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Did you know that even a perpetual motion machine will eventually come to a stop due to uncontrollable factors? Well, uncontrollable factors are at play in power plants, too. If you recall from our last article, heat rate is industry jargon for gauging how efficiently a coal-fired power plant is operating. We learned that heat rate can be affected by things like missing thermal insulation on pipes and equipment. Missing insulation is, of course, a thing that is under human control and easily corrected, but there are some things that affect heat rate that we just can’t do anything about. They’re called, appropriately enough, uncontrollable factors. Uncontrollable factors exist because anything devised and made by fallible humans who are beholden to the myriad laws of the universe cannot be 100 percent efficient. At their best utility coal fired power plants have an overall efficiency of between 30 and 40 percent. That means 60 to 70 percent of the energy available in the coal gets lost in the process of generating electricity. A terrible waste, right? And yet there’s nothing we can do to trim these losses until improvements in the present level of technology take place. Just as our ability to track microbes is dictated by the strength and accuracy of our magnifying equipment, so are we hampered by the tools we have at our disposal to deal with inefficiencies such as energy losses. So where does this energy get lost due to uncontrollable factors? The first and probably most obvious place to look is the smoke stack. Energy is also lost in three other ways: friction between equipment parts, auxiliary power consumption, and in a piece of equipment known as a condenser. Let’s look at each. In the most basic of terms, when coal is introduced into a power plant boiler it is combined with air and burned. This burning process releases heat energy, but it also forms gases that contain nitrogen and compounds like carbon monoxide and carbon dioxide. There’s also some water vapor formed by moisture in the coal and air. These gases and vapor absorb some of the heat energy released. To keep the combustion process going the gases and vapor must be removed from the boiler by powerful fans and sent up the smoke stack. Now, boilers are designed to absorb much of the heat energy from the gases and vapor that make their way to the stack, but they cannot possibly absorb it all. The result is that a significant amount of heat escapes up the smoke stack into the atmosphere along with the gases. Friction between parts is present everywhere in a power plant. It exists in the bearings on the shafts of motors, pumps, and steam turbines, slowing them down and hindering their operating capacity. Friction also exists where moving water and steam are present, impeding their ability to flow through piping systems. There is even friction working against the steam as it flows through parts in the turbine. Extra energy has to be expended to overcome this friction. This is energy that could be used to generate electricity. Now at some point in your life you’ve probably heard it said, “You need money to make money,” and this is very true. It takes a certain investment of resources to produce a profit-making enterprise. This investment principle holds true for the making of electricity, too. The bottom line is you need electricity to make electricity. Specifically, you have to use significant amounts of electricity to power machinery that is essential to move coal, air, combustion gases, and water through the process of making electricity in the power plant. This is called auxiliary power. It’s the electricity siphoned off by the various pieces of equipment in a power plant in its quest to generate electrical energy to be sold to customers. Another major factor at play in uncontrollable energy losses is in a piece of equipment integral to the very function of power plants: the condenser. It comes into play when water is boiled to make steam which then travels through the turbine, spinning its electrical generator and creating electric power. Unfortunately even the most efficient of steam turbines cannot use 100% of the heat energy coming at it from the steam. You see, after steam leaves the turbine, it’s turned back into water by a condenser so it can be sent back to the boiler to be turned into steam again. One of the reasons that this is done is so that the boiler does not have to be continuously filled with fresh, purified water. Water purification is necessary to keep minerals, seaweed, fish scales, and other nasty things from clogging up and damaging the boiler and steam turbine, and purified water is not as readily available as, say, lake water. The condenser acts as a heat exchanger that is hooked up to the steam turbine exhaust. It has tubes inside of it in which cold water flows, water which is drawn in from a nearby body of water, most often a river or lake. As steam blows across the outside of the cold water tubes in the condenser, it gives up its remaining heat energy and condenses into water again, then it is returned to the boiler to repeat its journey. The river water within the tubes of the condenser flows back into the river, carrying with it the heat energy removed from the steam. That wraps up our discussion about coal power plant efficiency. Next time we’ll discuss a new topic: coal fired power plant furnace explosions. _____________________________________________ |
Posts Tagged ‘condenser’
Coal Power Plants, Far From Perfect
Sunday, July 18th, 2010Thermodynamics In Mechanical Engineering, Part II, Power Cycles
Sunday, December 13th, 2009|
Last time we talked about some general concepts in an area of mechanical engineering known as thermodynamics. In this week’s article we’ll narrow our focus a bit to look at a part of thermodynamics that deals with power cycles. One mammoth example of a power cycle can be found in a coal-fired power plant. You can’t help but notice these plants with their massive buildings, mountains of coal, and tall smoke stacks. They’ve been getting a lot of negative press lately and are a central focus of the debate on global warming, but most people have no idea what’s going on inside of them. Let’s take a peek.
Figure 1 – A Coal-Fired Power Plant A power plant has one basic function, to convert the chemical energy in coal into the electrical energy that we use in our modern lives, and it’s a power cycle that is at the heart of this conversion process. The most basic power cycle in this instance would include a boiler, steam turbine, condenser, and a pump (see Figure 2 below).
Figure 2 – A Basic Power Cycle When the coal is burned in the power plant furnace, its chemical energy is turned into heat energy. This heat energy and the boiler are enclosed by the furnace so the boiler can more efficiently absorb the heat energy to make steam. A pipe carries the steam from the boiler to a steam turbine. Nozzles in the steam turbine convert the heat energy of the steam into kinetic energy, making the steam pick up speed as it leaves the nozzles. The fast moving steam transfers its kinetic energy to the turbine blades, causing the turbine to spin, much like a windmill (see Figure 3 below).
Figure 3 – The Inner Workings of a Steam Turbine The spinning turbine is connected by a shaft to a generator. The turbine works to spin the generator and thus produces electricity. After the energy in the steam is used by the turbine, it goes to the condenser, whose job it is to convert the steam back into water. To accomplish this, the condenser uses cold water, say from a nearby lake or river, to cool the steam down until it converts from a gas back to a liquid, that is, water. This is why power plants are normally found adjacent to a body of water. After things are cooled down, the pump gets to work, pushing the condensed water back into the boiler where it is once again turned into steam. This power cycle keeps repeating itself as long as there is coal being burned in the furnace, the plant equipment is functioning properly, and electrical energy flows out of the power plant. Thermodynamics sets up an energy accounting system that enables mechanical engineers to design and analyze power cycles to make sure they are safe, reliable, efficient, and economical. When all is said and done, a properly designed power cycle transfers as much heat energy as possible from the burning coal on one end of the cycle to meet the requirements for electrical power on the other end of the cycle. As was mentioned in last week’s blog, nothing is 100% efficient. Next time we’ll learn about being cool. No, I’m not going to talk about the latest cell phone gadget or who’s connected on Facebook. We’ll be covering refrigeration cycles. _________________________________________________________________ |
