Method and apparatus for providing a photobioreactor

The present invention provides for a photobioreactor for the cultivation of photosynthetic microorganisms and comprises a hydraulic circuit through which an aqueous solution containing culture of at least one type of photosynthetic organism circulates and gets exposed to a light source. The photobioreactor also comprises a means for feeding carbon dioxide into the system, a means for oxygen degasification, and a means for injecting nutrients into the system. The hydraulic circuit is comprised of two open receiving channels at the same relative elevation, a set of transparent or translucent tubes which connect receiving channels to each other, and at least one fluid moving device to move the fluid from the first to the second receiving channels through the tubes. Each receiving channel also comprises a dam which assists in maintaining two different surface levels of the aqueous solution within the upstream and downstream portions of each receiving channel.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates to the field of microalgae and cyanobacteria, specifically how microalgae and cyanobacteria may be cultivated to produce biofuels, food for humans and animals, organic fertilizers or pharmaceutical products.

2. Description of the Prior Art

Due to the increasing costs of fossil fuels as well as its negative impact for the environment (e.g. climate change) there is a growing interest for the use of biofuels such as biodiesel and bioethanol. So far, these biofuels have been obtained from traditional oleaginous and cellulose rich feedstock. However, these traditional crops are not cost effective and the biofuel production yield is not high enough to compete with petroleum and its derivates. Therefore, the production of biofuels from traditional crops may lead to negative side effects such as intolerable rises in food prices and other global issues.

One of the most promising solutions to solve the fossil fuel scarcity and its associated environmental issues is the production of biofuels from microalgae which are single cell photoautotrophic microorganisms. Photoautotrophic organisms (usually plants) carry out photosynthesis to acquire energy from sunlight to convert carbon dioxide and water into organic materials to be used in cellular functions such as biosynthesis and respiration. Importantly, single cell microalgae living in an aqueous solution transform light into chemical energy much more efficiently than any other organism due to their greater access to carbon dioxide and dissolved minerals. In addition, single cell microalgae are able to store lipids in higher density than any other plant or multi-celled organism which requires time, energy and nutrients to build support structures such as roots and stalks, light collector structures such as leaves, and lipid storing organs such as seeds. Therefore, to produce biofuels from microalgae presents the following advantages with respect conventional crops, if cultivated at a large scale:Production yields are 30 to 100 times higher for micro-organisms than any other known traditional crop.Microalgae can grow in soils and tolerate water that is not useful for conventional agriculture. They can even tolerate the use of waste water and saltwater.During its growth, enormous amounts of carbon dioxide as well as other contaminant gases are captured. In addition, vast amounts of oxygen are produced and liberated to the atmosphere.

Microalgae are single cell organisms that convert sunlight into chemical energy through photosynthesis:

Microalgae and cyanobacteria are the planet's most abundant organisms, having adapted to extreme conditions such as polar and volcanic environments. They constitute the core of the trophic chain that sustains life on Earth as well as of the natural carbon cycle. They produce 80% of the planet's biomass (phytoplankton).

Some algae and cyanobacteria mass concentrations of lipids that may achieve proportions in the range of 60-70% by weight on a dry basis. Therefore, they are ideal to produce biodiesel. In addition, algae constitute an effective and powerful carbon sink. It has been demonstrated that 100 tons of algae biomass will capture 170 tons of carbon dioxide.

In addition to light, carbon dioxide and water, photosynthesis requires inorganic salts which include essential elements such as nitrogen, phosphorous, iron and in some cases, silicon. The optimal temperature for microalgae growth is between 20 and 30° C. Therefore, to cultivate microalgae and cyanobacteria, the following is required:An aqueous media containing the algal culture, which can be fresh water, brackish water or saltwater depending on the organism type.A light source in order to develop the photosynthetic process.A carbon dioxide source to enhance the photosynthetic process.A system to extract the oxygen produced during the photosynthesis.Nutrients (mineral salts).A system to move or circulate the aqueous media containing the algal culture to enhance the photosynthesis.

Today, there are two main types of photobioreactors to grow microalgae on a large scale:

Raceways or open ponds. These systems are very simple. They are composed of a circulating pond or a set of circulating channels open to the atmosphere in which the aqueous solution circulates while capturing the sunlight. The biggest advantage of these photobioreactors is that they are very economical. However, open photobioreactors do not easily sustain the conditions for desired microalgae and cyanobacteria growth because the conditions of the algal culture can vary substantially over time due to water evaporation. In addition production is affected by contamination with unwanted algae and microorganisms that are deposited on the algal culture.

Closed photobioreactors have been developed in many different typologies to overcome the issues found in open ponds. Closed photobioreactors are based on closed hydraulic circuits, mostly tubular, through which the algal culture circulates. This type of system allows more intensive algae growth, requires less land surface and does not present contamination risks. However, with this technology, the oxygen produced during the photosynthesis, which is toxic to the algae and bacteria, may become an issue and might be hard to eliminate. Additionally, the cost of installation is around ten times higher than the cost of raceway photobioreactors. Due to the high costs, closed photobioreactors are not yet economically viable.

Current photobioreactors present important limitations to producing microalgae and cyanobacteria at a large scale. Whereas some of them require enormous amounts of energy to operate, others present prohibitive construction, installation and maintenance costs.

Today, the bottleneck for the production of massive amounts of microalgae and cyanobacteria is the photobioreactor itself. What is needed is a developed photobioreactor that makes it technically and economically feasible to produce microalgae and cyanobacteria on a large scale and ultimately makes producing biofuels competitive with that of fossil fuels.

BRIEF SUMMARY OF THE INVENTION

The illustrated embodiments of the invention respond to the specific need for mass, cost-effective microalgae and cyanobacteria biomass production. The illustrated embodiments of the invention are based on a concept which offers the advantages of both raceway and closed tubular photobioreactors, namely the low costs of construction, operation and maintenance for the open raceway technology and the advantageous features of the closed photobioreactors (i.e. high production rates and biological safety). The illustrated embodiments of the invention respond to the specific need for an economically viable mass production of biofuels. The illustrated embodiments of the invention include a closed photobioreactor which can operate in a continuous manner either in an outdoor or indoor environment.

In particular, one of the illustrated embodiments of the invention comprises a novel photobioreactor which comprises a hydraulic circuit through which the aqueous solution containing the culture with at least one type of photosynthetic organism circulates and gets exposed to the light source, a carbon dioxide feeding system, a zone to extract the oxygen from the aqueous solution, and a nutrient feeding system.

In one particular embodiment, the hydraulic circuit is comprised of two receiving channels in which the aqueous solution containing the algal/bacterial culture is exposed to the atmosphere, where both receiving channels are at the same elevation where a set of transparent or translucent tubes connect the first and the second receiving channels, and where at least one fluid moving device moves the aqueous solution from the first to the second receiving channel through the transparent or translucent tubes.

In another embodiment, the present invention comprises a hydraulic circuit open to atmospheric pressure in which the algal/bacterial culture may be protected from external pollution since the photosynthesis reaction occurs within and along the transparent tubes. As a result, in order to protect the algal/bacterial culture from external pollution, it is only necessary to cover the receiving channels, which surface is an insignificant portion with respect to the total surface of the photobioreactor. In addition, since the system is closed to contamination, it is very straightforward and cost effective to eliminate the oxygen and to feed the nutrients and carbon dioxide into it.

The oxygen concentration of the aqueous solution increases as it passes through the tubes where photosynthesis takes place. Thus, the length and diameter of the transparent or translucent tubes are calculated to predetermined levels in each application according to well understood design principles in order to avoid toxic oxygen concentrations. This generated oxygen is then eliminated in the next receiving or downstream channel.

Because the system is an open hydraulic system (i.e. at atmospheric pressure), moving the algal/bacterial solution throughout the photobioreactor is straightforward and inexpensive. In one embodiment, the fluid moving device induces a difference in the surface level between the sides of the moving device. In addition, the same device may help to eliminate the oxygen from the algal/bacterial solution (i.e. degasification). The fluid moving device may be a rotary direct lift device such as a paddle wheel, noria, scoop wheel, etc. which may or may not incorporate perforated blades to enhance degasification.

In one embodiment, each receiving channel includes at least one fluid moving device.

In another preferred embodiment, carbon dioxide and nutrients are fed into the culture from both receiving channels simultaneously.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Turning toFIG. 1, one of the illustrated embodiments of the invention provides for a tubular photobioreactor10open to atmospheric pressure which comprises a horizontal tube set3disposed between a first receiving channel1and a second receiving channel2designed for massive algal/bacterial production. An aqueous solution contained within the receiving channels1,2is circulated by a first fluid moving device11and a second fluid moving device21respectively as best seen inFIGS. 2 and 7.

In one embodiment of the invention, the horizontal tube set3of the photobioreactor10comprises a plurality of subsets or “ramps”31,32of tubes as seen inFIGS. 2, 3 and 7. For definitional purposes, ramps comprising the right-hand hemisphere of the horizontal tube set3as seen inFIG. 2are denoted with reference numeral31, while ramps comprising the left-hand hemisphere of the horizontal tube set3are denoted with reference numeral32. Each ramp31,32in turn comprises a plurality of tubes311,312,313,314,315,316seen in the magnified cross sectional view ofFIG. 4. Each individual tube311-316is preferably comprised of low-density polyethylene (LDPE) plastic with a thickness preferably between 800 and 2000 gauges (1 gauge=0.01 mil). Each tube311-316is placed horizontally and parallel to each other within each ramp31,32. Each tube311-316is also transparent or translucent enough to become an efficient sunlight collector. The tubes311-316are assembled in sets of six within each ramp31,32, however it is to be expressly understood that each ramp31,32may contain fewer or additional tubes311-316than from what is seen inFIG. 4without departing from the original spirit and scope of the invention.

Each ramp31,32is placed on a portion of leveled ground6which is covered by a plastic film (not shown). The plastic film is black or opaque on the bottom to prevent new vegetation growth beneath the ramps31,32, and white on the top to enhance light reflection along the surface area of the leveled ground6beneath the ramps31,32.

Each end of the plurality of tubes311-316within each of the plurality of ramps31,32are coupled individually to the first receiving channel1and second receiving channel2respectively as best seen inFIGS. 2 and 7. The receiving channels1,2collect the aqueous solution which contains an algal/bacterial culture5. The aqueous solution and algal/bacterial culture5travel in the same direction within each of the ramps31,32, specifically from the first receiving channel1to the second receiving channel2in the right-hand hemisphere of ramps31, and from the second receiving channel2to the first receiving channel1in the left-hand hemisphere of ramps32.

As the algal/bacterial culture5travels through the horizontal tube set3, incoming solar light crosses into the transparent tubes311-316within each ramp31,32and causes a photosynthesis reaction with the algal/bacterial culture5. Oxygen that is generated during the photosynthesis reaction along the tubes311-316is carried to the receiving channels1,2where it is eliminated by degasification. Carbon dioxide and other nutrients are fed into the receiving channels1,2which prepare the algal/bacterial culture5for a subsequent photosynthetic process to take place while it travels through the opposing set of ramps31,32in the opposite direction towards the other receiving channel1,2in which it came. Thus it can be seen that the receiving channels1,2and ramps31,32work together to form a unidirectional loop or circuit, namely from the first receiving channel1through the right-hand hemisphere of ramps31to the second receiving channel2, and then back to the first receiving channel1through the left-hand hemisphere of ramps32. In order to circulate the aqueous solution through the loop, each of the receiving channels1,2drives the aqueous solution with an inbuilt fluid moving device11,12. Each fluid moving device11,12may be any apparatus known in the art for moving a fluid such as a paddle wheel, noria, scoop wheel, or any other similar apparatus. The fluid moving devices11,12move the aqueous solution along the receiving channels1,2in the direction of the arrows shown inFIG. 2, namely from one set of tubes to the other set of tubes so that the aqueous solution may be driven towards the other respective receiving channel1,2.

According to the above description, it is preferred that the number of ramps31,32be even. In the preferred illustrated embodiment shown inFIGS. 2, 3, and7, the photobioreactor10comprises at least ten ramps31,32, five ramps31flowing into the second receiving channel2, and five ramps32flowing into the first receiving channel1. The length of the tubes311-316are preferably between 50 to 80 meters. Additionally, it is preferred the diameter of the tubes311-316to be between of 12.5 and 15 centimeters. It is to be expressly understood however that the length and diameter of each of the tubes311-316may be different from what is disclosed above and may be varied in order to avoid oxygen levels from reaching toxic conditions within the algal/bacterial culture5. The disclosed parameters of the tubes311-316may also be adjusted during the photosynthesis process according to changing microalgae or cyanobacteria growing conditions.

The receiving channels1,2may be built of different materials such as HDPP, HDPE, polyester, or a combination thereof. The dimensions of the receiving channels1,2are such that enough surface area is present to allow for degasification. Additionally, the depth of the aqueous solution in the downstream portion8of the receiving channels1,2is at least a few centimeters greater than that the diameter of the tubes311-316. In another embodiment, the receiving channels1,2are built from brick, although this is not the most optimal solution. In yet another embodiment, the receiving channels1,2may serve as sun collectors in addition to the tubes311-316if covered with transparent materials known in the art (not shown). In this embodiment, the photobioreactor10does not comprise any dark areas in order to allow the algal/bacterial solution5to react with the incoming solar light throughout its entire circulation through the photobioreactor10.

The fluid moving device11,12is disposed in the center point of each receiving channel1,2as seen inFIGS. 2 and 7. In one preferred embodiment, the fluid moving device11,12comprises a plurality of radially disposed blades113,114that are rotated by an electric motor111as best seen inFIG. 5. The electric motor111may be any type or model of electric motor now known or later devised and may have a power capacity as little as one horsepower (HP). Alternatively, the motor111may not be electric at all but rather a traditional gasoline or diesel motor as is known in the art. The motor111is coupled to the fluid moving device11,12via a driveshaft112, or other similar speed variation means through which the revolutions per minute of the fluid moving device11,12may be controlled in order to achieve an optimal fluid velocity along the receiving channels1,2. The optimal fluid velocity preferably avoids algal/bacterial culture deposition on the internal walls of the various components of the photobioreactor10including the receiving channels1,2and ramps31,32.

The pitch of the plurality of blades113,114of the fluid moving devices11,12are such that, when the fluid moving device11,12is in motion within the algal/bacterial culture5, a difference is created in the fluid level between an upstream portion7and an downstream portion8of the receiving channels1,2as best seen inFIG. 6. For definitional purposes, “upstream” refers to the flow of the aqueous solution and algal/bacterial culture5before it makes contact with the fluid moving device11,12, while “downstream” refers to the flow of the aqueous solution and algal/bacterial culture5after it has made contact with the fluid moving device11,12. The difference in fluid levels between the upstream portion7and the downstream portion8of the receiving channels1,2is maintained by a substantially wedged shaped dam4. As the aqueous solution and algal/bacterial culture5enter into the receiving channels1,2via the plurality of ramps31,32and their corresponding tubes311-316, the algal/bacterial culture5collects in the upstream portion7of the receiving channel1,2. As the fluid moving device11,12is set into a counterclockwise motion indicated by the arrow seen inFIG. 6, the algal/bacterial solution5is drawn to the plurality of blades113,114. As the fluid moving device11,12continues to spin, the plurality of blades113,114continually push the algal/bacterial culture5up and over the dam4and into the downstream portion8of the receiving channel1,2. The upstream surface of dam4may be curved to assist in the formation of wavelets in the culture5, like a sloped beach, as it is pushed by the blades113,114, which wavelets then crest or spill over the top of dam4to the downstream side. The dam4not only prevents the algal/bacterial culture5that has entered the downstream portion8from re-entering the upstream portion7of the receiving channel1,2, but it also prevents waves that have reflected off the inner walls of the receiving channel1,2from coming back and opposing the motion of the plurality of blades113,114and thus unnecessarily increasing the load on the motor111. Because the fluid level in the downstream portion8is higher than in upstream portion7, a sufficient fluid pressure differential is created which moves the aqueous solution and algal/bacterial culture5out of the original receiving channel1,2and along the ramps31,32towards the opposing receiving channel1,2where the entire process is repeated. With a fluid moving device11,12located in both the first and second receiving channels1,2respectively, it is ensured that a higher fluid level is maintained within the downstream portion8of each receiving channel1,2and it is in this manner that circulation of the algal/bacterial culture5throughout the entirety of the photobioreactor10is achieved for as long as the fluid moving devices11,12are in operation.

In addition to moving the algal/bacterial culture5within the aqueous solution, each fluid moving device11,12also facilitates in the gas exchange (i.e. the release of oxygen and the capture of carbon) as well as in the homogenous mixing of the algal/bacterial culture5with nutrients. As the algal/bacterial culture5is pushed along the transparent tubes311-316, the oxygen concentration increases as the photosynthesis reactions take place. As the algal/bacterial culture5enters the receiving channels1,2, degasification of oxygen takes place due to the turbulence created by the fluid moving device11,12as it makes contact with the algal/bacterial culture5. Additionally, each of the receiving channels1,2may comprise a means for injecting nutrients90as well as pure carbon dioxide or carbon dioxide streams91into the algal/bacterial culture5as it travels through the receiving channels1,2. In one embodiment, it is preferred that the nutrients90are injected into the receiving channels1,2just before the algal/bacterial culture5makes contact with the fluid moving device11,12as seen inFIG. 6so as to take maximum advantage of the turbulence and mixing effect created by the plurality of blades113,114. Therefore, in one embodiment, some or all of the plurality of blades113,114may include perforations or be made from a metal mesh so as to more effectively mix the algal/bacterial culture5with the nutrients and/or to aid in the exchange of gases. Preferably, the carbon dioxide is injected into the algal/bacterial culture5in the downstream portion8of the receiving channels1,2with a carbon dioxide injecting device91located near or at the beginning of each tube311-316in order to minimize losses.

The velocity of the aqueous solution and the algal/bacterial culture5as it travels within the photobioreactor10is regulated by varying the rotation speed of the fluid moving device11,12via the electric motor111, or by operating only one of the two fluid moving devices11,12contained within the photobioreactor10.

While it is thus apparent that the preferred embodiment shown and described provides certain advantages, many of the advantages of the present invention can nevertheless be realized in other configurations, and it will be appreciated that various modifications, changes and adaptations can be made, all of which are intended to be comprehended within the meaning and range of equivalents of the appended claims.

Many alterations and modifications may be made by those having ordinary skill in the art without departing from the spirit and scope of the invention. Therefore, it must be understood that the illustrated embodiment has been set forth only for the purposes of example and that it should not be taken as limiting the invention as defined by the following invention and its various embodiments.