On the Application of Fluid Power Transmission in Offshore Wind Turbines

Offshore wind energy is currently characterized by the high costs associated with installation and operation. Gearboxes in particular have been singled out as a key source of the high maintenance costs of offshore wind farms. For a given wind speed, the torque of the rotor increases cubically with the diameter of its swept area. As the maximum size of wind turbines continues to increase, mass reduction and reliability are of growing importance for the system’s economy. In any industry where robust machinery is required to handle large torques, hydraulic drive systems are applied. It is therefore almost the obvious solution for wind turbines. The main components of a fluid power drive train are (1) a positive displacement pump, which transforms the mechanical power of the aerodynamic rotor into a high pressure fluid flow, and (2) a hydraulic motor, which converts the hydraulic power back into mechanical power. The research presented in this thesis is centered around the questions of whether and how the application of fluid power technology is feasible as an alternative to currently applied drive train technologies for offshore wind turbines. The approach is to define several possible configurations of the fluid power transmission system. From these, a concept using seawater hydraulics for centralized electricity production within an offshore wind farm is subjected to further analysis. Through research, modelling and experiments, the feasibility of this concept, known as the Delft Offshore Turbine is analyzed. To make offshore wind a competitive source of electricity requires more than incrementally improving and scaling-up onshore turbines. The concept for power transmission as presented in this thesis is shown to be technically feasible and will significantly reduce the complexity of offshore wind energy technology. A way to further prove the functionality and demonstrate the possible use of such a drive train is by building and testing it, preferably in a real turbine, offshore.Hydraulic EngineeringCivil Engineering and Geoscience

Influence of the rotor nacelle assembly mass on the design of monopile foundations

In light of the developments of the offshore wind industry in terms of water depth and turbine size, the objective of the research presented in this paper is to gain insight in the applicability limits of the monopile support structure for large offshore wind turbines. This is done by demonstrating how the mass of the rotor nacelle assembly (RNA) of a turbine influences the design of monopile support structures. A fictitious 5MW class wind turbine with 126 m rotor diameter is used as reference case. The typical RNA mass of existing turbines in this class is around 400 tons. Here, the RNA mass is varied between 100 and 750 tons. For each variation, a design of the monopile is created with a first natural frequency of 0.29 Hz. The results are given in terms of mass, pile diameter and soil penetration depth for water depths of 30m and 50m. These are projected against the current industry limits for the production of monopiles and hoisting capacity on installation vessels. Furthermore, it is shown above which prestress of the RNA mass the size of the support structure is significantly influenced. The combined results substantiate that the monopile will remain the choice support structure type in coming years and that RNA mass reduction leads to significant economic gain for wind farm developers. Additionally, a solution which offers further perspective to the application of the monopile is briefly discussed.Hydraulic EngineeringCivil Engineering and Geoscience

Conceptual design of a DOT farm generator station

The Delft Offshore Turbine (DOT) is a DUWIND research project that focuses on reducing the cost of offshore wind energy by bringing a radical change in offshore wind turbine technology. The main concept is to centralize electricity generation by having individual wind turbines create a flow of pressurized seawater to a hydropower station. The idea behind the DOT is that the high power to weight ratio from hydraulic drive systems gives the opportunity for a reduced nacelle mass and increased reliability of components by eliminating the use of individual gear trains, generators and power electronics. Therefore, the ultimate goal of this project is not only to suggest an efficient way of exploiting offshore wind but to present a cost efficient assembly. The development of the hydraulic drive train of the individual turbines has been studied over the last 3 years. This paper builds on these results and shows the working of these systems on a wind farm level. The model is built up for a North Sea site with 5MW DOT turbines with a total installed capacity of 1GW. By investigating hydro turbines, the central hydro power station is designed and detailed in this paper.Hydraulic EngineeringCivil Engineering and Geoscience

Dynamic modeling of fluid power transmissions for wind turbines

Fluid power transmission for wind turbines is quietly gaining more ground/interest. The principle of the various concepts presented so far is to convert aerodynamic torque of the rotor blades into a pressurized fluid flow by means of a positive displacement pump. At the other end of the fluid power circuit, the pressurized flow is converted back to torque and speed by a hydraulic motor. The main advantage of a hydrostatic transmission over geared and direct drive systems is the possibility to vary the transmission ratio. Thus it is possible to operate with variable rotor speed (required for maximum energy extraction), whilst using a synchronous generator directly coupled to a grid, thereby eliminating the need for an AC frequency converter and a voltage transformer. Furthermore, hydraulic drives not only have a higher power density than electrical drives, but their use also allows for alternative arrangements of components. This provides the opportunity to significantly reduce the nacelle mass. Previous publications on fluid power transmissions for wind turbines have mostly been focused on the energy efficiency of the system, based on steady state simulations and mea- surements. Little has been mentioned about the dynamic behavior, especially regarding the inherent damping characteristics of fluid power transmissions. This paper presents a theoretical model of the fluid power transmission and the analysis of the influence of the main design parameters on the dynamic behavior of the system.Hydraulic EngineeringCivil Engineering and Geoscience

Wind tunnel experiments to prove a hydraulic passive torque control concept for variable speed wind turbines

In this paper the results are presented of experiments to prove an innovative concept for passive torque control of variable speed wind turbines using fluid power technology. It is demonstrated that by correctly configuring the hydraulic drive train, the wind turbine rotor operates at or near maximum aerodynamic efficiency for below rated wind speeds. The experiments with a small horizontal-axis wind turbine rotor, coupled to a hydraulic circuit, were conducted at the Open Jet Facility of the Delft University of Technology. In theory, the placement of a nozzle at the end of the hydraulic circuit causes the pressure and hence the rotor torque to increase quadratically with flow speed and hence rotation speed. The rotor torque is limited by a pressure relief valve. Results from the experiments proved the functionality of this passive speed control concept. By selecting the correct nozzle outlet area the rotor operates at or near the optimum tip speed ratio.Hydraulic EngineeringCivil Engineering and Geoscience

Wind tunnel experiments to prove a hydraulic passive rotor speed control concept for variable speed wind turbines (poster)

As alternative to geared and direct drive solutions, fluid power drive trains are being developed by several institutions around the world. The common configuration is where the wind turbine rotor is coupled to a hydraulic pump. The pump is connected through a high pressure line to a hydraulic motor and (synchronous) generator. However, in the concept presented here the high pressure line connects the pump to a nozzle. The nozzle converts the high pressure/low speed flow into low pressure/high speed jet, i.e. hydrostatic to hydrodynamic power. Converting the power in the jet to electricity can be done using an impulse hydro turbine, such as a Pelton turbine. However, this was beyond the scope of these experiments.Hydraulic EngineeringCivil Engineering and Geoscience

Dynamic Analysis of Fluid Power Drive-trains for Variable Speed Wind Turbines: A Parameter Study

In the pursuit of making wind energy technology more economically attractive, the application of fluid power technology for the transmission of wind energy is being developed by several parties all over the world. This paper presents a dynamic model of a fluid power transmission for variable speed wind turbines and shows a parametric study on the dynamic behaviour below rated wind speed. A pressure control strategy is proposed to achieve a variable speed operation. The rotor of the NREL 5 MW reference turbine is used to perform time domain simulations. Different values of the hydraulic line length, transmission efficiency and rotor mass moment of inertia are considered for the same wind conditions. The results show that the amount of oil in the system has a relatively large influence in pressure transients and controllability. Lowering the volumetric efficiency of the hydraulic motor leads to more damping of this pressure fluctuation, however its influence is minor and unlikely to be advantageous when compared to the power loss. A higher rotor mass moment of inertia implies a slower but smoother response of the system.Hydraulic EngineeringCivil Engineering and Geoscience

MicroDOT: Design of a 10W prototype of the Delft offshore turbine

The Delft Offshore Turbine (DOT) project is a research project within DUWIND. The main objective of the project is to reduce the costs of offshore wind energy through technical solutions. A defining characteristic of the DOT concept is the fluid power transmission. The next step for the DOT concept is to make a full preliminary design. A helpful tool to assist in this process of making a full scale preliminary design is to run experiments with a small scale prototype of the fluid power transmission, the MicroDOT. The aim of the MicroDOT project is to design and construct a fluid power transmission prototype of approximately 10kW. The focus of this paper is the design process of the prototype. During the design process the best options for the main components within the trans- mission are selected first. Then the components are sized such that the wind turbine rotor operates at its optimal tip speed ratio. This is where the rotor extracts energy from the wind most efficiently. Finally, a dynamic response analysis is carried out by making a model of the transmission design. The results from this analysis show that the damping of the system is very large. Even when the transmission is excited at its natural frequencies no problems are expected with the transmission, due to this large damping.Hydraulic EngineeringCivil Engineering and Geoscience

Extremum Seeking Control for optimization of a feed-forward Pelton turbine speed controller in a fixed-displacement hydraulic wind turbine concept

With the sustained drive towards higher power ratings for offshore wind turbines, the size of the turbine rotor and drivetrain components scale accordingly. Compact hydraulic transmissions are widely applied in high-load systems and form a business case for application in multi-megawatt offshore turbines. The Delft Offshore Turbine (DOT) is a hydraulic wind turbine concept replacing conventional drivetrain components with a single seawater pump. In the DOT concept, pressurized seawater is directed to a Pelton turbine-generator combination, located at a central electricity generation platform. An in-field test campaign is performed using a prototype DOT turbine with a retrofitted 500 kW hydraulic drivetrain, consisting of fixed-displacement components. As a result of this configuration, a feed-forward Pelton speed controller is derived and implemented for operating the Pelton turbine at maximum efficiency. However, the controller tuning is based on estimations of physical system properties, of which the resulting optimality is unknown. For verification of the implementation, the model-free, gradient-based and data-driven Extremum Seeking Control (ESC) optimization scheme is employed. Results show fast convergence of the algorithm and an average maximum power increase of 3 %. The algorithm is well suited for application to real-world systems, due to its simplicity and ease of tuning.Team Jan-Willem van Wingerde

Control design, implementation, and evaluation for an in-field 500kW wind turbine with a fixed-displacement hydraulic drivetrain

The business case for compact hydraulic wind turbine drivetrains is becoming ever stronger, as offshore wind turbines are getting larger in terms of size and power output. Hydraulic transmissions are generally employed in high-load systems and form an opportunity for application in multi-megawatt turbines. The Delft Offshore Turbine (DOT) is a hydraulic wind turbine concept replacing conventional drivetrain components with a single seawater pump. Pressurized seawater is directed to a combined Pelton turbine connected to an electrical generator on a central multi-megawatt electricity generation platform. This paper presents the control design, implementation, and evaluation for an intermediate version of the ideal DOT concept: an in-field 500 kW hydraulic wind turbine. It is shown that the overall drivetrain efficiency and controllability are increased by operating the rotor at maximum rotor torque in the below-rated region using a passive torque control strategy. An active valve control scheme is employed and evaluated in near-rated conditions.Team Jan-Willem van Wingerde