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As discussed in other lectures, energy storage comes in a variety of types and sizes. This can lead to different challenges such as which battery should be discharged first. This lecture discusses the coordination of storage resource and contains the following topics:

• A description of an optimal strategy for the use of storage resources in a particular scenario
• Other scenario’s where coordination of storage resources is required
• Challenges regarding charging strategies

Optimal strategy in a particular scenario

Consider the following example: Imagine there is a neighbourhood with a shared battery system. This includes a large battery with the following properties:

• Discharge power: 1 MW
• Storage capacity: 500 kWh
• Duration: 0.5 hours

Additionally, individual homes have a local battery storage system, which are smaller and have the following properties:

• Discharge power: 5 kW
• Storage capacity: 10 kWh
• Duration: 2 hours

A question that can arise in this example is the following: How should the battery usage be coordinated if there is total control?

In the scenario where only discharging is considered, there is a single satisfactory answer to the question. This scenario occurs when the batteries are tasked with supporting the system in a stress situation. The optimal strategy for this scenario is known as the “Greedy Greatest Discharge Duration First” policy. In this strategy the different batteries are ranked based on the duration. In the example, this would mean that the local batteries are ranked first, and the shared battery system is ranked second. Then the batteries with the longest discharge duration will be discharged first.

If the power request from the energy system is lower than the discharge power of the local batteries, only the local batteries will discharge. Since the local batteries are discharging, their duration will decrease. If the requested power increases above the level that the local batteries can supply, the shared battery system will also discharge, which means that the duration of all batteries will decrease. If the requested power would decrease, only the local batteries will supply the power again. This will happen until the duration of the local batteries equals the duration of the shared battery system, after which all batteries will discharge.

Other scenario’s where coordination is needed

The strategy described below is relatively simple. These simple solutions disappear when charging is considered as well. In this scenario, the decision of which battery should be charged or discharged includes different aspects.

An important aspect is the efficiency of storage technologies, since all practical energy storages have an efficiency lower than 100%. If the energy supply for charging is limited, a trade-off has to be made, since the most efficient storages may not be the most flexible one.

This problem can even occur, when it is not possible to charge batteries externally. It can be useful to transfer energy from one battery to another to divide the energy across the batteries. Since includes losses, so it needs to be considered whether this is worth the energy losses.

Different challenges occur when the amount of time for charging is limited. If two batteries have the same amount of stored energy, they can be charged simultaneously until they reach the maximum capacity. However, if the batteries have a different amount of stored energy, the battery with the most energy will reach its maximum capacity sooner. After this point, the maximum charge capacity will drop to the level of the other battery. Therefore, the time it will take to charge both batteries to the maximum capacity becomes longer.

The challenges regarding charging strategies

The examples above illustrate that there are many aspects involved in the strategy of charging batteries. This becomes especially difficult, when uncertainty is involved. This can be uncertainty about what will be asked from the storage facilities or the time and energy that is available.

Charging strategies occur in different applications. One example is the community storage system from the beginning, that can be mixed with home batteries and several electric vehicles. Another example is to use decentralized batteries together. An aggregator can collect the flexibility of different storage devices to use them as one virtual battery.

A different challenge occurs in the planning of the future energy systems. Even though it is clear that storage plays an important role in this system, it is not clear how this should be coordinated. There are many types of storage, ranging from small and efficient to large and inefficient.

Conclusion

This lecture discussed different cases where coordination of charging is required. The strategy “Greedy Greatest Discharge Duration First” is discussed, which is the optimal strategy for a scenario where only discharging is considered. Finally, different challenges that come with the coordination of charging strategies were introduced.