In the proposed hybrid power generator approach, shown in
Fig. 1b, the electric generator is moved down to ground/sea level so
that the shaft work from the rotor is instead fed into a hydraulic
pump in the nacelle linked to a hydraulic motor at the tower base
which can driven the electric generator. Hydraulic transmission
systems are normally used as continuously variable transmissions
[35], and hence this modification reduces the mass and complexity
of the nacelle drive components, and can eliminate the necessity of
a gearbox or a PMG direct-drive system [6]. Use of a hydraulic pump
and motor is preferred to that of a pneumatic pump and motor as
the former has a much high power density. Also, the electric
generator can be located at ground/sea level for ease of maintenance
and can be driven at a high uniform rpm to help eliminate
the complexity and cost of gearbox or PMG components. Perhaps
most importantly, the relocation of the electric generator allows the
tower head weight to be substantially reduced, allowing the
structural mass of the tower and its foundations to also be reduced.