Friday, October 19, 2012

First law of thermodynamics, dammit!

Here's an example of why we need education in math for sustainability.

The Independent (a major UK newspaper) has an exciting headline today. Exclusive: Pioneering scientists turn fresh air into petrol in massive boost in fight against energy crisis  According to the Independent, this is a " revolutionary technology that promises to solve the energy crisis as well as helping to curb global warming by removing carbon dioxide from the atmosphere."

Well, what have they done? According to the article, "Air Fuel Synthesis in Stockton-on-Tees has produced five litres of petrol since August when it switched on a small refinery that manufactures gasoline from carbon dioxide and water vapor...  'We've taken carbon dioxide from air and hydrogen from water and turned these elements into petrol,' said Peter Harrison, the company's chief executive."  (Petrol=gasoline for you USA types.)

In other words, they have reversed the combustion reaction
2C8H18 (1kg) + 25O2 (3.5kg) -> 16CO2 (3.1kg) + 18H2O (1.4kg) + 48 MJ
that powers our gasoline engines. This is a significant technical achievement (though, if I recall correctly, it is not the first time it has been done: wasn't this demonstrated at Sandia National Laboratories a few years back?)

But to reverse the reaction, we need an input of energy at least equal to the 48 MJ that are released when the reaction runs in the forward direction, that is, when we burn the fuel. (Probably a good deal more, because the process will not be especially efficient.)  In other words, what we have is a process for storing energy in the form of gasoline. This could be useful for some purposes, but it is hardly what is implied by the "turning fresh air into petrol" headline.

Let's see if the article acknowledges this. Sort of. It says, "The process is still in the early developmental stages and needs to take electricity from the national grid to work."  An interesting sentence! Neither clause is false, but many readers will receive the unstated suggestion that a fully developed version of the process would not need an energy input - and that is completely wrong.

I would hope that someone who had taken the "Math for Sustainability" course could read the article, perceive the basic issue that I have outlined above, and do for herself some simple calculations about the amount of (renewable!) energy that the process should draw if it were to have a significant impact on global CO2 levels.

I'd also like Independent journalists to be required to take my course!  Well, one can dream.

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