REALLY, JUST HOW HARD IS IT?
There are a lot of clever people in marine renewables and yet we are often asked "why hasn't it taken off yet, it seems like such a good idea?". Of course, it's a good idea, in comparison to fossil fuels, generating energy from the ocean means you don't have to pay for fuel, or transport of fuel, carbon taxation or the maintenance of the things transporting the fuel! So we thought we'd take a look at what makes it so difficult and there are a number of challenges; power density relative to fossil fuels, the cost of marine operations, working in a challenging environment, at present a lack of large farms or volume production, intermittency of the resource and a powerful oil and gas lobby.
We will start out with looking at the basic challenges of operating a device in the marine environment.
Mass and Buoyancy
Whatever device you have designed, it has a mass - this on it's own is not unsurprising, but the magnitude of this mass is actually quite important. The mass of the device dictates the vessels and cranes that can be used, and to some degree, the deployment and retrieval process. Dry mass is one thing, but there's a whole different ball game when you enter the water. If the device is surface floating or submerged, buoyancy is going to be important, not just the amount of buoyancy but the positioning for stability and teh reaction of the mass when being moved (dynamic factors). It would be less than ideal if you had a device that was due to be submerged and as it descended it flipped over due to the relative positions of mass and buoyancy - an extreme example I know - but a factor that has to be considered and can contribute to costs.
Another fundamental challenge of the marine environment is corrosion. A commonly favoured engineering material is steel, which, let's face it, is iron with a few other elements chucked in in very low volumes, so it has a fantastic ability to rust. "In rust we trust" you may say! Not when you have to put this device in the water for 20 years or more and could lose 0.5mm per year due to corrosion! This is because of a high oxygen content typically found in the energetic bodies of water that renewables devices are placed in. This oxygenated water aids the oxidation process that forms rust. That means all subsea external surfaces need to be protected in some form, be that through painting, coating or material choice - all of which add cost.
This is particularly important around connections and other more complex geometry features where the paint may struggle to adhere and the area may be crucial to performance, e.g. a sealed bolted flange or a key piece of geometry to aid installation.
Biofouling Another bi-product of the oxygenated water is that marine growth is prevalent. Everything from squishy sea squirts to harder barnacles will grow on a device whether you like it or not. This is not ideal if you require a certain hydrodynamic shape for the optimal performance of a device. Nor is it helpful in terms of the first point we touched on - mass and buoyancy - the extra layer of growth on the device adds mass to the structure and you have had no choice about its placement! Finally, these guys have an appetite! We've seen marine growth eat through 316L stainless steel no problem at all! Appropriate use of coatings, maintenance strategies and allowances in the design can help to mitigate these effects, but it's yet another unavoidable facet of operating in the marine environment.
Sealing Whenever you have a submerged interface between two components, water is trying to get in. This is even more challenging when that is a dynamic interface (think, a tidal turbine rotor, a blade pitch system or an attenuator connection between segments). The deeper you go, the harder it tries to get in (courtesy of the static pressure). Sealing to keep water away from the electrical components within a device is important or obvious reasons, but so is the ability to detect a loss or degradation of seal performance. As a result a lot of time and effort is put into demonstrating sealing solutions prior to real sea deployments - again this costs time and money. Unfortunately immersing something <50m underwater requires very similar sealing technology to the oil and gas solutions that work at 2000m underwater, which contributes considerably to the cost.
To quote an unnamed source "any numpty can put something in the water and make it spin"; the true challenge is being able to put something in the water over a period of 20+ years and make cost effective energy in the face of these challenges from the environment, which are all increasing the cost of the programme. Despite the capability and creativity of those in the marine renewables industry their core focus is often the concept, or the hydrodynamic performance rather than these more mundane considerations. However, if you can focus on the basic marine challenges at a fundamental stage, then the whole device has the potential to be more cost effective.