Electric storage system

Basic principle of batteries

A battery is a device that converts chemical energy directly to electrical energy. A battery is composed of numerous cells. Each cell consists of an electrolyte and two electrodes:
– The negative electrode from which anions (negatively charged ions) migrate is called anode.
– The positive one from which cations (positively charged ions) migrate is the cathode.
A redox reaction takes place inside the cell to convert chemical energy to electrical energy. When the cell is charged, cations are reduced at the cathode, while anions are oxidized at the anode during charging. Ions flow through the electrolyte, while electrons flow through an external electrical circuit.

There are two categories of battery: primary batteries, also called disposable batteries and secondary batteries or rechargeable batteries. A secondary battery can be recharged; its electrode reactions can proceed in either direction. A primary battery cannot be recharged and its energy is limited to the energy available from the reactants inside the cell.

 

When the battery is connected to an electric circuit, a redox reaction happens and electrons are flowing through an external circuit while ions are crossing the electrolyte.

Types of accumulators and their applications

A cell can be composed of different materials. Electrodes have to be as most conductive (electrically) as possible and they also have to be composed of good reducing and oxidizing agent such as metal.

Depending on the material used, each electrode has an individual redox potential. The Electromotive force (EMF) delivered by the cell is the difference between reduction potentials of the reaction.

The individual potential of a material depends on temperature, pressure and concentration. Higher is the individual potential of a metal, stronger it is as oxidizing agent and easier it could be reduced.
On the contrary, it would be weaker as reducing agent.
Different specifications (voltage, weight, lifetime, cost, efficiency …) can be obtained for a cell thanks to a combination between different metals which compose the electrodes. The table, bellow, compares different technologies.

 

Operating of Lithium Iron Phosphate (LiFePO4) battery

The operation of Lithium accumulators  is based on the exchange of lithium ions and the electron flow. While the battery is discharged, Lithium is released by the negative electrode in ion form (Li+), crosses through the electrolyte (which is an ion conductor) and joins with the crystalline mesh of the material making up the positive electrode.
The passage of each Li+ ion into the electrolyte is exactly compensated by the passage of an electron via the external circuit, thus causing an electrical current.
The reverse phenomenon occurs when the accumulator is being charged (1).

(1) M.C. Péra, D. Hissel, H. Gualous, C. Turpin, « Electrochemical Components », ISTE Ltd and Jon Wiley & Sons Inc., ISBN : 978-1-84821-401-9, 2013.

The chemical equations for the reaction involved are:

  • LiFePO4  nLi+ + ne + Li1-n FePO4

The lithium ions are split into cations Li+, who leave the cathode, and into electrons who create the electrical flow.

  • nLi+ + ne + C → LinC

The cations go through the electrolyte to reach the opposite graphic electrode where they are recombined.
When the unit is switched off and the battery is being recharged, the reverse reaction happens.
The recharging time depends on the speed of the ions and electrons traveling inside the cell.

Advantage of H2SYS hybrid systems :

H2SYS realizes an extended analysis of energy requirements and designs a hybrid system according to final use of the product.

The systems integrate different types of energy storage devices, such as battery or super capacitors to meet the load profile requirements.

This method allows to reduce the total cost ownership, improves the lifespan of the system and provides higher yields.