Hydraulic drive system for construction machine

A hydraulic drive system for a construction machine including a prime mover, a main pump of the variable displacement type driven by the prime mover, a plurality of actuators driven by hydraulic fluid delivered from the main pump, a plurality of flow control valves that respectively control the flow of the hydraulic fluid supplied from the main pump to the actuators, and a pump control device that performs load sensing control for the delivery flow rate of the main pump so that delivery pressure of the main pump becomes higher than maximum load pressure of the actuators by target differential pressure, comprising:

• a hydraulic motor arranged in a control hydraulic line connecting a hydraulic fluid supply line for supplying the hydraulic fluid from the main pump to the flow control valves, to a tank, the hydraulic motor being drivable by the hydraulic fluid delivered from the main pump;
• a generator connected with the rotating shaft of the hydraulic motor;
• a control device that performs power generation control of the generator so that the delivery pressure of the main pump becomes higher than a target control pressure determined by adding a preset value to the maximum load pressure with the rotation of the hydraulic motor;
• and
• an electricity storage device that stores electric power generated by the generator.