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Comment by mixedmath | Hacker News Reader

I'm impressed at the relatively high efficiency of the system. Pedalling at 150 watts and recovering 100 watts of electricity is higher than I would have expected from such a simple system.

I'm very curious how much better the system would perform if the flywheel were attached directly to the drive-train instead of via friction roller. In practice this might not give much higher efficiency, but I would guess that the chain and primary sprocket would wear out slower than a tire friction-running a flywheel.

(They mention that they don't do this because this because it would be harder to build. I believe that. But I'm still curious.)

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I've done enough bike wrenching to have raised an eyebrow on that.

What you need is two gears on the flywheel, rather than one, a gear rather than a friction wheel on the generator, and some way of tensioning the second chain which you run between them. This isn't the kind of harder to build that should stand in the way, although I will grant that the single gear was already on the wheel and changing that does involve, well, changing that.

This gets ~10% efficiency back, which for a generator is huge. It's probably the only efficiency gain left, other than a gear cassette to optimize power to a target voltage.

I'm wondering if the energy loss through the tensioning and/or internal gears is at all significant.

Instead, would it make sense to have a heavy flywheel, a fixie gear, and a way to slowly ramp up the power draw, so the power draw itself is the "gear" that allows you to go from a hard-to-start heavy wheel to a smoothly-running wheel?

> I'm impressed at the relatively high efficiency of the system. Pedalling at 150 watts and recovering 100 watts of electricity is higher than I would have expected from such a simple system.

It doesn't seem very surprising that motion can be converted to electricity at 66% efficiency. Normal power plants[1] can convert heat to electricity at 64% efficiency. It'll be much more interesting to see the end to end efficiency of this, ie. how many calories were consumed compared to electricity were generated.

[1] https://en.wikipedia.org/wiki/Combined_cycle_power_plant

A well-trained athlete can achieve about 33% efficiency in terms of calories used (not 'consumed' - we're talking about energy coming from glycogen and fat, in addition to whatever is eaten. The human digestive system can only process a couple hundred calories per hour before it starts diverting blood flow to the digestive system, which impairs athletic performance significantly.)

> but I would guess that the chain and primary sprocket would wear out slower than a tire friction-running a flywheel.

Yeah, direct drive trainers vs wheel-on trainers have the advantage of not ruining a tire or requiring a trainer specific tire.

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