Hydrogen: Sorting the Real Applications From the Hype

Hydrogen occupies a peculiar position in the proposed transition away from fossil fuels. It is simultaneously touted as a universal solution for decarbonisation and criticised as an inefficient diversion of renewable electricity. The tension arises because hydrogen is not an energy source but an energy carrier. Unlike coal or natural gas, which exist in high-density pockets ready to be extracted, pure hydrogen must be manufactured. The process of creating, compressing, and transporting it involves unavoidable thermodynamic costs that dictate where the technology is viable and where it is a liability.

The current industrial appetite for hydrogen is already substantial, though primarily satisfied by reforming methane. This process releases significant carbon dioxide as a byproduct. To achieve a low-carbon economy, the goal is to shift toward green hydrogen produced via the electrolysis of water. The central engineering challenge is that this shift is not a simple substitution of one gas for another. It involves a fundamental change in the physics of energy distribution.

Public discussion often conflates the chemical versatility of hydrogen with its practical utility as a fuel. While hydrogen can power a passenger car or heat a domestic boiler, these applications compete with direct electrification via batteries and heat pumps. In these contexts, hydrogen usually loses on the grounds of efficiency. However, in heavy industries such as primary steel production or long-haul shipping, the energy density requirements and chemical properties of the process make batteries physically insufficient. Understanding where hydrogen fits requires a precise look at its molecular constraints.

The physical properties of the first element

Hydrogen is the simplest and most abundant element in the universe, consisting of a single proton and an electron. In its molecular form, H2, it is a colourless, odourless gas. Its most significant trait in the context of energy is its high gravimetric energy density. One kilogram of hydrogen contains roughly 142 megajoules of energy, which is nearly three times that of petrol. This characteristic makes it an attractive candidate for applications where weight is a primary constraint, such as aviation.

However, this high energy per unit of mass is offset by an exceptionally low energy per unit of volume. At standard temperature and pressure, hydrogen is the least dense gas known. A cubic metre of hydrogen contains only about 11 kilojoules of energy, compared to the 35,000 kilojoules found in the same volume of natural gas. This disparity dictates the entire infrastructure of hydrogen handling. Because the molecules are so small and light, they are also prone to leaking through seals and diffusing into the crystalline structure of metals, a process known as hydrogen embrittlement. This can make steel pipelines brittle and prone to cracking over time, requiring specific alloys or internal coatings for transport.

The reactivity of hydrogen is another defining feature. It has a wide range of flammability when mixed with air, and its ignition energy is remarkably low. While this makes it an excellent fuel, it also means that a spark as weak as static electricity can ignite a leak. Unlike larger hydrocarbons, a hydrogen flame is nearly invisible to the naked eye and emits very little radiant heat. This makes fire detection a specialised technical requirement in industrial settings. These physical realities are not obstacles that can be "solved" by better software or manufacturing; they are fixed parameters of the periodic table.

Thermodynamics of water electrolysis

The production of green hydrogen relies on electrolysis, a process that uses an electric current to split water into its constituent parts, hydrogen and oxygen. This occurs within an electrolyser, which consists of two electrodes separated by an electrolyte or a membrane. When a voltage is applied, water molecules at the anode are stripped of electrons, producing oxygen gas and positively charged hydrogen ions. These ions move through the electrolyte to the cathode, where they combine with electrons to form hydrogen gas.

The efficiency of this process is governed by the laws of thermodynamics. The minimum amount of energy required to split water is defined by its Gibbs free energy, but in practice, additional energy is needed to overcome electrical resistance and the kinetic barriers of the chemical reactions at the electrodes. Most commercial electrolysers, such as Alkaline or Proton Exchange Membrane (PEM) systems, operate at efficiencies between 60 and 80 percent. The remaining energy is lost as heat.

The second law of thermodynamics ensures that every conversion step in the hydrogen cycle exacts a non-negotiable energy tax.

Current research focuses on high-temperature electrolysis using Solid Oxide Electrolyser Cells (SOEC). These systems operate at temperatures above 700 degrees Celsius. By using heat to provide some of the energy needed for the reaction, the electrical demand is reduced, potentially pushing electrical efficiency toward 90 percent. However, these systems require a constant source of high-grade industrial waste heat to be viable, and the ceramic materials used in the cells are susceptible to thermal stress. The engineering consensus suggests that while laboratory efficiencies are high, the durability of these systems under the fluctuating loads of renewable energy remains an unresolved challenge.

Volumetric density and the storage problem

Storing hydrogen is the most significant hurdle to its widespread adoption. Because of its low volumetric density, hydrogen must be compressed to extreme pressures or liquefied at cryogenic temperatures to be moved or stored in useful quantities. Standard industrial storage tanks typically pressurise the gas to 350 or 700 bar. To reach 700 bar, a compressor must exert force equivalent to the pressure at the bottom of the Mariana Trench. This compression process consumes a significant portion of the energy contained within the hydrogen itself, usually between 10 and 15 percent.

Liquefaction offers a higher density but comes with a steeper energy penalty. Hydrogen turns into a liquid at minus 253 degrees Celsius, only 20 degrees above absolute zero. Maintaining these temperatures requires complex, multi-stage refrigeration cycles. Roughly 30 percent of the energy in the hydrogen is spent simply turning it into a liquid. Furthermore, even in vacuum-insulated tanks, heat will eventually leak in, causing the liquid to boil off. This makes liquid hydrogen unsuitable for long-term storage unless the boil-off gas can be captured and used on-site.

Geological storage is the only plausible solution for large-scale, seasonal energy reserves. Salt caverns, which are already used to store natural gas, can be leached out to hold enormous volumes of hydrogen at high pressure. Unlike depleted oil and gas reservoirs, salt caverns are largely inert and non-porous, reducing the risk of the hydrogen reacting with microbes or leaking through the rock. While this is technically feasible and has been demonstrated in several locations, the number of suitable salt formations is geographically limited, meaning that hydrogen production and storage sites may be far removed from the end-users.

The efficiency penalty of the round trip

When considering hydrogen as a medium for storing renewable energy, the "round-trip efficiency" is the most critical metric. This refers to the percentage of electricity that is recovered after it has been converted to hydrogen, stored, transported, and then converted back into electricity via a fuel cell or turbine. Each stage of this journey involves losses. If we begin with 100 units of renewable electricity, roughly 75 units remain after electrolysis. After compression and transport, this might drop to 65 units. If this hydrogen is then put through a fuel cell to generate electricity, which typically operates at 50 to 60 percent efficiency, the final output is approximately 35 to 40 units.

This stands in stark contrast to lithium-ion battery storage, which typically boasts a round-trip efficiency of 85 to 90 percent. For applications like passenger transport or domestic heating, the three-fold loss of energy inherent in the hydrogen cycle makes it an expensive and inefficient choice. To provide the same amount of end-use energy, one would need to build three times as many wind turbines or solar panels to support a hydrogen-based system compared to a direct-electric one.

The efficiency penalty is less of a deterrent in sectors where no other options exist. In the manufacturing of ammonia for fertiliser, hydrogen is a chemical feedstock, not just a fuel. In steelmaking, hydrogen can replace carbon monoxide as a reducing agent to remove oxygen from iron ore. In these cases, the inefficiency of the hydrogen cycle is not compared against a battery, but against the carbon emissions of the current fossil-fuelled process. The debate is therefore shifting away from using hydrogen as a general-purpose fuel and toward its role as a targeted surgical tool for the most difficult parts of the industrial economy.

Decarbonising the primary steel industry

Steel production accounts for roughly seven to nine per cent of global carbon dioxide emissions. The vast majority of this comes from the blast furnace, a vessel where iron ore is heated alongside coke, a purified form of coal. The coke serves two purposes. It provides the heat necessary to melt the ore, and it acts as a reducing agent. As the coke burns, it produces carbon monoxide, which strips oxygen atoms away from the iron oxide in the ore. This chemical reaction leaves behind pure molten iron but releases substantial volumes of carbon dioxide as a byproduct.

To remove carbon from this process, the industry is testing the Direct Reduced Iron (DRI) method using hydrogen. In this configuration, the blast furnace is replaced by a shaft furnace where iron ore pellets are exposed to a stream of hydrogen gas. The hydrogen reacts with the oxygen in the ore to form water vapour rather than carbon dioxide. The resulting solid sponge iron is then melted in an electric arc furnace to produce steel. While this process is technically proven, the shift from carbon to hydrogen requires a total redesign of the plant's thermal management.

The primary constraint is the heat balance of the reaction. The traditional carbon-based reduction is exothermic, meaning it releases heat that helps sustain the process. In contrast, the reduction of iron ore with hydrogen is endothermic. It requires a constant input of external heat to keep the reaction moving at an industrial pace. Engineers must therefore find ways to inject high-temperature heat into the shaft furnace, often using electricity or by pre-heating the hydrogen stream. This adds to the overall energy demand of the plant, further increasing the amount of renewable electricity required for every tonne of steel produced.

Chemical feedstocks and the fertiliser gap

Modern agriculture is fundamentally dependent on the Haber-Bosch process, which synthesises ammonia for fertiliser. This process requires a steady supply of hydrogen. Currently, almost all industrial hydrogen is produced via steam methane reforming, where natural gas is reacted with steam at high pressure. This is a carbon-intensive method, yet it remains the global standard because it is inexpensive and the infrastructure is mature. Replacing this fossil-derived hydrogen with green hydrogen produced via electrolysis is a matter of direct substitution rather than mechanical invention.

The challenge lies in the scale of the transition. The Haber-Bosch process operates most efficiently at a constant, high pressure and temperature. Electrolysers powered by wind and solar produce hydrogen intermittently, fluctuating with the weather. To keep a fertiliser plant running around the clock, operators must either overbuild the electrolyser capacity to fill massive storage tanks or find ways to make the chemical synthesis more flexible. Neither option is cheap.

The transition to green hydrogen for fertiliser is less about new chemistry and more about the logistics of replacing an established, low-cost fossil supply chain.

Despite these hurdles, ammonia is a leading candidate for the first wave of large-scale hydrogen adoption. Unlike steel, which requires a new furnace design, an ammonia plant can be decarbonised simply by changing the source of its input gas. Furthermore, because ammonia is easier to liquefy and transport than pure hydrogen, some planners suggest that ammonia itself may become the primary medium for moving hydrogen across oceans. The chemical industry already possesses the ships, ports, and safety protocols to handle ammonia, providing a logistical head start that other sectors lack.

Challenges in heavy maritime transport

Shipping poses a specific problem for electrification. While a short-range ferry can operate on lithium-ion batteries, a container ship crossing the Pacific would require a battery so heavy and voluminous that it would displace a significant portion of the cargo. Hydrogen offers a higher energy density than batteries, but its volumetric density remains a problem. Even when compressed to 700 bar or liquefied at minus 253 degrees Celsius, hydrogen occupies roughly four to eight times the volume of heavy fuel oil for the same energy content.

Designing a ship to run on hydrogen requires significant trade-offs in vessel architecture. Liquid hydrogen tanks are typically spherical or cylindrical to manage pressure and insulation, making them difficult to pack into the rectangular hull shapes of modern ships. The boil-off rate is another concern. Even with advanced vacuum insulation, a small percentage of liquid hydrogen turns back into gas every day. On a long voyage, this gas must be either reliquefied, which consumes more energy, or used in the ship's engines to prevent pressure build-up.

Because of these storage difficulties, the maritime industry is investigating hydrogen derivatives like methanol or ammonia. These fuels are liquid at more manageable temperatures and have higher volumetric energy densities. However, each step of conversion, from hydrogen to methanol or ammonia, and then potentially back to hydrogen or directly burned in a modified engine, incurs an efficiency loss. The marine sector is currently a site of active competition between these different molecular carriers, with no single clear winner. The decision will likely depend on whether port infrastructure gravitates toward one specific fuel, creating a dominant standard for international trade.

Economic competition with battery chemistry

The most significant retraction in the hydrogen outlook has occurred in the transport sector. A decade ago, hydrogen fuel cells were seen as a viable competitor to batteries for passenger cars. That competition has largely ended. Battery technology has improved in energy density and decreased in cost at a rate that hydrogen has failed to match. The mechanical simplicity of a battery electric vehicle, which converts electricity to motion with roughly 80 per cent efficiency, is superior to the 30 per cent efficiency of the hydrogen pathway, which includes electrolysis, compression, transport, and reconversion in a fuel cell.

In heavy trucking, the debate is more nuanced but follows a similar trajectory. A heavy-duty battery electric truck requires large, heavy packs that reduce the payload capacity. Hydrogen fuel cells are lighter, allowing for more cargo. However, the rollout of megawatt-scale charging stations is proving to be more cost-effective than building a nationwide network of hydrogen refuelling stations. The infrastructure for electricity is already ubiquitous; the infrastructure for high-pressure hydrogen must be built from scratch.

Current engineering trends suggest that batteries will dominate any application where a vehicle can be recharged during a predictable dwell time, such as overnight at a depot. Hydrogen is being pushed into increasingly niche roles, such as ultra-heavy mining haulers that run 24 hours a day or heavy machinery operating in remote areas where the local grid cannot support high-power charging. The boundary between battery and hydrogen dominance is not fixed, but it has shifted steadily toward batteries as their chemistry and charging speeds improve.

A measured outlook for the hydrogen economy

The immediate future of hydrogen is unlikely to resemble the broad-based fuel economy once envisioned. It is established that hydrogen is a chemical necessity for fertiliser and a viable reducing agent for green steel. In these sectors, there is no serious electric alternative. It is also established that for light transport and short-duration energy storage, batteries are the more efficient and economical choice. The role of hydrogen in these areas is now largely considered speculative or obsolete by most energy analysts.

The contested territory lies in long-haul shipping, aviation, and long-duration seasonal energy storage. In these fields, the physics of energy density and the economics of infrastructure are still in conflict. For aviation, hydrogen combustion or fuel cells could work for regional flights, but long-haul travel may require synthetic liquid fuels derived from hydrogen. For the power grid, hydrogen stored in underground salt caverns could provide a buffer for weeks of low wind and sun, but the high cost of the round-trip efficiency makes this a last resort.

The success of the hydrogen economy depends not on its versatility, but on the falling cost of electrolysers and the availability of surplus renewable power.

Several factors could change this picture. A significant breakthrough in electrolyser durability or a drastic reduction in the cost of iridium and platinum, used as catalysts, would lower the entry price for green hydrogen. Conversely, if solid-state batteries achieve a doubling of energy density, they could displace hydrogen even in heavy trucking and short-haul shipping. At present, the most plausible path forward is a bifurcated energy system: electricity for everything that can be wired or easily charged, and hydrogen molecules for the heavy, hot, and distant tasks that electricity cannot reach. The hydrogen economy is not a replacement for the electric one, but a necessary and expensive supplement to it.