Volume 34, Issue 4,
August 2014, Pages 204–212
Progress in the studies on the greenhouse gas emissions from reservoirs
- Le Yanga, b, c,
- Fei Lub, , ,
- Xiaoping Zhoub, c, d,
- Xiaoke Wangb,
- Xiaonan Duanb, e,
- Binfeng Sunb, c
Received 14 April 2012, Revised 16
March 2013, Accepted 25 May 2013, Available online 25 July 2014
Abstract
The green credentials of hydroelectricity in terms of greenhouse-gas (GHG) emissions have been tarnished with the finding of the researches on GHG emissions from hydroelectric reservoirs in the last two decades. Substantial amounts of GHGs release from the tropical reservoirs, especially methane (CH4) from Brazil’s Amazonian areas. CH4 contributes strongly to climate change because it has a global warming potential (GWP) 24 times higher than carbon dioxide (CO2) on a per molecule basis over a 100-year time horizon. GHGs may emit from reservoirs through four different pathways to the atmosphere: (1) diffusive flux at the reservoir surface, (2) gas bubble flux in the shallow zones of a reservoir, (3) water degassing flux at the outlet of the powerhouse downstream of turbines and spillways, and (4) flux across the air–water interface in the rivers downstream of the dams. This paper reviewed the productions and emissions of CH4, CO2, and N2O in reservoirs, and the environmental variables influencing CH4 and CO2 emissions were also summarized. Moreover, the paper combined with the progress of GHG emissions from Three Gorges Reservoir and proposed three crucial problems to be resolved on GHG emissions from reservoirs at present, which would be benefit to estimate the total GHG emissions from Three Gorges Reservoir accurately.Keywords
- Carbon dioxide;
- Methane;
- Diffusion;
- Bubble;
- Turbine
1. Introduction
Carbon dioxide (CO2), methane (CH4),
and nitrous oxide (N2O) are the three principal greenhouse gases
(GHGs) in the atmosphere, and continuously increases in atmospheric
concentrations of three GHGs are closely related to global climate change [1].
The studies on the GHG emissions from reservoirs in the last two decades
indicated that hydroelectricity was not a green and clean energy as expected
that no GHG is emitted from the reservoir surface [2],
[3] and [4].
In fact, reservoirs are also an important GHG source in the terrestrial
ecosystems [5] and [6].
According to the natural belts that reservoirs located, the global reservoirs
could be divided into tropical reservoirs (e.g., reservoirs in Brazil, French
Guiana, and Laos) and temperate reservoirs (e.g., reservoirs in Canada,
Switzerland, and China). The global warming potential (GWP) of the GHG
emissions from Brazil’s reservoirs are amazing, which are even higher than that
from thermal power plants with similar installed capacity [2].
For example, Curuá-Una Reservoir in Brazil emitted 3.6 times more GHGs
than those would have been emitted by generating the same amount of electricity
from oil [7].
However, GHG emissions from Canadian reservoirs are relatively low [8],
which are lower than the GHG emissions compared with GHGs emitted by
fossil-fuelled electricity generation. Therefore, it cannot be generalized to
determine whether the development of hydroelectricity could reduce GHG
emissions, which should depend on the specific situation of reservoirs. The
geographic locations of reservoirs have an impact on the organic matter storage
and water temperature, and influence on CO2 and CH4
emissions subsequently [6].
However, CH4 emission fluxes from Lake Wohlen, a temperate reservoir
in Switzerland, are even higher than those from tropical reservoirs [9],
which cause the controversy on the development of hydroelectricity in the
middle Europe region [3].
Beside latitudes, CO2 emissions from reservoirs are also influenced
by reservoir ages [6],
wind speeds [10],
pH values [11],
precipitation [12],
chlorophyll-a concentrations [12] and [13],
and dissolved organic carbon in the water body [12] and [14],
while CH4 emissions from reservoirs are influenced by water depths [15],
water level fluctuations [16],
DO concentrations [17],
water velocities [16],
and wind speeds [10].
GHG emissions from reservoirs are different from the natural
water bodies, such as lakes and rivers, because the impoundment of the
reservoir has resulted in flooding of large areas of terrestrial and natural
aquatic ecosystems. CO2 and CH4 are the major end
products of the microbial decomposition of flooded organic matter [17],
which are transported to the atmosphere from the reservoir surface by diffusion
or bubbles. Turbines and spillways are unique to the dams, and turbines are
used to generate electricity by transforming potential energy of the storage
water into electric energy by the rotation of vane wheel; spillways are the
drainage channels to control the floods in the reservoirs. When the deep water
passes through the turbines and spillways, the dissolved gas (especially CH4)
in the hypolimnion before the dams would release into the atmosphere, becoming
a huge CH4 source, because of the abrupt change in temperature and
pressure, which is called “degassing” [18].
Besides, downstream fluxes are often higher than upstream ones because of the
strong disturbance to the water passing through the dams [19];
thus, the downstream emission fluxes should be paid attention. In conclusion,
there are 4 pathways for GHG emissions from reservoirs, i.e., diffusive
emission, ebullitive emission, degassing emission at turbines and spillways,
and downstream emission [20].
The CO2 emission from reservoirs is the largest,
the second is CH4 emission, and N2O emission is the
smallest. However, the GWP of the three gases is different. CH4 has
a GWP 24 times higher than carbon dioxide (CO2) on a per molecule
basis over a 100-year time horizon [3],
and nitrous oxide (N2O) has a GWP 298 times that of CO2[21].
Based on the studies on GHG emissions from reservoirs available, this paper
reviewed the 3 GHG emissions from the tropical and temperate reservoirs through
diffusion, ebullition, degassing, and downstream river. In addition, the
environmental variables influencing GHG emissions were also summarized.
2. CO2 emissions from reservoirs
2.1. CO2 production in reservoirs
In a broad sense, CO2 production in a reservoir
includes the carbon footprint of emissions from the use of fossil fuel, steel,
and cement during the construction phase of a dam [21],
which is related to the size of dam and the duration of creation. The Three
Gorges Dam (TGD) is a good example, with a length of 3035 m and a height
of 185 m, which lasted for 18 years to construct (1992–2009).
Although there is no study on CO2 emission during the construction
phase of the TGD, CO2 emission during the process cannot be ignored.
Besides, CO2 production in a reservoir also includes the CO2
emission when the dam operated normally, e.g., CO2 emission from the
fossil fuel combustion by shipping, and CO2 emission from the
turbines. Navigation and electricity generation are two important functions of
the Three Gorges Reservoir (TGR), but CO2 emission has not been
quantified during the two processes by far.
CO2 discussed in the paper is produced from the
decomposition of the flooded organic matter under the aerobic or anaerobic
conditions after the impoundment. Carbon sources in the reservoirs included the
flooded organic matter in the original forests, soils, vegetations,
allochthonous input from terrestrial ecosystems or the upstream rivers nearby,
and photosynthetic fixation by phytoplankton at the reservoir’s surface or
vegetations in the drawdown areas [21],
[22] and [23].
The flooded organic matter would decompose into CO2 and CH4
by methanogens under the anaerobic conditions at the reservoir bottom [23] and [24].
In fact, CO2 could also be produced at the aerobic conditions, e.g.,
the decomposition of dead trees left above the water surfaces [24].
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