WO2013079948A1

WO2013079948A1 - Photobioreactor

Info

Publication number: WO2013079948A1
Application number: PCT/GB2012/052951
Authority: WO
Inventors: John Douglas Mckenzie
Original assignee: Xanthella Limited
Priority date: 2011-11-29
Filing date: 2012-11-29
Publication date: 2013-06-06
Also published as: GB2511963A; GB201120550D0; GB201410312D0

Abstract

A photobioreactor is presented comprising a chamber for retaining aqueous media and a tubular circulation element positioned within the chamber, the circulation element defining a fluid circulation pathway within the chamber, wherein the tubular circulation element comprises a wall element which in turn comprises either or both a light source and/or a light guide for illuminating the chamber. The provision of a light source and/or light guide within a circulation element of a photobioreactor allows a larger volume of algae to be grown more cost effectively than photobioreactors in the art. In addition, a method of assembly of a circulation element for use within a photobioreactor is provided.

Description

PHOTOBIOREACTOR Field of the invention The invention relates to the field of photobioreactors, more specifically to algal photobioreactors. Background to the invention The supply of fossil fuels such as coal, crude oil and natural gas is of increasing concern. Demand is high and growing prompting concerns over the ability of existing reserves to match this demand. At the same time, concerns over the impact of carbon dioxide production on the world's climate through the burning of fossil fuels are creating a pressure to reduce the consumption of fossil fuels. One alternative to fossil-fuels that has shown promise is the production of oil from the cultivation of algae. Algae can be grown in water held in ponds or in bioreactors and fed with light, nutrients and carbon dioxide. Once the concentration of algae reaches a determined point, the biomass of the algae can be harvested and processed to yield oil from which fuels can be produced. One crucial limitation on the cultivation of algae is that of light penetration into the algae/water mixture. As algal densities increase, the turbidity of the water increases, commonly limiting the penetration depth of light to a few centimetres beyond which the algae near the surface of the light path shade those below, preventing them from photosynthesising. The length of the light path is thus critical to the operation of the bioreactor or pond. In order to produce oil on a large scale, algae is often cultivated in open, shallow ponds to allow sunlight to penetrate the water to the bottom of the ponds for the algae throughout the pond to absorb and photosynthesise. Ponds have the advantages of being simple and of relatively low capital cost. However, the open nature of these ponds results in them often being fouled by contaminants and they would be unsuitable for use with genetically modified algae because of the risk of these GMOs escaping into the wider environment. Evaporative losses may make them expensive to run where water is at a premium and they also require large areas of land that might otherwise be put to other uses, such as food production. Ponds have a poor efficiency footprint and the scope further technological advancement to improve them seems limited. The limitations of ponds has led to investigations of how enclosed bioreactors can be used so as to be able to produce the large amounts of algal biomass necessary for fuel production. Enclosed bioreactors offer solutions to the problems that afflict pond systems and growth conditions for the algae can be optimised. Bioreactors can achieve better growth rates than ponds making more efficient production of the algae possible. The main drawback of bioreactors is, however, the capital costs of the bioreactors compared with those of ponds. Bioreactors, however, offer much more scope for technological improvements to reduce costs than is the case for ponds. Bioreactors for growing algae under photosynthetic conditions are commonly termed photobioreactors (PBR). The water and growth media for the algae is held within the PBR housing and a source of carbon dioxide is passed through the algae/water mixture. Light is typically shone into the mixture from external light sources which may be artificial or direct sunlight. In externally lit bioreactors the usual solution is to give the bioreactor a large surface area to volume ratio (typically by having flat panels or thin tubes). To produce large volumes of algae these bioreactors need to be large to accommodate the large surface areas necessary for them to gather sufficient light. This makes such reactors very expensive. Bioreactors with high surface area to volume ratios can also suffer problems with photo-inhibition at the surface of the light path and over-heating. This often requires the algae/water mix to be pumped around the PBR which also raises the capital and operating costs of this type of PBR. An alternative approach is to have the lighting elements within the PBR. The external shape of the PBR housing becomes unimportant in an internally lit PBR, allowing the use of large, simple-shaped PBR housings with low surface area to volume areas. This considerably reduces the cost of the PBR housing. Light distribution within the PBR is still limited by the need for short path lengths but light transfer can be made much more efficient than is the case for externally lit PBRs. The balance of costs moves from the PBR housing to the light distribution apparatus. Accordingly, one aspect of the present invention is aimed to providing an improved, internally lit PBR that will be more cost effective than existing internally or externally lit PBR designs. Summary of the invention According to a first aspect of the invention there is provided a photobioreactor (PBR) comprising a chamber for retaining aqueous media and a tubular circulation element positioned within the chamber; the tubular circulation element comprising a wall element having an interior and an exterior and defining a fluid circulation pathway within the chamber, characterised in that the wall element comprises either or both a light source and/or a light guide for illuminating the chamber. Typically PBRs provide a means for introducing gas (generally carbon dioxide) into the aqueous media within the chamber (typically, at the base of the chamber) and to illuminate the aqueous media from external light sources. The turbidity of the aqueous media typically increases significantly during the cultivation of algae, such that the penetration depth of light into the aqueous media is severely limited. For example, the penetration depth in a dense algal culture may commonly be no more than 5 cm. Illuminating the PBR from external light sources has the effect that the volume of the PBR is limited by the short light path if the algae within the entirety of the PBR are to absorb light and photosynthesise efficiently. The provision of a tubular circulation element comprising a light source and/or light guide mounted within the PBR allows the dimensions of the PBR to exceed those of the penetration depth of the light, allowing a larger volume of algae to be cultivated per PBR thereby reducing costs per unit of oil produced from the algal biomass. The fluid circulation pathway defined by the tubular circulation element may extend in a loop through the interior of the wall element in a first direction, and then past the exterior of the wall element in a second direction, wherein the second direction is generally opposed to the first direction. For example, during use, the length of the tubular circulation element is typically oriented vertically such that gas released at the base of the tube rises up through it, creating an area of reduced pressure within the tube. This causes the aqueous media surrounding the tubular circulation element to enter the tube at its base then rise up the length of the interior of the tubular circulation element. The aqueous media exiting the top of the tubular circulation element then travels down the external surface of the tubular circulation element to once more enter into the tubular circulation element and thus completing the circulation cell. The volume, velocity and bubble size of the gas release at the base of the tubular element dictates the circulation rate of the fluid within the cell. The chamber may comprise a gas outlet that may release gas into the aqueous media of the chamber as a stream of bubbles. The chamber may be elongate. The chamber may be cylindrical. The chamber may have a height to width ratio of at least 3: 1 , at least 4: 1 or preferably, at least 5: 1. The provision of an elongate chamber maximises the part of the fluid circulation pathway passing through the tubular circulation element. The provision of a fluid circulation pathway extending in a loop defined by a tubular circulation element that also illuminates the aqueous media flowing through it allows a volume of aqueous media that is larger than the volume retained within the interior of the tubular circulation element to be periodically illuminated by the tubular circulation element, thereby providing for a PBR with external dimensions exceeding the penetration depth of light through the aqueous media. The tubular circulation element has a first end and an opposed second end. The tubular circulation element may have a first face which defines the interior of the tubular circulation element and a parallel second face which defines the exterior of the tubular circulation element. The tubular circulation element may be cylindrical. Preferably, the wall element comprises a tube and an illumination element, comprising the either or both a light source and /or a light guide, within the tube. Therefore, light from the light source and/or light guide will be directed into the interior of the tubular circulation element. The tube is typically opaque. The tube may have a reflective (e.g. white) internal surface to reflect light from the illumination element into the interior of the tubular circulation element In this embodiment, the illumination element may be retained within the tube. The tubular circulation element may be reconfigured by removing the illumination element from the tube, and replacing the illumination element such that it surrounds the tube such that the exterior of the tubular circulation element is defined by the illumination element and the interior of the tubular circulation element is defined by the tube. Alternatively, the tube may be opaque. Typically, in this embodiment the illumination element is retained within the tube such that the interior of the tubular circulation element is defined by the illumination element and the exterior of the tubular circulation element is defined by the tube. The light source and/or the light guide may illuminate the interior of the tubular circulation element. The light source and/or the light guide may illuminate the exterior of the tubular circulation element. The illumination element may comprise or be a flexible sheet. The flexible sheet may be sufficiently flexible to form a cylinder. The flexible sheet may be rolled into a cylinder. The flexible sheet may be rolled into a cylinder and placed within the tube. The flexible sheet may be retained within the tube by flexural tension in the flexible sheet. The diameter of the cylinder may be constrained by the wall of the tube. The flexible sheet may be rolled into a cylinder such that at least a part of the flexible sheet on a first side overlaps an opposed second side. The flexible sheet may overlap by at least 5%, at least 10% or at least 20%. The provision of an overlap ensures that there is no gap along the length of the tubular circulation element. The flexible sheet rolled into a cylinder, or the tube, may be mounted on a mounting within the chamber. The cross section of a cylinder so formed may be defined or constrained solely by the mounting such that overlap of the flexible sheet of the tubular circulation element is not required to be attached, bonded or sealed together. The provision of a an illumination element comprising a flexible sheet that may be rolled into a cylinder and that does not require to be bonded or sealed, for example, allows the tubular circulation element to be formed quickly and at a lower cost, than for tubular circulation elements requiring opposed edges of the flexible sheet to be bonded or sealed together, for example. In embodiments where the wall element comprises a light source only, the light source may extend across the majority of the surface area of the interior of the wall element. In embodiments where the wall element comprises a flexible sheet, the light source may extend across the majority of the surface area of the flexible sheet. The light source may be an electroluminescent material such as a light emitting polymer or an organic light emitting diode, for example. The wall element may comprise a plurality of light sources such as a plurality of light emitting diodes, for example. The plurality of light sources may be distributed across the surface of the interior of the wall element (e.g. on or in a said flexible sheet or other illumination element). The plurality of light sources may be mounted within the wall element. The plurality of light sources may be sealed within the wall element. At least one or all of the light sources within the plurality of light sources may be mounted in the plane of the wall element. At least one or all of the light sources within the plurality of light sources may be mounted normal to the plane of the wall element. At least one or all of the light sources within the plurality of light sources may be mounted at an angle to the normal of the interior surface of the wall element. Light emitted by at least one of or all of the light sources within the plurality of light sources may be incident to the surface of the interior of the wall element at an angle such that at least a fraction of the light is reflected from the surface of the interior of the wall element. At least one or all of the light sources within the plurality of light sources may be mounted within the wall element at an angle. The majority of light emitted by the or each light source within the plurality of light sources mounted at an angle may be totally internally reflected within the wall element such that the wall element acts as a light guide. In each case, the said angle may be at least 10° from normal, at least 20° from normal, or at least 30° from normal, for example. The provision of a wall element comprising a light source allows the interior to be controllably illuminated during operation. Therefore, exterior illumination is not required and external dimensions of the PBR are no longer limited by the penetration depth of the aqueous media. Accordingly, the volume of algal biomass held within by the PBR may be significantly increased. The plurality of light sources may form bands around the interior of the wall element (e.g. bands in or on a said illumination element). The bands may be spaced axially apart from each other such that the illumination of the interior of the tubular circulation element is non-uniform. The intensity of the illumination of the interior of the tubular circulation element may comprise one or more minima, the position each minima being between two maxima. Alternatively, the bands may be spaced evenly along the length of the interior of the tubular circulation element such that the interior of the tubular circulation element is illuminated substantially evenly along at least the majority (for example, at least 90%) or all of its length when the bands are emitting light. However, it may be that only some of the bands emit light at a given time such that the intensity of the illumination of the interior of the tubular circulation element may comprise one or more minima, the or each minima being between two maxima. Typically, algae require a rest period for efficient growth, whereby they are not illuminated. These "dark periods" may be automatically provided by a PBR comprising distinct bands of light sources, wherein only alternate distinct bands are activated at any one time, for example. The "dark periods" may also be supplied when the algae are external to the illuminated circulation element. The intensity and wavelength of the light delivered to the algae may be varied by altering the lighting elements used within the PBR or by altering the power supplied to the overall light delivery system or by selectively using only parts of the light delivery system or a combination of any or all of these elements. The PBR may have one or more light sensors within the illuminated portion that can detect changing light levels caused by changes in the density of the algae. The sensors may then be used to automatically alter the illumination so that the algae are kept within their optimal lighting conditions and the PBR operates in an optimal fashion when the algal density increases, for example. The intensity of the light delivered to the algae may be pulsed. The intensity of the light delivered to the algae may be varied in a daylight cycle. The PBR may comprise a heater. The heater may be located within the interior of the tubular circulation element. The heater may be located exterior to the tubular circulation element. The aqueous media within the interior of the tubular circulation element may be heated by the heater. The aqueous media surrounding the tubular circulation element may be heated by the heater. The aqueous media within the interior of the tubular circulation element and surrounding the tubular circulation element may be heated by the heater. The wall element may define at least part of the walls of the tubular circulation element. Accordingly, the wall element may define at least a part of a tube with a first end adjacent to the first end of the tubular circulation element and a second end adjacent to the second end of the tubular circulation element. Preferably, the wall element defines at least the majority of the tubular circulation element. For example, in embodiments where the tubular circulation element is cylindrical, the wall element may define the majority of the walls of the cylinder of the tubular circulation element. The wall element may extend continuously around the majority of the periphery of the tubular circulation element. The wall element may extend continuously around the periphery of the tubular circulation element. The light guide may extend continuously around the majority of the periphery of the wall element. The wall element may comprise a light guide for transmitting light into the chamber (typically to the interior of the tubular circulation element). The wall element may comprise a light guide and at least one light source, arranged such that the light from the at least one light source is transmitted through the light guide. The wall element may comprise an array of light sources. Each of the light sources within the array of light sources may be a light emitting diode (LED). The array of light sources may be arranged along the length of the wall element. For example, the array of light sources may extend in a line or a plurality of lines, along the length of the wall element such that the light emitted from the array of light sources is transmitted around the periphery of the wall element by the light guide. In embodiments where the array of light sources extend in two lines along the length of the wall element, the two lines may be adjacent to each other and the first line may emit light in one direction around the periphery of the light guide and the second line may emit light in the other direction around the periphery of the light guide, such that the intensity of light within the light guide may be at a minimum at the portion of the light guide opposed to the two lines of light sources. The array of light sources may be arranged around the first or second end of the wall element such that the light emitted by the array of light sources is transmitted along the length of the wall element by the light guide. The array of light sources may be arranged across the surface of the wall element. The majority of the light emitted by the array of light sources may be directed into the interior of the tubular circulation element. The wall element may comprise a first array of light sources arranged around the first end of the wall element and a second array of light sources arranged around the second end of the wall element. Light emitted by the first array of light sources and the second array of light sources may be transmitted by the light guide along the length of the wall element. The light guide may comprise a light guiding material. The light guide may comprise a sheet of a light guiding material. The sheet may comprise a plurality of layers of a light-guiding material. The light guiding material or layers of light guiding material may be transparent polymer, and/or resilient polymer, for example. By a transparent material, we refer to a material through which visible and near- visible light is transmitted without being absorbed by the material to a significant degree. Some of the layers within the plurality of layers may have a different refractive index. For example, a layer with a high refractive index may be between two layers of lower refractive index. Each layer within the plurality of layers may have a different refractive index. Each layer within the plurality of layers may have the same refractive index. The light guide may be provided with a coating. The coating may be a membrane. The coating may be a surface layer. The coating may have a lower refractive index than that of the light guide. The coating may have a significantly lower refractive index than that of the light guide. The coating may be provided on the interior surface of the light guide. The coating may be provided on the exterior surface of the light guide. The coating may be provided on both the exterior surface and the interior surface of the light guide. The coating may be a continuous layer formed on the surface of the wall element. The coating may be a porous layer formed on the surface of the wall element, and may be a microporous polymer, such as a polyester (the resin polyethylene terephthalate, PET, for example) or microporous polytetrafluoroethylene (PTFE), for example. The pores within the porous layer may extend to the surface of the light guide. The porous layer may be hydrophobic such that the porous layer may trap air in the pores, further reducing the refractive index of the porous layer. The light guide may comprise a reflective coating such that light incident upon the reflective coating from within the light guide is reflected back into the light guide. The light guide may comprise a plurality of illumination features, that is to say, features that cause some light which would otherwise be reflected internally back into the light guide to escape from the light guide. The plurality of illumination features may be operable to allow light incident on the plurality of illumination features from within the light guide to be transmitted out of the light guide via the said plurality of illumination features. The plurality of illumination features may be located on the exterior of the light guide. Preferably, the plurality of illumination features is located on the interior of the light guide. In embodiments where the light guide comprises a plurality of light sources, one of the plurality of illumination features or some of the plurality of illumination features may be associated with (e.g. arranged to receive light from) one light source within the plurality of light sources. The majority of the illumination features in the plurality of illumination features may be associated with a different light source within the plurality of light sources. Each illumination feature in the plurality of illumination features may be associated with a separate light source within the plurality of light sources. The plurality of illumination features may comprise some illumination features which are associated with more than one of the plurality of light sources. Some of the plurality of illumination features may be associated with each respective light source within the plurality of light sources. The plurality of illumination features associated with each individual light source may be arranged in a graded fashion to improve the uniformity of illumination within the tubular circulation element. For example, the density of illumination features may increase the further away from each respective light source. At least one illumination feature of the plurality of illumination features may be a dispersive element. The at least one illumination feature of the plurality of illumination features may be a refractive element. At least one illumination feature of the plurality of illumination features may be a surface formation, such as a ridge raised on the surface of the light guide, for example. At least one illumination feature of the plurality of illumination features may comprise a feature printed onto the surface. At least one illumination feature of the plurality of illumination features may be etched or moulded onto the surface of the light guide. At least one illumination feature of the plurality of illumination features may comprise one or more particles adhered to the surface of the light guide. The one or more particles may be dielectric particles. At least one illumination feature of the plurality of illumination features may comprise a discrete area having a different optical density to that of the light guide. At least one illumination feature of the plurality of illumination features may comprise a discrete area having an optical density intermediate that of the light guide and the surrounding aqueous media. The plurality of illumination features may comprise a pattern of features. The features within the pattern of features may comprise a transparent material. The plurality of illumination features may comprise an interference pattern. The pattern of features may be printed in a textured ink on to the light guide. The textured ink may comprise particles that form an uneven surface once the textured ink has dried. The light guide may comprise an array of illumination features. The array of illumination features may be located on the exterior of the light guide. Preferably, the array of illumination features is located on the interior of the light guide. The distribution of the illumination features within the array of illumination features may be regular across the surface of the light guide. The distribution of the illumination features within the array of illumination features may be irregular across the surface of the light guide. The illumination features within the array of illumination features may be distributed over the surface of the light guide in a graded fashion. The density of illumination features within the array of illumination features may vary across the surface of the light guide. For example, the density of illumination features may be low on the surface of the light guide adjacent to the light source and increase with distance from the light source. Alternatively, there may be minima and maxima in the density of illumination features along the length of the light guide, resulting in an irregular or periodic illumination of the interior of the circulation element. Accordingly, in embodiments where the wall element comprises an array of light sources arranged around either the first or second end of the wall element, the density of illumination features present on the surface of the light guide may be low adjacent to the array of light sources and gradually increase along the length of the light guide to a maximum density at the end of the light guide opposed to the array of light sources. In embodiments where the wall element comprises an array of light sources arranged along the length of the wall element, the density of illumination features may be low adjacent to the array of light sources and gradually increase around the periphery of the light guide to a maximum at the point of the light guide furthest from the array of light sources. Within a light guide, the intensity of light within a given portion of the light guide diminishes the further away from the light source that given portion is. Therefore, the provision of an increasing density of illumination features along the length of the light guide allows the intensity of light escaping the light guide at any given point via the said illumination features to be approximately the same along the length of the light guide. That is, the intensity of light escaping the light guide adjacent to the light source (a high intensity of light escaping via a low number of illumination features) may be approximately equal to the intensity of light escaping the light guide furthest from the light source (a lower intensity of light escaping via a higher number of illumination features). The PBR may comprise a gas outlet located within the chamber (typically, at the base of the chamber). The PBR may comprise a gas conduit extending into the chamber and in fluid communication with the gas outlet. The gas conduit may be connected to the tubular circulation element. The gas conduit may extend along the length of the tubular circulation element. The gas conduit may be integral to the tubular circulation element. The gas conduit and wall element may define the periphery of the tubular circulation element. The gas outlet may be operable to release gas incident to the gas outlet via the gas conduit. The gas outlet may comprise a sparger. The gas outlet may comprise a ring. The gas outlet may comprise a nozzle. The gas outlet may be located adjacent to one of the ends of the tubular circulation element such that gas released by the gas outlet may rise through the interior of the tubular circulation element. The gas may be released in the form of bubbles. The gas rising through the tubular circulation element may induce fluid to flow along the fluid circulation pathway defined by the tubular circulation element. Preferably, the gas is carbon dioxide. The gas may be a carbon dioxide rich gas mixture, or the gas may be air. The rising gas may induce fluid to rise through the interior of the tubular circulation element. The induced flow of fluid through the tubular circulation element may draw fluid from the periphery into the region adjacent to the gas distribution means. The induced flow of fluid may create a fluid circulation of aqueous media along the fluid circulation pathway, in use. For example, fluid may rise through the interior of the tubular circulation element, radiate out above the tubular circulation element and sink adjacent to the exterior of the tubular circulation element. To produce a fluid circulation pathway within a body of water, water pumps or mechanical impellors are often employed. The use of mechanical force to produce flow in liquids is energetically expensive. Using the gas introduced into the PBR to induce flow within the PBR through passive utilisation of density differences allows good circulation and mass transfer to be effected more efficiently and thus at reduced cost. Alternatively, the fluid may rise through the interior of the tubular circulation element and sink adjacent to one region of the exterior of the tubular circulation element. For example, in embodiments where the tubular circulation element abuts a wall of the chamber, the fluid may sink adjacent to the region opposed to chamber wall. Accordingly, the PBR may be a gas uplift reactor, wherein aqueous media is circulated by flow of gas from gas outlet. In use, the aqueous media may comprise algae and minerals in water. The algae may be one or more of Chlorella vulgaris, Botryococcus braunii, Dunaliella tertiolecta and Scenedesmus TR-84. The minerals may comprise nutrients upon which the algae may feed. Typical media comprise distilled water; salts at concentrations designed to produce optimal osmolarity for the algal strains being cultured; nutrient sources such as nitrates, sulphates and phosphates; trace elements; vitamins and pH buffers. Alternatively natural sources of nutrition may be provided such as municipal waste water or waste water from industrial processes such as brewing and paper manufacture. Besides helping to feed the algae the algae help cleanse such waste streams of undesirable pollutants. The PBR may comprise a plurality of tubular circulation elements. The plurality of tubular circulation elements may be located within the same chamber, such that a plurality of fluid circulation pathways may be defined within the same chamber. Each tubular circulation element within the plurality of tubular circulation elements may have an associated gas outlet such that during use gas rises through the interior of each tubular circulation element within the plurality of tubular circulation elements. Accordingly, the chamber of such a PBR may be much larger than would be possible to illuminate with either a single tubular circulation element or with a standard PBR known in the art. Therefore, a greater volume of algae may be produced within one chamber, thereby making more efficient use of space than using a plurality of PBR's. The PBR may comprise a chassis. The chassis may support the tubular circulation element. The chassis may support the gas outlet. The chassis may support the tubular circulation element and the gas outlet. Preferably, the chassis is demountably retained in the chamber such that the chassis may be removed and then replaced within the chamber. In embodiments where the tubular circulation element and the gas outlet are supported by the chassis, removal of the chassis from the chamber also removes the tubular circulation element and the gas outlet from the chamber. In this way, the tubular circulation element and the gas outlet may be readily removed from the chamber for cleaning or replacing if malfunctioning, for example. In addition, the chamber itself may be readily cleaned once the chassis is removed. In embodiments where the chamber elongate and cylindrical, it is of particular benefit to provide a chassis that allows the tubular circulation element and gas outlet to be readily removed, as these components of the PBR would be otherwise difficult to access. Typically, the chassis extends along the majority of the length of the chamber. For example, in embodiments where the chamber is cylindrical, the chassis may extend along the majority of the length of the cylinder. The chassis may comprise a handle. Typically, the handle is located at one end of the chassis, such that when the chassis is installed in the chamber, the handle is located adjacent to the top of the chamber such that during use, the handle is located adjacent to or above the level of the retained aqueous media within the chamber. The chassis may be removed from the chamber in an upwards direction. The chassis may support electrical leads that provide electricity to the tubular circulation element. The chassis may support electrical leads that provide electricity to the gas outlet. The chassis may support a gas supply that provides gas to the gas outlet. The chassis may comprise a framework. In embodiments where the chassis supports the tubular circulation element, the framework may extend around the exterior of the tubular circulation element. The chassis may support a plurality of tubular circulation elements. The chassis may support a plurality of gas outlets. The chassis may support a plurality of tubular circulation elements and a plurality of gas outlets, where each of tubular circulation elements within the plurality of tubular circulation elements is associated with a gas outlet within the plurality of gas outlets. In this way, in embodiments where the chamber comprises a plurality of tubular circulation elements, the chassis may allow the removal of a plurality of tubular circulation element/gas outlet units at once from a chamber for cleaning or for replacing malfunctioning units. Accordingly, the provision of a chassis supporting a plurality of tubular circulation elements, a plurality of gas outlets, or both a plurality of tubular circulation elements and a plurality of gas outlets, allows faster and more efficient access to the components of PBR resulting in a more efficient PBR. The PBR may be coupled to an external light delivery apparatus. The external light delivery apparatus may comprise a light source and an external light guide. The external light guide may be operable to direct light produced by the light source to the light guide of the PBR, for example. The light source of the external light delivery apparatus may be a sunlight collecting apparatus. The light source of the external light delivery apparatus may be an artificial light source, such as a light emitting diode (LED), or a light filament, for example. According to a second aspect of the invention, there is provided a method of manufacturing a tubular circulation element for a photobioreactor, comprising the steps of;

providing a planar element, having either or both of a light guide and/or light source, and a gas conduit, the planar element extending between substantially parallel first and second edges, and the gas conduit extending along the first edge; and securing the second edge to the gas conduit so as to form a tubular circulation element. The method may comprise the step of securing the first edge to the gas conduit. Manufacturing elements for PBRs is a precise process and therefore potentially expensive. The use of a planar element comprising a light guide that is sufficiently flexible to be curved or bent to form a tubular circulation element allows any machining and processing necessary to provide the resulting tubular circulation element with required properties, such as the provision of a low refractive index coating or the provision of illumination features, to be applied to a planar substrate, rather than a tubular substrate. Processing a planar element is generally simpler than processing a tubular element, and therefore is typically cheaper. Therefore, the use of the provided method allows tubular circulation elements (and by extension, PBRs comprising said tubular circulation elements) to be manufactured for lower cost than would otherwise be possible. The planar element may have a first surface and a second surface parallel to the first surface. After the first edge and the second edge of the planar element have been secured to the gas conduit, the first surface may be on the interior of the tubular circulation element and the second surface may be on the exterior of the tubular circulation element. The planar element may comprise a light guiding material. The planar element may comprise a sheet of light guiding material. The planar element may comprise a plurality of layers of a light guiding material. The light guiding material or layers of light guiding material may be transparent polymer, and/or resilient polymer, for example. In embodiments where the planar element comprises a light guide, the light guide may comprise light guiding material. Some of the layers within the plurality of layers may have a different refractive index. For example, a layer with a high refractive index may be between two layers of lower refractive index. Each layer within the plurality of layers may have a different refractive index. Each layer within the plurality of layers may have the same refractive index. The planar element may be provided with a coating. The coating may be a membrane. The coating may be a surface layer. The coating may have a lower refractive index than that of the light guiding material. The coating may be provided on the first surface of the light guide. The coating may be provided on the second surface of the light guide. The coating may be provided on both the first and second surfaces of the light guide. The coating may be a continuous layer formed on the surface of the planar element. The coating or membrane may be a porous layer formed on the surface of the planar element, and may be a microporous polymer, such as a polyester (the resin polyethylene terephthalate, PET, for example) or microporous polytetrafluoroethylene (PTFE), for example. The pores within the porous layer may extend to the surface of the light guiding material of the planar element. The porous layer may be hydrophobic such that the porous layer can trap air in the pores, further reducing the refractive index of the porous layer. The planar element may comprise a reflective coating such that light incident upon the reflective coating from within the light guiding material of the planar element is reflected back into the said light guiding material. The method may comprise the step of applying a coating or membrane having a low refractive index to either or both of the first and second surfaces of the planar element. The step of applying the said coating or membrane may be prior to the step of securing the second edge of the planar sheet to the gas conduit. In embodiments where the planar element is provided separate to the gas conduit, the step of applying the said coating or membrane may be prior to the securing of the first or second edge of the planar element to the gas conduit. The method may comprise the step of processing the planar element such that a plurality of illumination features are provided on the surface of the planar element. The step of processing the planar element may be prior to attaching the first edge of the planar sheet to the gas conduit; and attaching the second edge to the gas conduit such that the surface comprising the plurality of illumination features is facing into the tubular circulation element. The plurality of illumination features may be provided on both of the surfaces of the planar element. At least one illumination feature of the plurality of illumination features may be printed onto the or each surface of the planar element. At least one illumination feature of the plurality of illumination features many be etched onto the or each surface of the planar element. At least one illumination feature of the plurality of illumination features may be moulded onto the one or each surface of the planar element. At least one illumination feature of the plurality of illumination features may deposited onto the or each surface of the planar element. According to a third aspect of the invention there is provided a method of assembling a tubular circulation element for a photobioreactor comprising the steps of providing a flexible sheet comprising either or both a light source and/or a light guide, rolling the flexible sheet into a cylinder and then inserting the so-formed cylinder into a tube, such that the exterior of the tube forms the exterior of the tubular circulation element, and the interior of the rolled flexible sheet forms the interior of the tubular circulation element. The flexible sheet may comprise a plurality of light sources. The plurality of light sources may be arranged across the surface of the flexible sheet. The plurality of light sources may be arranged along one edge of the flexible sheet such that when the flexible sheet is rolled into a cylinder, the plurality of light sources are arranged around the one end of the cylinder, or along the length of the cylinder. The tubular circulation element so formed may be reconfigured by removing the retained flexible sheet from the tube and wrapping the flexible sheet around the exterior of the tube to form a non-circulating photobioreactor within the tube. The invention also extends to a method of making a photobioreactor according to the first aspect comprising the step of making the tubular circulation element according to the method of the third aspect. The invention extends in a fourth aspect to a kit of parts for a photobioreactor comprising a chassis, a gas outlet, a tubular circulation element and a chamber; the gas outlet and tubular circulation element is configured to be demountably retained by the chassis and the chassis is configured to be retained within the chamber. The gas outlet may comprise a sparger. The gas outlet may comprise a ring. The gas outlet may comprise a nozzle. The tubular circulation element may comprise a wall element. The wall element may comprise a light source. The wall element may comprise a plurality of light sources. The plurality of light sources may be arranged across the surface of the wall element. The plurality of light sources may be arranged within the wall element. Optional features discussed in relation to any aspect of the invention are optional features of each aspect of the invention. Description of the Drawings An example embodiment of the present invention will now be illustrated with reference to the following Figures in which: Figure 1 is a plan view from the side of the PBR; Figure 2 is a cut-away perspective view of the wall element of the PBR; Figure 3 is a plan view of an array of LEDs that may be mounted onto the circular element of the PBR; Figure 4 is a schematic of the fluid circulation pathway; Figure 5 is two side views of the light guide; Figure 6 is a perspective view from the front of the circulation element comprising a wall element and a gas conduit; Figure 7 is a plan view of the circulation element; Figure 8 is a view of a wall element; Figure 9 is a perspective view of the wall element of Figure 8 rolled to form a circulation element; Figure 10 is a plan view from the side of an alternative PBR; and Figure 1 1 is a flow chart showing the steps of a method of manufacture for a circulation element. Detailed Description of an Example Embodiment First Example of a photobioreactor With reference to Figures 1 to 5, a photobioreactor 1 comprises a chamber 2, a circulation element 4, a sparger 6 (acting as a gas outlet) at the base of the chamber and an exhaust 8. The circulation element and sparger are provided within the chamber. The circulation element comprises a cylinder 10 (acting as a wall element), and a gas conduit 12 extending along the length of the cylinder. The cylinder mounted on support elements 14 and is arranged vertically within the chamber directly above the sparger. The circulation element is held centrally within the chamber by an array of fins extending radially from the circulation element to the walls of the chamber (not shown). The gas conduit is in fluid communication with the sparger and an external supply of carbon dioxide rich gas 16 such that carbon dioxide may flow from the external source to the sparger via the gas conduit. The cylinder comprises a first end 18, a second, opposed end 20 and a wall 22 extending between the first end and second end. The cylinder comprises an array of LEDs 24 (acting as an array of light sources) arranged around the first end of the cylinder. The wall of the cylinder comprises an interior surface 26 and an exterior surface 28 and comprises a light guide 30 extending from the first end to the second end. The light guide comprises an acrylic sheet 32 (having a refractive index of approximately 1.49) with a coating of PTFE 34 (having a refractive index of approximately 1.51) on both the interior and exterior surfaces. The light guide further comprises a pattern of features printed onto the light guide in textured ink 36 on the interior surface (acting as illumination features). The density of the pattern is low 38 at the top of the cylinder, as oriented within the chamber, and gradually increases to a higher density 40 at the bottom of the cylinder. During use, the chamber retains a volume of aqueous media comprising the algae Chlorella vulgaris 42 and nutrients such as minerals and vitamins. The volume of the aqueous media is typically smaller than that of the chamber such that a gas pocket 44 is located above the aqueous media to allow exchange of gases between the gas pocket and the aqueous media. Excess gas, including produced oxygen, is vented through the exhaust. Carbon dioxide enriched air is pumped from the external gas supply to the sparger via the gas conduit. The sparger releases the carbon dioxide into the aqueous media in the form of a stream of small bubbles 46. Due to their buoyancy, the stream of small bubbles rises through the aqueous media through the interior of the circulation element, to the surface of the aqueous media. The movement of the bubbles reduces the pressure within the circulation element, thereby causing the aqueous media to flow up 48 through the interior of the circulation element. The aqueous media flows out radially 50 once it reaches the top of the circulation element. The displacement of aqueous media adjacent to the sparger and within the interior of the circulation element produces a flow of fluid in from beyond the periphery of the circulation element towards the sparger 52, drawing fluid down from the exterior of the circulation element 54, thereby producing a fluid circulation pathway 56. Once fluid is flowing in the fluid circulation pathway, the array of LEDs is activated. Light produced from the array of LEDs is captured by the light guide and transmitted along the length of the light guide. When a ray of light 58 is incident on the interface between the acrylic sheet and the PTFE coating at an angle Θ above the critical angle, the light is totally internally reflected according to Snell's law. If the light is incident on the said interface at an angle φ above the critical angle, the light is transmitted out of the light guide. Accordingly, if the light is incident on one of the features within the pattern of features 60, the uneven surface of the feature increases the likelihood that the light is incident on the interface above the critical angle and therefore transmitted out of the light guide. As the pattern of features is printed onto the interior surface of the circulation element, the majority of light transmitted out of the light guide is transmitted into the interior of the circulation element. In addition, although the intensity of light transmitted within the light guide diminishes the further along the light guide the light travels, the increase in the density of features within the pattern of features towards the second end of the cylinder ensures that the intensity of light within the interior of the circulation element is generally uniform along the length of the light guide. In an alternative embodiment with reference to Figures 6 and 7 the circulation element 62 comprises a wall 64 and a gas conduit 66. The wall has a first longitudinal edge 68 and a second longitudinal edge 70. The first longitudinal edge abuts one side of the gas conduit. The wall curves around, forming a cylinder such that the second longitudinal edge abuts an opposed second side of the gas conduit. The wall comprises an interior surface 72 and an exterior surface 74 and comprises a light guide 76 extending from the first longitudinal edge to the second longitudinal edge. The light guide comprises an acrylic sheet 78 with a coating of PET 80 on both the interior and exterior surfaces. The light guide further comprises a pattern of features 82 printed onto the light guide in textured ink on the interior surface (acting as illumination features). The density of the pattern on the light guide is low adjacent to the gas conduit 84, and gradually increases to a higher density on the side of the cylinder opposed to the gas conduit 86. The circulation element further comprises a first array of LEDs 88 (acting as a first line of light sources) and a second array of LEDs 90 (acting as a second line of light sources) arranged along the opposing sides of the gas conduit abutting the first and second edges of the wall. Light emitted from the first and second array of LEDs may be captured by the light guide within the wall and transmitted around the circumference of the wall. In a further alternative embodiment, the circulation element comprises an array of LEDs (acting as an array of light sources) arranged along the length of the wall of the circulation element. Light emitted by the LEDs within the array of LEDs is transmitted around the periphery of the circulation element via the light guide within the wall of the circulation element. In an alternative embodiment the distribution of illumination features is periodic along the surface of the light guide. Accordingly, during operation, the light intensity along the length of the circulation element varies periodically and algae rising through the circulation element pass through regions of high light intensity followed by regions of low light intensity followed by regions of high light intensity etc. Second Example of a photobioreactor With reference to Figures 8 to 10, a photobioreactor (PBR) 200 comprises a chamber 202, a circulation element 204, a sparger 206 (acting as a gas outlet), a cradle 208 (acting as a chassis) and an exhaust 210. The circulation element comprises a flexible sheet 212 rolled to form a cylinder and a transparent tube 214 having a reflective outer layer 216. The PBR further comprises a heater 218, a temperature sensor 220 and a light sensor 222. The heater comprises a coil 224. The coil extends into the interior of the circulation element. The flexible sheet (functioning as the illumination element) comprises an array of light emitting diodes (LEDs) 226 (acting as light sources) arranged across the surface of the flexible sheet such that when the flexible sheet is rolled to form the cylinder, the array of LEDs are arranged across the interior surface of the cylinder. Each LED within the array is associated with a group of illumination features arranged to form a gradient 228 with a density increasing with distance from the LED to thereby increase the evenness of illumination. The flexible sheet is retained within the tube, such that the interior surface of the circulation element is defined by the flexible sheet, and the exterior surface of the circulation element is defined by the tube. The tube is mounted on to the cradle. The sparger is mounted onto the cradle such that when the cradle is stood upright, the sparger is located below the tube and flexible sheet to allow gas 230 from the sparger to rise through the cylinder formed by the flexible sheet. A gas conduit 232 is in fluid communication with the sparger and an external supply of carbon dioxide rich gas 234 such that carbon dioxide may flow from the external source to the sparger via the gas conduit. An electrical lead 236 extends to the flexible sheet to provide power for the LEDs within the flexible sheet. The cradle is mounted within the chamber to form the PBR. The cradle comprises a handle 238. The cradle comprises a plurality of spacers 240 arranged around the periphery of the cradle to define a separation between the walls of the chamber and the circulation element, thereby defining and preserving the circulation pathway around the exterior of the circulation element. The circulation element is demountably attached to the cradle. When the cradle is mounted within the chamber, the handle is located adjacent to the top of the chamber. The handle may be used to lift the cradle, and the sparger and circulation element mounted onto the cradle, from the chamber. This facilitates cleaning of the chamber and other components, such as the flexible sheet. The PBR of the present example operates in a similar way to the previous example. Once fluid is flowing in the fluid circulation pathway described in the previous example, the array of LEDs is activated. Light produced from the array of LEDs is directed into the interior of the circulation element. Light produced by the array of LEDs directed towards the exterior of the circulation element is reflected back towards the interior by the reflective layer. The heater heats the water within the chamber and maintains the temperature of the water dependent on the output of the heat sensor. The light level within the circulation element is monitored by the light sensor and when a change in the light level is detected, the intensity of light output by the LEDs of the flexible sheet is modified accordingly. For example, as the algae density increases, the turbidity of the water increases, leading to a reduction in the penetration depth of the light within the PBR. The light sensor detects this increase in turbidity as a reduction in the intensity of light received by the sensor, and the intensity of light emitted by the LEDs of the flexible is increased until the pre- determined light level is restored. In an alternative embodiment, the tube is transparent and the flexible sheet is wrapped around the exterior surface of the tube such that the interior of the circulation element is defined by the tube and the exterior surface of the circulation element is defined by the flexible sheet. Method of Manufacture of a circulation element of a photobioreactor With reference to Figure 1 1 , a cylindrical circulation element for use in a photobioreactor may be manufactured by the following method. A planar sheet of flexible light guiding material and a gas conduit are provided 100. A low refractive index coating of microporous PET is applied to both surfaces of the planar sheet 102 to ensure the light guiding properties of the planar sheet. Illumination features are printed onto the surface of the PET coating on one side of the planar sheet 104. A first edge of the planar sheet is secured to the gas conduit 106 such that the gas conduit runs along the length of the first edge of the planar sheet. The planar sheet is then curved around to form a cylinder 108 such that the illumination features are on the surface on the interior of the resulting cylinder and a second edge of the planar sheet parallel to the first edge is adjacent the side of the gas conduit opposed to that to which the first edge is secured. The second edge is then secured to the gas conduit 1 10. In an alternative embodiment, the circulation element may be assembled by the following method. A planar sheet comprising an array of LEDs is rolled into a cylinder such that the first edge forms an overlap past the second edge. The cylinder produced may be used as a tubular circulation element without requiring the step of securing the edges of the planar sheet and may retain its circumference within the PBR by being constrained by a cradle or chassis. For example, the cylinder may be retained within a transparent tube such that the diameter of the cylinder is defined by the internal dimensions of the tube and the interior of the circulation element is defined by the cylinder and the exterior of the circulation element is defined by the transparent tube. The circulation element may then be reconfigured such that the cylinder is removed from within the transparent tube and wrapped around the exterior of the tube and the interior of the circulation element is defined by the transparent tube and the exterior of the circulation element is defined by the cylinder. In this second configuration, the light emitted by the LEDs is transmitted through the transparent tube to the interior of the circulation element. In addition, in the second configuration, the tube can be sufficient to form a simple PBR without the need for a chamber, albeit without the provision of circulation of the aqueous media. In this way the circulation element manufactured using the above method provides a simple and low cost method of forming a basic PBR. Further modifications and variations may be made within the scope of the invention herein disclosed.

Claims

Claims 1. A photobioreactor comprising a chamber for retaining aqueous media and a tubular circulation element positioned within the chamber, the tubular circulation element comprising a wall element having an interior and an exterior and defining a fluid circulation pathway within the chamber, characterised in that the wall element comprises either or both a light source and/or a light guide for illuminating the chamber.

2. A photobioreactor according to claim 1 , wherein the fluid circulation pathway defined by the tubular circulation element extends in a loop through the interior of the wall element in a first direction, and then past the exterior of the wall element in a second direction.

3. A photobioreactor according to either one of claims 1 or 2, wherein the wall element comprises a flexible sheet.

4. A photobioreactor according to claim 3, wherein the tubular circulation element comprises a tube; the flexible sheet is rolled to form a cylinder and retained within the tube such that the interior of the tubular circulation element is defined by the flexible sheet and the exterior of the tubular circulation element is defined by the tube.

5. A photobioreactor according to claim 4, wherein the flexible sheet comprises a first edge and a second each, and the first edge overlaps the second edge in the cylinder.

6. A photobioreactor according to any one preceding claim, wherein the light source and/or the light guide illuminates the interior of the circulation element.

7. A photobioreactor according to any one preceding claim, wherein the wall element comprises a light source and the light source extends across the majority of the surface area of the interior of the wall element.

8. A photobioreactor according to any one of claims 1 to 6, wherein the wall element comprises a plurality of light sources and the plurality of light sources are distributed across the majority of the surface of the interior of the wall element.

9. A photobioreactor according to claim 8, wherein the plurality of light sources form bands around the wall element.

10. A photobioreactor according to any one preceding claim, wherein the wall element comprises a light guide and the light guide extends continuously around the majority of the periphery of the wall element.

1 1. A photobioreactor according to any one preceding claim, wherein the wall element comprises a light guide and a light source, arranged such that the light from the light source is transmitted through the light guide.

12. A photobioreactor according to any one preceding claim, wherein the wall element comprises an array of light sources.

13. A photobioreactor according to claim 12, wherein the array of light sources are arranged along the length of the wall element.

14. A photobioreactor according to claim 12, wherein the wall element comprises a first end and an opposed second end and the array of light sources are arranged around the first or second end of the wall element such that the light emitted by the array of light sources is transmitted along the length of the wall element by the light guide.

15. A photobioreactor according to any one of claims 1 to 12, wherein the wall element comprises a first array of light sources arranged around the first end of the wall element, and a second array of light sources arranged around the second end of the wall element.

16. A photobioreactor according to any one preceding claim, wherein the light guide comprises a plurality of illumination features.

17. A photobioreactor according to any one preceding claim, wherein the light guide comprises an array of illumination features.

18. A photobioreactor according to claim 17, wherein the density of the illumination features within the array of illumination features varies across the surface of the light guide.

19. A photobioreactor according to claim 18, wherein the density of illumination features within the array of illumination features varies across the surface of the light guide such that the interior of the tubular circulation element is illuminated approximately uniformly along its length.

20. A photobioreactor according to any one preceding claim, wherein the photobioreactor comprises a gas outlet located adjacent to one end of the tubular circulation element such that gas released by the gas outlet may rise through the interior of the tubular circulation element.

21. A photobioreactor according to claim 20, wherein the gas rising through the tubular circulation element may induce fluid to flow along the fluid circulation pathway defined by the tubular circulation element.

22. A method of manufacturing a tubular circulation element for a photobioreactor, comprising the steps of;

providing a planar element, having either or both of a light guide and/or light source, and a gas conduit, the planar element extending between substantially parallel first and second edges, and the gas conduit extending along the first edge;

and securing the second edge to the gas conduit so as to form a tubular circulation element.

23. A method according to claim 22, wherein the method comprises the step of applying a coating having a low refractive index to either or both of the first and second surfaces of the planar element.

24. A method according to either one of claims 22 or 23, wherein the method comprises the step of processing the planar element such that a plurality of illumination features are provided on the surface of the planar element.

25. A method of assembling a tubular circulation element for a photobioreactor comprising the steps of providing a flexible sheet comprising either or both a light source and/or a light guide, rolling the flexible sheet into a cylinder and then inserting the so-formed cylinder into a tube, such that the exterior of the tube forms the exterior of the tubular circulation element, and the interior of the rolled flexible sheet forms the interior of the tubular circulation element.

26. A kit of parts for a photobioreactor, comprising a chassis, a gas outlet, a tubular circulation element and a chamber; the gas outlet and tubular circulation element configured to be demountably retained by the chassis and the chassis configured to be retained within the chamber.

27. A kit of parts according to claim 26, wherein the chassis comprises a handle.

28. A kit of parts according to any one of claims 26 and 27, wherein the tubular circulation element comprises a flexible sheet and a tube.

29. A kit of parts according to claim 28, wherein the flexible sheet is rolled into a cylinder and retained by the tube to form the tubular circulation element.