Mythbuster Hyneman on weird energy
In the new Mother Jones, Jamie Hyeman of The Mythbusters looks at a handful of odd ideas in alternative energy sources, from cow shit to grape juice. From Mother Jones:
ENERGY SOURCE: RaindropsLink
HOW IT WORKS: The Atomic Energy Commission uses special plastic to convert raindrops' falling motion into electricity.
BUSTED: JH: "A when-pigs-fly kind of scenario. It's millions of times more efficient to collect hydroelectric power through a dam than raindrop by raindrop."
ENERGY SOURCE: Dirty diapers
HOW IT WORKS: A British company turns poop and plastic from diapers into gas and oil.
BUSTED: JH: "Are you really going to be able to isolate diapers in such huge volumes that you're running your entire country off of gasoline powered by diapers? No."
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I wonder how much power you could get off of a rooftop that channeled all its rainwater into a single vertical pipe, let the pipe fill up to give it some pressure, then converted it to electricity or something. Anyone have the formula for the potential energy of a column of water two stories tall?
And when you've got the electricity, pipe the water into a storage tank for watering your lawn later.
We're surrounded by energy. Some is concentrated, some less concentrated. Whatever has a chance of making a substantial contribution is worth considering.
As always, the energy we waste (cars waste 85% percent of the energy in gasoline; it will now pay to fix up poorly insulated and glazed houses) doesn't have to be discovered.
I've always wondered why human waste can't be harnessed to make methane or something. Practically every city in the US has a waste water treatment plant, so it seems the infrastructure is already in place and with a few modifications those waste water plants could also be energy producing plants.
The last one on their list, "potassium carbonate, which absorbs carbon dioxide that is then converted into methanol, gasoline, or jet fuel", sounds really unlikely.
Potassium carbonate is itself the product of potassium hydroxide and carbon dioxide; if that reaction completed, no further CO2 should be absorbed.
(Disclaimer: When I start doing practical chemistry, run like hell.)
GREGLONDON #1: It depends on what actual turbine or waterwheel you're using. One random example formula from a turbine maker is be Head[ft] X Flow[cuft/sec] / 12 = KW. Flow depends on the size of the pipe and how much it's filled by a typcial rainfall.. In using rainwater, you will probably have an extremely low flow for light rains, and high flow for a rainstorm but only for a short time. Two stories may be sufficient head, though once the flow is low enough it may not help... and most turbines are designed for medium or high flow, at a low flow it may be really inefficient or just not work at all... probably an impact pump might be the best use of the power (the water builds up in the device until there is enough pressure to push a piston that can do one push of mechanical work, usually pumping well water into a tank). If you're just collecting water from the roof of your house, it might be enough to recharge a small battery now and then. Or collect into a tank and use for a very infrequent high load need. One solution I've read about for having a continuous supply of power is to use water for PV power storage instead of a battery: overprovision PV panels, and during the day use the excess power to pump water up into a tank, and let it out slowly at night.
Is my math right here?
Say a 500 gallon tank on the roof of a 2 story building.
500*8.3/2.2=1,886kg of water * 6 (meters) * 9.8 (acceleration of gravity) = 111,000 Joules.
111,000/60(seconds)/60(minutes)=30.83 watt/hours, or enough energy to power a 30 watt light bulb for an hour.
With my limited knowledge of physics, I think your math is right, however, all that water does not hit at once, and the velocity and any energy gained by the mass-impact of the raindrop is dispersed by being distributed over time.
So, the reasons why human waste is not used for energy are twofold:
Regulatory: You have to overhaul government to get to do anything different than the status quo, which is a lengthy and expensive process;
Technical: The microbes that like to digest human waste are different for each type of waste, and don't fare so well in the presence of the other type of waste & bacteria. We need an infrastructure that separates the two types of waste.
A large metropolitan area could harness human waste for energy production if the types of waste were separated instead of combined.
"I wonder how much power you could get off of a rooftop that channeled all its rainwater into a single vertical pipe"
How much energy do you think it takes to raise all of your bath-water two stories off the ground?
@Bardfinn & Iguanoid: A lot of US water treatment plants already use digesters which produce methane in some quantity.
In my home town (Olympia, WA), the water treatment plant is out on the peninsula in the bay, and I grew about directly across the water. They just burned off the methane they produced--you could tell because they burn it in the atmosphere, so one of the stacks on the building is a giant torch that runs nearly continuously.
It probably would not run too large of a generator, but it would be something.
With my limited knowledge of physics, I think your math is right, however, all that water does not hit at once, and the velocity and any energy gained by the mass-impact of the raindrop is dispersed by being distributed over time.
Yeah, but that figure IS over time. 30 watts for 1 hour. You could theoretically get 111,000 watts for 1 second, or 3 watts for 10 hours. In other words, not much.
Strathmyer: Haha, good point. That's it!!! Tapwater powered generator turbines! Free energy! :)
When you put it that way, it really illustrates how little energy a "roof tank" could store.
More on the human waste.
Think of anaerobic digestors as giant stomaches. Put bad stuff in your stomach and you get out bad gas...Humans eat all the wrong foods. Mix in the T.P., dish soap, laundry soap, industrial waste and everything else that goes into a metro sewer system and it is hard to keep the bacteria happy. Then you get soloxanes from make-up and some fast foods and it turns to glass in the pistons of your gas fired generator.
Cows are perfect for this, though. Their stomaches are nature's best methane generators but they only use about 15% of the energy from the foods they eat. Put that in a digestor and everything is balanced pretty close to perfect for the microbes and then you can mix in some other waste from a food plant or similar and you get a pretty good methane supply. This gas can be used for electric production, heat or cleaned up and put back in the natural gas pipeline.
for #9 strathmeyer:
100,000 sq ft roof (typical big box roof top)
2 inch rainfall
100,000 sq ft x 1/6 ft = 16,667 cubic ft of water
Assume the building is 20 feet tall and we empty it all in 1 hour, according to the formula given in post #5:
Head[ft] X Flow[cuft/sec] / 12 = KW
20 ft x 16,667/(3600 sec)/12 = 7.7 kW or 7.7kwh delivered over an hour. Enough to run 128 60-watt lightbulbs for 1 hour
That's some electricity but really, how often do you get a 2 inch rainfall? Not as often as you think, I bet.
@ #3 Iguanoid
Waste Treatment Plants, that treat waste water before returning it to the water supply, always have flare stacks to burn methane (not sure about other gases). the Plants usually gather some of that gas to power the Plant itself. The stacks are then used to burn off excess gas. At least that's how we do it at the City of Calgary.
The second scenario in the article quote is a nonsequiter. Just because breaking down diapers won't power an entire country doesn't make it an unacceptable alternative energy source. No country gets its power from a single source.
The problem with almost every alternative energy source (and there are numerable possibilities) is that they are only useful if they produce more energy than they require for production. The dirty diaper scenario is a great example of this. Yes, you could break down dirty diapers (or any other organic product) into oil and gas, but how much oil and gas would the factories require to do this?
This is a common problem with recycling plastics. Yes, recycling plastics saves us from needing to use as much oil (as direct constituents), but how much oil is expended in the process of the recycling? It ends up being cheaper and more energy efficient to make it from scratch than to recycle it. The only reason recycling plastics has merit is that it prevents those plastics from sitting for eternity in a landfill.
A truly useful alternative energy source must be able to provide us with energy at a lower cost than what our current energy sources cost. As the cost of our current energy sources continues to rise, we may find previously costly sources to be acceptable alternatives.
@#4: Reading the original article, Hyneman does make the point that you need energy to convert the CO2 into something -- CO2 by itself can't be used to create energy (it's already one of the simplest, most stable compounds we know of). Assuming something like http://www.technologyreview.com/read_article.aspx?ch=specialsections&sc=solar&id=18582&a=
could be done efficiently, it would use solar energy to address two problems: reducing existing CO2 quantities and transforming otherwise transient solar energy into more stable chemical energy.
It occurs to me that one of things people always say about biodiesel is that "it smells like French fries" (or whatever the oil came from). For this reason alone, I think nobody wants fuel made from dirty diapers...
So, the chrysler building is a thousand feet tall, and has a footprint of about 15,000 square feet. New York gets about 44 inches of rain a year. that's 55,000 cubic feet of water (just under half a million gallons) a year, a thousand feet in the air.
How much power could you get out of the rain that hits a single skyscraper in a year?
9e9 joules???
oop, 5e9 joules,
1.4e6 watt/hours
at 5-15 cents a kilowatt hour, I think that works out to be about $70-$210 worth of energy for the entire year.
but you could keep a 150 watt light bulb on the entire year for the cost of plumbing.
Gah! I had nearly finished composing a sophisticated analysis of the energy content of rainfall - and then the power went out.
Basically, (working from memory) you get about 5 joules of energy per square meter during a rainy month, if that month is January in Corvallis Oregon, e.i. a month with about 7 inches of rain. Assuming your raindrops are about 3mm diameter - mass and size drastically change the terminal velocity, and thus the kinetic energy of the drops. If you have your rain-power plastic on the roof of a tall building, and collect all the rain that hits it for additional power production from potential energy, it may be worth it if the plastic is fairly cheap, and you were going to collect the water anyway.
WHERE'S MY PAINT-ON SOLAR CELLS? The flying car I can wait for
If you guys are interested in how to do the math of the energy available from different energy sources, check out David MacKay's book -
http://www.withouthotair.com/
it goes through the physics of solar, wind, heat storage and heat pumps, etc. The book grew out of a course on how to do numerical estimates, and it's really an eye-opener.
eg. how many watts per square meter do you get via wind / solar / coal / nuclear, and so what is the country going to look like if/when renewables become common place.