Microalgae and Artificial Photosynthesis for Renewable Energy

• Microalgae’s natural photosynthesis, which uses carbon dioxide in the presence of sunlight, is harnessed to produce electricity and biofuels for renewable energy.
• Artificial photosynthesis, biomimics, and natural photosynthesis use semiconductors to capture light and fix ambient CO2.
• Artificial photosynthesis systems produce two types of fuels: hydrocarbons (methane, methanol, and formic acid) and pure hydrogen.

Challenges like global warming and our energy crisis are due to over-reliance on fossil fuels. Therefore, a diversification of energy sources is necessary. The new energy sources also must be sustainable and zero-emission technologies. The latest efforts are applications of natural systems, namely photosynthesis. Find out the status quo of the two major photosynthesis technologies- microalgal and artificial.

Artificial Photosynthesis for Renewable Energy

Photosynthesis, the physiological process in all plants, algae, and many bacteria in nature, captures solar energy to support the entire food chain directly and indirectly. It is being applied to solve several problems people face. Two promising trends that apply this process have emerged that can help replace fossil fuel use:

• Microalgal photosynthesis converts solar energy into electrical and chemical energy, such as algae-based biofuels.
• Artificial photosynthesis system (APS) duplicates natural photosynthesis using biomimetic systems to produce chemical energy.

Sustainability Benefits

Since universal solar energy is being harnessed, both systems can be used anywhere in the world, offering the potential of renewable energy self-sufficiency to all nations. There are other benefits specific to each system, too:

• Algae-powered fuel cells can be decentralized and used in remote areas without electricity grids.
• Algae-based biofuels are economically and technically viable and require less water or land resources than conventional biofuel production methods.
• Unlike fossil fuels, biofuels are biodegradable and non-toxic.
• Both systems can fix carbon and help mitigate climate change during production.
• Unlike solar photovoltaics, both systems convert solar energy into chemical energy that can be stored.

However, both systems have limitations relating to technology and scaling that must be addressed before they are on the market.

Microalgae Potential

The rise of liquid biofuels, like biodiesel and bioethanol, especially in the transport industry, underscores a growing shift where algal photosynthesis can help. Microalgae biofuel can be a significant renewable energy source for sustainable development, potentially replacing fossil-based fuels.

Microalgae biofuels are hailed as promising third-generation renewable energy sources, offering advantages over first and second-generation biofuels, such as oil crops and lignocellulosic biofuels, respectively. They circumvent issues associated with oil crops and lignocellulosic biofuels, negatively impacting food security, land and water scarcity, and deforestation.

Table 1: “Comparison of oil content, oil yield, and biodiesel productivity of microalgae with the first and the second generation biodiesel feedstock source,” Medipally et al. 2015. (Credits: https://www.hindawi.com/journals/bmri/2015/519513/tab1/)

Algal photosynthesis uses various microalgae species, such as Botryococcus braunii, Nannochloropsis sp., Dunaliella primolecta, Chlorella sp., and Crypthecodinium cohnii, because of their ability to produce significant amounts of hydrocarbons and lipids. Botryococcus braunii, in particular, stands out for its high production of hydrocarbons compared to its biomass and the synthesis of other commercially essential compounds like carotenoids and polysaccharides.

The algae generate significant amounts of oil that far exceed the yield from conventional first- and second-generation biofuel sources, see Table 1. The oil content in microalgae species can reach up to 80%, with levels from 20 to 50% being typical.

• Chlorella has up to 50% lipids.
• braunii has the highest oil content, approximately 80%.

Microalgae biomass and biofuel production have two main phases- upstream and downstream (See Figure 1).

Upstream Processes

The upstream stage includes different cultivation technologies to produce biomass and strategies to maximize quantity and quality.

Microalgae cultivation technologies

Three culture systems- batch, semi-batch, and continuous systems are used to produce microalgae biomass.

The culture systems’ algal growth rate determines biomass production. The algal growth rate, which depends on the photosynthetic rate, is affected by biotic and abiotic factors.

• Light, temperature, nutrient stress, pH, O2, CO2, salinity, and toxic chemicals are abiotic factors.
• Biotic parameters influencing the process are competition from other algae, pathogens, and operational factors.

The four bulk microalgae biomass cultivation methods are phototrophic, heterotrophic, photoheterotrophic, and mixotrophic. So far, only phototrophic are commercially feasible for large-scale microalgae biomass cultivation.

Figure 1.: “Different strategies involved in microalgae biomass and biofuel production,” Medipally et al. 2015. (Credits: https://www.hindawi.com/journals/bmri/2015/519513/fig1/)

Measuring photosynthesis

Since biomass production by the microalgae depends on photosynthesis, monitoring its rate in cultures is necessary.

Microalgal photosynthesis is measured by oxygen production, carbon dioxide consumption, and chlorophyll (Chl) fluorescence. Precision tools like the CI-340 Handheld Photosynthesis System from CID BioScience Inc. are ideal for measuring microalgal photosynthetic rates in closed systems using special modules with capacities ranging from a quarter liter to four liters. 

Molecular strategies

Molecular strategies are used to increase microalgae biomass yield and biofuel production.

They are genetic engineering of microalgae, which is easy due to their unicellular nature. The availability of the microalgae genome sequences availability has also greatly facilitated the process.

Interactions with bacterial biofilms

Microalgae and bacteria interact to form symbiotic relationships and establish a “phycosphere” similar to the rhizosphere between plant roots and bacteria.

Microalgae provide extracellular products on their surfaces, which provide the right environment for bacteria to form biofilms.

Downstream Processes

The downstream stage involves harvesting technologies and biofuel production.

Harvesting and drying of biomass

Harvesting involves separating microalgae cells from water and preparing for downstream processing using one or more solid-liquid separation steps. Biomass harvesting and drying requires a significant energy consumption, accounting for 23-30% of costs, and is the focus of research to optimize the entire process.

Lipids extraction and purification

This step involves lipid fraction extraction and coextraction contaminants removal. The appropriate oil extraction process must be cost-effective, efficient, non-toxic, and straightforward. There are several processes for oil extraction from microalgal biomass, like osmotic shock, enzymatic extraction, and solvent extraction. Of all the available methods, the enzymatic extraction method is commercially viable.

Biomass conversion technologies

The energy in the biomass is converted to biofuels or electricity through various conversion processes:

• The power is converted into electricity through direct combustion.
• The chemical conversion or transesterification process converts extracted lipids into biodiesel.
• Biochemical processes produce methanol and ethanol.
• Thermochemical conversion could be pyrolysis to produce bio-oil and charcoal, gasification for fuel gas, and liquefaction for bio-oil production.

Advantages

Biofuels offer advantages over fossil diesel, including biodegradability, non-toxicity, and potentially lower greenhouse gas emissions. Microalgae biofuel, once ready, could be cost-competitive, requires no additional land, improves air quality by absorbing CO2, and uses minimal water. Ongoing research and technological advancements are crucial for determining the widespread viability of these alternative fuels.

Limitations

Commercial microalgae biofuel production faces challenges such as low biomass concentration, costly downstream processes, and low oil content. Harvesting microalgae from water is energy-consuming, and microalgae farming is generally more expensive and complex than conventional agriculture.

To address these issues, suggested strategies include genetic engineering for higher biomass and lipid content, biorefinery approaches, cost-effective harvesting technologies, and optimizing the symbiotic interactions between microalgae and bacteria. While artificial lighting can address sunlight fluctuations, it increases costs, and the focus should be on employing sunlight efficiently.

Artificial Photosynthesis System

Figure 2: “The diagram representing the fundamental process of artificial photosynthesis systems having an efficient light absorber to trap sunlight and efficient water oxidation and proton reduction catalysts,” Kathpalia and Verma 2022.   (Image credits: https://www.intechopen.com/online-first/87027)

The concept of artificial photosynthesis (APS) was proposed by Giacomo Ciamician in 1912. The idea lay dormant until 1972 when Kenichi Honda and his student Akira Fujishima developed a light-powered device to split water. This milestone is called the “Honda-Fujishima effect.”

APS mimics natural photosynthesis, utilizing biomimetic systems to convert water, carbon dioxide, and sunlight into oxygen and energy-rich compounds. APS, a scalable technology, generates two types of fuels- hydrocarbons (methanol, formic acid, and methane) and pure hydrogen. Hydrogen can be a clean option for various applications, replacing fossil fuels.

APS must replicate the three critical components occurring in natural photosynthesis- light capture, water splitting, and carbon dioxide reduction, as shown in Figure 2.

Light absorption

Chlorophylls and carotenoids capture red and blue wavelengths of light to excite electrons in natural photosynthesis, with the help of manganese as a catalyst. The pigments absorb light only between 400 to 700 nm, i.e., about half of solar radiation.

For APS, the aim was to design photosensitizers that absorb a more comprehensive range and aggregate more sunlight. Semiconductors like titanium dioxide photoanode or gallium nitride, silicon, and metal oxides like ferric oxide, zinc oxide, and bismuth vanadate can absorb more wavelengths. Of these materials, silicon is cheap and abundant.

Cobalt oxide is a catalyst that imitates natural photosynthesis and reduces energy consumption.

Figure 3: Comparison of natural and artificial photosynthesis pathways, Xie et al. 2022. (Image credits: https://doi.org/10.1016/j.biortech.2022.127830)

Water Splitting

Lysis or splitting of water occurs in natural photosynthesis by oxidizing water. In APS, water splitting is a thermodynamic challenge requiring large amounts of energy.

In the presence of sunlight absorbed by semiconductors, water is oxidized to produce oxygen, protons, and electrons. The electrons move to another part of the apparatus, while the proton is reduced to give hydrogen, see Figure 3.

Carbon dioxide reduction

During this stage, carbon dioxide is split using energy to break the bonds between carbon and oxygen. The process gives hydrocarbons like methanol (CH3OH), methane (CH4), formic acid (HCOOH), and carbon monoxide (CO).

Advantages

Artificial Photosynthesis (APS) through semiconductor devices offers a sustainable and versatile energy solution by capturing and storing solar energy as chemical fuel.

Its benefits include renewable energy generation, carbon negativity, energy storage, reduced dependence on fossil fuels, scalability, low environmental impact, and potential for technological advancements.

Limitations

Artificial photosynthesis faces several challenges, including the complexity of mimicking natural photosynthesis, the time-intensive nature of research and development, and the need for a biomimetic approach for industrial utilization.

• The quest for a self-sustained APS system is in its early stages, with many models having drawbacks related to efficiency, instability, and financial expenses.
• A significant bottleneck is the search for a cost-effective, efficient, and stable catalyst material, whether organic or metal-based. Catalysts may interfere with system equipment, and the overall financial expenses associated with APS development are notable.

Semi-artificial photosynthesis

Researchers are also exploring semi-artificial photosynthesis by combining advantageous features from both natural and artificial processes. This includes incorporating high quantum efficiency, selectivity, specificity, self-repair mechanisms from natural photosynthesis, and synthetic materials to broaden the light absorption spectrum and modify molecular chemistries. Photosystem I (PSI) and Photosystem II (PSII) photocatalytic properties are harnessed to develop devices that efficiently convert light energy into molecular hydrogen and carbon-based fuels.

The goal is to overcome the limitations of natural photosynthesis and enhance the overall efficiency of energy conversion.

Nascent Technologies

Both the photosynthetic technologies for energy generation are in their nascency. Microalgae are attracting attention from researchers, governments, and entrepreneurs. APS also represents a promising avenue for renewable energy research. Overcoming challenges in both technologies requires interdisciplinary collaboration, sustained funding, and a commitment to long-term research efforts to unlock its full potential for sustainable energy production.

Sources

Blanken, W., Cuaresma, M., Wijffels, R. H., & Janssen, M. (2013). Cultivation of microalgae on artificial light comes at a cost. Algal Research, 2(4), 333-340.

Kathpalia, R., & Verma, A. K. (2023). Artificial Photosynthesis an Alternative Source of Renewable Energy: Potential and Limitations. DOI: 10.5772/intechopen.111501

Masojídek, J., Gómez-Serrano, C., Ranglová, K., …… Acién-Fernándéz, F. G. (2022). Photosynthesis Monitoring in Microalgae Cultures Grown on Municipal Wastewater as a Nutrient Source in Large-Scale Outdoor Bioreactors. Biology, 11(10), 1380. https://doi.org/10.3390/biology11101380

Wang, Z., Hu, Y., Zhang, S., & Sun, Y. (2022). Artificial photosynthesis systems for solar energy conversion and storage: platforms and their realities. Chemical Society Reviews.

Xie, Y., Khoo, K. S., Chew, K. W., Devadas, V. V., Phang, S. J., Lim, H. R., … & Show, P. L. (2022). Advancement of renewable energy technologies via artificial and microalgae photosynthesis. Bioresource Technology, 363, 127830.