What Is the Carbon Footprint of Hydropower Energy? A Life-Cycle Assessment

What Is the Carbon Footprint of Hydropower Energy? A Life-Cycle Assessment

Grace Smoot

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Hydropower is a sustainable energy source that can reduce the effects of global warming by limiting global GHGs. However, no energy source is without flaws and hydropower has been shown to still produce GHG emissions. So, we had to ask: What is the carbon footprint of hydropower energy?

Hydropower energy has the fourth-lowest carbon footprint of all energy types. Per kWh produced, hydropower emits 24 grams of carbon dioxide (CO2) on a life-cycle basis. It combats climate change and has various environmental benefits, but is still responsible for some greenhouse gas emissions.

Hydropower makes up more than 60% of global renewable energy use and has various environmental implications. Keep reading to learn about the overall carbon footprint of hydropower energy and its carbon footprint throughout its life cycle. 

How is Hydropower Energy Defined

Hydropower contributes to the avoidance of GHG emissions from the burning of fossil fuels (e.g. coal) and is classified as a renewable energy source because the resource (water) naturally replaces itself over time. To harness energy from water, flowing water turns turbines and spins a generator to generate electricity. 

“Hydropower: hydroelectric power (= the production of electricity by the force of fast-moving water)”

Cambridge Dictionary

Hydropower can be divided into three main categories depending on how many megawatts (MW) of power are generated.

Category of HydropowerGenerating Capacity
Micro Hydropower100 kilowatts (kW) or less
Low-Impact Hydropower (Low Hydro)Between 100 kW and 10 MW
Large Hydropower (Large Hydro)30 MW or more

And those categories can be defined as one of three types of hydroelectric facility:

  • Run-of-river: A facility that channels flowing water from a river through a canal or penstock to turn a turbine which spins a generator to produce electricity.
  • Storage: A large system that stores water in a reservoir via the use of a dam. Water is released from the reservoir to turn a turbine which spins a generator to produce electricity.
  • Pumped Storage: A system that harnesses water which is cycled between upper and lower reservoirs by pumps. Water is released from the upper reservoir into the lower reservoir to turn a turbine which spins a generator to produce electricity.
  • Offshore: A system that uses tides or waves to generate electricity from seawater. This is the least established form of hydropower.

What is the Carbon Footprint of Hydropower Energy 

The carbon footprint is one of the ways we measure the effects of human-induced global climate change. It primarily focuses on the greenhouse gas (GHG) emissions associated with consumption, but also includes other emissions such as methane (CH4), nitrous oxide, and chlorofluorocarbons.

Carbon footprint: the amount of greenhouse gases and specifically carbon dioxide emitted by something (such as a person’s activities or a product’s manufacture and transport) during a given period

Merriam Webster

Basically, it is the amount of carbon emitted by an activity or an organization. This includes GHG emissions from fuel that we burn directly (e.g., heating a home, driving a car) and GHG emissions from manufacturing the products that we use (e.g., power plants, factories, and landfills). 

What Is the Overall Carbon Footprint of Hydropower Energy

On a life-cycle basis, hydropower emits 24 grams (g) of carbon dioxide (CO2) equivalent per kilowatt-hour (kWh) of electricity produced

Illustration of CO2 equivalent per energy source
World Nuclear Association: Average life-cycle CO2 equivalent emissions

Low hydro emits between 0.01 and 0.03 pounds (4.5 and 13.6 g) of CO2 equivalent per kWh, and large hydro emits approximately 0.06 pounds (27.2 g) in arid regions but maybe over 0.5 pounds (226 g) of CO2 equivalent per kWh in tropical regions.

Category of HydropowerCarbon Footprint
Micro HydropowerNo reliable data available
Low-Impact Hydropower (Low Hydro)4.5-13.6g of CO2 equivalent per kWh
Large Hydropower (Large Hydro)27.2-226g of CO2 equivalent per kWh
Our World in Data: Hydropower Generation

Hydropower has been one of the world’s oldest and largest sources of alternative energy. Today it accounts for more than 60% of all renewable energy generation, although the scale of that energy generation varies significantly depending on the country. 

Our World in Data: Hydropower Generation

The six largest hydropower-producing countries (amount per year) in the world are: 

  1. China – 1,302 terawatt hours (tWh)
  2. Canada – 398 tWh
  3. Brazil – 387 tWh
  4. United States – 274 tWh
  5. Russia – 190 tWh
  6. India – 162 tWh

Hydropower makes up more than half of global renewable energy generation. Because the amount of GHG emissions from hydropower depends on the scale, it is important to understand what its carbon footprint is and how its carbon emissions affect the global climate change process.

To understand the carbon footprint of hydropower energy, we must assess its life-cycle and each stage’s carbon footprint. This life-cycle assessment (LCA) is a method to evaluate the environmental impacts of products and materials. Over the years, companies have strategically used LCA to research and create more sustainable products. So, let’s have a look at the LCA of hydropower energy!

The life-cycle stages of hydropower energyEach stage’s carbon footprint
Building of hydropower energyConstruction and transportation of materials, emissions from reservoirs
Operating of hydropower energyLittle to no CO2 emissions or waste products
Building back of hydropower energyVirtually none as hydropower infrastructure is able to be maintained indefinitely

The total carbon footprint of hydropower energy would equal the carbon footprint from building + the carbon footprint from operating + the carbon footprint from building back.  

What Is the Carbon Footprint of Building Hydropower Energy

The carbon footprint of building hydropower energy includes emissions from the construction of the hydroelectric facility and reservoirs.

  • Construction: Transporting the materials to build the hydroelectric facility requires transportation methods that run on diesel fuel. Burning one gallon of diesel fuel produces 22.38 pounds of CO2.
  • Reservoirs: They produce GHG emissions as a result of the decomposition of flooded organic material. A study by the International Hydropower Association found that reservoirs had a global mean GHG emission intensity of 18.5 gCO2 per kWh. However, this is still substantially less than fossil fuels such as coal. 

CO2 emissions at this stage occur when constructing the hydroelectric facilities and when transporting the materials needed in that construction. 

What Is the Carbon Footprint of Operating Hydropower Energy

Hydropower plants operate in the following way:

  • Water is temporarily stored in the forebay, a basin-like area commonly referred to as a reservoir.
  • The intake structure, a gate-like structure with trash racks to filter out debris, collects water from the forebay and directs it to the penstocks, large pipes built on a slope.
  • The penstocks, made of either steel or reinforced concrete, transport water from the forebay to the turbines.
  • When water from penstocks hits the turbine blades, it rotates the shaft at the center and causes the generator to produce electricity.

There are very few CO2 emissions or waste products associated with operating hydropower, making the carbon footprint of this phase very low. 

What Is the Carbon Footprint of Building Back Hydropower Energy

Hydropower plants have had a historical life expectancy of 80 years, but they are known to last around 100 years. This makes them a long-term, reliable source of energy. 

And if hydropower plants are properly maintained, the civil engineering infrastructure should last almost indefinitely. If this is the case, there might not be a building back phase and there would not be a carbon footprint associated with this phase.

What Role Does Hydropower Energy Play in Combating Climate Change

For hydropower to contribute substantially in the fight against climate change, installed capacity must reach at least 2%. In 2020, overall hydropower installed capacity reached 1,330 gigawatts (GW), representing year-on-year growth of 1.6%. Although this is lower than the ideal amount of 2%, hydropower still contributes to climate change mitigation. 

Using hydropower energy instead of fossil fuel energy helps mitigate the following negative effects of climate change:

  • Melting of sea ice: Since 1979, arctic sea ice has declined by 30%. Sea ice plays a major role in regulating the earth’s climate by reflecting sunlight into space and providing habitat for animal species. If all of the glaciers on Earth melted, sea levels would rise by approximately 70 feet, effectively flooding out every coastal city on the planet. 
  • Changing precipitation patterns: Extreme weather events (e.g., hurricanes, floods, droughts) are becoming more common and more intense. Storm-affected areas will experience increased precipitation and flooding whereas areas located further from storm tracks will experience decreased precipitation and droughts.
  • Ocean Acidification: The ocean absorbs 30% of the CO2 released into the atmosphere, which decreases the pH (increases the acidity) of the ocean. In the past 200 years, the pH of oceans has decreased by 0.1 pH units, which translates to a 30% increase in acidity. Aquatic life unable to adjust to this rapid acidification will die off. A prime example of this is coral bleaching, where coral expel the algae (zooxanthellae) living in their tissues as a result of changes in temperature, light, or nutrients. 

The more we reduce CO2 emissions, the more we slow the rate of temperature rise, sea-level rise, ice melting, and ocean acidification. When these rates are slowed, the earth’s biodiversity does not have to struggle to adapt to temperature and pH changes. People will not be displaced due to the flooding of coastal areas. And icebergs will continue to provide climate regulation. 

In order to help keep global temperature rise below 1.5C, as outlined in the Paris Agreement, hydropower must provide 2,600 GW of capacity by mid-century. This means that we must ramp up our capacity in the next 30 years as much as we have done in the previous 100. 

How Environmentally Friendly Is Hydropower Energy

The overall environmental friendliness of hydropower depends on the scale at which it is generated. 

“Environmentally friendly: (of products) not harming the environment.”

Cambridge Dictionary

Micro and Low hydro are more environmentally friendly than large hydro because they produce fewer GHGs and have a minute environmental impact. However, the amount of energy that can be generated from low hydro is less than what can be generated from large hydro. 

Large hydro is less environmentally friendly than micro and low hydro because it produces more GHGs and can potentially alter the natural state of the environment. However, it can contribute significantly higher amounts of energy to the power grid because it operates on a larger scale. 

What Are the Environmental Benefits of Hydropower Energy

Here are three ways in which hydropower energy benefits the environment:

  1. Climate Change Mitigation: Hydropower has an average life-cycle CO2 equivalent emission value that is much less than coal, 24g of CO2 equivalent per kWh compared to 820g of CO2 equivalent per kWh, respectively. This reduction in CO2 emissions, in turn, reduces the effects of global climate change including increasing temperatures, rising sea levels, melting of sea ice, changing precipitation patterns, and ocean acidification.
  1. Energy Independence: Being able to produce our own electricity in the U.S. without the aid of foreign countries is an important step to help us become more self-sufficient. Former President George W. Bush signed the Energy Independence and Security Act of 2007 to reduce U.S. dependence on oil, expand the production of renewable fuels (and confront global climate change). 
  1. Employment Opportunities: Hydropower supported more than 143,000 jobs in engineering, manufacturing, construction/utility operations, and maintenance in 2016. And this number has likely increased because the renewable energy sector collectively employed 11.5 million people worldwide in 2019. It could potentially support more than 195,000 jobs in 2050. Renewable energy jobs continue to increase as we start to realize just how beneficial renewable energy is for our environment. 

Hydropower has the potential to reduce overall GHG emissions by 5.6 gigatons by 2050, which is equivalent to nearly 1.2 billion passenger vehicles driven in a year. This would also save around $209 billion in damages caused by climate change. 

What Are the Environmental Drawbacks of Hydropower Energy

On the flip side, dams that create reservoirs can obstruct fish migration, alter the water temperature and chemistry, divert river flow patterns, affect silt loads, and flood out adjacent lands. Some drawbacks of hydropower energy include:

  • Fish Passage: Hydropower can affect aquatic organisms by interfering with migration, blocking movement up/downstream, and altering water flow patterns and quality conditions. Fish that move seasonally between large rivers and smaller streams could find their route blocked by hydroelectric dams. 
  • Environmental Flow Releases: Hydropower reservoirs can retain so much water that the water body below the dam dries up. Diversion projects take water away from the river and return it to the river downstream. In both cases, stream flows and aquatic habitats can be negatively affected.
  • Water Quality: The water released from the dam may experience changes in temperature and dissolved oxygen concentration. Warm water may enhance the metabolic rate of aquatic insects and fish eggs, causing them to emerge before their time. Coldwater may do the opposite, slowing the growth rates and reducing fish and invertebrate productivity. It may also lead to the establishment of coldwater fisheries in areas of the river where the natural river is too warm. Thermal stratification causes decreases in dissolved oxygen concentrations because the stagnant water bottom is isolated from the water at the surface. Turbines that take away water from the bottom can discharge water low in dissolved oxygen concentration.
  • GHG Emissions: Depending on the age, size, and location of the hydropower facility, vegetation inundation and decomposition can release GHG emissions in the form of both CO2 and CH4. CH4 has a global warming potential 28-34 times that of CO2 and can make up 80% of the emissions from dam reservoirs, as a study from Washington State University found. 

Hydropower does come with some drawbacks that, if handled properly, can be avoided or at least mitigated. Ways to mitigate both GHG emissions and environmental impacts include installing small turbines in irrigation canals, water-treatment plant outfalls, and existing hydroelectric facilities.

Final Thoughts

Hydropower has emerged as a leader in the renewable energy game due to its low carbon footprint across its building, operating, and building back phases. The use of flowing water to generate electricity produces little to no GHG emissions or waste products, unlike coal which produces a lot of both. Hydropower benefits the environment by combating climate change, creating jobs, and promoting energy independence.

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