Availability of Future Renewable and Non-Renewable Energy Supplies

Michael Minn

rev. 11 March 2013

This report provides a introduction to, and summary of, current and potential future energy resources. While there is considerable uncertainty in the quantification of available resources due to significant technical and political limitations, this information is provided to summarize some parameters that can inform further research and debate.

Energy Basics

Energy is "a fundamental entity of nature that is transferred between parts of a system in the production of physical change within the system" (Merriam Webster, 2013). Using Sir Issac Newton's definitions, energy is the ability to do work, and work is the result of force moving something over a distance (Goldemberg and Lucon, 2010, 4). Energy is a key not only to physical processes but also to biological life itself.

Energy comes in many forms: light, heat, movement, electricity, etc. Energy is stored as potential energy. For example, photosynthesis is a biological process that stores solar energy in the carbohydrates that make up a plant. When that plant is burned, that stored energy is released as kinetic energy.

Measuring Energy

For each different form of energy, there are different units for measuring amounts of energy as well as the flow of energy (power).

Form Power (Rate of Use) Energy (Amount of Use)
Heat Watt Joule
British Thermal Unit
Motion (Kinetic) Horsepower Foot-Pound
Electricity Watt
Light Lumen Lumen-second
Food Calories per Day Calorie
Fossil Fuels (Potential) Barrels per Day Barrel of Oil Equivalent (BOE - Petroleum)
Thousand Cubic Feet (Tcf - Natural Gas)
Tons / Tonnes (Coal)

The amount of energy is the flow of energy multiplied by the amount of time that flow is ongoing. For example, a 60-watt light bulb burning for eight hours uses approximately 60 * 8 or 480 watt-hours (0.48 kilowatt hours).

Conversely, the length of time that a stored power source can be used is the amount of energy divided by the flow. For example, a flashlight that uses three 2.4 watt-hour AA batteries and a 100-milliwatt LED can theoretically burn for 72 hours (3 * 2.4 / 0.1), although batteries often provide less than their rated capacity depending on the type of load and environmental conditions.

Converting Energy

The thermodynamic principle of conservation of energy recognizes the equivalence of heat and mechanical work (Fermi 1937). Conversion between forms of energy is a fundamental task performed by both machines and living organisms. While energy from different sources is often not interchangeable (e.g. solar-generated electricity cannot currently be used to power commercial jet airliners), technological and social adaptation can often permit significant levels of substitution (e.g. trains replace airliners).

Energy Conversions (Smil, 2008, 14)

Measurements of energy in different forms and from different sources can be converted to common units for rough comparison. In this paper, the common unit for measuring amounts of energy is the British Thermal Unit (BTU), which is equivalent to the amount of heat needed to raise the temperature of one pound of water at sea level by one degree Fahrenheit. The following are examples of the amount of energy needed for some specific tasks:

Energy Task
300 BTU Typical fully charged laptop battery (14.8V / 5850 mAh)
2,000 BTU Brew a single pot of coffee
950,000 BTU Drive from St. Louis, MO to Kansas City, MO (247 miles) in a 30 MPG Toyota Camry
2,000,000 BTU Drive from St. Louis, MO to Kansas City, MO in a 15 MPG Lincoln Navigator SUV
3,000,000 BTU Burn a 100W light bulb continuously for a year
22,000,000 BTU Drive a loaded 40-ton GCW tractor trailer between Iowa City, IA to New Orleans, LA (1,000 miles, 6 MPG)
113,000,000 BTU Average annual electricity use in an American home in 2009 (EIA, 2010f)

Because the BTU is a fairly small unit, large-scale energy usage figures from government literature are commonly given in quadrillion BTU, or quads. Although various scales of Joules (MJ, EJ, etc.) are commonly used in scientific literature to conform to the International System of Units, BTUs are used in this document to eliminate unnecessary mental transformation when referencing source documents. Since American total energy use has hovered around 100 quads since the mid 2000s, use of quads also facilitates quick mental calculation to percents. Quads are also fairly close to exajoules (1 quad = 1.055 EJ), so there is a rough interchangeability with literature that uses exajoules.

Listed below are some theoretical conversion factors between different energy measurement units:

Source Destination Conversion Factor
1 BTU 1,055 Joules
1 Quad 1.055 Exajoules
1 Quad 1,000,000,000,000,000 BTU
1 Kilocalories (heat) 3.966 BTU
1 Foot-pound (kinetic) 0.0012851 BTU
1 Megaton TNT (destructive power) 3,800,000 BTU
1 kWh (electricity - thermal conv. factor 33%) 10,339 BTU (Davis et al 2009, B-6)
1 kWh (electricity - 100% efficiency) 3,412 BTU
1 horsepower for one hour 2,500 BTU

Heat Content of Different Fuels

The table below summarizes conversions between units and typical heat values representing the energy capacity of various fuels:

1 Tonne of Oil Equivalent (Toe) 42,460,000 BTU
1 Barrel Petroleum (EIA 2010e) 5,800,000 BTU (net)
1 Gallon of Diesel (Davis et al, 2012) 128,700 BTU (net)
1 Gallon of gasoline (Davis et al, 2012) 115,400 BTU (net)
1 Gallon of ethanol (Davis et al, 2012) 75,700 BTU (net)
1 Pound of Jet A Fuel (EIA 2010e) 18,600 BTU
Natural Gas
1 Cubic Foot Natural Gas 1,027 BTU
1 Therm Natural Gas 100,000 BTU
1 Trillion Cubic Feet Natural Gas (tcf) 1.027 Quads
1 ton coal - US Avg. 2009 (EIA 2011b) 19,973,000 BTU
1 ton anthracite coal (EIA 2011b) 22,000,000 - 25,000,000 - 28,000,000 BTU (low,avg,high)
1 ton bituminous coal (EIA 2011b) 21,000,000 - 24,000,000 - 30,000,000 BTU (low,avg,high)
1 ton subbituminous coal (EIA 2011b) 17,000,000 - 17,500,000 - 24,000,000 BTU (low,avg,high)
1 ton lignite coal - high (EIA 2011b) 9,000,000 - 13,000,000 - 17,000,000 BTU (low,avg,high)
1 kg Uranium (WNA 2011) 360,000 BTU
1 lb. air-dry wood 6,400 BTU
1 lb. agricultural residue (moist) 4,300 BTU
1 lb. agricultural residue (dry) 7,300 BTU
1 bushel of corn (EIA, 2012, 330) 392,000 BTU
1,000 cubic feet softwood (Haynes 1990) 198,400,000 BTU
1,000 cubic feet hardwood (Haynes 1990) 256,000,000 BTU

Conversion Considerations

Although conversion between measurement units is generally trivial, the transformation between physical availability of fuel and energy involves uncertainty due to variations in fuel constitution and energy density, as well as often significant losses in different conversion processes.

For example, most fossil fuels are burned, with useful kinetic energy released as a by-product of generated heat. As such, with current technologies, much of the potential energy in fossil fuels and biomass is lost as waste heat sent up cooling towers or vented in radiators. Efficiency ranges from 15% for spark ignition gasoline engines (DOE 2011), to 58% for combined-cycle natural gas plants, (Franco and Russo 2002). In large cities (and with the UIUC campus generating plant), the steam output from the generating turbines can be distributed to area buildings for heat and hot water. In this way a significant amount of the "waste" heat from the fuel can be recovered for useful purposes, and this process (co-generation) can increase the effective efficiency of the plant to over 80% (Rosen, Le and Dincer, 2005).

All energy sources (especially renewables) have some level of intermittency. Coal-fired generators must be taken off-line occasionally for maintenance. Solar power is not generated at night and wind power is unavailable when the wind slows or stops blowing. Therefore, the capacity factor of renewable generating facilities must be considered when calculating the amount of energy generated over a given period of time based on the rated maximum potential power output. Capacity factors for renewable sources is dependent both on technology and geography, with hydroelectric dams having capacity factors as high as 80% and wind farms having capacity factors as low as 20% (RERL 2011).

A related concept is load factor, which represents how effectively a system's capacity is utilized by customer demand. Capacity factor focuses primarily on supply while load factor represents the level of harmony between supply and demand. Load factor is commonly use in transportation to indicate the percent of maximum capacity used on an average basis, such as the average percentage of seats occupied on an airplane). A car with four seats but carrying only a solo driver has a load factor of 25%.

Conversion Examples

Using the BTU common unit, it is possible to compare the amount of energy used or produced in different forms. For example:

Going in the other direction:

In considering efficiency:

Time-Space Compression

A further consideration should be given to the difference between conversion efficiency and use efficiency. Modern jet airplanes use a tremendous amount of fuel, but they are actually quite efficient in terms of the amount of energy needed to transport a single person for a single mile (passenger-mile).

For example: On a 2011 vacation to Israel, I flew a Boeing 777 between Atlanta and Tel Aviv. On disembarking at both ends, I asked the pilots how much fuel we had used.

This is a demonstration of the geographic phenomenon referred to as Time-Space Compression (Harvey, 1990, 240-307). Humans generally perceive the length of travel in terms of time (or financial expense) rather than in terms of distance. Technology has permitted humans to harnass fossil energy and move very quickly (both on land and in the air), so the perceived distance of my Israel trip was actually quite short. The equivalent trip 300 years ago by sailing ship would have taken weeks and would have been a complex, expensive and dangerous endeavor.

One significant implication of time-space compression is that although developed countries often use energy efficiently in thermodynamic terms, they tend to use more energy in total than developing countries. The flip side of that is the use value of a gallon of Diesel to a farmer in the developing world (such as to get crops to a local market) is greater than the use value of that same gallon to an American (who would use that same gallon only to move a truck of lettuce six miles on its way from California). This is referred to as marginal value.

Resources vs Reserves

In looking at estimates for the amount of a resource that is available, it is important to distinguish between three different ways of looking at a resource.

These figures can change due to improvements in technology, changes in accounting standards, or increases in resource prices that make previously inaccessible resources economically viable. Adding to the uncertainty of these figures are commercial or political considerations that provide incentives to overstate or understate reserves. For example, OPEC production quotas are based on a country's reserves, which provides an incentive for countries to overstate their reserves.


All energy sources require an investment of energy to extract a larger amount of energy and Energy Return on Energy Invested (EROEI or EROI) is used in this paper to evaluate the actual amount of energy available from a resource after the energy investment needed to exploit that resource is subtracted. EROI is a ratio of energy invested to energy gained, with higher values generally being considered preferable. The term EROI appears to originate with Cleveland et al (1984) although the concept dates back at least to Hall (1972). The concept can also be applied to larger biophysical and economic perspectives.

Despite the increasing prominence of life-cycle energy analysis for specific commodities and products, Murphy and Hall (2010, 109) note that there has been a surprisingly small amount of peer-reviewed research on EROI since the heyday of federally-funded American energy research in the 1970s and early 1980s. As with all life-cycle evaluations, differing decisions of where to set the boundaries of analysis can result in significantly different results. In addition, the complexity and opacity of the energy industry make it largely impossible to set definitive, exact values of EROI for any specific energy source. Nevertheless, it seems fair to assert that high EROI has been one of the principle advantages of fossil fuels, that EROI is declining for fossil fuels, and that the comparatively low EROI for renewables is a fundamental factor limiting their adoption thus far.

Given these caveats, EROI values published by Murphy and Hall (2011) are summarized in the table below:

Source Year Low EROI High EROI
Oil and gas 1930 100 100
Oil and gas 1970 30 30
Oil and gas 2005 11 18
Discoveries 1970 8 8
Production 1970 20 20
World oil production 1999 35 35
Imported oil 1990 35 35
Imported oil 2005 18 18
Imported oil 2007 12 12
Natural gas 2005 10 10
Coal (mine-mouth) 1950 80 80
Coal (mine-mouth) 2000 80 80
Bitumen from tar sands n/a 2 4
Shale oil n/a 5 5
Other nonrenewable
Nuclear n/a 5 15
Hydropower n/a 100 200
Wind turbines n/a 18 18
Geothermal (Herendeen & Plant, 1981) 1981 9 17
Wave energy n/a n/a n/a
Flate plate solar n/a 1.9 1.9
Concentrating solar collector n/a 1.6 1.6
Photovoltaic n/a 6.8 6.8
Passive solar n/a n/a n/a
Ethanol (sugarcane) n/a 0.8 10
Corn-based ethanol n/a 0.8 1.6
Biodiesel n/a 1.3 1.3

Peak Oil

In 1956, Shell Oil geologist M. King Hubbert presented a paper at an American Petroleum Institute conference that observed that resource extraction tended to follow a logistic curve (similar to a bell-shaped curve), and asserted that US petroleum production would peak in the early 1970s, with a similar peak for world production between 1990 and 2000.

This is the concept of peak oil. Although half the resource would still be available after the peak, the inability to satisfy existing and new demands could result in a disruptive transition to new energy sources (in the best scenario) or conflict over access to a shrinking resource (in the worst case scenarios).

As shown in the figure below, Hubbert's predictions for the US were fairly close in timing, although the addition of the Alaskan oil fields in the late 1960s threw him quite a bit below the peak in volume. In addition, high prices and new technologies are resulting in a smaller secondary peak with unconventional resources, which demonstrate that reality generally does not follow the smooth curves of simple statistical models.

Hubbert's predictions for the world were similarly uncorrelated with actual production figures. There has been somewhat of a plateau in production since 2005, although the opacity of the oil industry makes it impossible to know whether this is a result of geological limitations, economic conditions, geopolitics, corporate conspiracy, or some combination of all these factors.

The combination of petroleum dependency and uncertainty about supply has given rise to a cottage industry in books and conferences dealing with the concept of peak oil. Notable books on the subject and its social effects include Deffeys (2005), Kunstler (2005), Rubin (2009) and the websites for the Association for the Study of Peak Oil and Gas (apousa.org) and The Oil Drum (www.theoildrum.com). Smil (2010, 55-78) stands as one of the most articulate and well-researched skeptics of the peak-oil hypothesis.

Smil (2010) notes that the concerns about the exhaustion of oil resources have been present from the beginning of the commercialization of oil in the late 19th century. Regardless, the fairly common consensus is that fossil fuels in general and petroleum in particular are finite resources. So the question is less about whether there will be a peak, than when it will occur and, more importantly, how humanity will adapt when that peak occurs.

Cheap oil has facilitated cheap transportation, which in turn has facilitated vast spatial growth and decentralization in the United States. In that way, oil is embedded in the spatial structure of the US. The multi-trillion investment in vehicles, and the built environment means that undoing that embeddedness will not be easy or painless. In addition, with the rapid spread of development and petroleum-dependent lifestyles around the globe (especially in highly-populous India and China), the potential for violent contention for dwindling petroleum resources poses significant concerns for the future.

The most significant characteristic of petroleum-based fuels is their liquid state. This offers the same unparalleled advantage in density and ease of handling that made gasoline dominant over other power sources at the dawn of the automobile age. Substitutes are available, but they are not as cheap or easy as oil was in its heyday.

Total World and US Energy Use

Total world energy use in 2011 was around 521 quads or around 75 million BTU per capita (BP, 2012; USCB, 2013).

In 2011, world oil consumption was around 32 billion barrels or 186 quads.

In 2011, world natural gas consumption was around 114 trillion cubic feet or 117 quads.

Total US energy use in 2011 was around 97 quads or around 311 million BTU per capita (EIA, 2012). That number has been comparatively flat since the mid 1990s. Americans use over 4 times as much energy as the world average (on a per capita basis) and with only about 4.5% of the world's population use around 19% of world's energy. The figure above is a Sankey diagram showing sources and destinations for US energy use.

Total US oil consumption in 2011 was around 6.87 billion barrels or 39.9 quads (BP, 2012). Around 70% of American petroleum consumption was for transportation (EIA, 2012, 117), and American transportation is almost totally dependent on petroleum, which accounts for around 94% of direct transport energy use (Davis, Diegel and Boundy, 2009, 1-1, 2-4).

Total US natural gas consumption in 2011 was around 24 trillion cubic feet or 25 quads (BP, 2012). Around 23 trillion cubic feet of natural gas was produced in the US, an all time high.


Conventional Petroleum

The EIA (2010c) cites private sources for proven remaining conventional petroleum reserves of 1,200-1,300 billion barrels, or 6,500-7,300 quads after considering EROI. A 1996 USGS study indicated that undiscovered resources might represent up to 3,000 billion barrels.

The amount of remaining petroleum is a subject of considerable debate and is highly uncertain. Although petroleum availability is best expressed as reserves, which is the estimated amount that can be profitably extracted from known discoveries using existing technology, media reports frequently cite resource numbers, which represent the estimate of the total amount of the resource in the ground and which are likely much higher than the amount of the resource that will ultimately be produced.

Petroleum reserve estimation is an inexact science that is complicated by powerful political and commercial forces. The foreign reserve estimates from the EIA (2009a) are accompanied by the caveat that such figures are, "very difficult to develop," and that they simply made foreign fuel estimates available but did not certify the figures. The EIA figures come from three commercial sources: the oil company BP, Oil & Gas Journal and World Oil.

OPEC figures are especially suspect, especially since 1985 when the cartel decided to link production quotas to reserves estimates, resulting in suspicious increases in reserves declarations (Rodrigue, Comtois and Slack, 2009; Ying, 2007). A Wikileaked 2007 cable from an Aramco engineer and board member indicated that Saudi estimates of total reserves of 900 billion barrels might be overstated by as much as 300 billion barrels and that production from aging fields would begin declining after only 64 billion barrels.

Tar Sands / Natural Bitumen

Tar sands are deposits of bitumen - complex hydrocarbon molecules that comprise a very thick form of petroleum mixed with sand and water. Alberta and Venezuela contain vast deposits of bitumen close to the surface. Extraction of the bitumen and synthesis into liquid fuel requires significant amounts of water and input energy (usually from natural gas), resulting both in severe environmental degredation and low EROI. Nevertheless, as petroleum has become less plentiful and more expensive, oil sands have become increasingly viable economically and will likely be a significant world source for energy for many years (Rubin 2009, 42-45; Smil, 2010, 69-72).

The total amount of tar sands in Alberta and Venezuela could represent almost 4,000 billion barrels of oil or 23,000 quads of energy, but the net energy available when considering EROI and technical limitations on extraction is more reasonably in the range of 2,000-3,000 quads. (USGS 2009; Government of Alberta, 2009, 2011)

While some North Americans look to the Canadian oil sands as a potential path to freedom from foreign energy dependency, fuels are traded on a global market and there is no assurance that all or even most of the oil extracted from the Canadian tar sands will end up powering American SUV's. The EIA (2011d) notes that Chinese firms are, " beginning to forge a potent presence in the oil sands." As Chinese and Indian economic development continues and automobility expands, many generations of Canadians will likely pay a heavy environmental price for only a few decades of continued world automobility.

Oil Shale

Oil shale is a precursor rock to petroleum that has never been deep or confined enough to form oil deposits. Extraction of oil from the rock requires water and some variation on heating, which converts the organic kerogen to usable oil. As with tar sands, this results in high environmental impact and low EROI. Although oil shale exists in many countries around the world and has been commercially exploited, low petroleum prices have thus far made it economically unattractive except in a handful of locations (Deffeys, 2005, 109-123).

Estimated recoverable world oil shale potential is around 1,200 billion barrels, or around 5,800 quads when EROI is considered (Calculated from Dyni, 2006 and Bartis, 2005). One of the world's largest oil shale deposits is the Green River formation that spans Colorado, Wyoming and Utah, which places total US recoverable oil shale in the range of 500-1,100 billion barrels, or 2,300-5,100 quads.

Given the vastness of this resource, oil shale may have a significant role in the closing decades of the fossil fuel era if some way of economically tapping the resource can be found.

Note that oil shale should not be confused with shale oil (or tight oil) which is conventional petroleum locked in dense rock formations that can be extracted using hydraulic fracturing.


Coal is the granddaddy of industrial-age fossil fuels and still has quite a bit of life left in it. In 2007, coal was the source for 27% of world energy consumption, with 64% of that used for electrical power generation. While coal consumption has been flat in developed countries for a number of decades, it's use in developing countries has spiked since 2000 and is expected to continue to grow significantly (EIA 2010c). Liquid fuels derived synthesized from coal using the Fischer-Tropsch process powered the German war machine in World War II and changing energy economics are reviving interest coal-to-liquid fuels as a drop-in substitute for dwindling petroleum supplies from often insecure sources (Kreutz et al, 2008).

The EIA (2010c) reported world recoverable coal reserves of 909 billion tons, or around 18,000 quads when EROI is considered. Although reserves at American producing mines is only around 18 billion tons, recoverable reserves are estimated at 262 billion tons, or around 5,000 quads. Although coal extraction is perilous to both miners and the environment, and the fuel has a high carbon content that makes a significant contribution to climate change, it's abundance, economy, ease of use and high EROI will likely make it a significant component of the world energy market for decades to come.

Natural Gas

Natural gas is primarily methane (CH4) with varying mixtures of more complex hydrocarbons. Although gas for fuel can be produced from other sources (notably coal or decaying municipal waste), since the mid 20th century most natural gas has come from wells. Conventional natural gas deposits are either dissolved in oil deposits, sit in a cap above oil or coal deposits, or come from depths below 15,000 feet where heat and pressure break hydrocarbons down to the simple single-carbon molecule (Deffeys 2005, 52-81).

More recently, higher prices and technological innovation (notably, hydraulic fracturing) have made extraction of gas trapped in sedimentary shale deposits both practical and lucrative. This development has opened vast new fossil fuel resources which Smil (2010, 56) argues will significantly prolong the fossil fuel era as a substitution for declining petroleum availability. However, the price of that prolongation may significant contamination and overuse of surface and subterranean water supplies (MITEI 2010, 14-16; Osborn et al 2011) along with exacerbation of anthropogenic climate change.

Given the relative novelty of shale gas extraction and the size of the potential resource, the range in estimates of technically recoverable reserves is also quite wide. The EIA (2010c) lists proven world reserves at 6,609 trillion cubic feet, or around 6,100 quads when EROI is considered. An MIT study (MITEI 2010) gave a range of estimated recoverable resource of 12,400 - 20,800 trillion cubic feet, or 11,500 - 19,200 quads. Shale gas dominates projections for America's energy production future, with an estimate of 423 - 1,230 trillion cubic feet of reserves, or 391 - 1,764 quads, compared to only 205 trillion cubic feet (189 quads) of conventional gas.

Although shale gas extraction would seem to be more energy intensive than conventional natural gas extraction due to the physical fracturing needed to free the gas from the rock, Murphy and Hall (2010) do not include a specific EROI for shale gas and there does not currently appear to be any peer-reviewed research specifically focused on shale gas EROI measurement. A non-peer-reviewed report posted on the peak oil site The Oil Drum (Friese, 2008) anticipated a fairly rapid EROI decline to break-even.

Methane Hydrates / Methane Clathrates

Methane hydrates are molecules of methane locked in water crystals - effectively natural gas in ice. These compounds are formed where methane and water are present at low temperatures and high pressure, such as the ocean sediments. There is considerable uncertainty about the global resource, with estimates ranging from 35,000 - 35,000,000 trillion cubic feet or 36,000 - 36,000,000 quads (NRC, 2012, 33). But there is unquestionably enormous potential - leading to the oft-stated dream of a 1,000 year supply (NGSA, 2012).

There are numerous ongoing research projects into methane hydrate gas production (USDOE, 2012) and the NRC (2012, 6) observed no insurmountable technical challenges to economically-viable commercial development. But for now, exploitation of this resource remains hypothetical, with ultimate production, as usual, likely to be considerably lower than whatever the global resource actually is. And the climate implications of releasing all that carbon into the atmosphere are chilling.


Although Lewis Strauss's promise of electricity that was, "Too cheap to meter" never came true (Smil, 2010, 31), electricity generated with nuclear fission still accounts for 14% of world electricity generation (EIA 2010c). Nuclear power provides 75% of France's electricity (WNA, 2011b) and 49% of the electricity in Illinois (NEI, 2010). While growth in the United States has been stalled by massive costs, waste disposal issues and a hostile public, nuclear fission remains a vital part of the debate over the future of energy. Both Deffeys (2005, 124-151) and Smil (2010, 31-43) find a rare point of agreement that nuclear belongs in the world's energy future.

In addition to the geologic uncertainties about exact quantities of extractable uranium, the actual amount of fission energy available will likely be increased by improved reactor and fuel processing technology in the future. The World Nuclear Association (2011) identifies reserves of 5.4 million tonnes, which is slightly above the 4.7 million tonne figure from the International Atomic Energy (IAEA, 2006). Using a figure of 360,000 kWh electricity per kg of uranium from the World Nuclear Association (2011) and an EROI range of 5 - 15 yields a nuclear potential of 4,600 - 6,200 quads. However, the IAEA indicates that as much as 35 million tonnes may be commercially exploitable, giving a potential energy availability of 34,000 to 40,000 quads. It remains to be seen whether the need for cheap electricity ultimately overwhelms all the negatives associated with nuclear power.

Renewable Energy Sources

In contrast to concentrated fossil energy sources, renewable sources tend to be more sparsely dispersed, making them much more difficult and costly to exploit.


Hydroelectric power is the most mature and reliable of the renewable energy sources, providing 16% of the world's electricity in 2007 (EIA 2010c) and much higher percentages in many countries (IHA 2000). In the U.S. hydropower in 2009 only supplied around 7% of electricity (EIA 2010h).

The International Hydropower Association (IHA, 2000) estimated that the world's technically feasible hydroelectric generating capacity is around 14,370 TWh per year, or around 49 quads, although the economically feasible amount is somewhat less at 8,082 TWh / year or around 27 quads per year. The US Department of Energy estimated American technically feasible capacity of 300 GW and economically feasible capacity of 170 GW, or 9 quads and 5 quads, respectively.

While the potential values are substantially above above 2008 world generation of 3,119 TWh (10.5 quads) and 2009 US generation of 272 TWh (2.8 quads), those numbers pale in comparison to availability from non-renewable sources and overall energy demand. Additionally, political and environmental considerations present severe barriers to construction of new hydroelectric plants (TCPA, 2008, 268). While there almost certainly will be a place for hydroelectric in many locations and at different scales, there is not enough untapped hydroelectric power to replace significant amounts of energy currently supplied by non-renewables.

Note that the EIA uses a fossil-fuel heat conversion rate when converting hydroelectric Watt-hours to BTU, but this document uses a direct conversion rate that reduces the BTU values and facilitates a more realistic comparison with other renewable electricity sources.


Wind is a rapidly-maturing renewable energy source that has become an increasingly important in recent years. In the United States, wind generation increased from a statistically insignificant amount in 1988 to 70.8 GWh in 2009, or almost 2% of American electricity generation (EIA 2010h). Globally, installed capacity grew from 18 GW in 2000 to 159 GW in 2009 with strong growth expected to continue well into the future (EIA 2010c).

Archer and Jacobson (2005) estimate that world wind generation is the equivalent of 6,995 - 54,000 Mtoe per year, or between 280 - 2,200 quads per year. A study by AWS Truewind (2010) for the National Renewable Energy Laboratory estimated annual onshore wind energy potential in the United States of 38,553 TWh, or around 124 quads per year.

There are significant limitations on the number of areas where the wind blows strongly and consistently enough to make utility-scale power generation practical. In the United States, most of the class IV or better areas are the Midwest and offshore, with few good areas in the Northwest or deep south (Archer and Jacobson, 2003). The intermittency of wind presents technical challenges, although Georgilakis (2008) indicates that impacts on the power grid are modest up to a penetration level of 20%, and Smith et al (2007) asserts that at penetration levels of up to 30%, the problems will be more economic than physical.

While domestic wind power could theoretically supply the total current energy needs of the United States, the geographic sparsity of the resource, the issue of intermittency, and the high expense present a significant challenge to exploiting that potential. For example:


Energy from the sun drives almost all biological and environmental processes on earth and is at the root of most energy resources. Fossil fuels are essentially ancient sunlight that has been preserved in the earth. Wind and waves are driven by solar heating of the atmosphere. The amount of solar energy that strikes the earth annually is around 5.2 million quads, or almost 10,000 times the amount of energy used by humans in machines. Yet, in 2007, solar power only accounted for around 6 TWh of worldwide electrical generation (0.062 quads) and the question remains unanswered as to how much of the massive potential can be economically captured for useful work.

Because of the broad, sparse distribution of solar energy across the earth's surface and the evolving technology for concentrating, capturing and converting that energy for useful work, it is impossible to give a firm estimate of how much solar energy will be available in the future for use with machines. However, a number of authors have imagined possible scenarios for increased solar power generation in the future.

Fthenakis et al (2009) estimated that there is at least 250,000 square miles of land in the sunny American desert Southwest that is suitable for construction of industrial-scale solar plants. This land receives around 4,500 quads of solar energy each year and converting just 2.5% of that energy to electricity would generate 110 quads per year, or more than annual American energy consumption from all sources.

Taking a cue from former vice president Al Gore, Jacobson and Delucchi (2009, 2010; Delucchi and Jacobson, 2010) proposed a complete transition of the United States to wind, water and sunlight by 2030 in a plan that included 49,000 300-MW concentrating solar plants, 40,000 300-MW photovoltaic solar plants, and 1.7 billion 3-kW rooftop photovoltaic systems. With a 15% average capacity factor and EROI of 6.8, this would produce around 122 quads per year. Smil (2010, 148) dismisses the proposal as "claptrap" not only for its exponentially unprecedented scope and schedule, but also for it's estimated $100 trillion price tag.

Aside from industrial-scale plants and rooftop photovoltaic arrays, solar energy can be used in smaller-scale applications. Homes can be and are designed to take advantage of passive solar heating. Rooftop flat-plate water heaters have been used for decades. Small photovoltaic arrays are a common site for powering remote equipment that cannot be conveniently connected to the power grid. At the opposite end of the complexity scale, solar satellite systems and solar chimneys have been proposed.

However, few of these systems are yet price competitive with non-renewable energy sources in the absence of subsidies. While it seems likely that solar will at some point become an important energy source, the time for large-scale solar would seem to still be some distance in the future.


The Department of Energy defines "biomass" as, "Any organic matter that is available on a renewable or recurring basis, including agricultural crops and trees, wood and wood residues, plants (including aquatic plants), grasses, animal manure, municipal residues, and other residue materials. Biomass is generally produced in a sustainable manner from water and carbon dioxide by photosynthesis." (Perlack et al 2005). Biomass and biofuels derived from biomass can therefore be thought of as indirect solar energy.

Biomass can be incinerated directly for heat, or converted to liquid fuels through a broad variety of chemical and thermochemical processes that are still evolving. Indeed, perhaps the biggest attraction of biomass is that it is currently the only renewable source of liquid transportation fuel that can be used with the existing carbon-fuel-based transportation fleet, fuel distribution network and industrial infrastructure. Accordingly, biofuels have been widely marketed as the green fuels of the future, and there is an extensive amount of ongoing biofuel / biomass research.

Estimates of total annual biomass growth on the planet vary widely. Groombridge and Jenkins (2002, 10-11) give an estimate of total global biomass at 560 billion tonnes, with an annual growth in the range of 100 to 320 billion tonnes, 45% to 55% of that growth in the ocean. Parikka (2004) estimates that there is total of 420 billion tonnes of woody biomass on the planet (equivalent to around 5,900 quads) and 52 billion tonnes (734 quads) in North and Central America. The resource is significant - the question is how much of that can be economically and sustainably harvested for use in generating energy.

In 2008, biomass accounted for around 10.2% of global energy use (50 quads), most of it burned for cooking and heating in developing countries (IPCC 2011, 5). Parikka (2004) estimates that 95 quads globally could be sustainably produced. Ladanai and Vinterbäck (2009) cite a range of 213 - 256 quads.

Specifically for the United States, the Department of Energy and Department of Agriculture in 2005 adopted a strategic goal of 1 billion annual short tons of biomass production (Perlack et al, 2005), which may be equivalent to as much as 9 quads. That figure included a broad range of crops and crop residues, as well as municipal waste and animal manure. However, since that only represents around 9% of 2009 American energy usage, biomass alone clearly can not provide the solution to America's sustainable energy future.

In addition, no generalized, universally-accepted EROI figures are available that consider the energy required to harvest, transport and process bulky biomass on an industrial scale. Accordingly, the net energy available from biomass may be significantly below those direct conversion figures given above. Ethanol from corn is a notable case, with some authors even asserting that the energy required to produce ethanol actually exceeds the resulting energy in the fuel (Pimentel and Patzek 2005). Smil (2010, 98-115) is especially scornful of biofuels, stating, "Using complicated, energy-intensive, environmentally disruptive, and actually nonrenewable processes to produce liquid fuels for oversized, highly inefficient machines -- which are operated all too often for dubious reasons -- adds up to compounded irrationalities."

Geothermal Energy

Geothermal energy is heat that flows from the earth's interior resulting from the slow decay of radioactive particles in rocks (EIA 2011e). The estimated continuous geothermal heat produced by the planet is 42 TW or 1,255 quads per year. This is resource with significant long-term potential since the interior of the Earth is expected to remain extremely hot for billions of years to come (GEA 2011). Geothermal power can be used directly for heating or, in locations where heat levels are adequate, to generate electricity.

Geothermal power use in 2010 was fairly small 67 TWh per year (around 0.23 quads) (Bertani 2010). Estimates of global potential vary so widely as to be almost useless. Bertani (2003) site sources with electrical generating potential in a range of 1 - 142 quads per year. Goldstein (2011) gives a more optimistic range of 112 - 1,051 quads per year, which begins to approach the published figures for the total heat generated by the planet.

The major problem with geothermal energy is that there are a limited number of geographic locations where there is enough of this heat concentrated close enough to the surface where it can be accessed and exploited with a practical amount of effort. Most American geothermal sites are located in the West: California, Nevada, Arizona, Idaho, Utah, New Mexico, Alaska, Nevada, Hawaii (USGS 2008). There is also an issue in that some geothermal injection wells have been linked to earthquakes (Glanz 2009). But while geographic and thermodynamic constraints make geothermal a relatively limited resource in terms of total power needs, exploitation makes sense in areas where the resources exist.

Ocean Energy

The world's oceans represent a vast sink and storehouse of energy. The UNDP (2008, 166) World Energy Assessment groups extractable energy into four categories with estimates from various sources of world annual potential:

Older studies on this issue are an order of magnitude more pessimistic than the UNDP. The U.S. Minerals Management Service (MMS 2006), cited research estimating world economically feasible wave potential at 0.5 - 2.6 quads per year, with the caveat that projected improvements in capture technology could double or triple that amount. Hammons (1993) speculated that world tidal potential is 0.3 - 1.7 quads per year. The MMS estimated the total amount of wave energy on U.S. ocean shores (including Alaska and Hawaii) to be around 7 quads per year, but EPRI (2010) only believes the economically practical resource to be in the range of 0.3 - 0.6 quads per year.

Given the wide range in estimates and significant technical hurdles yet to be overcome, it is not possible to assess how soon or to what extent ocean energy will make a meaningful contribution to energy needs outside of a few niche applications in isolated areas.


Discussion of energy issues in America is often colored by the magnitude of the situation and the seeming inadequacy of the alternatives. It is difficult to look at a crowded interstate and imagine how we could ever replace all that gasoline. The American Century (Luce 1941) was driven by cheap fossil fuel and we have built much of our infrastructure and culture around resources that will be exhausted well before the next century. In addition, use of fossil fuels is the primary cause of climate change, which threatens to significantly alter favorable weather patterns that have blessed North America with a largely hospitable living environment and ample agricultural resources. Reactions to this situation have ranged from denial to a cornucopian faith in technology to fatalist assumptions that we will have to return to the primitive agrarian lifestyles of our forefathers.

It is clear that the amount of fossil fuel is finite and that renewables will not provide simple plug-in replacements any time soon. Either supply or demand will have to adjust to a new equilibrium at some point in the future. However, even as the world begins to scrape the bottom of the fossil fuel barrel (Rubin 2009, 52), the reported numbers for nonconventional fossil fuel sources seem to indicate there are plenty of hydrocarbons left in the ground - albeit at a higher economic and environmental price.

Winston Churchill is commonly quoted as saying that, "Americans can always be counted on to do the right thing - after they have exhausted all other possibilities." It seems we are now in the process of exhausting those possibilities.


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God gave man a penis and a brain. And only enough blood to run one at a time. (Robin Williams)