Why Is It So Hard To Solve The World's Energy Crisis?

I'm teaching a class at MIT this semester called "Thermal-Fluids Engineering II."  The class is largely concerned with energy, and how it is generated using machines like internal combustion engines and power plants.  This post will provide some background on the current state of affairs regarding energy.

Recently, former NASA astronaut and current NASA climate scientist Piers Sellers was diagnosed with stage 4 pancreatic cancer.  Two weeks ago he wrote an op-ed in the New York Times where, rather than feel sorry for himself or wax sentimental about his life, he described (in broad terms) what humanity needs to do to avoid the potentially catastrophic consequences of runaway global warming, since it is now likely that he won't be around to see what happens.  After describing the initial challenges associated with making effective energy and environmental policy (which are substantial), he writes (emphasis mine),

Ultimately, though, it will be up to the engineers and industrialists of the world to save us. They must come up with the new technologies and the means of implementing them. The technical and organizational challenges of solving the problems of clean energy generation, storage and distribution are enormous, and they must be solved within a few decades with minimum disruption to the global economy.

Here we'll address the question that is probably on many people's minds: why are the technical challenges associated with generating cleaner energy so enormous?  Let's start by looking at the current ways in which we extract energy.

Consider a conventional gasoline-powered car like the Toyota Camry, one of the most common cars on American roads.  The gas mileage on such a car is, accounting for highway and city driving, about 30 miles per gallon.  Since there are 16 cups in one gallon, this means you get about 2 miles per cup, or one mile out of about this much gas:

Pictured: a one-mile drive in a Toyota Camry...from an engineer's perspective at least.
(SEE NOTE 1)

Let's say you put that much gasoline in the tank of a Toyota Camry and drive a mile.  Then, the car breaks down, and the nearest service station is a mile away.  Now imagine the amount of energy you would have to expend to push the car (which typically weighs about 3,240 pounds) for the entire mile to get there.  There's a LOT of energy in that half-cup of gasoline (which, at today's prices, would currently set you back a whopping 5 cents)!

As it turns out, gasoline has a very high energy density compared with many other substances.  Here's a comparison with a few other notable substances (SEE NOTE 2).

	Food calories per pound	Compared to TNT
TNT	295	1
Chocolate chip cookies	2,269	7.7
Coal	2,723	9.2
Butter	3,176	11
Gasoline	4,538	15
Natural gas	5,899	20
Uranium-235	9,000,000,000	32,000,000

(Yes, 1 pound of chocolate chip cookies has the energy of 7.7 pounds of TNT!)

Now let's compare gasoline with other sources from which we draw energy, like a AA battery.  How much energy is stored in it?

Batteries deliver energy in the form of electricity, which at the most basic level means moving electrons.  Batteries are typically characterized by the amount of current (essentially, number of electrons per second) they can deliver, and the voltage (essentially, the amount of energy each electron carries) at which they can deliver it.  Different applications require different amounts of each.  A typical alkaline AA battery can deliver around 2500 milliamp-hours at 1.5 V.  A milliamp is a unit of current, so in other words, it can deliver 2500 milliamps of current for 1 hour at 1.5 V.  Carrying out the calculations,

Power = Energy per Time = Current x Voltage (see this article for an explanation)

Energy = Current x Voltage x Time

Energy = 3750 Watt-hours, or 13,500 Joules.

Here I'm using the metric unit for energy, called the Joule (after James P. Joule, a pioneer in the area of thermodynamics).

So......is 13,500 Joules a lot of energy?  Let's compare this to the amount of energy in food.

What we call 1 food calorie is actually 1,000 "actual" calories, or 1 kilocalorie (long story - see this article), and 1 kilocalorie turns out to equal 4,184 Joules.  So a AA battery contains a little over 3 calories' worth of energy.  Compare that to a granola bar, which has around 200 calories in it.  So, a granola bar is equivalent to 62 AA batteries in terms of energy stored!

What about the equivalent amount of gasoline?  A granola bar weighs about 56 g.  From the table above and some simple arithmetic, 56 g of gasoline has about 567 food calories, or the equivalent of about 186 AA batteries!

But, you say, there's plenty of energy in the Sun!  We just need to harvest and store it, right?

The rate at which we receive energy (in the form of electromagnetic radiation) from the Sun turns out to be about 1000 Joules per square meter per second.  On a perfect sunny day, a patch of ground with an area of 1 square meter receives about 1000 Joules every second.  Sounds like a lot of energy, right?  Well, setting aside the fact that this is only true in direct sunlight (and so by definition only during daytime), the issue is that most solar cells are only about 10% efficient.  This means we can only extract about 100 Joules per second for every square meter of (generally expensive) solar cells that we build.   

So, let's say we buy a 1-square-meter solar cell and collect sunlight for 1 hr.  In the best case scenario, this will give us about 86 food calories' worth of energy.  This is equivalent to half a granola bar, or about 3/4 of a Tablespoon of gasoline!

Conclusion:

Burning stuff like gasoline or coal releases a LOT of energy, and it's going to be hard to find other energy sources that are as cheap and energy-rich.

But that won't stop us from trying.  In future posts, I will address the steps we are taking at MIT to try to come up with new ways to generate clean energy, and just as crucially, how to store and distribute it.

NOTES!
1. Some of you may be surprised that that much gasoline is actually required.  (I was.)  But the thing to keep in mind is that cars are actually quite inefficient when it comes to extracting energy from gasoline: for a typical auto engine, the average efficiency (amount of useful mechanical energy actually extracted divided by the total extractable amount) is around 18-20%.  In fact, thermodynamics shows us that the best we can ever hope for with this kind of engine is around 37%.  So, the "large" amount of gasoline required to drive one mile is not due to a limited amount of energy in gasoline, but due to the limitations on our capability to extract useful energy from it.

2. Source: Muller, R.A., Energy for Future Presidents.  New York: W.W. Norton, 2012.