Assumptions
Most of these numbers come from the CA-GREET model, which is used by the California Air Resources Board as part of their low-carbon fuel source (LCFS) program to estimate the total greenhouse gas emissions associated with producing different types of fuels. In this model, the various gases are weighted by their relative global warming potential and so the total amounts are expressed as grams of CO2 equivalent (gCO2e).
Gasoline: I'm assuming an energy density of 127 MJ/gal. The CA-GREET model reports a carbon intensity of 93 gCO2e/MJ. Only 75% of this is tailpipe emissions, with the other 25% incurred during fuel production (it's about evenly split between extraction and refining).
Diesel: Diesel is about 10% denser than gasoline, and I'm assuming 140 MJ/gal. The carbon intensity is basically identical to gasoline.
Electric charging and distribution: I am assuming an 85% charger efficiency when computing the pump-to-wheel efficiency based on the miles/kWh reported by the car. The CA-GREET model includes 6.5% transmission losses in its carbon intensity estimates.
Electric generation - natural gas: The CA-GREET model says 89% of the natural-gas fired electric generation in the Western interconnect are combined-cycle plants (i.e. a gas turbine where the still-hot exhaust gases are used to generate steam for a steam turbine) with an average thermal efficiency of 51%. The remaining 11% are split 6:3:1 between gas-fired steam boilers, simple-cycle gas turbines, and internal combustion engines, with an average efficiency around 33%. Overall we get a carbon intensity of 154 gCO2e/MJ, with 20% of that due to fuel production.
Electric generation - coal: There is a negligible amount of coal-fired generation in California, but we do import about 4% of our electricity from coal-fired plants elsewhere in the Western interconnect. Using a 35% thermal efficiency, we get a carbon intensity of 310 gCO2e/MJ, with only 5% of that due to fuel production.
Electric generation - other: I'm assuming that all other electric generation have zero emissions. This is not quite right: geothermal plants release greenhouse gases that were trapped the reservoir fluid, solar thermal plants burn natural gas during start-up, and we should account for the energy used in the production of nuclear fuel. These are included in the CA-GREET model, but I chose to ignore them because they were negligible compared to the carbon intensity of natural gas and coal plants.
Electric generation - coal: There is a negligible amount of coal-fired generation in California, but we do import about 4% of our electricity from coal-fired plants elsewhere in the Western interconnect. Using a 35% thermal efficiency, we get a carbon intensity of 310 gCO2e/MJ, with only 5% of that due to fuel production.
Electric generation - other: I'm assuming that all other electric generation have zero emissions. This is not quite right: geothermal plants release greenhouse gases that were trapped the reservoir fluid, solar thermal plants burn natural gas during start-up, and we should account for the energy used in the production of nuclear fuel. These are included in the CA-GREET model, but I chose to ignore them because they were negligible compared to the carbon intensity of natural gas and coal plants.
The carbon intensity of electricity over time
The California Independent Systems Operator (CAISO) is an organization that oversees most electric generation in California and publishes both real-time and historical reports of the prices and quantities of electricity produced. Check out http://www.caiso.com/TodaysOutlook/Pages/default.aspx and/or their app!
I'm using their "Renewables Watch" data that breaks down the electricity supply in 1-hour segments into categories [renewables, hydro, nuclear, thermal, imports]. I'm assuming zero emissions for [renewables, hydro, nuclear], 154 gCO2e/MJ for thermal, and 126 gCO2e/MJ for imports (i.e. a 28/25/47 mix of coal/natural gas/carbon-neutral generation).
Let's plot this for the last 30 days, color-coded by weekday (Mon-Fri) vs weekend (Sat,Sun).
Averaging over an entire year:
For our main analysis, I'm considering 2 potential charging strategies: weekday overnight (1am–5am) at 93 gCO2e/MJ, and weekend daytime (9am–3pm) at 53 gCO2e/MJ.
It's also interesting to look at how this changes over the course of a year:
Note that there's a chunk from Nov-Mar that looks like it's been shifted left by one pixel. That's because of daylight savings time (or more precisely, the lack thereof). I think the bright stripes in late Jun, Aug, and Oct are due to heat waves.
A couple caveats:
A couple caveats:
- CAISO doesn't track all generation in California. Rooftop solar just shows up as a reduction in demand, and I'm not sure how they're counting the electricity produced at cogen plants.
- I'm using the average carbon intensity for imports, but the actual import power mix varies quite a lot. Unfortunately, I don't know how else to deal with this.
- This is a California-wide average; shouldn't I be looking more locally? The limited capacity of the transmission grid means that different areas will get very different power mixes. Again, this is something that I don't really have data for. Individual utilities will publish a power content label indicating where they get their generation from, but these are averages over an entire year, and I thought it was more important to capture the daily changes rather than the geographical ones.
- These are average power mixes, shouldn't we be interested in the marginal power mix? In other words, if I plug in my car, what is the power plant that is going to increase production to meet this demand? I'm not sure this is the right question to ask, though, since your behavior does ultimately change the cost/benefit analysis of plant operators bidding on the day-ahead market or the people deciding whether to build a new power plant or transmission line. It's also very difficult to answer, though the marginal price of electricity can give you a clue as to what types of generation may be cost-effective to operate.
Results
Over the last 8 months, we've averaged 4.2 miles/kWh in our electric car, which comes out to 1.0 MJ/mi when you account for charger inefficiencies. Our previous car was a Diesel-powered Volkswagen Golf (2.0L TDI), which was breaking all kinds of NOx limits but averaged a very respectable 41 mpg. For comparison, I'm also including a conventional gasoline car (35 mpg) and grid-independent hybrid (50 mpg).
| Vehicle | gCO2e/mi |
|---|---|
| Conventional gasoline (35 mpg) | 340 |
| Conventional Diesel (41 mpg) | 316 |
| Hybrid gasoline (50 mpg) | 236 |
| EV (4.1 mi/kWh), coal only | 310 |
| EV (4.1 mi/kWh), natural gas only | 154 |
| EV (4.1 mi/kWh), weekday overnight | 93 |
| EV (4.1 mi/kWh), weekend daytime | 53 |
Using the weekend daytime charging strategy (which is feasible because the 230+ mile range allows us to charge once a week), and averaging 10,000 miles per year, we are saving 2.6 tons of CO2 per year compared to our previous car.
What about the carbon footprint of battery manufacture? There's quite a large range in the literature (see e.g. this review) as it is quite sensitive to the electric grid of the country manufacturing the batteries. Using a value of 175 kg CO2 per kWh of battery capacity, our 60 kWh battery represents 10.5 tons of CO2. So we would take 4 years to break even on the battery manufacturing.
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