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Source Ratings

Green Energy Guide: Source Ratings

November 8, 2000

Table 2. Rating the Sources: Renewability

Source How well does it meet the criteria? Rating
Wind Wind energy is a form of solar energy. Winds are caused by temperature differences among the land, sea and air, and by the overall temperature gradient that exists from the equator to the poles. Wind will be available as a source of electricity until the sun stops shining. 10
Solar The sun gives life to our planet, driving the water cycle and the winds, providing energy for plants and, in turn, virtually all other organisms. The solar energy supply is inexhaustible. Its potential for electricity generation is limited only by the efficiency at which we can capture it and the amount of land or surface area that we can dedicate to this purpose. 10
Biomass Biomass production relies on solar energy. However, to be truly renewable, biomass production must employ sustainable farming methods that do not erode soil sources or productivity. Most biomass being produced today does not employ such methods, and its renewability score reflects this. 8
Small Hydro Hydropower is another indirect form of solar power: the sun drives the planetary water cycle. The renewability of small hydro depends on the longevity of the dam involved. Although silt flowing downstream will eventually fill up the reservoir, it can be removed from a small hydroelectric system with some effort. Run-of-the river type systems, which basically put turbines in the flow of the river, have no such problems. 9
Large Hydro Although flowing water is a renewable resource, there has been some debate over whether large-scale dams truly generaterenewable electricity. Over time, the constructed reservoirs will eventually silt-in (the silt being highly expensive and difficult to remove), and dams will eventually fail. The time scale involved, however, is relatively long and depends on the silt road of the river being dammed. 7
Natural Gas Although natural gas is continually being formed, humans are extracting it at a rate many tens of thousands times higher than the formation process is occurring. New reserves are found and new techniques of extraction developed. However, with escalating energy use the resource will be depleted within the next two hundred years. 1
Coal Like natural gas, coal is being extracted at a rate astronomically higher than its formation rate. Coal stocks are more extensive than those of oil and gas, but are only expected to last two to four centuries at current consumption levels. 1
Nuclear Although nuclear power requires a small volume of fuel relative to the power produced, huge amounts of uranium ore must be processed, and uranium is ultimately a nonrenewable resource. 1

 

 

Table 3. Rating the Sources: Impacts on the Land

Source How well does it meet the criteria? Rating
Wind About 11,500 hectares of land are required to produce a billion kWh of electricity (enough to power a city of 100,000). However, only 233 of these hectares are actually taken up by the turbines themselves or by access roads and the rest of the land is highly compatible with other land uses such as grazing or agriculture. Although not appropriate in wilderness, in migratory pathways or areas important to threatened bird populations, the effects of well-sited wind farms on birds have been found to be much less significant than other factors. 8
Solar A solar electricity plant would require about 2,000 hectares to produce a billion kWh of electricity. Although this is a significant amount of land, the amount should decline as photovoltaic (PV) efficiencies increase, and can also be offset somewhat by the use of rooftops, sides of highways and other unusable land. Furthermore, PVs do no harm to the land itself and pose few threats to wildlife populations other than the land required. 7
Biomass Between 130,000 and 220,000 hectares of fertile land would be required to produce a billion kWh, depending on the type of biomass. Ignoring any other effects of biomass on terrestrial habitats, this fact makes large-scale biomass impossible. 1
Small Hydro

Large Hydro

The amount of land required to produce a billion kWh is extremely variable when it comes to hydropower. It is estimated that about 75,000 hectares on average are required. On the other hand, micro-hydro systems that use turbines in the flow of streams require basically no land at all. It also debatable whether the effects of many small scale dams would have significantly fewer effects than a single large dam. 4

2

Natural Gas Although data for the amount of land required for natural gas extraction per kWh of electricity produced are not available, it is safe to say that natural gas has the lowest land requirement of all the sources of electricity considered. However, even this may be significant if located in an important wild area such as the Arctic National Wildlife Refuge. Other terrestrial habitat effects depend on the amount of gas extracted along with coal vs. with oil or extracted alone. 8
Coal Over 350 hectares of land are required for coal extraction and plant construction, but this is a low estimate since it does not include land needed for the disposal of mine tailings or ash produced during combustion. The terrestrial habitat impacts of coal production are overwhelming. Coal mining transforms the landscape on a large scale by removing mountaintops, filling in valleys, and often failing to properly clean up the mine once closed. 1
Nuclear Nuclear power requires a relatively small amount of land (about 50 hectares) to produce a billion kWh of electricity. This number does not include the 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. The land used to store high-level radioactive waste (and in a large vicinity) is basically unusable for eternity. Furthermore, nuclear power places terrestrial ecosystem health at great risk in the case of an accident. 1

 

 

Table 4. Rating the Sources: Impacts on Water

Source How well does it meet the criteria? Rating
Wind The amount of water consumed during construction and operation of wind turbines is tiny. If pumped hydro were used to store wind-generated energy, the water utilization would be higher. Otherwise, water usage and effects to aquatic habitats by wind power are negligible. 10
Solar The small amounts of water for PV-produced energy are used for periodic washing of the panels; the water used during construction is negligible. Similarly, the effects on aquatic habitats are minimal. 10
Biomass A wood-fired steam electric plant would require about 8.5 acre-feet of water per GWh of electricity produced, but this number would be much higher if energy crops were planted that needed water for irrigation (and water is a major limiting factor in agricultural production). A more significant impact, however, would come from the runoff of soil, fertilizers and pesticides typically used in industrial agriculture which is causing significant problems for stream and river habitats around the nation. 2
Small Hydro

LargeHydro

A medium-sized hydroelectric plant uses about 66,000 acre-feet of water per GWh of electricity. This water is not consumed, but the impacts are nevertheless large because all organisms must be screened out before use. The major impacts, though, stem from the effects of dams on the upstream and downstream aquatic habitats. Many miles of a river basin are flooded with the construction of a large dam, adversely affecting any fish species that need flowing water for reproduction or those (like salmon) that must swim upstream to complete their lifecycle. Water quality downstream from a dam differs substantially from the water entering a reservoir, and the hydrologic regime of the river itself is also altered, being dictated by power demand rather than by seasonal changes in precipitation which have occurred naturally for eons. As for small systems, at this point it is unclear what the effects on aquatic habitat would be if small dams were constructed on a large scale. 4

1

Natural Gas A little less than one acre-foot of water per GWh is consumed during natural gas combustion, but about 600 acre-feet are used and recycled. This latter amount is significant if it is extracted from a lake or river and then returned. It is also important to consider the pollution to the oceans that takes place during off-shore oil and natural gas production, making natural gas less sustainable in terms of aquatic habitat impacts. 7
Coal Coal combustion consumes between 1.5 and 3 acre-feet of water per GWh, and uses about 600 acre-feet of non-consumptive water. Coal's aquatic habitat impacts are significant, however, because of the problems of acid mine drainage harming streams and rivers (in the case of surface mines) and poisoning groundwater (in the case of underground mines). 1
Nuclear Nuclear reactions require between about 2.5 and 4 acre-feet of water per GWh, and about 800 acre-ft of non-consumptive water. Studies have shown that this amount of water can have a significant impact on nearby aquatic habitats because of the number of organisms killed when the water is screened to remove debris (one study in Delaware, for instance, found a 31 percent reduction of the bay anchovy as a result of water consumption from a single reactor. The real impacts appear when you consider the risk to aquatic habitats and drinking water from underground radioactive waste storage or in the event of an accident. 1

 

 

Table 5. Rating the Sources: Impacts on Air and Climate

Source How well does it meet the criteria? Rating
Wind Wind power produces only a trace of air pollution per GWh over its entire lifecycle and the amount that is produced stems mostly from fossil fuel burning from construction and maintenance. It also emits less than 8 tons of CO2 per GWh of electricity, again mostly from fossil fuel burning (so emission levels would drop even more if cleaner sources were used). 10
Solar Like wind, solar power produces only a trace of air pollution per GWh over its entire lifecycle and the amount that is produced stems mostly from fossil fuel burning from construction and maintenance. Its emission levels of CO2 are even lower than wind, just 5.4 tons per GWh of electricity, again mostly from fossil fuel burning. 10
Biomass Biomass combustion emits very high levels of carbon monoxide (CO), volatile organic compounds (VOCs), significant levels of particulates, and lower but not negligible levels of nitrogen dioxide (NO2) and sulfur dioxide (SO2), all of which have significant effects on human health, with the latter two also contributing to the terrestrial and aquatic habitat-sickening problems of acid rain. If biomass is grown sustainably it would not contribute any net CO2 to the atmosphere aside from the (possibly significant) amount used for the energy required for fertilizers, pesticides, planting and harvesting. 6
Small
and
Large Hydro
Both small and large hydro produce only a trace of air pollution per GWh over the entire lifecycle; the amount that is emitted stems mostly from fossil fuel burning from construction and maintenance. Emission levels of CO2 are also low, ranging from 3 to 10 tons per GWh of electricity, again mostly from fossil fuel burning. 10
Natural Gas Natural gas produces much lower levels of SO2 and NO2 than coal, but these are still much higher than almost any other source. Natural gas also produces very high levels of particulates (though still lower than coal), which are some of the most hazardous to humans. It is the high emission levels of CO2, however, that particularly mark natural gas as unsustainable: between 500 and 600 tons of this greenhouse gas is produced per GWh of electricity. 2
Coal Coal is the out and out loser when it comes to air and climate issues. A typical coal plant produces almost 3 tons each of SO2 and NO2, and over 1.5 tons of particulates for every GWh. Throw in significant levels of CO and VOCs, plus a whopping 750 to 950 tons of CO2 emitted for every GWh of electricity produced, and coal is just as dirty as its reputation. 1
Nuclear Although nuclear power produces very little of the typical air pollutants we concern ourselves with, and only emits about 8 tons of CO2 per GWh, it can't be taken off the hook because of the risk it would pose in the case of an accident. 7

 

 

Table 6. Rating the Sources: Human Health and Safety

Source How well does it meet the criteria? Rating
Wind Wind power is an all-around very safe method of producing electricity. Both during normal operation and in the case of an accident, the occupational hazards are low, as are the hazards to the public. Wind power poses no threat to future generations. 10
Solar Solar power probably poses the least threat to human health and safety than any other source of electricity. The dangers posed to workers. the general public and future generations are close to zero, considering both normal operation and accident scenarios. 10
Biomass Aside from the air pollution produced (which is significant), biomass poses few threats to the general public during normal operation or in the case of an accident. In contrast, however,, agriculture and forestry both have extremely high rates of occupational illness and injury, significantly exceeding even those of underground coal mining. Biomass does not pose any threats to future generations. 6
Small Hydro

Large Hydro

Hydroelectricity is an interesting case in terms of human health and safety. On one hand, hydroelectric dams pose few threats to the general public during normal operation, and in fact can provide some measure of flood control for citizens living downstream (large dams more so than small ones). On the other hand, although the probability is low, the failure of a dam would be disastrous (again, large ones more so than small ones). Hydroelectricity poses few threats to future generations, but no data in available on occupational hazard rates. 9

7

Natural Gas Natural gas has an occupational illness and injury rate that is about half that of coal mining, and threats to the general public are relatively low aside from the air pollution produced. The high amount of carbon dioxide produced during electricity production, albeit again less than coal, is contributing to global climate change which threatens the health and well-being of future generations. 5
Coal With the highest levels of occupational hazards and air pollution emissions of any of the other sources of electricity, coal is a loser when it comes to human health and safety. Furthermore, coal poses a threat to future generations in terms of it carbon dioxide production affecting global climate, as well as its potential long term impacts on ground water supplies with continuing acid mine drainage. 2
Nuclear Nuclear power poses significant human health and safety hazards. Although occupational hazard data make nuclear power seem like a relatively safe option, the data do not reflect the fact that radiation-linked cancers and other illnesses often take many years to appear. An accident at a nuclear power plant, of course, would also put employees and the general public in huge danger. But even under normal operation, the low and high level radioactive waste pose a significant threat to the health and safety of the nations citizens of today, as well as that of many generations to come. 1

 

 

Table 7. Rating the Sources: Energy Return on Investment

Source How well does it meet the criteria? Rating
Wind The energy return on investment ratio for wind power is calculated to be 5:1 over its entire lifecycle. This ratio is likely to increase in the future as designs improve and lighter-weight materials are incorporated 7
Solar PV systems have a lifecycle 9:1 energy return ratio when their efficiencies are about 7.5%. This estimate is low, however, 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. 8
Biomass Biomass has a very low energy output to input ratio of 3:1 for natural forest biomass. 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. 1
Small
and
Large Hydro
Hydropower has by far the most favorable energy return on investment, with a value of 48:1. 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), though this continues to be significantly higher than the ratio for any other source of electricity. 10
Natural Gas Information on the energy investment for natural gas plants is not available. However, some natural gas is extracted along with coal, and would therefore have the same ratio. As for gas that is tapped along with oil or by itself, the ratio is likely to be somewhat lower than that of coal because the extraction is less energy intensive. 8
Coal The energy return for coal has been calculated to be 8:1. This is a significant overestimate, however, because it does not take into consideration the energy required to extract the coal. Although more efficient technologies may increase the ratio in the future, these gains could be balanced by the decreased plant efficiencies that come with more stringent pollution controls. 7
Nuclear The myth of nuclear power is that it has a very high energy return on investment. When you look at the amount of energy required to construct a plant compared to the amount of energy that is produced, however, the ratio is only 5:1. Furthermore, even this value is an overestimate because does not include the significant energy required for uranium mining and processing into fuel rods, the treatment of and storage of the radioactive waste produced and the eventual decommissioning of the nuclear reactor. 2

 

 

Table 8. Rating the Sources: Pricing

Source How well does it meet the criteria? Rating
Wind Wind power has a current market price of 5 to 7 cents per kWh of electricity, which makes it competitive with other more traditional methods of electricity generation. This price would be expected to drop further if wind power became more widespread. Furthermore, the estimated externality costs (which are monetary estimates of the impacts on human health, natural ecosystems, climate, agriculture, etc.) for wind are low: in he range of 0 to 0.4 cents/kWh, meaning that the current market price is in line with the true cost of the electricity produced. 8
Solar The largest drawback to solar power is price, with electricity from PV systems costing about 30 cents/kWh. Although this price has dropped considerably in the last few decades, and will continue to drop as it becomes more widespread, it is not very cost effective today. On the other hand, its externality costs are estimated to be in the range or 0 to 0.4 cents/kWh. 2
Biomass The price for biomass fueled electricity is on the high side, costing between 7 and 10 cents per kWh. This price could drop in the future if more efficient techniques were developed, but any drop would not be dramatic because the technologies already been long employed. The externality costs have been estimated as being as high as 0.7 cents/kWh. 4
Small
and
Large Hydro
Hydropower is the least expensive form of electricity with a price of just 2 cents/kWh because most large dams have been around for decades and once one is built the operating costs are very low. Of course, if a new dam is being constructed and this cost is factored in, the price of electricity would be significantly higher. There are no available estimates of externality cost per kWh, but they would be greater than zero. 9
Natural Gas Natural gas is an inexpensive source of electricity, with a price of 4 to 6 cents per kWh. The US currently imports about 15% of its natural gas (from Canada), so unlike oil, the price is not likely to be heavily affected by global politico-economic events. Over the long term, however, as domestic supplies become more scarce, this may change. The low price is also artificial: externality costs have been estimated to be as high as 2.8 cents/kWh. 7
Coal Coal is a slightly cheaper source of electricity that natural gas, with a current market price of 3 to 6 cents/kWh. This price is not expected to increase in the near future due to supply issues because the US gets all of its coal domestically; the price could increase, however, as more environmental controls on coal plants are installed. Yet coal's low price is extremely out of line with its true cost: externality values have been estimated to be as high as 25.8 cents/kWh. 6
Nuclear Despite the claim that nuclear power was going to be so cheap produce, that plants were going to be basically "giving electricity away," it was soon discovered that nuclear power was actually a very expensive source of electricity. Current market prices range between 5 and 21 cents per kWh; even higher are the externality costs which are estimated to be as much as 37.8 cents/kWh. Prices are expected to increase rather than decrease as the true costs of decommissioning and long-term waste storage are discovered. 1