1.4.5 - Electric vehicles and CO2 emissions

Version 3

    Whether they use batteries or fuel cells, electric vehicles are considered zero emission vehicles.  So…

    …how can a battery-operated electric vehicle produce CO2?

     

    An electric vehicle obviously does not produce CO2 when it is driven. However, a well-to-wheel analysis requires that we measure emissions related to the electricity production of the network that is used to recharge the battery. The same electric vehicle can therefore have different CO2 balances depending on how electricity is produced in that country.  Cf. Electricity and Hydrogen Production.

     

    For example, an electric vehicle with a 20 kWh battery consumes on average 13 kWh/100km. It thus has a range of 154 km. When it is recharged, the efficiency of the charger and the battery are each 0.93; the overall efficiency is therefore 0.86. The recharge thus consumed 20/0.86 #23 kWh. The well-to-wheel balance relates to the entire process from the primary energy source to the engine.

     

     

    Primary energy

    CO2 emissions

    (g CO2/kWh)
    (a)

    Well-to-wheel vehicle emissions

    g CO2/km
    = (a) x 23/154

    Coal

    800

    119

    Natural gas

    460

    69

    Biomass

    60

    9

    Nuclear power

    15

    1. 2.2

    European mix

    400

    60

    French or Brazilian mix

    100

    15

     


    Comparison between battery-powered electric vehicles and internal combustion engine vehicles

     

    The electric vehicle that was used for the above calculations consumes 130 Wh/km, which corresponds to the average of the 13 best selling electric vehicles in 2012.

     

    The useful energy at the wheel is in the order of 123 Wh/km and if we take an efficiency of 0.25 an equivalent internal combustion engine consumes about 500 Wh/km. Fuel consumption is therefore around 5 liters per 100 km which on average corresponds to approximately 125 g CO2/km. These emissions are of the same order of magnitude as those produced by the electric vehicle using electricity produced from a coal plant (* the respective efficiencies of an electric motor and internal combustion engine are 0.95 and 0.25).

     

    It is therefore essential when doing a calculation for CO2 emissions for an electric vehicle to take into account the primary energy source.

     

    Internal combustion engine vehicles consume around 300 Wh/km or 3 liters of diesel fuel per 100 km, and thus emit 95 g CO2 /km, which is better than the equivalent produced by electricity from coal plants.

    Any calculation relating to electric vehicles which ignores the primary energy source is thus pointless.

     

     

    Hydrogen fuel cell vehicles and CO2 emissions


    An electric vehicle powered by hydrogen fuel cells doesn't produce any more CO2 during use than its battery-powered counterpart. Doing a well-to-wheel analysis in this case requires taking account of emissions from the hydrogen production process.

    We'll use the same example of a vehicle that consumes 130 Wh/km, but this time equipped with a hydrogen tank with a pressure of 700 bar, which provides an average range of 400 km. The efficiency of the chain between the production of hydrogen by electrolysis and usable energy at the wheel is no longer 0.86 but 0.31.
    Cf. Developing Fuel Cells.

     

    Estimated well-to-wheel balance, hydrogen obtained by electrolysis


    Primary energy

    g CO2/kWh electricity network
    (a)

    g CO2 for
    1 kWh at the wheel (ŋ=0.31*)
    (b)

    Well-to-wheel emissions per km
    g CO2/km
    = (b) x 0.13

    Coal

    800

    2,581

    336

    Natural gas

    460

    1,484

    193

    Biomass

    60

    194

    25

    Nuclear energy

    15

    48

    6


    * Efficiency values: water vapor electrolysis 0.80; compressed hydrogen 0.90; fuel cell 0.46;
    engine 0.95; for an overall network-to-wheel efficiency of 0.31.


    Well-to-wheel balance, steam-reformed hydrogen

    Primary energy

    H2
    produced
    kg

    CO2 emissions kg

    (b)

    Energy
    at the wheel 33 x 0.39 * 
    kWh
    (c)

    Emissions for 1 kWh at the wheel
    g CO2/kWh
    (e)
    = (b) x 1,000/(c)

    Emissions per km

    g CO2/km
    =
    (e) x 0.13

    Coal

    1

    19

    13

    1,462

    190

    Natural gas

    1

    9

    13

    692

    90

    Biomass

    1

    1. 1.5

    13

    115

    15

    Nuclear energy
    **

    1

    1. 0.3

    13

    23

    3

     

    * Efficiency values: compressed hydrogen 0.90; fuel cell 0.46; engine 0.95. The overall efficiency is therefore equal to 0.39. Theoretical energy for 1 kg of hydrogen is 33 kWh/kg.

    ** For steam cracking of water vapor using the heat produced in a fourth-generation reactor or a future fusion reactor.

    The data above are only estimates, since there is a broad range of values for fuel cell efficiency. It is worth noting that the values for an equivalent combustion engine are 160g CO2 /km for gasoline and 120 for diesel.

    In 2013, steam reformed hydrogen costs two to three times as much as gasoline for the same quantity of energy. Hydrogen from electrolysis will certainly be even two to three times more expensive.

    Vehicle costs, energy costs, and the price of C02 all enter into the complex equation which forms the basis of the economic model for electric (and hybrid) vehicles: the so-called business case for electric vehicles.


    Battery or fuel cell?

     

    Whatever the primary energy source, using a hydrogen fuel cell to power electric vehicles is 2 to 3 times less efficient. Yet the fuel cell's efficiency rate of 46 % is a good average.

     

    However it's not at all certain that the advantages of hydrogen (greater range and quick recharge) compensate for lower efficiency and the disadvantages of hydrogen distribution, which requires more rigorous safety measures than electricity. Especially since considerable progress is still being made which could extend the range of batteries close to fuel cell levels.