Solar Energy Capture and Biohydrogen Production

Abstract

Solar radiation energy capture, conversion of photo energy to chemical energy, and biopolymers by a number of photoautotrophic organisms constitute the basis of life on the planet. Through complex, molecular machinery and processes for the efficient production of energy, photoautotrophs have efficiently converted solar energy into chemical energy for more than three billion years. Man-made technologies have not fully succeeded in harnessing these processes for the production of renewable energy but progress is quite promising. The water-splitting reaction in the second phase of photosynthesis; photosystem II (PSII) effectively occurs at room temperature.

Thermal dissociation reactions, in contrast, to the reactions in PSII require a very high thermal energy: 1550 K. However, with the current research methodologies and the successful high-resolution structure elucidation of PSII with particular emphasis on the Mn4Ca cluster, an invaluable blueprint has thus been provided for the design and development of future solar power technologies.

The presence of genetic engineering tools has enabled scientists to develop biotechnological strategies for the production of renewable CO2 energy from solar energy. This review has a particular emphasis on the use of cyanobacteria and microalgae for biohydrogen production with specificity to the potential size of the energy market, global warming, and how the natural photosynthetic processes can be modified for improved sustainable biohydrogen production.

Introduction

Man depends on natural processes for the provision of his energy needs. Energy from the sun is utilized by plants through the process of photosynthesis to generate the energy necessary for growth, development, and reproduction. It is an established fact that there are some algae that produce hydrogen from water with help from radiation energy from the sun. It is this phenomenon that drives the quest for the production of renewable energy from the sun without inflicting harm on the environment. Photosynthesis is one of the many complex processes that plants utilize in the production of energy.

The production of biohydrogen is one of the most promising forms of renewable energy for the future. The greatest challenge of our society today is the production and use of zero-carbon dioxide emission fuels. The rapid and continuing depletion of fossil fuels whose combustion increases anthropogenic carbon dioxide emissions is a cause of concern due to the effects of global warming and climate changes. Future developments that will replace fossil fuels will increasingly be expected to be free of carbon dioxide emissions.

To this end, there have been rapid developments of noncarbon dioxide emitting forms of energy. Nuclear power tops this quest but the exorbitant costs of installation advanced technology and maintenance make it very inaccessible to many nations. Moreover, the threat of nuclear proliferation and the development of arms race are fierce opposing effects of this technology. Renewable energy power sources such as solar energy, wind, hydroelectric, and geothermal power sources are widely accepted forms of alternative sources of energy. However, the future requires an energy source that is sustainable and able to provide energy security owing to its borderless distribution.

Only incident solar energy meets this objective and is by far the largest source of energy (178000 TW year -1)2. Solar energy has the potential of meeting and surpassing the global energy demand if fully harnessed. However, the energy source is diffuse and the relevant capturing technologies are expensive. These constraints drive the opportunities for the development of biotechnologies necessary for solar capture and conversion.

The global energy market in the 21st century

The complexity of analyzing and predicting accurate energy demand globally requires the use of ‘ballpark figures for its predicted size. These figures highlight the potential for the development of solar-powered technologies. From a global energy demand prediction taken in the year 2000, the energy demand will rise from 13TW in 2000 to an estimated level of 46TW in 2100. The global energy demand in 2100 is however dependent on the rate of population growth, improvement in energy intensity, and actual economic growth of nations. Additionally, emerging economies of India and China are currently exerting strong upward pressure on global energy demand.

The Constraints of Global Warming

For 400,000 years, the atmospheric carbon dioxide levels were relatively stable between 200 and 280 ppm. Recently from around 1850, the levels have sharply risen to 370 ppm. The potential effects of gradual increase are disastrous, for example, if the levels reach a peak of 450 ppm there would probably be severe irreversible damage to the coral reef. At 550 ppm, the West Atlantic ice sheet will melt making the levels of the sea rise by approximately six meters.

This would be followed by the extinction of 24% of all plant and animal species. At 650 ppm there would be the disruption of thermohaline circulation prompting major local climatic changes and the extinction of 35% of animal and plant species. These worries are the key tenets that drive the development and use of energy production technologies that ensure that carbon dioxide anthropogenic emission levels do not reach catastrophic levels.

The Potential of Solar Energy

Solar energy is evenly and relatively distributed. It is accessible to small or large and low tech or high tech systems. The main challenges remain the construction and development of photon conversion efficient systems and the ability to efficiently convert captured solar energy to chemical energy such as hydrogen. Large-scale solar systems are unlikely to be installed as their economic costs are still exorbitant compared to readily available fossil fuels. This calls for the utilization of photosynthetic organisms that are preferably more suited to meet this challenge. However this also has a major constraint; under maximum solar irradiance, the overall efficiency of conversion is much lower. This is due to natural losses orchestrated by photoprotection mechanisms and thermodynamic inefficiencies.

Improving Biohydrogen Production

The photosynthetic pathway is central to the production of biofuels. Such biofuels include bioethanol, biodiesel, and biohydrogen. These fuels constitute 66% of the global energy provision. All solar energy technologies, for example, photovoltaics and solar thermal systems, are only capable of producing heat and electricity. Microbial photoreactions used for the production of biofuels are more technically challenging but they can range from the simplest forms to more complex coil systems or the fully enclosed bioreactors that are illuminated by fiber optic lights. Anabaena variabilis, a cyanobacterium have been used in tubular photobioreactor systems for long-term outdoor production of hydrogen.

Control over variable factors such as light, nutrients, temperature, and water ensures improvement in photon conversion efficiencies hence optional hydrogen production. The reaction basically involves a hydrogenase (Ni-Fe, Fe, and (FeS) Cluster freehand types. Maximal hydrogen photoproduction occurs through three major pathways. Studies carried out for C. reinhardtii and pilot-scale culturing projects involving mutants are still progressing. The first pathway requires two photosystems. A complete electron transport chain brings the electrons from water to Fd (Ferredoxin)

H2O ⇒H2 +1/2O2

4 quanta per H2O are required in this pathway. This evolution occurs in a single temporal stage with absolutely no gas separation. If this pathway was to achieve maximal efficiency and occur at full pull activity, an oxygen tolerant hydrogenase would be required together with this will be a promoter to aid gene expression in such physiological conditions and an assembly of a hydrogenase that is oxygen insensitive. The second pathway involves electron and proton flow to the plastoquinone pool. This flow proceeds from fermentation reactions and oxidative carbon metabolism of stored photosynthesized carbon. The reaction is mediated by a dehydrogenase through PSI into Fd then to hydrogenase.

H2O +CO2⇒ Light [CH2O] Biomass

Dark + Anaerobiosis – Oxygen⇒ Hydrogen Induction Light⇒ H2+CO2

The reaction requires 6 quanta of hydrogen. Because of the limiting nutrition, the commercial hydrogen system is based on a continuous process. In this process, both PSII and HydA function simultaneously. The third pathway utilizes nitrogenase with electrons and protons sourced from ATP derived from photosynthesis

H20 ⇒ 1/2O2+2e- [as NADPH +ATP]

N2 + 6H+ +6e- –[12ATP] ⇒ 2HN3(12ATP+Pi)

2H+ + 2e- (4ATP) H2 (4ADP+Pi)

Quantum Efficiency of Photosynthesis and Approaches to Improve Bipolar Efficiency

To achieve efficient and sustainable biohydrogen production by oxygenic phototrophs; light energy transfer from the antenna electron competition from ferredoxin and proton competition from the stroma and the photochemistry in the centers of reaction. To engineer an efficient reduced antenna size, the gene encoding factor that controls the antenna expression system is deleted. This increases the light saturation threshold.

Nonphotochemical quenching is also reduced in the antenna hence reduction of loss from fluorescence and heat. To reduce oxygen sensitivity of hydrogenase reduction of intracellular oxygen concentration is pursued to avoid hydrogen inhibition during light activation improving hydrogenase expression hence its synthesis is a key factor in increasing the yield of oxygen. Through genetic engineering, high hydrogen production mutants are screened and used for the efficient and sustainable production of hydrogen.

Conclusion

Even though photosynthesis is still undervalued as a blueprint for the future provision of renewable energy sources, advances in the understanding of the biochemical pathway of light capture and production of biohydrogen are on the rise. The development of biotechnologies and systems that utilize solar energy capture to fuel the energy needs of the future remains major technological challenge for the 21st century

References

Olaf Kruse, Jens Rupprecht, Jan H. Mussgnug, G. Charles Dismukes and Ben Hankamer. Photosynthesis: A Blueprint for Solar Energy Capture and Biohydrogen production Technologies.Photochem. Photobiol. Sci., 2005, 4, 957 – 970. Web.