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There's a wonderful blog called Do The Math, written by a physics prof at UC San Diego. I'm going to link to one particular article here in a second, but before I do, I feel like I should promote him a little bit- just about every single thing he's written there has been pure gold. He goes into specifics on all of these energy topics, and like the titled of the blog suggests, he goes through an issue and does the math.
I'd encourage EVERYBODY who's interested in alternative energy to go to the beginning of his archive and read all his articles . They're a magnificent education. But I'll spoil the ending for you, too: the main answer to the world's electricity needs is a photovoltaic system on most roofs. The world's need for liquid fuels just has to shrink. That what he says, anyway.
Mike Cantrell wrote:
Richard Hauser wrote:
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The first usable tech is microhydro,
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The second tech is pneumatic storage
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The second tech points to the third tech, thermal storage.
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The last energy storage is the one we are most accustomed to, wood.
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There's a wonderful blog called Do The Math, written by a physics prof at UC San Diego. I'm going to link to one particular article here in a second, but before I do, I feel like I should promote him a little bit- just about every single thing he's written there has been pure gold. He goes into specifics on all of these energy topics, and like the titled of the blog suggests, he goes through an issue and does the math.
I'd encourage EVERYBODY who's interested in alternative energy to go to the beginning of his archive and read all his articles . They're a magnificent education. But I'll spoil the ending for you, too: the main answer to the world's electricity needs is a photovoltaic system on most roofs. The world's need for liquid fuels just has to shrink. That what he says, anyway.
Ok, back to the point. The author takes up these exact issues here:
http://physics.ucsd.edu/do-the-math/2011/09/got-storage-how-hard-can-it-be/
Here are some of the highlights.
Let's start small by considering the 3 W-h of energy stored in a AA battery, as computed above. One kWh of energy is 3.6x106 J of energy, so our AA battery stores 10,800 J of energy. A mass of m kilograms, hoisted h meters high against gravity at g=10 m/s^2 corresponds to E = mgh Joules of energy. If we were willing to hoist a mass 3 m high, how much mass would we need to replace the AA battery? Have a guess? The answer is 360 kg, or about 800 lb. A battery the size of your pinky finger beats the proverbial 800 lb gorilla lifted onto your roof!
The lesson is that gravitational storage is incredibly weak. A volume of water the size of our bedroom raised even 10 m above our home in a precarious threat to the neighbors would store 0.625 kWh. That’s enough for 30 minutes of typical household electricity consumption. You’ll forgive me if I ignore efficiency losses. It’s not even worth the effort.
Electrolysis for the production of hydrogen tends to range between 50-70% efficient. Then the fuel cell converts the stored energy back into electricity at 40-60% efficiency for a round-trip efficiency of 20-40%. If you happen to want some of the waste heat, then you might boost the efficiency estimate (true for any of these storage methods, actually). But in a straight-up apples-to-apples comparison, the hydrogen method is a very lossy storage option. If it were dirt cheap and low-tech, I might be more excited about its potential, despite the poor efficiency. But since the opposite is true, I’m not revved up over hydrogen storage.
I spent some time searching for a hydrogen fuel cell that I could buy today with a rating in the 10 kW range (appropriate for a home). I saw some production models achieving efficiencies ranging from 40-53%, but never a price tag. If you have to submit a query to learn the price, you probably can't afford it...
We could store energy in something akin to a spring by compressing air.
The efficiency for compressing the air and later turning a turbine for electricity generation may be less than what one might find for a flywheel. The storage itself is not the hard part. I could go out today and get some lab-sized cylinders (~50 liters), which could store 1.5 kWh each- about like a golf-cart battery, although heavier and bulkier. But I would have a very difficult time arranging an efficient pumping and extraction/turbine system. If not for that, I would find compressed air to be an attractive system compared to batteries: minimal maintenance; no apparent cycle limitations, reasonably low-tech, and perfectly tolerant of remaining at low charge indefinitely.