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Thread: 55 gallon batteries

  1. #1 55 gallon batteries 
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    Here's a fun mind experiment for you

    Say I wanted to use gravity to make a bunch of batteries to run my home
    so I buy a bunch of 55 gallon drums and dig a few holes.
    How many drums would I have to dig (and how deep would they have to go) to store enough energy to run a home for 24 hours by letting those drums fall and run a generator?

    Lets just say average home runs 900 kWh a month, so... we'd need the drums to store and release roughly 30kWh...

    A 55 gallon drum weighs roughly 39 lb
    Water is roughly 8.35 lb per gallon
    so that's around 498.25 lb per drum.

    Assume the rope's weight is negligible.

    Also:
    how much energy loss would we usually see when lifting and returning the drum?
    That's gonna dictate how high the drums have to go to return enough energy at the right speed to power the 1.25 kWh... The slower they move, the more drums you'll need.

    multiply the depth to supply 1.25 kWh with X drums by 24, and you've got your answer there....

    So, it looks like the answer will have 3 variables. Height, number of drums, and speed of descent (with energy loss as a constant or a possible 4th variable)

    So, What would you say the answer is? (btw, this isn't homework, just a random curiosity. I also realize that the complexity of creating a system that would transform and return the energy to a home's grid would be expensive enough to make the idea not worth while)


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  3. #2  
    Brassica oleracea Strange's Avatar
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    So the weight is about 225kg
    Energy = m g h

    So for 30kWh with one container you would need a hole about 50km deep.

    Not very practical, so how about 10,000 holes 5m deep.

    (Someone else should check that, the numbers look quite large!)

    Maybe easier to raise them up on towers than dig holes?


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  4. #3  
    exchemist
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    Quote Originally Posted by Strange View Post
    So the weight is about 225kg
    Energy = m g h

    So for 30kWh with one container you would need a hole about 50km deep.

    Not very practical, so how about 10,000 holes 5m deep.

    (Someone else should check that, the numbers look quite large!)

    Maybe easier to raise them up on towers than dig holes?
    No that looks right.

    In round figures each drum weighs 2,000 Newtons so you get 2kJ per drum per metre of fall.

    30kWh is 30 x 60 x 60 kW-secs i.e. kJ = 108, 000 kJ. So with one drum it is 54,000m.

    It is always surprising how much mechanical work needs to be done to generate what seems like a modest amount of electrical power. This was brought home to me vividly when I started doing rowing training on an ergo. You work bloody hard and then find out all you are doing is powering one light bulb!

    This particular problem provides an insight into hydroelectric power generation.
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  5. #4  
    Moderator Moderator Janus's Avatar
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    In terms of practicality, you would need have some type of secondary energy storage system.
    So far, only total energy usage (Kwh) has been discussed, but you also would have to account for power usage (Kw).

    Your system would have to be designed to provide enough wattage to provide for peak load.( while 30 Kwh /24hr = an average demand of 1.25 Kw, In real life, the actual wattage at any given time of the day can be more or less than this.

    So imagine that your drums falling turn a generator. As the electrical load increases, the current flow increases, this in turn creates a counter force in the generator, which acts like a brake for the falling drums. The trick here is that you need the weight of your drums to equal the magnitude of this back force at peak load. Then the drum falls at a constant speed( neither accelerating nor decelerating) providing a constant wattage. You also have to make sure that the voltage output of your generator meets your supply voltage requirement.

    But now consider what happens if the load suddenly decreases (AC shuts off, Fryer cycle ends, etc.). The current demand drops, the back force on the generator decreases, and the drums start to fall faster. This turns the generator faster, increasing the voltage output. This type of voltage spike isn't good either. So what you want to do is maintain as constant a load on the generator as possible. You can do this by use of a regulator that shunts the excess somewhere. This could be a "dummy load"( just something that uses up the unneeded generator output), or better yet, a storage system. This storage system could be another set of drums. Essentially, if the load on your main system drops, the system gets rid of the excess by using it t raise the secondary drums. When the load grows, the energy stored in these drums take up part of the load. ( you really aren't increasing the total number drums you are using, you are just dividing them up into two groups that you can shuttle energy between.)

    This complicates the system, and introduces more losses, but maintains a more steady power source for your home.
    "Men are apt to mistake the strength of their feelings for the strength of their argument.
    The heated mind resents the chill touch & relentless scrutiny of logic"-W.E. Gladstone


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