Background
In 2008 the Delft University wind energy research institute (DUWIND) launched the Delft Offshore Turbine project. The aim of this project is to re-evaluate the way in which wind energy offshore is extracted on a large scale and converted to electricity onshore.
In any industry where robust machinery is required to handle large torques, the hydraulic drive systems are a common choice. It is therefore almost the obvious solution for wind turbines.
The Delft Offshore Turbine (DOT) concept for the drive train of offshore wind turbines is to have the rotor shaft directly coupled to an oil-hydraulic pump in the nacelle. The hydraulic motor is located at the base of the turbine tower, where it is coupled to a seawater-hydraulic pump. The pressurized flow of seawater from each turbine converges to a hydro-power-like generator station where it is converted to electricity using Pelton turbines.

Objective
The approach so far has been to stepwise study, model and demonstrate the different aspects of the design. From the results it was concluded that with off-the-shelf the DOT drive train is feasible for turbines up to 1MW of rated power production.

The 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 offshore wind turbine.
In the spring of 2013 a consortium was formed to realize precisely this ambition. The plan is two-fold. Part one is the construction and testing of a DOT-type fluid power transmission system. Part two is the implementation in a secondhand turbine and installation of the whole assembly offshore on an existing foundation. In this location, the system will be tested for up to two years.

Approach
The first step in the design of the DOT demonstrator is to analyze the bearing and load capacities of the existing offshore support structure to be used. Based on these results and on available data, a variable speed turbine with pitch to feather blades is selected. The rotor of this turbine and the dimensions of the nacelle and tower are the critical inputs for the design of the fluid power transmission system.

Results
This paper presents the contemporary design of the DOT demonstrator, including specifications of the main components, the expected performance, remaining design challenges and an overview of the key steps for this project.

In the past decade wind turbines have developed into an important source of electrical energy. Due to the high ratio of wind power in the grids during strong wind conditions, requirements of the grid code have become more and more stringent. To improve the quality of the electricity produced an increasing number of systems tend to use a generator running at constant speed and are thus capable of connecting to the grid without using a frequency converter. Consequently, a constant generator speed requires the transmission, connecting the variable speed turbine shaft to the generator, to operate with a variable transmission ratio.
Different concepts using hydrostatic as well as hydrodynamic components have been developed to fulfill this requirement. These components can either transfer the entire power of the turbine or be arranged in a power split transmission with a parallel mechanical gearbox. The selection of a specific drive train not only has a large influence on power production but also on controllability and reliability. When selecting one concept, the site where the system is to be placed should also be considered.
In this paper different concepts for wind power transmissions with a variable transmission ratio are compared. The way in which each concept influences system controllability and overall efficiency is analyzed. In addition, their applicability to different types of turbines, e.g. onshore, offshore or even floating turbines will be presented. Finally, one example of a fully hydrostatic transmission is presented including all auxiliary components and the operation strategy.

An average power in the order of 200 to 500 GW could potentially be utilised from ocean waves /Cru08/. Compared to the world wide installed wind power in 2012 of 282 GW wave energy converters (WEC) could significantly con-tribute to the futures demand for electricity /GWE13/.
However, efficient and robust power take-offs (PTO) are needed for wave energy converters to become a commercially viable technology. Fluid power is a favourable solution since hydrostatic drive trains (HDT) are well proven, mass produced and can be considered as state of the art.
A new promising technology are the dielectric elastomers (DE) which can gain a significant share in the sector of wave energy. This technology offers the possibility to directly convert relative motion into electrical power via the change of electric charge due to compression or elongation.
At first, this paper discusses hydrostatic drive trains for wave energy converters (WEC) with different control strategies and describes their operational behaviour and efficiency. It is shown how requirements for force control and energy storage influence system design and overall efficiency.
Dynamic wave-to-wire (W2W) simulations – of an offshore heaving buoy and a nearshore bottom hinged flap – taking into account the efficiency of the PTO components are introduced to assess and optimise the performance. As locations for the nearshore concept Yakutat (Alaska, USA) and for the heaving buoy Reedsport (Oregon, USA) are chosen. In order to enter discussions, results of the coupled simulations are presented in the form of matrices containing power output, capture factors and PTO efficiency.
Intended to go further than an outlook this paper introduces DE generators for the application in WEC. Requirements and boundary conditions as well as the governing physics for the use in WEC are discussed. An exemplary topology of a wave farm and the differences to hydrostatic transmission are shown. Especially in smaller applications with power levels of 10 to 100 kW in remote locations DE can be promising. Beyond that, the used materials offer the possibility of new shapes of WEC. Due to the theoretically formability the DE-generator can be fitted to different fields of applications.
DE generators can be a reasonable complement and help to make wave energy accessible.

/Cru08/ - J. Cruz, Ocean Wave Energy, Springer Verlag, Germany, 2008
/GWE13/ - Global Wind Energy Council, Global Wind Report 2012

Wave Energy Converters harness the hydrodynamic energy of sea water waves. Commonly a buoyant body, which moves with the waves in an oscillating manner, is used. Here the movement behaviour depends on the type of the converter. The oscillating movement is typically transformed into electrical energy. The Power-Take-Off providing this transformation consists of a linear or rotational electric generator. Apart from the generator the transformation may include mechanical or hydrostatic components. Thus, the Power-Take-Off provides the transformation of wave energy into electric energy and is able to rectify and to smooth the incoming power.
The requirements on Wave-Energy-Converters contain economic aspects such as life-cycle costs, efficiency and availability as well as technical aspects. These technical aspects include the need to meet the specifications for connecting the converter to the electrical grid as well as safety measures. The specifications are merged in the grid code, the so called transmission code in Germany. In the paper various Power-Take-Off are presented. Diverse parts are modified to find as many layouts as possible. The focus is not only put on the generator but also on rectifying and smoothing elements. The influence of the diverse components on the efficiency, the costs and the durability as well as on the grid code compliance is investigated subsequently. Thus, it is possible to compare various Power-Take-Off to each other with respect to the mentioned aspects.