What algae need to grow

Light: Light is essential for algae. Access to light limits the size and shape of the reactor. Algae absorb different wavelengths of light depending on their color.

Carbon: Algae use carbon, usually in the form of CO2 gas, to grow and make oil. The amount of CO2 available to the algae is therefore a design constraint for algae reactors.

Other nutrients: Algae also need other nutrients to survive and flourish, such as elements in seawater or silica.

Temperature: Algae are usually sensitive to temperature and cannot get too hot or too cold. This is an important issue for bioreactors that will be located outside.

Why algae?

In the beginning, there were algae, but there was no oil.

Then, from algae came oil.

Now, the algae are still there, but oil is fast depleting.

In future, there will be no oil, but there will still be algae.

So, doesn’t it make sense to explore if we can again get oil from algae?

-Oilgae.com

Algae is more efficient than other biofuels such s soy or corn oil in terms of nutrients and land. It has the potential to become the fuel of choice for the future, but for this to happen, an economically efficient reactor must be designed.

Designing an economically efficient reactor capable of growing algae for use in biofuel production

Sponsored by

Design objectives

•Low capital and operating cost•High conversion of nutrients to lipids•High reproduction rate•Temperature fluctuations mediated•Durable•Ease of operation•Limited additional infrastructure needed

Biofuel Processing

Once the algae is grown, it needs to be processed in order to harvest the oil. Currently it is only feasible to harvest oil from algae with very high lipid content. Researchers at OSU are investigating possibilities of harvesting ethanol from the algae’s cell walls. This would make processing lower lipid content algae economically feasible, and make growing algae in open reactors possible since species contamination would no longer be as much of an issue.

Plate ReactorBag Reactor

Features

1’ x 2’ x 1/2’ clear acrylic box, with a center plate dividing the box in two.One end of the box is hinged and can be opened for periodic cleaning.Air is bubble in one side at a high flowrate to cause the media to flow. A smaller flow of air is bubble in the opposite direction to keep the algae from sticking to the top side of the reactor.

Features

3’ x 1’ x 1’ clear vinyl bag set into the groundAir bubbled through pipe attached to floor of bag and released through a valve on the far sideAgitation system consisting of a mechanical arm which periodically pushes down on one end of the bag, creating a wave motion.

Benefits

Bag material very inexpensiveGround provides temperature mediationAir source need not be high pressure as agitation does not depend on pressure.

Benefits

Based on well understood triangle reactorNo additional agitation system neededEasy maintenance

Acknowledgements

The oilgae team would like to acknowledge Dr. Yokochi, Dr. Harding, and Dr. Murthy for their generous contributions to this project. Without their help, neither of these reactors would have been built. We would also like to thank several supportive community members who helped us find a location for our reactors.

School of Chemical, Biological, and Environmental EngineeringOilgae Team: Joshua Bruce, Sarah Herzog, Michael Robinson

Results

The bag reactor succeeded in mitigating temperature fluctuations, as shown in the graphs to the right. However, leaks were frequent and difficult to repair. A more sturdy material should be investigated in future testing.

Results

When compared to the triangle air lift reactor, the plate reactor has a much lower cost, takes up less space, and has a lower pressure head. Temperature mitigation strategies should be investigated in the future, in addition to more suitable materials of construction.

Thermal Experimentation

Temperature mitigation is a vital constraint for any reactor design. Most algae are very sensitive to temperature fluctuations, so keeping the reactor warm when it is cold outside and cool when it is hot is very important. This task can also be very difficult.

In hot weather, most of the reactor’s heat comes from energy absorbed from the sun rather than from ambient air temperature. The bag reactor design uses the earth as a heat sink in hot weather, dissipating some of the energy taken in by the sun. The algae team ran several tests to observe temperature change in each of the reactors.

The graphs to the left show the temperature change of the two reactors with respect to time of day, ambient temperature, light, and total light energy received by the reactors.

The results were as expected: The bag reactor heated up more quickly but had a lower peak temperature. It cooled more quickly, but stayed warmer at night.

These results show that the bag reactor design helps to mitigate temperature fluctuations.