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Introduction+fundamentals

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الكلية كلية الهندسة/المسيب     القسم هندسة الطاقة     المرحلة 2
أستاذ المادة واثق ناصر حسين الشمري       04/03/2018 07:25:43
energy source
references
1-steam plant operation by everett b. woodruf et al
2-handbook of energy engineering by albert thumann et al
3- renewable energy a first course by robert ehrlich
4-energy source by b. viswanathan 2006.

1st lecture
energy is the property that must be transferred to an object in order to perform work on, or to heat, the object. energy is a conserved quantity the law of conservation of energy states that energy can be converted in form, but not treated or destroyed. the si unit of energy is the joule, which is the energy transferred to an object by the work of moving it a distance of 1 metre against a force of 1 newton.
what are the units of energy?
the fact that energy exists in many forms is part of the reason why there are so many different units for this quantity—for example, calories and british thermal units (btus) are typically used for heat joules, ergs, and foot-pounds for mechanical energy kilowatt-hours for electrical energy
and million electronvolts (mev) for nuclear energy. however, since all these units describe the same fundamental entity, there must be conversion factors relating them all. to make matters more even confusing, there are a whole host of separate units for the quantity power, which refers to the rate at which energy is produced or consumed, i.e.,




example 1
suppose during a test of a nuclear reactor its power level is ramped up from zero to its rated power of 1000 mw over a 2 h period, and then after running at full power for 6 h, it is ramped back down to zero over a 2 h period. calculate the total energy generated by the reactor during those 10 h.
solution
we shall assume here that during the time which the power is ramped up and down it varies linearly, so that the power the reactor generates varies accordingly during the 10 h test as shown in figure 1.2. based on equation 1.1, and the definition of the integral as the area under the power–time curve, the energy must equal the area of the trapezoid in figure 1.2 or 8000 mwh (table 1.2).



table 1.2 some common prefixes used to designate
various powers of 10 terra (t) 1012
giga (g) 10+9
mega (m) 10+6
kilo (k) 10+3
milli (m) 10?3
micro (?) 10?6
nano (n) 10?9
pico (p) 10?12


example 2
example 1-2. a portable electric generator that is powered by diesel fuel produces 7 kwh of electricity during a single period of operation. (a) what is the equivalent amount of energy measured in mj? (b) suppose the fuel consumed had an energy content of 110 mj. if the device were 100% efficient, how much electricity would it produce in wh?
solution
(a) 7 kwh × 3.6 mj/kwh = 25.2 mj
(b) 110 mj/3.6 mj/kwh = 30.6 kwh

what is an energy source?
some energy sources are either stores (repositories) of energy such as coal, oil, or uranium typically chemical or nuclear that can be liberated for useful purposes. other energy (second source) sources are flows of energy through the natural environment that are present in varying degrees at particular times and places. an example, is wind or solar energy. consider the question of electricity—is it an energy source or not? electricity does exist in the natural environment in the extreme form of lightning, and therefore it can be considered to fall into the second category. in fact, lightning lightning strike could be captured and stored (in a capacitor) and then later released for useful purposes. anyone watching a storm is likely to marvel at the awesome power of a lightning bolt, which is indeed prodigious— typically about 1 tw (1012 w). this amount is equal to the power output of a thousand 1000 mw nuclear reactors—more than exists in the entire world! such a comparison may prompt the thought: great! why not harness lightning as an energy source? the problem is not figuring out how to capture the lightning, but rather that while the power is very high, the energy lightning contains is quite small, since a lightning bolt lasts such a short time—around 30 ?s = 3 ×10?5s, so by equation 1.1, the energy contained is around 1012 × 3 ×10?5 = 3 ×107 j = 30mj. thirty million joules may sound impressive, but suppose we designed a “lightning catcher” that managed to capture say 10% of this energy. it would only be sufficient to light a 100 w light bulb for a time t = e/p = 3 ×106 j/100w = 3000 s, which is just under an hour—hardly a useful energy source, considering the likely expense involved. what about electricity that humans create—can it be thought of as an energy source? hardly! any electricity that we create requires energy input of an amount that is greater than that of the electricity itself, since some energy will always be lost to the environment as heat. thus, human-created electricity, whether it be from batteries, generators, or solar panels is not an energy source itself, but merely the product of whatever energy source that created it. in the case of a generator it would be whatever gave rise to the mechanical energy forcing it to turn, while in the case of a solar panel it would be the energy in the sunlight incident on the panel. in order for an energy resource to be reliable, it must, first of all, deliver the service that the consumer expects. secondly, it must be available in the quantity desired, when the consumer wishes to consume it (whether this is electricity from a wall outlet or gasoline dispensed from a filling station). lastly, the resource must be available at a price that is economically affordable.

what exactly is the world’s energy problem?
1-all sources of energy have some environmental impact, but as you
are aware the impacts of different sources vary considerably.
2-the energy sources people worry the most about are fossil fuels (coal,
oil, and gas) as well as nuclear, while the renewable (“green”) energy
sources are considered much more benign—even though they too have
some harmful impacts.
3-moreover, the environmental impact of fossil
fuel and nuclear energy usage has gotten worse over time, as the
human population has grown, and the energy usage per capita has
also grown—an inevitable consequence of the rise in living standards
worldwide.



problems
1-how many kwh would a 1000 mw nuclear power plant generate in a
year?
2-consider a nuclear power plant whose power level is ramped up from
zero to a maximum 1000 mw and then back down to zero over a 10 h
period of time. assume that the power level varies as a quadratic function
of time during those 10 h. write an expression for the power as a
function of time, and then find the total energy generated by the plant
during the 10 h period.
3-compare the direct costs to the consumer of using a succession of ten
100-w incandescent light bulbs with an efficiency to visible light of
5%, a lifetime of 1000 h, and a price of 50 cents with one compact
fluorescent lamp giving the same illumination at 22% efficiency, a
lifetime of 10,000 h, and a price of $3. assume a price of electricity
of 10 cents per kwh.



2nd lecture
why has renewable energy and conservation been neglected until fairly recently?
there are many reasons aside from simple inertia why moving away from fossil fuels and toward renewable energy has and will continue to be a challenge. first, the awareness of the environmental problems associated with fossil fuels has come very gradually, and views on the seriousness of the threat posed by climate change vary considerably. moreover, in times of economic uncertainty long-term environmental issues can easily take a backseat to more immediate concerns, especially for homeowners.
second, compared to fossil fuels there are problems with renewable sources, which may be very dispersed, intermittent, and expensive— although the cost differential varies widely, and often fails to take into account what economists refer to as “externalities,” i.e., costs incurred by society as a whole or the environment. the intermittency poses special problems if the renewable source is used to generate electricity at large central power plants connected to the grid. one can cope with this problem using various energy storage methods, and upgrades to the electric power grid, but of course both have costs, see table 1.4.



example 2: which solar panels are superior?
suppose that ten type a solar panels produced enough power for your electricity
needs, had a lifetime of 30 years, cost only $1000, but they had an efficiency of only 5%. five type b panels cost $5000 but they had an efficiency of 10%, and lasted only 15 not 30 years. which panels should you buy?
solution
obviously, the more efficient panels would take up only half the area on your roof than the type a panels, but who cares if they both met your needs. the cost over a 30 year period would be $1,000 for the type a panels, but $10,000 for the more efficient type b panels that produced the same amount of power (since they last only half as long), so clearly you would opt for the less efficient choice in this case. as a general rule, as long as the fuel is free, and there are no differences in labor or maintenance costs, your primary consideration would almost always be based on cost per unit energy generated over some fixed period of time—usually the lifetime of the longer-lived alternative.

example 3: how the usage of wind power to offset coal-fired plants can generate more emissions not less.
suppose that a certain fraction of the power produced by a 500 mw coal
plant is offset by wind power. assume that when the coal plant runs at its
constant rated power it has an efficiency of 35%, but that when it needs
to be ramped up and down to compensate for the wind power variations
its efficiency is reduced by according to e = 0.35 ? 0.00001p2, where p is
the amount of wind power. find the percentage increase in emissions that
results when 90 mw of the 500 mw is generated by wind power instead
of coal.
solution
in order to generate the full 500 mw by itself, the coal plant requires 500/0.35 = 1429 mw of heat flow from the coal. if the wind power is 90 mw the efficiency of the coal plant is reduced to e = 0.35 ? 0.00001(90)3 = 0.269, and the heat flow required to generate (500 ? 90) = 410 mw is therefore 410/(0.269) = 1524 mw. the percentage increase in emissions is the same as the percentage increase in the heat flow to the coal plant, i.e., 6.7%.

problem 1: using the data in example 3, find the amount of wind power that could
be used with a 500 mw coal-fired plant that would result in the least
amount of emissions.

fossil fuels
most fossil fuels, which include coal, oil, and natural gas, were formed from the remains of ancient life over the course of tens to hundreds of millions of years—hence the adjective fossil. today, fossil fuels account for totally 85% of the world’s primary energy usage, with nuclear and hydropingower comprising 8% and 3% and the renewable sources of geothermal, solar, tidal, wind, and wood waste amounting to a bit over 1% collectively. an obvious question is what has made fossil fuels so attractive as an energy source in the past as well as today and why is it so difficult to move away from them despite the mounting evidence of the environmental problems they pose. the answer is for example, coal, oil, and gas have at least 200 times the energy per kilogram that is stored in a lead acid car battery. fossil fuels represent highly concentrated stores of energy compared to the much more dilute concentrations typical of renewablecheaply collected, stored, shipped, and used where and when desired than most renewable energy sources.

coal
composition of coal coal is a combustible sedimentary rock. it differs from other kinds of rocks, which are generally made of minerals, and hence inorganic by definition. coal, however, is mostly carbon made primarily from plant material and is therefore organic. while carbon may be its primary component, it does contain minor amounts of hydrocarbons, like methane, and inorganic mineral material that are considered impurities. coal does not have a specific chemical composition, because the precise mixture of sulfur, oxygen, hydrogen, nitrogen, and other elements comprising it varies according to the particular rank or grade of coal, and even within a grade. for example, for anthracite, the highest and hardest rank of coal, its composition includes 0%–3.75% hydrogen, 0%–2.5% oxygen, and up to around 1.6% sulfur. although the number of coal ranks depends on the classification system, one system that is widely used is based on the four grades listed in table 2.1.
coal is generally cheaper than oil, whereas natural gas is more expensive. the difference in price reflects the different costs of recovery, storage, and transport. nuclear fuel refined for use in nuclear electric power plants is less expensive than fossil fuels, per unit of heating value.
the energy believed to be present in the world’s coal supply dwarfs all
other fossil fuels combined, and it has been estimated at 2.9 × 1020 kj, most of which is not economically exploitable.


example: energy content of coal an empirically determined formula for the energy content of coal based on the elemental abundances of carbon, hydrogen, oxygen, and sulfur is
e = 337c + 1442(h ?o/8) + 93s (2.1)
where e is in units of kj/kg and the symbols stand for the mass percentages of the elements c, h, o, and s. use equation 2.1 and the information provided earlier about anthracite, i.e., h = 0%–3.75%, o = 0%–2.5%, and s = 1%, to estimate the highest value, lowest value, and average value of the energy content of anthracite assuming that no elements besides c, h, o, and s are present.
solution
based on the values of the constants in equation 2.1, the maximum
energy density requires h be as high as possible and o is as low as possible, and the minimum energy requires the opposite. thus, using the data from table 2.1, we have
as a check, we note that these values are fairly close to those provided in
table 2.1 for anthracite.
figure below shows energy consumption by the world in 2008

figure … world consumption of energy 2008

figure ..


المادة المعروضة اعلاه هي مدخل الى المحاضرة المرفوعة بواسطة استاذ(ة) المادة . وقد تبدو لك غير متكاملة . حيث يضع استاذ المادة في بعض الاحيان فقط الجزء الاول من المحاضرة من اجل الاطلاع على ما ستقوم بتحميله لاحقا . في نظام التعليم الالكتروني نوفر هذه الخدمة لكي نبقيك على اطلاع حول محتوى الملف الذي ستقوم بتحميله .
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