We need to talk about dunkelflaute

By Lukas Bergmann, Sunamp’s head of product management and regulatory affairs | December 2022


If you’re not already familiar with the term “dunkelflaute”, there’s no doubt it will be tripping off your tongue before you know it.

In the UK we are forging forward into a new future of renewable energy and the decarbonisation of heat. And with new technology, comes new vocabulary. Dunkelflaute is a German word meaning “a dark lull” or in the specific context of renewable energy, a “lull in wind generation”.

And this is important because, pivotal to the whole project of renewables generation, is the challenge of dealing with intermittency issues – extended periods when the wind doesn’t blow or the sun doesn’t shine.

Even in windy Scotland, there was a recent spell of sunny, still weather resulting in nine days of wind generation at just 5% of the average output.

Demands on the electricity grid

But the vagaries of the British weather are not the only challenge for renewable energy innovators.

By decarbonising heat and moving towards the electrification of heating using heat pumps run on electricity generated by wind turbines and photovoltaic (PV) panels, a considerable new demand is being made on the country’s electricity infrastructure.

At the moment, the UK’s electricity network operates on the principle that not everyone will use all of their electrical appliances at the same time. It has allowed for the “diversity factor”, assuming that some people will be on holiday, working shift patterns and sleeping during the day, that a certain percentage of users will be retired, working from home or working long hours in an office. And this complex calculation, based on a statistical analysis of usage behaviour, has generally worked – bar the odd outage in the past at Christmas when the entire population, in festive synch, put their turkey in the oven and watched the Queen’s annual speech to the nation on television.

Paradigm shift

People embracing renewables and electrifying their heat – who would previously have been gas users – will now be creating a significant new load on the electricity grid and disrupting all the existing carefully calibrated usage calculations and stressing capacity.

In addition to the new “renewables” cohort of electricity users with their range of behavioural demand patterns, the electricity grid will also be challenged by an increase in load governed by simple physical external influences as these new electricity consumers look to the network to supply energy when it’s cold or windy and their homes need heating.

So, if we are to progress the electrification of heat, we have an urgent need to find solutions which will dissociate the demand for heat from the demand for electricity.

We need a paradigm shift from a model where power plants respond to an increase in load by an increase in generation. That simply can’t be done with the intermittent nature of wind and PV power.

We need to find ways to harness wind and solar power when it’s in plentiful supply and have it ready and waiting for when demand kicks in.

Storage and the EXTEND project

And the only way to do this is to start using effective storage. Sunamp’s Phase Change Material (PCM) thermal batteries are being recognised as a key way of addressing this challenge.

Recently, Sunamp won major research and development funding from the UK Government’s Department for Business, Energy and Industrial Strategy (BEIS), through the Longer Duration Energy Storage Demonstration programme, part of the Net Zero Innovation Portfolio which provides finance for low-carbon technologies and systems. Sunamp’s competitive pitch-winning proposition, EXTEND, is a project exploring how Sunamp heat batteries – 20,000 units of which have already been installed by customers to replace hot water cylinders in homes – can be optimised for space heating applications which require more storage capacity but at much lower power.

This is ground-breaking work in the evolution of heat batteries.

Whilst there are currently some large heat batteries on the market for space heating, they are not compatible with heat pumps and depend on resistive heating. Sunamp’s EXTEND project is assessing the feasibility of developing three different sizes of heat storage batteries optimised for space heating and heat pumps:

  • a 50 kilowatt hours model that’s dishwasher sized
  • a 100 kilowatt hours model that’s refrigerator sized, and
  • a 300 kilowatt hours model that’s American fridge-freezer sized.

These batteries will benefit from a very high energy density in a relatively small amount of space, ensuring they are highly practical and cost effective in the average UK home.

The EXTEND project brings Sunamp together with some of the UK’s leading innovators in electrification including Ripple Energy providing customers with the option of part ownership of a wind farm, myenergi providing smart logic applications and Fischer Future Heat providing installation resources.

Supporting flexible demand

The new generation of Sunamp thermal batteries will allow homeowners to decarbonise their homes, benefit from savings through on- and off-site renewables generation and benefit from lower cost tariffs for flexible demand or participation in grid-supporting measures.

And the large capacity thermal storage of these innovative heat batteries will take the pressure off the grid, by storing energy to be delivered on demand. Even during a dunkelflaute.


I did say dunkelflaute was an important word. But maybe it’s even more than that? Perhaps it’s at the very heart of the renewables zeitgeist.

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