How Everything Works
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QUESTIONS AND ANSWERS
 
Question 1119

I understand that for a steam engine to produce useful work, you need a difference in temperatures. My question is whether the difference in temperatures between cold glacier ice and the warmer air could be used to drive a steam engine and generate electricity. — LNH & AJH, Juneau, Alaska
As you clearly recognize, any heat engine—a machine that converts thermal energy into work—can only do its job while heat is flowing from a hotter object to a colder object. That limitation is imposed by the second law of thermodynamics—a statistical law that observes that the disorder of an isolated system can never decrease. A heat engine's theoretical efficiency at turning thermal energy into work improves as the temperature difference between its hotter and colder objects increases. Since the air temperature is hotter than the glacier temperature, there is the possibility to convert some of the air's thermal energy into work as heat flows from the air to the glacier. In short, what you suggest could be done.

Unfortunately, most practical heat engines work best when the hotter object is really hot. For example, a steam engine works best when the hotter object is hot enough to produce very high temperature, high pressure steam. To operate a steam engine with outside air as the hotter object and cold ice as the colder object, the steam engine would have to operate at very low pressure. In fact, it would operate well below atmospheric pressure in a carefully sealed environment. Steam might not even be the best choice for a working fluid—you might do better with a refrigerant such as the various Freon replacements. In effect, your heat engine would be an air conditioner run backward—providing electric power rather than consuming it. Although this could be done, it would probably not be cost effective. The heat exchangers needed to obtain heat from the air and to deliver most of that heat to the glacier, as well as all the machinery of the heat engine itself, would probably make the electricity you generated too expensive. Just because something can be done doesn't mean that it's worth doing. Until other sources of energy become more expensive, this one won't pay for itself.

         

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