Energy Consumption and Conservation

Introduction

Information in this unit, particularly that relating to future energy options, draws heavily on a Science article by Hoffert et al (2002). The reader is encouraged to consult this article for more details.

Current rates of increase in uses of fossil fuels will create atmospheric concentrations of 550 to 950 ppm of CO₂ by the end of the 21st century. This compares with the pre-industrial value of about 275 ppm and exceeds atmospheric CO₂ levels in the Earth's atmosphere in the last 400,000 years and perhaps the last 2 million years. The current atmospheric CO₂ level is about 370 ppm and rising at about 1 ppm per year. Supplying future global energy needs with reduced contributions to atmospheric CO₂ requires a combination of more efficient energy use and major advances in non-fossil-fuel sources of energy.

The change in global temperatures produced by 550 ppm would be comparable to but opposite sign with the magnitude of cooling that created the last ice age. A ceiling of 450 ppm is needed if we expect to prevent major bleaching of global ocean coral reefs, shutdown of global ocean thermohaline circulation, and sea level rise from disintegration of the West Antarctic Ice Sheet.

Global Power Consumption

Global power use today is about 12 TW (one Terrawatt is 1,000,000,000,000 = 10¹² Watt), with about 10 TW being derived from fossil fuel, the primary source of increases in atmospheric greenhouse gases. The Kyoto Protocol called for a 5% reduction in emissions of greenhouse gases below 1990 levels by 2008 to 2012, but meeting this goal seems highly unlikely, since it is viewed by the US as being too great of an economic burden. However, even if this goal is met, stabilization (allowing to reach but not exceed) of CO₂ concentrations at 450 ppm seems unlikely. Meeting target goals of reducing emissions without major negative economic consequences will require revolutionary advances in technology of energy production, distribution, storage and conversion. Currently known methods, either in use or pilot demonstration, are insufficient to stabilize climate (Hoffert et al, 2002).

If we set a goal of stabilizing CO₂ at 450 ppm we need some combination of increased fuel efficiency, use of "carbon-neutral" fuels (i.e., fuels that put only as much CO₂ back into the atmosphere as was taken to create them, such as wood), and capturing CO₂.

Contributors to Emissions

Energy conversion (chemical energy of fossil fuels to electrical or mechanical) systems account for, by far, the largest contribution to emissions of carbon into the atmosphere as is shown in Figure 1. Other industries, agriculture, and waste processing account for modest amounts, and the forest industry is a negative contributor, being a net sink for atmospheric carbon dioxide.

The distribution by economic sector of carbon dioxide emissions (Figure 2) shows that utilities are the largest contributor, followed by transportation and industrial. Residential and commercial emissions are small by comparison with the previous three. Utilities (stationary combustion sources) primarily use coal, while transportation (mobile combustion sources) use petroleum almost exclusively. Industrial sources use primarily petroleum and natural gas, with a small amount coal.

Of the various types of energy consumed in the US in 2001 (Figure 3), as given by the US Energy Information Administration, petroleum accounts for about 39%, followed by natural gas at 24% and coal at 23%. Nuclear electric power contributes 8% and renewables contribute 6%.

Worldwide Energy Use

All nations have contributed to the growth in use of energy, as is shown in Figure 4. The centrally planned economies include China, Russia, and other communist countries, and the nations making up the Organization for Economic Development and Cooperation are listed in Figure 4. The Carbon Dioxide Information Analysis Center provides global and regional contributions to fossil fuel CO ₂ emissions.

The Energy Information Administration provides a wide variety of energy-related statistics from the US Government. Figure 5 converts the national carbon emissions into the fractional contribution of each nation to greenhouse warming. This pie chart shows that the US is the major single contributor, accounting for 21% of the total. The former USSR and the European Economic Community are approximately the same at 14% and China is fourth at 7%. More recent figures likely will show China to be a much larger contributor, however.

Growth in Emissions

The consequential rise in atmospheric carbon dioxide (Figure 6) attributable to growth in use of fossil fuels (i.e., CO₂ production rate) will continue indefinitely unless the production rate is diminished by about 2% per year. Growth in the emissions since the 1860s shows that since 1950 we have gone from producing about 1 gigaton/yr of carbon to over 6 gigatons/yr. Finally, to demonstrate the measures needed to stabilize the atmospheric CO₂ concentrations at current levels, Figure 7 shows that a reduction in excess of 75% of current emissions is needed - a very unlikely scenario.

The annual review of renewable energy put out by the Energy Information Administration of the US Department of Energy provides a good overview of the forms and contributions of renewable energy to the US energy consumption picture.

Fuel Efficiency

Fuel-use efficiency has improved substantially in recent years but there still is room for improvement. Fossil-fuel power plants are about 33% efficient in delivering electrical power, which is close to the theoretical limit for such plants. Conventional internal combustion engine automobile power use (18-23%) could be improved to 21-27% for battery-electric cars, 30-35% for hybrid gasoline-electric cars, and 30-37% for fuel cell-electric cars. Personal choices by consumers in developed countries could accelerate use of more efficient vehicles, but personal choices by consumers in developing countries (principally India and China) could offset or reverse gains in fossil fuel reduction due to an increased number of vehicles (Hoffert et al, 2002).

Keeping CO₂ Out of the Atmosphere

Carbon dioxide may be kept from release to the atmosphere in power generation by (1) decarbonization or (2) sequestration. Decarbonization refers to reducing the amount of carbon emitted per unit of primary energy generated. Sequestration refers to either capturing CO₂ emitted by fuel combustion before it reaches the free atmosphere or capturing CO₂ already in the atmosphere. A trend from coal to oil to natural gas to hydrogen is a trend toward decarbonization, since each type releases progressively less carbon per unit power generated, with use of hydrogen being carbon free. Unfortunately, Earth does not have geological reservoirs of hydrogen. Also, converting fossil fuels to hydrogen produces more CO₂ than burning the fossil fuel directly. Creating H₂ by electrolysis of water with energy supplied by renewable or nuclear power is not cost effective.

Possible reservoirs for sequestering CO₂ include oceans, forests, soils, depleted natural gas and oil fields, deep saline aquifers, coal seams, and solid mineral carbonates. Capture of CO₂ by plant matter is the simplest form of sequestration, but methods and timescales for keeping the CO₂ from reverting to the atmosphere through decay must be considered. Forests at temperate latitudes may sequester up to 3 billion tons per year although as warming intensifies, soil respiration (loss of CO₂) may increase to the point where forests are net sources instead of sinks of CO₂.

Injection of CO₂ into the deep ocean also has been considered. Such massive amounts of CO₂ added to the ocean would make it somewhat more acidic (carbonic acid). Eventually the CO₂ would diffuse back to the surface after a time depending on how deep and where it is injected.

From these considerations, if CO₂-free fuels for 10-30 TW of power are not available by 2050, enormous quantities of CO₂ will need to be sequestered to reach CO₂ stabilization.

Sources and Forms of Energy

Fuels, or primary sources of energy for conversion to other forms, include fossil fuels, renewable fuels, and nuclear fuels.

Renewable energy sources include the following:

Biomass

Wood
Alcohol (from corn, sugar cane, etc.)
Waste (municipal solid waste, paper, manufacturing waste, refuse-derived fuel, and methane recovered from landfills)
Switchgrass, corn stalks, wheat straw, or other agricultural waste

Direct Solar

Passive thermal (solar home, no moving parts)
Active solar (uses pumps for moving solar-heated fluid around)
Photovoltaic (converts solar energy to electrical energy)

Wind (could be considered a form of solar energy, since heating and cooling of the earth by the sun sets up wind patterns)

Hydroelectric (could also be considered a form of solar energy, since solar energy evaporates water from oceans and moves it to high elevations where it condenses out as rain or snow and eventually flows downhill)

Geothermal (drawing heat from deep in the earth)

Ocean thermal (drawing heat from the ocean)

Ocean tidal (using the incoming tide or other low-frequency oscillation of sea level to raise an object, thereby creating potential energy)

Statistics on US use of renewable fuels reveal that about half of all renewable energy comes from hydroelectric.

Another way of categorizing energy is by its form:

Electrical

Chemical (batteries, chemical bonds in fossil or biomass fuels)

Nuclear

Radiant (solar or infrared radiation)

Potential (water behind a dam, spring in a mechanical watch)

Kinetic or mechanical (moving air, spinning flywheel)

Chemical, nuclear, potential, and kinetic forms of energy can be used for storage, whereas electrical and radiant cannot.

Energy Density

An important issue with regard to any primary source of energy is how "dense" it is. A second consideration is how transportable it is, and a third issue is how easily it can be changed into other forms. By these measures, oil is remarkably versatile, since it can be burned directly or in other forms, it can be transported in pipelines, and it can be transformed to electrical energy, which is perhaps the most versatile form of energy since it can be transmitted over stationary power lines and used for so many purposes.

By these measures, wind energy has a low density (we need a large turbine to capture sufficient kinetic energy of moving air to convert it to electricity). Solar energy also has low density. We can use photovoltaic cells to convert it directly to electrical energy or plant crops or trees to capture it and use it to convert atmospheric CO₂ to plant carbon (e.g., wood) which then can be burned to give electricity. For comparison, 10 TW would require area of 220,000 km² (about 1.5 times the area of the state of Iowa) for solar collectors. If we looked to biomass for this 10 TW per year, we would need more than 10% of the land area of the Earth, which is comparable to all that currently is used for agriculture (Hoffert et al, 2002).

Hydroelectric energy is a more dense form of primary energy. Since water is about 1,000 times as dense as air, it takes a wind turbine of area 1,000 times larger than a water turbine to get energy out for the same fluid speed.

Renewable sources of energy contribute less than 1% to global total energy consumption if we exclude wood and hydroelectric. Geothermal energy is a possible source in isolated locations where high temperature water is accessible from the Earth's surface, such as in Iceland. Other primary forms of energy (e.g., ocean thermal and ocean tidal) are very diffuse and difficult or expensive to concentrate.

Space-Based Solar Power

Space-based solar energy collection takes advantage of the higher density (intensity) of solar radiation above the influence of the Earth's atmosphere (approximately a factor of 8) if we consider that a fixed point on the Earth's surface has night, clouds, and atmospheric particles to reduce its daily total. If a sufficient amount of power could be captured in space, it would be beamed to Earth by microwave transmission. Collector size would be less than for Earth-based systems, although costs for launch and assemble of such systems must be factored in (Hoffert et al, 2002)

Nuclear Power

Nuclear power includes fission and fusion. For nuclear fission, the nucleus of uranium molecules , U²³⁵, can be split to release large amounts of energy. About 500 power plants world wide now use this technology. If we looked to nuclear power for 10 TW/year, known reserves of U²³⁵ would last only 6-30 years.

Nuclear fusion relies on a large energy release when neutrons or heavy forms of hydrogen are combined with each other or with helium or lithium to add or remove neutrons. Forcing fusion requires nuclear particles to be confined for a minimum amount of time at high density. This form of primary energy has been researched for decades, and, while progress has been made, substantial hurdles remain before this source becomes commercially available.

Reference:

Hoffert M. I., K. Caldeira, G. Benford, D. R. Criswell, C. Green, H. Herzog, A. K. Jain, H. S. Kheshgi, K. S. Lackner, J. S. Lewis, H. D. Lightfoot, W. Manheimer, J. C. Mankins, M. E. Mauel, L. J. Perkins, M.E. Schlesinger, T. Volk, and T. M. L. Wigley, 2002: Advanced technology paths to global climate stability: Energy for a greenhouse planet. Science 298, 981-987

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3-2: Energy Consumption, Conservation, and Future Options