Intro to Power, Part 3: Efficiency
This is Part 3 of our launching of Engineerography Blog. If you haven’t checked out Part 1 and Part 2 already, I recommend that you do. I talked about the fundamentals to electricity generation and steam turbine power plants.
Efficiency is the hot word of the news today, but the science and engineering of it is actually pretty dry, I’ll admit. Consumers worry about fuel efficient cars and trucks, but do they think about fuel efficient power plants? Power plants generate about 20% of the world’s greenhouse gases, while all modes of transportation weigh in at around 15%.
What is efficiency anyway? In thermodynamics, efficiency is a ratio of the work extracted from a system to the heat or energy put into the system. Output over input.
Still with me? To increase efficiency, we want to essentially use less energy in for a certain amount of work done by the system. Makes sense, doesn’t it? If we look at the formula above, the math holds true to common sense.
Efficiency Leads to Complex Plants
In a power plant, overall efficiency can range from 20-50%. There are efficiency-ratings of every component in a power plant. For example, a boiler may be 80% efficient, while the generator might be up to 95% efficient. Why is the overall efficiency so low? There are many factors. Whenever energy is converted from one for to another (in this case, chemical to thermal to kinetic to electrical), energy is lost in the actual conversion. In addition, there can be leaks and insulation problems. Nothing is 100% efficient; it is an unattainable goal.
There are a lot of things power plants do to make the overall system more efficient. In fact, this is the what people in the field of power engineering do. They develop systems and processes to make the basic teakettle example run more efficiently.
I will mention just one type of idea, that is reheating and preheating the water and air via heat exchangers. Let’s assume a two-stage turbine system. (There is usually more than one co-axial turbine to get more energy from the boiled steam.) The steam that leaves the boiler and enters the first turbine is called the “main steam”. Instead of releasing the left-over steam to the atmosphere, we capture it and reuse it. It is still plenty hot and it returns to the boiler for a second run, bringing it back to the optimal max temperature. It is then called “reheat steam” and then enters the second turbine. The warm exhuasted steam is collected and reheated with stage heaters before returning into the boiler to eventually become “main steam” again. After all, less energy is required to boil hot water than is required to boil cold water!
The same can be done with air. Regardless of the fuel that is being used, the burning of it (called combustion) requires oxygen. Lots of it. Of course, the cool air is taken from the surroundings. When it’s used in the boiler, where it’s hot, a noticeable portion of the boiler’s energy goes into heating the air before it’s being used. In order to combat this, the dirty and hot exhaust air is used to preheat the fresh incoming air so that less energy from the boiler is needed.
What all this reheating and preheating does is to lower the amount of energy required for electricity (or work). Remember that the efficiency formula is energy out divided by energy in. If we can lower the amount of energy we need, we can drastically increase the overall efficiency of the plant! Different fuels have different amount of energy and the power plant’s overall efficiency is a direct result of how “good” the fuel actually is.
That’s it! Remember that these ideas refer to steam turbine plants. Renewable energy like solar and wind and hydrodams don’t follow steam turbine system.
And finally, for the last installment of “Intro to Power”, I promise we’ll talk about something more interesting. Promise!
(Plant image from my Flickr.)
Congratulations again Kevin!
Another aspect of efficiency is how we use the fuel. The use of air compressors, for exemple, pushes the molecules of air more tightly against the fuel, creating a better contact between the fuel and the oxygen of the air, resulting in a more complete combustion that is, more complete conversion of the chemical energy into heat. Reducing the fuel to as-thin-as-possible particles (for exemple spraying petroleum into a fine mist or grinding coal into a very fine dust) increases the contact area between air and fuel, allowing for still better efficiency. Yes, compressors and high pressure fuel injectors cost energy to operate, but they increase efficiency so much that the net gain is tremendous. For example, using compressed air and fine coal dust can almost triple the amount of steam produced with a same quantity of coal when compares to a furnace-type boiler that burns coal blocks into air at atmospheric pressure.
To these, of course, we add the efficiency “tricks” you mention above and the efficiency obtained in a well-designed power plant just cannot be compared with that of a coal, wood, oil or gaz stove heating a tea kettle!