Saturday, January 19, 2008

Rain Water

I consider myself an environmentalist. I try to do the basic things around the house that help reduce my carbon footprint (you can calculate yours here) like installing compact florescent lights, installing power strips to prevent appliances like TVs from pulling current while "off", driving the minimum possible, eating vegetarian, and drinking soy milk instead of the dairy variety (cows are surprisingly bad for the environment). And I'm always on the look-out for something crazy and interesting to try for the environment. An idea I've been tossing around in my head (especially now that I live in rainy Puerto Rico) is harvesting power from rainwater. Part of my recent interest in rainwater, I must admit, has to do with the fact that we average about one water-outage per month here; I want backup water. However, my original interest, a year or two ago, was actually in generating electricity from the rain falling on my roof.

Here are some order-of-magnitude calculations for what I might expect to get from this. Quick research indicated that average annual rainfall here is 62 inches (about 150 cm). This means that each square meter of rooftop collects about:

150 (cm) x 100 (cm) x 100 (cm) x 1 (g/cm^3) / 1000 (g/kg) = 1500 kg

of water per year. If a story of a building is 5 m high, then the potential energy in that water is:

Force x Distance = Mass x Gravity x Distance = 1500 (kg) x 10 (m/s^2) x 5 (m) = 75,000 (J) = .02 (kWhr)

So PR generates .02 (kWhr/m^2/story) annually. My apartment building is about 10 (m) x 20 (m), and is 4 stories high, bring the energy of rainfall to 16 kWhr annually. If electricity costs $0.10 per kWhr, I could save a whopping $1.60 off my power bill annually. That probably won't ever offset what it would take to build the generator. Sigh.

Of course, dams do exactly the above, but with an enormous collecting area (not just a rooftop--an entire drainage basin). It looks like rain on my roof isn't going to power my computer, though. But I still might try to use the water to flush my toilet when the water goes off next weekend.

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