https://doi.org/10.37815/rte.v36n1.1117

Original Articles

Comparative study of dynamic response by three
types of energy storage systems for grid studies: A Microgrid Laboratory
experimental-based study

Estudio
comparativo de la respuesta dinámica de tres tipos de sistemas de
almacenamiento de energía para estudios de red: Experimentos reales en un
laboratorio de Micro-Red

Danny Ochoa-Correa1 https://orcid.org/0000-0001-5633-1480,

Edisson Villa-Ávila1  https://orcid.org/0000-0002-2766-5913, Vinicio Iñiguez-Morán1 https://orcid.org/0000-0001-7494-191X, Patricio
Astudillo-Salinas1 https://orcid.org/0009-0000-3485-3472

1Universidad
de Cuenca, Cuenca, Ecuador

[email-masked], [email-masked], [email-masked], [email-masked]

Sent:       2024/01/12

Accepted:       2024/06/06

Published:       2024/06/30

Abstract

Summary: Introduction,
Materials and Methods, Results and Discussion and Conclusions.

How to
cite: Ochoa-Correa, D., Villa-Ávila, E., Iñiguez-Morán,
V. & Astudillo-Salinas, P. (2024). Comparative study of dynamic response by three types of energy
storage systems for grid studies: A Microgrid Laboratory
experimental-based study. Revista Tecnológica - Espol, 36(1),
13-31.

https://rte.espol.edu.ec/index.php/tecnologica/article/view/1117

Implementing
Energy Storage Systems (ESS) is increasingly significant in power electrical
systems. This is attributed to their ability to store surplus electricity
generated by renewable energy sources such as wind and solar, contributing to
the balance between generation and demand. Literature studies indicate that
this practice enhances network stability and diminishes the need for expensive
redesigns in infrastructure. This paper presents an exhaustive
experimentation-based study of the dynamic response provided by three energy
storage system technologies: supercapacitors, Lithium-Ion batteries, and
Vanadium redox flow batteries, technologies with significant academic,
research, and industrial interests nowadays. The objective of this research is
the experimental evaluation of the performance of such technologies in real
electrical system operations, focusing on determining the efficiency of charge
and discharge, as well as the tracking of active and reactive power achieved
through the associated grid interface. The experimental tests performed in a
microgrid laboratory show these technologies’ advantages and limitations in
different grid-integration applications, with the Lithium-Ion battery-based ESS
demonstrating the highest efficiency and faster power response. The results
achieved and reported in the article can serve as an essential input for
researchers and technology developers to feed their models with more precise
parameters and data that might provide results closer to the actual behaviour of the studied prototypes. The methodological
framework used in this research is descriptive, experimental, and quantitative.

Keywords: EESs,
Lithium-Ion batteries, Vanadium-redox flow batteries, Supercapacitors,
Microgrids.

Resumen

La implementación de sistemas de almacenamiento de
energía (SAE) está adquiriendo una creciente relevancia en los sistemas
eléctricos de potencia. Esto se debe a su capacidad para almacenar el excedente
de electricidad generado por fuentes de energía renovable, como la eólica y la
solar, y contribuir al equilibrio entre la generación y la demanda. Estudios
documentados en la literatura muestran que esta práctica mejora la estabilidad
de la red y reduce la necesidad de realizar costosos rediseños en la
infraestructura. En este artículo, se presenta un estudio exhaustivo basado en
la experimentación de la respuesta dinámica proporcionada por tres tecnologías
de SAE: supercapacitores, baterías de iones de litio y baterías de flujo de
Vanadio redox; tecnologías de gran importancia en la academia, investigación e
industria en la actualidad. El objeto de esta investigación es la evaluación
experimental del desempeño de tales tecnologías en operaciones reales de un
sistema eléctrico, centrándose en la determinación de eficiencia de carga y
descarga, así como en el seguimiento de potencia activa y reactiva conseguido
por la interfaz de conexión a la red asociada. Los resultados de los
experimentos realizados en un laboratorio de microrred muestran las ventajas y
limitaciones de estas tecnologías en diferentes aplicaciones, demostrando ser
la más eficiente y rápida en respuesta de potencia el SAE basado en baterías de
iones de litio. Los resultados alcanzados y reportados en el artículo sirven
como insumo esencial para que los investigadores y desarrolladores de
tecnología alimenten sus modelos con parámetros y datos más precisos que
brinden resultados más cercanos al comportamiento real de los prototipos
estudiados. La metodología utilizada en esta investigación es descriptiva,
experimental y cuantitativa.

Palabras
clave: SAEs, baterías de iones de
litio, baterías de flujo de Vanadio redox, supercapacitores,
microrredes.

Introduction

Overview

The
production of electricity from renewable sources  such as solar
and wind energy
is highly dependent
on weather conditions. Solar energy is available only from dawn to dusk,
and cloudy conditions can cause variations in energy production during this time. Likewise,
in the case of wind energy, wind speed is highly variable
in both on-shore and off-shore wind farms, resulting in varying levels of energy production. This
variability is a problem for
electrical power systems since energy must
be supplied to consumers (residential, commercial, and primarily industrial) in compliance with
rigorous quality standards concerning voltage level and frequency.

In this
context, there is a clear need to mitigate the
variability intrinsic to intermittent renewable energy sources (IRE) and improve
the reliability of the electricity supply in terms of its continuity. One of the ways currently used to fulfil these
purposes is to employ energy
storage systems (ESS) interconnected
to
the electric-electronic equipment of the generation plants that take advantage of
IRE and at specific strategic points of the electric grid (Ochoa & Martinez, 2021);
(Silva, 2018).

According
to (Shukla et al., 2000), two fundamental ways to
store electrical energy exist. The
first is indirect, involving batteries, where
the storage (charge)
is in the form of chemical
energy and is recovered (discharged)
through a redox process of the electroactive reagents of the device. The second is direct since the energy
is stored in electrostatic
form as positive and negative charges on the plates of a capacitor with very
high capacitance (hundreds or thousands of Farads), known as a supercapacitor or ultracapacitor.

Figure 1 shows the uses of batteries,
supercapacitors, and other energy storage
mechanisms employed at different scales within electrical
systems (Aneke & Wang, 2016).

Figure 1

Usage of energy storage systems at different
scales in electrical systems. Source: own elaboration
based on [4]

State-of-the-art
review

There is an important academic, research, and industrial interest in
energy storage, given its performance and technological maturity, which is why
it is possible to find many scientific contributions that have led to countless
applications in the literature. In (Hassan et al., 2022), an evaluation study of
using supercapacitors as a fast response energy storage unit to improve energy
self-consumption and self-sufficiency to compensate for intermittent
photovoltaic generation is presented. The results show that this combined use
improves specific parameters related to power quality and leads to an
appreciable increase in the total monthly energy supplied by the installation.

In this line, research reported in (Şahin et al., 2022) and (Gao et al., 2023) shows that the performance
of supercapacitors can be considerably improved when hybridized with other
battery-based storage technologies. Apart from providing a better energy
cushion, the hybrid solution is an interesting alternative from a techno-economic
point of view due to its increased lifetime (Subasinghage et al., 2022);
(Dikmen &
Karadağ, 2022).

In addition, these works explore the opportunities provided by the
concept of “second life” of electrochemical systems decommissioned in their
original applications (e.g., discarded batteries of electric vehicles) with the
consequent reduction of the carbon footprint (Mostert et al., 2018). Today, the predominant
applications of storage based on lithium-ion batteries are in the
electromobility sector and grid energy support (Şahin et al., 2022). The latter is the subject
of interest for this research, providing support for the grid in contingency
and loss of supply risk situations (Lee et al., 2022) and the contribution to the
proper integration of intermittent renewable generators to the grid, thanks to
the fast response in power terms and the high storage capacity offered by
lithium-ion batteries.

This property facilitates the absorption of fluctuations of wind and
photovoltaic generators in weak and isolated power systems, improving the
reliability and quality of the power supply (Mancuso et al., 2019). A third storage
technology that has attracted current scientific attention is the storage
system based on Vanadium Redox Flow Batteries (VRFB). Its high storage capacity
and long (theoretically infinite) electrochemical life cycle (Arribas et al., 2016) make this technology very
attractive for reliable energy supply applications in isolated microgrids (Suvire et al., 2017) and
for backup power supply of critical loads in industries and hospitals(Do Nascimento
Ricardo & Fthenakis, 2017); (Li et al., 2017).

Apart from the applications mentioned here, energy storage systems
provide a more comprehensive range of implementation opportunities in
electrical power systems to provide support in fundamental activities such as
frequency and voltage control in a power grid (Zecchino et al., 2021); (Ochoa et al., 2022), power factor correction (Niwas & Singh,
2016), power smoothing (Benavides et al., 2023), improving the
resilience of the power system through the concept of virtual or synthetic
inertia (Pazmiño et al., 2021),
(Pazmiño et al., 2022),
flexible alternating current transmission systems or FACTS (Kljajić et al., 2020), among many others. Figure 2 schematizes the
main applications of battery energy storage systems (BESS) found in this
literature review.

Figure 2

Applications
of battery energy storage systems in power systems

Aim of the
study

This research aims to experimentally evaluate the performance of ESS
technologies in actual power systems operations, explicitly focusing on
determining the efficiency of charge and discharge processes and analysing the active and reactive power set-point tracking
dynamics achieved through the associated network interface.  The study involves an exhaustive examination,
based on experimentation, of the dynamic response provided by three energy
storage systems (ESS) working on a power system: ESS based on supercapacitors,
ESS based on lithium-ion batteries, and ESS based on Vanadium Redox Flow
Batteries, three technologies of great current interest in the academic,
research and industrial fields. The laboratory tests are designed to yield
characteristic curves and times of the charge and discharge processes of the
different technologies, as well as active and reactive power set-point-tracking
dynamics.

Materials and
Methods

First of all, this section describes the
materials used for conducting the proposed validations. Subsequently, the
developed methodology and the design of the experiments, which will lead to the
achievement of the research objectives, are outlined.

Materials:
Energy storage systems under study

Microgrid
Laboratory of the CCTI-B

The
experimental work is conducted in the Microgrid Laboratory of the Centro Científico Tecnológico y de Investigación Balzay (CCTI-B) of
the Universidad de Cuenca. This laboratory has a variety of energy generation,
consumption, and storage agents and can work connected to the main distribution
network or in island mode. Figure 3 shows a schematic representation of
the main components of the laboratory.

Figure 3

Schematic representation of components of the
Microgrid Laboratory: 1. Main grid, 2. Grid connection bus bar (B-01), 3.
Energy storage components, 5. Programmable loads and sources, 6. Other loads,
7. Solar photovoltaic generation, 8. Wind generation, 9. Thermal generation, 10. Hydraulic generation

Within the
scope of the present study, the Microgrid Laboratory of Universidad de Cuenca
has the following energy storage systems, which constitute the materials used
in the current research (Espinoza et al., 2017):

·
ESS
based on supercapacitors (Supercapacitors- ESS)

·
ESS
based on lithium-ion batteries (Li-Ion BESS)

·
ESS
based on Vanadium Redox Flow Battery (Vanadium Redox Flow - BESS)

Supercapacitors–ESS

Supercapacitors
(SCs) are storage devices capable of holding more energy (charge) than a
conventional capacitor and releasing energy (discharge) much faster than a
battery (Castro-Gutiérrez et al., 2020). Additionally,
their high number of charge/discharge cycles and long-term stability position
SCs as an attractive option for implementing ESSs. Compared to batteries, their
main disadvantage is their low specific energy or gravimetric energy density
(watt-hours per unit mass - Wh/kg). SCs are
classified into three categories (Libich et al., 2018):
Electric double-layer capacitors (EDLC) with liquid electrolyte make up the
first. They store energy by electrostatic interaction in the Helmholtz electric
double layer at the phase interface between the electrode surfaces and the
electrolyte (known as the electrostatic storage principle), which gives them a
high-power density (very high discharge current). They have high durability and
a lifetime (cyclability) of millions of charge/discharge cycles. They are also
the most common and widely marketed.

The second
category is pseudo-SCs or faradaic SCs, which do not store energy in the double
dielectric layer. They are like batteries since redox reactions and energy
transfer between electrodes and the electrolyte occur during the charging and
discharging. This feature provides approximately twice as much higher specific
energy than EDLCs (Libich et al., 2018).
However, the electrodes degrade faster, causing an increase in the SC’s
internal resistance and lower stability and cyclability. Their use and
commercialization are limited.

Finally,
hybrid SCs are in the third category. These combine the properties of EDLCs and
pseudo-supercapacitors through a faradaic reaction at one electrode and an
electrostatic interaction at the other. It is the most modern and advanced
technology, providing higher specific energy (approximately 36 times more than
EDLCs and 18 times more than pseudo-SCs, according to (Libich et al., 2018)
and higher power density. In a study of trends in hybrid (SC-Battery) energy
storage devices (Benoy et al., 2022), the
authors state that hybrid SC applications are increasing significantly in the
industrial sector of energetically hybrid vehicles.

Among the
most critical applications of SCs are: load-levelling function; hybridization
with rechargeable batteries in drive systems and regenerative braking systems
for partial recharging in electric vehicles; short-term power backup (for safe
shutdown of systems or switching to permanent backup systems); control of
transients or power fluctuations (power smoothing), among others.

In the
Microgrid Laboratory, a supercapacitor bank comprises ten electric double-layer
capacitors (EDLC) connected in series of 130 F and 56 VDC each,
BOOSTCAP model BMOD0130, from the USA manufacturer Maxwell Technologies. The
bank’s terminal voltage is 560 VDC, and its total energy storage
capacity is 0.57 kWh. The power converter connected to this equipment is 30 kW
rated (Figure 4).

Figure 4

Supercapacitors-ESS of the Microgrid
Laboratory: supercapacitor bank (left), a cabinet containing the power
electronic converter (right)

Li-Ion
BESS

Lithium-ion
(Li-ion) battery cells consist of a cathode, anode, electrolyte, and separator
(a safety component that prevents a short circuit between the anode and
cathode, which is permeable to lithium ions). They use non-metallic lithium in
the cathode, i.e., chemical compounds called lithium metal oxides capable of
providing lithium ions (Li+), hence their commercial name and their varieties.
The ions are transported through the electrolyte between two electrodes in a
reversible chemical reaction from the cathode to the anode during charge and
discharge (Zubi et al., 2018). They are characterized by
having an “unmatched combination of high energy density and high power density”
(Nitta et al., 2015).

There are
six commercially available lithium-ion batteries (Beltran et al., 2020), with significantly
different technical characteristics depending on the lithium metal oxide they
employ. These are lithium cobalt oxide (LCO), lithium manganese oxide (LMO),
lithium iron phosphate (LFP), lithium nickel cobalt aluminium
oxide (NCA), lithium nickel manganese cobalt oxide (NMC), and lithium titanate
(LTO).

According
to (Zubi et al., 2018), LCO lithium-ion batteries
have high specific energy (up to 190 Wh/kg), long
life (up to 1000 cycles), and their main disadvantage is their low level of
safety due to the thermal instability of cobalt oxide. Their applicability is
in mobile electronic devices. Lithium-Ion LMO has a relatively high-power
density, higher durability (up to 1500 cycles), lower specific energy (up to
140 Wh/kg), and the higher thermal stability of
manganese oxide makes them safer. Their applications are in e-bikes, medical
devices, and power tools. Lithium-Ion LFP has higher durability (2000 cycles),
is safe, relies on environmentally friendly materials, and tolerates operation
over a wide range of state of charge (SoC) 15-100% with consistent performance.
However, it has low specific energy (up to 140 Wh/kg).
It has the potential for application in electric power systems (EPS) and has
been successful in e-bikes. Lithium-ion NCAs stand out for their high specific
energy (250 Wh/kg) and high-power density. They have
high durability (up to 1500 cycles). They are used in electric vehicles (EVs),
particularly in Tesla models. Lithium-Ion NMC has lower specific energy (up to
200 Wh/kg) and higher durability (up to 2000 cycles);
the proportions of their chemical components can be modified according to the
application to provide tailor-made solutions. They are the dominant technology
in EVs and plug-in hybrid electric vehicles (PHEV). Finally, Lithium-ion LTOs
are characterized by using nanotechnology to integrate lithium titanate
nanocrystals on the anode surface (Campoverde-Pillco et al., 2024). They have
very high-power density, which allows them to deliver a discharge current that
is ten times higher (approx.) than that provided by other lithium-ion
technologies. They have high durability (20,000 cycles) and safely support fast
charges (in the order of minutes). Regarding its applications, it has great
potential for energy storage in EPSs integrating IRE.

Electrical
energy storage in lithium-ion batteries currently has three main applications (Zubi et al., 2018). The first one is in mobile
electronic devices (cell phones, tablets, and laptops, among others), where it
is the dominant technology. The second is in electric vehicles (EVs, e-bikes,
and e-scooters, among others) as the primary energy source. Finally, they can
contribute to integrating intermittent renewable energies into the system in
EPSs. In this last application, the batteries store surplus energy to be
injected into the grid when the natural resource is unavailable (sunlight or
wind) or is technically and economically convenient.

The
rechargeable lithium-ion battery bank of the Microgrid Laboratory shown in  Figure 5 consists of 11 cells of 58.36 VDC,
connected in series, model ELPT392- 0002, Samsung brand. The bank’s output
voltage is 642 VDC, with a power of 88 kW and an energy storage
capacity of 44 kWh. The bank is connected to an 80-kW power converter.

Figure 5

Li-Ion BESS from the Microgrid Laboratory: cell
bank (left), a cabinet containing the power electronic converter (right)

Vanadium
Redox Flow – BESS

A redox
flow battery (RFB) consists of two reservoirs, one with charged electrolyte and
the other with discharged electrolyte, an electrochemical energy conversion
system (stacks connected in series or parallel), a hydraulic pumping system to
make the electrolyte flow through the conversion system, and the electrical
system for interconnection with the load and the electrical power source (Skyllas-Kazacos et al., 2011).
The current state-of-the-art identifies two types of RFBs: 1) standard or pure
flow and 2) hybrid. Figure 6 shows the Vanadium Redox Flow
Battery bank of the Microgrid Laboratory.

Figure 6

Vanadium Redox Flow - BESS of the Microgrid
laboratory: Container housing the vanadium cells (left), double cluster of
power electronic converters (right)

Standard or
pure flow batteries, among which the vanadium redox flow battery (VRFB) stands
out, provide the independence or decoupling between storage capacity and power
as one of the leading technical characteristics (Lourenssen et al., 2019).
Their capacity depends on the amount of electrolyte stored in the system, the
concentration of active components, the voltage of the electrochemical cells,
and the number of stacks. In contrast, the power depends on the active
components’ behaviour and the electrodes’ size. This
decoupling provides advantages such as modular and flexible design, excellent
scalability, moderate maintenance costs, and extended durability. Its energy
density is 25-35 Wh/L, which is still very low
compared to lithium-ion batteries (250 Wh/L or more).
Its cyclability is far superior to other types of batteries, with a range of
15000-20000 complete cycles (Sánchez-Díez et al., 2021).
According to market statistics, VRFB is currently the most commercially
successful (Transparency Market Research, 2023).

On the
other hand, hybrid RFB saves at least one solid electroactive material, such as
the zinc-bromine redox flow batteries (ZBFB), the second most important
technology on the market (Transparency Market Research, 2023). Their
decoupling between power and energy storage capacity is limited due to the
presence of the solid electrode. Commercially available models have a specific
energy range of 60-85 Wh/kg, with an estimated
lifetime of 11-14 years (Suvire et al., 2017).
Their main application is large-scale energy storage in EPSs for IREintegration, where they are intended to meet the
requirements of a long lifetime (cyclability and calendar years), low cost, and
high round-trip efficiency.

The
Microgrid Laboratory has a VRFB model CELLCUBE FB 20-100 from the Austrian
manufacturer GILDEMEISTER Energy Solutions, with an output power of 20 kW and
an energy storage capacity of 100 kWh in 12 electrochemical stacks. The battery
is equipped with two clusters of power converters (3 converters per cluster,
one master, and two slaves) to connect to any of the two three-phase bus bars
available in the Microgrid Laboratory.

Methodology:
Design of the laboratory test bench

Figure 7 shows the schematic representation
of the test bench set up in the laboratory. All the energy storage agents under
study will be connected to the main bus bar of the microgrid (B-01), which is
connected, in turn, to the main distribution network.

Figure 7

One-line
diagram of the test bench

The ESSs
will all be subjected to the same operating conditions to facilitate a
comparative analysis between the different storage technologies. Due to the
significant amount of energy handled in such tests, both the charge/discharge
tests and the dynamic tests of active and reactive power set point tracking
will operate one technology at a time. The following is a description of the
test plan to be carried out in this study, which constitutes the methodological
basis of this research:

Charge and
discharge tests. Each technology will be subjected to charge and discharge
tests under constant active power conditions. This first experiment aims to
determine the storage capacity and energy backup time provided by each system
studied. With this information, the round-trip efficiency will be calculated, a
parameter of great importance in ESSs since it quantifies the electrical energy
that can be extracted from an ESS compared to the amount of electrical energy
spent in its charging process. In addition, to respect the maximum and minimum
operating limits of all ESS components, it was ensured that the charging and
discharging tests were performed with an SoC between 20 and 80 %.

Active and
Reactive Power tracking tests. To evaluate the short-term response in terms of
active and reactive power provided by the different ESS technologies under
study, these will be forced to follow the same power time profile in charge and
discharge situations. The control of the set point signals applied to the power
electronic converters (PEC) associated with each ESS will be performed from the
laboratory’s Supervisory Control and Data Acquisition (SCADA) system. This test
will gauge the efficiency of each ESS technology in tracking the imposed power
set points in terms of its response time, power tracking error in dynamic and
steady-state, and the effect of the power tracking on the SoC.

Results and Discussion

Charge and discharge tests

This section presents the results obtained in
the laboratory during the charge and discharge tests of the different ESS
technologies considered. To have comparable cases in the three experiments,
each ESS is subjected to a constant power condition of 10 kW so that the
corresponding SoC varies from 20 to 80% (charging stage) and then from 80 to
20% (discharging stage) in a sustained manner. In Figures 8, 9, and 10, black
curves represent the active power at which the ESS is charged/discharged (in
kW), and the blue curve represents the evolution of its state of charge (in %).

Tests applied to Supercapacitors-ESS

Figure 8 shows the complete charging and discharging process of the
Supercapacitor-ESS. In this test, the correct operation of the power electronic
converters associated with this technology to track the pre-set active power is
evident: ±10 kW constant. In its charging stage, the SoC of the
Supercapacitor-ESS goes from 20 to 80%, in a sustained manner, in 72.6 seconds.
This process is associated with an expenditure of 0.202 kWh of electrical
energy from the grid. Once this ESS reaches 80% of the load, the power setpoint
applied to the inverter is reversed to start the discharging stage, as shown in
the figure. The tests show that going from a state of charge of 80% to 20% with
a power of 10 kW takes 67.1 seconds, delivering 0.189 kWh of electrical energy.
The findings reveal that, in this type of ESS, to extract 0.189 kWh of
electrical energy, it is necessary to use 0.202 kWh during battery charging.
The ratio between the first and second quantities is called round-trip
efficiency (RTE), which in the case of the Supercapacitor-ESS is 92.4%, an
expected value in terms of efficiency for this technology. An intriguing
observation from the results is that the dynamics presented by the SoC in the
charging and discharging process have a linear nature.

Figure 8

Charge and discharge test performed on the
Supercapacitor-ESS

Tests applied to Li-Ion BESS

The complete charging and discharging process
of the Lithium-Ion BESS is shown in Figure 9. It is observed that, in its charging stage, the SoC goes from 20 to
80% in 107.8 minutes, requiring 17.96 kWh of electrical energy. Likewise, in
the previous case, it is observed that once this ESS reached 80% of the load,
the power setpoint is reversed to start with the discharge process to 20% of
SoC at a constant power of 10 kW. The discharge, on the other hand, lasts 105.2
minutes and delivers 17.54 kWh of electrical energy. In this case, the RTE
efficiency is 97.7%, meaning the electrical energy extracted from this ESS
technology penalizes only 3% of the energy needed to charge the battery. Note
that the dynamics adopted by the SoC of this ESS in its charging and
discharging process have a linear characteristic.

Figure 9

Charge
and discharge test applied to Lithium- Ion BESS

Tests applied to Vanadium Redox Flow – BESS

The Vanadium Redox Flow-BESS presents a charge
and discharge cycle, as shown in Figure 10. Its charging process (SoC from 20 to 80%) takes about 13.5 hours, and
the temporal evolution of the SoC presents a non-linear dynamic. As expected,
sustaining a charging power of 10 kW for a long time involves an investment of
135.3 kWh of electrical energy. This characteristic makes this technology the
one with the highest storage capacity of the three studied here. The results
reveal that this battery can sustain a power supply of 10 kW for at least 7.8
hours, representing an energy extraction of 77.8 kWh. It is evident here that,
although this battery provides a reasonably large energy cushion compared to
the previous two, its RTE efficiency is the lowest: 57.5%.

Figure 10

Charge and discharge tests applied to Vanadium
Redox Flow-BESS

Figure 11 shows the results obtained in this first part of the tests, including
the round-trip efficiency calculations for the three ESS technologies studied.

Figure 11

Round-trip efficiency of the three ESS
technologies studied

Figure 12 (a) shows the charge and discharge cycles of the ESSs studied over the
same time scale. In this, the short-term response of the Supercapacitor-ESS
compared to that of the Li-Ion BESS is evidenced, and how, in turn, the time
response of the latter is short if compared to the Vanadium Redox Flow-BESS.
Complementarily, a superposition of the three charging and discharging
characteristics of the ESSs on a logarithmic time scale is given in Figure 12 (b).

Figure 12

Comparison of charge and discharge
characteristics of ESSs under constant 10 kW

Finally, for completeness, the tests described
above have been repeated to obtain the charging and discharging characteristics
of the ESSs but for different power values. Table 1 shows the results obtained in the experimental tests. In each case, the
round-trip efficiency becomes a characteristic value of each ESS technology and
varies very little for each charging and discharging power level. Thus, the
round-trip efficiency of Supercapacitor-ESS is 92%, the Li-Ion BESS is between
95-97%, and the Vanadium Redox Flow-BESS is 57%.

Table 1

Charge and discharge tests applied to the ESS
studied: 20-80-20% SoC-trip

Active and Reactive Power setpoint tracking
tests

Next, the studied ESSs are subjected to
variable active and reactive power conditions to evaluate their short-term
dynamic response. For this purpose, power profiles have been prepared, taking
the values 0, 5, 10, and 15 kW (and kVAR) in steps of
20 seconds each. These profiles were loaded into the algorithm commanding the
inverters associated with the ESS in the laboratory’s SCADA.

Figure 13 (a) depicts the response provided by the Supercapacitor-ESS. The graph
demonstrates the correct performance of this ESS since it successfully reaches
the active power setpoint values in both transient and steady-state regimes.
However, it is worth noting the volatility of the SoC of this technology in
terms of active power injection and absorption requirements. This
characteristic is expected since the Supercapacitor-ESS is the technology with
the lowest energy storage capacity of the three studied here. In addition,
regarding reactive power tracking (Figure 13 (b)), the Supercapacitor-ESS correctly achieves the setpoints imposed
with a negligible consumption of useful energy (SoC). With these two tests, the
usefulness of this ESS in applications requiring fast power response and its
low energy consumption is evident.

Figure 13

Power tracking of the
Supercapacitor-ESS

Next, regarding the response presented by the
Li- Ion BESS, Figure 14 (a) shows the high effectiveness in active power setpoint tracking of
the lithium-ion battery in transient and permanent regimes, shown in Figure 14 (b).

In this test, it is evident that when the ESS
is subjected to an active power requirement of short duration, the useful
energy handled is small and almost does not affect the SoC level for the
plotted time scale. A similar situation occurs in the reactive power control (Figure 14 (b)), in which the reactive power tracking is performed very
efficiently while the SoC does not show an appreciable reduction over time. In
this second test, the inverter associated with the ESS presents a correct
performance in short-term power tracking tasks and, in turn, provides operation
for relatively long times due to its significant energy storage capacity.

Figure 14

Power tracking of the Li-Ion BESS

Finally, Figure 15 shows the performance of the
Vanadium Redox Flow-BESS in power-tracking tasks. Figure 15 (a) shows the poor response of this technology in terms of short-term
active power. On the other hand, when long-term power transitions are required,
this ESS can respond adequately to such demands (See Figure 10). Finally, regarding reactive power control, Figure 15 (b) shows a null response from the ESS. This is because the actual
prototype in the laboratory does not have the reactive power control enabled by
default.

The results presented here offer insights into
the advantages and limitations of the three types of ESSs examined in terms of
their required time response. In addition, they confirm the different
applications of such technologies in their interaction with the electric power
system, as discussed in section 2 of this paper.

Figure 15

Power tracking of the Vanadium
Redox Flow-BESS

Conclusions

In this
study, a comprehensive study has been carried out to obtain the dynamic
responses of three ESS technologies: supercapacitors, lithium-ion batteries,
and Vanadium redox flow batteries. The work has been done on actual commercial
models with industrial characteristics available in the Microgrid Laboratory of
Universidad de Cuenca. In align with the research objectives aimed at
evaluating the performance of energy storage systems in terms of their
efficiency and dynamics of active and reactive power for applications in power
systems, two tests were devised: 1) charge and discharge and 2) active and
reactive power set point tracking. The most relevant results are presented
below:

Supercapacitor-ESS:
the results of charge and discharge tests showed that this energy storage
medium has the lowest storage capacity: 1/90 of the capacity of Li-Ion BESS and
1/670 of the capacity of Vanadium Redox Flow-BESS but with very high current
density since it allows swift responses in terms of active and reactive power
with practically imperceptible error margins. From these tests, it is concluded
that this ESS is ideal for applications whose power demands have a very
short-term variability, such as power smoothing, frequency and voltage control
in electrical systems, and voltage dips suppressor when switching from a
primary supply source to a backup source during a contingency. Its round-trip
efficiency is around 92%.

Li-Ion
BESS: Tests performed on this commercial model showed that its storage capacity
allows it to sustain significant power for at least a few hours, which, added
to its high current density, allows practically instantaneous transitions of
active and reactive power, allowing this ESS technology to be implemented in
applications similar to that of Supercapacitor-ESS. In addition, its long
autonomy allows it to be used as a backup system when the primary energy supply
is lost in an electrical installation, widening the range of applications
concerning the previous technology. Experimental results showed this technology
has a 95-97% round-trip efficiency.

Vanadium
Redox Flow BESS: after conducting charge and discharge tests on this type of
battery, it was found to have the highest capacity of the three ESSs studied.
This technology enables the sustenance of significant power load requirements
during tens of hours of uninterrupted operation. From the experimental results,
it is concluded that this ESS is ideal to be implemented as a reliable and
long-duration power backup system in applications where the primary supply
source presents a high risk of unavailability or in base load applications.
However, this ESS is unsuitable for applications where high-speed tracking of
load power is required due to the electrochemical constraints of the tested
commercial model. The round-trip efficiency is around 57%, the lowest of the
three technologies studied.

In addition
to the conclusions presented here, the results of this study provide charge and
discharge charts of the different ESSs and values of charge and discharge times
at different power levels. These data enable the elaboration of characteristic
power vs. charging time curves, which help feed mathematical ESS models.
Integrating mathematical ESS models in the grid studies facilitates obtaining
numerical results that closely resemble the actual operation of these
prototypes, thereby enhancing accuracy when sizing the energy storage
facilities for various applications within the electrical power system
framework.

Acknowledgments

The authors
thank Universidad de Cuenca for easing access to the facilities of the
Microgrid Laboratory of the Centro Científico Tecnológico y de Investigación Balzay (CCTI-B), for allowing the use of its equipment, and
for authorizing its staff the provision of technical support necessary to carry
out the experiments described in this article.

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