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Technologies supporting the integration of renewables into the energy mix
Storage systems allow energy to be accumulated and make it available for use when it is needed. When paired with technologies that use renewables, they help overcome intermittency by flexibly ensuring the required energy supply to the grid. Storage solutions are therefore particularly important in addressing the discontinuity and unpredictability of electricity generation from solar and wind sources.In this respect, developing powerful and reliable energy storage systems is essential for the penetration of renewables into the global energy mix.
Eni’s research also in this field adopts an approach that combines evaluating existing technologies with the development of innovative solutions and implements them after identifying those with the greatest growth potential. In addition to strengthening internal expertise, Eni is building a network of collaborations with other companies and international research groups.
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Main features
Energy storage
Batteries without critical materials: research
Flow batteries: research
Thermal storage: pilot
Flow batteries: electrical storage
Thermocline: thermal storage
Electricity storage
Eni’s research on electrical energy storage focuses on two types of medium-sized rechargeable batteries: flow batteries and sodium-ion batteries.
Thermal energy storage
For thermal energy storage, Eni’s researchers are developing a system known as thermocline, which stores heat in a solid vessel where a heat-transfer fluid circulates.
Electricity storage
Electricity storage is the primary area of focus for Eni’s research. Electrical storage systems, commonly known as batteries, rely on the ability to convert electrical energy into chemical energy and vice versa. At the core of their functioning are oxidation-reduction reactions, where the chemical substances found at the battery’s two poles or electrodes become ionised and exchange electrons. During the charging phase, the flow of electrons of the electric current passing from the positive pole to the negative pole of a battery triggers an oxidation reaction in the positive electrode (cathode) and a reduction reaction in the negative electrode (anode), thereby creating an electric potential difference. During the discharging phase, the process is reversed, with the potential difference triggered by a reduction reaction in the cathode and an oxidation reaction in the anode, which simultaneously generates a flow of electrons (electric current) from the negative pole to the positive pole. This electric current passing through the external circuit powers the grid.
There are different types of batteries depending on how they are built and on the chemical substances they use. Eni’s research focuses on flow batteries and sodium-ion batteries.
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Flow batteries - Structure
Eni’s research focuses on increasingly efficient storage technologies capable of contributing to a more sustainable and resilient energy system.
The main characteristic of flow batteries is that they decouple energy and power. This is possible because the reactive substances are dissolved in the electrolyte and are therefore in liquid form, whereas in static batteries (such as lithium batteries), the reacting chemical species are contained in the electrodes. Electrolytes are stored in separate tanks and are pumped into the so-called stack, which consists of a series of electrochemical cells, where they remain separated by a selective membrane. The geometric characteristics of the stack (active area, number of cells) determine the power of the battery, while its energy is related to the size of the tanks. The most common technology in flow batteries uses aqueous systems with vanadium salts.
The technology behind flow batteries is characterised by significant flexibility and scalability in design, enabling customised solutions tailored to specific needs. Moreover, other advantages of these rechargeable batteries are their long lifespan, minimal self-discharge, high recyclability of materials, and the absence of risks of explosion or flammability. Due to their low energy density, these systems are large and suitable for stationary storage applications, such as home or industrial use.
Unlike lithium-ion batteries, flow batteries can easily be used for medium- to long-duration storage, providing a significant advantage for integration with renewables.
Eni's research
In the field of flow batteries, Eni’s research focuses in particular on improving the membranes that separate the two electrolytes, as they currently represent a major cost factor in the production and widespread adoption of this storage system. Eni has patented more efficient and cost-effective membranes and is studying organic electrolytes with better theoretical performance than vanadium salts and fewer issues related to the availability of mined resources. We are currently developing a project for the installation of a storage system using vanadium flow batteries with a capacity of several MWh.
Among the most promising energy storage technologies are sodium-ion batteries, whose main advantage is the abundance and widespread availability of sodium compared to lithium.
Due to this characteristic, sodium-ion batteries are among the most advanced technologies in the field of electrical storage. They are particularly suited for stationary applications but also show potential in mobility, especially for two-wheeled vehicles, and in the future for other types of vehicles as well. Although sodium-ion batteries currently do not achieve the same energy density or stability as lithium-ion batteries, they are characterised by a supply chain based on non-critical materials, which reduces supply risks and lowers costs. Additionally, the production, use, and disposal of sodium-ion batteries have a lower environmental impact.
Eni's research
In the field of sodium-ion batteries, Eni is developing several projects. At present, the research is focused on electrode materials, with an emphasis on various stabilisation mechanisms that could enable a high number of charge-discharge cycles at higher energy densities. New electrolytes, both organic and aqueous, are also being investigated, alongside battery configurations obtained from recycling. Eni is also studying and testing the integration of storage and conversion systems with various renewable energy sources.
Thermal energy storage
Thermal storage enables the accumulation of excess energy in the form of heat, which can then be utilised when most required. Heat can either be reused directly—for instance, for heating buildings or for industrial processes—or converted into electricity by generating steam to power turbines.
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Thermocline storage technology
Eni’s research is developing a thermal storage system called thermocline, consisting of a steel tank filled with concrete slabs used to store energy as heat. The concrete is heated by a hot fluid (such as diathermic oil or molten salt) that flows directly through it from top to bottom. During the charging phase, the storage material gradually heats from the top down while the fluid cools as it passes through. When needed, the fluid, which has become cool as a result of being used, is pumped back upwards into the tank through the concrete, absorbing its heat and making the recovered thermal energy available. This energy can then be used to generate electricity via turbines or to provide heat for industrial facilities.
The benefits of using concrete lie in its intrinsic properties: it is inexpensive, easy to shape and capable of storing heat with high efficiency. The thermocline system is flexible, scalable and uses a single storage tank instead of the typical two-tank configuration - one for hot fluid and one for cold - found in most thermal storage systems, further reducing costs.
The applications of thermocline storage include industrial processes, CSP plants and power stations.
The advantages of thermal storage systems over electrochemical systems, due to the decoupling of thermal energy generation and supply and the resulting greater flexibility in energy dispatching, include higher overall system efficiency, significantly lower costs, and a longer expected lifespan. Additionally, these systems allow for the storage and reuse of residual heat from industrial processes, thereby reducing CO2 emissions.
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The prototype
Some data on thermocline technology
4
metres
in height
80 kWh
thermal energy
system capacity
400°C
max temperature with diathermic oil
550°C
max temperature with molten salts
4
metres
in height
80 kWh
thermal energy
system capacity
400°C
max temperature with diathermic oil
550°C
max temperature with molten salts
"Thermocline" thermal storage prototype at the Novara Research Centre
Global storage capacity
Some data on currently installed global stationary electrical storage and projections for 2030.¹⁾²⁾
215
GW
global installed stationary electrical storage power
165
GW
hydroelectric storage (77% out of a total of 215 GW)
43
GW
lithium-ion batteries (20% out of a total of 215 GW)
620
GW
global electrical storage power expected by 2030
220
GW
hydroelectric storage expected by 2030 (out of a total of 620 GW)
400
GW
storage from other technologies expected by 2030 (out of a total of 620 GW)
- (1) DOE US Department of Energy, 2022.
- (2) Wood Mackenzie, Global energy storage market outlook update: Q1 2023.
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