LW - microwave drilling is impractical by bhauth

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Welcome to The Nonlinear Library, where we use Text-to-Speech software to convert the best writing from the Rationalist and EA communities into audio. This is: microwave drilling is impractical, published by bhauth on June 13, 2024 on LessWrong. 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 roc...