Energy From Raindrops 144
conlaw writes to share that according to Discovery.com scientists have found a way to extract energy from rain. A new technique could utilize piezoelectric principles of a special kind of plastic to generate power from falling water in rainstorms or even commercial air conditioners. "The method relies on a plastic called PVDF (for polyvinylidene difluoride), which is used in a range of products from pipes, films, and wire insulators to high-end paints for metal. PVDF has the unusual property of piezoelectricity, which means it can produce a charge when it's mechanically deformed."
I can't believe... (Score:0, Interesting)
Re:I can't believe... (Score:4, Interesting)
http://www.uic.edu/aa/college/gallery400/notions/histories.htm [uic.edu]
"The hospital of Bicêtre, France boasts a prodigiously deep well underneath, dating from 1735. The horizontal wheel that pumped the water was turned
initially by twelve horses, then, starting in 1781, by 72 men, taking shifts on a 24 hr day. These workers were eventually replaced by epileptic
patients and "madmen" in residence at the hospital."
I would also challenge the notion that fluorinated plastics can be produced energy efficiently enough to actually produce an energy surplus by collecting raindrops. I might be wrong
though, but out of laziness I'll leave the proof to somebody else.
Re:Let's do the math on this one (Score:3, Interesting)
Re:Let's think about this for a second... (Score:4, Interesting)
Now, if you could build a completely frictionless waterwheel and put it underneath each raindrop, you would get a lot more energy than if you caught the same raindrop in a bucket and then let it drip onto the water wheel (which is effectively what a hydroelectric plant does). There are two problems with this idea. The first is that rain falls over a large area. The total energy from all of the raindrops is a lot, but the individual energy is quite small. The reason hydroelectric seems like a good idea is that, although you only capture a small fraction of the energy from each drop, water falls in to the reservoir from the surrounding hills, so you are capturing rain drops from a very wide area. Once you concentrate rain enough, your losses to friction become a lot less (try building a waterwheel that will spin when a single raindrop hits it, then try building one that will spin when you pour a bucket of water on it and see which is easier).
This piezoelectric idea is quite neat, because it allows you to capture a significant proportion of the energy from each rain drop and convert it directly into electricity (although you'll probably lose a lot transforming it into anything that you can draw a stable current from). It has the same problem that the hypothetical rain-powered water wheel had, however, and the same problem solar power has: You need a lot of surface area to get a decent amount of energy out. If we assume that it is twice the power output per unit rain of a hydroelectric plant (water falling more than twice the height, but lower efficiency power conversion. Entirely made up number, but probably within an order of magnitude) then it will need half the area of the hydroelectric plant to generate the same amount of power. Note that this isn't just the area of the reservoir, it's the total area that rain falls.
Some more back-of-an-envelope calculations:
Annual rainfall where I live is around 1-3 metres (more slightly inland than on the coast). Let's say 2m as an average. Cumulous cloud (the kind that typically causes rain) forms at 2-16km. Picking a number somewhere in the middle, let's say 8km for the average distance rain drops fall. That means, every year, two cubic metres of rain fall 8km per square metre of ground. That's 2,000 litres, which means roughly 2,000 kg. The total energy in this is calculated as mgh, so: 2,000 x 9.8 x 8,000 = 156,800,000 J.
That sounds like a big number, so let's break it down. Electricity is usually sold in kWh. One W is one J/s, so one kWh is 3,600,000J. That means this gives us 43.5kWh/year energy generation for every square metre of land we allocate for it (note: I am assuming 100% efficiency here, while I would be really surprised if it got 20% in the real world). The average household uses something in the range of 3-4MWh of electricity per year, so you would need 1,000 m^2, or roughly a 30x30m area of land per house. Assuming a more reasonable efficiency, you're looking at somewhere closer to 60x60m, which is still under an acre. Of course, you could probably combine this with solar energy, since solar power is pretty useless when it's raining and so you wouldn't need to supply the entire house's electricity with just this. If they can get efficiency to the 10-20% range, it seems feasible for a lot of uses.
Re:Let's do the math on this one (Score:1, Interesting)
To recap: inch/hour, miles/hour, gallons/(day*acre), pounds/(acre*day), (foot*pound)/(day*acre), (foot*pound)/second, horsepower, watts, kilowatt*hour, watt*hour/day
Sure, with metric measures the list would be just as long, but you would find the same units used everywhere instead of switching between inches, miles, feet, etc.
Re:why rain? (Score:2, Interesting)
Re:Let's think about this for a second... (Score:2, Interesting)
From velocity=acceleration*time, you can infer that the rain drop is reaching that velocity in about a second, after falling about 10 meters(OK, so it would be going slightly slower than 10 m/s and have fallen somewhat less than 10 meters, whatever). So your availability calculation should be 2000*9.8*10=196,000 J, a factor of 800 less than what you stated. So if your device is 100% efficient, my still generous estimate is that you would need 800,000 square meters, not 1000, which is more like 900 meters on a side, and more like 200 acres.
Re:meh (Score:3, Interesting)
Hospital Falling Wastewater Toilet Flush Energy (Score:0, Interesting)
I remain convinced that all 3-story and up hospitals could turn a generator, saving one month a year worth of electric bill. But this is where everybody makes the big mistake, thinking the electricity produced has to be regulated and purified to main power grid-acceptable electric current. It does not! You don't send the electricity into the power grid; you run a straight wire carrying the variable current directly over to the hospital's water heater element. Hmm, as a picture this makes a good Record of Invention Update for 2/10/2008 [newpath4.com].