November 18, 2009
CO2 Might Boost Geothermal Energy Efficiency
Fracturing rocks deep underground so that water can be heated up doesn't work well for generating geothermal energy. The US Department of Energy has decided to fund some national labs to develop an approach for geothermal energy capture involving carbon dioxide as a substitute for water. The approach offers the additional benefit of sequestering CO2.
In 2000, Los Alamos National Laboratory physicist Donald Brown proposed replacing water with supercritical carbon dioxide, a pressurized form that is part gas, part liquid. Supercritical CO2 is less viscous than water and thus should flow more freely through rock. Brown noted that a siphoning effect should help cycle the carbon dioxide, thanks to the density difference between the supercritical CO2 pumped down and the hotter gas coming up, slashing power losses from pumping fluid. Plus, Brown argued, instead of using precious fresh water resources, a carbon dioxide-based project could sequester the equivalent of 70 years worth of CO2 emissions from a 500 megawatt coal power plant.
In the on-going debate about substitutes for fossil fuels the main candidates are solar, wind, and nuclear. Geothermal just doesn't get much attention. Anyone know why?
"Geothermal just doesn't get much attention. Anyone know why?"
If you are not in a volcanic zone like Iceland or California, meaningful amounts of energy are way too far down for economic use of current drilling technology. I have read of various deep drilling experiments, but I don't think any of them have gotten all that far.
Did Fat Man mean the pun in his last sentence?
When I looked into geothermal power I was given to understand that the plants had problems with minerals leaching out of the water and clogging the pipes. Also the water contained acids that were hard on the turbines.
Those would not seem to be problems for supercritical carbon dioxide.
Hot (and supercritical) water is a solvent, but I'll bet that hot supercritical CO2 is also. If it reacts with rocks to be sequestered, that means it's chemically reactive. No panacea there, just differences. Useful differences? Good question.
Drilling through hot rock is a problem, because drill bits soften and wear faster at high temperatures. That's not going to be any easier whether the fluid to be pumped through the hole is hydrogen oxide or carbon dioxide. Also, fractures in the rock will tend to heal faster the softer the rock is, and rock softens with temperature. If you can't keep fractures open, you're SOL.
Oh, one more thing: conversion of minerals to carbonates will increase their bulk, and cause fractures to close up. The idea of carbon sequestration isn't a bad one, but it does conflict with fluid flow and heat transfer.
If hot water, which in it's pure form has neutral pH, damages the works due to acidity, wouldn't C02, which is an acid, damage them even more?
"Oh, one more thing: conversion of minerals to carbonates will increase their bulk, and cause fractures to close up."
Have deep sea heat vents ever stopped up, perhaps the genius in you could solve why they haven't, by the way, whats your assessment of realclimate alerting hadley the jig is up?