The vulnerability of green power – The Dunkelflaute

Aris de GrootArticles, News, Research

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Solar and wind energy generation are highly dependent on the weather. A consequence of this dependence is that energy supply to the community may become vulnerable to weather and climate variations. How do we ensure continuity of power supply during a long cold period of wind and solar scarcity?

In winter and especially during the so-called Dunkelflaute days an acute power shortage problem arises.

Research into our vulnerability to climate variations

The KNMI (Royal Dutch Meteorological Institute) is labouring for researching and mapping out our vulnerability to large-scale climate variations. A large part of north-west Europe including the North Sea goes through long periods of cold and windless weather. This type of weather is connected with the North Atlantic Oscillation (NAO). The NAO consists of a positive and a negative phase. The positive phase provides soft and wet winters with a lot of wind, the negative phase particularly brings cold and dry weather.

During a negative NAO nearly all European countries experience wind shortage. Due to this, disappointing wind energy yields in the North Sea cannot be easily compensated. The import of solar energy from southern Europe is difficult too. A negative NAO results in stormy and cloudy weather around the Mediterranean Sea, causing disappointing solar energy yields. The vulnerability to climate variations such as the NAO increases, because a period of low wind energy yields often coincides with a period in which the demand for energy is high.

Dunkelflaute – dark and windless weather

Besides periods of cold and windless weather there are periods of dark and windless weather: the Dunkelflaute. A Dunkelflaute on average occurs once every two years and lasts about 10 days.

Dunkelflaute: German term for the co-occurrence of ‘Dunkelheit’ (darkness) and ‘Windflaute’ (windlessness).

Dunkelflaute in the cold winter months

Dunkelflaute mostly occurs during winter. This is because days are shorter and temperatures are lower. For one thing, this causes the demand for energy to rise, and for another, it results in less electricity generation mainly through solar panels. When it is dark, misty or when there is snow on the solar panels and hardly any wind besides, wind turbines and solar panels can no longer fulfil the demand for electricity. In Belgium, during the winter of 2016-2017, nine of these Dunkelflaute days occurred. Also in other countries several days of Dunkelflaute were measured.


Source photo: Agora Energiewende

In more and more countries people start realizing that periods of Dunkelflaute may soon pose a threat to the supply of electricity. Afer all we, en masse, switch to wind turbines and solar panels, which hardly supply energy when there is no wind or sufficient sunlight.

How can we manage periods of Dunkelflaute?

How can we manage periods of Dunkelflaute?
Ever more coal, gas and nuclear power stations are closing. This happens under political pressure, but also because they simply are no longer cost-effective compared to subsidized green alternatives. Moreover, most companies are reluctant to continue investing in something that is likely to disappear in the years to come. In previous winters the Dunkelflaute did not yet have serious consequences, there are still sufficient conventional power stations that are able to take care of the energy shortage.

When more power stations are closing, we have nothing to fall back on. That is why many countries take measures to have a minimal capacity available for use if the need arises. In England energy supply is bought by auction years in advance, so that power stations are guaranteed of their revenues and stay open.

Apart from the expense there is much objection to keeping the polluting power stations open. But can we change? Which other possibilities are there that enable the use of power during days of Dunkelflaute?

The energy transition demands a systems approach that focuses on 2050

In 2035 moet een kwart van de energie in Nederland duurzaam worden opgewekt. Dit zal in de jaren daarna alleen maar verder worden uitgebreid. Deze plannen sluiten aan bij een wereldwijde beweging in de richting van duurzame energie, de energietransitie. Vervuilende energiecentrales die fossiele brandstoffen verbranden moeten wijken voor alternatieven die niet of minder schadelijk zijn voor het milieu. In Nederland wordt daarbij vooral ingezet op windmolens en zonnepanelen. De ‘brandstof’ voor deze energie-opwekkers is gratis en stoot niets uit. Dat klinkt ideaal, maar er kleeft ook een groot nadeel aan deze manier van energie opwekken: je bent afhankelijk van het weer. Geen wind of zon betekent geen stroom. Als in 2035 onze energie voor een groot deel uit wind en zonlicht moet komen, dan kan de winter en met name een Dunkelflaute voor een tekort aan elektriciteit zorgen.

Backup power by means of backup power stations

For the time being the use of a conventional coal, gas or nuclear power station is the most reliable solution that is easiest to fit in regarding a Dunkelflaute. Partly due to this many countries have introduced laws that need to keep a minimum number of these power stations open to prevent shortages. Eventually we want to get rid of these power stations, but as long as there are no better alternatives, we remain reliant on them. All the same, we can choose to deploy the least polluting type of power stations as backup power stations. Mostly, there is a strong preference for gas, because it is cleaner than coal. In the years to come important decisions will have to be made with respect to the future of these power stations, with the inclusion of the role they play in counterbalancing a Dunkelflaute.

The short term versus the long term problem

The Dunkelflaute problem is recognized by practically everyone involved in renewable energy. But what is the best solution to this problem? Many measures are suggested, regarding which attention should be paid to the impact of the short-term choice versus the impact of the long-term choice. The Flexibility Roadmap 2050, a report recently published by TenneT, points the way.

Heating sector is a sleeping giant

Many people only associate renewable energy with renewable electricity, whereas the need for renewable energy is determined by both electricity and heating or cooling. The heating sector is responsible for 50% of the European energy use, which makes it Europe’s largest energy sector. Nonetheless, there is much room for improvement. Optimization of this sleeping giant is vital for achieving the goals as described in the Climate Agreement and reducing the use of fossil fuels and carbon dioxide emissions.

In the recently published report of Aalborg University in Denemark “Renewable heating strategies and their consequences for storage and grid infrastructures comparing a smart grid to a smart energy systems approach” researcher Henrik Lund reaches the conclusion that a smart energy system with large thermal storage is by far the most affordable solution.

“The conclusion is that the “smart grid” pathway requires a 2 – 4 times expansion of the electricity grid and significant investments in electricity storage capacities, while the “smart energy systems” pathway can be implemented with relatively few investments in affordable minor expansions of existing grids and storage capacities.” aldus Lund.