Yet Another Blogpost about EVs

By Alex Bujorianu.

Tesla Model S

For all the attention electric vehicles (EVs) have garnered in the press over the last decade—in both the environmental media (Clean Technica), popular science publications (Wired, Ars Technica) and even the Wall Street Journal—a surprising amount of misinformation can be found on the topic. Political scientists have increasingly taken note of the Internet’s ability to promote facts, and at the same time promulgate damned lies. In the case of something as enormously complex as climate change, it’s understandable that the public may be confused; the layperson can hardly be expected to know about feedback loops or radiative forcing.

The case of electric cars is different, because in the end the concept of an EV is quite simple: it has much greater wells-to-wheel efficiency than a traditional heat engine, and electricity can be generated from less carbon-intensive sources than oil. The engineering, of course, can be incredibly sophisticated. But I don’t need to understand the finer points of engineering to appreciate how a bridge works.

Why, then, is there still so much confusion about EVs? Why do serious publications continue to hire know–nothing hacks to write pieces about electric cars, without so much as doing a technical edit or looking up a source?

The economist in me can answer this question: it’s because provocative pieces written by professional trolls attract advertising revenue. The loss of truth is just a price that journalists are willing to pay if the incentive is great enough. Still, the purpose of this blogpost is not to write a polemic about the state of the media. Rather, it is to inform the discussion using facts and physics principles, in the hope that the good information will eventually push out the bad.

It’s the Physics, stupid

How do electric cars work, and why are they “better” than internal combustion engined vehicles? To understand the answer, it is first necessary to understand why ICE cars produce so much CO2 to begin with—the IPCC (2014) estimates that transport accounts for about 14% of total anthropogenic greenhouse gas emissions.

At the basic level, it’s about the chemistry. An engine reacts a hydrocarbon mixture with oxygen, thus producing carbon dioxide and water. Long-chain hydrocarbons emit more CO2 per given MJ of energy because more of the energy comes from the formation of C–O bonds than H–O bonds, compared to a lower hydrocarbon like methane.

Graphic of carbon combustion

(We’ll ignore the fact that petrol engines don’t always run a stoichiometric fuel-air mixture, and that a bit of carbon monoxide is produced. Or that imperfect flame-front propagation leads to the emission of air pollutants. The primary GHG agent is CO2.)

This is the carbon-intensiveness problem. The other problem is efficiency: heat-engines all have a theoretical Carnot efficiency limit given by their operating and input temperatures.

Efficiency (eta) = Qhot – Qcold / Qhot

From this equation, we know that efficiency increases with higher operating temperature (Qhot). Of course, heat engines can only tolerate so much heat before the materials begin to melt or deform, and we obviously have no control over the ambient temperature. If you subtitute 1120K (a reasonable maximum temperature) and 290K into this equation, you get a Carnot efficiency limit of about 60%.

Unfortunately, real-world engines have nowhere near this efficiency. The reasons for this are rather complex; it is mainly because the flame-front propagation in a cylinder is not ideal. Automotive engineers have come up wiith all sorts of clever ways to deal with this, the most notable being fuel-injection technology and variable-valve timing.

In addition to this, there are obviously friction losses—this is part of the reason why engines with fewer cylinders are more efficient—and gearing losses, since ICEs require numerous gears to stay in their power band. The fuel also has to be processed from crude oil using fractional distillation. The actual well-to-wheels efficiency of an ICE car is between 13–25% (see this paper by Moghbelli et al., 2007).

By comparison, electric cars do not face any theoretical limit on their efficiency. Electric motors can, and nearly are, 100% efficient. Electric cars see losses primarily from the battery’s internal resistance. The same paper found that EVs have a WTW efficiency of about 70%. Furthermore, EVs can be powered from clean electricity, most notably wind and solar power.

Common Myths Debunked

Although the physics is well-understood and can be explained by a high school student (yours truly did just this in an A level physics project) there are still a number of myths floating around about EVs.

Electric cars produce more CO2 when they are made.

While this is true, the vast majority of CO2 emissions are produced during driving. LCAs in the literature support the view that EVs have substantially lower life-time CO2 emissions than ICE cars. (Hawkins et al., 2013)

Coal power makes EVs more polluting than petrol cars.

This is actually true—in countries with very high coal power in the grid, electric cars may produce CO2 emissions comparable to a petrol car (see the Australian study by Sharma et al., 2013). Globally, however, natural gas is the primary input source for electricity. EVs have lower CO2 emissions when run with typical grid power in most countries around the world.

Batteries syphon electricity when not used.

This depends on the battery chemistry. Phantom losses are pretty small in lithium ion batteries. (And most EV owners are going to be driving their car every day—it wouldn’t make economic sense to buy an EV otherwise.)

Electric cars are dangerous.

This is a common scare tactic used to frighten potential EV owners. While lithium ion batteries do have a nasty tendency to burn or explode (see this video of a Samsung phone) proper packaging mitigates this. Tesla has patented a very sophisticated safety mechanism for their battery.

Also, you should consider the reverse question: isn’t petrol highly volatile and flammable?

Bibliography

Hawkins, T. R., Singh, B., Majeau-Bettez, G., & Strømman, A. H. (2013). Comparative environmental life cycle assessment of conventional and electric vehicles. Journal of Industrial Ecology, 17(1), 53–64.

IPCC, 2014: Climate Change 2014: Synthesis Report. Contribution of Working Groups I, II and III to the Fifth Assessment Report of the Intergovernmental Panel on Climate Change [Core Writing Team, R.K. Pachauri and L.A. Meyer (eds.)]. IPCC, Geneva, Switzerland, 151 pp

(PDF) New generation of passenger vehicles: FCV or HEV? (n.d.). http://dx.doi.org/10.1109/ICIT.2006.372392

Sharma, R., Manzie, C., Bessede, M., Crawford, R. H., & Brear, M. J. (2013). Conventional, hybrid and electric vehicles for Australian driving conditions. Part 2: Life cycle CO2-e emissions. Transportation Research Part C: Emerging Technologies, 28, 63–73. https://doi.org/10.1016/j.trc.2012.12.011

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