Electric cars are a pipe dream

Discussion in 'General Science & Technology' started by Syzygys, May 20, 2010.

  1. Billy T Use Sugar Cane Alcohol car Fuel Valued Senior Member

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    I'm all for "combinded cycle" use of fuel, if there is a local need for heat, but don't see how this enters into the question as to which is better (more efficient) fuel for an ICE. Running an ICE car on pure NH3 was demonstrated some decades ago, but with the old (non-fuel injection) carborators that was tricky to do* and only few farmers did it. (They had tanks of NH3, and paid no tax on their car fuel.)

    To make your statement true, you need to claim the heat produced as an efficiency credit. I don't think it true if the CH4 is used to run an electric generator to charge a battery and the heat just discarded. Then using the CH4 as the car fuel is surely more efficient than losing ~ 2/3 of the chemical energy when generating electric energy for the battery. You could do the same "combinded cycle" with NH3 as the burnt fuel. Again it does not address the question as to which (NH3 or CH4) has the least losses of energy to drive a mile down the road.

    * NH3 tends to explode rather than burn.
     
    Last edited by a moderator: Aug 20, 2013
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  3. billvon Valued Senior Member

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    Nope. Well-to-wheels for the cycle "oil to power plant to EV charger to wheels" is still higher than the cycle "oil to internal combustion engine to wheels." This is due to the very high efficiencies of fossil fuel power plants, power transmission networks, battery charging and EV power usage. The 20-30% efficient internal combustion engine just can't keep up. (NG is very similar.)
     
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  5. Billy T Use Sugar Cane Alcohol car Fuel Valued Senior Member

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    Can you document that chemical energy is converted to electric energy at the power plant so much more efficiently than the modern car's ICE converts it to spinning shaft energy that the extra losses in transmission lines, AC to lower voltage DC conversion, the charging and discharge loses of the battery (at economically reasonable rates with electrode polarization loss due to high current densities, not ideal "trickle" charge and discharge rates), and then the electric motor loses do not "eat up" all the advantage of the power plant's greater efficiency conversion of chemical energy to high quality energy (electric energy or spinning shaft energy)? What efficiency are your crediting the power plant with?

    A quick search gave this end of 2011 Stanford university paper that said:

     
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  7. ElectricFetus Sanity going, going, gone Valued Senior Member

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    This is what I mean by combined cycle: http://en.wikipedia.org/wiki/Combined_cycle

    Efficiency of ~60% conversion of heat to electricity have been achieve in full size power plants. I was not talking about Co-gen. The use of a car size ICE no matter the fuel being NH3 or CH4 have an efficiency of 18-20%!

    Avoiding the inefficiency of manufacturing NH3, just burning CH4 in and high efficency power plant, and charging electric cars would allow for road transport to use HALF as much fuel as they presently do! http://thinkprogress.org/romm/2008/...as-plan-makes-no-sense-and-will-never-happen/
     
  8. KitemanSA Registered Senior Member

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    624
    "Combined Cycle" is using a Brayton Cycle (gas turbine) top end and a Rankine Cycle (steam turbine) bottoming cycle. Together they can be 60% efficient. The term you want is CHP (combined heat and power).
     
  9. KitemanSA Registered Senior Member

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    624
    The main reason the PP/EV route is more efficient is that braking energy is captured and reused in this cycle. With Hybrids, the "well to wheels" MPG is a lot more equal.
     
  10. KitemanSA Registered Senior Member

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    624
    The concept being discussed, as far as I was aware, was the use of UL carbon fuels. Since fossil CH4 is NOT an UL carbon fuel, continued discussion of it is basically off topic. So, given the choice of SYNTHETIC ULC fuels, what is the best?
     
  11. Billy T Use Sugar Cane Alcohol car Fuel Valued Senior Member

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    Ok Yes one can get higher over all conversion efficiency by using the exhaust heat of the upper stage as the input heat for a second lower stage, but doing that adds to the capital cost, which is the main part of the electric bill.

    You say this has been done - where? and for what price do they sell their electric energy? The efficiency achieved is no greater than a single engine operating between the higher temperature heat source and the lower temperature heat sink.; however there are no single "working fluid" that can span the same temperature range that two different cascaded thermal engines with two different working fluid can.

    Also I must ask you why you credit the ICE with less than half the efficiency that modern car ICEs have. Do you dispute the Stanford University study I quoted and gave link to? If you do, I can find many others than show more than twice the efficiency you state is possible. Lets have a little supporting MODERN references, instead of just claimed numbers.

    Summary: co generation, when there is a need for heat, is very economical and is very widely used, but rarely if ever, I think is Combined cycle used commercially, because you have the capital cost of two power plants for only a modest increase in fuel conversion efficiency.

    I note you second link is from 2008 when car's ICEs were not so efficient: Here is a graphic from 15 April13 article based on British Governent tests:

    Please Register or Log in to view the hidden image!

    and here is some text form the article:
    and as I mentioned in post 3163 that 38% power plant output is further reduced due to "extra losses in transmission lines, AC to lower voltage DC conversion, the charging and discharge loses of the battery (at economically reasonable rates with electrode polarization loss due to high current densities, not ideal "trickle" charge and discharge rates),..."

    * The Stirling cycle is the only one than can CONCEPTUALY get to the Carnot efficiency limit, but is much too expensive - too many heat eachangers recuperators etc. The combind cycle also has twice as many of those - it is two separate heat engines - That is why I asked if it is ever actually used in a commercial market where cheaper power per watt is easily achieved.
     
    Last edited by a moderator: Aug 21, 2013
  12. billvon Valued Senior Member

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    In the Prius, regen braking captures a tiny fraction (a few percent) of total power. It's definitely not the primary reason for its efficiency. (The primary reason is the use of the very efficient Atkinson cycle engine, made possible by the hybrid system.)
     
  13. billvon Valued Senior Member

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    Heck yes! The 42% is around the efficiencies of the best cars on the road today. (Prius for example currently at around 38%; 42% is the best they can do in the lab, and they hope to push it to 45%.)

    If you compare the best car efficiency in the world to the worst thermal efficiency in the world you get those numbers. But that's not a very accurate (or honest) way of presenting them.
     
  14. ElectricFetus Sanity going, going, gone Valued Senior Member

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    Billy T,

    First off Methane can't be burned in a diesel cycle engine, at least without an ignition fuel mixed with it, so I don't see what use there is in bring up diesel.
    Second transmission line inefficiency (93%), battery charging/discharging (>80%) and electric motor inefficiencies (~95%) all very small. Add all those together and burning methane in CC power plant matches your diesel engine, which mind you represent an ideal diesel engine and not the actual inefficiencies experienced on the road.

    For example a Tesla Model S consumes 19.8 Kwh/100km (making it the "gas guzzler" of electric cars) via EPA testing. Diesels Cars often achieve actual efficiencies of 30-50 MPG or 78-46 KWh/100km.
     
  15. Billy T Use Sugar Cane Alcohol car Fuel Valued Senior Member

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    23,198
    I think you have mixed my two links. The Stanford Un one says modern CARs get 35 to 40% efficiency. It is the British study that claims 42 is possible and demonstrated for the diesel.

    Although I don't believe Electric's >80% battery energy recovery reflects actual charge/ discharge cycles used (instead of slow charge of greater than 10 hour and moderate discharge rates – not when car is going up hill or faster than 60mph on level ground or passing) due to polarization losses near or at the electrode surfaces, I'll first use that and other factors Electric gave to compute the over all efficiency of the power plant to wheels EV car. However, I add in the losses in at least two, usually three, transformers. (Very high transmission line voltage down to local substation voltage, then down from that to 110V by the local transformer on your neighborhood power pole, then down from 110V to a little more than battery voltage, but still AC) for at best (100 -3 -3 -3 = 0.91% Plus at least 1% loss in the AC to DC conversion and 0.5% self discharge loses as car sits in garage of parking lot > 20 hours each day. I also doubt the typical coal fired power plant gets 60% efficiency (In England they get only 38% on average. China's super critical steam coal fired plants – the world's best / most efficient / they barely get 0.6 efficiency when making full rated out put and less with smaller demand as the thermal loses from hot metals remain the same.)

    I. e. 0.6x 0.93 x 0.91 x 0.99 x 0.8 x 0.995 = .500 only 50% efficiency with two very generous asssumptions. Now lets take the power plant efficiency as the average in England and the world's best in China – I.e (.38 + .60)/ 2 = 49% and actual battery recovery as < or = 0.75:

    I. e. 0.49x 0.93 x 0.91 x 0.99 x 0.75 x 0.995 = 40.8% or less than 1% better than best current cars and 1% worse than best current diesels.
     
  16. billvon Valued Senior Member

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    That makes sense as long as CAR means Prius. Normal cars get 25-30% efficiency from their engines.

    Slow (<.5C) charge and moderate (1C) discharge rates give you efficiencies well over 90%. When you factor in a lot of hard accelerating and fast charging you might drop to 80% - but most EV's won't be anywhere near that. Lithium ion batteries are very close to 100% coulombically efficient, and energy efficiency depends entirely on charge and discharge rates - but since their ESR is low it is not a factor unless charge and discharge rates are very high.

    OK so now you have 40% using your pessimistic assumptions; let's go with that. Now let's go back to ICE cars. Let's start with 28% (average thermodynamic efficiency for the average car.) Now calculate a few additional losses:

    Energy taken by drilling the oil
    Energy required to refine the gasoline
    Energy required to transport the gasoline to the pump
    Energy required to pump the gas

    Multiply those four numbers by 28% and you will get a number that is . . . way less than 28%.

    Currently the only vehicle that comes close to overall energy efficiency of pure EV's is the Prius line and some lab experiments.
     
  17. ElectricFetus Sanity going, going, gone Valued Senior Member

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    This is not a matter of calculated guesstimates here: Real word data has electric cars below 20 KWh/100km and real world diesels are using 78-46 KWh/100km.
     
  18. Billy T Use Sugar Cane Alcohol car Fuel Valued Senior Member

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    Why not go back in time to the inefficient Standly Steamer? - We are discussion what is the best choice for future car fuel systems. A 42% diesel exists and ICE car efficiency are increasing 2.5% per year, but some of that is they are lighter now. Carring around heavy batteries or metal for H2 adsoption is a step backwards for that trend.

    To billvon: yes it takes energy to get oil or NG or make NH3, but it also takes energy to make a Li-ion battery and dispose of it later with safety, or recycle parts, and energy to transport all from the source
     
  19. joepistole Deacon Blues Valued Senior Member

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  20. billvon Valued Senior Member

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    Because it is inefficient.

    't

    And carrying around heavy inefficient ICE engines isn't?

    Right now batteries are the big problem for EV's. Just fix one thing with batteries - cost - and EV's blow away everything else on the road. If battery costs allowed you to produce a Tesla for $26,000, you would see a pretty rapid takeover of a significant part of the market.

    Agreed. And it takes energy to make an ICE engine (lots of heavy steel and machining) and it takes energy to drill, refine, store and transport gasoline.
     
  21. ElectricFetus Sanity going, going, gone Valued Senior Member

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    Again real world efficiencies are much lower than 42% and also thermodynamics dictates that increase ICE efficiency will hit fundamental physical limits.

    Than why are EV so much more efficient?
     
  22. Gage Registered Senior Member

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    Out of curiosity what are motorcycles getting in terms of motor efficiency or "ICE"?
     
  23. ElectricFetus Sanity going, going, gone Valued Senior Member

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    ICE means Internal Combustion Engine. Motorcycles are generally more efficient then cars because of the lack of mass, but only achieve MPG of 40's-50's
     

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