Getting to the nitty gritty of tidal engineering: are 2MW-plus system packages the industry sweet spot?
Many types of tidal turbine are under development with individual efficiencies varying from less than 30% to significantly better than 40%. Consultancy Fraenkel-Wright offers some insights into the real issues associated with engineering a product for the tidal environment.
By Ritesh Gupta on Sep 8, 2014
The tidal energy industry continues to witness developments pertaining to validation and testing of large scale prototype of emerging turbine foundation structures. As technology specialists make progress, they are increasingly looking at corroboration of the design process and tools. Plus, the focus is also on offering some support for increased power output.
For instance, QED Naval recently gained the financial support from Kelvin Capital and the Scottish Investment Bank. The move will help the Edinburgh, Scotland-based company to move ahead with development of its Subhub tidal turbine foundation structure. The immediate plan is to work out tank testing of two models of increasing scale.
Apt performance evaluation
Peter Fraenkel, director at renewable energy consultancy Fraenkel-Wright says the over-riding factor when it comes to achieving respectable performance from a tidal turbine is the gross swept area of the rotor (or rotors) exposed to the current since the energy capture is directly proportional to rotor area; also there are large fixed overhead costs so just like with the wind industry, “bigger tends to be better”.
“It seems likely to me that ultimately the most successful technology would need to be deployed in 2MW or larger individual system packages,” says Fraenkel, who was co-founder, inventor of the technology and until March 2012, technical director for Marine Current Turbines, a UK company, now owned by Siemens.
He continues: “It needs to be emphasised that this will be 2MW at sensible current velocities of around 2.5m/s - some developers like to quote power at unrealistically high velocities as a marketing ploy to exaggerate the performance of their device. This is for the same reason that modern wind turbines are much larger these days than they used to be, typically 5MW or more offshore, as part of getting generating costs down. By the way wind turbines invariablly have their rated power quoted for 12 or 13m/s but tidal developers seem to use a spread of different assumptions in this respect.”
Although the key requirement for any electricity generator is cost-effectiveness and considering that big tends to be better so from that point of view, efficiency is another key requirement. At present there are many different types of tidal turbines under development and the efficiencies of different systems probably vary from less than 30% to significantly better than 40%. Clearly a 30% efficient system will need to be 33% larger than a 40% efficient equivalent to do the same job - and size usually equates with cost.
“Positioning of the turbine is also critical since the energy availability is proportional to the cube of the velocity; in other words only a 10% variation in velocity can represent a 33% variation in energy capture (1.1 cubed = 1.33),” says Fraenkel.
This also implies that it is best to have the rotors near the surface where the water moves fastest; in many situations 70 to 80% of the energy is in the top 50% of the water column due to the effects of velocity shear (surface water moving faster than water lower down).
“The implication is that a rotor with 3m clearance above it can capture 20% or so more energy than if the same rotor was mounted 10m below the surface,” he explains.
So how confident is the industry about engineering a product tailored for the tidal environment?
If you ask any developer about their confidence they will obviously tell you they have no doubts about engineering a system to work reliably in the marine environment.
But, as Fraenkel points out, tide races are very aggressive locations, normally avoided by mariners and other offshore operators, so the engineering requirements are demanding and there is limited experience in dealing with them. Very few tidal turbines have so far operated for any length of time so we will need to wait to see how well placed most of the developers’ confidence is, although one machine has already functioned for over five years and generated nearly 10GWh into the grid, says Fraenkel.
For its part, Energy Technologies Institute(ETI) asserts that industry confidence is being supported by some excellent learning from first-of-type machines like Alstom’s ReDAPT. Also, this learning is supplemented by on-shore testing at NaREC’s Nautilus facility, where drivetrain accelerated life testing provides insight into expected reliability issues and assumptions. Overall, the industry is working hard to provide reliable, safe and secure power from UK waters.
As for major challenges in engineering a product for the tidal environment, Fraenkel refers to three of them: “First, it’s about achieving an efficient and reliable rotor and power train - this is the bit which fixes how much energy can be collected. Then it’s about mounting it on a structure which can survive a sensible period, such as 25 years, without fatigue, resonance, corrosion or other adverse effects. Also, lastly, one has to come up with design features to facilitate reasonably economical installation and maintenance - at present many systems under development remain exceedingly costly and difficult to install and to access.”