How green is hydropower?

By Fergal McEntee

Head of Sustainable Energy at Sustain Europe   |   London


Austria's Kölnbrein Dam has recently installed the new pumped storage power plant Reisseck II

IMAGE: Luis del Rosario

Hydroelectric power (hydro)  is classed as a renewable energy due to the fact that it relies on the Earth’s natural water cycle's kinetic energy to generate electricity. With its 90% efficiency in converting the kinetic energy to electricity, and the fact that no fuels are burnt and no direct emissions are released into the atmosphere, it is often considered as a very clean form of electricity generation.


And hydro is popular, too. In 2016, hydro supplied a staggering 71% of all renewable electricity generated, accounting for 16.4% of the entire world’s renewable and hydrocarbon electricity generation. In the EU-28 countries, hydro accounts for over 14% of all prime electricity and 70% of all hydropower is from five main countries - Sweden, France, Italy, Austria and Spain. Impressively Norway gets 99% of electrical energy from hydro.


In many ways these figures are not surprising. The damming of water is one of the oldest forms of renewable energy harnessed by mankind.


However, as renewable energy is coming of age we are starting to increasingly notice the effects that certain technologies are having on the planet. So it is about time for us to be more technologically minded and look closer at the benefits versus the impacts of damming our natural waterways in our quest for clean energy. Is hydro all it stacks up to be?


Types of hydro and dams


There are several types of hydropower plants, and they are all powered by the kinetic energy of flowing water as it moves downstream. Dams are used to create a head of water and store for use as and when required. About 75% of the existing 45,000 large dams in the world were built for the purpose of flood control, irrigation, navigation and urban water supply, whilst around 25% of large reservoirs are used for hydropower and multi-purpose reservoirs purposes.


Run-of-river hydroelectricity (ROR) also known as run-of-the-river is where little or no water is stored or dammed. With there being no water storage at all is subject to seasonal river flows, thus the plant will operate as an intermittent energy source. To be an ROR the normal course of the river is not too materially altered in anyway. They are normally small plants and have a low environmental impact.


Pumped-Storage hydroelectricity (PSH) is a method of converting excess electrical energy into stored energy by pumping water vertically into a storage pond for later use. It is mainly used by electric power grids for load balancing.  Low-cost surplus off-peak electric power is used to pump water from lower elevation reservoir to a higher elevation.


Reservoir hydroelectricity is the most common type of hydropower plant uses a man-made dam on a river to store water in a reservoir. Water released from the reservoir flows through a turbine, spinning it, which in turn activates a generator to produce electricity. It is this damming of the river to create a reservoir that causes much of the environmental damage.


The impacts of dams


Damming rivers, be it for the purposes of flood control, hydroelectricity, irrigation, water storage or navigation, causes vast amounts of environmental damage. The most obvious of which is caused to the local flora and fauna, but there are other negative ramifications for people and planet due to increased greenhouse gas emissions, the displacement of local populations, earthquakes and it can even go so far as to effect the rotation of the Earth.




Many species of fish migrate upriver to reproduce and have done for hundreds of thousands of years. Dams block their migration route, which often results in a great decline in the number of fish - or worse yet - extinction of the species. The standard solution is to install fish ladders allowing the fish to jump from small pond to small pond. However, developing the right technique to attract the fish in order to climb the ladder is a complicated matter, whilst finding a good location is oftentimes very challenging.


Climate change


Climate change is adding uncertainty to hydropower plants. Changing rainfall patterns and prolonged droughts make it difficult to assess future river flows, and this can cause problems with energy supply. For example, Brazil’s energy market, which is highly dependent on hydroelectricity, is suffering from prolonged periods of drought which are causing debilitating electrical shortages. Furthermore, the increasingly frequent storms which we are experiencing as a result of accelerated climate change can leave dams unsafe. An unsafe dam is potentially a big and dangerous problem.




The Three Gorges dam in China required a staggering 1.3 million people to be relocated from 1,600 villages and 13 cites to allow the flooding of the region. At 600km long, this massive volume of raised water is actually slowing the rotation of the Earth by 0.06 microseconds per day as calculated by NASA scientists.


Energy costs


The levelised costs of energy (LCOE) represents the lifetime costs of production including construction without subsidies or any intervention. This is a good method of comparing different energy types over the lifetime of power plants. Hydro power has the lowest LCOE different forms of renewable energy (World Energy Council report, 2016). What is particularly interesting is the downward costs of onshore wind and phenomenal rate of reduced costs of solar PV.


The below chart shows the cost of electricity taking into account construction costs including profits the complete life cycle cost of kWh of energy. In 2015 the cost of electricity from hydro was 0.046 USD  per kWh.


The technical potential of renewable energy technologies to supply energy services exceeds current demands many times over. The LCOE of solar PV is plummeting and is set to become the cheapest form of global renewable energy available.


In 2018, the LCOE of utility scale solar is now around 0.04 USD and data from Bloomberg New Energy Finance (BNEF) shows that the average price of solar energy in almost 60 countries with high levels of solar irradiance (including the southern European countries) is the cheapest form of energy.  Solar is fast emerging as the clear winner.


Fossil fuel subsidies


There is not a level playing field for us to be able to compare the like-for-like cost of renewable energy versus fossil fuels due to the vast amount of subsidies the fossil fuel industry receives directly and indirectly which do not get factored in costings. Also fossil fuels are a finite resource which is destroying our ecosystem, compared to renewable energy which is inexhaustible.


A new report from Climate Action Network (CAN) and the Overseas Development Institute (ODI) shows that Public banks and EU financial instruments subsidized gas and oil production with an average of over €3 billion per year. The European Investment Bank (EIB) and the European Bank for Reconstruction and Development (EBRD) invested over €8 billion in fossil fuel projects between 2014 and 2016. And then add a further €20 billion a year on tax breaks.


Greenhouse Gas


We can apply methods to accurately calculate the impact using lifecycle GHG emissions (gCO2 eq/kWh). We know that vast amounts of concrete and energy are used for the construction of hydro-dams; resulting in a very high carbon footprint prior to even generating a single kilowatt of electricity.


What we have come to understand more over the past 15 years or so is that GHGs are also created in the reservoirs by decomposing organic matter, resulting in the formation of CH4 (methane) and CO2. Some early studies (Fearnside, 2006) claimed that certain hydro plants in Brazil were emitting up to three times more GHG than coal fired plants. Further extrapolation of this data has led some people to arrive at the conclusion that hydro alone is responsible for a staggering 4% of total global GHG emissions.


As we now have a greater understanding of how GHG emissions and reservoirs work, this has since been proven not to be the case. The emissions released depend upon factors such as the

decomposition rates, the length of time the water is being stored for, along with the age and latitude of the reservoir The decomposition rates will also be further effected by aspects such as the shape of the reservoir (including its volume and depth), the quantity and type of vegetation which is being flooded, and even its geographical location and the surrounding temperature.


Reservoirs located at low latitudes in the tropics produce higher GHG emissions due to higher biodegradation levels. In fact, studies suggest that compared with colder boreal regions (such as Russia and Canada), reservoirs in tropical regions can produce up to 20 times more GHG emissions (Steinhurst et al., 2012). Thus we can say that reservoirs in tropical regions produce more GHG than in colder climates.


So where does this leave us?  Well, for the first few years at least, the hydropower reservoir will emit higher levels of GHG emissions than the dam will actually save by generating its renewable energy. Such levels of GHG emissions may be even higher than the annual emission rates from certain fossil fuel sources. As the reservoir age increases and subsequently most of the decomposition has run its course, then the reservoir’s levels of GHG emissions will decrease. This is a simplistic explanation, and more studies need to be carried out to define exactly by how much, but what we do already know is that we can mitigate a lot of these GHG emissions by reducing the size of the area being flooded and removing terrestrial vegetation where possible.


What is important to note is that currently none of these emissions are being included in global greenhouse gas inventories. The Intergovernmental Panel on Climate Change in the 6th report in 2022 are going to be adding GHG emissions from hydro as a documented source and intend to carry out further in-depth studies to this end.


Pumped storage and renewables


On any nice and sunny, bright and windy day, grids across Europe are producing vast amounts of renewable energy. However, this solar and wind power is an intermitted supply of energy. Balancing the grid and storing the excess energy is a conundrum that we need to solve. A clever solution is to convert old coal mines into pumped-storage hydropower (PSH) facilities to store the excess energy and use it when required.


North-Rhine Westphalia, a region in north-western Germany, is set to turn its disused Prosper-Haniel hard coal mine into a 200-MW pumped storage hydroelectric reservoir. It will act as a super-giant battery with enough capacity to power over 400,000 homes for up to 4 hours when needed.


600,000 to 1 million cubic meters of water could be used to drive turbines at the foot of the mine shaft 600 metres below the surface. The University of Duisburg-Essen (UDE) is working with mine owner RAG AG to make the project a reality. Once successfully up and running, this is something that could potentially be modelled and rolled out Europe-wide.


Furthermore, the continent is scattered with thousands of abandoned coal mines that could be converted to pumped storage facilities, thus helping to provide local jobs where generations of workers formerly relied on fossil fuel for their livelihoods. There would also be very little red tape involved, as the facilities would be on existing industrial mining sites with relatively little value. By addressing and solving the storage problem, it would pave the way for the rapid expansion of renewable energy by helping to maintain electrical capacity even when the sun doesn’t shine or the wind doesn’t blow.


The potential for Pumped Storage technology to help balance the grid is enormous. There are a massive amount of disused coal mines all across Europe which can be converted and used as a vital means of storing electricity from intermitted renewable energy generators such as wind and solar.  Additionally, they will benefit from a life cycle of up to 75 years or more, which equates to a three to five times longer life cycle than that of utility-scale batteries.




The multiple benefits of reservoirs, including for irrigation, water-supply and hydro purposes (rapid response to grid demand fluctuations due to peaks or intermittent renewables), are hard to dismiss quickly.


However, as a form of renewable energy generation, we are gaining a much greater understanding of the complex relationship between different sources of energy and the serious consequences to the planet.


We now know that the damming of rivers causes immense strain on the local biodiversity, creates unwanted GHG emissions and kills off aquatic life in vast numbers, not to mention the massive inconvenience to local communities and indigenous tribes who are forced out of their lands and homes. The effect of dams on ecosystems is an increasingly complex issue and one which until relatively recently never received the public attention it truly deserves.


Ultimately, the goal of renewable development must be to minimize our ecological footprint and if the social and ecological costs are too damaging then that is something which needs to be addressed now, not at some point down the line.  We now have the necessary tools to help assess sites for hydro and extra-careful consideration needs to be given when planning and building new plants.


Unfortunately, humanity’s demand for energy is relentless and we still are not producing enough renewable energy. First we need to stop all European subsidies to the fossil industry. Instead, if we spend the same amount of funding on developing renewable energy and combine it with giant pumped-storage hydro-power systems using the abandoned coal shafts found scattered all across Europe, we can go a long way towards decarbonising electricity, creating jobs and slowing climate change. The means are available to us to do something about it and apply the appropriate action, but the fact remains that we’ll never get anywhere without ending all subsidies to the fossil fuel industry first.


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