Estimates of GHG emissions by hydroelectric reservoirs: The Brazilian case

Abstract

This article reports and discusses the application of an analysis of reservoir greenhouse gas (GHG) emission using the net emission approach (difference between post-impoundment and pre-impoundment emissions) to assessments of GHG emissions from hydropower reservoirs compared with GHG emissions from thermal plants producing electrical energy. Reservoirs were chosen from a representative set of different Brazilian biomes and climatic regimes. A field campaign program was performed between 2011 and 2013 to estimate net GHGs emissions from 8 Brazilian representative hydropower plants. Four field campaigns to each hydropower plant were scheduled within two-month intervals to measure CH4, CO2 and N2O diffusive fluxes on air-water interfaces (reservoir surface area and downstream river, CH4 and CO2 ebullitive fluxes on the air-water interfaces of the reservoir and degassing rates at powerhouse turbines). Permanent carbon burial rates were also measured in the reservoirs. Statistical analyses of the data were representative of post-impoundment annual values for GHG emissions for each pathway and for permanent carbon burial rates. Values for pre-impoundment representative annual GHG emission were calculated taking into account previous land cover mapping data from the period of the creation of the reservoir, and reference GHG flux values established by the literature for each land cover class. To compare with thermopower generation, annual values obtained were expressed as GHG intensity in g of CO2eq.kWh−1 considering a Global Warming Potential value for 100 years; the factor 44/12 was considered as the permanent carbon burial rate necessary to transform carbon into carbon dioxide; and production of plant firm energy was also considered. Annual net GHG emissions estimates were expressed as GHG intensity in g of CO2eq kWh−1.In all cases analyzed, hydroelectric plants emit less greenhouse gases than natural gas and coal-fired thermoelectric plants. The exception was the Balbina hydroelectric dam, which has a low annual power generation and a very large reservoir: it emits more gas per unit of energy than the two thermoelectric plants abovementioned.