Abstract:
The present subject matter provides examples of a photobioreactor for producing biomass material using a continuous flow of liquid growth media, the apparatus comprising a channel including a bottom portion, a first sidewall portion connected to the bottom portion, and a second sidewall portion connected to the bottom portion opposite the first sidewall portion, wherein a length of the channel is at least twice as long as a width of the channel; the length of the channel is measured along the bottom portion of the channel and parallel to the first and second sidewall portions of the channel, and the width is measured as a shortest distance between and perpendicular to the first and second sidewall portions of the channel, a transparent cover adapted to provide a seal with the first sidewall and second sidewall opposite the bottom portion along a substantial length of the channel, a liquid growth media input at a first end of the channel, a biomass and liquid media output for harvesting biomass material at a second end of the channel, and a gas backflow barrier positioned adjacent the transparent cover and between the first and second sidewall portions, the gas backflow barrier adapted to limit gas from passing from an interior portion of the channel through the liquid growth media input when the liquid growth media is flowing.

Description:
CLAIM OF PRIORITY AND RELATED APPLICATION 
       [0001]    This application claims the benefit under 35 U.S.C. 119(e) of U.S. Provisional Patent Application Ser. No. 60/968,490, filed Aug. 28, 2007, the entire disclosure of which is hereby incorporated by reference in its entirety. 
     
    
     TECHNICAL FIELD 
       [0002]    This application relates to photobioreactors and more particularly to continuous flow photobioreactors. 
       BACKGROUND 
       [0003]    Photobioreactors provide an artificial environment for growing biomass materials, including algae, for a variety of research and commercial uses. Such uses include, but are not limited, to development and production of alternative fuel sources. The design of a photo bioreactor depends on the application for which the biomass is produced and, therefore, must consider the specific requirements of the biological system used. Exposure of the biomass material to adequate light, nutrients and carbon dioxide are a few of the basic requirements applicable to most bioreactor designs. 
       SUMMARY 
       [0004]    The subject matter of this document provides an efficient and low cost photobiorector apparatus and method for high capacity production of biomass material. In various embodiments, the apparatus provides a closed system, shallow channel photo-bioreactor with an input and an output to allow continuous production of biomass material. In various embodiments, the photobioreactor allows light to access the growing biomass material using a transparent cover exposed to either natural or artificial light. In various embodiments, at least one gas discharge tube extending substantially the full length of the photobioreactor is used to expose the biomass material to carbon dioxide (CO 2 ) rich gas emitted through perforations extending the length of the gas discharge tube. One embodiment of the subject matter of this document provides a photobioreactor system using multiple, closed system, shallow channel bioreactors for use as a high capacity method of biomass production. 
         [0005]    This Summary is an overview of some of the teachings of the present application and is not intended to be an exclusive or exhaustive treatment of the present subject matter. Further details about the present subject matter are found in the detailed description. The scope of the present invention is defined by the appended claims and their equivalents. 
     
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         [0006]      FIG. 1A  is a side view of a photobioreactor according to one embodiment of the present subject matter. 
           [0007]      FIG. 1B  shows a prospective view of a photobioreactor according to one embodiment of the present subject matter. 
           [0008]      FIG. 2  is a cross-section of a closed system, shallow channel photobioreactor according to one embodiment of the present subject matter. 
           [0009]      FIG. 3  illustrates a detailed view of a gas discharge tube near the transparent cover of a photobioreactor according to one embodiment of the present subject matter. 
           [0010]      FIG. 4  shows a side view of a photobioreactor with gas release ports according to one embodiment of the present subject matter. 
           [0011]      FIG. 5  illustrates a photobioreactor system according to one embodiment of the present subject matter. 
           [0012]      FIG. 6  is a flowchart for producing biomass material with a closed, shallow channel photobioreactor according to one embodiment of the present subject matter. 
       
    
    
     DETAILED DESCRIPTION 
       [0013]    The following detailed description refers to subject matter in the accompanying drawings which show, by way of illustration, specific aspects and embodiments in which the present subject matter may be practiced. These embodiments are described in sufficient detail to enable those skilled in the art to practice the present subject matter. References to “an”, “one”, or “various” embodiments in this disclosure are not necessarily to the same embodiment, and such references contemplate more than one embodiment. The following detailed description is, therefore, not to be taken in a limiting sense, and the scope is defined only by the appended claims, along with the full scope of legal equivalents to which such claims are entitled. 
         [0014]      FIGS. 1A and 1B  show a closed system, shallow channel photobioreactor  100  according to one embodiment of the present subject matter.  FIGS. 1A and 1B  show a channel  101  with a liquid growth input  102  at one end, a liquid and biomass output  103  at the other end, a transparent cover  104 , a gas discharge tube  105  and various liquid flow barriers  106 . In various embodiments, the length of the channel from the liquid growth media to the liquid and biomass output is a substantially greater distance than the width of the channel as measured between the channel sidewalls. For example, the channel length should be at least 2 times greater than the channel width. In various embodiments, the channel length is more than 1800 times greater than the channel width. In various embodiments, the channel has a U-shaped cross-section, but is not so limited. In various embodiments the channel is formed of rigid water tight materials such as plastic, metal or concrete. In various embodiments, an earthen channel is lined with a poly film, such as polyethylene, or membrane material, such as Duro-Last® roofing membrane made by Duro-Last® Roofing, Inc. 
         [0015]    The cover  104  of the channel is a flat, rigid, transparent sheet material and is connected to the sidewalls of the channel so as to provide a water-tight seal between the cover and the sidewalls. Rigid materials from which the cover may be made include, for example, plastic polymers and glass. In various embodiments, the cover material is a thin transparent film and is supported during operation of the photobioreactor by the liquid growth media inside the photobioreactor channel. In some embodiments, the cover is weatherproof and connected to the channel such that it resists damage due to wind, rain, hail or combinations thereof. 
         [0016]    A gas discharge tube  105  discharges gases to promote growth of the biomass material within the photobioreactor channel. The gas is discharged through openings  107  in the gas discharge tube  105 . The openings  107  in the gas discharge tube  105  extend the length of the tube and are positioned at various spacing with respect to each other. In various embodiments, the gas discharge tube extends along substantially the entire length of the channel and is located near the cover  104  of the channel. In various embodiments, a channel contains a plurality of gas discharge tubes extending substantially the entire length of the photobioreactor channel. A gas backflow barrier plate  108  is located near the liquid growth input  102  to prevent gas bubbles, discharged from the gas discharge tube  105 , from exiting the channel at the liquid growth input  102 . The gas barrier plate  108  ensures the gas bubbles move with the flow of the liquid growth media and biomass through the entire channel allowing as much gas as possible to be absorbed into the liquid growth media. Any unabsorbed gas or gas produced through the biomass growth process, is allowed to exit the covered channel at the liquid and biomass output  103 , minimizing backpressure. In various embodiments, the gas discharge tubes are connected to a gas source in which the gas is rich in carbon dioxide (CO 2 ). 
         [0017]    Operation of a photobioreactor reactor according to one embodiment of the present subject matter, commences with filling the channel completely to the top cover with liquid growth media. The channel is orientated such that it is longitudinally horizontal, less than 1% incline or decline, and the transparent cover will receive adequate light. The liquid growth media is then seeded with biomass material. Growth is encouraged by exposing the biomass material to light and CO 2  rich gas. The light is provided either naturally, such as the sun, or artificially, through the transparent cover of the channel. The CO 2  rich gas is provided through at least one perforated gas discharge tube. After the initially seeded biomass material reaches a predetermined density, the process becomes continuous by providing a continuous flow of liquid growth media through the liquid growth input  102  and allowing liquid and biomass material to exit the photobioreactor through the liquid and biomass outlet  103 . In various embodiments, flow of the liquid growth media is monitored and controlled using level controls located at the liquid growth input  102  and the liquid and biomass output  103 . 
         [0018]    To maximize the growth of the biomass material, various embodiments provide features to reduce stagnation of the movement of the biomass material through the channel. Stagnated movement of the biomass material results in a minimal amount of gas being absorbed by the liquid growth media. Stagnated movement of the biomass material also blocks light to biomass material not adjacent to the transparent cover, thus reducing the overall growth potential of the channel. Additionally, stagnated movement reduces the availability of nutrients to biomass material not adjacent to the flow of liquid growth media. In various embodiments, flow barriers  106  are provided in the channel to induce turbulence of the liquid growth media by diverting the flow of the media in various directions. The turbulence disrupts stagnant biomass material and creates a continuous and even mixture of the liquid growth media and biomass material throughout the length of the photobioreactor channel. In various embodiments, actuated devices, such as impellers, are used to create turbulence and mix the liquid growth media and the biomass material. An efficient mixing of the liquid growth media flow and the biomass material provides more probability that all the biomass material will be exposed to adequate light, gas and nutrients for sufficient growth as the biomass moves with the flow of liquid growth media from the liquid growth input  102  to the liquid and biomass output  103 , where the biomass is captured for further processing. 
         [0019]      FIG. 2  is a cross-section of a closed system, shallow channel photobioreactor  200  according to one embodiment of the present subject matter.  FIG. 2  illustrates a U-shaped channel  208  with a bottom portion  209 , two adjoining side wall portions  210 , a transparent cover  204  and a plurality of perforated gas discharge tubes  205  positioned near the transparent cover  204 . In various embodiments, each gas discharge tube  205  is capped at one end and connected to a CO 2  rich gas source at the other. Examples of CO 2  rich sources of gas include gases produced from fuel combustion, fermentation processes and digestation processes. 
         [0020]      FIG. 3  illustrates a detailed view of a gas discharge tube  305  near the transparent cover  304  of a photobioreactor according to one embodiment of the present subject matter.  FIG. 3  shows a gas discharge tube  305  with openings  307  directing the discharge of CO 2  rich gas bubbles  311  toward the transparent cover  304  of the photobioreactor channel. In addition to introducing CO 2  for absorption by the liquid growth media, the gas discharge tubes  305  direct the gas bubbles  311  at the transparent cover  304  in a direction that creates shear forces that prevent and reduce the amount of biomass material accumulating on the cover  304 . Biomass material that accumulates on the transparent cover  304  reduces the amount of light available to other biomass material, therefore reducing the efficiency of the photobioreactor. Additionally, the turbulence of the liquid growth media created by the discharged gas bubbles  311  near the transparent cover  304  provides vertical mixing of the biomass material and the liquid growth media providing opportunity for the liquid growth media to circulate into close proximity of the light coming through the transparent cover  304 , thus providing light energy to a greater volume of liquid media. 
         [0021]      FIG. 4  shows a side view of a photobioreactor  400  with gas release ports according to one embodiment of the present subject matter.  FIG. 4  shows a channel  401  with a liquid growth input  402  at one end, a liquid and biomass output  403  at the other end, a transparent cover  404 , a gas discharge tube  405  and various liquid flow barriers  406 . In various embodiments, the length of the channel from the liquid growth media to the liquid and biomass output is a substantially greater distance than the width of the channel as measured between the channel sidewalls. For example, the channel length should be at least 2 times greater than the channel width. In various embodiments, the channel length is more than 1800 times greater than the channel width. In various embodiments, the channel has a U-shaped cross-section, but is not so limited. 
         [0022]    The illustrated embodiment of  FIG. 4  includes one or more gas release ports  412  and gas release backflow barriers  413  along the length of the transparent cover  404 . The illustrated gas release ports  412  include a gas release valve  414  mounted on top of a gas vent  415 , such as a pipe, for example. In various embodiment, the gas release valve  414  is a one-way valve allowing gas from inside the vent to escape without allowing any ambient air, including potential airborne contaminants to enter the bioreactor. The gas release ports  412  prevent a continuous layer of gas to form above the biomass material and growth media. The gas release ports  412  assure the underside of the transparent cover  404  is in contact with the biomass and growth media. The turbulance of the moving biomass material, growth media and gas bubbles at or near the transparent cover provides shear forces that reduce the adhesion of biomass material to the transparent cover. As a result, more favorable production conditions exist as light energy entering the chamber is not reduced by material accumulating to the transparent cover. Additionally, the gas release ports allow oxygen generated from the growth of the biomass material and of little benefit to the continued growth of the biomass material to escape the bioreactor. In various embodiments, such as the embodiment illustrated, gas backflow barriers  413  are positioned down-flow from each gas release port  412  to increase the residence time of freshly discharged CO 2 . The gas backflow barriers  413  reduce the release of CO 2  discharged near each gas release port from the one or more gas discharge tubes  405 . In various embodiments, the gas release ports  412  are spaced evenly from each other along the length of the bioreactor channel. The number of gas release ports is determined by the length of the bioreactor and the amount of gas produced by the biomass material. In various embodiments, the gas release vents  415  extend to a height above the transparent cover  404  equal to, or greater than, the height of the fluid level of the liquid growth media. 
         [0023]      FIG. 5  illustrates a photobioreactor system according to one embodiment of the present subject matter. The system includes a plurality of closed, shallow channel photobioreactors  520  as described above. In various embodiments, the photobioreactors  520  are positioned parallel to each other and spaced to allow maintenance while at the same time minimizing area occupied by the reactors to reduce capital costs. In the illustrated embodiment, the gas discharge tubes of each channel are connected to a gas source  524  using a gas distribution manifold  521 . In the illustrated embodiment, the liquid growth input of each channel is connected to a source of liquid growth media  525  using a liquid growth media distribution manifold  522 . In various embodiments, flow of the liquid growth media is monitored and controlled using level controls located at the liquid growth input and the liquid and biomass output. In various embodiments, the flow rate of the liquid growth media and the discharge gas are controlled using manual controls. In various embodiments, the flow rate of the liquid growth media and the discharge gas are controlled automatically using a controller and automatic controls. In various embodiments, the flow rate of the liquid growth media and the discharge gas are controlled using both manual and automatic controls. In various embodiments, the flow of the liquid growth media at each channel is controlled at a rate sufficient to sustain continuous biomass production at the liquid and biomass output while at the same time not exceeding a flow rate that would provide insufficient residence time of the biomass material resulting in low production rates in proportion to the liquid growth media input or a rate that would eventually deplete biomass material from the channel. The illustrated embodiment of a photobioreactor system according to the present subject matter of  FIG. 5  shows the liquid and biomass output of each channel of the system connected to common biomass collection system  523 . The collection system allows for collection and transport of harvested biomass to a common location  526  for further processing and/or distribution. 
         [0024]      FIG. 6  is a flowchart for producing biomass material with a closed, shallow channel photobioreactor according to one embodiment of the present subject matter. The process is initiated by providing a lengthy, closed, shallow channel photobioreactor  630  and then fully filling the channel with liquid growth media  631  and seeding the media with biomass material  632 . In various embodiments, a shallow channel is one between 4 inches and 5 feet in depth. Growth of the seeded biomass material is encouraged by providing light  633  and CO 2  rich gas to the photobioreactor  634 . In various embodiments, light is transmitted to the chamber through a clear channel cover exposed to natural light. In various embodiments, the clear channel cover is exposed to artificial light. In various embodiments, CO 2  rich gas is added to the channel using at least one gas discharge tube extending the length of the channel and located inside the channel and adjacent the transparent cover. CO 2  gas is prohibited from exiting a first end of the photobioreactor using a gas barrier near the first end of the photobioreactor  635 . 
         [0025]    After the seeded biomass material has grown to a predetermined density, the process becomes continuous by providing a flow of liquid growth media to the first end of the photobioreactor  636 . Adding new liquid growth media to the first end of the channel creates hydraulic pressure and subsequent flow through the channel to a second end of the channel. In various embodiments, the flow rate of the photobioreactor is controlled using level sensors located at the first and second ends of the channel  637 . As the biomass material proceeds with the flow of the liquid growth media from the first end of the photobioreactor to the second end of the photobioreactor, the liquid growth media and biomass material are mixed together using flow barriers positioned along the length of the photobioreactor  638 . Unabsorbed gas is allowed to exit the reactor at the second end of the photobioreactor  639 . As a sufficient positive pressure and flow of growth material is established in the photobioreactor, excess liquid and biomass material are allowed to exit the reactor at the second end of the photobioreactor  640 . As the biomass material exits the photobioreactor, it is collected for further processing and/or distribution  641 . 
         [0026]    This application is intended to cover adaptations or variations of the present subject matter. It is to be understood that the above description is intended to be illustrative, and not restrictive. The scope of the present subject matter should be determined with reference to the appended claims, along with the full scope of equivalents to which such claims are entitled.