Estimating greenhouse gas emission via degassing and modeling temperature profiles in tropical reservoirs.

Abstract: The aim of this project was to quantify the greenhouse gas (GHG) emissions from the degassing process ofhydroelectrical reservoirs in tropical regions.Reservoirs represent 25 % of the total area of man-made freshwater systems and are a source of GHG emissions tothe atmosphere. There are plans to construct an additional ca 3700 medium and large hydropower dams with theaim to double the current global energy production by hydropower. The majority of these are planned to be constructed in tropical regions. By understanding the processes controlling GHG emissions from these hydropowerreservoirs, the design of new hydropower plants can be developed to minimize the emissions.This project were designed to investigate GHG emissions from the turbines of two reservoirs in Brazil, as partof the larger ”Hydrocarb” project that investigates the total emissions from a number of reservoirs in Brazil. Toestimate the GHG emissions from the degassing process, a sampling campaign in the reservoir Chapeu D’Uvaswas conducted in April 2020 .Water samples from the entire water column at the water inlet, and directly afterthe dam were taken by using a sampling technique that involved a newly developed deep-water sampler. Themethane concentration was then analyzed for each depth of the water column and in the water directly after theoutlet. The results showed that at the deep layers with low oxygen concentration in the water column containedhigh concentrations of methane. These high methane concentrations were also found in the water at the outlet.This method was also planned to be used for the hydropower reservoir Funil, but due to the global COVID-19pandemic the campaigns were canceled. A modeling approach was instead constructed with the aim to modelthe methane concentration at the intake of the water in Funil, and to estimate the degassing as the water passesthe turbines. The first stage of this modeling approach was made within this study, where temperature profiles ofthe reservoir were modeled. The predicted profiles matched the observed temperatures profiles with a root meansquare error of 1.5 ◦C. The study concluded that the method of collecting methane concentrations throughout thefull water profile using the sampler were successful and can be used to examine methane concentration at the levelof the water inlet in reservoirs. The methane emission from the outlet at Chapeu D’Uvas was estimated to below contributing to 1.1 % of the total greenhouse gas emissions from the reservoir. For the modelling of methaneconcentration in water columns, the first part of the method to model daily temperature profiles that can be usedto implement empirical models of oxygen demand and methane production in the model.