Hybrid renewable energy systems (HRES) are becoming more commonplace, are well-developed and well defined today. A critical component of the HRES system is the energy storage system (ESS). Most ESS are designed for a single function, whereas the emerging requirements for HRES storage systems are to perform multiple functions. The hybrid energy storage system provides an elegant solution to the problem.
HRES are a well-established but still developing technology, finding application in more and more areas. Originally applied to remote stand- alone off grid systems to integrate power from diverse sources such as wind, solar and other renewable sources, with that from prime mover or other non-renewable sources, HRES has extended to such areas as behind the meter grid connected systems, microgrids (MG) and other applications. Early systems made limited use of electricity storage systems (ESS), possibly because of high costs and technology limitations, and the function of the ESS was limited to bridging short term variations in output of the other devices.
Development of ESS has produced improved performance, a wide variety of technologies as well as a great reduction in cost. This has led to the ESS playing a much larger role in HRES, to the point where the ESS, together with the associated energy management system, is a core component of any advanced HRES. Price reduction and technology development in ESS have allowed HRES to become less dependent on liquid fuel driven prime movers, to the point where HRES consisting solely of renewable energy sources and ESS are economically viable as well as being technically achievable.
This however, places more complex requirements on the ESS, which must now perform numerous functions other than simply storage, a requirement which cannot be met by using a single technology. Meeting these requirements needs a device which can deliver a high-power level for short periods, (high power density) and can also deliver a lower power for long periods (high energy density). Most ESS cannot simultaneously meet the requirements in both power density and energy density, and hybrid energy storage systems have been developed as a compromise scheme in many applications. Using different ESS technologies, together with advanced energy management systems (EMS), in a hybrid configuration allows both requirements to be met, while providing high level management and optimisation of the energy flows in the HRES. A typical HRES system using HESS is shown in Fig. 1.
The concept of the hybrid energy storage system (HESS) relies on the fact that heterogeneous ESS technologies have complementary characteristics in terms of power and energy density, life cycle, response rate, and other characteristics. A HESS is characterised by a beneficial coupling of two or more energy storage technologies with complementary or supplementary operating characteristics. In a typical HESS one storage unit is dedicated to cover “high power” (HP) demand, such as transients and fast load fluctuations and therefore is characterised by a fast response time, high efficiency and high cycle lifetime. The other storage will be the “high energy” (HE) storage with a low self-discharge rate and lower energy specific installation costs.
It is beneficial to hybridise ESS technologies in the way that synergise the functional advantages of two or more heterogeneous existing ESS technologies. This hybridisation provides excellent characteristics not offered by a single ESS unit . Many of the techniques applied for control of HESS systems were developed in the EV industry and have been effectively adapted for HRES applications.
The main advantages of a HESS are:
To take full advantage of the characteristics of the complementary ESS requires an energy management and control system, which in advanced systems also controls the energy flows in the HRES. In essence the HESS has moved from a subsidiary part to become the heart of the HRES. To understand the concept of the HESS, it is necessary to consider hybridisation principles and proposed topologies, power electronics interface architectures, as well as control and energy management strategies.
HESS design and overview
Designing a HESS system is fairly complex process and requires a number of issues to be taken into account. Fig. 2 gives an overview of the process.
Performance requirements of HESS in HRES
Current HRES system design places a number of functions on the HES, in addition to basic energy storage, particularly in stand-alone or islanded mode. These functions are performed by the ESS management system using stored energy. The two main active functions of the HESS are:
Many different technologies have been considered for HESS, but not all are suitable for the range of operations contemplated for HRES, being more suited for large grid or distribution system applications . Table 1 lists the common technologies used in HESS systems [1, 2].
|Technology||Power rating (MW)||Power density (W/l)||Discharge time||Energy Density (Wh/l)||Lifetime (cycles)|
|Fuel cell||<50||0,2-20||Sec-24 hr2||600||>106|
|Super capacitor||<0,3||(4-12)*104||ms -1hr||10-20||>106|
|VrB||0,03-3||0,5-2||sec-10 hours||20-70||>10 000|
1. SMES was developed mainly for large power (grid) applications, in the MW range and does not normally feature in Hybrid systems. There is a move to develop the technology for smaller (kW) ranges.
2. Fuel cells can run indefinitely depending on the fuel supply, but are not rechargeable unless part of renewable hydrogen plant.
As can be observed from Table 1, ESS technologies can be classified into two main categories namely high power (HP) and high energy (HE) technologies. High power storage systems supply energy at very high rates, but characteristically for short time periods. Superconducting magnetic energy storage (SMES), super capacitor, flywheel, and high-power batteries belong to this category. Technologies used for the HE component include lead-acid, Li-ion and other chemical types, flow batteries and fuel cells. Fuel cells (FC) are and would not normally be incorporated into a HRES as a storage unit as they are not rechargeable unless part of renewable hydrogen plant.
Table 2 categorises ESS technologies based on this classification. It should be noted that battery technologies can be employed either in high power or high energy applications devices due to their extensive characteristics range.
|High power devices (fast response)||High energy devices (slow response)|
|Super capacitor||Fuel cell|
|Flywheel||Flow battery (VRB)|
In a hybrid system the high power device should supply short term power needs, while the high energy devices meet the long term energy needs. Based on this idea, possible combinations are listed in Table 3. It should be noted that, all combinations shown in the table are not feasible in terms of technical and some system level limitations.
|HESS technology||HESS application|
|Energy supplier (HE)||Power supplier (HP)||Renewable energy|
|Battery||Supercapacitor (SC)||General microgrid|
|Flow battery (FB)||Supercapacitor||General microgrid|
Batteries, particularly Li-ion batteries, play a key role in many HESS-applications. They can be utilised both as the “high energy” or the “high power” storage. Super capacitors (SC) and flywheels are characterised by even higher power densities, efficiencies and cycle lifetimes compared to batteries. Redox-flow batteries are a promising technology due to their storage immanent decoupling of power and stored energy (similar to the hydrogen and power-to-gas storage path) and due to their good cycle lifetime and recycling capability. Renewable hydrogen (H2) and methane (CH4) are both very promising options for long-term energy storage. Also heat storage and power-to-heat concepts will gain importance in the context of future HESS-applications.
The most common choice for small systems is a lead-acid, Li-ion battery or other battery in combination with a super- or ultra-capacitor. Other common configurations include FC/SC, FC/B and FB/SC or FB/B combinations. Flywheels and SMES are not generally used in smaller HRES applications but find application in larger microgrids.
Out of all HESS technologies, the hybridisation of batteries and supercapacitors is the most common and has been studied by many researchers. Supercapacitors are also called ultra- capacitors or in some literature electrochemical double-layer capacitors (EDLC) are energy storage devices with high specific power, typically above 10 kW/kg and low specific energy, typically below10Wh/kg. Also, they possess very high cycle life typically above 500 000 cycles. The combination of flow battery/SC or FB/B would be applied to larger HRES systems with a requirement for long duration backup and would more likely find application in the microgrid sector.
The first HESS systems operated on a simple parallel connection of the high power (HP) and high energy (HE) energy systems. Subsequent research has shown that improved performance can be achieved by adopting other configurations. Several are discussed in the following section (Fig. 4).
There are different ways for the coupling of the energy storages in a HESS. A simple approach is the direct DC coupling of two storages. (Passive topology) The main advantage is the simplicity and cost-effectiveness. Moreover, the DC-bus voltage experiences only small variations. The main disadvantage is the lack of possibilities for power flow control and energy management and a resulting ineffective utilisation of the storages (e.g. in a SC/B HESS) with direct coupling only a small percentage of the SC capacity can be utilised when operated within the narrow voltage band of the battery).
The second energy storage coupling architecture in a HESS is via one bidirectional DC/converter (Semi-active topology). The converter can either be connected to the HP or to the HE storage. In the latter case the HE storage can be protected against peak power and fast load fluctuations. The DC/DC converter then operates in current-controlled mode. A drawback of this solution is the fluctuation of the DC-bus voltage, which is identical to the voltage of the HP.
The third and most promising coupling architecture consists of two DC/DC-converters. Here the parallel converter topology is very common. The DC/DC converter associated with the HP storage manages voltage regulation of the DC-bus. It helps to operate the HP storage in a broader voltage band, and hereby the available storage capacity is better utilised.
Besides the parallel converter topology also a serial, cascade-type of converter topology is possible, which is generally more expensive and more difficult to be controlled. Disadvantages of the two-converter coupling architecture are higher complexity and slightly higher costs. There are isolated and non-isolated DC/DC converter topologies available for HESS-applications (e.g. buck/boost, half-bridge, full-bridge) with the trend to highly efficient and cost-effective multi-port converters with a reduced number of conversion stages .
HESS capacity sizing
One of the most important issues in HESS applications is to determine the appropriate storage capacity. Various methods have been proposed for storage capacity sizing. Some methods are developed to determine the HESS capacity of a particular technology and some other, regardless of technology, can be used for sizing all types of storages. In HESS sizing procedure the total cost and the reliability of the system should be considered. Sizing methods based on the purpose of HESS application may be different. Storages capacity sizing techniques can be classified into analytical methods (AM), statistical methods (SM), search-based methods (SBM), pinch analysis method (PAM) and Ragone plot method (RPM) as shown in Fig. 5 .
HESS management and control systems
Designing and implementing a proper control system is the most important issue in HESS. The choice of the appropriate control method for HESS depends on different parameters: The purpose of the use of HESS (such as storage life extension, power quality, intermittency improvement and etc.), the type of system (DC MG, AC MG, grid-connected), the cost of the control method, the control method response time, the hybridisation architecture etc. To achieve the safe, stable, and efficient operation of HESS, a reasonable power-sharing strategy is essential.
Generally, the energy management and control system of HESS can be classified into two parts: Firstly, the underlying control unit that controls the current or power flow of HESS elements based on the reference signal generated by energy management control unit. Secondly, the energy management unit which performs power allocation between the HESS storages to enhance system dynamics, reach high overall efficiency, monitor SoC, reduce the loss and cost of system, minimise operational cost of the system, frequency regulation etc. Fig. 6 shows the classification of energy management and control methods developed for HESS.
In general, energy management methods can be categorized into intelligent control methods and classical control methods. Classical control methods can be divided into the rules-based controller (RBC), droop-based control (DBC), and filtration-based controller (FBC). These control methods are sensitive to changes in parameters and the exact mathematical model of the system is required. In RBC, power allocation of HESS is performed according to the pre-defined rules. RBC can be divided into thermostat, state machine, and power follower control methods. The RBC controller is simple and easy to implement and is an effective method for real-time energy management. Nevertheless, the sensitivity to parameters variation is the disadvantage of this method. In FBC, a filter is used to decompose net power to the high and low frequency. In most MG applications power-sharing between hybrid storages is based on FBC control. In this method, the storages reference powers are determined based on the filter parameters. Scheduling the reference current among ESS is characterised by different ramp rates, low pass filters, and high pass filters are normally utilised to share power demand into low/high frequency components.
Conventional droop control methods cannot be directly used in HESS control. In recent researches about HESS control, various types of DBC methods such as: high-pass filter-based droop control, extended droop control strategy , adaptive droop-based control , integral droop (ID) control , virtual capacitance droop , virtual resistance droop, virtual capacitance droop with SoC recovery , adaptive droop , secondary voltage recovery droop , and virtual impedance droop have been considered .
To control nonlinear and complex systems, optimisation based methods have become more popular. These methods can be categorised as the artificial neural network (ANN), fuzzy logic control (FLC), evolutionary algorithms such as genetic algorithm, dynamic programming, linear programming (if the system is convex and could be mathematically represented via a set of linear functions), and model predictive control (MPC) .
 R Hemati: “Emergence of hybrid energy storage systems in renewable energy and transport applications: A review”, Renewable and Sustainable Energy Reviews, November 2016.
 T Bocklish: “Hybrid energy storage systems for renewable energy applications” Energy Procedia, 2015.
 S Hajiagashi: “Hybrid energy storage system for microgrids applications: A review”, Journal of Energy Storage, 2019.
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