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energies
Article
Phase Change Material Selection for Thermal Energy
Storage at High Temperature Range between 210 ◦C
and 270 ◦C
JoséMiguel Maldonado 1, Margalida Fullana-Puig 1,2, Marc Martín1ID , Aran Solé3,
Ángel G. Fernández 4, Alvaro de Gracia 1,2 and Luisa F. Cabeza 1, *ID
1GREiA Research Group, INSPIRES Research Centre, Universitat de Lleida, Pere de Cabrera s/n,
25001 Lleida, Spain; jmmaldonado@diei.udl.cat (J.M.M.); mfullana@diei.udl.cat (M.F.-P.);
marc.martin@udl.cat (M.M.); alvaro.degracia@udl.cat (A.d.G.)
2CIRIAF—Interuniversity Research Centre on Pollution and Environment Mauro Felli, Via G. Duranti 63,
06125 Perugia, Italy
3Department of Mechanical Engineering and Construction, Universitat Jaume I, Av. Vicent Sos Baynat, s/n,
12071 Castellón de la Plana, Spain; sole@uji.es
4
Energy Development Center, University of Antofagasta, Av. Universidad de Antofagasta, 02800 Antofagasta,
Chile; angel.fernandez@uantof.cl
*Correspondence: lcabeza@diei.udl.cat
Received: 23 February 2018; Accepted: 27 March 2018; Published: 8 April 2018
Abstract:
The improvement of thermal energy storage systems implemented in solar technologies
increases not only their performance but also their dispatchability and competitiveness in the energy
market. Latent heat thermal energy storage systems are one of those storing methods. Therefore,
the need of finding the best materials for each application becomes an appealing research subject.
The main goal of this paper is to find suitable and economically viable materials able to work as
phase change material (PCM) within the temperature range of 210–270
◦
C and endure daily loading
and unloading processes in a system with Fresnel collector and an organic Rankine cycle (ORC).
Twenty-six materials have been tested and characterized in terms of their thermophysical conditions,
thermal and cycling stability, and health hazard. Two materials out of the 26 candidates achieved the
last stage of the selection process. However, one of the two finalists would require an inert working
atmosphere, which would highly increase the cost for the real scale application. This leads to a unique
suitable material, solar salt (40 wt % KNO3/60 wt % NaNO3).
Keywords:
thermal energy storage (TES); Fresnel collector; thermal stability; cycling stability;
thermophysical properties; health hazard; myo-inositol; solar salt; infrared spectroscopy (IR);
differential scanning calorimetry (DSC)
1. Introduction
Tackling greenhouse effect emissions by decarbonising energy production is one of the European
Commission’s main concerns. Therefore, solar energy has been highly promoted to globally reduce
the carbon footprint. Among other solar energy technologies, concentrated solar thermal energy
could cover up to 7% of world energy demand by 2030 and 25% by 2050 if considering advanced
development in its efficiency [1].
To increase solar thermal power plants’ competitiveness against fossil fuels power plants,
their dispatchability
needs to be enhanced so that they can dispatch power on grid demand or
according to market needs. Thermal energy storage (TES) systems can accomplish that requirement,
allowing solar thermal plants to generate energy consistently at times when sunlight is not available,
Energies 2018,11, 861; doi:10.3390/en11040861 www.mdpi.com/journal/energies
Energies 2018,11, 861 2 of 13
which makes the power plant more cost effective. Thermal energy storage also allows greater use of
the power-block components and increase grid flexibility.
The improvement of thermal energy storage systems is continuously targeted by researchers,
and this is studied in any of the three technologies of TES: sensible heat, latent heat, or sorption and
chemical reactions (also known as thermochemical energy storage). Among TES technologies, latent
storage using phase change materials (PCM) offers an effective solution since it has high energy storage
density and the melting and solidification processes undertaken by PCM are almost isothermal [2].
Research and development efforts within the solar energy industry are focused on lowering
the high operation and maintenance cost of solar plants. The choice of PCM has a strong influence
on such costs; not only the material itself but also indirect costs such as its container and the safety
measurements that the material requires. Therefore, it is crucial to perform an exhaustive phase change
material selection process considering materials cost, availability, and their facilities requirements.
Differential scanning calorimeter (DSC) and thermal gravimetric analysis (TGA) are widely used
techniques to characterize PCMs [
3
–
7
]. Haillot et al. [
3
] characterized 11 materials using DSC and
TGA coupled with a quadrupole mass spectrometer (QMS). The study checked solid–solid transition
materials, sugar alcohols, polymeric hydrocarbons, and aromatic hydrocarbons among other materials
that can be used in applications with interest in the 120–150
◦
C temperature range. The analyses were
performed before and after five thermal cycles, concluding that only a few organic materials showed
potential for thermal storing purposes and further investigations are needed regarding long-term
stability. Their study pointed out the relevance of the measurement conditions over the results.
Del Barrio et al. [
4
] evaluated the key thermal and physical properties of five sugar alcohols and
three eutectic blends; their aim was to compare them with the PCM currently used within 70
◦
C and
180
◦
C. The study concluded that sugar alcohols seem to be suitable materials for latent heat storage.
However, several issues, such as sugar alcohol thermal endurance and stability, must be faced by
further research.
Bayón et al. [
5
] performed DSC and TGA analysis, coupled with polarized light microscopy and
rheological measurements, on liquid crystals to check their feasibility as latent heat storage material.
They concluded that, despite the promising results shown by those materials, their long-term thermal
stability should be further studied.
Miro et al. [
6
] and Gasia et al. [
7
] also included health hazard analysis to take into account the
PCM effect on living beings and their impact on the facilities maintenance and design. Miro et al. [
6
]
added, to the latter analysis, thermal stability and thermal cycling tests to check the aging of the PCM.
Five different materials were tested within a 150
◦
C and 200
◦
C temperature range, concluding that
the suitable materials at that temperature range are benzanilide and D-mannitol; the authors also
considered hydroquinone when used in a closed system. Gasia et al. [
7
] characterized 16 materials;
their phase change temperature comprised from 120 to 200
◦
C and cycling stability up to 100 cycles.
They concluded that adipic acid and high-density polyethylene are fit candidates as PCMs for the
studied temperature range.
The overall objective of this study is to find suitable phase change materials to store thermal
energy in a temperature range from 210 to 270
◦
C, which fits the real application requirements.
As sundown takes place, the energy source is cut out. To keep-up with production, the TES takes over
as the energy supplier. Therefore, the PCM has to endure daily phase changes. The first stage of this
research is to perform a materials screening, considering both organic and inorganic materials in order
to select suitable candidates according to literature. Afterward, their health hazard was evaluated,
and their thermophysical properties where characterized by means of DSC. This process led to the
final candidates that were deeply characterized with thermal stability and cycling stability tests under
real operation conditions. Finally, results were discussed according to the defined requirements.
Energies 2018,11, 861 3 of 13
2. Materials Screening
In order to select the appropriate material a preliminary screening was done. Considered materials
were divided into inorganic and organic groups in the following subsections.
2.1. Inorganic Materials
At the temperature range of a solar plant output application, different inorganic materials can be
considered for use as PCMs. Common examples of inorganic materials are several technical salts and
binary eutectic mixtures of such salts shown in Table 1.
Table 1.
Inorganic salt phase change material (PCM) candidates with phase change temperature
between 210 ◦C and 270 ◦C.
Inorganic Salt Price (€/kg) Melting Temperature (◦C) Melting Enthalpy (J/g) Reference
80 wt % NaOH/20 wt % LiOH 52.3 215 280 [8,9]
40 wt % KNO3/60 wt % NaNO333 222 100 [10–12]
NaNO3/NaNO2* 39.3 226–233 n.a. [13]
45 wt % Ca(NO
3
)
2
/55 wt % NaNO
340.3 230 ~110 [10]
61 wt % NaOH/39 wt % NaNO232.9 232–265 250–300 [14]
87 wt % NaNO2/13 wt % NaOH * 33.2 230–232 206–252 [14]
Ca(NO3)2/LiNO390.6 235 n.a. [15]
LiNO3147 252 380 [16]
NaNO233.25 270 200 [17]
* Complex phase diagram, PCM with different compositions feasible; n.a.—not available.
From the economic point-of-view, several materials are expensive due to the presence of Li
(>150
€
/kg for pure lithium). In contrast with inorganic salts, pure metals and alloys have high
thermal conductivity and small volume change during phase change process. A TES system based
on these materials could provide fast thermal response and high operational power according to
recently performed studies [
18
,
19
]. Nevertheless, their energy density, fast thermal response, and high
operational power do not pay off the high cost of the material. Classified by phase change temperature,
metal PCMs can generally be divided into three categories: low temperature (0–30
◦
C), middle
temperature (40–200
◦
C), and high temperature (>200
◦
C) [
20
]. In this case, high-temperature phase
change material is required in the 210–270
◦
C temperature range (Table 2). However, as stated
by Fernández et al. [
21
], metal PCM present several issues to be considered: vapor pressure,
subcooling, corrosion or other undesired reactions, segregation, and changes in composition and
microstructure due to thermal cycling. Therefore, metal PCMs are excluded from this study since
further research is needed before these materials can be used as PCMs in thermal energy storage
systems at industrial scale.
Table 2. Metal PCM candidates with phase change temperature between 210 ◦C and 270 ◦C.
Metal Price (€/kg) Melting Temperature (◦C) Reference
Lead—Antimony alloys 2–3 251–254 [22]
Lead, babbitt metall alloy (Cu, Pb, Sb, etc.) 2.5–4 237–272 [22]
Lead—Tin alloys 5–6 183–277 [22]
Lead—Magnesium eutectic alloy n.a. 249 [23]
Tin—Lead alloys 13–14 181–296 [22]
Tin, babbitt metall alloys 16–19 241–354 [22]
Tin—Antimony alloys 7–7.5 236–256 [22]
Tin (pure) 7–7.5 227–232 [22]
Tin—silver alloys 35–40 221–222 [22]
Selenium (pure) 37–43 221 [22]
n.a.—not available.
Energies 2018,11, 861 4 of 13
2.2. Organic Materials
Organic PCMs offer advantages compared with inorganic PCMs, such as practically null corrosion
and low or no subcooling. In contrast, they generally show lower thermal conductivity and phase
change enthalpies as well as higher flammability. Nevertheless, three organic materials have been
found with promising properties in the chosen temperature range. Table 3shows the two sugar
alcohols and the polymer, respectively.
Table 3. Organic PCM candidates with phase change temperature between 210 ◦C and 270 ◦C.
Organic
Material
Molecular
Formula Price (€/kg) Melting Temperature
(◦C)
Melting
Enthalpy (J/g) Reference
Pentaerythritol C5H12O49.7–13 * 258–260 n.a. [24]
Myo-Inositol C6H12O68–10 * 220 190 [25]
PBT (C12H12 O4)n2.14–2.36 220–267 n.a. [20]
PCTFE (CF2CClF)n87.7–105 206–226 n.a. [20]
* Min. order 1000 kg.
Sugar alcohols are relatively new materials in the TES field [
25
]. They show potential to be
implemented as PCM since they have high phase change enthalpy, low cost, and are non-toxic [
26
].
Pentaerythritol also has a solid-solid phase change transition at 182–183
◦
C largely studied for use as a
PCM. For example, Hu et al. [
27
] included it in a composite with nano-AlN confirming improvements in
the crystallization process. A study comparing charging/discharging process with different fin volume
ratios was carried out by Khonjera et al. [
28
]. A novel point-of-view is given in this cited paper since
there is a lack of pentaerithritol studies as a PCM concerning its solid-liquid phase change transition.
Myo-inositol thermal cycling stability has been broadly studied by Soléet al. [
25
]. Although some
chemical changes were identified by FT-IR analysis, acceptable thermal performance was observed
after 50 cycles between 150
◦
C and 260
◦
C. However, new crystalline structures were observed when
thermal cycling range was extended to 50–260 ◦C.
Thermoplastic polymers present potential to be used as PCM since they have semi-crystalline
structure and, therefore, noteworthy energy related to their phase change. Based on this, high-density
polyethylene (HDPE) has been extensively studied and several times proposed as PCM for 100–150
◦
C
temperature range applications [
3
,
7
]. Nevertheless, no polymeric PCMs have been found in the
literature for the temperature range (210–270
◦
C) of interest in this paper. Thus, two new polymeric
materials are considered for their application as phase change materials since they have semi-crystalline
structure and suitable phase change temperature. Polybutylene terephthalate (PBT) is an engineering
polymer typically used in electric and electronics industries; it presents low price and, due to its
regular radical distribution in the aromatic rings, it ensures a high degree of crystallinity and has low
shrinkage during solid-liquid phase change transition. Polychlorotrifluoroethylene (PCTFE) finds its
applications due to its non-flammable properties and high chemical stability.
3. Materials and Methodology
3.1. Materials
Nine phase change materials with melting temperatures between 210
◦
C and 270
◦
C were selected
and their thermophysical properties studied. They belong to different PCM groups: inorganic,
sugar alcohol, and polymer.
The preselected materials were five inorganic compounds: 98% purity KNO
3
from VWR prolabo
(Llinars del Vallès, Spain), 98% purity NaNO
2
from Sigma Aldrich (Saint Louis, MO, USA), and Panreac
(Castellar del Vallès, Spain) provided with 98% purity NaOH, >99% purity NaNO
3
and 99% purity
LiOH. Two organic compounds were preselected, more specifically two sugar alcohols: 99% purity
Pentaerythritol from Sigma Aldrich and >98% purity Pentaerythritol from Xi’an lyphar biotech Co.
Energies 2018,11, 861 5 of 13
LTD (Xi’an, China), from the food industry under the US NF12/FCCV standard. Finally, a polymer
was also preselected PBT from BASF (Ludwigshafen, Germany).
Eutectic mixtures were prepared by mixing both solid components in the eutectic proportion and
melting the mixture at a temperature above their respective melting temperatures. Following this
methodology, several eutectic compounds were not able to be prepared. Therefore, both components
were dissolved in water, afterwards water was evaporated while the eutectic mixture remained. Table 4
shows the PCM melting temperature and enthalpy of the considered materials.
Table 4. Literature thermophysical characterization of PCMs.
PCM Melting Temperature (◦C) Melting Enthalpy (J/g) Reference
40 wt % KNO3/60 wt % NaNO3222 100 [10,11]
61 wt % NaOH/39 wt % NaNO2232–265 250–300 [14]
87 wt % NaNO2/13 wt % NaOH 230–232 206–252 [14]
80 wt % NaOH/20 wt % LiOH 215 280 [8]
70 wt % NaOH/30 wt % LiOH 215 280 [9]
NaNO2270 200 [17]
Myo-inositol 220 190 [25]
PBT 220–267 n.a. [20]
3.2. Health Hazard
Health hazard is studied to detect potential operational and personal risks of the selected PCM.
In this application, personal risks are not a critical parameter since the phase change material will be
stored in an enclosed tank. Nevertheless, health hazard has to be taken into account considering that it
indicates the standard procedures that need to be followed during the handling and operation of the
selected PCM and the degree of personal protective equipment.
In this study, health hazard was evaluated by means of the National Fire Protection Association
(NFPA) 704 diamond standard (Figure 1). The “NFPA 704: Standard system for the identification of
the hazards of materials for emergency response” is a standard which has been developed by the
National Fire Protection Association (NFPA). This standard visually provides the riskiness of common
chemical products by means of a colored diamond. This diamond is divided in four indicators:
flammability, health hazard, chemical reactivity, and special hazards. In this study, the blue indicator
that corresponds to health hazard is followed, which is graded from 0 to 4, being 0 non-hazardous
substances and 4 the ones that could cause death or major residual injury by very short exposure.
Figure 1. National Fire Protection Association (NFPA) 704 diamond standard [29].
Energies 2018,11, 861 6 of 13
Furthermore, the materials were also evaluated and labelled according to the Globally
Harmonised System (GHS) of classification, labelling, and packaging of substances or mixtures (CLP).
GHS classification
is provided by 1392 companies from 20 notifications to the European Chemical
Agents (ECHA).
According to 1272/2008/EC, a substance or mixture classified as hazardous and contained in
packaging shall have a label including different information such as the name and contact of the
supplier, the nominal quantity of the substance, and product identifiers, if applicable, such as hazard
pictograms, signal words, hazard statements, appropriate precautionary statements, etc. [
30
]. Therefore,
the hazard statements of the studied materials were also analysed. The hazard statements are classified
in three sub-groups: physical, health and environmental hazards. All statements are identified with
an uppercase H followed by a number comprised in the range of two, three or four hundred for each
sub-group respectively.
3.3. Thermophysical Characterization
The equipment used for themophysical characterization was a DSC 822e (Mettler Toledo,
Columbus, OH, USA). The amount of sample used was around 10 mg and experiments were performed
under a N
2
flow. Sugar alcohol and polymer samples were located into 40
µ
L cold-welded aluminum
crucibles. In order to avoid undesired chemical reactions with inorganic salt, these samples were
analyzed using a reusable 30
µ
L gold plated crucibles in the first stage selection, and then the
selected materials, after assuring that their nature would not react with the crucibles, were located in
cold-welded aluminum crucibles. The methodology followed to obtain the phase change temperature
and enthalpy of the PCM is based on dynamic temperature programs from 50
◦
C under their theoretical
phase change temperatures to 50
◦
C above it. The equipment precision is
±
0.1
◦
C for temperature and
±3 J/g for enthalpy results.
3.4. Thermal Stability
Thermogravimetric analyses (TGA) were carried out in order to characterize PCM thermal
decomposition. The equipment used was a TA Instrument Simultaneous SDTQ600 (New Castle, DE,
USA), which allows DSC-TGA measurements up to 1500
◦
C and has a balance sensitivity of 0.1
µ
g.
The analyses were performed under a 50 mL/min air atmosphere to simulate real boundary conditions.
The heating rate used to perform the thermogravimetric analysis was 10
◦
C/min from 40 to 600
◦
C.
Opened 100
µ
L alumina crucibles used were filled with around 1/3 volume of material leading to
average sample masses of around 22 mg. This technique provides maxim working temperature and
final degradation temperature of the analyzed materials.
3.5. Cycling Stability
Thermal cycling stability tests were performed to study changes in the thermophysical properties
of the PCM after a certain number of solidification-melting cycles within the operating temperature
range of the process in which the TES material will be implemented. The candidate materials were
exposed to 10 and 50 complete solidification-melting cycles. The thermal cycles were performed
in a muffle-type furnace N31H (NABERTHERM, Lilienthal, Germany) between 200
◦
C and 250
◦
C,
simulating real operating conditions. Samples of 10 g
±
1 g of each material were enclosed in crucibles
in air atmosphere. Porcelain crucibles were used for both materials. Furthermore, closed PTFE crucibles
were also used in the case of myo-inositol in order to simulate closed systems that may affect material.
Afterwards, phase change temperature and enthalpy were evaluated using DSC technique under the
same conditions described in Section 3.3.
The chemical characterization was carried out using a Fourier transform infrared (FT-IR)
spectroscopy with attenuated total reflectance (ATR), which analyses the PCM chemical degradation
caused by thermal cycling. The advantage of ATR is the possibility of obtaining the spectra directly
from the sample, without any specific sample preparation. The partial or total disappearance of
Energies 2018,11, 861 7 of 13
the characteristic peaks and/or the appearance of new peaks can indicate that the material is being
oxidized or degraded. This analysis was carried out with a PIKE MIRacle™ ATR sampling accessory
with a Diamond/ZnSe ATR base, FT-IR 6300 (Hachioji, Tokyo, Japan). It allows analysing substances
in solid and liquid states. It was optimized by a wavelength range between 4000 and 650 cm
−1
, and its
standard spectral resolution is 4 cm
−1
accounting for 64 infrared scans for each analysis; the data
recorded are their means. Its functionality is based on the characteristic wave numbers at which the
molecules vibrate in infrared frequencies.
4. Results and Discussion
4.1. Health Hazard
Table 5shows the health hazard characterization of the considered inorganic candidates, in fact
of the chemicals used to formulate the compounds produced. Following the NFPA 704 standard,
potassium and sodium nitrates are hazardous. Moreover, sodium nitrite, sodium and lithium
hydroxides can cause serious or permanent injury. Despite KNO
3
and NaNO
3
have the same HFPA
value (2), GHS method labels sodium nitrate as more dangerous to the human beings since it causes
damage to organs through prolonged or repeated exposure.
Table 5.
Health hazard values according to NFPA 704 and Globally Harmonised System (GHS) of the
considered inorganic materials [24,31].
Material NFPA 704 GHS
KNO3
H272 May intensify fire; oxidizer
H315 Causes skin irritation
H319 Causes serious eye irritation
H335 May cause respiratory irritation
NaNO3
H272 May intensify fire; oxidizer
H320 Causes eye irritation
H341 Suspected of causing genetic defects
H370 Causes damage to organs
H372 Causes damage to organs through prolonged or
repeated exposure
LiOH
H290 May be corrosive to metals
H302 Harmful if swallowed
H314 Causes severe skin burns and eye damage
H318 Causes serious eye damage
H412 Harmful to aquatic life with long lasting effects
NaNO2
H272 May intensify fire; oxidizer
H301 Toxic if swallowed
H319 Causes serious eye irritation
H341 Suspected of causing genetic defects
H361
Suspected of damaging fertility or the unborn child
H362 May cause harm to breast-fed children
H370 Causes damage to organs
H373 Causes damage to organs through prolonged or
repeated expose
H400 Very toxic to aquatic life
H410 Very toxic to aquatic life with long lasting effects
NaOH
H290 May be corrosive to metals
H314 Causes severe skin burns and eye damage
H315 Causes skin irritation
H318 Causes serious eye damage
H319 Causes serious eye irritation
Energies 2018,11, 861 8 of 13
Table 6shows the health hazard evaluation of the considered materials following both the NFPA
704 diamond standard method and the GHS methods. Organic materials studied have low health
hazard ratio according to the NFPA 704, considering them as slightly hazardous; while the GHS
standard shows that those materials cause irritation on living beings.
Table 6. Health hazard values according to NFPA 704 and GHS of the considered organic materials [31,32].
Material NFPA 704 GHS
Pentaerythritol H319 Causes serious eye irritation
Myo-Inositol
H315 Causes skin irritation
H319 Causes serious eye irritation
H335 May cause respiratory irritation
Polybutylene
terephthalate PBT
(C12H12 O4)n
-No need for classification according
to GHS criteria for this product
If LiOH, NaNO
2
, or NaOH were selected as ideal candidates, a specific safety study would be
required. This study could lead to some system design modifications, so their economic impacts
should also be considered.
4.2. Thermophysical Characterization
To confirm the literature phase change temperature and enthalpy of fusion of the studied materials,
DSC analyses were carried out for each material listed in Table 4. Table 7shows the comparison between
the experimental melting enthalpy and temperature of the considered inorganic materials, obtained
following the protocols described in Section 3.3, and the values reported by the literature.
Several inorganic salts mixtures have complex phase diagrams such as NaNO
2
/NaOH mixtures.
It is difficult to ensure homogeneous eutectic composition along the whole material, especially for large
quantities (>1000 kg) [
33
]. Along the DSC experimental tests, it was noticed that 70 wt % NaOH/30
wt % LiOH did not show phase change, and this is probably because the mixture resulted is not
homogeneous (not a proper eutectic mixture). However, despite of the adopted composition 80 wt %
NaOH/20 wt % LiOH experimental phase change temperature is in concordance with the values found
in literature, phase change enthalpy is much lower than expected. The eutectic phase solidification is a
kinetic process that is governed by multiple hardly controllable factors, for that reason the DSC tests
showed differences with the enthalpy values reported in the literature. Similar difficulties as the ones
encountered for the latter material were faced while the characterization of 87 wt % NaNO
2
/13 wt %
NaOH and 39 wt % NaNO2/61 wt % NaOH was carried out.
Unlike the previous materials, NaNO
2
and 40 wt % KNO
3
/60 wt % NaNO
3
DSC results showed
almost the same values as the literature, suitable latent heat storage capacity, and sharp phase
change temperature. However, sodium nitrite (NaNO
2
) experimental phase change temperature
exceeds the maximum acceptable temperature fixed by the output temperature of the solar field.
All in all, the so-called “solar salt” is the only feasible inorganic material which meets the required
thermophysical properties.
Energies 2018,11, 861 9 of 13
Table 7. Differential scanning calorimeter (DSC) results of inorganic salts and eutectic mixtures [8–12,14,17].
Inorganic Materials Melting Temperature (◦C) Melting Enthalpy (J/g)
Observations
Literature Experimental Literature Experimental
40 wt % KNO3/60 wt % NaNO3222 221 100 94 √No subcooling
61 wt % NaOH/39 wt % NaNO2232–265 232 250–300 60 ×Poor eutectic phase
formation
87 wt % NaNO2/13 wt % NaOH 230–232 232 206–252 35 ×Poor eutectic phase
formation
80 wt % NaOH/20 wt % LiOH 215 214 280 8 ×Poor eutectic phase
formation
70 wt % NaOH/30 wt % LiOH 215 - 280 - ×No eutectic phase
formation
NaNO2270 281 200 178
×Phase change
temperature out of range
√
High latent heat storage
capacity
Table 8shows the experimental melting temperature and enthalpy of the considered organic
materials obtained from DSC tests. Pentaerythitol solid-liquid phase change enthalpy was lower than
equipment resolution, so it could not be measured accurately. On the other hand, DSC tests confirm
the phase change temperature reported in the literature for myo-inositol (C
6
H
12
O
6
) [
25
] as well as for
polybutylene terephthalate (PBT) ((C
12
H
12
O
4
)
n
) [
20
]. The measured melting enthalpy of myo-inositol
was 223 J/g, higher than the one reported in previous studies [
25
]. However, measured PBT melting
enthalpy was very low (49 J/g) and was not reported in the literature to have a comparison.
Table 8. DSC results of organic materials (polymer and sugar alcohol) [20,24,25].
Organic
Material
Melting Temperature (◦C) Melting Enthalpy (J/g) Comments
Literature Experimental Literature Experimental
Pentaerythritol 258–260 - - - ×Did not show solid-liquid phase
change peak
Myo-Inositol 220 235 190 223
×High difference between solidification
and melting temperature (>35 ◦C)
√High latent heat storage capacity
√Phase change temperature within
the range
Polybutylene
terephthalate
(PBT)
220–267 227 - 49
×High difference between solidification
and melting temperature (>25 ◦C)
×Low latent heat storage capacity
√Phase change temperature within
the range
Therefore, given the thermophysical results obtained in the laboratory, three materials, 40 wt %
KNO
3
/60 wt % NaNO
3
, myo-inositol, and PBT, were preselected and deeper characterized in the
following section.
4.3. Thermal Stability
Thermal gravimetric analyses were performed on the two inorganic salts (40 wt % KNO
3
/
60 wt % NaNO
3
and NaNO
2
) and two organic materials (PBT and myo-inositol) shown in Figure 2.
Fernández et al.
[
34
] defined as maximum working temperature, the one for which the studied material
lost 3% of its initial mass, considered the thermal decomposition starting point. Solar salt TGA results
(Figure 2a) show that it starts losing mass at 500
◦
C. However, as this loss was below 3% no thermal
decomposition is considered, making it suitable for the desired final application.
Energies 2018,11, 861 10 of 13
Figure 2.
Thermal gravimetric analysis of (
a
) solar salt (40 wt % KNO
3
/60 wt % NaNO
3
),
(b) myo-inositol, and (c) polybutylene terephthalate (PBT).
On the other hand, the organic materials suffered a severe decomposition. Figure 2b shows how
myo-inositol loses 88% of its initial mass between 230
◦
C and 430
◦
C. The thermal decomposition keeps
on until 600
◦
C at a lower rate. The same behavior can be observed for PBT in Figure 2c. PBT mass loss
goes from 288
◦
C to 454
◦
C. The mass of PBT keeps dropping down at a lower rate between 454
◦
C and
600
◦
C. Both organic materials experienced rather a combustion reaction or a severe decomposition
at high temperatures, which make them unsuitable candidates. Nonetheless, due to its high phase
change enthalpy and low cost, in the next section myo-inositol is put through the cycling stability in a
close crucible.
4.4. Cycling Stability
Table 9shows the thermophysical properties of the two pre-selected candidates, 40 wt % KNO
3
/
60 wt % NaNO
3
and myo-inositol, after being cycled 10 and 50 times. The solar salt neither shows any
kind of temperature hysteresis nor subcooling. Additionally, its phase change enthalpy hardly varies
after the cycling process, and that variation does not affect its storing capacity.
Table 9. Cycling stability tests results.
Material Cycling
System Type Cycles Melting
Enthalpy (J/g)
Solidification
Enthalpy (J/g)
Melting
Temperature (◦C)
Solidification
Temperature (◦C)
40 wt %
KNO3/60 wt %
NaNO3
Open
0 94.84 92.72 220.75 219.70
10 93.07 91.02 222.07 221.21
50 94.68 80.31 222.18 221.37
Myo-Inositol Closed, in air
atmosphere
0 249.76 195.00 223.26 189.91
10 109.15 65.65 219.14 166.61
50 40.66 n.a. 215.31 n.a.
n.a.—not available.
Energies 2018,11, 861 11 of 13
On the other hand, myo-inositol did not stand the thermal cycles as well as the solar salt. Despite
the fact that its phase change temperature does not vary, its phase change enthalpy decreases along
the cycling process. After 10 cycles, myo-inositol lost more than half of its storing capacity (66%) but
was still over the discarding threshold. However, when characterizing the 50 cycles myo-inositol
sample, the results showed a melting enthalpy 83.6% lower than the initial one. Because of the
final application working conditions, the selected material should endure daily phase change cycles;
therefore, myo-inositol does not meet the requirements.
An infrared spectrometry is carried out to check the stability of the material samples after thermal
cycling. Figure 3shows the solar salt and myo-inositol spectrums, respectively after 0, 10,
and 50 cycles
.
The solar salt infrared spectrum (Figure 3a) reveals the same chemical structure presence on the
three different samples. Therefore, in combination with the results obtained for its thermophysical
characterization, it can be concluded that solar salt bears cycling processes successfully.
Figure 3.
Infrared attenuated total reflectance (IR-ATR) spectrums after 0, 10, and 50 thermal cycles of
(a) solar salt; and (b) myo-inositol.
On the other side, Figure 3b reveals the alterations in myo-inositol chemical structure when
being cycled. In spite of the closed crucible for this experiment, the remaining oxygen still oxidizes
myo-inositol and other degradation reactions might take place. Hence, myo-inositol would need an
inert atmosphere, which would increase the required cost for the design and maintenance of a real
application plant. For that reason, myo-inositol must be discarded.
5. Conclusions
A deep material screening was performed in the literature finding a total of 26 interesting PCMs in
the temperature range of 210–270
◦
C. From that first selection, nine materials were analyzed according
to their health hazard and preselected to be thermophysically characterized; measuring their phase
change enthalpy and temperature and studying their thermal behavior. This selection was based on
their enthalpy found in the literature and their price. From the DSC results, three candidates were
pre-selected to be deeply studied, the eutectic called “solar salt,” 40 wt % KNO
3
/60 wt % NaNO
3
,
myo-inositol, and PBT.
Those three materials were tested through thermal stability by means of TGA. Myo-inositol and
PBT suffered severe thermal decomposition from 230–288
◦
C respectively. Due to myo-inositol thermal
characterization and cheap price, it was thermally cycled in a closed system, but it was not stable after
50 cycles. All in all, from 26 candidates, including inorganic salts and organic materials that can be
used as PCMs in applications with a working temperature range from 210 to 270
◦
C, even though its
limited enthalpy of fusion, solar salt succeeded on every performed test.
Energies 2018,11, 861 12 of 13
Acknowledgments:
This work was partially funded by the European Union’s Horizon 2020 Research &
Innovation Programme under Grant Agreement 723596 with reference name Innova MicroSolar. This work
was partially funded by the Ministerio de Economía y Competitividad de España (ENE2015-64117-C5-1-R
(MINECO/FEDER) and ENE2015-64117-C5-3-R (MINECO/FEDER)). The authors would like to thank the Catalan
Government for the quality accreditation given to their research group (2017 SGR 1537). GREA is certified agent
TECNIO in the category of technology developers from the Government of Catalonia. JoséMiguel Maldonado
would like to thank the Spanish Government for his research fellowship (BES-2016-076554). Aran Soléwould like
to thank Ministerio de Economía y Competitividad de España for Grant Juan de la Cierva, FJCI-2015-25741. Alvaro
de Gracia has received funding from the European Union’s Horizon 2020 research and innovation programme
under the Marie Sklodowska-Curie grant agreement No. 712949. Angel G. Fernández would like to acknowledge
the financial support provided by GIZ “Programa de pasantía en el extranjero en tecnologías de concentración
solar para investigadores” and CONICYT/FONDAP 15110019 “Solar Energy Research Center” SERC-Chile.
Author Contributions:
Luisa F. Cabeza and Alvaro de Gracia conceived and designed the experiments;
Marc Martín and Margalida Fullana-Puig performed the experiments; Ángel G. Fernández,
JoséMiguel Maldonado
and Aran Soléanalyzed the data; JoséMiguel Maldonado, Margalida Fullana-Puig and Marc Martín wrote the
paper. All authors contributed to discuss the obtained results. The article was reviewed by every single co-author.
Conflicts of Interest: The authors declare no conflict of interest.
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