Hybrid emissions: Facts and numbers
Having laid out my views in part I, let me turn to the actual data regarding hybrids — both from an environmental and economic perspective.
How do carbon emissions per mile driven compare for various cars? The Volt is expected to be “less than $30,000″ with a 1.0L engine. Compare this to the Corolla, with a 1.8Lengine (peak hp of 126; 31 mpg) and a price of $14,400. It’s worth noting that this is in the optimistic, no-gasoline-use scenario for the Volt, computed below along with carbon emissions for the Volt running on cellulosic ethanol and gasoline, and emissions for comparable-sized ICE cars. Questions on the Volt’s actual usage patterns remain: how many people will recharge everyday? What percentage of total miles will be on the grid, and what percentage on gasoline?
|2010||2017||2017 (with 50% increase in ICE mpg efficiency)|
|Car||CO2 Emissions — grams/ mile||Monthly Cost (Car+Fuel)||CO2 Emissions — grams/ mile||Monthly Cost (Car+Fuel)||CO2 Emissions — grams/ mile||Monthly Cost (Car+Fuel)|
|GM Volt On Electricity||144||$623||144||$623||144||$623|
|GM Volt Cellul. E85||–||–||55||$641||37||$628|
|GM Volt Gasoline Only||219||$661||219||$661||146||$641|
|GM Volt Gas+Elec (1:1)||182||$642||181||$642||145||$632|
|Toyota Corolla (ICE Engine)||353||$385||353||$385||235||$353|
|Toyota Corolla (Hypothetical FFV version)||282 (Corn E85)||$387||88 (Cellul. E85)||$355||58 (Cellul. E85)||$334|
Note: A hypothetical “plug-in Prius” with a Volt-sized 16kWh battery would probably cost more than the Volt. Hence the above comparison of a plug-in Volt and a hybrid Prius is unfair to the GM Volt. The monthly cost includes the monthly amortized cost of purchase (7.5 percent loan — completely financed over 5 years) + cost of fuel (1,000 miles/month). Battery cost of $7,600 (at $400kWh + $1,200 control) in 2010 and $4,000 ($200kWh + $800 control) in 2017 is included in the Volt purchase price of $30,000 — these battery cost estimates are 40-60 percent lower than current estimates of $700-1000kWh and automaker margin is not included. Fuel cost assumptions of $0.11c/kWh electricity (U.S. average per EIA) and 5 miles per kwh for the Volt, $3.00 gasoline cost to consumers (roughly just the material cost of “oil” in gasoline at $100 oil price, before taxes — actual costs are likely to be higher for consumers if oil prices stay high), $2.25 per gallon corn E85 to consumers before taxes ($1.75 production cost per gallon), and $1.50 per gallon cellulosic E85 (based on $1.00 production cost before taxes) in 2017. A 25 percent mileage discount is used with ethanol (equal to current average EPA de-rating for E85). Does not include expected improvement on E85 mileage relative to gasoline or the carbon emissions from battery manufacturing, which are likely to make Prius and battery numbers about 10-20 percent worse on carbon emissions. None of the costs account for subsides or taxes, which we assume will be zero or equal for all technologies by 2017. No vehicle attribute changes are assumed.
The numbers are necessarily estimates, and apples-to-apples comparisons are difficult. Notably, they do not include the carbon emissions for battery manufacture amortized over the assumed 100,000-mile life of the battery. In addition, speculation persists that the GM Volt battery will be leased to consumers — and that the $30,000 price tag is not inclusive of any leasing costs. I suspect these errors are material, and make the electric numbers look materially better than they are. (Can somebody provide a source for a reasonable estimate?)
The assumptions behind this table are as follows: the Volt gets 5 miles/kwh. Given U.S. electrical grid emissions of approximately 1.35lbs per kWh (EIA estimates), that gives us a per-mile emissions level of roughly 0.32 lbs/mile (after adjusting for an electrical roundtrip storage efficiency at 85 percent for the battery, and assuming it is running on battery alone) or about 144 grams of carbon dioxide per mile. On gasoline alone (assuming no battery charging from electricity) the same car’s emissions would be 219 grams per mile.
Using only cellulosic ethanol, the same car would have 75 percent lower emissions, or 55 grams per mile (assuming trucks, etc., supplying biomass, and transportation still running on fossil fuel). We have modeled gasoline emissions for tank-to-wheels to be 80 percent of that from well-to-wheels emissions (roughly what the EIA uses). Any help in refining these numbers would be appreciated.
The percentage of coal in the U.S. grid is expected to go up, not down. Contrary to most forecasts, I think we can do better than that and limit coal-powered electricity to the point where its percentage will decline (we have investments to make renewable grid electricity cheaper than next-gen IGCC coal plants), but the decline will be gradual given typical power plant lifespans. Despite what we might wish for — driving on solar or wind power — the reality is likely to be different. For those of you who want to compute solar PV panels on your roof, its effective cost is between $0.25 (Low-cost panels in sunny Arizona?) to $0.50 (Foggy Seattle?) per kWh, depending upon your cost and the location of the panels. In a few locations wind might be cost-effective, but that would be for a small minority of car owners.
What about the cost of driving a mile? When we get to the incremental clean-grid costs, renewable electricity is expected to cost about $0.10-0.15c/kWh (prior to taxes; $0.06-0.10 per kWh delivered to utilities) delivered to the consumer at any large scale, or about $0.02-0.03 per mile from a vehicle of the Volt class. A production cost of $1.00 per gallon of biofuel (I suspect lower costs are likely in 10-15 years) will likely result in a $1.50 consumer price point (prior to any taxes, which vary by state), so one would have to get 50mpg (very doable; essentially the GM Volt gas-only mileage) in a flex-fuel car to get a similar variable cost per mile driven.
Yes, I do expect — within the decade — a good flex-fuel engine to get the same mileage on biofuels as it does on gasoline (for example, an ethanol-capable engine running at a compression ratio of 16 — ethanol’s higher octane rating means that today’s engines are not optimized for it). That will increase ethanol mileage by another 25 percent, which is not figured into our monthly cost reduction calculations. I should be clear that all numbers are necessarily approximations, probably to within 25 percent.