Greenhouse gas emissions from hydropower reservoirs: A synthesis of knowledge

Abstract

ydropower reservoirs, like other natural water bodies, have intense biota ranging from microorganisms to aquatic vertebrates. The most important effects of dam impoundment are potential changes in the movement of water by flow-ing much slower than the original river. Slower water movement conditions favor the establishment of phy-toplankton and nutrient increases in which methano-genesis replaces the oxidative water, generating anaer-obic conditions. Microorganisms (bacteria) decompose organic mat-ter and produce biogenic gases under water as a by-product. Some of these biogenic gases, such as methane, carbon dioxide and nitrous oxide, are effec-tive in terms of their global warming potential.

Full text

Hydropower reservoirs, like other natural water
bodies, have intense biota ranging from
microorganisms to aquatic vertebrates.
The most important effects of dam impoundment are
potential changes in the movement of water by flow-
ing much slower than the original river. Slower water
movement conditions favor the establishment of phy-
toplankton and nutrient increases in which methano-
genesis replaces the oxidative water, generating anaer-
obic conditions.
Microorganisms (bacteria) decompose organic mat-
ter and produce biogenic gases under water as a by-
product. Some of these biogenic gases, such as
methane, carbon dioxide and nitrous oxide, are effec-
tive in terms of their global warming potential.
1. Hydropower and greenhouse gases
Like most manmade projects, hydropower dams have
multiple effects on the environment that have been
studied in some depth over the past two decades
(REFERENCES?). The slow flow rates of reservoirs
favors the appearance of phytoplankton as nutrients
increase, with anoxic water replacing oxidative water
and generating anaerobic conditions.
Although research during the late 1990s highlighted
potential problems caused by greenhouse gas emis-
sions from reservoirs, many factors remained unstud-
ied.
Some biogenic gases cause global warming, includ-
ing methane, carbon dioxide and nitrous oxide. In
order to draw up an accurate assessment of the net
emissions caused by hydropower dams, significant
improvements are needed in carbon budgets and stud-
ies of representative hydropower dams located in trop-
ical, boreal, arid, semi arid and temperate climates.
Comparisons of GHG emissions from hydropower
reservoirs and from thermal power stations should be
calculated and characterized based on per unit gener-
ation of energy. Also to be considered in the compari-
son of hydropower emissions with alternatives are the
different types of fuels, the varying technology effi-
ciency levels of steam turbines, coal, and fuel oil and
natural gas turbines and combined cycle plants.
2. Conceptual model of greenhouse gases from
hydropower dams
There are three distinct and important phases of GHG
emissions derived from hydroelectric reservoirs. The
first phase is prior to dam construction and reservoir
formation. This pre-reservoir filling phase is charac-
terized by fast flowing water (turbulent flow), high
rates of dissolved oxygen, the presence of suspended
solid material, coarse and low autochonous produc-
tion, and gas flow predominantly by diffusion.
The second phase, after reservoir filling, is charac-
terized by significant changes to the chemical, physi-
cal and biological processes. The speed of water flow
is also much lower, rates of respiration and gross pri-
mary production are balanced, and there is a high
presence of labile organic material. During this stage,
deep anoxic layers begin to form in the reservoir.
Hydropower & Dams Issue Four, 2011 1
Greenhouse gas emissions from
hydropower reservoirs: a synthesis
of knowledge
M.A. dos Santos and L.P. Rosa
Energy Planning Program, COPPE/UFRJ, Brazil
Greenhouse gas emissions from hydro reservoirs are a subject of strategic importance and are often compared, using varying
methodologies, to emissions from other types of electric generation such as thermal power plants.
This paper offers the hydropower community state-of-the-art knowledge and helps to clarify some important issues
and challenges that remain to be solved going forward.
Fig. 1. Pre-
impoundment
phase of GHG
emissions
Fig. 2. After dam
impoundment
phase of GHG
emissions

As a result of the presence of submerged terrestrial
plant biomass, a process of degradation of organic
matter begins which results in the greater consump-
tion of oxygen in the water column.
This phase represents a peak in emissions of green-
house gases, and there is also strong production of
organic matter in the water column. Flows of gases
occur both by diffusion and by ebullition.
The third phase, reservoir stabilization referred to
the more mature phase where the processes will be
more balanced. Much of the terrestrial biomass that
was submerged was decomposed leaving only the
woody portion, such as trunks, branches and roots. As
the labile biomass was already decomposed the main
source of organic matter is from external sources.
The bottom of the reservoir has high amounts of fine
sediment along with organic material that precipitates
through the water column. The water column remains
relitivly anoxic and varies in depth depending on the
prevailing hydrodynamic conditions.
At this stage, the gross primary productivity is much
higher than respiration rates in the reservoir. The
autotrophic production is significant but the heterotro-
phy dominates from the material coming from the
drainage basin.
Fig. 4 shows the funnel for capturing bubbles from
the bottom underwater reservoir.
3. Previous studies and recommendations
The issue of greenhouse gas emissions from
hydropower reservoirs is a matter of relatively recent
concern. In the early 1990s, researchers identified sig-
nificant flows of CO2, CH4and N2O from air-water
interfaces (Rudd et al 1993; Rosa and Schaeffer, 1994;
Duchemin et al 1995; Galy-Lacaux et al 1999).
Other important contributions in this field are the
work of Galy-Lacaux, 1999; Delmas et al, 2001; Rosa
and Schaeffer, 1995, Rosa et al 1996.
Four major reports presented recommendations for
research into greenhouse gases and hydropower dams:
the HydroQuebec Meeting in Montreal Report, the
World Commission on Dams Report, 2000, 2000a,
2000b, the Rio GHG Working Team Report, 2001 and
the Lannemezan Working Group Report, 2004.
Recently, the International Hydropower Association
(IHA), with UNESCO has organized several meet-
ings. Hosted by the IHA in collaboration with the
International Hydrological Programme (IHP), a GHG
research project aims to improve understanding of the
impact of dams on natural GHG emissions in a river
basin, obtaining a more detailed overview of current
methodologies and bridging gaps in current knowl-
edge.
The IHA publication, in 2010, of the GHG
Measurement Guidelines for Freshwater Reservoirs
represents an important milestone in the search for
knowledge and as an attempt to standardize some pro-
cedures.
More recently, the International Energy Agency,
under the Hydropower Implementing Agreement on
Hydropower Programmes and Technologies, created a
new annex called “Managing the Carbon Balance in
Freshwater Reservoirs”. This initiative has the objec-
tive of developing broadly accepted procedures and
protocols for the measurement of GHG emissions
from hydropower reservoirs and best practices for
minimizing these GHG emissions.
Brazil is leading this major initiative with universi-
ties and research institutions from, Japan, Norway,
Finland, and the USA. Canada agreed to make contri-
butions to the work of the Annex and other contribu-
tions are presently expected from Portugal and France.
The objectives are to:
• increase knowledge of the processes;
• establish guidelines for planning studies on the car-
bon balance in reservoirs;
• standardize GHG flux evaluation methods.
4. Gross emissions associated with
hydropower compared with Thermo Power
Until now only gross emissions have been studied in
several parts of the world. Few experiences of net
emissions are underway.
It is important to note that before land is flooded, sig-
nificant natural emissions occur from areas such as
rivers, lakes, grassland, and forest. Calculations of
gross emissions do not subtract natural sources from
already existing emissions.
CO2emissions may be attributed to the natural car-
bon cycle between the atmosphere and the dam.
The discussions on the carbon dioxide cycle for
reservoirs are not yet concluded.
The Net Ecosystem Exchange (Net ecosystem
exchange, commonly known as NEE, is a measure-
ment of how much carbon is entering and leaving the
ecosystem) from pre-flooding emissions must be
taken into account.
2Hydropower & Dams Issue Four, 2011
Table 1: Comparison of GHG Emissions by Hydro and Natural Gas Power Plants
Type of Plant Emissions (t C eq./ MWh)
Natural Gas Power Plant
Simple cycle (A) 0.233
Combined cycle (B) 0.1399
Average emissions by 10 Hydropower Plants in Brazil
Measured surface emissions (C) 0.0127
Including estimated upper 0.0254
limit of downstream emissions (D)
Figure 3 below
provides a
conceptual
framework of the
third phase.
Fig. 4. Set of
funnels placed at
the reservoir
surface.

In 2006, the International Panel on Climate Change
(IPCC) noted that current data on GHG emissions
from hydropower dams does not give an accurate
account and concluded that there are currently no reli-
able data from several countries with a significant
number of dams.
The sampling did not consider the net flux of emis-
sions from manmade dams because it did not account
for the natural fluxes of gases (mainly CO2) that may
come from the atmosphere. This makes it harder to
separate anthropogenic emissions from emissions that
would have occurred in the absence of the manmade
dam.
This same IPCC Report points out that dams in
warm temperate, moist, tropical wet, and tropical dry
areas produce higher emissions.
Recent measurements taken at ten hydropower com-
plexes in Brazil totaling 28 537 MW are compared to
emissions by thermal power plants fueled by natural
gas producing the same amount of electricity [Santos
et al, 2006].
Gross GHG emissions from each of the selected
dams were assessed through sampling, with subse-
quent extrapolation of the findings to obtain a value
for the total dam area. A methodology was developed
for obtaining a representative average gas flux, taking
spatial and temporal variations into account.
A wide variation in the intensity of the emissions
was noted, indicating the influence of many factors
including external organic matter washed out of the
soils and down the slopes of the catchment basin.
Other researchers who have measured emissions at
the Balbina hydropower complex in Central
Amazonia (Kemenes et al, 2007) concluded that
downstream methane emissions are of the same order
of magnitude as emissions from the water surface of
the dam.
The findings of the studies to date are highly sensi-
tive, requiring careful analysis and interpretation.
However, it seems appropriate to assume that the ten
hydropower complexes studied have an upper limit of
downstream emissions that are roughly equal to their
surface emissions.
While research into improved measurement tech-
niques continues to progress, results so far indicate
that GHG emissions from hydropower dams per MWh
are lower, on average, than those emitted by power
plants fuelled by natural gas for seven of the ten
hydropower complexes with a total installed capacity
of 27 675 MW that have been studied.
Even if the surface emissions are multiplied by two,
assuming the upper limit of downstream emissions,
the resulting average emissions from the ten dams,
expressed in carbon-equivalent mass per MWh (t C
eq/MWh), are lower than those emitted by simple
cycle or combined cycle power plants fuelled by nat-
ural gas, as summarized below:
Based on the data presented above, average
hydropower plant emissions are lower than those
emitted by power plants fueled by natural gas as
shown through the following percentages (Table 2).
Research findings to date indicate that it is safe to
say that GHG emissions from hydropower plants are
less than those emitted by power plants fuelled by nat-
ural gas on a per MWh basis.
As shown above, in the worst-case scenario, emis-
sions from hydropower plants are 10.9 per cent of
those emitted by simple cycle and 18.2 per cent of a
combined cycle plant. A comparison with coal would
be considerably more dramatic.
5. Conclusions
Hydropower is a renewable and extremely low-carbon
energy resource with significant untapped potential.
Moreover, many developing countries are endowed
with ample potential that can be developed while
avoiding serious environmental implications.
While hydro-based resources, like all other energy
resources, are subject to limitations and face barriers,
the benefits in terms of extremely low operating and
essentially zero fuel costs are overwhelming. At a
time when humankind is looking for clean and afford-
able energy with a small carbon footprint, renewable,
clean and plentiful hydropower options cannot be
ignored.
Painful lessons learned from insensitive develop-
ments in the past that failed to consider the environ-
mental impacts of massive dams and reservoirs must
guide governments and policymakers in decisions of
how hydro resources are developed and managed in
future.
The lack of consistent long-term research about
reservoir-based GHG emissions in many areas of the
world is a major obstacle towards achieving an accu-
rate global estimate.
The temperate and sub-arctic zone have been the
focus of many studies, but there are also studies in
tropical reservoirs from Brazil and French Guyana.
The emission of CH4by hydroelectric reservoirs is
always unfavorable as it has a very high global warm-
ing potential.
Emissions of CO2can be attributed, in part, to the
natural carbon cycle between the atmosphere and the
water of the reservoir. Another part could be attributed
to the decomposition of organic material, caused by
the hydroelectric dam.
Questions still remain to be answered by further
research and these include the following.
• There are emissions that are not fully anthropogenic.
How can only emissions from hydropower be calcu-
lated? In some countries, untreated sewerage and
other high carbon loading from agricultural practices
that include the acceleration of soil erosion and defor-
estation, contribute to emissions.
• How can a good statistical representation of season-
ality be obtained? What is the minimum sampling fre-
quency to achieve representative data?
• How should projects be evaluated considering the
synergy of some cascade schemes?
• Good exploratory data analysis such as probability
density function analysis and statistical models can be
used to extrapolate data for GHGs estimations. What
is the best practice to obtain reliable data?
The following recommendations are given as areas
for further research.
• More measurements on a wider range and diversity
of reservoirs;
• More measurements on a wider range and diversity
of natural environments in countries that are currently
building dams;
• Improve the understanding of the role of transient
carbon in reservoirs and natural lakes;
Hydropower & Dams Issue Four, 2011 3
Table 2: Natural Gas Power Plant x Hydropower Plant Emission
C is 5.4% of A
D is 10.9 % of A
C is 9.1% of B
D is 18.2% of B

• Stimulate the inter-comparison of methods and tech-
niques for sampling and analysis in terms of accuracy
and representativeness.
• Study the cycle of carbon in an undammed catch-
ment compared to a dammed catchment.
Acknowledgement
The authors are grateful to Carlos Henrique Eça D´Almeida
Rocha for design of figures. This manuscript was completed
with the support of CNPq in the form of scholarships and a
National Institute of Science and Technology Global Change,
Emissions of Lakes and Reservoirs project.
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4Hydropower & Dams Issue Four, 2011
M. A dos Santos
L. P. Rosa
Marco Aurélio dos Santos is Associated Professor of
Energy Planning Program of COPPE/UFRJ. He is research
of National Council of Research in Brazil – CNPq. He is
coordinator of scientific team that study greenhouse gases
from hydropower reservoir in Brazil.
Luiz Pinguelli Rosa is Full Professor of Energy Planning
Program and Director of COPPE/UFRJ. He is research of
National Council of Research in Brazil – CNPq. He is
former coordinator of scientific team that study greenhouse
gases from hydropower reservoir in Brazil.
Programa de Planejamento Energético (COPPE, UFRJ)
Centro de Tecnologia, Bloco C, sala 211, Cidade
Universitária, Rio de Janeiro. Brasil