Hydrogen — carrier, not magic source

Producing, compressing, transporting and using hydrogen without confusing the tank with the source.

The color is not in the gas

A carrier, not a source

Hydrogen is not a primary energy source like the sun, wind or uranium: it is an energy carrier, that is, a means of transporting and storing energy that first has to be produced from another source, spending energy in the process. Before it can be used for anything, hydrogen has to be produced, and there are two main routes with completely different environmental costs. It can be chemically separated from natural gas through a process called steam reforming, which generates CO₂ as a byproduct unless an effective, large-scale carbon-capture system is installed. Or it can be obtained from water through electrolysis, a process that uses electric current to split water molecules into hydrogen and oxygen, consuming a considerable amount of electricity in the process.

Why color is just an origin label, not a measurement

The industry has popularized a color classification — gray hydrogen if it comes from natural gas without carbon capture, blue if it comes from natural gas with partial capture of the emissions, green if it comes from electrolysis powered by renewable electricity — that quickly summarizes the production route, but does not at all replace measuring the real emissions of the whole process. Hydrogen labeled "green" but manufactured with electricity that, in practice, comes mostly from a grid still dominated by fossil fuels at that specific time and place, can have a carbon footprint far higher than its label suggests. Color is a useful summary of the chosen technological route; the real figure for emissions, methane leaks in the case of natural gas, electricity consumed and water used is the only thing that allows an honest comparison.

Electrolysis loses energy at every conversion

Producing hydrogen via electrolysis is not a perfect process: current commercial electrolyzers reach electricity-to-hydrogen conversion efficiencies of around 70-80%, meaning a significant share of the input electricity is lost as heat in the separation process itself. And that is only the first conversion: if that hydrogen is later used to generate electricity again via a fuel cell — the so-called full "electricity to hydrogen and back to electricity" cycle —, the total round-trip efficiency drops considerably, because every conversion from one form of energy to another loses an additional fraction.

After producing it: compress, transport, convert again

Once produced, hydrogen has to be compressed to high pressure, liquefied at extremely low temperatures, or chemically converted into another, easier-to-handle molecule — such as ammonia — to be practically stored and transported. Each of these additional steps requires specialized equipment and consumes extra energy, further reducing the whole system's net efficiency. Finally, to make use of that stored energy, the hydrogen has to be converted again into heat through direct combustion, into motion through an adapted engine, or into electricity through a fuel cell — and this last conversion has its own limited efficiency, typically between 50% and 70% depending on the cell design. In a fuel-cell vehicle, besides the pressurized hydrogen tank, there is the cell itself (which usually contains platinum as a catalyst, a scarce and expensive metal), specialized membranes, an auxiliary battery to manage power peaks, and electric motors — a considerably more complex set of components than a simple battery-electric vehicle.

Useful, but not for everything

None of this chain of losses makes hydrogen useless as a technology: its ability to store energy over very long periods — weeks or months, something no chemical battery does economically at large scale — and to power industrial processes that are genuinely hard to electrify directly — steel production, ammonia manufacturing for fertilizers, certain long-haul heavy transport — can be very valuable. What does make hydrogen unsuitable is using it in applications where a simpler, more efficient direct electric route already exists, with fewer conversion losses — heating a home with a heat pump, or moving a car with a battery, in the vast majority of cases entirely avoid this long chain of conversions.

Where it makes sense to ask about hydrogen

It can be relevant for fertilizers, steel, chemicals, certain heavy transport and long-duration storage. For heating a home or moving a car, a heat pump or a battery usually avoid conversions and use less electricity at the source.

Before celebrating a "hydrogen engine," ask: is it combustion or a fuel cell?, where does the H₂ come from?, how much energy travels through the whole chain?, are there nitrogen-oxide emissions?, what tank and infrastructure does it need?, what direct alternative provides the same service?

Researching it is reasonable; presenting a prototype as a universal solution is not.

The numbers, with their source

How much is lost at each conversion

Current commercial electrolyzers convert electricity into hydrogen with an efficiency of roughly 70-80% (typical commercial units are around 79%, on the order of 50 kWh of electricity per kilogram of H₂ produced); more advanced systems under development target above 80%. If that hydrogen is converted back into electricity through a fuel cell, with a typical efficiency of 50-70%, the total round-trip efficiency — electricity to hydrogen and back to electricity — falls to a figure that, in the most common systems, is only around 37-40%, though advanced high-end configurations can approach 60-70% under optimal conditions.

Sources: California Energy Commission — Ultra-high Efficiency, Lower-Cost Green Electrolytic Hydrogen; Wikipedia — Roundtrip efficiency.

Compared with a battery, the difference is enormous

A modern lithium-ion battery has a round-trip efficiency (charging then discharging) typically above 85-95% — far above hydrogen's typical 37-40% full-cycle figure. This difference explains why, for hour-or-a-day storage applications — absorbing midday solar surplus to use at night, for example —, a battery is usually the more efficient option. Hydrogen regains the advantage precisely where a battery does not compete well: very long-duration storage, over weeks or entire seasons, where low conversion efficiency matters less than the ability to store a lot of energy for a long time without the self-discharge losses any chemical battery suffers at those timescales.

What this table is missing

None of these efficiency figures include the energy cost of compressing, liquefying or transporting hydrogen once produced — additional steps that subtract even more efficiency from the whole system — nor the cost of manufacturing the electrolyzer or fuel cell itself, which depend on materials like platinum, with their own extraction chain and their own environmental cost.

Sources