Hydrogen is considered an important energy carrier in a future decarbonized world. In fact, many are advocating for a hydrogen economy
that relies on hydrogen as the commercial fuel that would deliver a substantial fraction of a nation's energy and services (see Figure 1).
Hydrogen is a versatile energy source than can be used by:
- burning it directly: Pure hydrogen gas reacts with oxygen to release energy and form water as a byproduct. Burning in atmospheric air instead of pure oxygen typically yields small amounts of nitrogen oxides along with the water vapor. Opportunities to use direct burning of hydrogen include: heavy industry (such as steel making), heavy transport (trains, long-haul trucking), shipping, home heating, and power generation.
- using it in a fuel cell to generate electricity: A fuel cell is a device that converts the energy stored in chemical bonds to electrical energy (see Figure 2). In a hydrogen fuel cell the hydrogen reacts with oxygen across an electrochemical cell similar to that of a battery to produce electricity, water, and small amounts of heat. Currently, Toyota is producing the Mirai automobile which runs on hydrogen via a fuel cell utilizing a hydrogen tank in place of the traditional gas tank (see OWOE What is a fuel cell vehicle?. Although there are very few such vehicles actually in operation in the US, the push for hydrogen vehicles is strong in Japan and Korea. More widespread is the use of hydrogen fuel cells in light transport vehicles such as fork lifts.
An additional promising aspect of hydrogen as an energy source is that it has the largest energy content of any fuel, up to 120MJ/kg, and is very stable, which makes it ideal for energy storage
. A relatively small amount of hydrogen can store significant amounts of energy to be used through either of the above methods when needed, including as a dispatchable energy source to produce electricity. Figure 3 illustrates that hydrogen can be more effective than pumped hydro storage
and significantly more effective than traditional batteries as an energy storage medium. The stable chemistry of hydrogen also means you can store energy longer than any other medium.
Challenges to using hydrogen as an energy source include the cost required to produce pure hydrogen, the energy required to produce hydrogen that could possibly displace other energy uses, the cost of new equipment to utilize this source of energy, and infrastructure to transport and store hydrogen in either gas or liquid form.
In addition, both approaches require pure hydrogen. Although hydrogen is the most common element in the universe
, on earth it is tied up in water, natural gas, ammonia and all organic compounds. Energy is required to break down the molecule into its constituent elements, releasing the hydrogen. There are a number of ways to do this, and they are given a color that indicates how "green" the process is, i.e., how much greenhouse gas emissions are generated in the process to create the hydrogen. The hydrogen color spectrum
ranges from "black", which is the most environmentally unfriendly, to "green".
- "Black" hydrogen: Hydrogen can be produced though gasification of a carbonaceous (carbon-based) raw material, such as coal, into a fuel gas, also known as synthesis gas, synthetic gas, or syngas for short. Oxygen (or air) and steam are directly contacted with the coal or other feed material in a high temperature / high pressure vessel, which results in a series of chemical reactions that convert the feedstock to syngas and ash/slag (mineral residues). Syngas is composed primarily of carbon monoxide and hydrogen. It is typically further converted to hydrogen and carbon dioxide by adding steam in the presence of a catalyst. The use of the term black implies using black coal such as anthracite for the feedstock.
- "Brown" hydrogen: similar to black hydrogen, except that the geologically younger brown coal, also known as lignite, which contains a considerably higher percentage of oxygen and hydrogen, is used as the feedstock.
- "Gray" hydrogen: Hydrogen can be produced by steam methane reforming (SMR) in which heat and pressure are used to convert the methane in natural gas to hydrogen and carbon dioxide. This has come to be known as gray hydrogen since natural gas has a higher hydrogen-to-carbon ratio than coal, and, thus, the process produces less carbon dioxide than coal gasification.
- "Blue" hydrogen: Increasingly, many propose using carbon capture and storage to reduce emissions from the creation of black, brown or gray hydrogen, producing so-called blue hydrogen. This is frequently promoted as low emission hydrogen. However, far from being low carbon, greenhouse gas emissions from the production of blue hydrogen are quite high, particularly due to the release of methane during natural gas production, transportation and storage, as well as during the SMR process. In addition, the process assumes that carbon dioxide can be captured and stored (See OWOE: What is carbon capture and storage) indefinitely, which is both optimistic and unproven.
- "Turquoise" hydrogen: is made using a process called methane pyrolysis to produce hydrogen and solid carbon (see Figure 4). It is a new entry in the hydrogen color chart and production has yet to be proven at scale.
- "Pink" hydrogen: is generated through electrolysis powered by nuclear energy. Nuclear-produced hydrogen can also be referred to as "purple" or "red" hydrogen. Although this is a low carbon process, utilizing nuclear power removes a very important low emissions source of power from the grid. If that power needs to be replaced with power generated from burning fossil fuels, it does not address the problem with hydrogen-based energy.
- "Green" hydrogen: When the electricity to electrolyze water and create hydrogen is produced by a renewable energy source such as hydro, wind, or solar, the hydrogen is termed green hydrogen.
- "Yellow" hydrogen: is a relatively new phrase for green hydrogen that is made specifically through electrolysis using solar power.
- "White" hydrogen is a naturally occurring geological hydrogen found in underground deposits and created through fracking. There are no strategies to exploit this hydrogen at present.
Currently (2021), most hydrogen (95% in the US and 75% globally) is generated from fossil fuels, particularly from SMR of natural gas but also from coal gasification.
In recent years, energy companies, including major oil companies, have been increasingly advocating the use of blue hydrogen as a means to provide the energy the world needs while cutting CO2 emissions. Although these companies acknowledge that this form of H2 is not zero-carbon, they argue that it is a good transition fuel to help build demand and associated infrastructure for hydrogen deployment. At some point in the future when green hydrogen can be produced cost effectively, it will be an easy transition.
However, a recent study published in the Energy Science and Engineering journal titled "How green is blue hydrogen?"
demonstrated that burning hydrogen made using natural gas actually emits more CO2 than does burning natural gas or even coal directly. While burning of the hydrogen fuel itself is zero-emission, the process of producing it in the first place produces carbon dioxide emissions which must be captured and stored, which processes also require electricity, as well as fugitive CO2 and methane emissions directly to the atmosphere. The authors conclude that, even in the best-case scenario for producing blue hydrogen using renewable electricity, there is no role for blue hydrogen in a carbon-free future. They speculate that "...blue hydrogen is best viewed as a distraction, something than may delay needed action to truly decarbonize the global energy economy...", and further note that "...much of the push for using hydrogen for energy since 2017 has come from the Hydrogen Council
, a group established by the oil and gas industry specifically to promote hydrogen, with a major emphasis on blue hydrogen... since even more natural gas is needed to generate the same amount of heat."
A second study
published shortly after the first by researchers from the Australian National University concluded that both the emissions and financial costs from producing H2 using fossil fuels and storing captured carbon emissions was greater than that of producing green hydrogen with renewable energy. Given such science that argues strongly against the case for blue hydrogen, the current state of technology would indicate that the best path forward for maturing the hydrogen economy and displacing fossil fuels is to concentrate on green hydrogen.