Too much torque and not enough action

March 6, 2014 by  
Filed under Solar Energy Tips




Peter Fraenkel

Marine renewable energy technologies have taken longer to come to fruition than many people expected. Today we have reached the stage where a number of potential commercial systems are operational as full scale prototypes, but the first commercial arrays or “farms” of tidal turbines (and for that matter, wave devices) which are necessary to achieve the economies of scale for cost-competitive generation, are only at the planning stage. In one or two cases quite advanced planning has been completed and the first projects seem likely to be commissioned by 2016, possibly 2015.

In the background, reduced scale devices of various kinds are being tested and an even larger number of models and concepts are claimed to be under development. Between 100 and 200 websites exist to promote companies claiming to be developing novel tidal turbines or wave energy devices. This is not as good as it might seem considering that the market seems unlikely to sustain much more than about 10 key players at most.

Slower than expected

Although good progress has been achieved, at least by a few technology developers, the atmosphere at present seems more subdued than it was a few years ago. Progress has been slower than expected. Part of the reason is that as with all new technologies many developers in their enthusiasm to raise finance make over optimistic claims and raise expectations too high.

Most new technologies tend to converge on a “technology of choice” as they mature and become commercially viable; for example virtually all megawatt scale wind turbines use three-bladed up-wind axial flow rotors with pitch control and the rotor and nacelle almost always are placed on a tubular steel tower. Most wind turbines are even painted the same colour, grey, to minimize visual impact. There are smallish variations on this theme such as some wind turbines use gearboxes between the rotor and the generator whereas others use slow speed direct drive generators. Unfortunately wave energy technology still seems to have a different type of device from each developer, although there are trends towards a more limited number of basic principles of energy conversion, and tidal turbine technology is beginning to converge on the use of axial flow pitch controlled rotors, but with a variety of other factors such as the number of rotors, support structures submerged, floating or surface piercing, etc. So we have to accept the marine renewables are still immature and early stage.

Engineering and financing challenges

Two factors seem to be largely responsible for delaying the development of marine renewables technologies: the engineering challenge and difficulties in securing finance at the scale needed for full size projects.

The engineering challenge stems largely from the fact that wave, tidal and to some extent offshore wind technologies need to interact with relatively slow moving resources; wave orbital movements are in the order of 1m/sec; tidal currents drive turbine rotor blades at typically10m/sec tip velocities and the wind drives wind turbine rotor blade tips at 60 to 80m/sec. One of the immutable laws of physics is that power is the product of force and velocity – a classic definition of power is that it is produced when a force moves its point of application. Therefore a rarely discussed issue with these technologies is that relatively low driving velocities need proportionately large forces to produce a given level of power.

The table below shows the speed and torque (rotational force) for various typical power producing devices in order to produce 1MW of shaft power.


This shows how typically a tidal turbine produces 1000x the torque of a gas turbine of the same power, (and a wave energy converter can produce 10,000 times as much torque). The problem is that the torque is a measure of the loads that need to be resisted by the device, so clearly these new marine technologies inherently have to resist very large forces, which in turn demand strong and massive structures. “Strong and massive” equates with cost and also with challenging structural design issues, especially with respect to dynamic loads that can cause major fatigue and resonance issues. These problems have largely been solved for wind turbines, although there is of course always room for improvement, and they are close to being under control for tidal turbines. But wave energy devices capable of MW scale power outputs remain dauntingly large and difficult to construct in relation to their power capability. This perhaps explains the sequence in which these technologies have appeared, first wind, then tidal and despite an early start, lastly wave.

The second key factor that seems to be delaying development is lack of investment in the sector, a problem exacerbated by uncertainties until recently on what the government would offer. All new energy technologies require some form of government support; steam turbines, gas turbines and nuclear power were originally developed on a “money no object” basis for military purposes. The so called “Valley of Death” between demonstrating prototypes and achieving the scale of production necessary to get costs down can only be crossed given government financial support or some form of subsidy as otherwise early projects will not be sufficiently low in cost to be profitable in competition with mature generating technologies.

Less interest?

The future looked brighter when marine renewables were briefly “flavour of the month” or even “flavour of the year” perhaps around 2010, but at least in the UK an easily distracted government, once proudly claimed by the Prime Minister to be the Greenest Government ever, is suddenly pursuing nuclear power and fracking for gas as the great new hopes for future energy supplies. The major utilities therefore are tending to take less interest in risky and expensive new renewable energy technologies even though we are obviously likely to need them if we are to phase out fossil fuels (including the gas generated from fracking).

The UK government has revealed “strike prices” for wave and tidal generation of £305/MWh from 2014 to 2019 as part of the UK government’s EMR (Electricity Market Reform). However this subsidy level is probably just about adequate to enable tidal projects to go ahead , but for reasons alluded to earlier, wave energy looks like it may start significantly more costly than tidal and there has to be a question as to whether the strike price on offer for wave is sufficient to encourage the necessary investment.

Marine renewable technologies are of unique importance, representing huge clean energy resources that hardly require the use of scarce land. The engineering problems, at least for tidal are close to resolution but financing problems still remain. Short of some game-changing technical breakthrough it will need more action and less rhetoric from government to ensure these vital technologies are ready when they are needed.

About the author

Peter Fraenkel has been closely involved with tidal turbine development from the earliest days of this young technology, the early 1990s. From 2000 until the spring of 2012, he was Technical Director of one of the leading companies in this field, Marine Current Turbines Ltd (MCT).

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