so why doesnt it work?

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

  1. jbehr
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    jbehr Junior Member

    why is it that pure electric propulsion simply isnt feasible at all? after reading through these forums its become blatantly obvious that it doesnt but why is it that it doesnt?

    simply isnt an efficient enough engine or is it that enough electricity cant be produced/stored or both? just wondering where the problem is specifically
     
  2. daiquiri
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    daiquiri Engineering and Design

    Biggest problems: efficiency (meant as overall, from the power plant to the prop), the energy storage technology, the cost.
     
  3. Submarine Tom

    Submarine Tom Previous Member

    I respectfully dissagree. A prop can be made 80 - 90% efficient and an electric motor 90%.

    The inefficiencies come from producing and storing the energy, including monetary cost.

    Batteries are expensive, heavy, and not terribly eco-friendly when you consider manufacturing and replacement/recycling.

    And where is the energy going to come from?

    Boats put a high demand on motors and electrical supply.

    Now, if you could produce that energy through nuclear fission SAFELY, then you would be on your way to reducing your carbon footy-print.

    -Tom
     
  4. fcfc
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    fcfc Senior Member

    You are wrong.

    It works.

    Exaclty like new coal fired steam engine launches. I happen to know some who are absolutely delighted with their steam engines. And telling them to replace the steam engine with a way more practical, cheaper, safer outboard is a bad joke, if not an insult. Waiting fot the pressure to built up, handling coal bags, cleaning ashes, oiling the engine IS part of the game.

    For electric engine, it is the same. You have people found of electronic, electricity that have some running, and they love them, and are deligthed.

    Now, if you are not insterested in steam or electricty, and just want practical boat power for ordinary leisure craft, theses setup, electric or steam, are just complete mess.




    Simply burning a mere gallon of fuel in a diesel engine will give you between 16 to 20 hp at the prop for one hour.



    18 hp is 13 Kw. ( 1hp = 736 w).


    To supply 13 Kw at 12 volt (typical lead battery voltage) need 13000/12 = 1080 amps.

    So, to supply 13 Kw for one hour, like the diesel engine, you need 1080Amps for one hour.

    A typical lead battery can only supply half of its rated capacity. You need special batteries you want deeper discharge.


    So, a 120 AmpsH battery, someting weigthing in the 80 pound range can only supply 60A for one hour.

    You need 1080A, so you need 1080/60 = 18 batteries, that will weight 1440 # to store the same energy as a ONE gallon tank. This tank being able to be filled within seconds at most dock for a few $.


    This is just for a ONE gallon tank. A ten gallon tank will of course require 180 baterries weighting 14,400 pounds to store the same energy.



    That's all.

    PS : People telling you that electric engine are more suited to turn prop that diesel engines is simply plain ********. If diesel rotation speed was not suited for prop, there are a bunch of automotive gearbox manufacturer that would be more than delighted to sell manual multispeed gearboxes or automatic transmission to adapat engine speed to prop, and to sell them at marine prices, instead of automotive supplier prices.
     
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  5. daiquiri
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    daiquiri Engineering and Design

    What I meant is - up to the prop, or to the SHP. A prop doesn't care who drives the shaft, so it's out of the analysis.

    Electric motor is efficient, but an electric motor taken alone is nothing but a dead object. It requires batteries, batteries require a power source (either a PV pannels and inverters, or an external electrical grid). They have to be taken all together in order to be compared to a correspondant thermal-cycle engine. That's where a purely electrical system, unfortunately, loses.

    Fcfc has expressed it in numbers so simply and clearly.
     
  6. PAR
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    PAR Yacht Designer/Builder

    Simply put, range and weight are the biggest problems with a marine, electric propulsion setup. Cost is a factor, but all new technology has this issue, so it can be somewhat ignored for discussion sake.

    The bottom line is, when you reduce the weight of the system to match that of conventional arrangements, so that retro fits are possible or so you don't have to draw up fatter hulls for new builds, then the electric setup doesn't have the range necessary to be competitive with internal combustion.

    Advances have been made in batteries and motors, but more importantly in controllers, though a bilge full of batteries still is costly in terms of weight. Then comes the electricity generation end of the equation, which adds more weight. PV panels of sufficient area just aren't that practical yet, though I can see a day when most any surface could have a PV panel incorporated into it, which will be a great help. Maybe PV coatings, much like a paint job, that could generate enough electricity, though this is likely well off yet. Wind generation is another possibility, but again practicality comes to play when you start making enough watts to be significant.

    Jbehr, this has been an on going set of issues with electric storage. In the 1960's when I first became interested in electric stuff, the trade magazines all said in the next decade or two, advances to storage will make electric stuff common place. Well, the advances never came, though in the last decade we now have finally seen some.

    There is one method that would work and work very well. It would provide generation and this system has been tested and proven in the most inhospitable of conditions. These are the C-4 canisters used on space craft. These are Thermo-photovoltaic devices and work for years, but people have problems accepting a radioactive decaying device in their boat. Then of course is the hydrogen fuel cell, though unless you live in California and had a portable tank you could carry to the local gas station, you're pretty much screwed of fuel supply outlets.
     
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  7. jehardiman
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    jehardiman Senior Member

    I think Fcfc and Par have summed it up pretty well. Power density and cost weighed against range and speed. This is why for ships wind was supplanted by coal which was supplanted by oil which was supplanted by nuclear until that became a political hot button.

    For a small powerboat, the ICE is very hard to beat in terms of power density per unit cost.
     
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  8. PAR
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    PAR Yacht Designer/Builder

    I'd really like to get my hands on one of those C-4 canisters. No noise, no pollution, no vibration, just a steady stream of electrons comming out. If I remember well enough, the bigger ones are 5 KW, which alone is enough for a one tonner at displacement speeds.
     
  9. daiquiri
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    daiquiri Engineering and Design

    WickedGood, could you please attach the picture, rather than inserting it in the text?
    It is too big, page buttons are all out of the screen.
     
  10. daiquiri
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    daiquiri Engineering and Design

    PAR, I have read this review of the current technology (by NASA):

    http://gltrs.grc.nasa.gov/reports/2005/TM-2005-213981.pdf

    which says that current devices have efficiencies still below 20-25% and the power density is around 4 W/kg. 5 kW would mean 1.25 t of machinery. Am I missing something there?
     
  11. jehardiman
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    jehardiman Senior Member

    Not missing something, but you need to pull the thread to the end. Lets use data from here. So a 5 kW aux diesel weighs about 100 kg installed and has a SFC of 251 g/kW-hr for a consumption of 1.25 kg/hr. so for 1250 kg installed weight (not counting tankage) gets 920 hr of operation. If the vessel is ~4 tons, 5kw is ~ 4 knots so range is ~ 3680 NM. With the RTG, range is unlimited. This is what makes nuclear power attractive, no bunkerage.
     
  12. PAR
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    PAR Yacht Designer/Builder

    I became aware of this technology type during the Apollo program. A lot of research was underway, with continued manned flight, satellite and space exploration expected. With Apollo 17 being the last and funding cuts, much of this research went on the back shelf. The Shuttle program used fuel cells, though I suspect some of these canisters have made it to the ISS (my assumtion). I know some deep space probes and satellites have them, but folks tend to get pissy when you mention decaying isotopes, which is childish. Now they are revisiting the research and also coming up with alternatives. We'll see . . .
     
  13. CDK
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    CDK retired engineer

    In the distant past I acquired 3 such devices to power radio beacons for a marine positioning system, to be used in Alaska by a large dredging company. It was extremely difficult to find them, buy them and even more so to ship them between countries. To do that legally was plain impossible.

    The devices supplied 12 V @ 2.4 Amp but had a weight of approx 140 lbs, mainly because of the amount of lead to absorb radiation.
    Using this technology for propulsion of any type of vessel doesn't seem feasible to me.
     
  14. jbehr
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    jbehr Junior Member

    so electric propulsion systems would need an increased efficiency by over ten-fold before even starting to become feasible
     
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  15. baeckmo
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    baeckmo Hydrodynamics

    .....Supply chords don't come long enough.......
     
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