Intro to Power, Part 2: Steam Turbine Power Plants
This is Part 2 of our grand launching of Engineerography Blog. If you haven’t checked out Part 1 already, I recommend that you do. I talked about the fundamentals to electricity generation there.
The simplest form of a steam turbine power plant consists of a boiler and a turbine driving an electric generator. Typically, the axis of the turbine and the axis of the generator (i.e. the spinning magnet) shares a common shaft. So, if the turbine spins, then the magnet spins, and then we generate the power we need. Still quite simple, huh?
GE's LM6000 Turbine
At this point, we have switched over from the invisible magnetic field stuff and electric currents to something we can physically deal with. In fact, if we can get someone to run really, really fast in a circle and turn the generator’s shaft, we can also generate electricity. Unfortunately, we can never run quite fast enough. Instead, we let steam do it for us.
Why steam, anyway? Why not ammonia or some special hydrocarbon? Well, water is rather nonvolatile, meaning it doesn’t react with other components so easily, and if there happens to be a leak somewhere, it wouldn’t be too problematic. Most importantly, though, water’s properties are just too good to pass up. It has the chemical properties of vaporizing at just the right temperatures and pressures we need. Plus, it’s “pure,” right? You wouldn’t clean your kitchen counter with orange juice’s citric acid, for example.
What the hot and pressurized steam does is turn the turbine. You can think about wind mills, where the wind turns blades on a shaft to do kinetic work. It’s also like a jet engine in reverse: instead of using gas to turn the blades and cut the air, it’s analogous to having high-pressured air turn the blades to generate electricity. Sort of …
Turbine-Generator Assembly
Now the trick is getting the steam. Anyone who drinks tea would know that we can get steam from heating up water. We use fire as a heat source and the teakettle as the boiler. The steam that’s produced turns the turbine, which turns the generator, which produces electricity.
That’s the fundamental process of a power plant. It’s fairly straight-forward.
The question you might ask is: If a power plant is so basic and simple as the teakettle example, why do we have these complex monstrosities full of dirty piping, smoke stacks, and tanks? The answer is simple too: Efficiency. The amount of electricity a teakettle can produce in this system is practically none; the efficiency is essentially zero.
The different kinds of plants differ primarilly in the kind of fuel that’s used. Coal, natural gas, fuel oil, and nuclear sources are the more popular ones. They all use the same steam system of heating up water and turning the turbine! The difference is the kind of boiler that’s used; in other words, how we are getting from water to steam. Of course, there are inherent trade-offs. Now, it’s just a question of which kind of fuel. Of the fossil fuels (coal, natural gas, and petroleum), coal offers the most energy per pound. It is a relatively dense solid and is made up of complex chemical bonds which is broken up to release energy. Petroleum (fuel oil) is a thick liquid, and natural gas, is, well, a gas (made mostly of methane). Certain fuels release more energy than others when they are burned.
Please keep in mind that none of these are considered “clean” fuel. There are always by-products and waste. Coal, is by far, the worst of them. (I will address “clean-coal” in the future.) Fuel oil is made up of the black, tar-like junk stuff that isn’t good enough to go into your car, and natural gas is the cleanest of the fossil fuels. But remember, coal is mined from the earth, petroleum is pumped from the earth, and natural gas is released from the earth. All of these are burned in the boiler to heat up water for steam. Even nuclear power plants just use radioactive material whose chemical energy becomes thermal energy to heat up the same water for steam. (There is only the added issue of radioactive waste.) Everything else can be more or less the same. Even biomass and corn ethanol are merely just fuels. The process outside of the boiler is the same.
Does all this make sense? Let me know if you have any questions in the comments.
For next time, the secret word is “efficiency”.
(LM6000 image from Aventi Technology. Assembly image from World Of Energy.)
Brilliant read Kevin. You communicated what is a largely misunderstood concept into something clear and readable. Kudos.
Good work Kevin! I agree with “Steam Turbine Generators”.
There is one erre however. You say
“Even nuclear power plants just use radioactive material whose chemical energy becomes thermal energy to heat up the same water for steam. (There is only the added issue of radioactive waste.)”
Not quite true. Nuclear plants do not use CHEMICAL energy, that is energy that links atoms together into molecules : they use NUCLEAR energy, that is energy that links protons and neutrons together to form an atom’s core or “nucleus” (hence the name “nuclear”). Nuclear energy is millions of times more dense than chemical energy : a few pounds of uranium (or a few ounces of hydrogen when fusion processes will be controlled) generate as much energy as tons of coal, petroleum or natural gaz (or biomass or ethanol).
For the rest, you are right : nuclear reactors produce heat to give steam that will turn turbines which turn electrical generators (or alternators).
VERY good description of how a steam power plant works.
There are a few errors in this piece, however. First of all, GE’s LM6000 turbine is NOT a steam turbine. It is, in fact, a gas turbine. Gas turbines, of course, are also used to generate electricity for electrical ultilities. But they operate on a completely different principal. Secondly, coal does NOT have the highest energy content per pound when it comes to fossil fuels. Natural gas (which is mostly methane) is actually the winner here. Granted, if we are talking about energy per unit volume, this changes things considerably. But as far as energy per pound natural gas is the highest and coal is actually the lowest. Finally, speaking of such things as natural gas and gas turbines, it is actually quite rare to burn natural gas in a purely steam power plant. Gas turbine plants are MUCH more common. However, steam plants are typically used to utilize the energy in the hot gas turbine exhaust in order to increase overall efficiency of a natural gas power plant. Such a combined steam and gas turbine power plant is referred to simply as a combined cycle power plant. Given the high efficiency of a combined cycle power plant, natural gas is seldom used without a gas turbine to generate electricity.