CHAPTER-3

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ENGINEERING DESIGN PROCESS
Education Transfer Plan
Prepared by
Seyyed Khandani, Ph.D.
skhandani@dvc.edu
August 2005
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TABLE OF CONTENTS
PREFACE 3
BACKGROUND 4
ENGINEERING DESIGN 4
THE DESIGN PROCESS 5
1. DEFINE THE PROBLEM 6
2. GATHER PERTINENT INFORMATION 9
3. GENERATE MULIPLE SOLUTIONS 11
4. ANALYZE AND SELECT A SOLUTION 12
5. TEST AND IMPLEMENT THE SOLUTION 19
REFERENCE 23
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PREFACE
This document is designed as an introduction to how engineering products are designed.
It is intended for use in an introductory design course in engineering with the objective of
providing some hands-on experience for people interested in exploring engineering
disciplines.
This document is prepared based on the experience of the author* while completing a
summer fellowship at Solectron Corporation in Milpitas, California. This fellowship was
coordinated by the Industry Initiatives for Science and Math Education (IISME) in 2005.
I would like to specially thank Mr. Hoshang Vaid, as my principal mentor, at Solectron
Corporation whose continuous support and guidance has made my fellowship experience
very productive and educational. Furthermore, I would like to extend my appreciation to
the other members of the Design and Engineering department at Solectron Corporation
for making my experience pleasant.
__________________________________________________________________ _____
Seyyed Khandani has a Ph.D. in Mechanical Engineering from MIT and is currently a
professor of engineering at Diablo Valley College in Pleasant Hill, California.
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BACKGROUND
If you take a moment to observe your surroundings, you will see examples of
technological creativity. The physical objects you see, whether they are telephones,
automobiles, bicycles, or electric appliances, all came into being through the creative
application of technology. These everyday inventions did not miraculously appear but
originated in the minds of human beings and took time to develop. Engineering is the
creative process of turning abstract ideas into physical representations (products or
systems). What distinguishes engineers from painters, poets, or sculptors is that engineers
apply their creative energies to producing products or systems that meet human needs.
This creative act is called design.
ENGINEERING DESIGN
Most engineering designs can be classified as inventions-devices or systems that are
created by human effort and did not exist before or are improvements over existing
devices or systems. Inventions, or designs, do not suddenly appear from nowhere. They
are the result of bringing together technologies to meet human needs or to solve problems.
Sometimes a design is the result of someone trying to do a task more quickly or
efficiently. Design activity occurs over a period of time and requires a step-by-step
methodology.
We described engineers primarily as problem solvers. What distinguishes design from
other types of problem solving is the nature of both the problem and the solution. Design
problems are open ended in nature, which means they have more than one correct
solution. The result or solution to a design problem is a system that possesses specified
properties.
Design problems are usually more vaguely defined than analysis problems. Suppose that
you are asked to determine the maximum height of a snowball given an initial velocity
and release height. This is an analysis problem because it has only one answer. If you
change the problem statement to read, "Design a device to launch a 1-pound snowball to
a height of at least 160 feet," this analysis problem becomes a design problem. The
solution to the design problem is a system having specified properties (able to launch a
snowball 160 feet), whereas the solution to the analysis problem consisted of the
properties of a given system (the height of the snowball). The solution to a design
problem is therefore open ended, since there are many possible devices that can launch a
snowball to a given height. The original problem had a single solution: the maximum
height of the snowball, determined from the specified initial conditions.
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Solving design problems is often an iterative process: As the solution to a design problem
evolves, you find yourself continually refining the design. While implementing the
solution to a design problem, you may discover that the solution you've developed is
unsafe, too expensive, or will not work. You then "go back to the drawing board" and
modify the solution until it meets your requirements. For example, the Wright brothers'
airplane did not fly perfectly the first time. They began a program for building an airplane
by first conducting tests with kites and then gliders. Before attempting powered flight,
they solved the essential problems of controlling a plane's motion in rising, descending,
and turning. They didn't construct a powered plane until after making more than 700
successful glider flights. Design activity is therefore cyclic or iterative in nature, whereas
analysis problem solving is primarily sequential.
The solution to a design problem does not suddenly appear in a vacuum. A good solution
requires a methodology or process. There are probably as many processes of design as
there are engineers. Therefore, this lesson does not present a rigid "cookbook" approach
to design but presents a general application of the five-step problem-solving methodology
associated with the design process. The process described here is general, and you can
adapt it to the particular problem you are trying to solve.
THE DESIGN PROCESS
The basic five-step process usually used in a problem-solving works for design problems
as well. Since design problems are usually defined more vaguely and have a multitude of
correct answers, the process may require backtracking and iteration. Solving a design
problem is a contingent process and the solution is subject to unforeseen complications
and changes as it develops. Until the Wright brothers actually built and tested their early
gliders, they did not know the problems and difficulties they would face controlling a
powered plane.
The five steps used for solving design problems are:
1. Define the problem
2. Gather pertinent information
3. Generate multiple solutions
4. Analyze and select a solution
5. Test and implement the solution
The first step in the design process is the problem definition. This definition usually
contains a listing of the product or customer requirements and specially information
about product functions and features among other things. In the next step, relevant
information for the design of the product and its functional specifications is obtained. A
survey regarding the availability of similar products in the market should be performed at
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this stage. Once the details of the design are clearly identified, the design team with
inputs from test, manufacturing, and marketing teams generates multiple alternatives to
achieve the goals and the requirements of the design. Considering cost, safety, and other
criteria for selection, the more promising alternatives are selected for further analysis.
Detail design and analysis step enables a complete study of the solutions and result in
identification of the final design that best fits the product requirements. Following this
step, a prototype of the design is constructed and functional tests are performed to verify
and possibly modify the design.
When solving a design problem, you may find at any point in the process that you need to
go back to a previous step. The solution you chose may prove unworkable for any
number of reasons and may require redefining the problem, collecting more information,
or generating different solutions. This continuous iterative process is represented in the
following Figure.
This document intends to clarify some of the details involved in implementing the design
process. Therefore a description of the details involved in each step of the design process
is listed below. Although the descriptions of the activities within each step may give the
impression that the steps are sequential and independent from each other, the iterative
nature of the application of the process should be kept in mind throughout the document.
1. DEFINE THE PROBLEM
You need to begin the solution to a design problem with a clear, unambiguous definition
of the problem. Unlike an analysis problem, a design problem often begins as a vague,
abstract idea in the mind of the designer. Creating a clear definition of a design problem
is more difficult than, defining an analysis problem. The definition of a design problem
may evolve through a series of steps or processes as you develop a more complete
understanding of the problem.
Generate
multiple
solutions
Test and
Implement
solution
Define the
Problem
Analyze
and select
a solution
Gather
Information
Engineering
Design
Process
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Identify and Establish the Need
Engineering design activity always occurs in response to a human need. Before you can
develop a problem definition statement for a design problem, you need to recognize the
need for a new product, system, or machine. Thomas Newcomen saw the need for a
machine to pump the water from the bottom of coal mines in England. Recognizing this
human need provided him the stimulus for designing the first steam engine in 1712.
Before engineers can clearly define a design problem, they must see and understand this
need.
Although engineers are generally involved in defining the problem, they may not be the
ones who initially recognize the need. In private industry, market forces generally
establish the need for a new design. A company's survival depends on producing a
product that people will buy and can be manufactured and sold at a profit. Ultimately,
consumers establish a need, because they will purchase and use a product that they
perceive as meeting a need for comfort, health, recreation, transportation, shelter, and so
on. Likewise, the citizens of a government decide whether they need safe drinking water,
roads and highways, libraries, schools, fire protection, and so on.
The perceived need, however, may not be the real need. Before you delve into the details
of producing a solution, you need to make sure you have enough information to generate
a clear, unambiguous problem definition that addresses the real need. The following
example illustrates the importance of understanding the need before attempting a solution.
Example: Automobile Airbag Inflation - How Not to Solve a Problem
A company that manufactures automobile airbags has a problem with an unacceptably
high rate of failure in the inflation of the bag. During testing, 10 percent of the bags do
not fully inflate. An engineer is assigned the job of solving the problem. At first the
engineer defines the problem as a failure in the materials and construction of the inflation
device. The engineer begins to solve this problem by producing a more robust inflation
device. After considerable effort, the engineer discovers that improving the inflation
device does not change the failure rate in the bags. Eventually, this engineer re-examines
the initial definition of the problem. The company investigates the airbag inflation
problem further and discovers that a high degree of variability in the tightness of folds is
responsible for the failure of some bags to inflate. At the time the bags were folded and
packed by people on an assembly line. With a more complete understanding of the need,
the engineer redefined the problem as one of increasing the consistency in tightness of the
folds in the bags. The final solution to this problem is a machine that automatically folds
the bags.
Often the apparent need is not the real need. A common tendency is to begin generating a
solution to an apparent problem without understanding the problem. This approach is
exactly the wrong way to begin solving a problem such as this. You would be generating
solutions to a problem that has never been defined.
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People have a natural tendency to attack the current solution to a problem rather than the
problem itself. Attacking a current solution may eliminate inadequacies but will not
produce a creative and innovative solution. For example, the engineer at the airbag
company could have only looked at the current method for folding airbags-using humans
on an assembly line. The engineer might have solved the problem with inconsistent
tightness by modifying the assembly line procedure. However, the final solution to the
problem proved to be more cost effective and reliable, in addition to producing a superior
consistency in the tightness of the folds.
Develop a Problem Statement
The first step in the problem-solving process, therefore, is to formulate the problem in
clear and unambiguous terms. Defining the problem is not the same as recognizing a need.
The problem definition statement results from first identifying a need. The engineer at the
airbag company responded to a need to reduce the number of airbag inflation failures. He
made a mistake, however, in not formulating a clear definition of the problem before
generating a solution. Once a need has been established, engineers define that need in
terms of an engineering design problem statement. To reach a clear definition, they
collect data, run experiments, and perform computations that allow that need to be
expressed as part of an engineering problem-solving process.
Consider for example the statement "Design a better mousetrap." This statement is not an
adequate problem definition for an engineering design problem. It expresses a vague
dissatisfaction with existing mousetraps and therefore establishes a need. An engineer
would take this statement of need and conduct further research to identify what was
lacking in existing mousetrap designs. After further investigation the engineer may
discover that existing mousetraps are inadequate because they don't provide protection
from the deadly Hantavirus carried by mice. Therefore, a better mousetrap may be one
that is sanitary and does not expose human beings to the Hantavirus. From this need, the
problem definition is modified to read, "Design a mousetrap that allows for the sanitary
disposal of the trapped mouse, minimizing human exposure to the Hantavirus."
The problem statement should specifically address the real need yet be broad enough not
to preclude certain solutions. A broad definition of the problem allows you to look at a
wide range of alternative solutions before you focus on a specific solution. The
temptation at this point in the design process is to develop a preconceived mental
"picture" of the problem solution. For example, you could define the better mousetrap
problem as "Design a mousetrap that sprays the trapped mouse with disinfectant." This
statement is clear and specific, but it is also too narrow. It excludes many potentially
innovative solutions. If you focus on a specific picture or idea for solving the problem at
this stage of the design process, you may never discover the truly innovative solutions to
the problem. A problem statement should be concise and flexible enough to allow for
creative solutions.
Here is one possible problem definition statement for our better mousetrap problem:
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A Better Mousetrap: Certain rodents such as the common mouse are carriers and
transmitters of an often fatal virus, the Hantavirus. Conventional mousetraps
expose people to this virus as they handle the trap and dispose of the mouse.
Design a mousetrap that allows a person to trap and dispose of a mouse without
being exposed to any bacterial or viral agents being carried on the mouse.
Establish Criteria for Success
Criteria for success are the specifications a design solution must meet or the attributes it
must possess to be considered successful. You should include criteria in the problem
statement to provide direction toward the solution. At this point in the design process, the
criteria are preliminary. As the design solution develops, you will most likely find that
the initial criteria need to be redefined or modified. Preliminary criteria must not be too
specific so they allow flexibility through the design process.
The criteria that apply to a particular design problem are based on your background
knowledge and the research that you've conducted. Since each problem or project is
unique, the desirable attributes, or criteria, of the solution are also unique. Some criteria
are unimportant to the success of the design. The list of criteria is developed by the
design team. The design team is made up of people from various engineering
backgrounds that have expertise pertinent to the problem. This team may also include
people from backgrounds other than engineering, such as managers, scientists, and
technicians. The design team must evaluate each criterion and decide if it is applicable to
the design effort. Later in the design process, value judgments must be applied to the list
of criteria. Therefore, it makes little sense to include those criteria that will be of
relatively low priority in the evaluation of design solutions. For example, if you were
designing a critical life support system, you would not include the criterion of "must be
minimum cost," because cost is not an important factor in evaluating this design.
The following is a list of preliminary criteria for a better mousetrap design. This list
would be included in the problem definition statement.
· The design must be low cost.
· The design should be safe, particularly with small children.
· The design should not be detrimental to the environment.
· The design should be aesthetically pleasing.
· The design should be simple to operate, with minimum human effort.
· The design must be disposable (you don't reuse the trap).
· The design should not cause undue pain and suffering for the mouse.
2. GATHER PERTINENT INFORMATION
Before you can go further in the design process, you need to collect all the information
available that relates to the problem. Novice designers will quickly skip over this step and
proceed to the generation of alternative solutions. You will find, however, that effort
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spent searching for information about your problem will pay big dividends later in the
design process. Gathering pertinent information can reveal facts about the problem that
result in a redefinition of the problem. You may discover mistakes and false starts made
by other designers. Information gathering for most design problems begins with asking
the following questions. If the problem addresses a need that is new, then there are no
existing solutions to the problems, so obviously some of the questions would not be
asked.
· Is the problem real and its statement accurate?
· Is there really a need for a new solution or has the problem already been solved?
· What are the existing solutions to the problem?
· What is wrong with the way the problem is currently being solved?
· What is right about the way the problem is currently being solved?
· What companies manufacture the existing solution to the problem?
· What are the economic factors governing the solution?
· How much will people pay for a solution to the problem?
· What other factors are important to the problem solution (such as safety,
aesthetics and environmental issues)?
Search for Information Resources
As an engineering student in the 2000s you have many more sources of information
available to you than engineers did only 20 years ago. This section discusses some of the
most current resources available, but because our world is witnessing an information
explosion, by the time you read this many more resources will be available that are not
mentioned here.
Traditional publications are still an essential source of information to engineers and
scientists. However, electronic information transfer and retrieval are quickly becoming a
standard source for engineers and scientists. When you begin a search for information
relating to a design problem, you must be prepared to go to many different sources. The
library is still the primary source of information for an engineering student. Your success
as an engineer and student will be enhanced if you are able to use the library effectively.
For specific help on using our library, you should consult the library staff at the college;
they probably offer courses or seminars on library usage.
Some of the common resources available at a university library are discussed below:
Scientific encyclopedias and technical handbooks. These sources are a good place
to start when you are investigating an area or problem that is new to you. An
encyclopedia or handbook provides a brief general overview by an authority in a
particular field and includes references for more detailed information. The
McGraw-Hill Encyclopedia of Science and Technology covers all scientific fields.
Technical handbooks, such as the Electrical Engineers Handbook or Mark's
Handbook of Mechanical Engineering, cover various fields such as chemical, civil,
electrical, or mechanical engineering. The information contained in these
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handbooks is presented in a very concise form and can be good starting points for
an in depth search.
Electronic catalog. Electronic catalogs give a listing of all the sources available at
your library. The catalogs allow search by subject matter, author, or title. They
give a brief summary of the book's content, including the title, author, publisher,
copyright date, and total number of pages. The call number tells you where to
locate the book in your library.
Indexes. Indexes categorize current works in various disciplines. They list the
subject, title, and author of recent articles in technical and trade journals under
various subject headings. Some indexes include brief abstracts of articles. Most
indexes are updated monthly, so a complete search through an index may be
tedious. A familiar index to scientists and engineers is the Index of Applied
Science and Technology. It lists articles from 335 journals and is updated monthly.
The Engineering Index is another popular index for engineers. It selects articles
from approximately 2700 journals and periodic publications and includes an
abstract of each article.
The Internet. There is a wealth of information on the Internet from a variety of
sources. Manufacturers, professional and trade organizations, suppliers of
products, and many government agencies have valuable resources on their
websites. Search engines such as Google(www.google.com) and
Teoma(www.teoma.com) offer tools to locate relevant information quickly and
efficiently.
3. GENERATE MULTIPLE SOLUTIONS
The next step in the design process begins with creativity in generating new ideas that
may solve the problem. Creativity is much more than just a systematic application of
rules and theory to solve a technical problem.
You start with existing solutions to the problem and then tear them apart-find out what's
wrong with those solutions and focus on how to improve their weaknesses. Consciously
combine new ideas, tools, and methods to produce a totally unique solution to the
problem. This process is called synthesis. Casey Golden, age 13, did this when he
invented the BIOtee. Casey noticed that discarded and broken wooden golf tees littered
golf courses, damaging the blades and tires of lawn mowers. He decided to design a new
biodegradable tee. After experimenting with different mixtures, he devised a recipe made
of recycled paper fiber and food byproducts coated with a water-soluble film. When the
film is broken, moisture in the ground breaks down the tee within 24 hours. As a result of
his creative efforts, Casey's family started a company to manufacture BIOtees producing
several million tees per year.
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Psychological research has found no correlation between intelligence and creativity.
People are creative because they make a conscious effort to think and act creatively.
Everybody has the potential to be creative. Creativity begins with a decision to take risks.
Listed below are a few characteristics of creative people. These are not rigid rules to be
followed to experience creativity. You can improve your creative ability by choosing to
develop these characteristics in yourself.
· Curiosity and tolerance of the unknown. Creative people have a positive curiosity
of the unknown. They are not afraid of what they don't understand.
· Openness to new experiences. Creative people have a healthy and positive attitude
toward new experiences.
· Willingness to take risks. Creative people are not afraid to take risks and try new
experiences or ideas, knowing that they may be misunderstood and criticized by
others. They are self-confident and not afraid to fail.
· Ability to observe details and see the "whole picture." Creative people notice and
observe details relating to the problem, but they also can step back and see the
bigger picture.
· No fear of problems. Creative people are not afraid to tackle complex problems,
and they even search for problems to solve. They seek solutions to problems with
their own abilities and experience if possible. They have the attitude of "if you
want something done, you'd better do it yourself."
· Ability to concentrate and focus on the problem until it's solved. Creative people
can set goals and stick to them until they're reached. They focus on a problem and
do not give up until the problem is solved. They have persistence and tenacity.
Solutions to engineering design problems do not magically appear. Ideas are generated
when people are free to take risks and make mistakes. Brainstorming at this stage is often
a team effort in which people from different disciplines are involved in generating
multiple solutions to the problem.
4. ANALYZE AND SELECT A SOLUTION
Once you've conceived alternative solutions to your design problem, you need to analyze
those solutions and then decide which solution is best suited for implementation.
Analysis is the evaluation of the proposed designs. You apply your technical knowledge
to the proposed solutions and use the results to decide which solution to carry out. You
will cover design analysis in more depth when you get into upper-level engineering
courses.
At this step in the design process, you must consider the results of your design analysis.
This is a highly subjective step and should be made by a group of experienced people.
This section introduces a systematic methodology you can use to evaluate alternative
designs and assist in making a decision.
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Analysis of Design Solutions
Before deciding which design solution to implement, you need to analyze each
alternative solution against the selection criteria defined in step l. You should perform
several types of analysis on each design. Every design problem is unique and requires
different types of analysis. The following is a list of analysis that may need to be
considered; bear in mind that the importance of each varies depending on the nature of
the problem and the solution.
· Functional analysis
· Industrial design/Ergonomics
· Mechanical/Strength analysis
· Electrical/Electromagnetic
· Manufacturability/Testability
· Product safety and liability
· Economic and market analysis
· Regulatory and Compliance
The following paragraphs provide details of some of these analysis types.
Functional analysis. This part determines whether the given design solution will
function the way it should. Functional analysis is fundamental to the evaluation and
success of all designs. A design solution that does not function properly is a failure even
if it meets all other criteria. Consider for example the invention of the ballpoint pen. This
common instrument was first invented and manufactured during World War II. The
ballpoint pen was supposed to solve the problems of refilling and messiness inherent to
the fountain pen. Unfortunately, this new design had never been evaluated for
functionality. The early pens depended on gravity for the ink to flow to the roller ball.
This meant that the pens only worked in a vertical upright position, and the ink flow was
inconsistent: Sometimes it flowed too heavily, leaving smudgy blotches on the paper;
other times the flow was too light and the markings were unreadable. The first ballpoint
pens tended to leak around the ball, ruining people's clothes. An elastic ink developed in
1949, allowed the ink to flow over the ball through smooth capillary action. Not until the
1950s did the ballpoint pen finally become a practical writing instrument, thanks to
proper ink and engineering. Economy, appearance, durability, and marketability of a
design are unimportant if the product does not function properly.
Ergonomics. Ergonomics is the human factor in engineering. It is the study of how
people interact with machines. Most products have to work with people in some manner.
People occupy a space in or around the design, and they may provide a source of power
or control or act as a sensor for the design. For example, people sense if an automobile
air-conditioning system is maintaining a comfortable temperature inside the car. These
factors form the basis for human factors, or ergonomics, of a design.
A design solution can be considered successful if the design fits the people using it. The
handle of a power tool must fit the hand of everybody using it. The tool must not be too
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heavy or cumbersome to be manipulated by all sizes of people using the tool. The
geometric properties of people-their weight, height, reach, circumference, and so on-are
called anthropometric data. The difficulty in designing for ergonomics is the abundance
of anthropometric data. The military has collected and evaluated the distribution of
human beings and published this information in military standard tables. A successful
design needs to be evaluated and analyzed against the distribution of geometry of the
people using it. The following Figure shows the geometry of typical adult males and
females for the general population in millimeters. Since people come in different sizes
and shapes, such data are used by design engineers to assure that their design fits the user.
A good design will be adjustable enough to fit 95 percent of the people who will use it.
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Product Safety and Liability. The primary consideration for safety in product design is
to assure that the use of the design does not cause injury to humans. Safety and product
liability issues, however, can also extend beyond human injury to include property
damage and environmental damage from the use of your design. Engineers must also
consider the issues of safety in design because of liability arising from the use of an
unsafe product. Liability refers to the manufacturer of a machine or product being liable,
or financially responsible, for any injury or damage resulting from the use of an unsafe
product.
The only way to assure that your design will not cause injury or loss is to design safety
into the product. You can design a safe product in three ways. The first method is to
design safety directly into the product. Ask yourself, "Is there any probability of injury
during the normal use and during failure of your design?" For example, modern downhill
ski bindings use a spring-loaded brake that brakes the ski automatically when the ski
disengages from the skier's boot. Older ski bindings used an elastic cable attached to the
skier's ankle, but this had a tendency to disconnect during a severe fall.
Inherent safety is impossible to design into some products, such as rotating machinery
and vehicles. In such cases you use the second method of designing for safety: You
include adequate protection for users of the product. Protection devices include safety
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shields placed around moving and rotating parts, crash protective structures used in
vehicles, and "kill" switches that automatically turn a machine off (or on) if there is
potential for human injury. For example, new lawnmowers generally include a protective
shield covering the grass outlet and include a kill switch that turns the motor off when the
operator releases the handle.
The third method used in considering safety is the use of warning labels describing
inherent dangers in the product. Although this method does not implement safety in
design, it is primarily used as a way to shift the responsibility to the consumer for having
ignored the safety guidelines in using the product. In most cases, however, a warning
label will not protect you from liability. Protective shields or other devices must be
included in the design.
A product liability suit may be the result of a personal injury due to the operation of a
particular product. The manufacturer and designer of a device can be found liable to
compensate a worker for losses incurred during the operation or use of their product.
During a product liability trial, the plaintiff attempts to show that the designer and
manufacturer of a product are negligent in allowing the product to be put on the
market. The plaintiff's attorney may bring charges of negligence against the designer.
To protect themselves in a product liability trial, engineers must use state-of-the-art
design procedures during the design process. They must keep records of all calculations
and methods used during the design process. Safety considerations must be included in
the criteria for all design solutions. The designer must also foresee other ways people
could use the product. If a person uses a shop vacuum to remove a gasoline spill, is the
designer responsible when the vacuum catches fire? The courts can decide that a design is
poor if the engineer did not foresee improper use of the product. It is imperative that you
evaluate all of your alternative solutions against safety considerations. Reject or modify
any unsafe elements of your design at this stage in the design process.
Economic and Market Analysis. The net result or purpose of most engineering designs
is to produce a product that generates a profit for the company. Obviously, each
alternative design has to be evaluated against criteria such as sales features, potential
market, cost of manufacturing, advertising, and so on. Large companies often conduct
marketing surveys to obtain a measure of what the public will buy. These surveys may be
conducted by telephone interviews with randomly selected people, or they may be
personal interviews conducted with potential users of a product. Our society is based on
economics and competition. Many good ideas never get into production because the
manufacturing costs exceed what people will pay for the product. Market analysis
involves applying principles of probability and statistics to determine if the response of a
selected group of people represents the opinion of society as a whole. Even with a good
marketing survey, manufacturers never know for certain if a new product will sell.
Mechanical/Strength Analysis. Engineering analysis of a preliminary design often
include the analysis of its mechanical features. The engineer conducts mechanical
analysis to answer questions such as, "Will the device or structure support the maximum
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loads that it will be subjected to?" You must also determine the effect of shocks and
repetitive or dynamic loading over the life of the product. Many systems generate heat, so
you need to determine if the design can dissipate all of the heat being generated during
normal operation. Thermal analysis is an area important to the design of electronic
equipment. Many pieces of electronic equipment fail prematurely due to inadequate heat
transfer. For example, the early releases of Intel's Pentium microprocessor could not
operate at their rated speed due to overheating. The production of this microcircuit was
delayed while engineers figured out ways to dissipate the excess heat.
You need to perform strength calculations to determine whether the design alternative
will be able to support the specified mechanical loads. As a mechanical system is
subjected to applied loads, it will deform or deflect.
Many products contain several subsystems and, quite often, the evaluation is done on
each of the subsystems rather than the complete product itself.
The Decision Process
After analyzing your alternative solutions, you need to decide and document which
design solution is the best. You will refine and develop the best solution in more detail
during the later stages of the design process. At this stage, to evaluate each solution
objectively against the stated design criteria or requirements, you need a quantitative
basis for judging and evaluating each design alternative. One widely used method to
formalize the decision-making process is the decision matrix. The decision matrix is a
mathematical tool you can use to derive a number that specifies and justifies the best
decision.
The first step in creating a decision matrix is for the design team to rank, in order of
importance, the desirable attributes or criteria for the design solution. These attributes can
include factors such as safety, manufacturing considerations, the ease of fabrication and
assembly, cost, portability, compliance with government regulations, etc. You then assign
to each attribute or criteria a value factor related to the relative importance of that
attribute. For example, suppose you decide that safety is twice as important to the success
of your design as cost. You would assign a value factor of 20 for safety and a value factor
of 10 for cost. You assign value factors on a basis of 0 to 100, representing relative
importance of each criterion to the decision.
Next you evaluate each design alternative against the stated criteria. A rating factor is
assigned to each solution, based on how well that solution satisfies the given criterion.
The rating factor is on a scale of 0 to 10, with 10 representing a solution that satisfies the
given criterion the best. To make an accurate evaluation, you need as much information
as possible. Unfortunately, engineers seldom have enough information to make a
"perfect" evaluation. If you have done the analysis phase of the design process properly,
those results can provide a basis for evaluation. Computer models and prototypes can also
yield valuable information to assist in the decision phase. In most cases you must use
IISME/Solectron/2005 Seyyed 18 Khandani
engineering judgment, and the decision is subjective. The following example illustrates
the use of a decision matrix in deciding the best alternative design for a can crusher.
Example: Aluminum Can Crusher
Students are being asked to design a simple device to crush aluminum cans. A student
design team proposes four solutions to the problem. They develop six criteria that are
important to a successful design. The student team agrees that the most important criteria
(or desirable attributes) of the design and assigned weights are
· Safety: 30 percent (30 points)
· Ease of use: 20 percent (20 points)
· Portability: 20 percent (20 points)
· Durability and strength:10 percent (10 points)
· Use of standard parts:10 percent (10 points)
· Cost:10 percent (10 points)
This team also proposes four alternative solutions to this problem, which are illustrated in
the following Figure. They are
1. A spring-loaded crusher
2. A foot-operated device
3. A gravity-powered dead weight crusher
4. An arm-powered lever arm crusher
IISME/Solectron/2005 Seyyed 19 Khandani
After analyzing each solution against the six criteria, the team evaluates each design
alternative. After assigning a rating factor to each design alternative for each of the
specified criteria, the team multiplies the rating factor by the value factor. The product of
the value and rating factors is then summed down the column for each design alternative.
The total sum at the bottom of each column determines the best design alternative. The
results of this decision matrix are illustrated in the following Table.
IISME/Solectron/2005 Seyyed 20 Khandani
Although rating each design against the six stated criteria is subjective, the rating factor
for each design alternative is assigned according to the consensus of the design team. The
results of an analysis are used to evaluate and rate each design. The rating factor R is
assigned according to the following scale:
· Excellent 9-10
· Good 7-8
· Fair 5-6
· Poor 3-4
· Unsatisfactory 0-2
Design 4 was chosen the best design largely due to the rating assigned for safety,
criterion l. The team felt that the chances of human injury were negligible for this design.
Since safety is the most important factor (30% of the total weight), the high safety rating
for design 4 gives it the highest overall score (9 x 30, or 270).
5. TEST AND IMPLEMENT THE SOLUTION
The final phase of the design process is implementation, which refers to the testing,
construction, and manufacturing of the solution to the design problem. You must consider
several methods of implementation, such as prototyping and concurrent engineering, as
well as distinct activities that occur during implementation, such as documenting the
design solution and applying for patents.
Prototyping. The first stage of testing and implementation of a new product, called
prototyping, consists of building a prototype of the product-the first fully operational
production of the complete design solution. A prototype is not fully tested and may not
work or operate as intended. The purpose of the prototype is to test the design solution
under real conditions. For example, a new aircraft design would first be tested as a scale
model in a wind tunnel. Wind tunnel tests would generate information to be used in
constructing a full-size prototype of the aircraft. Test pilots then fly the prototype
extensively under real conditions. Only after testing under all expected and unusual
operating conditions are the prototypes brought into full production.
Concurrent Engineering. Traditional design practices are primarily serial or sequential:
Each step in the process is completed in order or sequence only after the previous steps
have been completed. The implementation of the design occurs after a prototype or model
is created from engineering drawings. A machinist working from the engineering
drawings generated by a drafter, or an engineer, makes the prototype. Only after creating
a prototype of the design would the engineer discover that a hole was too small, parts
didn't mate properly, or a handgrip was misplaced. The part would have to be redesigned
and the process completed until a satisfactory solution was reached.
IISME/Solectron/2005 Seyyed 21 Khandani
In the competitive manufacturing climate of the 1990s, the serial practice of design
proved inadequate. In a matter of months, a manufacturer may find that factors such as
markets, material prices and technology, and government regulations and tax laws may
have changed. This competitive environment required a company to design high-quality
products faster, better, and less expensively than their competitors. One solution to the
traditional design paradigm was concurrent engineering.
Concurrent engineering is the ability to implement parallel design and analysis in which
safety, manufacturability, serviceability, marketability, and compliance issues are
considered early on and during the process. Concurrent engineering is however possible
through the application of modern computer-aided design (CAD), analysis, and
manufacturing software. A designer starts with an idea of a new product in which the
above factors are considered and uses the CAD software to create a preliminary design.
With the appropriate software, the preliminary design can also be analyzed for
functionality as the design is being created. Using the results of this analysis, the designer
then makes any necessary modifications and reanalyzes the computer model. An engineer
designing a bicycle frame, for example, would use concurrent engineering to minimize
the weight and maximize the supported loads in a new frame design. The engineer would
first create a design and model the physical behavior of the frame on the computer before
actually manufacturing the frame.
The next stage in concurrent engineering is called rapid prototyping or sometimes called
"art to part." Here the three-dimensional computer model of the finished design is used
with computer-aided manufacturing (CAM) software to drive appropriate machinery to
physically create the part. The entire design cycle therefore becomes nearly paperless.
Engineers can go from design to prototype in a matter of days, instead of weeks or
months as with earlier serial design practices. Since design is an iterative process,
concurrent engineering significantly shortens the time between iterations. A product can
therefore get to market much quicker, at a lower cost, and with a higher quality.
Documentation. One of the most important activities in design is documenting your work,
clearly communicating the solution to your design problem so someone else can
understand what you have created. Usually this consists of a design or technical report.
Communicating the solution to a design problem through language, both written and oral,
is a vital part of the implementation phase. Many people you will be communicating with
do not have technical training and competence. They may be the general public,
government officials, or business leaders. Successful engineers must possess more than
just technical skills. The ability to communicate and sell a design solution to others is
also a critical skill.
You can use graphs, charts, and other visual materials to summarize the solution process
and present your work to others. Multimedia techniques, including Power Point
presentations, slides, sounds, videos, and computer-generated animations, are often used
to clearly communicate the solution to a design problem.
IISME/Solectron/2005 Seyyed 22 Khandani
Applying for Patents. If you develop an original and novel solution to a design problem,
part of the implementation phase may include applying for a patent on your solution. A
patent will not protect you from someone else copying your solution, but it does give you
specific rights to make and sell your design for a specified period. A patent is an
agreement made between you-the designer or inventor-and the U.S. government.
Through a patent document you agree to make public all the details and technology of
your invention. You agree to provide an invention disclosure, which provides enough
details to allow anyone to build a working model of your invention. Most large libraries
now have files of issued patents, which are available for anyone to see. These can be a
good source of ideas for engineering design solutions. In return for making your
invention or design solution public, the U.S. Patent Office grants you the exclusive right
to your invention for a specified period of time.
Pursuing a patent is not a trivial process and may take a long time, costing hundreds or
even thousands of dollars. Before considering a patent you should have a general
understanding of patent requirements and what can be patented. Ideas by themselves
cannot be patented. To obtain a patent, you must prove that your idea can be applied to
produce a "new and useful process, machine, manufacture, or composition of matter, or
any new and useful improvement thereof." These categories include just about everything
made by people and the processes for making them.
Most engineering design problems fall into the patentable categories of utility patents or
design patents. All mechanical and electrical devices fall into the category of utility
patent, which is granted for 20 years. At the end of the patent period, your protection
expires, and anyone can copy, manufacture, and sell your invention without giving you
credit or payment. A design patent is granted to protect the styling or ornamental features
of a design. A design patent is only granted for the appearance of an item, not for how it
works or is made. For example, if you invent a telephone that looks like a shoe, you
might apply for a design patent. The design patent would be granted on the appearance of
the phone, not on the electronic and mechanical workings of the phone. Design patents
are granted for 3-1/2, 7, or 14 years, depending on the patent fee paid. The fees range
from $200 to $600.
Patents are only granted to the inventor of a device. However, the inventor can assign the
rights to the patent to another party. If you develop an invention while working as an
engineer for a company, you will probably be required to assign the patent rights to that
invention to your employer.
Once a patent is granted, there is no guarantee that someone else will not try to copy the
invention. The U.S. Patent Office does not enforce patent rights. It is the responsibility of
the patent holder or a patent attorney to police the patent and make sure no one else
copies it while it is in effect. Since a patent makes all information about your design
public, some people choose not to pursue a patent, but rather keep the details of the
invention secret. If no one else learns how the invention works, you will have protection
until another inventor figures it out. For example, the formulas for Coca-Cola and Silly
IISME/Solectron/2005 Seyyed 23 Khandani
Putty have never been patented, and the secrets are only known to selected company
officials.
To apply for a patent, you need to prepare and include the following items:
A written document clearly describing your invention and stating that you are the
original inventor. Enough information must be provided so that someone else can
make your invention from the information you provide. You must also make
claims about your invention which describe the features which distinguish it from
already patentable material.
Engineering drawings that follow the format documented in Guide for Patent
Draftsmen, which is available from the U.S. Patent Office.
The filing fee. This is a basic fee of at least $ 150 that must accompany the patent
application. If the patent is granted, you will be charged an additional patent issue
fee. The total charges for obtaining a patent can be hundreds of dollars.
A patent is granted only after an extensive review process of the U.S. Patent Office. The
office will first search the nearly 5 million existing patents to determine whether your
design has been previously patented or infringes on an existing patent. This process can
take several years and be very expensive. Many inventors employ patent attorneys or
agents to conduct a preliminary patent search. Most large libraries have records of all the
patents filed with the U.S. Patent Office. This information is also available on a CDROM
database at many libraries. You can look at this database and read the patent
applications filed under the same product category as yours. This will give you a good
idea on how an application is written and might help you improve your own design.
Before spending more time and money pursuing a patent, it is a good idea to find out if
someone else has already patented your invention.
Testing and Verification. Testing and verification are important parts of the design
process. At all steps in the process, you may find that your potential solution is flawed
and have to back up to a previous step to get a workable solution. Without proper testing
at all stages in the process, you may find yourself making costly mistakes later.
IISME/Solectron/2005 Seyyed 24 Khandani
REFERENCE
1. Ertas, A., Jones, J. C., The Engineering Design Process, John Wiley and Sons, New
York, 1996.
2. Lumsdaine, E., Lumsdaine, M., Shelnutt, J. W., Creative Problem Solving and
Engineering Design, McGraw-Hill, Inc., New York, 1999.
3. Sanders, M. S., McCormick, E. J., Human Factors in Engineering and Design,
McGraw-Hill, Inc., New York, 1993.
4. Dym, C. L., Little, P., Engineering Design: A Project-Based Introduction, John
Wiley, New York, 1999.
5. Hyman, B., Fundamental of Engineering Design, Prentice Hall, New Jersey, 1998.

CHAPTER-2

Chapter-2

Energy Sources

Introduction:

Man has needed and used energy at an increasing rate for his sustenance and well-being since he came on the earth a few million years ago. Primitive man required energy primarily in the form of food. He derived this by eating plants and animals he hunted. Subsequently he discovered fire and his energy needs increased as he started to use wood and other biomass to supply the energy needs for cooking as well as for keeping himself warm.  With the advent of science and technology demand for energy increased at a very fast rate and with the availability of cheap coal, petroleum oil and natural gas, the dependence on these energy sources also increased at an alarming rate. Due to ever increasing rate of consumption of energy day by day, the rate of depletion of these sources has been very rapid resulting in their reserves reaching very low levels. This irreversible situation of today in the energy front has directed us to search for the alternate sources of energy like solar, wind, biomass, tidal, ocean, geothermal, etc.,.

Energy and its forms:

Energy is the capacity to do work or it can also be defined as the ability to cause change.

Energy exists in various forms like kinetic energy (energy possessed by bodies in motion), potential energy (energy possessed by bodies at an elevation), chemical energy (internal energy i.e., the energy possessed by virtue of motion and forces of individual atoms and molecules of the system).  The various other forms of energy are mechanical, thermal, electrical, chemical (storage batteries), radiant and atomic. All forms of energy are inter-convertible by appropriate process.

Energy can be classified into several types based on the following criteria as:

• Primary and Secondary energy

• Commercial and Non commercial energy

• Renewable and Non-Renewable energy

Primary and Secondary Energy sources:

Primary energy sources are those that are either found or stored in nature. Common primary energy sources are coal, oil, natural gas, and biomass (such as wood). Other primary energy sources found on earth include nuclear energy from radioactive substances, thermal energy stored in earth’s interior, and potential energy due to earth’s gravity. The major primary and secondary energy sources are shown in Figure 1.1

Primary energy sources are mostly converted in industrial utilities into secondary energy sources; for example coal, oil or gas converted into steam and electricity. Primary energy sources can also be used directly as well.

Commercial Energy

The energy sources that are available in the market for a definite price are known as commercial energy. By far the most important forms of commercial energy are electricity, coal and refined petroleum products. Commercial energy forms the basis of industrial, agricultural, transport and commercial development in the modern world. In the industrialized countries, commercialized fuels are predominant source not only for economic production, but also for many household tasks of general population.

Examples: Electricity, lignite, coal, oil, natural gas etc.

Non-Commercial Energy

The energy sources that are not available in the commercial market for a price are classified as non-commercial energy. Non-commercial energy sources include fuels such as firewood, cattle dung and agricultural wastes, which are traditionally gathered, and not bought at a price, used especially in rural households. These are also called traditional fuels. Non-commercial energy is often ignored in energy accounting.

Example: (i)Firewood, agro waste in rural areas; (ii)solar energy for water heating, electricity generation, for drying grain, fish and fruits;(iii) animal power for transport, threshing, lifting water for irrigation, crushing sugarcane; (v) wind energy for lifting water and electricity generation.

Renewable and Non-Renewable Energy

Renewable energy is energy obtained from sources that are essentially inexhaustible. Examples of renewable resources include wind energy, solar energy, geothermal energy, tidal energy and hydroelectric energy.  The most important feature of renewable energy is that it can be harnessed without the release of harmful pollutants.

Non-renewable energy is the conventional fossil fuels such as coal, oil and gas, which are likely to deplete with time.

Advantages

1.     Available in plenty in nature and inexhaustible.

2.     These can be built as close as possible to the point of consumption and transmission losses can be minimised.

3.     The available technologies are more flexible and highly diverse.

4.     A good amount of variation is possible in the energy quality.

5.     Locally available renewable energy can be fully utilised.

Disadvantages

1.     Concentration of these resources is limited to certain regions.

2.     Supply is intermittent and also varies seasonally.

3.     Cost of the equipment to harness these resources is comparatively higher than those of the conventional energy sources.

4.     Some of the components like solar cells, auto tracking systems, concentrators etc., all require very high technology.

5.     Need for storage system to store energy when the supply is available in plenty and to use when the supply is inadequate or absent.

Another way of classifying the energy sources is conventional and non-conventional energy sources. Fossil fuels, hydro and nuclear energies are conventional whereas direct solar energy, tidal, geothermal, wind energy and ocean thermal energy are non-conventional.

Fossil Fuels:

Fossil fuels are energy rich substances that have been formed from long-buried plants and micro-organisms. Fossil fuels include petroleum, coal and natural gas. Chemically fossil fuel consists largely of hydrocarbons, which are compounds of hydrogen and carbon. Hydrocarbons are formed from ancient living organisms that were buried under layers of sediment millions of years ago. As accumulating sediment layers exerted increasing heat and pressure, the remains of the organisms gradually transformed into hydrocarbons.

Coal Formation : Coal is a solid fuel formed from ancient plants- including trees, ferns and mosses that grew in swamp and bogs or along the coastal shore lines. Generations of these plants died and were gradually buried under layers of sediment. As the sedimentary overburden increased, the organic material was subjected to increasing heat and pressure that cause the organic material to undergo a number of transitional states to form coal. The mounting pressure and temperature caused the original organic material, which was rich in carbon, hydrogen and oxygen, to become increasingly carbon-rich and hydrogen and oxygen-poor. The successive stages of coal formation are peat, lignite, bituminous and anthracite.

Coal Reserves:

The proven global coal reserve was estimated to be 9,84,453 million tonnes by end of 2003. The USA had the largest share of the global reserve (25.4%) followed by Russia (15.9%), China (11.6%). India was 4th in the list with 8.6%.

Petroleum and Natural Gas formation:

Petroleum is formed chiefly from ancient microscopic plants and bacteria that existed in the ocean and seas. When these micro-organisms died and settled to the seafloor, they mixed with sand the silt to form organic –rich mud. As layers of sediment accumulated over this organic ooze, the mud was gradually heated and slowly compressed into shale, chemically transforming into petroleum. The petroleum fills the tiny holes within nearby porous rocks. The liquid petroleum and gases which are less dense than water and lighter move upwards through the earth’s crust. A portion of these petroleum eventually encounter an impermeable layer of rock which traps the petroleum, creating a reservoir of petroleum and natural gas. Because of its low density relative to petroleum, natural gas forms a layer over the petroleum.

Oil Reserves:

The global proven oil reserve was estimated to be 1147 billion barrels by the end of 2003. Saudi Arabia had the largest share of the reserve with almost 23%.  (One barrel of oil is approximately 160 litres)

Gas Reserves:

The global proven gas reserve was estimated to be 176 trillion cubic metres by the end of 2003. The Russian Federation had the largest share of the reserve with almost 27%.

Reserves/Production (R/P) ratio- If the reserves remaining at the end of the year are divided by the production in that year, the result is the length of time that the remaining reserves would last if production were to continue at that level.

The Reserves-to-production ratio (or R/P) is the remaining amount of a non-renewable resource, expressed in years. While applicable to all natural resources, the R/P is most commonly applied to fossil fuels, particularly petroleum and natural gas. The reserve portion (numerator) of the ratio is the amount of a resource known to exist in an area and to be economically recoverable (proved reserves). The production portion (denominator) of the ratio is the amount of resource used in one year at the current rate.

R/P = (amount of known reserves) / (amount used per year)

World oil and gas reserves are estimated at just 45 years and 65 years respectively. Coal is likely to last a little over 200 years

This ratio is used by companies and government agencies in forecasting the future availability of a resource to determine project life, future income, employment, etc, and to determine whether more exploration must be undertaken to ensure continued supply of the resource.

Advantages and disadvantages of conventional energy sources

Advantages:

1.     Fully developed technology is available to harness this energy.

2.     Cost of generation has been brought down to affordable levels.

3.     They can be easily transported to any place.

4.     Ideal for small applications.

Disadvantages

1.   They are polluting because of their emissions.

2.  Their availability is reducing as they are in limited quantity in nature and these sources are depleting at a fast pace.

3.   They are leading to lot of ecological imbalances.

4.   They are usually far off from the point of consumption.

Solar Energy:

Solar energy is a very large, inexhaustible source of energy. The power from the sun intercepted by the earth is approximately 1.8 x 10¹¹MW, which is many thousands of times larger than the present consumption rate on the earth of all commercial energy sources. Hence solar energy can supply all the present and future energy needs of the world on a continuing basis. This makes it one of the most promising renewable source of energy.

The sun provides earth with the radiant energy which has two distinctive properties viz, lighting and heating resulting from nuclear fusion reactions at its core. Some solar equipment is designed to use the light property of solar radiation while few others are designed to use the heating property. 

The sun constantly delivers 1343 W/m² of power on an average to the earth out of which a maximum of 1000 W/m² of power is received on the earth’s surface after passing through the earth’s atmosphere. It is environmentally clean source of energy and is freely available. However the solar energy is very diffuse, cyclic and often undependable. Therefore it needs systems and components that can collect and concentrate it efficiently.

Solar energy conversion:

The solar energy can be utilised in direct form as well as indirect form.

The direct form of utilisation of solar energy are (i) Helio–electrical and (ii)Helio- thermal  processes.

The indirect forms of solar energy are (i) Biomass energy (Helio-chemical) (ii) Wind energy (iii) Tidal energy (iv) ocean thermal energy (v) Hydel energy etc.,

Helio-electrical  Process :

Solar energy can be directly converted into direct current by photovoltaic cells.

The devices used in photovoltaic conversion are called solar cells. When solar radiation falls on these devices it is converted directly into DC electricity. The principal advantages associated with solar cells are that, they have no moving parts, require little maintenance and work quite satisfactorily with beam and diffuse radiation.

Description and Principle of working of solar cell

Single Crystal silicon cell:

 

Figure 2(a):  Structure and principle function of a solar cell                        Figure 3: Photovoltaic cell.

Figure 2(b)

Single crystal silicon cells are thin wafers about 300µm in thickness, sliced from a single crystal of p-type doped silicon. The silicon with added impurity such as boron or gallium is called p-type Silicon. A shallow junction is formed at one end by diffusion of the n-type impurity. The silicon with added materials such as arsenic or Phosphorus is called n-type silicon. Metal electrodes made from- Ti-Ag solder are attached to the front and back side of the cell. On the front side, the electrode is in the form of a metal grid which permit the sunlight to pass through, while on the back side, the electrode completely covers the surface.  A typical cell develops a voltage of 0.5 to 1 V and a current density of 20 to 40 mA/cm². In order to obtain higher voltages and currents, individual cells are connected in series and parallel to form a module. In turn, a number of modules are interconnected to form an array.

Principle of working of a solar cell.

When a p-n junction of a semiconductor is exposed to sunlight some of the photons are absorbed in the vicinity of p-n junction. The photons absorbed at the p-n junction will have high energy to dislodge an electron from the fixed position in the material and gives it enough energy to move freely in the material. The electron evicted from its customary bond can travel through the entire crystalline solid and capable of responding to electric field and other influences. The bond from which the electron was ejected is short of one electron creating a hole which is also mobile. Thus the ejected free electron and the hole form an electron –hole pair. The electrons and the holes being of opposite charge will be pushed in different directions by the electric field which already exists in the vicinity of the junction if they come into the region near the p-n junction. The permanent electric field which is already built-in near the p-n junction pushes the hole into the p-region and the electron into the n-region. Thus p-region becomes positively charged and the n-region becomes negatively charged. If an external load is applied, this charge difference will drive a current through it. The current will flow so long as the sunlight keeps generating the electron-hole pairs.

Helio-thermal Process.

The heating property of solar radiation is used in the devices to meet the thermal energy needs.  It is necessary to collect and concentrate the solar radiation in an efficient manner to arrive a reasonably high-temperature heat source. The collectors gather the sun’s energy and direct it onto receivers that contain the working fluid.

Basically two types of collectors are used and they are flat plate collectors and concentrating collectors.

Figure 4: Flat plate collector

Figure 4: Flat plate collector (cut-view)

In flat plate collectors the incident solar radiation is absorbed by the collectors surface itself, which are usually coated with black paint (usually electroplated), covered with transparent glass cover on top and insulated all around to prevent the heat loss from the collector surface. The black collector surface gets heated up and then in turn transfers the heat to the fluid passing through the tubes which are either welded or soldered or are integral part of the collector plate. Flat plate collectors are usually sloped and oriented in one particular direction and are capable of collecting both diffuse and beam radiation. Since there are no moving parts in it, the repair and maintenance cost is also nil or negligible. A maximum of 100°C can be easily achieved using flat plate collectors and are more popularly used in solar water heating applications and solar air heaters as they are relatively cheaper as compared to the cost of concentrating collectors.

In concentrating collectors the incident solar radiation falls on a large curved surface from where it is reflected and focused on to focal point or line depending upon the type of the geometrical construction of the concentrating collector.  When temperatures higher than 100°C are required, it becomes necessary to concentrate the radiation. This is achieved using focusing or concentrating collectors. A schematic diagram of a typical concentrating collector is shown in figure 5. The collector consists of a concentrator and a receiver. The concentrator shown is a mirror reflector having the shape of a cylindrical parabola. It focuses the sunlight onto its axis, where it is absorbed on the surface of the absorber tube and transferred to the fluid flowing through it. A concentric glass cover around the absorber tube helps in reducing the convective and radiative losses to the surroundings. In order that the sun’s rays should always be focussed onto the absorber tube, the concentrator has to be rotated. This movement is called tracking. In the case of cylindrical parabolic concentrators, rotation about a single axis is generally required. Fluid temperatures upto 400°C can be achieved in cylindrical parabolic focussing collector systems.  The generation of still higher temperature is possible by using paraboloid reflectors (shown in figure) which have a point focus. These require two-axis tracking so that the sun is in line with the focus and the vertex of the paraboloid.

One of the major problems associated with the utilisation of solar energy is its variability. For this reason, most applications require some type of energy storage system. The purpose of such a system is to store when it is in excess of the requirement of an application and to make it available for extraction when the supply of solar energy is absent or inadequate.

Figure 5: Concentrating collectors

Helio-chemical Process / Photosynthesis

The most important chemical reaction on the earth is the reaction of sunlight and green plants. Radiant energy of the sun is absorbed by the green pigment chlorophyll in the plant and is stored within the plant in the form of chemical bond energy.

The visible light having wavelength below 700A° is absorbed by the green chlorophyll which becomes activated and passes its energy on to water molecules.  A hydrogen atom is released and reacts with the carbon dioxide molecules, to produce H₂CO and oxygen. H₂CO is the basic molecule forming carbohydrate.  The oxygen librated is from H₂O molecule and not from CO₂.  This process is called as carbon fixation or carbon assimilation.

The process of photosynthesis has two main steps:

(i) Splitting of H₂O molecules into H_2­ and O₂ under the influence of chlorophyll and sunlight. This phase of reaction is called the light-reaction. In this reaction, light absorbed by chlorophyll causes photolysis of water. O₂ escapes and H₂ is transformed into some unknown compounds. The solar energy is converted into potential chemical energy.

(ii) In the second step, hydrogen is transferred from this unknown compound to CO₂ to form starch or sugar. Formation of starch or sugar is dark reaction not requiring sunlight.

However, photosynthesis concepts is less attractive as the average efficiency of solar energy conversion in plants is about 1% and overall efficiency of the conversion of sunlight to electricity would be about 0.3% compared to 18 to 21% for photo-voltaic cells. Still worldwide photosynthetic activity can store more than 15times as much energy as consumed by all nations of the world.

Wind Energy:

Wind energy is an indirect form of solar energy. It is caused by the uneven heating of the earth’s surface by the sun. Since the earth’s surface is made of very different type of land and water, it absorbs the sun’s heat at different rates. During the day, the air above the land heats up more quickly than the air over water. The warm air over the land expands and rises, and the heavier, cooler air rushes in to take its place, creating wind. At night, the wind is reversed in direction because the air cools more rapidly over the land than over water. Although solar energy is cyclic and predictable, wind energy, however is erratic, unsteady. However, there are many locations where the wind direction and velocity are relatively constant over long periods.

Wind possesses energy by virtue of its motion. Any device capable of slowing down the mass of moving air, like a sail or propeller, can extract part of the energy and convert it into useful work. Three factors determine the output from a wind energy converter:

i)        The wind speed,

ii)      The cross-section of wind swept by the rotor and

iii)     The overall conversion efficiency of the rotor, transmission system and generator or pump.

The wind turbines or windmills are used to convert the kinetic energy of the wind into mechanical work, which may be converted into electrical energy. No device, however well-designed, can extract all the wind’s energy because the wind would have to be brought to a halt and this would prevent the passage of more air through the rotor. The most that is possible is for the rotor to decelerate the whole horizontal column of intercepted air to about one-third of its free velocity.

The power in the wind can be computed by using the concepts of kinetics. The wind mill works on the principle of converting kinetic energy of wind to mechanical energy. We know that the power is equal to energy per unit time. The energy available is the kinetic energy of the wind.

The kinetic energy of any particle is = ½ mV²   joules

Where m = mass of the particle and

           V = velocity

The amount of air passing in unit time, through an area A, with velocity V, is = A.V  m³/s

and its mass m is equal to its volume multiplied by its density ρ of air, or

i.e.   m = vol X ρ   = ρ AV                   kg/s

(m is the mass of air traversing the area A swept by the rotating blades of a wind mill type generator)

Substituting this value of the mass in the expression for the kinetic energy, we obtain, power available in wind = ½ ρ AV. V² watts

                                                = ½ ρ A V³ watts.

Since A = (π D²) / 4, where D = diameter of the rotor.

Available wind power =

                                            = watts

Hence the wind power is proportional to square of the diameter of the rotor and to the cube of wind velocity.

Figure 6 shows the schematic diagram of a horizontal axis windmill. It consists of tall tower with a large propeller on the top. The wind blows the propeller to rotate, which turns a generator to produce electricity. Some of the generally used propeller for horizontal axis windmills is: Multiblade type, sail type and propeller type. The vertical axis windmill propellers are: Savonius type and Darrieus type. One advantage of vertical windmills is that they operate in all wind directions and thus need no yaw adjustments. But they have relatively low tip-to wind speed ratios and lower power outputs per given rotor size, weight and cost.

Figure 6: Windmill

Multi-blade Windmill: Usually consists of 12 to 20 blades made from metal sheets as shown in figure 7. Windmill having many blades is usually of low speed system. The Multiblade arrangement will be very heavy but develops a high torque. It is mainly used to drive water pumps. Example: Sail type, Propeller type

Fig.7: Multiblade windmill                 Fig.8: Sail type              Fig.9: Propeller type

  Figure 10: Savonious rotor                                     Figure 11: Darrieus Rotor

Advantages and disadvantages of wind energy

Advantages:

1.    It is a renewable resource.

2.    Wind energy is free, non-polluting and inexhaustible energy.

3.     Windmills are highly desirable to the rural areas, which are far from the existing grids

Disadvantages:

1.    Large size conversion machines are necessary due to dilute form of energy.

2.    The wind velocity is neither constant in magnitude nor in direction and also velocity varies from the bottom to top of the large rotor. This imposes cyclic loads on the turbine blades.

3.    Necessitates the energy storage device. i.e., store the energy when the wind is good and use it when it is inadequate or absent.

4.    High initial cost and low power co-efficient

Ocean Thermal Energy Conversion

The solar energy stored as heat in the ocean can be converted into electrical energy by making use of the temperature difference between the warm surface water and the colder deep water. The operation of Ocean Thermal Energy Conversion plant is based on the thermodynamic principle. It is possible to run a heat engine (prime mover), by utilising the temperature difference, if a heat source at higher temperature and a heat sink at lower temperature are available. The prime mover can convert a part of the heat taken from the source into mechanical energy and hence into electrical energy. The residual heat is discharged to the sink at lower temperature. Warm surface water is the heat source and the deep colder water provides the sink in OTEC systems.

The tropical oceans acts as built-in solar collectors and solar radiation is absorbed by the top layers of sea water and constitutes an infinite heat storage reservoir. Solar energy absorption by water takes place according to Lambert’s law of absorption, which states that each layer of equal thickness absorbs the same fraction of light that passes through it.

Mathematically,

                             I_(x) = I₀ e^-kx

Where I₀ and I_(x) are the intensities of radiation at the surface (x = 0) and at a distance x below the surface. K is an extinction co-efficient that has the unit L^-1. K has values of 0.05m^-1 for very clear fresh water, 0.27m^-1 for turbid fresh water and 0.5m^-1 for very salty water. Thus the intensity decreases exponentially with the depth and depending upon the K, almost all of the absorption occurs very close to the surface of deep waters. Because of the heat and mass transfer at the surface itself, the maximum temperatures occur just below the surface. There will be no thermal convection currents between the warmer, lighter water at the top and deep cooler, heavier water. The temperature difference between warm surface water and deep cool water can exceed more than 25 K.

The surface temperature vary both with latitude and season, both being maximum in tropical, subtropical and equatorial water making these the most suitable for OTEC systems.

Ocean thermal electric power generation using closed cycle system

It consists of an evaporator, condenser, turbine, pump and electric power generator. The ammonia (or propane or a Freon) is used as the working fluid which executes a closed cycle.

Fig.16: Schematic layout of closed cycle Ocean Thermal Energy Conversion plant

The warm surface water is pumped to an evaporator (a surface heat exchanger) where the working fluid is evaporated to high pressure vapour to drive the turbines which are essentially very small in size as compared to that of open cycle system turbine which handles low pressure steam. The exhaust of the turbine is condensed by the cold water drawn from the deep ocean through a pump. The condensate is pumped at high pressure to the evaporator to re-execute the rankine cycle. The schematic layout of the plant is as illustrated in the figure16.

The electricity produced could then be transmitted inexpensively to land by submarine cables or can be utilised at the plant site to produce energy-intensive materials in case of offshore OTEC plants.