Clean energy has both low greenhouse gas emissions and is renewable. It is also worth comparing the advantages and disadvantages of clean energy sources to explore what they cost and what effect they might have on the environment around them.
Clean energy definition
As defined by Cornell Law School, clean energy is renewable energy that does not emit greenhouse gasses (GHG). Depending on the criteria on which you judge clean energy, sources might be wind, solar, tidal, geothermal, nuclear, hydro, or biomass. Criteria might include environmental side effects, like air pollution, deforestation or hazardous waste in addition to low carbon and renewable resources.
Renewable energy or renewables come from resources that are not depleted when in use (Oxford dictionary). Non-renewable energy comes from resources that are finite and could run out. Oil, gas and coal are examples of non-renewable energy.
Greenhouse gasses are carbon dioxide, methane, nitrous oxide, fluorinated gasses and water vapor (National Resource Defense Council). 76% of global human-caused emissions are carbon dioxide (CO2).
Releasing these gasses into the atmosphere contributes to the greenhouse effect. The natural greenhouse effect is beneficial, it keeps the planet warm by trapping the sun. Greenhouse gasses add to the natural levels and so the world is getting warmer, causing all sorts of issues with our climate..
Clean energy examples
Based on just these two criteria, renewable and low carbon, clean energy examples are wind, solar, hydro and nuclear power (EDF). The energy sources with the lowest greenhouse gas emissions are renewables,
- concentrating solar,
- biomass and
So these energy sources are all clean energy, based on the initial definition.
The highest greenhouse gas emitters are fossil fuel energy sources; coal, oil and gas, in that order respectively. They emanate up to 100x the greenhouse gasses of renewables (Intergovernmental Panel on Climate Change). This makes them dirty fuel. This will come as no surprise. There are lots of reports about how we must move away from these energy sources.
We know that we need to move away from using fossil fuels. Ideally, we want to be using clean energy instead. Not all clean energy is created equal, and there are pros and cons to every energy source.
Renewable ocean energy
There are three types of renewable ocean energy; tidal energy, wave energy and ocean thermal energy. The energy is generated from the movement of waves and tides using turbines. Ocean thermal energy is created by the difference in temperature between surface water and deep ocean.
Ocean energy is one of the cleanest types of energy source, in terms of carbon emissions but it is also one of the most expensive to tap into (U.S. Government Accountability Office). It has the potential to help remote communities generate clean power for themselves and contribute up to 500 million tons of energy by 2050. One of the cons of ocean energy is the lack of knowledge about how these systems could impact oceanic ecosystems. Due to this concern, and the uncertainty about return on investment, ocean energy schemes often have difficulty getting financial backing and regulatory permissions.
Hydro power is one of the oldest and most established forms of renewable energy (Office of Energy Efficiency and Renewable Energy, U.S. Department of Energy). It uses the difference in height between the water behind a dam wall and that of the river below. As water flows downhill, the water flows through a turbine and the turbine drives a generator, which produces electricity.
Water is renewable through the water cycle and water for hydroelectricity can be artificially recycled using pumps to return it to the dam to start the hydro cycle once again.
Hydroelectric power is economical. Although dams are costly to construct they are long lasting. Dams can be large, like the Hoover Dam or small regional dams and even tiny individual schemes.
Hydropower is the largest renewable electricity source worldwide and produced 4 418 TWh of electricity in 2020 (International Energy Agency). To reach net zero by 2050, we must increase hydropower YoY by 48GW.
One of the objections to increasing hydropower is that we must clear more land for it. Owners and residents of houses that are in the dam flood zone will have to be relocated. We will lose the natural surroundings like forest, farming or grazing lands. In countries where land is at a premium, this is a major issue but existing dams could be underutilized.
In the U.S. there are approximately 80,000 dams but less than 3% of them are powered. We could scale up hydropower simply by retrofitting the existing dams with hydroelectric technology. No further relocations or deforestation would be necessary for this to happen.
Concentrated solar power
Concentrated solar power systems are the next ‘cleanest’ energy source, based on greenhouse gas emissions. Despite this, they do emit nitrous oxide (N₂O) which can damage the ozone layer. CSP plants require more materials to build than fossil fuel plants, they are expensive to build, the materials are not recyclable, and they can generate hazardous waste.
As opposed to photovoltaic solar power which uses the sunlight, they use mirrors to convert energy from the sun into heat and produce electrical power (Brunel). As they use thermal energy storage, they can continue to supply electricity even when the sun is not shining on them.
The other primary issue with concentrating solar power systems is the space they need. To find space they are often set up in arid desert regions but they require water towers to cool thermal heat and water is scarce in arid areas. Globally, 130 plants produce 5,500 MW of electricity.
One good reason to continue to research and improve concentrating solar power plants is that the IEA estimates that we will need to remove Co2 from the atmosphere by the end of the century and CSP has the potential to be able to do that and generate energy from the air.
Renewable wind power
The costs of wind power are falling, leading to it being one of the fastest growing renewable energy sources (International Renewable Energy Agency). Second only to hydropower, onshore wind power had a capacity of 769,196 MW in 2021. This is more than triple the 2011 capacity of 216,346 MW. It is now the largest source of renewable energy in the United States and one of the lowest costing 1-2 cents per kilowatt-hour after production tax (Department of Energy). Wind, of course, is also free.
Wind power is generated by wind blowing through turbines and uses the kinetic energy of the wind. It is actually a source of solar energy, as it is the sun’s heat that causes winds to blow.
The optimal speed of the wind is 30 miles or 48 kilometers per hour (Bloomberg). This brings us to the first issue with wind power – inconsistent wind speeds. High wind speeds can cause damage and even collapse of wind turbines.
Too much electricity can be produced by wind turbines in high winds and if there is no capacity in the national grid and nowhere to store it in batteries, as recently happened in the UK, they have to be turned off.
Wind turbines cause noise and can interrupt the landscape. Communities living near onshore wind farms need to be encouraged with incentives like cheap electricity. Spinning turbines can be harmful to birds and animals like bats.
The inconvenience of local wind farms blotting the landscape is out-passed by local nuclear power plants. Nuclear energy is a low carbon energy source. There is some debate about whether it meets the criteria for being renewable as it is dependent on resources like a rare type of uranium (U-235) (National Geographic).
Nuclear power plants are expensive to build, ‘relatively cheap’ to maintain and expensive to safely decommission. Nuclear reactors take up to a decade to construct. Nuclear energy costs between $112 and $189 per megawatt hour (MWh) versus $36-$44 for solar power and $29-$56 for onshore wind (Reuters)
There is the problem of nuclear waste which will remain radioactive for hundreds and potentially even thousands of years. Despite progress in vitrification of nuclear waste, there still appears to be little knowledge of what materials would be non-corrosive in the longer term. This is essential to assure safe storage deep in underground facilities (Chemical & Engineering News).
Geothermal energy is any heat derived from the ground (British Geological Society). Even a few hundred meters below ground, the ground itself acts like a battery, storing heat from the sun. This is called ground source energy. Systems draw ground source heating and cooling for domestic homes from this depth.
Deep geothermal energy is defined by the UK Government as below 500m. Deep geothermal heat was formed when the earth formed. A proportion of it is also contributed to by any radioactive elements decaying.
Hydrothermal systems directly use this heat, tapping into deep aquifers. Petrothermal systems tap into deeper, hotter granite. Enhanced or Engineered Geothermal Systems (EGS) fracture the granite, increasing the natural permeability. Water is then injected into the rock and is heated.
The disadvantage to geothermal energy is finding sites that produce enough steam to be economic. Geothermal plants are visible, noisy and expel water and gas. Advantages are that they can produce energy 24/7 whereas sun or wind farms only produce energy 30% of the time. Geothermal energy is competitive at between 5-10 cents per kilowatt-hour (Britannica).
Biomass energy uses the energy from living things (National Geographic). It has been used by men since the earliest cavemen burnt wood for fires. Today, biomass materials like corn or soy are burnt to produce heat or electricity.
Biomass materials are dried, using a process called torrefication, and then shaped into briquettes. The steam from burning the briquettes drives a turbine and turns a generator to produce electricity. Pyrolysis is a method of heating biomass materials without oxygen so they don’t combust. Biochar can be produced using pyrolysis and this is an excellent carbon sink.
Biofuel is a renewable fuel from fermenting biomass materials to make ethanol (from fats and oils) or biodiesel from carbohydrates. They can be used to fuel vehicles but are not as efficient as gasoline.
The cost of biomass electricity is between 8-15c per kilowatt-hour. Biomass energy uses less land than other renewable energies, but it does require land to produce crops that can be used. There is a risk of deforestation, unless materials are used from what would otherwise be waste. The main disadvantage to burning biomass materials is the emission of carbon monoxide, carbon dioxide, nitrogen oxides, and other pollutants.
Photovoltaic solar power
The final renewable energy is photovoltaic solar power. It is separated out from concentrated solar power because the greenhouse gas emissions are higher. Photovoltaic solar energy is generated by solar panels on domestic homes and solar farms with large areas of solar panels that produce electricity commercially.
The cost of solar energy is dropping and photovoltaic solar power is economical at 3 cents per kilowatt-hour on a utility scale (DoE). The U.S. launched a SunShot initiative in 2011. They estimate that low cost solar energy in combination with low cost storage ($100/kWh for an 8-hour battery by 2040) could enable PV solar energy to supply a large share of U.S. electricity needs by 2050.
One of the disadvantages of photovoltaics is the finite natural resources that go into making up the solar panels. Read more in “Is solar power renewable or non-renewable?” for more details about the materials used in solar panels. Many of these materials are not recyclable and produce hazardous waste so there is some work to be done to find alternative materials that are just as efficient.
Replacing fossil fuels
PV solar power is the least clean energy in this list due to the higher number of GHG emissions but it is still significantly lower than fossil fuels. We know we need to replace fossil fuels as soon as possible and any of these alternative renewable energy sources would be cleaner and healthier for the planet.
Ocean energy, hydropower, solar power (concentrated or photovoltaic), wind power, geothermal energy and biomass energy all have the potential to replace fossil fuels and put the world on the path of reducing our carbon emissions so that we can reverse the damage done to our climate. They are all renewable and low carbon. The exception to this is nuclear energy, which though low carbon, is not renewable and has some significant disadvantages.
There are advantages and disadvantages to every renewable energy, but the benefit of reduced greenhouse gas and carbon emissions has to be our goal at this point in history.