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I thought I’d try my hand at envisioning what a 200 megawatt carbon-negative power plant might look like. It would have to be low-tech, simple, and based on current technologies. Now, I can’t claim that this design is very efficient or that it would make a dent in the carbon that’s already in the air, but it wasn’t hard to come up with. A little more thought and we could probably do a lot better.

First of all: SCAF

The starting point for this design is SCAF, the Solar City Air Filtre. SCAF units would be house- to stadium-sized, and based on the technology of solar updraft towers. Basically you have a tall open tower standing atop a glass greenhouse-like structure. In the case of an urban SCAF, air flows in through air filters in the sides of the greenhouse, is warmed by the greenhouse effect, spins a vertical wind turbine to create power that you can sell, and exits (clean and green) out the stack. The SCAF site suggests planting these units in the middle of traffic circles, as a way of mitigating urban pollution.

The technology behind SCAF, the solar updraft tower, is well understood. A 200 meter high test unit was built in Spain in 1982 and produced power for a number of years. Although they’re not very efficient, solar towers have the advantage of being simple. Air comes in at the edges of a vast gently sloping greenhouse; is heated; and turns fixed wind turbines before exiting up the stack. Such towers can produce power quite reliably, and with the addition of heat buffers (plastic-sheet covered ponds under the glass roof, for instance) they can even produce power 24 hours a day. The largest towers envisioned would stand a full kilometer in height, and would produce 200 megawatts of power essentially forever (assuming you replace parts that wear out).

Interesting as they are, solar towers aren’t as compelling as some other renewable designs. Even ordinary windmills can probably out-compete them in most jurisdictions. But, with the addition of one low-tech feature, they could be carbon-negative. I call this design SuperSCAF.

SuperSCAF

A key ingredient in mortar and cement is a substance called quicklime. Quicklime is made very easily, by heating substances like limestone. It has an interesting quality: it absorbs CO2, and as it does it turns back into limestone.

So here’s how SuperSCAF works: we dump powdered quicklime into the air stream after it’s passed through the fixed wind turbines. The cloud of dust rises in the vortex within the central stack of the solar tower, and as it does it scrubs the air of CO2. Limestone dust precipitates out along the sides of the stack and is carried by conveyer belt to a solar oven that resides under the greenhouse roof. There the CO2 is released and piped away (to the sequestration method of your choice) and the product is… quicklime. What we then get (minus efficiency losses from the dust slowing the airflow) is a multi-megawatt power plant that actively sucks CO2 out of a rising column of air up to 130 meters in diameter, and does it 24 hours a day.

Would this work? Well I have absolutely no idea. The scheme depends on finely ground quicklime being able to absorb the CO2 at a respectable rate; and even if the quicklime removed 100% of the CO2 present in the air column, it would be removing it at only a tiny percentage of the rate that one modest-sized coal plant can pump it into the air. This is because a coal stack is a concentrated CO2 source, where SuperSCAF has only the highly diffuse (380 ppm or so) atmospheric concentration to work with. So you’d have to process truly gargantuan amounts of air this way to make a difference; the precondition to even bothering to try it would be a pre-existing global moratorium on coal-fired power.

Solar towers can process truly gargantuan amounts of air; hence this design. And what would happen if an international consortium subsidized the building of at least four such units in every country on Earth? What would 1000+ SuperSCAFs accomplish after running flat-out for ten years?

I don’t have the math to figure it out. But maybe one of our readers does–and I’d love to see the result. I will be neither surprised nor disturbed if the answer is that such a massive effort wouldn’t even scratch the surface of the gigatons of excess CO2 in the atmosphere.

Even so: it’s possible to conceive of carbon-negative power. That in itself is something.

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