6 Types of Renewable Energy

Naturalist filmmaker David Attenborough calls renewable energy, “energy that will never run out.” Technically, all energy is renewable; the law of conservation of energy states energy can never be created or destroyed, only converted from one form to another. But some energy-converting methods rely on resources that definitely do run out.

Fossil fuels, including coal, oil and natural gas, are the most important examples of non-renewable resources. Humans burn them to utilize the energy locked within them. All that combustion creates emissions that pollute the air and warm the planet. Plus, the fuels themselves are finite. When they’re gone, they’re gone.

The sun is a much more reliable energy source, continuously pumping out all the energy needed to make life possible on Earth and meet human needs. The Earth is another source of renewable energy, thanks to natural processes that occur underground, in the atmosphere and under the sea.

At present, 20% of electricity in the United States comes from renewable sources. That number promises to increase with improving technologies. Let’s review some promising sources of renewable energy.

Photovoltaic Energy

Sunlight is loaded with high-energy ultraviolet radiation that can generate an electrical voltage in certain combinations of semiconducting elements.

Most people think of photovoltaic (PV) panels when they think of solar energy. Large flat panel arrays (aka solar farms) can supply electricity to entire communities, while smaller arrays can power individual households or even personal electronic devices.

Present-day solar panels are only 25% efficient at best, and manufacturing them consumes non-renewable fossil fuels. So by themselves, PV panels aren’t the final answer to today’s energy needs. They currently supply less than five percent of electricity consumed globally, though that’s expected to increase and possibly overtake coal as soon as 2026. Things are moving fast.

Solar Thermal Energy

Everyone knows the sun generates heat. Some of the ways people can put this to good use include:

• Solar cooking;
• Passive solar heating for homes;
• Solar water heating;
• Desalination of ocean water.

The technology also exists to generate electricity from the heat of the sun. Most solar thermal electricity generators rely on concentrating sunlight on a fluid, superheating it via flat or curved reflectors. The hot fluid circulates through a coil and exchanges its heat with water, which turns to steam and drives a turbine.

Wind Energy

The sun drives Earth’s winds, so wind power is technically a form of solar energy. The sun’s energy has been assimilated by the atmosphere and converted to mechanical energy that can drive turbines to produce electricity. Wind turbines have efficiencies of up to 50% and currently supply almost seven percent of the world’s electricity.

Collections of wind turbines that generate electricity on a large scale are known as wind farms, on land (onshore) or in coastal seas (offshore). Smaller turbines that supply electricity to individual homes are becoming increasingly common. But because the wind is unreliable, most homeowners use them to supplement power from other sources.

Hydro Energy

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When water falls from a high elevation to a lower one, its kinetic energy can spin a turbine. Today, hydropower is the No. 1 form of renewable energy on Earth, supplying 17% of the world’s electricity.

Most hydropower comes from large dams or major waterfalls, like Niagara Falls, the birthplace of the world’s electric grid. Seven countries — China, Brazil, the U.S., Canada, India, Japan and Russia — produce the bulk of the world’s hydropower, but many others generate significant amounts. Homeowners with fast-moving streams on their properties can also use small turbines to supplement their electrical needs.

Tidal Energy

The sun’s energy, the Earth’s rotation and the gravitational forces of the sun, moon and planets combine to produce ocean currents. They have enough mechanical energy to produce significant amounts of electricity.

Several technologies exist to harness this energy, but the most common are:

• Tidal barrages: Placed in the mouths of large rivers or in lagoons function like dams. They force tidewater to back up to create a height differential, then use the spillover to spin turbines.
• Underwater tidal turbines: Similar to wind turbines, they generate electricity by spinning.

Today, tidal energy doesn’t account for a significant portion of the world’s electricity. But with developing technologies, it could supply as much as 10% in the future.

Geothermal Energy

Earth’s core is as hot as the surface of the sun. Just below the Earth’s surface, the temperature is a steady 57 degrees. That’s enough heat for ground-source heat pumps to provide warm air for buildings.

But that’s just the beginning. At greater depths and near fissures, geysers and volcanoes, hydrothermal reservoirs are far hotter — around 360 degrees and above. Technologies exist to tap into this heat and use it directly for warming, or to boil water for steam to drive turbines.

Geothermal energy accounts for less than one percent of the world’s energy supply, but it’s largely an untapped resource. In the U.S., its availability has been growing two percent per year since 2016. Electricity from U.S. geothermal plants is projected to triple by 2050.