Identification of future emerging technologies in the ocean energy sector
Part 2 - Wave Energy
Continuing our review of the findings of the European Commission's workshop on the identification of future emerging technologies in the ocean energy sector (link), we look at the common themes and challenges associated with wave energy converters.
Similar to our previous blog about tidal energy converters (link), the commission notes the huge variety in device types and the lack of convergence in this sector. The first phase of wave energy converters typically stemmed from mathematical analysis of the complex phenomenon that is a wave. As a result, the design focus was very much on optimising hydrodynamic performance without a consideration for overall system integration and manufacturability. This is a recurring theme in early devices, that whilst the hydrodynamics had been analysed in great detail, the integration of the PTO, the foundation/mooring arrangement and the manufacturing of the device often caused issues.
The second generation devices considered in this report target wave orbital velocities (such as Marine Power Systems WaveSub) and material flexibilities (Checkmate Anaconda) rather than the conventional fixed bodies being displaced by the waves. These devices are less mature and have more to prove in terms of their scalability. What is often found in second stage devices and the majority of more recently developed concepts through programmes such as Wave Energy Scotland, is a better system level design and a consideration of a wider set of environmental design conditions.
That leads nicely on to the biggest challenge facing all wave energy converters; the issue of survivability. Whilst designing for typical wave conditions, wave energy converters must be capable of remaining structurally intact during extreme storm conditions. These conditions typically result in significantly larger forces on the structure than normal operating conditions and will drive the design requirements. In the development of some devices the designers have opted to have a "survival mode" in which the device can be left in a condition that greatly reduces the loads. Trying to mitigate these loads using device or control system settings typically is more cost effective than adding mass to the structure to react the loads from waves that occur once in 50 (or more) years.
The other common challenge is that of PTO selection, once again there is no consensus of opinion and no obvious choice of PTO that is suited to the output of a wave energy converter. Trying to convert what is essentially random oscillatory motion into a consistent power output represents a significant challenge and may require some different thinking - something we will explore later in this blog series. This report dovetails nicely with the work that Wave energy Scotland have been doing recently, with their landscaping studies into PTO technology, scaling wave energy devices and moorings / electrical connections. Clearly there is some consensus on the key issues to be overcome with wave energy and structured funding to find the best solutions.