Study shows hydropower’s greenhouse gas footprint

24 May 2018

A new study of the greenhouse gas footprint of almost 500 reservoirs worldwide, which applied the G-res Tool for assessing net emissions, indicates that hydropower is one of the cleanest energy sources.

The greenhouse gas footprint of hydropower has long been questioned in both scientific and policy spheres, especially with regard to emissions caused by the creation of a reservoir. There has been a lack of scientific consensus on how to quantify this footprint, and this uncertainty has proved a significant obstacle for policy and decision makers concerning the financing of hydropower projects and whether they achieve the designation of being climate-friendly.

The Intergovernmental Panel on Climate Change (IPCC), in its Fifth Assessment Report published in 2014, noted that only onshore and offshore wind and nuclear power have lower median lifecycle greenhouse gas emissions than hydropower. However the panel cautioned that few studies had appraised the net emissions of freshwater reservoirs, allowing for pre-existing natural sources and sinks and unrelated human emission sources.

The challenge of assessing net climate emissions

Over the years, a number of researchers have measured gross reservoir emissions at sites around the world, but the results of each study cannot be reliably applied to other reservoirs, even in the same region. The biochemical processes leading to emissions from a reservoir are highly complex, and life-cycle emissions are very specific to the siting and design of each hydropower facility.

Emissions relating to the construction and operation of a dam, due to fossil fuel combustion and cement/steel production, can vary depending on its type and size. Once filled, factors such as a reservoir’s depth and shape, the amount of sun reaching its floor, and wind speed, affect the different biogeochemical pathways by which CO2 and CH4 are created and released to the atmosphere.

The process of taking measurements to determine the greenhouse gas (GHG) footprint of a hydropower facility or reservoir can be cumbersome or prohibitively expensive. Calculating the net change in emissions caused by a reservoir is highly challenging.

Development of the G-res Tool

Against this backdrop, the GHG Reservoir (G-res) Tool was developed by IHA and UNESCO in cooperation with researchers from the University of Quebec at Montreal (UQÀM) in Canada, the Norwegian Foundation for Scientific and Industrial Research (SINTEF) and the Natural Resources Institute of Finland (LUKE). This research was supported by the World Bank and sponsors from the hydropower sector.

The tool was devised to enable companies, investors and other stakeholders to more accurately estimate the net change in GHG emissions attributable to the creation of a specific reservoir. It takes into account the state of the land pre-impoundment, considering naturally occurring emissions and emissions related to other human activities over the lifetime of the reservoir. It also provides a method for apportioning the net GHG footprint to the various freshwater services that a reservoir provides, such as water supply for irrigation and cities, flood and drought management, navigation, fisheries and recreation.

The G-res Tool was formally launched, after more than a decade of development work, at the World Hydropower Congress in Addis Ababa, Ethiopia, in May 2017.

Worldwide study of hydropower reservoirs

During 2017, researchers from IHA undertook a study of 498 reservoirs worldwide using the G-res Tool. The study looked at reservoirs in boreal, temperate, subtropical and tropical climates more than 50 countries in North and Central America, South America, Europe, Africa, South and Central Asia, East Asia and the Pacific.    

The study used the G-res Tool to estimate the GHG footprint of 178 single purpose hydropower reservoirs and 320 multipurpose reservoirs, excluding emissions caused by construction activity. This data was coupled with project-specific installed hydropower capacity and average annual generation data to obtain the emissions intensity of each site’s hydropower operations.

The global median GHG emission intensity of the hydropower reservoirs included in the study was 18.5 gCO2-eq/kWh; this is the grams of carbon dioxide equivalent per kilowatt-hour of electricity generated allocated to hydropower over a life-cycle. The majority, or 84 per cent of reservoirs, exhibited emissions less than 100 gCO2-eq/kWh. For a comparison with the median values of other electricity sources, see figure 1.

Temperature is one of the variables that has, in theory, a significant effect on reservoir emissions. However mean annual temperature is only one of many variables that influence GHG emissions. The G-res Tool includes other input variables such as the soil carbon content of the reservoir, depth of the thermocline, reservoir drawdown area and the catchment annual run-off. The second figure above shows the emissions intensity attributable to hydropower reservoirs categorised by their respective climate zones.

The IHA study confirms the majority of hydropower reservoirs studied are producing very low-carbon power; although some reservoirs in every climate category can potentially have high emissions exceeding 100 gCO2-eq/kWh (defined by the Climate Bonds Initiative to be an important threshold).

Figure 2 shows the relationship between the GHG emissions intensity (gCO2-eq/kWh) plotted against the power density of the projects (W/m2). High emissions intensities are possible from hydropower reservoirs, even on the same order of magnitude as fossil fuel generators, but only at extremely low power densities. Low power density however does not necessarily translate to high emissions intensity, as many projects with low power densities have emissions intensities well below 100 gCO2-eq/kWh (left of the red line).

It bears noting that the emissions intensity identified from this study applies only to hydropower projects with large reservoirs; many hydropower projects, often run-of-river, do not flood significant areas of land and consequently will have even lower emissions. It should also be noted that hydropower facilities equipped with reservoir storage provide many other valuable power and water benefits. By storing water in a reservoir, a project can offer balancing and ancillary services, delivering dispatachable power when needed. A reservoir also provides water for vital non-power uses such as flood control and drought management, and water supply for municipalities and agriculture.

This article is featured in the 2018 Hydropower Status Report.

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