Grey – Blue – Green … Clean
As of 2020, the majority of hydrogen (∼95%) is produced from fossil fuels by reforming natural gas, partial oxidation of methane and coal gasification, which all release a lot of CO₂ (grey hydrogen).
Approx. 70 million tons of hydrogen were produced worldwide in 2019 for various uses, such as, oil refining, and in the production of ammonia and methanol, and also as a fuel in transportation. The hydrogen generation market is expected to be valued at US$115.25 billion in 2017.
There are four main sources for the commercial production of hydrogen: natural gas, oil, coal, and electrolysis; which account for 45%, 33%, 15% and 6% of the world’s hydrogen production respectively. Specifically, bulk hydrogen is usually produced by the steam reforming of methane or natural gas.
As an intermediate step, low-CO₂ (blue) hydrogen plays an important role, with most of the CO₂ emissions, up to 90%, being captured during the production of grey hydrogen and stored, for example, in empty gas fields (Carbon Capture and Storage, CCS).
The main way to make green hydrogen is electrolysis, where water is split into hydrogen and oxygen while using electricity generated from renewable energy sources. Three predominant types of cells are being used:
- alkaline electrolysis cells (AEC)
- polymer electrolyte membrane cells (PEM)
- solid oxide electrolyzer cells (SOEC).
Traditionally, alkaline electrolyzers are cheaper in terms of investment but less efficient; vice versa, PEM electrolyzers are more expensive but more efficient with a large potential to reduce cost when produced in large volumes.
SOECs operate at high temperatures, typically around 800 °C. The thermal energy provided at these high temperatures allow to reduce the amount of electrical energy required for electrolysis, thus having the potential to reduce the overall cost of hydrogen produced. This technology is also called High Temperature Electrolysis (HTE). The heat energy can be provided from a number of different sources such as waste industrial heat as an example.
PEM electrolysis cells typically operate below 100 °C. These cells have the advantage of being comparatively simple and can be designed to accept widely varying voltage inputs which makes them ideal for use with renewable sources of energy such as solar PV.
AECs optimally operate at high concentrations electrolyte (KOH or potassium carbonate) and at high temperatures, often near 200 °C.
Functional diagram: AEM
Functional diagram: PEM