Power control systems and grid interface for renewable energy production

The doctoral thesis "Marine renewable energy conversion - on and off-grid modeling" investigates the technical challenges in connecting ocean wave energy devices to the power grid. Unlike most other renewable and non-renewable energy sources, which can produce stable power over short periods, wave energy devices produces a pulsating power output due to the natural oscillations of ocean waves. These fluctuations lead to poor utilization of submarine cables and power components, and may also cause undesirable fluctuations in the power grid. Several methods involving different types of energy storage has traditionally been used to solve this problem, but require expensive components leading to increased energy prices. As part of the necessary cost reduction in wave energy, this dissertation endeavors to find solutions that do not involve costly power systems. The method used is based on spatial distribution of several wave energy devices so that the produced power can be directly evened out between the devices. Through clever positioning of the devices in respect to the typical wave patterns and wave directions, this work demonstrates that even small power plants can achieve relatively smooth power output without the use of energy storage. The detailed device analysis in this thesis is based on the Fred. Olsen's wave power system and presents detailed cost analysis and potential savings for the power system. The thesis also investigates autonomous systems that are not grid-connected, which can be interesting for offshore systems such as aquaculture, surveillance and offshore installations for oil and gas. The analysis shows that such systems based on the wave energy can be competitive with existing diesel-based solutions at the current technology level. Together with the presented opportunities for device improvements and cost reductions, the thesis demonstrates the potential for wave power as a commercial energy source for several potential niche markets.

The Ph.D. project will move in parallel with Fred Olsens wave energy project Bolt2Wavehub and will exchange knowledge with the project. The deliverables from the Ph.D. project is directly relevant to the project. Valuable background work has already been done in collaboration with Siemens and ABB. Furthermore, specific analysis and background deliverables were performed during the EU FP6 sponsored "Seewec" project (2005-2009), in particular from ABB, Chalmers university and Ghent University. Fred Olsens WEC concept is based on direct mechanical to electrical conversion of wave energy. Given the irregular nature of sea waves, this places demanding requirements on the control and conversion system. Also, the power produced from such a system is of a very irregular nature. This Ph.D. project will explore the overall performance of a farm system (multiple interconnected and grid connected wave energy producers) with focus on system optimization: -Optimal WEC design with focus on peak to average power rati o -Optimal farm design with focus on power quality based on geographical smoothing and practical constraints -Investigation of power quality improvements by using energy storage -Design of power collection system from WECs and grid interface