NexSys Research Summaries provide short synopses of work published by NexSys researchers and are designed to be easily accessible to a wide readership.
In Summary
- Adding solar PV generation to a wind-energy dominant power system reduces RES drought exposure.
- Solar PV generation shortens energy droughts: the return period of RES droughts is drastically reduced.
- Winter stays critical: solar PV generation cannot mitigate winter RES droughts, which coincide with the highest electrical demand in northwestern Europe.
- Use validated local generation and capacity data: generic datasets underestimate the frequency and duration of RES droughts.
- Validated datasets can be used to identify the risk level of future capacity installations.
Glossary of terms
- RES (Renewable Energy Sources): Electricity derived from wind, solar, etc.
- Availability: Electricity that a source is able to provide to the grid at any given time
- Generation: Electricity that a source provides to the grid given the technical constraints of the system
- PV: Solar photovoltaic, electricity obtained from solar panels.
- RES drought: A multi-day period with unusually low combined wind and solar PV electricity generation.
- Capacity factor: Actual generation divided by installed capacity averaged over a given time period (ranges 0–1); a measure of how much of the generation potential is being realised.
- Return period: How often an event of a given duration is expected to occur on average (e.g., “once in 5 years”).
- System adequacy: Capacity of generation sources and the grid to meet demand needs
- Energy security: Ability to maintain supply of power and handling possible disruptions
Introduction
Countries in Northern Europe are rapidly increasing their electricity generation from renewables, with large contributions from wind energy and growing volumes of solar PV (photovoltaic). Ireland is no exception here. This brings a key reliability challenge: weather variability can result in multi-day periods of unusually low renewable output, often called “RES (Renewable Energy Sources) droughts”. These events can threaten system adequacy and energy security if not properly planned for, especially as reliance on renewable energy sources increases.
However, there isn’t a single standard way to define or measure RES droughts. This makes any assessment of RES droughts sensitive to the definition used, but also to the data sources and even the modelling choices. In this context, two practical questions arise for planners and policymakers, both in Ireland and other similar wind-dominated systems in Northwestern Europe:
- Are widely used, generic renewable generation datasets accurate enough to quantify RES drought events for a specific country?
- How does adding substantial solar PV capacity to a wind-energy dominated system change the frequency, duration, and seasonality of RES droughts?
A team of NexSys researchers examined these questions in the journal publication Reducing RES droughts through the integration of wind and solar PV (Morin et al., 2025). This research summary is intended to provide a concise overview of the study’s key findings with insights for policymakers.
Data and methods
Our study compared three alternative datasets for estimating hourly wind and solar PV generation for Ireland over a multi-decade period. Instead of directly using generation data, the capacity factor is used: a measure of actual generation divided by the installed capacity (ranges between 0 and 1), which indicates how much of the energy potential of the wind and solar farms is being fulfilled at any given time. The datasets used are two standard Copernicus C3S-Energy products (a national aggregate and a gridded version) and a tailored, regionally validated dataset built specifically for Ireland. For each dataset, a 45-year hourly record of wind and solar PV capacity factor was constructed, enabling a like-for-like assessment of how the choice of data, and the assumed balance between wind and solar PV capacity affect the RES drought risk and, by extension, planning needs for backup generation, storage, demand-side measures, and interconnection to neighbouring countries.
All datasets draw on the same underlying weather information (ERA5 reanalysis, which provides the best estimate for the state of the atmosphere at any given time), but they differ in how they translate weather data into electrical power generation. The C3S products apply the same modelling assumptions across all of Europe, but the gridded C3S product is combined with the actual wind and solar farm locations, enabling a better representation of the actual Irish scenario. In contrast, the tailored dataset uses Irish wind and solar PV plant locations and technology choices that were tested against local observations. This regional calibration aims to better capture Irish conditions, such as coastal winds and typical siting, improving the accuracy of the power estimates.
To explore how the technology mix influences results, the analysis considered a present-day, wind-dominated capacity mix along with a more balanced future mix with substantially more solar PV capacity installed. No offshore wind capacity was considered, as the timing of its future deployment is still uncertain.
RES droughts were identified using a fixed, transparent rule: an event occurs when the 24-hour moving average of the combined wind-plus-solar PV capacity factor remains below a threshold of 0.1. Using the moving average approach allows us to look at events lasting at least one day in duration, without needing to worry about when exactly an event begins, and the fixed threshold ensures that events are comparable across datasets and capacity mixes.
This setup was used to quantify the frequency, duration, seasonality, and return periods of RES droughts for Ireland, and to test how sensitive these metrics are to (a) using an off-the-shelf dataset versus a regionally validated one, and (b) shifting from a wind-dominant to a more balanced wind–solar PV system.
Key findings
- Balancing wind energy with solar PV energy reduces the number of RES droughts substantially. Shifting from today’s wind-dominant mix (~91% wind and 9% solar PV capacity) to a more balanced mix (~57% wind and 43% solar PV capacity) halved the typical number of RES-drought events across datasets. In the balanced case, the median number of hours per year during an active RES drought drops markedly: from 14% to 5% for the regionally validated dataset, and from 8-9% to 3-4% for the other two off-the-shelf datasets.
- Extreme multi-day droughts become less common with increased solar PV capacity. A 5-day RES drought that typically recurs about every ~6 months in the wind-dominant system is only expected to occur every ~4 years with the balanced wind–solar PV mix using the regionally validated model. This change is qualitatively consistent for the other considered models.
- Despite the overall reduction in multi-day RES drought frequency with added solar PV, winter remains the critical stress period. Solar PV generation reduces the total number of hours with a RES drought during summer, smoothing seasonality (even modest shares of PV significantly lower the total summer RES-drought hours); but it cannot remove the winter drought risk. Winter is the period with peak electricity demand in Ireland, and Northern Europe in general, making this a very critical period to address as part of long-term system planning.
- Dataset choice meaningfully shifts the numbers. Non-validated, generic datasets tend to underestimate the count of RES droughts, and overestimate the return periods for longer events compared with a regionally validated dataset. Hence, careful model selection is necessary to avoid underestimating the risks associated with RES droughts.
- Validation of wind generation shows that it is possible to identify a single wind turbine power curve which represents the nationally observed onshore wind generation. This is supported by a 10-year record of quality observations of wind generation and availability. PV farms have only recently been installed in Ireland, which results in more uncertainties when validating PV generation, as just over one year of validation data was available, a situation that is expected to improve going forward.
Policy implications
A more balanced wind–solar PV system materially reduces exposure to RES droughts, but it does not eliminate the winter risk. System planning therefore needs to combine diversification with targeted measures for those critical peak demand weeks of the year. The presented study points to four practical implications for Irish (and similar Northwestern European) planners:
- Balance the renewable mix. Increasing solar PV generation alongside existing wind generation tends to reduce the number and duration of RES-drought events, and extends the return period for long multi-day events. Accelerating PV capacity build-out to complement wind energy might be helpful, especially to reduce RES droughts present during the summer months.
- Plan for winter adequacy explicitly. Even with more PV capacity, winter remains the critical stress season. Reserve capacity (flexible plant, storage, demand response) should be sized for winter conditions, not annual averages. Relying on generic datasets risks underestimating the risk of winter droughts and the impact on adequacy needs.
- Use regionally validated datasets for decision-making. Off-the-shelf RES datasets can understate event counts and overstate return periods for long droughts; technology representation (e.g., wind turbine power curves) matters more than simply placing generation capacity at actual locations. Investment cases, adequacy studies, and security-of-supply assessments should, therefore, emphasise validated, localised datasets (or calibrate generic datasets) before setting decision thresholds, or reserve margins.
- Set evidence-based reliability standards and responses. Use the duration/return-period curves based on the validated dataset to stress-test system plans (e.g., “cover at least a 5-day winter RES drought with a 1-in-5-year frequency”) and to design the corresponding operational response and contingency plan.
Conclusion
Ireland’s growing reliance on weather-dependent generation makes prolonged low-output periods (RES droughts) a major system adequacy concern. Our study showed that diversifying today’s wind-rich portfolio with more solar PV generation can substantially reduce the frequency and duration of RES droughts, and makes long, multi-day events rarer. However, diversification does not remove the core challenge: winter remains the critical stress season, when low renewable output can coincide with peak demand. The manner by which RES output is modelled also matters. Regionally validated datasets, especially with realistic wind turbine power curves, result in more accurate and credible estimates of RES-drought risk than generic, off-the-shelf products.
In practical terms, Ireland should pair accelerated PV deployment with measures that explicitly cover the winter tail-risk. Adequacy assessments and investment cases should rely on (or be calibrated against) validated local datasets, and planning standards should be framed against quantified drought durations and return periods. Together, these steps translate the study’s evidence into a resilient pathway for a high-renewables power system. Looking further ahead, planning should also account for offshore wind expansion and climate-driven shifts in both electricity demand and renewable generation, as these factors will increasingly shape system risk and adequacy.
References
- Morin, B., Maimó Far, A., Flynn, D., & Sweeney, C. (2025). Reducing RES Droughts through the integration of wind and solar PV. Renewable Energy, 123392. https://doi.org/10.1016/j.renene.2025.123392
NexSys is funded by Research Ireland Grant no. 21/SPP/3756 (NexSys partnership). Opinions, findings, and recommendations in this paper are those of the author(s) and do not necessarily reflect the views of NexSys affiliated research organisations or industry partners.
The authors would like to thank Andrew Keane and Terence O’Donnell for their constructive review of this Research Summary.