BESS: Battery Energy Storage System

BESS (battery energy storage system) is an electrochemical energy storage system, which is a plant consisting of subsystems, equipment, and devices necessary for energy storage and bidirectional conversion of the same into medium voltage electrical energy. These systems are essential for reducing dependence on fossil fuels and improving the performance of transmission and distribution networks.

The integration of storage systems in photovoltaic plants allows storing the surplus energy produced and not self-consumed, bringing numerous advantages both from an environmental and economic point of view. To fully exploit these advantages, it is essential to correctly size the battery capacity based on energy consumption.

An inadequately sized storage system may not meet energy needs, making the investment ineffective. To avoid this risk, you can use specific solar design software capable of effectively supporting  choices related to the type of storage system and its sizing.

What is meant by BESS

BESS stands for battery energy storage system and is a system that uses electrochemical batteries to convert electrical energy into chemical energy during the charging phase and then convert it back into electrical energy during the discharge phase.

These systems are renowned for their ability to respond quickly to both energy absorption and release and are based on rechargeable batteries that can be charged multiple times.

They combine the battery storage capacity with artificial intelligence techniques and machine learning algorithms to coordinate energy production and computerized control systems. These systems determine when to store energy to create reserves or release it into the grid.

How battery energy storage works

The operation of a BESS system is based on the use of one or more batteries for energy storage. These batteries can be charged in various ways, such as using excess electricity produced by wind or photovoltaic plants, or by connecting them to the power grid during periods of low demand. When integrated with advanced software, BESS systems become platforms capable of harnessing the battery storage capacity along with artificial intelligence techniques and machine learning algorithms to coordinate energy production and computerized control systems.

These systems determine when to store energy to create reserves or release it into the grid. The stored energy is then released during peak demand periods, helping to keep electricity costs low and ensuring a stable supply.

In the energy production chain, BESS systems are placed before the energy transmission section and operate in parallel with various components for managing and distributing electrical energy.

During peak demand periods, the stored energy is released from the battery storage system, helping to keep electricity costs low and ensure a stable supply. In the energy production chain, BESS systems are placed before the transmission section and operate in parallel with various components for managing and distributing electrical energy.

What are the different types of batteries used in BESS systems

The electrolytic cell of a battery pack in a BESS system can be made using different technologies, which differ in:

• the pair of electrochemical species involved in the reaction;
• the type of electrolyte used;
• the constructive characteristics of the system.

The main types of electrolytic cells include:

• batteries with aqueous electrolyte:
  • lead-acid batteries;
  • nickel/cadmium batteries;
  • nickel/metal hydride batteries;
• lithium-ion batteries;
• flow batteries:
  • flow batteries
• high-temperature batteries:
  • sodium/sulfur batteries;
  • sodium/nickel chloride batteries.

BESS Battery Types

Lithium-ion batteries

Lithium-ion batteries represent the most advanced technology in the field of electrochemical storage thanks to their high specific power. Their main disadvantage is the high cost due to the need to implement safety systems to prevent overcharging.

There are many variants of lithium batteries, with different cathode compositions, but all share a common basic structure. These devices include an anode generally made of graphite and a cathode made of a metal oxide, forming a layered or tunnel structure that facilitates the insertion and extraction of lithium ions. The electrolyte, which can be either liquid or polymeric, connects the positive and negative electrodes, separated by an electronic insulating layer, usually made of polyolefin.

The electrochemical reactions vary depending on the cell type, but the open circuit voltage is between 3.6 and 3.85 V.

Lithium batteries require careful management to avoid issues such as thermal leakage and the release of flammable vapors. For this reason, they are equipped with a battery management system (BMS) and are enclosed in metal containers.

Finally, another critical aspect is their degradation over time, leading to a progressive reduction in capacity compared to factory data, even in the absence of charge/discharge cycles.

Lead-acid batteries

Lead-acid batteries are made up of a lead electrode and a lead dioxide electrode immersed in a sulfuric acid solution.

There are different types of lead-acid batteries, mainly divided into two categories:

• open batteries, which have openings through which hydrogen and oxygen produced during charging are released;
• sealed batteries, which require less maintenance, take up less space, and limit hydrogen emissions.

In lead-acid batteries, self-discharge is caused by parasitic reactions that gradually consume the stored charge, leading to complete discharge over time. Normally, self-discharge causes a decrease in charge of about 2-3% per month.

The service life of lead-acid batteries can vary from 6 to 15 years, but it is significantly reduced if the battery is subjected to high power demands. The main advantages of lead-acid batteries include relatively low investment costs, a well-established technology, and efficient recyclability. This type of battery is widely used in vehicles and other applications that require high load currents.

Nickel-cadmium and nickel-metal hydride batteries

Nickel-cadmium batteries operate similarly to lead-acid batteries, but their use has been banned since 2006 due to the toxicity of cadmium used as an anode. These energy storage systems are based on electrochemical charge and discharge reactions that occur between:

• a positive electrode containing nickel hydroxide as the active material;
• a negative electrode composed of metallic cadmium.

A permeable membrane separates the electrodes, allowing ion flow, and they are immersed in an aqueous potassium hydroxide electrolyte, which remains relatively stable during operation.

During discharge, nickel hydroxide combines with water to produce nickel hydroxide and hydroxide ions, while the negative electrode generates cadmium hydroxide. It is important to note that nickel-cadmium batteries may be subject to memory effect, meaning that their performance may decrease with repeated use.

Alternatively, cadmium can be replaced with an alloy capable of absorbing hydrogen, creating nickel-metal hydride batteries. These batteries offer a capacity two to three times higher than nickel-cadmium batteries of the same size and have a less pronounced memory effect.

Flow batteries

Flow batteries store energy using two liquid electrolytes, one positive and one negative, which circulate through pumps between tanks and a central unit (stack) with cells and ion exchange membranes.

These membranes allow selective passage of ions and are essential for operation but expensive. Power is determined by the pump flow and membrane surface area, while energy capacity depends on tank size.

These batteries are ideal for applications that require fast responses and improve efficiency through the use of inverters to control the pumps. However, temperatures outside the optimal range (0-40°C) can compromise electrolyte circulation and energy efficiency.

Although theoretically having unlimited lifespan, component degradation limits their operational life to about 10,000 cycles, equivalent to 20 years of use.

High-temperature batteries

High-temperature batteries include sodium/sulfur batteries and sodium/nickel chloride batteries. These systems are characterized by the high internal operating temperature, reaching around 300°C, necessary to keep the electrodes in a liquid state and improve electrolyte conductivity.

Compared to lead-acid batteries, high-temperature batteries offer a longer lifespan, but require adequate safety systems due to the high operating temperatures.

They are appreciated for their high energy density, high energy efficiencies, and good longevity. However, their performance is generally lower than that of lithium-ion batteries.

What are the components of a BESS system

The main components of a BESS system are:

• container containing the batteries;
• BMS necessary to ensure the safety and reliability of the batteries during operation;
• PCS for BT/MT transformation;
• MT/AT step-up transformer.

Container containing the batteries

The container is a self-supporting metal structure, suitable for outdoor installations, made with profiles and insulated panels. This configuration allows transporting and installing the entire system without disassembling the various components, except for the battery modules that can be removed and transported separately if needed.

Each container is equipped with environmental sensors to constantly monitor internal temperature and humidity. Upon request, it can be equipped with air conditioning and ventilation systems to maintain optimal operating conditions and a liquid cooling system. The internal temperature is controlled using thermocouples to detect any fire risks.

BMS

Within the battery subsystem, there is a battery management system (BMS) that is responsible for monitoring, protecting, and maintaining the safety and optimal operation of the battery modules. The BMS is generally structured hierarchically, including a BMS for the battery module, a BMS for the battery string, and a BMS for the entire battery system.

The BMS of the battery module, usually integrated into the module itself, monitors the voltages and temperatures of the cells. In particular, at least two temperatures in different areas of the module are detected, along with the voltage and current of the module, electrical insulation resistance, and the connection status of the module.

PCS

Power conversion systems (PCS) are bidirectional devices that operate with the power grid, known as Grid Connected Power Converters (GCPC). These systems include transformers, control systems, and protection equipment to ensure the safe and efficient operation of the power conversion modules.

The main functions of PCS are battery charge and discharge management, battery block control, and converter protection.

Each PCS consists of input interfaces for direct current (DC) voltage, bidirectional DC/DC and DC/AC converters, and output interfaces for three-phase alternating current (AC) voltage.

GCPCs are designed to enter a safety state in case of emergency and can synchronize with the AC grid, providing active and reactive power according to operational needs and indications from the energy monitoring system.

Transformer

The MT/AT transformer, located inside the user station positioned centrally within the BESS area, allows the connection of the BESS system to the NRT at the established connection point. The transformer has a three-winding configuration.

The windings consist of entire solenoidal coils, usually in multiple layers arranged coaxially in the radial direction, especially for the “high voltage” part, or they can consist of partial coils in multiple layers also coaxial and superimposed in the axial direction and possibly equipped with cooling channels.

Components of a BESS system

Integration of a BESS system in building design

The integration of a BESS system within a building is not simply about “adding a battery,” but a true design process that involves systems, spaces, safety, and energy management. For this reason, in modern architectural and plant design, the BESS is increasingly considered a structural component of the building, on the same level as the thermal power plant or the main electrical panel.

Plant requirements, technical spaces, and integration with building systems

The installation of a BESS system requires careful planning of both spaces and integration with the building systems. During the design phase, it is necessary to identify a dedicated technical room or an adequate external space, often constructed using certified prefabricated containers. Among the elements to consider are:

• accessibility for maintenance interventions and periodic inspections;
• controlled ventilation or cooling systems to maintain stable internal temperature;
• insulation and soundproofing to prevent overheating and reduce thermal leakage risks;
• safety distances from other systems, combustible materials, or sensitive areas;
• escape routes and facilitated access for technical personnel and emergency teams.

The BESS, moreover, is not an isolated system but interacts with various building systems. It is essential to ensure:

• integration with HVAC systems, such as ventilation or liquid cooling, to limit thermal stress on the batteries;
• gas and fire detection, through specific sensors and automatic extinguishing systems;
• continuous environmental monitoring (temperature, humidity, vibrations) to ensure optimal operating conditions;
• correct electrical interface, with panels and protections compliant with CEI regulations and distributor requirements.

The importance of integration with safety systems increases in high-traffic buildings such as schools, hospitals, and shopping centers, where fire compartmentalization and separation of systems must minimize the effects of potential incidents.

EMS (Energy Management System) for building management

The true value of the BESS emerges when it works together with an Energy Management System (EMS). The EMS analyzes in real-time the data on production, consumption, weather forecasts, electricity rates, and storage capacity, and decides:

• when to charge the batteries;
• when to discharge them to support internal consumption;
• how much energy to feed into the grid;
• how to optimize the self-consumption of photovoltaic systems;
• how to reduce demand peaks and contain costs.

In the design of next-generation buildings, the EMS is an integral part of the energy efficiency strategy and allows for achieving advanced objectives such as:

• reduction of operating costs;
• smart balancing between renewables, the grid, and BESS;
• improvement of self-sufficiency;
• increased resilience in case of blackouts.

How do BESS storage solutions support grid stability and microgrids?

BESS storage systems play a fundamental role in maintaining a stable and efficient modern electrical grid, increasingly characterized by a strong presence of intermittent renewable sources. Their rapid response and ability to absorb or release energy in real-time make them an indispensable tool for ensuring continuity, quality, and safety of the electrical service.

Grid services: peak shaving, frequency regulation, black start

In the context of the electrical grid, BESS allows for the provision of a series of advanced services:

• Peak shaving: involves reducing demand peaks by absorbing energy during high load periods. This reduces stress on the grid and allows users (both residential and industrial) to decrease costs related to committed power.
• Frequency regulation: batteries react in milliseconds to frequency variations in the grid, stabilizing it when the power injected or withdrawn changes rapidly. It is an essential service in electrical systems with high penetration of photovoltaic and wind energy.
• Black start: in the event of a blackout, a BESS can provide the initial power needed to restart power plants or sections of the grid. Thanks to the rapid availability of energy, they play a key role in restoring service.
• Reduction of congestion and improvement of voltage quality: BESS can absorb energy in overloaded areas and release it where needed, helping to stabilize voltage and reduce grid congestion.

BESS in microgrids and off-grid systems

BESS systems are essential for the operation of microgrids, which are local electrical networks that can operate both connected to the national grid and in “island” mode.

In microgrids, the BESS ensures:

• continuity of supply during disconnections from the main grid;
• optimal integration of local renewables, such as photovoltaic and wind;
• intelligent management of energy flows to minimize costs and maximize self-consumption;
• resilience in emergency situations, blackouts, extreme weather events.

In areas not reached by the electrical grid (islands, shelters, isolated infrastructures), the BESS allows for the creation of reliable off-grid systems, powered by renewable sources, reducing or eliminating the use of diesel generators.

The role of BESS and renewables in zero-emission buildings (NZEB)

BESS systems are fundamental elements in the realization of nearly zero energy buildings (NZEB) and, increasingly, zero-emission buildings (ZEB). Their main function is to maximize the use of energy produced by renewable systems such as photovoltaic or micro-wind, compensating for their natural variability.

Thanks to the BESS, energy generated during peak production hours can be stored and used when the building truly needs it, increasing self-consumption and significantly reducing dependence on the grid. This translates into lower indirect emissions, greater stability of consumption, and improved resilience of the building in case of blackouts or sudden variations in renewable production.

In modern residential, commercial, or office buildings, storage allows for fully utilizing photovoltaic systems, making energy available even in the evening hours and optimizing the overall energy balance. The result is a more efficient, more sustainable building aligned with European decarbonization goals.

For an overview of the main sources of renewable energy and their contribution to the energy transition, you can also consult the in-depth article on what renewable energies are and what they contribute.

Designing hybrid systems with photovoltaic + BESS integration

The combination of photovoltaic systems and BESS storage systems represents the most effective solution today for increasing self-consumption and ensuring a constant availability of energy even when solar production decreases. By integrating PV modules, inverters, and batteries into a single coordinated system, it is possible to optimize energy flows and significantly reduce dependence on the grid.

One of the main advantages of BESS is the ability to use energy produced from renewable sources, compensating for the shortfalls due to the intermittency of photovoltaic and wind energy. Excess energy is stored in the batteries, allowing access to this reserve when the solar system does not produce enough energy, reducing dependence on the electrical grid.

In a hybrid system, photovoltaic energy is primarily used to power the building’s consumption. The surplus is stored in the BESS and made available in the evening hours or during peak demand times, improving the continuity and quality of electrical supply. The entire process is managed by a control system that decides when to charge and discharge the battery based on solar production, consumption, and grid conditions.

From an operational standpoint, the logic is simple:

• first, internal loads are powered;
• then the BESS is charged;
• finally, only when both PV and BESS are full, can energy be fed into the grid.

This approach allows for significantly increasing self-consumption, improving system stability, and reducing energy costs. In this way, a balance is created between solar energy and grid energy, optimizing energy costs.

The photovoltaic + BESS integration is now the most widespread configuration: according to ANIE, 97% of DER BESS units are combined with a solar photovoltaic system, confirming that electrochemical storage is now an essential element in the design of renewable energy-based systems, both in residential and industrial contexts.

Operating logic of the hybrid system PV + BESS

Safety protocols and regulations for large-scale BESS installations

The installation of large-scale BESS systems requires a clear regulatory framework and specific safety procedures to ensure reliable operation and minimize risks related to fires, gas emissions, and technological incidents.

Battery life cycle management and recycling strategies for BESS

The management of the battery life cycle is a central aspect to ensure sustainability, safety, and efficiency of BESS systems. Batteries do not maintain the same performance throughout their existence: over time, a gradual degradation occurs that reduces storage capacity and overall efficiency. This phenomenon is natural and must be managed through continuous monitoring, adequate maintenance, and a proper planning of module replacement.

When the remaining capacity drops below a certain threshold — typically between 70% and 80% — the batteries can be destined for a new operational phase: the so-called second life. In this second life, batteries no longer suitable for vehicular use find application in stationary storage systems, such as BESS for buildings, microgrids, or industrial plants. This practice helps to reduce the overall costs of storage systems, extend the useful life of materials, and contribute to a more circular resource management model.

At the end of the second life, batteries are sent for recycling, a fundamental step to recover critical metals such as lithium, nickel, cobalt, manganese, and copper. Recycling not only reduces the environmental impact of disposal but also helps alleviate the pressure on the demand for rare materials, which is increasingly high due to the spread of electric vehicles and storage systems. Advanced hydrometallurgical and pyrometallurgical treatment technologies are increasing recovery rates, making the sector more sustainable.

For lithium batteries, however, the processes are more complex and rapidly evolving, but European regulations are pushing for ever-increasing recovery rates for all critical components.

The management of the battery life cycle — from the operational phase to the second life and then to recycling — is therefore a strategic element to ensure that BESS systems are not only efficient and safe but also truly sustainable and aligned with the goals of the energy transition.

Battery life cycle

The future of BESS technology in smart cities and sustainable infrastructures

The future of BESS systems is closely linked to the development of smart cities, smart grids, and sustainable urban infrastructures. As the integration of renewable energies increases, so does the need for technologies capable of stabilizing the grid, reducing production variability, and making buildings energy self-sufficient. Electrochemical storage systems therefore represent one of the pillars of the energy transition, thanks to their ability to provide flexibility, rapid regulation, and support for service continuity.

In Europe, the largest battery energy storage system has recently been put into operation. Located in the United Kingdom, near the world’s largest offshore wind farm, Dogger Bank, this system has enough capacity to power approximately 300,000 homes for two hours. Every year, projects of increasing scale are being developed, from large utility-scale systems to modular plants for energy communities and smart neighborhoods. BESS are becoming a strategic component for smart grids, electric mobility, fast charging stations, energy communities, and critical infrastructures such as hospitals and data centers.

BESS are no longer an emerging technology, but a structural element of urban energy infrastructures. New technologies, such as solid-state batteries and AI-based control systems, will make future systems even safer, more efficient, and sustainable.

FAQ – BESS: Battery Energy Storage System

What is a BESS?

A BESS (Battery Energy Storage System) is an electrochemical storage system that stores electrical energy in chemical form and returns it when needed. It consists of batteries, management systems, and conversion components that allow connection to the grid and ensure system reliability.

How does a BESS work in an energy plant?

The system stores energy through rechargeable batteries, charging them when there is excess energy available and releasing it during peak demand periods. Software and control systems coordinate charging and discharging based on grid conditions and renewable production, contributing to the stability of the electrical system.

What types of batteries can be used in BESS systems?

The most commonly used technologies include:

• lithium-ion (high energy density, greater diffusion);
• lead-acid (cost-effective and recyclable);
• nickel-cadmium / nickel-metal hydride;
• flow batteries;
• high-temperature batteries such as sodium/sulfur and sodium/nickel chloride.

Each technology has specific characteristics regarding lifespan, performance, and costs.

What are the main components of a BESS?

A BESS consists of:

• containers housing the battery modules;
• BMS for cell management and safety;
• PCS for conversion between direct current and alternating current;
• transformer for interfacing with the medium or high voltage grid.

How is a BESS integrated into building design?

Integration requires the provision of suitable technical spaces, proper ventilation or cooling, fire detection systems, and an electrical interface compliant with regulations. The BESS works alongside the building’s EMS, optimizing self-consumption, energy costs, and resilience in case of blackouts.

What advantages does a BESS offer in microgrids and off-grid systems?

In a microgrid, the BESS ensures continuity of supply, intelligent management of energy produced from photovoltaic and wind sources, cost reduction, and stability during grid outages. In off-grid systems, it allows for the replacement or reduction of diesel generator use.

Why are BESS essential in NZEB and ZEB buildings?

They allow for the full utilization of energy produced by renewable systems, storing excess energy and making it available when consumption is high. This increases self-sufficiency, reduces indirect emissions, and improves the energy resilience of the building.

How does a photovoltaic + BESS hybrid system work?

Solar energy first powers the building’s loads; the surplus is stored in the BESS and used during low production hours. Only when both resources are saturated is the energy fed into the grid. This maximizes self-consumption and reduces costs.

How is the lifecycle of a BESS battery managed?

Batteries degrade progressively, and once their capacity is reduced, they can be designated for a second life in stationary applications. At the end of their useful life, they are sent for recycling processes to recover critical metals such as lithium, nickel, and cobalt, contributing to the sustainability of the system.

What are the future prospects for BESS systems?

BESS are set to become key elements of smart cities and smart grids. New technologies, such as solid-state batteries and advanced energy management systems, will increase safety, efficiency, and scalability, promoting greater diffusion in critical infrastructures and energy communities.