Novel photobioreactor for the sustainable production of algal biomass and electricity

Emeka G. Nwoba

Mass outdoor microalgal cultures for the production of low priced bio-based commodities (food, feed, fuels) and high-value bioproducts (polyunsaturated fatty acids, pigments, therapeutic agents) require stable and commercially-viable biomass production technologies. The classical open raceway pond, the traditional commercial-scale technology for mass biomass production has significant limitations; low productivity rates due to a long culture depth, high risk of contamination, and lack control of environmental conditions. To produce high biomass density microalgal cultures, closed photobioreactors are preferred due to a better operational control of culture conditions, environmental variables and contamination. However, the operation of solar closed photobioreactors under outdoor scenarios requires sufficient cooling (in summer) and heating (in winter) technologies for guaranteed production of biomass (products) throughout the year. Heating and cooling operations are not only expensive and energy-intensive but require both grid electricity and precious freshwater (already limited) for their effectiveness, thus imposing a sustainability challenge. Therefore, next-generation algal photobioreactor designs must address these challenges of cost, energy and land-use efficiencies, while offering optimum biomass production. To this end, we have developed for the first time a hybrid thermally-insulated photobioreactor that is based on illumination spectral filtering for passive temperature control and integration with photovoltaic panels for electrical energy generation geared towards grid-independent operation. The novel photobioreactor has the illumination surfaces constructed of spectrally-selective low-emissivity film, which reflects >90% of non-photosynthetic photons (ultraviolet and infrared wavelengths) and transmits >70% of photosynthetically-beneficial visible photons (wavelengths spanning 400 to 700 nm) and its double glass units allow for high thermal insulation. A semi-transparent cadmium telluride photovoltaic cell that transmits 40% of the captured sunlight was glued to the top of the photobioreactor. To assess the viability and effectiveness of the novel photobioreactor design in thermoregulating microalgal cultures, the growth and photophysiological responses of two microalgae species Nannochloropsis sp. MUR 267 and Arthrospira platensis MUR 126 were investigated under laboratory conditions. Experimental results show that the maximum culture temperature in the novel photobioreactors was similar to the conventional water jacket system and 23-33% lower than that in the controls without temperature control system. The biomass productivity of Nannochloropsis culture in the insulated photobioreactors (112.47±3.36 mg L-1 d-1) was only 10% lower than that attained in the water jacket reactor, and no net growth was seen in the control without thermoregulation due to a high temperature. Chlorophyll a fluorescence measurements show that both microalgae cultures in the cultivation systems were not thermally stressed. This proof-of-principle study clearly demonstrated that infrared blocking films can significantly reduce heat gain in flat plate photobioreactors without a dramatic reduction in culture performance. At this point, a pilot-scale spectrally-selective insulated-glazed photovoltaic (IGP) flat panel photobioreactor (1.2 m length x 1.5 m height, 10 cm optical depth, 140 L working culture volume) capable of co-producing microalgal biomass and electricity, while eliminating the need of cooling water was developed. The viability of this novel system for culturing Nannochloropsis sp. was compared to similar flat panel photobioreactors based on freshwater passive evaporative cooling (PEC), infrared reflecting thin-film coating (IRF), and an open raceway pond (ORP). Maximum culture temperature (33.8 ± 2.9 ˚C) was highest in the IRF reactor while no significant difference was seen between IGP and PEC photobioreactors. Specific growth rate and biomass productivity of Nannochloropsis sp. was similar in all closed photobioreactors; however, ORP showed significantly lower productivity. Algal cultures in these cultivation systems were not thermally stressed. Interestingly, electricity generated from IGP photobioreactor during this period was 2.5-fold higher than the mixing energy requirement. Investigating the impact of the temperature control strategies on macromolecular content and fatty acid profile of Nannochloropsis sp., the normalized biochemical composition of the biomass showed a general trend of lipid > protein > carbohydrate, with no large variation of each across treatments. Besides C16:0, which was 24% higher in the photobioreactors than ORP, no other significant shift in major saturated and monounsaturated fatty acid components of this alga were seen among cultivation systems. The highest eicosapentaenoic acid (EPA, C20:5n-3), 16% and ϒ-linolenic acid (C18:3n-6), 8% of total fatty acid were found in ORP with the lowest average culture temperature and diel temperature variation than photobioreactors. Among all photobioreactors, IGP has the least diel temperature changes with an EPA content that was 21% higher than PEC, indicating that constructing photobioreactors with spectrally-selective materials is a viable strategy for managing the internal temperature, with no significant negative impact on biochemical and fatty acid profiles of microalgae. When a cold-intolerant microalga, Arthrospira platensis was cultured in the thermally-insulated IGP (no heat supplementation) during austral winter and compared with PEC under a cycle of heating (13-hour night) and thermostat-regulated cooling, and a continuously heated ORP, the average temperature in the IGP (21.0±0.03˚C) was similar to the heated PEC. Experimental results indicated that biomass productivity of Arthrospira in IGP photobioreactor was 67% higher than ORP and significantly lower than PEC. Phycocyanin productivity (16.3±1.43 mg g-1 d-1) showed no variation between photobioreactors but significantly less in the ORP. During this winter operation, electrical energy output of IGP photobioreactor exceeded mixing energy need by 75%. Finally, from the energy efficiency perspective, the net energy ratio of a 1-ha IGP facility used to cultivate Nannochloropsis sp. without freshwater-based cooling reached 3.0, a value comparable to agricultural bio-oil crops such as Jatropha and soybean. The annual biomass productivity was 66.0-tons dry weight ha-1, equivalent to overall energy output of 1,696 GJ ha-1. The integrated semi-transparent photovoltaic panels generated an additional 1,127 GJ ha-1 yr-1 (313 MWh ha-1 yr-1). Energy demands from plant building materials, machinery, fertilizers, plant operations, and biomass harvesting constituted total energy input with a combined value of 707 GJ ha-1 yr-1. A comparison with a PEC photobioreactor requiring freshwater-based cooling showed that IGP had a 73% greater net energy ratio using the same plant size and system boundary. In conclusion, the above results suggest that developed IGP photobioreactor offers a reliable, energy-efficient platform for large-scale production of biomass and high-value chemicals from microalgae, with no requirements for extraneous cooling and heating systems, generating sustainable baseload electrical energy to energize production operations.

Novel photobioreactor for the sustainable production of algal biomass and electricity

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