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What is "clean energy"? Is any kind of energy completely clean?

“Clean energy” usually refers to energy sources that produce no climate-warming greenhouse gas emissions in their operation. That doesn’t mean they have zero impact on the environment. 

 

May 7, 2024

The group of technologies widely considered to be “clean energy” include hydropower, geothermal, solar, wind, nuclear, bioenergy (at least in some circumstances), and even some extremely nascent technologies like ocean wave power. These energy sources are “clean” with regard to climate change because—unlike fossil fuels—when they produce energy they do not emit greenhouse gases, the type of pollution that is warming our planet. The most important of these gases is carbon dioxide (CO2), so “clean” technologies can more precisely be referred to as low-carbon or carbon-free.

Clean energy technologies are in many ways very different from one another, but none directly emit CO2.1

“A key word there is directly,” says Jennifer Morris, a principal research scientist at MIT’s Joint Program on the Science and Policy of Global Change and the MIT Energy Initiative. Even if they do not produce emissions during operation, clean energy technologies all have some “embedded emissions,” like those associated with producing their equipment.

“When you start getting into life cycle assessments and you backtrack through all of the steps that it takes to get to the point that you're producing energy or electricity, then of course there are emissions involved in the different steps along the way,” says Morris. “There's no such thing as a true, perfectly clean energy source.”

Despite the emissions involved in their manufacture and setup, all types of clean energy do far less to warm the planet than fossil fuels. “When you factor in the life cycle assessment, fossil fuels are incredibly more emissions intensive than these clean energy options. So even if they're not perfect, they're significantly better,” says Morris. 

All in, a typical coal plant releases about 1,000 grams of CO2 per kilowatt hour of energy produced, according to the the National Renewable Energy Laboratory (NREL), and natural gas releases almost 500 grams.2 By comparison, solar energy typically releases less than 50 grams of CO2 per kilowatt hour, and wind not much more than 10 grams. And these embedded emissions may fall even further as we invent cleaner ways to mine metals, produce steel, cement and other needed materials, and transport and assemble wind turbines, dams and nuclear plants. “There's a lot of room in those areas to reduce emissions,” says Morris.

Fossil fuels also degrade the environment and human health in other ways. Coal-burning power plants emit chemical-laden smoke that worsens air quality. Mining coal can pollute waterways and land. And pulling natural gas from the ground can contaminate groundwater. Air pollution in particular is such a major health problem that coal and oil are estimated to lead to hundreds of times more deaths per kilowatt hour of energy than wind, nuclear or solar power.3

But just because a technology is “clean” in regard to climate change does not mean it’s free of all environmental tradeoffs. Nuclear plants create dangerous waste that must be disposed of. Mining the minerals needed to build batteries or solar panels can spur deforestation and water pollution and degrade the environment in other ways. Not everyone agrees that all low-carbon technologies deserve to be called “clean,” because of concerns with these other environmental impacts.

One final point of confusion is the difference between clean and renewable energy. “Renewable energy” simply means energy that comes from an effectively infinite source, like wind or sunlight. There’s plenty of overlap between clean and renewable power, but they are not identical. Nuclear energy, for instance, is fueled by uranium, of which there is a finite amount on earth. So although it’s “clean” in regard to climate change, it’s not a renewable resource—even if uranium is plentiful. “It's not typically considered an amount we need to worry about exhausting, at least not in this century, but it's not replenishable,” says Morris.

 

Thank you to Barbara MacLellan of Nanaimo, Canada, for the question.

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Creative Commons Attribution-NonCommercial-ShareAlike 4.0 International license (CC BY-NC-SA 4.0).
Footnotes

1 With the exception of bioenergy, because burning plant matter does emit CO2. Here, the idea is that plants take CO2 out of the atmosphere when they grow, and burning them simply puts the same carbon back into the air, for no net increase in atmospheric CO2.

2 U.S. Department of Energy, National Renewable Energy Laboratory: "Life Cycle Greenhouse Gas Emissions from Electricity Generation: Update," September 2021.

3 "What are the safest and cleanest sources of energy?" Hannah Ritchie, Our World in Data, February 10, 2020.

Want to learn more?

Listen to this episode of MIT's "Today I Learned: Climate" podcast on the social and environmental side effects of clean energy.

Transcriptions

LHF: [00:00:00] Hey, real quick before we begin the episode, we want to know if this podcast is making a difference for you. We have a quick survey we’d love for you to fill out. It would be so valuable to us, plus, two lucky people who fill it out will win a $50 gift certificate to Better World Books, which uses book sale profits to fund literacy programs. To take the survey, go to tilclimate.mit.edu/survey. OK back to the episode.

Hello and welcome to a bonus episode for season two of TILclimate, the podcast where you learn about climate change from real scientists and experts. I’m Laur Hesse Fisher from the MIT Environmental Solutions Initiative.

In season two, we talked a lot about how different energy technologies affect climate change.

But there are other consequences to the ways we make energy today.

Some of these consequences make the news, like oil spills that kill wildlife and devastate fishing industries; and mines that have collapsed on coal miners. And then there are things we don’t hear as much about, like some parts of the U.S. where people have lit their tap water on fire because it’s been contaminated by extracting natural gas nearby; or the fact that nearly 1 in 5 long-term American coal miners have black lung disease; or that millions people die every year from air pollution created by burning fossil fuels.

Our world is now in the midst of a huge energy transition in order to emit fewer greenhouse gases. Technologies like solar panels and batteries help us slow down climate change, but they’re not inherently perfect. They also require mining and processing toxic materials which sometimes is done in a way that’s dangerous and harmful.

As we make a conscious and dedicated effort to massively scale up clean tech, we have a chance to do it in a way that protects people’s rights, health and safety. If we don’t, then even after we have clean energy, we’re still left with a lot of problems.

To help us navigate this, we spoke with MIT’s Suzanne Greene who is an expert in supply chains and understanding the impacts of where our stuff comes from.

SG: [00:02:38] I work at the MIT center for transportation and logistics, and I manage our sustainable supply chains initiative.

LHF: [00:02:45] The term “supply chain” refers to all the materials and activities that go into making, transporting, using, and disposing of something.

SG: [00:02:54] We look at the stuff that we see in our everyday life and then trace it back to the ingredients and where they come from, from all around the globe.

LHF: [00:03:04] Because of our globalized world, many products’ supply chains are far more complex than you might expect. Take for example, something that seems simple, like a banana.

SG: [00:03:19] The Center for Transportation and Logistics did a study on the banana and that was an interesting study because actually the company that we worked with, a banana company didn't fully know its own supply chain. And it's interesting because fertilizer and chemicals, that was actually one of the biggest impacts in the banana’s supply chain.

And so when you're eating a banana, you're not thinking someone mined something out of the ground for this. But that’s a fact … fertilizers, many of them are mined.

LHF: [00:03:49] From a climate change perspective, a company can look at the supply chain to understand how much greenhouse gas your product took to produce, so you can start to reduce it. But the supply chain can also help us create a more just and equitable world.

SG: [00:04:04] We as people on the planet, we might have certain ethics that we apply to the things we want in our lives that we buy, right? So we might say, you know, “we want to see fair trade and fair labor.” So that comes into the banana discussion: Was this picked by someone that's making a fair salary? We vote with our dollars, right? What are we paying for? So, in order to understand that, you need to look down the supply chain and see if all of these things agree with your ethics.

Companies do the same thing. They decide on a set of ethics, you could call it, for their suppliers and certain standards that they need to meet. And some companies are very strict on that. And others less.

LHF: [00:04:46] Yeah, this conversation extends way beyond bananas. We wanted to understand this clean energy industry that’s poised to grow very fast--and making sure we take care of our water, air, and other people as we grow this industry.

So we asked Ms. Greene about the supply chain of one of the fastest-growing energy technologies: a solar panel.

SG: [00:05:10] Okay, so solar panels have a huge variety of ingredients that need to be assembled from around the world.

Aluminum, indium, silicon, cadmium, iron, silver, copper, lead, tellurium, gallium, nickel, tin, germanium, selenium, and zinc.  So all of these things need to be gathered, they need to be dug out of the earth.

How many of th ose have you heard of?

LHF (from interview): [00:05:37] Ah, five? I don't know.

SG: [00:05:39] Yeah, so there's some major things, ™right? There's aluminum, And then silicon is maybe the thing we most associate with solar panels.

LHF (from interview): [00:05:46] Yeah

SG: [00:05:46] So that's like sand. So, that sounds more innocent than some of the other things that are quite rare. In 90% of solar panels, the part that actually turns light into electricity--what’s called the “semiconductor”--is made of silicon. Which is a material that is super abundant; in fact, silicon is the second most common element in the Earth’s crust. But you have to mine silicon, and that’s not always a clean process.

 Chemicals are often used to extract the materials and depending what part of the planet that this resource is from, the chemicals might not be properly disposed of. Right? The chemicals that are used here, we don't know if they're treated before they reach waterways. So that's a concern, right? We want to make sure that our water is clean after we extract these materials.

LHF: [00:06:41] This isn’t exclusive to solar panels. All electronics -- our computers, our cell phones, and the batteries that power them -- can involve some pretty toxic chemicals that need to be handled really carefully, which is why you cant just throw them in the trash when you’re done with them -- they need to be taken to a specific facility. And these materials are being mined all over the planet.

SG: [00:07:05] A lot of copper comes from Chile. A lot of steel comes from Australia and Brazil. A lot of the other metals and minerals are coming from Africa. There's things that are mined in Europe, you know? But a lot of it is in the developing world.

 So when we think about cobalt, for example, that's in a lot of [lithium] batteries. The biggest source of cobalt is the democratic Republic of Congo, which has a pretty bad reputation for forced labor and child labor in mines.

And not all mines in the Congo are bad, but some of them are. So that's the thing we're trying to get resolution on. Like, can you buy from the good mines and can you differentiate between them as an end user?

LHF: [00:07:50] This is a challenge already today. And as solar, wind, and battery technologies skyrocket, it’s going put a lot of pressure on mining companies to produce more. A lot more...

SG: [00:08:03] When we're thinking about electric storage batteries. So that's the batteries that we're going to need to store solar and wind energy for our grid, for our electric grids. The different metals and minerals that are involved in that—aluminum, cobalt, iron, lead, lithium, manganese, nickel—they're expecting a growth in demand by more than 1000% to reach our renewable energy goals. We're talking really big numbers.

For the mining companies, this is a huge opportunity. They are excited about the renewable energy transition. Okay. They are going to mine more. So this is a business opportunity for them.

LHF: [00:08:44] So as we build more solar and wind and batteries -- which we need to do to slow climate change -- it’s important to go into this transition with our eyes fully open to the environmental and social costs that are often hidden in the supply chains.

SG: [00:09:00] What we need to do is hold the companies that are producing these things accountable. And give them the space and the time to clean up the supply chain and make sure it fits all of our standards.

LHF: [00:09:11] Yeah, we’re all in this together. And there’s a lot more to consider in this transition than just CO2 emissions.

SG: [00:09:17] We have to think of the full equation when we're making this transition. You can't just think of eliminating coal. That's not the answer. This is about clean water, fair trade, fair labor, you know, people's rights on the planet, animals’ rights, all of these things are part of it. We want to bring everyone with us on this journey and raise everyone up together. We need to make sure we do it right this time.

LHF: [00:09:48] MIT Environmental Solutions Initiative is doing work in this area: our Here & Real program is helping coal mining communities adapt and thrive as coal leaves their counties; and our Metals, Minerals and the Environment Program, which Ms. Greene leads, is working with big mining companies to advance their sustainability practices. You can find more about these programs -- and sources for today’s episode, in our show notes.

Hey, and don’t forget to take our survey! We want to know what you think about these episodes and what we should be doing differently. Two lucky people will win a $50 gift certificate to Better World Books, which uses book sale profits to fund literacy programs. To take the survey, go to tilclimate.mit.edu/survey. Again that’s tilclimate.mit.edu/survey.

Thank you to Suzanne Greene for joining us on this bonus episode, and thank you for listening.