TWENTY-FIVE: TWENTY Fifty Integration of Variable Energy
Design and Operation of Energy systems with Large amounts of Variable energy

Objectives

The main objective of the Task is to provide information on, and methods to optimize, wind and solar dominated power systems for system security, adequacy, economic efficiency and resilience. The scope has progressed from only focusing on electrical power systems to include a broader approach on energy systems towards net zero emissions. For power systems, the focus has shifted from wind integration studies towards system impact studies looking at how grid support from wind and solar energy can be used for future systems.

The Task will filter information on the impacts and experience including also mitigation, towards best practices of study methodology and evolving solutions and mitigation measures. The task will work towards commonly accepted standard methodologies to be applied in system impact studies for wind and solar dominated power and energy systems, in the areas of planning, operations, stability and markets. The task will analyse and further develop the methodology to best include capabilities of wind power plants in energy and power system impact studies.

Participation

Task had 16 countries and more than 30 institutes participating in the work in 2025, see Table 1.

Progress, Results, and Impact in 2025

Year 2025 saw the dissemination activities of Recommended Practices for Wind/Solar Integration Studies report published at the end of 2024. Updated fact sheets and bibliography were posted to the Task webpage and four journal articles were published. The new Task 25/63 Twenty Fifty Integration of Variable Energy (TWENTY-FIVE) started in 2025, continuing most of Task 25 activities.

Through Task meetings, an international forum has been established for member countries including their Transmission System Operators (TSOs) to exchange knowledge of and experiences with power and energy system planning and operations with large amounts of wind and solar energy (Table 1). The two meetings in 2025 were mainly in-person, however allowing for online participation.

The spring meeting was hosted by NVE in Oslo, Norway. This was in conjunction with power market modelling workshop organized by the Transmission System Operator Statnett, NVE, and Statkraft. The fall meeting was hosted by TUBerlin in Berlin, Germany, in conjunction with the annual Wind and Solar Integration Workshop.

Dissemination of the evolving best practices for system impact studies (Figure 1) was continued in several events: Imperial College London (Feb 2025) and IEA Paris (July 2025) as well as at wind and solar integration workshop, and as an ESIG webinar (October, 2025). A dialogue presentation, comparing the recommendations with an actual study in Sweden created positive feedback. The updated Recommended Practices (RP16 Edition 3) is published at the IEA Wind TCP web page [1], and video recordings and shorter summary presentations are available at Task 25/63 web site [2].

IEA Wind Task 25/63 published a collaboration article presenting a comprehensive global review of national and regional modelling practices for net‑zero emission electricity and energy systems [3]. Persistent challenges on net-zero pathways are related to renewable energy integration, long-duration flexibility, system adequacy, and grid stability. The analysis highlights significant differences in assumptions, time horizons, technology portfolios, and the treatment of grid reliability, flexibility, and seasonal balancing. The review concludes that integrated planning across sectors, detailed operational modelling, coordinated infrastructure investment, medium- to seasonal-scale flexibility, and context-specific policy design are essential to ensure that net‑zero pathways are technically feasible in real‑world power systems.

The second collaboration article published in 2025 explored the role of storage, transmission, and sector coupling in high renewable energy systems. Optimal investment solutions were found to have a mix of transmission and energy storage – at all levels of sector coupling. Storage cost uncertainty significantly impacts system cost as the level of energy sector coupling increases, while restricting transmission expansion increases system costs across all sector coupling levels [4].

In addition, work from the year 2024 on dynamic line ratings and the CO2 reduction benefits of wind energy were published in 2025 [5; 6]

The main stakeholders are the system operators, joining Task 25/63 directly (DE, DK, FR, IT) and as observers (FI, IE, NO, PT, SE). IEA and IRENA have been frequent observers to Task meetings, and work in other relevant international networks, Global Power System Transformation (G-PST) and Energy System Integration Group (ESIG) and ENTSO-E are closely followed.

Task 25/63 continues the active inter-TCP collaboration: solar integration (PVPS TCP Task 19), and flexibility needs in future (Hydro TCP Task 8; Bioenergy Task 44) are important common topics for system studies, as well as net zero system modelling (ETSAP TCP) and transmission planning (ISGAN WG6). Task 25/63 joined the IEA TCP Coordination Group on Flexibility in 2024-25.

Next Steps

The Task has meetings planned for 2026 in Paris in April, and Beijing in November. The April meeting will host also a public workshop on AI in energy planning in collaboration with Task 51 Weather Forecasting. Joint journal articles will be worked on topics like negative market prices; representing distribution grids in power system wide modelling, and as collaboration with other Tasks:

• IEA PVPS Task 19: a collaborative article on curtailments
• IEA Hydro Task 8: a collaborative article on dunkelflaute events, and
• Task 53 Economics: joint article(s) regarding electricity markets.

References

1. Holttinen, H. Helistö, N. (ed.) 2024. Recommended Practices for Wind/PV Integration studies. RP16 Ed.3 issued by IEA Wind, available at; iea-wind.org/iea-publications/
2. Recommended practices summaries and videos, https://iea-wind.org/task25-63/
3. Rabiee, A., Nikkhah, S., Salehi, D., Holttinen, H, Helistö, N, Göransson, L, Koivisto, M.J., Yasuda, Y. (2026) From electricity to economy-wide Net-Zero: Comparative analysis of global energy pathways. Renewable and Sustainable Energy Reviews, Vol 231, 116771, ISSN 1364-0321, https://doi.org/10.1016/j.rser.2026.116771
4. Yamujala, S., Koivisto, M., Korpås, M., McPherson, M., Helistö, N., Lew, D., Tejada-Arango, D., Morales-España, G., Flynn, D., Frew, B., Heggarty, T., Kiviluoma, J. & Holttinen, H. (2025) Synergies and trade-offs between storage, transmission, and sector coupling in high renewable energy systems Energy. 334, 137701. https://doi.org/10.1016/j.energy.2025.137701
5. Estanqueiro, A. I., et al. (2025). Dynamic Line Rating Models and Their Potential for a Cost-Effective Transition to Carbon-Neutral Power Systems. WIREs Energy and Environment, 14(1), e70002; https://doi.org/10.1002/wene.70002
6. Holttinen, H., Lindroos, T. J., Lehtilä, A., Koljonen, T., Kiviluoma, J., & Korpås, M. (2025). Estimating the CO2 Impacts of Wind Energy in the Transition Towards Carbon-Neutral Energy Systems. Energies, 18(6), 1548; https://doi.org/10.3390/en18061548