CO2 Emissions, Gas Vs Electric

[msumm21]

TL;DR Summary

Some quick numbers at first look like BEVs pollute more than old ICE vehicles, wondering what I'm missing

I read that a kg of gas provides about 13kWh (47.5MJ) of energy and that burning a kg of gas emits about 3.15 kg of CO2. So presumably a gas-powered engine emits 3.15/13 or about .242kg of CO2 per kWh of work, right?

According to the EIA, the US electric grid emits about 0.85lb or 0.386kg of CO2 per kWh. So an electric motor powered by batteries charged on the grid would emit 0.386kg of CO2 per kWh.

Unless I messed up, this says a battery-electric vehicle (BEV) pollutes more than a gas powered car (per unit work). Is this true? If so, how does a BEV end up polluting less overall?

 

[Dale]

You are missing two important things. The first is the efficiency of the ICE. That substantially increases the CO2 emitted per kWh for the ICE. The second is that fixed plants can be equipped with much better emissions controls. That is not as easy to estimate.

 

[msumm21]

The first is the efficiency of the ICE. That substantially increases the CO2 emitted per kWh for the ICE

I figured there was likely an inefficiency in ICE to explain some/all of this. I just read here that indeed only 40% of the energy created by an ICE powers the car forward. If true, this would increase the .242kg up to .605kg for ICE. If an electric motors is 90% efficient (numbers I found seem to vary a lot, so this may be way off?) then the .386 goes up to .429kg for BEV.

Also forgot to account for weight. A BEV is evidentially about 25% heaver and hence would require 25% more force to accelerate and overcome rolling resistance, so let's say this adds 20% to the work we're now at .515kg for the BEV. If electric cars have regenerative braking that would lower this again, but very unsure how much from reading around.

The second is that fixed plants can be equipped with much better emissions controls. That is not as easy to estimate.

I don't understand this. The EIA estimated 0.386kg of CO2 per kWh on the grid in that link. I assume this already takes everything into account, right? Are you saying that estimate doesn't include something significant?

 

[Bandersnatch]

No, I'm pretty sure it's already taken into account. That comment, however, can be seen as highlighting the fact that the EVs 'emissions' are only as polluting as the grid. Petrol is set in stone, and the efficiency of ICUs is unlikely to substantially improve. On the other hand, as the energy mix in the grid transitions towards greener sources, either fully renewable or with better emission controls, the per kWh emissions of EVs will keep going down.

 

[Dale]

The EIA estimated 0.386kg of CO2 per kWh on the grid in that link. I assume this already takes everything into account, right? Are you saying that estimate doesn't include something significant?

I don’t know for sure what the EIA was estimating, so I am not sure if it is relevant to CO2. However, CO2 is not the only pollutant, and many of those other ones have much higher emissions per unit energy in a small vehicle power plant than a large fixed plant

 

[jrmichler]

A better comparison of gasoline vs electric cars uses real data. The Chevrolet Bolt has a 65 kWh battery and 259 mile EPA range. That divides out to 0.25 kWh per mile.

A similar size gasoline engine car will get about 35 MPG. Gasoline has about 116,000 BTU per gallon energy of combustion. Dividing the 116,000 BTU per gallon by 35 miles per gallon results in 3300 BTU per mile. Dividing that by 3412 BTU per kWh results in 0.97 kWh of gasoline per mile.

The electric car converts electric energy into distance driven four times better than the gasoline car converts fuel energy into distance driven.

Fossil fuel power plants range from 30% to 60% efficient depending on design, age, and maintenance. If the electric car is charged using only electricity from fossil fuel power plants, then the total energy consumption of the electric car is 0.25 kWh per mile divided by 0.30 to 0.60 = 0.42 to 0.83 kWh of fuel burned per mile driven. The electric car is more efficient than the gas car even if recharged by the oldest, least efficient, most poorly maintained power plant.

More than 42% of the electricity generated in the US is from renewable and nuclear power. See https://flowcharts.llnl.gov/content/assets/images/energy/us/Energy_US_2020.png for more details. Including this, along with average power plant efficiency, into the calculation makes electric cars look much better than gasoline when comparing total fuel efficiency. Good search terms to get more information on power plant efficiency are power plant efficiency.