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This lecture will explain the concept of “Power-to-X-to-Power” and it will discuss the following topics:

• The need of Power-to-X-to-Power
• The definition of Power-to-X-to-Power
• A Power-to-X-to-Power system for a Dutch wind park
• The challenges in Power-to-X-to-Power implementation

The need of Power-to-X-to-Power

A fully sustainable energy system can only function with large scale energy storage. It has been predicted by TNO that the Netherlands will need 100 tera watthour of energy storage in the year 2030. Large scale energy storage systems exist.

The most dominant one is pumped-hydro electrical storage (PHES), which uses the height difference in water levels to store energy. A big challenge in PHES is that a large volume basins and large height differences are needed, which are not available everywhere.

Another alternative is compressed air storage. To store large volumes of compressed air, natural salt caverns are needed. Unfortunately, those are not widely available either. In the Netherlands, this potential storage capacity is only half a tera watt hour. This technology is slightly more expensive, but still affordable, and it has an acceptable round-trip efficiency.

Because neither of these options can provide sufficient storage capacity, a final option is considered: Power-to-X-to-Power. The available storage capacity can be increased by storing gaseous fuels in the caverns that are mentioned above. In this way, the available capacity in the Netherlands is way more than the needed 100 tera watt hour, which means that this technology may be needed. The downsides are that the costs are high, and the round-trip efficiency is low.

The definition of Power-to-X-to-Power

In Power-to-X-to-Power, excess electricity is used to produce any type of molecule, labelled with X. These molecules are now the energy carriers that can be used to store or transport energy. When electricity is needed again, a fuel cell or turbine is used to produce power, which can be fed into the grid. Different types of molecules are possible, but most of them contain hydrogen, since the atoms are light, widely available, and it makes energy rich bonds. The different molecules that will be focused on in this lecture are hydrogen, methane and ammonia.

A Power-to-X-to-Power system for a Dutch wind park

Hydrogen

The first energy carrier that will be discussed is hydrogen. In the case of a 1 GW wind park in the Netherlands, the produced power is transported through the DC power lines to the grid connection point. In this connection point, most of the power will directly be fed to the grid, such that it has minimum conversion losses. The excess electricity will be converted into hydrogen by using an electrolyser. The electrolyser will use electricity and purified water to produce electricity.

The hydrogen can be transported through hydrogen pipelines to the nearest suitable salt caverns, such that it can be stored here. When power is needed again, the hydrogen is transported back to the grid connection point and a fuel cell is used to create electricity.

Ammonia

The second energy carrier is ammonia, which adds an additional step. In the same case as before, the hydrogen will be fed into a Haber-Bosch reactor together with nitrogen (which can be captured from air). Under high temperature, pressure and catalysts ammonia can be produced.

The difference between ammonia and hydrogen is that ammonia can be stored in tanks, because ammonia can relatively easily be stored as liquid. This increases the volumetric energy density. The regeneration can be done by using direct ammonia fuel cells.

Methane

The third energy carrier is methane. In the same case as before, hydrogen and CO2 (which can be captured from air) are fed into a Sabatier reactor, which produces methane under high temperature, pressure, and catalysts. Methane is the main component of natural gas, which means that the conventional gas grid can be used to store it. Methane can be transported to the nearest gas Injection point and existing gas turbined can be used to regenerate electricity. A big challenge for methane is to make sure that 100% of the emitted CO2 is also captured.

Challenges of Power-to-X-to-Power implementation

Simulations of Power-to-X-to-Power systems have shown several challenges. The first challenge is the CapEx efficiency (Capital Expenditures efficiency). Because expensive equipment is used, efficient systems are desired to reach the return of investments in time. In a flexible energy system, the CapEx intensive system only operates part of the time, which means that there is a longer investment time. This increases the levelized cost of storage.

Another challenge comes from ramping up and ramping down. To provide flexibility, the operation of the implementation will fluctuate. This is difficult with high temperature and high-pressure sensors. Ramping up and down too frequently will affect the efficiency and the lifetime of the equipment. Furthermore, not all reactors are designed to be turned on and off frequently.

Another aspect are costs, since many of the components are expensive. The simulation has shown a cost of storage of at least 26 eurocents per kWh, which is added to the electricity price. This would make the energy prices unaffordable in the future.

The round-trip efficiency is also a challenge, since each step of conversion and transport affects the efficiency. A typical round-trip efficiency is around 35%, which means that a lot of energy is lost. Even though it is excess electricity, an inefficient storage system means that there will be an increase in the required surface area to provide enough energy in energy scarce periods.

A different challenge is scaling up. All the needed technologies that are needed exist, but they are not large enough to store energy for a 1 GW wind park. Such a wind park would require a 450 MW electrolyser to use all excess energy. Systems with this size have not been created so far.

The last challenge is the availability of inputs. For example, the CO2 capture of air is a challenge and the same can hold for purified water in some countries. This challenge is often overlooked, but this will become a more important problem as climate change causes more serious water scarcity issues.

Conclusion

In this lecture, the concept of Power-to-X-to-Power is explained. It is an alternative energy storage option and there are different reasons for why it is needed. For three different energy carriers, an example has been shown. Also, different challenges that come with Power-to-X-to-Power have been discussed.