Thursday, October 11, 2007

Plug-In Hybrids, CAFE and LNG

One of my readers pointed out that I missed an opportunity in Tuesday's posting to consider the impact of plug-in hybrid cars (PHEVs) on an increase in the Corporate Average Fuel Economy Standard (CAFE.) It's a great question, and it leads in two different but related directions. First, there's the basic problem of how to tally the energy consumption of a PHEV or any other multi-fuel vehicle, compared to a car that uses only gasoline. But looking at the impact of PHEVs also requires at least a superficial discussion of their electric power needs, from both an energy and capacity perspective. This is another one of those topics that really demands something longer than a 600-word posting, but then that's true of many aspects of our highly-interconnected energy systems.

It's rare to see a reference to PHEVs that doesn't include claims for gas mileage in excess of 100 miles per gallon. Of course, that's only the gasoline consumption, and it's premised on the average distribution curve for car travel in the US, in which a large fraction of all trips would not exceed the 30 or 40 mile all-electric driving range of planned PHEVs, before their gasoline engines kick in. That might be a reasonable way to look at it, if all we cared about were oil. But it makes little sense to ignore the fuel consumption and emissions that occur externally to the vehicle, since those also contribute to our overall energy demand, criteria pollution, and greenhouse gas output. For the sake of brevity, I'll confine this posting to looking at the energy side of this, because the emissions picture is even more complex.

Although my reader asked about the specific impact in New York City, it was easier to find consistent power statistics for the entire state. In 2005 New York's 39,122 MW of summer capacity from all sources generated 147 billion kW-hrs, net, for an average utilization of about 43%. So there's plenty of spare capacity to recharge electric vehicles, if it's managed properly so as not to interfere with peak demand from other sectors. Assuming that the state's nuclear, hydropower and coal-fired generators are usually dispatched first, the incremental demand from PHEVs would be met by natural gas turbines, which make up 42% of total NY capacity. Now put a million PHEVs on the road in New York, driving the national average of 12,000 miles per year. If half those miles were on battery power, at a typical consumption of 3 miles/kW-hr, they would use about 2 billion kW-hrs of electricity per year. That would require 250 MW of generating capacity, well within the state's overall spare capacity. It would also increase New York's annual natural gas consumption by 14 billion cubic feet, or about 1.3%.

What does that mean for the true fuel economy of a PHEV, as distinct from its gasoline fuel economy? Well, assuming the same 50/50 split of battery and engine usage, a PHEV traveling 100 miles would burn 1 gallon of gasoline (if its non-PHEV characteristics were similar to a Toyota Prius) and a quantity of natural gas with an energy content equivalent to about 0.9 gallons of gasoline. That means our "100 mpg" PHEV really gets around 53 mpg overall, counting both its direct gasoline and indirect natural gas consumption.

The obvious conclusion from the above comparison is that the benefits of plug-in hybrid vehicles are highly dependent on the energy source for the electricity they consume, and that will vary significantly on a regional and local basis. Natural gas is close to a worst-case comparison, at least on cost, but it's also a very realistic comparison for the first production PHEVs we expect to see within a few years. Where they will draw on gas-fired power generation--which could be most places, initially--their energy benefits look modest, at best. (Emissions are a different story, for another day.) By the time New York state reaches its first million PHEVs, however, it should have much more renewable power available, in line with the state's Renewable Portfolio Standard. PHEVs look like an excellent outlet for the off-peak power from wind turbines--assuming enough of them are actually built.

In the meantime, if we are looking to PHEVs as a relatively painless way to achieve a new 35 mpg CAFE standard, then we need to think very carefully about the reasons we want a CAFE standard in the first place. Unless PHEVs are rolled out in parallel with large quantities of renewable power generation, we could eventually end up with 100 million of them consuming 1.4 trillion cubic feet of natural gas per year, compared to total current US gas demand of 22 trillion, with 100% of the shortfall translating into additional LNG imports. I'm not sure that would create quite the energy security benefits that many PHEV boosters expect to see.

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