PowerSource

Take a look at these two quotes, both regarding the amount of energy stored in a laptop battery pack. The first is from a NY Times article about solar thermal energy generation:

“The idea is to capture the sun’s heat. Heat, unlike electric current, is something that industry knows how to store cost-effectively. For example, a coffee thermos and a laptop computer’s battery store about the same amount of energy, [bolding added] said John S. O’Donnell, executive vice president of a company in the solar thermal business, Ausra. The thermos costs about $5 and the laptop battery $150, he said, and “that’s why solar thermal is going to be the dominant form.”

The second is from the IEEE Spectrum article on Christina Lampe-Onnerud, the president of lithium ion battery vendor Boston Power: “She ruffled a few feathers … by pointing out in a talk that the energy density of lithium-ion batteries used for laptop computers, at 40 watt-hours per kilogram, was already getting uncomfortably close to that of your basic hand grenade. [bolding added] That density, the amount of energy stored in a certain mass, had been going up like a rocket as manufacturers competed fiercely for a growing market.”

So here we have two ways of looking at energy storage in a battery pack: One model is of a warm-and-fuzzy coffee thermos, while the other is of a nasty, violent hand grenade. How can both be accurate models of a laptop battery?

The two systems are using energy to do two very different things. The Ausra executive is pretty dismissive about our knowledge of how to store electrical energy, but it’s also much easier to store energy at a relatively low temperature. On the other hand, it’s hard to get any work out of low-temperature energy.  Plus, energy stored as heat can only be made to do work through a heat engine (you need a cold sink too), and you’re limited to the Carnot efficiency. A battery, on the other hand, stores energy chemically and isn’t limited by Carnot efficiency.  

The hand grenade analogy makes the point that this is an amount of energy that can be confined to your lap, and yet can be extremely destructive – especially when it’s in the sensitive environment of a lap rather than a battlefield.  The difference here is the rate at which energy is released. A grenade’s chemistry is such that the total amount of energy isn’t huge, it can do so much damage because of the rate at which it’s released. Under normal operating conditions, the laptop battery releases energy slowly. Practically speaking, a laptop battery isn’t going to go boom like a grenade because a grenade is designed to release its energy nearly instantaneously. So that analogy is a bit inaccurate also.

So, two very different models of one common energy storage device. We’re entering a time when there is no one-size-fits-all energy storage, and we will have to tailor our energy sources – and their storage – very carefully to their application in order to wring out the maximum efficiency, in a cost-effective way.