Can solar hydrogen be used as a source of clean, sustainable energy?
Currently, trends in energy supply and demand are not environmentally, socially, or economically sustainable. Fossil fuels and oil are the primary sources of energy worldwide, and their continued use raises concern about their long-term supply, price, and emissions. Scientists are working on green energy solutions to improve the long-term supply of energy and reduce their impact on the environment .
The term green energy is applied to a product that uses green chemistry principles. These principles involve the minimising of waste material, the use of safer and renewable inputs, energy efficiency and maximizing the use of energy and materials provided as inputs to the process (atom economy). Atom economy indicates how much of the reactant atoms form the final product.
Green energy options include solar, solar thermal, geothermal, wind and biomass. Of these options, solar has a high potential to generate long-term, low-cost energy options. Solar energy is available worldwide, with current solar energy received by the earth’s surface totalling over 10,000 times the current world energy demand.
Issues with current solar energy production include a low energy content per volume, high cost to develop infrastructure and the subsequent greenhouse gas emissions created during production. To overcome these issues, multiple different solar hydrogen production systems have been investigated. Examples of chemical synthesis methods of solar hydrogen can be split into four chemical production systems; concentrated solar thermal processing, photovoltaic, photoelectrochemical and photobiological. These systems can be sorted by the temperature requirement (high or low), and by the process required to separate the products (e.g. thermolysis, electrolysis, cracking). Only two of these processes produce carbon dioxide as a product of the reaction, with the rest producing a combination of green products; hydrogen, oxygen and carbon.
Comparison between the techniques highlights problems with feasibility of large-scale energy production. This includes high energy requirements to:
- catalyse the reactions, for example, solar thermolysis has clean outputs however requires temperatures of above 2,900K to trigger the reaction, with only 35% dissociation achieved
- unpredictable efficiency, for example in a concentrated photovoltaic system, the efficiency of this system varies between 18-24.8%
- production of undesirable products based on incomplete reactions, for example in water electrolysis, there is a small amount of NOx formation that cannot be avoided.
More research and development is required to make the different solar‐to‐hydrogen techniques greener, feasible, and cost-effective to meet the worlds growing energy needs in a changing market.
Adapted from: Hosseini, S. E., & Wahid, M. A. (2020). Hydrogen from solar energy, a clean energy carrier from a sustainable source of energy. International Journal of Energy Research, 44(6), 4110-4131.
Questions for students:
- Describe what is meant by green chemistry.
- Explain why it is important to minimise the production of unwanted products in energy reactions.
- Explain, giving examples, why it is important to consider both the economic and environmental impacts of chemical reactions.
Key words:
- Sustainable
- Environmental
- Social
- Economic
- Green energy
- Green chemistry principles
- Efficiency
- Atom economy
- Atom
- Solar energy
- Solar thermal energy
- Geothermal energy
- Wind energy
- Biomass energy
- Chemical synthesis
- Thermolysis
- Electrolysis
- Cracking
- Catalyse