Wednesday, 17 May 2017

What are we spending money on that doesn't generate power?

When we generate power from a renewable source, we don't pay for the fuel. We pay for the build and operation of the plant. So conversion efficiency doesn't tell us much. Instead we are interested in how the project costs translate into the energy generated.

As energy = stroke x load, we can also ask which of the project costs are directly associated with the stroke and load that occur when energy is captured. What are we spending money on that doesn't directly lead to energy capture, and how can we design to reduce this?

We have known for a while that costs are associated with maximum strokes and loads, while revenues are associated with mean strokes and loads. Can we say more?


How do loads/stroke associated with CapEx and OpEx differ?

The expected maximum loads tell us the ultimate strength our design must bear (CapEx)

The expected mean loads and strokes inform decisions about sub-system ratings, such as the maximum travel of actuators. Sub-system ratings influence the ability to capture power and the initial plant cost (CapEx), as well as O&M costs, particularly the vessel size, and hence installation (CapEx) and maintenance (OpEx) costs. These mean loads also influence fatigue (OpEx and CapEx).

However it is the loads experienced, rather than expected, that have the most bearing on OpEx.
The experienced maximum and fatigue loads determine component lifetimes. When there is a difference between experienced and expected values (of maximum and fatigue loads, as well as system strength and behaviour) things break.

Other ways in which costs don't directly translate into revenues

Strokes and loads associated with costs and revenues differ with:
  • operational state (e.g. available, storm-weathering)
  • axes
  • sub-systems



What does this tell us about opportunities for using control to reduce cost of energy?

This was the subject of a 10 minute quick-fire seminar I gave last week at All Energy. The pdf of the slides is available for download.


  1. " We pay for the build and operation of the plant " ........ i would suggest , the build / operation is the relatively easy bit . It ' s the design , development & evaluation is the real problem . The front end cost to get to the fifth generation , where the thing works often & screws up / falls over & sinks little . I was thinking , other than the output of power , how much coastal erosion could be reduced by wave farming . If magnox can be built to generate power / bomb fodder , should wave farming be perceived as coastal barriers with power output ? & if they are relatively close in @ least the copper / aluminium webs should be short . Do i know anything really , not much other than a cubic meter of water ways a tonne & if it ' s going 25 miles an hour , i 'll get out of it ' s way , 'cos it ' s only a little better than being hit by a Ford Cortina !

  2. Hi Bob! I have heard the idea of using wave power as coastal protection a few times in conferences, so I think there is some interest. Part of the thinking behind this is to find a way to fund early commercial development as you suggest. There are several ways a wave energy converter can deduce the waves that reach the coast: it can disipate power by wave breaking or turning wave energy into eddies, it can reflect waves, and it can absorb waves.

    There are a couple of challenges here - things that detract from the attractiveness of using wave energy for coastal protection. If devices 'look big' to the waves they will reflect waves whether they are absorbing power or not. So a row of ground-referencing flaps like the Oyster would act as coastal protection. Unfortunately, reflecting waves results in large reaction forces on the device and doing this in the types of seas where coastal protection is needed has costs in terms of the build (making it strong enough) and operation (fixing stuff that breaks). At the moment these costs are so high that it doesn't make economic sense as coastal defence, even for the Dutch! Which is not to say that in the future people might be willing to pay more for coastal defense. It's all a matter of economics.

    I do not think there is politial will to give coastal barriers the same priority as national defence. I think this comes down to how our brains perceive risk. We tend to underestimate risk if changes happen over several generations;our judgements about ethical questions are biased towards inaction and maintaining the status quo (the trolley problem).

    Another thought about coastal protection is something that is covered by the talk I did for All Energy. My current understanding is that for economic wave power we must have a storm-protection mode: i.e. we must do everything we can to avoid loads in the most damaging seas. We will need to limit how much of the energy we capture in damaging seas; we might even reduce the waves we reflect. While this will be good for cost of energy, it won't be helpful for coastal protection.