Energy Matters: Is hydro really dirtier than coal?

Once again, when it comes to energy choices and the environment, it is not sufficient to say something is good or bad, right or wrong. There are no absolutes here. If we hope to arrive at sensible policy to save our atmosphere, we need to pay attention to quantitative comparisons. The Numbers may set us free!
Jonathan Carter of the Forest Ecology Network recently wrote that Hydropower is dirtier than coal (Franklin Journal 3/12/21). Is it? Let’s investigate.
We know coal plants are dirty in a number of ways, emitting an array of polluting gases and particulates and producing huge amounts of solid waste (see our last column). Obviously, they emit a significant amount of CO2 and N2O–and hydropower is worse? Yikes!
Here we want to address the quantity of greenhouse gases (GHG) generated in the process of producing hydro-electricity. We’ll use “grams of carbon dioxide (CO2) equivalent” per kilowatt-hour of electricity (gCO2e/kWh.) The “equivalent” here refers to other gases with warming effects, such as methane and nitrous oxides.
So, what does hydropower emit? In fact hydropower does emit GHG. First, there is energy involved in dam construction, and this produces CO2. But in a life cycle analysis the amount (less than 2 gCO2e/kWh) is not significant. The energy required for construction is paid back in as little as six months.
The main source of hydropower’s greenhouse gases is the area flooded to make the reservoirs. These areas contain vegetative matter that begins to decompose over time, producing biogenic CO2 and methane. Ecological shifts caused by the flooding can complicate things, but emissions are limited to these two gases.
How do coal and hydro compare in terms of total GHG? Coal will produce anywhere from 900 to 1500 gCO2e/kWh, depending on the type of coal, the efficiency of the power plant, and the combustion method. This range is well established in the energy literature.
The amount of GHG from hydropower is not so well established, as it is more difficult to measure and predict. In recent years there have been a number of studies on this, with results that vary hugely, from as low as 5 gCO2e/kWh to as high as several times that of a coal plant.
The calculation depends on two quantities: 1) the amount of GHG produced from the reservoir, and 2) the energy produced by the stored water. We calculate the GHG per energy by dividing the first by the second.
The amount of GHG from a given reservoir depends on factors such as topography, density of the biomass that is flooded, temperature of the water, and age of the reservoir. In colder regions like Quebec, the GHG produced is much less than in tropical regions. A peer-reviewed life cycle analysis of the reservoirs in Quebec, which included many thousands of measurements over several years, concludes that the average GHG produced is around 35 gCO2e/kWh. (A. Levasseur et.al., in “Renewable and Sustainable Energy Reviews” 136 (2021) 110433). Again, coal ranges from 900-1500 gCO2e/kWh.
In his letter Mr. Carter mentioned an MIT study that said that a number of Hydro Quebec dams were among the dirtiest in the world. First, there is no such MIT study. He was referring to testimony against the NECEC power line to the Maine PUC by Professor Hager from MIT who cited some of the other (not his) research about GHG from hydropower. Second, as a basis for his results Dr. Hager referred to an article that used a worldwide statistical model (not measurements) and applied it to Quebec. Third, the data he chose to use in his testimony was cherry-picked to show the most extreme values.
In a couple of cases Hager had serious errors in his data. One value he used for energy output was six times smaller than the actual output, giving a GHG result six times too big. In another case the GHG from a large reservoir was double counted. In the end, Dr.Hager conceded that the average value is around 175 gCO2e/kWh. But even this number is not based on the measurements and is likely five times too high.
Now it is true that one can find some specific reservoirs for which these large GHG values are technically correct. But it isn’t that they produce a huge amount of GHG, rather that they happen not to produce very much energy. If you divide by a smaller number you get a bigger result. The Gouin reservoir in Quebec is in this category. It produces very little energy so the calculated result is in fact more GHG per unit of energy than a coal plant. But it is also a popular lake for recreational use that has been there since 1918. Its primary function now is to regulate flow downstream. Most lakes produce some GHG whether or not they produce power.
As with any scientific result based on real world data, there are uncertainties here. But the range of possibilities for the Hydro-Quebec electricity are far below that of either coal (30 times worse) or natural gas (10 times worse.)
Can we begin the serious task of de-carbonizing our energy economy by agreeing that no energy source is completely good or bad? The scale of our society’s energy demands necessitates a commitment to rational quantitative examination of the problem. Here, truth will more likely emerge from numbers than from passions.
Paul Stancioff, PhD., is a professor of Physics at the University of Maine Farmington who studies energy economics on the side. He can be reached at pauls@maine.edu. Cynthia Stancioff is a nature-lover who likes to re-word things. Previous columns can be found at https://paulandcynthiaenergymatters.blogspot.com/.