Would this carbon sequestration/biofuel idea work?

Cities have higher ambient CO2 than other areas. They also have lots of tall buildings. I'm thinking of some sort of automated algae-collecting system that could either generate biofuel, or simply sequester carbon, with fairly minimal expense and effort. The system would be about like this. You have a... show more Cities have higher ambient CO2 than other areas. They also have lots of tall buildings. I'm thinking of some sort of automated algae-collecting system that could either generate biofuel, or simply sequester carbon, with fairly minimal expense and effort.

The system would be about like this. You have a tank on the sunny side of a tall building. It's clear, to let in light, and has a solar-powered pump to pump in air (in fact, all the mechanisms are powered by solar panels). It also has a mechanism that will periodically drop some of the water a few feet down a shaft to a filter. After the water goes through the filter (to collect some of the algae), it is pumped back to the top, adding a bit more if water levels are low. Building greywater and the like could be used, rather than using fresh tap water. Periodically, an automatic scraper will clean off the filter, and collect the resulting algae in something like a tiny trash compacter. Once the compactor is full, it squeezes out the excess water (as much as it reasonably can), then drops the brick of compressed algae into a hopper to dry the rest of the way. The hoppers are emptied periodically (likely once every few months), and the contents are processed (as much as practical) to remove phosphorus and the like (which get added back into algae systems, or used as fertilizer), then the remainder is either burned for energy or buried.

Any fatal flaws you see with the idea? Do you think it would be practical with present technology? Any other thoughts?
Update: Basically, the pump would only need to work when the algae are growing, which would be... when there's sun.

The drying was mostly to reduce the cost of transport, I figured including the mechanisms to actually *process* the algae in each little rooftop system would be too expensive...
Update 2: Realist: or, some of the algae would die each night, which might make them easier to collect. It's not like there'd be *no* oxygen in the tank...
Update 3: I suspect my response to my Best Answer will be too long to fit, so I'll put it as additional details. 1....The algae system needs to be 1/12 of the cost to be competitive. Part of the goal is energy generation, and part is, in essence, spot CO2 reduction. Also, is your 3% figure considering just a... show more I suspect my response to my Best Answer will be too long to fit, so I'll put it as additional details.

1....The algae system needs to be 1/12 of the cost to be competitive.

Part of the goal is energy generation, and part is, in essence, spot CO2 reduction.
Also, is your 3% figure considering just a "sheet" of algae, or all the algae that could grow in a tank? Several inches of water might well have algae floating all through it. It wouldn't surprise me if you ended up with efficiencies within shooting distance of a standard solar panel.

4. There are higher value uses for the algae than fuel.

Even mixed, who-knows-what-species "junk" algae?

5. ...How about putting up buildings with concave shapes to act as giant focussing mirrors? ...

The main point is something that could be stuck onto existing buildings, fairly cheaply.

6. Other plants are easier to harvest. I have done experiments with Lemna (duckweed). The doubling time is 27 hours and it can be
Update 4: ... used as a substitute for corn mash in ethanol production. You would need to make the water slightly acidic pH~6 for optimal production. But you'd have to seed the tanks with duckweed, and potentially keep seeding it. If you leave a tank of water sitting in the sun, with any access to air, it will... show more ... used as a substitute for corn mash in ethanol production. You would need to make the water slightly acidic pH~6 for optimal production.

But you'd have to seed the tanks with duckweed, and potentially keep seeding it. If you leave a tank of water sitting in the sun, with any access to air, it will eventually end up growing algae. Also, algae (at least, as a category) aren't as picky about things like water salinity, and grey water can be fairly saline sometimes.

7. The temperature of the water tank needs to be regulated.

Somewhat. But, again, algae can grow in a lot of different conditions.

8. Your scheme would be more economic growing tomatoes and green peppers, etc. instead. You would need to do an assessment of contaminants in the grey water.

Which would be one advantage of *not* trying to do this with a high-value food crop. You don't need to assess the quality of the water if all you're doing is growing something you're going to burn, bury, or use a
9 answers 9