A battery energy storage system (BESS) is a power system used to supply power to an electrical installation, comprising a battery system, power conversion equipment (e.g. Inverters and charge controllers), and balance of system components (e.g. cabling, protection devices etc.). The BESS can be connected to a stand-alone power system, or connected to the grid. This reruns that the BESS can be designed to operate
Completely separate to the grid, parallel to the grid, or mixture of both. Similarly, BESS can be connected to only DC loads, only AC loads, or both DC and AC loads.
The BESS can be a single pre-assembled unit that contains the battery, inverter, and everything in-between, or a mix and match of different brands of inverter, charger and battery system working together to safely charge and discharge the battery. The former is known as a pre-assembled integrated BESS.
The most versatile way of storing this energy is chemically via rechargeable batteries. Certain sites operate on time of use tariffs and experience different electricity prices that are dependent on the time of day. For these sites, the ability to capture unused PV generation with a battery will have greater value than sites with flat tariffs, as the stored energy can be used to offset high electricity prices during peak demand usage periods. Additionally, a battery energy storage system can, in some cases, allow the PV array to continue to operate during a grid failure.
Example of a grid-connected PV system without battery storage. This shows an example residential PV generation and consumption. In this case the system owner uses the PV generation during the daytime when electricity from the grid is cheaper (during the shoulder period of a time-of-use pricing structure). Any excess PV generation must be exported where, in this case, there is only a small return. They still need to pay peak prices for their electricity use during the evening.
Battery System
The battery system within the BESS refer to the battery bank and any other components needed for the battery cell(s) or module(s) to be safely connected to the inverter or charge controller. The additional components can include battery management modules, battery management systems, pumps, and auxiliary equipment. Auxiliary equipment can be in the form of battery alarms, fire suppression systems, or items such as inbuilt cooling system and specific battery system communication devices. The features can be endless, but are all there to ensure the correct and safe operation of the battery cells.
What is the BESS's Function?
For designing a grid-connected PV system with BESS, it is important to understand the requirements of the system owner and the conditions in which the system must, operate. What do they want the battery energy storage system to do and why? Do they need back-up power supply because of frequent power outages? DO they want a battery to store all the excess solar energy? Do they just want to use batteries to minimize electricity bills?
Below are a few possible functions of battery storage systems for grid-connected PV systems:
1. Grid backup: the system provides an alternate supply for some or all of the site's loads during abnormal grid conditions, such as a blackout. This is also known as emergency power supply (EPS).note that these systems generally cannot provide continuous power to sensitive loads; an uninterruptable power supply
2. Grid as back-up: The system is designed in such a way that the site is primarily powered by a combination of PV and battery and the use of electricity from the grid is minimized. This could be for-financial, environmental, or other reasons. In these cases, the grid could be seen as a backup supply for the site.
3. Tariff arbitrage: The system charges its batteries from PV generation (or the grid, if PV is unavailable) to power loads that operate during times of high electricity cost to reduce electricity bills. This only applies for sites with time of use tariff (as shown in ).
4. Minimize PV export: The system charges its batteries from excess PV production. The stored energy can be discharged whenever desired. This function would be desirable if there are network limitations or if PV export attracts very little feed-in tariff.
5. Virtual Power Plant participation: The system's battery and PV inverter responds to an offsite controller to export or charge from the grid, with the intention to help relieve pressure experienced by the grid. In return, the system owner receives some monetary compensation.