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Basic concepts (I)

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)

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Basic concepts (I)

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  1. Basic concepts (I) How do you define energy?

  2. 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)

  3. Force of Gravity • On earth, we are constantly under the force of gravity. What types of energy does gravity produce? • Acceleration of falling objects • Altitude and depth pressure gradients of the atmosphere and the seas • Part of the fusion of the earth’s core F

  4. 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

  5. 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

  6. 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

  7. Forms of energy • Energy can take many forms • kinetic (movement of a mass) • electric, magnetic (movement of charges or electromagnetic fields radiating) • Electricity • Radiation (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?

  8. 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)

  9. Other common energy units http://www.onlineconversion.com/energy.htm

  10. Prefixes

  11. 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

  12. Basic concepts (II) How do you use energy?

  13. What is energy for? How do you use energy? Examples of: • Kinetic • Electro-magnetic • Electricity • Radiation (light) • Chemical • Potential • Heat (thermal) ?

  14. 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

  15. Properties of energy • In any process, energy can be transformed but is always conserved • Fuel + oxygen: heat, light + new compounds • Moving objects collide: heat + work on objects • Falling water+turbine: electricity + heat

  16. Basic concepts (III) Energy conversion, conversion efficiency

  17. 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

  18. Why is this important? 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 • What is efficiency?

  19. Efficiencies (2) Source: Smil 1999

  20. Efficiencies (3) Source: Smil 1999

  21. 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)

  22. Chain of conversion efficiencies:Light bulb t r t c e e r Source: Tester et al 2005 Etotal = E1 x E2 x E3 = 35% x 90% x 5% = 1.6%

  23. Example 2: diesel irrigation Losses: t t t,r t,m

  24. Example 3: Drive power

  25. 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.

  26. Efficiency of human-powered motion kcal/mile

  27. 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

  28. Importance of consumer behavior/lifestyle • EU energy label vs. temperature of washing

  29. USA EnergyGuide label • EnergyStar ratings exist, but are not A,B,C grades • Instead, appliances have EnergyGuide labels (usually without EnergyStar ratings)

  30. Basic concepts (IV) Thermodynamics and entropy

  31. 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.

  32. 3+1 laws of thermodynamics • 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). • Energy is always conserved. • The entropy of an isolated system not at equilibrium will tend to increase over time. • As temperature approaches absolute zero, the entropy of a system approaches a constant.

  33. Paraphrases of 2 laws of thermodynamics • You can’t get something from nothing. • You can’t get something from something. • You can't get anything without working for it. The most you can accomplish is to break even. • You even can't break even. (economics) There is no such thing as a free lunch.

  34. 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

  35. Entropy The entropy function S is defined as S = kBlog(W) • kB = Bolzmann’s constant = 1.38× 10−23 =Joule/Kelvin • W=Wahrscheinlichkeit = S possible states • Increases with increasing disorder For instance: • vapor, water, ice • expanding gas • burning fuel

  36. 2nd law of thermodynamics

  37. 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”

  38. Basic concepts (V) Applications of thermodynamics: heat engines, Carnot cycle, maximum and real efficiencies.

  39. Performance of energy conversion machines (Carnot cycle) • Heat engine (cycle) • Heat, cool engine fluid • Diesel, internal combustion • Reversible processes: • Entropy remains constant • DSc = - DSh • Irreversible processes • Real world • Heat losses, no perfect insulator • Heat leakage • Pressure losses, friction

  40. The Carnot Cycle (the physics) Ideal cycle between isotherms (T=constant) and adiabats (S = constant). dE = dW - dQ where dW = PdV dQ = TdS Loop integral over dE = 0. The total work from one cycle of the engine is The heat taken from the warm reservoir is : theoretical maximal for heat engine. The efficiency is

  41. 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%

  42. Comparison of heat engines

  43. Coal power plant Typical generating capacity: 500 MW 250 tonnes of coal per hour

  44. 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

  45. Wind power • Power = 0.47 x h x D2 x v3 Watts • h = efficiency ~ 30% (59% theoretical maximum) • D = Diameter (40 meters) • v = wind speed (13 m/s) • P = 500 kW

  46. Hydroelectricity (hydro) Uses difference in potential gravitational energy of water above and below dam • E = m x g x D h + m x D v2 / 2 • P = h x r x g x D h x (flow in m3/s) • r is the density of water = 1000 kg /m3 • Efficiency h can be close to 90% D h

  47. Power plant & fuel cell efficiencies % Efficiency Source: Miroslav Havranek, 2007

  48. 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”)

  49. Basic concepts (VI) Georgescu-Roegen and entropy applied to the economic system.

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