Even if the world could instantly switch over to an alternative energy source to minimize global warming, it wouldn’t be obvious which ones fit the bill. We examine the principles that could help make the choice.
Some sources release little CO2 but produce other greenhouse gases whose impact may be more severe. Others emit few greenhouse gases when burned, but the process of producing them releases a lot of GHG-emissions. High emissions aside, such a high energy intensity process could not sustainably and cost-effectively produce enough fuel replace fossil fuels on a global scale.
Making accurate comparisons among fuels demands a life-cycle analysis (LCA), examining all environmental impacts due to the fuel from cradle to grave. This includes extracting or growing the fuel's raw materials; production; transportation to its site of use; combustion of the fuel; and sometimes even disposal of byproducts.[1] (Henceforth I will use "production" to encompass all pre-combusion stages.) A complete LCA assesses greenhouse gas emissions, water contamination, general air pollution, forest destruction and much more.[2]
A life-cycle carbon analysis focuses on GHG emissions. It is helpful to examine it alongside a life-cycle energy analysis (LCEA) that considers the energy inputs to a fuel's production process, as the two are intertwined.[3] All else being equal, reducing the energy needed to produce a fuel almost always reduces GHG emissions.[4] However, a fuel may do well on one count and poorly on the other. For example, its production process might use up a lot electrical energy, but keep GHGs low by getting electricity from wind power.
This article examines key concepts for understanding LCEAs and carbon LCAs. We will focus on liquid fuels, but the concepts can be adapted for other alternative energy technologies such as solar, wind, hydro, geothermal, and nuclear power.
Global Warming Potential: what is a “unit” of greenhouse gas
To compare the effects of fuel life-cycles that release diverse greenhouse gases, we need to calculate the “global warming potential” (GWP) of each gas. GWP estimates the capacity of a greenhouse gas to trap heat in the atmosphere over time. CO2 is assigned a GWP of 1, and the values for other gases tell us how many times more heat is trapped by that gas over a given time period compared to a release of the same mass of CO2.
For example: methane traps far more heat than an equal mass of CO2, but within a few years' time nearly all of the methane breaks down into CO2 and hydrogen gas.[5] So, after 20 years methane has a GWP of 62, but over 100 years (the time frame used in the Kyoto Protocol), the GWP is only 23. The original release of methane has led to only about 23 times as much global warming as CO2 released at the same time.
{"CO2" from here on will imply "CO2 or equivalent quantity of other GHG over the time period of interest."}
Energy Intensity of Production (and what to not mistake it for)
The process of getting any fuel—be it fossil fuel or alternative fuel-- into a usable form for a given application requires additional energy inputs, such as electricity to operate an oil refinery plant, or gasoline to drive the tractors on a farm that grows corn for biofuels. A fuel production process should increase the total available energy, and/or convert available energy into a form needed for a given application (e.g.: burning coal to produce electricity that can run machinery to refine liquid fuel for a car—since you can’t just stick coal in your car engine.[6]) Otherwise we’d be better off just using the original fuel in the final application.[7]
The amount of energy inputs it takes to extract and refine one energy-unit of fuel and transport it to the point of use is called energy intensity of production. (Many studies use the inverse, energy ratio, energy in output fuel/energy inputs.) The “inputs” here only include energy powering the production processes, not the energy contained in the feedstock, such as crude oil.[8]
Energy return on energy invested, energy efficiency, energy balance, and net energy value can also be obtained directly from energy intensity. These metrics include the same information content, but they are not the same. Politicians that compare the energy ratio of one fuel to the energy efficiency of another may mistakenly (or deviously) advocate the less climate-friendly or efficent choice.[9] A glossary of these terms, with sample calculations, is included at the end of the article. Keep in mind that they are sometimes defined differently from one source to another, and may have a broader meaning outside the fuel LCEA context. Pay careful attention to a source's stated definitions.
One serious variation is that not all authors include both low-carbon and high-carbon emissions inputs in calculating these metrics. Following Lenzen (2008), the energy ratio would be the same whether the electrical plant running your ethanol refinery is coal-fired or wind-powered.[10] However, Gnansounou (2005) only counts fossil fuel consumption in the energy ratio, and Wang (2002) defines net energy value as the difference between the consumable energy in a fuel and its fossil fuel production inputs.[11] These fossil-only calculations are helpful in assessing the climate impact of a fuel.
Going further and distinguishing between different types of fossil fuel inputs is key for nations concerned about energy security as well as climate change. For a nation rich in coal but low on oil reserves, a high intensity coal-fueled process may still be worthwhile if it provides vehicle-ready biofuel, even if we end up with less energy than was embodied in the coal.[12]
Life-Cycle Greenhouse Gas Intensity
Greenhouse gas intensity calculations measure the greenhouse gases released per unit of something. The GHG intensity of a national economy is measured in kilograms of CO2 per unit GDP. In life-cycle analysis for a fuel, greenhouse gas intensity generally measures the greenhouse gases released in production, combustion, and disposal of the fuel per unit of energy provided by it.[13]
For a nuclear-powered electricalplant, we measure the grams of CO2 equivalent released per kilowatt-hour of electricity provided. In a combustion engine, it would be the CO2 released per Btu of heat energy released. GHG intensity may be measured per unit volume of fuel produced (grams CO2/Liter of fuel) when comparing alternate production methods for a particular fuel.[14] As always, verify the units when comparing energy alternatives.
Ethanol: a Sample Analysis
Fossil fuels currently have a much lower (better) energy intensity than ethanol. With one Btu of processing energy, you can extract and refine almost 5 Btu’s of gasoline, compared to an optimistic estimate of 2.5 for ethanol.[15] According to some studies, corn ethanol contains less energy than the fossil fuel burned in producing it![16] However, most scientists think bioethanol has at least a little more energy than its inputs--hopefully true, given how much money has been poured into the ethanol industry.[17] In addition, renewable fuel production processes are still improving in efficiency, while for non-renewable fossil fuels, extracting them is likely to take more and more energy as the easiest sources run dry.[18]
For reducing climate change, the key metric is the relative life-cycle greenhouse gas intensity of an alternative fuel versus the fossil fuel it is replacing in a particular application: biodiesel for diesel in a truck for example.[19] For the GHG intenstiy of a biofuel to be higher than that of gasoline or natural gas, the energy intensity would have to be so high that the fossil fuel burned to produce it releases more GHG than the production and combustion combined for the liquid fossil fuel. When biofuels burn, they release CO2 which was pulled out of the atmostphere by the plants as they grew only a short time before, so its not included in the emissions accounting.[20] Fossil fuel combustion releases carbon dioxide into the atmosphere that had been locked away under the earth, so it must be counted.[21]
What exactly is included in the estimates of the greenhouse gas emissions and energy consumption attributable to production of a particular fuel? Accurate and cross-comparable estimates demand detailed and consistent procedures. Between the definitions of the International Panel on Climate Change and the industry life-cycle analysis standards of the International Organization for Standardization (ISO), we are moving towards a better accounting system for LCA and LCEA, but the tasks remain costly, complex, and imperfect.
Future articles in the series will explore this process in more detail, and explore how LCEA and carbon life-cycle analysis concepts apply to other types of alternative energy.
Formulas and Sample Calculations for Energy Intensity and Related Terms
As an example, let’s take ethanol.
Say to make 1 Btu of ethanol takes .75 Btu of energy to run production processes. (Rapier and Wang)
Energy intensity of fuel production= Energy Return on Energy Invested
Defined in Lenzen (2008) and wikipedia "EROEI", respectively as:
(processing energy inputs)/(energy embodied in final fuel)=
.75/1=
.75, or 75% or 3:4
Energy ratio
Defined in Lenzen (2008) as:
Energy in final fuel/processing energy inputs=
1/.75= 1.3 or 130% or 4:3
Energy efficiency
Defined in Wang/Rapier (2006) as:
(energy in final output fuel)/(Energy embodied in final fuel + processing energy inputs)=
1/1.75 = .58, or 58% (wang/rapier)
Energy balance=net energy value (per Btu fuel)
Defined in wikipedia "energy balance" as:
(energy in final output fuel- energy in processing inputs)/energy in final fuel=
(1-.75)/1 = .25
Note 1: Sometimes these figures are calculated counting only fossil fuel inputs; this is especially frequent for energy balance/net energy value.
Note 2: Sometimes these values are calculated for an output not measured in energy units. For example, the energy efficiency per gallon of a fuel (rather than per Btu in the fuel) when making comparisons among different production methods for the same substance; or the energy intensity of an entire economy per dollar of output.
Note 3: These terms may have very different meanings in non LCEA/LCA contexts.
Always check how a given source is using the term.
References
[1] Tan, R.R. Culaba, A.B., “Life-cycle Assessment of Conventional and Alternative Fuels for Road Vehicles” (Proc.,50th National Convention of the Philippine Society of Mechanical Engineers, Manila, Philippines. , 2002).:2.[1]
[2] European Commission- Joint Research Centre, “LCA Tools, Services, and Data” (June 25, 2009).[2]
[3] "Life Cycle Assessment," Wikipedia (last modified February 2010). [3]
[4] At least until our energy supply is 100% fossil-fuel free.
[5] "Global Warming Potential," Nationmaster.com Encyclopedia.
[6] Robert Rapier, Michael Wang, Victor Khosla, “Postscript with Wang and Khosla,” R-Squared Energy Blog (September 2, 2006). [6]
[7] Hosein Shapouri, James A. Duffield, and Michael Wang, “The Energy Balance of Corn Ethanol: An Update”, Agricultural Economic Report Number 813 (Washington, D.C.: United States Department of Agriculture, July 2002): iii [7]
[8] Manfred Lenzen, “Life cycle energy and greenhouse gas emissions of nuclear energy: A review” Energy Conversion and Management 49 (2008): 2179. [8]. The difference between the production energy and the energy in the fuel is the energy balance or net energy gain.
[9] Rapier and Wang (2006).
[10] Lenzen (2008): 1179 and 2196.
[11] Shapouri et al (2002); E. Gnansounou and A. Dauriat, "Energy Balance of Bioethanol: A Synthesis," (European Biomass Conference, Paris, France, 2005).[11]
[12] Shapouri et al (2002): 1-2.
[13] Lenzen (2008): 2179.
[14] “Life Cycle Assessment of Renewable Fuel Production from Canadian Biofuel Plants for 2008-2009 ” uses this measurement.
[15] Shapouri (2002): 10.
[16] David Pimental, "Ethanol Fuels: Energy Balance, Economics, and Environmental Impacts are Negative," Natural Resources Research Vol. 12, No. 2 (June 2003).[16]
As mentioned above, a fuel production process can be worthwhile despite a negative energy balance if it gives us energy in a necessary different form. But here, the natural gas often used as a production input could easily be used directly in a vehicle, making the production of ethanol an exercise in absurdity.
[17] Gnansounou (2005).
[18] D. Elcock, “Life-Cycle Thinking for the Oil and Gas Exploration and Production Industries,” U.S. Department of Energy commissioned report by Argonne National Laboratory (Argonne, IL: U.S. Department of Energy, September, 2007): 17-18. [18]
[19] Gnansounou (2005).
[20] "Sins of Emission," Wall Street Journal Opinion Journal, October 29, 2009. [20]
[21] Ibid
Image(s) courtesy
Thomas Hawk Flickr
Dept of Energy and Climate Change Flickr
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