What is a Hybrid Energy Storage System (HESS)?

A hybrid energy storage system (HESS) is defined by the combination of two or more energy storage technologies within one operating system. This helps combine the benefits of the different technologies as well as resolve the issues faced by the individual energy storage solutions. An energy storage system must be reactive and flexible depending on demand which can vary considerably. As a result, within a fit for purpose HESS system there are storage components dedicated to “high power” demand such as supercapacitors and others dedicated to “high energy” demand such as batteries. A well balanced HESS improves overall performance, enhances efficiency, increases the lifespan of the system, and reduces operational costs.

Types of Energy Storage Technology

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There are many types of energy storage technology that have been investigated and developed by researchers across the globe. While modern research has focused on the harnessing of renewable energy, current energy generation relies on fossil fuels. In order to successfully integrate renewable energy sources into our complex powergrids reliably energy storage systems must be used. This section describes the major types of energy storage systems and their advantages:

Electrical energy storage systems (EESS) provide storage of electrical energy so that it can be used later. They include battery energy storage systems (BESS) such as lead-acid, flow and lithium-ion batteries. As well as electrostatic systems such as supercapacitors and superconducting magnetic energy storage.

Mechanical energy storage systems (MESS) are based on some of the oldest forms of energy manipulation. The technology includes systems that store potential energy including pumped hydro storage and compressed air storage. Another example is flywheel energy storage which stores kinetic energy.

Chemical energy storage systems (CESS) exploit the energy that is stored in the chemical connections between atoms and molecules. Specific CESSs include fuel cells, as well as hydrogen, ammonia, biofuel and aluminum storage.

Thermal energy storage systems (TESS) store heat (or cold) in an insulated tank that contains some sort of energy transfer media. Thermal energy can be stored in the form of latent heat, sensible heat and reversible thermochemical reactions.

High-power components

Flywheel, superconducting magnetic energy, BESS, supercapacitor energy storage are all considered high-power components within storage systems. They have fast response times (few sec/min) and high power density. They can used for voltage stabilization and improve the power system dynamic response.

High-energy components

Pumped hydro, compressed air energy storage, fuel cells and BESS are all considered high-energy components within storage systems. They have slow response times but higher energy density so can be used for long-term energy systems.

Principles of Hybrid Energy Storage Systems

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All energy storage systems are designed to harvest energy from various sources, transforming and storing the energy until it is needed. As mentioned, there are many types of energy storage systems that each have different advantages. There is yet to be one energy storage system that can cope with the real-world flux in demand. Therefore we must consider all the features of each storage system and which combination of systems can work together most efficiently both to meet energy demand and cost feasibility.

The features that need to be considered when selecting the appropriate combination of energy storage technologies includes:

Feature	Definition
Capacity	Total amount of energy a system can store
Power	How quickly the stored energy can be discharged (or charged)
Efficiency	Amount of energy lost during the storage period or during energy transfer
Reaction time	How long it takes between turning on to discharge starting
Storage period	How long energy can be stored
Lifetime	How long the energy storage system works for
Cycle Lifetime	How many times the energy storage system can be cycled
Cost	Manufacturing and operational costs

The table below compares some of the most popular energy storage systems based on some key features:

	Electrical			Chemical	Mechanical		Thermal
Feature	Supercapacitor	Superconducting Magnet	Battery	Hydrogen	Compressed Air	Flywheel	Sensible heat storage
Power Density	High	High	Low	High	Low	High	Low
Energy Density	Low	Low	High	High	High	Moderate	High
Costs	Moderate	High	Low	Low	Low	Moderate	Low
Reaction time	Fast	Fast	Fast	Slow	Slow	Fast	Slow
Cycle Lifetime	High	High	Moderate	Moderate		High	High
Efficiency	Moderate	High	Moderate/High	Low	Moderate	High	High
Type of Demand	Short-term	Short-term	Short/Mid-term	Long-term	Long-term	Short-term	Short/Mid-term

From the comparison of these key features the best combinations for the energy and power demands of a specific grid can be established. Batteries broadly perform the best as energy storage systems but sometimes need some support when it comes to short and intense power bursts or long-term supply.

Popular Hybrid Energy Storage System Combinations

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Supercapacitor and Battery

The combination of supercapacitor and battery has been widely investigated as a method to extend the lifetime of batteries. The supercapacitor can provide short but intense bursts of power, reducing the number of required battery cycles. Even as battery prices fall, the integration of a supercapacitor within a HESS containing a battery significantly reduces costs. Such HESSs are investigated for use in electric vehicles.

Superconducting Magnet and Battery

Typically, the superconducting magnetic energy storage (SMES) component has a higher power density and faster response time. The battery component has a higher energy density and longer discharge time. A properly designed and controlled SMES/battery HESS performs better under both short and long-term variations, minimizing the total investment costs and maximizing the system's reliability.  

Flywheel and Battery

The flywheel component represents the power dense energy storage component that can handle power fluctuations with a low volume and high frequency. This again is balanced by the battery components ability to fulfil high energy and low frequency power fluctuations. This type of HESS is used in grid frequency regulation and renewable energy support.

Compressed Air and Supercapacitor

This hybrid system combines compressed air energy storage (CAES), which stores energy mechanically by compressing air, with supercapacitors, which provide fast bursts of power and quick response times. Together, they balance high energy storage capacity (from CAES) with high power density and rapid response (from supercapacitors). This combination is well-suited for grid balancing and renewable energy smoothing.

Thermal Energy Storage and Battery

This system pairs thermal energy storage (TES), which stores energy as heat, with batteries, which store energy electrochemically. Thermal storage is useful for long-duration, low-cost energy storage, while batteries offer faster response times and higher round-trip efficiency. Combining them allows for both cost-effective bulk energy storage and fast-response power delivery, making it useful for applications like solar power plants and industrial energy management.

Timeline of Energy Storage Systems

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The range of energy storage systems available to use in combination has dramatically increased in the last 50 years. The selection has expanded along with the range of energy production methods. The image below shows a timeline of the development of energy storage systems:

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