Meeting the Entropy Challenge: The Second Law and Energy

International Thermodynamics Symposium in honor and memory of Joseph Keenan

MIT4 October, 2007

Thermodynamics began as the scientific basis of how to turn heat

energy into mechanical work.

It is the scientific foundation of how we harness energy… and the basis

of the Wealth of Nations.

Thermodynamics began with the

analysis of the Steam Engine.

Newcomen invented the first practical steam engine, used to pump water from coal mines.Without power, the position of the engine is shown.

•Valve V is opened. Low pressure steam produced by a boiler to fill space B.

• Valve V is closed, and valve V' briefly opens to send a spray of cold water into the cylinder.

• The cold water condenses the steam, creating a partial vacuum.

•. The pressure differential with the atmosphere drives the piston down in the power stroke.

• V and V′′ open, allowing the piston to come to atmospheric pressure. The weight of the pump pulls the piston up into its initial piston.

The Newcomen engine was inefficient:

• The water used to condense the steam in cylinder cooled the cylinder walls. In the next cycle, steam energy is used to heat the cylinder so that steam would no longer condense and fill the chamber.

• Heat losses scaled as the surface area, while useful work scaled as the volume. The larger the engine, the more efficient it was.

• In 1763, James Watt was given the task to fix a scaled-down model Newcomen engine at the University of Glasgow. The smaller model amplified the short comings of Newcomen engine.

• While wandering across Glasgow Green in 1765, Watt thought of separating the condensation system from the cylinder.

The Watt Steam EngineIn the top position (shown), the valve to the condenser is opened.

The condenser is kept cold. The cylinder remains hot.The top of the piston is sealed so the pressure of steam –slightly higher than atmospheric pressure improves the power stroke.

During the power-stroke, the warm condensate is drawn out of the piston. It is sent to a hot water well before heat energy is transferred to the cold water reservoir. The hot condensate was recycled as feed-water for the boiler.

James Watt (August, 1765):

“ I have tried my small model of my perfect enginewhich hitherto answers expectation. and gives great, I may say greatest, hopes of success … in greater model now far advanced; in short, I expect almost totally to prevent waste of steam and consequently to bring the machine to its ultimatum”.

• Is there a theoretical limit to the efficiency of a heat engine?

• Why should we care? In the end, it is actual performance we care about!

The First Law of Thermodynamics:

Energy is conserved.

• Heat engines:Thermal energy ⇒ mechanical energy (work)

• Friction and thermal losses ⇒ conversion of heat into mechanical energy will never be perfect.

• The 1st Law invalidates all patents on perpetual motion machines.

Suppose it is possible to extract mechanical energy from a very large heat reservoir at constant

temperature. For all practical purposes, a perpetual motion machine is possible.

The Second Law of Thermodynamics:1. The conversion of Heat ⇒ mechanical energy at

constant temperature is impossible. (Kelvin)

2. Spontaneous transfer of heat from a cold body to a hot body is impossible. (Clausius)

The Carnot cycle

Volume

Pre

ssur

e Thot

Tcold

A

B

C

D

( )FW F x A x P VA⎞⎛∆ = ×∆ = × ⋅∆ = ×∆⎜ ⎟

⎝ ⎠• In A → B → C → D, mechanical work done by expanding gas

= area enclosed by the cycle.

• Net energy flow = Qhot – Qcold. During adiabatic expansion and compression, there is no net flow of energy in or out of the engine.

Volume

Pre

ssur

e Thot

Tcold

A

B

C

D

At the core of Carnot’s anaylsis is the assumption:

A series of infintestimally small changes from thermal equilibrium states are reversible transformations.Thermodynamically reversible transformations are the mechanical equivalent of “frictionless motion”.

1. The efficiency of Carnot’s engine:

hot cold

hot

Net work done Q Qinput energy Q

η −= =

2. A frictionless engine is as good as it gets!

3. Without a temperature difference, Thot – Tcold ≠ 0,

no work can be done.

All reversible engines operating between Thotand Tcold have the same efficiency.

cold cold

hot hot

Q TQ T

=It can be “easily” shown that

hot cold hot cold

hot hot

Q Q T TQ T

η − −= =

The basic goal of heat engine design:

1. Minimize mechanical friction and “thermodynamic friction” (departures from reversible transitions).

2. Maximize Thot/Tcold

3. Take advantage of large “frree” thermal reservoirs.

The Watt Steam EngineIn the top position (shown), the valve to the condenser is opened.

The condenser is kept cold. The cylinder remains hot.The top of the piston is sealed so the pressure of steam –slightly higher than atmospheric pressure improves the power stroke.

During the power-stroke, the warm condensate is drawn out of the piston. It is sent to a hot water well before heat energy is transferred to the cold water reservoir. The hot condensate was recycled as feed-water for the boiler.

In a Carnot Cycle, hot cold

hot cold

Q QT T

= If we define heat received as +Q, and heat ejected as -Q,, 0hot cold

hot cold

Q QT T

+ =

Volume

Pre

ssur

e Qhot

Qcold

A

B

C

D

For any arbitrary closed path for A → C → A,

we can divide it into a connected set of

infinitesimal Carnot cycles, tiled together.

0,dQT

=∫

0,dQT

=∫ This implies that for any path A → Cthe integral is the same.0

C

A

dQT

=∫

In compete analogy to mechanical energy,

If we set S0 =0 as the entropy at T=0 [The

Third Law]

0A

A

TdQS T=

≡ ∫

Pre

ssur

e

A

C

Volume

The Laws of Thermodynamics

1. Energy is conserved.2. dW ≤ dQ ≤ TdS + Sdt

[Condensed form for experts]3. S=0 at T=0. 1. You can’t win.2. You can’t even break even.3. You can’t leave the game.

1. There is no free lunch.2. The lunch will always cost more than you think.3. You have to eat.

A → B: Reversible isothermal expansion of the gas at TH(Heat is added to do work)

B → C: Reversible adiabatic expansion. (No heat transfer, but addition work is done) The gas continues to expand, doing work on the surroundings and cooling to TC.

C → D: Reversible isothermal compression at TC. Work is done on the gas. Heat is transferred to the low temperature reservoir.

D → A: Reversible adiabaticcompression of the gas. Work is done to compress and heat the gas. Since the system is thermally isolated, there is no net transfer of heat energy.

The Carnot Cycle

(Temperature vs. Entropy)

The Rankine Cycle1 → 2: Water is pumped from low to high pressure. As a ~incompressional fluid, the pump requires little energy.

2 → 3: Water is heated at constant pressure to a saturated vapor.

3 → 4: vapor expands through a turbine, generating power. Temperature and pressure of the vapor decreases.

3 → 4: Vapor enters a condenser and cooled at a constant low pressure to a liquid.

The Efficiency of Coal Burning Plants

Japan (~40%)

China

India(~30%)

U.S.

50% may be possible with Supercritical Steamboilers, but new, temperature resistant metals are needed.

The same technology can allow oxygen-burn boilers that will make at-the-stack retro-fit capable CO2 capture.

Natural gas is 60% efficient. (80% with co-generation)

IGCC can also use turbine technology (~60%), but capital costs are becoming prohibitive.

A combined cycle power plant combines employs tow or more thermodynamic cycles

Entropy Engines that can generate sustainable (carbon-

free) energy sources

Three principles in energy generation:

1.Minimize dissipation: mechanical friction and “thermodynamic friction” (departures from reversible transitions).

2.Maximize Thot/Tcold

3.Take advantage of large thermal reservoirs.

Silicon PhotocellsLight quanta excite electrons into the conduction

band. The diode action produces an electric current that flows toward the positive terminal.

II p-type silicon n-type silicon

negative terminalpositive terminal

The p-n junction is a one-way door forthe electrons—they can only flow toward the p-type side

currentp n

Con

duct

ion

band

Val

ence

band

Solar photovoltaicSolar thermal

• Reduction of costs by a factor of ~ 3 is needed for roof-top deployment without subsidy.• A new class of solar PV cells at ~ 1/10cost is needed for wide-spread deployment.

th current

~ 0.2 – 0.3% of the non-arable land in the world would be need to generate current

electricity needs (~ 4 TW) with solar electricity generation at 20% efficiency.

Cost of electricity generation(1990 dollars/kilowatt hour)

Photovoltaics

Windmills

Gas turbines

2005

2005

High Efficiency Solar CellsIII-V multi-gap PV can be >50% efficient …

Schockley-Queisser limit

~30%

Nanotechnology-based solar cells:The benefits of going small

“ .. A diamond of double the weight costs

around 4 times more.”

• Perfect building blocks at low cost

• Atomic level control of essential interfaces

• New physics and chemistry

Limiting sizes for distributedjunction nano-solar cells

(Creation of electrons and holes by one nano-structure;charge transport to electrodes with another.)

Absorption

Charge Transfer

Charge Transport

Exciton Diffusion

Exciton Diffusion

Length

Polymer

ITO

e-h+

Al

20 nm100 nmAbsorption

Depth

CdSe Nanorods

S n

P3HT

Modest but stable fiscal incentives were essential to stimulate long term development of

power generation from wind

3 MW capacity deployed and 5 MW generators in design(126 m diameter rotors).

The Betts Limit:

Assuming conservation of mass for incompressible flow and conservation of momentum,Maximum kinetic energy delivered to a wind turbine

= 16/27 (½)mv2

~ 0.59 of kinetic energy The biggest turbines capture ~ 5/6 of this amount.

va vb vb vc

Aa, Pa

Ab, PbD

Ac, Pc

Ab, PbU

Wind sites in the US

The War of Currents

George WestinghouseThomas Edison Nikola Tesla

Edison carried out a campaign to discourage the use of alternating current. He ordered his technicians to use AC

electricity to kill animals, primarily stray cats and dogs, but also unwanted cattle and horses for the consumption of press.

Edison's series of animal executions peaked with the electrocution of Topsy the Elephant …“ He as Westinghoused".

Sunlight to energy via Bio-mass

SunlightCO2, H20, Nutrients

Biomass Chemical energy

More efficient use of water, sunlight, nutrients.

Drought and pest resistant

Improved conversion of cellulose into fuel.

New organisms for biomass conversion.

Additional sunlight energy has to be supplied to capture CO2 at 380 ppm and concentrate the carbon

into high density fuel (Decreased entropy).

Feedstock grasses (Miscanthus) is a largely unimproved crop.Non-fertilized, non-irrigated test field at U. Illinois can yield

10x more ethanol / acre than corn.

50 M acres of energy crops plus agricultural wastes (wheat straw, corn stover, wood residues, etc. ) can produce half to

all of current US consumption of gasoline.

Advantages of perennial plants such as grasses:• No tillage for ~ 10 years after first planting

• Long-lived roots establish symbiotic interactionswith bacteria to acquire nitrogen and mineralnutrients.

• Some perennials withdraw a substantial fractionof mineral nutrients from above-ground portions ofthe plant before harvest.

• Perennials have lower fertilizer runoff than annuals.(Switchgrass has ~ 1/8 nitrogen runoff and 1/100the soil erosion of corn.)

3.75

2.48

2.89

0.901.14

0.84

2.79

1.030.91

1.20

0

0.5

1

1.5

2

2.5

3

3.5

4

Current and projected production costs of ethanol

Source: BP Analysis, NREL, CERES, NCGA

etha

nol p

rodu

ctio

n co

st ($

/gal

lon)

EU Sugar Beet

Brazilian Sugar Cane

US Corn

US Switch-grass

Ligno-cellulosic Fermentation

Conventional Fermentation

US Corn Stover

Base case

10 year plausible technology stretch

Courtesy Steve Koonin, BP Chief Scientist

Cellulose 40-60% Percent Dry WeightHemicellulose 20-40%

Lignin 10-25%

What grasses are made of

Economic Impact of Potential R&D Advances

Increase hydrolysis yield

Halve cellulase loading

Eliminate pretreatment

Consolidated bioprocessing (CBP)

0% 10% 20% 30% 40% 50%

3%

13%

22%

41%

Processing Cost Reduction

Improved feedstock sugars

Simultaneous C5 & C6 Use

Increased fermentation yield

Increased ethanol titer

Improved sugarsproduct

6%

2%

11%

Increased ethanol titer following CBP 6%

Courtesy Lee Rybeck Lynd

Mono- &oligomers

Glucose, fructose,sucrose

Acetate

Fermentationpathways

H2 & CO2

Fermentationpathways

CellulasesHemicellulases

Termites have many specialized microbes that efficiently digest lignocellulosic material

Improving Nature’s Design

Chlamydomonas reinhardtii

Can the efficiency and/or rate of energy conversion be increased in algae or other micro-organisms?

Can we make algae that thrive at •High density• Continuous process for hydrogen production• Very high CO2 concentrations

What if we built light-concentrating, algae systems that operate at 500,000 ppm CO2 concentration?

Climate change during the last 500 million years.

0.45% CO2

0.00038% CO2

Man first learned to fly by imitating nature

Is it possible to engineer a artificial photo-synthetic system that is powered by either

sunlight or electricity?

H2

O2CO2

How does Nature split water?The OEC Active Site of PSII (Imperial College structure)

PCET

Multi-e

H

MnIVMnIV

O

MnIVMnV

O

OCa

O

OO

Cl

H+

e–

acidic O

basic O

acid + base = neutral

O O=O••••

••••O

••••

2++

2–

Multiple BondAtom Transfer

WaterAssembly

EXAFS High Resolution Structure of the Natural Mn4Ca Cluster

Yachandra et al., Science 314, 821 (2006)

Turnover rate of photo-system II: 300 sec-1

Hydrogenase enzymes: H2 oxidation 1500-9000 sec-1

Highest turnover rate of biomimetic catalysts:Mn dimer, 10 hour-1; Ru dimer, 50 hour-1

Helios: Lawrence Berkeley Laboratory and UC Berkeley’s attack on the energy problem

CellulosePlantsCellulose-degrading

microbesEngineered

photosynthetic microbesand plants

ArtificialPhotosynthesis

ElectricityPV Electrochemistry

MethanolEthanolHydrogenHydrocarbons

Wind, waves, nuclear

Energy Biosciences Institute$50M/ year for 10 years

Univ. California, BerkeleyLawrence Berkeley National LabUniv. Illinois, Urbana-Champaign

Joint Bio-Energy Institute (JBEI)LBNL, Sandia, LLNL, UC Berkeley, Stanford, UC Davis

$25M / year for at least 5 years

“Is Life Based on the Laws of Physics? ”

“…from all we have learnt about the structure of living matter, we must be prepared to find it working in a manner that cannot be reduced to the ordinary laws of physics [not because] there is any ‘new force’, … but because the construction is different from anything we have yet tested in the physical laboratory.

Erwin Schrödinger, 1944

Man-made machines work where friction is minimized.

In an organism, the molecular machinery is imbedded in a viscous

fluid. Friction and thermal fluctuations are huge.

The molecular machinery of life is imbedded in water.

If Newton were the size of a bacteria, his “Laws of Motion” would be different

1) An object in motion will quickly come “rest”.

2) An object with no net motion (at “rest”) will jiggle constantly.

3) F≠ ma!Force ~ surface area x velocity

DNA relaxing after hydrodynamic flow is turned off

Possible configurations of a polymer

Which configuration is more likely to occur?

1) Systems evolve to the lowest state of enthalpic energy, H

2) Systems evolve to a state of highest entropy, S

Gibbs Free Energy = H – TS

= H – kBT log(Ω)

Systems evolve into a state of lowest free energy.

When atoms move slowly, they become big-fuzz balls

λ

d

Einstein’s prediction: When λ > d, all the atoms will condense into the lowest energy quantum state long

before the Boltzmann factor exp(-∆E/kBT) demands it.

Eric Cornell, Carl Weiman, JILA Wolfgang Ketterle, MIT

vx

vy

Quantum mechanics: λ ~ h / p ~ h/(mkBT)1/2

d

As the atom fuzz balls get bigger, the number of accessible states decreases, decreasing the entropy S.

“Is Life Based on the Laws of Physics? ”

“…from all we have learnt about the structure of living matter, we must be prepared to find it working in a manner that cannot be reduced to the ordinary laws of physics [not because] there is any ‘new force’, … but because the construction is different from anything we have yet tested in the physical laboratory.

Erwin Schrödinger, 1944

Hairpin Ribozyme: an example of how nature uses new design rules at the molecular scale

A

B

Variants designed to cut

HIV virusZhuang, Kim, Pereira, Babcock, Walter, Chu, Science 296, 1473-1476 (2002).

Structure of the docked hairpin ribosome

calcium ions

Bond to- be-cut lies between the two gold colored nucleotides

Rupert and Ferre-D’Amare, Nature 410, 780 (2001).

The Free Energy solution to cleaving RNA

Universe = System + (Universe – system)

For a local decrease in entropy (such as the freezing of water to ice), the rest of the Universe gains entropy. The result is a net gain in total entropy. The steady march of entropy to more disorder seems to solve the problem of

the direction of time. Ice in a glass of water melts because it is in the direction of higher entropy.

Evolution to states of higher entropy works equally well when discussing the most likely time evolution

of a system going backwards in time.Why?

(1)The fundamental laws are time reversal invariant, so going forwards and backwards makes no difference based on these laws.

(2) The Second Law of Thermodynamics is due to the properties of large numbers.

If we look backwards in time (and we have no additional information about the system!) then we should expect that the system will also evolve to a state of higher entropy.

Running the movie backward in time, we would see a glass of water spontaneously form ice cubes!

Where did we mess up in the reasoning?

We did not go wrong.

The currently highly ordered state we see the universe in today is a relic of the Big Bang: at the time of the creation of the universe, the entropy was very low and has been increasing ever since.

The formation of galaxies and stars decreases the entropy of the material that makes up the stars, but the formation process and the burning of nuclear energy increases the total entropy of the universe.

The end state may cold dispersed matter and a collection of black holes. A black hole of a given mass turns out to have the maximum amount of entropy of any arrangement of matter.

September melt, 1979

September melt, 2002

The data from different instruments:

• Multi-channel microwave radiometer (Nimbus 7 satellite)

• Microwave imagers attached to the Defense Meteorological Satellite Program’s F8, F11, and F13 satellites.

More recent Arctic melting data

Earth Rise Earthrise from Apollo 8 (December 24, 1968 )

“Human prosperity has been intimately tied to our ability to capture, collect and harness energy. The control of fire and the domestication of plants and animals were two of the essential factors that allowed our ancestors to transition from a harsh, nomadic existence into stable, rooted societies that could generate the collective wealth needed to spawn civilizations…*

* … and support physicists!

Despite these developments, relative wealth in virtually all civilizations was fundamentally defined by access to and control over energy, as measured by the number of animals and humans that served at the beck and call of particular individual.”

“The industrial revolution and all that followed have propelled an increasingly larger fraction of humanity into a dramatically different era …. We go to the local market in automobiles that generate the pull of hundreds of horsesand we fly around the world with the power of a hundred thousand horses…We take for granted that their homes will be warm in the winter, cool in the summer, and lit at night.

The widespread use of energy is a fundamental reason why hundreds of millions of people enjoy a standard of living today that would have been unimaginable a mere century ago.”

Steven Chu and Jose GoldembergCo-chairs Preface to the InterAcademy Council Report

“Lighting the Way: Toward a Sustainable Energy Future”

There is a catch to the our exploitation of energy:

The cost of keeping the equivalent of a billion horses working 24/7, 365 days a year has a modern-day equivalent of keeping the horse stables clean. That modern day equivalent is the control of Carbon Emissions.

“We believe that aggressive support of energy science and technology, coupled with incentives that accelerate the concurrent development and deployment of innovative solutions, can transform the entire landscape of energy demand and supply. This transformation will make it possible, both technically and economically, to elevate the living conditions of most of humanity to the level now enjoyed by a large middle class in developed countries …

What the world does in the coming decade will have enormous consequences that will last for centuries; it is imperative that we begin without further delay.”

“On December 10, 1950, William Faulkner, the Nobel Laureate in Literature, spoke at the Nobel Banquet in Stockholm,

… I believe that man will not merely endure: he will prevail. He is immortal, not because he alone among creatures has an inexhaustible voice, but because he has a soul, a spirit capable of compassion and sacrifice and endurance.’

With these virtues, the world can and will prevail over this great energy challenge.”

Steven Chu (USA)José Goldemberg (Brazil)

Conclusion of the Co-Chairs preface

Cost of electricity generation(1990 dollars/kilowatt hour)

Photovoltaics

Windmills

Gas turbines

2005

2005

Efficiency Metric: Total Power/IT Power

Total Data Center Power/IT Power

1.00

1.50

2.00

2.50

3.00

3.50

1 2 3 4 5 6OSF 20

03 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22OSF 20

05 24CRT

Average of Facilities Measured: 1.8

CRT Goal = 1.2

050

100150

200250300

350400

450500

Hrs/YR

28 35 42 49 56 63 70 77 84 91 98 105 112Temperature (F)

Berkeley Weather

hr/yr0 10 0

11 20 021 30 331 40 50441 50 232551 60 315361 70 154271 80 84681 90 29791 100 75

101 110 15111 120 0

Range F

85%

15%

A → B: Reversible isothermal expansion of the gas at TH (heat addition) causes the piston to do work. The gas expansion is from the absorption of heat from the high temperature reservoir.

B → C: Reversible adiabatic expansion. The piston and cylinder are thermally insulated, so that no heat is gained or lost. The gas continues to expand, doing work on the surroundings and cooling to TC.

C → D: Reversible isothermal compression at TC. Work is done on the gas, causing heat to flow out of the gas to the low temperature reservoir.

D → A: Reversible adiabatic compression of the gas. The piston and cylinder are thermally insulated. Work is done on the gas, compressing it and causing the temperature to rise to TH.

Versions of The LawThere are many statements of the second law which use different terms, but are all equivalent. (Fermi, 1936) Another statement by Clausius is:Heat cannot of itself pass from a colder to a hotter body.An equivalent statement by Lord Kelvin is:A transformation whose only final result is to convert heat, extracted from a source at constant temperature, into work, is impossible.The second law is only applicable to macroscopic systems. The second law is actually a statement about the probable behavior of an isolated system. As larger and larger systems are considered, the probability of the second law being practically true becomes more and more certain. For any system with a mass of more than a few picograms, the second law is true to within a few parts in a million.[1]There are many ways of stating the second law of thermodynamics, but all are equivalent in the sense that each form of the second law logically implies every other form (Fermi, 1936). Thus, the theorems of thermodynamics can be proved using any form of the second law.The formulation of the second law that refers to entropy directly is due to Rudolf Clausius:

In an isolated system, a process can occur only if it increases the total entropy of the system.

Thus, the system can either stay the same, or undergo some physical process that increases entropy. (An exception to this rule is a reversible or "isentropic" process, such as frictionless adiabatic compression.) Processes that decrease total entropy of an isolated system do not occur. If a system is at equilibrium, by definition no spontaneous processes occur, and therefore the system is at maximum entropy.Also due to Clausius is the simplest formulation of the second law, the heat formulation:

Heat cannot spontaneously flow from a material at lower temperature to a material at higher temperature.

The Newcomen design inefficient (expensive to operate). After the cylinder was cooled to create the vacuum, the cylinder walls were cold enough to condense some of the steam as it was sprayed in.

This meant that a considerable amount of fuel was being used just to heat the cylinder back to the point where the steam would start to fill it again. As the heat losses were related to the surfaces, while useful work related to the volume, increases in the size of the engine increased efficiency. Newcomenengines became larger in time. Efficiency did not matter very much within the context of a colliery, where coal was freely available.

Attempts were made to drive machinery by Newcomen engines, but these were unsuccessful, as the single power stroke produced a very jerky motion.

Newcomen's engine was only replaced when James Watt improved it to avoid this problem (Watt had been asked to repair a model of a Newcomen engine by Glasgow University. The model exaggerated the scale problem of the Newcomen engine.).

In the Watt steam engine, condensation took place in a separate container, eliminating the cooling of the main cylinder, and dramatically reduced fuel use. It also enabled the development of a reciprocating engine, with upwards and downwards power strokes more suited to transmitting power to a wheel.

Watt's design, introduced in 1769, did not eliminate Newcomen engines immediately. Watt's vigorous defence of his patents resulted in the desire to avoid royalty payments as far as possible.

The Rankine Cycle

describe the operation of steam heat engines most commonly found in power generation plants. By taking advantage of the phase change of water, the cycle can almost achieve iso-thermal heat addition and rejection. The cycle is sometimes referred to as a practical Carnot cycleas, when an efficient turbine is used, the TS diagram will begin to closely resemble the Carnot cycle.

In gas turbines a significant fraction of the work generated by the turbine will go to driving the compressor and so limits net work output and efficiency. The Rankine cycle on the other hand does not face this problem. By condensing the steam to water, the work required by the pump will only consume approximately 1% of the turbine power. (Liquids are far less compressible they require only a fraction of the energy needed to compress a gas to the same pressure. Work = Force x distance

The efficiency of a Rankine cycle is usually limited by the working fluid. Without the pressure going super critical the temperature range the cycle can operate over is quite small, turbine entry temperature is typically 565°C (the creep limit of stainless steel) and condensor temperatures are around 30°C. This gives a theoretical Carnot efficiency of around 63% compared with an actual efficiency of 42% for a modern coal fired power station. This low turbine entry temperature (compared with a Gas turbine) is why the Rankine cycle is often used as a bottoming cycle in combined cycle gas turbine power stations.The working fluid in a Rankine cycle follows a closed loop and is re-used constantly. Water vapour seen billowing from power plants is evaporating cooling water, not working fluid. (Note that steam is invisible until it comes in contact with cool, saturated air, at which point it condenses and forms the white billowy clouds seen leaving cooling towers).

Advantages of HVDC over AC transmissionThe advantage of HVDC is the ability to transmit large amounts of power over long distances with lower capital costs and with lower losses than AC. Depending on voltage level and construction details, losses are quoted as about 3% per 1000 km. High-voltage direct current transmission allows use of energy sources remote from load centers.In a number of applications HVDC is more effective than AC transmission. Examples include:Undersea cables, where high capacitance causes additional AC losses. (e.g. 250 km Baltic Cable between Sweden and Germany[9]). Endpoint-to-endpoint long-haul bulk power transmission without intermediate 'taps', for example, in remote areas. Increasing the capacity of an existing power grid in situations where additional wires are difficult or expensive to install. Allowing power transmission between unsynchronised AC distribution systems. Reducing the profile of wiring and pylons for a given power transmission capacity. Connecting remote generating plant to the distribution grid, for example Nelson River Bipole. Stabilizing a predominantly AC power-grid, without increasing maximum prospective short circuit current. Reducing line cost since HVDC transmission requires fewer conductors (i.e. 2 conductors; one is positive another is negative) Long undersea cables have a high capacitance. While this has minimal effect for DC transmission, the current required to charge and discharge the capacitance of the cable causes additional I2R power losses when the cable is carrying AC. In addition, AC power is lost to dielectric losses.HVDC can carry more power per conductor, because for a given power rating the constant voltage in a DC line is lower than the peak voltage in an AC line. This voltage determines the insulation thickness and conductor spacing. This allows existing transmission line corridors to be used to carry more power into an area of high power consumption, which can lower costs.[edit] Increased stability of power systemsBecause HVDC allows power transmission between unsynchronised AC distribution systems, it can help increase system stability, by preventing cascading failures from propagating from one part of a wider power transmission grid to another. Changes in load that would cause portions of an AC network to become unsynchronized and separate would not similarly effect a DC link, and the power flow through the DC link would tend to stabilize the AC network. The magnitude and direction of power flow through a DC link can be directly commanded, and changed as needed to support the AC networks at either end of the DC link. This has caused many power system operators to contemplate wider use of HVDC technology for its stability benefits alone.[edit] DisadvantagesThe required static inverters are expensive and have limited overload capacity. At smaller transmission distances the losses in the static inverters may be bigger than in an AC transmission line. The cost of the inverters may not be offset by reductions inline construction cost and lower line loss.In contrast to AC systems, realizing multiterminal systems is complex, as is expanding existing schemes to multiterminalsystems. Controlling power flow in a multiterminal DC system requires good communication between all the terminals; power flow must be actively regulated by the control system instead of by the inherent properties of the transmission line.

War of Currents

Thomas Edison Nikola TeslaGeorge

WestinghouseEdison carried out a campaign to discourage the use of alternating current. He ordered his technicians to use AC electricity to kill animals, primarily stray cats and dogs, but also unwanted cattle and horses for the consumption of press. Edison's series of animal executions peaked with the electrocution of Topsy the Elephant …"Westinghoused".

Harold Brown (who was secretly paid by Edison) constructed the first electric chair for the state of New York in order to promote the idea that alternating current was deadlier than DC.

An example of PWM: the supply voltage (blue) modulated as a series of pulses results in a sine-like flux density waveform (red) in a magnetic circuit of electromagnetic actuator. The smoothness of the resultant waveform can be controlled by the width and number of modulated impulses (per given cycle)

A switch is rapidly switched back and forth to allow current to flow back to the DC source following two alternate paths through one end of the primary winding. The alternation of the direction of current in the primary winding of the transformer produces (AC) in the secondary circuit.

4,400-horsepower locomotive more environmentally conscious? With pure ecomagination.

GE engineers are designing a hybrid diesel-electric locomotive that will capture the energy dissipated during braking and store it in a series of sophisticated batteries. That stored energy can be used by the crew on demand – reducing fuel consumption by as much as 15 percent and emissions by as much as 50 percent compared to most of the freight locomotives in use today. In addition to environmental advantages, a hybrid will operate more efficiently in higher altitudes and up steep inclines.

Shukhov also turned his attention to the development of an efficient and easily constructed structural system for a tower carrying a large gravity load at the top - the problem of the water tower. His solution was inspired by observing the action of a woven basket holding up a heavy weight. Again, it took the form of a doubly-curved surface constructed of a light network of straight iron bars and angle-iron.

Coronas can generate audible and radio-frequency noise, particularly near electric power transmission lines. They also represent a power loss, and their action on atmospheric particulates, along with associated ozone and NOx production, can also be disadvantageous to human health where power lines run through built-up areas. Therefore, power transmission equipment is designed to minimise the formation of corona discharge. Corona discharge is generally undesirable in:Electric power transmission, where it causes:

Power loss Audible noise Electromagnetic interference Purple glow Ozone production Insulation damage

Inside electrical components such as transformers, capacitors, electric motors and generators. Corona progressively damages the insulation inside these devices, leading to premature equipment failure. Situations where high voltages are in use, but ozone production is to be minimised

he thyristor has three p-n junctions (serially named J1, J2, J3 from the anode).

Layer Diagram of ThyristorWhen the anode is at a positive potential VAK with respect to the cathode with no voltage applied at the gate, junctions J1 and J3 are forward biased, while junction J2 is reverse biased. As J2 is reverse biased, no conduction takes place (Off state). Now if VAK is increased beyond the breakdown voltage VBO of the thyristor, avalanche breakdown of J2 takes place and the thyristor starts conducting (On state).If a positive potential VG is applied at the gate terminal with respect to the cathode, the breakdown of the junction J2 occurs at a lower value of VAK. By selecting an appropriate value of VG, the thyristor can be switched into the on It should be noted that once avalanche breakdown has occurred, the thyristor continues to conduct, irrespective of the gate voltage, until either: (a) the potential VG is removed or (b) the current through the device (anode−cathode) is less than the holding current specified by the manufacturer. Hence VG can be a voltage pulse, such as the voltage output from a UJT relaxation oscillator.These gate pulses are characterized in terms of gate trigger voltage (VGT) and gate trigger current (IGT). Gate trigger current varies inversely with gate pulse width in such a way