Applications Tide and Ocean Current Energy Converters: Going with the Flow Whether with tide or a natural ocean current: As with wind energy it is the flow that drives the rotors. Water, however, is 1,000 times denser than air and generates extremely high forces, even at low flow rates. This requires specifically thought-up concepts. Underwater rotors are able to pick up energy effectively with a diameter that is considerably smaller compared to wind turbines. Even at low speeds high forces act on the complete system. Generator gear technology, which is already known from wind energy, is one direction of development, and one in which Rexroth is firmly established as the world’s leading supplier in the renewable energy sector. Another direction that the development is taking is demonstrated in the use of hydraulic converters. This straightforward and extremely robust concept converts rotational movement into a hydraulic flow, which drives a closed-loop hydraulic motor coupled to a high efficiency generator. Following is a brief explanation. On the pump side, a slow-running hydraulic pump picks up the rotor speed, which generates a volumetric flow. This energy feeds a variable displacement, fast-running, axial piston motor, which is capable of driving the generator at grid frequency. This way the hydraulic converter can dispense with the need for costly closed-loop control electronics and frequency converters. One particular benefit is as follows: Thanks to the variable displacement options offered by the axial piston motor, the smooth adjustment of the transmission ratio not only absorbs peak de- mand, it also adapts to different current speeds, even without rotor adjustment. The hydraulic converter is thus able to achieve an optimum degree of efficiency across the complete tide cycle. Above all, this solution simplifies the overall design as the variable displacement motors pick up directly the various directions of rotation at ebb and flow with the securely fixed systems. The dispersed construction method of hydraulics furthermore reduces the number of components under water: Only the robust rotor pump for the generation of the flow is positioned directly inside the installation. The system operator can install the hydraulic motorgenerator assembly above water: This reduces maintenance, at the same time also noticeably increasing availability.