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Comment by pg_bot | Hacker News Reader

I think the limiting factor on this type of turbine is the material you use to make the disks. As the disks get bigger the rotational stresses increase. To be practical the stress induced would need to be below the ultimate tensile strength of the material. Since these turbines need to spin at extremely high rpms and need to be fairly large to be efficient, there's no practical material that they can be made of.

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> and need to be fairly large to be efficient

This statement is not true. Actually the inverse is true, the smaller you can make the disks, the better the efficiency.

See https://onlinelibrary.wiley.com/doi/full/10.1002/ese3.1417

> As disc outer diameter decreases from 100 to 50 and 20 mm, the isentropic efficiency of the miniaturized turbines increases significantly from 21.46% to 30.78% and 32.21%, respectively.

Not Op, but I suspect the use of "efficient" here is used more in terms of "usefulness of output" rather than pure turbine efficiency.

For example, you might have a VERY efficient turbine, but if they're constrained to be small (and therefore low-output), you then need to have a squillion of them, which is not an efficient use of money or effort expended, in terms of output power total.

Now there's an interesting optimisation problem...

We've all played enough Factorio/Satisfactory to know what the end result is going to look like.

Given that the point he makes is that they've got high torque at low RPMs and that's what makes them interesting, I'm not sure that's relevant here. Frustratingly it doesn't go into any more details than that.

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