Currently, the electricity production sector is evolving rapidly, with a wide diversity of energy sources: conventional, renewable, and others. To support this evolution, power transmission and distribution networks are also being modernized. In order to manage the increasing diversity of production sources, it has become essential to accelerate the development of smart grids.
Globally, there is a strong trend toward energy transition, with the aim of limiting climate change and global warming. These phenomena are largely caused by the massive use of natural resources such as coal, natural gas, and liquid fuels.
In contrast, renewable energy sources such as solar (photovoltaic and thermal), wind, and hydro are clean and environmentally friendly. They represent the energy sources of the future, with the potential to support the energy transition and the gradual phasing out of conventional sources.
Challenges in Integrating the Energy Transition
In general, there is strong commitment among decision-makers and experts regarding the energy sources that can support the energy transition. Take nuclear energy, for example: international opinion is divided. Some consider it a low-emission and even renewable energy source that produces fewer greenhouse gases, while others view it as dangerous, posing risks not only to the environment but also to humanity.
Another energy source that is rapidly expanding not only in the electricity sector but also in others is hydrogen. Although not new, hydrogen has gained significant attention in recent years thanks to its advantages. There is broad consensus that green hydrogen can make a real difference and support the large-scale integration of renewable energy sources.
If we analyze Figure 01, we observe a clear dominance of fossil fuels (coal, gas, and oil) compared to hydro, wind, and solar power. However, a positive sign is that coal and oil curves have been decreasing in recent years, while solar and wind energy have shown growth.
At present, it is not logical to judge the contribution of renewable energy by comparing it directly to conventional energy sources, due to differences in capacity, maturity, and development. Instead, it would be wiser to focus on identifying the types of energy that can support renewable sources in order to accelerate the energy transition.
Figure (01) : Share of electrcity production by source, world
When it comes to integration into the power grid, renewable energy sources present certain technical limitations. For example, photovoltaic solar energy lacks electromechanical inertia because it uses inverters to inject power into the grid. In contrast, conventional sources such as thermal or hydroelectric power plants are equipped with generators coupled to turbines, which provide them with significant inertia. In the event of frequency variations on the grid, the speed governors of these plants can quickly act on the turbine control valves to accelerate or decelerate the generators and thus mitigate fluctuations.
Moreover, renewable power plants generally operate at full capacity to maximize their efficiency. As a result, they do not maintain spinning reserves (or hot reserves) for load and frequency regulation. On the other hand, conventional power plants are operated based on grid demand, sometimes below their maximum capacity. This allows them to maintain available capacity (hot reserve) to respond to demand fluctuations.
Therefore, renewable energy sources contribute relatively little to frequency regulation and power reserve services compared to conventional sources. For this reason, it is necessary to strengthen the power system with other facilities capable of ensuring these essential functions. One possible solution is to store energy during periods of low demand and low cost, and release it during peak hours when demand and electricity prices are high.
Different Types of Energy Storage
Electric energy, by nature, cannot be stored in large quantities directly. It must be distributed instantly after production. The most practical way to store energy at large scales is to store primary energy sources. However, with technological advancements, energy storage systems have been developed to store electricity often by converting it into other forms, as illustrated in Figure 02.
- Compressed Air Energy Storage (CAES)
This system stores energy by compressing air into underground caverns or high-pressure tanks using electric compressors during periods of low electricity cost. During peak demand periods, the compressed air is released, heated, and used to drive a turbine that generates electricity.
Investments in this type of installation remain limited due to energy losses during the decompression process, which reduces overall efficiency. However, some systems can reach capacities of up to 300 MW.
- Flywheel Energy Storage
This method stores kinetic energy through the rapid rotation of a mass (typically a cylinder or disc), set in motion by an electric motor. When energy is needed, the motor operates in generator mode to feed electricity back into the grid.
Flywheel systems can achieve efficiencies of up to 80% and capacities of 20 to 30 MW. They are especially useful for applications requiring rapid response times.
- Fuel Cells
The operating principle is the reverse of electrolysis. A chemical reaction between a fuel (usually hydrogen) and an oxidant generates electricity, heat, and water.
Fuel cells can provide power outputs of up to 50 MW, depending on the technology used. Although still relatively uncommon, the growing interest in green hydrogen produced from renewable sources has renewed attention to this solution due to its low pollutant emissions.
- Pumped Storage Power Plants (PHS – Pumped Hydro Storage)
Currently, these facilities are the most widely used for large-scale electricity storage. They operate by pumping water to an upper reservoir using electric pumps during periods of low demand. During peak demand periods, intake valves are opened to release the water back down, driving hydraulic turbines that generate electricity.These stations are known for their fast response time, making them highly effective in meeting urgent grid demands.
-
Figure (02): Different Types of Electrical Energy Storage
Battery Energy Storage Systems (BESS) to Support Renewable Energy
In addition to the storage options mentioned in the previous section, there is another widely used technology across various sectors—particularly for startup applications and as backup or emergency power sources: battery storage systems.
The advantage of BESS compared to other technologies lies in the simplicity of their installation, which shortens implementation time. Their operation and maintenance are straightforward, and overall costs are relatively low.
Figure (03) shows the 20 countries with the highest installed capacities, with China and the United States leading the list—having around 300 GWh installed in 2024. According to the same source, this figure is expected to grow to 965.8 GWh by 2027.
Furthermore, according to a report published by Zion Market Research, the global BESS market size is expected to reach USD 7.99 billion in 2024, with a compound annual growth rate (CAGR) of 8.80%. This highlights the significant role that BESS is set to play in the global economy in the coming years.
Figure (03) : Top 20 Countries by Battery Storage Capacity (Benchmark minerals)
In power grids, battery energy storage systems (BESS) are increasingly being deployed on a large scale thanks to advances in storage technologies and battery development. They are especially useful for compensating for fluctuations in renewable energy production and for contributing to frequency and voltage regulation. Today, BESS power plants are found around the world, with the largest located in the United States, boasting a capacity of 700 MW. In some countries, total installed capacity reaches several gigawatts, and the number of BESS modules continues to grow within the electricity production sector.
BESS plants are becoming essential particularly in light of the recent blackout that affected Spain and Portugal on April 28, 2025. Although the definitive causes of the incident still require further investigation, early analysis indicates that solar production peaked at 18 GW around 12:35 p.m., just before the event, alongside wind production of 3.6 GW. This reflects an almost total dominance of renewable energy at that moment, resulting in near-zero system inertia, which led to a cascading failure of several generation units following the opening of the circuit breaker connecting to the French grid.
One of the underlying causes of this blackout was the dominance of renewable energy, which is characterized by low inertia and the near absence of spinning (hot) reserves. On their own, renewables are unable to provide adequate frequency regulation during system disturbances. However, with the integration of storage systems coupled with renewables, the grid becomes more stable and flexible, and better equipped to handle disruptions—particularly frequency deviations caused by the sudden loss of large loads.
If the Spanish grid had had enough BESS capacity installed, the impact of the blackout could have been mitigated. According to the load curve for April 28, it appears that BESS contribution was nearly zero before the incident.
A similar model to Spain, in terms of renewable energy dominance during the day, is seen in California as illustrated in Figure (04). According to this figure, solar energy dominated from 7:00 a.m. to 6:00 p.m., peaking at 21 GW at midday. This high contribution forced fossil fuel generation to reduce its output. Meanwhile, BESS installations operated in charging mode during the day and began discharging between 6:00 p.m. and midnight, reaching a peak contribution of 9.8 GW. This clearly shows that BESS installations play a crucial role in California’s load curve and significantly contribute to ancillary services within the power grid.
Figure (04): Daily Demand and Production Curve for the State of California (according to Engaging Data)
Recommendations and Benefits of BESS
Energy storage is a technology that must be further developed, and authorities should encourage investment in such systems. The growing demand for electricity is putting increasing pressure on grid operators and, in some cases, challenging the reliability of power systems. BESS installations are among the key options that can accelerate the energy transition not only in the electricity sector but also in other areas, such as transportation, which is undergoing a radical transformation through electrification.
BESS systems are considered essential tools for large-scale integration of renewable energy. On the one hand, they can absorb fluctuations in renewable generation, and on the other, they can contribute to frequency and voltage regulation.
In recent years, the rapid development of battery technology has begun to challenge the traditional notion that electricity cannot be stored at scale. With BESS, some countries are already reaching capacities in the gigawatt range, suggesting that this definition may change in the coming years. Large-scale investment and ongoing technological advancements can help reduce the cost of these systems and encourage countries that have not yet adopted BESS to begin investing in this promising solution.
By Merahi Reda, Doctor and Energy Researcher
Référence :
[1] Hannah Ritchie and Pablo Rosado (2020) - “Electricity Mix” Published online at OurWorldinData.org.[2]: Battery Energy Storage System (BESS) Market Size, Share, Trends, Growth and Forecast 2034
[3] Site web Engaging data: California Electricity Generation