Assessing Lulls in Variable Renewable Generation

What happens when the sun doesn’t shine and the wind doesn’t blow?

Experts have long debated the feasibility of achieving deep decarbonization primarily with solar and wind energy technologies. So-called intermittent or variable renewable energy (VRE) technologies, wind turbines and photovoltaic panels, obviously only generate electricity when their respective renewable resources are available.

As is often reported, after decades of deployment, wind or solar (or the two combined) often supply significant majorities of some grids’ power. On the flip side, sometimes they generate no power at all. We wondered how often these “lulls” in VRE generation occur — in particular, how often wind and solar combined supply zero (or close to zero) electricity, and how long these lulls tend to last.

We found that absolute lulls — in which wind and solar combined generate almost nothing — are, already, reasonably rare. In a world where wind and solar still only provide 7 percent of global electricity, countries with high renewable penetrations mostly get by with at least some contribution from VRE most of the time.

But beyond that, there is significant variation in the experience different countries have with wind and solar. Certain grids still suffer significant lulls in VRE generation. And solar and wind generating significantly “below average” is more common than absolute lulls across all the countries we looked at. Finally, even at this relatively early stage, countries are already encountering challenges related to overgeneration — that is, having “too much” solar and wind electricity for a grid at a given time.

In our analysis, we looked to countries with some of the highest VRE shares in the world. What explains the different levels of success that these countries have had in integrating VRE resources? Specifically, to what degree have they been able to minimize the effects of underproduction, and what characteristics of their grid infrastructure and energy mix account for it?

The countries that are best managing higher penetrations of solar and wind have some factors in common. We propose a framework for understanding the factors that enable countries to smooth lulls, mitigate overgeneration, and integrate solar and wind, which we abbreviate FAR:

  • Flexibility: the ability to shift electricity generation and demand in response to variability.
  • Area: the geographic size of the interconnected area in the grid.
  • Resource: the physical potential of diverse renewable sources.

These three factors help determine how “far” variable renewables can go in the energy transition. For large grids with diverse resources and flexibility in their supply and demand, the integration of VRE will be relatively feasible, all else equal. But for grids constrained on these factors, decarbonization will require either improvements in FAR, heavier deployment of technologies like long-duration storage, or alternative clean sources of electricity like nuclear and gas with CCS.

On Lulls

There are six European nations which generate between 20% and 50% of their annual electricity from VRE, with onshore and offshore wind accounting for the majority of this production. These are some of the highest shares of electricity from VRE in the world. Table 1 shows the six nations in our dataset and their shares of VRE with respect to total yearly generation.

2018 IEA Electricity System Data of 6 European nations

Table 1: Six counties with high VRE shares

But annual averages don’t tell the full story of VRE integration. They obscure seasonal changes in solar availability or extended periods of low wind production. To examine variability at a more granular scale, we analyzed hourly VRE generation and other electricity system data from these six countries. Our data runs from March 2018 through February 2019 to capture a full modern year of VRE performance. While a year has been shown to be fairly representative for long-term production and weather patterns, we also wanted to see how lulls were managed in present grid operating conditions. By comparing a rolling average generation to the hourly data, we can identify:

  • The total hours in lulls in the whole year (e.g. 80 hours over a year)
  • The number of lulls (e.g. 12 periods in a year for a total of 80 hours)
  • The longest lull over the time period (e.g a single lull of 18 hours)

Table 2 shows our results at four lull standards: below 5, 10, 25, and 50 percent of rolling average generation.

Table 2: Analysis of lulls in variable renewable energy generation

Our results show that lulls in VRE generation vary significantly across countries.

In Spain, Germany, and the UK, generation from VRE was relatively consistent, as less than one percent of the hours in the year suffered lulls at the 10% rolling average standard. But Ireland, Denmark, and Portugal are a different story. These smaller countries have it worse by an order of magnitude: lulls at the 10% standard occured for 1-6% of the hours in the year. In particular, Ireland suffered nearly fifty lulls with virtually no renewable generation at all, during which production was under 5% of the rolling average.


What determines the ability of grids to integrate variable renewables? From the European experience with VRE, we identify three key factors. They are the flexibility of supply and demand on the grid (F), the geographical size of the interconnected area (A), and the quality of diverse VRE resources (R).


The first consideration in integrating VRE is the flexibility of the surrounding grid. Demand management, flexible generators, and the availability of storage all impact how flexible a grid is in response to variance in VRE production. The more flexible the grid, the more efficiently it can respond to the challenges posed by higher shares of VRE and reach higher levels of VRE penetration.


The larger a grid’s interconnected geographic area, the better chance it has for VRE resources to balance out variability in any one location. Larger areas have more options for VRE integration, as well as increasing likelihood for greater VRE potential. High-quality VRE resources are often located far from industry and population centers. Interconnections are needed to transport electricity from where it is produced to where it is consumed. A grid with adequate transmission and a larger interconnected area can do so more easily, and thereby more easily integrate VRE.


The third element to consider is the diversity and quality of the physical resource. If a region is consistently sunny and has particularly favorable 50m wind speeds, it can more easily integrate higher portions of VRE onto its electricity grid. While falling costs of renewables decrease the necessary resource threshold for VRE deployment, higher resource potential will nonetheless facilitate a higher capacity to integrate VRE. Having a diverse mix of high-quality VRE resources also enables resource balancing, as wind speed increases when solar production is absent at night. Diverse types of variability profiles from VRE generators help to offset the risk exposure to lulls in production from a single source.

FAR is a framework to help assess how much a grid can rely on VRE for decarbonization. We intend it to be a rough heuristic to understand the readiness of a grid to incorporate VREs. These elements also complement one another, as larger interconnected areas also present more options for resource mix and flexibility. A grid’s FAR is not fixed, however. Governments can invest in improving elements of their FAR — in the form of flexible generators, long-distance transmission, and diverse clean resources.

Each high-VRE nation has taken different steps to integrate renewables, with varying degrees of success. To demonstrate how the FAR factors work in practice, Denmark, Germany, and Ireland offer sharply contrasting case studies of different strategies to manage variability.

Denmark: Flexibility and interconnections help integrate high VRE share

Denmark is a unique and perhaps anomalous example of a country able to reach a high VRE share, when compared against larger European nations. Though it has relatively high lulls, Denmark has redesigned its electricity system to adjust to its wind-dominated 50% VRE share of generation. Its flexible domestic electricity demand, excellent interconnection capacity to much larger electricity markets, and high onshore and offshore wind resource potential have allowed it to manage its high VRE share without serious underproduction-related issues.

Fig. 1: Denmark’s FAR elements

Denmark’s emphasis on flexibility has focused on district level combined heat and power plants. The country changed its tax structure for CHP plants to make their operation more responsive to fluctuating electricity prices. In periods of high VRE generation and low electricity prices, CHP plants are incentivized to produce heat exclusively. During lulls, peaker plants and retrofitted coal plants with higher ramping capability begin to run in order to ensure grid reliability.

Though Denmark has made its grid remarkably flexible, its foundation for VRE management is built on its interconnections.

Denmark’s 6.4 GW transmission interconnection with nearby EU countries is the crucial tool in dealing with underproduction from Danish wind, and it exemplifies how interconnections can be used to balance variability from renewables. Given that this transmission capacity is greater than Denmark’s entire peak demand of 6.1 GW, it is clear that Denmark can handle periods of unusually low demand by purchasing electricity from surrounding nations. Its connection to nearby power markets in Germany, Sweden, and Norway is another geographic advantage that helps interconnections to mitigate variability. This advantage is primarily what sets Denmark apart, as a small nation with larger neighbors that it can buy and sell electricity from.

This interconnection also helps Denmark deal with its overproduction. Having access to Norway’s considerable and flexible hydroelectric infrastructure allows Denmark and the greater Nord Pool Spot network to “store” VRE electricity.

Denmark has some of the best onshore and offshore wind resource potential in Europe, and has relied on wind to produce nearly half of its electricity. Denmark’s substantial share of offshore wind (~13% of all electricity) helps bolster its onshore resources because offshore wind usually has higher capacity factors. Because most of Denmark’s offshore and onshore wind is in the western part of the nation, they follow similar trends in generation — which exacerbates lulls. However, the higher capacity factor of offshore wind means that it does not vary as drastically as onshore wind, which somewhat reduces the number of lulls.

Germany: Reliance on large-scale VRE deployment and continental interconnection

Germany is currently undertaking an ambitious energy initiative, the Energiewende, which aims to reduce emissions by 95 percent by 2050 and bring renewable energy up to 80 percent of the country’s gross power production by 2050.

Though the Energiewende did slow the pace of emissions reductions due to the premature closure of Germany’s nuclear plants, it has been successful in bringing large amounts of VRE online. In 2018, renewable resources surpassed coal as the leading source of generation in Germany. For one day in April 2019, 77 percent of the country’s net public electricity supply came from renewable energy, which included 40 percent from wind, 20 percent from solar, and 10 percent from biomass. By developing grid flexibility via policy supports, increasing its interconnected area, and using diversified VRE sources, Germany has managed its lulls fairly well.

Fig. 2: Germany’s FAR elements

Germany is investing in a much larger and more flexible grid that will expand onshore transmission capabilities and connect to planned offshore wind generation. Like Denmark, Germany’s current flexibility relies on the massive pumped hydro capacity of Norway to balance VRE. Germany is also planning to establish a 2 GW capacity reserve in order to deal with supply and demand imbalances via its Capacity Reserve Regulation. The reserve will be established via a public tender held by the German transmission system operators, and generating plants, storage facilities and variable loads will all qualify. These steps are also intended to help mitigate the effects of overproduction from German renewables, which has resulted in disruption to the European power markets.

Integrating the large VRE share in Germany has been challenging due to the limited domestic transmission between Germany’s population, industrial centers, and renewable generators. However, its central location in Europe gives Germany an advantage in being able to utilize continental interconnections with its neighbors.

Developing further domestic and international interconnections is a priority for Germany. While expanding domestic transmission is challenging, Germany passed the Network Expansion Acceleration Act for the Transmission Grid in 2019, which aims to simplify and speed up the expansion, reinforcement, and optimization of its power lines. To increase its ability to take advantage of Norway’s hydroelectricity, Germany plans to complete its 1.4 GW HVDC subsea cable connected to Norway later this year. The expansion of transmission is a tool for building flexibility into the power system in the event of extreme variability.

Germany solar’s large seasonal variance balances reasonably well with wind: onshore wind tends to fall during the high solar production months. As Figures 3 and 4 below illustrate, these diverse seasonal generation trends partially offset one another.

Figure 3: Seasonal changes in solar generation in Germany for 2018.
Figure 4: Seasonal changes in wind generation in Germany for 2018.

This counterbalancing can occur in smaller intervals as well, as wind production often increases at night. Figure 4 demonstrates how a diverse mix of VRE resources can mitigate variability challenges over a representative week in April 2018.

Figure 5: German wind and solar balancing in April 2018

As VRE penetration increases, however, it will strain the complementarity of solar and wind and make managing overgeneration harder and harder. A 2016 analysis of Germany’s Energiewende showed that, lacking any seasonal storage technologies, overproduction of solar electricity in particular will require significant curtailment, balancing from grid interconnections, redundant generation infrastructure, or all of the above.

Ireland: Poor resource diversity and interconnections exacerbate variability challenge

If Denmark and Germany offer examples of successful integration of VREs, Ireland offers an example of failure. Ireland is not going to meet its near-term clean energy targets, and it suffers from long lulls in VRE production. In order to reach higher VRE penetration cost-effectively, Ireland will need to address its lack of diversity in VRE generation and the poor flexibility of its grid. If it does not, Ireland will hamper its progress on climate action and make the integration of future renewables even more difficult than it already is.

Fig. 6: Ireland’s FAR elements

Ireland has not supported flexibility measures for its grid, and has only recently begun to take policy steps to begin meeting its potential for VRE. One challenge the nation has faced is creating new electricity management and cooperation between its local and national grid operators. Because of these limited measures, integrating VRE is particularly difficult.

Ireland is disadvantaged by its location and limited interconnected area. It is separated from mainland Europe and has limited transmission capacity to the United Kingdom. Its current 500 MW East-West connection to the UK is insufficient for its plans for greater VRE; for comparison, national peak electricity demand is 6.5 GW. To support the transition to more solar and wind, Ireland is proposing to develop the Celtic Interconnection, a 700 MW HVDC submarine power cable between the southern coast of Ireland and the northwest coast of France. Unless more transmission capacity is developed both on the island and with other nations, it will be lacking one of the FAR elements that makes VRE integration and lull management more successful.

Onshore wind is the only VRE source in Ireland currently deployed at scale. Because Ireland’s VRE is not diversified, there is a large variability risk in its energy system that requires especially accurate weather predictions and careful grid management. While Ireland’s solar resource potential is poor, it has yet to seriously invest in its substantial 4GW of offshore wind potential. This means that potential balancing from either resource is eliminated, and variability is worsened. While Germany benefits from seasonal and daily balancing between solar and wind, Ireland does not.


As the world evaluates how to reduce emissions and meet clean energy goals, the FAR framework provides insight into the factors required to successfully do so with variable renewables. It’s clear from the Denmark and Germany case studies that managing lulls is quite possible today. And with the right arrangement of technologies and resources, countries and states may be able to continue to push their VRE penetrations up considerably.

But the FAR framework also reveals the remaining constraints on solar and wind. Germany or Denmark may be able to offset the variability of their relatively high-VRE grids reasonably well thanks to their interstate interconnections. But this advantage dissipates when there is nowhere to buy electricity from, or sell it to, as the case of Ireland illustrates. As countries move past dealing with under-generation and lulls, overgeneration looms on the horizon. Without seasonal storage technologies, the realities of curtailed electricity and redundant infrastructure entail significant added cost.

While exploiting FAR makes integrating more variable renewable technologies possible, taking FAR for granted poses its own risks. Area cannot be expanded indefinitely, after all. Wind and solar are complementary, except when they’re not. At some point the tradeoff between deep decarbonization and maximizing VRE deployment tilts towards the former. Various countries and grids will manage that tradeoff in different ways. Before committing to narrow decarbonization pathways, they should consider both FAR and other options. Those options include seasonal storage technologies, higher-capacity-factor VRE resources like offshore wind and geothermal, and firm low-carbon power sources like nuclear and carbon capture, which can do as much or more than FAR to soften the constraints on deep decarbonization.