GB2614560A

GB2614560A - Algae cultivation apparatus

Info

Publication number: GB2614560A
Application number: GB2200172.1A
Authority: GB
Inventors: John Richards Stephen
Original assignee: Nature Based Solutions Global Ltd
Current assignee: Nature Based Solutions Global Ltd
Priority date: 2022-01-07
Filing date: 2022-01-07
Publication date: 2023-07-12
Anticipated expiration: 2042-01-07
Also published as: GB2614560B; WO2023131785A1

Abstract

An algae cultivation apparatus 10 is provided comprising an algae-cultivation vessel 12 defining a fluid-receiving chamber 16 for receiving algae-cultivation medium 18, an algae-harvesting outlet 20a, 20b connected to the fluid-receiving chamber 16. A gas inlet 5 28 at or adjacent to a base 24 or in-use lower end of the algae-cultivation vessel 12, the gas inlet 28 having a controllable valve for altering a flow of gas into the algae-cultivation vessel 12 through the gas inlet 28. A sensor device 30a, 30b is configured to measure a characteristic of an algae-cultivation medium 18 in the fluid-receiving chamber 16, the controllable valve being automatically controllable in response to an output of the sensor 10 device 30a, 30b. The sensor may be a temperature sensor device and the apparatus may further comprise an electrical flocculation means. The vessel may have a diameter and/or depth of 500mm. Also disclosed is an algae cultivation system using the apparatus and a method of improving the yield of algae farming

Description

Algae Cultivation Apparatus The present invention relates to an algae cultivation apparatus which has an improved algae cultivation vessel. The invention further relates to an algae-cultivation system comprising a plurality of said apparatuses. The invention yet further relates to a method 5 of improving the yield of algae farming utilising such a system.

The production of biofuel, that is, fuel provided from biomass, is a potential solution to the problem of excess greenhouse gas production, and particularly for reducing carbon dioxide (CO2) emissions. Improved biofuel production techniques allow for the replacement of fossil fuels in the green energy production cycle.

Many biofuels have been manufactured in a laboratory setting, but the scaling up of production to commercially-viable levels capable of producing large quantifies of biofuel has proven difficult in terms of scale and efficiency.

Algae farming is one avenue of exploration for biofuel production. This requires a growth medium which provides sufficient nutrients for algal growth, as well as sufficient sunlight for photosynthesis. Algae is currently farmed in raceway circulating man-made rivers, since algae need to be well circulated in order to not become stagnant and die. Such raceways work to provide continuous flow, with light penetrating the surface of the medium to a depth of around 300mm. As such, in order for the raceway to operate well, a large area is required to ensure that good photosynthesis properties can occur. Photo-bioreactors have also been trialled.

Neither of these techniques have been successfully scaled to produce harvestable algae in sufficient quantities for use as biofuel.

The present invention seeks to provide an improved vessel for algal cultivation, as well as an improved method for cultivating algae, in order to overcome the above-described limitations.

According to a first aspect of the invention, there is provided an algae cultivation apparatus comprising: an algae-cultivation vessel defining a fluid-receiving chamber for receiving algae-cultivation medium; an algae-harvesting outlet connected to the fluid-receiving chamber; a gas inlet at or adjacent to a base or in-use lower end of the algae-cultivation vessel, the gas inlet having a controllable valve for altering a flow of gas into the algae-cultivation vessel through the gas inlet; and a sensor device configured to measure a characteristic of an algae-cultivation medium in the fluid-receiving chamber, the controllable valve being automatically controllable in response to an output of the sensor device.

The present invention allows for the automatic control of the temperature of the algae-cultivation medium by changing the rate of gas flow through the gas inlet. This can significantly change the turbulence in the system which in turn alters the rate of evaporation. With a vessel of this type, improved yields of algae growth can be harvested, since this solves a considerable issue in the art; how to maximise the absorption of natural light in an open-topped vessel without having problems with temperature regulation.

Optionally, the sensor device may be a temperature sensor device.

Since the gas flow through the vessel permits temperature regulation by modulation of the rate of evaporation, a temperature sensor device is the most important of the types 15 of sensor to be considered as part of the present apparatus.

The algae cultivation apparatus may further comprise a controller which is communicatively coupled with the controllable valve and the sensor device, the controller automatically operating the controllable valve in response to the output of the sensor device.

A dedicated controller can be used to not only provide the automatic control of the opening and closing of the valve, but also may allow for control of the other feedback options for the apparatus, in connection with the other monitorable characteristics.

The algae cultivation apparatus may further comprise a gas source in communication with the gas inlet, wherein the gas source comprises CO2-enriched air.

Not only is CO2 necessary for photosynthesis, and therefore the activity and growth of the algae, but it also provides a suitable means of providing temperature control to the algae-cultivation medium.

The algae cultivation apparatus may further comprise an algae-cultivation medium inlet which is fluidly communicable with the fluid-receiving chamber.

The ability to top up the algae-cultivation medium is very important for the present vessel, since the modification of the rate of evaporation has significant consequences for the fluid level therein.

Optionally, the algae-cultivation medium inlet may be controllable in response to an 5 output of the sensor device.

It is preferred that the topping up of the algae-cultivation medium be performed automatically, as soon as certain pre-determined threshold conditions are met. This eliminates the prospect of human error.

Preferably, the algae-cultivation medium inlet may be connected to any or all of an 10 algae-cultivation-medium reservoir; a nutrient-enriched algae-cultivation-medium reservoir; and a nutrient reservoir.

It is preferred that various different options for topping up the algae-cultivation medium be provided, particularly in respect of the nutrient content thereof, so as to allow for full control over the growth conditions therein.

Optionally, a plurality of sensor devices may be provided, each sensor device monitoring a different characteristic associated with the algae-cultivation apparatus. Preferably, the said different characteristics may include any of dissolved 02; dissolved 002; pH; electrical conductivity; salinity; nitrogen content; and phosphorous content.

A multi-purpose sensor configuration allows for many or all of the relevant characteristics 20 of the algae-cultivation medium to be controlled.

In a preferable embodiment, the algae-cultivation vessel may have a flat base.

A flat-based vessel may be simpler to manufacture and transport, which may simplify the assembly of an algae cultivation plant on a large scale.

In an alternative embodiment, the algae-cultivation vessel may have a sloped base.

An at least in part sloped base has the advantage of allowing for bottom-flocculation of algae, which may be advantageous for the ease of harvesting from a large-scale vessel.

The algae-cultivation vessel may have a rigid support frame and walls formed of a flexible material forming the fluid-receiving chamber.

A simple vessel structure of this type can be manufactured cost-effectively, allowing for large-scale rollout to be conducted very quickly.

The algae-cultivation apparatus may further comprise a dosing system.

Dosing can be used to improve the state of the algae-cultivation medium, for example, if 5 there are pH imbalances, or if chemical flocculation is desired.

The algae-cultivation apparatus may further comprise an electrical flocculation means.

Electric flocculation may allow for easier bottom-flocculation within the vessel, which has the advantage of enabling simpler harvesting for the user.

Optionally, the algae-cultivation vessel may be at least in part light-transmissible.

Light-transmissible walls of the vessels allow for increased light penetration of sunlight into the algae-cultivation medium, which in turn allow for improved growth conditions.

Preferably, the algae-cultivation vessel may have a diameter and/or depth of at least 500mm.

Large-scale vessels have to date not been viable to provide economic temperature control thereto, which is now possible within the scope of the present invention. As noted above, it has been previously deemed problematic in the art for vessels to have a depth of greater than 300mm, since light cannot penetrate through the medium to allow photosynthesis to occur in this scenario. By using the gas flow in the medium, the algae in the medium is cycled so that all algae is exposed to sufficient levels of sunlight to photosynthesise and grow. This eliminates the need for a large-footprint raceway, as used in the art.

According to a second aspect of the invention, there is provided an algae-cultivation system comprising a plurality of algae-cultivation apparatuses in accordance with the first aspect of the invention.

Optionally, the algae-cultivation vessels may be open-topped vessels.

It has been difficult to use open-topped vessels in the art for algae growth to date, since there is too great a risk of contamination from, for example, the ingress of contaminating matter. Dead birds and snakes are common forms of contamination in raceways. By providing individual small-footprint vessels as in the present invention, the risk of contamination of the entire algae-cultivation plant is reduced in the event of a single contamination incident; one dead bird will only contaminate a single vessel, rather than the entire raceway.

According to a third aspect of the invention, there is provided a method of improving the yield of algae farming, the method comprising the steps of: a] providing an algae-cultivation apparatus according to a first aspect of the invention; b] growing algae in the algae-cultivation vessel when the fluid-receiving chamber contains a liquid algae-cultivation medium; and c] introducing a controllable flow of gas into the algae-cultivation apparatus via the gas inlet from the base of the algae-cultivation vessel to encourage circulation of the liquid algae-cultivation medium.

Optionally, the flow rate of the gas may be adjustable in response to the output of the sensor device.

The automatic control of the gas in response to the sensor provides several benefits, 15 including the control of the CO2 level in the algae-cultivation medium for photosynthesis, as well as possible temperature benefits. The gas also prevents stagnation of the algae-cultivation medium.

Preferably, the sensor device may be a temperature sensor device, the flow of gas being used to modulate the temperature of the algae-cultivation medium.

Temperature control of the algae-cultivation medium by using the gas flow therein allows for the use of open-topped vessels which have not previously been deemed feasible within the art. A greater photosynthetic boon can be achieved in hot and sunny climates. The ability to achieve temperature control of the vessels for optimum growth enables much greater yields of algae production. In conjunction with the feedback control of the other parameters of the algae-cultivation vessel, explosive growth of the algae is readily achieved, thereby enabling higher yields.

Optionally, the gas may be a CO2-enriched air.

CO2 is required for algal growth, as a necessary input for photosynthesis, but it also has the advantage of being able to modulate the temperature of the algae-cultivation medium 30 via changes to the rate of evaporation from the algae-cultivation vessel.

According to a fourth aspect of the invention, there is provided an algae cultivation apparatus comprising: an algae-cultivation vessel defining a fluid-receiving chamber for receiving algae-cultivation medium; an algae-harvesting outlet connected to the fluid-receiving chamber; a gas inlet to the algae-cultivation vessel, the gas inlet having a controllable valve for altering a flow of gas into the algae-cultivation vessel through the gas inlet; and a sensor device configured to measure a characteristic of an algae-cultivation medium in the fluid-receiving chamber, the controllable valve being controllable in response to an output of the sensor device.

It may be advantageous to provide a manually controllable apparatus, in which there is 10 no direct feedback from the sensors to the gas inlet valve.

The invention will now be more particularly described, by way of example only, with reference to the accompanying drawings, in which: Figure 1 shows a diagrammatic representation from the side of a first embodiment of an algae-cultivation vessel in accordance with the first aspect of the invention; Figure 2 shows a diagrammatic representation from the side of a second embodiment of an algae-cultivation vessel in accordance with the first aspect of the invention; and Figure 3 shows a diagrammatic representation from the side of a third 20 embodiment of an algae-cultivation vessel in accordance with the first aspect of the invention.

Referring to Figure 1, there is illustrated an algae-cultivation apparatus, referenced globally at 10, having an algae-cultivation vessel 12. The algae-cultivation vessel 12 is here formed as a cylindrical or substantially cylindrical tank, which will allow stacking of 25 multiple algae-cultivation vessels 12 during the construction of an algae farm.

In a preferred embodiment, the walls 14 of the algae-cultivation vessel 12 may be formed from a flexible liner-type material. This could be formed from rubber, or a similar flexible plastics material. It is preferable that the walls 14 be fully or partially light-transmissible, allowing light to pass into a fluid-receiving chamber 16 defined by the algae-cultivation vessel 12 to aid photosynthesis of algae therein. In this scenario, the algae-cultivation vessel 12 is formed from a rigid support frame with the walls 14 being formed of a flexible material forming the fluid-receiving chamber 16.

A dimension of the algae-cultivation vessel 12 preferably exceeds 500mm in diameter. An indicative algae-cultivation vessel 12 anticipated for use is 2400mm in diameter, though vessels up to 3000mm in diameter may be feasible. The height of the algae-cultivation vessel 12 is anticipated as being at least 300mm in height, and more preferably in the range of 2400mm to 3000mm.

The fluid-receiving chamber 16 is fluid-tight, and an algae-cultivation medium 18 is receivable therein. Such an algae-cultivation medium 18 may be fresh or salt water for 10 instance.

At least one algae-harvesting outlet 20a, 20b is provided which is fluidly communicable with the fluid-receiving chamber 16. The depicted embodiment shows both an upper algae-harvesting outlet 20a which is positioned towards an upper, preferably open, end 22 of the algae-cultivation vessel 12, and a lower algae-harvesting outlet 20b which is positioned towards a base 24 of the algae-cultivation vessel 12. Both algae-harvesting outlets 20a, 20b may be provided, or the upper or lower algae-harvesting outlets 20a, 20b only may be provided, depending on whether the algae strain used is a top-or bottom-flocculating variety, or depending on the flocculation means used. The base 24 is shown as a flat base, which allows for deposition of bottom-flocculated algae thereon without disturbance.

At the base 24 of the algae-cultivation vessel 12, here shown centrally in the algae-cultivation vessel 12, there is a gas inlet conduit 26 having a gas inlet 28, through which a gas flow can be introduced into the fluid-receiving chamber 16. A controllable valve is provided at the gas inlet 28 for controlling this gas flow. The gas flow is firstly used to encourage circulation, via the movement of the gas through the algae-cultivation medium, and also provides CO2 for photosynthesis to the algae to encourage growth. This can be enhanced further by the gas from a gas source being provided as a CO2enriched air. The cycling of the algae within the medium ensures that all algae is exposed to sufficient sunlight for photosynthesis, so that algae never remains in a low-light part of the vessel, that is, more than 300mm from a light-transmissible surface thereof, for long. A greater volume of algae can be produced in a much smaller-footprint vessel than a comparable raceway.

The gas source itself may preferably be a CO2-enriched air from other carbon producing activities on-site, such as from CO2 emissions from an associated biomass reactor which burns the biomass produced from the harvested algae, either in the form of algal biomass, or algae oil.

The gas inlet conduit 26 may be provided as a rigid pipe, such as a metal gas flow pipe, or could be provided as a flexi-pipe. This may allow the gas inlet conduit 26 to also act as an initial fluid inlet into the algae-cultivation vessel 12 on initial top-up, or as a drain to empty the fluid-receiving chamber 16.

At least one sensor device 30a, 30b is provided which allows for the determination of at least one condition of the algae-cultivation medium. A ball float sensor device 30a is illustrated which provides a fluid-level indication within the fluid-receiving chamber 16. Furthermore, a combined sensor device 30b is illustrated which is submerged in the algae-cultivation medium 18. This sensor device 30b may be configured to monitor any or all of: temperature; dissolved 02; dissolved CO2; PH; electrical conductivity; salinity; nitrogen content; and phosphorous content. In most arrangements, temperature monitoring will be the primary sensed characteristic, as will be discussed in more detail below. However, the monitoring of a plurality of, and preferably all of, the above-referenced characteristics will synergistically allow for the maintenance of an optimum environment for explosive growth. It is noted that the optimum conditions may be dependent on the strain of algae used, which may differ for different strains, and specific values for these characteristics are not outlined for brevity. The optimum values will, however, be predetermined for each algae strain in the given environment.

Monitoring of these characteristics also allows for feedback control to automatically change the algae-cultivation medium from one in which there are high-growth conditions, for which the predetermined characteristics will be known for a given algae strain, and a high-stress condition, for which the predetermined characteristics will be known for a given algae strain. The high-stress condition will slow the rate of algal reproduction, but will encourage lipid production instead, which is crucial for the production of algal oil, and thus for the production of biofuel. Modification of the nitrogen content, for instance, is a means of inducing high-stress conditions.

Further monitoring of the sensors and the monitored characteristics will provide data regarding the optimum time to harvest the algae for biofuel production, which can then be calculated. Automated harvesting conditions can therefore be set to remove manual intervention requirements.

The or each sensor device 30a, 30b may be communicable with the controllable valve associated with the gas inlet 28, such that the controllable valve can be operated automatically in response to feedback control from the or each sensor device 30a, 30b, based on the measurement of the sensed characteristics. It will be appreciated that this communication may be effected via an intermediate controller 32, which may have a processor configured to make decisions regarding actions to take in response to said sensed characteristics.

The algae-cultivation vessel 12 further includes an algae-cultivation medium inlet 34 which is fluidly communicable with the fluid-receiving chamber 16. There may be a controllable valve associated with the algae-cultivation medium inlet 34 which is in communication with the controller 32 or a further controller based on a sensed characteristics of the algae-cultivation medium 18. In this instance, it would be anticipated that the sensed characteristic would be a level of the algae-cultivation medium 18, and therefore a measured level from the ball float sensor device 30a could be used.

The algae-cultivation medium inlet 34 is here connected to any or all of: an algaecultivation-medium reservoir 36; a nutrient-enriched algae-cultivation-medium reservoir 38; and a nutrient reservoir 40. These respective reservoirs allow for: introduction of additional untreated algae-cultivation medium, for topping up the level of algae-cultivation medium 18 in the fluid-receiving chamber 16; introduction of additional nutrient-enriched algae-cultivation medium, which made be appropriate where no change to the nutrient concentration is desired; and introduction of nutrients directly into the existing algae-cultivation medium 18.

Such nutrients would typically be in the form of organic matter containing nitrogen, phosphorus, carbon, and some trace metal elements. This can be provided in the form of waste products, such as human or animal faeces, which may provide additional synergistic benefits where the apparatus is co-located with an agricultural facility. It will of course be possible to provide the various components of the overarching system on separate sites.

A dosing system may also be provided, preferably associated with the algae-cultivation medium inlet 34, but a dedicated alternative inlet could equally be supplied. Such a dosing system may be used to provide other required changes to the algae-cultivation medium 18, for instance, a means of altering pH or salinity. Other treatment systems could be considered which do not involve dosing, for example, an ultraviolet irradiation system configured to sterilise the algae-cultivation medium. The dosing system could also be used to provide chemical flocculants for encouraging flocculation of the algae for harvesting.

An electrical flocculation means is shown in the fluid-receiving chamber 16, comprising an anode 42 and cathode 44, via which an electrical current can be passed through the algae-cultivation medium 18. Timing the electrical current using the controller 32 or a further controller will cause the algae to deposit on the base 24 of the algae-cultivation vessel 12. The activation of the electrical flocculation means may be triggered simultaneously with the closing of the controllable valve associated with the gas inlet 28, since the circulatory effect of the gas flow inhibits deposition of flocculated algae.

Figure 2 shows a second embodiment of algae-cultivation vessel 112 which has a centrally positioned algae-harvesting outlet 120 at the base 124. The base 124 is then sloped towards the algae-harvesting outlet 120 so that flocculated algae 146 can be easily harvested. The gas inlet is not shown in this embodiment, for clarity.

Figure 3 shows a third embodiment of algae-cultivation vessel 212 which has an algae-harvesting outlet 220 positioned at one side of the base 224, the base 224 again being at least in part sloped towards said algae-harvesting outlet 212. A flat portion 248 of the base is provided which may then encourage settlement of the flocculated algae 246 without causing a blockage.

Both of the second and third embodiments of the algae-cultivation vessel 112; 212 are highly suited towards use with bottom-flocculating algae, and/or with electrical flocculation.

Within the scope of the invention, it is therefore possible to provide an algae-cultivation system comprising a plurality of algae-cultivation apparatuses 12; 112; 212 as previously 30 described. It is preferred that these algae-cultivation apparatuses be provided as open-topped vessels Open-topped vessels allow significant amounts of light in for photosynthesis, but lose heat to the atmosphere rapidly compared with sealed vessels.

In such an arrangement, the gas flow from the gas inlet of the algae-cultivation vessel additionally provides a means of temperature modulation, which can be automatically controlled using feedback from the sensors in the apparatus. Temperature modulation is achieved by using the gas flow to increase or decrease the rate of evaporation from the algae-cultivation vessel. Increasing the turbulence in the algae-cultivation medium will increase the rate of evaporation, and decreasing the turbulence will decrease the rate of evaporation.

It may also be feasible to change the rate of evaporation by increasing the atmospheric CO2 concentration by input of liquid CO2 through the gas flow, whilst also cooling the gas flow bubbled through the algae-cultivation medium. It is noted that this may also have the effect of generating carbonic acid in the algae-cultivation medium, decreasing pH, which may need to be countered with the addition of a base.

In some circumstances it is advantageous to induce stress to encourage algal growth. In the case of cultivation to produce biofuels, a high lipid content within the algae is desired. Lipid production can be induced and increased through environmental and nutrient stress. By precisely controlling the environment of the algae-cultivation medium used, it is possible to alter the characteristics of some strains of algae as well as providing the optimum conditions for explosive growth and reproduction of the algae. This allows the apparatus defined herein to achieve much higher yields than apparatuses in the art. Such biofuel has the advantage of being a direct replacement for diesel, without requiring modification of the vehicle to utilise the biofuel.

It will be appreciated that a ball float sensor has been described as the indicative arrangement of a level sensor within the above disclosure. Other types of fluid-level sensor will be apparent to the skilled reader. Furthermore, which a combined sensor package for monitoring other characteristics may be preferred, any number of individual or combined sensors could be provided in any combination to provide sensor output data relating to the full spectrum of monitorable characteristics which the user is intending to observe.

Whilst the gas inlet is described as being at or adjacent to the base of the vessel, it will be appreciated that any gas inlet could be provided as long as the desired turbulence effects could be generated.

It is therefore possible to provide an algae-cultivation apparatus which is suitable for providing higher yields of algae growth for biomass production, particularly in areas of the world with little temperature variation over time. The gas flow arrangement of the apparatus allows for improved temperature control effects to be introduced by automatic modulation of the rate of turbulence, and therefore evaporation, within the algae-cultivation medium.

The words 'comprises/comprising' and the words 'having/including' when used herein with reference to the present invention are used to specify the presence of stated features, integers, steps, or components, but do not preclude the presence or addition of one or more other features, integers, steps, components or groups thereof It is appreciated that certain features of the invention, which are, for clarity, described in the context of separate embodiments, may also be provided in combination in a single embodiment. Conversely, various features of the invention which are, for brevity, described in the context of a single embodiment, may also be provided separately or in any suitable sub-combination.

The embodiments described above are provided by way of examples only, and various 20 other modifications will be apparent to persons skilled in the field without departing from the scope of the invention as defined herein.

Claims

Claims 1. An algae cultivation apparatus comprising: an algae-cultivation vessel defining a fluid-receiving chamber for receiving algae-cultivation medium; an algae-harvesting outlet connected to the fluid-receiving chamber; a gas inlet at or adjacent to a base or in-use lower end of the algae-cultivation vessel, the gas inlet having a controllable valve for altering a flow of gas into the algae-cultivation vessel through the gas inlet; and a sensor device configured to measure a characteristic of an algae-cultivation medium in the fluid-receiving chamber, the controllable valve being automatically controllable in response to an output of the sensor device.
2. An algae cultivation apparatus as claimed in claim 1, wherein the sensor device is a temperature sensor device.
3. An algae cultivation apparatus as claimed in claim 1 or claim 2, further comprising a controller which is communicatively coupled with the controllable valve and the sensor device, the controller automatically operating the controllable valve in response to the output of the sensor device.
4. An algae cultivation apparatus as claimed in any one of claims 1 to 3, further comprising a gas source in communication with the gas inlet, wherein the gas source comprises CO2-enriched air.
5. An algae cultivation apparatus as claimed in any one of the preceding claims, further comprising an algae-cultivation medium inlet which is fluidly communicable with the fluid-receiving chamber.
6. An algae-cultivation apparatus as claimed in claim 5, wherein the algae-30 cultivation medium inlet is controllable in response to an output of the sensor device.
7. An algae-cultivation apparatus as claimed in claim 5 or claim 6, wherein the algae-cultivation medium inlet is connected to any or all of: an algae-cultivation-medium reservoir; a nutrient-enriched algae-cultivation-medium reservoir; and a nutrient reservoir.
8. An algae-cultivation apparatus as claimed in any one of the preceding claims, wherein a plurality of sensor devices is provided, each sensor device monitoring a different characteristic associated with the algae-cultivation apparatus.
9. An algae-cultivation apparatus as claimed in claim 7, wherein the said different characteristics include any of: dissolved 02; dissolved 002; PH; electrical conductivity; salinity; nitrogen content; and phosphorous content.
10. An algae-cultivation apparatus as claimed in any one of the preceding claims, wherein the algae-cultivation vessel has a flat base.
11. An algae-cultivation apparatus as claimed in any one of claims 1 to 8, wherein the algae-cultivation vessel has a sloped base.
12. An algae-cultivation apparatus as claimed in any one of the preceding claims, wherein the algae-cultivation vessel has a rigid support frame and walls formed of a flexible material forming the fluid-receiving chamber.
13. An algae-cultivation apparatus as claimed in any one of the preceding claims, further comprising a dosing system.
14. An algae-cultivation apparatus as claimed in any one of the preceding claims, further comprising an electrical flocculation means.
15. An algae-cultivation apparatus as claimed in any one of the preceding claims, wherein the algae-cultivation vessel is at least in part light-transmissible.
16. An algae-cultivation apparatus as claimed in any one of the preceding claims, wherein the algae-cultivation vessel has a diameter and/or depth of at least 500mm.
17. An algae-cultivation system comprising a plurality of algae-cultivation apparatuses as claimed in any one of the preceding claims.
18. An algae-cultivation system as claimed in claim 17, wherein the algae-cultivation vessels are open-topped vessels.
19. A method of improving the yield of algae farming, the method comprising the steps of: a] providing an algae-cultivation apparatus as claimed in any one of claims 1 to 16; b] growing algae in the algae-cultivation vessel when the fluid-receiving chamber contains a liquid algae-cultivation medium; and c] introducing a controllable flow of gas into the algae-cultivation apparatus via the gas inlet from the base of the algae-cultivation vessel to encourage circulation of the liquid algae-cultivation medium.
20. A method as claimed in claim 19, wherein the flow rate of the gas is adjustable in response to the output of the sensor device.
21. A method as claimed in claim 20, wherein the sensor device is a temperature sensor device, the flow of gas being used to modulate the temperature of the algae-cultivation medium.
22. A method as claimed in any one of claims 19 to 21, wherein the gas is a CO2-enriched air.