My question essentially is how much water to power green house LED lights. Specifics bellow :

Say I have a warehouse with a flat roof with a bunch of water tanks or the whole thing being a water tank. Using say 30 feet High and 20'Lx20'W. How much power could I generate utilizing the roof to store water?

One lighting fixture could run 400 watts.

I was pondering these last few days on this, I've googled a couple of equations but not sure I used the right ones. Thanks for your input in advance.

  • $\begingroup$ I'm trying to understand the logic behind the scheme. Do want to establish a mini hydroelectric system using a roof as a rainwater catchment that would drive a small Pelton wheel type device connected to an electrical generator 30 ft below the roof? If so, electricity would only be generated during significant rainfall events. Have you considered having batteries to store any electricity that may be generated for times when there is no significant rainfall? It seems like a system that would be used intermittently. I think you might be better off to install solar panels instead. $\endgroup$
    – Fred
    Oct 16 '19 at 3:12

Not very much.

One liter of water has a mass of about 1kg. The acceleration due to gravity is 9.8m/s^2. One Joule is one Newton exerted over one meter. So one liter of water falling one meter loses 9.8 Joules of potential energy.

Your roof is 9 meters high, and has an area of 36 meters (6m * 6m). If you cover that with a meter of water that's 36 cubic meters, or $36 \cdot 10^3$ liters. The average drop is 9.5 meters, so you get $w = (9.8 \mathrm{m/s^2})(9.5 \mathrm{m})(36 \cdot 10^3 \mathrm{l})(1 \mathrm{kg/l}) = 3.35 \mathrm{M J}$.

Which sounds pretty impressive, until you do just a bit more math and discover that's 0.93 kilowatt-hours, or enough power for your 400W panel for about two hours, twenty minutes -- and that's assuming 100% energy conversion, where less than 50% is probably more realistic.

There's some $x^2$ terms in there, but you're talking about needing a water tank that's taller than your greenhouse, and enough rain to keep it filled.

  • 2
    $\begingroup$ One meter of water on a roof would be impressive to see. That would be one serious rainfall or inundation event! With 36 tonnes of water on the roof, one would hope the structure supporting the roof would be strong enough. $\endgroup$
    – Fred
    Oct 16 '19 at 3:00
  • $\begingroup$ @Fred I got to the "how much rainfall do you need?" part of my analysis and was so impressed I decided to leave out the "how strong a building do you need?" Wind or solar plus batteries -- maybe yes. But not water, unless you live under a waterfall. $\endgroup$
    – TimWescott
    Oct 16 '19 at 16:26
  • $\begingroup$ Awesome, Thanks for doing the math! About the same results I came to as well. Not much. Yes I know solar panels would work, It just feels like if you had a bunch of water on top of your roof acting like a Hydroelectric dam it would be enough supply some serious power. Guess thats why dams are so big!! haha well thanks everyone for chiming in. $\endgroup$
    – Danube219
    Oct 16 '19 at 17:00

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