Before discussing battery energy storage system (BESS) architecture and battery types, we must first focus on the most common terminology used in this field. Several important parameters describe the behaviors of battery energy storage systems.
Capacity [Ah]: The maximum electric charge that the system is able to provide to the attached load at a reasonable voltage. The battery's technology has a significant impact on this parameter, whose value is set for a particular discharge current and temperature.
Nominal Energy [Wh]: This is the total energy produced between the states of full charge and full discharge. It is equivalent to the battery voltage times the capacity. Temperature and current have an impact as well, since capacity determines it.
Power [W]: Defining a BESS's output power is difficult because it relies on the attached load. Nonetheless, nominal power represents the power in the most typical discharge scenario.
Specific Energy [Wh/kg]: This indicates the battery's energy storage capacity in relation to mass.
The scale used to determine charge and discharge durations is called the C Rate. The discharge current will drain the battery completely in an hour at 1C.
Charge/discharge/charge is the cycle. There is no agreed-upon definition of what a cycle is.
A battery's cycle life is the total number of cycles it can produce.
DoD: Discharge depth. Complete discharge is 100%;
State-of-charge (SoC,%): A battery's charge level is indicated by this number.
The term "coulombic efficiency" refers to the battery's ability to transmit charge efficiently. It is the proportion of charge required to return to the original state of charge to the charge quantity (Ah) released during the discharge phase. With the exception of lead-acid technology, most ordinary batteries have an efficiency that is comparable to this.
The Main Types of Electrochemical Energy Storage Systems
Numerous battery systems exist, each based on a unique combination of chemical components and processes. Lead-acid and Li-ion batteries are currently the most widely used types, but flow, nickel, and sulfur-based batteries also have a place in this market. We'll quickly review the key benefits of the most popular battery technologies.
We use these batteries on a regular basis. This battery's base cell is composed of a bi-oxide or lead positive electrode and a negative lead electrode. The electrolyte is a sulfuric acid solution in water.
These batteries' primary benefits are their affordability and advanced technological state.
Nickel–Cadmium (Ni–Cd) Batteries
Before lithium battery technology was widely used, this type of battery served as the primary power source for portable devices for a number of years.
These batteries provide a high power output and a quick recharging time.
An improvement on these batteries is represented by Nickel-metal-hydride (NiMH) technology, which can provide about 40% higher specific energy than the standard NiCd.
Lithium-Ion (Li-Ion) Batteries
Of all the metals, lithium has the highest specific energy and is the lightest. Lithium metal anode rechargeable batteries have the capacity to provide incredibly high energy densities.
There are other restrictions as well. For instance, the development of dendrites on the anode during cycling is a pertinent restriction. It may result in a power outage, which could raise the temperature and harm the battery.
The Composition of a BESS
Different "levels," both logical and physical, make up a BESS. Every unique physical part needs its own control system.
Here's a rundown of these key stages:
The battery system is made up of various battery packs and numerous batteries that are connected to one another in order to achieve the desired voltage and current levels.
The battery management system regulates each cell's appropriate functioning to enable the system to function within a voltage, current, and temperature range that is safe for the batteries' excellent health rather than the system as a whole. Additionally, the status of charge in each cell is adjusted and balanced by doing this.
To convert the power into AC, the inverters are attached to the battery system. A specialised power electronic level known as the PCS (power conversion system) is present in every BESS. It is typically grouped in a conversion unit together with all the auxiliary services required for appropriate monitoring.
The system and energy flow monitoring and control (energy management system) are the following steps. The supervisory control and data acquisition system, or SCADA system, often includes general monitoring and control functions. On the other hand, the energy management system is specifically designed to monitor power flow in accordance with application requirements.
The medium-voltage/low-voltage transformer connection and, based on the system's size, the high-voltage/medium-voltage transformer at a dedicated substation are the last connections.
PV Module and BESS Integration
Renewable energy sources are poised to have a significant impact on electrical systems in the future, as discussed in the first piece of this series. Both the electrical system and the renewable power plant may benefit from the integration of a BESS with a renewable energy source.
The following explains the various ways in which a BESS could assist a power plant:
In order to achieve a more steady and predictable generation curve, this would offset the "volatility" of the generation profile under cloud cover or abrupt spikes in power. The contrast between a PV plant's generation curve on a cloudy day and one with a clear sky is displayed in Figure 4. The generation would exhibit less "flickering" with the integration of a BESS, yielding a more regular curve.
The generation curve will "smooth" as a result of peak shaving (for more on peak shaving, read the previous article).
With regard to grid support and ancillary services, the BESS can play a significant role in the power plant's integration into the electrical grid by offering frequency regulation and voltage management (together with reactive power compensation) with significantly less of an impact on the electrical system.
Apart from the aforementioned services, there exist more potential collaborations between photovoltaic modules and battery energy storage systems, commencing with the exchange of point of connection (POC). Since a BESS is frequently installed to "complement" the PV module, its presence couldn't necessitate extra power at the POC.
Additional potential collaborations stem from decisions made in the architecture of how PV modules connect to a BESS. At least three primary options exist:
DC Coupling: In this option, a particular DC/DC converter is used to link the BESS and PV on the DC side of the batteries and PV modules in order to stabilise the voltage. With this method, all of the AC side of the plant will share the inverters between the PV module and BESS (the inverter in this scenario will be able to operate in all 4 quadrants of the P-Q diagram).This choice is quite common for residential applications, or in the case of a small plant (kW). In the case of a large-scale plant, the BESS will be distributed along the field. It will, however, require specific and expensive logic to control the DC voltage and the charge of each battery pack.
AC Coupling After the Inverter: This method is comparable to the preceding one, but it places the BESS and PV module coupling point after the inverters. In this instance, the BESS and the PV module will each have their own dedicated inverter. Because there is no need for the additional control logic for the DC coupling, this method is also popular for residential applications and could be used in large plants to create a distributed BESS.
AC Coupling at the POC: In this solution, the PV module and BESS share only the interconnection facility, while they have completely separated sections at plant level.