Solar-powered Algae Bioreactor

Creating a solar-powered, no running cost, algae bioreactor.

Step 1: Finished Bioreactor

I apologize for not getting any pictures during construction of my solar-powered algae bioreactor, but I only decided to make an instructable out of it after the fact. There are many diy's about making biodiesel out of oil, and algae has been discussed far and wide as a great source of vegetable oil for conversion. However, I've only seen a few homebrew solutions for actually GROWING the algae. There's a great one here on instructables, but it uses CO2 bottles to provide the algae with the carbon dioxide which they need to grow. As a proof of concept that's fine, but I don't think it makes any sense in the long run -- bottled CO2 is probably more expensive both cash-wise and environmentally than the algae grown from it. There's CO2 available in the air, but only a flat, shallow pool of water has adequate surface area to absorb enough CO2 for algae growth. Here's my solution: a solar-powered small aquarium air pump, which aerates the water inside a five-gallon water bottle. Notice that the water is cloudy after only a couple of days; that's the algae starting to grow. If you look closely you can clearly see the air bubbles which prove that the air pump is still running; the sun has set on the system in this early-evening shot, but the rechargeable batteries are keeping the pump running.

Step 2:

I bought the battery-powered aquarium air pump (complete with tube and air stone) on eBay; IIRC it cost around $8 with free shipping. I constructed the solar panel from five 3" x 3" solar cells (handcut from 3" x 6" cells as you can probably see in the main picture). This gives an open-cell voltage of 2.75V, and a short-circuit current of 1.75A, according to the solar cell specs, and verified by measurement after construction. I didn't get fancy with the construction; I just cut two pieces of plexiglass about 12" x 9" from some scraps I had from another project, glued the cells down with hot glue, then glued the pieces of plastic together with more hot glue. I glued the pump to the back of the panel with some five-minute epoxy I had lying around, and drilled holes for a wire to hang the panel from the neck of the bottle. Cost for materials:

$8 air pump
~$7 solar cells (from a pack of 50 I bought on eBay)
$16 NiMH D-cell batteries (2 @ $8 each)

Stuff I had lying around that other people might have to pay for:

hot glue gun & hot glue
5-minute epoxy
2 pieces 1/8" plexiglass 9" x 12"
electrical wire to attach solar panel to pump
picture-hanging wire
Schottky diode (see next step)
5-gallon water bottle

Step 3:

Inside the body of the air pump, you can see the two NiMH D-cell batteries which charge off of the solar panel and keep the pump operating after the sun goes down. The pump draws 0.55A (measured), so the ~1.75A supplied by the solar panel in direct sun can charge the batteries fast enough to run for twice a many dark hours as sun hours (for example, 8 hours of sun will keep the pump running on batteries for 16 hours). The batteries are rated at 9.5 Amp-hours each, so between the two of them they'll keep the pump running for almost forty hours without recharging. Longer dark periods (like a stretch of bad weather) will cause the pump to stop, but a lack of sun also means the algae aren't growing much if at all anyhow. I drilled a small hole in the side of the pump to bring in the wires from the solar panel (which reminds me I need to get the hole caulked up now that it's wired), and you can also see the Schottky diode I added to keep the solar cells from discharging the batteries when there's no sun. And that's all there is to it!

Step 4: Project Update

Unfortunately, before I was ever able to see any significant algae growth, my cheap battery-powered air pump failed. I guess these things are primarily intended to be used for a few hours when you're moving your aquarium, not to run continuously for weeks/months on end. Either that, or my solar charging circuit (consisting of one Schottky diode) wasn't up to the task. I was hoping that the NiMH batteries would limit the overvoltage experienced by the pump motor, but it may not have worked. When the sun is shining fully on the solar panel, it puts out close to two amps, and the batteries and the motor may not be sufficient load to keep the voltage under control, especially after the batteries are fully charged. I was hoping an extremely minimal charging circuit would be enough -- no regulators or anything -- but I guess all I can say at this point is 'lesson learned'. I'm an EE so a fully-designed system is well within my reach; I was just hoping that I wouldn't have to spend that much money, time and effort on a rigorously complete solution.

In any event, the air pump quit just a few days after I put the unit out in the back yard. There wasn't enough time to find the optimum amount of sun exposure (for the algae, not the solar cells -- the solar cells want as much exposure as they can get!), nutrients, or anything else.

Someday I hope to get back to this project, but my time these days is totally occupied by other, hopefully paying, projects. Maybe they'll show up on instructables once I get them working!