Iss2/Extra Hydroelectric power emits more greenhouse gases than fossil fuels

In the podcast version of Chapter 1, Charley asked why we hadn’t included hydroelectric power as a green energy candidate, and this led to an interesting conversation. Hydroelectric power uses the force of running water to push turbines and generate electricity. At first glance, hydroelectric seems like it should be a serious candidate for replacing fossil fuels. Not only is hydroelectric renewable, it excels in the area where all of our green energy candidates are weak: hydroelectric is dispatchable, meaning electricity generation can be turned up or down at a moment’s notice according to demand for electricity. This is because of the massive reservoir of water behind the dam: operators can fully open the gates of the dam to let maximum water through and maximize electrical output; operators can close the gates entirely, stopping all electricity generation; or choose any level of output in between, according to demand, and all at a moment’s notice.

As discussed in Chapter 1, the fact that none of our green energy candidates are dispatchable poses one of the most vexing challenges of the green energy transition. Because they are baseload (geothermal, nuclear) or intermittent (wind and solar), all four require massive utility batteries to avoid blackouts when demand is high. This is a serious problem because utility batteries require an enormous amount of minerals. Having a dispatchable source of green energy would greatly reduce the amount of utility batteries required for a green energy transition, and thus dramatically reduce the need for mining.

However, there are two key reasons why hydropower is not a viable option for replacing fossil fuels.

First and most importantly, hydroelectric plants are net emitters of greenhouse gases (peer-reviewed life cycle analysis review article; paywalled). This is not true for the wind, solar, nuclear, or geothermal. While all four require fossil fuels to manufacture, overall the carbon emissions necessary to construct, maintain, and run them are tiny relative to the amount of electricity they produce. By contrast, hydroelectric plants emit so much greenhouse gas that some hydroelectric plants “potentially have higher GHG [greenhouse gas] footprints than fossil-based energy generation technologies.”

Why would hydroelectric power generate as much or more greenhouse gas as a fossil fuel power plant? There are two sources. First, hydroelectric dams require a great deal of concrete and steel, and as we discussed in Chapter 1, we currently do not have the technology to make either without fossil fuels. But up to 90% of the greenhouse gas emissions of hydropower come from another source: the reservoir. The body of water behind the dam generates an astounding amount of carbon dioxide, nitrous oxide (N₂O), and methane, all potent greenhouse gases. These gases are primarily created from the decay of plant material when the once-dry land is flooded to create the reservoir.

The issue of reservoirs emitting extraordinary amounts of greenhouse gases can be mostly eliminated by run-of-river designs. Run-of-river hydroelectric plants do not use a reservoir at all: they divert some amount of a river’s flow to generate electricity miles away, then return the water to the river.

In addition to eliminating nearly all of the carbon emissions of hydropower, run-of-river designs have other advantages. Reservoir-based hydroelectric dams impose severe social and environmental costs because anything behind the dam – from villages, towns, and farms to sensitive ecosystems – is wiped out by the reservoir. Additionally, because dams stop the flow of water, everything downriver is desiccated as the dam deprives it of water. For an extreme example, the Three Gorges Dam in China displaced 1.2 million people and wiped out so many ecosystems  that several species are believed to have gone extinct. Similarly, 33 million people are estimated to have been displaced by all of India’s dam projects. Because they lack a reservoir and do not cut off the flow of a river, run-of-river hydroelectric plants have negligible environmental and social impact.

But run-of-river designs give up the biggest strength of hydropower: they are not dispatchable. Run-of-river plants can only generate electricity when water levels are high. No electricity can be generated when there is a drought or when water levels are seasonally low. In other words, run-of-river designs are intermittent and would not reduce the need for utility batteries.

The second reason we did not include hydroelectric power: as climate change and environmental degradation stress our freshwater supplies, it is not clear that we can spare freshwater for generating electricity. Similarly, it is doubtful that we can even count on hydroelectric plants to be operable in a climate-changed world. For example, the amount of electricity that the Hoover dam can generate has substantially decreased over the past 30 years as the water levels of its reservoir – Lake Mead – have steadily fallen. Indeed, water levels may fall so low that the Hoover Dam cannot generate any electricity within a few years. This problem is even more acute for run-of-river designs: because there is no reservoir, they cannot build up a store of water during floods or seasons with above-average precipitation.

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Dam you straight to h*ck!

In sum, hydroelectric is not a viable replacement for fossil fuels. Hydroelectric plants generate as much or more greenhouse gas emissions as fossil fuel power plants, and the water they rely on may not even be around in a climate changed-world.