Modeling of biomass productivity in tubular photobioreactors for microalgal cultures: effects of dilution rate, tube diameter, and solar irradiance

A macromodel is developed for estimating the year-long biomass productivity of outdoor cultures of microalga in tubular photobioreactors. The model evaluates the solar irradiance on the culture surface as a function of day of the year and the geographic location. In a second step, the geometry of the system is taken into account in estimating the average irradiance to which the cells are exposed. Finally, the growth rate is estimated as a function of irradiance, taking into account photoinhibition and photolimitation. The model interconnects solar irradiance (an environmental variable), tube diameter (a design variable), and dilution rate (an operating variable). Continuous cultures in two different tubular photobioreactors were analyzed using the macromodel. The biomass productivity ranged from 0.50 to 2.04 g L-1 d-1, and from 1.08 to 2. 76 g L-1 d-1, for the larger and the smaller tube diameter photobioreactors, respectively. The quantum yield ranged from 1.1 to 2.2 g E-1; the higher the incident solar radiation, the lower the quantum yield. Simultaneous photolimitation and photoinhibition of outdoor cultures was observed. The model reproduced the experimental results with less than 20% error. If photoinhibition was neglected, and a growth model that considered only photolimitation was used to fit the data, the error increased to 45%, thus reflecting the inadequacy of previous outdoor growth models that disregard photoinhibition. Copyright 1998 John Wiley & Sons, Inc.     

A closed solar photobioreactor for cultivation of microalgae under supra-high irradiance: basic design and performance

An account is given of the setting up and use of a novel type of closed tubular photobioreactor at the Academic and University Centre in Nove Hrady, Czech Republic. This "penthouse-roof" photobioreactor was based on solar concentrators (linear Fresnel lenses) mounted in a climate-controlled greenhouse on top of the laboratory complex combining features of indoor and outdoor cultivation units. The dual-purpose system was designed for algal biomass production in temperate climate zone under well-controlled cultivation conditions and with surplus solar energy being used for heating service water. The system was used to study the strategy of microalgal acclimation to supra-high solar irradiance, with values as much as 3.5 times the ambient value, making the approach unique. The cultivation system proved to be fully functional with sufficient mixing and cooling, efficient oxygen stripping and light tracking. Experimental results (measurement of the maximum photochemical yield of PSII and non-photochemical quenching) showed that the cyanobacterium Spirulina (= Arthrospira) platensis cultivated under sufficient turbulence and biomass density was able to acclimate to irradiance values as high as 7 mmol photon m^–2 s^–1. The optimal biomass concentration of Spirulina cultures in September ranged between 1.2 to 2.2 g L^–1, which resulted in a net productivity of about 0.5 g L^–1 d^–1 corresponding to a biomass yield of 32.5 g m^–2 d^–1 (based on the minimum illuminated surface area of the photobioreactor).     

Effect of dilution rate on eicosapentaenoic acid productivity ofPhaeodactylum tricornutum utex 640 in outdoor chemostat culture

Summary Eicosapentaenoic acid (EPA) volumetric productivity from an outdoor chemostat culture ofPhaeodactylum tricornutum UTEX 640 in a 50-l tubular photobioreactor varies with dilution rate, reaching a maximum of 47.8 mg l^–1 d^–1 at D=0.36 d^–1. Continuous culture at high dilution rates' is proposed as the most adequate operating mode to maximize polyunsaturated fatty acid production.     

Modeling of eicosapentaenoic acid (EPA) production from Phaeodactylum tricornutum cultures in tubular photobioreactors. Effects of dilution rate, tube diameter, and solar irradiance

A model for the prediction of eicosapentaenoic acid (EPA) productivity from Phaeodactylum tricornutum cultures that takes into account the existence of photolimitation and photoinhibition of growth under outdoor conditions is presented. The effects of the external irradiance on the culture surface, the average irradiance inside the culture, and the light regime at which the cells are exposed on pigments and EPA content are studied. The chlorophyll content decreases exponentially with the average irradiance, whereas the carotenoids content increases linearly with the external irradiance due to a higher extension of photoinhibition. A decrease in the fatty acid content of the biomass with irradiance on reactor surface is observed when photoinhibition becomes relevant. The average irradiance within the culture mainly influenced the fatty acid profile of the biomass. As the average irradiance becomes higher, percentages of saturated and monounsaturated fatty acids decrease, increasing the portion of EPA. By taking into account the different relationships among pigment and EPA content with the irradiance, the variation in EPA productivity over the year can be simulated as a function of average and external irradiance. For the two photobioreactors employed the maximum EPA productivity is attained in spring and fall (30 mg L(-1) day(-1) for tube diameter 0. 06 m and 50 mg L(-1) day(-1) for tube diameter 0.03 m). In winter, the biomass productivity is limited by low light availability although the EPA content is maximum. In summer, the biomass productivity is higher although the EPA content diminished by photoinhibition; the higher the dilution rate, the lower the minimum. Thus, the conditions that increase the biomass productivity and the polyunsaturated fatty acids content are in opposition, the optimum being reached by operating under photolimitation with high growth rates in order to produce a high proportion of polyunsaturated fatty acids.     

Tubular photobioreactor design for algal cultures.

Principles of fluid mechanics, gas-liquid mass transfer, and irradiance controlled algal growth are integrated into a method for designing tubular photobioreactors in which the culture is circulated by an airlift pump. A 0.2 m(3) photobioreactor designed using the proposed approach was proved in continuous outdoor culture of the microalga Phaeodactylum tricornutum. The culture performance was assessed under various conditions of irradiance, dilution rates and liquid velocities through the tubular solar collector. A biomass productivity of 1.90 g l(-1) d(-1) (or 32 g m(-2) d(-1)) could be obtained at a dilution rate of 0.04 h(-1). Photoinhibition was observed during hours of peak irradiance; the photosynthetic activity of the cells recovered a few hours later. Linear liquid velocities of 0.50 and 0.35 m s(-1) in the solar collector gave similar biomass productivities, but the culture collapsed at lower velocities. The effect of dissolved oxygen concentration on productivity was quantified in indoor conditions; dissolved oxygen levels higher or lower than air saturation values reduced productivity. Under outdoor conditions, for given levels of oxygen supersaturation, the productivity decline was greater outdoors than indoors, suggesting that under intense outdoor illumination photooxidation contributed to loss of productivity in comparison with productivity loss due to oxygen inhibition alone. Dissolved oxygen values at the outlet of solar collector tube were up to 400% of air saturation.     

Overall assessment of Monodus subterraneus cultivation and EPA production in outdoor helical and bubble column reactors

The outdoor production of Monodus subterraneus wasstudied in bubble column and helical reactors, mainly analysing the influenceofdilution rate, air flow rate and solar irradiance on growth rate andbiochemicalcomposition. Photoinhibition and photo-oxidation phenomena were also analysed.The cultures were stressed at high solar irradiance and dissolved oxygenconcentrations. A clear relationship between stress of the cultures and thefluorescence from PSII measurements was observed, the Fv/Fm ratio being lowerinthe helical reactor than in the bubble column. Growth rate and biomassproductivity were both a function of the average irradiance and the Fv/Fmratio;maximum values of 0.040 h^–1 and 0.54 gL^–1 d^–1 were measured. The influenceofphotoinhibition and average irradiance was modelled, the model also fitting theexperimental data reported by another author. The chlorophyll contenthyperbolically decreased, whereas the carotenoid content decreased linearlywiththe average irradiance. The higher the dilution rate the higher the protein andcarbohydrate content of the biomass, and the lower the lipid content. Theeicosapentaenoic acid (EPA) content ranged from 2.3 to 3.2% d.wt, the higherthe dilution rate, the lower EPA content, although the higher the EPAproportion. Maximum EPA productivity was only 9 mg L^–1d^–1, due to the stress to which the cultures wereexposed.     

Carbon dioxide uptake efficiency by outdoor microalgal cultures in tubular airlift photobioreactors

The influence of solar irradiance and carbon dioxide molar fraction of injected CO(2)-air mixtures on the behavior of outdoor continuous cultures of the microalga Phaeodactylum tricornutum in tubular airlift photobioreactors was analyzed. Instantaneous solar irradiance, pH, dissolved oxygen, temperature, biomass concentration, and the mass flow rates of both the inlet and outlet oxygen and carbon with both the liquid and gas phases were measured. In addition, elemental analysis of the biomass and the cell-free culture medium was performed. The oxygen production rate and carbon dioxide consumption rate increased hyperbolically with the incident solar irradiance on the reactor surface. Carbon losses showed a negative correlation with the daily variation of the carbon dioxide consumption rate. The maximum CO(2) uptake efficiency was 63% of the CO(2) supplied when the CO(2) concentration in the gas supplied was 60% v/v. Carbon losses were >100% during the night, due to CO(2) production by respiration, and hyperbolically decreased to values of 10% to 20% in the midday hours. An increase in the carbon fixed in the biomass with the solar cycle was observed. A slight daily decrease of carbon content of the cell-free culture medium indicated the existence of carbon accumulation in the culture. A decrease in CO(2) molar fraction in the injected gas had a double benefit: first, the biomass productivity of the system was enhanced from 2.05 to 2.47 g L(-1) day(-1) by reduction of CO(2) inhibition and/or pH gradients; and second, the carbon losses during the daylight period were reduced by 60%. The fluid dynamics in the reactor also influenced the carbon losses: the higher the liquid flow rate the higher the carbon losses. By using a previous mass transfer model the experimental results were simulated and the usefulness of this method in the evaluation and scale-up of tubular photobioreactors was established.     

Continuous production of green cells of Haematococcus pluvialis: Modeling of the irradiance effect

This paper presents a model for the continuous production of green cells of the microalga Haematococcus pluvialis, in both indoor and outdoor conditions. To develop this model, the influence of irradiance and dilution rate on the performance of continuous cultures of H. pluvialis was studied in the laboratory but simulating outdoor conditions. Characterization of the cultures included biomass productivity, fluorescence of chlorophylls, pigment content, elemental composition of the biomass, cell density, cell size, homogeneity and nitrate consumption rate. Results showed that the optimal dilution rate was 0.04 h⁻¹, and that higher external irradiance resulted in higher biomass productivity in all cases, with a maximum value of 0.58 g L⁻¹day⁻¹. Continuous cultures were stable for more than 3 months, in spite of photoinhibition at noon, producing homogeneous biomass with a stable biochemical composition and cell morphology at each steady state. Astaxanthin accumulation was not observed in spite of the high levels of irradiance essayed, and cells remained in the flagellated-palmeloids green form whatever the culture conditions. High dilution rates produced small cells of 22 μm diameter, with a high nitrogen content of up to 10.0% d.wt. The average irradiance within the reactor was the main factor determining the behaviour of the cultures, although the external irradiance impinging on the reactor surface also influenced the results, indicating the existence of photoinhibition. The influence of both external and average irradiance on the growth of H. pluvialis was modelized. The accuracy of the model obtained was verified on a 0.22 m³ outdoor tubular photobioreactor operated in both discontinuous and continuous mode, obtaining a maximum biomass productivity of 0.68 g L⁻¹day⁻¹. The model reproduced the experimental data of biomass concentration and productivity, cell size and nitrate consumption, providing to be a powerful tool for optimizing the design and operation of outdoor photobioreactors for the production of green cells of H. pluvialis.     

Comparative evaluation of compact photobioreactors for large-scale monoculture of microalgae

Engineering analyses combined with experimental observations in horizontal tubular photobioreactors and vertical bubble columns are used to demonstrate the potential of pneumatically mixed vertical devices for large-scale outdoor culture of photosynthetic microorganisms. Whereas the horizontal tubular systems have been extensively investigated, their scalability is limited. Horizontal tubular photobioreactors and vertical bubble column type units differ substantially in many ways, particularly with respect to the surface–to–volume ratio, the amount of gas in dispersion, the gas–liquid mass transfer characteristics, the nature of the fluid movement and the internal irradiance levels. As illustrated for eicosapentaenoic acid production from the microalga Phaeodactylum tricornutum, a realistic commercial process cannot rely on horizontal tubular photobioreactor technology. In bubble columns, presence of gas bubbles generally enhances internal irradiance when the Sun is low on the horizon. Near solar noon, the bubbles diminish the internal column irradiance relative to the ungassed state. The optimal dimensions of vertical column photobioreactors are about 0.2 m diameter and 4 m column height. Parallel east–west oriented rows of such columns located at 36.8°N latitude need an optimal inter-row spacing of about 3.5 m. In vertical columns the biomass productivity varies substantially during the year: the peak productivity during summer may be several times greater than in the winter. This seasonal variation occurs also in horizontal tubular units, but is much less pronounced. Under identical conditions, the volumetric biomass productivity in a bubble column is 60% of that in a 0.06 m diameter horizontal tubular loop, but there is substantial scope for raising this value.     

Minimization of carbon losses in pilot-scale outdoor photobioreactors by model-based predictive control

The optimization of carbon use in pilot-scale outdoor tubular photobioreactors is investigated in this study. The behavior of a 0.20-m(3) tubular photobioreactor was studied, with and without algae, by steady-state and pulse dynamic-response analysis experiments. A model of the system was obtained and implemented in a programmable control unit and was used to control the reactor under normal production conditions. Results showed that, using and on-off control, the mean daily CO(2) flow in the reactor was 0.86 g min(-1), 19.7% of this being lost. By using a predictive control algorithm the mean daily CO(2) flow was reduced to 0.74 g min(-1), with losses being reduced to 15.6%. In this case, pH tracking was not adequate, especially at the beginning and end of the daylight period, because the variation in solar irradiance was not considered. Taking solar irradiance into account resulted in better performance, with mean daily CO(2) flow reduced to 0.70 g min(-1), and carbon losses reduced to 5.5%. pH tracking was improved and valve actuation was reduced. Improvement of pH control reduced pH gradients in the culture, which increased the photosynthesis rate and biomass productivity of the system. Biomass productivity increased from 1.28 to 1.48 g L(-1) day-(1) when on-off control was replaced by model-based predictive control plus solar irradiance effect mode. Implementation of this methodology in outdoor photobioreactors can increase productivity by 15% and reduce the cost of producing biomass by >6%. Clearly, application of effective control techniques, such as model-based predictive control (MPC), must be considered when developing these processes.     

Comparative analysis of the outdoor culture of Haematococcus pluvialis in tubular and bubble column photobioreactors

The present paper makes a comparative analysis of the outdoor culture of H. pluvialis in a tubular photobioreactor and a bubble column. Both reactors had the same volume and were operated in the same way, thus allowing the influence of the reactor design to be analyzed. Due to the large changes in cell morphology and biochemical composition of H. pluvialis during outdoor culture, a new, faster methodology has been developed for their evaluation. Characterisation of the cultures is carried out on a macroscopic scale using a colorimetric method that allows the simultaneous determination of biomass concentration, and the chlorophyll, carotenoid and astaxanthin content of the biomass. On the microscopic scale, a method was developed based on the computer analysis of digital microscopic images. This method allows the quantification of cell population, average cell size and population homogeneity. The accuracy of the methods was verified during the operation of outdoor photobioreactors on a pilot plant scale. Data from the reactors showed tubular reactors to be more suitable for the production of H. pluvialis biomass and/or astaxanthin, due to their higher light availability. In the tubular photobioreactor biomass concentrations of 7.0 g/L (d.wt.) were reached after 16 days, with an overall biomass productivity of 0.41 g/L day. In the bubble column photobioreactor, on the other hand, the maximum biomass concentration reached was 1.4 g/L, with an overall biomass productivity of 0.06 g/L day. The maximum daily biomass productivity, 0.55 g/L day, was reached in the tubular photobioreactor for an average irradiance inside the culture of 130 microE/m2s. In addition, the carotenoid content of biomass from tubular photobioreactor increased up to 2.0%d.wt., whereas that of the biomass from the bubble column remained roughly constant at values of 0.5%d.wt. It should be noted that in the tubular photobioreactor under conditions of nitrate saturation, there was an accumulation of carotenoids due to the high irradiance in this reactor, their content in the biomass increasing from 0.5 to 1.0%d.wt. However, carotenoid accumulation mainly took place when nitrate concentration in the medium was below 5.0mM, conditions which were only observed in the tubular photobioreactor. A similar behaviour was observed for astaxanthin, with maximum values of 1.1 and 0.2%d.wt. measured in the tubular and bubble column photobioreactors, respectively. From these data astaxanthin productivities of 4.4 and 0.12 mg/L day were calculated for the tubular and the bubble column photobioreactors. Accumulation of carotenoids was also accompanied by an increase in cell size from 20 to 35 microm, which was only observed in the tubular photobioreactors. Thus it may be concluded that the methodology developed in the present study allows the monitoring of H. pluvialis cultures characterized by fast variations of cell morphology and biochemical composition, especially in outdoor conditions, and that tubular photobioreactors are preferable to bubble columns for the production of biomass and/or astaxanthin.     

Growth physiology of a marine nitrogen-fixing cyanobacterium (Nodularia harveyana) in outdoor culture

The performance ofNodularia harveyana, a N₂-fixing cyanobacterium isolated from seawater, has been studied outdoors in two different culture systems: open pond (OP) and tubular photobioreactor (TPR). The productivity in both devices was influenced by areal density. The maximum yield obtained was 12.0 g (d.wt) m^–2 day^–1 in OP and 14.0 g (d.wt) m^–2 day^–1 in TPR in August, corresponding to the highest solar radiation received. In a month-long experiment with the cyanobacterium cultivated in TPR at high circulation speed, a net increase in productivity was obtained over that at low circulation speed. The influence of temperature on the productivity of the cultures grown in open ponds and tubular photobioreactors has been investigated. The higher productivity obtained in TPR compared to OP was attributed to its better controlled temperature conditions. In outdoor culture the maximum nitrogenase activity did not coincide with the maximum light intensity, but occurred in early afternoon. The amount of carbohydrate accumulated during the day probably influenced the rate of dark nitrogenase activity and its duration in the night.     

Photolimitation and photoinhibition as factors determining optimal dilution rate to produce eicosapentaenoic acid from cultures of the microalga Isochrysis galbana

Eicosapentaenoic acid (EPA) productivity from continuous cultures of the marine microalga Isochrysis galbana was studied, taking into account the irradiance on the reactor surface, that is, the photolimitation/photoinhibition regime to which the cells are exposed. Experiments were conducted under a wide variety of operating conditions. The dilution rate ranged from 0.005 h⁻¹ to 0.040 h⁻¹ at five external irradiances (820, 1620, 2050, 2450 and 3270 μmol photons m⁻² s⁻¹) covering photolimited to photoinhibited growth. Under these conditions, the specific growth rate (μ) was found to be the main factor influencing EPA content (ranging from 2.35% to 5.23% dryweight) and productivity (up to 0.88 mg l⁻¹ h⁻¹). The fatty acid content was not significantly affected by the external irradiance, but was influenced by the state of growth of the microalga, depending on whether the light regime was photolimiting or photoinhibiting. It might be suggested that light should no longer be considered an isolated factor affecting EPA synthesis, but an indirect influence through the photolimitation/photoinhibition regime and growth rate. At a given dilution rate, EPA content and biomass concentration are lower under photoinhibiting external irradiances than those corresponding to photolimiting conditions, and consequently EPA productivity decays. Since the effect of photoinhibition is less marked at high biomass concentration, a strategy to optimize EPA productivity from microalgal cultures could consist of reducing the dilution rate when the external irradiance increases above the phoinhibition threshold. Received: 16 January 1998 / Revised revision: 27 March 1998 / Accepted: 27 March 1998     

Mixotrophic growth ofChlorella sorokiniana in outdoor enclosed photobioreactor

Chlorella sorokiniana was cultured in heterotrophic or mixotrophic mode in outdoor enclosed tubular photobioreactor. The culture temperature was maintained at 32–35 °C. At night, theChlorella culture grew heterotrophically, and 0.1 M glucose was completely consumed. The biomass growth yield of glucose was 0.35 ± 0.001 g-biomass g-glucose^–1. During the day, the algal culture grew mixotrophically and the biomass growth yield was 0.49 g-biomass g-glucose^–1 in low density culture (initial biomass concentration, X_o = 2 g l^–1), 0.56 g-biomass g-glucose^–1 in medium density culture (X_o = 4 g l^–1) and 0.46 g-biomass g-glucose^–1 in high density culture (X_o = 7 g l^–1). The daily area productivity of the culture, with X_o = 4 g l^–1 corresponded to 127 g-biomass m^–2 d^–1 during the day and 79 g-biomass m^–2 d^–1 during the night. In all the cultures, the dissolved O₂ concentration increased in the morning, reached the maximum value at noon, and then decreased in the afternoon. The dissolved CO₂ concentration remained at 3 mBar in the morning and increased in the afternoon. Glycolate was not found to accumulate in culture medium.     

Biomass and icosapentaenoic acid productivities from an outdoor batch culture of Phaeodactylum tricornutum UTEX 640 in an airlift tubular photobioreactor

An outdoor tubular photobioreactor, based on an external-loop airlift, has been used to cultivate the diatom Phaeodactylum tricornutum UTEX 640, rich in icosapentaenoic acid (EPA). The system was operated in batch mode. The specific growth rate in the exponential phase, averaged over the whole day, was 0.254 day^–1 · A 25 gl^–1 maximum biomass concentration was obtained by the end of the culture period. The dissolved oxygen generated never rose over 210% of air saturation nor was there impediment to biomass generation or inhibition of photosynthesis. Furthermore, around 2 × 10¹⁷ quanta cm^–2 s^–1 light intensity seemed to saturate the photosynthetic activity. Biomass losses during the night could be reduced to less than 5% by maintaining the culture at 16°C at night. The maximum concentration and productivity of EPA were 423 mgl^–1 and 13 mgl^–1 day^–1, respectively, at the end of the culture, although a 21-mgl^–1 day^–1 net EPA generation rate was obtained at the linear growth phase.     

Comparison of four outdoor pilot-scale photobioreactors

Background

Microalgae are a potential source of sustainable commodities of fuels, chemicals and food and feed additives. The current high production costs, as a result of the low areal productivities, limit the application of microalgae in industry. A first step is determining how the different production system designs relate to each other under identical climate conditions. The productivity and photosynthetic efficiency of Nannochloropsis sp. CCAP 211/78 cultivated in four different outdoor continuously operated pilot-scale photobioreactors under the same climatological conditions were compared. The optimal dilution rate was determined for each photobioreactor by operation of the different photobioreactors at different dilution rates.

Results

In vertical photobioreactors, higher areal productivities and photosynthetic efficiencies, 19–24 g m^?2 day^?1 and 2.4–4.2 %, respectively, were found in comparison to the horizontal systems; 12–15 g m^?2 day^?1 and 1.5–1.8 %. The higher ground areal productivity in the vertical systems could be explained by light dilution in combination with a higher light capture. In the raceway pond low productivities were obtained, due to the long optical path in this system. Areal productivities in all systems increased with increasing photon flux densities up to a photon flux density of 30 mol m^?2 day^?1. Photosynthetic efficiencies remained constant in all systems with increasing photon flux densities. The highest photosynthetic efficiencies obtained were; 4.2 % for the vertical tubular photobioreactor, 3.8 % for the flat panel reactor, 1.8 % for the horizontal tubular reactor, and 1.5 % for the open raceway pond.

Conclusions

Vertical photobioreactors resulted in higher areal productivities than horizontal photobioreactors because of the lower incident photon flux densities on the reactor surface. The flat panel photobioreactor resulted, among the vertical photobioreactors studied, in the highest average photosynthetic efficiency, areal and volumetric productivities due to the short optical path. Photobioreactor light interception should be further optimized to maximize ground areal productivity and photosynthetic efficiency.

     

Irradiance optimization of outdoor microalgal cultures using solar tracked photobioreactors

Photosynthetic activity and temperature regulation of microalgal cultures (Chlorella vulgaris and Scenedesmus obliquus) under different irradiances controlled by a solar tracker and different cell densities were studied in outdoor flat panel photobioreactors. An automated process control unit regulated light and temperature as well as pH value and nutrient concentration in the culture medium. CO₂ was supplied using flue gas from an attached combined block heat and power station. Photosynthetic activity was determined by pulse amplitude modulation fluorometry. Compared to the horizontal irradiance of 55 mol photons m^?2 d^?1 on a clear day, the solar tracked photobioreactors enabled a decrease and increase in the overall light absorption from 19 mol photons m^?2 d^?1 (by rotation out of direct irradiance) to 79 mol photons m^?2 d^?1 (following the position of the sun). At biomass concentrations below 1.1 g cell dry weight (CDW) L^?1, photoinhibition of about 35 % occurred at irradiances of ≥1,000 μmol photons m^?2 s^?1 photosynthetic active radiation (PAR). Using solar tracked photobioreactors, photoinhibition can be reduced and at optimum biomass concentration (≥2.3 g CDW L^?1), the culture was irradiated up to 2,000 μmol photons m^?2 s^?1 to overcome light limitation with biomass yields of 0.7 g CDW mol photons^?1 and high photosynthetic activities indicated by an effective quantum yield of 0.68 and a maximum quantum yield of 0.80 (F _v/F _m). Overheating due to high irradiance was avoided by turning the PBR out of the sun or using a cooling system, which maintained the temperature close to the species-specific temperature optima.     

Light/dark cyclic movement of algal culture (<Emphasis Type="Italic">Synechocystis aquatilis</Emphasis>) in outdoor inclined tubular photobioreactor equipped with static mixers for efficient production of biomass

Synechocystis aquatilis SI-2 was grown outdoors in a 12.5cm diam. tubular photobioreactor equipped with static mixers. The static mixers ensured that cells were efficiently circulated between the upper (illuminated) and lower (dark) sections of the tubes. The biomass productivity varied from 22 to 45g m^–2d^–1, with an average of 35g m^–2d^–1, etc which corresponded to average CO₂ fixation rate of about 57 g CO₂ m^–2 d^–1. The static mixers not only helped in improving the biomass productivities but also have a high potential to lower the photoinhibitory effect of light during the outdoor cultures of algae. Revisions requested 27 July 2004; Revisions received 12 November 2004     

A two-stage solar photobioreactor for cultivation of microalgae based on solar concentrators

A novel two-stage experimental photobioreactor (PBR) with a total volume of 450 L and based uniquely on solar concentrators—linear Fresnel lenses—has been constructed and tested. Daily courses of irradiance, and also its distribution inside cultivation tubes, were studied in two unit types. The supra-high irradiance units in the ‘roof’ achieved a maximum summer value above 6 mmol photon m⁻² s⁻¹, while irradiance in the vertical-facade units was lower than ‘ambient’. In model cultivations, cultures of the cyanobacterium Arthrospira platensis were cultivated at much higher solar irradiances than those usually recorded outdoors in summer, indicating that this organism is resilient to high-irradiance (photoinhibition). Starting from a biomass density of 0.5 g L⁻¹ at optimum temperature, the cultures grew exponentially. A two-stage cultivation process of the green microalga Haematococcus pluvialis was investigated with respect to correlations between photochemical activities and astaxanthin production. The culture was first grown in low-irradiance units, and then exposed to supra-high irradiance when the rate of astaxanthin production was 30–50% higher than in the culture exposed to ‘ambient’ irradiance. Within 4 days, the astaxanthin content reached 3% of dry weight, whereas under ambient irradiance the astaxanthin content was 25% lower.     

Outdoor production of <Emphasis Type="Italic">Tisochrysis lutea</Emphasis> in pilot-scale tubular photobioreactors

In this paper we study the outdoor production of Tisochrysis lutea in pilot-scale tubular photobioreactors (3.0 m³). Experiments were performed modifying the dilution rate and evaluating biomass productivity and quality, in addition to the overall performance of the system. Results confirm that T. lutea can be produced outdoors on a commercial scale in continuous mode, obtaining productivities of up to 20 g m^?2 day^?1 of biomass, which are rich in proteins (45 % d.wt.) and lipids (25 % d.wt.). The utilization of this type of photobioreactor allows one to control the levels of contamination and pH within the cultures, but daily variations in solar radiation impose elevated dissolved oxygen concentrations and insufficient temperature conditions on the cells inside the reactor. Excessive dissolved oxygen reduces biomass productivity to 68 % of that which is maximal, whereas inadequate temperature reduces it to 63 % of maximum. Thus, by optimally controlling these parameters, biomass productivity can be almost doubled. These results confirm the potential for producing this valuable strain on a commercial scale in optimally designed/operated tubular photobioreactors as a viable biotechnological industry.