A new project — I want to learn advanced technologies from scratch, and share what I learn. You can learn from scratch too!
This page is my first notes on geothermal energy.
What I learned from “geothermal energy for kids” on YouTube.
It’s really hot inside the earth. There are hot rocks and sometimes water near the hot rocks. So if you have a hole down to where the rocks are hot… … you can use the heat and the water! And even to make electricity! Now you can call it geothermal power, not just geothermal energy. We mostly use the water from next to the nearest hot rocks, but now we have some new ideas too.
Different ways to use the hot rocks.
If the hot rocks are really close you can just use the heat. Humans have been doing this for a really long time (think hot springs). In some places you can run water through pipes nearby the rocks to heat the water, and then heat a building — call this geothermal heating (versus geothermal power) — either through conduction or a heat pump. This is like Iceland level of accessible geothermal, though.
If the hot rocks aren’t quite that close, then you can drill a hole down to where the rocks are hot.
If the hot rocks are wet, you can use that water/steam in a couple different ways:
- If there’s steam near the rocks, you can just let that flow directly to a turbine to generate electricity (dry steam plant)
- You can also pump hot water from down below up to the surface, let it cool quickly in a tank to “flash vaporize” and use the vapor to spin a turbine. (It goes from water → steam as it cools because it’s hot and high-pressure as a liquid, then at the surface the pressure decreases so it quickly turns to steam. Remember
PV = nRT?) (this is a flash steam plant) - You can also pump out hot water from underground and use a heat exchanger to heat a second fluid which then vaporizes quickly and spins a turbine (because you picked a fluid with a lower boiling point than water or whatever the first one was) (this is a binary cycle plant) Most geothermal power plants also put new water back in to replenish the reservoir.
If the hot rocks are dry, that’s where it gets interesting. Enhanced geothermal systems (EGS) involve the creation of geothermal systems by pumping water down where there wasn’t already a reservoir. This involves fracking techniques borrowed from the oil & gas industry — injecting water into the ground to create fractures in the rock — allowing that water to heat up, and then pumping it back to the surface.
Spinning turbines make electricity. Wait, what?
Steam is hot (water in gaseous state, so it’s gotta be hot). So If we direct that steam towards a turbine blade we can spin the turbine. Sort of like a windmill. Combine that with a generator and you get electricity.
Note that steam turbines aren’t unique to geothermal power systems. We can also heat water using nuclear reactions, or by burning coal or natural gas. That’s the primary way to generate electricity from coal or nuclear. In 2022 about 43% of US electricity was generated by (single cycle) steam turbines.1
But how does the generator work? An electric generator uses magnets and coils of wire to convert mechanical energy (from a shaft rotating because of the turbine) into electric power.
There’s a part of the generator that rotates, and a part of the generator that stays still. You put a magnet on one part and a coil of wire on the other. When the coil of wire rotates in the magnetic field (or the magnet rotates around the coil of wire) that induces current in the wire.
It’s hard for this not to feel like magic, even if you’ve taken AP Physics…let’s leave generators there for now but come back to it later.
The rocks are hot and there’s a lot of them.
There are two primary sources of heat within Earth
- Residual. The core and mantle are still hot from when Earth formed (sometimes called primordial heat)
- Radioactive decay. Radiogenic heating caused by the decay of reactive elements down there releases heat.
There’s lots of other processes too but this is the bulk of it. For example, there’s some tidal dissipation heat when as the moon pulls on the ocean, generating heat from friction along the ocean floor that dissipates. Anyway there’s plenty of weird challenges in estimating heat inside the earth, because, well, we can’t go there very easily.
Sometimes people think that gravity creates heat by making it so high pressure down there. That’s sort of part of it in that higher pressure raises temperature all else equal… but mostly the high pressure helps trap and sustain the heat that is there already.
Stanford’s Global Climate and Energy Project (GCEP) estimated the total thermal energy stored in the Earth’s crust at 15 million zettajoules.2 Global primary energy consumption is about 180,000 terawatt-hours, or 648 exajoules.3 So that’s enough for about 20 million years. A lot of hot rocks!
Geothermal is exciting.
24/7. The earth’s core doesn’t seem to sleep, so geothermal energy is available 24/7. Like burning oil/gas/coal, and unlike solar or wind, this means geothermal can match electricity demand.
Built-in batteries. Solar and batteries might be a winning combination. But that requires skilling up two tech trees at the same time. The reservoir of a geothermal site basically is a giant battery. If you let the water get hotter, the steam will just blow the turbine faster (to a certain extent).
Ready to scale? EGS and other recent breakthroughs in geothermal energy borrow a lot of technology, talent, and techniques from the oil & gas industry. This might mean that learning curves in oil & gas happen faster than you might expect, and that workforce development is easier compared to other new industries.
Low marginal cost. Most of the cost is in the drilling. Unlike solar panels, which need to be replaced, you can imagine the marginal cost of power generated by geothermal getting really low. Or at least cost-competitive. And most of the regulation is ported over from oil and gas.
Geothermal is exciting now.
In July 2023 geothermal startup Fervo demonstrated commercial viability at it’s Project Red site in Nevada with 63 liters per second and 3.5 MW of generation.4 Fervo is using techniques from EGS and also drilling horizontally to collect heat over larger area. Other next-generation geothermal concepts like closed-loops, heat-roots, or supercritical EGS could mature as well.5
The promise of geothermal is basically inexhaustible, cheap, firm power anywhere in the world. And there’s not much in the way other than learning how to make it happen.