Hydropower- A Catalyst for Climate Change Mitigation
Large hydropower plants make up 16% of the global energy supply, in some newly industrializing and developing countries the proportion of hydropower accounts for nearly 100% of the total energy generation of the country. In the context of climate change there is a growing awareness towards the potential of hydropower to contribute to future energy demands by simultaneously moving economies to a lower-carbon future. The lifecycle energy payback ratios1 for well-performing hydropower plants (high energy yield pro water surface) still reach the highest values of all energy technologies (SRREN 2011).
However, studies show that hydropower is not always carbon neutral. Reservoirs can be sources of green house gas (GHG) emissions, such as carbon dioxide (CO2) and methane (CH4) resulting from the decomposition by bacteria of the biomass that was either submerged due to impoundment, that enters the reservoir from upstream inflows or that grows within the reservoir itself.
Up to date it is not clear how strong GHG emission from reservoirs contribute to the global GHG emissions. Hydroelectric reservoirs cover an area of 3.4x105 km² and comprise 20% of all reservoirs worldwide. Barros et al. (2011) studied 85 globally distributed reservoirs that account for 20% of the global area of these systems in order to estimate the global GHG emissions of reservoirs. The global GHG emission rates derived by them represent 4 % of the estimated global carbon emissions from inland waters. This is less than those estimated by Luis et al (2000) (an equivalent of 7% from CO2 and CH4) based on a more limited number of sampled reservoirs. Luis et al. (2000) estimated as well that 90% of the whole CH4 emissions by reservoirs are from reservoirs located in the tropics and according to CO2 it is about 40 %. Generally the emissions of both gases are much higher in tropical reservoirs than from reservoirs located in the boreal zone. This is very likely because of higher temperatures and higher amounts of biomass (Kosten 2010). Fearnside () estimated that a tropical reservoir covered before with tropical forest (like the Belo Monte complex which also had a low power density) could need nearly 41 years to pay off the initial emission debt.
GHG emission could be minimized by logging trees before flooding or removing the plants which re-grow during low level conditions, in order to reduce the organic material for decomposition. Furthermore long residence times of the water should be avoided and the outflow of the reservoir should not be located in the CH4 enriched depths of the reservoir. In case a reservoir emits relevant amounts of methane, the counter- measures should be designed on the basis of a limnological study, which identifies the limiting factors. Measures would be the land management of the upstream catchment, up-stream waste water treatment inflows, cleaning of the reservoir, regulating the draw-down to discharge nutrients, and regulating the times of residence. The elements of such an anti-eutrophic management are well known, but have to been determined in every single case based on investigations.
Hydroelectric reservoirs all over the world vary a lot in their conditions. They differ in size and productivity as well as in their composition of the flooded area and the water residence time. All these factors have an influence on GHG production and emission. The emissions of greenhouse gases, calculated according to the rules of a life cycle assessment (LCA), are still among the lowest of all forms of renewable energy and of course dramatically lower than for fossil fuel electricity generation (SRREN 2011).
Noting the scientific uncertainties concerning GHG emissions from reservoirs and that these uncertainties will not be resolved in the short term, a simple and transparent criteria, based on thresholds in terms of power density (W/m2), are to be used to determine the eligibility of hydroelectric power plants for Clean Development Mechanism (CDM) project activities. The UNFCCC classified three categories:
1. Hydroelectric power plants with power densities (installed power generation capacity divided by the flooded surface area) less than or equal to 4 W/m2 cannot use current methodologies;
2. Hydroelectric power plants with power densities greater than 4 W/m2 but less than or equal to 10 W/m2 can use the currently approved methodologies, with an emission factor of 90 gCO2eq/kWh for project reservoir emissions;
3. Hydroelectric power plants with power densities greater than 10 W/m2 can use current approved methodologies and the project emissions from the reservoir may be neglected 2
1 the ratio of total energy produced during a system’s normal lifespan, divided by the energy required to build, maintain and fuel it
2 UNFCCC/CCNUCC- Executive Board 23, Report, Annex 5, page 1
