Search

Microalgal industrial production requires high surface area, resulting in production costs currently unacceptable. A possible optimization of land use for the cultivation of Scenedesmus obliquus is proposed, by conjoint production of biomass and electricity using photovoltaic cells (PV). For this purpose, biomass cultivation in a continuously operated 1 ha open pond placed inside a greenhouse was considered, at two different Italian latitudes, as case studies. The greenhouse roof surface was partially covered with commercial PV modules, resulting in a reduced average irradiation. The light profiles and the average temperatures inside the greenhouse were simulated for different seasons, and the corresponding microalgal productivities were calculated based on a validated growth model. The partial pond shading limited photoinhibition in summer at Southern location, resulting in higher productivities. On the other hand, the loss of sunlight, due to the partial roof covering, resulted in a lower productivity in the other cases and for the Northern location. The presence of PV, however, allowed a better exploitation of light to produce electricity, which supports the energy duties of the process, with an additional net electricity production. Finally, an economic analysis was carried out showing a reduction of biomass production costs when PV is present. [ABSTRACT FROM AUTHOR]

One of the main limitations to large-scale production of biofuels derived from microalge is the lower efficiency of sunlight conversion. The maximum theoretical value for photosynthetic efficiency is hardly achieved in real outdoor cultivation systems, mainly due to inefficient light utilization, in addition to photosaturation and photoinhibition phenomena that take place at high irradiances. This work is focused on testing different possibilities aimed at improving the overall photoconversion efficiency of microalgal production in photobioreactors. Two strategies were followed: the first one increases the portion of spectrum available for photosynthesis employing luminescent spectral-converter filters on the photobioreactor surface, the second one integrates microalgae reactors with photovoltaic panels, producing electrical energy together with biomass. Experiments were carried out both in batch and continuous laboratory scale flat-plate photobioreactors, at different light intensities and regimes, with two different species ( Nannochloropsis salina and Scenedesmus obliquus ), measuring the growth rate, pigment content, biomass concentration and photosynthetic efficiency. Results show that spectral-converters do not substantially improve the growth rate, while an integrated PV and PBR system could be a valid way to improve energy conversion performances. [ABSTRACT FROM AUTHOR]

• A novel kinetic model for oxygen inhibition was implemented to simulate tubular PBRs. • The biomass concentration at the inlet is the key variable to minimize oxygen inhibition. • An optimum tube length was found, minimizing the effect of biomass concentration. • The measurement of O 2 concentration alone is not a reliable index of the overall productivity. Tubular photobioreactors (PBRs) guarantee high microalgal productivities but suffer from oxygen accumulation. It is known that the tube length must be limited to prevent build-up of high oxygen levels, but the combined effect of other variables (light intensity and biomass concentration) was not fully addressed. In this work, a mathematical model is developed to understand the influence of oxygen on biomass productivity in a continuous tubular PBR. Material balances are applied to investigate the behavior of a single tube reactor and of a complete process flowsheet of a commercial plant. Biomass concentration at the inlet resulted the key variable to minimize oxygen inhibition, confirming the solid retention time (SRT) as the main operating variable. However, an optimized length of the tube can minimize the effect of biomass concentration. Finally, it was observed that measuring the O 2 concentration alone is not a reliable index of the overall productivity in a PBR. [ABSTRACT FROM AUTHOR]

• SRT is the key variable to maximize the biomass productivity. • At the optimum value of SRT, the HRT does not affect the biomass productivity. • Operating with SRT > HRT does not improve productivity in light-limited cultures. • Operating with SRT < HRT allows to strongly reduce water and nutrients consumption. In conventional continuous bioreactor operations, it is a common practice to decouple the hydraulic retention time (HRT) from the solid retention time (SRT). When dealing with photosynthetic microorganisms, light intensity plays a major role in the growth kinetics; thus, the effect of HRT and SRT on biomass productivity may differ from that in chemotrophic bioreactors. In this study, we assessed the role of uncoupled SRT and HRT on biomass productivity in continuous photobioreactors (PBRs) based on simple mass balances, light transfer, and kinetic models to highlight the specific features of PBRs. Under a non-limiting nutrient supply, SRT is the fundamental variable used to maximize productivity and control the biomass concentration and light penetration in a PBR. Operating at SRT > HRT does not enhance the productivity. If the goal is biomass production, it is instead convenient to operate at SRT < HRT, thereby significantly reducing the water and nutrient consumption. [ABSTRACT FROM AUTHOR]

Flashing light is a promising strategy to increase light penetration in microalgal cultures, even though its actual advantage is still a matter of discussion, in particular in batch cultures. In this work, we demonstrated that the cultivation of Arthrospira maxima in steady-state continuous photobioreactors under high frequencies (up to 3700 Hz) of red-and-blue LED light pulses resulted in an increased productivity with respect to the control, carried out under continuous light. Pulse duration between 10 and 1000 μs and pulses intensity up to 70,000 μmol m−2 s−1 were tested at different values of integral light intensities and residence times. At both 120 and 300 μmol m−2 s−1 of integral light an increase of biomass concentration under light periods between 100 and 200 μs was observed. By managing the pulse intensity at a constant light period of 100 μs, biomass concentration can be up to three times higher than the control, but only if the residence time is properly set. Thus, residence time is one of the main variables affecting productivity under pulsed light. Finally, even though the efficiency of the LED apparatus under pulsed regime was lower than in continuous regime, the loss was compensated by the increase of productivity under certain conditions. • Flashes with light periods between 100 and 200 μs enhance biomass concentration. • Increasing pulse intensity, wash out and productivity maximum shift to longer residence times. • Biomass productivity can be doubled if proper light regime and residence time are set. • Flashing light can be energetically convenient with respect to continuous light. [ABSTRACT FROM AUTHOR]

[Display omitted] • In continuous PBR, a comparable biomass productivity is achieved under R/B and white LED. • Acclimation to the light source is needed to obtain reliable kinetic parameters. • Biomass growth under tailored spectra can be predicted by the mathematical model. • R/B LEDs have advantage in terms of both photosynthetic and energy efficiency. Irradiation with light emitting diodes (LEDs) is proposed as a method to increase the photoconversion efficiency in large-scale microalgal cultivation. Although several authors have reported the possibility of using colored LEDs in batch systems and for many species, the overall energy efficiency of a continuous system integrated with fine-tuned LED technology is still not available. In this work, the cyanobacterium Arthrospira maxima was cultivated under a R/B (red/blue) LED lamp at increasing light intensities, in order to study the effects of light quality on microalgal performances. Kinetic growth parameters were retrieved from respirometric tests and implemented in a mathematical model to examine the effects of process variables on microalgal growth and to predict maximum biomass productivity in the multi-wavelength spectrum employed, which was also included in the simulation tool. The efficiency of both photosynthetic and LEDs were examined for energy evaluation. It was found that integrating tailored LEDs in microalgal cultivation increases process efficiency by reducing light energy waste. Moreover, integrating tailored LEDs was also found to be more efficient than white LED source, due to their higher energetic efficiency. [ABSTRACT FROM AUTHOR]

• Photorespirometry can be applied to assess oxygen inhibition in microalgal growth. • Biomass concentration and light intensity affect sensitivity to oxygen concentration. • A comprehensive model including photorespiration, basal respiration and inhibition is proposed. • The model is able to reproduce well experimental results under different conditions. Microalgae cultivation has been the object of relevant interest for many industrial applications. Where high purity of the biomass/product is required, closed photobioreactors (PBRs) appear to be the best technological solution. However, as well as cost, the major drawback of closed systems is oxygen accumulation, which is well known to be responsible for growth inhibition. Only a few quantitative approaches have attempted to describe and model oxygen inhibition, which is the result of different biological mechanisms. Here, we have applied a photorespirometric protocol to assess and quantify the effect of high oxygen concentration on photosynthetic production rate. In particular, the effects of light intensity and biomass concentration were assessed, resulting in different maximum inhibitory oxygen concentrations. Literature models available were found not to fully represent experimental data as a function of concentration and light. Accordingly, a new formulation was proposed and validated to describe the photosynthetic rate as a function of external oxygen concentration. [ABSTRACT FROM AUTHOR]