How do Battery Energy Storage System (BESS) Work?

Energy Storage Systems (ESS) store energy from the source for future use. The systems are particularly useful to store energy harvested from renewable sources of energy. While there are many types of ESS that use different storage technologies, Battery Energy Storage System (BESS) uses electro-chemistry to store the energy in specially designed batteries .

It is common knowledge that renewable sources of energy currently struggle to meet dynamic power demand consistently. Hence, the basic principle of any energy storage system (let alone BESS) is not different from the famous ant and grasshopper story. Similar to the ants, we store the harvested energy when available and accordingly use it to meet our seemingly primal needs. This prevents any excess stress on the power supply system particularly during peek demand.

Composition of Battery Energy Storage Systems

Although a BESS is designed for large-scale commercial applications, going by first principles, it is not different from a simple power bank. Firstly, let us look at the components of a simple power bank module:

1. A portable battery to store energy
2. A charging module to charge the battery
3. A voltage converter to discharge the battery at the desired output voltage
4. A switch to control the charging and discharging operation

Fig 1 — Circuit Diagram of the DIY Power Bank [1]

To know more about how the circuit operates, visit DIY Power Bank Circuit by Arvind Raghupathy.

The most essential component of a BESS is, well, the battery system. Unlike a power bank, a BESS comprises multiple battery packs collectively packed into a humongous building like structure. And, each battery pack further consists of numerous batteries uniquely arranged to store maximum possible energy. Hence, the term “battery system”. However, since they are still batteries!

Similar to the charging module in the power bank, an energy management system monitors and controls the charging mechanism of the entire battery system. The major difference is in the complexity of operation. The energy management system handles charging as well as detects the state-of-charge and battery health. For more details on the functionality, read Battery Management System (BMS) : The Manager and Battery Management System in Smartphones.

Most electrical appliances used in our daily run on alternating current (AC). However, batteries supply direct current (DC). which unfortunately is not suitable to power our home. Thus, a power conversion system (analogous to the voltage converter in the power bank) typically consisting of inverters and transformers are used. While inverters convert DC to AC, transformers convert and set the AC energy to appropriate voltage and current levels.

Fig 2 — Components of BESS [2]

Types of Batteries Used

Lead Acid Batteries

• Flooded Type — The electrolyte is an aqueous solution of sulphuric acid.
• Valve Regulated Lead Acid (VRLA) — Unlike the freely flowing electrolyte in flooded type, the electrolyte is not liquid and immobilized. This increases the longevity of the cell. Furthermore, the pressure regulating valve dictates the output voltage.

Lithium Based Batteries — Depending on the cost, energy and power densities, different BESS plants use different types of lithium batteries . Some common examples are lithium-ion, lithium-air, lithium-sulphur, Lithium Nickel Manganese Cobalt Oxide (NMC) and lithium iron phosphate (LFP).

Metal Air Batteries — Anodes comprise high energy density metals that release electrons when oxidized. Cathodes are porous graphite/carbon rods with suitable catalysts enabling air flow. Thus the term “metal air”.

Flow Batteries — This type consists of two large tanks containing two different electrolytes. The electrolytes are then pumped through a chamber consisting of electrodes (cathode and anode) to generate electric current. Flow batteries are generally used for renewable energy storage applications like energy from solar photovoltaics and wind.

• Vanadium Redox Flow battery
• Regenerative Fuel Cell
• Zinc Bromin (ZnBr)

The primary factor governing battery selection is the purpose of installing battery energy storage system plant. To put it in another way, which application will the BESS plant be used for? Moreover, the capacity and physical construction of BESS dramatically vary with the type of battery. Other factors include regulatory issues concerning the installation site, safety, available financial and human capital and any socio-political issues.

Due to the high storage capacity lithium based batteries are increasingly becoming the de facto choices for most BESS plants. However, the batteries are extremely sensitive to changes in the neighbouring surroundings thereby demanding first-rate charge management and power conversion systems.

What to Ask an Expert and How to Judge a Battery Energy Storage System?

In a power bank, what would make you say, “This one is good. I’ll buy it!”

It’s a no brainer that you start by asking the capacity (in kW-h) and how long it lasts after one full charge? Likewise, there are standard parameters which describe a BESS plant.

1. Round Trip Efficiency — It is the ratio of energy (in MW-h) put into the system to energy (in MW-h) extracted from the system. Obviously, a higher ratio indicates a lower loss of energy and an efficient system. Typically, grid engineers consider any value above 80% as good.
2. Response Time — The time taken for the system to react to a change either in the input energy or the output demand. A faster response time depicts a flexible system that can adapt quickly to changes.
3. Ramp Rate — It is the rate at which the system’s output power varies or can be varied — ramping up (increasing) or ramping down (decreasing). Generally, the output varies due to variation in the power demanded from the system. Ideally, a BESS plant with a high ramp rate and low response time (a few seconds) is a job well done!
4. Energy Retention Time — It indicates the amount of time that the system retains its energy, particularly during no or negligible power demands. Systems with higher retention time, store energy for longer time-periods thereby requiring less energy for recharging.
5. Battery Parameters — Since the crux of BESS is a battery, battery characteristics play a major role in determining the system’s performance. I’ve described the following parameters in detail in Pick the Best: Metrics to Select Batteries.

• Life Span — For BESS, it is the total number of charge-discharge cycles beyond which batteries lose their storage capacities and become unusable.
• Energy and Power Density
• Self-Discharge Rate
• Ambient Temperature

Economic Feasibility of a Battery Energy Storage System

No matter how good a technology is, if it cannot be realized, it is as good as a museum artifact.

1. Cost of the storage element — Although battery costs have been constantly declining they are still not cheap!
2. Integration with existing technology — Every new technology has to be backward compatible. This dictates the cost associated with companies licensed to distribute electricity.
3. Availability of commercial services to increase revenue streams thereby enabling continuous running of the plant.
4. Appropriate policy developments to prevent accidents and ensure safety of all the stakeholders involved.

Risks Involved

The biggest risk accompanying a battery storage energy system is explosion of batteries. Even with massive improvements in protection and safety systems, this risk cannot be eliminated completely. One way is to enforce strict policies to ensure overall safety without compromising performance.

Another challenge is the deluge of batteries. While developing batteries for renewable energy storage cuts emissions, it also demands complete recycling of batteries to prevent the birth of a new conundrum.

Conclusion

Despite the risks, battery energy storage system is one of the best techniques to store energy reliably. Moreover, they accelerate transition towards carbon-free energy systems thereby cutting carbon dioxide emissions.

Currently, the commercial realization of BESS plants is still in its infancy primarily due to the high admission cost. However, as the battery prices continue to fall and reach an acceptable range, the technology will redefine energy storage.

Thank you for your time!

References

[1] Arvind Raghupathy, “DIY Power Bank Circuit”, Circuit Digest, Jan, 2018

[2] “Handbook on Battery Energy Storage System”, Asian Development Bank, Dec, 2018