I used my own title. Because I like it better. This is the first time I (personally) can recall seeing a discussion of exergy on the intertubes.
Still, the problems with net energy and economic triage both ultimately rest on thermodynamic issues, because the exergy available from solar energy simply isn’t that high. It takes a lot of hardware to concentrate the relatively mild heat the Earth gets from the Sun to the point that you can do more than a few things with it, and that hardware entails costs in terms of net energy as well as economics. It’s not often remembered that big solar power schemes, of the sort now being proposed, were repeatedly tried from the late 19th century on, and just as repeatedly turned out to be economic duds.
Consider the solar engine devised and marketed by American engineer Frank Shuman in the first decades of the 20th century. The best solar engine of the time, and still the basis of a good many standard designs, it was an extremely efficient device that focused sunlight via parabolic troughs onto water-filled pipes that drove an innovative low-pressure steam engine. Shuman’s trial project in Meadi, Egypt, used five parabolic troughs 204 feet long and 13 feet wide. The energy produced by this very sizable and expensive array? All of 55 horsepower. Modern technology could do better, doubtless, but not much better, given the law of diminishing returns that affects all movements in the direction of efficiency, and most likely not enough better to matter.
Does this mean that solar energy is useless? Not at all. What it means is that a relatively low-exergy source of energy, such as sunlight, can’t simply be used to replace a relatively high-exergy source such as coal. That’s what Shuman was trying to do; like most of the solar pioneers of his time, he’d done the math, realized that fossil fuels would run out in the not infinitely distant future, and argued that they would have to be replaced by solar energy: “One thing I feel sure of,” he wrote, “and that is that the human race must finally utilize direct sun power or revert to barbarism.”
He may well have been right, but trying to make lukewarm sunlight do the same things as the blazing heat of burning coal was not the way to solve that problem. The difficulty – another of those awkward implications of the laws of thermodynamics – is that whenever you turn energy from one form into another, you inevitably lose a lot of energy to waste heat in the process, and your energy concentration – and thus the exergy of your source – goes down accordingly. If you have abundant supplies of a high-exergy fuel such as coal or petroleum, that doesn’t matter enough to worry about; you can afford to have a great deal of the energy in a gallon of gasoline converted into waste heat and pumped out into the atmosphere by way of your car’s radiator, for example, because there’s so much exergy to spare in gasoline that you have more than enough left over to send your car zooming down the road. With a low-exergy source such as sunlight, you don’t have that luxury, which is why Shuman’s solar plant, which covered well over 13,000 square feet, produced less power than a very modest diesel engine that cost a small fraction of the price and took up an even smaller fraction of the footprint.
This is also why those solar energy technologies that have proven to be economical and efficient are those that minimize conversion losses by using solar energy in the form of heat.
That’s the secret to using low-exergy sources: heat is where exergy goes to die, and so if you let it follow that trend, you can turn a relatively diffuse energy source to heat at very high efficiencies. The heat you get is fairly mild compared to (say) burning gasoline, but that’s fine for practical purposes. It doesn’t take intense heat to raise a bathtub’s worth water to 120º, warm a chilly room, or cook a meal, and it’s precisely tasks like these that solar energy and other low-exergy sources do reliably and well.
http://www.energybulletin.net/51901
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