Energy alternatives series: Power plants



I have spent a good deal of my life in school, educated in the physical sciences, first in physics, then in chemistry. Natural law and scientific method are, to me, ingrained and second-nature. Some things I don’t even recognize any other way. But there are parts of the natural world that still fill me with the deepest, unalloyed awe.

Pretty much anything to do with magnetic fields is like this. The thought that my lights work and my computer turns on because some magnetic objects are spinning inside a coil of wire is still kind of miraculous to me. Why the electrons in my house care what the twirling magnetic fields are doing downtown is, I suppose, understood to some fine extent by modern physics. But we also often confuse being able to describe something with understanding it. It is a deep, enduring mystery that turns out to have myriad delightful and useful implications.

With the exceptions of things like solar power, fuel cells, and thermoelectric generation, most power sources we rely on involve just this- a magnet twirling in a coil of wire, which induces a current in the wire coil. The motive power that we use to get the twirling to happen is the issue here, and where we often find ourselves in contention.

Most of the time, some fuel is burned to produce heat, which then boils water to steam, which runs through a turbine. This sets our magnets a-twirl, and power is generated. We can burn coal, cord wood, natural gas, pellets of waste paper, all sorts of things. The quantity of heat we can derive from these is knowable, and will tell us how good a fuel this will be. We will consider pollution of various kinds in a later installment. We neglect it now, recognizing that the process is the same, though the end products that are produced along with heat are very important to the environment. We just need to separate these for a little while to get a sense of what goes in to generating power.


Nuclear power works, in most cases, exactly the same way, only the fuel is not burned; heat is produced by the process of nuclear decay. Ultimately, this boils water to make a turbine spin, too, so from a fundamental point of view, we are discussing the same process.

Nuclear waste is a kettle of fish all to itself, which we can consider later, especially when we discuss trade-offs of various pollutants, what the earth can best accommodate, and whether the only sane thing to do is to shiver, in the dark, or make hard choices that do the most good with the least harm. It gets sticky, because values come into play. But it cannot be avoided, and the details, if only a few and simple ones, will help navigate this.

The class of machines that use heat from burning or nuclear decay like this for power generation, or for locomotion, are called heat engines. So if you play along, you will discover some things that will also help a bit later when we look at transportation, as well as illuminating power generation.

There are thermodynamic laws at work in heat engines that tell us, unequivocally, what we can expect, at best, to get out for an amount of fuel (energy) in. While the process is different in the case of the generation methods we mentioned that don’t involve twirling magnets (and we will look at these next time), thermodynamics is not something we can escape. The rules are a bit different there, but they still apply.

The design of a modern power generating turbine is immensely complicated, from the precise shapes of the component parts that must mate with astonishing accuracy, to what materials one can trust to withstand the enormous pressures involved. Yet the efficiency maximum can be calculated by merely knowing the operating temperature of the heat engine, and the temperature of the surroundings where it will put the heat.

This theoretical maximum (known as the Carnot efficiency) is an iron-clad upper limit. We can certainly do worse (and do, as a rule) but you cannot do any better. Period. (This measure is named for Sadi Carnot, a French engineer, who worked this all out before thermodynamics was developed. His short but scientifically heroic life is worth a look at Wikipedia.)

I’ll show you how to calculate this next time, and we will see some interesting implications with the simplest of math.