Energy efficiency and sustainability are two indispensable factors driving the transition from the present fossil fuel–based economy to a circular economy. A circular economy is a renewable circular sustainable fuel utilization cycle that will characterize the highly efficient engineering technological choices of the 21st century. The world is presently experiencing the dawning of hydrogen energy in all sectors, including energy production, storage, and distribution; electricity, heat, and cooling for buildings and households; the industry; transportation; and the fabrication of feedstock. Hydrogen energy end-use technologies are not yet mature, unlike conventional technologies. However, they offer potentially significant advantages in terms of low or zero emissions and flexibility in fuel sources.
More than half of the world’s population lives in areas with high levels of air pollution, negatively affecting public health. The chief causes are nitrogen oxides and hydrocarbons. Hydrogen-powered vehicles emit none of these but only H2O and warm air. It can be seen as a long-term replacement for hydrocarbon fuel for air and land transportation and comes with a per-unit-weight-advantage over gasoline and other fuels.
Thermal processes for hydrogen production typically involve steam reforming, a high-temperature process in which steam reacts with a hydrocarbon fuel to produce hydrogen. Many hydrocarbon fuels can be reformed to produce hydrogen, including natural gas, diesel, renewable liquid fuels, gasified coal, or gasified biomass. Today, about 95% of all hydrogen is produced from steam reforming of natural gas. Solar-driven processes use light as the agent for hydrogen production. Photobiological processes use the photosynthetic activity of bacteria and green algae to produce hydrogen. Photoelectrochemical processes use specialized semiconductors to separate water into hydrogen and oxygen. Solar thermochemical hydrogen production uses concentrated solar power to drive water splitting reactions, often along with other species such as metal oxides. Biological processes use microbes such as bacteria and microalgae and can produce hydrogen through biological reactions. In microbial biomass conversion, the microbes break down organic matter like biomass or wastewater to produce hydrogen, while in photobiological processes the microbes use sunlight as the energy source.
Over the long term, hydrogen energy will be used in many everyday situations, and hydrogen energy technologies will replace many conventional technologies. The economics of such hydrogen technologies will improve as they enter widespread use, and the added value of low pollution should further enhance the value of such systems.