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The efficient use of light for the cultivation of Nannochloropsis SPP.

Vadiveloo, Ashiwin (2017) The efficient use of light for the cultivation of Nannochloropsis SPP. PhD thesis, Murdoch University.

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Due to their distinct advantages over terrestrial plants, microalgae biomass is currently sought after for various applications, ranging from human health, aquaculture, high value chemicals and also as a feedstock for bioenergy production. However, low biomass productivity and inflated production cost have restricted the commercialization of commodity end-products (e.g. biofuel) derived from microalgae. Among the primary factors limiting the growth and productivity of microalgae is the availability and distribution of light (quantity and quality). Identifying these shortcomings, the combination of spectrally selective photovoltaic filters above microalgae cultivation systems for the full exploitation of available sunlight has been previously proposed. Through this system, wavelengths of incoming sunlight most efficient for the growth and productivity of microalgae would be selectively filtered in and transmitted to the algae culture through the photovoltaic filter while all remaining ineffective wavelengths such as infrared (IR), green-yellow and ultraviolet (UV) would be captured and converted into electricity by the photovoltaic apparatus.

Thus, in order to evaluate the viability and efficacy of such integrated systems, it is first vital to identify the most efficient light spectra for the targeted microalgae. In this work, the response of two Nannochloropsis spp. (MUR 266& MUR 267) grown and acclimated under various light spectra were evaluated. Two distinct laboratory controlled experimental setups (filtered and non-filtered) were carried out to evaluate the impact of different light spectra on the growth, biomass productivity, biochemical composition and also photosynthetic response of the selected microalgae.

The first study involved the selective filtration of incident light, bringing forward changes in both spectral distribution and the concomitant reduction in irradiance. The filtered light studies were exactly in accordance to the proposed PV-microalgae system in which incoming sunlight would be selectively filtered. The irradiance level of each filtered light spectra was relatively equalized according to its distribution and proportional number of photons found in white light.

Under such conditions, light acclimatized cultures of Nannochloropsis MUR 266 and MUR 267 showed the highest light to biomass conversion efficiency when blue light (BL, 400-700nm) (1.93 ± 0.14 mg L-1 d-1 per photon) and white light (WL, 400-700nm) (1.87 ± 0.08 mg L-1 d-1 per photon) were respectively used. Total lipid content was significantly enhanced under BL for both species of Nannochloropsis when compared to the other treatments of blue-green (BGL, 450-600nm), red (RL, 600-700nm), pink (PL, 400-525nm and 600-700nm) and WL.

As the biomass productivity of microalgae is directly governed by the efficiency of photosynthesis, the photosynthetic response of both Nannochloropsis spp. under the different filtered light spectra was evaluated using both chlorophyll a fluorescence and oxygen evolution based methods. In this study, most parameters (αETR, ETRmax, α O2 P-I, O2 Pmax) measured using both methods inclined to be higher under PL and WL for Nannochloropsis MUR 266 and MUR 267 respectively than the other treatments.

Building up on the results of the filtered light study, a narrower band of blue light (LEDB, 430-490nm) was introduced in the second experimental setup to optimize the proposed PV-microalgae system for higher electricity production. In order to eliminate the effects of light irradiance as observed in the previous study and to elucidate the impact of spectral quality, the radiant flux of each treatment was standardized (non-filtered) in this study. Biomass productivities trended to be higher for both MUR 266 and MUR 267 cultures acclimated under BL, LEDB and WL when compared to PL and RL. Total lipid yield was significantly higher in cultures grown under BL (0.26 ± 0.02 g L-1) and PL (0.15 ± 0.01 g L-1) for MUR 266 and MUR 267 respectively when compared to the positive control of WL. The photosynthetic response (Fq′/Fm′, αETR and ETRmax) of MUR 266 and MUR 267 measured using chlorophyll a fluorescence tended to be higher under BL and LEDB when compared to RL, PL and WL.

Based on the results of both the filtered and non-filtered light studies, it can be concluded that blue wavelengths (BL and LEDB) and WL was most efficient for MUR 266 and MUR 267 respectively. Direct comparison between both species of Nannochloropsis identified MUR 266 to be a more suitable candidate for application in the PV-microalgae system due to enhanced growth, lipid content and photosynthesis under short wavebands of light such as blue (BL and LEDB). Modelling of the PV-microalgae technology indicated between 150 and 210 W m-2 of electrical energy can be potentially generated if only blue wavelengths (BL and LEDB) are selectively filtered and channelled to MUR 266 from incoming sunlight while converting all the remaining portions of available light into electricity through photovoltaic cells.

According to the PV-microalgae concept of dividing incident light for two distinctive purposes, a novel flat plate photobioreactor (insulated glazed photobioreactor, IGP) with its illumination surface area customized with insulated glazing units (Tropiglas/ClearVue) was also designed and developed as part of this dissertation. The IGP design transmitted more than 50% of visible light through it while blocking 90% of UV and IR radiations. The major advantage of the IGP design was its ability to capture and convert the untransmitted wavelengths of light into electricity through the integration of external photovoltaic cells.

In order to validate the feasibility of the IGP design, the growth and photosynthesis of Nannochloropsis sp. (MUR 267) in the IGP was compared against conventional flat plate photobioreactors (hot light photobioreactor, HLP) and also externally cooled photobioreactors (cool light photobioreactor, CLP) subjected to the full spectrum of halogen light. High temperature (up to 42°C) resulted in no growth of the microalgae in the HLP. Volumetric biomass productivities of Nannochloropsis sp. in the CLP (90.5 ± 8.16 mg L-1 d-1) was significantly higher than the IGP (52.7 ± 5.70 mg L-1 d-1) due to higher light transmission and lower temperature profiles recorded in the CLP. Nevertheless, no external cooling system (e.g. freshwater) was required for the IGP design in sustaining the growth of Nannochloropsis (unlike the HLP and CLP). This would make the large scale operation of the IGP independent of freshwater cooling, reducing overall production cost.

Total lipid content of Nannochloropsis was higher when grown in the CLP while protein content was higher in the IGP. A basic energy analysis was used to model the amount of electricity that could be potentially produced by the IGP design from the untransmitted wavelengths of light. Modelling indicated up to 25.2 W m-2 of electrical power can be generated by the IGP system in an outdoor scenario through the addition of external photovoltaic cells.

In conclusion, this study clearly shows the efficient use of available light spectra for the cultivation of Nannochloropsis spp. and also the potential generation of electricity using highly efficient photovoltaics.

Publication Type: Thesis (PhD)
Murdoch Affiliation: School of Veterinary and Life Sciences
Supervisor: Moheimani, Navid, Parlevliet, David, Cosgrove, Jeff and Borowitzka, Michael
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