## weight batteries

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### weight batteries

Something I have wondered about for a while and finally getting around to start investigating...

Would it be possible to lift giant boulders (rocks) and use them as a type of battery?

Like say you have a solar panel array that runs a DC motor connected to a very geared down pully system that slowly but surely lifts an incredible amount of weight (say 5 tons). Then at night time (or whenever) you release the motor lifting the weight and then connect an alternator and release the 5 tons so that it slowly drops running the alternator.

Is that even feasible?

I just started looking up the calculations for this. Please check my math.

For an example I take my laptop
It says 4.74 amps at 19V.
That would be 90 watt hours. right?
If I ran my laptop 10 hours a day that's 900 watt hours.

using google 900 watt hours = 663 foot pounds per second (663 pounds moving 1 foot every second?)

5 tons = 10,000 pounds
10,000/663 = 15
Does that mean I would need 5 tons moving 1 foot every 15 seconds to generate enough power to power my laptop for 10 hours? This is the point I am at.

How do I calculate this?

Would I go
90 watts per hour to power my laptop/60 minutes / 60 seconds = .025 watts per second

.025 watts = .02 foot pounds per second

would that be .3 inch pounds per second?

How do I get to something that makes sense?

Is it 0.02 feet * 1 lbs / seconds? or is it 0.02 lbs * 1 foot / seconds? I'm a little confused here.

zetreque
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### Re: weight batteries

I suppose you could do that, but I've never heard of anyone actually doing so.

Something that is done on a large scale in some places is a system that involves pumping water to a large reservoir at a higher elevation, and the using hydroelectric generations to release same at night. However, such systems work best for large scale power generation at the municipal or regional level, not for individual homeowners.

Darby
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### Re: weight batteries

God, Robert C. Moses, Thomas Jefferson's slaves, a thousand CBI veterans, and about a gazillion bike riders have done something like this. A "gut estimate" tells me you need, within a factor of ten, 50 pounds moving 1 ft per second to do this, i.e., between 5 pounds and 500 pounds. If you want something more exact, use the metric units; calculations are easier.
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### Re: weight batteries

So, don't trust my "gut estimate"; I was flim-flammed by the ole velocity squared trap.

So use metric - it's easier. Take your 90 W and convert it to kg-m^2/s^3. Set that equal to kinetic energy, arbitrarily pick a velocity and solve for mass. 1 kg weighs about 2.2 pounds.

Your original estimate was close. Don't worry about ft-lb vs lb-ft; they're the same thing.
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### Re: weight batteries

http://www.scientificamerican.com/artic ... el-design/

I saw this in SciAm, couple years ago. Seems more doable, at the domestic level.

Braininvat

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### Re: weight batteries

Hah I was actually designing a cat toy with a weight battery in it.

About a week back I hammered a nail into the ceiling with a string dangling from it. Attached a cat toy to the end, and the kittens loved it!

I started looking for ways to improve it, e.g. make the toy more responsive. I figured that there's a ceiling fan nearby that I never really use, so I was going to tie some floss to it (since floss is smooth with a high tensile strength) and put an offset loop on the ceiling near it. This would allow the fan to basically be a mechanical energy pump; it'd pull the string back-and-forth, once per rotation (attached with two strings tied together at the end so that it actually pushes and pulls upon rotation, rather than going slack for half of the rotation).

Then I figured that I could recover oscillatory motion by running the floss through, say, plastic drink bottle with its top and bottom cut off. Then I could use that to motion in something hanging down for the cats to play with. The huge perk would be that I could turn the toy on and off using the wall switch that's already there for the fan.

Then this got me all interested in what other stuff I could do with the power of a string that's moving back and forth. I was trying to treat it like electric current, so I tried making various circuit components for it. For example, the weight battery was going to be a capacitor for the system; it'd store up energy, then release it in some dizzying display of cat-toy-suddenly-burst-into-rotation for good ol' kitten fun.

I know this is silly, but it was really a lot of fun. I almost wanted to write a small article on it, "The Engineering of String Theory, for Cat Toys". It'd actually be sufficiently difficult to merit publication, e.g. designing units which transform arbitrary string motion from one function to another in the general case, so if anyone wants to go for it I'd personally love to read that paper!
Natural ChemE
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### Re: weight batteries

Btw, I'd suggest using WolframAlpha for calculations.

A weight battery would store about
$E_{\text{stored}}=g_{\text{gravity}}{\times}m_{\text{battery weight}}{\times}{H_{\text{battery height}}}$,
so if you lifted a weight of, say, $50\text{kg}$ to power your $90\text{W}$ laptop for an hour, then you'd need to lift the battery by about
$H=\frac{90{\text{W}}{\times}1{\text{hr}}}{9.81\frac{\text{m}}{{\text{s}}^2}{\times}50{\text{kg}}}{\approx}661{\text{m}}$,
which you can get from the WolframAlpha query (90 W * 1 hr) / (50 kg * (9.81 m / s ^2)).

WolframAlpha also gives you comparisons. So, say hypothetically, you built that weight battery by lifting 50kg up nearly by over half a kilometer. Then it'd have an energy density of about (90 W * 1 hr) / (50 kg), which WolframAlpha reports to be about 0.7% to 1.5% of what a typical lithium battery stores (in terms of energy stored per weight of battery's mass used used for the storage).

'course not all energy is created equal. The mass battery would store mechanical energy while a lithium battery stores electrical energy. There's a huge efficiency loss in converting either way, so your laptop would probably need much more stored mechanical energy to convert to the full amount of required electrical energy.
Natural ChemE
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### Re: weight batteries

Last night's NOVA was all about the search for the superbattery, and touched on pretty much all the battery techs out there being developed, including "pump hydro," which is the largescale reservoir version of a "weight battery." Excellent show, look for it at pbs.org, should be a link. I was fascinated by the "crustal abundance" approach this one guy took, making batteries out of elements very common in the earth's crust and using saltwater as the electrolyte solution. And it explains, very clearly, why lithium ion batteries are so prone to fires and explosions, except for this new type that replaces the electrolyte solution with a solid plastic electrolyte sheet that can still pass ions. You will also learn what dendrites are, in the safety woes of older lithium batteries. Truly outstanding episode.

Braininvat

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### Re: weight batteries

I don't have TV anymore but I keep up with what's coming out on their youtube. They posted a couple full length videos about the battery stuff a couple days ago.

If only money went into research this stuff, instead of the backward energy policies we have.

zetreque
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### Re: weight batteries

http://www.pbs.org/video/2365946487/

complete episode

yeah, largescale storage will be so important for intermittent forms of power to really come of age.

Braininvat

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### Re: weight batteries

If you are looking for low tech energy production methane seems the most practical to me. All you need is a few milk cows and something to convert the waste to methane in then use internal combustion to generate electricity.

Although the critics of alcohol from grain are right that it is a horrible idea that may not be true for a small farm. You can run internal combustion engines on pure alcohol as they do in Brazil and alcohol has other uses. You can even use the waste as feed for livestock.

I have a friend that partially heats his house from compost. It is very low tech and produces a useful byproduct.

None of the low tech options appeal to me personally because they require a change in lifestyle. They could be a source of entertainment if you have the right personality.

Addressing the original concept I think that BV has answered the question. It is a matter of reversing the flywheel concept. Instead of a light weight at very high speed you need a very heavy weight at low speed. I think a house would do as the heavy weight. Lift the house using solar power and let it fall to generate electricity. A reversible hydraulic pump could be used to both lift the house and generate electricity. An average house weighs 120,000 lbs. If you made the house out of something heavy like concrete and stone you would need a house that weights 720,000 lbs lifted four feet to generate 1000 watts for one hour not considering losses. Not very practical. We need something more massive than a house :-) A structurally liftable swimming pool 10ft deep a 100ft long and 25ft wide would be heavy enough to generate 1000 watts for 8 hours lifted 4 feet at 50 percent loss in efficiency. Still not very practical we need something heavier. Unfortunately I can't think of anything useful that would be heavier. Oh well it was fun to contemplate.

For years I have considered using windmills to lift water to a holding pond to generate electricity. Classical Dutch style windmills are aesthetically pleasing and if you build one on a hill and placed a generator at the bottom you don't need a lot of unsightly or distracting high tech or noisy devises like wind generators. The holding pond could perhaps be part of the landscaping with a nice little waterfall produced by the windmill. You can easily live in the windmill with a lot of cool architectural options to explore including a viewing deck. Not particularly practical but it creates an interesting living space that you could build entirely on your own outside of the generator and turbine. I don't know how much fluctuation in water level you could have and still raise fish in the top and bottom ponds but I see the ponds as a big bonus and the architecture justifies the expense of the ponds. Did I tell you I like raising fish :-) outdoor fish are more fun than an aquarium. If you eat the fish you even save on food cost lol

wolfhnd
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