Internally Illuminated Photo Bioreactor with Light Pipe for photo-reactive microorganism culture Technical Field

[0001] The invention pertains to the general technical field of CO2 capture and culture of photo-reactive microorganism, in particular capture of CO2 and use thereof to aliment photo-reactive microorganisms. More particularly, the invention relates to a photo-bioreactor (PBR) comprising photo-reactive microorganism being illuminated by internal light pipe (LP). The present invention also relates to a method for capturing CO2 and producing photo-reactive microorganism. The present invention finally relates to the transformation of the photo-reactive microorganism obtained by the present method or within the photo-bioreactor.

Technical Background

[0002] How to reduce CO2 emissions and to utilize captured CO2 are issues of main interests in the academic and industrial fields. CO2 capture, sequestration, and utilization strategies adopted so far can be broadly divided into physical and biological means. Physical methods including capturing, transporting and storing CO2 are very expensive processes. Biological utilization of CO2 emission by photo-reactive microorganism culture proves to be a promising alternative to physical methods. Photo-reactive microorganism culture can convert CO2 through photosynthesis into organic chemicals such as carbohydrates, proteins, lipids, and other high added-value biofuels, which cannot only help mitigate global warming effects due to CO2 emission, but can also alleviate the energy crisis.

[0003] Currently, photoautotrophic culture is the only technically and economically feasible way to produce photo-reactive microorganism biomass at commercial scale. Two main types of photo-reactive microorganism farming systems can be distinguished: open systems (the cultivated photo-reactive microorganism are in contact with the open air), and closed system, for example in photo-bioreactors (PBRs) (the cultivated photo-reactive microorganism are in a closed container).

[0004] Open systems are difficult to control because the parameters such as light availability, mixing, pH, temperature and nutrient concentration cannot be easily controlled. In contrast, closed PBRs allow great and diversified control of the parameters.

[0005] Either in the open or closed systems, light availability proves to be the most limiting factor for photo-reactive microorganism growth.

[0006] One major issue in photo-reactive microorganism culture is, thus, to provide sufficient and uniform quantity of light to each photo-reactive microorganism because the light is quickly absorbed by the surface-exposed photo-reactive microorganism cells. The higher the concentration of photo-reactive microorganism, the more light is absorbed. As a result, photo-reactive microorganism cells close to the light source receive far more light than photo-reactive microorganism in darker regions. The photo-reactive microorganism in the darker regions cannot get sufficient light for growth.

[0007] One could think about increasing the light intensity so as to decrease the volume of darker regions. However, this solution should be avoided because photo-reactive microorganism are limited in photosynthesis and oversaturated illumination can lead to irreversible damage due to photo-inhibition effects.

[0008] One could therefore think of drastically limiting the depth of the PBR to several millimeters to centimeters. Such limitation leads to very thin and long reactors, and, in consequence bulky system which requires large areas to be operated industrially. Moreover, gas transfer and mixing in such thin and long reactors is insufficient, which decreases the efficiency of the photo-reactive microorganism production. [0009] There are two main ways to enhance the light distribution inside a photo-reactive microorganism farming. One way consists in increasing the mixing of the photo-reactive microorganism inside the system. The mixing enhances the photo-reactive microorganism cell circulation between the illuminated and dark regions. Such a mixing can be done for example by means of gas bubbling, installation of stirrers and mixers. However, inherent sensitivity of photo-reactive microorganism to strong shear stresses has limited further improvement on photo-reactive microorganism production by using the mixing solution.

[0010] Another way to enhance the light distribution inside a photo-reactive microorganism farming consists in bringing light inside the PBR as close as possible to the photo-reactive microorganism. By internally illuminating the photo-reactive microorganism, the light is not delivered via the reactor surface by light-emitting structures outside the reactor, but via light-emitting structures disposed directly inside the reactor. The reactor geometry is not limited to very small layer thickness unlike the externally illuminated PBRs, and can thus be designed to be easily scaled up to industrial application.

[0011] Internally illuminated PBRs (IIPBRs) can be divided into two main categories: PBRs with internal light guides and PBRs with internal light sources.

[0012] PBRs with internal light sources have light sources inside the PBRs itself. Internal illumination via internal light sources leads to much lower light losses but higher volume losses. However, the heat released by internal light sources will result in temperature rise of the culture medium (21) and even cause damages to the photo-reactive microorganism cells.

[0013] PBRs with internal light guides comprise an external light source and light guides coupled to the external light source and transmitting the light from the outside into the reactor. For example, types of light guides can either consist of optical fibers or optical plates. Such a PBR has low volume losses since the volume of a light source is larger than the volume of a light guide, but substantial light losses will occur during light guiding process. Another issue is that the scale-up and the following maintenance process may be expensive and laborious.

Technical Problem

[0014] As aforementioned, there is a plurality of systems for producing photo-reactive microorganism. However, open systems are difficult to control and are subject to technical issues relating to the mixing of gas with the photo-reactive microorganism. Close systems are more easily operated. However, photo-reactive microorganisms need to be consistently and sufficiently illuminated without being overheated or over-illuminated. Additionally, close systems for large scale production of photo-reactive microorganism still need to be developed.

[0015] There is therefore still a need for developing a photo-reactive microorganism farming enabling large-scale production of photo-reactive microorganism with low energy consumption, being cost-effective and being less bulky than large open system.

[0016] Also, there is still a need for developing a system for illuminating photo-reactive microorganism with efficient amount of light.

Summary of the invention

[0017] The invention relates to a photo-bioreactor comprising photo-reactive microorganism being alimented by carbon and nutrients injection and artificially illuminated by light pipes. In particular, the photo-bioreactor is incorporated within a wider system aimed at capturing CO2 emitted by an industrial facility to produce photo-reactive microorganism-based biomass. This biomass is then transformed, for example into biofuel. [0018] The invention is therefore directed to a light pipe comprising a tubular external structure with a length, a liquid filling the tubular external structure and a light inlet, wherein the refraction index of the liquid and the refraction index of the material of the tubular external structure are chosen so that light input through the light inlet is uniformly distributed through the liquid and all over the tubular external structure. The liquid filling the tubular external structure comprises a chemical compound having optical properties.

[0019] Such light pipe makes it possible to have a uniformly distributed light profile along the length of the tubular external structure, notably, the intensity of the light transmitted to the outside by the tubular external structure is Itot/L ± 20%, preferably ± 10 %, with hot the total intensity provided by the light source or light sources through the light inlet or inlets when the light pipe comprises a plurality thereof and L the length of the tubular external structure.

[0020] The tubular external structure may further comprise a gas inlet to provide gas bubbles inside the tubular external structure in operation.

[0021] The reflecting chemical compound may have biocide properties, notably against bacteria and/or algae. Additionally or alternatively, the reflecting chemical compound may be chosen from the group consisting of: sodium hypochlorite (NaCIO), calcium hypochlorite (CaCIO) chlorine compounds such as chlorine dioxide (CIO2), hydrogen peroxide (HOOH), sodium peroxide (NaOONa), bromine (Br) and their mixtures. Preferred chemical compounds are: sodium hypochlorite, chlorine dioxide, hydrogen peroxide, sodium peroxide and their mixtures.

[0022] The liquid filling the tubular external structure may comprise scattering solid particles. In such case, the scattering solid particles may be chosen from the list consisting of: metal particles, glass particles, plastic particles, ceramic particles and their mixtures.

[0023] The equivalent diameter of the scattering solid particles may be 10 pm to 10 mm, preferably 50 pm to 2 mm, still preferably 100 pm to 1 mm. The equivalent diameter being the diameter of a sphere of same surface as the solid particles.

[0024] The concentration of the scattering solid particles may be 0.1 and 1 g/L, preferably below 0.75 g/L, still preferably below 0.5 g/L.

[0025] The tubular external structure may comprise a liquid inlet to refill the tubular external structure during operation with the liquid. Additionally, the tubular external structure may comprise a liquid outlet to create a liquid flow between the liquid inlet and the liquid outlet during operation.

[0026] The tubular external structure may be made of a single wall or of two walls which are parallel to each other.

[0027] The single wall or the two walls may be made of material chosen from the list consisting of strengthened glass, poly(methyl methacrylate), fluorinated ethylene propylene and polycarbonate. In particular, the material can be an allow of some or all of the aforementioned components.

[0028] The single wall may have a thickness of 2 to 8 mm. Alternatively or additionally, the ratio between the length of the tubular external structure and the thickness of the single wall may be 250 to 2500, preferably below 1500, still preferably below 1000.

[0029] The two walls may be made of different materials or a same material. [0030] The light pipe may comprise two light inlets, one placed at a first end of the tubular external structure and the other end at a second end of the tubular external structure opposite the first end. Additionally or alternatively, the light pipe may comprise a plurality of light inlets placed along the length of the tubular external structure.

[0031] The light pipe may comprise a light source, for example a LED, a laser, a xenon lamp, or a halogen lamp; preferably a LED. A light source may be provided for each light inlet or may be common to a plurality of the light inlets. Both solutions can be used in the same light pipe.

[0032] The liquid may be chosen in the list consisting of water, alcohol.

[0033] The liquid may preferably be water further comprising an antimicrobial compound.

[0034] According to another aspect of the invention, a light pipe grid comprises a plurality of light pipes provided with a light source as described above.

[0035] The light pipes of the plurality of light pipes may be located in concentric circles. Alternatively, the light pipes may be placed in a triangular or squared pattern.

[0036] The choice of composition of the liquid filling each tubular external structure may be function of its radial position within the grid. Alternatively or additionally, the choice of material for each tubular external structure may be a function of its radial position within the grid.

[0037] The light pipe grid may further comprise a controller configured to control the light distribution pattern by controlling each concentric circle individually and independently from each other. In such case, the controller may be configured to control the light distribution pattern by controlling each light pipe individually and independently from each other.

[0038] The controller may be configured to control the light emission pattern. In such case, the controller may be configured to control the light emission pattern by changing, for each light pipe, at least one of: the light properties of a corresponding light source, the flow of the liquid through the tubular external structure, and the flow of gas through the tubular external structure. For example, the controller may be configured to control the light properties of the corresponding light source by changing at least one of its light emission spectrum and its intensity. Alternatively or additionally, the controller may be configured to control the flow of the liquid through the tubular external structure by changing at least one of: the liquid composition and the liquid flow rate. Alternatively or additionally, the controller may be configured to control the flow of gas through the tubular external structure by changing at least one of: the gas composition and the gas flow rate.

[0039] According to another embodiment, the invention is directed to a photo-bioreactor comprising:

- a tank with a gas inlet fluidly connected to at least one nozzle, and a culture volume comprising a culture medium and at least one species of photo-reactive microorganism;

- the light pipe grid described above.

[0040] The at least one nozzle may be at the bottom of the tank.

[0041] The culture medium may be chosen from the group consisting of river water, Arnon medium culture and a combination thereof

[0042] The photo-reactive microorganism may be chosen from bacteria or micro-algae. The photo-reactive microorganism is preferably a microalga that may be chosen from the group consisting of the following genera: Botryococcus, Chlorella, Phaeodactylum, Scenedesmus, Synechocystis, Ankistrodesmus, Scenedesmus, Synechococcus, Synechocystis, Anabaena, Spirulina, Nostoc, and Calothrix, preferably Scenedesmus or Synechocystis.

[0043] According to another embodiment, the invention is directed to a method for producing photo-reactive microorganism with a photo-bioreactor of the present disclosure, comprising:

- connecting the gas inlet of the photo-bioreactor to an industrial chimney,

- illuminating the photo-reactive microorganism via the light pipes,

- regulating at least one of:

- the light pattern of the light pipe or light pipe grid according to the photo-reactive microorganism concentration inside the tank; and

- the gas flowrate and gas holdup within the tank according to the photo-reactive microorganism concentration inside the tank;

- collecting the cultivated photo-reactive microorganism when the photo-reactive microorganism concentration in the PBR reaches a predetermined value.

[0044] The concentration of the photo-reactive microorganism inside the tank may be maintained between 0.2 and 10 g/L, preferably between 0.5 and 6 g/L, still preferably between 1 and 5 g/L, still preferably between 1 and 3.5 g/L.

[0045] Regulating the light pattern of the light pipe or light pipe grid may comprise regulating, for part or all light pipes of the light pipe grid, at least one of:

- the light properties of the corresponding light source according to the photo-reactive microorganism concentration inside the tank;

- the flow of the liquid through the tubular external structure according to the photo-reactive microorganism concentration inside the tank; and

- the flow of the gas through the tubular external structure according to the photo-reactive microorganism concentration inside the tank.

[0046] Regulating the light properties of the corresponding light source may comprise regulating at least one of: the light emission spectrum and intensity thereof.

[0047] Regulating the light properties of the corresponding light source may comprise regulating at least one of the liquid composition and the liquid flow rate.

[0048] Regulating the flow of gas may comprise regulating at least one of the gas composition and the gas flow rate.

[0049] Finally, the invention is directed to a system comprising a photo-bioreactor as presently described and an industrial facility comprising a gas exhaust chimney coupled to the gas inlet of the photo-bioreactor, and a heat exhaust coupled to the light pipe grid of the photo-bioreactor to provide energy to the light source(s) through a fatal heat recovery process during operation.

Brief description of drawings

[0050] Other features, details and advantages will be shown in the following detailed description and on the illustrative figures, on which:

[0051] Fig. 1 is a longitudinal section of a PBR according to the invention, the different elements not being true to scale. [0052] Fig. 2A to 2D show examples of LPs. Fig. 2A and 2B show two examples of LP with one light source at each end thereof. Example of Fig. 2B, unlike Fig. 2A, uses scattering solid particles. Fig. 2C and 2D show two examples of LP with a plurality of light sources located along the length of the LP. Example of Fig. 2D, unlike Fig. 2C, uses scattering solid particles.

[0053] Fig. 3 show a schematical graph of a light pipe grid of the PBR according to the invention.

[0054] Fig. 4A to 4C are graphs of a cross-section of the PBR Light distribution profile for a 5 m long LP with 2 light sources on both ends with a photoreactive concentration of 1.5 g/L (Fig. 4A); 3 g/L (Fig. 4B); and 4.5 g/L (Fig. 4C).

[0055] Fig. 5 is a graph showing light intensity along the length of the LP when illuminated (A) from one end only at 100 % of the power of the light source, (B) from both ends at 50 % of the power of the light source, and (C) from both ends at 100 % of the power of the light source.

[0056] Fig. 6A to 6I are a cross-sectional view of different configurations of the LPs in the PBR. Fig. 6A to 6C show LPs disposed in circles; Fig. 6A with one circle, Fig. 6B with two circles and Fig. 6C with three circles. Fig. 6D to 6F show LPs disposed in a triangular pattern, with the size of the smallest triangular unit decreasing from Fig. 6D to 6F; the LPs of Fig. 6D forming one hexagon, of Fig. 6E two concentric hexagons, of Fig. 6F three concentric hexagons. Fig. 6G to 6I show LPs disposed in a square pattern with the size of the smallest square unit decreasing from Fig. 6G to 6I.

[0057] Fig. 7 is a scheme of a photo bioreactor coupled to the chimney of an industrial facility.

Description of embodiments

Lightpipe 30 (LP)

[0058] According to an object of the present invention, the inventors have developed a light pipe 30, which will be described in further details with reference to Fig. 2A to 2D.

[0059] The LP 30 may be considered as a light pipe distributor or provider. It is configured to let injected light to efficiently penetrate the outside medium, such as the reacting volume inside a bio-reactor.

[0060] The LP 30 comprises a tubular external structure 31 , a liquid fully or partially filling the tubular external structure 31 and a light inlet, wherein the refraction index of the liquid and the refraction index of the material of the tubular external structure 31 are chosen so that light input through the light inlet is uniformly distributed through the liquid and all over the tubular external structure 31. The liquid filling the tubular external structure 31 comprises a chemical compound having optical properties.

[0061] The LP30 addresses the problems of substantial light losses and heat generation inside a PBR. Such a LP 30 is cost effective, easy to manufacture and to clean.

[0062] Refraction indexes of the tubular external structure 31 and of the liquid are chosen so that the tubular external structure 31 can transmit an efficient light distribution toward the photo-reactive microorganism 200 in operation. The light coming through the light inlet of the LP 30 is reflected and scattered by the liquid and its constituents and the interface between the liquid and the tubular external structure 31 all along the tubular external structure 31. Also, every time a light ray reaches the interface, part of the light can be transmitted radially so that the photo-reactive microorganism 200 receive sufficient light. An efficient transmission of light by the LP can be described as a transmitted intensity of Itot/L ± 20% all along the tubular external structure 31, wherein hot is the overall light intensity provided by the light source(s) at the light inlet(s) and L is the length of the tubular external structure 31. [0063] Total internal reflection (TIR) is the phenomenon which occurs when a propagated wave strikes a medium boundary at an angle larger than a particular critical angle (TIR angle) with respect to the normal to the surface. TIR effects only occur when the light is transmitted from a denser medium, with a larger refractive index, to a thinner medium, with smaller refractive index.

[0064] It has been found that the larger the difference of refractive indexes between two adjacent media, the smaller the critical angle for the TIR phenomenon. A compromise between TIR effect and the transmission value of the tubular external structure 31 has to be found. Indeed, if the TIR angle is too low, for example lower than about 30°, most of the light will stay inside the tubular external structure 31. In contrast, if the TIR angle is too high, for example above about 70°, most of the light coming from the light inlet will be transmitted through the part of the tubular external structure 31 closest to the light inlet.

[0065] Thus, two characteristics have a significant impact on the light intensity and uniformity in an LP 30: The first one is the amount of light staying inside the LD due to the TIR phenomenon, which will determine the light uniformity and also the light intensity inside the LP 30. The second one is the light transmission through the tubular external structure 31 which will determine the outside light intensity. The TIR phenomenon and light transmission characteristics are controlled by the dimension of the tubular external structure 31 and the combination of the refraction indexes of the materials used for the tubular external structure 31 and the liquid.

[0066] The refraction index of the liquid may be adjusted by directly modifying its composition, for example by choosing the compounds having optical properties. The optical properties may be for example a light reflecting property or a light scattering property.

[0067] The chemical compound with optical properties may have in addition biocide properties. For example, the chemical compound with optical properties may have an antimicrobial property, a bactericide property and/or against algae. This is advantageous to keep the inside of the LP 30 free from bacteria or algae, which by growing on the surface of the tubular external structure 31 can prevent light from being efficiently transmitted towards the outside of the LP 30.

[0068] The chemical compound may be chosen from the list consisting of: sodium hypochlorite (NaCIO), calcium hypochlorite (CaCIO) chlorine compounds such as chlorine dioxide (CIO2), hydrogen peroxide (HOOH), sodium peroxide (NaOONa), bromine (Br) and their mixtures. Preferred chemical compounds are: sodium hypochlorite, chlorine dioxide, hydrogen peroxide, sodium peroxide and their mixtures.

[0069] Also, the solvent of the liquid may be changed. For example, the solvent of the liquid may be chosen from the list consisting of water, alcohol and their mixtures. When the liquid is water, it is preferably de-ionized water.

[0070] The use of water and a chemical compound with biocide properties or of alcohol or a mixture of water and alcohol makes it possible, in additional to the optical properties, to have a (at least almost) germ-free liquid and in particular free of germs that can color the liquid and/or absorb a portion of the light propagating through the liquid.

[0071] The liquid is inert to the material of the tubular external structure 31.

[0072] The liquid may fully or partially fill the tubular external structure 31.

[0073] The tubular external structure 31 may comprise a liquid inlet 36 to refill the tubular external structure 31 during operation with the liquid, in particular when the tubular external structure 31 is open and evaporation of the liquid occurs. Although, it is still possible to refill the tubular external structure 31 without such liquid inlet 36 by pouring liquid into the open tubular external structure

31. [0074] The tubular external structure 31 may further comprise a liquid outlet 37, thus, enabling the creation of a liquid flow between the liquid inlet 36 and the liquid outlet 37 during operation.

[0075] Several advantages can be linked to the flow. First, it is possible to change the composition of the liquid inside the tubular external structure 31 by injecting a liquid with a different composition. For example, the liquid inlet 36 is in fluid communication with a chemical compound reservoir and a solvent reservoir. By changing the flow rates from the chemical compound reservoir and the solvent reservoir, the chemical compound concentration within the liquid can be changed. The liquid inlet 36 may further be in fluid communication with a plurality of chemical compound reservoirs, each reservoir containing a different chemical compound with optical properties. Changing the flow rates from each of the chemical compound reservoir makes it possible to have a large choice in composition for the liquid. Flowever, even without such liquid inlet 36 and outlet 37, the liquid can be changed by pumping out and pouring in a liquid with different composition into the open tubular external structure 31.

[0076] Second, it is possible to control the temperature of the LP 30 by providing fresh cooler or warmer liquid through the liquid inlet 36 into the tubular external structure 31, the heated or cooled liquid within the tubular external structure 31 can be evacuated through the liquid outlet 37.

[0077] Further, the tubular external structure 31 may comprise a gas inlet 38 to provide gas bubbles inside the tubular external structure 31 in operation. A gas outlet 39 may be provided to guide exhaust of the gas out of the tubular external structure 31.

[0078] The gas inlet 38 may be configured to provide bubbles of certain diameter, for example from 0.1 to 5 mm, preferably 0.5 to 3 mm, still preferably 0.5 to 1 mm. The diameter in question here is the diameter of a sphere with the same volume as the bubble.

[0079] The gas injected into the liquid filling the tubular external structure 31 may be air, which can be compressed air.

[0080] The injection of gas generates bubbles 32 inside the liquid in the tubular external structure 31 provides gas-liquid interfaces which have scattering properties. Scattering light redistributes the received light. The bubbles 32, thus, help to obtain the efficient light distribution.

[0081] The liquid filling the tubular external structure 31 may further comprise solid particles 33. These solid particles 33 may have scattering and/or reflective properties. These solid particles 33 may be metal particles, glass particles, plastic particles, ceramic particles, and their mixtures. The solid particles 33 help in obtaining the efficient light distribution. The equivalent diameter of the scattering solid particles 33 may be 10 pm to 10 mm, preferably 50 pm to 2 mm, still preferably 100 pm to 1 mm. The equivalent diameter being the diameter of a sphere of same surface as the scattering solid particles 33.

[0082] The concentration of the scattering solid particles 33 may be 0.1 and 1 g/L, preferably below 0.75 g/L, still preferably below 0.5 g/L.

[0083] In certain conditions, the gas bubbles 32 created inside the tubular external structure 31 help to regulate the temperature within the LP 30, by either cooling or warming the liquid inside the LP 30.

[0084] Further, the scattering solid particles 33 can also deform gas bubbles 32 within the tubular external structure 31 when the scattering solid particles 33 contact the gas bubbles 32. By deforming the bubbles 32, the scattering solid particles 33 increase even more the scattering effect of bubbles 32.

[0085] Preferably, the scattering solid particles 33 are homogeneously distributed in the liquid. In such case, the LP 30 may further comprise a mixer for mixing the liquid with the scattering solid particles 33 inside the external tubular structure 31, so that distribution of the particles is even throughout the external tubular structure 31 and the distribution of the light is enhanced. [0086] Alternatively or additionally, one user may adjust the refraction index of the tubular external structure 31 by modifying its geometry and/or its material.

[0087] The tubular external structure 31 may be closed or open. When it is said that the tubular external structure is closed or open, the gas inlet 38, gas outlet 39, the liquid inlet 36 and liquid outlet 37 (when they are provided), are not taken into consideration.

[0088] The longitudinal section of each of the outer and inner surfaces of the tube forms a pattern. The pattern may be a straight line, sinusoidal line, serrated line or a line made from a random variation between two maxima; the line extending collinearly with the longitudinal axis of the tube. The lines formed by both the outer and inner surfaces may be in phase (maxima of the outer surface are at the same level as the maxima of the inner surface and minima of the outer surface are at the same level as the minima of the inner surface) or in opposite phase (the maxima of the outer surface are at the same level as minima of the inner surface and vice versa). More generally, the lines of the outer and inner surfaces may be offset from one another. Such pattern may enhance the radial distribution of light toward the culture chamber.

[0089] The tubular external structure 31 is then obtained by a rotation of the lines about the longitudinal axis of the tube.

[0090] If the pattern is a line, the tubular external structure 31 is a right cylinder.

[0091] The tubular external structure 31 may also be a tube with a cross-section which defines a polygon, for example a hexagon. In this case, the tubular external structure comprises facets participating in the efficient distribution of the light to the outside.

[0092] The tubular external structure 31 may also comprise protrusions randomly distributed all over its surface or homogeneously arranged. The protrusions protrude from the outer surface of the tubular external structure 31 towards the outside thereof. That is to say that the protrusions are within the tank, in contact with the culture medium and the photo-reactive microorganism. The protrusion may enhance the mixing of the photo-reactive microorganism and, if present, the bubbles. The protrusions may therefore decrease the time spend by the photo-reactive microorganism in darker regions.

[0093] The tubular external structure 31 may be made of one or more walls.ln one example, the tubular external structure 31 is made of a single wall. The single wall can be made of a material independently chosen from the list consisting of strengthened glass, poly(methyl methacrylate) (PMMA), fluorinated ethylene propylene, and polycarbonate (PC). Preferably, the single wall is made of strengthened glass and/or PC. The material can be an alloy of two or more of strengthened glass, poly(methyl methacrylate) (PMMA), fluorinated ethylene propylene, and polycarbonate (PC).

[0094] Such materials reduce adhesion of the photo-reactive microorganism 200 onto the outer surface of the tubular external structure 31 avoiding proliferation of the photo-reactive microorganism 200 thereon which would prevent the light from being efficiently distributed to the culture chamber. It makes it also easier to clean the outer surface of the tubular external structure 31.

[0095] The single wall may have a thickness of 2 to 8 mm. Alternatively or additionally, the ratio between the length of the tubular external structure and the thickness of the single wall may be 250 to 2500, preferably below 1500, still preferably below 1000.

[0096] To adjust the refraction index, the tubular external structure may be made of two walls which are coaxial to each other.

[0097] To adjust the index refraction of a tubular external structure comprising two walls, the two walls may be fixed with each other or vacuum may be made between the two walls.

[0098] Preferably, the two walls are made of different materials or a same material. [0099] Preferably, each of the walls of the tubular external structure is made of a material independently chosen from the list consisting of strengthened glass, poly(methyl methacrylate) (PMMA), fluorinated ethylene propylene (FEP) and polycarbonate (PC), preferably PMMA.

[0100] The length of the cylinder is configured for industrial culture of photo-reactive microorganism 200. The inside volume of the tubular external structure is preferably comprised between 0.05 m ³ and 5 m ³ , preferably between 0.5 and 1 m ³ .

[0101] The tubular external structure 31 and the liquid are preferably chosen so that the assembly they form are not configured to float in water or medium for cultivating photo-reactive microorganism 200. The material is preferably nontoxic for the photo reactive microorganism 200 culture.

[0102] Additionally, one or more coatings or films can be applied to the wall(s) of the tubular external structure 31 to provide it with further properties. For example, a light reflective coating or film can be applied to at least part or whole of a wall of the tubular external structure 31. It is also possible to apply such coatings or films to all walls of the tubular external structure. The same or different coatings or films can be applied to the walls.

[0103] The LP 30 comprises a light inlet that makes the tubular external structure couplable to a light source.

[0104] The light inlet may be a single opening at one end of the tubular external structure 31. The light inlet may be an opening made on a side of the tubular external structure 31. The light inlet may also be an opening part of the plane perpendicular to the longitudinal axis of the tubular external structure 31, while being at its outermost end.

[0105] The light inlet may also comprise lens or optical system that enables to concentrate the light from an external light source 35 into the interior of the tubular external structure 31.

[0106] The light inlet may also have a connector that would allow an external light source 35 to be attached into it, for example by screwing or clipping.

[0107] The tubular external structure 31 may be closed at one end and comprises a light inlet at the other end.

[0108] If the concentration of photo-reactive microorganism 200 is too high, it may be advantageous to use two light inlets per

LP 30, each at one end of the LP 30. Both light inlets may be coupled to a single light source 35 or each light inlet may be coupled to its own light source 35.

[0109] Fig. 5 is a graph showing the light distribution profile for a 5m long LP 30 with one single light source 35 being switched on at 100% (curve A), with two light sources 35 being switched on at 50% (curve B), and two light sources 35 being switched on at 100% (curve C). The intensity is expressed in pE.m Ts ^·1 , measured at the outer surface of the tubular external structure 31 of the LP

30.

[0110] Curve A has a minimum value of 700 pE.mAs- ¹ at the end with the light source turned off, and a maximum value of 2200 pE.nr ² .s- ¹ at the end with the light source turned on. The decrease from the maximum value to the minimum value is not linear. The LP 30 is, however, able to deliver a predetermined amount of light necessary for the growth of the photo-reactive microorganism. The different between the maximum of intensity and the minimum of intensity, normalized by the maximum of intensity, is 67 %.

[0111] Curve B has a minimum value of approximately 1220 pE.nr ² .s- ¹ at the middle of the length of the tubular external structure 31, and a maximum value of approximately 1440 pE.nr ² .s- ¹ at both ends thereof. The different between the maximum of intensity and the minimum of intensity, normalized by the maximum of intensity, is 15 %. [0112] Curve C has a minimum value of approximately 2440 pE.rrv ² .s- ¹ at the middle of the length of the tubular external structure 31, and a maximum value of approximately 2800 pE.nr ² .s- ¹ at both ends thereof. The different between the maximum of intensity and the minimum of intensity, normalized by the maximum of intensity, is 14 %.

[0113] As it can be seen in Fig. 5, when both external light sources are switch on, the light at the ends of the tubular external structure 31 may reach almost 3000 pE.mTs-L At the middle of the tubular external structure 31, the light reaches almost 2700 pE.nr ² .s- ¹ .

[0114] Further, it can be seen that the different between the maximum of intensity and the minimum of intensity is higher when only one light source is switched on than when two light sources at opposite end of the LP 30 are switched on.

[0115] A light pipe 30 that has two or more light sources may decrease the amount of light needed at each end of the tubular external structure 31. Indeed, the power of the light source needs not be too high in order for the light to reach all the length of the LP 30 when there is a light inlet provided at both ends of thereof. If only one light inlet is provided, in order for the light to reach the other end of the LP 30, the power of the light source must be increase, which increases heat generation at the end where the light inlet is provided.

[0116] Alternatively or additionally, the LP 30 may comprise a plurality of light inlets placed along the tubular external structure

31.

[0117] The LP 30 may also comprise a light sensor to sense the amount of light inside the LP 30. The LP 30 may also comprise a light sensor movable inside the tubular external structure 31. The LP 30 may comprise a light sensor fixed to the external surface of the tubular external structure 31.

Lightpipe grid 300 (LPG)

[0118] Another aspect of the present invention is a light pipe grid 300 (LPG) comprising a plurality of LP 30 as described above with at least one light source. Such LPG 300 will be described in more details hereafter with reference to Fig. 3 and Fig. 6A to 6I.

[0119] With respect to the light source 35, one light source 35 may be provided for each light inlet of the LP 30 of the LPG 300, or one light source 35 may be provided for a group of light inlets of the LP 30 of the LPG 300. For example, a light source 35 may be provided for each LP 30 irrespectively of the number of light inlets. In another example, a light source 35 may be provided to each inlet of a LP 30. A light source 35 can also be provided for a group of LP 30. However, the preferred embodiment provides a light source 35 for each light inlets of the LPG 300. Indeed, the more light source 35 the more flexible the LPG 300.

[0120] The light source 35 is preferably chosen from the group consisting of a LED, a laser, a xenon lamp, and a halogen lamp, preferably a LED. Using LED makes it possible to control the light emission spectrum of the light source 35 in addition to its intensity, this makes it possible to scale the LPG 300 for any size of photo-bioreactor.

[0121] The light source 35 may be internal or external to the external tubular structure 31. In particular, when the light inlets are provided at ends of the LP 30, the light source 35 may be internal or external, with external being preferred. When the light inlets are provided along the length of the external tubular structure 31, the light source 35 are preferably internal to the external tubular structure 31.

[0122] The light source 35 may also comprise a passive or active heat management system. An example of passive heat management systems is a radiator. An example of active heat management systems is an air cooler. Also, a fan can be used and placed close to the light source to cool them. [0123] The light source 35 may also be connected to a light concentrator, notably when the light source 35 is external to the external tubular structure 31. One end of the concentrator is fixed to the source of light 35 while the other end is fixed to the light inlet of the LP 30. The concentrator may be a funnel made from mirrors or metal sheets that guide the light from the light source 35 with one end of the tubular external structure 31 though the light inlet.

[0124] When external, the light source 35 may be distant from the light inlet so that the heat produces by the external light source 35 is managed. For example, the concentrator may be sufficiently long so that the external light source 35 and the upper surface of liquid are distanced of at least 5 cm.s

[0125] As depicted in Fig. 3, preferably, the longitudinal axes of all LP 30 within the LPG 300 are all parallel to one another. They may be positioned so that their lengths are vertical or horizontal.

[0126] The LP 30 of the LPG 300 may be homogeneously arranged. For example, as shown in Fig. 6A to 6C, the LP 30 of the LPG 300 may be arranged in concentric circles, i.e. their cross-sections form together circles which are concentric with the center of the LPG 300. If n is the radius of the smallest circle, the radius of the n ^th circle counted form the center may be r _n = nxn.

[0127] As can be seen in Fig. 6A, in one example, seven LP 30 are evenly distributed so that one LP 30 is at the center and six LP 30 are evenly distributed in a circle Ci concentric with the one at the center. The distribution of the LP 30 has therefore a rotational symmetry of 60°. This example can also be seen as a homogenous triangular distribution (see Fig. 6D).

[0128] As can be seen in Fig. 6B, in another example, thirteen LP 30 are distributed so that five are evenly placed in a first smaller circle Ci concentric with the center of the LPG 300 and eight are evenly placed in a second bigger circle C2 concentric with the center of the LPG 300.

[0129] As can be seen in Fig. 6C, in still another example, there are thirty-four LP 30 distributed over three circles concentric with each other and with an LP 30 placed at the center of the LPG 300. Six LP 30 are evenly placed in a first circle Ci, twelve LP 30 are evenly placed in a second circle C2 greater than the first circle Ci and eighteen LP 30 are evenly placed in a third circle C3 greater than the second circle C2. The radius of each circle is n < G2 = 2xn < r3 = 3xn.

[0130] The LP 30 of the LPG 300 may also be arranged in a triangular pattern as illustrated by Fig. 6D to 6F with smallest unit labelled MT. Fig. 6D is the same as Fig. 6A. Fig. 6E shows thirteen LP 30 distributed so that one is in the center of the LPG 300, six LP 30 surrounds the central one forming a smaller hexagon Hi and six other LP 30 surrounds smaller hexagon Hi by forming a bigger hexagon H2. Fig. 6F shows thirty-seven LP 30 distributed so that one is in the center of the LPG 300 and the other form three concentric hexagons Hi, H2 and H3. In each of these embodiments, each LP 30 is separated from its closest neighbors by a distance which is the same for all LP 30 and their closest neighbors.

[0131] The LP 30 of the LPG 300 may also be arranged in a square pattern as illustrated by Fig. 6D to 6F with smallest unit labelled Me. In Fig. 6G, there are six LP 30 distributed over two “rows” and three “columns”. In Fig. 6H, there are ten LP 30 distributed over four “rows” and three “columns”, the last row having only one LP 30. Fig. 6I shows twenty-nine LP 30 distributed over five rows and five columns to form a square and the middle row and middle column having two additional LP 30, one at each side of the square.

[0132] The choice of composition of the liquid filling each tubular external structure 31 of the LPG 300 may be function of the radial position of the LP 30 within the LPG 300. [0133] Alternatively or in addition, the choice of material for each tubular external structure 31 may be a function of the radial position of the LP 30 within the LPG 300.

[0134] The LPG 300 may comprise a controller 40 configured to control the light distribution pattern of the LPG 300 by controlling groups of LP 30 individually and independently from each other. For example, each concentric circle or hexagon formed by the LP 30 is controlled individually and independently from the other concentric circle or hexagon. In another example, each row or column formed by the LP 30 is controlled individually and independently from the other row or column. Alternatively, the controller 40 may be configured to control the light distribution pattern by controlling each one of the LP 30 individually and independently from each other. The controller 40 may be configured to control the LP 30 by groups and each LP 30 individually giving the choice to the operator.

[0135] The controller may be configured to control the light emission pattern of the LPG 300 by changing, for each LP 30 or group of LP 30, at least one of: the light properties of a corresponding light source 35, the flow of the liquid through the tubular external structure 31, and the flow of gas through the tubular external structure 31.

[0136] The controller 40 may be configured to control the light properties of the corresponding light source(s) 35 by changing at least one of its (their) light emission spectrum and intensity. In such case, the preferred light source 35 is LED which makes it possible to scale the LPG 300 to any size of photo-bioreactor.

[0137] The controller 40 may be configured to control the flow of the liquid through the tubular external structure 31 by changing at least one of: the liquid composition and the liquid flow rate. For example, with regard to the flow rate, the controller 40 may act on a valve at the liquid inlet 36. With regard to the composition, the controller may act on valves placed between reservoirs and the liquid inlet 36. The valves may be ON/OFF valves with only two positions, a closed position and an open position. Alternatively, the valves may be gradual valves enabling adjustment of the size of the opening from a closed position to a fully open position. A combination of both types can also be used. For example, the valves are electro-valves.

[0138] The controller 40 may be configured to control the flow of gas through the tubular external structure 31 by changing at least one of: the gas composition and the gas flow rate. For example, with regard to the flow rate, the controller 40 may act on a valve at the gas inlet 38. With regard to the composition, the controller may act on valves placed between reservoirs and the gas inlet 38. The valves may be ON/OFF valves with only two positions, a closed position and an open position. Alternatively, the valves may be gradual valves enabling adjustment of the size of the opening from a closed position to a fully open position. A combination of both types can also be used. For example, the valves are electro-valves.

Photo-bioreactor (PBR)

[0139] According to another object of the present invention, the inventors have developed a photo-bioreactor comprising:

- a tank 20 with a gas inlet 50 and a culture volume comprising a culture medium 21 and at least one species of photo-reactive microorganism 200;

- a LPG 300 as described above provided inside the tank 20.

[0140] The inventors have demonstrated that such a PBR can reach an average light intensity of at least 100 pEnr ² s- ¹ to 2000 pErrv ² s- ¹ , with a biomass concentration of 1 g/L to 5 g/L. Such results show that the PBR of the present invention can be used for an industrial production of photo-reactive microorganism. [0141] Fig. 1 is a longitudinal view of a photo-bioreactor 1 according to one embodiment of the present invention. The PBR 1 comprises a tank 20, a plurality of LPG 300 and external light source 35. The tank 20 is full of a culture medium 21. The culture medium 21 is a specific medium that enables the photo-reactive microorganism 200 to growth.

[0142] The length of the LP 30 of the LPG 300 may be the same as the one of the tank 20. For example, the tank 20 and the tubular external structures 31 have a length of at least 3m, at least 4m, or at least 5m.

[0143] Each LP 30 may be fully immerged in the tank 20 or partially immerged. Preferably, the tubular external structure 31 of the LP 30 is fully immerged so that the upper surface of the culture medium 21 in the tank 20 almost reaches the upper surface of the tubular external structure 31. In such an embodiment, when the light source 35 is external, it is distant to the culture medium 21. Heat produced by the external light source 35 is not conducted into the culture medium 21.

[0144] Preferably, the LPG 300 is centered with respect to the tank 20 of the PBR 1. The lengths of the LP 30 may be parallel to the longitudinal axis of the tank 20 or perpendicular thereto.

[0145] The number of LP 30 in the LPG 300 may be chosen according to the capacity of the tank 20. Too many LP 30 would decrease the volume of cultivated photo-reactive microorganism but enhance the amount of light in the culture medium 21. On the contrary, too few LP 30 would increase the volume of cultivated photo-reactive microorganism but decrease the efficiency of light all over the culture medium 21 because they would be too far from each other so that the decrease of light intensity of one LP 30 cannot be compensated by the light intensity of an adjacent LP 30.

[0146] Preferably, the ratio of the illuminating surface area of the at least one tubular external structure 31 of the LP 30, over the culture volume is comprised between 2 and 15 nr ¹ and preferably between 5 and 13 nr ¹ .

[0147] The LP 30 may be independently operated. For example, when homogeneously distributed in circles (cf. Fig. 6) only LP 30 in the center may be operated while the LP 30 on the edge are switch off.

[0148] Preferably, the tank 20 has a cylindrical shape with a longitudinal axis and wherein the LP 30 has a cylindrical shape with a longitudinal axis, the longitudinal axis of the tank 20 and the longitudinal axis of the light pipe distributor 30 being collinear or perpendicular.

[0149] However, the shape of the tank 20 is absolutely not limited to a cylindrical shape. The tank 20 may be a three-dimension container defined by a base and a length. The base may be polygon, a circle or a random shape in order to optimize the mixing of the photo-reactive microorganisms 200 within the tank 20

[0150] The tank 20 may be open or closed. Preferably, the tank 20 is closed so that only light from the LP 30 is distributed to the photo-reactive microorganism 200. A closed tank 20 enables one operator to easily operate the different parameters of the PBR 1, i.e. amount of light, mixing, temperature, pH.

[0151] If the tank 20 is closed, the tank 20 comprises a cover. The light source 35, when external, may be fixed to the cover and be placed in order to face light inlet of the LP 30.

[0152] Also, the cover may have holes of the same dimension of the one of the tubular external structure 31 so that one extremity of the LP 30 is outside of the inside of the tank 20.

[0153] The cover may be made in a heat conducting material so that heat from external light source is dissipated.

[0154] The cover may further comprise a radiator to enhance the heat dissipation. [0155] If the tank 20 is open, apart from the possibility for the light inlet to be connected to an external artificial source of light, it is also possible to connect it with a light concentrator for concentrating natural light inside the LP 30.

[0156] The tank 20 is preferably in a non-transparent material. The tank 20 may comprise a window made of a transparent material so that it is possible to have a look of the inside of the tank 20 while it is not needed to remove the cover.

[0157] The tank 20 is preferably made from metal or rigid plastic.

[0158] Additionally, the tank 20 may also comprise stirrers and mixers to increase the mixing of the photo-reactive microorganism.

[0159] The gas inlet 50 makes it possible to bring gas inside the tank 20, which is necessary for the photo-reactive microorganism’s growth.

[0160] The tank 20 may further comprise at least one nozzle fluidly connected to the gas inlet 50. The nozzle enables to distribute gas from the gas inlet 50 to the photo-reactive microorganism 200.

[0161] The at least one nozzle is preferably placed at the bottom of the tank 20. It makes it possible to move about the photo reactive microorganism in the tank 20 thanks to the bubbles of injected gas moving upward from the bottom of the tank 20 so that each cell of photo-reactive microorganism 200 receives an efficient quantity of light.

[0162] Preferably, the gas inlet 50 comprises a plurality of gas nozzles uniformly distributed all over the bottom of the tank 20, so that the photo-reactive microorganism 200 are uniformly mixed by the gas bubbles 22 inside the culture medium 21 , as it can be seen in Fig. 1.

[0163] The gas inlet 50 may also comprise a regulator to be operated to increase or decrease the volume of gas. For example, it could be needed to increase the volume of gas when the photo-reactive microorganism concentration is high.

[0164] The gas inlet 50 may further comprise a valve that makes it possible to choose which nozzle to fluidly connect to the gas inlet 50.

[0165] Preferably, the culture medium 21 is chosen from the group consisting of river water, Arnon medium culture and a combination thereof

[0166] Preferably, the photo-reactive microorganism is a micro-algae is chosen from the group consisting of the following genera: Botryococcus, Chlorella, Phaeodactylum, Scenedesmus, Synechocystis, Ankistrodesmus, Scenedesmus, Synechococcus, Anabaena, Spirulina, Nostoc, and Calothrix, preferably Scenedesmus and Synechosystis.

[0167] The PBR 1 may also comprise at least one light sensor placed inside the tank 20 in the culture medium 21, for example at a point of the horizontal plane passing through the middle of the tank 20. The light sensor may also be movable so that a plurality of zones inside the tank 20 may be measured.

[0168] The PBR 1 may further comprise sensors for other parameters, such as temperature, pH or concentration of a specific chemical compound.

Method for producing photo-reactive microorganism

[0169] According to another object of the invention, the inventors have developed a method for producing photo-reactive microorganism with a photo-bioreactor of the present disclosure, comprising: - connecting the gas inlet 50 of the PBR 1 to an industrial exhaust pipe,

- illuminating the photo-reactive microorganism through the LPG 300,

- collecting the cultivated photo-reactive microorganism when the photo-reactive microorganism concentration in the PBR reaches a predetermined value.

[0170] The method may further comprise regulating at least one of:

- the light pattern of the LPG 300 according to the photo-reactive microorganism concentration inside the tank; and

- the gas flowrate and gas holdup within the tank according to the photo-reactive microorganism concentration inside the tank.

[0171] Regulating the light pattern of the LPG 300 may further comprise regulating, for part or all LP 30 of the LPG 300, the light properties of the corresponding light source 35 according to the concentration of the photo-reactive microorganism 200 inside the tank 20. For example, the light emission spectrum may be regulated, notably by increasing intensity of one wavelength or one waveband and/or diminishing intensity of one wavelength or one waveband of the light source 25. Thus, this makes it possible to match the emission spectrum with the culture stage of the photo-reactive microorganism 200. For example, the light source spectrum is close to the solar spectrum, i.e. for each wavelength, the relative intensity (wavelength intensity over total intensity) is at least 85 % of the relative intensity of the solar spectrum. Also, the relative intensity of the light source in specific wavelength ranges can be adjusted.

[0172] Alternatively or in addition, intensity of the light source 35 may be regulated, notably through increase or decrease of the overall intensity of the light source 35.

[0173] Regulating the light pattern of the LPG 300 may further comprise regulating, for part or all LP 30 of the LPG 300, the flow of the liquid through the tubular external structure 31 according to the concentration of the photo-reactive microorganism 200 inside the tank 20. For example, the liquid composition may be regulated, notably by regulating the inflow of the components of the liquid. Alternatively or in addition, the liquid flow rate may be regulated by increasing or decreasing it.

[0174] Regulating the light pattern of the LPG 300 may further comprise regulating, for part or all LP 30 of the LPG 300, the flow of the gas through the tubular external structure 31 according to the concentration of the photo-reactive microorganism 200 inside the tank 20. For example, the gas flow rate may be regulated by increasing or decreasing it.

[0175] The method may further comprise regulating the temperature within the tank 20. Regulating the temperature within the tank 20 may comprise regulating the temperature of the culture medium 21, notably through injection of fresh culture medium 21 into the tank 20. Alternatively or in addition, regulating the temperature within the tank 20 may comprise regulating the temperature of the LP 30 of the LPG 300. For example, fresh liquid may be injected into the tubular external structure 31 of the LP 30 and/or bubbling of gas into the tubular external structure 31 of the LP 30 may be carried out.

[0176] The industrial exhaust pipe may be a chimney 1100 of a waste incinerator. Preferably, the gas of the industrial exhaust pipe is a C02-rich gas.

[0177] As for the source of power for the light source 35, an industrial heat exhaust coming from the same industrial facility as the industrial exhaust pipe or a different industrial facility may be used and coupled to the LPG 300 to provide energy through a fatal heat recovery process. [0178] The concentration of the photo-reactive microorganism 200 inside the tank 20 is advantageously maintained between 0.2 and 10 g/L, preferably 0.5 and 6 g/L, preferably 1 and 5 g/L, preferably 1 and 3.5 g/L.

[0179] Fig. 7 is a scheme of a system comprising an industrial facility (1000), for example a waste incinerator, and a photo bioreactor 1200 coupled to the industrial chimney 1100 of the industrial facility 1000

[0180] According to another object of the present disclosure, the inventors have developed a system comprising a photobioreactor and an industrial facility 1000 comprising an exhaust pipe or chimney 1100, wherein the gas inlet 50 of the photobioreactor is coupled to the exhaust pipe or chimney 1100

[0181] The system has the advantage of using the exhaust fumes coming from the industrial chimney 1100. The system also has the advantage of cultivating high added value photo-reactive microorganism that can be used in a lot of different fields, such as bio energy.

[0182] The gas exhausted by the chimney 1100 may contain gas that is consumed by the photo-reactive microorganism, such as C02. While the gas is treated and the bad gas for the atmosphere is consumed, the photo-reactive microorganisms are cultivated and the concentration in the tank 20 increases. When the concentration of photo-reactive microorganism in the tank 20 reaches a predetermined value, the photo-reactive microorganisms are collected and transformed, for example in bio energy. Thus, the photoreactive microorganism collected thanks to the system of the present invention are of high added value.

[0183] The gas of the industrial exhaust pipe may be treated before it enters the PBR via the gas inlet 50. For example, the gas of the industrial exhaust pipe is doped with a gas compound useful for the photo-reactive microorganism culture. For example, the gas of the industrial exhaust pipe is doped with CO2 before it enters the gas inlet 50.

[0184] The gas inlet 50 may be coupled to a valve. The valve enables to control the quantity of gas to be injected inside the tank 20. Also, the quantity of gas injected inside the tank 20 can be continuously controlled according to the concentration of the photoreactive microorganism inside the tank 20

[0185] The culture of the photo-reactive microorganism in the PBR may be a continuous culture. In that case, the concentration of the photo-reactive microorganism inside the tank 20 is maintained between 0.2 and 10 g/L, preferably between 1 and 5 g/L.

[0186] The culture of the photo-reactive microorganism may also be in batches. In that case, each time the concentration of the photo-reactive microorganism in the tank 20 reaches a predetermined value. The culture is stopped and the photo-reactive microorganism are collected.

[0187] The illumination of the photo-reactive microorganism may be continuous or discontinuous. For example, the LP 30 may be operated so that they illuminate the photo-reactive microorganism for a predetermined time and turns off for another predetermined time. Also, the way of illuminating the photo-reactive microorganism could follow a predetermined pattern of ON/OFF periods and of different light powers. For example, when the concentration of photo-reactive microorganism is low, the LP 30 are operated at high low power for a predetermined time. Then, the light power increases while the concentration of photo-reactive microorganism increases.

[0188] Also, the concentration of photo-reactive microorganism in the tank 20 could be inhomogeneous. In that case, and if the tank 20 comprises a plurality of LP 30, it could be advantageous to increase the illumination in the tank 20 where the concentration is high, or to decrease the illumination where the concentration is too low. Each of the external sources of light would be therefore operated according to the concentration of photo-reactive microorganism in the tank 20 close to them. [0189] When switched on, the external light source emits light that is conducted from the light inlet to the top and bottom of the LP 30 through the liquid and all over the tubular external structure 31 of the LP 30. As the light went from the upper to the lower end of the LP 30, the light is also radially distributed in the culture medium 21 to the photo-reactive microorganism 200.

[0190] The light source 35 may be further configured to be operated in order to change its emission spectrum, thus making it possible to match the emission spectrum with the culture stage of the photo-reactive microorganism. For example, the light source spectrum is close to the solar spectrum, i.e. for each wavelength, the relative intensity (wavelength intensity over total intensity) is at least 85 % of the relative intensity of the solar spectrum. Also, the relative intensity of the light source in specific wavelength ranges can be adjusted.

[0191] Each external source of light 35 may be independently operated. For example, the concentration of photo-reactive microorganism in the lower part of the tank 20 can be higher than that in the upper part. In that case, it may be beneficial to enhance the illumination of the photo-reactive microorganism in the lower part in comparison with the upper part. Thus, the light power of the light external source is in the lower end of the LP 30 can be increased.

[0192] The system may further comprise a heat exhaust coupled to the LPG 300 of the PBR 1 to provide energy to the light source(s) 35 through a fatal heat recovery process during operation, the heat exhaust being part of the same industrial facility as the gas exhaust chimney 1000 or another industrial facility

[0193] The collected photo-reactive microorganism 1300 may be transformed in order to make bio plastic, bio energy, food supplements, fish or pet food, bio cosmetic, medicine or all of the potential use corresponding to the specific cultivated photo reactive microorganism.

Example [0194] Fig. 4A, 4B and 4C are three graphs showing the propagation of light from the tubular external structure 31 of a LP 30 in the culture medium 21 for three photo-reactive microorganism concentrations in the tank 20: respectively 1.5 g/L, 3 g/L and 4.5 g/L.

[0195] According to this embodiment, the LP 30 is a cylinder of 5m, both ends of the cylinder have a light source 35 that makes the light profile in the culture medium 21 homogeneous. The sources of light may be operated accordingly to the concentration of photo-reactive microorganism 200, the more the concentration of photo-reactive microorganism. [0196] As it can be seen in Fig. 4A, 4B and 4C, the higher the concentration of photo-reactive microorganism in the tank 20, the lower the propagation of light in the culture medium 21 is.