Work and potential energy
Potential energy is
closely linked with forces. If the work done by a force on a body
that moves from A to B does not depend on the
path between these points, then the work of this force measured from A assigns
a scalar value to every other point in space and defines a scalar potential field.
In this case, the force can be defined as the negative of thevector gradient of
the potential field.
If the work for an
applied force is independent of the path, then the work done by the force is
evaluated at the start and end of the trajectory of the point of application.
This means that there is a function U (x), called a
"potential," that can be evaluated at the two points xA and xB to
obtain the work over any trajectory between these two points. It is tradition
to define this function with a negative sign so that positive work is a
reduction in the potential, that is
where C is
the trajectory taken from A to B. Because the work done is independent of the
path taken, then this expression is true for any trajectory, C,
from A to B.
The function U(x)
is called the potential energy associated with the applied force. Examples of
forces that have potential energies are gravity and spring forces.
Compressed air energy storage
is a way to store
energy generated
at one time for use at another time using compressed
air. At utility scale, energy generated during periods of low
energy demand (off-peak) can be released to meet higher demand (peak load)
periods. Small scale systems have
long been used in such applications as propulsion of mine locomotives. Large
scale applications must conserve the heat energy associated with compressing
air; dissipating heat lowers the energy efficiency of the storage system.
,
where , and so, . Here, is the absolute pressure, is the volume of the vessel, is the amount of substance of gas (mol) and is theideal gas constant
Potential
energy for a linear spring
A
horizontal spring exerts a force F = (−kx, 0, 0) that
is proportional to its deflection in the x direction. The work
of this spring on a body moving along the space curve s(t) =
(x(t), y(t), z(t)), is
calculated using its velocity, v = (vx, vy, vz),
to obtain
For
convenience, consider contact with the spring occurs at t = 0,
then the integral of the product of the distance x and the x-velocity, xvx,
is x2/2.
The
function
electrical-energy
storage
1-Mechanical storage systems
The most common mechanical storage
systems are pumped hydroelectric power plants (pumped hydro storage, PHS),
compressed air energy storage (CAES) and fl ywheel energy storage (FES).
Pumped hydro storage (PHS)
With over 120 GW, pumped hydro
storage power plants represent nearly 99 % of world-wide installed electrical
storage capacity [doe07], which is about 3 % of global generation Capacity
Compressed air energy storage
(CAES)
Compressed air (compressed gas)
energy storage is a technology known and used since the 19th century for
different industrial applications including mobile ones.
2-Electrochemical storage systems
In this section various types of
batteries are described. Most of them are technologically mature for practical
use. First, six secondary battery types are listed: lead acid, NiCd/NiMH
Li-ion, metal air, sodium sulphur and sodium nicke chloride; then follow two
sorts of fl ow battery.
Waste Heat Recovery
Waste heat losses arise
both from equipment inefficiencies and from thermodynamic limitations on
equipment and processes. For example, consider reverberatory furnaces
frequently used in aluminum melting operations. Exhaust gases immediately
leaving the furnace can have temperatures as high as 2,2002,400°F
[1,2001,300°C]. Consequently, these gases have highheat content, carrying away
as much as 60% of furnace energy inputs. Efforts can be made to design more
energyefficient reverberatory furnaces with better heat transfer and lower
exhaust temperatures; however, the laws of thermodynamics place a lower limit
on the temperature of exhaust gases. Since heat exchange involves energy
transfer from a hightemperature source to a lowertemperature sink, the
combustion gas temperature must always exceed the molten aluminum temperature
in order to facilitate aluminum melting. The gas temperature in the furnace will
never decrease below the temperature of the molten aluminum, since this would
violate the second law of thermodynamics. Therefore, the minimum possible
temperature of combustion gases immediately exiting an aluminum reverberatory
furnace corresponds to the aluminum pouring point temperature 1,2001,380°F
[650750°C]. In this scenario, at least 40% of the energy input to the furnace
is still lost as waste heat (Appendix A: Documentation of Waste Heat Estimates)
Band with of sun radiation
In a point at the top of Earth’s atmosphere,
the beam of nearly parallel incident
sunrays is referred to as extraterrestrial
radiation (ETR). ETR fluctuates about
6.9 % during a year (from 1412.0 Wm-2
in January to 1321.0 Wm-2 in July) due to the Earth’s varying
distance from the Sun
Wind turbine
A wind turbine is a device that converts kinetic energy from the wind into electrical power. The term appears
to have migrated from parallel hydroelectric technology (rotary propeller). The
technical description for this type of machine is an aerofoil-powered generator.
The result of over a
millennium of windmill development and modern engineering, today's wind
turbines are manufactured in a wide range of vertical and horizontal axis
types. The smallest turbines are used for applications such as battery charging
for auxiliary power for boats or caravans or
to power traffic warning signs. Slightly larger turbines can be used for making
contributions to a domestic power supply while selling unused power back to the
utility supplier via the electrical grid. Arrays of large turbines,
known as wind farms, are becoming an increasingly
important source of renewable energy and are used by many countries as part
of a strategy to reduce their reliance on fossil fuels.
Types
Horizontal axis
Horizontal-axis
wind turbines (HAWT) have the main rotor shaft and electrical
generator at the top
of a tower, and must be pointed into the wind. Small turbines are pointed by a
simple wind vane, while large turbines generally use a wind sensor coupled
with a servo motor. Most have a gearbox, which turns the slow rotation of the
blades into a quicker rotation that is more suitable to drive an electrical
generator
Vertical axis design
Vertical-axis
wind turbines (or VAWTs) have the main rotor shaft arranged vertically. One
advantage of this arrangement is that the turbine does not need to be pointed
into the wind to be effective, which is an advantage on a site where the wind
direction is highly variable. It is also an advantage when the turbine is
integrated into a building because it is inherently less steerable. Also, the
generator and gearbox can be placed near the ground, using a direct drive from
the rotor assembly to the ground-based gearbox, improving accessibility for
maintenance.
Breeder reactors
238U is not
usable directly as nuclear fuel, though it can produce energy via
"fast" fission. In this process, a neutron that has a kinetic energy
in excess of 1 MeV can cause the nucleus of 238U to
split in two. Depending on design, this process can contribute some one to ten
percent of all fission reactions in a reactor, but too few of the about 1.7
neutrons produced in each fission have enough speed to continue a chain
reaction.
238U can be
used as a source material for creating plutonium-239, which can in turn be used
as nuclear fuel. Breeder reactors carry out such a process of
transmutation to convert the fertile isotope 238U into
fissile Pu-239. It has been estimated that there is anywhere from 10,000 to
five billion years worth of 238U for use
in these power plants. Breeder technology has been used
in several experimental nuclear reactors.
By December 2005, the only
breeder reactor producing power was the 600-megawatt BN-600 reactor at the Beloyarsk
Nuclear Power Station in
Russia. Russia has planned to build another unit, BN-800, at the Beloyarsk
nuclear power plant. Also, Japan's Monju breeder reactor is planned to be
started, having been shut down since 1995, and both China and India have announced
plans to build nuclear breeder reactors.
The breeder reactor as its
name implies creates even larger quantities of Pu-239 than the fission nuclear
reactor.
The Clean And Environmentally Safe Advanced Reactor (CAESAR), a nuclear reactor concept
that would use steam as a moderator to control delayed neutrons, will potentially be able to
burn 238U as fuel
once the reactor is started with LEU fuel. This design is still in the early
stages of development
Coal world
consumption per day
According to a Greenpeace analysis, between
the months of January and September 2015 coal use around the world was down by
at least 2.3% and by as much as 4.6% versus the same period last year.
This historic fall was caused by a ‘perfect
storm’ of circumstance: dire fossil fuel economics, rising renewable energy
uptake, slowing global energy demand, and China’s crackdown on air pollution.
Because this fall doesn’t happen without China,
by far the largest coal consuming country in the world.
After its rapid coal growth made a dramatic
u-turn in 2014, China has seen a massive decline in its use so far this
year.
The country’s 3-5% fall in the first half of
2015 (43-69Mtce) accounts for more than half of the world’s coal consumption
reduction.
And yes, this analysis uses the coal stats recently reported on by the New York Times (but covered by us six months ago).
Things are changing elsewhere as well; coal use
in the US, for instance, is way down in 2015, with a string of coal plant
closures and mine retirements pushing production to its lowest point in three
decades.
These shifts have more than offset coal gains
in other countries around the world, most notably India, where consumption has
grown by around 5% (8-13Mtce)
What’s also remarkable is that coal has
continued to trend downwardsdespite global benchmarks falling to record lows.
With the
UN climate summit in Paris to kick off at the end of November, you’d
imagine all this might come up
Exothermic process
In thermodynamics, the term exothermic process (exo- : "outside")
describes a process or reaction that releases energy from
the system, usually in the form of heat,
but also in a form of light (e.g. a spark, flame, or flash), electricity (e.g.
a battery), or sound (e.g. explosion heard when burning
hydrogen). Its etymology stems from the Greek prefix έξω (exō, which means
"outwards") and the Greek word θερμικός(thermikόs,
which means "thermal").The term exothermic was first coined by Marcellin Berthelot.
The opposite of an exothermic process is an endothermic process,
one that absorbs energy in the form of heat.
The concept is frequently
applied in the physical sciences to chemical reactions,
where as in chemical bond energy that
will be converted tothermal energy (heat).
Exothermic (and
endothermic) describe two types of chemical reactions or systems found in
nature, as follows.
Simply stated, after an
exothermic reaction, more energy has been released to the surroundings than was
absorbed to initiate and maintain the reaction. An example would be the burning
of a candle, wherein the sum of calories produced by combustion (found by
looking at radiant heating of the surroundings and visible light produced,
including increase in temperature of the fuel (wax) itself, which with oxygen,
have become hot CO2 and water
vapor,) exceeds the number of calories absorbed initially in lighting the flame
and in the flame maintaining itself. (i.e. some energy produced by combustion
is reabsorbed and used in melting, then vaporizing the wax, etc. but is (far)
outstripped by the energy produced in breaking carbon-hydrogen bonds and
combination of oxygen with the resulting carbon and hydrogen).
On the other hand, in an
endothermic reaction or system, energy is taken from the surroundings in the
course of the reaction. An example of an endothermic reaction is a first aid
cold pack, in which the reaction of two chemicals, or dissolving of one in
another, requires calories from the surroundings, and the reaction cools the
pouch and surroundings by absorbing heat from them. An endothermic system is
seen in the production of wood: trees absorb radiant energy, from the sun, use
it in endothermic reactions such as taking apart CO2 and H2O and
combining the carbon and hydrogen generated to produce cellulose and other
organic chemicals. These products, in the form of wood, say, may later be
burned in a fireplace, exothermically, producing CO2 and water, and releasing energy in the form
of heat and light to their surroundings, e.g., to a home's interior and chimney
gasses.
Endothermic process
In thermodynamics, the term endothermic process describes a process or reaction in
which the system absorbs energyfrom its surroundings; usually, but not
always, in the form of heat.
The term was coined by Marcellin Berthelot from the Greek roots endo-, derived from the word
"endon" (ἔνδον) meaning "within" and the root "therm"
(θερμ-) meaning "hot." The intended sense is that of a reaction that
depends on absorbing heat if it is to proceed. The opposite of an endothermic
process is an exothermic process,
one that releases, "gives out" energy in the form of (usually, but
not always) heat. Thus in each term (endothermic & exothermic) the prefix
refers to where heat goes as the reaction occurs, though in reality it only
refers to where the energy goes, without necessarily being in the form of heat.
The concept is frequently
applied in physical sciences to, for example, chemical reactions,
where thermal energy (heat) is converted to chemical bond energy.
Endothermic (and exothermic)
analysis only accounts for the enthalpy change
(∆H) of a reaction. The full energy analysis of a reaction is the Gibbs free energy (∆G), which includes an entropy (∆S)
and temperature term in addition to the enthalpy. A reaction will be a spontaneous process at a certain temperature if the
products have a lower Gibbs free energy (anexergonic reaction)
even if the enthalpy of the products is higher. Entropy and enthalpy are
different terms, so the change in entropic energy can overcome an opposite
change in enthalpic energy and make an endothermic reaction favorable.
electromagnetic waves
Electromagnetic radiation (EM radiation or EMR)
is the radiant energy released by certain electromagneticprocesses. Visible light is one type of electromagnetic radiation;
other familiar forms are invisible electromagnetic radiations, such as radio
waves, infrared light and X rays.
Classically,
electromagnetic radiation consists ofelectromagnetic waves, which are
synchronizedoscillations of electric and magnetic fields
that propagate at the speed of light through a vacuum. The oscillations of
the two fields are perpendicular to each other and perpendicular to the
direction of energy andwave propagation,
forming a transverse wave. Electromagnetic waves can be
characterized by either the frequency or wavelength of
their oscillations to form the electromagnetic
spectrum, which includes, in order of increasing frequency and
decreasing wavelength: radio waves, microwaves, infrared radiation,visible light, ultraviolet radiation, X-rays and gamma rays.
Electromagnetic waves are
produced whenever charged particles areaccelerated, and these waves can subsequently
interact with any charged particles. EM waves carry energy, momentum and angular momentum away from their source particle and
can impart those quantities to matter with
which they interact. Quanta of EM waves are called photons, which are massless, but they are still affected by gravity.
Electromagnetic radiation is associated with those EM waves that are free to
propagate themselves ("radiate") without the continuing influence of
the moving charges that produced them, because they have achieved sufficient
distance from those charges. Thus, EMR is sometimes referred to as the far field. In
this language, the near field refers to EM fields near the charges
and current that directly produced them, specifically,electromagnetic
induction and electrostatic induction phenomena.
In the quantum theory of
electromagnetism, EMR consists of photons, theelementary particles responsible for all electromagnetic
interactions. Quantum effects provide additional sources of EMR, such as the transition of
electronsto lower energy levels in an atom and black-body radiation.
The energy of an individual photon is quantized and is greater for photons of higher
frequency. This relationship is given by Planck's equation E=hν, where E is the energy per
photon, ν is the frequency of the photon, and h is Planck's constant.
A single gamma ray photon, for example, might carry ~100,000 times the energy
of a single photon of visible light.
The effects of EMR upon
biological systems (and also to many other chemical systems, under standard conditions) depend both upon the
radiation's pow٢دer and
its frequency. For EMR of visible frequencies or lower (i.e., radio, microwave,
infrared), the damage done to cells and other materials is determined mainly by
power and caused primarily by heating effects from the combined energy transfer
of many photons. By contrast, for ultraviolet and higher frequencies (i.e.,
X-rays and gamma rays), chemical materials and living cells can be further
damaged beyond that done by simple heating, since individual photons of such
high frequency have enough energy to cause direct molecular damage.