# Is it possible to convert rain drops to energy?

Would it be possible to convert water drops into energy using a dynamo? A heating mechanism could then be used to transform the water back to a vapor to start the cycle again.

• They actually carry very little energy. I've seen calculations and it's not worth it. Google should find some data for you Commented Nov 9, 2016 at 17:55
• Also what's the point of the heater and turning them back into drops? Commented Nov 9, 2016 at 19:58
• @ChrisH That sounds very much like him thinking about a perpetuum mobile
– user2451
Commented Nov 10, 2016 at 12:32
• @JanDoggen it does. And that's why I just commented rather than working on an answer before knowing Commented Nov 10, 2016 at 12:51
• I dunno, it's how the whole hydrological cycle works. You could use black surfaces to speed it up if you wanted to.
– Móż
Commented Nov 12, 2016 at 22:10

It's possible, but the best way to do it was devised over a century ago: hydro power. Let the high land collect and channel lots of rain, and then put a turbine on the channel.

The problem is that raindrops, as they fall, have very little energy - very little mass, and pretty low velocity too. Once they've fallen on high land, they've still got a lot of gravitational potential energy; and as they're channeled by the lay of the land, the combined mass becomes high, and there's enough velocity, to make the kinetic energy sufficiently high to justify investing in the infrastructure to harvest the energy.

• More accurately "once they fall on high land and you collect a lot of them, the collection has a lot of energy". Hitting the ground doesn't add energy to the drop.
– Móż
Commented Nov 10, 2016 at 0:29
• Comment only: Raindrops dissipate even more energy than one may expect when falling because they do not fall in the drop shapes that everyone shows in drawings but as wider and more flattened 'blobs' with greater frontal areas and so lower velocities. Commented Nov 24, 2016 at 12:38

Would it be possible to convert water drops into energy using a dynamo? A heating mechanism could then be used to transform the water back to a vapor to start the cycle again.

Any rainfall can be used to supply energy but the energy available for typical volumes and "heads" is so small as to make the exercise not worthwhile.

An exception is if you are in an extremely rain-blessed environment with a large collection area and can channel the water so it falls in a controlled manner over a significant "head", or at good volume and flow rate but little 'head'. In the first case you end up with a microhydro system and in the second something more akin to a traditional waterwheel.

The problem is the available energy compared to available rain and typical energy needs (or desires). Below I'll start with 100% efficiency assumed. Anywhere I say "=" or equals it usually means 'close enough to = that's it's not worth saying "about" all the time.

A litre of water weighs close enough to 1kg. A US gallon is close enough to 4 kg for this exercise. A litre of water falling through a 'head' of 1 meter (about 3 feet) acquires a kinetic energy content of about 10 Joule = 10 Watt seconds.

Watts seconds = litres x metres x 10 x efficiency

That is enough to light
a 10 Watt bulb for 1 second or
a 1 Watt bulb for one second or
a single small bright high efficiency white LED (at 50 milliwatts) for 200 seconds.

A single modern very high performance white LED run at 16mA x 3V = 50 milliWatts will produce about 10 lumens of light. That's enough to read a book by comfortably. Above we said that 1 litre.metre of water at 100% efficiency would operate this LED for 200 seconds. To light it for an hour requires 3600 seconds/200 = 18 litre metres at 100%.

If we have a 10 foot/3 metres fall from gutter to generator we need 18/3 = 6 litres per hour to do this at 100% efficiency. If roof area collecting water was 1000 square feet = 100 square metres we need only 6/100 mm/hour at 100% efficiency.

Real world efficiency overall of such a system is liable to be around 20% - over 50% may be possible with great care and much less than 20% is easy :-). At 20% we need 0.3mm/hour of rain to run our LED.
That's doable.
Alas, our real world desires soon catch up with use.
A mains LED bulb of 5 Watts is "small". That's 5/.05 = 100 x larger than our LED
= 30mm/hour of rain or a bit over 1 inch.
You CAN get more than 1" per hour of rainfall in some places just occasionally, but if you've ever seen it you'll remember :-).

If it takes extremely heavy rain on a 1000 square foot roof to light a single 5 Watt bulb, larger power levels will be unachievable by this means.

If you can manage a good flow rate (small stream or better) AND a good head then useful power can be provided.
1 US gallon per second x 30 foot head gives about 100 Watts output.
Not enough for heating but useful for light or maybe a small TV (but probably not both).

To supply say 10 kW to a home with a mix of cooking, heating, air conditioning, lighting, TV etc requires 10,000/10 = 1000 litre.metres of water flow. If a dam has say 100 feet = 30 metres of head that's about 30+ litres = 8+ gallons per second x 100 foot head to power your home. Or more.

This DOES get supplied by falling raindrops and DOES get returned by solar heat in due course and is not a "perpetuum mobile" as someone suggested, as the sun is non sustainable, but the scale is utterly too vast for all but a very few individuals.