Hydrogen gas does not exist in nature. Unlike natural gas and oil, it is therefore not a source of energy. Instead, production of hydrogen requires a lot of energy and is terribly inefficient. Roughly half of the input energy is lost in the process. Hydrogen production is in fact a very fast way to waste both fossil and renewable energy. Hydrogen as a fuel always yields less than 50% of the energy that was needed to make it. It is therefore unsuitable, and actually counterproductive for CO2 reduction.

Integral analysis
Hydrogen is a simple chemical substance, with formula H2. It burns readily in air, producing its oxide H2O (water). Hydrogen is carbon-free, so it does not produce any CO2 at the place of use. That seems attractive, but we must look further than the place of use. An integral analysis of the hydrogen production and distribution chain reveals that the upstream part of that chain causes more CO2 emission than is saved at the end. This is true for grey, blue and green hydrogen, since hydrogen is not there to start with and every process step in the production chain comes with substantial energy losses.

Green hydrogen
Even green hydrogen appears to be counterproductive, increasing global CO2 emission instead of decreasing it. This seems a paradox at first, as green hydrogen is made of CO2-free energy from wind turbines and solar panels. However, using these scarce renewable energy sources for hydrogen production implies that there is less renewable electricity on the public grid for direct use. At any given electricity demand, with any given mix of fossil and renewable supply on the grid, starting a green hydrogen production plant will increase the amount of fossil electricity production. Solar and wind sources cannot be tuned up on demand, so fossil (and perhaps nuclear) power plants are the only source of the extra electricity needed for green hydrogen production. As half of the input energy is lost in the process, the extra CO2 emission so caused will always be higher than the CO2 savings when the hydrogen is subsequently used. In addition, green hydrogen is very expensive and therefore a waste of money.

Energy waste
Put the other way around, switching off a green hydrogen production plant lowers the amount of fossil electricity needed on the public grid. Analyzing the marginal CO2 effects of starting and stopping a hydrogen production plant, therefore yields the inevitable conclusion that we should never start one and stop all running ones. That minimizes global CO2 emission at any time. Wind and solar energy presently (2022) produce some 5% of the global energy demand. They are time, weather and season dependent, and we can best use all this scarce renewable energy directly. Storage in hydrogen (or perhaps metals, or synthetic fuels) inevitably comes with large energy losses, in the order of 50% and more. So, the more electricity (renewable, fossil or nuclear) we store, the more we lose and the more we have to generate. This will only change when we can structurally produce excess renewable energy that would otherwise not be needed for direct use. This will not happen before 2040 (if ever), so until then we should not produce hydrogen on a large scale for energy applications.

Increased CO2 emission
This conclusion remains true for plans and realization of new green hydrogen plants with accompanying solar and wind parks. As soon as the solar and wind parks are there, their renewable electricity can best be supplied to the public grid for direct use. This will lower the total amount of fossil electricity on the public grid, and thereby lower total CO2 emission. The subsequent hook on of a hydrogen production plant does not again lower CO2 emission. Instead, it increases fossil electricity supply to the grid again, and thereby total CO2 emission. Like electric cars, heat pumps or any other electricity user, hydrogen production plants by themselves can never reduce CO2 except by consuming less gross energy. Electric cars and heat pumps hardly do this compared to fuel cars and gas fired central heating, and hydrogen plants even do the opposite as they actually increase gross energy consumption.

Back up
Green hydrogen may locally be beneficial to absorb a momentary local excess of renewable energy. In most developed countries rich enough to build large wind and solar parks as well as hydrogen production and distribution infrastructure, this only happens on windy and sunny Summer days. And even then, increasing regional grid capacity to handle excess renewable electricity is much more effective than wasting half of it by making hydrogen. Green hydrogen can at best be a local momentary back up, and is counterproductive on a large continuous scale.

Blue hydrogen
Blue hydrogen is no better. It is made of natural gas, thereby producing as much CO2 as direct combustion of natural gas. And again, roughly half of the input energy is wasted in the process. Blue hydrogen therefore yields less than half of the original energy content of the natural gas it was made of. Supporters of blue hydrogen argue that the emitted CO2 is captured and stored underground. Apart from the linear non-sustainability of CO2 capture and storage (CCS), this is a truly false argument. CO2 emission from burning natural gas in e.g. power plants and industrial processes can also be captured and stored. The net result of blue hydrogen is therefore always a waste of energy (natural gas in this case), a waste of money, and more CO2.

Low energy content
An important additional downside of hydrogen is its low volumetric energy content. As a gas, it contains only 11 megajoule (3 kilowatt-hours) per cubic meter, three times lower than natural gas. It must therefore be compressed to very high pressures (500-1000 bar) for practical storage and use in e.g. cars. Compression requires a lot of energy, and heavy steel storage tanks. An alternative would be liquefaction. This also requires a lot of energy, as hydrogen gas has a very low boiling point of -253 degrees C (20 degrees Kelvin). Comparison with liquified natural gas (LNG, boiling point -162 degrees C, 111 degrees K) does not improve things for hydrogen.

Hydrogen is inefficient to make, and problematic to store. It is not a source of energy, and not the applauded holy grail of an energy transition. We should not waste our precious (green) energy to make it on a large scale, as this increases CO2 emission in every scenario until at least 2040. It is good to do research on hydrogen technology, but counterproductive to produce it on a large scale in the coming 20 years.