Geothermal energy is renewable.
Electricity produced from geothermal is “renewable” in a couple different senses — first, the potential scale is orders of magnitude bigger than current energy consumption. Second, the carbon emissions from geothermal are significantly lower than, say, burning coal. And third, the Earth’s core is still releasing more heat.
We could use geothermal energy faster than it replenishes.
The Earth’s surface constantly releases something like 44 to 47 terawatts of power in the form of heat.1 Global primary energy consumption is approximately 180,000 terawatt hours per year. 2 So hypothetically, total geothermal energy flow is roughly twice the total energy that we humans use in a year.
But not all the heat created in a power plant becomes electricity. This called the heat rate, and heat rates for most power plants are such that about one third of the heat is converted to electricity (~35% for nuclear and coal, a bit better for natural gas and a bit worse for oil).3 Plus global energy consumption is growing at about 1 or 2% per year, which would be a doubling by 2060. Maybe faster with broader electrification and increasing demand for data centers.4
So we could quite plausibly use geothermal energy at a rate faster than it replenishes.
But the rocks are already really hot.
Wait, but then why is there any hype?
Geothermal energy is about “mining” the rocks that are already hot, not about harnessing the net flow of heat from the earth’s core. You may have heard a stat that says something like ” there’s 50,000 times more energy available in geothermal than all our fossil fuels”. This stat comes from Stanford’s Global Climate and Energy Project (GCEP), which estimated the total thermal energy stored in the Earth’s crust at 15 million zettajoules.5
Geothermal is not a sci-fi silver bullet.
Compared to nuclear (fission or fusion) geothermal is orders of magnitude less “renewable”. Global primary energy consumption is about 180,000 terawatt-hours,6 or ~648 exajoules per year. 15 million zettajoules would last about 20 million years at current primary energy demand. That’s a lot of hot rocks!
But what if primary energy demand grows? Right now the growth rate is modest at about 1-2% per year6 — but even that adds up over time, doubling roughly ever 35 to 70 years.
Geothermal runs out. if primary energy demand grows at +1-2% per year, we could use up all the thermal energy in the earth’s crust in 650 to 1,200 years (my calculation). Exponential growth is wild!
In comparison there is plausibly billions of years of energy supply in fissible material on earth7 — now that’s still ignoring growth rates, but it’s 2 or 3 orders of magnitude more than geothermal.
What’s exciting about geothermal is that there is a plausible path to a meaningful amount of reliable, 24/7, clean energy over the next 5, 10, and 20 years — not that it is a sci-fi silver bullet.
Geothermal is (relatively) waste free
There is relatively little waste from geothermal power plants compared to, say, burning coal.
It does take water to run the system, but most systems collect the steam-to-water and pump the cooled off water back into the reservoir to replenish it. And you don’t need to use potable water so there’s potentially less of an impact on local water supply.
The hot water from the reservoirs often contains lots of minerals which are sometimes disposed of as hazardous waste. But a lot of these minerals are actually marketable byproducts like lithium.8
There are some emissions from gasses in the wells. But many systems (closed-loop in particular) inject those back into the well not into the air — and even for open-loop systems it’s 30X less than coal-powered plants.9
Geothermal is just as green as solar
Just like other forms of renewable energy, there are greenhouse gas emissions from the lifecycle of a geothermal power plant — you have to build and ship panels for solar, you have to drill wells for geothermal.
The IPCC has a 2012 report with a literature review on the lifecycle emissions of 11 different electricity generation technologies. Geothermal estimates range from about 8 to 80 grams of CO2 equivalent per kilowatt hour, depending on the study and type of system, which for reference is about 30 times less than coal. You have to squint to see much of a difference across renewables, so geothermal is on par.10 (As an aside, this table lists nuclear as one of the non-renewable electricity generation technologies, which is a longstanding technical choice in climate literature but also a bit confusing as always).
Most of the lifecycle emissions are from drilling a well and building the power plant and pipelines. This implies that productivity improvements can drive the lifecycle emissions further, especially anything that extends the lifetime of the power plant or well.
Lifecycle estimates are tricky — there are a lot of well-intention and hopefully well-informed assumptions stacked on top of each other. And the “right” assumptions can change over time — if the electricity mix gets greener, than that would drive down the construction emissions of the marginal power plant… The National Renewable Energy Laboratory (NREL, part of the Department of Energy) maintains and updates these estimates. The most recent 2021 fact sheet doesn’t look too different. What stands out to me is how wide the ranges still are — what makes these studies have numbers from like 0 to 200 gCO2eq/kwhr for solar??? — and how much closer carbon capture and storage (CCS) gets fossil fuels to parity.
Footnotes
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Pollack, Hurter, & Johnson (1993) or Davies & Davies (2010) ↩
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Frequently Asked Questions (FAQs) - U.S. Energy Information Administration (EIA) and Table 8.1. Average Operating Heat Rate for Selected Energy Sources ↩
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Global electricity demand set to rise strongly this year and next, reflecting its expanding role in energy systems around the world - News - IEA ↩
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x.com. I don’t really have a better source on this stat, it might be made up. But I think the point stands that it’s easier to find more material for fission than to get net-new geothermal energy. ↩
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pangea.stanford.edu/ERE/pdf/IGAstandard/SGW/2013/Bakane2.pdf ↩
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Environmental Impacts of Geothermal Energy | Union of Concerned Scientists ↩
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SRREN Report - IPCC-WG3, pg 732 in chapter 9 for the chart comparing electricity generation sources. ↩