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Thread: Repeatable energy storage

  1. #1 Repeatable energy storage 
    Forum Isotope (In)Sanity's Avatar
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    This may be a physics question. It may not.

    I have a real world problem that I would like some feedback on. I have a 60 watt solar panel that I would like to apply to a practical application. I would like to use this 60 watts to power my desktop PC. Let's say however my desktop PC takes 90 watts of power. Most of you would say, well that's just not going to work..you'll need more solar panels. Let's say however that I was willing to settle with powering the PC from the solar panel in time segments. For example, store the energy in some fashion until it reaches a threshold and then apply that energy to the PC until it reaches some cutoff point and repeat the cycle.

    I've already done this exact project once before. The solar panel would charge a bank of sealed lead acid batteries. These batteries were connected to a UPS backup unit with it's charging circuit disabled. Once the batteries were fully charged a circuit would disconnect the AC power feeding the UPS and it would run off the batteries powering the PC. This same circuit would then re-enable the AC power when the voltage in the batteries got to a set point. This cycle would continue over and over again all day long while the sun was shinning. The part I didn't count on however was the very limited amount of times you can cycle lead acid batteries. They failed in just a few short months making my idea completely cost ineffective.

    So, the question I have for the forum is can you figure out another way to store this energy and then release it to a UPS backup unit much like I had already done? It would need to be a method with a cycle limit that would last for years. This solution could be direct electrical storage, or it could be some mechanical storage that converts back to electricity. For example using the solar panel to get a large flywheel turning and then dumping that energy in to a generator to power the UPS, repeating that cycle over and over again. The more efficient of course the better. Using capacitors came to mind, having a capacitor the size of a refrigerator to get just 60 watts of power is not however ideal.

    Let's also not think of this question in terms of just 60 watts. Let's say I had 2000 watts and wanted to power my central air conditioner part of the time. The same solution if we can come up with one might be used in that case as well.

    Thanks,

    IS


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  3. #2  
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    What voltage did you let the batteries discharge to? It may not be the number of cycles that was the problem, but how deep you were discharging them. Also car batteries used for starting (if that is the type of battery you are using) do not do well if they are deep discharged. Something like a marine battery or golf cart battery may do better,
    Deep Cycle Battery FAQ


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  4. #3  
    Forum Isotope (In)Sanity's Avatar
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    I was using AGM sealed batteries down to around 50% capacity. I only had about 20 ah of capacity however. So the number of cycles per day ended up being significant. If I used a much larger capacity solid plate (aka deep cycle) battery and only skimmed say 10% off the top the project might work out. The problem is eventually the battery will fail and the cost will exceed any possible savings. So I was looking for a solution that would be guaranteed to last as long as the panels.

    The PC I'm typing from right now uses a 19 volt brick style power pack. I could just boost the 16.8 volts from the solar panel to 19 volts or so and using a couple diodes mix it in with the power being feed to the PC. I think this might have the highest overall efficiency. The problem is the PC only draws 30 watts of power and I have 60 watts to use. Naturally that 60 watts would fall due to transmission loss and any type of conversion. Still I would prefer to utilize all of the power.

    Another solution, boost the 16.8 volts to 200 + volts and tie it directly in to the power supply of a PC or other device. While I've done DC to DC converters in this range, it's not at all my favorite thing to do.

    I could also convert AC to DC (12 volts or so) and mix in the solar power, that would then feed an inverter which powers a device. The problem with that scenario is the conversion efficiency. I would be lucky to get 50%.
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  5. #4  
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    I think you've just come up against a problem that's been extensively researched by people with budgets of billions of dollars.
    The answer is that there is no efficient way of storing energy. Only inefficient ways.
    We do store energy, not because it's efficient, but because it's convenient.
    Starting a car with a battery isn't particularly energy efficient, but it's the most convenient way.
    Same with torches, you could save energy by using a mains torch, but it would be a pain.

    If you want to store energy and then use it to do work, then that is the most inefficient of all.
    That's why electricity is so costly to make, because it's highly concentrated energy.

    So I would say your task is impossible. You would need much more than 60 watts, to allow for the inherent losses in any system.
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  6. #5  
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    You have discovered firsthand experience why terrestrial solar will not power world anytime soon. May you prosper in your search and keep us posted of your progress in this area.

    On small scale such as solar watch or calculator, seems feasible, but quickly inherent limitations arise as summarized by mistermack.

    With wind derived energy and crazy fluctuations in frequency as well as amperage, problem would be worse, of course.
    The bravest are surely those who have the clearest vision of what is before them, glory and danger alike, and yet notwithstanding go out to meet it.- Thucydides
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  7. #6  
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    There might be another way to run the computer. There are only a few DC voltages used in the innards, usually +12, +5 and 3.3 Volts. You could muck around with providing separate sources, but that would be an extremely challenging engineering effort.
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  8. #7  
    Forum Bachelors Degree Waveman28's Avatar
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    Ignore everyone above, for they have obviously not heard of....... Supercapacitors!

    Electric double-layer capacitor - Wikipedia, the free encyclopedia

    They have an extremely long life and are unaffected by even hundreds of thousands of charge cycles, not to mention modern developments have given them an energy storage density greater than lead-acid batteries as well. This could well be the solution to your problem.

    Man, haven't been here for over a year! Where have all the cool peeps gone? Dr. Rocket? MagiMaster?
    "Doubt is the origin of Wisdom" - Rene Descartes
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  9. #8  
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    Quote Originally Posted by Waveman28 View Post
    Ignore everyone above, for they have obviously not heard of....... Supercapacitors!

    Electric double-layer capacitor - Wikipedia, the free encyclopedia

    They have an extremely long life and are unaffected by even hundreds of thousands of charge cycles, not to mention modern developments have given them an energy storage density greater than lead-acid batteries as well. This could well be the solution to your problem.

    Man, haven't been here for over a year! Where have all the cool peeps gone? Dr. Rocket? MagiMaster?
    They are expensive, and don't hold a charge for long enough.
    They are great for when you only need to store energy for a very short period, but no good for storing energy from solar panels to run through the hours of darkness.
    They self discharge, and also don't give up all of the charge that they do keep.
    They are getting better though, so they are finding more and more uses as they improve.
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  10. #9  
    Reptile Dysfunction drowsy turtle's Avatar
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    How about using gravity to store your energy?

    First Hydro Company Pumped Storage
    "The major difference between a thing that might go wrong and a thing that cannot possibly go wrong is that when a thing that cannot possibly go wrong goes wrong it usually turns out to be impossible to get at or repair." ~ Douglas Adams
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  11. #10  
    Reptile Dysfunction drowsy turtle's Avatar
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    Quote Originally Posted by Waveman28 View Post
    Where have all the cool peeps gone? Dr. Rocket? MagiMaster?
    We ate them. They tasted bitter and sarcastic.
    "The major difference between a thing that might go wrong and a thing that cannot possibly go wrong is that when a thing that cannot possibly go wrong goes wrong it usually turns out to be impossible to get at or repair." ~ Douglas Adams
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  12. #11  
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    Quote Originally Posted by drowsy turtle View Post
    How about using gravity to store your energy?

    First Hydro Company Pumped Storage
    ON THE SCALE BEING DISCUSSED THIS APPROACH DOES NOT SEEM TO BE AN OPTION, sorry, did not notice caps lock was on...

    As for pumped storage, at seacoast locations brine lagoons can be constructed to drain into brine "lakes" on ocean floor, facilitating energy storage for cities in vicinity. From where comes brine? Nuclear desalination.
    The bravest are surely those who have the clearest vision of what is before them, glory and danger alike, and yet notwithstanding go out to meet it.- Thucydides
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  13. #12  
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    Another option is compressed air at depth, not as elegant, but feasible, or do any disagree?
    The bravest are surely those who have the clearest vision of what is before them, glory and danger alike, and yet notwithstanding go out to meet it.- Thucydides
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  14. #13  
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    I'm with drowsy turtle. For embodied cost, environmental impact, and longevity, raising water into a container is hard to beat.

    Let the eaves-troughs feed into your watertower, and you'll gain energy from rain as well.
    A pong by any other name is still a pong. -williampinn
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  15. #14  
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    Perfect. And where there are no elevated basins, say, at the seacoasts?
    The bravest are surely those who have the clearest vision of what is before them, glory and danger alike, and yet notwithstanding go out to meet it.- Thucydides
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  16. #15  
    Forum Isotope (In)Sanity's Avatar
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    Well I managed to put my solar panels back to use again. Right now I'm typing this while drawing pretty much nothing off the grid.

    The solution was crazy simple. Two Schottky diodes with a 0.23 volt drop and one switching regulator. Power comes from the normal 20 volt power pack and and is feed to one of the diodes. The power from the solar panels comes in to a switching regulator and is down converted to 20.5 volts. That is then feed to the other diode. Most diodes in a sense mix the power together and that feeds to this little PC I'm using. So when the sun is shining the regulator produces 20.5 volts and powers the machine. When the sun dips, it's cloudy, etc the normal power packs voltage takes over.

    While this is not the perfect solution it at least is allowing me to use the power I had sitting around collecting dust. My next move is to build a more efficient regulator and perhaps figure out a way to store the excess wattage.

    Thanks for all the feedback,

    IS
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  17. #16  
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    Even thought I have my solar panels doing something useful again I still have great interest in the original topic.

    How much energy is wasted in something like winding a clock spring, and how much energy is wasted trying to utilize a clock spring ? Compressed air would be highly ineffective due to the friction losses in producing the compressed air, lifting a heavy object to a high distance and then dropping it to produce energy I suspect would also be highly friction limited, although I suspect less so then compressed air. A clock type spring might have the same numbers. Pumping water in to a vessel and later releasing it to produce energy might work, with a big IF on how efficient the turbine is.

    So far batteries are almost out due to the limited cycle lifespan, super capacitors are definitely out..at least for now. Flywheels work, but are very costly to build with any efficiency.

    Water storage? Maybe.

    If anyone has any cost effective ideas...please let us know. I think identifying the most efficient method in use today would be a good start.

    IS
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  18. #17  
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    Pleased to meet you. Hope you guess my name
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  19. #18  
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    Well, I am really great to be here. I am thinking about making a project on home solar panels. Now a day’s people really need repeatable energy source to keep them efficient. All our energy sources are depleting year by years with a rapid rate. These kind of project will really help them to understand the basics of saving energy.. thanks for sharing it.
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