Friday, August 28, 2009


Setting the Bar Very Low

I have often stated that the tiny bit of power we get from the seriously intermittent sources of wind and solar will not save a single lump of coal in a nearby or back-up coal fired power plant, because they set the base power output without regard to small, seriously intermittent sources, like wind and solar. Difficult to know if I'm as right as I think I am.

So today I read in the Denver Post that Xcel is building a smallish coal power plant combined with a focused mirror solar plant to see how much coal, if any, the solar powered portion saves. Wow! Perfect.

It's always good to see what the not disinterested party thinks. Here is Xcel's prediction:
The solar unit is forecast to replace about 900 tons of coal a year at Cameo. The plant burns about 230,000 tons a year.
900 divided by (230,000 minus 900) is .0037 or .37%, a third of a percent. That's really tiny. .37% of The Drive against Cleveland would be less than 4.5 inches. Helpful, but not very helpful.

I still say if the experiment is honest, there will be no real savings of coal burned (even though the solar part is designed merely to pre-heat the water the coal fire will turn to steam and will not generate its own power per se). Almost all the energy used in turning water to steam occurs after the water is heated to 210 degrees F. So the savings may well be smaller than the slightly less than lavish amount expected.

I'll get back to you when the experiment has been preformed.


Energy cost to boil water is 1 calorie per gram/degree C plus 540 calories per gram to change from water to steam.

Assuming 10C water (a reasonable approximation that will vary with the time of the year), total energy cost will be 630 calories per gram, of which 90 calories per gram will be preheating the water. It looks like they're expecting a 2C (or so) increase in temperature from the solar preheaters.

Now I'm wondering whether they use warming ponds (passive solar, functionally) in the summer already. You should be able to get at least a 10C rise in temperature (average) when the air is warm.
I've never seen a warming pond at any coal fired plant. Have you?
The focused mirrors can make something in the focal point very hot. I think that such a system using molten salt would actually be a good system--much better than photovoltaic. Takes a lot of space though.
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