Posts Tagged ‘mechanical energy’

Superheating, Part 2

Sunday, August 25th, 2013

      Last time we added a piece of equipment called a superheater, positioned between the boiler and steam turbine, to our basic electric utility power plant steam and water cycle.   Its addition enables a greater and more consistent supply of heat energy to the steam which powers the turbine.   How much more?   Let’s look at Figure 1 to get an idea.

Coal Power Plant Engineering Expert Witness

Figure 1

 

      You may have noticed that our illustration lacks numerical representation.   That’s because power plants are designed differently, depending on fuels used and power output required.   So unless we’re talking about a particular power plant, number values would be impractical.   For example, I could specify a boiling point of 596°F at 1,500 pounds per square inch (PSI), and a superheater outlet temperature of 1,050°F at 1,200PSI, and I could make note of esoteric things like enthalpy (British Thermal Units per pound mass) values on the Heat Energy axis.    But to facilitate our discussion we’ll keep things simple and focus on the general process.

      Figure 1 shows in phase D the additional heat energy being added to the steam, thanks to the superheater.   This is significantly more than had been added by the boiler alone, as represented by phase C.   The turbine consumes heat energy added in phases C and D and converts it into mechanical energy to drive the generator, resulting in electrical energy being provided to consumers in the most energy efficient way possible.

      But increasing power output and efficiency isn’t the superheater’s only job.   The heat it adds during phase D ensures the turbine’s safe operation when it’s cranking at full capacity, as represented by the superheated steam zones of phases C and D.

      Next week we’ll discover how the superheater prevents a destructive process known as condensing from occurring inside the turbine.

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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.   

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Coal Power Plant Fundamentals

Sunday, January 23rd, 2011

     Several years ago I was asked by power producers within the electric utility industry to write and then present a training course on the subject of coal power plant fundamentals.  The finished product was a two day introductory course on the energy transformation process within a coal fired plant.

     Since that time my seminar, entitled Coal Power Plant Fundamentals, has been presented to a variety of audiences, including Mirant Corporation, Platte River Power Authority, and Integrys Energy Group, Inc.  Audience makeup has been diverse and has included equipment manufacturers, mining companies, power industry consultants, and regulatory agencies.

     This seminar, which I continue to present today in meeting rooms across the country, covers all major systems in a typical power plant, from coal handling when the coal first enters the plant, to its eventual end destination, the electrical switch yard which facilitates power transmission to customers.  My Power Point presentation is embellished with ample illustrations, including photographs that I have taken during the course of my career and diagrams which I created using CAD, or Computer Aided Drawing software, one of which is featured below.  In addition to the overhead slides, I provide a 150-page bound book which is distributed to seminar attendees.  They use it to both follow along with my lecture and have a source of refresher material to take home with them.  I’ve been told that having my illustrations in front of them makes a world of difference towards their understanding of the subject matter.

     The unique thing about my course is that it focuses on the simplified presentation of complex engineering concepts, much like my blogs do.  Of course it always helps to have an engineering background or scientific background of sorts, but I wrote the course to accommodate understanding of the subject matter by individuals without any technical background.  Accountants, salespersons, administrative staff, plant operating and maintenance workers, and journalists have all found the course to be easy to follow, interesting, and informative.

     So how do you get electricity from coal?  To answer this question and give you a sampling of my seminar material let’s take a look at Figure 1. 

Figure 1 – The Coal Power Plant Energy Transformation Process

     Following along from left to right, the coal is first burned in order to transform the chemical energy which it contains into heat energy.  That heat energy is then absorbed by water inside a nearby boiler, where it is converted into steam.  The heat energy in the steam flows through a pipe into a steam turbine where it is again transformed, this time into mechanical energy that enables the turbine shaft to spin.  The mechanical energy in the turbine is then transmitted by its shaft, enabling it to turn an electrical generator.  And, finally, the mechanical energy is transformed by the generator into electrical energy for our usage.

     Simple process, right?  Well, maybe, maybe not.  My illustration certainly helped to simplify things, but there are a lot of details that were purposely omitted so as not to “muddy the waters.”  It’s those details which have the potential to make things a lot more complicated, and next week we’ll begin to take a closer look at some of them.  

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Diesel Locomotive Brakes

Sunday, June 6th, 2010

     In the past few weeks we’ve taken a look at both mechanical and dynamic brakes.  Now it’s time to bring the two together for unparalleled stopping performance.

     Have you ever wandered along a railroad track, hopping from tie to tie, daring a train to come roaring along and wondering if you could jump to safety in time?  Many have, and many have lost the bet.  That’s because a train, once set into motion, is one of the hardest things on Earth to bring to a stop.  In this discussion, let’s focus on the locomotive.  A large, six-axle variety is shown in Figure 1.

Figure 1 – A Six-Axle Diesel-Electric Locomotive

     These massive iron horses are known in the industry as diesel-electric locomotives, and here’s why.  As Figure 2 shows, diesel-electric locomotives are powered by huge diesel engines.  Their engine spins an electrical generator which effectively converts mechanical energy into electrical energy.  That electrical energy is then sent from the generator through wires to electric traction motors which are in turn connected to the locomotive’s wheels by a series of gears.  In the case of a six-axle locomotive, there are six traction motors all working together to make the locomotive move.  So how do you get this beast to stop?

 

Figure 2 – The Propulsion System In A Six-Axle Diesel-Electric Locomotive

     You probably noticed in Figure 2 that there are resistor grids and cooling fans.  As long as you’re powering a locomotive’s traction motors to move a train, these grids and fans won’t come into play.  It’s when you want to stop the train that they become important.  That’s when the locomotive’s controls will act to disconnect the traction motor wires running from the electrical generator and reconnect them to the resistor grids as shown in Figure 3 below.

Figure 3 – The Dynamic Braking System In A Six-Axle Diesel Electric Locomotive

     The traction motors now become generators in a dynamic braking system.  These motors take on the properties of a generator, converting the moving train’s mechanical, or kinetic, energy into electrical.  The electrical energy is then moved by wires to the resistor grids where it is converted to heat energy.  This heat energy is removed by powerful cooling fans and released into the atmosphere.  In the process the train is robbed of its kinetic energy, causing it to slow down.

     Now you may be thinking that dynamic brakes do all the work, and this is pretty much true, up to a point.  Although dynamic brakes may be extremely effective in slowing a fast-moving train, they become increasingly ineffective as the train’s speed decreases.  That’s because as speed decreases, the traction motors spin more slowly, and they convert less kinetic energy into electrical energy.  In fact, below speeds of about 10 miles per hour dynamic brakes are essentially useless.  It is at this point that the mechanical braking system comes into play to bring the train to a complete stop.

     Let’s see how this switch from dynamic to mechanical dominance takes place.  A basic mechanical braking system for locomotive wheels is shown in Figure 4.  This system, also known as a pneumatic braking system, is powered by compressed air that is produced by the locomotive’s air pump.  A similar system is used in the train’s railcars, employing hoses to move the compressed air from the locomotive to each car. 

Figure 4 – Locomotive Pneumatic Braking System

     In the locomotive pneumatic braking system, pressurized air enters an air cylinder.  Once inside, the air bears against a spring-loaded piston, as shown in Figure 4(a).  The piston moves, causing brake rods to pivot and clamp the brake shoes to the locomotive’s wheel with great force, slowing the locomotive.  When you want to get the locomotive moving again, you vent the air out of the cylinder as shown in Figure 4(b).  This takes the pressure off the piston, releasing the force from the brake shoes.  The spring in the cylinder is now free to move the shoes away from the wheel so they can turn freely.  We have now returned to the situation present in Figure 2, and the locomotive starts moving again.

     Next week we’ll talk about regenerative braking, a variation on the dynamic braking concept used in railway vehicles like electric locomotives and subway trains.

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