The increasing infeed of fluctuating generation of the renewable energies presents huge challenges for the energy supply system. Those cannot be solved by grid expansion, generation and load management as well as existing storage technologies. At the moment, the share of renewable energies of gross electricity generation is around 30% (2016). If renewable energies reach (as politically determined) at least 40-50% in 2025, large capacities of short-term and long-term storage are needed, which cannot be covered by current technologies. Above all, the required long-term storage to cover longer lasting weather-related production gaps can be made possible with the aid of the power-to-gas concept, the coupling of electricity and natural gas networks.
1. How does Power-to-Gas work?
With the power-to-gas process a surplus of power, for example in the case of overproduction of wind energy, is used to separate water into hydrogen and oxygen in a solution by electrolysis. In a second step, the hydrogen is further processed with CO2 to form methane.
First step Electrolysis: 2H2O + Energy ↔ 2H2 + O2
Second step Methanation: 4H2+ CO2 → CH4 + H2O
The obtained methane is referred to as a natural gas substitute and has nearly identical properties as conventional natural gas. The produced gases (H2 and CH4) can be fed into the natural gas network.
2. Which gas offers the greater advantages?
- Hydrogen (H2) is characterized by a higher conversion efficiency and the independence of CO2 due to an avoided conversion step. Therefore, the immediate use of hydrogen is to be preferred as long as the critical limit values and associated loads of the natural gas network do not occur.
- Natural gas (CH4) offers advantages in terms of the almost unlimited storage potential via the natural gas network and the compatibility with downstream plants (eg gas power plants). In addition, the use as substitute gas in the natural gas network is realizable, so that fossil natural gas can be replaced and a spatial and temporal decoupling of energy generation and demand can be made possible. This reduces the currently high import dependency of natural gas.
3. When is the use of Power-to-Gas useful?
Optimal, since with the least storage losses, is the immediate use of the generated power. If the grids and as well as the generation and load management reach the limits, storage becomes unavoidable. First, all available short-term and then long-term storage must be filled. If their capacity has also reached their limits, only the operation of Power-to-Gas systems and limiting of the power plant remain as the last options. This means that Power-to-Gas is only a supplementary measure which should be considered when all the other measures (with less loss) are exhausted. However, facing the increasing infeed of fluctuating generation of the renewable energies the importance of Power-to-Gas will likewise increase.
Figure 1 shows a classification of power-to-gas to the various measures of the adjustment of the changing energy system.
4. Can Power-to-Gas contribute to the security of supply?
In general, the operation of combined heat and power plants and also combined gas and steam power plants will be necessary for the use of the long-term storage capacity of the natural gas grid. In the long term, these two technologies will form the cornerstones of a sustainable energy supply. In order to achieve this goal, the expansion of such plants is to be promoted nowadays. In addition to the use as flexible back-up power plants, the secured power can be increased together with the fluctuating production – existing conventional basic load power plants can be displaced. The grid-stabilizing effect of Power-to-Gas contributes to the prevention of power failures – an hour-failure already results in economic losses in billions of euros. In this respect, Power-to-Gas can in any case contribute to the security of supply. The usage of renewable energy generation surpluses supports their integration, so that the Energiewende is actively supported. Power-to-Gas brings together all the requirements for a functioning, efficient and safe energy system, dominated by renewable energies.
However, the Power-to-Gas concept, can neither prevent grid expansion nor be understood as the single solution for an energy system dominated by renewable energies. The focus should not be on the electricity sector alone (since it accounts for only one fifth of total energy consumption) while heat and mobility account for around 60%. In this respect, the potentials in the heat and mobility sector should be exploited with Power-to-Gas, for example by power-heat coupling or the provision of fuel. In the future, Power-to-Gas in combination with gas storage, flexible gas and combined heat and power plants will be used for the long-term storage of renewable energies and the replacement of fossil and nuclear power plants.
5. What are the challenges for Power-to-Gas?
Two key challenges are still to be overcome on the road to market launch. On the one hand, the technical components have to be optimized so that a flexible use and higher efficiency are possible. On the other hand, further cost savings as well as additional business models are necessary in order to achieve economical operation of a power-to-gas system. The creation of a regulatory framework is essential. For example in Germany, the Renewable Gas Feed and Storage Act (Erneuerbares Gas Einspeise- und Speichergesetz) has been contributing to the market introduction of Power-to-Gas since 2012. There are also other technical challenges, eg the research of the hydrogen compatibility of the natural gas network and downstream technical units.
6. In summary: What are the advantages and disadvantages of Power-to-Gas?