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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

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.

Outdoor industrial-scale microalgae cultivation is limited by several factors, among which a low efficiency of overall light energy conversion plays a key role, mainly due to photosaturation and photoinhibition under high irradiations. This work aims at improving the overall photoconversion efficiency in a microalgal production photobioreactor (PBRs), by exploiting an advanced photovoltaic (PV) technology. A semi-transparent dye sensitized solar cells module (DSC) is placed on the irradiated surface, thus absorbing part of the incident light to produce electricity, while transmitted photons are used by algal cells for photosynthesis. Experiments are carried out in a continuous laboratory scale flat-panel PBR, at different constant light intensities and under a day-night irradiation regime, to ascertain the performances of this combined PV-PBR system in terms of biomass productivity and overall photoconversion efficiency, compared to traditional transparent PBRs. The results obtained show that the configuration proposed, combining biomass production with innovative photovoltaics technology, could be a valuable way to improve light energy utilization and efficiency in microalgal production.

A profitability analysis of a large scale supercritical fluid extraction (SFE) process to obtain natural extracts from rocket salad is presented in this work. An industrial-scale SFE plant is designed using process simulation techniques based on experimental data. Afterward, the effect of operative parameters on the process costs is evaluated. At the optimum conditions, the economic and financial analysis is performed, and the influence of the price of rocket salad on the price of the natural extract is assessed. Results indicate that the raw materials account for 67% of the total production costs. The proposed industrial application could be economically viable with a pay-back period of 10 years if the price of the rocket salad extract is larger than 21.9 US$ kg−1, considering the current wholesale price in the market, or 17.0 US$ kg−1 if using non-saleable rocket salad. Both scenarios make the process economically sustainable and competitive with the current price of natural extracts obtained by extraction with organic solvents.