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!
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:
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.