Diurnal Pumped‐Storage Operation Minimizes Methane Ebullition Fluxes From Hydropower Reservoirs

Abstract

Hydropower is considered green energy and promoted to reduce greenhouse warming. However, hydropower is typically generated using reservoirs and reservoirs are known to emit substantial amounts of the greenhouse gas methane (CH 4) to the atmosphere. In many reservoirs ebullition is the dominant pathway of CH 4 emission. We show that continuous diurnal pumped-storage operation, which combines water pumping into the reservoir typically during the night and water drawdown during high demand of electricity, is beneficial for reducing CH 4 ebullition associated with hydropower generation. This conclusion is based on ebullition fluxes and water levels measured over 3 months in Schwarzenbach reservoir located in Germany. The reservoir was managed using three modes of operation: (1) diurnal pumping and turbination, (2) no pumping and no turbination, and (3) diurnal turbination. Cross-correlation analysis indicates that ebullition fluxes predominantly occur during diurnal water level decrease associated with turbination. Consistently, average ebullition fluxes of CH 4 were negligible during Mode (2) and substantial during Modes (1) and (3). During Mode (3) the average CH 4 ebullition flux was~197 mg m −2 day −1 ,~12 times larger than during Mode (1) (16 mg m −2 day −1). Our data indicate that overall CH 4 ebullition is about 3 times larger during 51 days of operation consisting of 38 days of no turbination followed by 13 days of diurnal turbination than during 51 days of continuous diurnal pumped-storage operation. This suggests that continuous diurnal pumped-storage operation leads to reduced CH 4 ebullition from reservoirs and is therefore advantageous compared to modes of operations involving long-term, large-amplitude turbination cycles. However, inland waters and thus also reservoirs are important components of the regional and global carbon cycle (

Bayesian Calibration Points to Misconceptions in Three‐Dimensional Hydrodynamic Reservoir Modeling

“…The SR is located in the Northern Black Forest (Germany, EPSG 3857: 48.654842°N, 8.329314°E) and serves as the upper reservoir of a hydroelectric pump‐storage scheme (cf. Figure 1 and Encinas Fernández et al., 2020). The reservoir has a length of 2.2 km and a maximum depth of 47 m at its highest operating level of 668.5 m a.s.l., leading to a total storage capacity of 14.4 million m 3 .…”

“…While the background horizontal eddy viscosity $${\nu }_{h}^{\mathit{back}}$$ and diffusivity $${{\Delta }}_{h}^{\mathit{back}}$$, as well as the initial water temperature T tow were chosen as constraining calibration parameters, there are many other unconstrained (constant) parameters. We made and had to make simplifications about environmental processes, which are necessary for modeling hydrodynamics of pump‐storage reservoirs (Bermúdez et al., 2018; Encinas Fernández et al., 2020; Salehi, 2017). Other factors influencing lake stratification could be, for instance, biotic parameters such as nutrients, carbon, or oxygen concentration (Chanson, 2004).…”

“…Furthermore, the reservoir is influenced by seasonal thermal stratification. Hence, the reservoir is separated into an epilimnion, metalimnion, and hypolimnion during summer, while turnovers in fall and spring cause full vertical mixing (Encinas Fernández et al., 2020).…”

“…The hydrographs and water levels were not used for model calibration in this study. Vertical profiles of flow velocity in the reservoir were measured with an acoustic Doppler current profiler (ADCP) between July and October 2016 (Encinas Fernández et al, 2020). The position of the ADCP is marked by a star at the center of the reservoir in Figure 1.…”

“…To investigate and test the two hypotheses, we ran a Bayesian calibration of a large‐scale, complex hydrodynamic Delft3D‐FLOW (Deltares, 2022) model of the pump‐storage Schwarzenbach reservoir (SR) in Germany (Encinas Fernández et al., 2020). The calibration considered two bulk parameters (background horizontal eddy diffusivity and viscosity) and an initial guess for the vertical water temperature profile in the reservoir (assumed to be a constant over depth).…”