Dunkelflaute

When I first heard about this phenomenon, I explained it to myself as occurring when there was a cold, cloudy, windless winter's day.  Which isn't too bad a quick descriptor.  But here is a more informative Wikipedia article: 

 

In the renewable energy sector, a dunkelflaute (German: [ˈdʊŋkəlˌflaʊtə] ⓘ, lit. 'dark doldrums' or 'dark wind lull', plural dunkelflauten)[1] is a period of time in which little or no energy can be generated with wind and solar power, because there is neither wind nor sunlight.[2][3][4] In meteorology, this is known as anticyclonic gloom.[5]

Meteorology

Unlike a typical anticyclone, dunkelflauten are associated not with clear skies, but with very dense cloud cover (0.7–0.9), consisting of stratus, stratocumulus, and fog.[6] As of 2022 there is no agreed quantitative definition of dunkelflaute.[7] Li et al. define it as wind and solar both below 20% of capacity during a particular 60-minute period.[8] High albedo of low-level stratocumulus clouds in particular – sometimes the cloud base height is just 400 meters – can reduce solar irradiation by half.[6]

In the north of Europe, dunkelflauten originate from a static high-pressure system that causes an extremely weak wind combined with overcast weather with stratus or stratocumulus clouds.[9] There are 2–10 dunkelflaute events per year.[10] Most of these events occur from October to February; typically 50 to 150 hours per year, a single event usually lasts up to 24 hours.[11]

In Japan, on the other hand, dunkelflauten are seen in summer and winter. The former is caused by stationary fronts in early summer and autumn rainy seasons (called Baiu and Akisame, respectively),[12] while the latter is caused by arrivals of south-coast cyclones.[13]

Renewable energy effects

These periods are a big issue in energy infrastructure if a significant amount of electricity is generated by variable renewable energy (VRE) sources, mainly solar and wind power.[14][1][15] Dunkelflauten can occur simultaneously over a very large region, but are less correlated between geographically distant regions, so multi-national power grid schemes can be helpful.[16] Events that last more than two days over most of Europe happen about once every five years.[17] To ensure power during such periods flexible energy sources may be used, energy may be imported, and demand may be adjusted.[18][19]

For alternative energy sources, countries use fossil fuels (coal, oil and natural gas), hydroelectricity or nuclear power and, less often, energy storage to prevent power outages.[20][21][8][22] Long-term solutions include designing electricity markets to incentivise clean power which is available when needed.[19] A group of countries is following on from Mission Innovation to work together to solve the problem in a clean, low-carbon way by 2030, including looking into carbon capture and storage and the hydrogen economy as possible parts of the solution.[23]

Renewables naysayers say that because of dunkelflauten, we can't use wind and solar to power our economies.  But let's have a look at how often they happen:  a maximum of 150 hours a year, or, about 1.7% of the time.  

Because battery storage is getting so cheap, we soon won't need to use gas peaking plants.  The problem with dunkelflauten is that they last much longer than the 4 to 6 hours when electricity demand exceeds supply, the sort of shortage that can easily be covered by batteries.  For now, until cost-effective long-term storage is invented, we will need to use gas to make sure the lights don't go out.  However, we can make synthetic natural gas from green hydrogen using the Sabatier process, or we may store hydrogen, which we can burn in gas plants for the 150 hours a year when we need it.  These won't add net CO2 to the atmosphere. 

Even if we used fossil gas to provide 100% of the power during dunkelflaute events, and renewables/nuclear the rest of the time, we could still cut emissions from power generation by 98%.  

Because we won't be using dunkelflaute gas plants for most of the year, the cost of their electricity per MWh will be high, because the cost of interest payments and depreciation will be spread over only a few hours of usage.  But, by the same token, taken over the whole year, the occasional high cost per MWh will be spread over thousands of hours of electricity generation.

Dunkelflaute is not an insoluble problem.  We can fix it, and still switch to renewables and cut our emissions.

Making hydrogen electrolysers super efficient

The traditional electrolysis process is relatively inefficient.   If you use surplus green electricity to produce hydrogen and then burn the hydrogen to make electricity, its round-trip efficiency is low, much lower than alternative energy storage techniques:

 

Flora noted that converting power to hydrogen and then using the fuel to generate power has a relatively low round-trip efficiency. Round-trip efficiency is the percentage of electricity retrieved after being stored.

The technology to convert power to hydrogen and back to power has a round-trip efficiency of 18%-46%, according to data that Flora presented from the Massachusetts Institute of Technology and scientific journal Nature Energy. In comparison, two mature long-duration technologies, pumped-storage hydropower and compressed air energy storage, boast round-trip efficiencies of 70%-85% and 42%-67%, respectively. Flow batteries, a rechargeable fuel cell technology that is less mature, have a round-trip efficiency of 60%-80%. 

(Source: S&P Global --- Hydrogen technology faces efficiency disadvantage in power storage race
)

[Incidentally, Elon Musk claims that's Tesla's lithium-ion batteries have a round-trip efficiency of 93%]

But an Australian start-up, Hysata, is developing a process which enormously increases the efficiency of electrolysis.    I've already talked about this company and their super efficient hydrogen electrolyser, here.  

This update is from ARENA (Australian Renewable Energy Agency)

A pioneering, all-Australian hydrogen electrolyser technology is getting the chance to prove itself at a commercial scale.

If it works, the project has the potential to transform the economics of renewable hydrogen production.

ARENA’s support has helped develop this new technology since it was a concept in a University of Wollongong laboratory. That work saw a spin-off company, Hysata, established to commercialise the development.

Now, Hysata will receive $20.9 million ARENA funding as part of a $47.5 million project. Hysata will build and test a 5 MW system at its new Port Kembla manufacturing facility.

The plan then is to move the entire system to Rockhampton in Queensland, for installation and trials next to the Stanwell Power Station.

Queensland government-owned power company Stanwell Corporation is providing the site and facilities, and also backing the project with $3 million.

ARENA CEO Darren Miller says the project is a crucial step to enabling purchase orders for the technology.

“Hysata’s electrolyser technology could be a game-changer for renewable hydrogen,” Mr Miller said.

“The demonstration at Stanwell’s site will be key to unlocking commercial demand for Hysata’s product by proving the technology works at scale.

Currently, the production cost of renewable hydrogen (using renewable energy) is at least twice that of hydrogen produced from fossil fuels. Hysata says its technology will slash costs and produce hydrogen “well below” a competitive target price of $2 per kilogram (approx. US$1.50/kg).

FYI, if there’s one number you should remember, it is that price of $2 per kilogram. That’s the key to competing with fossil fuel-derived hydrogen and fully unlocking renewable hydrogen’s industrial and energy future.

It’s all in the bubbles.   All electrolysers work by passing an electric current from electrodes through H2O – water. The current splits the water into its two parts, hydrogen and oxygen. That process takes energy.

Now, if the entire process were 100 per cent efficient, all that energy would go into splitting the water. Nothing else.

But, until now, electrolysers have also produced a lot of heat. That’s because, just like an electric heater at home, they have electrical resistance.

The heat generated is not only wasted energy, but it must also be removed. Electrolysers need a lot of cooling and that uses even more energy.

So, if you can reduce resistance, a greater proportion of energy is available to split the water. Also, the system generates far less far less heat, which in turn requires less cooling.

Hysata has tackled the problem by completely redesigning their electrolyser to remove all the main sources of electrical resistance.

It turns out, that means eliminating hydrogen and oxygen bubbles. When bubbles form on the electrolyser’s electrodes, they reduce the surface area available for electrolysis and increase resistance.

In fact, Hysata says it has completely eliminated bubbles from its system and cut electrical resistance to virtually zero. As a result, Hysata says it expects a fully operational electrolyser will stay cool through good air ventilation alone.

The combined effect is what has raised the overall efficiency of a Hysata electrolyser to around 95 per cent. That’s a huge jump on current technologies, which operate with efficiencies closer to 75 per cent.

To put that in context, to make renewable hydrogen competitive with its fossil-fuel derived alternative, the International Renewable Energy Agency (IRENA) in 2020 set an electrolyser efficiency target of up to 85 per cent … by 2050.

I'm not sure that hydrogen by itself is in fact the future.   To transport it, you need to compress it and refrigerate it, which takes additional energy, further reducing its round-trip efficiency.  Also, it makes gas pipes brittle, and, because its molecules are so small, it easily escapes through the gaps in the molecular lattices of gas pipes or storage tanks.   But if you convert it to methane, using the Sabatier process, it's the equivalent of natural gas, and in fact is called synthetic natural gas (SNG, which is a bit of an oxymoron, no?)  And then you can use the existing gas distribution system and gas storage system, as well as existing gas turbine electricity generators.  On the other hand, to make SNG, you need a source of CO2, and unless you use the escape gases from a gas-turbine power plant flue, you have to produce CO2 using direct air capture, which is still very expensive.

We will need seasonal (or long-term storage)  to reach 100% renewables, and hydrogen, or more probably, SNG, will be how we fill that gap.  So, if this can be commercialised, it will be a huge step forwards towards a 100% green energy system. 

A near 100% renewables grid is feasible.

From RenewEconomy

(I talked about this analyst's simulations here and here)

There have been many simulations of a 100% renewable electricity grid for Australia, including some ground-breaking studies from Beyond Zero Emissions, The University of New South Wales and the ANU.

Even the recently released Integrated System Plan from the Australian Energy Market Operator exceeds 97% renewable in the 2040s.

So, what is the point of another one?

Well, this simulation differs from the others in a couple of ways:It uses actual generation and demand data rather than relying on synthetic traces for those quantities
It is being conducted in near real-time

The benefit of using actual generation and demand data is that some people are sceptical of synthetic wind and solar traces. They may also be dubious when you start modifying demand.

The benefit of the near real-time modelling is that people tend to be more concerned about recent events. If a recent day had very little wind and solar generation, some will take that as proof that you cannot run an electricity grid on renewables. A study based on data from a few years ago is unlikely to change their minds.

Another aspect of near real-time modelling is that it is one thing to optimise a simulation when you have all the data in advance, it is another when you design the simulation before you get the data.

With that in mind, exactly one year ago I started running a simple simulation of Australia’s main electricity grid to show that it can get very close to 100% renewable electricity with approximately five hours of storage (24GW/120GWh).

Each week, I would download demand and generation data from OpenNEM. I left demand unchanged.

The generation data for wind, rooftop and utility solar data was rescaled to supply ~60%, 25% and 20% of demand respectively over the year. For example, over the last year utility solar generation has met 5% of demand. The target for utility solar was 20%, so I rescaled the last 7 days of utility solar data by 4x (ie, 20% divided by 5%).

Note that the sum of 60%, 25% and 20% is greater than 100%. This is important. Any optimised model of a highly renewable grid will have significant amounts of over-generation.

It is better to over-generate and have some curtailment than to generate exactly what you need over the year with significant shortfalls during some months requiring huge amounts of storage or backup. As will be seen later in this article, this simulation ended up having 18% excess generation over the year.

The decision to use 60% wind, 45% solar was based on rough optimisation experiments. A mixture reasonably close to 50:50 takes advantage of the fact that wind and solar are negatively correlated with each other.

Wind tends to generate above average during the night and during winter, complementing the solar generation. I have a bias to wind as it requires less short-term storage, which is used primarily to shift solar generation from the day to the evening and night.

My simulation used the 24GW/120GWh of assumed storage and existing hydro to firm up the wind and solar and match demand.

Both the hydro and storage were assumed highly flexible. Note that I did not use the actual hydro generation data. I completely changed the dispatch of hydro so that it had minimal generation on days when it wasn’t needed, and elevated levels whenever there was a day with significant shortfalls of wind and solar relative to demand.

This is reasonable as most of the hydro capacity on the NEM is associated with large storage dams, making the hydro highly dispatchable. However, to maintain consistency with historical generation, hydro generation was also subject to the following constraints:

Hydro generation was kept between 200 MW and 6,000 MW
Weekly hydro generation was kept above 168 GWh
Annual hydro generation was targeted at between 6% and 9% of demand, though ideally closer to 15,000 GWh, or about 7.5% of demand.

If the wind, solar, storage and hydro was unable to meet demand, then the model supplements generation with ‘Other’. ‘Other’ was deliberately left undefined. It could be gas generation. Indeed, in the short to medium term it is likely to be existing gas peakers that will help firm renewables along with storage and hydro.

But longer term, ‘Other’ could be a highly flexible dispatchable generator running on renewable fuels such as biofuels or green hydrogen, or it could be long-term storage such as Snowy 2.0. When calculating the renewable percentage of the simulation, I have assumed ‘other’ is not renewable, even though it is hoped that in the future ‘other’ will become renewable.

Each week I posted the results of the simulation of the previous seven days to my Twitter account. On Wednesday of this week, I posted the 52nd week, marking a full year of simulations.

I’ve copied the simulation below. It is fitting that the renewable penetration of 99% for the final week of the simulation very closely matched the renewable penetration over the entire 52-week period, 98.8%

Key results from the 52 weeks of simulations are summarised as follows:

• Renewables met 98.8% of demand over the year, with the remaining 1.2% met by ‘Other’
• ‘Other’ generation peaked at 6.59 GW on the night of July 12. Over the year its average capacity factor was 4.3%.
• Hydro met 6.9% of demand. This was lower than my target of 7.5%, and also less than actual hydro generation of 8%. This means that dam storage levels in my simulation would have ended the year higher than they did in the real world.
• 17% of the wind and solar generation was in excess of requirements and ended up being curtailed.
• 11% of wind and solar generation went into storage. Storage discharge met 10% of demand.
• 82% of demand was directly powered by wind and solar without having to pass through storage or be curtailed

The wind and solar generation ended up slightly exceeding the targets of 60%, 20% and 25% for wind, utility solar and rooftop solar respectively.

It is impossible to know in advance if the year would be above or below average, so it is not surprising that they did not exactly hit their target. However, the methodology used to rescale the wind and solar data meant that there was a high probability that they would exceed their targets.

The graph above shows the weekly fraction of demand that was met by ‘Other’. Levels of ‘Other’ were essentially zero for almost seven months from September to late March. However, by late April, the simulation started to become more ‘interesting’.

Most weeks from late April through to the present required some levels of ‘Other’, due to the inability of wind, solar, storage and hydro to entirely meet demand throughout the week. The week starting on June 29 proved to be the most difficult week of the simulation, with ‘Other’ having to provide 8.1% of demand that week.

The graph illustrates clearly that late autumn and winter will prove to be the most challenging periods for a mostly renewable grid in Australia. Solar generation in late June and early August can often be as low half the annual average.

And while wind tends to be above average during winter, there are often stretches of two or three days in a row that have significantly below average wind. This can leave a significant shortfall in generation that cannot be entirely filled by existing hydro.

The challenge of matching supply and demand during winter will be even more difficult as we start to electrify much more of the gas heating that is present in the southern states, particularly Victoria. Doing so will elevate winter demand much more than summer demand.

It is important to note that wind in Queensland is not well correlated with wind in the southern states. That means that when it is calm in South Australia, Victoria, Tasmania and NSW, it is often windier than average in Queensland. For this reason, it is unfortunate that wind only makes up 3% of Queensland demand, or about one-quarter of the NEM average of 12%.

More wind in QLD will greatly help to improve the geographic diversity of renewable generation, making it easier to match supply and demand over the year. However, it will not completely solve the problem. There will remain many days with poor renewable supply in both the southern states and in Queensland. Increases in Queensland wind generation will make it easier to get closer to 100% renewable electricity, but is unlikely to significantly reduce the peak requirements of ‘Other’.

It is interesting to note that the ISP is predicting that approximately 9GW of peaking gas or liquids will need to be retained in the NEM’s generation mix out to 2050. This is more than the 6.6GW required so far in this study.

However, the ISP is a much more sophisticated model than the simulation I have done here, with increased demand due to increased electrification. It has modelled many years of generation, ensuring that supply stays secure and reliable. It is quite likely that some winters may prove more challenging in a high renewable world than the winter of 2022 simulated here.

For countries without hydro, the results of this simulation for Australia suggest that to cover winter demand, "other" (natural gas, for now; SNG later) would need to be ±15% of the generation mix.  Of course, this would only be for part of the year.  The average over the year would still be modest, so even if we have to use gas, we would still cut emissions substantially.  But the simulation highlights the need for gas peaking/back-up.  If we use surplus green electricity to create synthetic natural gas (SNG) then we could in principle run a 100% green grid.

US LNG exports are booming

 An interesting video from the FT (Financial Times, of London) about the boom in LNG exports from the US, mostly to Europe to substitute for the losses of gas from Russia.  It explains the process of creating LNG from natural gas, and shows how the US is now the world's largest exporter, with further increases likely.

Is the world locking in gas?  Prolly not, or at least not in the quantities implied in the question.  A grid powered by wind and solar will still need gas for cold, gloomy, windless periods  ("Dunkelflaute").    In countries with enough hydro, gas may not be needed, but even then gas will be necessary as back-up.  Until power-to-gas, i.e., converting surplus electricity to methane, becomes widespread, we will still need natural gas.  Producing surplus green electricity will require overcapacity in wind and solar, and we are a long way away from that now.  But by 2030, the need to curtail renewable output will be frequent, and in order not to waste it, we'll use it to make synthetic natural gas via the Sabatier process.  At that point, natural gas production will start to fall, and its place in the grid will be replaced by SNG.

It's dark, it's still, it's dunkelflaute

 From Energy Networks

Whether you’ve heard of it or not, dunkelflaute (dunk-el-flout-eh) is a challenge our energy systems will need to manage. Dunkelflaute is a German word that literally means dark doldrums or dark lull. It describes events where there is minimal or no sunshine and wind for extended periods, usually occurring during winter. Dunkelflaute is a specific problem of low electricity output that occurs in highly-renewable electricity systems. The challenge it presents is obvious – how to guarantee electricity supply when the dark lull descends?

In Australia, this has been referred to as a renewable drought. A recent lull in wind generation in South Australia is a small-scale snapshot of what could become a much larger problem in future.

AEMO data (via Open NEM) shows that across 11 and 12 June, wind power (represented by green in Figure 1 below) generated fewer than 4,800 MWh of a total demanded 55,000 MWh, only 8.7 per cent of total generation. This is compared with 9 and 10 June when wind power generated 46,000 MWh out of a total demanded 73,000 MWh, contributing 63 per cent to generation.

 

Germany is in a similar position as South Australia in terms of renewable penetration. Renewable electricity in Germany contributed 45.4 per cent of electricity consumption in 2020, more than coal, oil and gas combined. Germany also has significant transmission connection with the EU, possessing more interconnectors than any other country in Europe.

In Germany there is a growing fear of dunkelflaute as the share of renewable generation increases and displaces dispatchable generation. The type of event to cause dunkelflaute doesn’t have to be severe weather like we saw in Texas in February. It can be as benign as several still winter days in a row. 

How do we manage dunkelflaute?

A recent Grattan Institute report Go for net zero referenced dunkelflaute as ‘the winter problem’. In the document, Grattan notes that an energy system with 90 per cent renewable electricity would reduce emissions by 105 million tonnes at a cost of less than $20 per tonne. The final 10 per cent, however, is much trickier to achieve because the electricity system must increasingly rely on firming options.

The immediately available electricity storage option that might come to mind is batteries – but batteries tend to be best suited to managing hourly fluctuations across the day, charging from the midday sun and then discharging to help with the evening peak. Today’s batteries are not well placed to manage longer durations, with most having less than four hours of storage. The Victorian 300MW Big Battery project in Geelong is slated to be able to provide electricity to 400,000 households for one hour at full charge. That may be big but managing dunkelflaute will require a much bigger battery.

Broadly, there appear to be three options that could assist the transition from 90 to 100 per cent renewables.

Lots of renewable generation and transmission

The first is building a diverse renewable generation fleet all across the country in hopes that the wind is blowing or sun is shining somewhere, while ensuring sufficient interconnection to transport large quantities of electricity all across the country. This option would result in a large amount of electricity being ‘wasted’, along with lowering the utilisation of interconnection, while still leaving room for dunkelflaute in severe cases.

There is a positive correlation between solar energy across the National Energy Market (NEM) . When the sun is shining in one area, it is also likely to be shining in others, and visa versa. The absence of solar energy in one region may not be easily replaced by solar in another as different regions can be affected by similar weather systems.

Deep storage

The second option is building deep storage, like pumped hydro, that by its nature is well placed to provide storage capacity. Snowy 2.0 for example will be able to provide 2000 MW of generation capacity for 175 hours at full capacity. Grattan has modelled that across a 10-year period, up to 9GW of storage capacity might be required to bridge the largest gap between renewable generation and demand over 14 days. That’s about nine Snowy 2.0’s assuming they all start at full capacity.

This type of deep storage solution is likely to sit idle most of the time and could be challenging to finance, with Grattan rightly noting that many optimal sites for pumped hydro have already been developed. Additional interconnection would also be required to connect this deep storage, which may again be poorly utilised.

Developing this much deep storage is likely to be incredibly costly and unlikely to be in customer’s best interests.

Zero emissions dispatchable energy

The third and most promising option is building zero-emissions dispatchable energy, consisting of renewable gas usage in gas powered generation plants. Natural gas already provides a similar role in today’s generation mix and renewable gas will allow much of the current infrastructure to be utilised to support high levels of variable renewable electricity generation.

Frontier Economics examined the role of gas powered generation in South Australia during renewable droughts to support a highly-renewable system and found that using gas powered generation could reduce the overall system cost by between 28 to 35 per cent per year, depending on the extent of the renewable drought during winter.

Figure 3 – Indexed systems cost for 2030 and 2035 – South Australia (Source: Frontier Economics (2021), Potential for gas-powered generation to support renewables)

The optimal level of gas generation was found to be seven per cent of total generation. If natural gas can be substituted by renewable gas into the future, it’s likely that full decarbonisation can be achieved by utilising existing infrastructure and lowering overall costs.

 

Managing the winter lull

Dunkelflaute is a challenging problem that requires detailed planning and mapping of the electricity system and usage throughout the year, rather than relying on averages that are more commonly talked about.

There are a range of technical options available to manage dunkelflaute. Batteries and pumped hydro can be good options for managing hourly and daily fluctuations in demand, but there are questions over longer durations. Shorter-term storage is likely to best be complemented by renewable gas electricity generation to manage longer periods of low variable renewable generation.