How efficient are Open Raceway Ponds, Plastic Bags and Tubular Glass Photobioreactors in algae production?
The productivity of a photobioreactor is one of the most important indicators to evaluate how economically algae can be cultivated, but what is the productivity of a PBR? And how do different reactor systems differ in their productivity? This article intends to provide some simple answers to these questions.
Here we compare three different algal cultivation methods: Tubular Glass Photobioreactors, Open Raceway Ponds and Plastic Bags are the best-known methods to grow algae: Open Raceway Ponds (ORP) are the most widespread method and originate from natural ponds. Tubular Glass Photobioreactors (PBR) have been reliably in operation for almost 20 years, indoors and outdoors. With systems made of disposable polymers the plastic bag is particularly popular. Green Wall Panels (GWP) are a variant of the plastic bag systems.
A note in advance:
The statements made in this article are based on scientific studies and calculations made for Open Raceway Ponds (OWP), Tubular Glass Photobioreactors (PBR) and Green Wall Panels (GWP). The information on Green Wall Panel (GWP) can be applied to other plastic bag systems.
- Surface to volume ratio
The production of biomass by photosynthesis is proportional to the volume of liquid solution that is exposed to photons (e.g. sunlight or artificial light) with optimal wavelength. High biomass production rate is determined by the “light volume”, which is within 20 – 30 mm from the surface of the liquid solution and the concentration of algae in the liquid. The key figure which expresses this requirement is the relation of: Surface (through which light can reach the liquid) to volume (of total liquid).
Comparison of the three distinct production systems lead to the following results:
Thanks to its geometric features, a tube as a container for the liquid solution is especially suited, since light can reach the interior of a tube from all sides, therefore there are no dark areas where photosynthesis cannot take place. In addition to a greater surface area, there is also a wide variety of reflections that ensures optimum light conditions. It can be viewed that tubular glass PBRs possess the highest surface-to-volume ratio.
Green Wall Panels also display good surface-to-volume ratio. In contrast to other hanging or floating polymer bags used in algae cultivation, GWPs have a significantly smaller diameter, allowing for tighter setup. At the same time, GWPs have a larger surface through which light can reach the interior. There are different types of GWPs, which can reach between 50% and 100% of the surface-to-volume ratio of a tubular PBR system.
Compared to these two systems, Open Raceway Ponds have a significantly lower surface-to-volume ratio. The ratio is 15-25 times lower than GWPs or PBRs. Why is that? Most ORPs are about 30 cm deep and only 7 – 10% of the total liquid volume is “light volume”, as photons can only reach the upper 20 – 30 mm of the liquid. This is where the algae grow and the volume underneath is dark, not allowing for photosynthesis.
- Areal productivity
Yet the key figure for discovering the best productivity of algae cultivation is areal productivity. Decisive in the end is how much biomass can be produced with the different methods on a specific area over a specific period of time. In order to determine areal productivity, two features of the methods must be weighed against one another:
2.1 Photoactive volume per area
Photoactive volume describes the volume of liquid solution exposed to sufficient light for photosynthesis to take place. The key figure “active volume per area” measures the size of the photoactive volume in a specific area. Surface-to-volume ratio is a good indicator for the active volume per area, as both are proportional to each other.
With regard to active volume per area, the following picture emerges for the three systems:
2.2 Volumetric productivity
Volumetric productivity involves the growth rates possible in the photoactive volume of each procedure. The following information involves measured values from studies and experience from industrial operation.
Algae growth is indicated in grams per liter and day:
It is obvious that the productivity of tubular glass PBR and Green Wall Panel is more or less identical. There is a dramatic decline, however, for Open Raceway Ponds.
2.3 Areal productivity is the most important indicator
Areal productivity is the product of the two key figures before.
The graph below shows how many tons of biomass (dry mass) can be produced in one year with the three methods:
Tubular glass PBRs typically have the highest areal productivity of the three methods, since they have high surface-to-volume ratio and can achieve high growth. Green Wall Panels are 30 – 60% less productive than tubular glass PBRs. Open Raceway Ponds show dramatically lower areal productivity in comparison. In ORPs only 10 – 30% of the algae can be cultivated per year compared to a tubular glass PBR of the same area. ORPs have a lower photoactive volume per area and a lower volumetric productivity than PBRs.
This article is an extract from our e-book “Comparison of commonly used technologies for the cultivation of algae: Open Raceway Ponds, Tubular Glass Photobioreactors and systems with disposable polymer materials“. If you would like to inform yourself in detail about the advantages and disadvantages of the different technologies in algae cultivation, you can download the e-book for free.