Hydraulic-electric hybrid wind turbines: Tower mass saving and energy storage capacity

This study investigates concept of introduction of a hydraulic motor in the nacelle to convert rotor shaft work into hydraulic power that is transmitted to the electric generator at ground/sea level. This combination of hydraulic and electric power generation can help simplify or even eliminate the gearbox, and significantly reduce the head weight mass that the tower needs to support. Also, this hybrid concept allows energy storage in the tower which can reduce electric generator size. The analytical technique for tower mass savings employed herein was validated and used to show that 33%-50% of the tower mass may be saved through decreased tower thickness. In addition, the hydraulic-electric generator concept is compatible with employing isothermal CAES in the tower. Analysis based on cross-over pressure for the design limit indicates that this energy storage concept provides more than 24 h of energy storage if one considers S-glass towers of 10 MW or more. To accompany the above engineering analysis, a CAPEX cost model was developed based on recent production wind turbines and system designs. The hydraulic-electric hybrid system with CAES was estimated to yield a total CAPEX savings of 17% due to a substantial decrease in generator and electrical infrastructure costs.