Abstract:
A raceway pond for circulating microalgae in a fluid medium includes a plurality of interconnected channels. Each channel is straight and has a structured gradient, due to tilt or terracing, that moves the fluid medium along the raceway. In operation, the concentration of microalgae in the fluid medium is maintained substantially constant, and the depth of the fluid medium in the raceway is maintained below a pre-determined level.

Description:
FIELD OF THE INVENTION 
       [0001]    The present invention pertains generally to plug-flow reactors (PFRs) having a circulating raceway pond for growing microalgae in a fluid medium. More particularly, the present invention pertains to PFRs that provide conditions for producing microalgae having an oil content as high as 60%. The present invention is particularly, but not exclusively, useful as a PFR that relies on gravity for moving microalgae in a fluid medium along the length of its raceway. 
       BACKGROUND OF THE INVENTION 
       [0002]    The growth rate of microalgae in a liquid environment is dependent on several disparate factors. For one, it is known that the fluid medium in which the microalgae grows (i.e. liquid environment) must be circulated to provide for mixing and exposure of the microalgae to light for photosynthesis. For another, each algae species has an optimal concentration for consumption of all, or nearly all, of the available resources in the fluid medium. The import here is that with a high consumption of available resources by the microalgae, the time available for growth of weed algae, bacteria or predators that would otherwise diminish algae production, is limited. Yet another factor concerns the depth of a circulating microalgae pond. Indeed, pond depth has been determined to be a very important factor affecting microalgae growth. 
         [0003]    Heretofore, conventional thinking has been that an increase in the net production of microalgae in a circulating pond could be achieved only by an increase in the depth of the fluid medium in the pond. It has been determined, however, this is not the case. Contrary to earlier conclusions, shallow circulating pond depths of around 7.5 to 10 cm have proven more efficient and more productive than deeper ponds. A problem with shallow ponds, however, is that typical means used for circulating the fluid medium are generally ineffective when pond depths are in the 7.5-10 cm range. For example, paddle wheels are typically not effective for this purpose with pond depths less than 20 cm. 
         [0004]    Another factor for consideration, when designing a system that will be used to grow microalgae for commercial purposes, is the volume of microalgae that can be produced. On this point, it is clear that the amount of biomass that can be produced is directly proportional to the volume of fluid medium that can be used. There must, of course, be compliance with the pond depth and concentration considerations mentioned above. Nevertheless, although a shallow depth for the fluid medium is crucial, the width of fluid channels that are constructed for the circulating pond is not so limited. 
         [0005]    Yet another factor for consideration, when using a conventional raceway circulating pond having a paddle wheel to grow microalgae, is that the pond size is limited to be under several acres to maintain relative evenness. On this point, the larger the area, the more the unevenness in depth across the culture area, which can be a drawback in terms of productivity. 
         [0006]    In light of the above, it is an object of the present invention is to provide a circulating pond that is dimensioned to provide conditions for optimal growth of microalgae. Another object of the present invention is to provide a circulating pond with raceways that avoid dead zones, and consequently uneven fluid flow. Still another object of the present invention is to provide a circulating pond for promoting microalgae growth that relies on gravity as the primary force for moving a fluid medium through the pond. Yet another object of the present invention is to provide a circulating pond that is relatively simple to manufacture, is easy to use, and is comparatively cost effective. 
       SUMMARY OF THE INVENTION 
       [0007]    A raceway pond that is used to circulate a fluid medium for the purpose of growing algae includes a pair of substantially straight, elongated channels. The channels are generally juxtaposed, side-by-side to each other, with both of their respective ends in fluid communication with each other. Several structural aspects of the channels are of particular importance. For one, the fluid medium in the channels has a substantially constant and relatively shallow depth (e.g. 7.5 cm). For another, the channels have a structured downstream gradient that allows fluid to flow continuously from the upstream end of one channel to the downstream end of the other channel under the influence of gravity. 
         [0008]    In detail, each elongated channel of the raceway pond has a first (upstream) end and a second (downstream) end, with a substantially flat floor and opposed sidewall portions extending between the ends. For disclosure purposes, one channel is referred to hereinafter as the first channel, and the other channel is referred to as the second channel. A transfer section connects the second (downstream) end of the first channel in fluid communication with the first (upstream) end of the second channel. Importantly, this transfer section provides for a gravity flow of the fluid medium from the first channel to the second channel. At the second (downstream) end of the second channel, a lifting device is provided to lift water from the second (downstream) end of the second channel, back into the first (upstream) end of the first channel. For purposes of the present invention, the lifting device can be of any type well known in the pertinent art and is, preferably, selected from a group consisting of an Archimedes pump, a conveyor, a bucket lift, a paddle wheel, a sealed paddle wheel or an electro-mechanical pump. 
         [0009]    As indicated above, a structured downstream gradient is provided for each channel that will cause the fluid medium to flow through the raceway pond under the influence of gravity. In one configuration for this structured downstream gradient, the floor of the channel is provided with an incline. For example, a 1.3 foot height difference between the ends of a 2500 foot long channel would provide an adequate incline for the present invention. Alternatively, the structured gradient can be accomplished by constructing steps along the length of the floor of a channel. If steps are used, each step could be formed with a height “h” of approximately 3 cm, with a distance “s” between steps of approximately 100 m. Further, for either floor configuration, a plurality of vortex generators can be mounted on the floor in the channel to create turbulence in the fluid medium that will assist algae growth. 
         [0010]    In addition to the structural aspects of the channels mentioned above, the sidewall portions of the first channel can be tapered with an increasing downstream width established by a taper angle “α” that is equal to approximately 0.002 radians. With this taper, the first channel will establish a logarithmic growth stage for microalgae in the raceway pond. The sidewalls of the second channel can then be oriented substantially parallel to each other to provide for an oil accumulation stage for the microalgae. 
         [0011]    An important aspect of the present invention is its scale. In particular, this aspect concerns the physical dimensions of the first and second channels. For example, each channel can have a length of approximately 2,500 m, and a width that can be greater than about 100 m. Further, regardless of other dimensions for the raceway pond, it is important that the depth of fluid medium in the channels be maintained below a level of about 15 cm. And, preferably, the depth of fluid medium will be around 7.5 cm. 
         [0012]    At least one injector can be provided with the raceway pond to add fluid medium to the pond at a selected point(s) along the length of the raceway. The purpose for adding the fluid medium is two-fold. First, the addition of fluid medium is done to maintain the depth of the fluid medium substantially constant in the channels (e.g. 7.5 cm). Second, the controlled addition of fluid medium, together with the tapered construction of the first channel, provide for the maintenance of a pre-determined concentration of microalgae in the fluid medium (e.g. approximately 1.5 grams per liter). These considerations, along with the dimensions and structural aspects given above for the channels, are intended to ensure an operational net oil productivity from algae grown in the raceway pond that is in a range of 15-50 g/m 2 /day. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0013]    The novel features of this invention, as well as the invention itself, both as to its structure and its operation, will be best understood from the accompanying drawings, taken in conjunction with the accompanying description, in which similar reference characters refer to similar parts, and in which: 
           [0014]      FIG. 1  is a top view of a circulating pond in accordance with the present invention; 
           [0015]      FIG. 2A  is a side cross-section view of a structured gradient for the raceway of the present invention as seen along the line  2 - 2  in  FIG. 1 ; 
           [0016]      FIG. 2B  is a side cross-section view of an alternate embodiment for the structured gradient of the raceway, as would be seen along the line  2 - 2  in  FIG. 1 ; and 
           [0017]      FIG. 3  is a top view of an alternate embodiment for a circulating pond for the present invention. 
       
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0018]    Referring initially to  FIG. 1  a raceway pond in accordance with the present invention is shown and is generally designated  10 . Specifically, in  FIG. 1  it can be seen the pond  10  includes a first channel  12  and a second channel  14  that are shown juxtaposed in a side-by-side relationship with one another. Further, it is shown that the channels  12  and  14  are in fluid communication with each other and that a fluid medium  16  flows continuously from one to the other. As will be appreciated by the skilled artisan, the arrangement of the channels  12  and  14  shown in  FIG. 1  is only exemplary. Depending on topography of the terrain where the pond  10  will be used, and the ability to satisfy other requirements of the present invention, the channels  12  and  14  can have any of various arrangements. 
         [0019]    In greater detail,  FIG. 1  shows that the fluid medium  16  flows in the first channel  12  from an upstream end  18  to a downstream end  20 , as indicated by the arrow  22 . After flowing through the first channel  12 , the fluid medium  16  transitions through a transfer section  24  from the first channel  12  to the second channel  14 , as indicated by the arrows  26   a  and  26   b . In the second channel  14 , the fluid medium  16  flows from an upstream end  28  to a downstream end  30 , as indicated by the arrow  32 . At the downstream end  30  of the second channel  14 , the fluid medium  16  enters a collection trough  34 . A lifting device  36  is then used to lift the fluid medium  16  from the collection trough  34  (channel  14 ) and into a distribution trough  38  (channel  12 ). As envisioned for the present invention, the algae culture will pass through the circulation pump (e.g. lifting device  36 ) every 2-4 hours. As the cell size is generally small (1-20 μm dia.) and may have a thick cell wall, the shear stress generated by the pump (lifting device  36 ) has little or no effect on growth. However, to insect larva, the shear stress is significant as larva is generally large in size (10 mm) and has no cell wall. Therefore, such design also helps prevent contamination of the algae culture by insects. Moreover, for an open body of water, such design is environmentally friendly due to this insect control mechanism. Thus, fluid medium  16  is transferred from the downstream end  30  of the second channel  14  to the upstream end  18  of the first channel  12  for a re-circulation of the fluid medium  16  through the raceway pond  10 . Preferably, the lifting device  36  is of a type well known in the pertinent art, such as a conveyor, a bucket lift, a paddle wheel, a sealed paddle wheel or an electro-mechanical pump. 
         [0020]    As implied above, except for the lifting device  36  between the collection trough  34  (channel  14 ) and the distribution trough  38  (channel  12 ), the fluid medium  16  flows through the entire pond  10  under the influence of gravity. For purposes of the present invention, this gravity flow is accomplished using a structured gradient. A preferred embodiment of a structured gradient for use with the pond  10  is shown in  FIG. 2A . There it will be seen that the respective floors  40  of channel  12  and  14  are formed with a plurality of steps  42  (the steps  42   a  and  42   b  are exemplary). In detail, the steps  42  are defined by a height “h” of approximately 3 centimeters, with a distance “s” between the steps  42  being preferably on the order of approximately 100 meters.  FIG. 2A  also shows that a plurality of vortex generators  44  can be positioned along the respective floors  40  of the channels  12  and  14  for the purpose of providing turbulent flow for the fluid medium  16 . 
         [0021]    In an alternate embodiment of a structured gradient as shown in  FIG. 2B , a floor  46  is provided with an incline. For example, the slope of this incline will be “e/L”, as indicated in  FIG. 2B . And, “e” will preferably equal about one meter, and “L” will equal about 2,500 meters. Importantly, although the dimensions of the incline can change, a desired volumetric flow rate is provided by the incline in all instances. Again, vortex generators  44  can be employed. Impliedly, the dimensions given here are approximate, and are given to provide a notion of scale for the invention. Accordingly, actual dimensions can be selected to suit the individual needs of the raceway pond  10 . 
         [0022]    An important aspect of the raceway pond  10  for the present invention will be appreciated with reference to  FIG. 2A , and again to  FIG. 1 . This aspect is that the depth “d” of the fluid medium  16  in the channels  12  and  14  needs to be rather shallow (i.e. less than about 15 cm, and preferably around 7.5 cm). To maintain this depth “d”, however, it may be necessary to replenish the fluid medium  16  along the lengths “L” of the channels  12  and  14 . This may be for any of several reasons (e.g. evaporation losses). Regardless of the reason, however, replenishment can be done by appropriately positioning injectors  48  along the channels  12  and  14  (injectors  48   a ,  48   b  and  48   c  are only exemplary). 
         [0023]    For an operation of the present invention, microalgae (not shown) are to be grown in the pond  10 . For this purpose, it is necessary the pond  10  have a logarithmic growth stage (i.e. channel  12 ), as well as an oil accumulation stage (i.e. channel  14 ). The logarithmic growth stage, however, needs to be constructed with a configuration that will accommodate growth of the microalgae. Accordingly, the side  50  of channel  12  can be slightly angled relative to the side  52  of the channel  12 , to thereby provide an increasing taper for the channel  12  from its upstream end  18  to its downstream end  20 . It happens that, due to the relatively extreme length of the channel  12 , the magnitude of the taper angle “α” that is needed to do this will be on the order of only approximately 0.002 radians. With this in mind, the purpose of adding fluid medium  16  from injectors  48  into the logarithmic growth stage (i.e. channel  12 ) becomes two-fold. In addition to maintaining a substantially constant depth “d” of fluid medium  16  in channel  12 , the addition of fluid medium  16  can be controlled to maintain a pre-determined concentration of the microalgae in the fluid medium  16 . Preferably, this pre-determined concentration is approximately 1.5 grams per liter. 
         [0024]    Unlike the logarithmic growth stage provided by channel  12 , the oil accumulation stage provided by channel  14  is not concerned with microalgae growth, but rather with allowing the microalgae to mature. Accordingly, although the depth “d” needs to be maintained as discussed above, the main concern for channel  14  is to keep the fluid medium  16  moving. This can be done with the respective sides  54  and  56  of the channel  14  being constructed substantially parallel to each other. 
         [0025]    For a modification of the raceway pond  10  of the present invention, instead of a configuration for transfer section  24  as shown in  FIG. 1 , a transfer section  24 ′ as shown in  FIG. 3  can be provided. Specifically, the transfer section  24 ′ shown in  FIG. 3  provides for a continuous turn from channel  12  to channel  14 . Regardless of configurations, however, the depth “d” of fluid medium  16  in the raceway pond  10 , the pre-determined concentration of microalgae in the fluid medium  16 , and the volumetric fluid flow of the fluid medium  16  around the raceway pond  10  are each calculated to provide for an operational oil productivity from algae growth that is in a range of approximately 15-50 g/m 2 /day. 
         [0026]    While the particular Microalgae Growth Pond Design as herein shown and disclosed in detail is fully capable of obtaining the objects and providing the advantages herein before stated, it is to be understood that it is merely illustrative of the presently preferred embodiments of the invention and that no limitations are intended to the details of construction or design herein shown other than as described in the appended claims.