Green hydrogen technologies are; photobiological hydrogen production, steam reforming (SMR), photoelectric conversion (PEC), solar thermochemical production, and PV electrolysis.
This article discusses green hydrogen technologies, as follows;
1). Photobiological Hydrogen Production (as an example of Green Hydrogen Technologies)
Feedstock that are used with this technology for hydrogen production include water, biomass and biogas.
Also known as biophotolysis, it involves the photo-induced breakdown of water into oxygen and hydrogen, in the presence of a reducing agent.
Cyanobacteria and algae in the reactor may also contribute to the splitting, deriving their own energy from sunlight to enable them facilitate biochemical reactions that break down the substrate to release hydrogen.
Due to it relative immaturity, photobiological hydrogen production technology has not been clearly defined yet in terms of a generally-accepted series of steps and principles that govern the process.
However, all forms of this technology use sunlight as the primary driver of hydrogen isolation.
2). Steam Reforming (SMR) Hydrogen Generation
Steam reforming (also known as steam methane reforming; SMR), is a thermochemical technology or process, whereby natural gas is subjected to catalytic thermal splitting under controlled conditions to yield carbon monoxide and hydrogen gas mixture, also known as syngas .
It is one of the most common and widely accepted means of hydrogen production in general, and accounts for approximately 48% of total hydrogen production globally, alongside technologies like coal gasification .
This arrangement (renewable energy-dependence) can be challenging however, as reforming is energy-intensive and requires high temperature between 700 and 1,000°C.
3). Photoelectrochemical Conversion (as an example of Green Hydrogen Technologies)
Photoelectrochemical (PEC) conversion is usually a combination of photolytic and electrochemical mechanisms for splitting water molecules to yield hydrogen and oxygen.
Alternatively written as photo-electrochemical, this approach merges the fuel cell concept with photon capture and solar-electricity conversion; using an electrode-electrolyte assembly to perform light-induced electrochemical decomposition.
The light-sensitive component may be the electrode , although some solid electrolytes may also be used to capture photons.
While it also depends on solar energy, photoelectrochemical technology differs from PV electrolysis, which uses photovoltaic (rather than photolytic) conversion to isolate green hydrogen fuel.
4). Solar Thermochemical Hydrogen Production
Solar thermochemical hydrogen (STCH) production is a broad term that refers to all technologies that split water molecule into its component elements at high temperature, with solar as the primary energy resource.
The technology can be energy-efficient when solar concentrators are used to supply heat, provided the water-splitting reaction is divided into a series of cycles, collectively called the thermochemical water-splitting cycles (TWSCs).
While the exact mechanisms may vary, several of them involve an attempt at recycling heat in the water-splitting system. It is also not uncommon for materials like metallic oxides to be used as catalyst .
Steam reforming can be categorized under this technology when it depends solely on solar energy (alternatives like nuclear and waste heat energy can be used) alongside other sub-categories like gasification and cracking.
Feedstock used also may include others aside water, like petroleum and natural gas.
5). PV-powered Electrolysis (as an example of Green Hydrogen Technologies)
PV electrolysis is simply electrolysis where power that is used by the electrolyzer comes entirely from photovoltaic systems that may include solar panels, solar batteries, and power inverters among others.
For green hydrogen production, PV-powered electrolysis is one of the most common as well as simple technologies applicable to decompose feedstock and isolate hydrogen.
It must be acknowledged that PV electrolysis for green hydrogen production is mostly conceptual or hypothetical; as real-life scenarios are usually faced with challenges that include the low energy efficiency of existing solar technology, and the energy-intensive nature of electrolytic water splitting.
Therefore, solar is often used as a backup or supplementary energy resource, especially in large-scale hydrogen production.
Some hybrid concepts may combine multiple solar-powered mechanisms to boost efficiency and reliability. This includes the use of both solar concentrator and solar photovoltaic components, as in concentrator photovoltaic-electrolysis (CPV-E) .
Green hydrogen technologies are;
1. Photobiological Hydrogen Production
2. Steam Reforming (SMR) Hydrogen Generation
3. Photoelectrochemical Conversion
4. Solar Thermochemical Hydrogen Production
5. PV-powered Electrolysis
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