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Battery-supercapacitor hybrid energy storage system in
standalone DC microgrids: a review
Citation for published version:
Jing, W, Lai, CH, Wong, WSH & Wong, MLD 2017, 'Battery-supercapacitor hybrid energy storage system in
standalone DC microgrids: a review', IET Renewable Power Generation, vol. 11, no. 4, pp. 461–469.
https://doi.org/10.1049/iet-rpg.2016.0500
Digital Object Identifier (DOI):
10.1049/iet-rpg.2016.0500
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Peer reviewed version
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IET Renewable Power Generation
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Download date: 26. Aug. 2022
1
Battery-Supercapacitor Hybrid Energy Storage System in
Standalone DC Microgrids: A Review
Wenlong Jing
*
, Chean Hung Lai, S. H. Wallace Wong, M. L. Dennis Wong
Faculty of Engineering, Computing & Science
Swinburne University of Technology, Sarawak Campus, Malaysia
Email: wjing@swinburne.edu.my
Abstract: Global energy crisis and environmental pollution increasingly promote the application of
Renewable Energy Sources (RES). As a feasible option to overcome the issues of RES integration in
power system such as instability and fluctuation, large scaled Battery Energy Storage System (BESS) and
its associated Energy Management System (EMS) has become one of the most popular research area for
future RES power system. Despite many advantages of integrating BESS in RES based power system, the
highly dynamic fluctuations in generation and demand in standalone RES based Microgrid (MG) causes
damaging impact on lifespan of battery, which greatly increases the operating cost of the standalone MG.
In recent years, the novel concept of Battery-Supercapacitor Hybrid Energy Storage System (HESS),
which contains two complementary storage devices, is been developed to mitigate the impact fluctuating
power exchange on lifespan of battery. This paper critical reviews the latest works related to this area In
terms of topologies, Control and Energy Management System (EMS). A case study of Standalone PV MG
with HESS is presented to show its practicality and the benefits of using HESS are elaborated via Cost
Function. Among the various topologies and related EMS design, the review analysis and discusses the
importance of balancing the trade-off between the technological needs and economic feasibility when
designing a Battery-Supercapacitor HESS in Standalone RES based MGs.
1. Introduction
Global warming and its associated environmental impacts have accelerated the development of
Renewable Energy Sources (RES) and Smart-grid technologies aiming to improve energy efficiency and
to reduce carbon footprint [1][2]. As the increased penetration of RESs, low voltage Microgrid (MG) has
attracted increasing attentions from researchers and power industries due to its unique architecture that
allows highly efficient power generation and distribution in decentralised settings [3]. MG is a small-
scaled, decentralized and autonomous power grid system that may consist of multiple distributed
generations (DG) and/or RESs, end-use customers, Energy Storage Systems (ESS) and power electronic
converters that is operated either in standalone mode or interconnected to the utility grid [4][5][6].
Another advantage of MGs is that it allows power generation and supply to remote isolated
community without the need of long-distanced, costly and inefficient high-voltage transmission and
distribution infrastructures that is not economically feasible [7][8]. However, due to its relatively small
capacity and the intermittent nature of RESs, conventional operations practised in the utility grids may not
be applicable to MGs due to the highly dynamic supply and demand sides [9]. As a result, many works
2
have been carried out to improve the power quality and reliability of the MGs ranging from novel system
topologies [10][11][12] to intelligent power management and control strategies [13][14][15][16].
Unlike the grid-connected MGs that have virtually unlimited and strong support from the high
inertia power supply, standalone MGs rely heavily on ESS to balance the unmatched profiles of generation
and power consumption [17]. The ESS acts as buffer to absorb surplus energy and supply the stored
energy when power deficit. Furthermore, ESSs in standalone MGs also play an important role in
regulating instantaneous power variations, power quality and the system reliability [18]. The nowadays-
available ESS technologies for MG applications are been listed and compared in Table 1:
Table 1 Characteristics of Different ESS Elements [16][19]
Energy Storage System
Energy Density
Power Density
Cycle life
Response time
Cost
Chemical Battery
High
Low
Short
Medium
Low
Sodium-Sulfur (NaS) Battery
Medium
Low
Short
Slow
Medium
Flywheel
Low
High
Long
Fast
High
Supercapacitor
Low
High
Long
Fast
Medium
Superconducting Magnetic
Energy Storage (SMES)
Medium
High
Long
Fast
High
In standalone MG, the power flows in and out of the ESS elements varies widely depending on the
instantaneous power generation and load conditions [20]. In general, the power exchange in ESS can be
categorised into high-frequency components such as sudden surge in power demand or intermittent solar
power generation on a cloudy day, and the low-frequency components such as natural behaviour of RESs
or daily average energy consumption [21]. The high-frequency power variations generally require ESS
elements with high power density with fast response time, while low frequency or long-term power
variations prefer high energy density and low cost ESS elements.
Based on the characteristics of different ESS elements as shown in Table 1, none the ESS
technologies fulfils all the desired characteristics to respond to high and low frequencies power variations
in standalone MG applications [22]. Therefore, using single type of energy storage element in standalone
MG applications limits the potential of what ESSs can offer. Among all HESS combinations, Battery-SC
HESS has been the popular combination in HESS researches because of their wide availability, relatively
low cost compared to other ESS elements, similarity in working principle and most importantly, they
complement each other’s weaknesses rather beautifully.
HESSs have been actively investigated in other high energy demand applications such as Electric
Vehicles (EVs) and Hybrid Electric Vehicles (HEV) and have shown great performance improvement in
3
many aspects, for example, optimising the energy recovery from regenerative braking, improving the rate
of charging and prolonging the service life of battery by reducing the strain of deep discharge [23].
However, HESSs in grid scale applications are mostly still in research stage [24][25][26]. HESSs typically
couple to the power network via AC or DC coupling with the aids of power electronic converters to
control the power flow of different ESS elements [27][28][29]. Though modern power electronic
converters allow energy sources of different characteristic to hook up together, it also increases the system
complexity and cost [30]. Hence, the trade-off between economic feasibility and technical advantages exist
and it is crucial in determining the financial and technical sustainability of the system.
Various Battery-SC HESS topologies had been proposed in MG applications aiming to optimally
utilise the benefits of different ESS elements [31][32]. Besides having correct HESS topology and
appropriate sizing of different ESS elements, energy management and control strategy of HESS is another
key to improve system efficiency, maximise energy throughput and prolong lifetime of HESS
[33][34][35][36][37].
This paper reviews the current trends of Battery-SC HESS in renewable energy based standalone
MGs, including existing HESS topologies, energy management strategies and control algorithms. The rest
of the paper is organised as follows. Section II presents the different HESS topologies available today for
high power storage applications with a comprehensive analysis of HESS in standalone MGs. Section III
reviews existing energy management strategies including control goals, power allocation strategies and
safety measures. A case study that locates Kuching, Sarawak, Malaysia is presented in Section IV and it
shows the fundamental methodology that how to evaluate HESS via Cost Function. Section V gives a
thorough review of different control algorithms in energy management system and evaluation of their
effectiveness, economical and technical viability for HESS in MGs and its future trend are also included in
this section. Finally, the paper is concluded in Section V.
2. Battery-SC HESS Topologies
In Battery-SC HESS, the two complementary ESS elements are typically connected to a common
DC or AC bus [38][39][40]. For RES based standalone MGs with ESS, coupling through common DC bus
is the preferred choice due to many reasons [41][42]. Firstly, most of the common ESS elements and RESs
operate in DC voltage, thus minimises the needs of power converter [43]. Also, DC bus does not require
synchronization which greatly reduces the complexity of the overall system [44][45]. As a result, DC
coupling is more efficient and lower cost than equivalent AC bus systems because of lower power losses
4
and the needs of high power components [46][47][48]. In general, Battery-SC HESS can be categorized
based on their connection topology as shown in Fig. 1.
Battery-SC
HESS
Passive HESS
Semi-Active
HESS
Full Active
HESS
SC Semi-Active
Battery Semi-Active
Parallel
Cascaded
Fig. 1. Classification of the Battery-SC HESS Connection Topologies [49][50]
2.1. Passive HESS
Passive connection of battery and SC to the DC bus provides the simplest and cheapest structure of
HESS. Passive Battery-SC HESS has been demonstrated to effectively supress transient current under
pulse load conditions, increase the peak power and reduce internal losses [51][52][53][54]. As shown in
Fig. 2, the battery and SC are connected to the DC bus directly and they share the same terminal voltage
that depends on the State-of-Charge (SoC) and charge/discharge characteristic of battery. In some rural
MG applications, the battery capacity is decided assuming three to five days as reserve without any
external source of energy [55]. Consequently, the battery will be cycled approximately 20% Depth-of-
Discharge (DoD) and charged/discharged in a relatively low C-rate due to the large capacity. As a result,
the fluctuation in DC bus voltage will be minimal, ensuring a relatively stable system voltage.
Batt SC
DC Bus
Fig. 2. Passive HESS Topology
However, in passive connection of HESS, the system current will be allocated to these two ESS
elements based on their respective internal resistance [54]. In facing of the uncontrolled power flow, the