What is a dark lull? And how can we counteract it?
Dunkelflaute/dark lull: no sun, no wind
There is in fact really no comprehensive and uniform definition of a dark lull. Yet, if we stick closely to the term, it refers to a longer period of time during which there is neither wind nor sunlight. So, what we are looking at is darkness combined with a lull.
Consequently, neither solar panels nor wind turbine generators (WTGs) reach their output for power supply with renewable energy systems. Taking this a step further, the dark lull would therefore result in a noticeable dip in the power supply. This comes with certain challenges for a society which aims to sustainably cover its energy needs from renewable sources.
How often do dark lulls occur and how long do they actually last?
Particularly during winter, it becomes more likely that supply and demand for electricity are far apart. Energy requirements during cold and dark months are high simply because of the additional need for heating and lighting. At the same time, days are shorter. Consequently, the time during which sunlight is available is less. Add to that low-wind phases, that is when the German Federal Environment Agency uses the term “cold dark lull”.
Interestingly, these “cold dark lulls” take place in a clearly defined time window, on the basis of information from the environmental authority. Most often, this concerns a mere two-week period between January 23 to February 6. This was shown by measurements from 2006 to 2016 – demonstrated in conventional power stations that cover the increased demand for energy. According to Deutschlandfunk, the Science Media Center in Cologne reported specifically of three longer power lulls during the winter months in the years between 2015 and 2020.
Why are storage systems so important as a way to effectively counteract dark lulls?
Batteries and battery packs are among the best-known forms of energy storage. Yet, other options are also available to ensure electrical energy is durable. All storage types are based on the principle of converting an energy form so that it becomes “storage-capable” and can be used decoupled in time. In the process, volatile electricity is converted into chemical energy, for example.
Well-known examples of this include several battery types as well as the wide range of power-to-X with the electrochemical electrolysis of water to hydrogen and oxygen. Energy storage systems can also rely on location and kinetic energy as the operating principle, in the form of pumped storage plants, compressed air storage systems or rotary flying mass storage systems, for example. Thermal units such as ice storage systems are also becoming increasingly significant in the context of electricity-based heat transition.
What can batteries do to counteract a dark lull?
If wind farms are combined with an industrial battery storage system, the systems will receive a functional booster to counter a dark lull. Here, the storage system captures the energy generated, which is basically surplus, when there is wind and more current is produced than is required.
On the other hand, if there is a lull, electricity can be supplied from the storage system. As such, batteries act as buffers and are valuable for grid stability – despite a dark lull. This setup is particularly interesting for older generation wind farms that are no longer subject to government funding periods. The same operating principle applies just as much to the combination with PV systems. If the yield from sunlight is higher than the requirement, it is stored. If thick layers of cloud pull across the sky, the storage system supplies the grid specifically.
What does the dark lull have in common with load management?
The technical options available to limit the effects of a dark lull on the security of supply can be compared to those that currently building management and factory automation are focusing on under the heading of peak load management or peak shaving.
Peak loads are expensive and put a strain on the supply network. Energy storage can be used to smooth peak consumption – i.e. it is a way in which to shave down fluctuations in the power requirement. This is where the term “peak shaving” comes from. Storage systems are therefore a good way to create harmony in the grid. This becomes even more effective when power generation and electricity consumption are harmonized with sector coupling.
What is sector coupling capable of doing in an All Electric Society during dark lulls?
An effective energy network can be created by coupling sectors, for example by way of an energy and data-related connection of buildings, production systems, storage systems, and PV systems. This way, energy flows can be moved within a smart micro grid.
A balance is achieved given the close connection between energy consumers and producers. The entire thing can be compared to a self-leveling system. Moreover, sector coupling paves the way for controlling complete systems with their versatile cross connections and dependencies. Sector coupling is key to paving the way to an All Electric Society, while effectively combating dark lulls as well.
Summary
Dark lulls are natural occurrences can therefore not be prevented. Studies have shown that longer periods with no sun and wind only occur rarely when assessing the year as a whole. With intelligently coupled sectors, digital ways of obtaining information, and the use of energy storage in particular, the effects of dark lulls can be controlled effectively and with the use of standard technology that is already being applied today.
Connect with our experts
Relevant Articles
Energy storage systems connect sectors
The importance of innovative connection technology for the safe operation of energy storage systems and for the realization of the All Electric Society.
Green energy with security
Hauke Kästing and Phoenix Contact protect wind turbine generators against cybercrime.
DC grids for sector coupling
E-mobility, solar systems, and battery storage systems are revolutionizing industry.