microwave drilling startups
I’ve seen a bunch of articles about startups trying to do microwave drilling of rock for geothermal energy. Multiple people have asked me about Quaise Energy. (Here’s a popular video.) I’m tired of hearing about them, so I’m writing this post to explain some of the reasons why their idea is impractical.
vaporized rock condenses
When rock is vaporized, that rock vapor doesn’t just disappear. What happens to it? The answer is, it would quickly condense on the hole wall and pipe.
Initially, a lot of people working on microwave drilling didn’t even think about that. Once they did, they decided the solution was to use compressed air to condense the rock and blow the rock particles out. But as anyone familiar with drilling would know, that introduces new problems.
air pressure
Current drilling sometimes uses air to lift up rock particles, but “rotary air blast” (RAB) drilling has limited depth, because:
Air velocity at the bottom of the hole needs to be high enough to lift up rock particles. That means the bottom part of the hole needs a certain pressure drop per distance. So, the deeper the hole is, the higher the air pressure needs to be.
1 km depth requires about 300 psi, and obviously deeper holes require even higher pressure. Higher pressure means more gas per volume, so energy usage increases faster than depth. That’s why drilling of deeper holes uses liquid (“mud”) instead of air to lift rock particles. But here’s Quaise, saying they’re going to do ultra-deep holes with air. At the depths they propose, there are even more problems:
A pipe to contain 1000+ psi gas would be pretty thick and heavy.
At some point, the gas itself starts becoming a significant weight, and then required pressure increases exponentially.
I suppose the particle size of condensed rock could theoretically be smaller than RAB particles and thus require a lower pressure drop, but that’s not necessarily the case. Hot rock particles would stick together. Also, particle size depends on the mixing rate at the bottom, and fast mixing requires fast flow requires a significant pressure drop rate at the bottom of the hole.
energy payback
energy usage
Vaporizing rock takes ~25 kJ/cm^3, or ~7 MWh/m^3. That doesn’t include heat loss to surrounding rock, and microwave sources and transmission have some inefficiency.
In order to cool vaporized rock down to a reasonable temperature, you need a lot of air, perhaps 20x the mass of the rock. Supposing the air is 500 psi, the rock is granite, and compression has some inefficiency, that’d be another, say, 5 MWh per m^3 of rock.
thermal conductivity
Rock has fairly low thermal conductivity. Existing geothermal typically uses reservoirs of hot water that flows out the hole, so thermal conductivity of the rock isn’t an issue because the water is already hot. (It’s like drilling for oil, but oil is less common and contains much more energy than hot water.) Current “enhanced geothermal” approaches use fracking and pumps water through the cracks between 2 holes, which gives a lot of surface area for heat transfer. And then after a while the rock cools down.
With a single hole, thermal conductivity is a limiting factor. The rock around the hole cools down before much power is produced. The area for heat transfer is linear with distance from the hole, so the temperature drop scales with ln(time).
payback period
The heat collected from the rock during operation would be converted to electricity at <40% net efficiency. The efficiency would be worse than ultra-supercritical coal plants because the efficiency would be lower and pumping losses would be much higher.
Considering the efficiencies involved, and the thermal conductivity and thermal mass of rock, the rock around the hole would cool down before there was net power generation. I’m estimating...significantly over 10 years for energy payback, not including the production of equipment needed. Long enough to make the economics unworkable.
some other problems
waveguide losses
Quaise plans to have a microwave at the top of the hole, and a waveguide that goes down the hole.
With carefully designed and precisely machined waveguides, people have gotten 1 dB/km losses with microwaves. With a 10 km waveguide, that still means you’re losing 90% of the energy. There hasn’t been substantial progress in design of straight microwave waveguides for decades.
reverse heat transfer
Only the deep part of the hole is hot. Some fluid gets pumped down and heated up, but then it needs to go back up to the surface. As it goes back up, it transfers some heat back to the surrounding rock. This reduces the feasible fluid temperature.
hole collapse
When rock gets hot and pressures get high, a hole will slowly close as rock flows inward. This has been a limiting factor for attempts to drill as deep as possible, and Quaise has no solution to it.
one thing at a time
I once talked with a founder of a startup (which got funding from Bill Gates) trying to store grid energy in compressed air in composite tanks. Their calculations had much lower tank costs than market prices, and I told them that, if they could build tanks cheaply, they should start out by selling them for CNG transport, and then worry about energy storage after that. They didn’t take my advice, they didn’t have a cheaper way to make tanks, and the company failed.
If a company has a cheaper way to drill deep holes, that’s already valuable without developing a new approach to geothermal power at the same time. Just start with that.
solar power exists
Solar power is cheap. Yes, it’s inconsistent, but even if you add compressed air energy storage, the resulting LCOE is still much lower than energy from geothermal with microwave-drilled holes could plausibly be.
this is only an example
The amount of money Quaise Energy and other microwave drilling startups have raised is relatively small. There are much larger wastes of money. The reason I’m taking the time to write this post because startups like Quaise Energy are a condemnation of the technical due diligence of investors and government agencies, and of the approach journalists and youtubers take when covering new technologies. I’m using Quaise Energy as an example of a much larger overall trend—of the inability of investors to effectively evaluate technologies. The ability of investors to recognize good technical evaluations is the key thing that’s lacking in the economy today; there are plenty of good ideas and there’s plenty of investment capital.
I actually like new ideas and novelty and exploratory engineering. I’m more generous to radical new proposals than a lot of people are. But, like the Hyperloop, microwave drilling for geothermal power isn’t even interesting, let alone practical.
Just in case people aren’t aware of this, drilling wells the “old fashioned way” is a very advanced technology. Typically a mechanically complex diamond-tipped tungsten carbide drill bit grinds its way down, while a fluid with precisely calibrated density and reactivity is circulated down the center of the drill string and back up the annulus between the drill string and edges of the hole, sweeping the drill cuttings up the borehole to the surface. A well 4 miles long and 8 inches wide has a volume of over 200,000L, meaning that’s the volume of rock that has to be mechanically removed from the hole during drilling. So that’s the volume of rock you would have to “blow” out of the hole with compressed air. You can see why using a circulating liquid with a reasonably high viscosity is more efficient for this purpose.
The other important thing about drilling fluid is that its density is calibrated to push statically against the walls of the hole as it is being drilled, preventing it from collapsing inward and preventing existing subsurface fluids from gushing into the wellbore. If you tried to drill a hole with no drilling fluid, it would probably collapse, and if it didn’t collapse, it would fill with high pressure groundwater and/or oil and/or explosive natural gas, which would possibly gush straight to the surface and literally blow up your surface facilities. These are all things that would almost inevitably happen if you tried to drill a hole using microwaves and compressed air.
tl;dr, drilling with microwaves might sense if you’re in space drilling into an asteroid, but makes so no sense for this application.
Also, polycrystalline diamond (PDC) bits are pretty common these days.
ertical holesYes: conventional drilling requires much less energy—but takes 30 times longer!
To ensure stability—the two vertical boreholes and the bundle of connecting boreholes of the “heat exchanger” for the closed loop system must be installed in solid granite!
- - Advance there: conventional 1 to 1.5 m/h—with evaporation 35 to 40 m/h (for Diameter ~2 inch).
For a 1 GW-construct 4m /hr are realistic - so one of the two vertical holes can be drilled in 4000 hrs -
nearly half of a year ! Limit for rotation drilling is regulary at 9000 m !
Note additional : with increasing depth, the temperature increases by approx. 40k per km.
At 12 km it is already almost 500°C—plus the heat for the drilling process 150-200°C.
The heat for the drill head is then almost 700°C - ! No material can withstand that for a long time.
And: even the best diamond drill head wears out within a few days—then the entire string has to be completely pulled out EVERY TIME to change the bit.
Drilling are NOT be done with air, but with inert gas—e.g. liquid CO2 - in the heat on site it gasifies and thus increases its volume by almost 600 times (under normal pressure) and there is also the enormous increase in volume of the vaporized granite.
This means that the gasified rock does not have time to attach itself to the wall when it condenses until it has cooled down to simple dust!
Bringing the microwave power to the drill head is certainly a problem. The attenuation is actually enormous, especially since there is currently no concept for a FLEXIBLE wave guide several kilometers long.
An existing slim version of the gyrotron will probably have to be modified in such a way that it can be dragged a few 10 meters behind the drill head (heat protection).
Then all that is needed is the inert gas and the power cable to achieve this length - - - intermediate inert gas pumping stations can also be supplied with power via the cable. Since the diameter will be greater than 15 inches for an appropriate flow rate of supercritical water, there is sufficient cross-section for pumps and cables—as well as return flow.
There are certainly still problems to be solved - - and therefore the world’s specialized scientific institutions should definitely focus on these problems with the highest priority and appropriate financial resources!
If it weren’t for the enormous volume of orders with the insane profit of several hundred billion when converting the economy to wind & PV with the subsequent ruin and the even more profitable reconstruction of the economy - !
Because drilling into natural grown granite (stock) it is almost impossible to come across a gas or oil deposit—and since THIS non-contact drilling (no vibrations !) allows continuous sonar measurement, cavities and faults in the rock can be largely avoided!
Due to the fact that must be drilled in Granite - THIS rock does NOT flow into the borehole .
And if Granite will be vaporated, than any fluid anyway - that would be a coocking kettle .
And if the rock will be vaporated—the wall is melting and will get a thick and stabile rock glaze !
That stabilisizes and seals same time like an intruded tube!
Particle size seems like an important factor here. You don’t know it and I don’t know it either.
But presumably that’s one of the factors that QUAZE will be working on. Possibly something fancy like injecting a small electrostatic charge so the hot rock specks repel each other.
When rock gets very hot in a microwave beam, it expands. Pressure could get very high. Will it quickly flow outwards, meaning that removing the dirt isn’t needed?
Are you assuming that QUAZE, with their microwave tech, will use a less efficient form of 1 hole geothermal when you know that most geothermal plants use something more efficient?
In general, this is the sort of article that can be written, whether a tech is feasible or not.
You name engineering problems, and don’t discuss whether they are show stoppers or manageable. You guess at a way things might be done, do calculations, and declare it impractical. (The calculations are for a naive / stupidly designed version of the tech that is indeed impractical.)
I’m not sure what that’s supposed to mean.
You’re suggesting fracking between 2 holes? With supercritical water? In rock that’s hot and pressurized enough to have a little plastic flow?
I am probably better at designing drilling systems than the people at Quaise Energy. In general, my problem has been overestimating how competently startups are doing something. I’ve actually designed multiple things for drilling, such as electric DTH hammer systems; I just don’t post such things on my blog.
Why lift dirt when you can push it sideways.
IDK about physics but would it help to have another pipe that is a vacuum? (Like, hooked up to a vacuum pump stationed on ground level.) So then you don’t need such a high pressure at the bottom?
It wouldn’t help that much, because you only have one atmosphere of pressure to remove (which for reference is only enough to suck water up about 35 ft.).
I guess that’s right… what if you have a series of pumps in the same pipe, say one every kilometer?
That’s theoretically possible, but how do you install them? Power them? Deal with abrasive particles and lubrication issues?
What people have decided is more practical is: have a big bucket in the hole, and have compressed air blow the cuttings into the bucket. Then it’s periodically lifted up and emptied. But liquid drilling fluid has other advantages, like balancing pressure down in the hole.
Of course, with microwave drilling you can’t use liquid and need a large mass flow of air for cooling.
If the ambient temperature is already close to 500°C (at a depth of around 13 km), I cannot see how you want to work with a liquid—and there is (NOT YET) no drill at all working and generating additional heat! And your great diamond drill bit will be worn out in no time at such a temperature. The idea with the bucket will certainly work up to several hundred meters—if the drill and rods have been removed from the borehole beforehand—I am curious to see what the client says about a drilling system where the entire string has to be pulled back every 10 minutes to clear the drilling debris.
Incidentally, cooling and blowing out is NOT carried out with air but with inert gas—e.g. liquid CO2.
Since several cross connections between the two vertical deep boreholes have to be created for the “closed loop system” and provided with pressure-tight walls, it would be interesting to know how you would clear a horizontal, 0.5-1km long borehole at a depth of 15km with a bucket! And for several 100 MW, you need—a whole bundle - such cross pipes as heat exchangers—which also have to be tight for the 300 bar pressure of the supercritical water! Approx. 900 l/s (0.9688 kWh/kg ) for 1GW output power of the generators - - - which needs 3 MW of power for the approx. 15 high pressure pumps—to produce that stream!
e.g. https://de.starpumpalliance.com/pumpen/verdraengerpumpen/plungerpumpen/
>I’m using Quaise Energy as an example of a much larger overall trend—of the inability of investors to effectively evaluate technologies. The ability of investors to recognize good technical evaluations is the key thing that’s lacking in the economy today; there are plenty of good ideas and there’s plenty of investment capital.
Yeah, just look at Varda “manufacture drugs in space,” and Colossal “bring back the wooly mammoth.” It just doesn’t make sense for these to be profitable businesses.
I could imagine some billionaires being willing to pay a lot of money for cloned extinct animals, if they can do it.
Varda, they’re trying to make ritonavir crystallize in a different way, but even if they can do that in space, and even if there isn’t an easier way to do it...the crystals aren’t the active form of the drug, it has to dissolve before it can do anything. If you want it to dissolve faster you can use smaller crystals, and if you want it to dissolve slower you can encapsulate it. It’s totally meaningless. Earlier, I think they were trying to make ZBLAN optical fibers, but the only reason they were supposedly better in space was because there were fewer particulates than in labs without air filters, and also they’re not actually better than current optical fibers in practice.
Your arguments in “energy payback” apply to any form of geothermal energy. Why, then, are there apparently profitable geothermal plants? Is everyone in the industry a faker?
All of your calculations assume that the rock has to be vaporized. I don’t see why it wouldn’t suffice to melt the rock, or even just heat it quickly enough that it shatters while remaining in the solid state. The quaise.com website repeatedly says “vaporize”, but an article in IEEE Spectrum says “melt”.
No:
Drilling normally uses much less energy than vaporizing rock.
Using liquid drilling fluid uses much less energy than compressed air for deep holes.
Pumping out existing hot water isn’t limited by thermal conductivity, and conventional geothermal does that.
Quaise Energy is a startup that is specifically about vaporizing rock with microwaves.
Melting rock doesn’t help get it out of a hole. (What are you going to do, try to pump it?) And thermal cracking is pointless when you can use a normal drill. But yes, you can drill rock while keeping it in the solid state.
I know very little about Quaise Energy’s technology, but find it very intriguing because if it works it would revolutionize power generation. Anyhow, obviously people commenting here have engineering backgrounds so I can’t speak with much technicality. But, I was under the impression Quaise was going to use microwaves in a system that works just in front of a standard high-tech drill bit? That way the rock is merely cooked and fractured apart by the waves (not fully vaporized) and thereafter the standard drill bit above the microwave device....a bit system that’s the most cutting edge available, would perform heavy lifting to complete the job of overall boring. My basic understanding is the problem in drilling so deep is bits wear down too quickly unless something else is used in conjunction with them in order to successfully prevent the drill bit’s destruction to get a hole...is it about 12 miles deep to hit geothermal everywhere? If anyone can answer this inquiry of mine, i.e a microwave used in conjunction with a normal drill bit (?) as a solution I’d be interested to know. Thanks...Minot
Super useful post, thank you!
The condensed vaporized rock is particularly interesting to me. I think it could be an asset instead of a hindrance. Mining expends a ton of energy just crushing rock into small pieces for processing, turning ores into dust you can pump with air could be pretty valuable.
I was always skeptical of enhanced geothermal beating solar on cost, though I do think the supercritical water Quaise could generate has interesting chemical applications: https://splittinginfinity.substack.com/p/recycling-atoms-with-supercritical
In this context, the most important advantage of supercritical water is that it contains nearly SIX times as much energy per ton—e.g. at 300 bar and 600°C—than in 160 bar 300°C superheated steam.
As a result, almost 5 times less water has to be driven through the heat exchanger system at depth—whereby—due to the higher pressure—the pump load is about three times lower—and about five times the output is possible with the same borehole diameter. Stone is a poor conductor of heat. So after the initial heat loss to heat up the wall of the riser borehole, only a small part of the 600°C depth temperature at 15-16 km depth is lost, so that about 500°C reaches the turbines. Then the 300 liters per second are enough for about 1 GW production—with a pump output of about 0.1%