basic concepts (i) how do you define energy?. energy: definition related to physical forces...
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Basic concepts (I)
How do you define energy?
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Energy: definition related to physical forces
Definition of energy: in physics, energy is the work that a force can or could do.
Forces are:
gravitational (due to interaction between mass and energy concentrations)
electric (attraction and repulsion of charged particles)
magnetic (attraction and repulsion of magnetic objects)
chemical (driving chemical reactions: electro-magnetic)
nuclear (binding nuclei together or breaking unstable apart)
mechanic (impact of one moving object on another)
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Force of Gravity
On earth, we are constantly under the force of gravity. What types of energy does gravity produce?
Acceleration of falling objectsAltitude and depth pressure gradients of the atmosphere and the seasPart of the fusion of the earth’s core
F
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Mechanical Force
Mechanic forces are when one object hits another. What type of energy does this produce?
Acceleration / deceleration of interacting objects
Heat dissipation within the objects
Change of shape of objects v v
v v
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Electric & magnetic forces
Cause electrons to be attracted to nuclei in atoms -> basis for chemistry
Cause charges (electric current) to flow in electric circuits -> basis for energy used in electronics, lights, appliances
Cause needle of compass to point north
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Energy: definition, continued
Energy is can also be inherent in a system, without any forces acting on it.
Types of inherent energy are:
In a steadily moving particle: ½ mass x velocity2
In a mass: mass x (speed of light)2 = mc2
In a body at a certain temperature: (heat capacity of body) x temperaturefor water, heat capacity is, 1 calorie per gram per degree Celsius or Kelvin
In a chemical compound:
2 H2 + O2 -> 2 H2O , Enthalpy released = -571.6 kJ/mol
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Forms of energy
Energy can take many formskinetic (movement of a mass)
electric, magnetic (movement of charges or electromagnetic fields radiating)
ElectricityRadiation (light)
chemical (molecules with internal energy)
heat (thermal) (statistical expression of kinetic energy of many objects in a gas, liquid or solid - or even radiation)
potential (water above a dam, a charge in an electric potential or a battery)
Other examples?
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SI units for energy
The SI unit of energy is a Joule: 1 kg*m2/s2 = 1 Newton*m (Newton is the unit of Force)
mass * velocity 2
mass * g * height (on earth, g = 9.81 m/s2 )
for an ideal gas = cvkBT (cv =3/2 for a monatomic gas)
Power is energy per time: 1 Watt = 1 Joule/s = 1 kg*m2/s3
most commonly used in electricity, but also for vehicles in horsepower (acceleration time)
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Other common energy units
Energy conversion
Unit Quantity to Note
1 calorie = 4.1868000 Joule
1 kiloWatt hour = kWh = 3600000 Joule A power of 1 kW for a duration of 1 hour.
1 British Thermal Unit = btu 1055.06 JouleIt is a is a unit of energy used in North
America.
1 ton oil equivalent = 1 toe 4.19E+010 JouleIt is the rounded-off amount of energy that
would be produced by burning one metric ton of crude oil.
1 ton coal equivalent 2.93E+10 Joule
1 ton oil equivalent = 1 toe 1 / 7.33Barrel of oil
or 1 / 7.1 or 1 / 7.4 ...
1 cubic meter of natural gas 3.70E+07 Joule or roughly 1000 btu/ft3
1000 Watts for one year 3.16E+010 Joule for the 2000 Watt society
1000 Watts for one year 8.77E+006 kWh for the 2000 Watt society
1 horsepower 7.46E+002 Watts
http://www.onlineconversion.com/energy.htm
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Prefixes
Orders of magnitude
Name Quantity Prefix
thousand 1E+03 kilo
million 1E+06 mega
billion 1E+09 giga
trillion 1E+12 tera
quadrillion 1E+15 peta
quintillion 1E+18 exa
sextillion 1E+21 zetta
septillion 1E+24 yotta
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How to do energy conversions(quick reminder)
Given E = 5 kWh, what is value in MJ?
From table, 1 kWh = 3.6 MJ
5 kWh x (3.6 MJ / kWh) = 18 MJ
In other direction: 5 MJ = ? kWh
1 MJ = 0.28 kWh
5 MJ x (0.28 kWh / MJ) = 1.4 kWh
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Basic concepts (II)
How do you use energy?
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What is energy for?
Examples of:
Kinetic
Electro-magneticElectricity
Radiation (light)
Chemical
Potential
Heat (thermal)
?
How do you use energy?
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Practical energy: what is it for?
Energy in daily life: we use it to ...stay alive (food, oxygen: chemical) move faster (transportation fuel: chemical)keep warm (heating fuel: chemical)almost everything else (keep cold, preserve food, light and ventilate spaces, cook, run machines, communicate, measure, store data, compute,...): electricity
In industrial processes: we use it to …Extract (mechanical), refine (chemical), synthesize (chemical), shape (heat, mechanical), assemble (mechanical): produce
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Properties of energy
In any process, energy can be transformed but is always conserved
Fuel + oxygen: heat, light + new compoundsMoving objects collide: heat + work on objectsFalling water+turbine: electricity + heat
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Basic concepts (III)
Energy conversion, conversion efficiency
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Energy conversion
Energy conversion: from one type to another
Examples:Chemical to kinetic
Chemical to electric
Potential to electric
Thermal to electric
Chemical to thermal
Radiation to chemical
Radiation to electric
Radiation to thermal
Electric to thermal
Electric to chemical
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Why is this important? Efficiency
What is efficiency?
Output / Input
Energy out / energy in for an energy conversion process?
Energy out = energy in , so not very useful
Useful energy out / energy in
Physical work / Heat content of fuel
Electricity / physical work
Food / Inputs to agriculture
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Efficiencies (2)
Source: Smil 1999
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Efficiencies (3)
Source: Smil 1999
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More than one conversion process
The total efficiency is the product of all conversion efficiencies:
Etotal = E1 x E2 x E3 x E4 x E5 x E6 x …
Total losses can be (and are) tremendous
Most losses are in the form of radiated heat, heat exhaust
But can also be non-edible biomass or non-work bodily functions (depending on final goal of energy)
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Source: Tester et al 2005
Etotal = E1 x E2 x E3 = 35% x 90% x 5% = 1.6%
ec e r
t
r
t
Chain of conversion efficiencies:Light bulb
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Example 2: diesel irrigation
Losses: t t t,r t,m
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Example 3: Drive power
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Example 4: living and eating
Need 2500 kcal/day = 10 MJ/day or 2kcal/min.
2200 for a woman, not pregnant or lactating, 2800 for a man (FAO). EU: 3200 kcal/day.
Equivalent to 4.75 GJ/year vegetable calories in a vegetarian diet (including 1/3 loss of food between field and stomach)
Equivalent to 26.12 GJ/year vegetable calories in a carnivorous diet (1/2 calories from meat)
Vegetarians are 5.5 times more efficient in terms of vegetable calories.
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Efficiency of human-powered motion
kcal/mile
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EU Energy Label
A, B, C … ratings for many common appliances
Based on EU standard metrics for each appliance
kWh / kg for laundry
% of reference appliance for refrigerators
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Importance of consumer behavior/lifestyle
EU energy label vs. temperature of washing
kWh per cycle/Energy Rating A B C D E F
90°C wash 1.22 1.46 1.59 1.72 1.85 1.98
60°C wash 0.94 1.12 1.23 1.34 1.47 1.6
40°C wash 0.56 0.67 0.74 0.79 0.85 0.91
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USA EnergyGuide label
EnergyStar ratings exist, but are not A,B,C grades
Instead, appliances have EnergyGuide labels (usually without EnergyStar ratings)
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Basic concepts (IV)
Thermodynamics and entropy
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Conservation, but …
Energy is ALWAYS conserved
However, energy is not always useful: dissipated heat is usually not recoverable.
Useful energy is an anthropocentric concept in physics: from study of thermodynamics
Thermodynamics investigates statistical phenomena (many particles, Avogadro’s number = 6×1023): energy conversion involving heat.
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3+1 laws of thermodynamics
0) If systems A and B are in thermal equilibrium with system C, A and B are in thermal equilibrium with each other (definition of temperature).
1) Energy is always conserved.
2) The entropy of an isolated system not at equilibrium will tend to increase over time.
3) As temperature approaches absolute zero, the entropy of a system approaches a constant.
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Paraphrases of 2 laws of thermodynamics
1. You can’t get something from nothing.
2. You can’t get something from something.
1. (economics) There is no such thing as a free lunch.
1.You can't get anything without working for it. The most you can accomplish is to break even.
2.You even can't break even.
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History of thermodynamics (2nd law)
Nicolas Léonard Sadi Carnot (1796-1832)
Theory of heat engines, “reversible”Carnot cycle: 2nd law of thermodynamics
Ludwig Boltzmann (1844-1906)
Kinetic theory of gases (atomic)
Mathematical expression of entropyas a function of probability
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EntropyThe entropy function S is defined as
S = kB log (W)
kB = Bolzmann’s constant = 1.38× 10−23 =Joule/KelvinW=Wahrscheinlichkeit = possible statesIncreases with increasing disorder
For instance:
vapor, water, ice
expanding gas
burning fuel
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2nd law of thermodynamics
system isolated-nonan for 0
system isolatedan for 0
change, a undergoing system aFor
time)ofn (definitio over time increasesentropy 0
tenvironmensystem
system
SS
S
dt
dS
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2nd law of thermodynamics
Total entropy always increases with time.
An isolated system can evolve, but only if its entropy doesn’t decrease.
A subsystem’s entropy can increase or decrease, but the total entropy (including the subsystem’s environment) cannot decrease.
R. Clausius (1865): “Die Energie der Welt ist konstant.Die Entropie der Welt strebt einem
Maximum zu.”
Notion of “heat death of the universe”
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Basic concepts (V)
Applications of thermodynamics: heat engines, Carnot cycle, maximum and real efficiencies.
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Performance of energy conversion machines (Carnot
cycle)Heat engine (cycle)
Heat, cool engine fluidDiesel, internal combustion
Reversible processes:Entropy remains constant Sc = - Sh
Irreversible processesReal worldHeat losses, no perfect insulatorHeat leakagePressure losses, friction
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The Carnot Cycle (the physics)
Ideal cycle between isotherms (T=constant) and adiabats (S = constant).
dE = dW - dQ
where dW = PdVdQ = TdS
Loop integral over dE = 0.
The total work from one cycle of the engine is
The heat taken from the warm reservoir is
The efficiency is : theoretical maximal for heat engine.
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Common types of heat engines
Rankine cycle: stationary power system (power plant for generating electricity from fossil fuels or nuclear fission), efficiency around 30%
Brayton cycle: improvement on Rankine to reduce degradation of materials at high temperature (natural gas and oil power plants), efficiencies of 28%
Combined Rankine-Brayton cycle: for natural gas only, efficiencies of 60%!
Otto cycle: internal combustion engine, electric spark ignition, efficiency around 30%
Diesel cycle: internal combustion engine, compression ignition (more efficient than Otto if compression ratio is higher), efficiency around 30%
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Comparison of heat engines
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Coal power plant
Typical generating capacity: 500 MW250 tonnes of coal per hour
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Other types of power generation
Not based on heat (fossil combustibles or nuclear)
Use various types of energy (guess which?)
Hydraulic power: gravitational energy of water
Wind power: kinetic energy of air
Solar power: radiation from sun
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Wind power
Power = 0.47 x x D2 x v3 Watts
= efficiency ~ 30% (59% theoretical maximum)
D = Diameter (40 meters)
v = wind speed (13 m/s)
P = 500 kW
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Hydroelectricity (hydro)
Uses difference in potential gravitational energy of water above and below dam
E = m x g x h + m x v2 / 2
P = x x g x h x (flow in m3/s)
is the density of water = 1000 kg /m3
Efficiency can be close to 90%
h
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Power plant & fuel cell efficiencies
Source: Miroslav Havranek, 2007
% E
ffici
ency
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Energy, entropy and economy: some history
Austrian Eduard Sacher (1834-1903) Grundzüge einer Mechanik des Gesellschaft : economies try to win energy from nature, correlates stages of cultural progress with energy consumption.
Wilhelm Ostwald (1853-1932) “Vergeute keine Energie, verwerte Sie!” concerns due to rising fuel demands and realization of thermodynamic losses
Frederick Soddy (1877-1956) “how long the natural resources of energy of the globe will hold out”, distinguishes between energy flows in nature and fossil fuels (“spending interest” vs. “spending capital”)
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Basic concepts (VI)
Georgescu-Roegen and entropy applied to the economic system.
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Implications of entropy for economics
Geogescu-Roegen (1906-1994), Romanian economist, wrote The Entropy Law and the Economic Process in 1971.
Points out that economic processes are not circular, but take low entropy (high quality resources) as inputs and produce high entropy emissions (degraded wastes).
Worries about CO2 emissions from fossil fuel burning
Concludes that current entropy production is too high, advocates solar input scale for global economy.
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Georgescu-Roegen (1)“The economic process is nothing but an extension of biological evolution. Therefore the most important problems of the economy must be considered through this lens.”
Econo-my
Environment
Society
Brundtland’s 1987 vision of sustainable development
Economy
Society
Environment
G-R’s vision, taken up by H. Daly and ecological economics
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“(…) our whole economic life feeds on low entropy, to wit, cloth, lumber, china, copper, etc., all of which are highly ordered structures. (…) production represents a deficit in entropy terms: it increases total entropy (…). (…) After the copper sheet has entered into the consumption sector the automatic shuffling takes over the job of gradually spreading its molecules to the four winds. So the popular economic maxim “you cannot get something for nothing” should be replaced by “you cannot get anything but at a far greater cost in low entropy”.”
The entropy law and the economic process, p. 277-279key concepts:
Economic processes feed on low entropy, produce high entropy
Concentrated natural resources are gradually dispersed
Georgescu-Roegen (2)
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“[…] It is not the sun’s finite stock of energy that sets a limit to how long the human species may survive. Instead it is the meager stock of the earth’s resources that constitutes the crucial scarcity. […] First, the population may increase. Second, for the same size of population we may speed up the decumulation of natural resources for satisfying man-made wants, usually extravagant wants. The conclusion is straightforward. If we stampede over details, we can say that every baby born now means one human life less in the future. But also every Cadillac produced at any time means fewer lives in the future. ”
Key concepts: Solar energy will still be available in the future, howeverthe quantity (STOCK) of low entropy natural resources is limitedthus the responsibility to future generations.
The entropy law and the economic process, p. 304
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Global entropy – global population
Meadows (1971): There are limits to economic and physical growth of human societies.
Daly (1973): steady-state economy and population is a goal, but at levels supported by organic agriculture alone: population probably lower than today. Advocate of managed decline in population, economic growth.
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Origin of energy
How do we get energy? Where does it all come from? (not so simple...)
Energy system (primary, final, useful, energy services)
Basic concepts (VII)
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Origin of energy on earthFood? Solar (via photosynthesis)
Oxygen? Solar (via photosynthesis)
Wood for burning? Solar (via photosynthesis)
Fossil fuels? Solar (via photosynthesis and geological processes: geothermal heating, pressure)
Hydraulic or wind? Combination of solar and earth's rotation (Coriolis effect)
Geothermal? Combination of nuclear fission and gravitation.
Nuclear fission? Fossil supernova explosion energy.
How do we compare such different sources?
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Energy chain
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Origin of nuclear energy: supernova
Nuclear fusion, powered by gravity, is the fuel of stars. Fusion is only efficient up to iron creation (nothing heavier).
Some heavy stars burn to iron, then implode under the force of gravity. The shock wave is so strong it creates heavier atoms.
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Comparing energy types
1)Primary energy: energy initially extracted from nature
2)Final energy: transported, transformed, converted, ready to use (electricity, gasoline, bioethanol)
3)Useful energy: used by final consumer (light, heat, motion)
These concepts are mainly applicable to fossil energy systems.
Three main types of primary energy: fossil, solar-based (renewable) and nuclear
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Including biomass
Source: Haberl 2001
Also advocates an approach to energy accounting similar to material flow analysis:energy density of all materials (and wastes) should be included.
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Emergy
H. T. Odum
Embodied (and/or Emergent) Energy
“Emergy is the available energy of one kind previously used up directly and indirectly to make a product or service.”
Solar emergy for ecological systems.
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Exergy
Refers to a process analysis in which the material and energy flows are measured with respect to a “reference state”
Can be done at a large regional or global level, if “reference state” of materials is calculated relative to their earth averages.
Exergy studied and concept promoted by Robert and Leslie Ayres (many references).
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Calorific content: gross & net
Gross calorific value: include heat from exhaust water (C + H both burn with O, creating CO2 + H2O)
Net calorific value: exclude latent heat of water vapor.
Difference:
Gross is 5-6% larger than net for solid + liquid fuels
Gross is 10% larger than net for natural gas.
Worse if fuel is damp (has water trapped inside it)
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Traditional/commercialaccounting
International Energy Agency compiles national statistics (since 1960s for OECD and 1970s non-OECD)
Available online at
http://www.iea.org/Textbase/stats/index.asp
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Ene
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Energy system: services & scale
Lifestyle
Building envelope
Shared heat/cold facilitiesTechnology solutions at different geographic scales:
But where does infrastructure like rail/highway or urban density/diversity belong? Topographyof energy stream.the larger the scale, the bigger the potential savings.
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What is missing?
Source: Tester et al. 2005
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Example: Driving a car 1 km Smart Average Jeep
Useful energydisplacement 0.5 MJ 0.9 MJ 1.3 MJof car by 1 km
Final Energy Gasoline/diesel 1.7 MJ 2.9 MJ 4.5 MJconsumed by car
Primary Energy Extraction, 2.1 MJ 3.6 MJ 5.6 MJtransformation,transportation
(assuming 32 MJ/liter gasoline, 41 MJ/litre diesel, engine 1/3 efficient, 25% losses primary => final)