6.2 Renewable energy sources

A wide variety of renewable energy sources are available to provide energy for electricity, heating, and transportation. Renewable does not mean that these energy sources have no environmental impacts. Some, such as animal dung, can create significant health hazards and contribute to climate change. Here, we explore the major types of renewable energy and their environmental impacts including impacts on the sustainability of resources needed for their use.

Biomass

No standard definition exists for biomass, but typically it includes wood and wood waste, animal and human manure, crop residues, and natural material in municipal waste, including paper products, food waste,  yard waste, and compost. Usually, biomass is burned to generate heat. Processes also exist to generate liquid fuels such as alcohol and biodiesel, or gases such as methane – biofuels (next section).

The use of animal dung and charcoal as fuel in lower-income areas is often associated with significant particulate air pollution. Soot is a common byproduct, and not only creates a health hazard but also reduces albedo and contributes to climate change. Biomass burning in mountainous regions can hasten melt of snow and glaciers because the soot darkens the snow and ice.

Biomass is considered a clean fuel because it burns organic material (creating GHG) that would have decomposed naturally, also creating GHG. Thus, burning of biomass does not add more C to the atmosphere. In contrast, burning fossil fuel takes a carbon source that was buried deep in the earth and converts it to carbon in the aboveground carbon cycle, thereby increasing GHG.

Biofuel

Liquid (ethanol, biodiesel) and gas (methane) fuels derived from biomass are considered to be biofuels. Biofuel researchers describe 4 “generations” of biofuels, but the world uses only the first two. First-generation biofuels are made from the edible portion of food crops – for example, corn and soybeans. Second-generation biofuels are made from non-food plant biomass, including the inedible portion of food crops (for example, the stalk and leaves of corn, called corn stover, or the grains left over after fermentation for grain alcohols, called spent grain), forest wastes, and some non-food energy crops, often grown on degraded or marginal farmland. Both first- and second-generation biofuel require agricultural work to produce the crop and processing to produce fuel, both of which require energy.

Almost all of the present-day biofuels are first-generation biofuels (primarily corn, soy, and palm oil), and represent food that could feed people or food crops that are planted in areas that could be producing more useful food for people. This “food or fuel” conflict is acknowledged in the biofuels world and is one reason that second-generation fuels are in use. We will see more of this when we look at sustainable agriculture and food security.

Because of the market for sustainable energy, large areas have been converted from natural habitat to production of food crops to be used for biofuel. Because of the loss of natural vegetation and the impact of soil disturbance, these potentially renewable fuels often incur a carbon debt. More GHGs are produced in the process of clearing land and undertaking agricultural work to grow them than is saved by using them, and carbon that may have been in forms that will not enter the atmosphere soon (trees that may grow old, soil that may lock the carbon into unavailable forms) is moved into forms that will enter the atmosphere quickly. Some crops in some settings can “repay” their carbon debt in a few years, because much of the initial carbon debt is a one-time cost and the renewable nature of the fuel offsets it quickly. But carbon debt resulting from destruction of rainforest, particularly rainforest on carbon-rich, peat soil, may require many decades to offset.

Corn (for corn oil for corn ethanol), soy (for soybean oil for biodiesel), and palm fruit (for palm oil for biodiesel) have all been associated with major campaigns to open up new ground for their production, beginning with the early corn-ethanol boom in the midwestern US in 2007-2012, then increased clearing of Southeast Asian rainforest for palm oil (Fig 1), and, slightly more recently, destruction of the cerrado savanna of Brazil for soybean fields. All three of these biofuels are still used extensively.

Note that liquid biofuels (ethanol and biodiesel) are needed primarily for transportation, and could be replaced by electric vehicles powered with renewable energy.

Hydropower

Hydropower  is considered a clean and renewable energy source because it does not directly produce pollutants and because the power source is regenerated. Read here to learn more about the kinds of hydropower installations. Essentially all hydropower is used to generate electricity. Hydropower produces a reliable, consistent supply of electricity, in contrast to wind and solar sources which generate electricity intermittently.

Hydropower depends on a reliable supply of sufficient volumes of water for the energy needs. Depending on the amount of power to be generated, hydropower installations are also limited by the elevational gradient of the river flow and the geology of the region. Large dams require a geology that will support construction of a high dam that restrains a large volume of water to create a head – an elevation of water above the turbines – that will generate considerable force to turn turbines. Major hydropower dams typically have heads of at least 100 m (330 ft); pumped hydropower settings provide 100-200 m of elevation. By contrast, run-of-river dams may operate with only 1-2 m (3-6 ft) of head, or even less for micro-hydropower that provides small amounts of power for local uses.

Figure 2. A run-of-river micro-hydropower system. US Department of Energy. Public domain.

Run-of-the-river systems, which do not require large storage reservoirs, are often used for micro-hydro and sometimes small-scale hydro projects. For run-of-the-river hydro projects, a portion of a river’s water is diverted to a channel, pipeline, or pressurized pipeline (penstock) that delivers it to a waterwheel or turbine

Although hydropower is considered a clean energy source, it does have a carbon footprint. Hydropower infrastructure depends on concrete, often in large volumes, which, in turn, requires cement which takes a lot of energy to create, producing significant volumes of GHGs. When reservoirs are created, they submerge vegetation that generates methane as it decomposes, and lakes are usually ongoing sources of lesser amounts of methane. Reservoir creation also displaces people living along the river, downstream of the dam site, causing economic and social hardship. China’s Three Gorges Dam, operational since 2012, displaced over 1 million people.

All hydropower use modifies river environments to some extent. Dam-and-reservoir systems create the largest changes, blocking movement of aquatic organisms and sediment, creating GHG emissions, and affecting water volume, temperature, quality, and flow regimes (how the flow changes over a year) downstream. Depending on the extent of the changes, most aquatic species may be lost from the downstream waters for a considerable distance below the dam. Many fish species, including salmon, trout, and sturgeon, have been eliminated from rivers or had their populations severely reduced as a result of dam construction (not always for hydropower) that cuts them off from spawning and/or foraging grounds and may so modify the river environment that it no longer provides suitable habitat. Creation of the reservoir destroys terrestrial habitats as it creates aquatic ones.

Upstream of the dam, the reservoir soon resembles a lake more than a river, with limited flow and an increasingly sediment-laden bottom. Species that require flowing, well-oxygenated waters and gravel river bottoms are lost, and lake species and invasive species that can tolerate less oxygenated water and higher nutrient levels can become established.

Run-of-river dams can have all of the major impacts of hydropower dam – significant dewatering of portions of the river, changes in the natural flood regimes and sediment transport, emissions of GHG, and disruption to fisheries. Impacts vary depending on the proportion of flow extracted from the river and the nature of the instream construction.^{[1] [2]}

In addition to environmental impacts, dam and run-of-river hydropower can displace communities and destroy livelihoods. The Three Gorges Dam in China displaced more than 1 million people. Associated agricultural, industrial, and municipal lands may be flooded, and fisheries may be sharply reduced.^[3]

Dams have lifetimes – major dams are expected to continue in operation for 50-100 years. Decommissioning a dam responsibly involves removing the dam and restoring the river to as natural a state as can be achieved. Initially, all the sediment that has built up behind the dam will enter the river, potentially causing considerable ecological harm, at least in the short term. Water volume and flow regime will change again, along with water temperature and water quality. The total cost of removal and restoration of two major dams on the Elwha River in the state of Washington in the US, between 2011 and 2014, was estimated at $350 million. Many dams in the world are operating past their intended lifetimes. It’s also possible to replace major dams, but, to date, this has seldom happened (many major dams are still within their operating lifetimes). The 220′ (67 m) replacement for the Calaveras Dam in California, in the US, completed in 2018, in order to improve safety in an earthquake zone, cost $823 million.

Geothermal energy

Geothermal energy uses the heat of Earth’s subsurface to provide endless energy. Heat from the planet’s molten core spreads upwards through the planet’s mass and heats rock and deep aquifer. Where tectonic plates come together, heat may come rather close to the earth’s surface, but everywhere on the planet, deeper layers are warmer.

High-temperature geothermal systems may use naturally occurring steam, directly, to turn turbines. But because this natural water is often rich with minerals and salts, which corrode equipment and block pipes, other geothermal systems pipe secondary liquids with lower boiling points very close to naturally heated water to create vapor to turn turbines. Geothermal systems are most common near tectonic plate boundaries and areas of volcanic activity – they are constrained by the natural availability of very hot rocks and/or water found at shallow depths.

Low-temperature geothermal systems, such as heat pumps, can be employed anywhere. A field of pipes is laid at a depth of 4-6 ft (1.2-1.8 m), where the ground temperature is constant, year-round and water is circulated in them. Heat pumps use the constant temperature at that depth as a heat source (in the winter) or a heat sink (in the summer). The environmental impact of geothermal energy depends on how it is being used. Direct use and heating applications have minimal impacts on the environment.

Clean hydrogen

Studies of how to achieve 100% clean energy typically include clean hydrogen in the mix of solutions. Hydrogen is a clean-burning gas – it creates water vapor when burned. It is created by electrolysis of water – splitting water molecules with electricity – which produces hydrogen gas and oxygen gas. It can be used to fuel vehicles or, like natural gas, to generate electricity. Because hydrogen is readily stored and transported, it can be a useful fuel to fill in the gaps of availability of solar and wind energy and to even out energy supply among regions.

For utility-scale power, hydrogen gas must be created, which takes power. Clean hydrogen is created using another clean energy source, essentially converting one power source to another. The IPCC suggests that use of clean hydrogen should be primarily for electricity generation, as electricity is becoming the fuel of choice for transportation, and heat pumps are a more obvious choice for heating.

Solar energy

Watch this video for a quick introduction to photovoltaic (PV) solar energy. It’s over-technical for 2 minutes but has good basic information on limitations. Figure 3 shows the distribution of solar energy. Areas near the tropics have an obvious advantage in relatively constant day lengths, but moist tropical areas may often have cloud cover, which reduces solar availability. Drier areas, even into the temperate zones, also have high potential for solar installations.

Then watch click here for video for additional information on concentrated solar energy and issues associated with energy storage and transmission.

PV solar energy uses crystalline silicon, which is created using energy, but silicon is extremely abundant and easy to obtain. Copper and silver are less common, and are considered to be critical minerals – important minerals whose quantities are, or may become, limiting (see the next section). Higher-efficiency solar cells may require additional critical minerals. All of the minerals are mined, so solar energy is linked to environmental impacts of mining, but mining for renewable energy, overall, is anticipated to be less than mining for fossil fuels.^[9] Recycling of solar panels is required in the European Union and includes recovery of critical minerals.

Solar panels are increasingly deployed in large solar farms, both as PV solar and as concentrated solar. In this format, solar energy represents habitat loss for many species and can reduce land available for other uses, particularly agriculture. Agrivoltaics, solar panels that share the land with agriculture or grazing, and floatovoltaics, solar panels deployed as floating units on water, are recent developments that seek to reduce the land footprint of solar energy.

Panels in solar installations need to be washed (or rained on) to maintain their efficiency. More than 10 billion gallons of water are used each year to clean solar panels, equivalent to drinking water for 2 million people.^[10] Less water-intensive measures are being developed.

After their useful lives, photovoltaic panels become waste. Presently, only about 10% are being recycled. By one estimate, China, where the largest number of panels is deployed, could have 13-20 million tons of PV-associated waste by 2050.^[11] Regulations in the EU have led to PV-specific recycling centers, but even there, capacity lags behind need.

Solar panels deployed by individuals and small businesses can often be connected to energy grids fairly easily. But large-scale installations require construction of infrastructure, with associated impacts. If battery storage is to be added to improve the stability of the energy supply, additional impacts will occur, including impacts associated with supply chains and waste handling for the batteries.^[12] Alternative means of storing energy, including pumped hydroelectric and compressed air storage, offer cleaner solutions to grid stability but are not yet as widely deployed.^[13]

Wind energy

Figure 4. Areas with abundant and reliable wind power. Antonini, E.G.A., Virgüez, E., Ashfaq, S. et al., Fig 2. https://doi.org/10.1038/s43247-024-01260-7, licensed under CC BY 4.0

 

Because a wind turbine has a small physical footprint relative to the amount of electricity it produces, many windfarms are located on crop and pasture land. They contribute to economic sustainability by providing extra income to farmers and ranchers, allowing them to stay in business and keep their property from being developed for other uses. However, wind farms are prohibited in some areas for a variety of reasons including noise, aesthetics, safety, impact to property values, and military security.

Collisions with wind turbines kill large numbers of birds and bats that use wind for foraging and migration. In the US, they are contributing to the endangerment of three species of bats that were otherwise considered numerous and safe. Research is ongoing to find ways to reduce mortality, including coloring turbine blades, adding sound or light effects. Some mortality can be avoided by siting turbines away from key locations and routes, but these are often in high wind corridors – high-quality locations for wind energy. During migration season, significant mortality reduction can be achieved by programming the turbine to spin only at higher wind speeds. Birds and bats face lower mortality from stationary blades, and much less power is generated at low wind speed, so the mortality reductions do not cause high economic losses. However, wildlife safety measures are not required by law, and numbers of deployed wind turbines continue to increase. Of course, if wind turbines are not deployed, and global temperatures continue to rise, many wildlife species will face serious declines due to climate change.  

Offshore wind turbines have impacts to marine species and to seabirds, both positive and negative. Many species avoid offshore wind farms, which results in habitat loss. Sea birds die through collisions. Noise may interfere with marine-mammal communication. Evolving technologies  can dampen some of the construction and operation noise; these are required in the EU, but not universally. Once installed, offshore turbines can offer habitat for some species, and mitigation measures can reduce impacts to birds. Nevertheless, a recent review of impacts to migratory marine species (excludes birds) found that the present, limited knowledge suggests a net negative impact.^[14] More research on both impacts and mitigation is needed.^[15]

Wind technology employs several critical minerals for the most common turbine generator. As a result, wind relies on mining of critical minerals with the resulting environmental impacts, although, as noted above, less mining is anticipated to be needed for renewables than for fossil fuels. Many wind turbine parts can be recycled, and turbine blades can be made of recycled materials. Some European countries have targets for recycling of wind-power components, but no larger commitments to recycling exist at present.

As with solar energy (see above), connecting wind farms to existing power grids also involves development and environmental impacts. Choice of energy-storage technology, if it is used, affects overall environmental impacts.

Electric vehicles

Electric vehicles obviously aren’t a power source. But they are a means of transitioning away from liquid fossil fuels in the major sector for those – the transportation sector. The use of biofuels to replace gasoline and diesel fuels remains problematical because of arguments over how to assess emissions^[16] and because the vast majority of the volume of transportation biofuels comprises first-generation fuels that are also food crops. Use of these fuels can affect food prices and food security. Electric vehicles eliminate the need for liquid fuels, and can be powered by renewable energy, if that is available. New options for fast charging and increasing availability of charging stations improve affordability and accessibility. Although electric passenger cars remain a small proportion (<10%) of the world fleet, in China, in 2024, almost half of car sales were of electric vehicles, accounting for over 60% of such sales, worldwide.^[17]

The clean energy spatial footprint – an assessment for the US

The US National Renewable Energy Lab issued a report in 2023 examining options for achieving 100% clean energy in the US by 2035^[18]. They examined four scenarios that varied in their assumptions regarding changing cost and performance; advances in transmission technology; land availability for wind, solar, and biomass; transportation and storage costs; etc. Figure 5 shows the anticipated land needed, for the 4 scenarios, for solar energy, for direct, land-based wind energy (the footprint of the turbines), for land for wind (the footprint of the windfarms), and for transmission rights-of-way. The estimated land needed for solar energy is much less than the land currently used for biofuel corn ethanol, whereas the land needed for wind farms is greater, depending on the scenario in question. However, given that wind farms are a mixed-use land use, much of the footprint needed for wind energy could be on the land presently used for corn ethanol, or for livestock grazing and feed production. Although solar and wind have considerable land footprints, they are by no means impossible to accommodate.

Figure 5. For scenarios to achieve 100% clean energy in the US by 2035, the land needed for wind, solar, and transmission rights-of-way, compared to other major land uses. US National Renewable Energy Laboratory. Public Domain.

The colored lines around the utility-scale solar, wind (direct and spacing), and transmission RoW boxes show the box size for the four scenarios. The ADE demand case assumes that demand for electricity will accelerate during the clean-energy transmission as processes previously fueled by fossil fuels convert to clean electricity. Rights of way and wind farms include land that is available for other uses – this is shown by the dashed lines around the related boxes. Although solar farms can also, in theory, include additional land uses, these share a footprint with solar panels, and utility solar is not considered a mixed-use land use for purposes of this graphic.

Knowledge Check

Take a moment to complete the short quiz below to assess your understanding of this section. Read each question carefully and refer to the section content as needed. This quiz is not graded – it’s simply an opportunity for you to reflect on what you’ve learned and reinforce key concepts.