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Prime vs Volt Fuel Economy

Discussion in 'Prime Fuel Economy & EV Range' started by Prashanta, Nov 25, 2016.

  1. EV-ish

    EV-ish Active Member

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    MPGe DOES NOT equal MPG

    A graph of carbon emissions per distance would be interesting, but it would vary by each users electricity source(s).
    Perhaps three graphs: Clean, NG and coal. Most people could use either one of the graphs or eyeball themselves somewhere in between two of the graphs.

    I would use 1 lb/kWh for NG and 2 lbs/kWh for coal.

    Some grid utilities are transparent enough to report their fractional sources. The other (perhaps even more valid) approach is to use NG if charging during the day and coal if charging during the night based on typical marginal emissions for most of the US.
     
  2. Zythryn

    Zythryn Senior Member

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    Yes, sometimes there is.

    What you are saying is valid on the macro scale.
    On the neighborhood scale, behind the transformer for the neighborhood it is quite easy.
    Because the voltage being delivered too the transformer from the grid is higher than the voltage on the neighborhood side, electricity won't travel from the neighborhood side to the grid side of the transformer if there is any other choice.
     
  3. Zythryn

    Zythryn Senior Member

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    Yes, it does, for what it was designed to measure.
    MPGe measures the energy content (in kWh) of gasoline.

    It is not designed to measure CO2 content.
     
  4. Lee Jay

    Lee Jay Senior Member

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    If you have multiple sources and multiple nodes, which you do in every non-trivial grid, you really don't know where things go even if you know the current in every conductor, which you never do. This is because of nodes. When you inject current into a node, you can't tell how much went one way or another if there are multiple current sources, which there always are on any non trivial grid.

    The entire Western interconnect is synchronized and the flows are only known at the largest scale and through pathways that often have multiple routes.
     
  5. Zythryn

    Zythryn Senior Member

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    My former comment still stands.
    I may not know which of my neighbors is getting my excess solar power, but I do know that as long as there isn't more excess than my entire neighborhood is using, it is all being used within my neighborhood.
     
  6. EV-ish

    EV-ish Active Member

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    Here are graphs for carbon pollution by distance using clean energy, natural gas, or coal as the electricity source:

    I used EPA values

    Screenshot 2016-11-26 at 12.49.33 PM.png Screenshot 2016-11-26 at 12.49.15 PM.png Screenshot 2016-11-26 at 12.49.02 PM.png
     
    #26 EV-ish, Nov 26, 2016
    Last edited: Nov 26, 2016
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  7. Lee Jay

    Lee Jay Senior Member

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    Intuitively, that makes sense. It just happens to be wrong. In slang, "all electrons are identical". Your "power" could be flowing out and other power is flowing in in greater amounts. You can't tell.
     
  8. EV-ish

    EV-ish Active Member

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    Look at what the OP did with MPGe
     
  9. Zythryn

    Zythryn Senior Member

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    I did, what the OP did with MPGe is correct, as long as you are measuring the efficiency of the use of two different types of energy.

    When you start trying to use MPGe to measure CO2 emissions is where you get in trouble.
     
  10. Tideland Prius

    Tideland Prius Moderator of the North
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    My smart fortwo cost more than my Prius to insure even though the smart was half the price. It was because it was labelled as a "2-door, 2 seater" so it cost the same as a sports car/roadster (n). Transporting it via rail cost me "minivan/SUV" prices cause it was too tall to fit in to the "passenger car" crate o_O.
     
  11. EV-ish

    EV-ish Active Member

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    Look again, and read his captions. He is calculating miles per gallon of liquid fossil fuel. If he was calculating btu/mile you would be right, but that would be fairly useless, don't you agree ?
     
    #31 EV-ish, Nov 26, 2016
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  12. EV-ish

    EV-ish Active Member

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    Here is the tabular data used for the graphs posted above. I find it easier to use because we have mixed uses for the car: most days under 25 miles and 90 mile trips 2-3 times a week. The numbers are Lbs of CO2 per distance.

    Screenshot 2016-11-26 at 5.01.49 PM.png
     
    #32 EV-ish, Nov 26, 2016
    Last edited: Nov 26, 2016
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  13. Zythryn

    Zythryn Senior Member

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    I went back again to the OP to check again.

    Here it is...
    The original post is talking about fuel efficiency, not carbon intensity.
    The graph has "miles" along the X axis and "fuel efficiency" along the y axis.
    No where does it mention CO2, carbon intensity, or green house gases.

    Fuel efficiency, when electricity and gasoline are both used, needs to be converted to a single unit that can be compared at different mileages.
    This could be BTUs, Joules, kWh or whatever you like.

    So when measuring fuel efficiency, MPGe and mpg are the same thing. MPGe is simply a conversion of miles per (energy content in one gallon of gasoline). Or, miles per 33.7kWh.
    MPGe, however, is completely inappropriate to measuring carbon intensity.
     
  14. EV-ish

    EV-ish Active Member

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    He is talking about liquid fuel per mile, as his captions show.
     
  15. Lee Jay

    Lee Jay Senior Member

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    I don't agree.

    Petroleum is primary energy, electricity is not.
     
  16. Zythryn

    Zythryn Senior Member

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    Guys, you are both splitting hairs here.

    MPGe is a measurement of fuel efficiency.
    It doesn't matter if that fuel is gasoline, electricity or hydrogen.

    MPGe allows the direct comparison of the efficiency, nothing else.
    It doesn't measure CO2, nor cost, it measures how far the car will go on a unit of energy. The unit chosen is one gallon of gasoline, which has a mathematical equivalence to 33.7 kWh or (I believe) one kg of hydrogen.

    The OPs graph is 100% valid, in measuring energy efficiency.
     
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  17. Lee Jay

    Lee Jay Senior Member

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    If I put a gallon of gasoline into a power plant, it'll produce a lot less than 33.7kWh of electricity - more like 8-12kWh. To make 33.7kWh from gasoline would take something like 2.5-4 gallons.

    That's why such a comparison is fought with pitfalls.
     
  18. Zythryn

    Zythryn Senior Member

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    No one, ever, claimed that you could.
    If you thought MPGe meant that, you misunderstand the definition.

    The "e" is a mathematical conversion and accurately compares the inherent energy in a gallon of gas in kWh or hydrogen or potato chips or whatever you like. The MPG part tells you how far that particular model car will go on a standardized test.
     
  19. Lee Jay

    Lee Jay Senior Member

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    The implication of the chart in the OP is that MPG and MPGe are directly - and algebraically - compatible. But they aren't because of this reason. This should be pretty obvious from the fact that they are so different numerically for the same car.
     
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  20. Prashanta

    Prashanta Active Member

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    Alright, time to settle this once and for all! Let's figure out which vehicle releases less CO2 for X miles driven.

    I want to note that the information presented here will apply only to the US market. I'm Canadian but I understand most readers here are 'Mercan... I'm doing it for you!

    Assumptions

    Let us start with the assumption that the entire grid in the US is connected. Since coal represents 33% of electricity generated in the US, we will assume that regardless of where you are charging your car, 33% of the juice came from coal. This is obviously not true, of course, but I highly doubt most Americans research about the source of electricity in their home state before purchasing their vehicle. This site shows the generation share of each fuel source: coal: 33%, natural gas 33%, oil 1%.

    We will also disregard the CO2 emissions involved in mining/pumping and transportation of fuel, and the lost energy during transmission of electricity. I don't have the time or the energy to account for this.

    Not all coal is alike. Depending on the type of coal burned, the CO2 numbers would vary slightly. I simply assumed the US uses equal amounts of various forms of coal that can be burned.

    Electricity generated from a source other than coal, oil, and natural gas produces no net CO2 emissions.

    Analysis

    I decided not to work with real units here because we're not interested in the units. We are only interested in comparing Prius with the Volt. If you want to do this analysis with real units, I'd love to see them.

    We need to convert the MPG and MPGe numbers to a better unit for analysis (distance in denominator). I chose L/100km.
    Prius
    4.36L/100km gas
    1.77Le/100km electricity (Le = litres equivalent)
    Volt
    5.6L/100km gas
    2.22Le/100km electricity

    Not all hydrocarbons release the same amount of CO2 in the atmosphere when 1 unit of energy is burned. For example, burning 1 energy unit of coal releases nearly twice the amount of CO2 that natural gas does. Let us call this CO2 intensity. Since we are not concerned with units, we will normalise the numbers such that CO2 intensity of natural gas is exactly 1. This site lists the CO2 intensity of various fuel sources.

    Hybrid Mode

    The CO2 emission in hybrid mode = L/100km * CO2 intensity of oil. Oil (gasoline) releases 34% more CO2 in the atmosphere than natural gas for the same unit of energy burned. So Oil's energy intensity is 1.34.
    upload_2016-11-28_1-20-23.png

    Electric Mode

    The CO2 emission in electric mode is a little more complicated. Since the MPGe number only refers to efficiency of car and not the efficiency of the power plants that generated the electricity, we need to account for it here. This is called thermal efficiency. Imagine a power plant is only 50% thermally efficient. It would produce 2x more CO2 than you might expect if you only considered the energy content of the the fuel being burned. Let us call the inverse of the thermal efficiency of a power plant the thermal efficiency penalty. This PDF shows that the thermal efficiency of coal, natural gas, and oil is 36%, 48% and 43% respectively in the US currently.
    upload_2016-11-28_1-15-36.png

    The (relative) CO2 emission in electric mode from coal =
    Le/100km * Thermal efficiency penalty for coal * CO2 intensity for coal * Electric generation share for coal

    We can do similar calculations for natural gas and oil.

    So finally tabulation of electric CO2 emissions for Prius and Volt is as follows:
    upload_2016-11-28_1-19-30.png

    Notice that majority of the electric CO2 emissions come from the use of coal. Closing coal fired plants and "going electric" must go hand in hand.

    The thing to note here is that both Prius and Volt produce fewer CO2 emissions in electric mode than in hybrid mode. The Volt produces slightly less CO2 in electric mode than Prius in hybrid mode. It's very obvious that for short commutes and very long commutes, Prius produces fewer CO2 emissions. So the question is: does the Volt produce fewer emissions than the Prius for "medium" commutes?

    Now we're ready for the graphs. (The Red lines are Prius)
    upload_2016-11-28_1-32-34.png

    Answer is no. Even for a 53 mile commute, Prius produces slightly (very slightly) fewer CO2 emissions than the Volt.

    Since Canadian electricity is less carbon intensive (in general), I'd estimate that the Volt would have a slight edge on the Prius for commutes between 40 and 60 miles). If your electricity is generated predominantly from coal, there is no contest. Prius wins hands down.
     

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    #40 Prashanta, Nov 28, 2016
    Last edited: Nov 28, 2016