Intermittent nature and variable power levels of renewable energy sources requires their integration with energy storage systems. One of the energy storage systems is the hydro-pneumatic system with the hydraulic pump/motor unit and hydro-pneumatic accumulator. The paper deals with the mathematical modeling, design and simulation of such a system. The bond-graph model of the system is given, which leads to the control oriented mathematical model ready for computer simulation. The model includes thermal effects of the system. Simulation analysis of the model shows its functionality.