8.21 The Physics of Energy Fall 2009

Transcription

1 MIT OpenCourseWare The Physics of Energy Fall 009 For information about citing these materials or our Terms of Use, visit:

2 8.1 Lecture the Scales of Energy Use September 11, 009 1

3 Outline The basics: SI units The principal players: energy, power, force, pressure : From the macroworld to our world CO and other greenhouse gases: measurements, units, energy connection Perspectives on energy issues --- common sense and factors

4 SI International System MKSA = Meter, Kilogram, Second, Ampere Units Not cgs or English units! Electromagnetic units Charge Coulombs Current Amperes Electrostatic potential Resistance Ohms Volts Derived units Energy Joules Power Watts Pressure Pascals Force Newtons Thermal units Temperature Kelvin (K) More about these next... 3

5 Outline The principal players: energy, power, force, pressure 4

6 Force, Energy, Power and Pressure [ X ] means The units of X Basics: m kilograms, l meters t seconds, and Q coulombs For example: [ speed ] = l/t = meters/second We can get the units of any physical quantity by using a definition or physical law that relates it to something we already know... Newton s second law Force = mass acceleration [ force ] = [mass][acceleration] = ml/t = kilogram meter/second 1kgm/s =1Newton The force of gravity on you: = kg m/s F gravity = mg =80kg 9.8 m/s = 784 Newtons Kinetic energy Energy = 1 mass velocity-squared [ energy ] = [mass][velocity ]=ml /t = kilogram meter /second 1kgm /s =1Joule = 1 Newton-meter = kg-m /s Your kinetic energy walking at 3 miles per hour: 1 1 Ekinetic = mv = ( ) 80 kg 1609 meters 1 hours 3 miles hour mile 3600 second = 7 Joules 5

7 Power Power = energy per unit time de/dt ( ) [ power ] = [energy][time 1 ]= ml /t (1/t) = kilogram meter /second 3 = kg-m /s 3 1kgm /s 3 =1Watt = 1 Joule/second Power you exert climbing stairs at 0.5 meters per second ΔE P climbing = mgδh =80kg.5 m 9.8 m/s = 390 Joules = ΔE/Δt = 78Joules /1 second = 390 Watts Pressure Pressure = force per unit area df/da ( )( ) [ pressure ] = [force][area 1 ]= ml/t 1/l = kilogram/meter second = kg m 1 s 1kgm 1 s = 1 Pascal = 1 Newton/meter You, standing on the ground: p gravity = Force Area = 784 Newtons 36 in = = 34, 000 Pascals 6

8 Outline 7

9 Forms of Energy A long time to discover energy conservation. Energy disappears? No! Changing form... kinetic to potential to chemical to thermal to Einstein s rest energy... In fact it is conservation of energy, a single number characterizing a system or even each part of a system, that can be traced through time as it flows and changes, that makes energy so important in physics. and Units: length / All these time ) different forms must have the same units of mass Review a little 8.01 & 8.0 and see what s to come 8

10 KINETIC [mass] ENERGY [speed] 1 mv = m mass [speed] ( ) l l = m t t Energy manifest in motion WORK AND POTENTIAL ENERGY Fd force distance [force] [distance] = l l = m t t ml When a force acts on an object over a distance it does work that can show up as kinetic energy or be stored as potential energy 9

11 WORK BY PRESSURE PV pressure volume = force area volume [pressure] [volume] = l m 3 l = m lt t THERMAL ENERGY 1 1 NRT nk BT Thermal energy per degree of freedom at temperature T. Alternatively, N number of moles R the gas constant R = Joules mole K T temperature in Kelvins ( ) Joules [NRT] =moles mole K K = m Kinetic energy of all the molecules makes its appearance as heat l t n number of molecules 3 1 k B Boltzmann s Constant JK T temperature in Kelvins Note that the mole departs from MKS units: It is a gram molecular weight, the molecular weight of a compound expressed in grams. 10

12 ELECTRICAL Q CHARGE V ) THERMAL ENERGY ENERGY Q ELECTROSTATIC POTENTIAL V NRT Convenient, colloquial unit of energy: the BTU Electrostatic potential Electric field Force and the force does work on a charge that is moved. How to measure electrostatic potential in SI Units? nk B T 1 BTU = energy to heat 1 pound of H to 61 O from 60 F Q + ) V What potential do you have to move one Coulomb of charge through to get (or lose) 1 Joule of energy: 1 Joule 1 Joule/sec 1 Watt 1 Volt = = = 1 Coulomb 1 Coulomb/sec 1 Ampere 11

13 TWO FORMS OF ENERGY THAT MAY NOT BE AS FAMILIAR... QUANTUM ENERGY h is Planck s constant E = hν h = Js ν is the frequency of light. Energy of one quantum of light (photon). [energy] = [h] [frequency] ml 1 = [h] t t [h] = [energy] [time] = Js EINSTEIN S REST ENERGY: E = mc Einstein s relativity showed that mass itself is a form of energy with the speed of light as the factor... E = [E] = mc [mc ] = ml t 1

14 Outline Translations and scales: units and energy subcultures 13

15 There s no other quantity that different units than energy. is exajoules, quads, terawatt-years described tonnes of coal, tons of TNT, barrels of oil, therms, kilowatt hours INDUSTRIAL BTU, kilocalories calories, foot-pounds, Joules ergs, electron-volts ENERGY POWER UNITS UNITS in more GLOBAL STRENGTH HUMAN CHILD SIZED SIZED MICROWORLD UNITS UNITS UNITS UNITS UNITS Units of Energy and Power 1 electron volt (ev) 1.60 x J 1 ev per molecule kj mol -1 1 erg 10-7 J 1 foot pound J 1 calorie IT * (cal IT ) J 1 calorie th * (cal th ) J 1 BTU IT * kj 1 kilocalorie IT * (kcal) or Calorie IT * (Cal) kj 1 kilowatt-hour (kwh) 3.6 MJ 1 cubic meter natural gas 36 MJ 1 therm (U.S.) MJ 1 tonne TNT (ttnt) GJ 1 barrel of oil equivalent 5.8x10 6 BTU GJ 1 ton of coal equivalent 7 Gcal IT GJ 1 ton of oil equivalent 10 Gcal IT GJ 1 quad BTU EJ 1 terawatt-year (TWy) EJ 1 watt (W) 1 joule/sec 1 foot pound per second W 1 horsepower (electric) 746 W 1 ton of air conditioning kw definition *th thermochemical four significant figures * IT International Table actual value varies 14

16 ENERGY UNITS, GLOBAL UNITS UNIT EXAJOULE (EJ) QUADRILLION BTU (QUAD) TRILLION KILOWATT-HOURS TERAWATT-YEARS (TWYR) World Total Energy consumption (005) World Oil consumption (006) World Net Electricity consumption (005) U. S. Total Energy consumption (005) U. S. Oil consumption (006) U. S. Total Electricity consumption (005) U. S. Transportation energy consumption SI EQUIVALENT J J J J barrels 15.7 TkWh 101 quads barrels.8 TkWh 8.4 quads 488 EJ 190 EJ 56.5 EJ 106 EJ 4 EJ 10.1 EJ 30.0 EJ 005 HUMAN CONSUMPTION 488 EJ 463 quads 136 TkWh 14.5 TWyr INDUSTRIAL STRENGTH UNITS UNIT SI EQUIVALENT 1 KWH J THERM J MILLION BTU TON TNT BARREL OF OIL TONNEOFCOAL J J J J APPROXIMATE PHYSICAL SIGNIFICANCE 10 5 BTU Raise 1000 lb of H O 100 F Definition. Approx chemical energy in 000 lb of TNT Definition: BTU energy of 159 liters of oil Definition: 7 GCal Energy in 1000 kg of coal means it s a definition means it s given to 4 significant figures means it s a nominal value. Actual value varies, quoted value is often used. From U. S. DOE Energy Information Administration Website Notice how many units are close to 10 9 J not an accident close to one human year of work (see later!) HUMAN SIZED AND CHILD SIZED UNITS MICROWORLD UNITS UNIT SI EQUIVALENT 1 CALORIE J 1 FT-LB J 1BTU 1CALORIE OR KILOCALORIE J J APPROXIMATE PHYSICAL SIGNIFICANCE Energy needed to heat 1gramH O1 C Lift one pound 1 foot against earth s gravity Heat1lbH Ofrom 60 to 61 at one atm UNIT ERG EV SI EQUIVALENT 10 7 J J More on the electron volt and erg PHYSICAL SIGNIFICANCE cgs unit 1 gm-cm/sec 1 dyne-cm Move one electron through one volt Charge of 1 electron = (63) Coulombs 1 (electron charge) 1 Volt = 1 electron-volt =1eV 1eV= (63) Joules Energy of a single quantum (E = hν) of green light is.5 ev Present record computer efficiency is 36 flop/erg which is.78 kj/tflop The Green 500 website lists a Blue-Gene-L supercomputer at Daresbury Lab, GB, as the most efficient computer in

17 ENERGY GLOBAL UNIT E Q T T XAJOULE UNITS, UNITS (EJ) UADRILLION RILLION ERAWATT-YEARS BTU (QUAD) KILOWATT-HOURS (TWYR) SI EQUIVALENT J J J J HUMAN CONSUMPTION 488 EJ 463 quads 136 TkWh 14.5 TWyr World Total Energy consumption (005) World Oil consumption (006) World Net Electricity consumption (005) U. S. Total Energy consumption (005) U. S. Oil consumption (006) U. S. Total Electricity consumption (005) U. S. Transportation energy consumption barrels 15.7 TkWh 101 quads barrels.8 TkWh 8.4 quads 488 EJ 190 EJ 56.5 EJ 106 EJ 4 EJ 10.1 EJ 30.0 EJ means it s a definition means it s given to 4 significant figures means it s a nominal value. Actual value varies, quoted value is often used. From U. S. DOE Energy Information Administration Website 16

18 INDUSTRIAL STRENGTH UNITS UNIT SI EQUIVALENT 1 KWH J THERM MILLION TON TNT BARREL BTU OF OIL TONNEOFCOAL J J J J J APPROXIMATE PHYSICAL SIGNIFICANCE 10 5 BTU Raise 1000 lb of H O 100 F Definition. Approx chemical energy in 000 lb of TNT Definition: BTU energy of 159 liters of oil Definition: 7 GCal Energy in 1000 kg of coal Notice how many units are close to 10 9 J not close to one human year of work (see later!) an accident 17

19 HUMAN SIZED AND CHILD SIZED UNITS UNIT SI EQUIVALENT APPROXIMATE PHYSICAL SIGNIFICANCE 1 CALORIE J Energy needed to heat 1gramH O1 C 1 FT-LB J Lift one pound 1 foot against earth s gravity 1BTU J Heat1lbH O from 60 to 61 at one atm 1CALORIE J OR KILOCALORIE 18

20 MICROWORLD UNITS UNIT ERG SI EQUIVALENT 10 7 J PHYSICAL cgs unit SIGNIFICANCE 1 gm-cm/sec 1 dyne-cm EV J Move one electron through one volt More on the electron volt and erg Charge of 1 electron = (63) Coulombs 1 (electron charge) 1 Volt = 1 electron-volt =1eV 1eV= (63) Joules Energy of a single quantum (E = hν) of green light is.5 ev Present record computer efficiency is 36 flop/erg which is.78 kj/tflop The Green 500 website lists a Blue-Gene-L supercomputer at Daresbury Lab, GB, as the most efficient computer in

21 POWER UNITS UNIT WATT SI EQUIVALENT 1 J/sec (SI Unit) PHYSICAL SIGNIFICANCE One Ampere thru one Ohm resistance (for example) FOOT-LB/SEC W Just what it says! HORSEPOWER 746 W Just what it says! Power back to energy: 1 Watt-second = 1 Joule, or more commonly 1 kilowatt-hour = 1kWh= = Joules = 3.6 MJ 1MJ= 1 MegaJoule 5-10 hard-working human beings working for an hour 0

22 Outline : From the macroworld to our world 1

23 A guide for our tour: The workingman (or woman). Although we use Joules for units (as consistently as possible), it will be helpful to relate to a human scale. 100 Watts is an average value for power production by a manual laborer Watts for8hours aday, 00 daysayear THE HUMAN YEAR = HYR One = 100 Joules second hours 8 day seconds 3600 hour days 00 year = 580 MegaJoules A typical gamma-ray burst perhaps the collapse ofaneutron star to form a black hole Joules hyr Solar energy output ina year 3.8 ( Watts seconds/year =1 Joules J/year)/ (580 MJ/hyr) hyr/year 10 34

24 World annual human energy consumption in EJ = hyr Efforts of > 100 times the earth s population for a year Annual energy losses in U. S. (005) Efforts of ~ 15 times the earth s population 56 quads EJ/quad = 59 EJ for a year 59 EJ = hyr Rest energy of a person 80 kg ( m/sec) = J= hyr EJ Efforts of times the earth s population for a year Solar radiation arriving atmosphere per second at the top of the earth s Efforts of ~ U. S. population for a year J= hyr Annual output of typical nuclear power plant 1GW sec/year = EJ = hyr J Efforts of times California s population for a year 3

25 U. S. per capita yearly energy consumption in EJ / people = 350 GJ/person 350 GJ/person = 600 hyr/person Compare to world: 130 /person Per day? 1 GJ/(person-day) hyr/(person-day) Average yearly Northeast U. S. household energy consumption BTU Joule year BTU = 191 GJ/year 191 GJ/year = 39 hyr/household Energy_use_in_the_United_States 4

26 Energy required per person BOS-LAX via Boeing gallon/mile 600 miles = 6 gallons 14 MJ/gallon of Jetfuel 6 gallons = GJ 6.36 hyr GJ One tank of gasoline 15 gallons 3.79 liters/gallon = 56.9 liters 56.9 l 34.8 MJ/l = 1.98 GJ = 3.4 hyr 5

27 Recommended U. S. dietary energy intake,000,500 kcal/person-day 000 kcal/day J/kCal 365 days/year = 3.1 GJ/ (person-year) Result: One year s energy intake per person, GJ Yearly energy input per person amounts hyr to Or 10 MJ/(person-day) Energy input to produce average (where I could find data) diets Swedish Complex definitions of system boundaries Include: crop production, processing, storage, transportation, storage, preparation, cooking... Exclude: Capital cost of equipment, packaging, waste, human labor... Result: GJ/person-year 1 36 hyr/person The system boundaries in the study included farm production with production of farm inputs, drying of crops, processing, storage and transportation up to the retailer. They also included storage, preparation and cooking in households. The system boundaries excluded production of capital goods such as machinery and buildings, packaging material, waste treatment, transportation from the retailer to the consumer and dishwashing. The economic value of products and by-products was the basis for allocation of energy use during processes with multiple outputs. The energy use was calculated as process energy with no inclusion of production and delivery energy, and transmission losses. Only commercial energy inputs were considered, e.g., inputs derived from electricity, fuel oil, coal or gasoline. Energy inputs from the sun or from human labour were not considered 6

28 Outline CO and other greenhouse gases: measurements, units, energy connection 7

29 Measurement of CO (and other greenhouse gases) In later lectures we will have much to say about and relation to energy use and to CO. Here... global climate change, its origins Properties of CO. Important measures of CO. Other greenhouse gases. Properties of CO Atomic weight =44. Compared to air 78% N and 1% O So CO has density 44/8.6 = 1.54 density of air. Sublimes (directly from solid to gas) at -70 C at atmospheric pressure. No liquid state at pressures below 5 atm. At higher pressures CO Atomic weight melts at -57 C. 8

30 Important measures of CO In the atmosphere in parts per million (ppm) Transfer and production through human activity and in earth s carbon cycle usually in millions of metric tonnes (MMT) Emissions in energy processes: kilograms CO per Joule of energy produced (kg/j). (Usually energy produced at the source, not including losses and other inefficiencies. ) CO in the atmosphere Pre-industrial concentration: 80 ± 10 ppm. Present concentration: 383 ppm. Increase due to human activity For example: Find quoted CO emission per kwh of coal produced electricity. Does not include efficiency of power plant ( 35%) or transmission losses, or losses in application. 9

31 CO produced by human activity (millions of metric tonnes!) Some U. S. data Total CO in the atmosphere:,996,000 MMT World CO emissions from fossil fuel (007): 8,190 MMT Emissions as a fraction of total: 0.94% Leading emitters from fossil fuel (007): Region CO (MMT) United States 5,957 China 5,3 Europe 4,675 CO emission in U. S. by sector (006): Sector CO (MMT) Residential 1,04 Commercial 1,045 Industrial 1,650 Transportation 1,990 Electric Power,344 CO emissions per person in U. S. per year: tonnes persons =0, 000 kg/person Staggering! 30

32 CO emission in energy processes Different fossil fuels have different carbon and energy content. Data typically given in standard unit of kilograms of carbon per million BTU of energy produced at the source. We ll convert to kilograms of CO pergjofenergy. Fuel CO emission factor kg(c)/million (BTU) kg(co )/GJ Coal 6 90 Jet fuel, kerosene, gasoline Liquified petroleum gas Natural gas

33 Other greenhouse gases: Methane (CH 4 ), nitrous oxide (N O), and a variety of chloro- and fluorocarbons (collectively halocarbons) contribute significantly to the greenhouse effect. Current Atmospheric Increased Gas concentration GWP lifetime (years) radiative forcing (W/m ) Carbon dioxide 383 ppm 1 Variable (5 00) 1.66 Methane 1800 ppb Nitrous oxide 319 ppb Halocarbons 1 ppb , GWP is a measure of global warming potential (per unit mass) with CO defined as 1. Increased radiative forcing is the increase in the rate that energy is made available at the earth s surface due to this greenhouse gas, measured in W/m and can be compared with the solar energy input. Source: Carbon Dioxide Information Analysis Center, cdiac@ornl.gov. Water, while an important greenhouse gas, is omitted from the table. Atmospheric lifetime of CO is hard to estimate because so many different processes affect it. 3

34 Outline Perspectives on energy issues --- common sense and factors 33

35 1. The energy content of matter is enormous Matter equivalent of annual human energy budget: J c M = Mc = m/sec = / = kg not even heavy lifting! What does one gram buy? kg (3 10 m/sec) 13 =9 10 J enough to run a.86 megawatt powerplant for a year. 34

36 . The solar energy incident on the earth is enormous Sun s energy incident at the top of the earth s atmosphere varies between 131 and 141 watts/m throughout the year. Average of 1366 watts/m. Solar power incident on the earth 1366 W/m πr W =0.174 EJ/sec so in 488/(.174) seconds humanity in a year. = 47 minutes the sun delivers all the energy used by 35

37 3. Estimate what area of solar would provide U. S. annual energy budget at 100% efficiency? Find data: National Renewable Energy Laboratory has solar radiation atlas. ( Convert to SI units: Units are kwh/m /day, 1 day = 4 hours, 1 kwh/day =1kWh 4h/day AVERAGE POWER 4 W So insolation is 4 W/m times the factor, needed for E = J/year I, in the table. Compute area Take I =6.5 (looks typical for American southwest), J/year 7 ( sec/year) (6.5 4 W/m ) 4 =1. 10 km 36

38 Collector area 4 =1. 10 km Looks pretty promising. It s the reason why we believe solar energy is the best long-term energy source. Warning, big multiplication factor from inefficiencies (stay tuned). 37

39 4. What s the ratio of typical nuclear energy source? carbon based energy source to typical Atomic energy scales are ev energy necessary to ionize hydrogen is 13.6 ev. Nuclear energy scales are MeV energy necessary typical nucleus is 8 MeV. to remove a proton from a Energy of Energy of reaction for reaction for H + 1 O H O is 3 ev/reaction. fission of uranium is 00 MeV/reaction. Nuclear fuels have roughly 1 million times the energy density of carbon fuels. 38