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# Climate Solutions: Chapter 8

This article has been reviewed by the following Topic Editor: Peter Saundry

## Net Energy Value (NEV) and Energy Payback Time (EPBT)

In a world in which no biological ecosystem is free of human influence and no industrial ecosystem is free of biological influence, it is appropriate to abandon the artificial division between the two frameworks and develop a new synthesis—Earth system ecology—as the logical construct for all of Earth's ecosystems. [14]
—Thomas Graedel

Cradle to grave designs dominate modern manufacturing. [28: p. 27]
—architect Bill McDonough and chemist Michael Braungart

Energy analysis is the first step to determine and compare energy consumption and production of different options. This can involve complex life-cycle net energy analysis, but often the most useful energy analysis is done by calculating simple energy consumption or conversion efficiency on the back of an envelope. [34: p. 166]
—John Randolph and Gilbert Masters, 2008

The energy return on investment (EROI) value is a ratio and therefore has no specific dimension. When we use the EROI, we have to decide whether by-products (or coproducts) of the energy conversion process belong on the top or the bottom of the fraction. In the case of some energy systems, for example, ethanol production, the EROI result will be very different, depending on whether we consider the coproduced energy as a positive output—to be added to the numerator, on top, because it can be used “as is” without further conversion and it increases energy return—or a negative input—to be added in the denominator because it needs to be disposed of at some added energy cost and it shrinks energy return. In these cases, an alternative is to compute the net energy value in a system. For example, the production of ethanol creates not only liquid ethanol fuel but also usable animal feed and solid fuel that have energy value too. The total value of the net energy gain or loss in a system can be calculated as follows, which lets us avoid choosing whether a by-product is a gain or loss:

Equation 4: Net energy value (NEV) = Energy output - Energy input = (Energy in liquid ethanol + Feed by-product + Solid fuel by-product) - (Energy needed to product ethanol)

Net energy computes as an absolute value with a specific dimension, such as Btu per gallon (Btu/gal) in the case of ethanol or gasoline. One British thermal unit (Btu) is small unit of energy equal to 0.293 Watt-hours.

Notice the simple EROI expression did not have a time component. Very often we also want to examine the time value of energy conversions, that is, their break-even or payback period. For example, how much time will it take an energy system, such as a new geothermal system, a wind farm,a solar array, or even a new more-efficient oil burner, to recover its start-up costs? This is especially useful for systems that have one-time development costs and very low net operating costs, like wind or photovoltaic or solar thermal installations. For such systems, the energy payback time (EPBT) is the equivalent of the inverse of the energy return on investment value (1/EROI). All we need to do is add the time component—typically years—to the denominator on the bottom of the fraction:

Equation 5: Energy payback time (EPBT) = (Energy input of up-front one-time costs)/(Energy output) = Btu/(Btus per year)

where energy input is the up-front indirect energy cost of creating a system, which is divided by the energy output, which is usually written as the useful energy (or power) output per unit of time, for example, Btu/year.

For example, in the realm of photovoltaic technologies, the EPBT approach helps us compare the payback time for two different kinds of solar electric panels if we know the indirect up-front “embodied” energy costs of producing the system. The energy input for a crystalline silicon PV module is 5,600 kilowatt-hours per peak kilowatt-hour power output of the module, and for thin-film copper indium diselenide (CIS) modules the energy input is 3,100 (in the same units). So the new CIS module takes less energy to produce. Given that the Sun’s energy shining on two panels is going to be the same, 1,700 (in the same units), we can see that the thin-film module has a payback of 1.8 years versus 3.3 years for the silicon-based module. In reality, the actual net power performance of the panels is about 80% of the original because of system losses to the power after it leaves the module (line loss, inverter operations, etc.). So the energy payback times become 2.2 years for the thin-film and 4.1 years for the silicon modules.

Now, here is the interesting part. Both these technologies have an expected life of 30 years. So now we can compute the energy return on energy investment as follows, and learn that the newer thin-film module will have an EROI twice as high as the older silicon technology:

Equation 6:
EROIsilicon = 30 years/4.1 years = 7
EROIthin-film CIS = 30 years/2.2years = 14

Table 8.5 shows some current EROI values. Note the huge drop in energy return on US gas and oil production over the seven-decade time. Note also that the energy return on thin-film photovoltaic modules is only slightly lower that on nuclear power—without taking any of the actual economic costs of each into account.

 Table 8.5 Some Energy Return on Energy Investment Values for Different Energy Sources Energy source or system EROI* Research source US gas and oil production (1930) 100 Cleveland (2005) US gas and oil production (2000) 20 Cleveland (2005) Electricity from hydro with reservoir 205 Gagnon et al. (2002) Electricity from nuclear 16 Gagnon et al. (2002) Electricity from coal (with SO2 scrubbers) 5 Gagnon et al. (2002) PV modules (thin-film CIS) 14 Knapp and Jester (2001) PV modules (crystalline silicon) 7 Knapp and Jester (2001) US ethanol fuel from corn 0.78 Pimentel and Patzek (2005) US ethanol fuel from switchgrass 0.79 Pimentel and Patzek (2005) US ethanol fuel from soybeans 0.67 Pimentel and Patzek (2005) *Other researchers arrive at very different EROI values for ethanol, depending on whether the coproducts are counted as positive or negative values. Table Source: Adapted from [32: table 5.2] using data from [3, 13, 22, 33]

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## Action items

• Action 1: Green Buildings and Building Design
• Action 7: Biofuel Industry and CO2 Emissions?— Implications for Policy
• Action 9: How to Ensure Wind Energy Is Green Energy
• Action 10: Nuclear Energy?— Using Science to Make Hard Choices

## Instructor resources

 This is a chapter from Climate Solutions Consensus. Previous: Chapter 7: No Silver Bullet, Many Silver Wedges  |  Table of Contents  |  Next: Chapter 9: Multiple Intensity Disorder

## Citation

David Blockstein, Leo Wiegman (Lead Author);Peter Saundry (Topic Editor) "Climate Solutions: Chapter 8". In: Encyclopedia of Earth. Eds. Cutler J. Cleveland (Washington, D.C.: Environmental Information Coalition, National Council for Science and the Environment). [First published in the Encyclopedia of Earth June 16, 2010; Last revised Date May 7, 2012; Retrieved May 25, 2013 <http://www.eoearth.org/article/Climate_Solutions:_Chapter_8>

## The Authors

David E. Blockstein is a Senior Scientist with the National Council for Science and the Environment, a nonpartisan organization of scientists, environmentalists, business people, and policymakers working to improve the scientific basis of environmental decisionmaking. Dr. Blockstein joined the organization in 1990 and was its first Executive Director. Presently, he organizes NCSE's annual National Conference on Science, Policy and the Environment. Dr. Blockstei ... (Full Bio)

A former book publisher, Leo serves as Mayor of the Village of Croton-on-Hudson, New York, and is Vice Chair of the Northern Westchester Energy Action Consortium. Leo is the founder of E to the Fourth Strategic Communications, a firm dedicated to helping environmental groups communicate more effectively. Leo is co-author of The Climate Solutions Consensus with David Blockstein at the National Council on Science and the Environment and of the forthcoming, Heirlooms to Live In: H ... (Full Bio)