No, hydrogen is not considered an energy source. Hydrogen is an energy carrier that can be used to store energy similar to a battery. However, not all hydrogen molecules are created equal. Different production and transportation methods of hydrogen lead to varied environmental effects and costs.
Where does hydrogen come from?
Hydrogen sources mainly include water, hydrocarbons, and biomass. Hydrogen also exists naturally on its own underground although very rarely (Source).
Water molecules (H2O) can be split into hydrogen (H2) and oxygen (O2) molecules in a process known as electrolysis. Since this process requires energy in the form of electricity or heat, the produced hydrogen is considered an energy carrier. When hydrogen undergoes combustion, it releases this energy and produces water.
Hydrocarbons such as oil and gas are also sources of hydrogen molecules. The main difference between hydrocarbons and water is the presence of carbon molecules instead of oxygen. This results in carbon being released in one form or another when hydrogen is produced from hydrocarbons. For example, carbon dioxide is a product of the steam reforming of methane gas. Hydrogen produced from biomass through the reforming of ethanol also releases carbon monoxide and carbon dioxide (Source).
What is hydrogen used for?
Hydrogen is used for petroleum refining, chemical production, heating, cooking, and power applications including transportation.
For petroleum refining, hydrogen is used in two main processes: hydrotreatment and hydrocracking. Hydrotreatment increases the hydrogen content of crude oil and removes impurities (sulfur, oxygen, nitrogen). Hydrocracking is used to break the carbon-carbon bonds of hydrocarbon fuels thereby reducing their molecular weight.
Chemicals produced using hydrogen are mainly ammonia (NH3) and methanol (CH3OH). Ammonia production for fertilizers is the second-largest use of hydrogen globally. Methanol is used as a transportation fuel, hydrogen carrier, or for the production of other chemicals.
Hydrogen can also be used in domestic applications for heating and cooking. It can either be used directly or blended with natural gas with minor modifications to existing equipment. When blended with natural gas in pipelines, hydrogen can help reduce the carbon footprint of heaters and cooktop burners.
Finally, hydrogen is used in fuel cells to produce power and heat for stationary and portable applications. There are many different types of fuel cells that differ according to the used electrolyte and the operating temperature. Fuel cell vehicles also make use of hydrogen to generate the required power for their electric motors. These vehicles include cars, busses, trucks, and trains. Their advantage over battery-powered electric vehicles is lower costs when transporting heavy loads over long distances.
How can hydrogen be stored?
Hydrogen can be stored at high pressures, very low temperatures, or a combination of both. Hydrogen gas has a very low density which makes it ineffective to store it without any compression or cooling. Alternatively, hydrogen can also be stored using organic hydrogen carriers and various other chemicals.
Compressed hydrogen is usually stored at pressures between 150-700 bars (2,000-10,000 psi). These pressures result in a much higher density for hydrogen. Although some energy is required for compression (usually about 20% of the energy content of hydrogen), it ensures that a lot less energy is wasted during the transportation phase. Simply put, transporting less compressed gas cylinders requires fewer trucks and therefore less fuel.
When hydrogen is cooled below -253 °C (-423 °F) at atmospheric pressures, it becomes liquid. Liquified hydrogen also has a higher density than gaseous hydrogen which translates into transport savings. Similar to compression, cooling also requires the expenditure of energy (about 25-45% of hydrogen’s energy content) (Source). It is important to efficiently seal such storage vessels to keep the temperature as low as possible and avoid losses from the boil-off of hydrogen. The use of thermal insulation can help isolate the liquified hydrogen although it makes its use for transportation impractical.
Liquid organic hydrogen carriers (LOHC) such as toluene and cycloalkanes are compounds that can store hydrogen and release it when needed. By converting hydrogen into liquid chemicals, complexities arising from its transport and storage are eliminated. However, the dehydrogenation process by which the hydrogen is released is an endothermic reaction meaning it requires an input of heat. Other hydrogen storage chemicals include metal hydrides and metal-organic frameworks which have chemical and physical hydrogen sorption capabilities respectively.
How is hydrogen transported?
Hydrogen is transported in pipelines, tanker trucks, tube trailers, and ships. The transportation method depends on how the hydrogen is stored and how far it is traveling. How hydrogen is used can also affect the transport method of choice.
Tanker trucks are used to transport liquified hydrogen and can carry about 4000 kg per trip. Over long distances, they are more economical than tube trailers.
Tube trailers, on the other hand, are used for compressed hydrogen and can only carry about 300-1000 kg of hydrogen per trip. The hydrogen is stored in tubes which are then stacked in 20 or 40 ft. containers on top of trailers.
Pipelines and ships are mainly used for long distances. Natural gas pipelines can be used for transporting hydrogen although more cracking and corrosion are expected. An alternate solution is blending hydrogen into existing natural gas pipelines and then separating it downstream or using the mixture as is. Ships are also used for transporting hydrogen, with the first hydrogen-dedicated ships being commissioned in 2020. Liquified hydrogen transport on ships is similar to liquified natural gas (LNG) although more challenging due to lower temperatures.
What are the different colors of hydrogen?
Hydrogen is assigned a color based on how it was produced. Grey, blue, and green are the main colors of hydrogen. Several other colors such as brown, black, turquoise, and yellow are related to a specific source or production method.
Grey hydrogen is produced from hydrocarbons through conventional methods such as steam reforming and gasification. The main feedstocks and energy sources can be oil, gas, or coal and the hydrogen production process releases carbon emissions. These emissions are why the produced hydrogen is assigned a grey color.
Blue hydrogen is hydrogen produced from hydrocarbons with the addition of carbon capture and storage. Capturing carbon emissions avoids their effect on climate and global warming. Although this adds to the cost of the process, the environmental benefits are priceless. Carbon dioxide can be stored in underground caverns or used for enhanced oil recovery operations. Either way, it is locked up and never released into the atmosphere. Although blue hydrogen is better than grey hydrogen, the best option for the environment is always green. After all, hydrocarbons are finite resources that are not sustainable.
Green hydrogen is produced from water electrolysis using renewable energy sources. This avoids the release of harmful emissions completely as the only byproduct is oxygen. Since renewable energy sources such as solar, wind, and hydro release no carbon emissions, the whole process is considered clean. This means that the electrolysis is only as clean as the energy source used to produce electricity. Although hydrogen produced in this way is currently the most expensive option, renewables are steadily becoming cheaper. It is only a matter of time before green hydrogen becomes as cheap as its grey or blue counterparts. For example, wind turbines that can be used to generate electricity for electrolysis are becoming increasingly bigger and cheaper.
As mentioned earlier, other colors can also be used to describe hydrogen produced in specific ways from certain sources. Brown hydrogen is used for hydrogen produced from the gasification of lignite coal while black hydrogen refers to that produced from bituminous coal. Turquoise hydrogen is produced from the pyrolysis of natural gas with solid carbon as a byproduct. Pink hydrogen is a result of electrolysis from nuclear energy while yellow hydrogen gets its color from the use of solar energy.
Is hydrogen sustainable?
Hydrogen as an energy carrier is only sustainable if produced from renewable energy. When produced through the conventional methods of reforming, it cannot be considered sustainable due to the use of finite resources such as oil and gas even if carbon capture is included.
The source of hydrogen and its production method dictates how sustainable it can be. Therefore, it is important to keep in mind that hydrogen is only an energy carrier that can be produced through many different pathways. Hydrogen produced through electrolysis from renewable energy sources is more sustainable than that produced through the gasification of coal or natural gas.
Another important aspect is the use of water for electrolysis. In certain regions such as the Middle East, where water resources are scarce, hydrogen production through electrolysis can become unsustainable. This is mainly due to the consumption of valuable water resources especially when most of the freshwater is a product of desalination, a process that consumes large amounts of energy.
What are hydrogen vehicles?
Hydrogen can be used in fuel cells onboard electric vehicles to power their electric motors. Fuel cell electric vehicles (FCEV) use hydrogen to generate electricity and produce water vapor. In these vehicles, hydrogen is stored at high pressures in pressurized tanks. These tanks can be filled at hydrogen refueling stations.
Fuel cells have an anode, a cathode, and an electrolyte. At the anode, hydrogen is oxidized and electrons travel through a circuit to generate electricity. Positively charged ions travel through the electrolyte to the cathode where they combine with oxygen to produce water. Unlike conventional cars where gasoline is combusted, fuel cells do not have any carbon emissions.
Is hydrogen a better energy carrier than batteries?
When comparing electric vehicles, the source of energy can either be hydrogen stored in a fuel tank or lithium-ion batteries. Although hydrogen-powered vehicles require a fuel cell to convert hydrogen into electricity, there are many advantages to using hydrogen.
Hydrogen has a higher gravimetric energy density than batteries which translates into a lighter vehicle. If the range of battery electric vehicles is to be increased, the size of the battery and its weight is also increased. This can be disadvantageous since it adds to the total weight of the electric vehicle.
Another advantage is the refueling time and durability of hydrogen vehicles. It is a lot faster to fill up a hydrogen tank than it is to recharge a lithium-ion battery. This can save a lot of time for long-distance journeys since the refueling time is a lot less. Batteries also have a limited amount of charging cycles which reduces the lifetime of battery electric vehicles. Although fuel cells also have a limited lifetime, it is currently a lot longer (240,000 km) than batteries (160,000 km) (Source).
What are renewable energy sources?
Renewable energy sources are those methods by which energy is produced from clean sources. They mainly include solar, wind, hydropower, biomass, geothermal, and nuclear energy systems.
Renewable energy sources are intermittent in nature which makes the produced electricity fluctuate. For example, solar energy is only available during the daytime while wind may only be strong enough during certain times or seasons. This problem can be solved through energy storage. By using excess electricity to produce hydrogen, energy can be stored and consumed when needed.
Solar energy has long been used to produce clean energy in a sustainable way. After all, the sun is the source of most surface and underground energy sources on planet earth. By harnessing the power of sunlight, solar systems directly tap into the sun’s energy. The main technologies are photovoltaics (PV) which converts sunlight into electricity and concentrated solar power (CSP) which uses generated heat for steam power and electricity.
Wind power is another sustainable energy source that makes use of airflow to drive wind turbines. Wind farms are groups of turbines that can be installed onshore or offshore. By grouping them closer to each other, electricity can be distributed more efficiently. The same is true for producing hydrogen centrally before it is distributed by pipeline or trucks. For an idea of how much electricity can be generated from wind turbines, check our article on the topic.