ALGAE GROWTH
Written by Microsharp
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Light Management for Algae Growth

Microsharp has initiated a project, Solalgen, to develop novel optics to improve the productivity of algae culturing systems – both open pond and closed systems - and to demonstrate this optical system in a hybrid open/closed algae growth system to be developed within the project.

Microsharp has both filed a patent (Photobioreactor Illumination System) and been a finalist in the Shell Springboard innovation competition in 2011 for this algae illumination technology.

The goal of the illumination system for improved algae growth project is to design, develop, prototype, IP protect and commercialise an optical light distribution system that will  significantly increase the productivity of existing open pond algae cultivation plant designs while maintaining low capital and operating costs and consequently reducing the overall costs per unit mass of algae oil/biomass produced.

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Light Utilisation by Algae

Large scale production of microalgae biofuels must use sunlight as the only source of light energy. The available sunlight limits productivity. Therefore, efficient light use is critical. Especially in summertime and in sunny countries, sunlight intensities are high. At these high light intensities the algae are not able to use all the light they absorb, and part of the absorbed light will be wasted as heat. Therefore, the efficiency of light use is low, the algae culture overheats and productivity is low.

Light intensity on the actual algae throughout the growing medium is a key parameter affecting algae growth. Algae generally need light intensities between 30 W/m2 and 100 W/m2 - which is around 1/10 of the light intensity of direct sunlight. If light levels are too high there is poor additional growth or even actual photo inhibition.

Photosynthesis is proportional to number of suitable photons not energy (as in e.g. silicon solar cells). That is, for example blue photons are no more’ useful’ in terms of photosynthetic output than red photons. Photosynthesis only uses a specific window of the solar spectrum - photons between 400 and 700 nm (visible light) and this absorption spectrum is not uniform but has peaks at 450nm and 680nm

Microsharp Solution

In order to optimally utilize solar radiation to get the highest level of algal photosynthetic activity Microsharp proposes the following solution:

  • ·         Collect and distribute light throughout the volume of the algae growth medium such  that an increased proportion of the volume has optimal light intensities for algae growth
  • ·         Provide a system whose capital and running costs are more than compensated by the increases in algae growth productivity
  • ·         Provide a system which is useable in a range of approaches from open ponds to closed photobioreactors, including solutions for avoidance of biofouling and efficient cleaning
  • ·         Inclusion of conversion of light wavelengths to utilise light not used in photosynthesis to provide light with a wavelength optimised for algae photosynthesis
    • ·         Control direct solar heating of the algae growth medium through infrared filters, mirrors or up conversion
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  • A first prototype has been constructed. Lumogen Red 310 is used as luminescent dye suspended in thin film coating laminated to PMMA. A microlens light emitter with reflective backing is used as the emitting area and all edges are mirrored

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Growth Simulation with Optical System

Productivity equation from from Eilers and Peeters, 1988 [4]. Related to standard variables: initial slope of productivity, optimal light level and maximum productivity. Values used:

  • a = 0.01,
  • b = 3.16
  • c  =  735.76

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Photoresponse of simulated algae - productivity against light intensity

High density algae cultures are relatively impervious to light transmission. Ogbonna and Tanaka (1997) give the light extinction coefficient µ as 200 meters squared per kilogram . A 10g/l algae concentration would yield an 86% loss of light energy at 1mm depth,
 ln(I/Io) = µ x C x d
where,
I = light intensity at depth of penetration d
Io = initial light intensity
C = algae concentration, kg/cubic meter or g/l

Relatively deep penetration light due to low algae density. Productivity against transfer proportion – maximum at 55% light transported (50% efficiency). Overall the optical systems results in a maximum 61% productivity enhancement

At very low algae densities there is only small improvement (~20%) as light already reaches the pond bottom. At higher biomass densities (e.g. 7g/l) improvement remains similar (increases): up to 79%.

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Clearly the better the optical efficiency (% of PAR added at bottom compared to PAR removed at top) the better the performance, If we assume 55% PAR removed from top and we vary the efficiency of the light transported (so at 100% all the PAR removed at the top is released at the bottom), then above 40% efficiency we get more than 50% productivity improvement. Below 20% efficiency there is a poor performance. Overall the system needs to achieve >40% efficiency on this measure to be worthwhile 

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