If one considers the case where the tower is unchanged but the hydraulic-electric design allows a head weight equal to either 50% or 67% of the conventional head weight, the first tower mode frequency (f1) is increased to 0.40 Hz or 0.47 Hz, respectively. This increase in frequency indicates that the baseline tower might be over-designed once the head mass is reduced, which provides an opportunity to reduce the tower structural mass with proper design. This mass reduction can be achieved in various approaches: I) by reducing D with a fixed t, II) by reducing t with a fixed D, or III) reducing D and t with a fixed D/t. As shown in Fig. 3, all three mass reduction approaches show that an increased savings in tower mass (generally desirable) results in a reduction of tower frequency (generally undesirable), and the degree of sensitivity depends on which approach is employed. Proper design can be achieved by forcing the hydraulic-electric design to have the same tower frequencies as the original case, (while keeping the rotational frequency of the rotor and its third multiple are unchanged). In particular, the 1.4 safety factor is preserved such that the new tower natural frequency is the same as for the conventional tower. Based on Eq. (3a), the reduced head mass should then be accompanied by a reduced tower stiffness. As shown in Fig. 3, fixing D and reducing t is the most effective approach to reducing tower mass for a given reduction in head mass. When the head weight can be reduced by 50%, this case leads to a tower mass savings of also about 50%, which is very substantial. This indicates great potential for tower mass saving using a hydraulic-electric design.
If one considers the case where the tower is unchanged but the hydraulic-electric design allows a head weight equal to either 50% or 67% of the conventional head weight, the first tower mode frequency (f1) is increased to 0.40 Hz or 0.47 Hz, respectively. This increase in frequency indicates that the baseline tower might be over-designed once the head mass is reduced, which provides an opportunity to reduce the tower structural mass with proper design. This mass reduction can be achieved in various approaches: I) by reducing D with a fixed t, II) by reducing t with a fixed D, or III) reducing D and t with a fixed D/t. As shown in Fig. 3, all three mass reduction approaches show that an increased savings in tower mass (generally desirable) results in a reduction of tower frequency (generally undesirable), and the degree of sensitivity depends on which approach is employed. Proper design can be achieved by forcing the hydraulic-electric design to have the same tower frequencies as the original case, (while keeping the rotational frequency of the rotor and its third multiple are unchanged). In particular, the 1.4 safety factor is preserved such that the new tower natural frequency is the same as for the conventional tower. Based on Eq. (3a), the reduced head mass should then be accompanied by a reduced tower stiffness. As shown in Fig. 3, fixing D and reducing t is the most effective approach to reducing tower mass for a given reduction in head mass. When the head weight can be reduced by 50%, this case leads to a tower mass savings of also about 50%, which is very substantial. This indicates great potential for tower mass saving using a hydraulic-electric design.
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