Human Energy Use

-Today's Fuels

-Tomorrow's Alternatives

-Systems and Policy


"Here we aim to continue a path of uninterrupted progress in many fields... New technologies are proving that we can save energy without sacrificing our standard of living.  And we're going to encourage it in every way possible."
-Vice President Richard B. Cheney

"America must have an energy policy that plans for the future, but meets the needs of today. I believe we can develop our natural resources and protect our environment."
President George W. Bush

01/04/2006 format for printing

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Outline of Lectures

  1. Today’s state of global energy
    oil, gas, and coal
  2. Alternatives
    renewables and nuclear
  3. Systems and Decisions
    the sociopolitics of energy

Today's State of Global Energy

Energy is a central part of peoples' lives, and human ingenuity will almost certainly be able to guarantee that the supply of energy will continue to grow to meet our demands.  What you should realize by the end of these lectures is that it is the the environmental consequences of energy that are limiting to humans, and not the supply itself.  Since energy plays such a crucial role in economic development and policymaking, we have provided here some of the recommendations from the 2001 United States Report of the National Energy Policy Development Group:

National Energy Policy, 2001

  • Ease restrictions on oil and gas development on public lands.
    Open a portion of the Arctic National Wildlife Refuge in Alaska to drilling.
    Ease permit process for refinery expansion and construction.
    Speed license procedures for hydroelectric dams and geothermal plants.
  • Power Plants
    Streamline approval process for siting power plants.
    Give government authority to take property through eminent domain for power lines.
    Provide tax breaks for developing clean coal technologies.
    Ease regulatory barriers, including clean air rules, to make plants more efficient.
  • Nuclear
    Adjust regulations to speed relicensing of reactors and licensing of new plants.
    Pursue a national nuclear waste repository; Yucca Mountain is not singled out in the report.
    Tax breaks for purchase of nuclear plants. Reauthorize law that limits industry liability from a nuclear accident.
    Revive technology that allows spent fuel from nuclear reactors to be reused to produce electricity (abandoned in 1970s because it was consider a proliferation risk).
  • Renewable Energy
    Tax credits to encourage development of energy plants that use organic waste, or biomass.
    Continue tax credits for wind energy generation.
    Give tax credit of 15 percent for homeowners who purchase solar panels.
    Study whether to require automobiles to meet higher fuel efficiency standards.
    $5 billion in new spending, mostly tax credits, for renewable energy and conservation projects.
    Expand alternative fuels tax incentives to landfills capturing methane gas for electricity generation.
  • Conservation
    Tax credit for purchase of high-mileage, hybrid gas-electric vehicles.
    Tax benefits and regulatory relief for co-generation plants that produce both heat and electricity.
    Expand federal Energy Star program beyond businesses into schools, homes and hospitals.

Energy Laws

The most fundamental thing humans do is harness energy to support and improve life: food is really chemical energy that can be captured through metabolic pathways and subsequently used to power the cellular functions in our bodies; firewood is a source of chemical energy that can be readily converted to heat energy for warmth and cooking; fossil fuels are extracted from the Earth's crust and converted to electrical energy that can then be transported into our homes and used for refrigeration and many other appliances. Modern society has found technological means of harvesting massive amounts of energy and has become dependent on the primary energy sources. This lecture will summarize some of the important issues and attributes of energy usage in the modern world.

We start with a brief statement of two important energy laws. The first law states the principle of Conservation of Energy. It basically says that you can't get something for nothing. In other words:

Energy Law #1

The First Law of thermodynamics says that Energy can be neither created or destroyed, but can only be changed from form to form.

The first energy law (also know as the law of conservation of energy) deals with energy transformations. Important basic forms of energy are:

  1. Kinetic Energy: energy associated with motion
  2. Potential Energy: energy associated with position in a gravitational field
  3. Internal (heat) Energy: energy with the speed of atoms and molecules
  4. Radiant Energy: energy carried by electromagnetic radiation (light)
  5. Chemical Energy: energy contained within molecular bonds holding the elements of chemical substances together

Energy Law #2

The Second Law of Thermodynamics states that there is no such thing as a perfectly efficient energy transformation.

The second law of energy basically tells us that we can't even break even. This means that energy transformations always tend to make systems less well ordered. Things left alone tend to drift into a lower energy state (i.e., a more disorganized form). For the purposes of this discussion, the second law means that there is always unavoidable loss in energy any time we transform it from one type to another.

Energy Units

  • Work = Force x Distance (= m.a.d)
    = N.m = Joule

  • A British thermal unit (Btu) is the amount of heat required to raise the temperature of 1lb of water by one degree Fahrenheit.  Lighting a 100-watt light bulb for one hour requires 341.5 Btu.

  • Amounts of petroleum are measured in barrels, where one barrel contains 42 gallons.

  • Amounts of natural gas are measured in cubic feet. In terms of Btu energy equivalents, 6,000ft3 of gas equals one barrel of oil.  One thousand cubic feet of gas heats a typical home for one winter day. 

Energy Flows

Solar Energy

The ultimate source of almost all the energy we use is the Sun. Solar energy used to warm the planet and grow plants makes up 99% of all energy in our world. Humans supplement this with about 1%, made up of commercial energy (sold in the marketplace) and non-commercial energy (mostly firewood). Energy used for human applications is broken down into two types:renewable energy sources (solar, wind, water, geothermal, biomass) and non-renewable sources (mostly fossil fuels).

Solar energy transformations (Source: NASA Langley Research Center)

Figure 1 shows the flow of solar energy in our world. About 33% of the sun's energy is reflected right back out to space. About 42% goes into heating the atmosphere and the surface of the land and waters. Another 23% goes into the work needed to evaporate water from the oceans and a further 1% goes into driving the atmospheric circulation or winds. Only about 0.02% of the sun's energy is captured by plants in the process known as photosynthesis. This small amount forms the base of the energy pathways for nearly all life forms.

Human Use of Energy

Before the agricultural revolution, a human could only harness the energy from his/her own muscles and from fire (discovered long before by Homo erectus). The energy from human muscles is derived from the food we eat which, in turn, is provided by plant photosynthesis via a chain of pathways, involving plants and animals. Energy from human muscles is rather limited. For short periods, a fit person can deliver power at a rate of about 800 Watts (a little more than one horsepower, HP). Over a period of days, a hard working individual could probably maintain a pace yielding 0.3 HP.

Humans have long sought to control energy, but advances were initially slow in coming. The first giant leap occurred when animals were first domesticated. Animals, such as horses, oxen, mules, camels and elephants are capable of sustained power levels far in excess of those of human individuals. Also, wind energy was captured. There is evidence that the Egyptians used sailing boats around 3500 BC. A significant milestone was the invention of the horseshoe around 400 BC. This allowed horses to be directly used to plough stony fields without permanent injury. By the time of the Middle Ages, wind and water were regularly used to provide energy.

At the dawn of the industrial revolution, Britain was running out of timber for firewood. In 1765, James Watt invented the steam engine and this, coupled with the availability of plentiful coal resources, powered the rapid industrialization of Britain and initiated the related phenomenon of urbanization (For example, Britain was the only country prior to the first world war with more than half the population living in cities).

Energy use per capita for the past 150 years










Total Energy Use

The industrial revolution led to rapid development, population growth and the rush to uncover more energy sources from the earth's crust. The Figure shown above summarizes the increased use of energy globally, as well as normalized per capita. As can be seen, the total energy usage has increased at a rate greater than the overall increase in population - leading to a rate of energy usage per person that has exploded. As of 1990, the average use of energy per person is roughly equivalent to each person possessing the equivalent of 19 full-time slaves!

Where has all this energy come from to drive the industrial revolution and its aftermath? As we shall see below, by far the majority of the energy used is from non-renewable sources.

The non-renewable sources are mined from the earth's crust as shown below. Important sources include coal, oil, and natural gas. Since uranium ore is also mined from the crust, nuclear fuel may also be considered a non-renewable source, although not in the same manner as the others.

How about you (or your parents)?  Try this calculator:

Energy from the Earth's Crust

Where do we go to extract fossil fuels (figure below). Coal is found in seams that are associated with sediments laid down hundreds of millions of years ago in carboniferous swamps. Where these seams occur near the surface, environmentally damaging strip mining is a relatively cheap way to access the coal. Oil and gas are often found together and occur in natural geological bottles, where the rock formations form natural containment vessels. Oil pipelines are used to transport the liquid (though viscous) oil to other parts of the world or to ports for shipping and refinement. Sometimes, oil platforms are developed in coastal regions where the old reserves are readily accessible with economical techniques. Geothermal energy is obtained from the crust where access to hot rocks and water is possible (for example in Iceland).


This photograph is of the first oil well to be established anywhere. The photograph dates from 1859 and shows Edwin L. Drake (in the Top Hat) inspecting the facility in Pennsylvania.

 The importance of the new source of energy in oil was not first appreciated. In light of the best estimates we can make today, the total amount of energy available in the earth's liquid oil is only about 6% of that present in coal. However, its relative cleanliness, availability, and the large number of ancillary products that can be derived from oil quickly made it a major source. Modern transportation would be severely handicapped if it had to depend on coal and other solid sources (lignite and peat). While these other resources can, of course, be converted into liquid form, the conversion itself requires a wasteful amount of energy.

The mix of fuels and the trends over the last 40 years are shown in the Figure 5. The global use of energy is still rising rapidly, though not as rapidly as hitherto. The tapering off of the rise is due to increased energy conservation efforts in the developed world.

Energy Consumption by Fuel, 1970-2020

Source: Energy Information Administration, Annual Energy Outlook 2002

Since about the middle 1960's, oil has been the most important fossil fuel, with coal and gas a close second and third. The rate of increase in use of oil has diminished, due to the increasing costs of production and the limited known reserves. A similar plateau has been reached for gas for the same reasons, but coal has shown a steady rise due to the much larger known reserves and the relatively low cost, which makes it attractive to developing countries. Hydroelectric and nuclear power provide minor additional sources of energy.

Historical Fuel use in the U.S. (per capita, by proportion)

The right Figure illustrates the relative proportions of the different fuels used within the U.S., normalized to units of per capita. In the late 1800's, wood and water provided the bulk of the energy used by people. Coal became the most important source in the early part of the 20th century, followed by the combination of oil and gas. Each curve has a similar form - a bell shape with about a 50-year width. This picture suggests three things. First, people have previously been able to switch from one dominant energy source to another. Second, such switches take about 50 years to accomplish. Third, the era of oil and gas may be coming to an end.

The lesson from this figure is that we probably need to start the development now of the next dominant energy source if we want to avoid major social turbulence. The energy trends within the US mirror those of the developed world at large as can be seen by the next figure, showing the relative use of the various energy sources globally and within the US today.

Oil remains the largest single source, with coal the second most important source globally, followed by natural gas. Within the US, the position of natural gas and coal is reversed. This is because we make good use of natural gas, primarily because of highly developed transportation technologies.

Global energy use patterns


North America


The following table provides numerical data on energy use around the globe.

World Primary Energy Consumption, 1998. (Source: UNDP World Energy Assessment, 2000)

Within the US, the dependence of foreign sources of oil is projected to become more significant, as the relative size of the domestic oil production drops. This fact is behind the current vigorous discussion on whether to open the Alaskan Arctic Refuge for oil prospecting. It also explains the interest of the West in stability in the Middle East and continued access to the oil reserves there. The left Figure summarizes the recent history of domestic fossil fuel use. In the early 1970's there was a much larger fraction of the total provided for by domestic production than is currently the case.

Also, the importance of the Alaskan fuel reserves is evident as many of the original oil finds within the lower 48 states become depleted.  However, the effect of new drilling in Alaska may be small 

Where does this Energy Go?

A major fraction of the energy used within the US is consumed by industry for mining, milling, smelting, chemical industries, and the forging of primary metals. The next most important use in for space heating of buildings, air-conditioning, lighting and water heating. Transportation accounts for about 28% of the total, mostly for highway vehicles such as cars and trucks.

The figures below illustrates the breakdown by component.

U.S. Relative consumption. Source: WRI Earthtrends, 2001

Relative Consumption

The US uses much more energy per capita than countries in the developing world, as would be expected. If we lump the developing nations together and separate out North America (US and Canada), Western Europe, the former USSR, we can see the overall trends in energy use.

Annual per capita Commercial energy Consumption (Source: UNEP, 2000)

Although the West is still the dominant user of energy, the developing world is on a track to increase its use of energy and will surpass North America soon as the largest user. Both Western Europe and North America have more-or-less stabilized their usage of energy (albeit at a very high level). This has been achieved primarily through more vigorous conservation approaches. On the other hand, energy use in the developing nations is driven by two factors: population growth and economic growth. While many nations have adopted family planning methods to control their population growth, all developing nations are striving to reach full development and this latter factor drives their use of energy significantly. Clearly, to stabilize the global use of energy, we will need to reach a new pattern of economic development that allows for growth without the normally concomitant surge in energy use.

It will not be a surprise to see the relationship of energy use and gross national product, below.

This plot shows a near direct proportionality between wealth and energy usage. The more wealthy nations use more energy. There are some outliers on this curve that are instructive, however. We see that Japan has a relatively low use of energy per capita for its level of wealth. this is because Japan has essentially no domestic energy source and has developed aggressive conservation techniques to limit its bill for imported fuel. Also Sweden, Denmark, and Switzerland have well developed conservation programs. On the other side of the fitted line, Qatar has a huge energy usage due to its need for air conditioning and the massive desalinization plants to produce drinking water.

3. Energy Use Prospects

Reserves and Resources

It is important first to clearly distinguish between the terms Reserves and Resources. These are often confused and have quite different meanings.

The term Reserves is used to describe the proven, existing supplies of a commodity that can be extracted and sold economically at today's prices.

The term Resources is used to describe the total amount of the substance in nature - whether found or yet to be discovered - and irrespective of whether the costs of extraction would make it uneconomic to exploit at today's prices.

Location of resources

Location of Resources

The figure describes where coal resources are to be found. The former Soviet Union, North America and Asia are coal and gas rich, while the Middle East, FSU and Africa are oil rich.


The situation on reserves provides a cautionary tale. The coal reserve is the largest. Known sources of coal could last 200-300 years more at today's rate of consumption. The global oil reserve will be exhausted within 40 years from the present. If a reasonable allowance is made for those resources not yet discovered, then oil might last another 60 years. There is some expectation that oil from sandy tars might provide an additional leeway (perhaps doubling the timescale). Natural gas reserves will also be exhausted in about 60 years.

Energy Efficiency

Given this situation, it is clearly essential to develop alternative strategies for energy and, of course, this has been a main motivation for the development of the many renewable sources of energy and for the special case of nuclear power. Before discussing these alternatives, we should stress, however, the importance of conservation measures. If we consider the efficiency of certain appliances we can find much room for improvement. Any reduction in current fossil fuel burning rates, of course, will buy time to develop the successive energy systems and therefore is to be encouraged.

Fate of energy used

This diagram summarizes the sources of energy within the US and the ultimate fate of the energy used. The majority of the energy comes from fossil fuel burning. Only about 8% of the total energy acquired is used productively in commercial and non-commercial applications. A further 7% or so is used in the petrochemical industry. The remainder of the energy used is lost - either through unavoidable losses (remember the second energy law) - or through avoidable losses. The avoidable losses are in excess of 40% and their reduction represents the very best way to spend an energy dollar today. For example a dollar spent on energy conservation is worth 7 spent on the development of a nuclear power plant - which may well help to explain why nuclear plants are so much out of favor.

Avoidable losses include those due to poorly designed appliances, low efficiency incandescent lights, poor insulation, etc. etc. Simple strategies to save energy include converting incandescent lights to fluorescent lights for an efficiency gain from 5% to 22%.

A comparison between two methods of space heating provides another example. The first relies on electricity generated in a nuclear power plant. The second relies on direct sunlight entering though a well insulated window. In the case of the nuclear energy, only 14% of the actual energy becomes useful heat. The rest is lost in the multiple inefficient transformations (uranium processing, power plant losses, electrical transmission, and resistance heat losses). Solar heating, on the other hand, loses only a small fraction of energy in the single transformation due to transmission through the window. In general, the greater the number of energy transformations, the larger the fraction of wasted energy.

Energy available from conservation vs. the Arctic National Wildlife Refuge

Much of the required technology is in place to achieve substantial savings via efficiency programs. Realizable efficiencies include: refrigeration (87%); air conditioning (75%); electric water heating (75%); electric range (50%); gas furnace (59%); gas water heater (63%); and gas range (64%). Efficient appliances such as these, plus programs of improved insulation and lighting should lead to major reductions in energy use rates in the near future. As a stark example of what is possible, the figure compares realizable energy savings over the next two decades with the energy products that could accrue from an opening of the Alaskan Arctic Refuge. Clearly, greater gains are to be found in energy conservation than in further prospecting.

Other Resources

As discussed above the reserves of oil and gas will become depleted within the next half century or so. Less well known are similar reserve lifetimes for other resources. The following table provide a list of estimated depletion periods for various resources, based on two assumptions: 1) that current consumption rates are maintained and 2) that the developing world increases its rate of consumption to match the current US rates. In both cases, the resources become depleted within periods of tens to hundreds of years. In each case, alternative strategies will need to be adopted and/or pricing schemes will be instituted to dramatically lower consumption.

Tomorrow's Alternatives

Renewable Energy Sources

While much of the last discussion centered on non-renewable energy sources, there is growing interest in renewable energy sources. These include:

Solar energy through photovoltaic cells Photovoltaic cells convert light energy into electricity at an atomic level. PV systems can be constructed to any size based on individual energy requirements, and are low-maintenance. They are ideal for supplying power to homes far from utility power lines in remote areas.
Wind energy Wind energy was widely used as a source of power before the industrial revolution, but later displaced by fossil fuel use because of differences in cost and reliability.
Flowing water Hydropower plants use the kinetic energy of falling water to generate electricity. A turbine and a generator convert the energy from the water to mechanical and then electrical energy.
Biomass Biomass is the organic material that stems from plants, trees, and crops. Its largest contribution to energy consumption is found in developing countries, traditionally used as firewood for cooking and heating. Modern uses include combustion to produce energy in the form of electricity, steam, and biofuels.
Earth's internal heat Electricity can also be captured from Earth's internal heat. Geothermal resources range from those in shallow ground to hot water and rock several miles below Earth's surface, and even farther down to molten rock. There are three types: geothermal heat pumps, direct-use applications, and power plants. Pictured to the left, natural steam from production wells powers a turbine generator, producing white plumes of water vapor. View an online slide show on geothermal energy.

A summary of resource potential (below) shows that our entire nation may benefit from renewable resources, but that their regional relevance varies.  It has been estimated that 50-80% of our energy needs can be met by using renewable resources


U.S. Resource Potential for Renewable Energy (Source: NEP Report, 2001)

The costs of many of these alternatives has been dropping with time (see Figure for the reduction of costs in wind and photovoltaic energy in units of cents per kilowatt-hr). It is to be anticipated that a combination of these new sources will be needed to replace oil and gas within the first two or three decades of the next century. It is estimated that development of these renewable energy sources could meet 50-80% of the US energy needs by 2030. In the meantime, many countries have a petroleum tax structure which is designed to more closely reflect the real costs of oil. A comparison of gas taxes in various countries shows the US tax on gas is relatively low. These prices do not reflect the full costs of protecting oil supplies and cleaning the environment. In fact, there is a hidden subsidy for highway driving in the US.

Use and comparable cost of selected renewable energy technologies, 1998.
Source: UNDP World Energy Assessment, 2000.

Fuel Cells

One intermediate solution in the current energy structure is the introduction of fuel cells.  A fuel cell consists of two electrodes sandwiched around an electrolyte. Oxygen passes over one electrode and hydrogen over the other, generating electricity, water and heat. Hydrogen fuel is fed into the "anode" of the fuel cell. Oxygen (or air) enters the fuel cell through the cathode. Encouraged by a catalyst, the hydrogen atom splits into a proton and an electron, which take different paths to the cathode. The proton passes through the electrolyte. The electrons create a separate current that can be utilized before they return to the cathode, to be reunited with the hydrogen and oxygen in a molecule of water. A fuel cell system which includes a "fuel reformer" can utilize the hydrogen from any hydrocarbon fuel - from natural gas to methanol, and even gasoline. Since the fuel cell relies on chemistry and not combustion, emissions from this type of a system would still be much smaller than emissions from the cleanest fuel combustion processes.  However, the reliable production of H2, the energy source, requires methane or other hydrocarbons, so the technology is not a cure.


Systems and Policy

1. How long can fossil fuels last at present consumption levels?

2. What effect will various scenarios of energy diversification have on supplies and prices?

3. How should policymakers take the benefits of green energy into consideration?

Implications for the Future

Energy and Social Issues (Source: UNDP 2000)


A fundamental activity of humans involves the control of energy to support and enhance life. The energy laws dictate that energy cannot be created and/or destroyed, but can only be changed in form. Furthermore, energy transformations are always inefficient to some degree.

Fossil fuels represent the most important sources for energy in the modern world. Currently, oil and gas play the major role. Coal used to be the dominant energy source for the developed world in the early part of the 20th century and is once again becoming used more heavily due to considerations of cost and availability. The developing world's energy requirements are increasing at a rate that will soon outpace the energy needs of the developed world, where energy conservation programs are becoming more sophisticated. Current energy use is nearly directly proportional to gross national product.

The reserves for gas and oil will only last another 40-80 years, after which alternative energy sources will be needed to carry the load. Candidate technologies include renewable sources of energy, such as photovoltaic and wind energy sources.

Natural resources of all types are being consumed at rates that will lead to depletion in a few tens of hundreds of years. 

Suggested Readings

  • G. Tyler Miller, Jr., Sustaining the Earth: An Integrated Approach, Wadsworth, 1994

  • F. T. Mackenzie and J. A. Mackenzie, Our Changing Earth: An Introduction to Earth System Science and Global Environmental Change, Prentice Hall, 1995.

  • National Academy of Sciences: Policy Implications of Greenhouse Warming, 1991.

  • William P. Cunningham and Barbara W Saigo, Environmental Science: A Global Concern, Wm. C. Brown Publishers, 1990.


Energy and the Challenge of Sustainability. United Nations Development Programme, World Energy Assessment 2000.

Report of the National Energy Policy Development Group, May 2001.

State of the World 2000, Worldwatch Institute.

Pictured at the top: The Honda Insight, rated among the greenest of cars, is a "hybrid" vehicle that runs on regular gas but generates electricity as you drive it.


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All materials 2002 by the University of Michigan.