Low-emission energy sources like hydrogen have garnered the reputation of being a sustainable alternative to fossil fuels. However, hydrogen’s energy density is a significant roadblock to widespread usage. In this article, we will examine how the density of hydrogen impacts its practical applications and discuss potential storage and transport solutions.
What is energy density?
Energy density refers to the amount of energy that can be stored in a given volume or mass of a substance. It indicates how much energy is contained in a specific material or region of space, which provides valuable insight into how efficiently the energy can be used, stored, or transported.
Energy density can be measured in gravimetric energy density (per unit of mass) or volumetric energy density (per unit of volume). Gravimetric energy density is more relevant when comparing the energy efficiency of fuels, and volumetric energy density has a more significant impact on storage and transportation capabilities.
The energy density of hydrogen
The energy density of hydrogen can vary based on the physical state of hydrogen (gas, liquid, or solid) and the storage conditions (pressure and temperature). Below are approximate values for different states of hydrogen.
Gaseous Hydrogen:
At standard temperature and pressure, the volumetric energy density of gaseous hydrogen is around 0.09 kilograms per cubic meter (kg/m³). However, pressure changes can change the energy density significantly.
- Compressed hydrogen stored at high pressure (700 bar) can have an energy density of about 42 kg/m³.
- Compressed hydrogen at lower pressure (350 bar) can have a volumetric energy density of 28 kg/m³.
Liquid Hydrogen:
Stored at its boiling point of -253°C, liquid hydrogen has a higher volumetric energy density than gaseous hydrogen. The volumetric energy density of liquid hydrogen is approximately 70.8 kg/m³.
Hydrogen Fuel Cells:
The volumetric energy density of a hydrogen fuel cell system (including the hydrogen storage and the fuel cell stack) can vary. Still, it is generally lower than the energy densities of hydrogen gas or liquid. This is due to the additional volume occupied by the fuel cell components and the reaction products.
How does energy density impact practical applications?
Regarding fuels, energy density influences the amount of energy that can be extracted from a given volume of fuel. Generally, a higher energy density indicates that more energy can be stored in a smaller space or mass. Therefore, an energy-dense fuel will typically provide energy for a longer period.
Additionally, a higher density would allow for storing larger amounts of fuel in a smaller container, which is advantageous for global importing and exporting and smaller applications like fueling a car. However, there are other factors to consider when assessing the practicality of a fuel source.
Hydrogen has a substantial energy content by weight (gravimetric energy density) at atmospheric temperature, much higher than conventional fossil fuels like gasoline and diesel. This makes it a highly efficient energy carrier.
Gaseous hydrogen is also lightweight, which is advantageous for weight-sensitive applications. For example, lighter fuel storage systems in an aircraft can reduce its overall weight, providing performance benefits such as faster acceleration, better handling, increased fuel economy, etc.
However, hydrogen also has a low energy content by volume (volumetric energy density), meaning it would require substantial space to store large amounts of fuel (in the gaseous state). Additionally, hydrogen in its gaseous state is highly combustible, and a leak could lead to dangerous explosions. Fortunately, researchers have been working on various solutions to improve the efficiency, safety, and practicality of hydrogen storage and transport.
Solutions to more efficient hydrogen storage and transport
Compressing and liquifying hydrogen gas are two of the most prominent solutions for enhancing hydrogen storage and transportation capabilities. These are discussed below.
Compressed Hydrogen Gas Storage
Hydrogen can be stored in high-pressure tanks. Increasing the pressure of the gas increases its volumetric energy density, allowing for more energy to be stored in a given volume. Advances in materials science and tank design enable the development of lightweight and high-strength tanks capable of safely containing hydrogen at high pressures.
Liquefied Hydrogen Storage
Hydrogen can be cooled to extremely low temperatures to become a liquid. Liquid hydrogen has a much higher volumetric energy density than gaseous hydrogen, allowing for more efficient storage. However, the cryogenic temperatures required for liquefaction present challenges regarding energy consumption and maintaining low-temperature conditions.
Liquid hydrogen also has a much lower volumetric energy density than conventional fuels like gasoline and diesel (see Figure 1 and Figure 2). However, hydrogen’s environmental benefits and potential for renewable production make it an area of significant research and development for clean energy applications.
Figure 1:
A graphic comparison of the volumetric energy density of liquid hydrogen to other liquid fuels (at boiling point) in terms of kilograms per cubic meter (kg/m³).
| Fuel | Volumetric Energy Density (kg/m³) |
| Liquid Hydrogen (H2) | 70.8 |
| Gasoline | 720 – 780 |
| Diesel Fuel | 830 – 860 |
| Ethanol (C2H5OH) | 789 |
| Methanol (CH3OH) | 791.3 |
| Propane (C3H8) | 584 |
| Butane (C4H10) | 601.6 |
| Methane (CH4) as LNG | 430 – 470 |
| Ammonia (NH3) | 682 |
Figure 2:
A chart comparison of the volumetric energy density of liquid hydrogen to other liquid fuels (at boiling point) in terms of kilograms per cubic meter (kg/m³).
Other approaches for efficient hydrogen storage and transportation:
- Hydrogen Carriers: Hydrogen can be chemically bound to a carrier molecule, such as ammonia or liquid organic hydrogen carriers (e.g., baking soda). These carriers can store and transport hydrogen at ambient conditions, reducing the challenges associated with compression or liquefaction. The hydrogen can be released from the carrier through a chemical process when needed.
- Hydrides and Chemical Storage: Metal hydrides (e.g., magnesium hydride and alkali borohydride salts from shrimp waste) can absorb and release hydrogen through chemical reactions. These materials offer a reversible way to store and release hydrogen, but research is ongoing to develop hydrides with high storage capacities, fast kinetics, and reasonable operating conditions.
- Advanced Adsorption Materials: Porous materials like metal-organic frameworks and carbon-based structures (e.g., solar-powered device that produces hydrogen fuel from the air) that can adsorb hydrogen gas at high surface areas, increasing storage capacity. Research is focused on designing materials with high adsorption capacities and tunable properties for efficient hydrogen storage.
- Hydrogen Pipelines: For industrial-scale transport, pipelines can be used to move hydrogen over long distances (e.g., the China Petrochemical Corp pipeline from Mongolia to Beijing and the European undersea pipeline). However, pipeline considerations materials include compatibility, safety considerations, and minimizing hydrogen embrittlement.
- Hydrogen Production On-Site: Producing hydrogen directly at the point of use can minimize the need for extensive storage and transport infrastructure (e.g., the Coradia iLint, North America’s first hydrogen train)
- Hydrogen Infrastructure Development: A robust infrastructure that includes storage facilities, distribution networks, and refueling stations is essential for the widespread use of hydrogen. Government incentives (e.g., New York’s $10M Clean Hydrogen Initiative) and industry collaborations (e.g., Joint Forum to establish a global certification standard for the hydrogen industry) play a significant role in accelerating infrastructure development.
Continued research and development are essential to discovering new materials, technologies, and processes that can enhance the efficiency, safety, and practicality of hydrogen storage and transport. Finding innovative solutions to these challenges can lead to a cleaner, more sustainable energy future.