Electric Car: They Might Be Giants
Some time ago, the Chevy Volt attracted my attention. I think the plug-in hybrid concept hits the sweet spot for American drivers, and the Volt’s 35–40 mile electric-only range seemed to be the perfect number. A pure electric vehicle (EV) would not permit my wife’s periodic work-related jaunt to Pasadena, so any battery-powered solution for us must be of the plug-in hybrid electric vehicle (PHEV) variety. The problem, ultimately, was the high price tag (and the hump in the middle of the back seat occupied by the battery). Although I don’t self-identify as being in the “upper class,” our income edges us into the top quintile in the U.S. So for us to decide that the Volt costs too much—despite genuine enthusiasm—seemed to spell trouble (indeed, the average income of Volt owners was claimed to be $175,000). My conclusion was that electric/plug-in cars are out of reach, and could well remain so.
In April of this year, I became aware of the Ford plug-in, called the C-Max Energi (yes, with an “i” at the end!). The C-Max Energi has a 21 mile electric-only range, and gets an EPA rating of 43 miles per gallon (2.3 gal/100 mi; or 5.4 L/100 km). The price tag is approximately $6k cheaper than the Volt, and the back seat passed my wife’s approval. Nonetheless, after carefully considering the C-Max Energi as a replacement for our increasingly ailing car, we decided against springing for one: still too expensive. I was all set to write a Do the Math post to the tune of “Almost bit on a PHEV again.”
But the fact remained that our 11-year old 28 MPG car (bought used) has been costing us a fair bit in maintenance, its reliability increasingly dubious. Replacement loomed. Motivated by an upcoming long-haul road trip, we explored options again, looking at hybrids and the C-Max Energi. In the end—aided by a federal tax credit, a California rebate, and an unfathomably good offer that together knocked $9k off the MSRP—we drove an Energi off the lot under battery power.
It turns out that:
- the lifetime cost for the PHEV is still higher than other options we considered, but not prohibitively so given credits, rebates, and discounts;
- the CO2 emissions are cut in half in electric mode (considering upstream electricity production in our region);
- batteries still stink compared to liquid fuel, and likely always will.
Batteries fail—as certainly as death and taxes. Rechargeable batteries at least offer the possibility of repeating the cycle, so are in this sense more like recurrent taxes than death. But alas, the story cannot repeat indefinitely. One cheerful thought after the other, yes? But wait, there’s more… Add to their inevitable demise an overall lackluster performance in battery storage technology, and we have ourselves the makings of a blog post on the failure of batteries to live up to their promises.
To set the stage, the specific energy of gasoline—measured in kWh per kg, for instance—is about 400 times higher than that of a lead-acid battery, and about 200 times better than the Lithium-ion battery in the Chevrolet Volt. We should not expect batteries to rival the energy density delivered by our beloved fossil fuels—ever.
A recent article in APS News reported on an emerging view that batteries are failing to live up to our dreams in the electric car realm:
Despite their many potential advantages, all-electric vehicles will not replace the standard American family car in the foreseeable future. This was the perhaps reluctant consensus at a recent symposium focused on battery research.
The Do the Math blog series has built the case that physical growth cannot continue indefinitely; that fossil fuel availability will commence a decline this century—starting with petroleum; that alternative energy schemes constitute imperfect substitutes for fossil fuels; and has concluded that a very smart strategy for us to adopt is to slow down while we sort out the biggest transition humans have ever faced. The idea is to relieve pressure on the system, avoid the Energy Trap, and give ourselves the best possible chance for a successful transformation to a stable future. Since building this case, I have described substantial adaptations in our home energy use, but have not yet addressed the one that bears most directly on the immediate problem: transportation and liquid fuels. Let’s take a look at what can be done here.
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On Do the Math, three previous posts have focused on transportation efficiency of gasoline cars, electric cars, and on the practicalities of solar-powered cars. What about personal-powered transport—namely, walking and biking? After stuffing myself over Thanksgiving, I am curious to know how potent human fuel can be. How many miles per gallon do we get as our own engines of transportation?
Okay, the “miles” part is straightforward. And we can handle the “per.” But what’s up with the gallon? A gallon of what? Here we have all kinds of options, as humans are flex-fuel machines. But food energy is not much different from fossil fuel energy in terms of energy density.
A typical efficient car in the U.S. market gets about 40 MPG (miles per gallon) running on gasoline. A hybrid car like the Prius typically gets 50–55 MPG. In a previous post, we looked at the physics that determines these numbers. As we see more and more plug-in hybrid or pure electric cars on the market, how do we characterize their mileage performance in comparison to gasoline cars? Do they get 100 MPG? Can they get to 200? What does it even mean to speak of MPG, when the “G” stands for gallons and a purely electric car does not ingest gallons?
This post addresses these questions. Continue reading
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Since I was a teenager, I frequently heard stories that some guy had invented a car that could get 100 miles per gallon (MPG), but that powerful interests (often GM, Chevron, etc.) had bought rights to the idea and sat on it. We suckers were left to shell out major bucks for gasoline, when a solution was in hand and under wraps.
Leaving aside the notion that such a design would bring unbelievable prosperity to its holder (i.e., no real incentive to sit on it), let’s look at what physics says is possible.
We like cars because we can travel quickly from point A to point B. So let’s evaluate the energy requirements to make that journey at freeway speeds. We will use the somewhat awkward (although appropriate) speed of 67 m.p.h. because it conveniently maps to 30 meters per second. At these speeds, aerodynamic resistance is the dominant energy drain, so we will start by evaluating only this to get a lower bound on fuel efficiency, and find that we do a pretty good job! Continue reading