DISCUSSIONS OF INTERESTING TOPICS

Which vehicle puts more CO2 into the atmosphere per mile driven:  a Tesla Model S, or a Turbo Diesel?   The answer may well surprise you . . .  so I invite you to read on.

We are all concerned about the potential implications of Global Warming (aka Climate Change), but it crucial to always try and understand the bigger picture when evaluating one energy related alternative vs another.

Electric vehicles look great from from a CO2 emissions standpoint when they are charged from hydroelectric or nuclear power plants, but how do they look when their batteries are charged with electricity generated by burning fossil fuels?

Its not difficult to find a mid-sized turbo-diesel passenger car that will get 40MPG.  The Jetta 1.4T gets 40MPG for example.  Turbo diesels typically get 25% better mileage than gas cars – which is one of the reasons they have been so popular in Europe where fuel costs more.

So lets do some math . . .

Tesla advertises that an 85KWhr Model S goes about 250 miles on a charge .  But Tesla users report under real world conditions that the car consumes 375WHr/mile – so assuming that the battery is allowed to discharge to a 20% charge level, then the range is 180mi – which is a typical range actual users report (70mph, radio, A/C, 3 total passengers).  Hence to travel 40 miles requires ~15KWhr of battery energy.

So how much fuel does a fossil fuel power plant burn to generate 15KWhr?

Too keep thing simple I will just consider fuel oil, as the heat capacity is very similar to diesel used in vehicles, and the power plant ratios between oil and the other fuel sources (natural gas and coal) are well established.  We assume the energy costs of refining and providing diesel is similar to that for refining and providing fuel oil to a power plant.

The operating heat rate of oil in typical US power plants is 10,800 BTU/KWhr
https://www.eia.gov/electricity/annual/html/epa_08_01.html

The heat content for oil is ~138,000 BTU/gallon
https://www.eia.gov/electricity/annual/html/epa_07_03.html

Hence the amount of oil burned to generate 1KWhr is  0.077gal, or 1.17 gal for 15KWhr (or 27 lbs of CO2)

This should not be too surprising as operating efficiencies of US coal and oil fossil fuel power plants are in the 35% range (natural gas is 40%+) , while operating efficiencies of newer auto turbo diesel engines are in the 40-45% range.

So using oil to generate 15Kwh to travel 40 miles in a Tesla, is 17% less efficient than a turbo-diesel 4 passenger vehicle.  But we are not done yet because there is another ~10% loss in transmitting the power from the power plant to the residence, and ~8% loss in charging the battery (the loss is higher if supercharge is used as the battery heats up and the vehicle’s battery thermal management system has to run to keep the battery cool).

This means that that roughly 40% more fuel oil is used to travel the same distance in a Tesla whose electricity is generated by oil than in a turbo diesel.  This also means that roughly 40% more greenhouse gasses are emitted driving a Tesla that is fueled by oil generated electricity than a turbo-diesel.

Coal fired plants (which account for 30% of US energy generation) emit about 20% more greenhouse gas for the same amount of energy generated, or 65-70% more CO2 than a turbo-diesel.

Now natural gas fired plants (which account for 34% of US energy generated) emit ~50% less CO2 than coal fired plants for the same amount of electricity generated  That says the CO2 output for a Tesla powered by electricity generated by a natural gas fired plant is roughly 10%-15% less than the CO2 emitted by a turbo diesel.   Use of natural gas for electricity generation has increased rapidly in the last few years – mainly due the phenomenal success of fracking.

There are other factors to consider besides CO2 emissions when evaluating the environmental impact of an EV, including the environmental and energy impact of creating a 1,200 lb lithium battery pack, or the energy cost of smelting ~800lb of aluminum that goes into a Model S.  A recent article in the IEEE Spectrum magazine cited an Arthur D Little estimate that the manufacture of an EV generates 3x the environmental toxicity than the manufacture of a conventional vehicle – and is mostly due to the greater use of heavy metals.

So then it seems a bit disingenuous that EVs should prominently display a “zero emissions” sticker, and be granted the privilege of driving in the commuter lane, when they are far from a “zero emissions” vehicles.   As someone who has worked in the energy industry told me – you have to apply a full “well to wheel” analysis to know what you are really looking at.

=========================================================

As another interesting exercise – How many pounds of CO2 are dumped into the atmosphere to “mine” a single bitcoin?  Answer:  A current estimate (no doubt using current GPGPU technology) is that “mining” a single bitcoin requires a whopping 250KWhr of energy (see: BITCOIN-ENERGY-CONSUMPTION) , or about 3x the full charge of a Tesla P85.  This requires 19.5 gallons of oil for an oil fired powerplant to generate – which means 450lbs of CO2 are dumped into the atmosphere for every bitcoin “mined” when the electricity comes from burning oil.   But since Bitmain, one of the worlds largest bitcoin mines is located in Inner Mongolia, where power is generated by burning coal – for a bitcoin mined there the amount of  CO2 produced would be ~20% higher, or closer to 540lbs.

As always – I welcome reader comments, additions and/or corrections.