A battery energy storage system (BESS) is a system that uses battery technology to store energy which is then used at a later time. A battery is a self-contained device that produces electrons through electrochemical reactions. Inside each device are electrochemical voltaic cells that transform chemical energy directly into electrical energy. Each cell is divided into halves; a positive electrode, the cathode and a negative electrode, the anode. These sections are connected by a conductive electrolyte containing cations (positive charged ions) and anions (negative charged ions). Each half cell has its own electro-motive force, which provides its ability to transfer electric current outside the cell. The battery itself contains 2 or more cells, which are connected so that their power is combined.
The materials that make up a battery differ based on the intended usage, and new designs are introduced frequently as a result of evolving technology. However, one condition that remains consistent is that the electrodes in the cell must be composed of dissimilar materials that are both electrically conductive. One material should accept electrons and the other should give them up. If the materials inside your battery were the same, there would be no net flow of electrons, no current, and you would essentially have a battery that doesn’t work. When a battery is activated, there is a steady flow of electrons moving out of the battery over time. As the battery is used up, the chemical composition of the materials inside changes.
The full BESS is an integrated system of battery arrays, power conversion systems, transformers and energy management controls. The battery module consists of cylindrical or prismatic cells that contain the charging and discharging chemistry. Saturn Power’s BESS cells are composed of lithium iron phosphate, where LiFePO4 is the cathode material, and graphitic carbon with a metallic current collector grid is the anode. However, lithium iron phosphate’s performance is often limited by its low electrical conductivity. As a result, in order to enhance conductivity, nearly all LiFePO4 cathodes are integrated with carbon nanocomposites, and the cell is instead denoted as LiFePO4/C.
LiFePO4 batteries are among the longest-lived batteries in existence. The most recent design of these batteries that are currently in use are lasting approximately 4500 charging and discharging cycles. This number can vary depending on the rate they are charged, the surrounding temperature, how often they are used, and what they are used for. A key strength for LiFePO4 batteries is that they are composed of a non-aqueous solution, unlike lead-acid and other lithium-ion batteries. The advantage lies in the physical properties of the cell. Iron phosphate has a remarkably robust crystal structure, which doesn’t degrade under the packaging cycles of lithium ions during the charging and discharging process.
LiFePO4 batteries are unique because they have both high capacity and high power deliverance, making them the front-runner for large scale energy storage projects. They are also a safer option than other lithium-ion designs, including the LiNiMnCoO2 design, which is utilized in Tesla’s Powerwall 2. The main difference is the battery’s specific overcharge tolerance. If a battery is charged beyond its maximum overcharge tolerance, the higher voltage leads to the decomposition of the electrolyte and damages the cell. A LiCoO2 battery has an overcharge tolerance of around 0.1V while a LiFePO4 battery’s is 0.7V, making it a safer and more versatile design.
Stay tuned for the next part of our description of battery systems series, where we will be showcasing how battery systems are managed!