Wondering if geothermal energy is renewable? We break down the science.
In an increasingly green-oriented society, geothermal has become a key player in the renewable resources landscape and is growing increasingly central to discussions about the future of energy. Being generated from the Earth's internal heat, geothermal is a reliable and constant source of power, and crucially, is not dependent on external factors like sunlight or wind.
We'll explore the role geothermal energy can play in our global energy scenario, and whether or not it constitutes a truly "renewable" energy source.
At its "core" (pun very much intended), geothermal energy is the heat stored beneath the Earth's surface - as in, the molten core approximately 2,900 kilometers below the surface. As the hottest part of Earth, this heat is mostly produced by the decay of radioactive isotopes like potassium-40 and thorium-232.
The gradual change in Earth's temperature from surface to core is called the geothermal gradient, which averages 25° C per kilometer of depth. Magma, formed when underground rock formations are heated to about 700-1,300° C, heats nearby rocks and aquifers and can surface as geysers, hot springs, and mud pots, all sources of geothermal energy. The majority of Earth's geothermal energy remains in the mantle, creating pockets of high heat accessible through drilling.
For electricity generation, geothermal power plants rely on heat that's only a few kilometers beneath the Earth's surface. Generating power from geothermal energy involves three types of power plants: dry steam, flash steam, and binary cycle.
Dry-steam power plants use natural underground steam sources. Larderello, Italy housed the first dry-steam power plant back in 1911. The Geysers in California, U.S. provides about a fifth of the state's renewable energy.
Flash-steam power plants utilize hot water pumped into low-pressure areas. Iceland meets nearly all its electricity needs this way, and the largest single geothermal power plant is in Malitbog, Philippines.
Binary cycle power plants heat water underground to heat an organic compound with a lower boiling point to generate steam and electricity. The Beowawe Geothermal Facility in Nevada, U.S. operates on this principle.
There's also co-produced geothermal energy, which uses hot water byproduct from oil and gas wells. The U.S. produces about 25 billion barrels of such hot water annually. The steam generated from this byproduct has recently been used to sell back to the grid, and newer technologies have even made the process portable.
Enhanced Geothermal Systems (EGS) utilize drilling, fracturing, and water injection to create underground fluid reservoirs in hot but dry rocks. However, the injection process can cause seismic activity, as evidenced by a cancelled project in Basel, Switzerland in 2009. A recent study attempted to explain this phenomenon. The new study looked into three possible causes for these man-made earthquakes: changes in underground water pressure, the way stress spreads in water-soaked rocks, and how stress can cause rocks to slip.
The research suggested that changes in water pressure underground are a major cause of these earthquakes. But, this idea alone doesn't fully explain why faults (breaks in the rocks) act the way they do, especially at the Basel EGS site.
When the scientists took into account how stress spreads in water-soaked rocks and how it can make rocks slip, they got a model that made more sense with the observed man-made earthquakes. But, the model still isn't perfect. It can't explain why all faults act the way they do, especially Faults D and G. It also doesn't capture the biggest earthquake event, which happened at fault C just after they stopped injecting water.
Despite this, the study is a big step forward in understanding why these man-made earthquakes happen when using hydraulic stimulation (a process used in geothermal energy projects). This deeper understanding of what causes these earthquakes will help us predict them in a better way and come up with ways to lessen their impact. And these improvements are crucial for the successful creation and management of geothermal energy projects.
Geothermal energy has historically been used across a variety of applications. While its most obvious use is electricity generation, geothermal is also used for district heating, agriculture, and industrial processes.
For example, low-temperature geothermal energy, obtained from pockets of heat that factors at about 150° C, can be found a few meters below ground and is used for heating greenhouses, homes, fisheries, and industrial processes.
Geothermal heat pumps (GHPs) utilize Earth's heat with drilling depths between 3 to 90 meters. They are efficient in heating and cooling buildings, parking lots, and other areas. The largest GHP system, completed in 2012 at Ball State University, Indiana, replaced a coal-fired boiler system, saving an estimated $2 million a year in heating costs.
In New Zealand, natural geysers generate geothermal power to heat pools, homes, and greenhouses, and in Iceland, volcanic heat sources warm nearly 90% of homes.
Yes, geothermal energy is indeed renewable. This natural reservoir of power, produced through the continuous radioactive decay occurring in Earth's core, represents an energy source that has been radiating for roughly 4.5 billion years and will persist for billions more. As long as the Earth continues its internal radioactive decay, the planet will produce more heat than humanity can conceivably extract.
However, this reservoir is not invulnerable. Extraction sites, especially those where heat removal outpaces replenishment, may eventually cool. The once-thriving geothermal site at Larderello, Italy, the world's inaugural geothermal power plant, exemplifies this, with steam pressure falling over 25% since the 1950s. Reinjection of water could enhance site longevity, but it risks inducing seismic activity, even leading to site closure, as witnessed in Basel, Switzerland.
The environmental impact of geothermal systems is relatively minimal. Water usage in binary systems primarily serves as a heat transfer agent, reducing freshwater demands. But, when not properly contained and recycled, geothermal fluid can carry contaminants like arsenic, boron, and fluoride, posing risks to water sources and ecosystems.
Advantages of geothermal energy are numerous and significant. It's not only a renewable source, it's also universally accessible and relatively clean, with emissions primarily consisting of water vapor and minor amounts of sulfur dioxide, nitrous oxides, and particulates. Geothermal power plants have remarkable lifespans, potentially spanning centuries with careful management.
Unlike wind or solar power, geothermal systems offer constant energy production, unaffected by the time of year or daily weather changes. They are an around-the-clock solution. Moreover, geothermal power plants have a comparatively smaller footprint. A geothermal plant can produce a gigawatt-hour of energy, a substantial amount, using significantly less land than wind, solar, or coal plants.
However, the quest for geothermal energy is not without hurdles. Earthquakes triggered by high-pressure water injection, land subsidence, greenhouse gas emissions, and potential water contamination are among the environmental challenges. Further, the high upfront costs may deter developing nations from investing in geothermal technology, although international collaboration can enable progress, as evidenced by American-financed plants in the Philippines. The U.S., as the top geothermal energy producer, annually generates the equivalent energy of burning 25 million barrels of oil, primarily in western states, but the U.S. must drill at higher costs than a country like Iceland, which heavily relies on accessible underground hot water.
Costs have decreased (marginally) over the last decade, just not as much as solar and wind power have. and the U.S. DOE set a target to bring the costs for newer, enhanced geothermal systems down to $45 per megawatt hour by 2035 (which would be down 90% from where they are today).
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