# Geothermal instead of fossil and nuclear?

Why aren't we taking advantage of geothermal heating to produce electricity around the world, instead of burning fossil fuels and using nuclear?

Wouldn't it be a lot cheaper than building new power stations that cost billions?

If we can drill oil wells, why not wells deep enough for geothermal?

Apologies for all the questions, I have the thought in my head and wanted some logical answers.

• Because geothermal energy is not available everywhere. Iceland or other volcanically active countries? Yes. Other places? Not so much. Jan 24 at 12:18
• To extend on Gimelist, it is available everywhere, but the economics on deep holes to get sufficient temperature differences, isn't worth it. Also of note is the growing interest in using old oil/gas wells and coal workings for low grade geothermal systems. Not all are suitable but it is useful out-of-the-box thinking. Jan 24 at 14:24
• Geothermal is very expensive because of the corrosivity ( requiring $alloys) and the relatively low temperatures. Modern power plants produce steam at about 1000 F for efficiency. Jan 24 at 16:16 • Geothermal heating is growing increasingly popular, you don't need to go down nearly as deep (only a few metres for a usable heat gradient), and it has the potential to displace a fair amount of electricity (and other fuels) currently used for heating and hot water. But generally the energy you're extracting is from solar heating of the ground throughout the year, not from the mantle. Jan 24 at 23:26 • When you drill for oil you can sell what comes out for$80 a barrel. You're not going to get that kind of money pushing 55 gallons of water around, no matter how hot it is. Jan 25 at 3:04

By way of example, the Habanero geothermal pilot plant was investigated in Australia from 2003 to 2013. It as a 1 MW plant in the Cooper Basin, in the central region of Australia.

The heat source rock, with a temperature of 264 °C was in excess of 4600 m depth. The project cost AUD 105 million and was ultimately deemed uneconomic. The project involved drilling a number of holes to approximately 4600 m depth, injecting some with vast quantities of water and then collecting the steam generated via another set of holes drilled to 4600 m depth and using the steam to power a turbine to generate electricity.

The problems with the project were:

• The depth of drilling required - it was deep and expensive.
• Acquisition of sufficient supplies of water in a arid location.
• The site location was too remote from locations that might have been able to use any generated electricity. The establishment of electricity transmission lines would have been very expensive.
• Having to deal with unfavorable geology, such as fracture zones at shallower depths where the injected water would be lost.

In the picture below, the red areas are the only parts of Australia which has hot rocks to a depth of 5 km that could provide geothermal energy. The area is very small compared to the rest of the continent. Some rocks are deeper.

We certainly couldn't replace all of the world's energy consumption with geothermal; there literally isn't enough energy coming out of the Earth.

The mean thermal energy flow through continental crust from Earth's core is about 65 milliwatts per square meter. (We will assume that we will not have the technology to built geothermal plants on the ocean floor.) The land area of Earth is 148,940,000 km2. Multiplying these two together, we get a net flow of energy of $$9.7 \times 10^{12}$$ watts, or 9.7 terawatts. Over the course of one year, or 8760 hours, this would amount to about $$3.1 \times 10^{20}$$ joules, or 290 quadrillion BTUs. But the total energy consumption of the world in 2019 was about 600 quadrillion BTUs, which is over double this.

Of course, one might argue that we wouldn't want to replace all energy use with geothermal energy, just electricity generation. This is a smaller number: about 81 quadrillion BTUs. This is starting to look a bit more feasible, but the problem is that it is impossible — not just "we can't figure out how to do it", but "there's a law of physics that forbids it" — to extract 100% of the thermal energy present in a source as electrical energy. Typical efficiencies for current commercially viable geothermal electrical plants are around 7–10% instead, and those plants are built in the sorts of places where there are particularly hot rocks at particularly shallow depths.

Geothermal is still a useful energy source, of course, in those regions of the world where the geology allows for it to be viable. But it is unlikely to ever comprise a large part of the world's energy budget.

• Not to mention that even if it were viable all over the world... geothermal wells have a limited lifetime. They're not quite as fossil as coal, but on human time scales, they might as well be - a good spot might get 50-100 years of the very high efficiency around 10%, but then you have to wait another 200-500 years for the heat reservoir to replenish. If we magically changed all our power generation into geothermal overnight, we'd be creating a hell of a energy crisis for our kids :D Not every place sits right on top of a spreading boundary... Jan 25 at 7:47
– towe
Jan 25 at 14:06
• @towe: It's one of those units you have to get used to if you want to read about the US energy system, unfortunately. It even has a brief Wiki article. In terms of more sensible units, it's about 5% larger than an exajoule. Jan 25 at 14:11
• @MichaelSeifert didn't realize that the BTU and kilojoule were such similar units. Jan 25 at 18:42
• @WaterMolecule: Yup. That's about the only good thing about BTU. Jan 26 at 7:50

Asking whether we can replace the major energy sources we are using today today with one single source is a flawed question. It is obvious that there is not one single solution and anything that can be part of the energy mix should be taken into account. The all or nothing approach like the one in the answer by @Michael Seifert is the classic argument used to criticise all the possible solutions by those who don't want to invest anything on those solutions. If you take separately hydroelectric, geothermal, wind, biomass and other renewables none of them is the magic wand that would solve the energy problem. But also nuclear and fossil fuels are by themselves insufficient, they always worked as parts of the energy mix.

I don't know how much geothermal could actually contribute, but I know that it deserves a lot more attention. The example cited by @Fred shows that it is not a solution available anywhere, but on the other hand it seems a project designed to fail. They decided to evaluate cost returns by geothermal under the most difficult conditions. It is true that the high temperature they found guarantees a better efficiency, but it came at a cost because the cost of drilling a well increases exponentially with the depth. So a lower depth, would have meant a lower efficiency, but much lower startup cost and geothermal projects are capital intensive, the running cost is usually a small part of the total cost. I wrote usually because the water actually matters, choosing a desert for the first project doesn't seem a great idea. BTW the deepest wells of the longest running geothermal project in Larderello are not much deeper than 3000 meters.

So, even though we don't have a lot of places with the right temperature at low depths and enough water available for big projects in the style of Iceland there are many places suitable for small scale projects with small efficiency, but also smaller initial investment. Furthermore there are even more places suitable for direct heating. Simply expanding the use of geothermal energy for district heating around the world could cover a non negligible chunk of our energy demand, just forget the all or nothing approach, magic wands just don't exist.

• For the record, I do agree that geothermal will form a portion of whatever sustainable energy system we (hopefully) end up with in the long run. And I wholeheartedly agree with your last sentence. The framing of my answer was designed to critique the notion, present in the original question, that we could get rid of fossil and nuclear if we just used geothermal instead. Jan 26 at 21:43
• "but it came at a cost because the cost of drilling a well increases exponentially with the depth." I would think that it's at most quadratic. Jan 27 at 5:57

Only a few regions where hot water is available near the surface (e.g. Iceland) are suitable for producing electricity. But in order to store thermal energy and thus save or replace the consumption of fossil, but also renewable energies and green electricity, underground water volumes are ideal because 1m³ of water with a temperature difference of ΔT = 40°C (e.g. 15°C - 55°C) can store about 47 kWh of thermal energy.

Since the heat flow from the earth's core towards the surface is rather poor (~ 0.09W/m²), I suggest using the summer's radiant energy from the sun (up to ~ 1000W/m²) to use this unbeatably cheap energy storage. In order to absorb this heat mainly in summer, solar modules are a widespread, efficient tool today - water-cooled PV-T modules (electricity & heat production) would be even better, which, for whatever reason, are unfortunately still a niche product.

The power output, but also the longevity of almost all PV modules increases by ~0.4%/°C cooling, while the overall efficiency of a module / area can quadruple from ~20% to ~80%.

So the simplest and most logical solution for a large part of the energy sector would be to harvest summer heat and store it seasonally for use in winter. With the right heat pump, this heat store turns into an ice store in winter, which in turn can be used for cooling in summer.

There are countless caverns, former coal mines or other underground mines worldwide that are suitable as large-volume energy storage in the form of heat/cold storage, pumped storage or compressed air storage - ideally, the combination of the different types of use increases the overall efficiency of such investments.

Since we not only have to reckon with increased water shortages due to climate change, but also with heavy rain and flooding (drought & flood), such underground water reservoirs can also help to get these events under control much better.

Geothermal power has vast applications but it suffers several drawbacks....

1. Economics: Power has has a 20-30% failure rate at drill sites where potential sites become unrecoverable. So economic failure, Wells a mile deep can cost tens of millions of dollars and if it's loose, seismic rock regions the hole can collapse before even being exploited.
2. Availability: while abundant as an energy resource; it is highly diffused thru much of the world, accessing hot spots may require drilling several thousand meters, an expensive endeavor, especially with a 20-30 percent failure rate. And where it's ubiquitous may not be near population centers, thus demand very long transmission distances negating their advantage; many plants are small (never more than 10-100 MW) and not worth investment.
3. Depletion risk: Hot spots should be used cautiously before risking depletion. Dumping excess water for steam runs risk of depleting thermal source.
4. Energy Density/Thermodynamic efficiency: Average suitable temps of most wells; exceed seldomly 150 degrees Celsius......A fairly low temperature for a working fluid, compared to fossil fuels which can run as hot as 500-1000 degrees Celsius or nuclear which runs at 280-300(some experimental at 495-600), geothermal has a thermal efficiency of 7-10% (Nuclear 33-35%, Fossil: 33-90%)

However Other options for their exploitation involve using Low boiling point (below 100 degrees C) closed cycle engines. Instead of boiling water, they heat up ammonia or a hydrocarbon based refrigerant.

Geothermal can also be utilized for energy efficient space heating and greenhouse management thus mitigating significant fossil energy consumption.