Macro view including use cases
At the beginning of the energy transition, progress was essentially demonstrated by the technological optimization of power generation from renewable resources and their increasing share in total electricity production. The measures had focused solely on the electricity sector.
Today, it is becoming clear for decarbonization in countries like Germany, which have been on this path for some time, that further progress in the electricity sector alone can only be realized at sharply rising costs. For this reason, the other energy sectors have rightly come into focus in recent years. For highly inustrialized countries with temperate climate in the middle latitues, the heat and transport sectors are the most relevant; under other climatic and geographical conditions, sectors such as water desalination or cooling are worth mentioning.
The often high efficiency and automation of electrified sectors make sector coupling interesting. As already explained in Sec. 2.3.3, sector coupling here means in particular the coupling of the sectors heating or cooling supply and transport with the electricity sector. Systemic issues dominate at the latest in the optimal design of sector integration, so that macromodeling plays an important role.
A current example deals here with the “color” of hydrogen, i.e. its process-related origin.
Hydrogen color theory
Although hydrogen is a colorless gas, it carries different color designations depending on its origin in the production process:
Green hydrogen. Green hydrogen is produced by electrolysis from water, with the electricity required for electrolysis coming exclusively from renewable sources. Since the power required comes from renewable sources, hydrogen production – regardless of the electrolysis technology selected – is correspondingly CO2-free.
Gray hydrogen. Gray hydrogen is obtained from fossil fuels. In a process known as steam reforming, natural gas is converted into hydrogen and carbon dioxide under heat. As a rule, the carbon dioxide produced in this process is then released unused into the atmosphere. Unlike its green counterpart, gray hydrogen thus contributes directly to the global greenhouse effect.
Blue hydrogen. Blue hydrogen is gray hydrogen, but its carbon dioxide is captured and stored as it is produced. With this process, also known as carbon capture and storage (CCS) or carbon capture and utilization (CCU), the carbon dioxide generated during hydrogen production is therefore not released into the atmosphere. Thus, blue hydrogen is considered CO2-neutral in balance sheet terms. In practice, no technical process has perfect efficiency. Accordingly, a residual amount of carbon dioxide naturally always remains during separation.
Turquoise hydrogen. Turquoise hydrogen is actually hydrogen produced via methane pyrolysis, i.e. the thermal splitting of methane. Instead of gaseous carbon dioxide, solid carbon is produced. The prerequisites for the balance sheet CO2 neutrality of this process are, on the one hand, the heat supply of the high-temperature reactor from renewable energy sources and, on the other hand, the permanent binding of the resulting carbon.116 The process has already been demonstrated in the laboratory. Some development work is still required for practical, large-scale implementation.
Purple hydrogen. Purple hydrogen, like green hydrogen, is produced by electrolysis from water. The electricity required for electrolysis comes from nuclear power plants. Since the production of nuclear power is not associated with CO2 emissions, purple hydrogen is considered CO2-neutral and is currently being considered in some countries due to existing power plant parks. Regarding a discussion of the advantages and disadvantages of nuclear power, we refer to the relevant literature.
The future role of hydrogen in decarbonization and in our energy systems is currently the subject of intense and controversial public debate. Hopes for future-proof jobs and sustainable business models play a major role in this discussion. This is accompanied by particular interests and political influence, whereby transparency can easily be lost. This is precisely where macromodeling can help identifying key levers, as well as provide a projection of assumptions often formulated in scenarios to a possible future. Without question, macromodeling results cannot tell us what the future will be like. But they do provide a transparent and consistent basis for political and economic decision-making by showing the consequences of long-term policy goals on today and the near future. However, the algorithms and models cannot take the decision away from us.
[Digital Decarbonization, Chapter 5, Introduction]