By Watmore Makokoba
Large water reservoirs could be major contributors to climate change
Hydro-electricity is usually touted as a clean renewable energy source and favourably preferred as an alternative to carbon emitting thermal power stations.
However, these water bodies could silently be emitting tonnes of methane gas into the ozone layer and contributing significantly to global warming.
According to emerging research findings, electricity generation, besides its worth and indispensability to life, is the leading cause of industrial air pollution globally.
Ironically for countries like Zimbabwe which heavily rely on hydro-electricity, Lake Kariba’s contribution to local ozone toxic emissions could be more intriguing than previously cited.
“It has been noted that these reservoirs release methane gas from the decomposing organic matter like vegetation and dead animals from their reservoir bed. Methane is produced when organic matter is broken down anaerobically at the bottom of dams,” reports The Greenplanet report.
“The gases are emitted from the surface of the reservoir, at turbines, spillways and tens of kilometres downstream. Hydro plants with large reservoirs relative to their generating capacity can have a much greater impact on global warming than fossil fuel plants.
“Emission levels vary widely between different reservoirs depending upon the area and type of the ecosystems flooded, reservoir depth and shape and the way in which the depth is operated”, adds the report.
According to the Southern African Power Tool publication (2012): “Methane has more than 25 times the global warming impact compared to carbon emissions when released to the atmosphere.”
This means when methane finds its way into the atmosphere, its pollution rate is more devastating than that of other gasses on the ozone layer.
The International Panel on Climate Change (IPCC), says hydro-electric reservoirs are classified as land-use changes. As such, countries with large areas covered by reservoirs should work to reduce uncertainties regarding the role of hydropower in climate change.
“Emissions are especially high in large tropical reservoirs built over densely forested areas, where there is a large input of organic matter, the average temperature is high and there are large areas with anoxic conditions at the bottom of the water column,” reports IPCC.
Methane warming potential is 72 times greater than carbon dioxide over a 20 year period. Studies have shown the impact of hydro reservoirs on climate change through the decomposition of accumulated organic matter. In some cases, research has shown that greenhouse gas emissions per kWh of electricity generated could be higher for hydro-electricity than fossil fuel-based energy production.
Bongani Masuku, a manufacturing engineering graduate from National University of Science & Technology (NUST), says given the amount of methane produced and trapped underneath water bodies, Zimbabwe could capitalise on this and turn it into a climate smart initiative by harvesting methane and using it as fuel.
“Contributions of fossil fuel to global warming are well known. The impact of hydro-electric dams, in terms of greenhouse gas (GHG) emissions, is only now beginning to be uncovered.
“This alone shows the need to develop technologies which capture the methane before it reaches the turbines in electricity generation. Recent studies also relate … to methane variability and discuss the reduction of future anthropogenic climate change by aggressively maintaining methane emissions as proffered by Gavin Schmidt (2004),” says Bongani.
Bongani has developed a submersible methane harvester (S.M.H.), which is a mobile methane harvesting mechanism. This is done through following pre-defined path-lines to harvest methane from underneath the water. This could effectively be used in Zimbabwe to harvest methane from water bodies such as Lake Kariba.
“The Submersible Methane Harvester (SMH), aims to enable the recovery and use of the large and untapped underwater methane resources present in hydro-electric dams. Harvested methane will ultimately be purified in order to become a sustainable heat and energy source”, explained Bongani.
How it works.
According to Bongani, the methane harvester is built on a houseboat model that has submersible methane absorbers underneath water surfaces as illustrated in the diagram.
These specially made absorbers trap the gas before it finds its way to water surfaces, purifies it and compresses the gas for domestic and commercial uses.
“ More importantly, the fact that deep waters are almost saturated with methane explains why turbine degassing is so important in many large tropical reservoirs.
“ The cause of this scenario is quite simple, but has frequently been overlooked in estimates of reservoir emissions. Since water intakes are generally located well below the surface and CH4 concentration strongly increases with depth, much of the dissolved methane is quickly degassed when the pressure drops as the water passes through the turbines,” he said.
Hydro-electricity is regarded as a renewable energy source in Africa and the rest of the world hence its wide adoption. According to the Zimbabwe Power Company, hydroelectricity accounts for 51% of electricity produced in Zimbabwe.