WE CONSUME ENERGY IN DOZENS OF FORMS.
Yet virtually all of the energy we use originates in the power of the atom. Nuclear reactions energize stars, including our sun. The energy we capture for use on Earth comes largely from the sun or from nuclear forces local to our own planet.
Sunlight is by far the predominant source, and it contains a surprisingly large amount of energy. On average, even after passing through hundreds of kilometers of air on a clear day, solar radiation reaches Earth with more than enough energy in a single square meter to illuminate five 60-watt lightbulbs if all the sunlight could be captured and converted to electricity.
The sun’s energy warms the planet’s surface, powering titanic transfers of heat and pressure in weather patterns and ocean currents. The resulting air currents drive wind turbines. Solar energy also evaporates water that falls as rain and builds up behind dams, where its motion is used to generate electricity via hydropower.
Most Americans, however, use solar energy in its secondhand form: fossil fuels. When sunlight strikes a plant, some of the energy is trapped through photosynthesis and is stored in chemical bonds as the plant grows. We can recover that energy months or years later by burning wood, which breaks the bonds and releases energy as heat and light. More often, though, we use the stored energy in the much more concentrated forms that result when organic matter,
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WE CONSUME ENERGY IN DOZENS OF FORMS. Yet virtually all of the energy we use originates in the power of the atom. Nuclear reactions energize stars, including our sun. The energy we capture for use on Earth comes largely from the sun or from nuclear forces local to our own planet. Sunlight is by far the predominant source, and it contains a surprisingly large amount of energy. On average, even after passing through hundreds of kilometers of air on a clear day, solar radiation reaches Earth with more than enough energy in a single square meter to illuminate ﬁve 60-watt lightbulbs if all the sunlight could be captured and converted to electricity. The sun’s energy warms the planet’s surface, powering titanic transfers of heat and pressure in weather patterns and ocean currents. The resulting air currents drive wind turbines. Solar energy also evaporates water that falls as rain and builds up behind dams, where its motion is used to generate electricity via hydropower. Most Americans, however, use solar energy in its secondhand form: fossil fuels. When sunlight strikes a plant, some of the energy is trapped through photosynthesis and is stored in chemical bonds as the plant grows. We can recover that energy months or years later by burning wood, which breaks the bonds and releases energy as heat and light. More often, though, we use the stored energy in the much more concentrated forms that result when organic matter, 6
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after millions of years of elements, provides geothermal energy. At present, geological and chemical it is chieﬂy used in only a few places, such as activity underground, turns California and Iceland, where proximity to high temperature geothermal ﬁelds makes it practical.* into fossil fuels, such as coal, oil, or natural gas. Either way, we’re reclaiming THE HIGH COST OF CHANGE the power of sunlight. By the time energy is delivered to us in a usable form, it has typically undergone several conversions. Every The only other original source of energy on Earth’s time energy changes forms, some portion is “lost.” surface is found in more local nuclear reactions, It doesn’t disappear, of course. In nature, energy is where atoms of radioactive elements such as uranium always conserved. That is, there is exactly as much split apart into smaller atoms and liberate energy in of it around after something happens as there was the process. Harnessed as heat, the released energy before. But with each change, some amount of the boils water, producing steam that turns turbines, original energy turns into forms we don’t want or thereby being converted to mechanical energy can’t use, typically as so-called waste heat that is so that generates electricity. Nuclear energy currently diffuse it can’t be captured. provides 20% of total electricity generation in the United States. Reducing the amount lost—also known as increasing efﬁciency—is as important to our energy future Finally, the heat of Earth’s molten interior, itself as ﬁnding new sources because gigantic amounts largely the result of the nuclear decay of radioactive of energy are lost every minute of every day in conversions. Electricity is a good example. By the time the energy content of electric power reaches the end user, it has taken many forms. Most commonly, the process begins when coal is burned in a power station. The chemical energy stored in the coal is liberated in combustion, generating heat that is used to produce steam. The steam turns a turbine, and that mechanical energy is used to turn a generator to produce the electricity. *One exception to the solar and local nuclear origins of Earth’s energy promises only an exceedingly small contribution to our total energy picture at present: Some engineers are exploring methods for capturing energy from ocean tides, thus tapping into a gravitational source of energy. 7
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Example of energy lost during conversion and transmission. Imagine that the coal needed to illuminate an incandescent lightbulb contains 100 units of energy when it enters the power plant. Only two units of that energy eventually light the bulb. The remaining 98 units are lost along the way, primarily as heat. In the process, the original energy has taken on a it heats a thin wire ﬁlament until the metal glows, series of four different identities and experienced wasting still more energy as heat. The resulting light four conversion losses. A typical coal-ﬁred electrical contains only about 2% of the energy content of the plant might be 38% efﬁcient, so a little more than coal used to produce it. Swap that bulb for a compact one-third of the chemical energy content of the fuel ﬂuorescent and the efﬁciency rises to around 5%— is ultimately converted to usable electricity. In other better, but still a small fraction of the original. words, as much as 62% of the original energy fails to ﬁnd its way to the electrical grid. Once electricity Another familiar form of conversion loss occurs leaves the plant, further losses occur during delivery. in a vehicle’s internal combustion engine. The Finally, it reaches an incandescent lightbulb where chemical energy in the gasoline is converted to heat 8
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energy, which provides pressure on the pistons. That waste heat from a natural gas turbine and uses it to mechanical energy is then transferred to the wheels, power a steam turbine, resulting in a power plant that increasing the vehicle’s kinetic energy. Even with a is as much as 60% efﬁcient. Similar technologies are host of modern improvements, current vehicles use being developed for use in coal power plants. only about 20% of the energy content of the fuel as power, with the rest wasted as heat. The energy sources that power our most indispensable devices often reﬂect convenience as Electric motors typically have much higher efﬁciency much as efﬁciency. Energy can take many forms, ratings. But the rating only describes how much of but modern society prefers those that are easily the electricity input they turn into power; it does not produced, distributed, and stored. For example, reﬂect how much of the original, primary energy is American passenger cars are designed to hold lost in generating the electricity in the ﬁrst place and enough onboard energy to travel 300 miles or so at then getting it to the motor. a reasonable rate of speed. That’s easy to do with the relatively high chemical energy content of gasoline or Efﬁciencies of heat engines can be improved diesel fuel, despite the inefﬁciency of the engines. further, but only to a degree. Principles of physics place upper limits on how efﬁcient they can be. If a car is powered by electricity, however, the energy Still, efforts are being made to capture more of the has to be stored in batteries that have a much lower energy that is lost and to make use of it. This already energy density than gasoline does. To carry 300 happens in vehicles in the winter months, when heat miles’ worth of energy, an electric car would need a loss is captured and used to warm the interior for lot of very heavy batteries. Furthermore, it is difﬁcult passengers. In natural gas combined cycle, or NGCC, to deliver the energy needed to power an electric car power plants, we now have technology that takes the in an acceptably short time. Modern battery-powered 9
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Measuring Energy cars charge at a rate roughly a thousand times slower than the rate of refueling with gasoline, meaning overnight charging is required to store enough energy Energy exists in many forms, so there are many for a day’s worth of driving. For most Americans in ways to quantify it. Two of the most widely used the fast-paced 21st century, that’s an unacceptably for general purposes are the British Thermal Unit long time span. (BTU), which is a measure of energy content, and the watt, which is a measure of power, or how ENERGY AND THE INDIVIDUAL fast energy is used. Energy trade-offs and decisions permeate society, One BTU is the amount of energy needed to directly affecting everyday quality of life in many raise a pound of water by one degree Fahrenheit. ways. Some effects may be most noticeable at That’s not a very large amount. One cubic foot of home—or at least in household energy bills due natural gas contains around 1,000 BTUs. A gallon to the rising costs of heating oil and natural gas. of gasoline is about 124,000 BTUs, and a ton of Residential energy use accounts for 21% of total coal represents about 20 million BTUs. Enormous U.S. consumption, and about one-third of that quantities, such as total U.S. energy consumption goes into space heating, with the rest devoted, in in a year, are expressed in “quads.” One quad is decreasing proportions, to appliances, water heating, a quadrillion—that is, a million billion, or 1015— and air-conditioning. So our personal preferences BTUs. America consumed about 100 quads in 2006. One watt of power is equal to one ampere (a measure of electric current) moving at one volt (a measure of electrical force). Again, this is a fairly small unit. U.S. household electricity is provided at 120 volts. So a 60-watt lightbulb needs half an ampere of current to light up. For larger quantities, watts are usually expressed in multiples of a thousand (kilowatt), million (megawatt), or billion (gigawatt). A big coal, natural gas, or nuclear electrical plant can produce hundreds of megawatts; some of the largest generate one or more gigawatts. A typical wind turbine has a one megawatt rating, and the largest are now four megawatts when turning. An average U.S. household consumes electricity Percentage of energy consumed by each at the rate of a little more than one kilowatt, for economic sector in the United States in 2006.* an annual total of about 10,000 kilowatt-hours * Percentages do not sum to 100% due to independent rounding. (kilowatt-hours equal power multiplied by time). 10
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are intimately tied to, and immediately affect, the nation’s overall energy budget. Our individual automotive and public-transit choices also have a substantial impact, because transportation takes up 28% of all U.S. energy consumption (and about 70% of all petroleum use). Even the 50% of total U.S. energy consumption that goes to commercial and industrial uses affects every single citizen personally through the cost of goods and services, the quality of manufactured products, the CO2 emissions by U.S. economic sector and energy strength of the economy, and the availability of jobs. source in 2005. The condition of the environment also holds 40% since the beginning of the industrial revolution— consequences for all of us. Carbon dioxide (CO2) from 270 parts per million (ppm) to 380 ppm—and concentration in the atmosphere has risen about contributes to global warming and ensuing climate change. At present, the United States emits approxi- mately one-fourth of the world’s greenhouse gases, and the nation’s CO2 emissions are projected to rise from about 5.9 billion metric tons in 2006 to 7.4 billion metric tons in 2030, assuming no changes to the control of carbon emissions. Of course this is not just a national concern. Worldwide, CO2 emissions are projected to increase substantially, primarily as a result of increased development in China and India. Future decisions about whether and how to limit greenhouse gas emissions will affect us all. Before we can consider ways to improve our energy situation we must ﬁrst understand the resources we currently depend on, as well as the pros and cons of using each one. Energy usage in the U.S. residential sector in 2006. 11