Posts Tagged ‘electrical generator’

Enthalpy and Steam Turbines

Thursday, November 14th, 2013

      Last time we learned how the formation of condensate within a power plant’s turbine results in a vacuum being created.   This vacuum plays a key role in increasing steam turbine efficiency because it affects a property known as enthalpy, a term used to denote total heat energy contained within a substance.   For the purposes of our discussion, that would be the heat energy contained within steam which flows through the turbine in a power plant.

      The term enthalpy was first introduced by scientists within the context of the science of thermodynamics sometime in the early 20th Century.   As discussed in a previous blog article, thermodynamics is the science that deals with heat and work present within processes.   Enthalpy is a key factor in thermodynamics, and is commonly represented in engineering calculations by the letter h and denoted as,

h = u + Pv

where u is the internal energy of a substance, let’s say steam; P is the pressure acting upon a specific volume, v, of the steam; and P and v are multiplied together.   Pressure is force per unit area and is measured in psi, pounds per square inch.   For the purposes of our discussion, it’s the amount of pressure that steam places on pipes containing it.

      Looking at the equation above, simple math tells us that if we increase the pressure, P, the result will be an increase in enthalpy h.   If we decrease P, the result will be a decrease in h.   Now, let’s see why this property is important with regard to the operation of a steam turbine.

      When it comes to steam turbines, thermodynamics tells us that the amount of work they perform is proportional to the difference between the enthalpy of the steam entering the turbine and the enthalpy of the steam at the turbine’s exhaust.   What is meant by work is the act of driving the electrical generator, which in turn provides electric power.  In other words, the work leads to a useful outcome.   This relationship is represented by the following equation,

W = h1h2

      In terms of the illustration below, W stands for work, or potential for useful outcome of the turbine/generator process in the form of electricity, h1 is the enthalpy of the steam entering the inlet of the turbine from the superheater, and h2 is the enthalpy of the steam leaving at the turbine exhaust.

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      We’ll discuss the importance of enthalpy in more detail next week, when we’ll apply the concept to the work output of a steam turbine.


Superheater Construction and Function

Sunday, September 15th, 2013

      Power plants produce electrical energy for consumers to use, whether at home or for business, that’s obvious enough, but did you know that in order to produce that electrical energy they must first be supplied with heat energy?   The heat energy that power plants crave comes from a fuel source, such as coal, oil, or natural gas, by way of a burning process.   Once the heat energy is released from the coal through burning, it’s transported into a steam turbine by way of superheated steam, which is supplied to it by a piece of equipment named, appropriately enough, a superheater.

      So what is a superheater and how does it function?   Take a look at the illustration below.

Electric Utility Power Plant Superheater

      The superheater looks like a W.   It’s actually a cascading array of bent steam pipe, situated above a source of open flames which are produced by the burning of a fuel source.   A photo of an actual superheater is shown below.

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      So how many bends are in a superheater?   Enough to fill the needs of the particular power plant it is supplying energy to.   Since all power plants are designed differently, we’ll keep things in general terms.

      The many bends in the superheater’s pipes form a circuitous path for steam to flow as it follows a path from the boiler to the steam turbine.   The superheater’s unique construction gives the steam flowing through it maximum exposure to heat.   In other words, the bends increase the time it takes for the steam to flow through the superheater.   The more bends that are present, the longer the steam will be exposed to the flame’s heat energy, and the longer that exposure, the more heat energy that is absorbed by the steam.

      Superheating routinely results in temperatures in excess of 1000°F.   This superheated steam is laden with abundant heat energy which will keep the steam turbine spinning and the generator operating.   The net result is millions of watts of electrical power.

      As we learned in a previous blog, the superheater is designed to provide the turbine with sensible heat energy to prevent steam from completely desuperheating, which would result in dangerous condensation inside the turbine.

      The newly added superheater is a major improvement to a power plant’s water-to-steam cycle, but there’s still plenty of waste and inefficiency in the system, which we’ll discuss next week.


Superheating, Part I

Monday, August 19th, 2013

      Last time we learned that our power plant boiler as presently designed doesn’t do a good job of producing ample amounts of superheated steam, the kind of steam that turbines need to spin and power generators.   During the process of superheating the more heat energy that’s added to the steam in our boiler, the higher its temperature becomes.   However, our boiler can only produce a limited amount of superheated steam as it stands now.

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      So how do we get more heat energy into the superheated steam?   Take a look at the illustration below for the solution to the problem.

coal fired power plant expert witness

      You’ll note a prominent new addition to our illustration.   It’s called a superheater.

      The superheater performs the function of raising the temperature of the steam produced in our boiler to the incredibly high temperatures required to run steam turbines and electrical generators down the line, as explained in my blog on steam turbines.   The superheater adds more heat energy to the steam than the boiler can alone.

      In fact, the amount of heat energy in the superheated steam produced with our new design is proportional to the amount of electrical energy that power plant generators produce.   Its addition to our setup will result in more energy supplied to the turbine, which in turn spins the generator.   The result is more electricity for consumers to use and a more efficiently operating power plant.

      But inefficiency isn’t the only problem addressed by the superheater.   We’ll see what else it can do next week.


Heat Energy Within the Power Plant— Water and Steam Cycle, Part 2

Wednesday, August 14th, 2013

      Last time we learned that electric utility power plants must have water treatment systems in place to remove contaminants from incoming feed water before it can be used.   This clarified water is then fed to a boiler by the boiler feed pump as shown below.

utility power plant expert

      As it stands this setup will work to provide electricity, however in this state it’s both inefficient and wasteful.   We’ll see why in a minute.

      Boilers, as their name implies, do a great job of heating water to boiling point to produce steam.   They do this by adding the heat energy produced by burning fuel, such as coal, to water, then steam.   We learned in earlier blogs in this series that the energy used to heat water to boiling point temperature is known as sensible heat, whereas the heat energy used to produce steam is known as latent heat.   The key distinction between these two phases is that during sensible heating there is a rise in temperature, during latent heating there is not.   For a review on this, see this blog article.

      When water starts to heat inside the boiler, sensible heat energy is said to be added.   This is represented by phase A of the graph below.

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      During A, heat energy will raise the temperature of the water to boiling point.   As the water continues to boil in phase B, water is transforming into steam.   During this phase latent heat energy is said to be added, and the temperature will remain at boiling point.

      In phase C something new takes place.  The temperature rises beyond boiling point and only steam is present.   This is known as superheated steam.   For example, if the boiler pressure is at 1,500 pounds per square inch, steam becomes superheated at temperatures greater than 600°F.

      Unfortunately, boilers alone do a poor job of superheating steam, that is, continuing to raise the temperature of the steam present in phase C.   This is evident by the fact that phase C is quite small in comparison to phases A and B before it.   This inefficiency in producing ample amounts of superheated steam results in a small amount of useful energy being provided to the turbine down the line, which is bad, because steam turbines require exclusively superheated steam to run the generator.

      Next time we’ll see how to provide our steam turbine with more of what it needs to run the generator, more superheated steam.


Coal Power Plant Fundamentals – The Generator

Monday, March 7th, 2011
     When I was a kid I remember how cool it was to have a headlight on my bike.  Unlike the headlights that the other kids had, mine was not powered with flashlight batteries.  The power came from a little gadget with a small wheel that rode on the front tire.  As I pedaled along, the tire’s spinning caused the small wheel to spin, and voila, the headlight bulb came to life.  Little did I know that this gadget was a simple form of electrical generator, and of course I was oblivious to the fact that a similar device, albeit on a much larger scale, was being used at a nearby power plant to send electricity to my home.

     Over the last few weeks we learned how a coal fired power plant transforms chemical energy stored in coal into heat energy and then into mechanical energy which enables a steam turbine shaft to spin.  We’ll now turn our attention to the electrical generator.  It’s responsible for performing the last step in the energy conversion process, that is, it converts mechanical energy from the steam turbine into the desired end product, electrical energy for our use.  It represents the culmination in energy’s journey through the power plant, the process by which energy contained in a lump of coal is transformed into electricity.   

    To show how this final energy conversion process works, let’s look at Figure 1, a simplified illustration of an electrical generator.

Figure 1 – A Basic Electrical Generator

     You’ll note that the generator in our illustration has a shaft with a loop of wire attached to it.  When the shaft spins, so does the loop.  The shaft and wire loop are placed between the north (N) and south (S) poles of a horseshoe magnet.  It’s a permanent magnet, so it always has invisible lines of magnetic flux traveling between its two poles.  These magnetic lines of flux are the same type as the ones created by kids’ magnets, when they play with watching paperclips jump up to meet the magnet.  The properties of magnets are not completely understood, even to adults who work with them every day.  And what could be more mysterious than the fact that as the shaft and wire loop spin through the lines of magnetic flux in the generator, an electric current is produced in the wire loop.

     Now, this current that’s flowing through the spinning wire loop is of no use if we can’t channel it out of the generator.   The wire loop is spinning vigorously, so you can’t directly connect the ends of the loop to stationary wires.  A special treatment is required.  Each end of the loop is connected to a slip ring.  A part called a “brush” presses against each slip ring to make electrical contact.  The electrical current then flows from the loop through the spinning slip rings, through the brushes, and into the stationary wires.   So, if, for example, a light bulb is connected to the other end of the stationary wires, this completes an electric circuit through which current can flow.  The light bulb will glow as long as the generator shaft keeps spinning and the wire loop keeps passing through the magnetic lines of flux from the magnet.

     So we see that the key to the whole energy conversion process is to have movement between magnetic lines of flux and a loop of wire.  As long as this movement occurs, the electricity will flow.  This basic principle is the same in a coal fired power plant, but the electrical generator is far more complicated in construction and operation than shown here.  My Coal Power Plant Fundamentals seminar goes into far greater detail on this and other aspects of electricity generation, but what I have shared with you above will give you a basic understanding of how they operate.

     That concludes our journal with coal through the power plant.  This series of blogs has, you will remember, presented a simplified version of the complex material presented in my teaching seminars.  Next week we’ll branch off, taking a look at why electrical wires come in different thicknesses.   



Coal Power Plant Fundamentals – The Steam Turbine

Sunday, February 20th, 2011

     When I was a kid I didn’t have video games or cable TV to help me occupy my time.  Back then parents tended to be frugal, and the few games I had were cheap to buy and simple in operation, like the plastic toy windmill I’d play with for hours on end.  All I had to do to make it spin was take a deep breath, pucker my lips together, fill my cheeks with breath, then blow hard into the windmill blades.  Its spin was fascinating to watch.  Little did I know that as an adult I would come to work with a much larger and complex version of it, in the form of a power plant’s steam turbine.

     You see, when you trap breath within bulging cheeks and then squeeze your cheek muscles together, you actually create a pressurized environment.  This air pressure buildup transfers energy from your mouth muscles into the trapped breath within your mouth, so that when you open your lips to release the breath through your puckered lips, the pressurized energy is converted into kinetic energy, a/k/a the energy of movement.  The breath molecules flow at high speed from your lips to the toy windmill’s blades, and as they come into contact with the blades their energy is transferred to them, causing the blades to move.  A similar process takes place in the coal power plant, where steam from a boiler takes the place of pressurized breath and a steam turbine takes the place of the toy windmill.

     If you recall from my previous article, the heat energy released by burning coal is transferred to water in the boiler, turning it to steam.   This steam leaves the boiler under great pressure, causing it to travel through pipe to the steam turbine, as shown in Figure 1.

Figure 1 – A Basic Steam Turbine and Generator In A Coal Fired Power Plant

     At its most basic level the inside of a steam turbine looks much like our toy windmill, of course on a much larger scale, and it is very appropriately called a “wheel.”  See Figure 2.  

Figure 2 – A Very Basic Steam Turbine Wheel

     The wheel is mounted on a shaft and has numerous blades.  It makes use of the pressurized steam that has made its way to it from the boiler.  This steam has ultimately passed through a nozzle in the turbine that is directed towards the blades on the wheel.  This is the point at which heat energy in the steam is converted into kinetic energy.  The steam shoots out of the nozzle at high speed, coming into contact with the blades and transferring energy to them, which causes the turbine shaft to spin.  The turbine shaft is connected to a generator, so the generator spins as well.  Finally, the spinning generator converts the mechanical energy from the turbine into electrical energy.

     In actuality, most coal power plant steam turbines have more than one wheel and there are many nozzles.  The blades are also more numerous and complex in shape in order to maximize the energy transfer from the steam to the wheels.  My Coal Power Plant Fundamentals seminar goes into far greater detail on this and other aspects of steam turbines, but what I have shared with you above will give you a basic understanding of how they operate. 

     So to sum it all up, the steam turbine’s job is to convert the heat energy of steam into mechanical energy capable of spinning the electrical generator.  Next time we’ll see how the generator works to complete the last step in the energy conversion process, that is, conversion of mechanical energy into electrical energy.


Transformers – The Voltage/Current Trade-Off

Sunday, December 26th, 2010

     As a child I considered the reindeer Rudolph, with his nose so bright, to be a marvel of engineering.  Now an adult, I remain perplexed as to the mystery behind the self-generating power source behind his nose.  Did it ever overheat? I wondered.  Perhaps today’s discussion can shed some light on the matter.

     During the course of our discussion of electricity certain terms have been tossed about, like voltage and current.  For some the distinction between the two may be unclear, and that is what we’ll be addressing today.

     Electricity is a rather abstract phenomenon, but you may consider the flow of electrical current through a wire to be much like water flowing through a garden hose.  The water won’t flow unless there’s sufficient pressure behind it, and that pressure is supplied by pumps, either at your city water works or your personal well.  Take away the pressure, and the water stops flowing through the hose. 

     Electricity flows in much the same manner.  It requires a pushing pressure to get it on its journey from power plant to home, and that pressure is voltage.  Take away voltage, and the current stops flowing through the wire.  Voltage is, of course, produced by an electrical generator at the power plant.

      Last time we saw how an electrical transformer can reduce high voltage to low voltage and how this process also works in reverse.  But how can that be?  How can low voltage be turned into high?  Is it really possible to get “something from nothing”?  Let’s take a closer look.

     When a light bulb burns out in your home, you routinely look at the bulb to see how many watts it is so you can replace it with the same type.  But what exactly is a “watt”?  It’s a unit of power, and the markings on the bulb tell you how much electrical power it consumes when you use it.  Generally speaking, this electrical power is related to voltage and current by this formula:

Power = Volts × Electrical Current

     Knowing this, if I have a 60 watt bulb in a table lamp, and I plug it into a 120 volt wall outlet, then how much electrical current is the lamp going to draw from the outlet?  Using the formula above and a little algebra, we get:

Electrical Current = Power ÷ Volts

Electrical Current = 60 watts ÷ 120 volts = 0.5 amperes

     And believe it or not, this same formula that’s used to assess power  of a light bulb also applies to electrical transformers.  Basically, the power going into the transformer is equal to the power coming out.

     To see how this works, consider the example step-up transformer shown in Figure 1, which converts a low voltage to a higher one.  By the way, “step up” transformers have all sorts of applications.  For example, they are used by electric utilities to raise the voltage produced by a power plant to make it more economical to transmit to far away customers.  We’ll get into that in another article.

Figure 1 – A Step-Up Transformer

     In this example the input voltage on the primary coil is stepped up from 120 volts to 480 volts on the secondary coil, and this works according to the formula we learned about in last week’s blog:

NP ÷ NS = VP ÷ VS

where NP and NS are the number of turns of wire in the primary and secondary coils respectively, and VP and VS are the voltages of the primary and secondary coils respectively.  Plugging in the numbers we get:

50 turns ÷ 200 turns = 120 volts ÷ VS

[(200 turns ÷ 50 turns) × 120 volts] = VS = 480 volts

     Okay, for the sake of our example let’s say that an electric motor is connected to the 480 volt secondary coil.  We have an electric meter hooked up to the primary coil and we measure a 2 ampere (a.k.a. “amps”) electrical current flowing through it.  Without having the benefit of another electric meter positioned at the secondary coil, how can we measure how much electrical current is flowing through it?  The current flowing through the secondary coil is found by equalizing the power in the primary and secondary coils:

PowerP = PowerS

     Another way of stating this is to say that electrical power is equal to volts times current, so the equation becomes:

VP × IP = VS × IS

where IP and IS are the primary coil and secondary coil currents, respectively.  Plugging in the numbers and working a little algebra we get the electrical current in the secondary coil:

120 volts × 2 amps = 480 volts × IS

IS = (120 volts × 2 amps) ÷ 480 volts = 0.5 amps

     This shows us that the current flowing in the secondary coil is lower than that of the primary coil.  It is therefore obvious that the voltage increase in the secondary coil comes at the expense of electrical current that can flow through the secondary coil.  Squeeze down on current, voltage goes up.  Squeeze down on voltage, current goes up.  The power flowing through the transformer stays the same.

     Conversely, step-down transformers reduce the voltage coming in, and thereby produce the reverse effect.  There is an actual increase in current that can flow through the secondary coil.  This principle exemplifies the tradeoff process which is often present in science and engineering.

     Next time we’ll explore how both step-up and step-down transformers are used by electric utilities to transmit power from power plants to its customers tied into the utility grid.  As for Rudolph and his power source, that’s still under investigation.



Coal Power Plants, Far From Perfect

Sunday, July 18th, 2010

     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.