A decrease in solar power generation led to a series of rotating brownouts across The Philippines during May 2026. With fears power cuts would last for up to seven hours across two main grids, the National Grid Corporation of the Philippines issued a red alert.
According to the country’s Energy Regulatory Commission (ERC), the cushion between supply and an elevated demand in late afternoon shrank due to solar power generation tapering off leading towards the evening.
“We are not simply facing an afternoon and early evening shortage,” ERC Chairperson and CEO, Francis Saturnino C. Juan, explained. “We are facing a flexibility shortage in the hours after the sun goes down.”
As Suiee Suarez, Vice President for Corporate Affairs at AboitizPower said this isn’t “an indictment against solar power generation” but a lesson on having and sustaining a flexible and diversified power mix, where the limitations of one technology can be backed up by another technology that is ready, available, and dispatchable in a matter of minutes.
Earlier in 2026, in efforts to help build a resilient, inclusive, and forward-looking energy system, a consortium led by Aboitiz Renewables acquired the 797MW Caliraya-Botocan-Kalayaan Hydroelectric Power Plant (CBK HEPP) complex in Laguna from the Philippine government.
The so-called Thunder Consortium, composed of Aboitiz Renewables and Japanese firms Sumitomo Corporation and Electric Power Development Co, won the bidding process for privatisation in July 2025. The complex, particularly its Kalayaan pumped storage units, has been described as a critical energy buffer for the Luzon grid. Its ability to store energy and rapidly dispatch power strengthens system stability, provides essential ancillary services, and supports the integration of renewable energy sources.
Aboitiz Renewables says as the country expands its clean energy portfolio, the facility is expected to play a key role. While Philippine President Ferdinand Marcos Jr underscored the strategic importance of the handing over of the CBK HEPP complex as being “a deliberate step” towards an energy system that is steadier in operations, smarter in design, and future proofed.
Challenges
System integration has emerged as a central challenge in power systems with growing shares of variable renewable energy (VRE). However, according to Ahmet Hakan Özkan, high hydropower penetration does not automatically translate into smooth integration.
His recent study has examined why hydro-dominant electricity systems continue to face integration and flexibility challenges as wind and solar shares increase. It focuses on the fact hydropower operation is shaped not only by installed capacity, but also by hydrological conditions, transmission constraints, operational rules, and long-term planning priorities, all of which can limit real-time balancing capability.
In his research, Özkan develops a system-level conceptual framework that distinguishes between technical flexibility and effective system flexibility, highlighting temporal, spatial, operational, and institutional constraints that shape hydropower’s real-world balancing capability.
Using Canada as a case study, the analysis shows how seasonal hydrology, geographic separation between generation and demand, operational restrictions on hydropower assets, and planning inertia can limit the system’s ability to absorb VRE variability, despite favourable resource endowments.
Özkan claims his findings demonstrate that hydropower provides valuable but conditional flexibility and cannot function as a standalone solution for renewable integration. Effective scaling of wind and solar requires coordinated flexibility portfolios that combine hydropower with complementary infrastructure, storage, demand-side flexibility, and advanced system operation.
The author says the study contributes to smart energy research by reframing hydropower’s role in renewable electricity systems and emphasising the importance of system design.
Pumped sorage in Rwanda
Another study has explored the advantages of combining VRE with pumped storage hydropower for grid integration. It focuses on the 12MW Mukungwa hydropower reservoir in the northern province of Rwanda, which faces water level reduction due to weather variation in the dry season.
A major waterway in the Musanze River Basin, the Mukungwa river is endowed with an abundance of solar, water resources and a medium wind speed. Its mainstream and tributaries have an estimated 48.4MW of undeveloped potential.
Authors Niringiyimana et al say in their study published in Energy Strategy Reviews, that although hydropower plays a central role as a source of energy storage and balance to supplement highly intermittent and variable energy sources such as wind and PV energy, many reservoirs have already been burdened with multiple responsibilities for flood mitigation, water supply, power generation, the provision of downstream water flows for environmental protection, and recreation.
Consequently, they add: ‘managing a hydropower reservoir is already a complex mission and adding the new task of complementing wind/PV power represents both a significant opportunity and an outstanding challenge’.
However the authors believe the introduction of a pumped storage system will help provide adequate, stable and reliable power generation from Mukungwa hydropower, considering the intermittent nature of solar sources, wind sources and the demand fluctuations. Their analysis evaluated two distinct operational schemes: a baseline configuration without energy storage, and an enhanced configuration integrating pumped storage hydropower. A comparative techno-economic and environmental assessment was conducted to quantify the implications of storage integration.
With the availability of resources, proximity to hydropower infrastructure, and land constraints directly determining the least-cost technology mix, the authors’ results showed that the incorporation of pumped storage ‘yields a significant reduction in total system costs’, alongside a concurrent decrease in operational CO2 emissions. This dual benefit underscores the role of pumped storage in enhancing both the economic viability and environmental sustainability of power systems with high renewable penetration, Niringiyimana et al add.
Limitations of the study include considering various factors influencing the integration of pumped storage units, such as different ecological and geographical conditions (flat terrain, poor geology, or high sediment loads). It is also inapplicable in ecologically protected areas with legally binding environmental flow requirements, and distinct, varying load demand patterns which may result in different outcomes in different situations. Future research should also give consideration to evaporation losses due to atmospheric temperature variation.
Combined heat and power
The urgent need for sustainable energy solutions has led to a growing interest in the innovative integration of hydropower and thermal energy systems for combined heat and power (CHP) production.
Research by Elkelawy et al has shown how such hydro-thermal integration can achieve greenhouse gas emission reductions of up to 30%, significantly improving energy efficiency by 20-30% compared to conventional systems. Furthermore, integrating these systems can lead to cost savings of approximately 15-25% in operational expenses, primarily through enhanced fuel utilisation and reduced reliance on fossil fuels.
Through an analysis of case studies and recent advancements in technology, the authors claim their review offers valuable insights for policymakers, researchers, and industry stakeholders aiming to implement sustainable energy solutions.
However, they caution that various obstacles must also be overcome for this complex system design. To further reduce costs and emissions, their design and operational management requires sophisticated optimisation models, such as two-layer stochastic techniques that consider uncertainty in renewable energy and load demands. Because it entails managing start-up costs and operating restrictions that cannot be resolved by conventional approaches, the integration of mixed integer programming for hydro and thermal plants further complicates the planning process. The complex character of these energy solutions is highlighted by the need to balance operating efficiency, regulatory requirements, and environmental implications in the design of an effective hybrid system. Such obstacles can lead to high initial costs, technical complexity, and problems with water availability for hydropower generation.
Despite this, the authors claim such hybrid systems are becoming increasingly practical and affordable thanks to developments in energy storage, smart grid technology, and digital control systems. They add that hybrid hydropower-thermal systems have the potential to be a key component of the global shift to a more resilient and sustainable energy future, helping to meet rising energy demands while minimising environmental impact, provided that supportive policy frameworks, financial incentives, and ongoing technological innovation are in place.
