Why Hydropower is Not Clean Energy
9 Jan, 2007 02:53 pm
Dr. Philip M. Fearnside is a research scientist in the Ecology department at the National Institute for Research in the Amazon (INPA) in Manaus, Amazonas, Brazil.
“It’s baloney!” was the initial response of the industry, as voiced by a spokesperson for the U.S. Hydropower Association (see McCully, 2002). What had sparked the reaction was my calculation that Brazil’s Balbina Dam was worse than fossil fuels in terms of greenhouse-gas emissions (Fearnside, 1995). A Canadian group had also shown that northern reservoirs can release greenhouse gases (Rudd et al., 1993). This was only the beginning of the long debate that continues to this day. Large emissions from water passing through the turbines of tropical dams have been have confirmed by direct measurements of methane release immediately below the Petit-Saut Dam in French Guiana (April et al., 2005) and the Balbina Dam in Brazil (Kemenes et al., 2006).
In 2002, I published a paper in the journal Water, Air and Soil Pollution calculating that in 1990, Brazil’s Tucuruí Dam (then 6 years old) released even more greenhouse gases than the city of São Paulo (Fearnside, 2002). Once again, shock waves were set off. The head of ELETROBRÁS (Brazil’s government agency for promoting hydroelectric dams) claimed that the study showed that those who say that dams have high emissions (that is to say, me) are subject to “the lures of the thermo-power and nuclear-power lobbies.” (Rosa et al., 2004; See reply: Fearnside, 2004). In a follow-up attack (Rosa et al., 2006; see reply: Fearnside, 2006), they made the now-famous claim that the bubbles in a leisurely consumed bottle of guaraná (a Brazilian soft drink) would reveal the error in my use of Coca Cola as the illustration of Henry’s Law—the chemical principle that gases have higher solubility under increased pressure (see Giles, 2006; McCully, 2006). I had used the bubbles of CO2 released when a bottle of Coca Cola is opened to explain why so much methane (CH4) is released when water from the bottom of a reservoir emerges from the turbines. Unfortunately, it makes little difference whether the gas bubbles all emerge immediately or whether the process continues for half an hour or more (as with a bottle of guaraná). The important fact is that the water at the bottom of a reservoir is under high pressure and contains a high concentration of dissolved methane. When the pressure is suddenly released as the water emerges from the turbines, most of this methane is released.
Methane accumulates in the water near the bottom of the reservoir because the water column is thermally stratified (generally at a point less than 10 m below the surface), such that the colder deep water does not mix with the warmer surface water. Since the deep water (hypolimnion) has virtually no oxygen, decomposition ends in CH4 rather than CO2. Organic matter undergoing decomposition comes both from what was originally present in the vegetation and soil before the reservoir was formed and from carbon that enters the reservoir each year, one example being from the soft vegetation that grows on the mudflats that are exposed annually when the water level is drawn down, only to be flooded again when the reservoir is refilled. Unlike a natural lake where an outlet stream draws water from near the surface, a hydroelectric dam is like a bathtub where one pulls the plug at the bottom—outflow is through turbines and spillways that are located at depths where the water is loaded with methane. Although the emission is greatest in the first years after a reservoir is filled, the annual flooding of the drawdown zone can sustain an appreciable level of emission permanently (Fearnside, 2005). Since one ton of methane is equivalent to 21 tons of CO2 in terms of impact on global warming, according to the conversions adopted under the Kyoto Protocol, this gas release gives hydroelectric dams a significant contribution to the greenhouse effect.
Omissions of methane from the turbines and spillways is the main reason why my estimates of greenhouse-gas emissions from Brazilian hydroelectric dams are more than ten times higher than the official estimates Brazil submitted to the Climate Convention in its national inventory (Brazil, MCT, 2004, p.154; 2006). It is relevant to mention that the official responsible for Brazil’s national inventory confessed in a singularly public way that ELETROBRÁS had been invited to “coordinate” the portion of the report on hydroelectric emissions specifically because the agency would produce a politically convenient result that would avoid international pressure for Brazil to reduce its emissions (Brazil, MCT, 2002; see Fearnside, 2004).
The dispute over greenhouse gases from hydroelectric dams, as in many scientific disputes, is likely to cause people not involved in the dispute to assume that the truth must lie somewhere between the two sides, presumably at the midpoint. Unfortunately, while the central-limit theorem is a good guide to interpreting a series of measurements of the same type, such as a series of measurements of gas concentrations in the water at a given place and time, the theorem doesn’t apply when the differences are caused by omissions of important components of a problem, in this case the principal sources of methane release: the turbines and spillways. Both sides of the controversy are available in the “Amazonian Controversies” section at http://philip.inpa.gov.br.
The issue of hydroelectric dam emissions has gained greater public attention following the colorful exchange of “editorial comments” in the journal Climatic Change (Fearnside, 2004, 2006; Rosa et al., 2004, 2006). Outside experts invited to comment on the debate recognized the potential of dams to make substantial emissions through the turbines and spillways and called for the Intergovernmental Panel on Climate Change (IPCC) to prepare a special report on the subject (Cullenward and Victor, 2006). The United Nations Educational and Scientific Organization (UNESCO) convened a meeting in December 2006 to promote an intensification of research on the subject (see Giles, 2006).
The fact that hydroelectric dams have significant greenhouse-gas emissions has a variety of practical implications. One is the possibility of capturing some of the methane as a power source (Bambace et al., 2006). Another is the need to reduce the net benefit attributable to dams when calculating carbon credits that some of them are eligible to earn under the Kyoto Protocol. Most important is having a reasonably complete accounting of the impacts (as well as the benefits) of proposed development projects so that rational choices can be made in the best interests of society.
April, G., F. Guérin, S. Richard, R. Delmas, C. Galy-Lacaux, P. Gosse, A. Tremblay, L. Varfalvy, M.A. dos Santos and B. Matvienko. 2005. Carbon dioxide and methane emissions and the carbon budget of a 10-year old tropical reservoir (Petit Saut, French Guiana). Global Biogeochemical Cycles 19, GB4007, doi:10.1029/2005GB002457. (http://dx.doi.org/10.1029/2005GB002457).
Bambace, L.A.W., F.M. Ramos, I.B.T. Lima and R.R. Rosa. 2006. Mitigation and recovery of methane emissions from tropical hydroelectric dams. Energy. doi:10.1016/j.energy.2006.09.008. (http://dx.doi.org/10.1016/j.energy.2006.09.008).
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