The Industry 4.0 concept of a Digital Twin will bring many advantages for wind energy conversion systems, e.g., in condition monitoring, predictive maintenance and the optimisation of control or design parameters. A virtual replica is at the heart of a digital twin. To construct a virtual replica, appropriate modelling techniques must be selected for the turbine components. These models must be chosen with the intended use case of the digital twin in mind, finding a proper balance between the model fidelity and computational load. This review article presents an overview of the recent literature on modelling techniques for turbine aerodynamics, structure and drivetrain mechanics, the permanent magnet synchronous generator, the power electronic converter and the pitch and yaw systems. For each component, a balanced overview is given of models with varying model fidelity and computational load, ranging from simplified lumped parameter models to advanced numerical Finite Element Method (FEM)-based models. The results of the literature review are presented graphically to aid the reader in the model selection process. Based on this review, a high-level structure of a digital twin is proposed together with a virtual replica with a minimum computational load. The concept of a multi-level hierarchical virtual replica is presented.

Desalination is a well-established technology used all over the world to mitigate freshwater scarcity. Wind-powered reverse osmosis plants are one of the most promising alternatives for renewable energy desalination, particularly for coastal areas and islands. Wind energy can satisfy the high energy consumption of desalination while reducing costs and CO${}_2$ emissions. However, the mismatch between the intermittent availability of the wind resource and the desalination’s power demand makes the integration between the two technologies critical. This paper presents a review of wind-powered desalination systems, focusing on the existing topologies and technological advances. An overview of the advantages and disadvantages are analysed based on the theoretical and experimental cases available in the scientific literature. The goal of this work is to show the current status of wind-powered desalination and to present the technical challenges that need to be overcome in order to ensure a sustainable freshwater source.

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.