What Is Energy?

What is energy? INTRODUCTION Dictionary definitions of energy 1 often list the personal meaning (“she has a lot of energy” as the first meaning of the work. The scientific definition, such as “usable heat or power”, “a source of usable power, such as petroleum or coal”, or “(Physics) the capacity of a physical system to do work” are subsidiary definitions. It is difficult to find a good definition of energy. I remember what Justice Potter Stewart once said about the problems in defining pornography in a Supreme Court opinion: “I don’t know how to define it, but I know what it is when I see it going on.” That statement could serve as a definition of energy. It is hard to capture in an exact definition, but all of us have a general idea of what the topic means in the concepts used in this seminar. Before the 1973 oil embargo, there was little public discussion of energy, energy cost, energy resources, energy conservation, or technology for improving the efficiency of energy use. The sharp rise in oil prices caused by the embargo and the potential impact on national lifestyles and national security has made energy a significant part of national policy for nearly thirty years. The importance of energy in the spectrum of issues facing the country seems to vary with the price of gasoline or electricity. However, there is an ongoing need for engineers who have good understanding of the technical issues about energy supply and the implication that these technical issues have on national policy. Engineers and scientists first encounter the topic of energy in physics courses where the sum of the kinetic plus potential energy is defined as the negative of the work that is done in a “conservative” process (i.e., one without friction). This tells us that the dimensions of energy must be the same as those of work, name force times distance. Since force has the dimensions of mass times acceleration or mass times length divided by time squared, the dimensions of energy are mass times (length/time)-squared, written symbolically as MLT-2. Power, which is the rate of energy change, has the dimensions of MLT-3. In the SI system of units the units of energy are joules, the amount of energy required to life a mass of one kilogram a distance of one meter. The power rate of one joule per second is called a watt. A joule is not very much energy. Electricity use is billed in units of kilowatt-hours; this is 3,600,000 joules (J) or 3.6 megajoules (MJ). A May 2002 electricity bill from the Los Angeles Department of Water and Power charged about 10.5 cents per kilowatt-hour for electricity delivered to a home in the San Fernando Valley. This is about $3x10-8/J. The definition of energy as the amount of work done is expanded in thermodynamics to include processes with friction. In thermodynamics the internal energy, a property of the system, is defined such that the total work for any process change is the negative of the change in the energy sum. The energy sum is the sum of the kinetic energy, the potential energy, and the thermodynamic internal energy. Thermodynamics defines a new form of energy transfer, heat, which is defined as energy in transit due only to a temperature gradient. What is energy Page 2 Prior to the development of thermodynamics, there was no recognition that heat and work were both forms of energy, which means that they should have the same units. A separate set of units were developed for heat, the use of which persists today. These units, the calorie and the British thermal unit or Btu, are defined as the amount of heat required to raise a unit mass of water one unit of temperature. The exact definition of these energy units is considered further below. Thermodynamics, the science of energy and its transformations, provides two broad laws for energy processes. The first law of thermodynamics states that energy is conserved. The second law states that some energy (work) is better than other forms (heat) because heat cannot be completely converted to work in a cycle. Thus, work is a more valuable form of energy than heat. A practical example of the value of different forms of energy can be found by the comparison of their costs. The electrical energy charge of $3x10-8/J listed above is an example of the cost of energy in the form of heat. In contrast, one can examine the utility charge for fuel as a cost of heat. An example of a heat energy charge can be found from a bill by The Gas Company that charges its customers for each “therm” they use. A therm is defined as 100,000 Btu, which is approximately the amount of energy in 100 cubic feet of natural gas. (The conversion factor between Btu and joules is 1 Btu = 1,055.056 joules.) A June 2002 gas bill for a home in the San Fernando Valley charged $0.535 per therm or $5.35 per million Btu. This is a cost of $5x10-9/J or about 1/6th the cost of electricity. ENERGY UNITS As we have seen above, the Joule, which is the basic unit for energy in the SI system of units as a very small amount of energy compared to typical energy use rates. Typically one uses units of kJ, MJ, GJ, etc. to express practical amounts of energy. In addition to the use of Joules for representing energy there are two systems of units – calories and British Thermal Units or Btus that data back to the early 19th century before the realization that heat and work were actually two different kinds of energy and could be measured by the same units. Both the Btu and the calorie were defied as the amount of heat required to raise a unit mass of water one unit of temperature. Thus, the BTU and the calorie were defined in such a way that the heat capacity of water would be 1 cal/gm-K or 1 Btu/lbm-R. Since the heat capacity of water changes with temperature, we have to specify the temperature at which the calorie or Btu is defined. Depending on the temperature selected, one can obtain different definitions of the Btu or calorie. Engineers typically use a reference temperature of 15 C or 59 F for the definition of the Btu and calorie. This is tall the International Steam Table Calorie or IT calorie for short. One IT calorie is equal to approximately 4.1868 J. Another definition of the calorie, called the thermochemical calorie, is set be defining the calorie to be exactly 4.184 J, which is the heat capacity of water at about 17 C. The Btu is defined at a temperature of 15 C. Because the calorie and the Btu have been defined so that the heat capacity of water will be 1 Btu/lbm-R or 1 cal/gm-K. These two unit combinations are equal. This allows us to determine conversion factor between IT calories and Btu as follows: 1 Btu 1 ( IT )cal 1 K 453.59237 g = lbm − R g − K 1.8 R lb m From the two definitions of calories, we have 1 J = 1 IT calorie 4.1868 ⇒ 1 Btu = 251.995 ( IT )cal 1 thermochemical calorie , so that 1 thermochemical calorie= 4.184 4.1868 4.184 = IT calorie, so that 1 BTU = 251.8272 cal. The approximate conversion factor that 1 BTU = 252 calories is correct (to three significant figures) for either definition of the calorie. Using the conversion factor that a BTU = What is energy Page 3 251.995 (IT) calories and one (IT) calorie = 4.1868 joules, gives the conversion factor that 1 Btu = 1,055.056 J. The nutritional calorie used to measure the energy content of foods is actually 1000 (IT) calories. This is properly called a kilocalorie. Sometimes it is written as Calorie, with a capital C, to emphasize the difference between the original definition of the calorie and its use in nutritional applications. The usual metric prefixes, kilo-, mega-, etc. are not typically used with Btu. In engineering applications the abbreviation M is often used to indicate a factor of 1000. Thus, the abbreviation MBtu would indicate one thousand Btu and the abbreviation MMBtu would indicate one million Btu. The unit of one million Btu is a common energy unit in engineering analysis and the cost of fuel at current prices ranges between $2 and $10 per million Btu. Discussions of national and world energy use are often done in terms of quadrillion Btu, called quads for short, are used. World energy consumption in 1999 was 382 quads. A comparable unit is the exajoule; one exajoule is 1018 Btu. Because one Btu is about 1,055 J, the exajoule and the quad are approximately the same amount of energy. (Of course, the exact factor is that one quad is 1.055056 exajoules.) Large-scale electrical energy use is typically reported in units like terawatt-hours or terawatt-years (TWy). (One terawatt (TW) is 1012 watts; since there are 8,760 hours in the 365-day year, one terawatt year is 8.76x1012 kWh.) The annual world energy consumption is approximately 400 quads or 400 exajoules. The world consumption of electricity is about 1.5 TWy. Another way of reporting large scale energy use is to equate it to a certain amount of fuel. For example the International Energy Agency reports all energy use statistics in million metric tons of oil equivalent. We will consider this measure further after first considering the energy content of fuels. ENERGY CONTENT OF FUELS In calculating costs of heat energy above, we said that The Gas Company charges for the heat energy in the gas it sells. This is the combustion energy, which is a thermodynamic internal energy due to chemical change. Heat content of fuels is usually expressed as the enthalpy change when a mass of fuel reacts form ultimate combustion products at the same temperature as the fuel and oxidizer. The amount of oxidizer in the reaction used for calculating the heat content is called the stoichiometric amount; this is exactly the amount required for the complete combustion of the fuel into the ultimate combustion products. For example, methane, which is the principal component of natural gas, has a stoichiometric complete combustion reaction given by the following chemical balance equation. CH 4 + 2 O2 → CO2 + 2 H 2 O [1] The difference in enthalpy between the products – CO2 + 2 H2O (as liquid water) –and the reactants (CH4 + 2O2), at 25oC and atmospheric pressure is -55,496 kJ/kg. This enthalpy change for the reaction is negative, indicating that heat is released, as expected. The value of the enthalpy change for combustion is equal to -23,859 Btu/lbm. In the thermodynamic sign convention for heat, a negative heat transfer means that head is transferred from the system. Thus the combustion equation shown in [1] has a heat release of 23,859 Btu per pound of methane when the product is liquid water. The calculation when the water in the combustion products is a liquid is called the higher or gross heating value. If the water in the products is in the form of water vapor, a smaller amount of What is energy Page 4 energy is released in the reaction. The resulting energy release is called the lower or net heating value. The difference between the gross and net heating value is simply the amount of energy released when the water vapor in the combustion products is condensed. The reaction in equation [1] produces two moles (2*18.01528 lbm) of water for each mole (16.04246 lbm) of methane. Since the latent heat of water at 25 C is 2,442.79 kJ/kg, the difference between the gross and net heating value is found as follows: 2442.79 kJ 2(18.01528) kg H 2O 5486.4 kJ = kg H 2O 16.04246 kg CH 4 kg CH 4 Subtracting this energy from the gross heating value of 55,496 kJ/kg gives the net heating value as 50,010 kJ/kg or 21,500 Btu/lbm. As noted in the discussion of energy costs above, the therm, which is the unit used for natural gas sales, is approximately the energy content of 100 ft3 of natural gas. (The increment of 100 cubic feet was the traditional billing unit for natural gas delivered to residential and commercial users prior to the 1970s.) We can show this by using the density of methane, measured at a standard temperature of 60oF, which is 0.042269 lbm/ft3. Multiplying 23,859 BTU/lbm by this density shows that the chemical energy content of one cubic foot of methane (at 60oF and atmospheric pressure) is 1,008 BTU/ft3. Actual natural gas is slightly higher than this due to amounts of higher molecular weight hydrocarbons that have a greater heating value per unit volume. We see that this equivalency is based on the use of the higher heating value. Although we can compute the energy content of pure compounds exactly, the energy content of practical fuels such as oil, natural gas, and coal varies with the particular source of the fuel. Nevertheless, one often sees energy represented as an equivalent amount of fuel. This is based on some assumed average energy content of the fuel. For example, the International Energy Agency (IEA) uses “metric tons (tonnes) of oil equivalent” as a unit for reporting energy statistics. This IEA defines this as 107 (IT) kilocalories of gross (higher) heating value. This is 41.868 GJ or 39.6853 million Btu. Since one metric ton (or tonne) is 1000 kg, the implied lower heating value of the “equivalent” oil is 18,001 Btu/lbm. Energy units are often listed on a per-unit-volume basis as well. This requires the knowledge of the fuel density. Typical hydrocarbon liquid fuels have a density of about 7 pounds per gallon (U. S.). Thus a crude oil with a lower heating value of 18,000 Btu/lbm would have 126,000 Btu/gallon. In the petroleum industry, one barrel contains 42 gallons. Thus, the energy in a barrel of this fuel would be 5.3 million Btu. The table below shows some date on the energy content of fuels; these data are based on the higher heating value. As a rule to thumb, the lower heating value is about 93% of the higher heating value of typical liquid hydrocarbon fuels. Energy Information Administration Data on “Heat Rates” Coal 1 Production: million Btu per short ton 21.070 Consumption: million Btu per short ton 20.753 Coke Plants: million Btu per short ton 27.426 Industrial: million Btu per short ton 22.489 Residential and Commercial …... million Btu per short ton 23.880 Electric Utilities: million Btu per short ton 20.401 Imports: million Btu per short ton 25.000 Exports: million Btu per short ton 26.081 Coal Coke: million Btu per short ton 24.800 Crude Oil Production: million Btu per barrel 5.800 Imports: million Btu per barrel 5.948 Consumption: Btu per cubic foot 1,027 Petroleum Products Consumption 2: million Btu per barrel 5.336 Motor Gasoline 2: ... million Btu per barrel 5.204 Jet Fuel: million Btu per barrel 5.670 Distillate Fuel Oil: ... million Btu per barrel 5.825 Residual Fuel Oil: .. million Btu per barrel 6.287 Liquefied Petroleum Gas 2: million Btu per barrel 3.603 Kerosene: million Btu per barrel 5.670 Petrochemical Feedstocks 2 million Btu per barrel 5.545 Unfinished Oils: million Btu per barrel 5.825 Imports 2: million Btu per barrel 5.326 Exports 2: million Btu per barrel 5.749 Natural Gas Plant Liquids What is energy Page 5 Non-electric Utilities: Btu per cubic foot 1,028 Electric Utilities: Btu per cubic foot 1,019 Imports: Btu per cubic foot 1,022 Exports: Btu per cubic foot 1,006 Electricity Consumption: . Btu per kilowatt-hour 3,412 Production 2: million Btu per barrel 3.887 Natural Gas Production, Dry: ..... Btu per cubic foot 1,027 Consumption: Btu per cubic foot 1,027 Non-electric Utilities: Btu per cubic foot 1,028 Electric Utilities: Btu per cubic foot 1,019 Imports: Btu per cubic foot 1,022 Exports: Btu per cubic foot 1,006 Btu = British thermal unit. 1 Coal conversion factors vary from year to year. Values correspond to those published by EIA for 1999 and may differ slightly from model results. 2 Conversion factors vary from year to year. 2010 values are reported. Sources: Energy Information Administration (EIA), Annual Energy Review 2000, DOE/EIA-0384(2000) (Washington, DC, August 2001), and EIA, AEO2002 National Energy Modeling System run AEO2002.D102001B. SOME ENERGY MEASURES American Petroleum Institute Web Data Energy content or energy use Item Btu Calories A match 1.00E+00 2.52E+02 An apple 4.00E+02 1.01E+05 Making a cup of coffee 5.00E+02 1.26E+05 Stick of dynamite 2.00E+03 5.04E+05 Loaf of bread 5.10E+03 1.29E+06 Pound of wood 6.00E+03 1.51E+06 Running a TV for 100 hours 2.80E+04 7.06E+06 Gallon of gasoline 1.25E+05 3.15E+07 20 days cooking on gas stove 1.00E+06 2.52E+08 Food for one person for a year 3.50E+06 8.82E+08 Apollo 17’s trip to the moon 5.60E+09 1.41E+12 Hiroshima atomic bomb 8.00E+10 2.02E+13 1,000 transatlantic jet flights 2.50E+11 6.30E+13 United States in 1999 9.70E+16 2.44E+19 World in 1999 3.82E+17 9.63E+19 ENERGY RESERVES AND RESOURCES Look at what we know versus what we speculate may be there and consider if the particular commodity can be recovered economically. Known Unknown Economical to Recover Not Economic to Recover Reserves Resources Resources Resources What is energy Page 6 WHAT ARE THE CURRENT AMOUNT OF RESERVES The following data on oil and gas reserves are taken from the Energy Information Agency web site: http://www.eia.doe.gov/emeu/iea/table81.html. Region Country Russia Saudi Arabia Iran Iraq United Arab Emirates Kuwait Venezuela Qatar United States Nigeria Algeria Libya China Mexico Indonesia Other Malaysia Norway Canada Oman Egypt Australia Netherlands Brazil United Kingdom Turkmenistan Kazakhstan Angola Argentina India Yemen Uzbekistan Trinidad and Tobago Pakistan World Total Crude Oil (Billion Barrels) Oil and Gas Journal 48.6 261.7 89.7 112.5 97.8 World Oil 54.3 265.3 96.4 115 62.8 Natural Gas (Trillion Cubic Feet) Oil and World Gas Oil Journal 1,700.0 1,695.0 213.8 214 812.3 929.1 109.8 112.6 212.1 204.1 Region Energy estimate (quads) quads EEF ME ME ME ME 2000.9 1767.7 1419.4 782.0 681.6 96.5 76.9 13.2 22 22.5 9.2 29.5 24 28.3 5 0.3 3.9 9.4 4.7 5.5 2.9 2.9 0.1 8.1 5 98.8 47.6 5.6 22 24.1 12.7 30 30.6 26.9 9.7 10.5 5.1 10.1 5.6 5.8 3.6 2.8 0.1 8.5 4.7 52.7 146.8 393.8 177.4 124 159.7 46.4 48.3 30.4 72.3 0.8 81.7 44 61 29.3 35.2 44.6 62.5 8.2 26.8 56.6 147.6 400 177.4 125 155.6 46.4 42 41.4 146.9 237.1 81.7 41.8 62.2 30.3 50.6 44 58.4 7.8 26 ME SA ME NA A A A A&O NA A&O EEF A&O WE NA ME A A&O WE SA WE 630.5 514.3 452.3 307.1 261.9 222.2 221.8 206.1 198.7 152.9 150.8 108.2 100.4 92.0 63.1 62.1 61.1 61.0 56.9 55.0 0.5 5.4 5.4 3.1 4.7 4 0.6 0.7 0 0 9 3 3.3 2.1 0 0.7 101 65 1.6 26.4 22.8 16.9 66.2 21.4 0 0 4 27.4 15.9 17 0 23.5 EEF EEF A SA A&O ME EEF SA 52.0 48.4 45.3 44.9 42.9 34.9 34.9 26.6 0.2 1028.3 0.3 1004 21.6 5288.2 25.1 5450.2 A&O 24.8 11361.23 What is energy Page 7 The following data on coal reserves are taken from the Energy Information Agency web site: http://www.eia.doe.gov/emeu/iea/table82.html. World Coal Reserves (million short tons) Recoverable Anthracite and Bituminous 126,804 54,110 68,564 90,826 46,903 25,353 54,586 17,939 34,172 22,377 71 0 6,908 3,826 Recoverable Lignite and Subbituminous 146,852 118,964 57,651 2,205 43,585 47,399 0 19,708 3,307 2,050 17,849 13,149 420 3,425 Total Recoverable Coal 273,656 173,074 126,215 93,031 90,489 72,753 54,586 37,647 37,479 24,427 17,919 13,149 7,328 7,251 Area NA EEF A&O A&O A&O WE A EEF EEF EEF WE SA SA NA Estimated Energy (quads) 5,383 3,170 2,573 2,259 1,825 1,356 1,337 745 888 580 278 204 176 147 Czech Republic Indonesia Botswana Uzbekistan Turkey Greece Bulgaria Pakistan Iran 2,330 871 4,740 1,102 306 0 14 0 1,885 3,929 5,049 0 3,307 3,760 3,168 2,974 2,497 0 6,259 5,919 4,740 4,409 4,066 3,168 2,988 2,497 1,885 EEF A&O A EEF WE WE EEF A&O ME 118 100 116 78 66 49 46 39 46 United Kingdom Romania Thailand Mexico Chile Hungary Peru Kyrgyzstan Japan World Total 1,102 1 0 948 34 0 1,058 0 852 571,218 551 1,605 1,398 387 1,268 1,209 110 895 0 512,033 1,653 1,606 1,398 1,335 1,302 1,209 1,168 895 852 1,083,254 WE EEF A&O NA SA EEF SA EEF A&O 36 25 22 29 20 19 28 14 21 21,931 Region/Country United States Russia China India Australia Germany South Africa Ukraine Kazakhstan Poland Yugoslavia Brazil Colombia Canada What is energy Page 8 ENERGY USE DATA Period 1990 1991 1992 1993 1994 1995 1996 1997 1998 1999 2000 January February March April May June July August September October November December Total 2001 January February March April May Total Year to Date 2001 2000 1999 Amount of U. S. electricity generated by various energy sources (TWh) Coal Oil Gas 1,559,606 117,017 264,089 1,551,167 111,463 264,172 1,575,895 88,916 263,872 1,639,151 99,539 258,915 1,635,493 91,039 291,115 1,652,914 60,844 307,306 1,737,453 67,346 262,730 1,787,806 77,753 283,625 1,807,480 110,158 309,222 1,767,679 86,929 296,381 Nuclear 576,862 612,565 618,776 610,291 640,440 673,402 674,729 628,644 673,702 725,036 Hydroelectric 279,926 275,519 239,559 265,063 243,693 293,653 327,970 337,234 304,403 293,932 Geothermal 8,581 8,087 8,104 7,571 6,941 4,745 5,234 5,469 5,176 1,698 Other 2,070 2,050 2,096 1,994 1,992 1,664 1,980 1,993 2,030 2,018 Total 2,808,151 2,825,023 2,797,219 2,882,525 2,910,712 2,994,529 3,077,442 3,122,523 3,212,171 3,173,674 Coal 55.54% 54.91% 56.34% 56.87% 56.19% 55.20% 56.46% 57.26% 56.27% 153,871 137,477 135,329 122,437 134,171 145,722 150,690 156,643 139,802 137,211 134,200 149,065 1,696,619 4,771 3,184 2,974 3,110 5,743 7,395 7,004 8,689 7,488 5,758 4,914 11,150 72,180 18,152 16,166 20,186 20,937 29,146 29,226 35,077 38,381 27,366 20,693 17,332 18,054 290,715 66,214 60,053 58,704 54,514 59,864 62,973 64,538 62,905 54,521 49,097 52,841 59,209 705,433 22,811 20,253 23,997 25,830 24,755 22,636 21,920 19,875 15,783 15,434 17,288 17,613 248,195 14 13 13 13 13 13 13 13 11 12 12 13 151 158 177 194 191 198 164 180 176 165 185 177 125 2,090 265,991 237,324 241,397 227,031 253,890 268,128 279,421 286,682 245,137 228,389 226,765 255,229 3,015,383 57.85% 57.93% 56.06% 53.93% 52.85% 54.35% 53.93% 54.64% 57.03% 60.08% 59.18% 58.40% 56.27% 146,431 123,805 129,514 117,933 128,666 646,350 11,271 6,101 6,836 6,879 7,062 38,150 15,549 13,501 16,658 20,565 22,761 89,035 48,823 43,500 43,428 38,992 43,285 218,028 16,685 15,630 18,128 15,401 17,059 82,903 14 12 14 13 * 52 194 166 195 188 188 931 238,967 202,716 214,773 199,971 219,021 1,075,449 61.28% 61.07% 60.30% 58.98% 58.75% 60.10% 646,350 683,286 702,834 38,150 19,782 40,200 89,035 104,587 101,931 218,028 299,348 285,336 82,903 117,646 135,210 52 65 1,606 931 918 850 1,075,449 1,225,633 1,267,968 60.10% 55.75% 55.43% What is energy Page 9 Annual Energy Use in the US 120 Energy Use (quads) 100 80 60 40 20 0 1940 1950 1960 1970 Year 1980 1990 2000 What is energy Page 10 What is energy Page 11 What is energy Page 12 IEA Selected Energy Statistics for 1999 Region Population GDP Energy Net TPES Elec. CO2*** (PPP)* Prod. Imports Cons.** Emissions (billion (billion (Mt (million) 90 US$) 90 US$) (Mtoe) (Mtoe) (Mtoe) (TWh) of CO2) World (a) OECD Middle East Former USSR Non-OECD Europe China Asia (b) Latin America (c) Africa 5921.39 32445.29 39841.35 9722.55 1116.41 26446.42 23626.26 3790.28 1463.68 162.17 551.48 921.07 1218.94 -877.25 290.49 471.11 1490.39 1213.07 -301.34 58.18 GDP 120.56 323.74 57.22 34.86 1260.32 1112.84 4501.08 1057.01 42.27 1849.63 1621.73 4965.64 933.61 113.98 408.97 1551.85 2528.79 595.40 -146.23 775.23 569.30 1484.38 857.02 -350.15 9774.48 13502.41 5229.45 8753.51 343.18 395.26 915.59 1070.96 92.22 22955.87 12238.95 885.84 2281.06 146.05 231.95 1106.24 1179.13 1021.53 959.68 444.67 617.37 490.05 380.45 3051.11 1916.43 862.11 730.26 (a) World excludes Netherlands Antilles and DPR of Korea. (b) Asia excludes China and DPR of Korea. (c) Latin America excludes Netherlands Antilles. * Data for individual republics of Former Yugoslavia are not available. ** Gross production + imports - exports - transmission/distribution losses. *** CO2 emissions from fuel combustion only. Emissions are calculated using IEA's energy balances and the Revised 1996 IPCC Guidelines. Sources: Energy data: IEA (Energy Balances of OECD Countries; Energy Statistics and Balances of non-OECD Countries; CO2 Emissions from Fuel Combustion.) Population: OECD/World Bank GDP and GDP(PPP): OECD/World Bank/CEPII (Paris)