so why doesnt it work?

Discussion in 'Hybrid' started by jbehr, Sep 23, 2010.

  1. PAR
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    PAR Yacht Designer/Builder

    I suspect the shielding to weight ratio to go down as the units get more powerful, but yes, shielding is an issue.
     
  2. Stumble
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    Stumble Senior Member

    As mentioned before, but just to clarify. Electrical power is plenty efficient for use in a marine environment. The problem really is energy storage density. Currently there is no practical way of storing enough electrical power on board for even modest range requirements.

    Compare this to a car where it is expected that the car will be able to recharge daily, the moved weight is generally lower, and the overall efficiency of the system is higher (MPG vs GPM is an important distiction).
     
  3. WestVanHan
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    WestVanHan Not a Senior Member

    jbehr Ive been through all this and posted all kinds of stuff.

    I should write a post with all the info...can it be stickied???
     
  4. sparky_wap
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    sparky_wap Junior Member

    http://www.karvin-polska.com/

    The 5500 model makes 7.4hp but you need a big battery 120AH at 48 volts or about 150 lbs of LiFePo4. Will only last about 45 min at full speed. Not sure exactly how fast you could go with that power level but I guess about 20 mph with a light hull. Forget going fast on Pb.

    At slower speeds, you should last all day.

    Torqueedo (sp?) makes an overpriced unit with 4hp..

    Trolling motors will run all day at 3-4 mph on a jon boat with a big Pb marine battery. This is a cheap solution if you don't want to go fast. A couple of hundred dollars and you are on the water.
     
  5. WestVanHan
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    WestVanHan Not a Senior Member

  6. CDK
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    CDK retired engineer

    No, the efficiency is not the issue, just the storage density.
    Carbon hydrates pack more than 10 kW in 1 kg or 2 lb., chemical battery storage of the same amount takes 40-100 times that weight.
     
  7. daiquiri
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    daiquiri Engineering and Design

    These two quantities are related, so they are both an issue. For example, a hypotetical battery with the ideal discharging efficiency of 100% would have twice the storage density (in terms of energy per unit weight or volume) of a hypotetical battery of the same construction but with 50% efficiency. That's because you would need 2 batteries of the later type to get the same energy given by the former type.
     
  8. CDK
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    CDK retired engineer

    That depends on how you define efficiency.
    If you compare how much energy you put in with what you get out at the prop shaft, electric systems do far better than any internal combustion engine. If you put in 10 kW, you get 8-9 kW at the prop shaft, while even the very large diesels give you only 3-3.5 kW.

    The same definition applied to storage of electrical energy gives a near 100% efficiency: what you put in, you can retrieve (almost).

    But if you narrow efficiency down to cost-effective, space-effective or weight-effective, electrical drives do not stand a chance. It simply isn't possible to store the required amount within a reasonable space or weight.

    Your hypothetical batteries both exist: a Ni-Cd battery must be fully discharged to retain its capacity, but the energy density is just as lousy as lead-acid. Lithium has the highest voltage potential, but developments in that direction are a waste of time because there simply isn't enough for mass application.

    For electrical drives to become competitive, we need a device that packs a substantial amount of energy in a small space. That needs not be 10 kW in one kg, because the drive is more efficient, but 2-3 kW are required to be competitive.

    Unfortunately there is nothing on the market or under development that even comes close.
     
  9. daiquiri
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    daiquiri Engineering and Design

    I agree, but the energy count should start at the point where the power is generated. And it is not the battery terminals, but the power plant.

    Independently of where the prime energy comes from (from fossil or nuclear fuels, or from hydroelectric or other renewable resources), if you just consider the efficiency of electric generators in the power plant (95%), power transmission grid (93%), battery charge/discharge process (50%-70%) and mechanical output by the electromotor (85%-95%), you have an overall energy efficiency ranging between 40% and 55% (and I believe it's an optimistic estimate).

    Now, if you are using fossil fuels to run the generator at the beginning of the chain (at 45%-55% thermal efficiency), that means around 25% medium overall efficiency of the complete system, from the fuel at the power plant to the prop shaft. That's well below a 30% to 40% efficiency of common marine diesel engines (though it is comparable to gas engines), but at the much higher weigh penalty for a given range due to low energy densities of current batteries.

    That's why I have named the overall efficiency as an important issue, in my first post in this thread.

    Another thing is if the power plant uses renewable resources, like water dams or similar (except photovoltaics, which is out of competition due to actual ridiculous efficiencies). In that case the efficiency count of the power grid stops at the 40%-55% (imho, but I'm not sure), which is favourable for an electric system. But the battery weight issue remains.

    Cheers!
     
  10. WestVanHan
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    WestVanHan Not a Senior Member

    I'm not an engineer like you guys,but I know enough and where/how to look for info.

    Reason I have/like AGM is the low impedance/internal resistance of 2% or so.

    On flooded cells as you know there's a much larger (10-15% ?) loss on charging....one more thing to consider.
     
  11. CDK
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    CDK retired engineer

    Not untrue Daiquiri, but a bit too philosophical for my liking.

    When I buy electricity for 10 cent @ kW I need not consider the chain from the power plant to my wall socket. If there was any way to store a sufficient amount, I would gladly exchange my diesels and gearboxes for two electric motors and a pulse width controller. No noise, no oil, no diesel smell. And I could use at least 90% of the energy to turn the props.

    If I were to generate the energy from wind or solar, it would be even better. At the present I have no choice but to buy fuel at about the same price per kW and waste approx. 70% of that, pumped overboard as warm water and exhaust gases.

    I won't be around to see it happen, but some day the storage solution will be available in the form of a hybrid (or *******) between a battery and a capacitor, where the electrolyte will not take part in the process but only provides an electrical path between two isolated electrodes with immense surface areas.
     
  12. CDK
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    CDK retired engineer

    The 10-15% loss is only there because people are impatient. Charge a flooded cell with 0.01C (1% of its nominal current) and you will obtain almost 100% efficiency. The chemical process is the same as in AGM cells.
     
  13. daiquiri
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    daiquiri Engineering and Design

    I perfectly understand your reasoning, but I honestly can't tell if it is the most correct one.

    Basically, you are leaving the task of improving the efficiency of the grid behind your wall socket to power companies and (or) to the government. Which is theoretically correct, since that's their job - and a well paid one.

    But in practice, if you know that out of 100 liters of diesel consumed in a remote power plant only 25% of the contained energy will arrive to your prop if you use an electric propulsion, while it will be 35% if you use that gasoline to feed the on-board diesel engine, it is clear which technology is currently "greener" and should be preferred.

    Then again, if everyone refused to use the electric propulsion systems for that reason, there wouldn't be any incentives for the research aimed at improving their overall efficiency. So I really don't know which one is ethically a more correct choice... :rolleyes:
     
  14. jehardiman
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    jehardiman Senior Member

    Maybe, but I'm pretty sure that I'll live to see the day where electrons are stored on hydrogen atoms and stripped directly off into the wires (i.e. a fuel cell). Hopefully I'll live to see the day we finaly harness power the way the creator of the universe intended it....fusion: a hundred billion billion suns can't be wrong....;)
     

  15. apex1

    apex1 Guest

    I wish you a long life, a very long.
    Most probably you will need it to see efficient el. propulsion on boats.

    Regards
    Richard
     
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