Author: Lou Bloomfield

How much steam is required to produce a unit of power? — DKB, Dubai

There is no easy answer to this question, but for an interesting reason. First, “power” is a measure of energy per time (e.g. joules per second or BTUs per hour) so any answer would have to involve the amount of steam per time (e.g. kilograms per second or cubic meters per hour). But even recognizing that requirement, I can’t answer the question. First, I’d need to know the temperature of the steam. The hotter the steam, the more thermal energy it contains and the more energy it could provide. For more complicated reasons, I’d also have to know the pressure of the steam. But there is a fourth issue: even knowing the amount of steam involved and the temperature and pressure of that steam, the amount of useful energy that can be extracted from that steam depends on the existence of a colder object. You can’t turn thermal energy—the type of energy that steam contains—directly into useful work or into electric energy in a continuous manner. You must use the steam in a “heat engine”, converting a fraction of its thermal energy into work as that thermal energy flows as heat from the hot steam to a colder object. This requirement is established by the laws of thermodynamics and there is no way to get around it. The hotter the steam and the colder the object, the larger the fraction of the steam’s thermal energy you can convert to work. However, there is no way to convert all of the steam’s thermal energy into work continuously.

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.