The fundamental challenge of large torqueAt the quickfire seminar series at All Energy 2013, tidal energy veteran Peter Fraenkel (Fraenkel-Wright) gave his views on fundamental engineering challenges facing both tidal stream and wave energy: the lower the speed of the prime mover, the higher the torque, and the bigger the machinery. He estimated that tidal turbines were an order of magnitude slower, and WECs (wave energy converters) two orders of magnitude slower, than fossil fuel turbines.
Peter Fraenkel’s summation of the fundamental engineering constraints carried the implicit suggestion that wave energy would always be more expensive than tidal energy. Perhaps some insight might be gained from examining the reasons why tidal energy has progressed faster than wave energy. Of course I naturally feel compelled to defend wave energy as a matter of principle, so while I acknowledge that tribalism is a motivating factor for me, I shall do my best to prevent this from affecting my objectivity.
There are two issues surrounding low speeds in wave energy: one is the provision of PTO (power take off) equipment that can deal with large torques, and the other is the requirement for large primary wave activated bodies.
Do high torques necessarily make wave energy expensive?When the surfaces acted upon by the resource move slowly, the first stage PTO system must be strong enough to deal with high torques. In general these are large and expensive, and this would seem to indicate that wave energy is facing higher hurdles than tidal stream energy. Of course, another difference between these technologies is the directionality of forces in the first stage of power capture: for wave energy the forces on the PTO system are oscillatory and generally bi-directional, whereas for tidal stream the forces are unidirectional. Wave energy has the additional challenges of rectification and frequent operation at part load. Most PTO equipment (particularly electrical generators) are very inefficient when operating at part load.
If the first stage of PTO in a WEC was an electrical generator, then the problems of high torque, rectification, and inefficient operation at part load would result in expensive and unreliable (because gearing is a weak point) power generation. There are however alternatives to electrical PTO which would be better suited to wave power. There is an existing supply chain for hydraulic drives that are suited to high torque, low speed applications. Rectification using hydraulic drives is a well proven method, and hydraulic drives are widely considered to be more robust than systems comprising gears, electrical generators and power electronics.
Hydraulic systems are not immune to the problem of lower efficiency operation at part load. Technology initially designed to mitigate the part load problem in wave energy has been commercialised and successfully implemented for other engineering applications. Artemis are now developing a high part-load efficiency hydraulic drive for use in their parent company Mitsubishi’s large offshore wind turbines. Large scale wind turbines are another technology facing problems due to high torques. As torque increases with size, this is seen as a potential barrier for scaling up to even larger turbines. If the development costs of marinising this technology are funded by offshore wind, this puts wave power (and tidal stream for that matter) in a good position to benefit from this technology.
It is also interesting to note that for a WEC, it is not compulsory for the primary wave activated body to be directly damped in the first stage of power capture. It is possible for some form of gearing to be implemented prior to power capture. A well known example of gearing that is built into a design is an OWC (oscillating water column). The primary wave activated body is the column of water oscillating within a caisson. The surface of the water column activates the column of air above it. Before the air passes through the turbine, the internal shape of the caisson narrows, and as a result the air speeds up. Thus a system can be designed so that the PTO operates at a higher speed than the primary wave activated body.
Do big bits necessarily make wave energy expensive?The other problem with slow motions is the requirement for big surfaces for the resource to act on. In tidal energy, the moving parts acted upon by the resource (blades) are fairly sophisticated and expensive bits of engineering. In wave energy, the moving parts acted upon by the resource are relatively unsophisticated. Indeed, they are typically much larger than tidal turbine blades, but the difference in cost is not as large as the difference in mass. The primary wave activated body for a wave energy converter (WEC) is typically a large rigid body, and much of its mass could be gained from cheap ballast. The shell of the body has fewer requirements for precision engineering than a turbine blade.
There is certainly an opportunity for cost savings in wave energy by considering other materials for the primary wave activated bodies. It is interesting to note that Pelamis have announced that they are moving from steel to concrete tubes for cost reasons (Andrew Scott, speaking at the Wave&Tidal 7 session at All Energy 2013).
A key issue for WECs is that the size of a device is related to the size of the waves that can be caught. The larger the size of the wave activated body, the broader the bandwidth of capture, the longer the wave period at which capture efficiency begins to drop off, the higher the loads induced, and the higher the wave height at which load or power shedding takes place.
Clearly the size of a WEC with respect to a particular resource is an important determinant of cost of energy. Unfortunately not enough is known yet about the scaling laws for the cost effectiveness of WECs. Those who have considered device size (relative to resource) in cost models acknowledge the uncertainty due to insufficient validation data, and there is still plenty of debate about the most economically viable size. The group who have access to the most validation data for their cost model are Pelamis. Although results from their cost optimisation program have not been published, at All Energy they did give an indication of where the next generation Pelamis (P2e) was going. In short, the developer with the most operational and iterative design data thinks that, for the Scottish resource, they can bring down the cost of energy for their device by using larger wave activated bodies. The only thing we can say with certainty about economic scaling laws for wave energy is that they differ to those for tidal energy.
It is interesting to break down what advantages are bought with large wave activated bodies: broader bandwidth, higher resonant period, higher induced loads, and higher load shedding limits. This presents an opportunity for wave power: are there other, cheaper, ways that we can attain these goals? There is evidence that bandwidth can be broadened by using a multibody system, or capturing power from a DoF that is made up of two modes of motion (e.g. the sloped IPS buoy compared to the heaving IPS buoy). Power capture from more than one DoF also appears promicing. It has already been shown that power capture in heave has less potential than heave combined with surge or pitch. I would be interested to hear about academic work that explored the practical (using a non-reactive, damping only PTO) power capture potential of a wide range of combinations of DoF for single and multiple bodies. The question of whether there are ways to increase the allowable motions before load shedding comes into play follows on to another fundamental challenge raised by Peter Fraenkel.
The fundamental challenge of the average to extreme ratioAt the recent RenewableUK conference, I asked Peter Fraenkel why he thought wave energy was lagging behind tidal stream in terms of cost of energy. He answered that the extremes of a resource dictated the project costs, while the average of the resource determined the energy generated (income).
If this issue is indeed a fundamental cost driver, then the wave power industry should be giving this more consideration. Here are some questions that arise from this fundamental engineering challenge:
- We need to be able to express the project costs as a function of resource extremes. Is the maximum wave power transport directly related to the maximum load? There is anecdotal evidence that for WECs the highest loads occur at steep waves at the resonant period, which might occur during power generation, rather than during storm conditions at longer wave period where the device is less resonant. The questions that arise from this is whether peak loads can be reduced by hydrodynamic load shedding, or operational mitigations?
- We need to be able to express the revenue generated as a function of resource average. This is strongly dependant on the size of the device, but for a fixed size, nameplate rating is important. As the rating has an impact on capital costs, the choice of rating, and the resulting capacity factor, amounts to a CoE consideration. We do not yet know what capacity factor will result in optimal CoE - it might depend on the resource and the type of device. For a fair comparison of CoE in medium and high power resources, capacity factors optimised for the resource should be used.
- Are there ways to operate a WEC that could increase average power generation relative to maximum loads? For example, would it be worthwhile in low power sea states to actively moor a WEC in a high performance configuration that attracted high loads, while in high power sea states allow it to passively weathervane into a low performance configuration that shed loads? Perhaps there is an opportunity for semi-flexible materials that transmit low and medium loads, but deflect rather than transmitting high loads?
- It has been observed that sites with medium wave resource have a smaller gap between average and extreme resource. Do CoE models presently represent this difference? It would be useful to develop an understanding of how much the CoE is impacted by the ratio of average to extreme resource. A higher ratio could result in higher design tolerances, high factors of safety, reduced weather windows, and increased maintenance costs.
- Anecdotal evidence suggests that semi-sheltered testing sites have a greater ratio of average to extreme resource. If this is indeed the case, then this could result in overly pessimistic CoE estimations.
- If it is found that the average to extreme ratio has a significant bearing on CoE, perhaps the industry should consider developing early arrays at medium power sites. There is an additional synergy with early commercial projects for medium power sites: as large WECs are resonant, and medium power sites probably have lower period waves than high power sites, the most economical size of device in a medium site will be smaller than that for a higher power site. To incrementally reduce risk and increase technology readiness, it is more cost effective to use smaller devices for early projects and gradually increase device size with subsequent projects. This trend in upward sizing has been seen for both Aquamarine (Oyster 1->Oyster 800) and Pelamis (P2->P2e). This suggests that the early prototypes of both companies may have produced better CoE if designed and operated in medium power sites. Although power production would have been lower, there would probably have been cost savings on lower rated PTO equipment, as well as installation and maintenance.
Final comments from Peter FraenkelPeter Fraenkel responded to the ideas discussed above with a few additional thoughts. He encouraged an openness to new ideas, and a focus on overcoming the challenges of high torques and a high ratio between the average and extremes of the resource:
"there is always the chance someone comes up with a wave energy conversion system which is such a game changer as to overcome the fundamental disadvantages."
He thought there was an opportunity for devices that were simple, had low capital costs, or made use of alternative materials. Yet he also thought that as the technology matures, design complexity will be needed to make wave energy cost effective. He offered a comparison to another technology:
"Engineering developments have always been cleverer than anyone expected - even the designers of the first modern airliner - the DC3 - would have found a Boeing 747 or an Airbus A380 difficult to comprehend even though they share similar construction and principles of design with the DC3. It only took 30 years to go from the DC3 to the 747 yet DC3s are still in widespread use nearly 80 years after they were designed. But that is all a slight distraction from wave power other than to make the point that we will probably eventually be surprised at what becomes possible."
He gave an example of the type of innovation needed to overcome the fundamental technology changes facing wave energy capture. He thought that gearing implemented before power capture was important, as it would allow the use of higher speed, hence smaller and cheaper, PTO equipment. He suggested a primary absorber of reinforced rubber, which transmitted forces across a large area to compressed air, which would be damped when passing through a small area. Such a system would allow gearing, is fatigue resilient, and would not be as heavily stressed as rigid steel. He observed that several developers are already working on such systems.
'Sublime' by badjonni: http://www.flickr.com/photos/badjonni/3844800850/in/set-72157594290716209