At present, energy-efficient scheduling algorithm in High-Performance Computing (HPC) environment is becoming a research hotspot owning to its high operation cost and low reliability. In this paper, we investigate energy-efficient scheduling algorithm of data dependent tasks in DVS-Unable cluster system. The proposed ESTD algorithm efficiently integrates task clustering with task duplication technologies to reduce data transmission time and communication energy consumption. In order to decrease the static power of processing elements, it also uses one of the power-saving techniques in system level - Dynamic Power Management on the premise that application execution is non-preemptive and predictive. ESTD algorithm not only optimizes the makespan of task dependent graph, but also confines its energy consumption into a certain extent. Compared with EAD algorithm and PEBD algorithm, experimental results and case studies demonstrate that ESTD algorithm can reduce more energy consumption while not affecting scheduling performance of applications.