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This lecture will discuss the following topics:

• An intuitive explanation of what a hybrid energy storage system is
• The benefits and opportunities of a hybrid energy storage system
• The challenges of a hybrid energy storage system

Motivation for hybrid energy storage system

Before looking at a hybrid energy storage, first the purpose of energy storage all together is revised. Clean energy sources like wind and sun are dependent on the weather, which do not usually coincide with the demand for energy. Storage is therefore needed to allow a time-shift between when renewable energy is needed and when it is produced. This bridges the gap between availability and user demand.

In hydrogen energy storage, electricity is converted into hydrogen by using an electrolyser. This is then stored as compressed gas or liquid. Afterwards electricity can be generated from hydrogen in a fuel cell when needed. In a hybrid energy storage system, a battery is integrated with hydrogen such that the benefits of both technologies can be combined.

An example for hybrid energy storage system

Imagine a setup where an array of solar panels operates parallel to the grid. Because solar power has a cycle on two different time scales (daily and yearly), the availability of sun power does not match the demand of energy. Therefore, most of the energy is fed back to the grid in the summer, and in the winter, most of the energy has to be purchased from the grid. This would mean that on average most of the energy has to be purchased from the grid, despite having the solar cells.

If a battery is added to the system, basic time-shifting can be allowed. In the summer, the battery can be charged by the PV system during the day and discharged during the night. With this setup, a larger degree is the generated PV energy is used by the household. In the winter, however, most of the energy still needs to be purchased from the grid. The ideal solution for this problem would be to store energy from summer to winter.

This situation is where combining two technologies can be a viable solution. Because of the technical properties, batteries and hydrogen storage complement each other well. Batteries are more suited for daily time-shifting, whereas hydrogen storage is better for yearly time-shifting. In the summer, the battery is being charged by the solar panels. The surplus of electricity in the battery is stored as hydrogen instead of being sold to the grid. In the winter, the stored hydrogen can be used to generate electricity again, without having to purchase electricity from the grid.

Benefits and opportunities of a hybrid energy storage system

To discuss the benefits and opportunities, first the advantages and drawbacks of batteries and hydrogen storage will be listed. These advantages and drawbacks are shown in the table below.

First, batteries have a high round trip efficiency compared to hydrogen storage, where the efficiency (both for making hydrogen and using it) is quite low. Batteries, on the other hand, have a limited capacity, whereas hydrogen can be stored easily in large quantities.

Hydrogen also has minimal storage losses over time. Batteries have the problem of self-discharging, which means that some of the stored energy is lost over time. Another difference is that batteries perform better at shorter timescales, due to their fast charging and discharging rates, compared to a slow response capability of hydrogen.

Because of these advantages and disadvantages, batteries are better suited for daily time-shifting, and hydrogen storage is better suited for yearly time-shifting. In a hybrid storage system, the benefits of both technologies are used, such that it has the following advantages:

• It has a rapid response capability
• It has seasonal storage capability
• Full energy autonomy

An additional advantage of a hybrid system is that the round-trip efficiency of hydrogen systems increases. A lot of energy is lost by the generation of heat by the fuel cell. However, this heat can be used for domestic heating by cooling it with water, since the fuel cell would mostly operate in the winter.

Challenges of a hybrid energy storage system

Besides the advantages that are listed above, there are also some challenges regarding hybrid energy storage. The main challenge is cost. At the current prices for the hydrogen storage system, it is difficult to recover the investment over time. However, as with all technologies it is expected that the prices of the major components (such as electrolysers and fuel cells) will decrease majorly in the coming years.

Another challenge is complexity. Because there are more parameters to optimise (battery size, hydrogen storage capability, and fuel cell power) the design of the system becomes more complex. Furthermore, due to the increase of complexity, it is more likely that something will break and will cause the system to shut down.

The last major challenge is operational complexity. The control strategies for charging the battery versus producing hydrogen needs to be considered, to maximize hydrogen production, while at the same time ensuring there’s electricity available for the user. With technological advancements, better smarter controls can be developed to mitigate this complexity.

Conclusion

A hybrid system is a storage system where batteries and hydrogen are combined. By explaining the advantages of batteries and hydrogen storage, it was shown that batteries are more suitable for short-term time-shifting, and hydrogen storage is more suitable for long-term time-shifting. In the end, the main challenges of a hybrid storage system are discussed.