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Towards a technically and economically viable energy storage solution

The energy transition will depend on a significant increase in the production of renewable electricity, particularly generated by wind farms and photovoltaic panels. The distinctive feature of these technologies is that they are reliant on local weather conditions. Consequently, the amount of electricity generated is variable. It is possible to reduce production during peaks, but it is impossible to increase production in the absence of wind or sun! Hence a potential lack of synchronization between electricity production and demand.
    
In order to maximize the share of renewable energies in the energy mix, IFPEN proposes technological innovations for the development of the energy storage sector.
  

Energy storage as a source of flexibility

As long as the share of renewable energy across the electricity network remains low, the difficulties encountered are limited to forecasting production as well as system issues (network connection and integration). Currently, these are managed by frequency and/or voltage adjustment systems.
   
When the share of renewable energy increases, it can be difficult to maintain the balance between supply and demand. To restore the balance, several flexibility solutions exist:

  • interconnections,  
  • load management (paying consumers to reduce their demand),
  • consumption management as a function of production, 
  • the provision of flexible, often CO2-emitting production facilities, such as combustion turbines,
  • stationary electricity storage. This solution is often the most expensive but it is also the one that delivers the most services. Compared to combustion turbines, for example, storage makes it possible to manage surges and absorb production peaks.

While stationary electricity storage appears to be an obvious means of ensuring a balance between supply and demand across the electricity network, a number of technical, regulatory and economic obstacles still remain, hampering its roll-out compared to other flexibility solutions.
   
  
There are two types of storage systems:

  • “Power” systems that have been economically optimized to supply power for a short period of time, such as flywheel energy storage systems.   
  • “Energy” systems that have been economically optimized to supply a given amount of power for a long period. An example of these are pumped storage power plants.

Although better suited to power systems, Li-ion technology is present across both sectors.
  
  

Choosing the best solutions for each case

   

Managing energy systems

Energy storage can deliver a variety of services to the network (arbitrage or load management during peaks, for example) and must be promoted on the basis of economic models that are variable and can be potentially used in combination.
   
Depending on their specific characteristics (investment and operating costs, yield, acceptable depth of discharge, etc), the various storage technologies vary in their suitability to meet these needs.
   
IFPEN’s approach is aimed at identifying the best technical solutions in terms of their economic and environmental challenges. This multi-criteria analysis incorporates the simulation of the system concerned throughout its lifespan.
   
This simulation is based on the capacity to develop Energy Management Systems (EMS) in the form of IT systems designed to best manage energy systems, applying strategies making it possible to optimize a storage system within a given network.
   
Alongside this research, IFPEN is developing two “energy”-type technologies: AA-CAES compressed air energy storage and redox flow batteries.
 

Compressed air can breathe new life into storage

Compressed air energy storage has existed since 1978 (Huntorf power plant in Germany) in the form of enhanced gas plants with a maximum energy efficiency of 50%. In these old Compressed Air Energy Systems (CAES), the heat produced by compression is lost.
   
A more sophisticated concept is Advanced Adiabatic Compressed Air Energy Storage (AA-CAES). The advantage of this concept is the capacity to store compression heat and achieve a far higher degree of efficiency. The construction of a first demonstration platform forms the focus of the Adele project in Germany (2009-2018).
   
On the AA-CAES principle, IFPEN proposes a system partly based on existing components, such as compressors and turbines, but also new components such as TES (Thermal Energy Storage) heat storage systems.

  • Storage is founded on air compression and heat storage,   
  • Air is passed through turbines following re-heating to release the energy from storage.

This technology is particularly interesting since it does not require a cavity for air storage: it can thus be placed close to where it is required. Nevertheless, where large quantities of energy are required, cavity storage remains the favored option, and IFPEN is also working on this area.
 
IFPEN’s contribution to this technology is on three levels:

  • Process optimization: thanks to its experience in applied processes in a variety of fields, IFPEN has been able to develop an AA-CAES system that is technically and economically optimized. 
  • Heat storage: IFPEN has also used its process engineering experience to propose innovative heat storage technologies. These innovations have been the subject of several patents and are currently being developed; with laboratory trials under way.
  • Compressed air storage: the air tank is a crucial element in the system’s economic equation since it accounts for a significant proportion of the overall cost. To address this problem, IFPEN is proposing an innovative solution aimed at reducing the costs substantially.

 

Reducing costs by circulating energy

Among the various energy storage technologies, electrochemical storage meets a number of needs and delivers services for stationary applications. Redox Flow Batteries, for example, are the focus of growing interest for applications requiring the storage of large quantities of energy, since these systems, which resemble processes, deliver large-scale cost reductions.
   
Redox flow batteries are, in fact, rechargeable accumulators that have the capacity to store energy in the liquid phase. Two liquid electrolytes containing electro-active species in solution circulate within the two positive and negative compartments of an electrochemical reactor, these compartments being separated by an ion-exchange membrane. The electrolytes are stored in tanks. The unique characteristic of this technology lies in the separation between the power and the quantity of stored energy. The power is governed by the active section of the electrodes and the membrane (reactor size) and by the circulating electrolyte flow (pump flow rate). The energy stored depends on the quantity of electrolytes present in the tanks.

stockage électrochimique

The main components of a flow battery are:

  • stacks: individual cells are mechanically combined and electrically connected in series to achieve standard direct current voltages,  
  • electrolyte tanks,
  • pumps and pipes necessary to enable the electrolytes to flow,
  • the BMS (Battery management system),
  • the DC/AC conversion system (PCS / Power conversion system)

Redox flow batteries offer unique modularity since they can be scaled to the desired power and energy, and can thus be adapted for use in a large number of applications; However, the relatively low energy density of the electrolytes does represent an important drawback. The preferred fields of application for these batteries thus lie in the intermediate power ranges from around 10 kW to several MW, and storage times of between 2 and 10 hours.

IFPEN’s contribution to this technology concerns:

  • The development of innovative electrolytes to improve electrochemical performance (energy and power density) and reduce the overall cost of the technology.
  • The multi-physical and multi-scale modeling of a flow battery coupling electrochemical, exchange, hydraulic and electrical phenomena. The model can be used as a basis for dimensioning the battery, optimizing its use, simulating its function and optimizing its management.
  • The understanding of elementary mechanisms (electrochemical reactions, material exchange) via a multi-scale characterization (laboratory, pilot and demonstrator) of the components and complete battery.

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