Origin of Energy
From Thermal-FluidsPedia
The scientific theory of the creation of the universe is based on a huge explosion (big-bang), which took place some 12-15 billion years ago. The explosion resulted in the birth of a cosmic nuclei “egg” consisting of a very condensed mass of elementary particles. This nucleus led to the formation of our stars, galaxies, and planets. These processes were of a type which, today, we call thermonuclear, i.e. matter converted into energy. Hydrogen was the first (and simplest) element that was formed and is still the most common element in the universe. It also makes up the core of our sun, which is estimated to be at a temperature in excess of 15 million degrees and a pressure of many billions of atmospheres. Every hour approximately 16 billion tons of solar mass is converted into energy through fusion of hydrogen to heavier atoms. The cores of heavier and bigger stars are at even higher temperatures and pressures. This allows the fusion of other atoms to form heavier elements such as carbon, oxygen, silicon, and eventually iron.
Most of the energy known to us has its source in the sun, i.e. solar energy. The sun radiates energy in all directions in the form of electromagnetic waves, including heat and light. A tiny fraction of this energy is beamed toward the earth, of which one-third is reflected back to the sky before ever reaching the ground. Of the remaining two-thirds, some is absorbed by the atmosphere, causing it to heat. The rest is intercepted by the earth in the form of light and heat, of which a mere 0.03% is transformed into biomass through photosynthesis. Biomass can be used as food and consumed by animals and humans, directly burned, or used to produce secondary fuels such as methanol and ethanol. If biomass is shielded from air, it decays and under suitable conditions, can eventually turn into fossil fuel-- coal, oil, and natural gas.
Because of different vegetation and rock formations, solar radiation does not heat the land and the ocean uniformly. Differential heating of land and water surfaces results in wind patterns and the ocean waves, two major sources of renewable energy. The earth is also being affected by the gravitational forces of its neighboring celestial bodies, most notably the moon, which is the cause of tides and much of the wave patterns in open channels. Tidal power plants are constructed to exploit the variation in the tides’ potential energy in the same way that hydroelectric power stations use the potential energy from falling water.
Both nuclear fuel and geothermal energy have roots in the earth’s early stages of development. Uranium fuel is the remains of the radioactive decay of the heavy materials and isotopes formed during the early stages of cosmic nuclei cooling. Uranium can be extracted from earth and processed for use as fuel in a nuclear reactor. What remains inside the earth continues to produce heat and form magma that can find its way to the earth’s surface. Along the way it heats rocks, minerals, and water reservoirs which together constitute our geothermal resources.
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Forms of Energy
There are many different ways that we can classify energy. For example we can classify all forms of energy as potential or kinetic, external or internal, primary or secondary, renewable or non-renewable, or in terms of its application as thermal, mechanical, chemical, electrical, nuclear, etc.
Potential energy is stored energy. This energy can be stored in atomic nuclei, molecular bonds, or gravitation. Kinetic energy is energy in action such as microscopic motion of atoms and molecules (heat), or gross motion of matter (falling rock, wind or river).
External energy is defined with respect to some outside frame of reference. Energies contained in winds, waves, and falling waterfalls -- commonly referred to as mechanical energy -- are of this type. Internal energy is energy locked within the internal structure of atoms and molecules. Examples are mass, chemical, thermal, light, and nuclear energy.
Mass is energy of matter by virtue of its own existence. Until 1905, when Einstein formulated the general theory of relativity, mass was not considered a form of energy. Einstein’s famous formula E = mc2 (where m is the mass and c is the speed of light in vacuum) gives a relationship between mass and the energy associated with it. It expresses the amount of energy that is given off if matter is completely annihilated. Chemical energy is the energy locked in the molecules of various substances. The energy stored in a molecule of carbon dioxide is that energy which holds two atoms of oxygen with one atom of carbon together. Biomass and fossil fuel store their energy as chemical energy. Thermal energy, or heat, is the energy associated with the random motion of individual molecules of matter. It can be considered as kinetic energy at the microscopic level. Geothermal and a big portion of solar energy are in the form of thermal energy. Light energy (also called radiant or electromagnetic energy) is the energy associated with a quantum of energy called a “photon” as it travels through outer space. The sun is the source of light energy. Nuclear energy is the energy trapped in the nucleus of an atom. This energy can be liberated by splitting apart a large nucleus to form two or more lighter atoms (fission) or by combining two light atoms to form a heavier atom (fusion).
Depending on its source, energy can also be divided into primary or secondary. Primary energy sources are those sources that can be directly converted to heat or mechanical work. The human intervention is limited only to extraction, cleaning, and separation, without changing the physical or chemical characteristics of the sources. (1) There are five main sources of primary energy that we use today; these include fossil fuel, nuclear, geothermal, solar, and tidal.( a ) Secondary energy comes from transformation of primary energy. Electricity is a secondary source of energy that can be produced from any of the primary sources mentioned above. (Table 1).
Table 1. Energy Forms Primary Secondary Fossil fuel
Solar (Wind, Hydro, Biomass)
Tides
Geothermal
NuclearTown gas (from coal);
Gasoline, heating oil, diesel, and jet fuel (from crude);
Alcohol, synfuel, and charcoal (from coal, oil shale, biomass)
Hydrogen (from fossil, nuclear, solar)
Electricity (from anything)
Depending on their long-term availability, energy resources can also be classified as either renewable or non-renewable. Renewable resources are those that will replenish themselves naturally in a relatively short period. Examples of renewable energy are solar, wind, hydroelectric, ocean thermal, waves, and tides. Nonrenewable resources are fossil fuels and uranium. Geothermal hot water, steam reservoirs and some material such as peat (decayed vegetable matter) and wood can be considered renewable if the rate of usage is small enough so as to allow their natural replenishment over time.
Units of Energy
There are two major systems of measurement in the world today: The United States Customary System (USCS) and the International System of Units (SI or metric). The metric system was adopted by the General Conference on Weights and Measures as the international system of units to be used in all international commerce. The scientific community, for the most part, has adopted this system to report findings and carry out scientific calculations. The United States is the last major industrial nation which has not converted fully to the metric system, a delay that has put us at a disadvantage in world trade.
Each system has its own fundamental units and other quantities (including energy) can be expressed in terms of them. The fundamental units of the USCS are foot (for length), pound (for weight) and second (for time). All other units can be reduced to these units. For example, velocity is given in feet per second (ft/s), work and energy are given in pound-feet (lb-ft), and power is given in horsepower – equal to 550 lb-ft/s.
In the SI system of units, fundamental units of measurements are kilogram (for mass), meter (for length), and second (for time). Weight is a derived quantity defined as the force of gravity acting on an object of a given mass:
The unit of weight is newton, defined as the force of acceleration of a mass of 1 kg by 1m / s2(1N = 1kg.m / s2). In this equation, m is mass in kilogram and g is the gravitational acceleration. On earth g = 9.8m / s2 = 9.8N / kg. In other words, objects on the influence of the earth gravity will accelerate at the rate of 9.8 meters per second (22 miles per hour) for each second they fall. Alternatively, it can be concluded that objects experience a force of 9.8 newtons per each kilogram of mass they posses.
In Mechanical Energy, we will define work as the energy needed to displace a force by a certain distance; it is therefore reasonable to conclude that a newton-meter commonly referred to as joule, is an appropriate unit for both work and energy (1J = 1N.m). Other units that have been derived and are used in this text are the watt (1W = 1J / s) for power, the ohm (W) for electrical resistance, the volt (V) for electromotive force, and the ampere (A) for electric current. (2)
The fact that the SI system is based on 10 and is much easier to use than US customary units is not the reason the SI has found the popularity that it receives today. The main difference is the choice of weight in USCS and mass in the SI system as the fundamental units. A truck that has a mass of 2,000 kilograms in Detroit is still 2,000 kilograms in Los Angeles, or in Paris. In fact, if you load the truck and take it to the moon, it still has the same mass. On the contrary, a truck which weighs 4,000 pounds in Detroit will have a slightly different weight in Paris. Even the weight in Detroit varies by day, as the temperature and pressure changes. If you ship the same truck to space, it will have no weight at all.
In addition to the USCS and SI units, other units have been traditionally used (and unfortunately are still in use today). In the case of energy, these are BTU, therms( b ), quad( c ), barrels of oil( d ), and even tons of TNT( e ). In order to conform to the rest of the world, we will concentrate primarily on SI units in this text. Since many people, especially in the United States, are most familiar with the USCS units, we will include these units when we feel the digression aids in comprehension. The unit conversion tables are given in Appendix B.
Example: The latest data gives the US energy consumption in 2007 as 101 Quads. Express this in BTU, joules, barrels and cubic kilometers of oil. Solution: From the table of unit conversion given in Appendix B:
Or:
Example: Express the weight of a 180-pound person in newtons. Solution: A person weighing 180 pounds (we mean pound -force or lbf) has a mass of 180 pounds (we mean pound-mass or lbm). The same person has a mass of 180 / 2.2 = 82 kilograms and a weight of 82x9.8 = 800 newtons.
References
(1) OECD/IEA/Eurostat, Energy Statistics Manual, 2005. http://www.iea.org/Textbase/publications/free_new_Desc.asp?PUBS_ID=1461
(2) The NIST Reference on Constants, Units, and Uncertainty (http://physics.nist.gov/cuu/Units/index.html).
(3) Toossi Reza, "Energy and the Environment:Sources, technologies, and impacts", Verve Publishers, 200
Additional Comments
(a) Some consider waste as a primary source, as it is a surplus from any other process that has no further use for that particular process.
(b) Therm is used mainly in the United States for metering household natural gas (1 therm=100,000 BTU).
(c) A quad is 1015 BTU or 1.055 EJ.
(d) A barrel of crude oil: 5.8 million BTU.
(e) Trinitrotoluene commonly known as TNT is a major constituent of many explosives, with the energy equivalent of one million food calorie per ton (strange but true!). TNT has a lower energy density than gasoline (32.2 MJ/kg), or even sugar (17 MJ/kg). The main attractiveness of TNT is that it does not have to be mixed with air to burn and in its ability to release this energy rapidly.
Further Reading
Meadows, D. H., et al., The Limit to Growth, Universe Books, 1972. Also see, The Limit to Growth: The 30-Year Update, Chelsea Green Publishing Company, 2004.
Diamond, J., Collapse: How Societies Choose to Fail or Succeed, Penguin Group, USA, 2004.
Cleveland, C. J., Encyclopedia of Energy, Elsevier Direct Science, 2004.
The International Journal of Energy, Science Direct Elsevier Publishing Company.
Applied Energy, Elsevier Publishing Company.
Journal of Energy Resource Technology, ASME International.
The Energy Journal, The quarterly journal of the IAEE’s Energy Education Foundation, (http://www.iaee.org/en/publications/journal.aspx).
External Links
Energy Citation Database, US Department of Energy (http://www.osti.gov/energycitations).
Environmental Protection Agency (http://www.epa.gov).
US Department of Energy (http://www.doe.gov).
The NIST Reference on Constants, Units, and Uncertainty (http://physics.nist.gov/cuu/Units/index.html).
The Club of Rome (http://www.clubofrome.org).
The Sierra Club (http://www.sierraclub.org).