Hydroelectric power has huge potential to produce electricity at specific sites where water traverses a large height difference, and therefore loses gravitational potential energy. Through the driving of turbines this energy can be converted into electricity.
From purely a renewable electricity production view point hydroelectric has huge potential to form part of a sustainable future electricity mix. WorldWatch Institute state that 16.1% (3427 terawatt-hours) of global electricity production is currently from hydroelectric power. Five countries produce 52% of this 16.1%: China, Brazil, the United States, Canada and Russia. There are currently three hydropower dams larger than 10GW: The Three Gorges (China), Itaipu (Brazil) and Guri (Venezuela).
| Guri Dam, Venezuela (Source) |
| The Three Gorges Dam, China (Source) |
| Itaipu Hydroelectricity Power Plant, Brazil (Source) |
Four countries, although small, produced 100% of their electricity from hydroelectric power as far back as 2008! The countries leading the way on the hydroelectric front? Albania, Bhutan, Lesotho and Paraguay. Fifteen other countries generated over 90% from hydroelectric power. Unsurprisingly, the most hydroelectric power produced per/capita are countries with mountainous terrain and significant snow melt to drive the dam, New Zealand, Iceland and Norway.
So why are countries already opting to go full throttle on hydroelectric powers? Firstly, it's relatively cheap costing between 6-10p per kilowatt-hour for a hydro plant larger than 10MW. Hydro power also provides electricity security and it is produced 'in-house', unlike fossil fuels where many states rely on others for their electricity production. Hydropower also enables very fast increases in power production to deal with peaks in demand, whereas fossil fuel power stations can take hours and some other renewables cannot increase production at all. Due to these characteristics Kosnik concludes that a strong case can be made for using hydroelectric power in USA to fight global warming.
However, it's not all plain sailing for hydroelectric power. Damming majorly interrupts local and national ecosystems preventing the migration of fish. For rivers with multiple dams, this practically destroys any hope of a healthy ecosystem as species became increasing dispersed with ever-decreasing gene pools. Building of dams also displaces people and terrestrial wildlife, see the Three Gorges Dam for an example of this on a huge scale. A further issue with the Three Gorges especially is it flooded mines which results in leakage of toxic chemicals into the water. Dams also trap sediment. Firstly, this means that the capacity of the dam will decrease if it is not regularly dredged. Secondly, it deprives downstream communities of silt which is vital to keep land fertile. Furthermore, dams can stop flooding downstream (unless a natural flood regime is mimicked), which may be vital to local economies downstream. The immobility of hydroelectric dams is also an issue. As explored by Whittington and Gundry hydroelectric plants are often justified on long-term economic calculations. With global climate change altering precipitation patterns, it is worth considering in a proposal what might happen to such rainfall patterns in the future. Given the huge development of dams across the world it seems inevitable there will be abandoned dams where rains have moved on. For example, see how many dams have been constructed in the USA alone:
Amid all the above local impacts, I'd like to focus on a global impact of hydroelectric dams, and that's the emissions from dams. Dams flood vast areas of land that was not previously flooded. This newly created water-land interface is the location of chemical reactions and decomposition. Fearnside explains that carbon dioxide is emitted from the decay of trees left standing in the newly formed water mass, whilst methane is emitted from the decay of soft vegetation at the bottom of the lake (this is especially prevalent for macrophytes and plants in the drawdown zone which floods when the water level rises). Fearnside ascertains that because of this dissolved methane levels are highest at the bottom of the reservoir. Some methane is released through bubbling, but mainly from the water traversing the turbines and spill way. As water is drawn from sufficient depth, this results in large methane concentrations. Fearnside studied the Curuá-Una dam in Brazil. He found that in 1990 after 13years of production the Dam had 'emitted 3.6 times more greenhouse gases than would have been emitted by generating the same amount of electricity from oil'. Rudd et al. found similar patterns of high Carbon Dioxide and Methane emissions from a Quebec reservoir. At most sampling sites, Rudd et al. found that Carbon Dioxide were 2-3times equilibrium levels, whilst Methane levels had 'concentrations higher than in natural, stratified Canadian shield lakes'.
This is only two dams, so covering Barros et al.'s research in Nature on 85 globally distributed dams accounting for 20% of total hydroelectric dam area gives a broader perspective. Total reservoirs from hydroelectric dams cover 3.4×105 km2, about 20% of global reservoirs. Previous research from St Louis et al. suggests that 321Tg of Carbon/year are emitted from such reservoirs. Barros et al. estimate that 48Tg of Carbon as Carbon Dioxide is emitted from hydroelectric dams, and 3Tg of Methane. More limited data is cited as the reason for lower estimates than previously produced by St Louis et al. Barron et al. found that the highest emissions were from Amazonia, understandable for the mass of vegetation/biomass that will have been flooded to create dams.
Such evidence is damning (intended) for the case of hydroelectric power. Hydroelectric power plants are often marketed as positive for the environment, but this is not the case on a global scale. The common opinion that dams reduced greenhouse gas emissions, and hence global warming, is clearly misconceived. Dams produce significant amounts of Carbon Dioxide and Methane, whilst also having a very long list of negative impacts as listed above. The positives of dams for a single country however do remain larger, they produce electricity within the state at a low cost and production can be ramped up quickly. Hence, dam construction is likely to continue as governments place economic and energy security high on the agenda.
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