied by the sensor is used as an alarm signal for the processing means. The filter banks are connected in such a way that signals processed through the first filter are processed by the first 2-demultiplexing filter bank in a next-stage; signals processed through the second filter are processed by the second 2-demultiplexing filter bank in a next-stage.