One of my pet peeves is news stories about energy that say something like “Flokistan just added 100 MWs in solar panels to its grid. This is enough to power 150 homes,” with no further numbers. I always want to know the context, like how many MWs does Flokistan use? How many homes are there in Flokistan? How does 100 MWs in solar panels compare in cost with putting in 100 MWs of coal-fired powerplants, and how long will it take to pay back?
The energy field is full of numbers – cars emit 19 tons of CO2 per year, solar panels cost $0.20/kWh, PG&E just contracted for 800 MWs of solar power in California. They are used freely, but seldom put into context. So I was very happy to discover an interesting online resource over the weekend: Richard Muller’s U.C. Berkeley class “Physics For Future Presidents.”
In one semester, my goal is to cover the physics that future world leaders need to know (and maybe present world leaders too.
Chapter one, Energy and Power (and the physics of explosions), of his textbook is online (at least temporarily) and it’s fascinating. Most interesting is the table near the beginning comparing the energy content per gram of various substances, including TNT, gasoline, hydrogen, various batteries, an asteroid traveling at 30 km/sec, and even chocolate chip cookies.
For example, a gram of gasoline has about 15 times as much energy content as a gram of TNT, and about half again as much energy content as a gram of ethanol. Hydrogen has almost three times the energy content of gasoline per gram, but even as a liquid, hydrogen is only about 1/10 as dense as gasoline, meaning that per volume, it has about 1/3 the energy content. Muller’s paper is full of useful rules of thumb, such as the following:
Remember this: Compared to gasoline, liquid hydrogen has
- 3 x more energy per gram (or per lb)
- 3 x less energy per gallon (or per liter)
He then goes on to discuss the relative merits of different forms of energy for powering cars, noting that gasoline is a particularly desirable fuel given its very high energy content and ease-of-use compared to, say hydrogen. Or batteries – which have 100x less energy density than gasoline. He also explains why having a big meteorite or asteroid hit Earth would be a bad thing.
If you’re interested in the numbers around energy, and how to compare them, I can definitely recommend a reading this chapter. The course is also available via podcast – see here for links, and covers not just energy, but terrorism, nukes, space, and global warming.
I’d be interested in hearing your comments about Muller’s information. If you have sources that you use for energy-related numbers, let me know about those too. I’m putting together a reference site for these sources, which I’ll link to on this blog as a static page when it’s ready.