Friday 4 March 2016

Field trail adventures

In this instance, the misspelling of ‘field trials’ was intentional. This ‘alternative spelling’ amuses me, because it served as a regular reminder of my exceptional gift for switching letters. I once misspelled the name of the folder containing field trial data, and couldn’t change it because an external contractor had set up a data transfer to this folder. So I had plenty of opportunity to ponder over what a 'field trail' might possibly be. Surprisingly, I found it a very useful concept. After a discussion with a technology developer recently, I thought it was worth sharing with a wider audience.

Deciding on a development path for a new technology is a bit like hacking a trail through a jungle with a machete. Heading straight for our destination, say a mountain, may not be the best route. There may only be a couple of safe paths up the mountain, so we should be aiming for the start of one of these routes. Plus, it is wise to choose a path that follows a source of sustenance. The design of any field trial is part of a trail-building process. We should both work forward from our starting point and work backward from our destination. This might sound like I’m using different words to describe the more familiar ‘technology roadmap’- this is not the case. Roadmaps might set the goal as ‘lowering cost of energy’, but they don’t give insights into how this might be achieved. The method I am suggesting is to focus on the requirements for reducing the cost of energy. There are no roads to where we want to go; each must cut their own trail.

First we must identify the viable routes up the mountain; i.e. we need to know which of the challenges to achieving cost reductions we want have a shot at. We might not know exactly how we are going to reduce cost of energy, but we need to know the questions we are seeking answers for. The following list is a good starting point for a fruitful discussion about we will need to have figured out in ten year’s time:

  • What is our chosen method for increasing the ratio of revenue-earning loads to cost-incurring loads?
  • How do we avoid loads progressively during rated operation and as much as possible during storm mode?
  • What is the level of control complexity required for efficient capture in below-rated operation, yet effective load avoidance in storms?
  • What are the bits of kit needed to control below-rated, rated, and storm operational modes? What are the opportunities for using this equipment for other functions? For example, wind turbines use pitching blades for each of these three operating modes.
  • Which modes of hydrodynamic interaction are the best, not in terms of efficiency, but in terms of revenue to cost ratios (of loads, power and stroke). Considerations include modes of motion, reaction points, water depth and parasitic modes. These should be considered on the farm level, rather than device level.
  • What are the best relative sub-system ratings for an economic device, i.e. how big must the accumulators and generator be compared to the hydrodynamic working body?
  • What is the economic upper limit on the size of the hydrodynamic absorber? A bigger body will always give more power capture. However, it will also give greater loads that must be paid for; hence there is an economic limit to absorber size.
  • Given the upper limit on the hydrodynamic absorber, where do the upscaling opportunities lie in the rest of the system, i.e. how can the proportion of fixed costs be reduced by upscaling? Can the accumulators, generators, control and power electronics be shared between multiple absorbers?
  • How will the upscaled utility version of the wave farm be fault resilient? i.e. how do we make sure faults in small components don’t stop energy capture or cause faults in other sub-systems?
  • How will the upscaled utility version of the wave farm be maintained?
  • Which sub-systems are required to improve the revenue to cost ratio? The full system must be considered – e.g. when comparing direct drive to systems with power smoothing, it is not enough to compare the costs of the direct drive generator to those of the accumulator and associated motor/generator set. For a fairer comparison, the direct drive system should be assessed taking into account the higher reaction loads on the structure, the power electronics to attain equivalent smoothing, the loss of the ability to independently control hydrodynamic absorption and electricity production, and the loss of the upscaling option to connect a single large generator to several hydrodynamic oscillators whose size is limited by economics. Candidates for increasing the revenue to cost ratio are gearing, rectification, power smoothing, and the ability to isolate the hydrodynamic oscillator from the rest of the structure.

Once we have an idea of what we are working towards, the next task is to consider what projects are required to answer these questions, and to provide stepping stones to where we want to be. Just as wind turbines were initially stall- rather than pitch-controlled, wave power will probably need evolutionary half-way houses for commercial devices. It would be useful to have technologies that give greater availability, in order to allow experience to be gathered. There are more than enough reliability problems to sort out in a new technology, so designing for availability makes sense. Consequently early iterations may need to be simpler, smaller, closer to shore, or in more benign resources, than would be ideal for the mature technology. For example, even if a full system comparison showed that power-smoothing is preferable to direct drive for a given utility-scale concept, there may still be value in using a more reliable direct drive system in the early iteration of the concept.

Image credit
'Its not a waterpark' from

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