Wind and solar need storage diversity, not just capacity
The global energy landscape is undergoing a dramatic shift marked by the accelerating deployment of wind and solar technologies. Driven by compelling economics and intensifying decarbonization commitments, these renewables have transformed from supplemental sources into the backbone of new electricity systems. According to the International Energy Agency, the levelized cost of electricity for utility-scale solar photovoltaics has declined by over 80% since 2010, while the cost of onshore wind has fallen by nearly 60%. These improvements have made renewables environmentally imperative and economically competitive with fossil fuels.
In 2024, the world added 585 GW of new renewable energy capacity, an all-time high, with wind and solar accounting for 96.6% of the total. This surge has fueled the optimistic belief that continued expansion of these resources will suffice to meet national and global energy transition goals.2 However, a closer inspection reveals structural limitations of the current trajectory.
Unlike thermal generation, wind and solar are inherently variable, spatially distributed, and weather dependent. Their output fluctuates daily and seasonally, often peaking during periods of low demand. These characteristics introduce several challenges3: temporal mismatch between generation and demand, grid instability due to rapid ramping and frequency fluctuations, and curtailment when local generation exceeds grid capacity.
These issues have already become apparent in regions with high penetration of renewable energy. In California, for example, the “duck curve” illustrates a steep drop in net load during midday solar peaks, followed by a rapid increase in demand in the evening. Despite massive capacity additions, wind and solar curtailment rates have remained stubbornly high in northwestern China. Moreover, reliance on fossil fuel-based backup capacity persists even in systems that are heavily reliant on renewables, thereby undermining decarbonization objectives.
Similar operational challenges are emerging across diverse power systems globally. In Germany, the Energiewende policy has driven renewable energy penetration above 50%, yet the reliance on seasonal storage and electricity imports during winter underscores the need for long-duration balancing solutions. Nordic countries, while benefitting from abundant hydro storage, are also investing in hydrogen-based storage pilot programs to manage surplus wind generation. Meanwhile, in sub-Saharan Africa, off-grid solar projects are increasingly incorporating hybrid battery systems to enhance reliability at the community level. These examples suggest that the limitations of capacity-centric storage strategies are structural and globally pervasive.
This paradox—more renewables but persistent fossil support—stems from the disconnect between generation growth and system flexibility. Without complementary infrastructure and exceptionally diverse and integrated energy storage,4 the effective utilization of variable renewable energy remains fundamentally constrained.
