Basic concepts (I) How do you define energy?. Energy: definition related to physical forces...

Basic concepts (I)

How do you define energy?

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)

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

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

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

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

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?

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)

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

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

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

Basic concepts (II)

How do you use energy?

What is energy for?

Examples of:

Kinetic

Electro-magneticElectricity

Radiation (light)

Chemical

Potential

Heat (thermal)

?

How do you use energy?

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

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

Basic concepts (III)

Energy conversion, conversion efficiency

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

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

Efficiencies (2)

Source: Smil 1999

Efficiencies (3)

Source: Smil 1999

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)

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

Example 2: diesel irrigation

Losses: t t t,r t,m

Example 3: Drive power

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.

Efficiency of human-powered motion

kcal/mile

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

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

USA EnergyGuide label

EnergyStar ratings exist, but are not A,B,C grades

Instead, appliances have EnergyGuide labels (usually without EnergyStar ratings)

Basic concepts (IV)

Thermodynamics and entropy

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.

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.

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.

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

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

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

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”

Basic concepts (V)

Applications of thermodynamics: heat engines, Carnot cycle, maximum and real efficiencies.

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

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.

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%

Comparison of heat engines

Coal power plant

Typical generating capacity: 500 MW250 tonnes of coal per hour

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

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

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

Power plant & fuel cell efficiencies

Source: Miroslav Havranek, 2007

% E

ffici

ency

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”)

Basic concepts (VI)

Georgescu-Roegen and entropy applied to the economic system.

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.

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

“(…) 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)

“[…] 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

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.

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)

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?

Energy chain

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.

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

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.

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.

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).

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)

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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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.

What is missing?

Source: Tester et al. 2005

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)