By Prashant Kumar, Dr. Narale Pradip D. and Prof. Mahendra S. Seveda, College of Agricultural Engineering and Post-Harvest Technology (Central Agricultural University, Imphal), Ranipool, Gangtok, India.
Liquid hydrogen, abbreviated as LH2, is a colourless, odourless and highly flammable liquid that is widely used in diverse industrial and scientific applications. It is one of the lightest and most efficient fuels, with a high energy density and low molecular weight. In this review article, we will discuss the properties, production, storage and applications of liquid hydrogen.
Liquid hydrogen has a boiling point of -252.87 °C and a density of 70.85 kg per m3 at boiling point. It is the coldest liquid in existence and must be stored and transported at extremely low temperatures. Owing to its high heat of combustion, it releases a large amount of energy when burned. It also has very low viscosity, making it an excellent coolant for high-temperature applications.
There are several technologies for producing liquid hydrogen. These include:
- Steam methane reforming (SMR): This is the most common method for producing large quantities of liquid hydrogen. It involves reacting natural gas with steam in the presence of a catalyst to produce hydrogen gas and carbon dioxide.
- Electrolysis: This method uses an electric current to split water into hydrogen and oxygen gases. This process can be powered by renewable energy sources such as wind or solar power, making it a sustainable option for producing liquid hydrogen.
- Biomass gasification: This method involves heating biomass materials such as wood chips and agricultural waste to produce a gas that can be converted into hydrogen gas and other useful products.
- Pyrolysis: This is a process that involves heating a material in the absence of oxygen to produce hydrogen gas and other by-products.
- Coal gasification: This method involves heating coal to produce a gas that can be converted into hydrogen gas.
- Nuclear energy: Nuclear energy can also be used to produce hydrogen gas through high-temperature electrolysis or thermochemical water splitting.
- Photobiological water splitting: This is a process that uses photosynthetic organisms to split water into hydrogen and oxygen gases.
Each of these technologies has its own advantages and disadvantages, and the choice of technology depends on factors such as availability of resources, energy efficiency, and environmental impact. As the demand for liquid hydrogen continues to grow, researchers are exploring new and innovative methods for producing it sustainably and efficiently.
The production of liquid hydrogen involves a complex process that requires specialised equipment and expertise. SMR, for instance, is a highly efficient and the most common method for producing large quantities of liquid hydrogen. However, it requires a significant amount of energy, and the process also produces carbon dioxide, which is a greenhouse gas.
To address these issues and reduce greenhouse gas emissions and improve sustainability, researchers are exploring alternative methods for producing liquid hydrogen. These include electrolysis and biomass gasification, which use renewable energy sources and produce no carbon dioxide emissions. Scaling up and improving the production of liquid hydrogen are essential for its widespread use in various industries, including aerospace, automotive and energy production.
Storage of liquid hydrogen
The storage of liquid hydrogen is a major challenge because it must be done at extremely low temperatures and requires special containers and insulation materials. The most common storage method involves vacuum-insulated containers, designed to maintain a constant low temperature of the liquid. These containers are constructed from materials such as stainless steel and aluminium and are typically spherical or cylindrical in shape.
Liquid hydrogen has several advantages over compressed hydrogen, making it a more favourable option for certain applications:
- Energy density: Liquid hydrogen has a higher energy density than compressed hydrogen. This means that it can store more energy in a smaller volume, which is important for applications where space is limited, such as transportation.
- Range: Liquid hydrogen has a longer range than compressed hydrogen, making it more suitable for applications that require covering long distances, such as long-haul transportation.
- Efficiency: Liquid hydrogen is more efficient to transport over long distances compared to compressed hydrogen. This is because it requires less energy to transport liquid hydrogen in insulated tankers, whereas compressed hydrogen requires a significant amount of energy to compress it and transport it in high-pressure tanks.
- Safety: Liquid hydrogen is considered safer than compressed hydrogen because it has a lower flammability range and can be stored at lower pressures. Moreover, the risk of sudden pressure release is lower with liquid hydrogen since it does not require high-pressure tanks.
- Reliability: Liquid hydrogen is more reliable than compressed hydrogen since it does not require the same level of maintenance as high-pressure tanks. This means that liquid hydrogen has a reduced risk of leaks or other failures.
Although compressed hydrogen has advantages, such as lower capital costs and easier handling, liquid hydrogen is often preferred for applications that require higher energy density, longer ranges and more efficient transportation.
Applications of liquid hydrogen
The future applications of liquid hydrogen are vast and diverse due to its being a highly versatile and clean fuel source. Here are some of its potential future applications:
- Transportation: Liquid hydrogen can be used as a fuel for fuel cell vehicles, which produce electricity through a chemical reaction between hydrogen and oxygen, emitting only water as a by-product. This offers a clean and sustainable alternative to traditional gasoline-powered vehicles.
- Aerospace: Liquid hydrogen has been used as a rocket fuel for a long time due to its high energy density and efficient combustion. As the space industry continues to grow, the demand for liquid hydrogen as a rocket fuel is likely to increase.
- Energy storage: Liquid hydrogen can be utilised for energy storage as it can be produced when renewable energy sources, such as wind and solar power, are abundant and then used to generate electricity when demand is high.
- Industrial processes: Liquid hydrogen can be used in various industrial processes such as refining and petrochemical production. It can also serve as a coolant in power plants and other industrial applications.
- Heating: Liquid hydrogen can be used as a fuel for heating homes and buildings, and providing a clean and efficient alternative to traditional fossil fuels.
- Marine transportation: Liquid hydrogen can be used as a fuel for ships, providing a clean and sustainable alternative to traditional marine fuels.
Liquid hydrogen is increasingly being regarded as a promising fuel source for the future to achieve climate goals. It has the potential to reduce CO2 emissions in industry and transportation, and facilitate the storage and transportation of renewable energy. It offers a clean and sustainable alternative to traditional fossil fuels, as hydrogen only produces water when burned.
However, it is important to note that hydrogen is not a silver bullet for decarbonisation as its production can be carbon-intensive if it relies on fossil fuels. This is because the majority of hydrogen currently produced comes from steam methane reforming, a process that requires fossil fuels as feedstock. In addition, the production of hydrogen from renewable electricity can be expensive and inefficient as significant amounts of energy are required to produce hydrogen through the process of electrolysis.
Despite these challenges, there are reasons to be optimistic about the future of hydrogen. For one, the cost of renewable electricity is rapidly declining, which is expected to significantly reduce the cost of producing hydrogen from renewable energy sources. Additionally, several promising new technologies are being developed that could make hydrogen production more efficient and cost effective.
Furthermore, the potential applications of liquid hydrogen are vast and diverse, ranging from transportation to aerospace to industrial processes.
However, there are also potential risks associated with hydrogen, such as the risk of hydrogen leaks, which could have negative environmental and safety implications. In addition, there are concerns that the rush to invest in hydrogen fuel technologies could potentially make global warming worse if hydrogen production relies on fossil fuels or if the hydrogen leaks into the atmosphere.
Overall, the potential benefits of liquid hydrogen as a clean and sustainable fuel source are clear, and as research and development continue to advance, the potential applications of liquid hydrogen are likely to expand further. However, it is important to carefully consider the challenges and risks associated with hydrogen and develop a comprehensive strategy for its production, distribution and use in order to maximise its potential as a key component of a low-carbon future.