The Architecture of Battery Energy Storage Systems

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 amount of electric charge the system can deliver to the connected load while maintaining acceptable voltage. This parameter is strongly affected by the technology of the battery and its value is defined for specific temperature and discharge current.

Nominal Energy [Wh]: This is the energy generated from a full charge status up to complete discharge. It is equal to the capacity multiplied by the battery voltage. As it depends on the capacity, it is affected as well by temperature and current.

Power [W]: It’s not easy to define the output power for a BESS, as it depends on the load connected. However, nominal power  indicates the power during the most representative discharge situation.

Specific Energy [Wh/kg]: This specifies the amount of energy that the battery can store relative to its mass.

C Rate:  The unit by which charge and discharge times are scaled. At 1C, the discharge current will discharge the entire battery in one hour.

Cycle:  Charge/discharge/charge. No standard exists as to what constitutes a cycle.

Cycle Life: The number of cycles a battery can deliver.

DoD:  Depth of discharge. 100% is full discharge;

State-of-charge (SoC, %): Indicates the charge level of a battery.

Coulombic efficiency: This describes the charge efficiency with which electrons are transferred in the battery. It is the ratio between the charge quantity (Ah) released during the discharge period and the amount of charge needed to reset to initial state of charge. This efficiency is close to one for most common batteries, except, for example, lead-acid technology.

 

The Main Types of Electrochemical Energy Storage Systems

There are many different types of battery technologies, based on different chemical elements and reactions. The most common, today, are the lead-acid and the Li-ion, but also Nickel based, Sulfur based, and flow batteries play, or played, a relevant role in this industry. We will take a brief look at the main advantages of the most common battery technologies.

 

Lead-Acid Batteries

These batteries are very common in our daily lives. The base cell of this battery is made with a negative lead electrode and a positive electrode made of bi-oxide or lead, while the electrolyte is a water solution of sulfuric acid.

The main advantages of these batteries are low cost and technological maturity.

 

Table 1. Pro and cons of lead-acid batteries. Source Battery University

 

Nickel–Cadmium (Ni–Cd) Batteries

This kind of battery was the main solution for portable systems for several years, before the deployment of lithium battery technology. 

These batteries have strong power performance and require little time to recharge.  

 

 Table 2. Pro and cons of Nickel-Cadmium batteries. Source Battery University

 

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

Lithium is the lightest of all metals and provides the highest specific energy. Rechargeable batteries with lithium metal on the anode can provide extraordinarily high energy densities. 

There are also limitations, for example, one relevant limit is the production of dendrites on the anode during cycling. It can create an electric shortage with a consequent increase in temperature and damage for the battery.

 

Table 3. Pros and cons of Lithium batteries. Source Battery University.

 

The Composition of a BESS

A BESS is composed of different “levels” both logical and physical. Each specific physical component requires a dedicated control system. 

Below is a summary of these main levels:

• The battery system is composed by the several battery packs and multiple batteries inter-connected to reach the target value of current and voltage

• The  battery management system that controls the proper operation of each cell in order to let the system work within a voltage, current, and temperature that is not dangerous for the system itself, but good operation of the batteries. This also calibrates and equalizes the state of charge among the cells.

• The battery system is connected to the inverters, in order to convert the power in AC. In each BESS there is a specific power electronic level, called PCS (power conversion system) usually grouped in a conversion unit, including all the auxiliary services needed for the proper monitoring.

• The next level is for monitoring and control of the system and of the energy flow (energy management system). The general monitoring and control is usually included in the SCADA system (supervisory control and data acquisition system), while the energy management system has the specific purpose of monitoring the power flow according to the specific applications.

• Lastly, there is the connection with the medium-voltage/low-voltage transformer and according to the size of the system, the high-voltage/medium-voltage transformer in dedicated substation.

 

Figure 2. An example of BESS architecture. Source Handbook on Battery Energy Storage System

Figure 3. An example of BESS components - source Handbook for Energy Storage Systems

 

PV Module and BESS Integration

As described in the first article of this series, renewable energies have been set up to play a major role in the future of electrical systems. The integration of a BESS with a renewable energy source can be beneficial for both the electrical system and the renewable power plant. 

Below is an explanation of how a BESS could support a power plant in several ways:

• This would compensate for the “volatility” of the generation profile when clouds occur or when there are sudden peaks of power increase, in order to obtain a more predictable and stable generation curve. Figure 4 shows the difference of the generation curve of a PV plant on a cloud day versus a clear sky day. The integration of a BESS would reduce the “flickering” of the generation, resulting in a more regular curve.

 

Figure 4. PV Generation profile in cloud days and clear sky day. Image courtesy of Enel Green Power. 

 

• Peak shaving will “smooth” the generation curve (see the previous article for more information on peak shaving).

• For grid support with ancillary services, the BESS can contribute in a relevant way to the integration of the power plant into the electrical grid, providing voltage control (with reactive power compensation), frequency regulation, with much less impact in the electrical system.

In addition to the services mentioned above, there are also more possible partnerships between battery energy storage systems and PV modules, starting from the sharing of the point of connection (POC). The presence of a BESS couldn’t require additional power at the POC, because it is often installed to “complement” the PV module.

Other possible partnerships are derived from design choices regarding the coupling between PV modules and a BESS. There are at least three main possibilities:

• DC Coupling: With this choice, the BESS and the PV are interconnected on the DC side of the batteries and of the PV modules, by means of a specific DC/DC converter to stabilize the voltage. This solution allows sharing the inverters between the PV module and BESS (in this case the inverter shall be able to operate in all the 4 quadrants of P-Q diagram) and all the AC side of the plant will be in sharing. 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 solution is similar to the previous one, but with the coupling point between a BESS and a PV module after the inverters. In this case, there will be a dedicated inverter for the BESS and a dedicated inverter for the PV module. This solution is also common for residential applications and it would be applicable for big plants, resulting in a distributed BESS, because the limitations due to the additional control logic for the DC coupling are not needed.

• 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.