Choosing the Right Microalgal Photobioreactor Design

September 1, 2011 |

By Fadhil M. Salih, ClearValue Companies

Microalgae are Earth’s first forms of life and first forms of food for subsequent species. They now hold the potential to become the planet’s next major source of energy and a vital part of the solutions to climate change and dependence on fossil fuels.

Microalgae are a superior biofuel feedstock as opposed to plants such as corn and soy used today. They need substantially less water and fertilizer than other plant life and they are extremely resilient. Algae doubles its mass approximately every four hours or more depending on the type, and can be cultivated in a controlled environment. Microalgae are veritable miniature biochemical factories, appear more photosynthetically efficient than terrestrial plants and are efficient CO2 fixers.

The ability of algae to fix CO2 has been proposed as a method of removing CO2 from flue gases from power plants, and thus can be used to reduce emission of GHG. Such a fact has encouraged many scientists, technologists, researchers and investors to explore this tremendous capacity and work on improving productivity.

Accordingly, a number of photobioreactor designs have been put into work under different environmental conditions. So far none of the existing designs has fulfilled the dream of the interested parties due to inherent problems these designs are facing, such as control of light intensity, CO2 introduction, pH adjustment, oxygen collection, culture mixing, water loss, contamination, temperature control, nutrients addition, algae collection and algae aggregation and adhesion, in addition to the use of huge land area.

Most of these problems are either unresolvable or may require a lot of costly modifications that are not very much favored by microalgae producers. Among these designs are the open pond, raceway (racetrack) and tubular photobioreactor. Each design has its own problems which place the efficiency far behind theoretical calculations and wishful expectations, particularly when considering that solar radiation striking the earth on an annual basis is equivalent to 178,000 terawatts, i.e. 15,000 times that of current global energy consumption. It was estimated that efficiency could be improved four to five times in magnitude if sunlight is properly utilized and some of the problems of the current designs are taken care of.

Concerted efforts have been made to improve the efficiency of the existing bioreactor designs by incorporating new modifications and measures to minimize the impact of these problems.  None has so far fully fulfilled the anticipated improvement. However, the tubular design managed to relatively  overcome some of the problems, such as contamination, pH adjustment, harvesting and introduction of nutrients, but there still light intensity, mixing, aggregation and temperature control, in addition to large space requirement are not resolved.

Any photobioreactor design that can take care of some or all of the above problems will improve efficiency fairly well. A three dimensional photobioreactor (3dPBR) design that brings sunlight to the algae in a closed system instead of taking the algae to the sun in an open system could be the right solution. It should be able to optimize light intensity by tuning up the quantity of sunlight to suite a given experimental conditions, and contamination and water loss will be minimal.

Since it is a closed system, temperature should be easily controlled and mixing and aggregation will be idealized. Most importantly is that the 3dPBR does not require big land because it is upright and it only needs some extra space for sunlight collection. The size of the land required to install a facility like this will be negligible compared to the land size required to establish any of the existing bioreactor designs of a comparable capacity.

Technically, the 3dPBR design will only have one little technical concern which lies in the light transfer and distribution facilities. So far the available means of light transfer and distribution are not efficient enough, which could cause energy loss thus reducing the efficiency of the photobioreactor. In the events that this weakness is resolved the system will be a great achievement that will contribute in improving the biological conversion of solar energy to chemical products including biofuel and in mitigating atmospheric CO2 as well.

The 3dPBR would be an improved means of microalgae cultivation that would reduce land use, save capital, effort and offer an efficient, yet practical, means of converting solar energy to chemical energy, in addition to reducing humanity’s carbon footprint.

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