Using Gravity to run a Generator

Sorry if this isn't the place to ask this question (if anyone knows of a more appropriate one, please let me know)

I'm interested in seeing if a large weight, (several tons, at least) when raised up to a certain height and hooked up to a portable generator, could supply a decent amount of electricity for a decent length of time.

Let's say I raised a 2 ton weight 20 feet above the ground and attached it (using some kind of pulley system w/gears) to a 3500-5000 watt portable generator. (sans motor)

Obviously (even to a neophyte like me) there would be a lot of variables to consider, but I'm just looking for some rough estimates to see if it's even worth considering.

I'd really appreciate it If someone could give me some idea (an educated guess) on the amount of time/watts/ per feet I could expect to get.

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It's all simple physics! See any elementary physics book.

In this case you would have a potential energy of 2X2000 pounds at 20 feet = 80,000 foot pounds.

One foot pound per second =

0.0013558 kilowatts = ~ 108 KW for one second in your case (You could run your toaster for less than two minutes!)

0.0018182 horse power = ~ 145 horsepower for one second (Your refrigerator would run for about ten minutes!)

The above assumes 100% conversion efficiency, which is never true. You can see that the power we get from electricity, represents a lot of weight lifting! :-)

It's not a guess--it's just the facts! :-(

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Well, let's see.

1 horsepower is 33,000 lb-ft/min. 2 tons x 20 feet = 80,000 lb-ft.

So, in very round numbers, letting 2 tons fall 20 feet in 1 minute would generate 2.4 mechanical horsepower for 1 minute. Let's say that, allowing for efficiency issues, you could generate 2 electrical horsepower from 2.4 mechanical horsepower.

1 horsepower = 746 watts.

So, again in round numbers, lowering 2 tons through a distance of 20 feet will produce about 1500 watts for 1 minute.

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The heck with dropping the thing. Get it spinning somehow.

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... minus all the losses along the way

Basically you hauled 40 bags of sand up to the roof, hooked up your rope, threw them all off and you get 2 slices of free toast.

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Forget the generator. I'll give him two slices of toast just to watch. ;-)

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Paul Hovnanian P.E.

use the 100 watt toaster from the other thread :)

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Isn't that what hydroelectric generation is al about?


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Exactly.... Energy from the sun (in the form of heat) for the evaporation of water does all the heavy lifting...


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Well, I'll just pay the utility their 10 cents per Kw/h and let them deal with the sandbags (or however they produce the juice). John

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LOL, tha's rich! Thanks for the chuckle.

BTW, only the actual toasting would be free! You still had to buy the bread.

Forget the math though. How are you going to get your 2 ton weight to a height of 20 feet in the first place?

The answer is, you will need to exert ENERGY to do it.

How much?

Answer: A little more than you will recover by generating energy from dropping it! Friction losses, windage losses, heat losses etc.

Energy out = energy in minus losses. There ain't no free lunch.

Our entire world runs off of energy that has fallen onto out planet in one form or another and has been stored for us up to this point. Most of it is solar energy either in the form of chemical storage in billions of plants over millions of years decaying and making oil, or water being vaporized and falling back down at higher elevations. The rest is in the form of nuclear (sorry, nucular according to our fearless leader) energy from elements that became part ofthe planet back in the big-bang era. No other form of useable energy exists on this planet as far as we know.

Now if you could put anti-gravity paint on the 2 ton mass prior to lifting it and peal the paint off just before you drop it, THEN you would have another source of free energy!

Now where did I leave that can of anti-gravity paint? Oh yeah, right behind my Tom Swift book collection!

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But it makes the toast soggy! :-)


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" Forget the math though. How are you going to get your 2 ton weight to a height of 20 feet in the first place? "

I figured there'd be a way of using some type of mechanical advantage. (e.g. hydraulic jack)

Another option would be to use something that can hold a large amount of water as the weight. If it was used in an area that got alot of rain, you'd just need to wait til it rained enough to fill it up or use a water pump to refill the container. (draining it after it reached the bottom)

Also, there was a show on TV where a huge dome structure was created by pouring a cement form over a bladder that was then filled with air to raise it, and it only took a few pounds of pressure to lift the massive weight. That might be another possibility.

Or, if the weight were put into a large hole in the ground, then water (if it could float) or air could be used to bring it up. (or some type of mechanical advantage)

Finding a way to raise it didn't really concern me. Whether it would supply enough energy, by running a generator, to bother trying did.

Having been through several Hurricanes that caused long periods of power blackouts, I had hoped this would be viable source of power. (even if it was for a short period of time)

But it obviously wouldn't be worthwhile if it could only generate enough electricity to make two pieces of toast!

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