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I've understood the powerbanks are actually not that effiecient, some power is lost on the way. And lithium is problematic. The price of a dynamo probably comes more from the hours spent making the complicated parts. And you probably won't need to eat any more than you would otherwise as you're putting out as much power as you can anyway, you'll just be that much slower.
Samuli-
Agree Li is nasty. But the human body is only about 25% efficient so much worse than power banks.
Time spent making fiddly parts still has a carbon footprint: if it's people doing work, they need to eat and consume other stuff which all has a cost.
The only bit of the cost of the dynamo which doesn't have environmental cost is the IPR or value in its patents.The challenge with a lot of green technologies is to get them to the point that they use less energy in their manufacture than they save in their usable life. A bike dynamo has its advantages but it's far from obvious it passes that test!
frank9755
Yes, but there's no such thing as a free lunch. The energy has to come from somewhere. In this case you have to eat (even) more food (7 Days croissants) to generate the power, rather than taking it from a plug into a power bank. I reckon you are probably less efficient than the average European power station so your dynamo will have a higher carbon footprint per Watt.
And your dynamo took resources to manufacture. As do AAs and power banks. None of us know what the relative amounts are but, the rule of thumb is that more complex and more expensive something is, the more resources and energy it takes to produce it. So, I would be willing to bet that many times more resources went into producing your £x00 dynamo than my 4 power banks and 4-pack of AAs from Poundland!