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Green Energy Guide: Appendix

November 8, 2000

Appendix: Values Used in Calculating the Ratings

Fuel Mix for Electricity Generation in the US

Table A1. Sources of electricity used for electricity generation in 1998. Table adapted from EIA, 1999 a.

Electricity SourceBillion kWh% of Total
Natural Gas53214.7
Biomass: Wood a33.60.93
Waste: MSW and LFG b17.80.50
Geothermal 13.90.39
Other gas c12.80.35
Wind power3.50.10
Biomass: Other waste d3.30.09
Solar e0.90.003
Total3,620100 %

a Wood, wood waste, wood liquors, peat, railroad ties, wood sludge, and spent sulfur liquor
b Municipal solid waste and landfill gas
c Butane, methane, propane and other gases
d Agricultural waste, straw, tires, fish oils, paper pellets, tall oil, sludge waste and waste alcohol
e Photovoltaics, solar ponds, and solar thermal

Land Requirements

Table A2. Land required each year for facilities that produce 1 billion kWh/yr of electricity (enough to supply a city of 100,000 people). Some sources of electricity (such as hydro) require a one-time land allotment and no new land is disrupted each year. Other sources (such as coal) have one-time land allotments in the construction of the power plant, but also require new land to be disrupted each year for fuel extraction. In these cases, it is noted how much land is required for facility construction vs. fuel extraction (when such information is available). Table is adapted from Pimentel et al. (1994 a); data on fuel extraction vs. plant operation land requirements is from USDOE (1989 b); natural gas data is from Ottinger et al. (1991).

Source of electricityHectares per billion kWh
Coal363 a
Natural gas9 b
Nuclear48 c
Hydroelectric75,000 d
Wind233; 11,666 e
Photovoltaics1350 - 2700 f
Biomass132,000 -220,000 g

a Coal: Estimate includes coal power plant construction (33 ha) and coal fuel extraction (330 ha), but does not include land needed for disposal of coal mine tailings or ash produced during combustion.

b Natural gas: Estimate includes power plant construction only, and does not include land required for fuel extraction (which is likely to also be low).

c Nuclear: Estimate includes nuclear power plant construction (22 ha) and fuel extraction (26 ha), but does not include land needed for disposal of uranium tailings produced during mining, the waste produced during electricity generation, or the land requirements involved in the eventual decommissioning of the reactor.

d Hydroelectric: Estimate is based on a random sample of 50 large hydropower reservoirs in the US, ranging from 482 ha to 763,000 ha. The amount of land flooded depends on the particular topography of the region.

e Wind: It takes more than 11,500 ha of land to space out enough wind turbines such that they can produce a billion kWh of electricity. However, 233 of these hectares are actually taken up by the turbines themselves, or by access roads for their construction and maintenance. There is also an offshore wind farm that is being planned which would avoid terrestrial impacts altogether.

f Photovoltaics: Estimate includes land covered by a photovoltaic electricity generating facility. The 2700 ha number is based on an actual PV electricity plant with an overall 7.5% efficiency. This is somewhat an overestimate of the land required because the PV panels on the market have efficiencies of up to 15%, and those under development have efficiencies of up to 30%. A more modern PV plant, therefore, would cover about 1350 ha.

g Biomass: Estimate is based on the amount of natural forest that would have to be permanently set aside for sustainable forestry. If energy crops were grown, the amount of land required would drop to the still very high number of 132,000 ha.

Water Requirements

Table A3. Water consumption for fuel extraction, and the construction and operation of electricity-generating facilities in terms of acre-feet of water used per GWh of power produced. Data includes consumptive water use only (most of which is lost as evaporation from cooling systems) and does not include non-consumptive water use (which returned to the water body from which it was drawn), unless otherwise indicated.

Source of electricityAcre-ft water per GWh
Coal1.5 - 3.1 a
Natural gas0.8 b
Nuclear2.6 - 4.1 c
Hydroelectric66,000 d
Wind~0 e
Photovoltaics0.1 f
Biomass8.4 g
a Coal: The amount of water depends on the technology and type of coal used (USDOE, 1989 a). Non-consumptive water use for fossil fuel plants is much higher, averaging 590 acre-ft per GWh (Ottinger et al., 1991).

b Natural gas: The value of 0.8 acre-ft/GWh includes consumptive water use only (USDOE, 1983). Non-consumptive water use for fossil fuel plants is much higher, averaging 590 acre-ft per GWh (Ottinger et al., 1991).

c Nuclear: Nuclear reactions generate large amounts of heat, and therefore require large amounts of water for cooling purposes (USDOE, 1989 a). Non-consumptive water use for nuclear plants is much higher, averaging 806 acre-ft per GWh (Ottinger et al., 1991).

d Hydroelectric: The 66,000 acre-ft of water is calculated for small hydro plants (in this case, assumed to be less than 65 feet) and is non-consumptive (USDOE, 1983). Even though the water is not consumed, because all organisms must be screened out before use, the impacts of this water use are large.

e Wind: The water consumed during construction and operation of wind turbines is negligible (USDOE, 1983). However, if pumped hydro were used to store wind-generated energy, the water utilization would be much higher.

f Photovoltaics: The small amounts of water for PV-produced energy are used for periodic washing of the panels; the water used during construction is negligible (USDOE, 1989; Ottinger et al., 1991).

g Biomass: This water is used in a wood-fired steam electric plant (USDOE, 1983). Energy crops that require irrigation would consume much higher levels of water.

Air Pollution Emissions

Table A4. Emissions of 5 major air pollutants from electric power generation over the fuel lifecycle. Tr stands for trace (less than 0.01 tons/GWh). Adapted from USDOE, 1989 a. Externality values are from EIA, 1995.

Tons of pollutants per GWh
Source of electricity SO2 a NO2 b TSP c CO d VOCs e
Natural gas0.370.251.18trtr

a Sulfur Dioxide contributes to air pollution-related health effects by reacting in the atmosphere to form sulfates, which are believed to form a significant portion of total suspended particulates (TSP) in terms of both volume and toxicity to humans (Ottinger, 1990). The major effect of sulfur oxides on ecosystems, however, is their contribution to acid rain. Externality values have been assigned by six states, averaging $1582 per ton of SO2, with a range from $0 to $4,486 per ton; only some attempt to gauge acid rain costs.

b Nitrogen Oxides such as NO and NO2, are often considered as a group (abbreviated NOx) because they are commonly produced together and can incontrovert. In the atmosphere, NOx forms yellow-brown clouds which add to haze and smog, contribute to climate change when they convert in the atmosphere to form the potent greenhouse gas nitrous oxide (N20), react to form tropospheric ozone (O3), the major component of smog, and precipitate out as acid rain. The contribution of nitrogen oxides to ozone formation merits particular attention because O3 is a significant contributor to forest and crop damage, and has also been found to cause respiratory problems (90% of inhaled O3 is never exhaled) (Ottinger, 1990). Externality values have been assigned by six States, averaging $5008/ton, with a range from $850 to $9120 per ton.

c Particulates, abbreviated TSP (total suspended particulates), are very fine solid particles suspended in air; They have diverse sizes and chemical properties, with the smaller particles being particularly hazardous to human health since they remain in the air longer and can not be filtered out by the respiratory system (Ottinger, 1990). Some particulates (such as dust, soot, and radioactive isotopes) are emitted directly into the air, while others (such as sulfates) form after reactions between compounds in the atmosphere. Particulates typically remain in the air for a week to forty days before they are deposited by rain, or combine to form larger particles which settle out. A comprehensive review of US and European studies found a high correlation between mortality rates and TSP levels (Dockery and Pope, 1993 and 1994). Externality values have been assigned by six States, averaging $3036/ton, with a range from $333 to $4,608/ton.

d Carbon Monoxide is a gas formed from the incomplete combustion of fossil fuels. Carbon monoxide can cause headaches in people, and place additional stress on those individuals with heart disease (Ottinger, 1990). The two states that have assigned externality values to CO give it $960 and $1,012 per ton.

e Volatile Organic Compounds are actually a diverse class of air pollutants made up of hundreds of compounds which all contain hydrogen and carbon. Methane, the simplest of the volatile organics, is a potent greenhouse gas, but is unreactive in the atmosphere. The rest of the VOCs, on the other hand, are highly reactive and combine with nitrogen oxides to produce major components of smog and tropospheric ozone. Although volatile organic compounds are produced in small amounts, they have been assigned a high externality value of $3,085/ton by the one state that has assessed them.

Carbon Dioxide Emissions

Table A5. Carbon dioxide emissions per GWh of electricity produced over the total fuel cycle. A typical person uses about a tenth of a GWh of electricity in a year (Pimentel et al., 1994 a). Figures are adapted from USDOE (1989 b), and USEPA (1994).

Source of electricityTons of CO2/GWh
Coal751 - 964 a
Natural gas484 - 590 a
Nuclear7.8 b
Hydroelectric3.1 - 10.0 b
Wind7.4 b
Photovoltaics5.4 b
Biomasssee footnote c
a Coal and Natural gas: The combustion of all fossil fuels produce large amounts of carbon dioxide. Coal, however, contributes far more CO2 per GWh of electricity than other power sources. Coal-fired plants are responsible for 84% of electric utility carbon emissions in the US (Ottinger et al., 1991). Gasified coal produces slightly less CO2 per unit of electricity produced (751 tons/GWh), while natural gas that is not derived from coal produces only about half the amount that traditional coal combustion emits (484 tons/GWh).

b Nuclear, Hydroelectric, Wind, and Photovoltaics: These energy sources have no direct emissions of CO2 during electricity generation. Rather, the emissions stem from fossil fuel burning during other parts of the fuel cycle (uranium mining and processing, turbine construction, etc.). The carbon emissions would therefore be much lower if renewable energy sources were used which had small rates of CO2 emissions. The scale of hydropower affects the amount of CO2 produced: the lesser value corresponds to large hydro and the greater value to small scale hydro.

c Biomass: If forests were harvested sustainably, the use of biomass for electricity would yield no net CO2 increase in the atmosphere because the amount of carbon emitted during combustion equals the amount that would be removed from the atmosphere during regrowth. However, if energy crops were grown using industrial agricultural methods, the CO2 emitted per unit of electricity produced may be significantly greater than zero due to the energy required for fertilizers, pesticides, and planting and harvesting.

Occupational Health and Safety

Table A6. Occupational fatalities and lost work days for major sources of electricity. Data on natural gas, hydroelectricity, and biomass are not available. Adapted from Ottinger et al. (1991).

Source of electricityFatalities (deaths
per GW/yr)
Lost work days
(1000 worker days
lost per GW/yr)
Coal1.191.94 - 56.0
Natural gaslow alow
Nuclear0.16 - 2.10.15 - 1.95
Photovoltaicsless than 0.011.5
Wind0.28 - 0.440.82 - 1.36
Biomasshigh ahigh

a Parallel figures were not available for natural gas or biomass-fueled electricity. However, it has been calculated that the harvesting of forest biomass has an occupational injury and illness rate that is 14 times higher than underground coal mining and 28 times higher than oil and gas extraction, per kilocalorie of energy produced (Pimentel et al., 1984).

Energy Return on Investment

Table A7. Total energy inputs required for the construction of electricity-generating facilities that produce 1 billion kWh/yr of electricity. The energy return on energy investment is an energy input/output ratio: the amount of energy that is produced over the unit's 30-year life span, divided by the amount of energy consumed during construction. Energy to extract the fuel is not included in the figures, but is discussed in the notes on each fuel source.

Source of electricityEnergy
(kWh x 109)
Energy return
on energy investment
Coal0.12less than 8:1 a
Natural gasNANA b
Nuclear0.03less than 5:1 c
Hydroelectric0.02less than 48:1 d
Wind0.215:1 e
Photovoltaics0.11more than 9:1 f
Biomass0.303:1 g

a Coal: The energy return ratio of 8:1 (Pimentel et al., 1994 a) is a significant overestimate because it does not take into consideration the energy required to extract the coal. Although more efficient technologies may increase this value, these gains could be balanced by the decreased plant efficiencies that come with more stringent pollution controls.

b Natural gas: Information on the energy investment for natural gas plants is not available.

c Nuclear: The 5:1 ratio (Pimentel et al., 1994 a) for nuclear power is an overestimate for a few reasons. First, this value does not include the significant energy required for uranium mining and processing when this is included in the calculations. Second, the 5:1 ratio does not take into consideration the large amounts of energy required to process the radioactive waste and eventually decommission the nuclear reactor. Although other forms of electricity generation, especially coal plants, have additional energy requirements to dispose of their associated waste, the energy consumed is very small compared to what is required to deal with radioactive waste.

d Hydroelectric: Hydropower has by far the most favorable energy return on investment, with a value of 48:1 (Pimentel et al., 1994 a). However, such returns should not be expected from future dams because the most favorable sites have already been developed or set off-limits. The future ratio may be expected to be closer to 15:1 which has been calculated for Europe (Pimentel et al., 1994 a).

e Wind: The energy return on investment ratio of 5:1 is likely to increase for wind power as designs improve and lighter-weight materials are incorporated (Pimentel et al., 1994 a).

f Photovoltaics: PV systems have a 9:1 energy return ratio when their efficiencies are about 7.5% (Pimentel et al., 1994 a). However, this estimate is low because more recent PV systems are achieving 10-15% efficiencies in the field, and prototype modules are reaching much higher efficiencies in the laboratory. A more likely figure for energy return on investment, therefore, may be 17:1 (Pimentel et al., 1991).

g Biomass: Biomass has a very low energy output to input ratio of 3:1 for natural forest biomass (3 tons of woody biomass can be harvested per hectare, but 33 liters of diesel fuel per hectare are used in harvesting and transporting the wood, and 70% of the 13.5 million thermal kcal are lost in the conversion to electricity- a value similar to coal plants; Pimentel et al., 1994 a). Ratios for crops grown specifically for electricity production vary greatly but would probably be even lower. Although yields for energy crops would be higher per acre than natural forest biomass, these gains would be offset by the greater energy inputs associated with intensive agriculture (Pimentel et al., 1994 b).

Current Market Price Table A8. Current market price for sources of electricity in the US. Price ranges are due to differences in the technology employed, geographical region, subsidies, etc. Adapted from Pimentel et al. (1994 a), and Flavin and Lenssen (1994).

Source of electricityCurrent Market Price
Coal3 - 6
Natural gas4 - 6
Nuclear5 - 21
Wind5 - 7

Estimated Externality Costs

Table A9. Externality costs were adapted from Kennedy et al., 1991, and. The range cited is an indication of the large variations of estimates within the literature.

Source of electricityExternality Cost
Coal0.04 - 25.8
Natural gas0.09 - 2.8
Nuclear0.001 - 37.8
Hydroelectricity> 0
Wind0 - 0.15
Photovoltaics0 - 0.4
Biomass0 - 0.7

Impacts of Household Electricity Use

Table A10. Adapted from Brower and Leon (1999).

Impact typeAmount caused by all consumer activitiesPercent caused by electricity consumptionAmount caused by electricity consumption
Greenhouse gases28,461 lb/yr.20.8%5913 lb/yr.
Common air pollution1120 lb/yr.18%202 lb/yr.
Toxic air pollution64 lb/yr.0.7%0.4 lb /yr.
Water pollution1391 lb/yr.6.5%90 lb /yr.
Water use767 gallons/day2.5%7001 gallons/yr
Terrestrial habitat alteration88,920 ft20.3%267 ft2/yr.


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