Battery Energy Storage System (BESS) | The Ultimate Guide

Battery Energy Storage System (BESS) | The Ultimate Guide

How Does a Battery Storage System Work?

A BESS accumulates energy from renewable sources like wind or solar panels or from the electric grid and stores it utilizing battery storage technology. The batteries discharge to supply energy when required, such as during peak demand periods, power outages, or grid balancing. Besides the batteries, a BESS necessitates additional components to link to an electrical network.

A bidirectional inverter or power conversion system (PCS) is the central device that converts power between the DC battery terminals and the AC line voltage. This allows power to flow both ways to charge and discharge the battery. Another crucial component is an energy management system (EMS), which coordinates the control and operation of all system elements.

Contact us to discuss your requirements of bess technology. Our experienced sales team can help you identify the options that best suit your needs.

BESS Power and Energy Ratings

To design a battery energy storage system intelligently, it's essential to specify both power ratings in megawatt (MW) or kilowatt (kW) and energy ratings in megawatt-hour (MWh) or kilowatt-hour (kWh).

In circumstances where a significant amount of energy needs to be discharged in a short period, such as in frequency regulation applications, the power-to-energy ratio is generally higher. However, for pricing purposes, the usual measure is the energy rating.

What Is the Battery C-Rate?

A battery's C-rate is the rate at which it can be fully charged or discharged. For instance, a 1C charge rate means the battery is fully charged from 0 to 100% or discharged from 100 to 0% in one hour.

A C-rate higher than 1C signifies a faster charge or discharge, such as a 2C rate, which takes only 30 minutes. Conversely, a lower C-rate indicates a slower charge or discharge—for example, a 0.25C rate means a 4-hour charge or discharge.

The formula for calculating this is:

T = Time, Cr = C-Rate

T = 1 / Cr (hours)

T = 60 min / Cr (minutes)

AC vs. DC-Coupled BESS: The Pros and Cons

Solar panels can be linked to a battery either through alternating current (AC) coupling or direct current (DC) coupling. AC current rapidly flows on electricity grids both forward and backward, whereas DC current flows only in one direction.

DC current is generated by solar panels and stored by batteries, but since appliances use AC current, it must be converted via inverters.

Traditionally, AC-coupled BESSs were more common for residential and commercial solar setups, while DC-coupled systems were used for remote and off-grid installations. However, options for DC-coupled systems have increased, with new streamlined and standardized power electronics equipment being developed.

In recent years, inverter technology has advanced, leading to new AC-coupled and DC-coupled systems.

What Are AC-Coupled Systems?

In AC-coupled systems, solar panels and batteries have separate inverters. This setup enables flexible operation as both the solar panels and the battery can either be dispatched together or independently. They may share an interconnection to the grid or run separately.

AC BESSs typically consist of a lithium-ion battery module, inverters/chargers, and a battery management system (BMS). These units are compact and easy to install, making them popular for grid-connected sites.

However, AC-coupled systems require three current conversions, potentially reducing overall efficiency.

What Are DC-Coupled Systems?

DC-coupled systems usually utilize solar charge controllers to charge the battery from the solar panels, along with a battery inverter to convert the electricity flow to AC.

This setup uses a single inverter for both the solar panels and the battery, sharing the grid interconnection. This reduces equipment costs and power losses from current inversion.

Advantages and Disadvantages of BESS Systems

AC-Coupled Battery Systems

Advantages

• Retrofitting: Easy to add to existing solar panel systems.
• Flexibility: Compatible with any type of inverter.
• Resiliency: Multiple inverters reduce the risk of outages.
• Versatility: Batteries can charge from the grid as well as solar panels.

Disadvantages

• Cost: More expensive due to multiple inverters.
• Lower Efficiency: Multiple current conversions lead to energy loss.
• Supply Limitations: Not designed for off-grid use.

DC-Coupled Battery Systems

Advantages

• Affordability: Cheaper as only one inverter is used.
• Higher Efficiency: Fewer current conversions lead to less energy loss.
• Oversizing: Excess energy can be used for charging batteries or other utilities.

Disadvantages

• Limited Flexibility: Inverter must be close to the battery.
• Less Resiliency: Single inverter means a potential single point of failure.

Which System Should You Use?

For larger or utility-scale plants, AC-coupled systems are generally preferred. Despite being slightly less efficient at charging (90-94% vs. 98% for DC-coupled), they are easier to install, especially in existing setups.

The decision between AC-coupled and DC-coupled systems will depend on specific project needs. Use a PV plant software solution to run simulations and identify the best option.

Interested in learning how to hybridize your PV systems? Check out this webinar and get all the insights on AC-coupled BESS.

RatedPower's platform is designed to optimize and automate the design process, helping you evaluate AC- and DC-coupled BESS options. Contact us to learn how RatedPower can increase your project’s efficiency and profit margins.

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