PART 1 - TIDAL STREAM TURBINES
The European Commission has released the findings of its Workshop on Identification of Future Emerging Technologies in the Ocean Energy Sector. It’s a fairly chunky report, so we will go through it a section at a time in our latest blog series; starting off with tidal stream energy.
The report considers three “generations” of tidal energy converters:
Seabed mounted horizontal axis turbines (for example Nova Innovation's Shetland Array)
Floating generation systems employing rotational turbines (for example Orbital Marine's O2 turbine to be installed at Menter Mon)
Systems built on entirely different principles than horizontal or vertical axis turbines (for example Minesto's DG500 tidal kite deployed in Holyhead Deep)
The degree of maturity across these three categories varies considerably, yet the challenges they face can be grouped into some common areas.
One key recurring theme is operational expenditure (OPEX). For any device generating power in energetic tidal flows the maintenance windows are small and accessing the turbine can be costly. Whilst floating devices mitigate the need for heavy lift vessels, the conditions must allow crew transfer to the device or safe towing of the device back to a harbour. The overriding solution is to try and minimise device complexity and increase the reliability of all subsystems.
All devices are attached to the seabed in some form or another; this positional restraint has been a real challenge across device types. Structural foundations are an obvious source of significant CAPEX and challenging marine operations. Floating (and semi-submerged) solutions face their own challenges in the modelling of these structures and mooring lines as well as the operational challenge of installing anchors. The same challenges apply to tidal kites, which put significant dynamic loads through tether ropes and anchors, which need to be understood, quantified and designed for. Where possible, cross industry learning could be used to accelerate development here. Input from floating offshore platforms and other moored structures could accelerate learning in these areas.
Another challenge shared across all device types is numerical modelling to understand the loads on the device. For horizontal axis turbines the software exists for the most part, but there are still question marks about particular applications and the applicability of helicopter wake models to a tidal flow. In order to most efficiently design a device, the loads and environment need to be thoroughly understood such that the device is not over-designed (increasing cost), nor unsuitable for the environment (leading to public failure of the technology).
Control systems are the final common challenge for a whole range of tidal devices. These systems are essential in maintaining the integrity of the device, whilst maximising its power output. Since devices can be so different there is no one size fits all software, but we think there is a suitable one size fits all high-level philosophy. An adaptive control system with the objective of generating maximum power from the current environmental conditions within its prescribed loading limits. Give a controller an awareness of the loads on key parts of the system and the means to control those loads, and it can be used to find the right balance between maximising power output, maximising operational life and maintaining safe operational load margins.
Although not being addressed directly at the moment due to the immaturity of the industry, the issues surrounding scalability and tidal farms is fast becoming a serious challenge. Despite the fact that prototype machines are essential to demonstrate the technology, making the technology cost effective requires scaling up to multi-megawatt array scale installations. This is not really a challenge for current prototypes, it is a big challenge to be overcome in the next few years. Multi-machine foundations, managing assets within a farm to maximise generation and life, creating an O&M solution to allow machines to be maintained and replaced without increasing the risk to other devices, creating the right environment for other users of the ocean – these are all big problems that need innovative, collaborative and reliable solutions to be developed.