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Delegates are invited to meet and discuss with the poster presenters in this topic directly after the session 'Innovative concepts for drive train components' taking place on Thursday, 13 March 2014 at 11:15-12:45. The meet-the-authors will take place in the poster area.

Keysan Ozan University of Edinburgh, United Kingdom
Co-authors:
Keysan Ozan (1) F P Markus Mueller (1)
(1) University of Edinburgh, Edinburgh, United Kingdom

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Abstract

Sizing of electrical generators for combined wind and wave energy platforms

Introduction

A combined wind and wave energy platform which has a 5.2 MW wind turbine and ten independent 500 kW oscillating water columns (OWCs) is analysed in this paper. Simulations at different wind-wave conditions using the real measured data are used to estimate the performance of different generator sizes. The annual energy harvest is estimated for each power rating, in order to get an understanding of the relation between the generator cost and the electricity generation income.

Approach

- Data from two sites (the coast of Norway and Spain) are obtained to get a better representation of the wave and wind energy outputs. These data represent the 10 year measurements for the wind speed and wave height.
- Comprehensive set of Simulink models are developed to model the Wind turbines and Oscillating water columns (OWCs).
- Two different drive trains are used for the wind turbine: doubly fed induction generator coupled to a three stage gearbox and a medium speed permanent magnet generator coupled to a two stage gearbox.
- Well's turbines are used in oscillating water columns, which are connected to induction generators. The speed of the generators are adjusted to maintain maximum aerodynamic efficiency in the Well's turbines.
- The output of the generators are rectified using independent rectifiers and all these rectifiers are connected to a common DC-link to minimize the inverter and step-up transformer cost and to smooth out the power output.
- Simulations with different power ratings are performed and the annual energy generation is recorded in each simulation. These data will be used to find out the optimum generator rating (e.g. maximum energy output and minimun initial cost).
- In the full paper, it is planned to extend the electrical models with a coupled thermal model that will simulate the air-flow and temperature increase in the generators. By this way, it would be possible to utilize the improved thermal performance of a generator in a offshore platform due to increased air circulation and humid environment.


Main body of abstract

Deep water offshore wind and wave energy platforms have larger energy resources compared to onshore equivalents. The MARINA Platform Project, which is a 4.5 year funded EU FP7 project, aims to provide different combined offshore wind and wave energy platforms [1]. One of the challenges in offshore floating platforms is the high installation and operational costs due to challenging environment. The cost of the system can be minimized by optimizing the power rating of the drive-train.

In this paper, in particular the optimal power rating of the electrical generators will be investigated. This can be achieved in two ways: Firstly, the electrical generators in a wave energy converter operate at partial-loads almost all the time due to the nature of the resource. Thus, the power rating can be reduced without affecting the energy output. Secondly, the electrical generators are subject to increased air circulation with high humidity, which improves the thermal performance significantly [2]. Therefore, they can be over loaded for a short amount of time.

A 5.2 MW wind turbine coupled to ten independent oscillating water columns (OWCs) with Well's turbine are analysed. The rated mechanical energy input for each OWCs is 500 kW. It is possible to choose 500 kW generators, which is the usual practice, however, in this case the capacity factor of the generators would be very low. Alternatively, it is possible to use a smaller power rated generator, but this would reduce the energy harvest as the maximum power is limited. Simulations at different wind-wave conditions (i.e. high wind-low wave, low wind-high wave) are performed for different power rated generators. The measured data series from two sites (off the coast of Norway and Spain) are used the estimate the annual energy harvest of the platform for each power ratings. The main characteristics of the wave and wind resource are presented in Fig. 1-2 and Fig 3-4.

Probability density distribution of the significant wave height in the coast of Norway.

Probability density distribution of the wind speed in the coast of Norway.

Probability density distribution of the significant wave height in the coast of Spain.

Probability density distribution of the wind speed in the coast of Spain.

Typical power outputs of the wind turbine and one of the OWCs are presented in Fig. 5. Annual energy generation versus the generator power rating is presented in Fig. 6. The figure shows that if the generator rating is decreased by 60 % (to 200 kW), the energy harvest just reduces by 15 %. In this simulation, the increase in the thermal performance of the machine is not taken into account yet. In the full paper, it is planned to couple the electrical model with a thermal model thaht can simulate the airflow around the generator. In this way, it would be possible to overload the generator for a short amount time, and this will make if possible to get the same energy output from a smaller generator. Furthermore, it is planned to include power smoothing between OWCs (such as common DC-link), which will help to reduce the size of other components such as switches and cables.



Conclusion

This paper focuses on the optimum sizing of the electrical generators in a combined wind and wave energy platform. The floating platform has a single wind turbine and ten independent oscillating water columns. Simulink models are developed to simulate the energy output from the wind turbine and OWCs. The OWCs are connected to a common DC-link with separate controlled rectifiers. The wind turbines has separate inverters and step-up transformers.

Typical power output from the wind turbine and one of the oscilating water columns.

The annual energy generation versus the generator power rating (for a single oscilation water column).

Time series data from the coast of Norway and Spain are used to estimate the energy harvest for different generator ratings. The aim is to minimize the initial cost of the drive train without affecting the electricity generation income. The preliminary results show that the generator size can be reduced by 60% with a 15 % decrease in the wave energy output. It is also planned to include a coupled thermal model in the full paper, which simulate the operating conditions in a offshore platform more realistically.

Typical power output from the wind turbine and one of the oscilating water columns.

The annual energy generation versus the generator power rating (for a single oscilation water column).

Time series data from the coast of Norway and Spain are used to estimate the energy harvest for different generator ratings. The aim is to minimize the initial cost of the drive train without affecting the electricity generation income. The preliminary results show that the generator size can be reduced by 60% with a 15 % decrease in the wave energy output. It is also planned to include a coupled thermal model in the full paper, which simulate the operating conditions in a offshore platform more realistically.


Learning objectives
The paper presents Matlab/Simulink models of a wind turbine coupled to oscillating water columns. The thermal models of the generators will be also presented. These models will be shared as open-source, which will enable delegated to use the models for their own systems.


References
[1] Barrios, I. Martinez, J. Murphy, K. Lynch, and C. Lopez Pavon. "Methodology for assessing multiple combined wind and ocean energy technologies as part of the EU FP7 MARINA Platform Project." ICOE 2012 Proceedings, Dublin, Ireland (2012).
[2] Hodgins, Neil. "High speed electrical power takeoff for oscillating water columns." ,University of Edinburgh PhD Dissertation, (2010).