A hybrid energy storage system (HESS) means that two or more energy storage technologies or energy storage resources work together. The reason is that using the best qualities of each technology makes it possible to have a better power performance, energy capacity, efficiency, reliability or battery life than just one single storage technology would be capable of.

In the field of practical renewable energy projects, the term can be used in a wider meaning to refer to a system combining solar PV, battery storage, the electric utility grid and sometimes even the diesel generator. The exact configuration will depend on the project and its energy needs.

How Does a Hybrid Energy Storage System Work?

The principle of hybrid energy storage system is quite easy: different energy resources deal with different operational conditions.

For example, lithium-ion batteries are perfect for storing energy for several hours, but supercapacitors are great for responding quickly to short-term power changes. With a hybrid system there is no need to make any of energy storage technologies responsible for all the loads.

Typical HESS is made up of:

• Energy storage devices like lithium-ion batteries, supercapacitors or flow batteries
• Power conversion systems (PCS) controlling charge-discharge processes
• Battery management systems (BMS) for monitoring battery voltage, temperature, current and state of charge
• Energy management systems (EMS) for establishing how different energy sources should operate
• Control and protection equipment to ensure safe and stable system operation

As for research on HESS, one can note that control is always stressed to be as important in it as the hardware itself. The EMS establishes which storage technology will be responsible for dealing with specific power load.

Common Hybrid Storage Configurations

There is no universal configuration of a hybrid energy storage system. Several different combinations are possible.

Battery and Supercapacitor

It is one of the most popular and most researched HESS configurations.

The battery stores energy during several hours and the supercapacitor deals with fast changes in power load. This allows to avoid high-frequency charge and discharge of the battery.

This configuration is useful for the projects with frequent power fluctuations since supercapacitors are able to deliver and absorb power very quickly and resist a very high number of cycles.

Battery and Flow Battery

While lithium-ion batteries can provide high power density and flexibility, flow batteries are suitable for the applications requiring long-duration energy storage and cycling ability.

This kind of hybrid configuration can be chosen if the project has both short-duration power needs and energy-shifting requirements.

Solar PV and Battery Storage

Another meaning of hybrid energy storage system is a combination of solar PV with battery energy storage in commercial and industrial projects.

During the periods with high solar generation, extra electricity is charged in the battery. Stored energy can be used in cases of low generation of solar energy or high electricity rates.

The hybrid PV-BESS systems are studied quite widely in the applications including renewable energy integration, peak-load management, energy-shifting and improved power quality.

Why Use a Hybrid Energy Storage System?

The main advantage of such a system is that different technologies can complement each other.

One single battery system has to balance energy capacity, power output, cycling life, price and response speed. However, improving one aspect requires trade-offs in others.

The hybrid system allows dividing these aspects.

Key advantages of a hybrid system are:

1. Improved power response
   High-power storage technologies can respond quickly to the abrupt changes of load.
2. Reduced battery stress
   Secondary storage technology can deal with short high-power loads without unnecessary battery cycling.
3. Better integration of renewables
   Hybrid storage system can smooth the variability of solar or wind generation and improve stability of power supply.
4. More operational flexibility
   Different storage technologies can be managed depending on load conditions, electricity prices and renewable generation.
5. Increased life time of the system
   Correct energy management can decrease the stress on each storage technology.

Though, a hybrid system does not always mean the better one. Adding another storage technologies, additional converters, controls and protections increases complexity and cost of the system. The system should be designed based on actual load conditions.

Hybrid Energy Storage vs Hybrid Energy System

Although these two concepts are sometimes used synonymously, they do not always have technical equivalence.

Hybrid energy storage system usually denotes combining of several storage technologies like battery and supercapacitor.

Hybrid energy system, on the contrary, means the combination of different energy sources and storage technologies. For example:

Solar PV + Battery Storage + Grid + Diesel Generator

Such a configuration is especially useful for remote facilities, microgrids, temporary power, telecom power systems and commercial and industrial energy projects.

Solar power generates electricity during the day and the surplus is charged into the battery. The battery supplies the load in case of insufficient solar generation and the grid or diesel generator supplies it when the renewable and storage resources cannot satisfy the demand.

This configuration is much more relevant for practical commercial energy projects than just battery-supercapacitor combination.

Where Are Hybrid Energy Storage Systems Used?

Technology of hybrid energy storage system can be implemented in several situations:

• Commercial and industrial energy storage systems
• Renewable energy and microgrids
• EV charging infrastructure
• Remote and off-grid power systems
• Grid stabilization
• Peak shaving and load management
• Telecom power systems
• Backup power applications

In the recent researches, HESS application in microgrids is investigated again and again since combining of different storage technologies helps to solve problems connected with renewable intermittency and load variations.

How to Choose the Right Configuration

Based on my experience, it is not choosing of the most sophisticated storage technology which is important. It is choosing the correct combination taking into account actual load profile.

Prior to designing the hybrid energy system, engineers should evaluate:

• Peak and average load
• Backup duration
• Daily energy consumption
• Profile of solar generation
• Frequency of charge/discharge process
• Required response speed
• Grid conditions
• Tariff structure of electricity
• Space available for installation
• Expected operating life

In case of the project needing several hours of energy-shifting, traditional lithium iron phosphate (LiFePO4) battery system is enough. In case of frequent high-power fluctuations, the more advanced HESS is more reasonable.

Hybrid Energy Storage Solutions by Highjoule

In Highjoule, hybrid energy storage solutions can be provided in accordance with project needs and not just one fixed configuration.

Depending on the application, the solution can include battery storage, solar PV, grid power, diesel generation and energy management. The goal is to balance energy capacity, power output, reliability, cost of operation and life time of the system.

For commercial, industrial, microgrid, telecom and off-grid projects, the correct configuration should be selected based on actual load profile.