A photo of the Hoover Dam at sunset, which is a hydropower plant and a renewable energy resource.
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Adobe Stock
Renewables for the win

What Are the Five Main Renewable Energy Resources?

Steve Hansen
/
September 18, 2023

What Are the Five Main Renewable Energy Resources?

We are living in an amazing time as renewable energy continues its inexorable path to dominance in the energy industry. In fact, we’re actually returning to relying mostly on renewable energy, as for most of human history, renewable energy was the dominant energy source. Or the only source. We used to depend on wood, animal dung, and plant oils, for example, until the industrial revolution unlocked the power of fossil fuels.

Today, the U.S. Energy Information Administration (EIA) classifies five types of renewable energy sources. Here they are, along with the percentage of U.S. consumption compared to all the energy we consume:

  • Biomass: 4.9%
  • Wind: 3.8%
  • Hydropower: 2.3%
  • Solar: 1.9%
  • Geothermal: 0.2%

Nonrenewables still make up the majority of energy consumption in the U.S. because of all those miles we drive using gasoline and diesel fuel:

  • Petroleum: 35.7%
  • Natural gas: 33.3%
  • Coal: 9.8%
  • Nuclear: 8%

As you can see, the smallest player in nonrenewables—nuclear power—is still larger than the largest renewable energy source—for now.

When we look at electricity generation, the picture changes drastically because of the importance of petroleum in transportation:

  • Fossil fuels (total): 60.2%
  • Petroleum liquids: 0.6%
  • Natural gas: 39.8%
  • Coal: 19.5%
  • Nuclear: 18.2%

Renewable energy used for electricity generation:

  • Renewable energy (total): 21.5%
  • Biomass: 1.3%
  • Wind: 10.2%
  • Hydropower: 6.2%
  • Solar: 3.4%
  • Geothermal: 0.4%

Nuclear power is not a renewable energy resource, by the way. It’s been part of the conversation recently because it’s regarded as “carbon neutral” and doesn’t contribute to the climate crisis in the way that coal, oil, and natural gas do. That’s true, but it’s definitely not a renewable energy source, as uranium must be mined much as coal is mined.

But we’re talking about the future here, and that’s all about renewable energy, which is energy that is naturally replenished. Let’s look at each renewable energy source in more detail.

What is Biomass Energy?

Biomass energy comes from plant and animal sources, some of which are waste. People have been using this material for energy for literally thousands of years. Biomass can be used for heating, electricity production, and transportation fuels. Biomass fuels like wood can be burned directly—think firewood—and waste from logging and furniture industries can also be burned for heat and processed into products like wood pellets.

The agricultural sector also produces plenty of usable waste, or residues, from growing corn, soybeans, switchgrass, and sugar cane. Most of this waste goes to produce biofuels such as ethanol and biodiesel. Sometimes, the crop itself goes to producing the biofuel, as with corn ethanol.

Animal waste can be burned directly when dried, as humans around the world have done for centuries. It can also be collected with waste bedding material (wood products, straw, etc.) and processed in an anaerobic digester to produce biodiesel.

Human sewage can be harvested for biogas at sewage treatment plants. Biogas is mostly methane and carbon dioxide. The EIA estimates that in 2021, 57 sewage treatment facilities produced 895 Kilowatt hours (kWh) of electricity from such facilities. Biogas also occurs naturally in landfills, where it can be harvested for use. The Clean Air Act, in fact, requires landfills of a certain size to manage their biogas production. Some flare it off, meaning they burn it on-site. Others capture it and sell it.

What is Wind Energy?

Mass-scale wind production has been on a roll for several years now and keeps growing like mad. Most wind energy is collected from gigantic wind turbines that are part of wind farms constructed by utility companies. According to the U.S. Department of Energy, “Wind energy helps avoid 329 million metric tons of carbon dioxide emissions annually – equivalent to 71 million cars worth of emissions.”

Today, the U.S. has just under 73,000 wind turbines, according to the U.S. Geological Survey’s Wind Turbine Database, generating more than 145,000 megawatts (MW). That makes it the fourth-largest source of electricity, and when combined with solar power, it is larger than the amount that coal produces and very close to the amount that nuclear produces.

Two workers look over plans on a computer while working at a wind turbine plant
Source: Adobe Stock

Five states produce half the wind power in the U.S. Texas has the most turbines, with over 10,000, generating over 18,000 MW of power, about 26% of the U.S. total. Iowa, California, Oklahoma, and Illinois come next. Together, those four states produce just a bit more power than Texas.

Wind power has its challenges, of course. Like solar, it’s considered an intermittent supply of power rather than “always on” like a natural gas plant. That can be mitigated with giant on-site battery banks and pumped hydro storage. Turbines also get pushback from local people for the perceived visual and noise disturbance they cause. However, homeowners who let utilities place turbines on their land and get rental payments tend to favor turbines.

What is Hydropower?

Hydropower harnesses the energy of moving water, so really, we can thank gravity. Hydropower has a long history as it’s a relatively simple technology. It’s sort of like wind power in that moving water pushes a turbine that spins a generator. Hydropower can be one of three types:

  • Diversion: In this system, a canal diverts water from the river channel down through a pipe, or penstock, to spin a turbine. With this type of system, you don’t construct a dam or reservoir, so they run on the prevailing flow and can run dry. They may be considered an intermittent resource.
  • Storage or impoundment: Here, you can think of the big ones like Hoover Dam on the Colorado River, but they’re not all huge. These systems depend on a dam creating a body of water that feeds a constant flow of water through the penstock and through the turbines to spin a generator and produce electricity.
  • Pumped hydroelectric: In this type of system, pumps deliver water from a lower reservoir to a higher reservoir when electricity is plentiful, say in daytime hours. Then, at peak load later in the day, the utility can release water from the upper reservoir. It travels downhill through a penstock and spins a turbine, then is released to the lower reservoir. Then the water gets pumped back up to the upper reservoir again.

What is Solar Power?

There’s more than one type of solar power generation, but here we’re talking about solar photovoltaic, or PV, power. That means solar panels, whether they’re mounted on your roof or in giant utility-scale installations. There’s also community solar power, which is great for people who want to embrace clean energy but can’t do it on their own roof, for whatever reason.

Solar panels work by turning solar energy into electricity with specially processed silicon PV cells and copper conductors. The PV cells produce electricity, and the copper conductors send it all through wiring into your home or into the grid.

The American Clean Power Association says that utility-scale solar power projects are powering 18 million homes with 18 MW of clean electricity, which is growing steadily.

Two workers in orange vests working on solar panels at a large solar farm
Source: Adobe Stock

What is Geothermal Power?

Geothermal power is the least developed of the five renewable energy options but may have the most potential to provide 24/7 power at scale. Geothermal power uses the heat within the earth - which can range from lukewarm to scorching - to warm a liquid in a pipe that can then spin a turbine. Geothermal energy can also be “direct use,” which is heating buildings directly with water or a heat-transfer fluid heated by the earth. Until relatively recently, geothermal power was located in places like Iceland and other volcanic regions, where the earth’s heat is close to or right at the surface.

That’s no longer necessary. If you drill deep enough, the earth has enough heat everywhere to make geothermal energy worthwhile. Several companies are exploring “geothermal everywhere” thanks to the fracking revolution that unlocked a flood of natural gas worldwide. Advanced drilling technology from the oil and gas industry, it turns out, can precisely create the ground loop that a geothermal plant needs to circulate a heat-transfer fluid for a closed-loop plant.

Advantages include 24/7 power, zero emissions, a relatively low footprint compared to coal and natural gas plants, and easy scalability. Disadvantages include high upfront cost, as all that drilling is not cheap or particularly fast.  

So What’s Next?

More growth! Renewable energy is still just one-fifth of U.S. electricity consumption, so there is massive room for continued expansion. And since wind and solar are also the cheapest new types of electricity to bring online, it’s mostly those projects that are coming online and are being proposed. This will take a while, but we’re on the right track with renewables.

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