The National Academies: Advisers to the Nation on Science, Engineering, and Medicine

Sources and Uses

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, after millions of years of geological and chemical activity underground, turns into fossil fuels, such as coal, oil, or natural gas. Either way, we're reclaiming the power of sunlight.

The only other original source of energy on Earth's surface is found in more local nuclear reactions, where atoms of radioactive elements such as uranium split apart into smaller atoms and liberate energy in the process. Harnessed as heat, the released energy boils water, producing steam that turns turbines, thereby being converted to mechanical energy that generates electricity. Nuclear energy currently provides 20% of total electricity generation in the United States.3

Finally, the heat of Earth's molten interior, itself largely the result of the nuclear decay of radioactive elements, provides geothermal energy. At present, it is chiefly used in only a few places, such as California and Iceland, where proximity to high temperature geothermal fields makes it practical.[*]

The High Cost of Change

By the time energy is delivered to us in a usable form, it has typically undergone several conversions. Every time energy changes forms, some portion is “lost.” It doesn't disappear, of course. In nature, energy is always conserved. That is, there is exactly as much of it around after something happens as there was before. But with each change, some amount of the original energy turns into forms we don't want or can't use, typically as so-called waste heat that is so diffuse it can't be captured.

Reducing the amount lost – also known as increasing efficiency – is as important to our energy future as finding new sources because gigantic amounts 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.

In the process, the original energy has taken on a series of four different identities and experienced four conversion losses. A typical coal-fired electrical plant might be 38% efficient, so a little more than one-third of the chemical energy content of the fuel is ultimately converted to usable electricity. In other words, as much as 62% of the original energy fails to find its way to the electrical grid. Once electricity leaves the plant, further losses occur during delivery. Finally, it reaches an incandescent lightbulb where it heats a thin wire filament until the metal glows, wasting still more energy as heat. The resulting light contains only about 2% of the energy content of the coal used to produce it. Swap that bulb for a compact fluorescent and the efficiency rises to around 5% – better, but still a small fraction of the original.4

Example of energy lost during conversion and transmission.

Example of energy lost during conversion and transmission. Imagine
that the coal needed to illuminate an incandescent light bulb 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.

Another familiar form of conversion loss occurs in a vehicle's internal combustion engine. The chemical energy in the gasoline is converted to heat energy, which provides pressure on the pistons. That mechanical energy is then transferred to the wheels, increasing the vehicle's kinetic energy. Even with a host of modern improvements, current vehicles use only about 20% of the energy content of the fuel as power, with the rest wasted as heat.

Electric motors typically have much higher efficiency ratings. But the rating only describes how much of the electricity input they turn into power; it does not reflect how much of the original, primary energy is lost in generating the electricity in the first place and then getting it to the motor.

Efficiencies of heat engines can be improved further, but only to a degree. Principles of physics place upper limits on how efficient they can be. Still, efforts are being made to capture more of the energy that is lost and to make use of it. This already happens in vehicles in the winter months, when heat loss is captured and used to warm the interior for passengers. In natural gas combined cycle, or NGCC, power plants, we now have technology that takes the waste heat from a natural gas turbine and uses it to power a steam turbine, resulting in a power plant that is as much as 60% efficient.5 Similar technologies are being developed for use in coal power plants.

The energy sources that power our most indispensable devices often reflect convenience as much as efficiency. Energy can take many forms, but modern society prefers those that are easily produced, distributed, and stored. For example, American passenger cars are designed to hold enough onboard energy to travel 300 miles or so at a reasonable rate of speed. That's easy to do with the relatively high chemical energy content of gasoline or diesel fuel, despite the inefficiency of the engines.

If a car is powered by electricity, however, the energy has to be stored in batteries that have a much lower energy density than gasoline does. To carry 300 miles' worth of energy, an electric car would need a lot of very heavy batteries. Furthermore, it is difficult to deliver the energy needed to power an electric car in an acceptably short time. Modern battery-powered 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 for a day's worth of driving. For most Americans in the fast-paced 21st century, that's an unacceptably long time span.

Measuring Energy

Energy exists in many forms, so there are many ways to quantify it. Two of the most widely used for general purposes are the British Thermal Unit (BTU), which is a measure of energy content, and the watt, which is a measure of power, or how fast energy is used.

One BTU is the amount of energy needed to raise a pound of water by one degree Fahrenheit. That's not a very large amount. One cubic foot of natural gas contains around 1,000 BTUs. A gallon of gasoline is about 124,000 BTUs, and a ton of coal represents about 20 million BTUs. Enormous quantities, such as total U.S. energy consumption in a year, are expressed in “quads.” One quad is a quadrillion – that is, a million billion, or 1015 – 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 at the rate of a little more than one kilowatt, for an annual total of about 10,000 kilowatt-hours (kilowatt-hours equal power multiplied by time).6

Energy and the Individual

Energy trade-offs and decisions permeate society, directly affecting everyday quality of life in many ways. Some effects may be most noticeable at home – or at least in household energy bills due to the rising costs of heating oil and natural gas. Residential energy use accounts for 21% of total U.S. consumption, and about one-third of that goes into space heating, with the rest devoted, in decreasing proportions, to appliances, water heating, and air-conditioning. So our personal preferences are intimately tied to, and immediately affect, the nation's overall energy budget.

Percentage of energy consumed by each economic sector in the United States in 2006.

Percentage of energy consumed by each economic sector in
the United States in 2006.*7

* Percentages do not sum to 100% due to independent rounding.

Energy usage in the U.S. residential sector in 2006.

Energy usage in the U.S. residential sector in 2006.8

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 strength of the economy, and the availability of jobs.

The condition of the environment also holds consequences for all of us. Carbon dioxide (CO2) concentration in the atmosphere has risen about 40% since the beginning of the industrial revolution – from 270 parts per million (ppm) to 380 ppm – and contributes to global warming and ensuing climate change.9 At present, the United States emits approximately one-fourth of the world's greenhouse gases,10 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.11 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.

CO2 emissions by U.S. economic sector and energy source in 2005.

CO2 emissions by U.S. economic sector and energy source in 2005.12

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.

[*] 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.

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Introduction | Sources and Uses | Supply and Demand | Improving Efficiency | Emerging Technologies | Looking Ahead | References and Credits

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