Algae Cultivation Systems – Open Ponds vs PBR

By: Dr. John Kyndt – Head Scientist of the Renewable Energy Program at Advanced Energy Creations Lab.

Industrial cultivation of algae allows for the commercialization of numerous products including food, fertilizer, green chemicals, nutriceuticals and pharmaceuticals, or fuel. Whatever the target product is, if one is planning to cultivate algae for commercial production there will be a challenge of scaling the production system and inevitably the dilemma will emerge to either grow the algae in open ponds or closed systems.

Algae Farm – Open Ponds

Open ponds are the most “natural” way to cultivate the algae and are the option that is currently preferred in commercial settings due to its lower cost. Although the open pond technology was invented in the early 1940’s several variations on the basic concept have been engineered. The designs vary from circular ponds and lagoons to paddle wheel driven oval raceways. The main advantages over the closed systems are:

  • – Simple and cheap design
  • – Relative easy maintenance
  • – Cost efficient operation

However major disadvantages prevent this technology from being superior for certain applications:

– High contamination risk (other algae outcompete or grazers take over)
– Control of temperature, pH and nutrients can be more cumbersome given that the system is more exposed to environmental changes
– A long light path is necessary, since only the top 10 cm typically has enough light penetration for efficient light to energy conversion (photosynthesis)
– Depending on the place of growth the evaporation rate can be relatively high

The biomass yield obtained from any cultivation system is typically given in grams per surface area (m2) or volume (m3) per day. The most optimistic numbers for open pond systems are currently around 25-30 g/m2/day.

Closed Systems – Photobioreactors (PBR’s)

Closed systems are typically referred to as photobioreactors (PBR’s) and can be placed indoors or outdoors. Often these systems are placed inside greenhouses to reduce the cost of lighting. The major advantage of using these systems is the increased surface area for a certain culture volume. This means increased light exposure and typically results in higher biomass yields. Several new creative designs have been invented in the last two years, mainly with the purpose of increasing the light exposure per volume. The most common models vary from using tubular columns or movable clear panels to flexible bags, but there are some more futuristic looking designs like biodomes and even floating bags on ocean waters.

Biomass numbers for closed systems are currently around 50 g/m2/day, which is 40-50% higher than open pond systems. Other advantages are:

  • – Lower contamination risk
  • – Easier mixing which improves mass transfer
  • – Easier to control temperature, pH and nutrient supply

However, before you run off to build your own novel, potentially patentable PBR in your back yard, keep in mind that the high cost of installation and operation is generally what keeps these systems from being competitive. When growing algae for commercial applications we are in the order of growing thousands to millions of gallons (especially when targeting biofuel production).

The take home message is that there is currently no one single cultivation system that fits all needs and when deciding on a growth system, ask the question what fits your target molecule, what is your application, how scalable does it need to be, and what is your geographic location.

All of these will have an influence on your choice of system. However, further research is on the way where scientists and engineers are being creative to come up with cost-efficient “next generation” cultivation systems, for example in the form of hybrid systems.

One last remark is that whatever your target algae-based product is, the cultivation method is only one aspect and you should take into account the entire value chain. This includes strain selection, product yield from biomass, extractability of the products, chemical contamination, product stability and convertibility. Improvements in any of these sectors could have a significant impact on the economical feasibility of the overall production process.

Source: http://algaeforbiofuels.com/algae-cultivation-systems-open-ponds-vs-pbr/

Member of American Aquabiotech, Biofuels Revolution, Algae for Biofuels, Moura Enterprises and MyBeloJardim (ver. Portuguese) Group

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