A power generator for use in a unidirectional flowing fluid has a fixed part and a movable part, the movable part is mounted on the fixed part for reciprocal movement with respect to the fixed part between first and second positions. A valve element on the movable part is adapted to move between open and closed positions for relatively unimpeded and impeded flow. A valve actuation mechanism is connected to the valve element to move the valve element to the closed position when the movable element reaches the first position and to move the valve element to the open position when the movable element reaches the second position. An electrical generator is coupled to the fixed part and the moveable part and is adapted to generate electrical energy when the movable part moves between the first and second positions.