Abstract
This study illuminates the groundbreaking innovation and real-world
utility of Latent Heat Thermal Energy Storage (LHTES) systems,
unveiling an advanced and readily deployable solution for efficiently
storing and releasing thermal energy. The Latent Heat Thermal Energy
Storage (LHTES) system has been developed as a dispatchable solution
for storing and releasing thermal energy. LHTES units use phase change
materials (PCMs), which, through charging and discharging, store
energy in the form of thermal energy. LHTES devices are more practical
than alternative approaches because of their increased heat storage
capacity, a sizable array of PCMs, and virtually isothermal behavior.
LHTES systems also need one hermetic container with no salt pumps,
pipelines, or heating trace requirement. One of the major challenges for
such an LHTES system is the selection of proper PCMs to achieve the
targeted applications. Despite significant efforts to improve the LHTE
units, their efficacy and broader range of application remain limited. To
address these issues, researchers have explored alternate techniques to
enhance the efficacy of the PCM-based energy storage and exchange
units. This review provides a comprehensive analysis of LHTES based on
PCMs, focusing on exploring the potential of different techniques to
improve their efficacy for enhanced thermal performance. The paper
thoroughly scrutinizes the different aspects of phase change
materials (PCM), methods of improvement in their performance, and
different hybrid techniques. The present status of the PCMs-based
advanced energy storage system is also presented systematically. Finally,
challenges and future recommendations are also proposed for future
researchers. The review's outcome reveals that hybridization
techniques can potentially enhance the performance of PCMs-based
energy storage units. This review work also covers the PCM-based
energy storage system's economic aspects for long-term sustainability.
Introduction
Solar energy has gained widespread attention as a sustainable power
source; however, its intermittent nature poses a challenge to its
widespread implementation. Specifically, solar power is unavailable
during the evening and cloudy days, making it unreliable for continuous
energy supply. To address this limitation, Latent Heat Thermal Energy
Storage (LHTES) units have been developed as a dispatchable solution
[1].
The originality of this work lies in its comprehensive exploration of
Latent Heat Thermal Energy Storage (LHTES) systems, emphasizing
their innovative and practical aspects. The study provides insights into
the advanced nature of LHTES as a dispatchable solution for efficient
thermal energy storage and release, highlighting its unique features,
which include the use of diverse phase change materials (PCMs) and the
simplification of system design without the need for additional
components like salt pumps, pipelines, or heating traces. Furthermore,
the work addresses the ongoing challenge of selecting appropriate PCMs
for specific applications and suggests potential ways to enhance the
efficacy of PCM-based energy storage systems, mainly through
hybridization techniques. This in-depth examination of LHTES systems,
their practical advantages, material selection challenges, and economic
sustainability makes it an original and valuable contribution to thermal
energy storage.
LHTES units use phase change materials (PCMs), which, through
charging and discharging, store energy in the form of thermal. These
PCMs might be as basic as a container or as sophisticated as a
complicated unit with several upgrades. To guarantee a steady supply of
power, the energy that has been saved can then be released at off-peak
times [2]. Sensible heat storage (SHS) and thermo-chemical storage
(TCS) are two more forms of heat storage in addition to the LHTES unit
(Fig. 1). Utilizing materials that conserve and generate heat through
endothermic and exothermic processes, such as potassium oxide and
lead oxide, is a key component of TCS [3]. The material is heated to a
point where it splits into two pieces, each kept separately. The two
components are combined once more to discharge the energy. In
contrast, SHS stores heat using a storage medium, mostly water but
sometimes sand or rocks [4,5].
Although SHS is now the most popular heat storage technique, LHTES
units provide several benefits over other techniques. LHTES devices are
more practical than alternative approaches because of their increased
heat storage capacity, a sizable array of PCMs, and virtually isothermal
behavior. In particular, LHTES devices can store the same power level in
a smaller volume than SHS, and the steady temperature enables almost
isothermal heating and cooling. LHTES systems also need one hermetic
container, with no salt pumps, pipelines, or heating traces requirement.
Depending on the PCM, these systems can also function at various
temperatures. However, PCMs have a larger chance of leakage, and
LHTES units often have a higher initial cost than other storage
techniques [6]. LHS has a significant advantage over SHS, storing high
temperatures in a relatively small area. This is illustrated in Fig. 1, where
a single PCM is used in both SHS and LHS systems. For a small
temperature range that covers the phase change, the energy stored in the
LHS unit is three times greater than that of the SHS unit. However, for a
larger temperature range, the benefit of the LHS unit diminishes, with a
ratio of 6:4, equivalent to 1.5 [7].
LHS systems have three components: a suitable PCM, an encapsulating
container, and a heat exchange surface. LHS containers can be used in
heating or cooling systems with air or liquid as the heat transfer fluid.
Compact LHS systems have a shell-and-tube configuration and are ideal
for integration with water-warming units. Encapsulated units are
flexible in shape and have a greater SHS component, making them
suitable for both air and H O warming units [8]. Generally, LHTES
systems offer a promising solution for the intermittent nature of solar
energy by providing a dispatchable energy source using PCMs. While
these systems have some limitations, their advantages make them a
promising method for heat storage.
Generally, heat energy storage capacity of PCM-based LHS system
expressed [2] as�=∫����mC�dT+ma�Δℎ�+∫����mC�dTwhere the
symbol m, C , T, a and Δh corresponds to the storage material mass
(kg), specific heat capacity (kJ/kg K), temperature (K), fraction of
melted material and latent heat of fusion (kJ/kg). The sensible energy
storage by the material (during the initial heating at the solid state) is
given by the first term of the equation. The latent heat energy absorbed
or released (when phase change process initiates) is given by the second
term of the equation; while the third term of the equation is the rise in
the temperature in the liquid phase (during solid to liquid LHS).
In the configuration of an LTES unit, selecting an appropriate storage
medium is crucial for the desired application. The phase change material
(PCM) chosen must exhibit specific thermophysical, chemical, and
kinetic properties. The thermophysical properties should include high
2
p
m
m
thermal conductivity, high enthalpy of fusion, high density, and minimal
variation in size during phase change to minimize storage capacity.
Additionally, the storage medium should possess chemical stability,
consistency with materials used for containers, non-toxicity, noncorrosiveness, and minimal sub-cooling. Economic viability is also
important, with cost and abundance being major considerations [9,10].
PCMs are built on their critical temperature and products and are
generally categorized as organic, inorganic, and eutectic materials (Fig.
2). These classifications depend on the composition and nature of the
materials [11]. Table 1 compares the various features of organic,
inorganic, and eutectic PCMs. PCMs have gained attention as a
technology with potential for a wide range of uses, including in the
building sector for thermal energy storage and management, in
concentration technology for efficient solar power generation, in HVAC
systems for energy-efficient cooling and heating, in the cold chain and
packaging for maintaining temperature-sensitive products during
transport, in electronics for thermal management, and in textiles for
advanced thermal comfort (Fig. 2). PCMs offer unique merits, including
high energy storage capacity, high thermal conductivity, and thermal
stability, making them versatile and efficient solutions for various
applications. PCMs are anticipated to become more crucial as research
advances in fulfilling the rising need for clean and effective heat
solutions. Table 2, Table 3 represents the properties commercially
available PCM.
PCMs are compounds that, through a phase transition, may conserve
and emit much heat, such as melting or solidifying. The desirable
characteristics of PCMs depend on their intended application but
generally include [[12], [13], [14]]:

1.
Thermal Properties: The main desirable thermal properties of
PCMs include a high latent heat of fusion (the quantity of energy
gained or lost throughout a phase transition), a narrow
melting/freezing temperature range, and a high thermal
conductivity. These properties ensure that the PCM can absorb or
emit enormous quantities of power quickly and efficiently, with
minimal temperature fluctuations.

2.
Physical Properties: The physical properties of PCMs should
contain high thermal stability, low flammability, and non-toxicity.
The PCM should be able to maintain its chemical structure and
phase change behavior over multiple cycles without degrading or
decomposing. The PCM should not pose health or safety risks to
humans or the environment.

3.
Kinetic Properties: The kinetics of the Transition between phases is
also important, as a fast phase change rate can ensure efficient heat
transfer. The PCM should have a low hysteresis (the temperature
difference between the charging and discharging points) and a
common supercooling/superheating tendency, meaning it should
melt or solidify at its intended temperature without requiring
additional energy input.

4.
Chemical Properties: The PCM's chemical characteristics must be
considered, especially in applications where the PCM is in contact
with other materials or substances. The PCM should be chemically
stable and not react with other materials, including the container
or enclosure in which it is stored. The PCM should also be
compatible with any other chemicals or materials it may contact.

5.
Economic Properties: The economic properties of the PCM,
including its cost, availability, and ease of manufacturing, are also
important considerations. The PCM should be affordable, easily
accessible, and efficiently manufactured and incorporated into
various products and applications. Additionally, using a PCM's
energy savings and efficiency benefits should outweigh the initial
implementation costs.