Abstract

Wind energy is among the most important technologies for reducing our carbon footprint in the atmosphere. An analysis of the wind energy market over the last years shows a favor for wind turbine technology with a rather low nominal speed resulting in a frequent down-regulation or limitation of the harvested wind power by the commonly used pitch control mechanism. Two reasons for this development can be named. Firstly, it is economically infeasible to size the electric generator for a very high peak power that will not show up very often during operation. Secondly, setting the target towards rather low nominal wind speeds results in a situation where the wind power plants can operate very steadily, especially in situations with rather stable wind conditions like offshore plants. Hydraulic drive trains for wind turbines have been researched in the past mainly with the goal of getting rid of the expensive mechanical transmissions between wind rotor and generator, most remarkably by the Artemis company in Edinburgh, Scotland. Also hybrid technologies with hydropneumatic accumulators for buffering energy have already been presented, e.g. by the group around Prof. Kim Stelson at the University of Minnesota. This paper deals with the situation of a wind rotor without pitch control, i.e. with fixed blades, and a rotor speed floating free with the wind speed. A system of differently sized hydraulic pumps is used in a digital fluid power manner to feed the harvested energy into a high pressure accumulator. This accumulator powers a variable displacement hydraulic motor driving a synchronous generator at grid frequency. Basic concepts and a simulation study are presented in order to get insight into the potentials of this technology. A first prototype is under construction in cooperation with a wind energy startup company.

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