[dojomouse]

Love you for this! I had exactly the same “solar freaking roadways” thought, although at least that idea qualified by basic theoretical analysis of available energy and area for harvesting and conversion efficiency. It was an obviously terrible idea for other reasons :-) yet it still got a prototype…

I wasn’t sure about the droplet analysis so took your same numbers (25mm/h, 10m/s) and just worked out aggregate mass: 25mm over 1m^2 = 0.025m^3 = 25kg

0.5mv^2 => 1250J/h… so looks like we agree.

And to add a simple economic analysis of why this is such a dead-end idea:

Mawsynram, in India, is apparently the rainiest city in the world with roughly 10,000mm of annual rainfall - 10x the global average.

A given rain energy harvesting panel, deployed there, would generate 500,000J/yr… or 0.138kWh. That’s significantly less than what a typical rooftop 1m2 solar panel would generate in an hour on a sunny day. 0.138kwh is worth around 1.3cents at 10c/kWh.

A big roof might get you $1-$2/year. You couldn’t pay to clean your roof for that. You couldn’t even pay someone to answer an email enquiry about the install costs for your system for that. This solution would have to be VASTLY cheaper than paint to stand a chance of being viable.

There is a reason our existing systems to collect power from rainfall rely on vast existing landscapes and aggregation mechanisms (rivers) to concentrate the rainfall for us.

It is - in my view - a dead idea.

[itake]

If the raindrops were caught with a funnel, so the actual surface area of the device is very small, but the funnel is large, would that improve the economics? Maybe add in a water tank + hydro power to capture more gravitational potential energy from the water,

[CPLX]

Catch them with a patch of ground and use a river as the funnel and you’re really on to something.

[dojomouse]

It wouldn’t help with kinetic energy harvesting from the raindrops as that would go into the funnel as heat.

It might provide a way to harvest the remaining gravitational potential energy of the rain (possible funnel being your roof and guttering) but the only upside is that you could concentrate the energy with something that’s already there (and hence harvest over a smaller area). The amount of energy (and hence value) available would be even lower - unless you had a really high roof.

This is also the reason I abandoned my high school scheme of hydro turbines at the bottom of downpipes.

As the comments below say - you need to be working at the scale of a few major geographic features as a funnel before it starts to get really interesting.

[LeonB]

Given the explanation above, it would need to be a funnel that is cheaper than paint. If the analysis above Is accurate (I can’t vouch for it either way) then it’s hard to imagine a material strong enough at such a low price point.

It’s possible too that the proposed mechanism is related to electrostatic charge in which case funnelling would probably interfere.