Thursday, 25 February 2016

How do we make wave competitive with wind?

At low technology readiness level, before tank tests and numerical models have been done, it is very difficult to estimate the levelised cost of energy. Indeed, this is not so much an estimate as complete guess work– we don’t know the optimum system size, nor relative subsystem size.

I’d like to propose a different early stage metric: the revenue to cost ratio. Both revenues and costs can be expressed as engineering parameters (such as loads, power, stroke or time). The ratios of these parameters can be informative and insightful. The most well-known example of the gain to pain ratio had been around for decades: the ratio of mean to extreme. Revenue is a function of the mean; costs are a function of the extremes. This ratio can give insights into a whole bunch of design choices. For example, it indicates that low variability can be a more important consideration than average power when choosing a site for installation. As the gain to pain ratio can be applied to many types of renewable energy, we can also use this perspective to ask: what are the design requirements for wave energy to be competitive with wind?

There are several fundamental challenges we could consider, such as the reciprocating rather than constant loads, or the high loads / slow speeds. Here I will pick one illustrative example: the operating regime.

The ‘power curve’ of wind turbines can be roughly divided into three regions of interest (see above): below-rated, rated, and storm protection mode. In below-rated operation:
  • The curve is cubic; control involves power capture optimisiation via the tip speed ratio.
  • The wind turbine has a strong interaction with the resource.
  • Typically this might occur, say, three quarters of the time.

In winds above the rated windspeed:
  • Control involves keeping the power constant while minimising the loads. This is done by pitching the blades.
  • The wind turbine becomes less load-attracting as the wind speed rises.
  • Typically this might occur, say, a quarter of the time.

At the cut-out windspeed and above, the wind turbine operates in storm mode:
  • There is no power capture. The blades pitch and the rotor parks to minimise loads.
  • The wind turbine looks (relatively) transparent to the resource.
  • This occurs a very small percentage of the time.

Each operating regime contributes differently to revenues and costs. The storm mode contributes nothing to revenue but a lot to costs via the ultimate design strength. Operation at rated contributes to (say) half of the power production, but also to loads, in terms of fatigue and wear. Operation at below-rated has a negligible contribution to costs in terms of additional strength and wear, but contributes to (say) half the power production.

It might be tempting to conclude that we want to operate at below-rated more and in the storm mode less – this is absolutely not the case. If there are any doubts about this, it is insightful to consider that all makes of utility-scale wind turbines for a given class of resource have very similar values of rated- and cut-out-windspeeds. This is because design convergence has economically optimised these values over the last few decades. Any major changes to the rated- or cut-out- windspeeds would result in a lower ratio of revenue to cost.

So how does wave power compare to this? Investigations into control have been mainly focussed in the below-rated operating regime. Very few wave energy concepts are built around the requirement for load-shedding in the most extreme resources; where this has been considered, the degree of load shedding is much less than the ‘transparency’ achieved by wind turbines. Many don’t have a rated operation that progressively avoids loads. However, these are not nice-to-have after-thoughts. To be competitive with wind, it is essential that wave energy technologies have these three operating regimes, and that the transition between them is an economic optimisation.

1 comment:

  1. Yesterday I was chatting to someone who has worked in the wind industry, and he told me that most of the wear actually occurs in the below-rated regime. This is because fluctuations in windspeed lead to lots of blade pitching to remain at optimum c_p. I'm not going to change the graph in this version as I've already published it.