Tuesday, 25 November 2014

The cost-revenue conundrum

Here's a wave power conundrum: what must be done to reduce the cost of energy? The resource is free, so 'efficiency' is measured by how well the structure earns its keep. Back in 1980 the physicist Sir Hermann Bondi wrote (In 'Power from Sea Waves', edited by Brian Count) that you pay for the peaks and get paid for the averages. He had also noted that you pay for loads and get paid for power. Thus the intermittent, reciprocal, high force, slow moving, nature of sea waves is an intrinsic challenge. Consideration of the ratio of expenditure to revenue in terms of the loads is a view now acknowledged by many leading engineers.

I would like to extend Sir Hermann's observations, by considering all the steps between the cost incurring loads and the revenue generating power. An outline of this approach is shown in the diagram below (original pdf can be downloaded here; updated version here) The advantages and uses of such as approach are:
  • identify sources of cost ineffectiveness, e.g. low availability, a narrow range of sea-states in which operation is possible, collateral loads, and losses arising from running the generator at part load. 
  • identify design synergies and contradictions.
  • identify positive feedback, e.g. mass influences peak loads, and peak loads influence mass.
  • show that some cost incurring loads occur at the same place (degrees of freedom) and time (operational mode) as the valuable loads, e.g. fatigue loads which occur while operating in design seas. So there is a design challenge of how to reduce the cost incurring loads during operation.
  • show that many cost-incurring loads occur at different places or times to the valuable loads, e.g. loads which increase installation costs, or forces in directions that do not capture power. It is a separate design challenge to reduce these costs with respect to revenue. 
  • show how important it is to consider design holistically. 
  • identify problems specific to immature technology, e.g. unexpected loads, no failsafe mode, and no safe storm-weathering mode, all result in low availability.
  • test sensitivity of design parameters, and identify key design levers and promising clusters of design parameters.
  • investigate the requirements for being competitive with offshore wind. It could be argued that the ratio of peak resource power to average (operational) resource power is much higher for wave than for commercial wind sites. Even wind turbines employ adaptability: they pitch their blades. This suggests that in order to compete with offshore wind, wave energy technology must be adaptable; it must hinder small waves and appear transparent in big waves. 
  • this diagram could be developed into an equation that relates loads to the cost of energy.
  • Yes, the resolution is rubbish. A clearer version is available here

Request for feedback
This topic will be the main idea of my next EWTEC paper, and I would be very grateful to receive feedback.

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