Energy storage: the definitive solution to the challenge of Spain's duck curve

The energy transition is radically transforming our electricity system. In just five years, Spain has gone from 19%, in 2019, to exceeding 65% of renewable generation in 2024 with photovoltaics leading the mix for the first time in history this year [2024] and for four consecutive months. However, this success brings with it new technical challenges that require innovative solutions. Among them is the so-called “duck curve,” a phenomenon that energy storage is called upon to solve.

The duck curve, named for its distinctive shape, represents net electricity demand throughout the day. This curve occurs when solar generation dramatically reduces demand during the central hours of the day, creating a deep “valley,” followed by a sharp increase in the evening when solar output declines but demand remains high.

This phenomenon, initially observed in California, is becoming increasingly pronounced in Spain. Forecasts for 2030 indicate that it could even become a “canyon curve,” with even more marked differences between periods of maximum solar generation and times of greatest demand. Studies show that the ramps of variation in net demand will become increasingly pronounced, requiring greater flexibility from the electrical system.

This situation brings with it several critical challenges. Firstly, it generates high volatility in electricity market prices, with significant drops during the hours of maximum solar production and sharp peaks at dusk. In addition, it causes increasing renewable energy curtailment at the times of highest generation, wasting a valuable and clean resource and, in turn, preventing the hours of cheap energy from being extended. Far from showing improvements, analyses of the evolution of curtailment until 2030 show a worrying trend: without additional energy storage systems, renewable energy wastage could multiply significantly, especially in regions with greater solar resources. That would not only affect the efficiency of the grid system but would also directly impact the profitability of photovoltaic installations.

This is why UNEF proposes energy storage as a comprehensive solution to undo the duck curve and its negative consequences, both for bill prices and for a deployment of renewables that allows us to meet the PNIEC [Integrated National Plan for Energy and Climate] objectives for 2030. In other words, we are clear that energy storage is no longer a technology of the future but has become an essential reality, and that today, an energy storage solution is as fundamental for a photovoltaic plant as the solar panel itself.

We are talking about the only technology that enables solar energy to be transformed into a manageable resource, offering the same quality and predictability as old, conventional energy. Energy storage projects’ capacity to charge energy during periods of high solar generation and discharge it during evening demand peaks is essential to solving the problem of the duck curve. This flexibility not only optimizes the use of renewable energy but also stabilizes market prices and reduces the need for backup plants.

Energy storage also offers a variety of services to the electrical system, with diverse applications that bring with them multiple benefits. In the area of system adjustment services, energy storage provides primary, secondary, and tertiary regulation, as well as voltage control and deviation management, representing an additional source of income that improves the profitability of such projects. Energy storage systems also play a crucial role in grid stability, providing synthetic inertia and autonomous start-up, “black start” capacity in the event of blackouts. These functionalities are especially relevant in a context of the increasing disconnection of traditional synchronous generators.

In terms of economic and technological prospects, progress is ongoing. The economic viability of energy storage is constantly improving thanks to cost reductions and increased battery life. Storage costs have followed a similar trajectory to that experienced by photovoltaics, with a significant reduction in capex [capital expenditure] and a constant improvement in the number of useful life cycles. Projections indicate that the levelized cost of storage (LCOS) will continue its downward trend, making the technology’s implementation increasingly attractive.

As we can see, there are plenty of reasons to encourage the integration of energy storage in photovoltaic projects. Finally, we must bear in mind that it is not only an economic or environmental issue at a country level but it is also a profitable initiative for individual renewables projects themselves: energy storage will not only improve the manageability of plants but will also increase their profitability by enabling a better use of the energy generated.

Finally, let us think about the road ahead: energy storage represents the new frontier in the energy transition and its development is not only essential to solve the challenge of the duck curve but is also key to achieving a 100% renewable, efficient, and reliable electricity system. It is, therefore, essential to think about what we need to do to maximize its benefits and act in a decisive manner. Firstly, we should develop an appropriate regulatory and remuneration framework with specific auctions for energy storage that take into account both rated power and energy storage capacity. That will be crucial to provide the economic signals for energy storage development. We also need institutional responsibility for storage regulation based on scientific and technical criteria, which sends a message of legal certainty to potential investors, both national and foreign. Only in this way, with the commitment of all the actors involved, will we be able to enjoy the benefits of this technology and pave the way towards a more sustainable and economically viable energy future.