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What is a Hybrid Energy Storage System (HESS)?

energy storage systems

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.

Hybrid energy storage systems (HESS)
Hybrid Energy Storage System

Types of Energy Storage Technology


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:


Check MarkElectrical 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.

Check MarkHigh power density
Check MarkFast response times
Check MarkHigh efficiency


Check MarkMechanical 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.

Check MarkLow cost
Check MarkSystem voltage control
Check MarkHigh energy


Check MarkChemical 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.

Check MarkIncreased lifetime
Check MarkLow cost
Check MarkHigh efficiency


Check MarkThermal 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.

Check MarkClean energy
Check MarkLow cost
Check MarkMarket-ready technology


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


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 HighElectrochemical Cells. Green Tick HighElectrochemical Cells. Green Tick Low HighElectrochemical Cells. Green Tick Low HighElectrochemical Cells. Green Tick Low
Energy Density Low Low HighElectrochemical Cells. Green Tick HighElectrochemical Cells. Green Tick HighElectrochemical Cells. Green Tick Moderate HighElectrochemical Cells. Green Tick
Costs Moderate High LowElectrochemical Cells. Green Tick LowElectrochemical Cells. Green Tick LowElectrochemical Cells. Green Tick Moderate LowElectrochemical Cells. Green Tick
Reaction time FastElectrochemical Cells. Green Tick FastElectrochemical Cells. Green Tick FastElectrochemical Cells. Green Tick Slow Slow FastElectrochemical Cells. Green Tick Slow
Cycle Lifetime HighElectrochemical Cells. Green Tick HighElectrochemical Cells. Green Tick Moderate Moderate HighElectrochemical Cells. Green Tick HighElectrochemical Cells. Green Tick
Efficiency Moderate HighElectrochemical Cells. Green Tick Moderate/HighElectrochemical Cells. Green Tick Low Moderate HighElectrochemical Cells. Green Tick HighElectrochemical Cells. Green Tick
Type of Demand Short-term Short-term Short/Mid-term Long-termElectrochemical Cells. Green Tick Long-termElectrochemical Cells. Green Tick 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.


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


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:

Energy storage system timeline

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References


Contributors


Written by

Dr. Amelia Wood

Application Scientist

Diagrams by

Sam Force

Graphic Designer

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