Tidal Energy Harvesting is Anything but Neat & Tidy
The Sea is a Dynamic and Destructive Environment
By Jordan W. Taylor - @Jordan_W_Taylor
Tidal power still makes headlines all round the world, but especially in the UK, from Norfolk—
to Orkney—
Oft hailed as a renewable energy saviour, the premise of tidal power combines predictability with yield. In practice, however, this has not proven to be quite true, and is why solutions such as the OpenHydro turbine that I worked on didn't quite work, and remains stuck on the seabed of the Bay Of Fundy.
Tidal power has promise but is limited by geography, deployability and a very hostile environment. Firstly, there are two types of tidal power plant: A tidal barrage, and freestream tidal power.
A tidal barrage works similarly to a hydroelectric dam. There aren't many places you can deploy them, but they can generate a lot of power, though needing expensive offshore civil works. The main advantage is that power generators and gearboxes can be kept above the waterline and are thus maintainable.
Freestream tidal is an emplaced turbine in a naturally occurring tidal 'race', where currents are concentrated. This is what I worked on. You can in principle mass produce the turbines on land, but downside: All the sensitive gear is exposed to a hostile marine environment. I’d like to evaluate some specific issues and factors relating to tidal power.
Scalability of manufacture:
The barrage is mostly built in-situ: Negative. The tidal stream turbine and it's base can be built onshore and transferred to a deployment barge. Easier to mass produce: Positive.
Environment:
The barrage is exposed to a normally hostile offshore environment. The freestream turbine is exposed to an extremely hostile offshore environment! This is a highly negative factor.
Design & durability:
On the positive side, the barrage can minimize the amount of sensitive gear below the waterline: Generators, current management and gearboxes can be 'upstairs' (positive).
But the freestream turbine has everything exposed to corrosion and marine violence (highly negative).
Pre-deployment surveying:
Both are tricky. Seabed survey is expensive, though new droneships such as the one pictured are rapidly reducing the cost. (See XOcean which was founded by ex OpenHydro engineers after the tidal business went under, ironically.)
Deployment:
Barrage: Offshore civil works on a large scale is a highly negative factor.
Freestream: Dropping a turbine inside an hour with precision in the slack tide period of a tidal race (negative), but this could ultimately prove to be a positive factor with the right deployment barge setup. There’s good potential.
Maintainability:
Barrage: Positive. It can easily be designed for it, like a hydroelectric dam.
Freestream: Negative. In situ maintenance costs a fortune, but shipping to the shoreline isn't easy either. There’s some potential to improve with the right modular design.
Possible Niches:
Both are niche power sources, but the barrage is proven and works. Freestream will probably fizzle out due to the challenges mentioned, though some of the floating moored designs may have limited promise (see the Scotrenewables SR2000).
Conclusion.
Energy from the sea is a romantic image, but so is a sunset, and freestream tidal is unfortunately at that phase. The ocean is too dynamic and too destructive a place for easily managing the lifecycle of a powerplant. I’d happily be proven wrong about this.
More expensive, highly engineered, extremely low entropy collectors to grab high entropy random heat energy just won’t work (It’s SCIENCE!). This is just the offshore wind boondoggle all over again. Littering the ocean floor with wires and machines will be terrible for the marine ecosystem and we will be left wanting in the end.
Saying they can do it for $78/MWh is pure fantasy and can’t possibly include maintenance, short lifetimes, etc. Wishcasting of the highest order.
Nuclear that costs $98/MWh is a crime. It should be $1-10/MWh. Although I would bet even $98 looks good in the UK right now...
The ocean is an electrolyte solution. It corrodes metals quite aggressively. (A big difference from freshwater hydro plants.) This can be slowed by using plates made of rapid corroding metals like zinc. Zinc plates are standard on boats for this purpose. The ocean will thin plates on ships, particularly where there is vibration. Any metal object in the ocean is on a clock. The ocean is contaminated with tiny flakes of ship paint all over the world, because it's just another surface for life and chemistry to attack.
Sessile sea life colonizes anything placed in the water. If life doesn't bore into it, or grow in narrow crevices (that can start microscopic) it glues itself to the surface. Barnacles, mussels, seaweed, anemones, etc, colonize any surface. The faster the flow of water over that surface the better, as long as life can land and stick long enough to take hold. Tidal flows are perfect for this. Why? Because water flow brings nutrients and oxygen, and there are periods every day when the water is still and things can land and glue on.
There's a method from the tropics for making reinforced coral blocks fast by passing electrical current through the insulated wires in the water. They higher the voltage the better. Sea life doesn't just detect it, it uses the energy in its cells. We do the same. Our mitochondria are fuel cells that burn hydrocarbons and oxygen, putting out CO2 and water, creating an electrical potential out cells use to make ATP which is an energy currency in the cell. But life exists that harvests electricity directly. It stands to reason that something will evolve to bypass all the complex chemical processes cells use to make electric potential if there is current flowing.
https://en.m.wikipedia.org/wiki/Electric_bacteria
FWIW- there are deep sea cold water corals, and life isn't static. Life is constantly evolving, as we have seen with this still current pandemic. If anything could evolve a coral that thrives in upper ocean cold water, it would be miles and miles of high voltage AC current cables in the ocean.
Aside from such problems, harvesting tidal and ocean current flow for energy is great. It was proposed way back in the 1950's.