Sunday, April 20, 2008

thermal energy = electrical energy?

What's the problem with the following excerpt from this NYT article?

Solar power, the holy grail of renewable energy, has always faced the problem of how to store the energy captured from the sun’s rays so that demand for electricity can be met at night or whenever the sun is not shining.

The difficulty is that electricity is hard to store. Batteries are not up to efficiently storing energy on a large scale. A different approach being tried by the solar power industry could eliminate the problem.

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, 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 problem is that the energy in a cup of hot coffee, although equal to the energy in the battery, does not really equal the energy in the battery. What? Yes, it's true; the energies are equal (in the scientific sense), yet not equal (in the popular sense, meaning what use you can get out of them). What do I mean? Let's ask a historical figure for an explanation. Sadi Carnot once said,

the maximum amount of motive power gained by
the use of steam [in a Carnot cycle] is also the maximum
that can be obtained by any means whatsoever.

This means that a specific cycle that Carnot thought up would produce the most work possible from thermal energy (which also means the most electricity possible because electricity is produced by spinning a turbine, which is work). This "maximum work" is described through the Carnot efficiency, 1-Tc/Th. Tc is the cold reservoir temperature, where you dump heat (the atmosphere) and Th is the hot reservoir temperature (the coffee). As you can see (and can imagine), 1-Tc/Th isn't very large for a cup of coffee in an atmosphere at room temperature. What amount of work can you get from the energy in a battery? Almost all of it.

Thermal energy is the lowest form of energy, the least useful. It's 100% efficient to heat a house with thermal energy, but it can also be 100% efficient to heat a house with electrical energy because you can convert electrical energy with 100% efficiency to thermal energy.

There's nothing wrong with trying to store thermal energy, but it's misleading to talk about thermal energy as if it's on par with electrical energy.

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