The following examples illustrate more clearly the use of both short-term flexibility and long-term flexible capacity and the challenges associated with them:
Short-term Flexibility during the Course of the Day
The feed-in from wind and PV as well as the power consumption fluctuate sometimes strongly in the course of the day. Events such as sudden cloud cover and consequently less PV feed-in require the best possible forecasts and subsequent rapid compensation. The steeper the ramp of the event, i.e., the faster and more severe the unforeseen change, the faster the technology must react to provide flexibility to smooth the fluctuation, to basically fill the gap that has suddenly appeared. Solutions include batteries or flexible power plants whose flexibility potential is bundled in Virtual Power Plants, for instance, and thus made available to the balancing energy market. In Germany, for example, the integration of renewable energies in the short-term flexibility markets is already quite advanced.
Long-term Flexible Seasonal Capacity
Long-term flexible capacity is needed not for intraday but for seasonal balancing of fluctuations in feed-in and consumption. Particularly in the winter months, weather conditions can occur in which little wind blows and the sun hardly shines, resulting in low feed-in from wind and solar. The German language, inventive as ever, has now even its own word for this situation: Dunkelflaute (dark doldrums). Electricity consumption remains high during these phases, due to the winter season. The question of how to bridge the Dunkelflaute is the subject of emotional debate. The current approach in most countries, as in Germany, is to bridge the gap with the help of conventional power plants, because renewable energies are not yet able to cope with long-lasting dark doldrums on their own, given the current state of expansion. Opponents of the energy transition and coal lobbyists like to use this fact as proof that an electricity system without conventional power plants would be impossible to implement. We disagree with this statement and are of the opinion that the Dunkelflaute period is currently still a problem that renewables cannot solve alone. However, there are already numerous concepts, clean technologies and approaches to overcome the Dunkelflaute period, and an adequate power system that knows how to use these potentials will also be able to do without conventional energy in times when solar and wind power generation is low. Which solutions do we mean, you ask? More interconnectors to link different countries to use their generation and storage to balance out the Dunkelflaute, more flexible generation from renewables, more demand response schemes, and of course more long-time storage, be it hydrogen, battery storage, or pumped hydro where possible.
Flexibility and Space
In the discussion about flexibility in the power system, the spatial component often receives less attention. Yet the question in which spatial units we consider flexibility and how we ensure that flexibility also arrives physically where it is needed are essential.
The Spatial Unit of Flexibility
In most cases, the spatial unit in which we think flexibility is defined by the market. In many European countries, the defining spatial unit is still the nation. Why still? Because the EU aims to harmonize the European flexibility markets. The Electricity Balancing Guideline (EBGL) lay out these plans. Instead of looking at countries individually, it should now be possible to use flexibility throughout Europe. After implementation, flexibility will not be a national, but increasingly a European matter.
However, smaller spatial units are also present in the discussion, in which local flexibility balances out local grid imbalances. This consideration is central, for example, in smart city approaches, in which the city balances peak loads within the city spatial unit with smart and networked infrastructure.
The electricity grid cannot always carry out the spatial determination of flexibility by the market. Thus, with the harmonization of the European markets, a progressive grid expansion is essential. Only then, electricity traded on the balance sheet can also be physically delivered. Bottlenecks are currently occurring above all at the cross-border interconnection points, i.e. the transfer points of the electricity grids between the countries. If more electricity is traded on the exchange than can be supplied, this leads to negative effects such as the breach of supply contracts and the associated penalty costs or, from a technical point of view, to loop flows. Market and grid must therefore be coordinated, which leads us directly to the next question.
How can the Power Grid and the Market be Better Coordinated?
Grid bottlenecks do not only occur at border interconnection points. Within a country, an uneven spatial distribution of electricity generation also puts a strain on the grids. One example is wind power generation in Germany, which is primarily located in the north, while the largest centers of consumption are in the south. This results in grid bottleneck situations that are balanced with the help of grid-serving flexibility. Until now, the transmission system operators in Germany regulate this from above with system services called redispatch and feed-in-management of renewables. The problem is that these measures are expensive, but the market does not reflect these costs. Therefore, the question is: How can the market and the grid be better coordinated and the management of grid bottlenecks be carried out more effectively and more cheaply? Globally, there are already other approaches:
Nodal pricing uses dynamic grid usage fees to reflect the costs of using the grid on the market and is a possibility for preventive grid balancing management. Instead of separating the grid into zones, it is separated into so-called nodes (feed-in nodes). At each node, a different price prevails, based on the electricity price and the cost of electricity supply. Different prices between nodes in the same market system thus reflect different costs of (local, regional) network use. Advantages are that the market sends price signals to relieve the grid in making it more expensive to use the grid when it is already heavily used and vice versa. Another advantage is that it sends additional investment signals, which create incentives to add capacity in areas where networks have not been under much strain so far and therefore the grid usage is cheaper. New Zealand and some states of the USA use nodal pricing for instance. Whether nodal pricing really is a practicable solution is, however, controversial.
Criticisms include the fact that the market loses liquidity and that there is too little competition at the individual nodes. In addition, implementation is very complex and, as previous examples show, error-prone and unstable.
How much flexibility must regulation provide? What can the market contribute? Where is the perfect balance between a perfect “copperplate” and grid flexibility? Which market actors should participate? Our expert on grid-serving flexibility, Tobias Nitze, identifies the biggest challenges as follows:
Die Energiewende findet vor allem in den Verteilnetzen statt, doch die Verteilnetzbetreiber nehmen bisher nur eine untergeordnete Rolle in einer lokalen Netzengpassbewirtschaftung ein. Dies soll sich mit dem Redispatch 2.0 schrittweise ändern, der im Oktober 2021 eingeführt werden soll und dann das bisherige Einspeisemanagement in Deutschland ablöst. Vereinfacht bedeutet dies: EE-Anlagen werden in den Redispatch einbezogen und die ad-hoc Maßnahmen des Einspeisemanagements werden durch ein planbares Redispatch-Regime ersetzt.
Inwiefern eine stärkere Einbindung des Marktes auch bei dieser Ausgestaltung des Netzengpassmanagements sinnvoll sein kann, ist im Hinblick auf den neuen Redispatch umstritten. Wie viel Flexibilität muss regulatorisch bereitgestellt werden? Was kann der Markt beitragen? Wo ist das perfekte Gleichgewicht zwischen Kupferplatte und Netzflexibilität und welche Akteure sollen integriert werden? Unser Experte für netzdienliche Flexibilität, Tobias Nitze, benennt die größten Diskussionspunkte wie folgt: