Promising green energy storage technologies include energy carriers such as green hydrogen, hydrogen derivatives and rechargeable batteries. An energy carrier is not a source of energy, but a means to store it for later use. Energy stored in chemical bonds in these green energy carriers can be converted to electrical energy or vice versa, without CO2 emissions. Emission free fuels such as green hydrogen and green ammonia have gained interest as a method to reduce the carbon footprint of the maritime sector.
What is hydrogen?
Hydrogen (H2) is a molecule that consists of two hydrogen atoms (H). A water molecule contains one oxygen and two hydrogen atoms. Methane contains four hydrogen atoms. Hydrogen is therefore an abundant element on earth, while the hydrogen molecule (H2) must be produced. Hydrogen gas is used in many industrial processes, including the production of margarine and fertilizers.
When hydrogen reacts with oxygen, energy is liberated from chemical bonds – with water and heat as the only products. When this reaction occurs inside a fuel cell, chemical is converted to electric energy. Green hydrogen is considered an emission free energy carrier.
There are two main methods of production: Water electrolysis, where water is split into hydrogen and oxygen gas in an electrochemical process, and steam reforming, where methane reacts with water to produce hydrogen and CO2. Hydrogen is termed grey, blue or green, depending on the method of production. Grey H2 is hydrogen produced from steam reforming, blue is from steam reforming combined with carbon capture. Green hydrogen is a term used for hydrogen produced from renewable energy sources through water electrolysis. Both blue and green hydrogen is considered emission free energy carriers, but blue hydrogen requires the production of CO2 and subsequent capture and storage, while no CO2 is produced from water electrolysis.
Hydrogen has a low energy density under normal conditions. It must be compressed or liquefied to increase the energy density. Hydrogen derivatives – chemicals derived from hydrogen – can be produced to obtain emission free energy carriers with a higher energy density than compressed hydrogen. One example is green ammonia, which is viewed as a promising alternative for an emission free fuel for long distance shipping.