Sustainability is a key principle in natural resource management, and it involves operational efficiency, minimisation of environmental impact and socio-economic considerations; all of which are interdependent. It has become increasingly obvious that continued reliance on fossil fuel energy resources is unsustainable, owing to both depleting world reserves and the green house gas emissions associated with their use. Therefore, there are vigorous research initiatives aimed at developing alternative renewable and potentially carbon neutral solid, liquid and gaseous biofuels as alternative energy resources. However, alternate energy resources akin to first generation biofuels derived from terrestrial crops such as sugarcane, sugar beet, maize and rapeseed place an enormous strain on world food markets, contribute to water shortages and precipitate the destruction of the world's forests. Second generation biofuels derived from lignocellulosic agriculture and forest residues and from non-food crop feedstocks address some of the above problems; however there is concern over competing land use or required land use changes. Therefore, based on current knowledge and technology projections, third generation biofuels specifically derived from microalgae are considered to be a technically viable alternative energy resource that is devoid of the major drawbacks associated with first and second generation biofuels. Microalgae are photosynthetic microorganisms with simple growing requirements (light, sugars, CO 2 , N, P, and K) that can produce lipids, proteins and carbohydrates in large amounts over short periods of time. These products can be processed into both biofuels and valuable co-products. This study reviewed the technologies underpinning microalgae-to-biofuels systems, focusing on the biomass production, harvesting, conversion technologies, and the extraction of useful co-products. It also reviewed the synergistic coupling of microalgae propagation with carbon sequestration and wastewater treatment potential for mitigation of environmental impacts associated with energy conversion and utilisation. It was found that, whereas there are outstanding issues related to photosynthetic efficiencies and biomass output, microalgae-derived biofuels could progressively substitute a significant proportion of the fossil fuels required to meet the growing energy demand.

/pdf/biofuels-from-microalgae-a-review-of-technologies-for-1t1wwr5e69.pdf

Biofuels from microalgae—A review of technologies for production, processing, and extractions of biofuels and co-products

The use of fossil fuels is now widely accepted as unsustainable due to depleting resources and the accumulation of greenhouse gases in the environment that have already exceeded the “dangerously high” threshold of 450 ppm CO2-e. To achieve environmental and economic sustainability, fuel production processes are required that are not only renewable, but also capable of sequestering atmospheric CO2. Currently, nearly all renewable energy sources (e.g. hydroelectric, solar, wind, tidal, geothermal) target the electricity market, while fuels make up a much larger share of the global energy demand (∼66%). Biofuels are therefore rapidly being developed. Second generation microalgal systems have the advantage that they can produce a wide range of feedstocks for the production of biodiesel, bioethanol, biomethane and biohydrogen. Biodiesel is currently produced from oil synthesized by conventional fuel crops that harvest the sun’s energy and store it as chemical energy. This presents a route for renewable and carbon-neutral fuel production. However, current supplies from oil crops and animal fats account for only approximately 0.3% of the current demand for transport fuels. Increasing biofuel production on arable land could have severe consequences for global food supply. In contrast, producing biodiesel from algae is widely regarded as one of the most efficient ways of generating biofuels and also appears to represent the only current renewable source of oil that could meet the global demand for transport fuels. The main advantages of second generation microalgal systems are that they: (1) Have a higher photon conversion efficiency (as evidenced by increased biomass yields per hectare): (2) Can be harvested batch-wise nearly all-year-round, providing a reliable and continuous supply of oil: (3) Can utilize salt and waste water streams, thereby greatly reducing freshwater use: (4) Can couple CO2-neutral fuel production with CO2 sequestration: (5) Produce non-toxic and highly biodegradable biofuels. Current limitations exist mainly in the harvesting process and in the supply of CO2 for high efficiency production. This review provides a brief overview of second generation biodiesel production systems using microalgae.

https://www.springer.com/cda/content/document/cda_downloaddocument/BioEnergyResearch_Second+Generation+Biofuels+High-Efficiency+Microalgae+for+Biodiesel+Production.pdf?SGWID=0-0-45-1347846-0

Second generation biofuels: high-efficiency microalgae for biodiesel production

Cultivation, photobioreactor design and harvesting of microalgae for biodiesel production: A critical review

Photobioreactors for mass cultivation of algae.

The present hype in microalgae biotechnology has shown that the topic of photobioreactors has to be revisited with respect to availability in really large scale measured in hectars footprint area, minimization of cost, auxiliary energy demand as well as maintenance and life span. This review gives an overview about present designs and the basic limiting factors which include light distribution to avoid saturation kinetics, mixing along the light gradient to make use of light/ dark cycles, aeration and mass transfer along the vertical or horizontal main axis for carbon dioxide supply and oxygen removal and last but not least the energy demand necessary to fulfil these tasks. To make comparison of the performance of different designs easier, a commented list of performance parameters is given. Based on these critical points recent developments in the areas of membranes for gas transfer and optical structures for light transfer are discussed. The fundamental starting point for the optimization of photo-bioprocesses is a detailed understanding of the interaction between the bioreactor in terms of mass and light transfer as well as the microalgae physiology in terms of light and carbon uptake kinetics and dynamics.

https://onlinelibrary.wiley.com/doi/pdf/10.1002/elsc.200900003

Design principles of photo-bioreactors for cultivation of microalgae.

Invention of outdoor closed type photobioreactor for microalgae

For a photobioreactor for mass-culturing microalgae, it is known that flashing light effect enhances the efficiency of photosynthesis. A dynamic model for photosynthesis was developed to elucidate this effect. A particular feature of the model is that discrete RuBP particles circulate in the Calvin cycle and their speeds in the cycle are determined by the amount of ATP generated in the photon reception process. This can realise the light saturation under continuous light and the flashing light effect under fluctuating illumination. Laboratory experiments were conducted to obtain model parameters by curve-fitting for Chaetoceros calcitrans. The present model demonstrates the light flashing effect moderately well and elucidates its mechanism reasonably.

Dynamic discrete model of flashing light effect in photosynthesis of microalgae

A device (1) and a method for cultivating micro algae capable of providing a high productivity by realizing the sufficient stirring of culture solution and maintaining a high culture efficiency over a long period by preventing the micro algae from adhering to the wall surface of a culture container and from precipitating onto the bottom surface of the culture container, wherein, in the micro algae cultivating device (1) for cultivating micro algae in the culture container (2) by filling the culture solution (6) in the culture container (2) having a gas discharge opening part (17) at a top part and letting visible light beam into the culture solution (6) while blowing the gas containing carbon dioxide therein, the culture container (2) is formed in a parabola shape with inner and outer double curved surface walls (4, 5), at least the outer curved surface wall (5) is formed with a transparent material allowing visible light beam to transmit therethrough, and a gas inlet (5b) is opened to the inside bottom part of the culture container (2), whereby the swirl flow of the culture solution (6) rising along the lateral center thereof, distributed laterally at the top thereof, and lowering along the outer periphery thereof can be formed in the culture container (2) by blowing the gas from the gas inlet (5b) into the culture container.

Device and method for cultivating micro algae

To provide a microalgae culture device which permit sufficient stirring of a culture solution and thus can achieve high productivity and which can prevent the microalgae from adhering to the walls of the culture vessel or settling to the bottom of the culture vessel and thus can maintain high culture efficiency over a long period of time. A microalgae culture device 1, comprising a culture vessel 2 having an gas outlet opening 17 at a top part thereof, wherein a culture solution 6 filled in the culture vessel 2 is irradiated with visible light, while injecting a gas containing carbon dioxide therein, to culture the microalgae in the culture vessel 2 characterized in that the culture vessel 2 is formed into a parabolic shape by inner and outer curved walls 4 and 5, in that at least the outer curved wall 5 is made of a transparent material transparent to visible light, and in that a gas inlet 5b opens into the lower portion or the culture vessel 2. A vortex flow of the culture solution 6 that: progresses upward along the center across the width direction, separates on the right and left sides, and then lowers along the outer circumferential side is created in the culture vessel 2 by the aspiration of the gas through the gas inlet 5b.

Yutaka Kondo

Papers

Invention of outdoor closed type photobioreactor for microalgae

Dynamic discrete model of flashing light effect in photosynthesis of microalgae

Device and method for cultivating micro algae

Method of culturing microalgae and device therefor