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Memorandum

To: Karen Thompson

From: Ryan Chapman

Date: 22 February 2015

Subject: Writing to Define and Describe

This memo describes the cycle which the U of I Steam Plant uses to produce power, otherwise known as

the Rankine Cycle. I will be using a simplified version of this cycle due to space and knowing that trying

to explain the full complexity of the cycle would be far too in depth for my audience. The simplified

version I am using will still succeed in explaining the model of the Rankine Cycle and how it produces

power.

Audience

My intended audience is anybody who has achieved a basic high school education and have basic scientific

knowledge. I expect my audience to understand terms such as temperature, pressure, and efficiency.

The audience will be able to have a better understanding of how one of the biggest power sources on our

campus works.

Purpose

Every single time I used to walk past the Steam Plant I would always wonder how it generated power.

After taking a Thermodynamics class and learning about the Rankine Cycle I was finally able to get a basic

understanding of what happens inside. Many U of I students and faculty walk past the Steam Plant

regularly and have no idea how it produces power or heats the sidewalks. This technical description allows

the audience to understand the basics behind what goes on at the Steam Plant via use of the Rankine

Cycle. This is also aimed at Engineers who may have trouble explaining how certain thermodynamic cycles

to laymen.

Placement

The essay will consist of a technical definition at the beginning to introduce the reader to the purpose and

uses of the Rankine Cycle, followed by a technical description of how each step works. This text can be

used for people interested in the Mechanical Engineering program, an informational sheet that can be

used for Steam Plant tours, as well as part of a booklet describing different power generation cycles.

Choice of Visuals

The visuals chosen are to give the reader a better understanding of how a Rankine Cycle driven power

plant works. The two visuals consist of an overview of an entire power plant with the point of interest

highlighted and a closer view of the components of the cycle. The reader will be able to refer to these

visuals and will be able to follow the steps in the Rankine Cycle while it is being defined and described in

the piece.

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Technical Definition

The Rankine Cycle is one of many vapor power generation cycles. This is used in many large scale power

plants such as the Steam Plant on the University of Idaho campus. The Rankine Cycle can be defined as:

a thermodynamic heat engine that converts heat into mechanical work via a turbine in a closed loop

system. In other words the Rankine Cycle uses a fluid that runs in a continuous loop which is heated and

then transfers the energy from that heat into a turbine.

Rankine Cycle

This cycle is generally used in large scale power plants with access to fuel to provide a heat source and

water to act as the working fluid. As shown in the figure, the Rankine Cycle is only a small part of the

power plant, but everything outside of the box is not critical to the analysis, however, this information is

helpful to understand the inputs and outputs of the cycle. The Ideal Rankine Cycle consists of four

components: the boiler, the turbine, the pump, and the condenser.

Heat Exchangers

A heat exchanger is a device that transfers heat energy between two different mediums. In the Rankine

Cycle there are two heat exchangers, the boiler and the condenser. The boiler exchanges heat between

fuel and water in the closed cycle. The condenser exchanges heat between the water in the closed cycle

and cooling water.

Input Side

The boiler receives all the power input.

Water is heated via a fuel, in the case of

the U of I Steam Plant the boiler is heated

by wood chips. Other boilers can be

heated by natural gas, geothermal, or

coal. This heats the water or working fluid

up and then the exhaust from the fuel

combustion is expelled through a stack

outside of the plant. Often at the U of I

Steam Plant a thick plume of exhaust gas

from the stack.

Output 1

The turbine is the main power output of the Rankine Cycle. Power can be sent to generators to any form

of energy storage. Power plants that operate under the Rankine Cycle can power anything from factories,

to neighborhoods, or even small towns! One benefit of having so much energy come off the turbine is

that the power plant is able to sustain all of its energy needs that are not met by the fuel in the boiler.

Output 2

The condenser is cooled by water that is sent out of the power plant to be cooled. This can be by seawater

or a cooling tower like in the figure. At the U of I Steam Plant the cooling water is dispersed under the

sidewalks and as a result the sidewalks dont need to be shoveled in the winter!

Figure 1. A Simple Vapor Power Plant

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Technical Description

Temperature and Pressure

Temperature and pressure are the two governing principals of the Rankine Cycle. These changes the

water or another working fluid undergo are what allow the system to circulate through the loop and

transfer energy through it. Between each point in the system (or through each device) either temperature

or pressure will change, but never both at the same time.

The Boiler (4 1)

From point 4 to 1 water or another working fluid is pumped into the boiler, a heat exchanger, where it is

heated by a fuel source. The energy from this heat will be turned into mechanical energy to spin the

turbine after vaporization. Between these two points the temperature increases to change the water into

steam.

The Turbine (1 2)

This is where the system creates usable

energy. Steam causes the turbine to spin,

rotating a generator such as depicted in

Figure 1. It is critical that the water has been

heated up to steam, otherwise the water may

condense onto the turbines blades causing

them to corrode. Between points 1 and 2

pressure decreases, sending mechanical

energy through the turbine.

The Condenser (2 3)

The purpose of the condenser is to cool the

steam back down into a liquid so it can later be moved by the pump. The condenser is also a heat

exchanger which does the opposite of the boiler, it removes heat energy from the working fluid. This is

done via another fluid being pumped through the condenser. Generally the amount of fluid that accepts

heat from the working fluid is much greater in volume, imagine pouring a glass of hot water into a

swimming pool, the temperature of the pool wont change much.

The Pump (3 4)

For the Rankine Cycle circulation of the working fluid is required. Between points 3 and 4 the pump

increases the pressure to keep the fluid through the rest of the cycle. It is crucial at this stage that there

is no steam passing through the pump, otherwise it will not be able to move the working fluid as

efficiently. Even though the pump does require power, it doesnt even require 1% of what the turbine

outputs.

Working Fluid Selection

Fluids other than water can be used in the Rankine Cycle. Fluids that vaporize (turn into steam) at different

temperatures can be used to create a more efficient system. If a fluid is better at accepting and rejecting

heat in the heat exchangers then less energy is required to turn it into a vapor. Due to its age and the

location, the U of I Steam Plant uses water as its working fluid.

Figure 2. Rankine Cycle Components

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Visual Source

Both visuals came from Thermodynamics lecture slides created by Dr. Steve Penoncello, a professor at

the University of Idaho.