Hydropower is not carbon-free

Energetyka wodna

Contrary to oft-repeated opinions, there are no power plants that are indifferent to the environment and the organisms that inhabit it. So much has already been written about the multifaceted harm of fossil fuel facilities that there is probably no need to repeat it. Relatively little damage is done by some power plants that generate energy from renewable sources. But even they have turbines that prove deadly to birds and bats in the case of wind turbines and fish in the case of hydropower plants. It seems that, contrary to the popular narrative, modern nuclear power plants are the least burdensome when environmental costs per unit of energy gained are calculated. However, for various reasons, they are not able to completely replace other sources.

As mentioned above, hydropower plants are known for their negative environmental impacts, which can be most generally put down to habitat fragmentation (cross-barriers). It is considered by biologists to be the main cause of the biodiversity crisis. This impact can be reduced, but it cannot be eliminated. Among the arguments put forward when discussing the cost-benefit balance, there is the assertion that hydropower at least has the advantage of being greenhouse gas emission-free. However, the list of damages is so long that more than one article could be devoted to it. In this one, I will focus on the issue of alleged zero-emission.

Hydropower uses the potential energy of water falling from a height. In some cases, a power plant can be hydrologically identical to a waterfall. However, this requires a very high natural gradient of the river bed, so it is limited to mountainous regions. In most cases, however, the gradient is insufficient for power generation needs, and then damming is used to change the hydrological conditions of the river. A section of a volatile watercourse turns into a lenitic dam reservoir. Such habitats differ in many respects, including biogeochemistry. Stagnant water, as a rule, is less oxygenated, and this changes the decomposition of organic matter, in whose products the share of methane increases at the expense of the share of carbon dioxide.

Surface water is always a source of methane to some degree. The compound dissolves poorly in water and even under volatile conditions it is easy for the water to become supersaturated and the gas to permeate the atmosphere. According to a study of the German section of the Elbe, the concentration of methane in the water along its entire length makes the river an emitter during the summer. However, hydromorphology is not indifferent here – concentrations in the water, and consequently also above the surface, are highest at weirs and in ports, and even at spurs. The dependence of methane concentration on the speed of water flow is inversely proportional, and in slower flowing sections the concentration is ten times higher than in faster flowing sections. In reservoirs, ebullition, the release of gaseous bubbles, is also added to emissions by diffusion[1].

The issue is known not only to geochemists. For example, in the fall 2019 issue of the journal Hydropower, in the “From the World” section, the editors included a note about the publication of a new computational model to help Amazon hydropower plant designers choose a location promising the lowest possible gas emissions[2].

Initially, it seemed that this problem only affected intertropical regions. The first articles pointing to dam reservoirs as a source of methane and carbon dioxide emissions began to appear in the first half of the 1990s, and their review was summarized by St. Thomas. Louis et al. in 2000,[3] they showed that emissions, which are exceptionally low for conditions in intertropical America (the average emission is 300 mg-m-2-d-1), only happen to record-breakers in the temperate zone (average emission of 20 mg-m-2-d-1). At the time, it was assumed that in the north a bigger problem might arise if the reservoir was created by flooding the peatland, but otherwise relatively small. Subsequent studies have confirmed the rule that tropical dam reservoirs are significant emitters. For example, Brazil’s Curuá-Una power plant on the river of the same name still emitted 3.6 times more greenhouse gases a dozen years after its creation than an equally efficient oil-fired plant would have done[4].

However, the more research was conducted on temperate zone reservoirs, the less obvious this picture became. A study of the Wohlen Reservoir in Switzerland[5] proved to be a breakthrough for the perception of the role of extra-tropical dam reservoirs in methane emissions. In winter, at a temperature of a few degrees Celsius, methane diffusion was indeed not very high (magnitudes on the order of a few mg-m-2-d-1), but after exceeding 10°C, emissions increased sharply (exponentially) and ebullition began to dominate over ordinary diffusion. In the end, annual average emissions exceeded 150 mg-m-2-d-1.This is still less than values typical of intertropical areas, but not at all marginal. It should be noted that the reservoir was already 90 years old at the time of the measurements, so the high measurement results cannot be attributed to the decomposition of the vegetation of the just-flooded valley.

Little was missed, and Lake Wohlen’s place could have been taken by Lake Wloclawek. A year before the publication of DelSontro and co-authors, an article by Polish researchers under the direction of Trojanowska[6] appeared in the notebook “Teki Komisji Ochrony i Kształtowania Środowiska Przyrodniczego O.L. PAN”. It presents data from four Polish reservoirs: Sulejowski, Turawski, Siemianówka and Wloclawski. The ebullition of the first two differs by an order of magnitude, but is within the values typical of this climate zone – 4 and 42 mg-m-2-d-1, respectively. The value for Siemianowka is already another order of magnitude higher, reaching 401 mg m-2 d-1. In this case, it could probably be explained by the completely ill-considered location of the reservoir on the peatland, which was supposed to be a reservoir of drinking water (sic!). Emergency conditions, however, cannot explain the ebullition from Lake Wloclawek, which also exceeds 400 mg-m-2-d-1. Unfortunately, the article went virtually unnoticed.

Since then, more studies and review articles[7] have been published, showing that smaller dam reservoirs generally emit less methane per surface area than large ones, putting small hydropower plants in a better light. This phenomenon distinguishes dammed reservoirs from natural reservoirs, where usually the smaller the object (pond, pond), the greater the emissions per unit area[8]. Of course, estimates are emerging as to the global amount of methane emitted by dam reservoirs, as well as two other non-synthetic greenhouse gases, carbon dioxide and nitrous oxide. However, since these estimates change with data growth, there is no point in citing them now.

Ultimately, compared to coal power, hydropower may seem clean. For example, carbon dioxide-equivalent emissions from the Three Gorges Dam reservoir per energy gained are only 1.7% of the emissions of an equally efficient coal-fired power plant,[9] but that’s still, according to Yang and co-authors[10], a concentration of 7.93 mg-m-2-d-1. So calling it emission-free is wrong.

With this awareness, further discussion can be held. Hydropower is not carbon-free, but perhaps it can be described as low-carbon. One can consider whether such a cost is acceptable or not.

Another aspect of the discussion is the issue of different scales of the carbon cycle. Undoubtedly, a mitigating circumstance is the fact that emissions remain in the so-called “greenhouse”. short carbon cycle. Leaving aside, of course, the situation in which a dam reservoir causes the destruction of a peatland. In this regard, burning fossil fuels has much worse effects, as it releases carbon stored thousands (peat) or millions (gas, oil, coal) of years ago into circulation.

It should be acknowledged that in terms of greenhouse gas emissions, dam reservoirs resemble natural wetlands to some extent (in a broad sense, meaning watercourses, lakes, coastal waters and wetlands). The discussion should therefore take into account how the transformation of a given river and its valley will affect overall emissions and the temporal variability of this phenomenon. For example, on the scale of millennia, the accumulation of carbon in gyttos may outweigh its emissions. The point is that while in natural lakes the deposition of gyttis is a permanent process, in used dam reservoirs this process is seen as their degradation and reversed by de-silting. Moreover, while it makes sense to consider natural lakes on the scale of millennia (including predicting their landing and transformation into still carbon-storing peatlands), the future of dammed reservoirs is less certain. The question is also to what extent the gains in carbon accumulation predicted hundreds of years from now should be taken into account when positive feedbacks are causing unprecedented increases in greenhouse gas concentrations right now.

So the issue of greenhouse gas emissions accompanying the generation of electricity from hydropower is complex, but it cannot be said to be completely neutral in this regard.

In the article, I used, among other things. From the works:

[1] Bussmann I., Koedel U., Schütze C., Kamjunke N., and Koschorreck M. (2022). Spatial Variability and Hotspots of Methane Concentrations in a Large Temperate River. Frontiers in Environmental Science, 10, 833936. doi:10.3389/fenvs.2022.833936.

[2] A new computational model identifies the best dam locations in Amazon. (2019). Hydropower, 3, 11.

[3] St. Louis V. L., Kelly C. A., Duchemin É., Rudd J. W. and Rosenberg D. M. (2000). Reservoir Surfaces as Sources of Greenhouse Gases to the Atmosphere: A Global Estimate. BioScience, 50(9), 766-775 . doi:10.1641/0006-3568(2000)050[0766:rsasog]2.0.co;2.

[4] Fearnside P. M. (2005). Do Hydroelectric Dams Mitigate Global Warming? The Case of Brazil’s Curuá-una Dam. Mitigation and Adaptation Strategies for Global Change, 10, 675-691. doi:10.1007/s11027-005-7303-7.

[5] DelSontro T., McGinnis D. F., Sobe S., Ostrovsky I. and Wehrli B. (2010). Extreme Methane Emissions from a Swiss Hydropower Reservoir: Contribution from Bubbling Sediments. Environmental Science & Technology, 44(7), 2419-2425. doi:10.1021/es9031369.

[6] Trojanowska A., Kurasiewicz M., Pleśniak Ł. and Jedrysek M. O. (2009). Emission Of Methane From Sediments Of Selected Polish Dam Reservoirs. Teka of the Commission for the Protection and Shaping of the Natural Environment of the O.L. of the Polish Academy of Sciences, 6, 368-373.

[7] Gibson L., Wilman E. N. and Laurance W. F. (2017). How Green is ‘Green’ Energy? Trends in Ecology and Evolution, 32(12), 922-935. doi:10.1016/j.tree.2017.09.007.

[8] Rosentreter J. A., Borges A. V., Deemer B. R., Holgerson M. A., Liu S., Song C., Eyre B. D. (2021). Half of global methane emissions come from highly variable aquatic ecosystem sources. Nature Geosience, 14, 225-230. doi:10.1038/s41561-021-00715-2.

[9] Zhao Y., Wu B. F. and Zeng Y. (2013). Spatial and temporal patterns of greenhouse gas emissions from Three Gorges Reservoir of China. 10(2), 1219-1230. doi:10.5194/bg-10-1219-2013

[10] Yang L., Lu F., Wang X., Duan X., Song W., Sun B., Zhou Y. (2013). Spatial and seasonal variability of diffusive methane emissions from the Three Gorges Reservoir. JGR Biogeoscience, 118, 471-481. doi:10.1002/jgrg.20049.

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