An energy extraction device and method for extracting energy from a fluctuating energy flow from a renewable energy source. A hydraulic pump is driven by a rotating shaft, driven in turn by a renewable energy source. A hydraulic motor drives a load and a high pressure manifold communicates between the pump, motor and an elastically deformable fluid retaining body. The hydraulic pump and hydraulic motor comprise working chambers displacing a volume of working fluid selectable on each cycle of working chamber volume by the control of electronic valves. The pressure in the high pressure manifold is measured and the net rate of displacement of working fluid by the hydraulic pump is selected responsive thereto to regulate the torque applied to the said rotating shaft. The net rate of displacement of working fluid by the hydraulic motor is selected to smooth the energy flow to the load. The net rate of displacement of working fluid by the hydraulic motor is selected to regulate the pressure in the high pressure manifold to remain within an acceptable pressure range. Instead of tightly controlling the pressure in the high pressure manifold, it is allowed to vary and, by allowing the pressure to vary, a smooth output can be produced with high efficiency despite fluctuations in the energy flow from the renewable energy source. The pressure is more strongly controlled at the top and/or bottom of the acceptable pressure range than closer to an optimum working pressure.