A 30 MW battery storage system operated by Energy Australia. (Source: Victoria State Government.)

Batteries are playing a vital role in the transition from fossil fuels to renewable energy. One such role is in battery energy storage systems (BESSs), which bank battery energy and release it when needed.

The BESS market has been rapidly growing worldwide. The last 10 years have seen mass grid-scale BESS installations in both the automotive and power industries. According to BloombergNEF, 9 GW / 17 GWh of BESS were deployed as of 2018 and 1,095 GW / 2,850 GWh are expected to be deployed by 2040.

In this article, we’ll examine how BESSs work and how they’re proving valuable in a number of applications.

How a BESS Works

The main components of BESS are a battery string, a control system, an inverter, and a battery management system (BMS).

A battery string is an array of batteries in parallel and/or series connections to provide the required capacity and voltage.

A control system is the brain of the BESS. It determines when the batteries should be charged and when they should provide power to the loads. The control system must interact with devices outside of the BESS to coordinate these operations.

Since batteries provide DC voltage, an inverter is used to convert it to AC voltage suitable for consumers.

Among other responsibilities, the BMS provides thermal management to control the temperature of the batteries. This is critical, because the heat generated from the chemical reactions during battery operation can damage the system and be harmful to personnel. Controlling the temperature is important for all battery types, but especially for lithium-ion batteries, as their operation is very dependent on the temperature. Inappropriate temperatures can cause thermal runaway of the batteries, leading to fire and explosion.

Most Suitable Battery Types for a BESS

Older BESSs mostly employed lead-acid or nickel-cadmium (Ni-Cd) batteries. However, in the last five years, lithium-ion (Li-ion) batteries have quickly gained market share. Li-ion is now the most common battery used in new BESS installations.

Lead-acid battery technology has the best price in the market, and the technology is mature and reliable. However, it is considered obsolete. Lead acid batteries have a poor specific energy rate when compared to Li-ion technology, which means the system must be heavy to provide sufficient energy. There are two lead-acid battery types: flooded vented lead-acid (VLA) and valve-regulated lead-acid (VRLA). VLA has a lower price, but VRLA is more popular because it has fewer maintenance requirements, no risk of acid leaking, and mounting flexibility.

VRLA batteries are still often used as energy backup in electrical substations. (Source: DV Power.)

When compared to lead-acid, Ni-Cd flooded batteries are a better choice for high current applications, such as starting engines. They also have a good service lifetime and maintain a nearly constant voltage during discharging. Thus, they can be used in critical stationary applications where a high level of security is required. However, Ni-Cd is highly toxic, and it is quickly being replaced by more efficient and environmentally friendly Li-ion and nickel-metal hydride (Ni-MH) technology.

Ni-MH batteries are superior to lead-acid with double the specific energy, reducing the size and weight of the batteries. However, the specific energy of Ni-MH is significantly lower than Li-ion batteries.

The main reason for the remarkable success of Li-ion batteries is the progressive development of the technology that has resulted in competitive prices and excellent technical specifications. Li-ion batteries have both technical and practical advantages over traditional lead-acid batteries. Li-ion battery systems with built-in battery management systems provide nearly maintenance-free operation and longer lifetimes. Li-ion batteries have a very high energy density compared to other battery types, allowing for more energy in less space. Between 2013 and 2018 in Germany, residential Li-ion BESS installations rose from under 30 percent market share to over 95 percent, effectively squeezing out lead-acid systems.

Residential BESS based on Li-ion batteries. (Source: Samsung SDI.)

More about these different battery types can be found in the article What You Need to Know About Batteries for Electric Vehicles.

BESS Applications for the Power Grid

Fossil fuels are falling behind and society is switching to greener energy production. Consumers are using more electricity than ever, such as when charging their EVs. These changes present a challenge for grid management, and BESSs are a promising and versatile solution.

BESSs have found applications in many segments of the electricity supply chain, including generation, transmission, distribution, and consumption. They play an important role in the integration of renewable energy in the power grid. BESSs compensate the fluctuation in electricity generation from renewable sources by absorbing energy during peaks and providing it during low energy production.

BESSs can provide power grid balancing to ensure stable and efficient operation of the grid. Power generation and consumption must be balanced at all times. However, the balance can be disturbed by many factors, such as failures in power plants and transmission lines or mismanagement of renewable power generation, leading to surfeits or deficits in electricity.

Proper power management—system balancing—should keep frequency and voltage within acceptable limits until regular operation is restored. Frequency control and reactive power management are traditionally performed in coal power plants that use large generators. However, as fossil fuel power plants are phased out, new solutions will be required.

BESSs can provide a quick deployment time, extremely fast response, and good scalability for power management. According to PV Tech, the power grid requires a BESS to be able to ramp up to their nominal power in less than 30 seconds and sustain a constant power output for 15 minutes. Modern Li-ion batteries fulfill those requirements.

Because of their extremely fast response times and good scalability, BESSs are an effective solution for reducing peak loads. This requires BESSs to provide power for four hours. In combination with the renewable energy sources, this could be an efficient replacement for gas-fired power plants used to meet peak power demand. However, this BESS application increases the complexity of grid automation.

A 25 MW / 50 MWh BESS co-located with the Gannawarra Solar Farm in Australia. This BESS stores 100 percent renewable energy, smoothes the output of the solar farm, and provides power at peak times. (Source: Victoria State Government.)

Another application is to deploy a BESS for transmission grid flexibility. Transmission grid reliability standards require that the system remains in a stable state even during contingency events in the grid, when a line or key substation is down. The transmission system must be capable of carrying full capacity when all transmission facilities are in service, and it should incorporate future load growth patterns. Thus, traditional transmission systems are typically oversized and underutilized. Since the peak load occurs rarely, transmission systems usually only utilize about 55 percent of their capacity.

BESS deployments can increase the operational capacity of existing transmission lines without building additional towers and lines. BESSs are flexible and can be re-located and scaled as needed. During contingency events, they are capable of automatically injecting power into the system and providing grid stability. If the grid goes down, the BESS can immediately provide the energy backup and support the restart of critical generators. BESSs can also absorb energy and act as a fast-acting load, helping manage the stability of power supply and demand. The BESS can be charged during low load hours and discharged during peak load time.

A BESS can also increase the efficiency of the distribution network. Today, the distribution grid is burdened with new loads such as EV chargers or distributed energy generation. BESSs can be charged during intermittent local generation and serve power during sudden local load spikes, decreasing the stress on distribution infrastructure. They also participate in the compensation of reactive energy, maintaining the required energy quality in terms of the voltage and frequency and reducing power losses at the same time.

A Second Life for EV Batteries

EVs have been commercially available for more than 10 years, and they are expected to be the dominant vehicle in the future market. But their batteries have a finite lifespan, and it is a challenge to manage old EV batteries. When these batteries are not sufficient for EVs any longer, they are still capable of powering other, less demanding applications. BESSs are a suitable way for old EV batteries to find a second life. These BESSs can be used in applications with less demand for energy and power density.

The primary reason BESSs have not been used on a massive scale thus far is the high cost of investment. The payback periods could exceed the expected service life of the BESS. However, advancements in battery technologies over the last decade have led to decreased costs and enabled the rise of BESS technology. Utilities including power production, transmission, and distribution are the most important BESS users. BESSs are a solution for peak demand, short outages, and new network challenges such as the integration of intermittent renewables.

Although BESSs are still at the edge of profitability in many applications today, they will play a key role in enabling the energy transition from fossil fuel to renewable energy. The experience gained in installing BESSs in the power grid will provide the knowledge and data to help manage the growing numbers of EVs that will tax the grid. It will be interesting to see which innovative BESS applications will be open up in the coming years.