Water evaporation-driven toy motor

For practical purposes you need to consider energy returned on energy invested not just power flow rates, although power flow rates can be an issue. Less cost and less environmental damage to just use a battery.

A simple optical sensor could sense rotation, and there's probably some weird meteorological situation or chemical plant process meter that would find a rotating optically detected non powered humidity sensor to be useful.

For chemical process plant work something that measures humidity would be super boring because there's a zillion competing technologies, all mostly unpatented by now. The fun would begin with selective uptake of "weird stuff" non-water substances using the same physical design. Maybe you could detect H2S leaks or hydrocarbon leaks or ...

I was the only poster to the previous thread about this at: https://news.ycombinator.com/item?id=9732609.

Here is what I wrote:

I also work in this space, so the headline caught my attention. This is far from the first. Some other evaporation driven engines include:

https://en.wikipedia.org/wiki/Drinking_bird

https://en.wikipedia.org/wiki/Energy_tower_%28downdraft%29

https://en.wikipedia.org/wiki/Barton_evaporation_engine

The solar updraft tower and vortex engine also operate in a similar space, meaning that all of them operate using the temperature difference between ground and tropopause which feeds the water cycle.

It is my personal opinion that evaporation-driven engines (or something quite similar) are the most promising energy source (on this planet). Essentially, they are an alternative source of solar energy which doesn't have the problem of needing the sunlight to be present to work.

The main issues, as with most new energy sources, are efficiency and cost. The Columbia motor costs $5 for 50 microW, or $1x10^5 dollars per watt. You generally want something nearer 1$/watt for primary energy use. The efficiency in terms of water or thermal energy is not stated, but is likely similar to drinking birds, which have a thermal efficiency around 0.01%. The theoretically possible efficiency for the evaporative half of the water cycle is on the order of 12%, so there is a lot of room for improvement.