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Electric cars without clean energy supply...

  1. May 13, 2016 #1
    Hello all. I'm pretty disorganised and not very clever so I thought I'd ask you all...

    If you charge an electric car from a power station run on fossil fuel is it cleaner than a petrol car???

    I'm wondering after looking at the energy loss in converting fossil fuel to electricity and then in transmission -vs- just putting petrol in a car. Or does it require so little electricity to power the car to drive the same distance that electric cars (and bill) are a no brainer vs petrol.

    So, say you were driving 300 miles in each.... which is best for the environment???
    (Of course I realise that with green energy an electric car is preferable)
     
  2. jcsd
  3. May 13, 2016 #2

    Borg

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    It's hard to say because of the many variables that have to be taken into account.

    The power plant's effeciency + power line loss needs to be compared to the effeciency of the gasoline powered car.
    You also have to take into account the fact that most electric cars use regenerative braking and they also shut down when stopped. This gives a big boost to efficiency in city traffic but not at highway speeds.
    Another item that sometimes comes into play is temperature. For example, a Prius will get worse mileage in the winter because the software attempts to keep the catalytic converter hot enough for the emmisions to be in the proper range. Therefore the engine tends to run more and use more gas. For a pure electric vehicle, this obviously isn't a consideration of course.
     
  4. May 13, 2016 #3

    russ_watters

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    It is a potential issue, yes. It also depends on what you are comparing it to. The calculations to figure this out are not very difficult though. I'll move this to mechanical engineering and we can run through it.

    The basic methodology goes like this:
    Pick a couple of cars to compare and a driving style. Let's say a Tesla S and a comparable sedan. In highway driving, the gas sedan might get 30 mpg. The Tesla S...can be looked up. Google will also tell you the carbon content of gasoline.

    For the carbon content of the electricity, that can be easy or difficult, depending on where you want to go with it. The EPA publishes regional carbon content data for electricity generation. You can just use that or you can calculate (or perhaps find...) a carbon content based on a certain chosen source. For example, you might choose to say that since most of the new electricity generation added in the US is made with natural gas, you might consider comparing only the natural gas...

    Why don't you come up with a more specific scenario for us that you are interested in and we can help you analyze it...
     
  5. May 13, 2016 #4

    berkeman

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    Also, the electricity to charge the car can come from non-fossile-fuel sources, like Wind, Hydro, Fission, etc. That will up the overall efficiency...
     
  6. May 13, 2016 #5
    It depends on which fossil fuel is used and the overall mixture, and the specific plant.

    Gas cars generally get get around 25 mpg
    However, a tesla gets ~95 mpge (miles per gallon of gasoline equivalent)

    Car engines also have a thermal efficiency of roughly 25%, meaning 25% of the energy released by the gasoline is actually going to do what you want to do
    Oil power plants typically have somewhere between 35-40%

    so, using a gallon of gas for both systems
    the gas car obviously goes 25 miles
    the plant takes the gas and turns it into 0.4 units of electricity

    the Tesla takes that 0.4 gallons of gasoline equivalent electricity
    and with it's extremely high milage, turns that into 38 miles
    so in this scenario, the tesla won out.

    Of course, there's a hundred thousand other factors at play here, but unless you're powered by coal, it's better per mile driven to use electric.
     
    Last edited by a moderator: May 16, 2016
  7. May 13, 2016 #6
    ok, let's say that my car, a Mazda 6 is getting 300 miles per 60L tank. This is a variety of roads and speeds....typically, ludicrous speed.

    Let's also look at a purely electric car...the Tesla Model 3 say....or whichever model comes with the ludicrous speed upgrade. If you charge this from a coal burning power station (I started using a Sankey diagram)...
    ...how much energy is required to completely charge the car and drive 300 miles?
    How much coal needs to be burnt to generate that energy.
    How much energy is lost in transmission to a home?
    What is the resultant carbon footprint vs the petrol car?
     
    Last edited by a moderator: May 13, 2016
  8. May 16, 2016 #7
    With coal its no contest, the petrol car is better. It honestly can't even compare
    Coal is something like C137H97O9NS
    Meanwhile, gasoline is primarily hexane, heptane, and octane. So C6-8Hlike 16 or something
    And actually, hydrogen oxidation is more exothermic than carbon oxidation, so petrol actually releases more energy per mol, and less CO2 per mol.
    Only when you're down to things in the realm of hexadecane does it actually become a competition between electric and petrol.
     
  9. May 17, 2016 #8

    jim hardy

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    Ahh new posts appeared while i typed.....

    Here's a method to figure actual fuel burned to push a Tesla down the road . It'll work for coal or gasoline and you can see where i accounted for power plant efficiency.


    @Praestrigiator
    I'm glad you showed the energy consumption of the Tesla corrected for how much fuel goes into the power plant to create the electricity consumed by the Tesla .

    95 mpge ? What the heck is a mpge ?

    Hmmmmm....
    A gallon of gas has about 115,000 BTU's
    and 33.7 kwh = 115,700 BTU so that part of their conversion is certainly close enough for demonstration purposes.

    I too think the true energy consumption of the Tesla should be judged by how much fuel went into the power plant to make that 38 kWh the Tesla used covering that 100 miles.

    If a coal plant with heat rate 9,000 BTU/kwh is burning coal with heat content of 9,000 BTU/lb(those are not unrealistic numbers) ,
    it's making 1kwh/lb of coal.
    Now the Tesla takes 38kwh to go 100 miles
    so it'll take 38 pounds of coal into the power plant to push the Tesla that hundred miles
    now ## \frac{100 miles}{38 lbs coal}## = 2.63 ## \frac{ miles}{ lb coal}##
    so the Tesla gets 2.63 miles per pound of coal .
    Were the power plant burning not coal but gasoline with EPA's heat content of 115.700 BTU/gallon
    ## \frac{115700 BTU/gallon }{9000BTU/kWh}## equals 12.85 kWh/gallon
    and ## \frac{38kWh/100miles }{12.85kWh/gallon} ## = 2.96 gallons/100 miles = 33.8 mpg

    89 mpg ? Bilgewater, By your reckoning it gets 38.
    By mine it's just shy of 34, exactly same as my secondhand Ford Escort got on a recent 3500 mile trip. And i only paid $2000 for my Escort.


    BUT - at two bucks a gallon , my 3500 mile trip fuel cost would be 3500m/34mpg X $2/g = $205. About what it actually did cost,
    At 15cents a kwh the Tesla would cost 3500m X 38kwh/100m X 0.15$/kwh = $199.50
    (Better check my arithmetic for i'm notoriously awkward.)

    Most people wouldn't look past that 89 mpge figure.
    I think EPA is acronym for Egregiously Pompous Airheads .
     
    Last edited: May 18, 2016
  10. May 18, 2016 #9
    Mpge (properly written MPGe)
    Is miles per gallon electrical.
    Which means the amount of energy contained in 1 gallon of gasoline, in electrical form, approx 33.7 kWh.
     
  11. May 18, 2016 #10

    jim hardy

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    okay so 33.7/38 = .887
    and that rounds off to their 89 mpge


    ........it'd be easy to turn that into sophistry to deceive people

    your post #7 points out that in coal country a Tesla may actually be "worse for the environment" than a gasoline economy car ?

    Thanks for those posts - you improved my understanding.

    Can't beat hydro, it's clean and predictable .
     
  12. May 18, 2016 #11
    Try the calculation for the winter.
    Where an engine burning gasoline will provide "free heat" for the driver and occupants.

    An electric car doesn't have the free heat option.
    Or you can freeze and frost up the car windows and save the environment at the same time.
     
  13. May 18, 2016 #12

    jim hardy

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    interesting thought.
    Having grown up in S Florida where cheap cars didn't even have heaters i tend to overlook the obvious.

    I worry more about those high energy batteries - does a Chevy Volt refuse to go when the airconditioner quits and can no longer control battery temperature ?
    http://gm-volt.com/2013/05/03/volt-battery-thermal-management-system-in-the-hot-arizona-sun/
     
    Last edited by a moderator: Apr 19, 2017
  14. May 18, 2016 #13
    Hey thanks everyone, particularly Jim, for the responses on this. I had no idea where to get the figures from to do these calculations.

    Electric cars are a no brainer if you can charge them from green energy sources. If not, well, you're as well off buying a very economical petrol/diesel car.

    Just after listening to people bang on about electric cars and composting being good for the environment it makes you want to question these things and it's annoying when you can't find the answers...
     
    Last edited: May 18, 2016
  15. May 18, 2016 #14
    Question is " How green is green?"
    Even though for example a hydro-electric dam is touted as being "clean energy" in reality there is an environmental cost associated with it.

    Cement making is one of the most intensive C02 producing industries. To make 1 ton of cement, 1 ton of C02 is released into the air, from the chemical conversion of limestone and from the use of fossils fuels used in production. To dam a river with concrete, requires 100's of thousands of tons of cement to be poured, so one can see that there is an initial high CO2 cost associated with the hydro plant, to be spread over the life of the plant. The artificial lake behind the dam floods vegetation that will by its decompositon release methane into the atmosphere. Other environmental costs can include elevated values of mercury, displaced flora and fauna, whatever have you to the list.
    https://en.wikipedia.org/wiki/Environmental_impact_of_concrete

    In comparison though, from this site, if the figures are reliable,
    http://www.ucsusa.org/clean_energy/...nvironmental-impacts-hydroelectric-power.html
    I am not sure if those figures include the production of a distribution network necessary to get the electricity to market, or if only for the plant itself. Although that would not change for the type of electric production plant, but might be important to come to a conclusion of an electric car vs petrol car.

    Didn't somebody up top say there are a lot of variables.
     
    Last edited: May 18, 2016
  16. May 18, 2016 #15
    I'm pretty sure they did.

    I didn't bring up what the additional footprint is for getting the fuel out of the ground and to power stations etc either.

    What's greenest I'd imagine is the lowest impact to the environment device which offers renewable energy at the highest rate. As you observed, hydro isn't good if you're damming a river... though a barrage would be good. Solar probably has high production costs though I'm not sure if they're higher or more materials intensive than wind turbine technology. I'd like to see more wind tech coming in to play and have always like the idea of nuclear. Rolls Royce have some great ideas at the moment.

    Is nuclear necessary though? It's constant and manageable and not dependent on the elements but also costly and potentially very dangerous....

    I wonder how many wind turbines we'd need to power the country and what the cost would be vs nuclear.
     
  17. May 18, 2016 #16

    Randy Beikmann

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    I'll just add a couple of comments:

    As alluded to above, the MPGe is a very misleading value, that considers the "thermodynamic value" of 1 kJ of electrical energy stored in a battery equal to 1 kJ of thermal energy in gasoline. Of course it's not. One kJ of fuel burned at the power plant, however, IS equal in value to the 1 kJ in the gasoline. Assuming 33% efficiency at the power plant, 1 kJ of energy in the gasoline is equivalent to 0.3 kJ in the battery. In other words, electrical energy (which can ideally be 100% converted to work) is about 3 times as valuable as heat released from fuel (which here is only converted to work at a 33% efficiency). So to be fair, the MPGe values should be divided by 3 in order to be meaningful.

    My book (mentioned in my signature below) covers the exact gasoline vs. electrical car comparison you mention. They do come out about equal.
     
  18. May 25, 2016 #17

    mheslep

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    List of EV emissions (grams carbon) when charged by the grid average in various countries. Compare that to the emissions from the average combustion vehicle which the EPA keeps somewhere ...

    Edit: EPA uses some 8.8 kg emitted per gal gasoline, so for a 25 mpg car that's 220 grams per km, the units used below. There are some other details, such as the 15% energy overhead for refining petrol, etc.

    Electric-Car-Emissions.gif

    The US grid is rapidly getting cleaner with combined cycle gas fired plants, which are replacing coal plants, hitting 60% efficiency. Oil fired electricity is almost completely gone in the US.

    With respect to all vehicle emissions and not just co2 ie, NOx, volatiles, particulates, etc, it's long been shown that EVs are a large improvement for urban air quality even if the grid is all coal.
     
    Last edited: May 25, 2016
  19. May 25, 2016 #18

    mheslep

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    US combustion power plants can be as high as 60% efficient, and then a third of the US electric grid is hydro, wind or nuclear with zero emissions.
     
  20. May 25, 2016 #19
    Well it certainly is cleaner for the community you live in.
    So that is a good start.
    Electric or hydrogen cars do need an energy resource somewhare which probably does in involve some carbon burning, but if hydro dams are possible locally that has to be better.
    Sometimes though that does involve having to relocate populations which is awkward politically.
    Handling emission problems at the source does make sense to me.
     
  21. May 26, 2016 #20

    mheslep

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    Last edited: May 26, 2016
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