mcld.co.uk

Britain has good stocks of renewable energy - plenty of waves, plenty of wind, plenty of tide - but not much sun. The one thing we do have plenty of is drizzle. Could we power the national grid by harvesting the energy in light rain? If so, Manchester would be lit up like Blackpool Illuminations!

You can actually work out how much power we might get out of the rain. Say you have a house with a roof that's got 10m² of surface area, for the rain to fall on. Now imagine you have some kind of special covering on that roof which takes the kinetic energy from all those falling raindrops and turns it into electricity. How much electricity would you get?

• Average rainfall varies around the country and around the year. But the total annual rainfall at Manchester Airport is 800mm, which is 2.2mm per day. So you multiply that by the surface area of the house and you get 0.022 m³ of rain falling on the house every day.
• How fast are the raindrops going when they hit the roof? The terminal velocity of rain varies according to how fine the rain is, between about 2 m/s and 9 m/s. Let's use a speed of 4 m/s.
• So how much energy is there? Remembering back to physics at school, kinetic energy is half times mass times speed². The mass of the water is the volume times the density (about 1 million grams per cubic metre). So the energy is 0.5 * 0.022 * 1000000 * 4². That's 176,000 joules.
• So now we convert energy-per-day into a power measured in Watts. The grand total: 2.037 Watts!

Hm. Great. You also have to take into account the inefficent conversion - no device could really capture all the energy from the rain. So let's say that optimistically you could get about 1 Watt of power from the rain falling on your house roof. That's certainly not going to power your lightbulbs. So that's why there aren't companies selling drizzle-power systems...