Availability of Future Renewable and Non-Renewable Energy Supplies
rev. 3 January 2016
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 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 fundamental 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 heat 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)|
British Thermal Unit
|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.
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).
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:
|300 BTU||Typical fully charged laptop battery (14.8V / 5850 mAh)|
|2,000 BTU||Brew a single pot of coffee|
|125,000 BTU||Energy in one gallon of gasoline|
|950,000 BTU||Drive from St. Louis, MO to Kansas City, MO (247 miles) in a 30 MPG Toyota Camry|
|2 million BTU||Drive from St. Louis, MO to Kansas City, MO in a 15 MPG Lincoln Navigator SUV|
|3 million BTU||Burn a 100W light bulb continuously for a year|
|22 million BTU||Drive a loaded 40-ton GCW tractor trailer between Iowa City, IA to New Orleans, LA (1,000 miles, 6 MPG)|
|75 million BTU||World per capita annual primary energy use (BP 2015)|
|113 million BTU||Average annual electricity use in an American home in 2009 (EIA, 2010f)|
|302 million BTU||US per capita annual primary energy use (BP 2015)|
|98 quadrillion BTU (98 quads)||Total US primary energy use (BP 2015)|
|549 quadrillion BTU (549 quads)||Total world primary energy use (BP 2015)|
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 Lumen-hour (light)||0.005 BTU (Atkinson et al 2007, 12-28)|
|1 kWh (electricity - 100% efficiency)||3,412 BTU|
|1 horsepower for one hour||2,500 BTU|
|1 kWh (electricity - thermal conv. factor 33%)||10,339 BTU (Davis et al 2015, B-6)|
|1 Megaton TNT (destructive power)||3.9 T BTU (Hall et al 180)|
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 (gross)|
|1 Gallon of Diesel (Davis et al, 2015)||138,000 BTU (gross)|
|1 Gallon of gasoline (Davis et al, 2015)||125,000 BTU (gross)|
|1 Gallon of ethanol (Davis et al, 2015)||84,600 BTU (gross)|
|1 Pound of Jet A Fuel (EIA 2015; Imperial Oil, 2015)||20,260 BTU (gross)|
|1 Cubic Foot Dry Natural Gas (EIA 2015, A-4)||1,032 BTU|
|1 Therm Natural Gas||100,000 BTU|
|1 Trillion Cubic Feet (tcf) Natural Gas||1.032 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 lb Uranium (WNA 2015b)||166 MM BTU|
|1 lb. dry wood (Foote 2013)||8,600 BTU (gross)|
|1 cord (1.25 tons) fuel wood (EIA 2015, Table D-1)||20,000,000 BTU (gross)|
|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)||248,000,000 BTU|
|1,000 cubic feet hardwood (Haynes 1990)||320,000,000 BTU|
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).
There are two heating values that can be given for fuel combustion. A High (gross) value considers the energy that vaporizes the water resulting from combustion. A Low (net) value ignores that energy. Low values are commonly used reports from Europe. High values are used by the US Energy Information Administration and this document follows that convention in using high heating values (EIA 2015).
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%.
Using the BTU common unit, it is possible to compare the amount of energy used or produced in different forms. For example:
While high-performance athletes can work at levels up to 2.5 horsepower for brief spurts, over extended periods, humans can only generate the equivalent of 0.1 to 0.3 horsepower over extended periods:
0.1 horsepower * 2500 BTU/hp = 250 BTU 0.3 horsepower * 2500 BTU/hp = 750 BTU 250 BTU/person * 8 hours / farm-worker-day = 2,000 BTU 750 BTU/person * 8 hours / farm-worker-day = 6,000 BTU A farm worker exerts 2,000 - 6,000 BTU per working day 125,000 BTU/gallon-gasoline / 2,000 BTU/worker-day = 63 days 125,000 BTU/gallon-gasoline / 6,000 BTU/worker-day = 21 days One gallon of gasoline contains the equivalent of 21 and 63 days of human labor!
Going in the other direction, A drive from Urbana, IL to Naperville, IL in a compact sedan is around 130 miles:
130 miles / 30 miles/gallon-gasoline = 4.3 gallons of gasoline 4.3 gallons-gasoline * 21 days-labor/gallon = 90 days human labor 4.3 gallons-gasoline * 63 days-labor/gallon = 271 days human labor A drive from Urbana to Naperville in a compact sedan requires the energy equivalent of 90 - 271 days of human labor! This does not consider the energy used in the manufacture of the car, construction or maintenance of the highway, etc.
In considering efficiency, a conventional 60-watt incandescent light bulb is rated at emitting 800 lumens. Leaving that light on for an hour:
800 lumen-hours * 0.005 BTU/lumen-hour = 4 BTU 60 watt-hours * 3.412 BTU/watt-hour = 205 BTU Efficiency = output / input = 4 / 205 ~= 2% An incandescent bulb only converts 2% of electricity to light The other 98% of the energy is released as heat
An equivalent compact florescent bulb emitting the same amount of light would use around 15 watts:
800 lumen-hours * 0.005 BTU/lumen-hour = 4 BTU 15 watt-hours * 3.412 BTU/watt-hour = 51 BTU Efficiency = 4 / 51 ~= 8% Compact florescent bulb is 4x as efficient as comparable incandescent
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 in pounds:
ATL -> TLV = 240,000 pounds of Jet A fuel TLV -> ATL = 465,000 pounds of Jet A fuel (240,000 + 225,000) / 2000 lbs/ton = 232.5 tons! 465,000 lbs-fuel * 20,260 BTU/lb = 9.42 trillion BTU 9.42 trillion BTU / 125,000 BTU/gallon-gasoline = 75,400 gallons-gasoline-equivalent The trip was a total of around 12,800 statute miles 12,800 miles / 75,400 gallons = 0.17 MPGe for a B777 The B777 holds around 300 passengers and both of my flights were full 300 passengers * 12,800 miles / 75,400 gallons ~= 51 passenger-miles/gallon-gasoline-equivalent Typical compact sedan = 30 MPG Taking a B777 is more energy efficient than driving alone in a typical compact sedan
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 harness 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.
- Resource or In-Place figures give the estimate of the total amount of resource that is in the ground. Because these resources can only be measured indirectly, there is always some uncertainty about the accuracy of such figures.
- Potential or Technically Feasible figures give the amount of a resource that might be extractable with current technology or potential future technology. These figures are always lower than the In-Place figures since it is generally impossible to extract every last drop of a resource.
- Reserves are the actual amount of a resource that can be extracted with existing technology at a profit. This figure is lower than the potential figure both due to technical limitations and the portion of the potential resource that would require so much expense to extract that it would not be profitable.
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
- EROI is declining for fossil fuels (diminishing returns)
- The comparatively low EROI for renewables is a fundamental factor limiting their adoption thus far
Murphy (2011) asserts that concern should be focused as EROI starts to fall below 8. Above EROI of 8, the amount of energy needed to get more energy is fairly small, so the difference between and EROI of 80 and 40 is not that great in practical terms (2.5% vs. 1.2%). However, at an EROI of 2, half your energy is being spent to get more energy, which radically changes the economics and energetics of an energy-dependent society.
Given these caveats, EROI values published by Murphy and Hall (2011), with others as noted, 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|
|World oil production||1999||35||35|
|Shale gas (Yaratani & Matsushima)||2014||12||23|
|Bitumen from tar sands||n/a||2||4|
|Geothermal (Herendeen & Plant)||1981||9||17|
|Flate plate solar||n/a||1.9||1.9|
|Concentrating solar collector||n/a||1.6||1.6|
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.
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 conventional oil production in the lower 48 states based on a 200 billion barrel ultimate resource fit very closely to actual production over the next 60 years.
However, with the addition of production in Alaska (which began in earnest in the mid 1960s and grew rapidly with the opening of a the Trans-Alaska pipeline in 1977), a significant growth in natural gas plant liquids production (which were insignificant in 1956), and, most notably, the explosion of tight oil production in the early 2010s, total US liquid fuels production far exceeded Hubbert's expectations. As with Malthus's predictions, technological change and resource substitution changed the math that was the foundation of Hubbert's assumptions. Whether the optimistic EIA models prove to be more aligned with the ultimate reality remains to be seen.
In 1969, Hubbert made a similar projection for a world production peak between 1990 and 2000 based on ultimate global production of 1.35 and 2.1 trillion barrels, respectively. In this case, the fit is much weaker, with the geopolitical oil crisis and economic downturn of the early 80s a clear deviation from the logistic curve. While there was a leveling of demand in the late 2000s that led some catastrophists to assert that the peak had been reached, production resumed increasing in the 2010s. Whether this is a trend or transient remains to be seen.
(Data and R Script for the analysis above)
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, and 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 Annual Energy Consumption
Depending on the source, the Americans use three to five times the amount of energy on a per capita basis than the global average.
|Annual Amount||BTU||Per Capita||US % of World|
|World Primary (BP 2015)||549 Quads||75 MM BTU|
|US Primary (BP 2015)||98 Quads||302 MM BTU||18% of world|
|World Oil (BP 2015)||33.6 B Barrels||195 Quads||4.6 Barrels|
|US Oil (BP 2015)||6.95 B Barrels||40 Quads||21.5 Barrels||21% of world|
|World Natural Gas (BP 2015)||120 Trillion Cubic Feet||126 Quads||16,400 Cubic Feet|
|US Natural Gas (BP 2015)||26.8 Trillion Cubic Feet||26.4 Quads||83,000 Cubic Feet||21% of world|
|World Coal (BP 2015)||9,000 MM tons||167 Quads||1.23 tons|
|US Coal (BP 2015)||998 MM tons||21.6 Quads||3.1 tons||11% of world|
|World Electricity (BP 2015)||23,500 tWh||243 Quads||3.2 mWh|
|US Electricity (BP 2015)||4,300 tWh||44.4 Quads||13 mWh||18% of world|
|World Population (USCB 2015)||7,296 MM|
|US Population (USCB 2015)||323 MM||4.4% of world|
Annual consumption and years left figures are based on overall consumption for that particular class of fuels (e.g. liquid fuels, gas) rather than the current rates of production of that particular type of fuel. The intention is to place the reserves in a broader context and demonstrate that nationalist hopes for American non-renewable energy independence for any extended period of time face may be unrealistic.
|World Proved Reserves (BP 2015)||1,700 B Barrels||9,860 Quads||33.6 B Barrels||51 Years|
|World Proved Reserves (Davis 2015, 1-2)||1,656 B Barrels||9,605 Quads||33.6 B Barrels||49 Years|
|US Proved Reserves (BP 2015)||48.5 B Barrels||281 Quads||6.95 B Barrels||7 Years|
|US Proved Reserves (Davis 2015, 1-2)||36.5 B Barrels||211 Quads||6.95 B Barrels||5 Years|
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).
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. A 1996 USGS study indicated that undiscovered resources might represent up to 3,000 billion barrels.
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
|Venezuela Resource (USGS 2009)||1,300 B Barrels||7,540 Quads||33.6 B Barrels/year||38 Years|
|Venezuela Reserves (USGS 2009)||513 B Barrels||2,980 Quads||33.6 B Barrels/year||15 Years|
|Alberta Resource (Govt. of Alberta 2009)||1.71 T Barrels||9,918 Quads||33.6 B Barrels/year||50 Years|
|Alberta Reserves (Govt. of Alberta 2009)||173 B Barrels||1,000 Quads||33.6 B Barrels/year||5 Years|
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 degradation 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).
While some 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.
|World Resource (Dyni 2006)||2,826 B Barrels||16,400 Quads||33.6 B Barrels/year||84 Years|
|World Reserves (Dyni 2006; Bartis et al 2005)||1,700 B Barrels||9.800 Quads||33.6 B Barrels/year||50 Years|
|US Resource (Dyni 2006)||2,085 B Barrels||12,000 Quads||6.95 B Barrels/year||300 Years|
|US Reserves (Dyni 2006; Bartis et al 2005)||1,250 B Barrels||7,260 Quads||6.95 B Barrels/year||192 Years|
Oil shale is a precursor rock to petroleum that has never been deep or confined enough to form oil deposits. One of the world's largest oil shale deposits is the Green River formation that spans Colorado, Wyoming and Utah. 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).
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. But 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.
The 60% net recovery factor is from Bartis et al (2005).
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.
|World Coal Reserves (BP 2015)||981 B tons||18,700 Quads||165 Quads/year||113 Years|
|US Coal Reserves (BP 2015)||261 B tons||5,000 Quads||19 Quads/year||263 Years|
|US Coal Recoverable (EIA 2015b)||257 B tons||6,200 Quads||19 Quads/year||324 Years|
|US Coal Reserve Base (EIA 2015b)||480 B tons||11,500 Quads||19 Quads/year||600 Years|
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).
A challenge with assessing coal reserves and consumption is that quality and heat content of coal varies widely depending on the resource. The gold standard is Anthracite that can contain up to 98% carbon and have a heat content of 28 MM BTU/ton. At the other end of the spectrum is lignite, which can contain as little as 25% carbon and have a heat content of 9 MM BTU/ton. BP (2015) reports reserves in high and low groups: Anthracite/Bituminous and Sub-bituminous/lignite. In the calculations above Anthracite/Bituminous is converted at 24 BTU/ton and Sub-bituminous/lignite at 15 BTU/ton.
Further, the EIA groups reserves as recoverable at producing mines, estimated recoverable reserves, and demonstrated reserve base. Estimated is demonstrated minus considerations for land use restrictions and assumed recovery rates. Economic feasibility is not considered, so although the US clearly has a vast resource, the amount that can be extracted at a profit in the face of environmental concerns is an open question.
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. And there may be a historical symmetry as the first fossil fuel becomes the last fossil fuel as oil and natural gas reach their inevitable depletion.
|World Resource (MITEI 2010, 7)||12,400 - 20,800 tcf||12,2700 - 21,400 Quads||120 tcf/year||103 - 173 years|
|World Reserves (BP 2015)||6,610 Trillion Cubic Feet||6,780 Quads||120 tcf/year||55 Years|
|US Resource (MITEI 2010, 10)||1,440 - 2,100 tcf||1,480 - 2,160 Quads||26.8 tcf/year||55 - 80 years|
|US Reserves (BP 2015)||345 Trillion Cubic Feet||354 Quads||26.8 tcf/year||12 Years|
|US Dry Gas (EIA 2015c)||369 Trillion Cubic Feet||378 Quads||26.8 tcf/year||14 Years|
|US Shale Gas (EIA 2015c)||200 Trillion Cubic Feet||200 Quads||26.8 tcf/year||8 Years|
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 amount of gas ultimately recoverable at a profit is uncertain.
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, Yaratani and Matsushima (2014) actually give a mean EROI of 12, which is slightly better than the overall natural gas numbers provided by Murphy and Hall (2010). 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
|Global Resource (NRC 2012, 33)||35,000 - 4,200,000 tcf||36,000 - 4,300,000 Quads||120 tcf/Year||291 - 35,000 Years|
|Gulf of Mexico Resource (NRC 2012, 36)||11,000 - 34,000 tcf||11,300 - 35,000 Quads||26.8 tcf/year||421 - 1,300 Years|
|Alaska Technically Recoverable (NRC 2012, 36)||25.2 - 158 tcf||26 - 162 Quads||26.8 tcf/year||1 - 6 Years|
|Japan (NRC 2012, 44)||40 tcf||41 Quads||4.0 tcf/year||10 Years|
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 are orders of magnitude of uncertainty about the global resource, and since there has been no commercial hydrate production, reserves are only wild guesses. 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 (ironically) chilling.
|Global Resource (IAEA 2006)||38 MM tons||13,900 Quads||74,800 tons/year||500 Years|
|Global Resource (WNA 2015b; 2015c)||15.7 MM tons||5,733 Quads||74,800 tons/year||209 Years|
|Global Reserves (WNA 2015b; 2015c)||6.5 MM tons||2,370 Quads||74,800 tons/year||86 Years|
|US Reserves (WNA 2015b; 2015c)||228,000 tons||76 Quads||24,700 tons/year||9 Years|
|US Reserves (EIA 2010i)||614,000 tons||203 Quads||24,700 tons/year||24 Years|
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, 2011) 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. And the development of commercially-viable fusion would be a game changer for the planet. But it remains to be seen whether the need for cheap electricity ultimately overwhelms all the negatives associated with nuclear power.
Fuel to BTU conversions are based on BTU calculated backwards from electrical output using a thermal conversion rate divided by the US share (33%) of nuclear fuel use (WNA 2014b-c). The calculated figure of 35,400 kWh/kg is an order of magnitude below below the WNA (2015a) figure of 360,000 kWh/kg. And this is substantially lower than the theoretical amount of heat available with complete consumption of all fissile material in the fuel.
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.
Annual electrical energy is converted to BTU using the thermal power plant heat rate of 10,339 BTU/kWh to make the numbers comparable to the non-renewable sources that will need to be replaced.
|Annual Generation||BTU||% Electricity||% Total Energy|
|World Technically Feasible (IHA 2000)||14,370 tWh||149 Quads||61% World||27% World|
|World Economically Feasible (IHA 2000)||8,082 tWh||83 Quads||34% World||15% World|
|World Current (BP 2015)||3,880 tWh||40 Quads||16% World||7% World|
|US Technically Feasible (USDOE 2004)||1,300 tWh||14 Quads||30% US||14% US|
|US Economically Feasible (USDOE 2004)||745 tWh||8 Quads||17% US||8% US|
|US Potential (Lopez et al 2012)||300 tWh||3.1 Quads||7% US||3% US|
|US Current (USDOE 2004)||350 tWh||3.6 Quads||8% US||4% US|
|US Current (BP 2015)||261 tWh||2.7 Quads||6% US||3% US|
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).
Additional potential hydroelectric power pales 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.
|Annual Generation||BTU||% Electricity||% Total Energy|
|World Potential (Archer and Jacobson 2005)||28,700 - 222,000 tWh||297 - 2,290 Quads||120% - 943% World||54% - 420% World|
|World Current (BP 2015)||706 tWh||7.2 Quads||3.0% World||1.3% World|
|US Potential (AWS Truewind 2010)||38,600 tWh||400 Quads||900% US||400% US|
|US Onshore Potential (Lopez et al 2012)||32,700 tWh||338 Quads||760% US||340% US|
|US Offshore Potential (Lopez et al 2012)||17,000 tWh||176 Quads||400% US||180% US|
|US Current (BP 2015)||183 tWh||1.9 Quads||4.2% US||1.9% US|
Wind is a rapidly-maturing renewable energy source that has become an increasingly important in recent years. In the United States, wind generation increased dramatically since the late 1980s.
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:
- A typical wind turbine has a 2,000,000 watt nameplate capacity
- At a capacity factor of 30%, over a year (8,760 hours) that turbine would generate 5,256 mWh or around 54 billion BTU
- Completely replacing 98 quads of US energy usage would require 1.8 million wind turbines
- Wind turbines cost around $3.5 million apiece to install (Windustry, 2013)
- The total installation cost would be around $6.3 trillion dollars or 36% of total 2014 US GDP (BEA 2015)
- This does not consider maintenance, repair and renewal costs, or the cost of support infrastructure
|Annual tWh||Annual BTU||% Electricity||% Total Energy|
|US Urban Utility PV (Lopez et al 2012)||2,200 tWh||22.7 Quads||50% US||23% US|
|US Rural Utility PV (Lopez et al 2012)||280,600 tWh||2,900 Quads||650% US||300% US|
|US Rooftop PV (Lopez et al 2012)||800 tWh||8.3 Quads||19% US||8% US|
|US Concentrating Solar (Lopez et al 2012)||116,100 tWh||1,200 Quads||2,700% US||1,200% US|
|Arizona Scenario (Fthenakis et al 2009)||10,600 tWh||110 Quads||248% US||112% US|
|US Scenario (Deluchi & Jacobson, 2010)||11,800 tWh||122 Quads||274% US||124% US|
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 industrialized society as primary energy. 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, the complexities of land use, 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 could supply all US energy needs
- Jacobson and Delucchi (2009, 2010; Delucchi and Jacobson, 2010) took a cue from former vice president Al Gore and 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, which could supply all US energy needs. 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.
- Lopez et al (2012) used GIS land-use modeling to assess total technical potential from renewables across the 50 states. The potential numbers are stunning, but no attempt was made to model political or economic constraints
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.
|Annual BTU||% Total Energy|
|World Potential (Parikka 2004)||95 Quads||16% World|
|World Potential (Ladanai and Vinterbäck 2009||213 - 256 Quads||36% - 43% World|
|World Current (IPCC 2011)||50 Quads||8% World|
|US Potential (Perlack et al 2005)||9 Quads||9% US|
|US Potential (Lopez et al 2012)||5.2 Quads||5% US|
|US Current (EIA 2015)||4.8 Quads||5% US|
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.
Biomass still accounts for a significant amount of global energy use, primarily burned for cooking and heating in the global south (IPCC 2011, 5).
In 2005 the Department of Energy and Department of Agriculture 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."
|Annual Generation||BTU||% Electricity||% Total Energy|
|World Potential (Bertani 2003)||1,000 - 42,000 tWh||10 - 434 Quads||4% - 180% World||2% - 80% World|
|World Current (Bertani 2010)||67 tWh||0.69 Quads||0.3% World||0.1% World|
|US Hydrothermal Potential (Lopez et al 2012)||300 tWh||3 Quads||7% US||3% US|
|US Enhanced Geothermal Potential (Lopez et al 2012)||31300 tWh||320 Quads||730% US||330% US|
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. However, estimates of technical potential are speculative and, accordingly, vary by orders of magnitude.
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.
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:
- Tidal (75 quads): Tides are variations in sea level caused primarily by the gravitational pull of the moon, with additional help from the sun and the rotation of the earth. Therefore, tidal power is essentially indirect lunar power. Tides can be captured using barriers (two-way dams), lagoons and in-current turbines. Tidal energy facilities have been used for centuries and industrial-sized facilities are in operation, but the environmental impacts of barriers have constrained expansion of tidal power.
- Wave (62 quads): Ocean waves are small disturbances in the ocean surface caused by the force of blowing wind. Since the source of wind power is heat from the sun, wave power is essentially indirect solar power. A wide range of different devices for capturing wave energy have been developed or are in development, with most involving some kind of floating device that translates the kinetic undulations of waves into electricity.
- Ocean Thermal: Temperature differentials between warm surface waters and cold deep waters can be used with a heat engine to create kinetic energy to turn electrical generators. Although the potential power available is quite massive (6,800 quads per year) the practicality of large-scale facilities has yet to be demonstrated. High temperature differentials and low energy output requirements are desirable, which makes ocean thermal attractive on islands and in developing countries.
- Osmotic Power: Osmotic pressure differentials between ocean salt water and fresh water from rivers can be exploited to turn turbines and generate electricity. As with ocean thermal, technologies to exploit this resource are at an early stage in development and estimates of potential (80 quads per year) are highly speculative.
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.
Alaska Department of Revenue. 2013. Alaska Oil Production: Production History FY 1959-2012. http://www.tax.alaska.gov/sourcesbook/AlaskaProduction.pdf (accessed 30 December 2015).
Alberta Energy and Utilities Board (AEUB). 2007. Alberta's energy reserves 2006 and supply/demand outlook 2007-2016. http://www.ercb.ca/docs/products/STs/st98-2007.pdf (accessed 6 May 2011)
Alberta Energy Resources Conservation Board (ERCB). 2008. Alberta's energy reserves 2007 and supply/demand outlook 2008-2017. http://www.ercb.ca/docs/products/STs/st98-2007.pdf (accessed 6 May 2011)
American Physical Society (APS). 2011. Energy units. http://www.aps.org/policy/reports/popa-reports/energy/units.cfm (accessed 25 April 2011)
Archer, Cristina L. and Mark Z. Jacobson. 2003. Spatial and temporal distributions of U.S. winds and wind power at 80 m derived from measurements. Journal of Geophysical Research 108 (D9).
Atkinson, Barbara, Andrea Denver, James E. McMahon and Robert Clear. 2007. Energy-efficient lighting technologies and their applications in the commercial and residential sectors. In, Handbook of Energy Efficiency and Renewable Energy, ed. D. Yogi Goswami, Frank Kreith. Boca Raton, FL: CRC Press. https://books.google.com/books?id=cxDNBQAAQBAJ&pg=SA12-PA28.
AWS Truewind. 2010. Estimates of Windy Land Area and Wind Energy Potential by State for Areas >= 30% Capacity Factor at 80m. http://www.windpoweringamerica.gov/wind_maps.asp (accessed 7 May 2011).
BP. 2012. BP Statistical Review of World Energy, June 2012. http://www.bp.com/assets/bp_internet/globalbp/globalbp_uk_english/reports_and_publications/statistical_energy_review_2011/STAGING/local_assets/pdf/statistical_review_of_world_energy_full_report_2012.pdf (accessed 25 November 2013).
Bartis, James T., Tom LaTourrette, Lloyd Dixon, D.J. Peterson, and Gary Cecchine. 2005. Oil Shale Development in the United States Prospects and Policy Issues. Report MG-414-NETL. Santa Monica, CA: RAND Corporation. http://www.rand.org/pubs/monographs/2005/RAND_MG414.pdf (accessed 28 March 2011)
Bertani, Ruggero. 2003. What is geothermal potential? Extracted from IGA News 53, 1 - 3. http://www.inference.phy.cam.ac.uk/sustainable/refs/geothermal/potential.pdf (accessed 10 May 2011).
Bertani, Ruggero. 2010. Geothermal Power Generation in the World 20052010 Update Report. Proceedings World Geothermal Congress 2010, Bali, Indonesia, April 25-29, 2010. http://geotermia.org.mx/geotermia/pdf/WorldUpdate2010-Ruggero.pdf (accessed 10 May 2011).
Bureau of Economic Analysis (BEA). 2015. Gross Domestic Product: Fourth Quarter and Annual 2014 (Advance Estimate) http://www.bea.gov/newsreleases/national/gdp/2015/gdp4q14_adv.htm (accessed 31 December 2015).
Cleveland, Cutler J., Robert Costanza, Charles A. S. Hall, and Robert Kaufmann. 1984. Energy and the U.S. economy: A biophysical perspective. Science 225, pp 890-897.
Cleveland, Cutler. 2011. Energy return on investment (EROI) The Encyclopedia of Earth http://www.eoearth.org/article/Energy_return_on_investment_(EROI) (accessed 24 April 2011)
Davis, Stacy C., Susan W. Diegel, and Robert G. Boundy. 2009. Transportation Energy Data Book: Edition 28. ORNL-6984. Oak Ridge, TN: Oak Ridge National Laboratory.
Davis, Stacy C., Susan W. Diegel, and Robert G. Boundy. 2012 Transportation Energy Data Book: Edition 31. ORNL-6984. Oak Ridge, TN: Oak Ridge National Laboratory.
Davis, Stacy C., Susan W. Diegel, and Robert G. Boundy. 2015 Transportation Energy Data Book: Edition 34. ORNL-6991. Oak Ridge, TN: Oak Ridge National Laboratory. http://cta.ornl.gov/data/index.shtml (accessed 30 December 2015)
Deffeyes, Kenneth S. 2005. Beyond Oil : The View from Hubbert's Peak. New York : Hill and Wang.
Deffeys, Kenneth S. 2010. When Oil Peaked. New York, NY: Hill and Wang. http://books.google.com/books?id=P5Y96u9CZj8C (accessed 6 May 2011)
Delucchi, Mark A. and Mark Z. Jacobson. 2010. Providing all global energy with wind, water, and solar power, Part II: Reliability, system and transmission costs, and policies Energy Policy 39 (3), March, 1170-1190.
Dyni, John R. 2006. Geology and resources of some world oil-shale deposits: U.S. Geological Survey Scientific Investigations Report 20055294. http://pubs.usgs.gov/sir/2005/5294/pdf/sir5294_508.pdf (accessed 6 May 2011).
Electric Power Research Institute (EPRI). 2007. Assessment of Waterpower Potential and Development Needs (1014762). Palo Alto, CA. http://epri.com (accessed 10 May 2011).
Fermi, Enrico. 1937. Thermodynamics. Upper Saddle River, NJ: Prentice-Hall. http://books.google.com/books?id=VEZ1ljsT3IwC
Foote, Sam. 2013. How much heat is there in one pound of wood? http://www.mha-net.org/docs/v8n2/docs/WDBASICS.pdf (accessed 30 December 2015).
Franco, Alessandro and Alessandro Russo. 2002. Combined cycle plant efficiency increase based on the optimization of the heat recovery steam generator operating parameters. International Journal of Thermal Sciences 41 (9), 843 - 859.
Friese, Jon. 2008. North American natural gas production and EROI decline. http://www.theoildrum.com/node/3673 (accessed 7 May 2011)
Georgilakis, Pavlos S. 2008. Technical challenges associated with the integration of wind power into power systems. Renewable and Sustainable Energy Reviews 12, 852-863.
Geothermal Energy Association (GEA). 2011. Basics. http://geo-energy.org/Basics.aspx (accessed 10 May 2011).
Goldemberg, José and Oswaldo Lucon. 2010. Energy, Environment and Development. London and Sterling, VA: Earthscan. http://books.google.com/books?id=mn-pfeCFHvsC
Goldstein, Barry, Gerardo Hiriart, J.W. Tester, Ruggero Bertani, Chris Bromley, Luiz C.J. Gutierrez-Negrin, Ernst Huenges, Á. Ragnarsson, M.A. Mongillo, M.A. Muraoka, and Vilnus Zui. 2011. Great expectations for geothermal energy to 2100. Proceedings, Thirty-Sixth Workshop on Geothermal Reservoir Engineering (SGP-TR-191) Stanford University, Stanford, California, January 31 - February 2, 2011. http://ere.stanford.edu/pdf/IGAstandard/SGW/2011/goldstein1.pdf (accessed 10 May 2011).
Government of Alberta. 2009 . Responsible actions: A plan for Alberta's oil sands. http://treasuryboard.alberta.ca/ResponsibleActions.cfm (accessed 6 May 2011)
Government of Alberta. 2011. The oil sands story: The resource. http://www.oilsandsdiscovery.com/oil_sands_story/resource.html (accessed 6 May 2011)
Hall, Carl W. and George W. Hinman. 1983. Dictionary of Energy. New York: Marcel Dekker, Inc. https://books.google.com/books?id=3ykBDaKKk1YC (accessed 30 December 2015)
Hall, Charles A. S. 1972. Migration and metabolism in a temperate stream ecosystem. Ecology 53 (4), pp 585-604.
Groombridge, Brian and Martin Jenkins. 2002. World Atlas of Biodiversity. Prepared by the UNEP World Conservation Monitoring Centre. Berkeley, CA: University of California Press. http://books.google.co.nz/books?id=_kHeAXV5-XwC (accessed 10 May 2011)
Hall, Charles A.S. and Cutler J. Cleveland. 2005. EROI: definition, history and future implications. ASPO-US Conference, Denver, CO November 10. http://www.esf.edu/efb/hall/talks/EROI6a.ppt (accessed 24 April 2011)
Hammons, Thomas James. Tidal power. Proceedings of the IEEE 8 (3), 419 - 433.
Harvey, David. 1990. The Condition of Postmodernity: An Enquiry into the Origins of Cultural Change. Cambridge, MA: Blackwell.
Haynes, Richard W. 1990. An analysis of the timber situation in the United States: 1989-2040 General Technical Report RM-199. Fort Collins, CO: U.S. Department of Agriculture, Forest Service, Rocky Mountain Forest and Range Experiment Station. http://www.fs.fed.us/rm/pubs_rm/rm_gtr199.pdf (accessed 24 April 2011)
Haynes, Richard W. 2003. An analysis of the timber situation in the United States: 1952 to 2050. Gen. Tech. Rep. PNW-GTR-560. Portland, OR: U.S. Department of Agriculture, Forest Service, Pacific Northwest Research Station.
Heinberg, Richard. 2009. Searching For A Miracle: "Net Energy" Limits & The Fate of Industrial Society. San Francisco, CA and Santa Rosa, CA: International Forum on Globalization and the Post Carbon Institute. http://www.postcarbon.org/new-site-files/Reports/Searching_for_a_Miracle_web10nov09.pdf (accessed 4 April 2011).
Herendeen, Robert A. and Randall L. Plant. 1981. Energy analysis of four geothermal technologies Energy 6 (1), Pages 73-82.
Hoogwijk, Monique and Wina Graus. 2008. Global potential of renewable energy sources: A literature assessment. Utrecht, The Netherlands: Ecofys Group. http://ecofys.net/com/publications/documents/Report_global_potential_of_renewable_energy_sources_a_literature_assessment.pdf (accessed 10 May 2011).
Hubbert, M. King. 1956. Nuclear energy and the fossil fuels. In, Spring Meeting of the Southern District, Division of Production, American Petroleum Institute, San Antonio, TX, 7-9 March. http://www.hubbertpeak.com/hubbert/1956/1956.pdf (accessed 21 July 2012).
Hubbert, M. King. 1969. Energy resources. In, Resources and Man. San Francisco: W.H. Freeman and Company. http://www.hubbertpeak.com/hubbert/resourcesandman.pdf (accessed 30 December 2015).
Imperial Oil. 2015. Aviation turbine fuel Jet A-1 conversion chart. http://www.imperialoil.ca/Canada-English/products_conversion.aspx (accesed 30 December 2015)
Intergovernmental Panel on Climate Change (IPCC). 2011. Summary for Policymakers: Special Report on Renewable Energy Sources and Climate Change Mitigation http://srren.ipcc-wg3.de/ (accessed 10 May 2011)
International Atomic Energy Agency (IAEA). 2006. Global uranium resources to meet projected demand. http://www.iaea.org/newscenter/news/2006/uranium_resources.html (accessed 7 May 2011).
International Energy Agency (IEA). 2000. Hydropower - A Key to Prosperity in the Growing World http://www.ieahydro.org/uploads/files/hydropower_a_key_to_prosperity.pdf (accessed 7 May 2011).
International Hydropower Association. 2000. Hydropower and the World's Energy Future. http://www.ieahydro.org/reports/Hydrofut.pdf (accessed 7 May 2011).
Islandsbanki. 2011. U.S. Geothermal Industry Overview (February). http://www.islandsbanki.is/english/geothermal-energy/datamarket (accessed 10 May 2011).
Jacobson, Mark Z. and Mark A. Delucchi. 2009. A path to sustainable energy by 2030. Scientific American, 301 (5), November, 58-65.
Jacobson, Mark Z. and Mark A. Delucchi. 2010. Providing all global energy with wind, water, and solar power, Part I: Technologies, energy resources, quantities and areas of infrastructure, and materials. Energy Policy 39 (3), March, 1154-1169.
King, Carey W. 2010. Energy intensity ratios as net energy measures of United States energy production and expenditures. Environmental Research Letters 5 (044006).
Kreutz, Thomas G., Eric D. Larson, Guangjian Liu, Robert H. Williams. 2008 Fischer-Tropsch Fuels from Coal and Biomass Paper presented at the 25th Annual International Pittsburgh Coal Conference Pittsburgh, Pennsylvania, 29 September 2 October. http://web.mit.edu/mitei/docs/reports/kreutz-fischer-tropsch.pdf (accessed 7 May 2011)
Kunstler, James Howard. 2005. The Long Emergency: Surviving the End of Oil, Climate Change, and Other Converging Catastrophes of the Twenty-First Century. New York: Atlantic Monthly Press.
Ladanai, Svetlana and Johan Vinterbäck. 2009. Global Potential of Sustainable Biomass for Energy (Report 013, ISSN 1654-9406) Uppsala: Swedish University of Agricultural Sciences, Department of Energy and Technology http://www.worldbioenergy.org/system/files/file/WBA_PP-1_100122final10.pdf (accessed 10 May 2011)
Lawicki, Dave. 2002. Jet Fuel Characteristics. Boeing Aircraft Corporation. http://www.smartcockpit.com/download.php?path=docs/&file=Jet_Fuel_Characteristics.pdf (accesed 30 December 2015)
Lopez, Anthony, Billy Roberts, Donna Heimiller,Nate Blair, and Gian Porro. 2012. U.S. Renewable Energy Technical Potentials: A GIS-Based Analysis. Golden, CO: National Renewable Energy Laboratory. http://www.nrel.gov/docs/fy12osti/51946.pdf (accessed 1 January 2016).
Luce, Henry. 1941. The American century. Life Magazine, 17 February. http://books.google.com/books?id=I0kEAAAAMBAJ (accessed 12 November 2010).
Merriam Webster. 2013. Energy. http://www.merriam-webster.com/dictionary/energy.
Meyer, Richard F. and Emil D. Attanasi. 2003. Heavy Oil and Natural Bitumen -- Strategic Petroleum Resources. U.S. Geological Survey, Fact Sheet 70-03. http://pubs.usgs.gov/fs/fs070-03/fs070-03.html (accessed 28 March 2011)
Minerals Management Service (MMS). 2006. Technology White Paper on Wave Energy Potential on the U.S. Outer Continental Shelf. http://ocsenergy.anl.gov/documents/docs/OCS_EIS_WhitePaper_Wave.pdf (accessed 7 April 2011).
MIT Energy Initiative (MITEI). 2010. The future of natural gas: An interdisciplinary MIT study. http://web.mit.edu/mitei/research/studies/naturalgas.html (accessed 7 May 2011)
Mórrígan, Tariel. 2010. Peak Energy, Climate Change and the Collapse of Global Civilization: The Current Peak Oil Crisis. Oil Crisis. Global Climate Change, Human Security & Democracy, Orfalea Center for Global & International Studies, University of California, Santa Barbara. http://www.econ.utah.edu/~mli/Economics%207004/Morrigan_2010_Energy_CC4.pdf (accessed 7 May 2011)
Murphy, David. 2011. The Energy Return on Investment Threshold. http://www.resilience.org/stories/2011-11-26/energy-return-investment-threshold (accessed 31 December 2015)
National Renewable Energy Laboratory (NREL). 2010. Estimates of Windy Land Area and Wind Energy Potential by State for Areas >= 30% Capacity Factor at 80m. http://www.windpoweringamerica.gov/pdfs/wind_maps/wind_potential_80m_30percent.pdf (accessed 5 April 2011).
National Research Council (NRC). 2012. Realizing the Energy Potential of Methane Hydrate for the United States. Washington, DC: The National Academies Press. http://www.nap.edu/openbook.php?record_id=12831&page=1 (accessed 9 March 2013).
Natural Gas Supply Association (NGSA). 2012. Understanding the size of U.S. Natural Gas Resources. http://www.ngsa.org/Assets/understanding the size of natural gas resources in the united states.pdf (accessed 9 March 2013).
Nuclear Energy Institute (NEI). 2010. Nuclear energy in Illinois. http://www.nei.org/filefolder/Illinois_Fact_Sheet.pdf (accessed 7 May 2011).
Osborn, Stephen G., Avner Vengosh, Nathaniel R. Warner, and Robert B. Jackson. 2011. Methane contamination of drinking water accompanying gas-well drilling and hydraulic fracturing Proceedings of the National Academy of Sciences of the United States of America 1100682108v1-5. http://www.pnas.org/content/early/2011/05/02/1100682108.full.pdf+html (accessed 10 May 2011)
Parikka, Matt. 2004. Global biomass fuel resources. Biomass and Bioenergy 27 (6), 613-620.
Perlack, Robert D., Lynn L. Wright Anthony F. Turhollow, Robin L. Graham, Bryce J. Stokes, Donald C. Erbach. 2005. Biomass As Feedstock For A Bioenergy And Bioproducts Industry: The Technical Feasibility Of A Billion-Ton Annual Supply. Oak Ridge, TN: Oak Ridge National Laboratory. http://www1.eere.energy.gov/biomass/pdfs/final_billionton_vision_report2.pdf (accessed 7 April 2011).
Pimentel, David and Tad W. Patzek. 2005. Ethanol production using corn, switchgrass, and wood; Biodiesel production using soybean and sunflower. Natural Resources Research 14 (1), 65 - 76.
Renewable Energy Research Laboratory, University of Massachusetts at Amherst (RERL). 2011. Wind Power: Capacity Factor, Intermittency, and what happens when the wind doesn't blow? http://www.ceere.org/rerl/about_wind/RERL_Fact_Sheet_2a_Capacity_Factor.pdf (accessed 24 April 2011)
Rodrigue, Jean Paul, Claude Comtois and Brian Slack. 2009. Changes in Major Crude Oil Reserves, 2001-2006. In The Geography of Transport Systems (second edition). Hofstra University, Department of Global Studies & Geography. http://people.hofstra.edu/geotrans/eng/ch5en/appl5en/oilreserves.html (accessed 6 May 2011)
Rosen, Marc A., Minh N. Le, and Ibrahim Dincer. 2005. Efficiency analysis of a cogeneration and district energy system Applied Thermal Engineering 25 (2005), 147159.
Rubin, Jeff. 2009. Why Your World Is About To Get A Whole Lot Smaller: Oil and the End of Globalization. New York: Random House
Rudolf, John Collins. 2011. Leaked cables reveal U.S. concerns over Saudi 'Peak Oil.' http://green.blogs.nytimes.com/2011/02/10/leaked-cables-reveal-u-s-concerns-over-saudi-peak-oil (accessed 6 May 2011)
Smil, Vaclav. 2008. Energy in Nature and Society: General Energetics of Complex Systems. Cambridge, MA: MIT Press.
Smil, Vaclav. 2010. Energy Myths and Realities: Bringing Science to the Energy Policy Debate. Washington, DC: American Enterprise Institute.
Smith, J. Charles, Michael R. Milligan Edgar A. DeMeo and Brian Parsons. 2007. Utility wind integration and operating impact state of the art IEEE Transactions on Power Systems 22 (3), 900-908.
Texas Comptroller of Public Accounts (TCPA). 2008. The energy report 2008. http://www.window.state.tx.us/specialrpt/energy (accessed 7 May 2011).
U.S. Census Bureau. 1975. Chapter M, Minerals. In, Historical Statistics of the United States: Colonial times to 1970. Washington, DC. http://www.census.gov/compendia/statab/past_years.html (accesed 12 December 2013)
U.S. Census Bureau (USCB). 2015. Population Clock. http://www.census.gov/popclock (accessed 30 December 2015).
U.S. Department of Energy (DOE). 2004. Water Energy Resources of the United States with Emphasis on Low Head/Low Power Resources. http://www1.eere.energy.gov/water/pdfs/doewater-11111.pdf (accessed 31 December 2015).
U.S. Department of Energy (DOE). 2011a. Advanced Technologies and Energy Efficiency. http://www.fueleconomy.gov/feg/atv.shtml (accessed 24 April 2011).
U.S. Department of Energy (DOE). 2011b. U.S. Installed Wind Capacity and Wind Project Locations http://www.windpoweringamerica.gov/wind_installed_capacity.asp (accessed 25 April 2011).
U.S. Department of Energy (DOE). 2012. U.S. and Japan Complete Successful Field Trial of Methane Hydrate Production Technologies (press release). http://energy.gov/articles/us-and-japan-complete-successful-field-trial-methane-hydrate-production-technologies (accessed 9 March 2013).
U.S. Department of the Interior (USDOI). 2005. Hydroelectric Power. www.usbr.gov/power/edu/pamphlet.pdf (accessed 7 May 2011).
U.S. Energy Information Administration (EIA). 2009a. International Petroleum (Oil) Reserves and Resources. http://www.eia.doe.gov/international/oilreserves.html (accessed 4 April 2011)
U.S. Energy Information Administration (EIA). 2010a. U.S. Total Crude Oil Proved Reserves, Reserves Changes, and Production. http://www.eia.doe.gov/dnav/pet/pet_crd_pres_dcu_NUS_a.htm (accessed 4 April 2011)
U.S. Energy Information Administration (EIA). 2010b. U.S. Crude Oil, Natural Gas, and Natural Gas Liquids Reserves. http://www.eia.doe.gov/oil_gas/natural_gas/data_publications/crude_oil_natural_gas_reserves/cr.html (accessed 4 April 2011)
U.S. Energy Information Administration (EIA). 2010c. International Energy Outlook 2010. Report #DOE/EIA-0484(2010). http://www.eia.doe.gov/oiaf/ieo/index.html (accessed 4 April 2011)
U.S. Energy Information Administration (EIA). 2010d. International Energy Statistics. http://www.eia.gov/cfapps/ipdbproject/IEDIndex3.cfm?tid=5&pid=53&aid=1 (accessed 4 April 2011)
U.S. Energy Information Administration (EIA). 2010e. Annual Energy Outlook 2011 Early Release. Report Number: DOE/EIA-0383ER (2011). http://eia.doe.gov/forecasts/aeo/index.cfm (accessed 5 April 2011)
U.S. Energy Information Administration (EIA). 2010f. Electric Sales, Revenue and Price: Table 5A. Residential Average Monthly Bill by Census Division and State. http://www.eia.doe.gov/cneaf/electricity/esr/table5_a.html (accessed 6 May 2011)
U.S. Energy Information Administration (EIA). 2010g. Renewable Energy Annual 2008. http://www.eia.doe.gov/cneaf/solar.renewables/page/rea_data/rea.pdf (accessed 7 May 2011).
U.S. Energy Information Administration (EIA). 2010h. Annual Energy Review 2009. http://www.eia.doe.gov/totalenergy/data/annual (accessed 7 May 2011).
U.S. Energy Information Administration (EIA). 2010i. U.S. Uranium Reserves Estimates. http://www.eia.gov/uranium/reserves/ (accessed 31 December 2015)
U.S. Energy Information Administration (EIA). 2011a. World Shale Gas Resources: An Initial Assessment of 14 Regions Outside the United States. http://www.eia.gov/analysis/studies/worldshalegas/ (accessed 14 April 2011)
U.S. Energy Information Administration (EIA). 2011b. Annual Coal Report 2009, DOE/EIA-0584 (2009). Washington, DC: Office of Oil, Gas, and Coal Supply Statistics, U.S. Energy Information Administration, U.S. Department of Energy. http://eia.doe.gov/cneaf/coal/page/acr/acr.pdf (accessed 25 April 2011)
U.S. Energy Information Administration (EIA). 2011c. Annual Energy Outlook (DOE/EIA-0383 2011). http://eia.doe.gov/forecasts/aeo (accessed 6 May 2011)
U.S. Energy Information Administration (EIA). 2011d. Canada. http://www.eia.doe.gov/countries/cab.cfm?fips=CA (accessed 6 May 2011)
U.S. Energy Information Administration (EIA). 2011e. Geothermal explained. http://www.eia.doe.gov/energyexplained/index.cfm?page=geothermal_home (accessed 10 May 2011).
U.S. Energy Information Administration (EIA). 2012. Annual Energy Review 2011. http://www.eia.doe.gov/totalenergy/data/annual (accessed 7 May 2011).
U.S. Energy Information Administration (EIA). 2014. Tight oil production pushes U.S. crude supply to over 10% of world total. http://www.eia.gov/todayinenergy/detail.cfm?id=15571 (accessed 30 December 2015).
U.S. Energy Information Administration (EIA). 2015. Monthly Energy Review, September. http://www.eia.gov/totalenergy/data/monthly/ (accessed 25 September 2015).
U.S. Energy Information Administration (EIA). 2015b. Annual Coal Report 2013. http://www.eia.gov/coal/annual/pdf/acr.pdf (accessed 31 December 2015).
U.S. Energy Information Administration (EIA). 2015c. U.S. Crude Oil and Natural Gas Proved Reserves, 2014. https://www.eia.gov/naturalgas/crudeoilreserves/ (accessed 31 December 2015).
U.S. Geological Survey (USGS). 1996. U.S. Geological Survey World Petroleum Assessment 2000 – Description And Results U.S. Geological Survey Digital Data Series - DDS-60. http://pubs.usgs.gov/dds/dds-060/ESpt2.html (accessed 4 April 2011)
U.S. Geological Survey (USGS). 2008. Assessment of Moderate- and High-Temperature Geothermal Resources of the United States, September 2008, http://pubs.usgs.gov/fs/2008/3082/pdf/fs2008-3082.pdf (accessed 10 May 2011).
U.S. Geological Survey. 2009 An Estimate of Recoverable Heavy Oil Resources of the Orinoco Oil Belt, Venezuela. http://pubs.usgs.gov/fs/2009/3028/pdf/FS09-3028.pdf (accessed 7 April 2011).
United Nations Development Programme (UNDP). 2008. World Energy Assessment. http://www.undp.org/energy/activities/wea (accessed 10 May 2011).
World Energy Council (WEC). 2010. 2010 Survey of world energy resources. http://www.worldenergy.org/documents/ser_2010_report_1.pdf (accessed 10 May 2011).
World Nuclear Association (WNA). 2011. Nuclear power in France. http://www.world-nuclear.org/info/inf40.html (accessed 7 May 2011).
World Nuclear Association (WNA). 2015a. The Economics of Nuclear Power http://www.world-nuclear.org/info/Economic-Aspects/Economics-of-Nuclear-Power/ (accessed 31 December 2015).
World Nuclear Association (WNA). 2015b. Nuclear share figures, 2004-2014. http://www.world-nuclear.org/info/Facts-and-Figures/Nuclear-generation-by-country/ (accessed 31 December 2015).
World Nuclear Association (WNA). 2015c. Supply of Uranium. http://www.world-nuclear.org/info/Nuclear-Fuel-Cycle/Uranium-Resources/Supply-of-Uranium/ (accessed 31 December 2015).
World Nuclear Association (WNA). 2015d. Uranium Markets. http://www.world-nuclear.org/info/nuclear-fuel-cycle/uranium-resources/uranium-markets/ (accessed 31 December 2015).
World Nuclear Association (WNA). 2015e. Uranium production figures, 2004-2014 http://www.world-nuclear.org/info/Facts-and-Figures/Uranium-production-figures/ (accessed 31 December 2015).
Williams, David R. 2010. Earth fact sheet. NASA National Space Science Data Center. http://nssdc.gsfc.nasa.gov/planetary/factsheet/earthfact.html (accessed 5 April 2011).
Windustry. 2013. How much do wind turbines cost? http://www.windustry.org/how-much-do-wind-turbines-cost
Yaritani, Hiroaki and Jun Matsushima. 2014. Analysis of the Energy Balance of Shale Gas Development Energies 7(4), 2207-2227. http://www.mdpi.com/1996-1073/7/4/2207/htm (accessed 31 December 2015)
Ying, Gao. 2007. News Analysis: Why does Kuwait keep its oil reserves secret? http://news.xinhuanet.com/english/2007-07/12/content_6366187.htm (accessed 6 May 2011)
Zirin, Harold. 2011. Solar constant. Encyclopedia Britannica. http://www.britannica.com/EBchecked/topic/552889/solar-constant (accessed 5 April 2011)
All these woes shall serve for sweet discourses in our time to come. (William Shakespeare)