Category: Environmental Issues

Electrification and Carbon Emissions

Railvolution reports FTA numbers that say the average CO2 emissions of the New York City Subway are 0.17 pounds per passenger-mile (48 grams per passenger-km). That’s the equivalent of 114.6 passenger-mpg of gas, if you prefer to think in those terms. The presentation gives average seat occupancies, which we can also confirm with the NTD; it works out to about 4 car-mpg of gas. Other agencies can have somewhat different numbers, based on train efficiency and especially the local sources of power generation, e.g. BART has very low emissions coming entirely from the fact that the Bay Area has ample hydro power resources.

New York’s emission number, 4 mpg, may be familiar to you as roughly the emission-efficiency of regional diesel trains. Per ton of car mass the regional diesel trains do slightly better, since the regional train in question weighs 40 tons vs. 33-39 for New York’s subway cars, but this comes from making fewer stops. At agencies with very dirty power generation, such as the Chicago L, and even ones without very dirty power, such as the energy-hungry Washington Metro, the numbers are even lower, even though they’re electric and the regional diesel trains are not.

What we see is then that railroad electrification does not add too much to fuel economy. The question is then why the situation for cars is so different. The Nissan Leaf’s EPA-rated fuel economy equivalent rating is 99 mpg – almost as good as the New York City Subway, better than nearly all subway systems in the US. But if we try to break it down based on energy consumption, we get other numbers; the EPA just massaged the numbers to make plug-in hybrids look good.

The Leaf’s energy efficiency is 0.34 kWh per vehicle-mile, pardon the mixed units; the FTA’s numbers for major US subways range from 0.186 kWh per passenger-mile in high-seat-occupancy New York to 0.388 in low-seat-occupancy Chicago. This is not 99 mpg, unless one uses a fairly clean mixture of fuels; with the New York mixture, it’s 63 vehicle-mpg. So right off the bat, the official numbers underestimate the Leaf’s CO2 emissions by 36%, and overestimate its CO2 efficiency by 57%.

But even that doesn’t take care of inefficiencies in generation. Well-to-wheels, plug-in electric cars have about the same emissions as regular hybrids. This confirms the rough numbers we’ve seen from trains. The Tesla Roadster, a very fuel-efficient car, gets even better energy-efficiency even wells-to-wheels, but it also has much lower electricity consumption, and to get the right numbers it assumes electricity is generated from natural gas rather than coal.

Bear in mind, all of this assumes certain things about the grid mix. At the current US grid mix, on average electrification does not impact carbon emissions. Of course, since people need electricity for reasons other than transportation, any regime in which carbon emissions fall is one in which electricity becomes lower-carbon, and this would tilt the field in favor of all-electric vehicles, both cars and trains.

So, why electrify, if there’s no carbon emission benefit, why electrify? Two answers: air pollution, and, for trains, performance. Electric trains outperform diesel ones, and also cost less to operate in terms of both energy and maintenance. But electrification should be sold only on grounds that are in fact correct.

Carbon Costs May Be Far Higher Than Previously Thought

A pair of economists at Economics for Equity and the Environment (E3) have just released a study positing that the social cost of carbon is far higher than previous estimates, by up to an order of magnitude. The official estimate used by the US government is $21 per metric ton of CO2 as of 2010, and various estimates go up to about $100-200, e.g. the Swedish carbon tax is 101 Euros per ton, and James Hansen recommended $115 per ton. In contrast, the E3 study’s range, using newer estimates of damages, goes up to $900 per ton of CO2 as of 2010, escalating to $1,500 in 2050, when the discount rate is low and the price is based on a worst case scenario (95th percentile) rather than the average.

One should bear in mind that the discount rate used to get the high numbers is 1.5%, in line with what was used by the economists at Bjorn Lomborg’s Copenhagen Consensus to arrive at the conclusion that climate change mitigation was a waste of time. It’s not a radical estimate, although some commentators have wrongly confused it with zero discount rate; it’s in line with the long-term risk-free bond yields. Even using average rather than worst-case damages (but still averages coming from the newer, higher estimates) would give a carbon tax of $500 as of 2010, escalating to $800 by 2050.

The carbon content of gasoline is such that a $900/ton tax would be almost to $8 per gallon of gasoline, or $2 per liter. For diesel, make it $9 per gallon. Good transit advocates are engaging in fantasy if they think this, even together with other costs such as air pollution, would eliminate driving; however, it would severely curtail it, inducing people to take shorter trips, switch some trips to public transportation, and drive much more fuel-efficient cars. All three are necessary: not even in Switzerland has the transit revival gotten to the point of abolishing the car. However, the current US car mode share – 86% for work trips – is unsustainable and has to go down under any scenario with a high carbon tax.

More intriguing would be the effect on electricity consumption and generation. Current coal-fired plants in the US would see an average tax of about $0.89 per kWh; natural gas plants would be taxed $0.49 per kWh. Cities already have an advantage there – New York City claims 4,700 kWh of annual electricity consumption per capita, while the current US average is about 13,000. Obviously, in both cases, fossil-fired electricity consumption would crash, while solar and wind power would become a bargain, but it would be easier to do this in large cities. But again, urban revival has its limits; suburban houses would still exist, just with much more passive solar design and extensive solar panels.

Yes, Transit is Green

I’ve just found a post by Brad Templeton arguing that US mass transit is less green than high-efficiency cars, at least when compared per passenger-km. (He agrees that transit is overall better because it is more efficient when used more extensively, as in Europe and especially East Asia.) The analysis of how this can be given the numbers is cogent, but the numbers themselves are suspect, and are worse for transit than other numbers I’ve seen.

Better numbers can be found in this FTA presentation, on pages 10-11; the data is sourced to the National Transit Database. They’re expressed in pounds of CO2 per passenger-mile; if you’re more used to thinking in terms of passenger-miles per gallon of gasoline equivalent, then convert x pounds per passenger-mile to 19.374/x passenger-miles per gallon. The New York City Subway gets the equivalent of 114 passenger-mpg, versus 47 on Templeton’s page. Even FRA-regulated commuter rail does significantly better than cars – the low efficiency of the trains cancels out with the fact that there’s almost no off-peak traffic.

Another piece of evidence Templeton’s transit numbers are too low: he lists JR East’s energy use as equivalent to about 78 passenger-mpg. In reality, JR East claims much lower emissions, about 13 grams per passenger-km (400 passenger-mpg equivalent) or 19 (280), depending on whether one counts the emissions of the company’s buildings or just transportation emissions. It could be that Japanese power generation is that efficient; but given that Japan’s overall per capita emissions are not low by non-US developed country standards, I doubt it.

Finally, although it appears as if technology is about to make cars much more efficient, in reality technology is expensive if you’re a driver and cheap if you’re a transit agency. Take hybrids: the market share of new hybrid car sales is in the single digits, about 300,000 out of 8 million light vehicles sold in the US in 2008, but the market share of new hybrid bus orders was 22% in 2007. Electrified trains are also gaining efficiency, perhaps more slowly but the important thing for them is to transition to low-carbon power generation; if their emissions are nontrivial thirty years from now, then we have bigger problems than transportation to worry about.