Hydrogen is an ideal, clean, carbon-free carrier of energy that produces only water vapor as a waste product and has potential applications in automobiles, airplanes, and also in home-heating.
The projected mean power generation for the years 2050 and 2100 has been estimated to be 28 and 46 terawatt (TW), respectively. By far, sunlight provides the largest of all carbon-neutral energy sources. In fact, 14 TW of solar energy falls on the earth every hour. Thus, more energy from sunlight strikes the earth in 1 h than all the energy consumed on the planet in a year.
The most successful technologies taking advantage of this resource are photovoltaics (PVs; solar electricity), a $10 billion industry that is currently growing at a rate of 35–40% each year. Continued growth of the PV sector at a rate of ~25% would increase the production level from 1.7 GW in 2005 to 380 GW in 2030, and thus would satisfy a significant fraction of the world energy demand. Moreover, among the renewable sources, PVs have the highest potential to reduce the costs compared to biomass, geothermal, wind, and solar thermal.
Photoelectrolysis of water and especially the use of semiconductor–electrolyte interfaces illuminated with sunlight for the production of hydrogen from water and other suitable solvents have been reviewed in several excellent publications. At present, only about 5% of the commercial hydrogen production is primarily via water electrolysis, whereas the other 95% is mainly derived from fossil fuels. This does not represent inhouse consumption of hydrogen such as oil refineries and ammonia plants where the bulk of hydrogen is consumed.
When determining how much electricity is needed to produce H2 by solar energy, the energy requirements of generation (electrolysis), compression, liquefaction, storage, and transportation all have to be considered and added up. The energy content of 1 kg of H2 is 39.3 kWh. In order to generate 1 kg of H2 by the electrolysis of water, about 50 kWh of electric energy is required. Therefore, the efficiency of H2 generation is about 66%.
Once a kilogram of H2 is produced, it is either compressed or liquefied before storage or distribution. If handled in the high-pressure gas form, about 3 kWh of energy is required for its compression and 2.5 kWh is required for its transportation over each 100 km distance. Therefore, a total of about 6 kWh is required to compress and transport the gas over a distance of 100 km.
This energy corresponds to about 15% of the higher heating value (HHV) of the gas. As the transportation distance increases, this percentage also rises. Therefore, when transportation over long distances is required, it is more economical to transport the H2 in liquid form by trucks, rails, or ships. If handled as a cryogenic liquid, about 12 kWh is required to liquefy each kilogram of H2 and about 1 kWh is needed to store and transport it, for a total of about 13 kWh, which is about 33% of the HHV of the liquid.
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