Ask an engineer for the best solar cell and, if they are honest about what "best" means, they will ask a question back: best under what constraints? That return question is the subject of this third essay in my techno-crofting series. Most of the difficulty in building anything at the edge — a Highland croft, an island, a research station, eventually a habitat off-world — comes from being slightly sloppy about which constraints we are actually holding fixed. If we get precise about the constraints, I think a surprising amount of the techno-croft idea turns from musing into a more generalisable method. As with all of my essays, this is a live document that will change and evolve as I explore how useful this line of thinking is. Two kinds of best There are two optima worth discussing explicitly, because we (by which I mean engineers) constantly confuse them. The global optimum is the best solution that physics allows if you ignore material constraints. Assume any material, any...

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