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This lecture discusses the benefits that energy storage can bring to our energy system. This will be done by covering the following topics:

• The need for energy storage
• The alternatives for energy storage
• The common technical characteristics of energy storage

The need for energy storage

The production of renewable energy, such as solar and wind, varies with time. Solar energy has variations in the different time scales of minutes:

• There is a variation in the time scale of minutes due to the passing of clouds.
• There is a pattern in the order of hours due to the day and night cycle.
• There is variation between months due to the seasonal differences.

Wind energy has similar behaviour but has two main differences. First, both cycles (day and night, and seasonal) are reversed, since wind energy is generally stronger during the night and in the winter. Second, the seasonal variation of wind is a factor of 2, compared to the seasonal variation of solar which has a factor of 10.

These different properties can be used by coming the wind and solar energy to reduce the variability. However, there are still limits. Both solar and wind are dependent on weather spells, such as long irregular periods without sun or wind. To compensate for this, resources from different locations can be combined, since the weather patterns might be different at different locations. This can reduce the volatility.

The electricity demand also varies at similar time scales, such as daily, weekly and seasonally. The daily pattern is present in our activities, since we share the time between work and home. In the commercial sector (such as offices, shopping malls and stadiums), daily and weekly patterns can be recognized. The seasonal pattern is originated mostly from heating and cooling.

The fluctuations of both demand and supply have an impact on the price of electricity. When there is more demand than supply, the electricity price will likely go up. In the other way, when there is more supply than demand, the electricity price will go down. It is even possible for the electricity price to be negative. This happens when it is more economical to pay consumers for consuming electricity than shutting down the production facilities.

The alternatives for electrical storage

In conventional power systems, these variations are compensated by controlling the generation. For a sustainable energy system, an alternative has to be found. The production of wind and solar can be varied, but this has two downsides: it is a waste of energy, and this can only be done in one way. If the solar or wind generators are operating at their maximum point, the power production can only be reduced. The production can only be varied in both ways if the generators are operating below their maximum point.

Another option is to adjust the controllable demand, such that it follows the supply. However, this is well below the level of renewables that are planned for in the grid.

The best alternative to handle the surplus of electricity is to store it into a different form of energy. Then this energy can be converted back into electricity at a different time when it is needed.

The characteristics of energy storage

Generally, variations with higher frequencies and smaller magnitude require smaller storage. On the other hand, variations with a higher magnitude and lower frequencies require larger storage. Therefore, there is a need for both short-term storage, such as hourly, daily or weekly, and long-term storage, such as seasonal storage. This requires different storage technologies of different energy capacities.

We can also consider that there are times when a higher energy conversion rate is needed, for example when clouds shade an entire solar farm in the order of seconds. At other times, the energy conversion can be lower, such as when the speed of the wind gradually decreases.

The storage capacity and the energy conversion rate are the two characteristics of the storage technology that needs to be considered when deciding which technology to use.

Conclusion

This lecture has outlined the need for energy storage in sustainable energy systems. Different reasons for energy storage have been listed, which are variations in renewable energy, demand, and the electricity price. Also, alternatives for storage have been discussed. Finally, the main technical characteristics of storage that need to be considered are discussed.