This paper presents a mathematical model of an innovative offshore wind turbine with fluid power transmission. The proposed concept is a variable-speed, pitch controlled turbine which differs from conventional technology by using fluid power technology as a medium to transfer the energy from the wind. The final aim is to use several turbines to centralize electricity generation. Unlike conventional variable speed concepts, the proposed turbine comprises a passive-torque control method which allows the turbine to operate at optimal aerodynamic performance for different wind speeds. A numerical model of a single turbine is developed and time-domain simulations are used to analyze the dynamic response of the different operational parameters to a turbulent wind speed input. The results are compared with those of a reference offshore wind turbine with similar characteristics. It is shown that operation below rated wind speed with a passive control is possible for a single turbine with a better dynamic performance than the reference in terms of transmission torque. However, the efficiency of the energy transmission is reduced throughout the operational range. The addition and simulation of more turbines to the hydraulic network is necessary to determine to which extent the benefits of a centralized wind farm compensate for the relatively lower efficiency.

Thermal cycling is one of the major reasons for failure in power electronic converters. For submerged tidal turbine converters investigating this failure mode is critical in improving the reliability, and minimizing the cost of energy from tidal turbines. This paper considers a submerged tidal turbine converter which is passively cooled by seawater, and where the turbine has fixed-pitch blades. In this respect, this study is different from similar studies on wind turbine converters, which are mostly cooled by active methods, and where turbines are mostly pitch controlled. The main goal is to quantify the impact of surface waves and turbulence in tidal stream velocity on the lifetime of the converter IGBT (insulated gate bipolar transistor) modules. The lifetime model of the IGBT modules is based on the accumulation of fatigue due to thermal cycling. Results indicate that turbulence and surface waves can have a significant impact on the lifetime of the IGBT modules. Furthermore, to accelerate the speed of the lifetime calculation, this paper uses a modified approach by dividing the thermal models into low and high frequency models. The final calculated lifetime values suggest that relying on passive cooling could be adequate for the tidal converters as far as thermal cycling is concerned.

Currently, the techno-economic performance of Wave Energy Converters (WECs) is not competitive with other renewable technologies. Size optimization could make a difference. However, the impact of sizing on the techno-economic performance of WECs still remains unclear, especially when sizing of the buoy and Power Take-Off (PTO) are considered collectively. In this paper, an optimization method for the buoy and PTO sizing is proposed for a generic heaving point absorber to reduce the Levelized Cost Of Energy (LCOE). Frequency domain modeling is used to calculate the power absorption of WECs with different buoy and PTO sizes. Force constraints are used to represent the effects of PTO sizing on the absorbed power, in which the passive and reactive control strategy are considered, respectively. A preliminary economic model is established to calculate the cost of WECs. The proposed method is implemented for three realistic sea sites, and the dependence of the optimal size of WECs on wave resources and control strategies is analyzed. The results show that PTO sizing has a limited effect on the buoy size determination, while it can reduce the LCOE by 24% to 31%. Besides, the higher mean wave power density of wave resources does not necessarily correspond to the larger optimal buoy or PTO sizes, but it contributes to the lower LCOE. In addition, the optimal PTO force limit converges at around 0.4 to 0.5 times the maximum required PTO force for the corresponding sea sites. Compared with other methods, this proposed method shows a better potential in sizing and reducing LCOE.

In the pursuit of making wind energy technology more economically attractive, fluid power technology for the transmission of wind energy is being developed by several parties all over the world. This study presents a dynamic model of a fluid power transmission for the variable speed wind turbines and shows a parametric study on the dynamic behaviour below the rated wind speed. A pressure control strategy is proposed to achieve a variable speed operation of the rotor while using a fixed speed generator. The rotor of a five megawatt reference turbine is used to perform the time domain simulations. Different values of the hydraulic line length and transmission efficiency are considered for the same wind conditions and the results are compared with the response of the reference gearbox drive train. The results show that the amount of oil in the hydraulic drivetrain has an important influence on the first natural frequency of the transmission and the pipeline dynamics become more relevant for the longer transmission lines. Lowering the volumetric efficiency of the hydraulic motor leads to an additional damping of the pressure fluctuation, however, its influence is minor and unlikely to be advantageous when compared with the power loss.

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