The present invention pertains generally to systems and methods for growing microalgae. More particularly, the present invention pertains to the use of a system that can grow microalgae in a cold climate area. The present invention is particularly, but not exclusively, useful as a system for growing algae in a cold climate area that uses heat byproducts from power plants, and an underground sump, to maintain a temperature conducive to algae growth.
As worldwide petroleum deposits decrease, there is rising concern over petroleum shortages and the costs that are associated with the production of carbon-based fuel sources. As a result, alternatives to products that are currently processed from petroleum are being investigated. In this effort, biofuel has been identified as a possible alternative to petroleum-based transportation fuels. In general, a biodiesel is a fuel comprised of mono-alkyl esters of long chain fatty acids derived from plant oils or animal fats. In industrial practice, biodiesel is created when plant oils or animal fats are reacted with an alcohol, such as methanol.
Apart from using animal fats, the creation of biofuels from plant oils has gained wide attention in recent years. The process of creating biofuel from plant oils, of course, necessarily begins by growing and harvesting plants such as algae cells. In particular, algae is known to be one of the most efficient plants for converting solar energy into cell growth, so it is of particular interest as a biofuel source.
In an algae cultivation system, the algae cells are typically grown as part of a liquid medium that is often exposed to sunlight to promote photosynthetic growth. Further, the algae cell growth process normally requires the liquid medium to be circulated through the system. Due to heating requirements for cell growth, geographic areas with warmer climates and higher degrees of solar insolation are preferred locations for algae cultivation systems. In particular, locations with warmer climates allow the temperature of the liquid culture to remain sufficiently warm, for a sufficient period of time, to promote efficient algae cell growth. On the other hand, freezing or near-freezing conditions will cause serious algae cell growth problems. Cold temperatures will greatly inhibit, or even stop, the growth of algae cells. Clearly, slowing or stopping the growth of algae cells is detrimental to an algae cultivation system. And, to produce biofuel in a cost effective manner as compared to carbon-based fuel products, disruptions in algae cultivation cannot occur. Consequently, any stopping or slowing of algae growth will make an algae growth system economically unsustainable.
Like most plants, algae cells do not grow effectively in cold weather. At the present time, the predominant methods used to grow algae for use in biofuel production are limited to geographic areas with warmer climates. As a consequence, many suitable sites in cold climate areas are not being efficiently exploited. Thus, by developing an algae growth system that is optimized for cold climate regions, the geographic footprint available for biofuel production facilities could be increased dramatically.
In light of the above, it is an object of the present invention to provide a system and method for growing microalgae for biofuel production in cold climate areas. Another object of the present invention is to provide a system and method for growing microalgae that expands the geographical footprint of areas suitable for biofuel production. Still another object of the present invention is to mitigate pollution by recycling heat and CO2 byproducts produced by power plants to grow microalgae. Yet another object of the present invention is to provide a system and method for growing microalgae for biofuel production in cold climate areas that is simple to implement, easy to use, and comparatively cost effective.
In accordance with the present invention, a system and method for cold climate algae growth is provided. As envisioned for the present invention, the system is constructed in a cold climate area and is co-located with a power plant that produces heated cooling water and CO2 as byproducts. Structurally, the system comprises an expanding Plug Flow Reactor (ePFR) connected to an underground sump. The underground sump is provided for storing the algal culture during periods of extreme cold temperature. In an operation of the present invention, the algal culture can be transferred from the ePFR to the sump, and vice versa, as required to ensure algae growth is not hindered by cold temperatures.
As mentioned above, the system of the present invention begins with a plug flow reactor (PFR) that is used to grow an algal culture. In further detail, the PFR comprises a plurality of individual ponds. Preferably, the ponds are each elongated in shape and form a raceway type cultivation pond with a configuration that is well-known in the trade. Collectively, the plurality of individual ponds creates an expanding PFR (ePFR), meaning that the ponds are arranged in order of increasing capacity, with the first pond being the smallest and kept under sterile conditions. Importantly, each pond is in fluid communication with adjacent ponds to facilitate transfer from one pond to the next larger pond as required.
For their construction, each pond of the ePFR is preferably constructed with a sloped bottom portion that provides for gravitational fluid flow through the pond to facilitate the mixing of algae cells with nutrients. Furthermore, the bottom portion is positioned between opposite sidewalls to form a shallow fluid flow channel that will maximize the exposure of the algae to sunlight. A light-transmitting, insulating cover can be attached to each pond to extend between the sidewalls, and the cover is positioned opposite the bottom of the pond. Further, the light-transmitting cover should be transparent or translucent, and constructed with lightweight plastic to allow for floatation on top of the algal culture. To further promote floatation, the plastic used to construct the cover may include sealed air cells. By constructing the cover in this manner, the cover is dual-purpose as solar energy required by the algae cells for photosynthesis can still enter the system, and the cover provides an insulative effect. In addition to the light-transmitting cover, an insulation liner is constructed on top of the bottom and the sidewalls of each pond. For a preferred embodiment of the present invention, the insulation liner is sprayed onto the bottom and sidewalls during construction of the ePFR to prevent heat losses due to thermal conduction to the ground.
In addition to the ePFR, the present invention includes an underground sump that is connected by a pipe to the ePFR. In one embodiment, the underground sump may be divided into separate chambers, with each chamber receiving algal culture from a dedicated cultivation pond of the ePFR. In an alternate configuration, one underground sump may be provided for each of the individual cultivation ponds. As contemplated for the present invention, the underground sump is connected to the downstream end of the ePFR by a pipe having a valve. In this configuration, the valve can be opened to allow for gravity flow of the algal culture from the ePFR during periods of extreme cold temperature. Most often, these periods of extreme cold temperature occur at night. In a preferred embodiment, only one pipe is used to move the algal culture into the sump and from the sump back into the ePFR. Configurations using multiple pipes, however, may also be used. While gravity flow may be sufficient to move the algal culture from the ePFR to the sump, a pump is necessary to transfer the algal culture from the sump back to the upstream end of the ePFR. Furthermore, the pump may also be configured to move the algal culture from the ePFR to the sump, if necessary.
The system of the present invention also adds heat from the power plant to the underground sump. To do this, the power plant is connected to the underground sump by a water pipe. This water pipe carries heated cooling water from the power plant to a first heat exchanger placed in the underground sump. Once the heated cooling water reaches the first heat exchanger, the heat from the heated cooling water is transferred into the stored algal culture in the underground sump. To facilitate the addition of heat to the growing algal culture, a second heat exchanger is provided and placed into the ePFR. The water pipe is constructed with a directional valve that can close to stop the flow of heated cooling water. And, the directional valve can be configured to direct the heated cooling water into either the first heat exchanger or the second heat exchanger. When heat is required in the ePFR, heated cooling water is directed to the second heat exchanger which will transfer heat from the heated cooling water into the culture in the ePFR. The cooled cooling water effluent from the heat exchanger flows back to the power plant.
Flue gas produced by the power plant is recycled into the system of the present invention. Once the flue gas leaves the power plant, it is piped to a CO2 absorber through a gas pipe. Makeup media is also piped from an algae processor to the CO2 absorber. The makeup media is created in the algae processor by separating and removing mature algae cells from the algal culture and has a high concentration of sodium carbonate. This makeup media will act as an absorbent for CO2 and heat present in the flue gas. Once absorption has occurred, makeup media is enriched with bicarbonate. At this point, the makeup media is added to the ePFR to act as a heat and carbon source for the growing algal culture.
In operation, the light-transmitting, transparent/translucent insulating cover is attached between the sidewalls of the ePFR. This attachment can occur prior to the introduction of algal culture into the ePFR or after the introduction of algal culture into the ePFR. Both heat losses and evaporation losses are minimized by placing the cover onto the ePFR. When the algal culture is introduced into the ePFR, it remains in continuous motion due to: (1) the sloped configuration of the individual ponds of the ePFR and (2) a mixing means, such as a paddle or a pump. While the algal culture is being mixed within the ePFR, byproducts from the power plant are being collected. As mentioned previously, these byproducts are heated cooling water and flue gas. The heated cooling water is piped directly from the power plant through the second heat exchanger and into the ePFR to provide heat to the growing algal culture. In addition, flue gas from the power plant is piped to the CO2 absorber where it is absorbed by makeup media. After absorption, the makeup media is fed into the ePFR through a conduit to both nourish and heat the growing algal culture.
During periods of extreme cold temperature, the valve of the underground sump is opened to allow for the algal culture to flow from the ePFR into the underground sump. While stored in the underground sump, the algal culture will be protected from the type of growth disruptions that may be caused by cold weather. This is accomplished in several ways: (1) heated cooling water from the power plant is piped to the underground sump via the first heat exchanger to warm the stored algal culture, (2) heat losses due to environmental conditions are minimized by the insulative properties of the surrounding soil, and (3) the surface area of the algal culture is reduced when exposed to ambient air. Once the period of extreme cold temperature has passed, the algal culture is pumped from the underground sump back to the ePFR where algae cells can continue to grow.
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:
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While the particular Microalgae Cultivation System for Cold Climate Conditions 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.