The latest Technology Review has a great information graphic showing the sources and uses of energy in the United States.
The most important take-away? That almost 45% of energy input is lost as waste heat. And, of course, that almost 85% of energy inputs come from oil, natural gas, and coal.
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Given the realities of the Third Law of Thermodynamics, it’s no surprise so much energy goes out as waste heat.
It’s been known since the time of Carnot that any heat engine is limited to an efficiency no greater than 1 – Ts/Tr where Ts is the absolute temperature of the heat source and Ts is the absolute temperature of the heat sink. If your heat sink is at room temperature, 300 degrees Kelvin, then achieving an efficiency of, say, 75% requires that the heat source be at least four times room temperature, or 1200 degrees Kelvin — that’s 900 degrees Centigrade or 1620 degrees Fahrenheit.
Try building a practical commercial boiler that operates at 1620 Fahrenheit.
And that’s just the theoretical limit; there are also all sorts of practical inefficiencies that even very smart engineers have to work very hard to overcome.
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