A simulation study by PhD candidate Antonio Jarquín Laguna suggests that hydraulic wind parks will have only a slightly lower efficiency than the electric standard.
Hydraulic windparks are attractive in their simplicity. The wind turbines involved don’t have gearboxes and generators in the top of the tower, like their all-electric counterparts. A high-pressure oil pump is enough. In the bottom of the tower, the pressure is transferred to a water pump by hydrostatic transmission. Pressure lines from several turbines converge into a central powerhouse where they drive a Pelton turbine, the workhorse from hydropower plants.
This simple concept, locally known as the Delft Offshore Turbine or DOT, cuts back the number of generators, needs no vulnerable gearboxes and reduces the amount of steel required.
The first experimental hydraulic wind turbines were built in the 1980s. One of the reports, by Jacobs Energy Research, concluded that ‘this system would not be practical to market in the small output range.’
Less efficient
The idea that hydraulic wind energy systems are less efficient than electric systems has remained. PhD candidate Antonio Jarquin Laguna challenges that view, pointing out that hydraulic drives have recently improved in efficiency as well as reliability.
An earlier study, in 2013, by Dr. Niels Diepeveen focussed on a single hydraulic turbine and calculated an overall efficiency of about 80 per cent (wind to power).
Jarquin Laguna simulated a cluster of five hydraulic wind turbines coupled to a common generator. Depending on the power transfer, the total efficiency is 68-70% or 77-81%. That is only marginally lower than the efficiency (82-84%) of an entirely electric system.
The lower option is achieved by pointing the nozzles from all five turbines at different angles at the same Pelton wheel. Combining the output of multiple turbines into one high-pressure line with a single nozzle results in a higher efficiency, but it also requires an active speed control of the rotors.
Thus far, only Mitsubishi/Vestas combination seems to be the only industry that actively develops hydraulic wind turbines. (It’s an option on their eight-megawatt version). Integration of hydraulic accumulators to minimize fluctuations in the electrical power output may convince others to go hydraulic as well, says Jarquin Laguna.
Antonio Jarquín Laguna, ‘Centralized electricity generation in offshore wind farms using hydraulic networks’, PhD supervisor Dr. Andrei Metrikine (CiTG), April 4, 2017.
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