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.