Thursday, September 27, 2007

Chevrolet Volt, a plug-in polluter

In January, 2007, General Motors announced an electric vehicle, the Chevrolet Volt. At that point, GM had no working product, just a concept with a set of performance estimates. The vehicle was to be powered by a 121 kW peak (162 HP peak) electric motor drawing from a 16 kWh battery that could be discharged up to 70 percent. GM estimated that the vehicle would travel 40 miles at highway speeds on one such battery charge. Batteries can be charged from the electrical power grid. The vehicle was also to include a 53 kW (71 HP) gasoline engine driving a generator, estimated to yield 50 miles per gallon when it supplies the electricity to power the vehicle.

From these performance estimates, one can compare fuel costs and greenhouse gas emissions powering this vehicle from the gasoline pump versus the electrical power grid. Multi-stage inverters charging batteries from the electrical power grid are about 80 percent efficient in storing grid energy. Consider the estimated 40-mile range from one battery charge:

Gasoline using the generator: 40 miles / 50 miles per gallon = 0.8 gallon
Electricity using the power grid: 70 percent * 16 kWh / 80 percent = 14 kWh

For June, 2007, U.S. Department of Energy data show average retail prices of $3.06 per gallon for regular gasoline and $0.111 per kWh for residential electricity. Energy costs for 40 miles of travel:

Cost of gasoline: $3.06 * 0.8 = $2.45
Cost of electricity: $0.111 * 14 = $1.55

The U.S. Environmental Protection Agency estimates carbon dioxide emission from burning gasoline is 19.4 pounds per gallon and estimates U.S. average carbon dioxide emission from generating electricity is 1.37 pounds per kWh. Greenhouse gas emissions for 40 miles of travel:

Carbon dioxide from gasoline: 0.8 * 19.4 = 15.5 lb
Carbon dioxide from electricity: 14 * 1.37 = 19.2 lb

If this vehicle were driven 10,000 miles per year (about 40 miles per workday), powering it from the electrical grid instead of the gasoline pump would save about $200 per year in energy costs but emit about 925 pounds more carbon dioxide. The vehicle is estimated to cost about $5,000 to $10,000 more than a conventional gasoline-powered vehicle, so the cost recovery time from using household electricity would be at least 25 years.

Powering the Chevrolet Volt from the electrical grid is mainly burning coal instead of gasoline. The gasoline efficiency estimated for the vehicle is more than a match for the efficiency of electrical power generation, transmission and conversion. The energy cost difference comes mainly from not paying taxes on gasoline and instead burning largely untaxed coal. When powered from the electrical grid, the Chevrolet Volt becomes a small energy cost saver and a net polluter.

8 comments:

ted said...

i'd like to see your equations repeated when the electricity is generated with renewable sources (eg, solar, wind). We can make our own electricity a lot easier than making our own gasoline.

Craig Bolon said...

That's a possible approach. There will be another post on the topic soon. The cost of photovoltaic power would currently be around $1,300 per year, as compared to about $600 per year for gasoline.

Docjest said...

This math is based on incorrect data. The point of the electric engine is that it can travel 40 miles without using any gas. So, for a 40 mile trip, remove all of the references to gasoline, it wouldn't use any! So, remove $2.45 per trip and 15.5 lbs of greenhouse gas emissions. It sounds kinda nice now, huh?

Craig Bolon said...

In fact, it sounds like another version of the old "free lunch."

You can run the car on gasoline, using its generator, or you can run it using power plant energy that was stored in the battery, which usually means running on coal instead of gasoline.

If you run with the battery, the carbon dioxide was emitted while the battery was being charged. As most people learn by the age of 10 or so, there really is no "free lunch."

gsinvestor said...

Craig - Your analysis is badly flawed on two points resulting in a conclusion that is off by a factor of 6.

The 1st error in your analysis is that you don't take into account the difference between electricity usage patterns and auto usage patterns. These vary considerably - due to factors such as industry, environment and population. For instance Wyoming produces lots of dirty coal electricity to mine coal but doesn't account for much driving. Because you don't adjust your kWh/lb number to reflect this you are off by over 50% on your estimation of net pollution by the Volt on this fact alone.

Details: The 1.37 figure per kWh is a U.S. average (its actually 1.363 but I digress) - it comes from the EPA's eGrid which totals electrical output and emmissions by type on a per state basis. The average number you quote is the sum total of kWh produced in each state divided by sum total of lbs of CO2 emitted by each state. This number varies widely by state and predominant energy - e.g., Washington is mostly hydro so its CO2 lbs/kWh is very low - while Wyomings is very high.

The problem for your analysis is that each state's share of the total US electricity and electrical emmissions market as represented by that US average you use has no relationship to each state's share of miles driven in the U.S. and therefore no relationship to a reasonable estimation of what percentage of the total electricity drawn by Volt drivers will come from each state.

According to Federal Highway Administration data, California represents about 11% of the miles traveled in the U.S. and Wyoming is 0.3%. But in the EPA eGrid analysis Wyoming represents about 1.9% of total CO2 emissions while CA represents 2.5%. This means that Wyoming's percentage of CO2 emitted relative to the kWh that would be used by Volt drivers on average (assuming an average distribution of the cars by population across the U.S.) is wildly overrepresented in that 1.37 lbs number that you use. Vice versa with the the CO2 used by CA drivers.

California is 30 times more important than Wyoming for your analysis from driving standpoint but for deriving the average lb of CO2/kWh you use to prove the Volt is a net polluter they are treated nearly 1 to 1.

When adjusted on a state basis for share of miles driven given different state CO2 lbs/kWh rates as expressed by the EPA, the U.S. average for CO2 lbs/kWh is about 0.81 pounds - so the Volt would emits 11.3 pounds of CO2 on average under electrical power for that 40 mile trip v. 15.5 pounds for gas before - even before you look at the impact of renewables.

The second error in the analysis is comparing the gas generator and electric motor straight up. In order for your assertion of a net polluter to be correct you would have to believe that someone would buy a Volt with only a 71 horspower gas motor - which no one would because it would be pig slow.

By comparing the two CO2 emmissions numbers in the fashion that you do, you compare apples and oranges and ignore how the vehicle works on an integrated basis. The electric motor has much better performance than the 71 hp gas engine - and even if you are using the car with the gas motor recharging the batteries - you get the performance of a 162hp gasoline car because the car can tap all 162hp for starting up or bursts of passing and then settle back into net charging mode while cruising on the highway (which uses about 17hp at 70mph.)

Accordingly in order for your statement to be true you would need to multiply the gasoline emmissions number by 2.32 to get an equivilent performance numbers in a gasoline car.

The correct CO2 emission numbers for equivilent 40 mile trips are:

Carbon dioxide from gasoline: 0.8 * 19.4 * 2.32 = 36.0 lb
Carbon dioxide from electricity: 14 * 0.81 = 11.3 lb

- or electricity is 3x more efficient from a CO2 basis than gasoline in the Volt given the same performance.

Craig Bolon said...

A comment from anonymous "gsinvestor" is similar to what we have seen in recent years from promoters of plug-in pollution. The usual dodge is to ignore environmental costs of the electrical fuel cycle. This comment augments that confusion by trying to bound pollution with state lines. However, every state now exchanges substantial electrical power with other states. California in particular is a notorious hog, and it got into financial hot weather during 2000 and 2001. Enron and friends -- remember those days?

Physics and chemistry of pollution from electrically powered vehicles is well established. Studies from the early 1990s at Lawrence Berkeley and other scientific centers differ little from more recent ones. Electrical power usage has long been highly efficient, while coal-powered generation technology changes slowly.

Our electrical grids draw power as needed from the lowest cost source with available capacity. Nearly all the time, almost everywhere, low-pollution sources are fully committed. Only if and when we build enough solar, wind or nuclear power to satisfy the current minimum base load -- probably decades from now -- will there be low-pollution power to recharge electrical vehicle batteries. Until such a distant time, electrical vehicles will all be plug-in polluters, adding to rather than reducing environmental risks.

gsinvestor said...

1. My comment is not any more or less anonymous than "ted" or "docjest" - besides the anonyminity has no relevance on the fact that your analysis is flat wrong. If you must know who I am to believe my analysis you can read my blog at Green Street Investor

2. You argue that I ignore the fuel cycle for electricy - but that's wrong. In fact the whole reason I wrote that post was to correct your analysis of the fuel cycle. The problem is that you completely ignore the fact that demand for electricity to power the Volt will not match one to one with each state's share of electrical production for a myriad of reasons. Your use of the total production of electricity is a proxy for total U.S. electricity consumption and only works for an analysis of Volt electrical consumption if the state level data of electricty production matches state level miles driven proportionally - which it doesn't in the least. This seemingly small issue has a huge impact on the outcome of your arguement - yet you completely ignore it.

The place where this ignorance really wrecks your analysis are states with small populations but big power hungry heavy industries - such as coal mining or steel production - these states use dirtier fuel (they tend to use the fuel on hand) and consume lots of power that is produced locally because of the volume of the demand - it takes a lot of population to make up for the energy usage of a single steel mill or large coal mine - and since steel mills or coal mines don't drive cars at the same rate as the general population - you are going to get the wrong answer with regard to the true impact of fueling the U.S. Volt fleet by electricity when using the average of all electrical power produced in the U.S. for your analysis.

3. Your point about me trying to bound pollution by state lines is sorta correct - what you really mean is that electrical consumption doesn't match production on a state by state basis so adjusting the national average using state level driving and electrical production data as proxies to reflect the average CO2 lbs/kWh consumption by Volt drivers is incorrect.

This, BTW, is not the same thing bounding pollution by using state lines - which is irrelevant - it is an attempt to match consumption better to emissions by recognizing the consumption associated with driving will be closer linked to miles driven in each state than amount of electricity produced in each state. Anyway, yes, because of the structure of the market some states are net exporters of electrical power and some are net importers. And while you protest in the methodology I use, you fail to point out this point - one that I will concede, that ignoring exports and imports may have some impact on my analysis because of differing levels of CO2 emissions per state based on production resulting in higher or lower emissions in states with net imports of power. And since not all power is created equal - even net export/imports will have some flaws because if a state is importing some power of one type and exporting another that wouldn't be reflected in a net number.

This export problem, however, is far less important than ignoring the impact of driven miles on the analysis. For instance, CA imports roughly 30% of its electricity - so even if all that imported power came from extremely dirty Wyoming you are talking about changing CA's average CO2 lbs per kWh consumed by 100% which sounds big until you note that CA power is a clean 0.70 lbs/kWh compared to other states. When extrapolated out to projected Volt consumption based on driving data you are talking about adding 0.7 lbs per kWh for about 10% of the projected kWh consumption associated with driving the Volt - or less than 7% on that 0.81 lb average I estimated (and this is a worst case scenario which assumes the imports come from really dirty WY which is highly unlikely - rather than cleaner than CA Oregon or WA - which is far more likely - and which would actually lower the 0.81 lb average).

By ignoring driving you overstate Wyoming's estimated CO2 lbs per kWh of consumed power for driving by about 3000% in gross CO2 lbs produced to be used in the U.S. average. The reality is that even though we cannot precisely track exports and imports to match with consumption, we do know that not all the export power used is dirty power, for instance, when CA had rolling blackouts a big chunk of the extra power came from Washington and Oregon hydro - which is zero emission and would lower the CA state CO2 lbs/kWh of consumption relative to its CO2 lbs/kWh of produced power.

State production numbers are not perfect, but they are better thn a flat U.S. average. Take a look at state electrical production numbers in eGrid - its not a co-incidence that high population Southern states and heavy industrial states have disproportionate share of the electrical production market compared to miles driven (which is the consumption driver of importance for this analysis) the two are so far apart its impossible to assume that the gap could be closed solely by exporting power to other states where it would be consumed by drivers in more populous importing states (e.g., its unlikely that Wyoming exports 86% of its power generated - which is what would need to occur for its contribution to the national average production number you use to not be overstated based on how much people drive in Wyoming). Furthermore, the U.S. average number you quote, because its a production number, ignores the impact of Canadian hydro power production that is consumed the Northeast and Midwest - which given the population served is not insignficant either (as a mea culpa I too missed that in my analysis because I couldn't use consumption numbers matched to prodution either).

Unfortunately, its near impossible to properly correct for the export/consumption issue - assuming you can even find the data to actually figure out what power is being exported to where on a plant by plant basis so you can then match it to state level consumption. I actually considered this and then dismissed it as non solvable and irrelevant to my conclusion even though I knew it would raise the level of error in my analysis. I figure I capture most of the impact of differing consumption and production data through the state level production proxy - so even if the proxy I used was off by the real consumption data by 10 or 15% the conclusions would be correct. I figure that given the impact of transmission losses most power gets used close to home, and a disproportionate amount of the dirtiest power, coal that powers heavy industry is almost certainly locally produced and consumed - often in co-gen plants on site. Therefore it would be more accurate to use state level data adjusted by driving share than the blanket production average that you used to get a sense of the true pollution impact of Volt driving.

This isn't meant to confuse to make a point, I was merely pointing out that you are using a simplistic analysis of a situation that requires a more sensitive and complex analysis. Thereby ultimately resulting in an incorrect conclusion. In reality, my analysis is wrong too, but yours is wrong based on a fundemental error in how you approach the linkage between auto usage and CO2 emissions from electrical power production. As a result you draw a conclusion that is 180 degrees from reality.

My wrong analysis, by comparison, stems from a lack of available details rather than flawed reasoning. It does address that fundemental linkage between consumption associated with driving and CO2 emmission associated with producing electricity - albeit with a somewhat blunt proxy instrument. Where I might be off by 10-15% due to a lack of available data to project you are off by as much as 80% on this issue alone. But that is merely the small error you have made in your overall assertion that the Volt is a net polluter.

The really big mistake you make, however, is when you equate the gasoline engine emissions to the electric engine emissions without adjusting for the difference in performance between those two motors - and therefore the utility of the two compared items. In order for your analysis to have any validity, it requires that from a value standpoint the performance alternative to a 162 hp electric motor is a 71 hp gasoline motor. This is not remotely realistic.

If you don't make this adjustment, you might as well make the argument that a Volt emits more CO2 than a man riding a bicycle - which is true and valid until you assign a value to the utility of an auto over a bicycle. A 71 hp gas powered car does not equate to a 162 hp electric powered car - given torque characteristics of an electric motor the equivilent gas vehicle would need more than 162 hp to deliver the same performance. To get a valid comparison you need to adjust the gasoline motor performance to match the electric motor performance - which is at least 2.32x - this means you understate equivilent gasoline emissions by more than a factor of two.

You could argue that the Volt is power hungry - that we don't need that much performance - and then compare the Volt to a lower performing gasoline equivilent - but a.) you don't make that argument, and b.) that assigns a value of equal utility based on your opinion to the difference between a 162 hp v. 71 hp motor which may or may not be correct - I would say not or we would all be driving 71 horsepower cars.

I am not certain about your point about pollution studies - I don't beleive I ever made the argument that electric vehicles don't polute - they just aren't net poluters compared to a gasoline engine as you wrongly argue.

To the degree that these studies you mention defend your analysis methodology of using average U.S. production emissions as a proxy for a projected average U.S. consumption from driving, I would like to see the citations.

Lastly with your point about incremental electrical demand being from the cheapest and dirtiest sources I assume you are making the argument that you should calculate the pollution impact of a Volt based on emissions of the marginal kWh produced and not the average because that PHEV driven demand is incremental and the last power on-line is the dirtiest so in real terms PHEVs create more pollution than the average consumed kWh.

1.) If you are making that argument, you don't actually make that argument when you argue "net polluter".

2.) Your statment "cheapest available" isn't exactly true - the make up of power on the grid is driven by estimated peak demand. Coal is part of that peak demand. The problem is that when demand is not peak, coal power plants cannot be modulated over the short term like natural gas power plants or a wind farm - so they run flat out 24/7 regardless of overall demand - thereby offsetting lower polluting options that can be "turned down" when power demand is low. Because they are needed for peak power and can't be turned on and off like a switch that power (while being the last to be added to the peak calculation) is the first power into the grid (along with nuclear) - and not the last. This has implications on what happens at non-peak time when cleaner but more flexible plants are turned off or down. Additionally, in some cases where the imbalance between peak demand and off peak is large enough, the grid stores excess power from these plants in batteries which then feeds that dirty power back into the grid when demand grows in the daytime. What is important is where the PHEV demand comes on line. To the degree that it drives peak usage it woud drive coal utilization as the incremental kWh - but that's not the likely scenario - since the cars will be plugged in at night and will drive incremental non-peak demand - which isn't coal. In that case, the PHEVs are using power that is already going to be produced no matter what including some that is stored and put back into the grid at a later date (offsetting lower polluting capacity), this means that the excess demand for charging the PHEV will come from turning back on lower polluting but easier to modulate sources (either to increase capacity off-peak or because its no longer offset by stored coal power generated at night)

What this means that the incremental capacity to charge a PHEV isn't coal, its natural gas or some other less polluting.

Craig Bolon said...

Regardless of what individuals may hope for, power grids respond with what they have available at lowest cost. Until they have surplus renewable power available at lowest cost, they will usually respond with coal-fired power.

No one can make any electricity "easily." Currently it costs about five to six times as much at wholesale to make photovoltaic electricity as it costs to make coal-fired electricity.

As with the "ease" of producing electricity, there is romantic lore in state bounds of pollution. In fact, power plant pollution goes with the winds. The U.S. Environmental Protection Agency tried a fraudulent approach to regulation, but federal courts sh**canned their garbage. See "A game of regulation with jokers in the deck," July 12, 2008, at http://ergament.blogspot.com/2008/07/game-of-regulation-with-jokers-in-deck.html.