What the Sea Pushes Against: Choosing a coastal energy system when the site, not the technology, makes the decision

A coastal machine should be selected by what it pushes against, and paid for by what it produces — and electricity is rarely the best product.

Coastal and island communities with weak grids or none pay some of the highest electricity prices on earth and sit beside one of the largest untapped energy resources on earth. The obvious response is to build a machine that turns waves into electricity. That framing has produced four decades of prototypes and very few installations, because there is no box to buy: a wave machine is at least three things — a structure that takes the load, a conversion and transmission chain, and a store — and each is chosen independently, from different constraints, by different people. Change the site and any one can flip while the other two stay put.

This document is therefore a selection guide rather than a design. It organises converters into three families by what they react against — BEDROCK against the seabed, BULWARK against the shore, DRIFTER against itself — gives a compatibility table showing which conversion chains and stores each family can actually take, and screens sites twice: once on what the site can supply, and once on what the community will pay for.

The second screen is the argument the work exists to make. A coastal community rarely wants kilowatt-hours as such; it wants cold so a catch can be stored for sale rather than lost, fresh water, a coast that stays where it is, and a sheltered place to land a boat. Storing energy as ice or water costs a fraction of storing the equivalent in batteries, does not degrade with cycling, and takes a delivery burden off infrastructure that is already inadequate. Deep cold water halves the electricity per tonne of ice, and in a microgrid sized to peak, halving a major load releases capacity.

A cost chapter ranks every parameter by measured sensitivity. The ranking is not where the engineering effort usually goes: capacity factor and conversion cost dominate, the entire electrical layer sits at the bottom, and two decisions — rating the conversion chain below peak, and procuring the structure as coastal-defence infrastructure rather than as a power plant — move the levelised cost more than any efficiency gain available.

An appendix records nine approaches that were evaluated and deprecated, each with the number that killed it and what replaced it, on the principle that in a field whose diagnosed problem is a proliferation of concepts and a failure to converge, a documented list of what does not work is worth more than another concept.

Novelty is not claimed. Two attribution passes found prior art for most elements and the paper states which of its claims were consequently withdrawn or downgraded. The contribution offered is an integration and a decision procedure, not a discovery. An interactive selection tool accompanies the paper: a single self-contained HTML file, no installation and no network required, in which every cost assumption is an editable field carrying a provenance badge, so that a reader with better numbers can replace ours and get their own answer.