Turbine design turns the tide for renewables

At any given point in the future it would be impossible to say with any degree of certainty how strong the wind or how much sunshine there will be. Renewable energy dependent on these uncontrollable natural elements is therefore impossible to calculate or rely on. At that same given point we will know exactly what the state of the tide will be and, within a certain error margin, the flow it is producing as it goes through its lunar-induced cycle. Why then has this entirely dependable resource been left relatively unplundered in comparison to its solar and wind counterparts? Basically it is more difficult and more expensive. Beyond the pressure and corrosion of the underwater environment, the costs of installing and maintaining subsea installations has proven to be prohibitive in the past.

Sitting on the Pembroke docks (at the time of writing) is DeltaStream, which is one of a number of embryonic projects aimed at turning the tide in favour of tidal energy. DeltaStream is a prototype device by Tidal Energy that is designed to generate up to 400kW from the tidal flow in Ramsey Sound off the southwest Welsh coast.

Although the reason it is still above and not below the water is down to a wait for the commercial pieces to fall into place, which they hopefully will do so during 2015, having this 200 tonne device ready for action on the quayside is a demonstration of why other forms of renewable energy harvesting are easier to progress than tidal turbines.

“You tend not to put anything underwater till you're pretty confident it's going to work,” said Peter Bromley, engineering manager with Tidal Energy. “So we have to invest a lot more time upfront doing the development, rather than trying things to suck it and see.”

Designing the device started with an understanding of tidal flow. The site at Ramsey Sound is interesting. There is a reef of rocks called the Bitches that go out across the channel, which have the effect of accelerating the water from the south to the north, so the flow in that direction is much faster than in the opposite direction. They also make this northwards flow more turbulent and an 80m trench, along with surface-breaking rock structures, mean the site is complex in terms of flow and therefore an ideal spot to put the prototype through its paces.

An ADCP (Acoustic Doppler current profiler) was used to measure water velocity at different heights in the water column to give a profile of the conditions at any stage of the tide. Unlike wind turbines that have to operate, or be protected from, wind conditions from calm to gale force, the undersea conditions are far more predictable, but there can be factors such as storm surges, waves and weather systems that will add a small amount of variability.

The strength required of the device can therefore be calculated accurately and the design driver becomes more about fatigue – the cyclic loading over its 25 year projected working life.

But the first thing most people will notice looking at the picture of the DeltaStream is that the turbine’s blades are not as long and slender as those in a wind turbine. “The main thing is that the energy density underwater is much greater than in air - water is approximately eight hundred times as dense as air,” said Bromley. “But we get much more energy for a smaller rotor disc, hence the stubby nature of our blades. But actually, our blade design is an even stranger shape compared to a lot of our competitors', and that's specific to the performance of our rotor and what we've done with our speed control patent that we hold.”

The DeltaStream waiting to take the plunge is fitted with a 12m rotor, with an upgrade path using 15m and 18m versions on the cards. Bromley explained: “The important thing that you're working on as a designer is the Tip Speed Ratio, and that's a ratio of the speed of the tip versus the flow of the water coming into the rotor. That's important, because it drives the pitch angle of the blade.” It also means to have an optimal Tip Speed Ratio a larger rotor rotates slower and a smaller one faster – approximately 12rpm for the 12m rotor in this case. The 15m rotor will rotate at 10rpm to produce the same generating capacity. The 18m would be slower still although it is likely that larger generators will be fitted so that the capacity and rotor speed can both be increased.

Rotor diameter of 18m seems a realistic size limit at the moment as bigger rotors would require deeper water and that typically means that the tidal flows tend to be less strong. But in the case of DeltaStream there is also the issue of ‘thrust control’, as it is a lightweight, gravity-based system. From an engineering perspective, driving piles down into the seabed is obviously the most robust solution, but it is also both costly and also has considerable environmental impact. Having a system that is gravity-based (i.e. it sits unanchored on the seabed) gets round these problems and also makes it easier to redeploy or decommission.

Dealing with the highest flows, harvesting a competitive amount of energy and keeping the gravity-based structure stable was therefore a challenge to Bromley. He explained: “The way we deal with it is we have a large rotor, but we allow the rotor to over-speed once we get to a rated power, with the aim being that we hit rated power as early as possible in the tidal cycle. The particular thing about our blade design is that not only does it cap power production, but at that point, by over-speeding, it actually reduces the coefficient of thrust. This limits the thrust on the whole device, which is important as there's a theoretical maximum thrust the device can take before it starts moving on the seabed. We want to keep well below that.”

The original design for DeltaStream is for a three turbine device, one turbine mounted on each corner giving maximum energy capture for a single installation. Bromley said: “With this first unit we decided it was more pragmatic to only put in a single turbine. So if there were any issues with the drivetrain or the nacelle or the rating design, we at least only made the mistake once rather than three times.”

However, most components are standard with varying levels of marinisation. The generator is actually designed for mining applications and would normally sit within a water jacket for cooling. “However, we've got plenty of water,” observed Bromley, “so we've just removed the water jacket, allowing it to cool in the seawater.”

It is unlikely to overheat off the Welsh coast this winter!

The danger to marine mammals

One area of environmental concern surrounds the potential damage to marine mammals, but as it stands there seems little evidence to support these concerns as yet. A report on the subject by the Natural Environment Research Council in 2013 concluded: “Our understanding of the potential impacts on marine mammals of wave and tidal devices is developing, yet a large degree of uncertainty still surrounds the potential impacts of many devices, demonstration and commercial scale arrays.” Which sums up that there may or may not be a problem but it will be difficult to provide evidence either way.

Bromley commented: “The reality is that we know very little about what goes on underneath the surface of our seas, especially the mammals and creatures that live there. We really don't understand their behaviour fully. And so it makes it very difficult to quantify that risk. Real experience from units in water is that there have been no known collisions or certainly no marine deaths directly attributable to a marine turbine. These creatures are not stupid. Seals, dolphins, porpoises - they're very intelligent animals, very aware of the environment that they're in and they've got a great capability to avoid things.”