Journal of Undergraduate Research 5, 1 (2015)
A Review of Latent Heat Thermal Energy Storage for Concentrated Solar Plants on
the Grid
Edwin Flores
University of Illinois at Chicago, Chicago, IL 60607
Thermal Energy Storage has improved the dispatch ability of Concentrated Solar Power Plants
(CSP), a renewable source of energy on the grid. Furthermore, Latent heat Thermal Energy Storage
(LTES)shows potential as a storage technology by further reducing costs and improving efficiency
for CSP plants. This papers reviews the goals for LTES, the developments in phase change materials
for LTES, the types of system configurations possible, and the challenges that LTES face. From
the scientific literature available, LTES systems can meet TES goals, and research is progressing in
making it a scalable technology for CSP plants.
Introduction
Thermal Energy Storage (TES) has gained traction
over the years as a way to store energy from renewable
sources. Particularly, it has gained more attention in the
application of Concentrated Solar Power (CSP) plants.
CSP plants concentrate the suns rays to heat a heat
transfer fluid that is fed it into a turbine system connected to a generator to produce electricity (see Figure
1). TES can improve the dispatch ability of CSP plants
by allowing the plant to store the suns heat during offpeak times and discharge it during peak times in the
absence of the sun or during shorter cloudy times, and
thus, increases CSP plants annual capacity factor. For
an example, TES systems with 7 hours of storage can increase a CSP plants capacity factor from 25% to as high
as 43%.1 As a comparison to other renewable sources
without storage, onshore wind farms have been shown to
have capacity factors from 30% to 45% and solar PV have
16% to 28%.2 For this reason, increased research efforts
have focused on the relationship between TES and CSP.
National efforts have grown in support of high temperature TES in conjunction with CSP technology. In 2011,
the U.S. Department of Energy launched the SunShot ini-
FIG. 1: Illustration of CSP plant layout consisting of a
solar field, TES, and power block subcomponents.
Adapted from Reference 3.
tiative as a collaborative project to make unsubsidized solar energy cost competitive by the year 2020.4 The goal is
to reduce the installed cost of solar energy systems about
75% of 2010 costs in order to increase large-scale adoption of the solar technology.4 One focus of the program
involves CSP technology with TES. Its specific milestone
is to achieve a subsidy-free levelized cost of electricity of
0.06 kW/h by the end of 2020.4 Current records for 2013
show an achievement of 0.13 kW/h, which is about 62%
reduction from 2010s 0.21 kW/h.4 The Sun-Shot Initiative is benefiting research in lowering costs and improving
performances of TES in CSP plants.
With regard to TES technology, three types of energy storage exist:sensible heat, latent heat and thermochemical. Sensible heat Thermal Energy Storage
(STES)uses either a solid or liquid as storage media
by increasing the temperature of the medium as more
thermal energy is stored. Latent heat Thermal Energy
Storage(LTES or LHTES) stores energy by converting
storage material from one phase to another (typically
a solid to liquid to reduce volumetric costs). Materials used in LTES are commonly referred to as Phase
Change Materials (PCMs). Thermochemical Thermal
Energy Storage stores and discharges energy by making and breaking chemical bonds. Each Thermal Energy
Storage type has different research projections, with sensible heat TES (STES) being the most mature technology
for CSP plants, followed by LTES, and thermochemical
TES as being the least mature technology.1
Various reviews have focused on TES with regard to
different design types and their techno-economic challenges.1,3After the successful commercialization of Sensible heat TES, Latent heat Thermal Energy Storage(LTES) is the next concept that can hold better performance for CSP plants, if designed well. This review
paper will focus on current trends in LTES for application in CSP plants. The general workings of LTES
systems and their ideal qualities for TES are explained,
and the current status and research on storage media
for LTES is reviewed below. Possible design types, challenges and future research possibilities for LTES are also
presented. This paper will present the reader with the
status of LTES and its potential in improving CSP plants.
Journal of Undergraduate Research 5, 1 (2015)
TABLE I: Key requirements for developing TES
systems for CSP plants.Table data taken from
References 1,6
1 High energy density capability of storage material
2 Efficient heat transfer between the storage material and HTF
3 Fast response to load changes in the discharge mode
4 Low chemical activity of storage material and HTF towards the
materials of construction
5 Good chemical stability of storage material/HTF and temperature
reversibility in a large number of thermal charge/discharge cycles
comparable to a lifespan of the power plant, 30 years
6 High thermal efficiency and low parasitic electric power for the
system
7 Low potential contamination of the environmental caused by an
accidental spill of large amount of chemical used in the TES system
8 Low cost of storage material, taking into account the embodied
energy (carbon)
FIG. 2: T-s diagram for steam production in a power
plant with thermal storage process. Adapted from
Reference 5
9 Ease of operation and low operational and maintenance costs
10 Feasibility of scaling up TES designs to provide at least 10 full
load operation hours for large-scale solar power plants of 50 MW
electrical generation capacity and larger
General LTES Systems Workings and Goals
Phase Change Materials and Enhancement Methods
Latent heat Thermal Electric Storage(LTES) works
through an isothermal process in which the storage material changes phases. This characteristic can provide
enhanced storage capacity when compared to Sensible
heat Thermal Energy Storage (STES)systems of the same
temperature range since STES is limited by the need to
store at higher temperatures to achieve the desired output temperature for the turbine systems. Also, LTES
can potentially enable a smaller, more efficient, and lower
cost alternative to STES due to storage capacity being
related to enthalpy of phase change of the material (see
Figure 2). For this reason, LTES commonly uses phase
changes between solid and liquid states, due in large part
to its lower volumetric expansion, compared to liquidgas, and its high latent heat compared to solid-solid transition.
After years of research development in overall TES
technology, beginning with sensible heat TES, various
characteristics have emerged to define an ideal TES.
DOEs requirements for TES under the SunShot Initiative seeks to improve heat transfer and thermal energy
storage medium, lower the systems costs to less than 15
kW/h, increase the exegetic efficiency to greater than
95%, and lower material degradation due to corrosion
to less than 15 m/year.4 Research in Europe proposed
a methodology that summarized the key requirements
for developing a thermal energy storage system for CSP
(see Table I).6 These are currently the targets that researchers in TES would like to achieve. LTES systems
show potential in fulfilling several of these categories including reduction of component materials and costs and
increasing high energy densities capabilities when combined with thermo-enhancing elements.
Phase Change Materials (PCMs)are the storage
medium for LTES systems. Their energy storage capability depends on the thermal properties of the materials. For PCMs, a high latent heat of fusion is desired
in order to function within the high operating temperatures for the power block (or turbine system of the CSP
plant). Additionally, high thermal conductivity for materials is desired since it positively affects the dynamics of
heat transfer and the performance of the system. It was
shown that a reduction of heat transfer tubes for concrete
TES systems was possible for concrete with higher thermal conductivities.7 Table II presents some of the potential PCMs for LTES systems. Although thermal physical
properties vary across material types, ways to improve
some of these properties to fit the desired ranges for a
CSP plant do exist.
Some research has focused on improving the low thermal conductivities of PCMs. Recent experiments have
demonstrated that adding high thermal conductivity materials such as graphite, metal fibers or metal/ceramic
matrices to PCMs can increase heat transfer.10 A recent study that added graphite foam to the PCM material, Magnesium Chloride (MgCl2 ),similarly indicated
that heat transfer was greatly enhanced due to graphites
high thermal conductivity, processing ability and chemical stability.11 Some of the key findings for graphite foam
LTES were that round trip energy efficiencies increased
from 68% to 97% and that the number of heat transfer pipes decreased by a factor of eight, which can lead
to significant cost reductions.11 By improving material
properties of PCMs, greater system performances can be
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©2015 University of Illinois at Chicago
Journal of Undergraduate Research 5, 1 (2015)
tween the tanks (common in sensible heat thermal energy
storage), or passive systems, where the storage medium
is solid, contained in one tank, and the heat transfer
fluid passes through the storage material only for charging and discharging.Most Latent heat Thermal Energy
Storage (LTES)systems are passive system types to ensure that phase changes occur in a controlled environment. The systems in this review are:embedded structures LTES,packed bed/thermocline LTES, and cascading LTES1,3 ,each with their own unique characteristics
in improving the overall performance of LTES.
TABLE II: Published data on potential latent heat
storage materials. Table data taken from from
References 1,8,9.
Tmelt Tmelt Material
[◦ C] [◦ F]
Latent
heat of
fusion
[J/g]
Thermal
cond.
[W/(m
K)]
307
585
NaNO3
177
0.5
318
604
77.2 mol% NaOH-16.2% NaCl-6.6% 290
Na2 CO3
320
608
54.2 mol% LiCl-6.4% BaCl2 -39.4% 170
KCl
335
635
KNO3
88
340
644
52 wt% Zn-48% Mg
180
348
658
58 mol% LiCl-42% KCl-33% NaCl 170
380
716
KOH
380
716
45.4 mol% MgCl2 -21.6% KCl-33% 284
NaCl
381
718
96 wt% Zn-4% Al
397
747
37 wt%
Li2 CO3
443
829
59 wt% Al-35% Mg-6% Zn
310
450
842
48 wt% NaCl-52% MgCl2
430
0.96
470
878
36 wt% KCl-64% MgCl2
388
0.83
487
909
56 wt% Na2 CO3 -44% Li2 CO3
368
2.11
500
932
33 wt% NaCl-67% CaCl2
281
1.02
550
1022
LiBr
203
632
1170
46 wt% LiF-44% NaF2 -10% MgF2 858
1.20
660
1220
Al
398
250
714
1317
MgCl2
452
149.7
Na2 CO3 -35%
0.5
0.5
Embedded Stuctures LTES
138
K2 CO3 - 275
One concept of embedded structures LTES employs
pipes embedded in the storage medium to transfer heat
effectively as the heat transfer fluid passes through the
pipes. One study used a shell and tube heat storage
unit as a LTES because of its high efficiency and relatively smaller volume.13 In this particular unit, the Phase
Change Material (PCM) fills the annular shell space
around the tube while the heat transfer fluid flows within
the tube and exchanges the heat with the PCM.
2.04
Another embedded structure LTES uses a sandwich
concept for PCM using different fin materials made from
graphite or aluminum.13 The fins are mounted vertical to
the axis of the tubes and the PCM is placed between fins.
The fins enhance the heat transfer between the PCM and
heat transfer fluid.13 Systems such as these have increased
the thermal power densities to 10-25 kW m−3 .1
achieved, leading to cost reductions in other components
such as heat exchangers.
Other methods to improve the heat transfer of PCM
materials are macro- and microencapsulation.
Encapsulation involves putting PCMs in capsules made
from higher conducting materials as a way to overcome difficulties of low heat transfer of large masses
of PCM. Moreover, encapsulation of PCMs reduces
the PCMs reactivity with the surrounding environment
and controls the changes in the PCMs volumes as
the phase changes occur.1 Macro-encapsulation involves
larger sized capsules, but requires labor-intensive manufacture processes.3 On the other hand, microencapsulation involves smaller sizes and cheaper batch manufacturing processes. For this reason, micro-encapsulation is
investigated more often, particularly for radius sizes such
as 0.5cm.12 By encapsulating PCMs, less storage medium
is needed while thermal conductivity increases, thus leading to potential cost reductions for LTES systems.
Another concept under consideration is the employment of embedded gravity assisted Heat Pipesor thermosyphons between a PCM and heat transfer fluid in
order to improve heat transfer (see Figure 3). Many researchers have focused their efforts on investigating the
system properties of these systems.14-17Heat pipes have
a high thermal conductivity, allowing them to increase
the rate of heat transfer between the heat transfer fluid
and PCM.18Additionally, heat pipes can be modified to
operate passively in specific temperatures ranges and can
be fabricated in a variety of shapes.
Of all these variations for embedded structures LTES,
heat pipes LTES has been studied closely for integration to a modeled Concentrated Solar Power (CSP) plant.
One study found that longer heat pipe condenser length
increased the surface area available for the phase change,
enabling higher energy storage.14 However, a tradeoff between storage costs and heat exchange rates between the
heat transfer fluid and PCM emerged due to optimum
longitudinal spacing between heat pipes.14 This study is
critical for understanding the methodology to determine
the optimum design for LTES embedded with heat pipes
when working in conjunction to CSP plant operation.
Latent Heat Thermal Energy Storage System Types
In order to achieve the desired efficiencies and energy capacities for Thermal Energy Storage (TES), various system designs have been proposed. In general,
these systems are categorized in two groups: active systems, where the storage medium is a fluid and flows be30
©2015 University of Illinois at Chicago
Journal of Undergraduate Research 5, 1 (2015)
FIG. 4: Schematic illustration of: (a) latent thermocline
storage system (LTES), (b) capsule cross-section.
Adapted from Reference 14
FIG. 3: Schematic illustration of: (a) close up view of
HTFs pipe with gravity heat pipes (b) gravity heat
pipes embedded latent thermal energy storage
(HP-TES). Adapted from Reference 14.
different temperature ranges to convert liquid to gas (an
isothermal process) and a higher temperature to superheat the steam in order to increase the turbine efficiency
(see Figure 2). A cascading system would utilize certain
PCM materials in discharging energy during the liquidgas phase of the steam Rankine cycle. Then other PCMs
would provide the energy at the appropriate high temperature to superheat the steam in order to increase the
turbine efficiency.
Currently several options exist for LTES systems that
improve heat transfer between PCMs and heat transfer
fluid and lead to material and cost reductions. Moreover,quantitative studies have been performed on some
systems to estimate the LTES performance on the storage system by itself or connected to a CSP plant.14,15
These studies are paving the way to better understand
the performances and feasibility of LTES systems before
actual construction, investment, and testing of a fullscale LTES system connected to CSP plant.Continued
research on LTES system designs will help overcome the
challenges in construction of full-scale LTES system.
Packed bed systems
Packed bed systems use a storage tank that contains
material elements, in this case PCMs, positioned in various shapes and sizes. These elements help transfer heat
from the heat transfer fluid as it flows between these elements and stores the thermal energy for later use.These
systems act similar to a thermocline system, in which
thermal gradient develops within the tank (see Figure
4). The hot heat transfer fluid is pumped to the top
in charging mode and pushes the colder fluid to the bottom. These elements act as filler materials to maintain an
ideal thermal gradient and reduce the natural convection
within the liquid. These systems are mostly single tanks,
which reduce costs compared to sensible heat Thermal
Energy Storage systems that use two tank systems to
establish a thermal gradient.
Packed bed systems with encapsulated PCMs have
been studied in detail with regard to their dynamic
performance when incorporating operational conditions
found in aCSP plant.Researchers found that the use of
smaller PCM capsules (.0075 m or less) can greatly reduce the tank volume by up to 40% compared to similar
systems with sensible materials.15 Overall, the numerical
study showed the effectiveness of packed bed thermocline
systems and provided a procedure to design an optimum
system with operational requirements for a CSP plant.
Challenges of Latent Heat Thermal Energy Storage
Although research has established tools for understanding and evaluations on the potential of Latent
heat Thermal Energy Storage (LTES) systems, various
challenges still exist in the deployment of LTES systems.These challenges have been reviewed by Steklie et.
al, and are elaborated in this section on tank Phase
Change Material (PCM) TES systems and encapsulated
TES systems.3 Some of the setbacks are attributed to
poor heat performances, manufacturing difficulties, and
durability of LTES systems. Nonetheless, opportunities
do exist for future research to investigate these setbacks.
Challenges for Tank PCM TES systems include poor
heat transfer, low energy efficiency, and added costs of
Cascading Systems
Cascading systems utilize more than one PCM in a single tank in order to maximize the exergetic efficiency.16,17
Since CSP plants utilize a steam Rankine cycle, they need
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©2015 University of Illinois at Chicago
Journal of Undergraduate Research 5, 1 (2015)
to overcome poor heat performances of PCMs, to discover manufacturing methods for microencapsulation,
and to test the durability of LTES after various charging/discharging cycles. Moreover, determining and lowering the costs for design and construction processes
need further investigation before a full scale LTES is
implemented to a CSP plant.A study by Nithyanandam
et al. provides one example of research analyzing the
techno-economic feasibility of LTES systems and performance when coupled to CSP plants. This study found
that Encapsulated PCM TES and LTES with embedded
heat pipes with a two-tank Sensible heat thermal energy
storage system can meet SunShot requirements in costs
(6 ¢kWh−1 ), energetic efficiency, and performance for
Supercritcal-CO2 cycles rather than steam cycles.12 With
regard to steam Rankine cycle, the Levelized Cost Of
Electricity (LCOE)for both systems were higher, ranging
in 9-10 ¢kWh−1 .12 Research like this shows promise for
LTES systems to become a competitive TES technology,
even despite the above named challenges.
cascading system variations. Typically, large tanks filled
with a PCM (commonly a salt) suffer from a low heat
discharge rate due to a buildup of solids near the heat
transfer area.3 The poor energetic efficiency is due to the
fact that TES systems incorporating one PCM can only
operate at a given temperature. This limits the TESs
operation to specific parts of the steam Rankine cycle
for a Concentrated Solar Power (CSP) plant. Various
solutions have been proposed to counter these challenges,
although bringing with it an added cost to the system
as a whole. Further research is needed to ascertain if
the added cost of improvements to the tank PCM TES
systems pays off in the performance of the overall CSP
plant while reducing costs in other components such as
pipes, heat exchangers and PCM.
Encapsulated PCM TES systems has its own set of
challenges, which focus on the manufacturing methods
needed to create a durable capsule for high temperature
CSP plant. Since encapsulation of PCM requires high
temperature durability and a void space to allow for expansion of the material, methods of manufacturing encapsulated materials for these applications need further
studies.1,3 Moreover, higher costs are assumed in developing such manufacturing methods and testing their quality control to ensure the encapsulated PCMS last for 30
years as required for CSP plants. Further research on
analyzing manufacturing methods, testing the reliability
of the production, and lowering its costs is called for in
order for Encapsulated PCM TES systems to be a viable
model.
Other challenges that LTES systems face,in common
with other sensible heat TES, are corrosion at high temperatures and loss of mechanical strength at high temperatures. Standard materials of containment construction
such as carbon and stainless steel have corrosion rates
higher than 15 µm yr−1 at 650 ◦ C when containing solar
salt.1 Corrosion measurement techniques for high temperature applications are also time intensive in gathering
data (months to years) and limited to testing a set of
conditions at a time.3 Additionally containment materials lose mechanical strength at high temperatures. Methods such as internal insulation and shot peening (a type
of strain hardening)do exist to mitigate these strength
losses, but further research is suggested in evaluating
any side effects when containing various PCM for many
charging/discharging cycles and any associated costs for
maintenance.
This section clearly indicates more research is needed
1
2
3
Conclusion
The status of LTES systems in the areas of requirements, material components, systems types, and challenges has been reviewed above. Research on LTES systems continues to look toward enhancing PCMs thermophysical properties,while trying to be cost effective.
Moreover, several types of LTES systems indicates that
they are capable of increasing thermal storage and heat
transfer and at the same time offsetting other system
costs.The challenges for LTES present opportunities of
future investigations toward developing better tank storage materials with less corrosion and improving manufacturing practices for encapsulated phase change materials. Although full-scale LTES systems have not yet
been built, numerical models estimate that encapsulated
phase change materials TES and heat pipes TES have
potential in meeting performance and cost requirements
compared to todays sensible heat TES technologies. A
pilot plant for LTES is needed to validate numerical models and show further insight on the feasibility of LTES.
Research in LTES systems in general point toward material research and manufacturing research in order to set
the stage to build a full scale LTES system and prepare
it for commercialization in the future.
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