Solvothermal method as a green chemistry solution for micro- [611982]

REVIEW
Solvothermal method as a green chemistry solution for micro-
encapsulation of phase change materials for high temperaturethermal energy storage
Albert Ioan Tudor1, Adrian Mihail Motoc1, Cristina Florentina Ciobota1, Dan. Nastase Ciobota1, Radu Robert
Piticescu1,*, and Maria Dolores Romero-Sanchez1,2
1National Institute for Nonferrous and Rare Metals-IMNR, Blvd. Biruintei no.102, Pantelimon, Ilfov, Romania
2Instituto Tecnologico del Calzado y Conexas-INESCOP, Elda-Alicante, Spain
Received: 15 November 2017 / Accepted: 18 January 2018
Abstract. Thermal energy storage systems using phase change materials (PCMs) as latent heat storage are one
of the main challenges at European level in improving the performances and ef ficiency of concentrated solar
power energy generation due to their high energy density. PCM with high working temperatures in thetemperature range 300 –500 °C are required for these purposes. However their use is still limited due to the
problems raised by the corrosion of the majority of high temperature PCMs and lower thermal transfer
properties. Micro-encapsulation was proposed as one method to overcome these problems. Different micro-encapsulation methods proposed in the literature are presented and discussed. An original process for the micro-encapsulation of potassium nitrate as PCM in inorganic zinc oxide shells based on a solvothermal method
followed by spray drying to produce microcapsules with controlled phase composition and distribution is
proposed and their transformation temperatures and enthalpies measured by differential scanning calorimetryare presented.
Keywords: phase change materials / thermal energy storage / micro-encapsulation
1 Introduction
Due to the advantages offered by latent heat thermal
energy storage, such as low temperature variationduring charging and discharging cycles, small unit size,
high storage density, relativ e l yc o n s t a n th e a tt r a n s f e r
fluid (HTF) temperature dur ing the discharge process
and versatility, the use of PCMs as energy storage
materials is growing in many different applications. In
particular, PCMs with melting temperatures between300 and 500 °C are needed in the storage of the heat
obtained from high temperature concentrated solar
thermal power plants. Micro-encapsulation of hightemperature PCMs is a key process in reducing
corrosion and improving thermal stability but problems
related to thermal stability, mechanical resistance dueto volume change during thermal cycling and compati-bility between PCMs and encapsulation materials must
be overcome.2 Actual state of thermal energy storage
using high temperature PCMs
Development of green energy solutions and achieving the
20% energy savings by 2020 using energy storage systems is
one of the main objective of the European Energy Strategy
2020 [ 1]. Key energy storage technologies can be classi fied
in two main categories depending on the energy productionmethod: chemical energy storage and thermal energy
storage [ 2]. Thermal energy systems (TES) developed using
these technologies are classi fied in three groups: i) sensible
heat storage that is based on storing thermal energy by
heating or cooling a liquid or solid storage medium (e.g.
water, sand, molten salts, rocks), with water being thecheapest option; ii) latent heat storage using phase
change materials (PCMs )(e.g. from a solid state into a
liquid state); and iii) thermo-chemical storage (TCS)
using chemical reactions to store and release thermalenergy. In Table 1 the advantages and disadvantages of
these three types of thermal energy systems are presented.
Developers from more European countries work on thermalenergy storage (TES) systems for buildings and transpor-
tation of thermal energy [ 3].
*e-mail: rpiticescu@imnr.roManufacturing Rev. 5, 4 (2018)
©A.I. Tudor et al., Published by EDP Sciences 2018
https://doi.org/10.1051/mfreview/2018004
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However, costs, performance and reliability should be
verified, since these applications are at the beginning of
commercialization. A thermal energy storage system
combined with the existing equipment cooling capacity,
for example, could be a solution for energy savings insteadof adding more mechanical and electrical equipment likechillers, pumps, condensing equipment [ 4].
Different problems related to thermal stability and
mechanical resistance due to volume change duringthermal cycling, compatibility between PCMs and encap-
sulation materials and easiness of the encapsulation
process, must be overcome.
Due to the advantages offered by latent heat thermal
energy storage, such as low temperature variation during
charging and discharging cycles, small unit size, highstorage density and relatively constant HTF temperature
during the discharge process, the use of PCMs as energy
storage materials is growing in many different applications,thanks also to the versatility offered by the differentmelting temperature range.
In particular, PCMs with melting temperatures
between 300 and 500 °C are required to be used for the
storage of the heat obtained from high temperature
concentrated solar thermal power plants.
Concentrated solar power (CSP) provides electricity
relying on fossil or nuclear fuels (as most other technolo-
gies). They generate medium to high temperature heat,
which is then used to operate a conventional power cyclethrough a steam or gas turbine.
CSP is a technology which produces electricity by
concentrating solar energy in a single focal point, whileproducing no greenhouse gas emission, so it could be a keytechnology for mitigating climate change. This concen-
trated energy is then used to heat up a fluid, produce steam
and activate turbines that produce electricity. The use ofthermal energy storage technology solves the time
mismatch between solar energy supply and electricity
demand. When CSP combines with thermal storagecapacity of several hours of full-capacity generation,
CSP plants can continue to produce electricity even when
clouds block the sun, or after sundown or in early morningwhen power demand steps up [ 5].
Thermal storage increases the value of the plant by
providing guaranteed capacities. Storage can be used to
extend the electricity generation after sunset, whenelectricity loads remain high [ 6]. Storage could also serve
round-the-clock, base-load generation, displacing, e.g.
high-CO
2emitting coal plants.Simple and non-polluting CSP technologies can be
deployed relatively quickly and can contribute substan-tially to reduce carbon dioxide emissions. Current CSP
power plants are proving to be ef ficient and cost-effective.
Today, sensible heat materials in the form of synthetic oiland molten salt are the most widely used storage materialsin large-scale CSP systems with limitations commented
above, while systems that utilize latent heat or thermo-
chemical still being developed [ 7].
Innovation in energy technology is crucial to meet
climate mitigation objectives while also supporting
economic and energy security objectives. Cost-effectivetechnologies are what will make the energy system
transformation possible. However, continued dependence
on fossil fuels and recent trends such as unexpected energymarket fluctuations reinforce the role of governments,
individually and collectively, to stimulate targeted action
to ensure that resources are optimally aligned to accelerateprogress.
Last researches and developments in the solar energy
sector by different authors indicate the necessity to reduce
the high costs of the operation and maintenance of the solarplants. In this sense, one of the most important research
lines is addressed to the development and characterization
of new PCMs to be used for energy storage. It is necessaryto improve TES materials to build less expensive and more
profitable solar plants, mainly avoiding corrosion phenom-
ena at high temperatures, due to the high temperatureconditions necessary to maintain the mixtures flowing, as
well as the storage temperature, around 390 °C in parabolic
trough collectors and 565 °C in central receivers [ 8,9].
Although thermal energy storage is a key issue for CSP,
the available and mature technologies of TES do not match
all the actualized criteria for the properties required. As
studied by Guillot et al. [ 10], alternative approaches have
to be identi fied and developed to guaranty the expected
extension of CSP implementations with respect to the
International Energy Agency (IEA) 2050 scenario.
Energy Research Knowledge Centre (ERKC), funded
by the European Commission to support its Information
System of the Strategic Energy Technology Plan (SETIS)includes in its report CSP 2013, the 9 Priority Areas for the
Energy sector. R&D challenges in TES for CSP plants,
according to ERKC include: increasing temperature limits,
both the higher and lower limits, since most HTFs (e.g.,molten salt mixtures) have a high melting point;
maximizing heat transfer properties; enhancing compati-
bility with containing materials (e.g., tank walls); exploit-Table 1. Advantages and disadvantages of TES [ 3].
TES Advantages Disadvantages
Sensible heat
storageRelatively inexpensiveLarge volumes are necessary
Low energy density
Design to discharge thermal energy at constant temperatures
PCM and TCS3 times and 5 times higher
energy density comparing
to sensible heat storage;reduce CO
2emissionsExpensive; feasible for a high number of cycles
Barriers to entry on the market: cost improvements in terms of
stability of storage performance and materials properties arenecessary for both PCM and TCS.2 A.I. Tudor et al.: Manufacturing Rev. 5, 4 (2018)

ing new material properties (e.g., phase change behaviour of
mixtures, nanoparticles); increasing the life of materials
subject to cyclic thermal loads (e.g., concretes); introducingnew forms of storage (e.g., thermochemical reactions);
reducing the overall costs of TES capital expenditure of
commercial systems in the range of 25-40EUR/kWh. Table 2
includes a list of the last representative projects funded bythe EU having as main objective the thermal energy storage
in solar plants working in high temperatures range.
Additional alternative thermal energy storage
approaches are needed, but they have to ful fil a list of
essential properties such as: low cost, high availability, long
life time expectancy, thermal stability up to1000 °C, large
storage capacity and good thermal conductivity, easy to be
used, low CO
2emissions and renewable energy content. For
latent heat storage, the high temperature levels of thefuture CSP plants (200 –1000 °C) require the study of PCM
(inorganic salts) with good stability at high temperature
and high speci fically heat. However, considering that the
corrosion produced by molten salts could be signi ficant,
micro-encapsulation of high temperature PCMs is pro-
posed as a solution to reduce/avoid corrosion problems.
In the next few years, and according to the IEA Solar
PACES Implement Agreement, research and development
efforts will be supported (mainly by the EU) for improve-
ments in all components of CSP plants. The focus will be ontroughs with direct steam generation, increasing the overall
efficiency and development of Phase-change materials and
concrete for thermal storage. Also for towers manyinnovative designs are currently proposed, with heat fluids
and storage options. Currently, the use of PCMs as latent
heat materials is still reduced and consequently its
investment cost too.
For the next years, IEA points out an increase in the
development of CSP based on the technologies offering new
opportunities for energy storage. Typically, storagerepresents about one fifth of the total cost of a CSP plant
with storage.
PCMs are products with a hig h potential for thermal
management solutions in thermal energy storage sys-
tems (TES) due to their cap acity to store and release
thermal energy during the me lting and crystallization
process [ 11]. The main requirements for these materials
can be grouped in three distinct categories: physical,
technical and economical [ 12]. The temperature of the
phase transformation and the higher melting enthalpyrepresent two of the most important requirements that a
PCM must ful fil to store and to release heat. Depending
on the working temperature range where PCMs must beused in TES, different organic, inorganic or metallic
PCMs have been proposed. For high tempearute range,
inorganic salts were reported as best candidates. Themain storage properties of some PCMs are presented inTable 3 [13–20].
3 Encapsulation of high temperature PCMs
Encapsulation of PCM may increase their exchange surface
required for a good heat transfer [ 21,22]. Moreover, it is
important to notice that micro-encapsulated PCMs canstore 10 –15% more energy per unit volume than current
molten salt systems. The possibility of using micro-
encapsulated PCMs with different melting temperaturesalso increases ef ficiency of the energy transfer and a
reduction of the residual waste heat.
Though macro-encapsulation ( >1 mm) of high temper-
ature PCMs has been reported by different authors inliterature, nothing has been established to simultaneously
solve the three problems encountered for the encapsulation
of high temperature PCMs compared to low temperaturePCMs: (1) liquid metal and molten salt generally exhibit
high chemical corrosion in the presence of some shell
materials (i.e. metals); (2) PCMs with solid to liquidtransitions commonly show volume expansion and conse-
quently they suffer thermal stress; (3) to reduce these
problems, thick shells are used producing the drop of heatstorage density [ 23].
An interesting study, carried out by the Lehigh
University [ 24], about high temperature PCMs macro-
encapsulations, proposes Zn or Al as core materials macro-encapsulated into stainless steel containers. Although
software simulations with these systems demonstrate the
thermal energy storage capacity of the PCMs, they alsoshow a decrease in the latent heat storage capacity with
thermal cycles or after long-term exposure to high
temperatures. The reason seems to be the inter-metallicdiffusion between the metal materials and the encapsula-
tion metals during high temperature melting/solidi fication
process. Thus, it is conceivable that metal PCMs materialsmacro-encapsulated by certain materials such as Al or steelare not suitable for TES. Using electroplating method,
other authors have obtained particles (3 mm diameter)
with lead-nickel core-shell structure suitable for heatrecovery of high temperature waste heat [ 25].
More simple methods can also be used for the
encapsulation of metal PCMs such as indium (meltingtemperature = 156 °C) by using silica as shell by sol-gel
procedure and obtaining nm size particles [ 26]. However,
this method requires the melting of the metal, withincreased dif ficultness for handling when using metals with
higher melting temperature. LiNaCO
3has been prepared
as eutectic by Ge et al. [ 27] to be used as PCM with melting
temperature ca. 500 °C. Carbon allotropes have been used
to enhance the thermal conductivity, finally obtaining
composites with good physical and chemical stability and
high thermal conductivity. Sol-gel processes using organicpolymers with high melting temperature (i.e. polyimide)
and organic-inorganic hybrids shells are proposed for the
micro-encapsulation of PCMs in this project. For example,in this sense, stainless steel has been coated by sol-gel
procedure of hybrid coatings for corrosion protection with
interesting results, forming a physical barriers towardscorrosion [ 28,29].
Storing thermal energy in PCMs, such as inorganic salt
mixtures, latent heat of fusion can increase the energy
density for storage by 50%. However, a major issue that hasprevented the commercial use of PCM-TES for CSP is that
it is dif ficult to discharge the latent heat stored in the PCM
melt at speci fied heat rates. This is because when heat is
extracted, the PCM-melt which has low thermal conduc-
tivity solidi fies onto the heat exchanger surface decreasingA.I. Tudor et al.: Manufacturing Rev. 5, 4 (2018) 3

Table 2. List of some research projects funded by the European Commission related with PCM thermal energy storage.
Project Programme Description
Energy Storage for
Direct Steam SolarPower Plants
(DISTOR)FP6-2002-ENERGY-
1. 2004 –2007Development of thermal storage systems using phase change
materials (PCM) in the temperature range from 230 °C to 330 °C
for systems using steam between 30 and 100 bar
Novel Ef ficient Solid
Storage for H2 (NESSHY)FP6 Integrated
project. SES6-518271
(2006 –2011)The project aims at developing novel materials, storage methods
and fabrication processes that provide the energy density and the
charge/discharge, storage/restitution rates necessary for mobile
applications with spin-offs in stationary systems. To identify themost promising solid storage solutions for such applications.
New Solar Collector
FP7 2010 –2013.
Grant agreement:
256830The project is based on the research and development of a solar
receiver that will increase the parabolic-trough ef ficiency,
reaching an operating temperature of the heat transfer fluid
above the current limit of 400 °CConcept for High
Temperature Operation
in CSP applications(HITECO)
Innovative con figuration
for a fully renewablehybrid CSP plant
(HYSOL)FP7-ENERGY-2012-
1-2STAGE. Grantagreement: 308912
(2013 –2016)Renewable energies have often problems in order to provide a
stable and reliable power supply, as they often depend onmeteorological circumstances that have a variable or stochastic
component. This fact is often used by their detractors to favour
the use of other alternatives such as fossil fuels.
High temperature thermal
energy storage by
Reversible thermochemicalReaction (STORRE)FP7-ENERGY-2011-
1. Grant agreement:
282677 (2012 –2016)Proposal concerns the area of thermal energy storage by chemical
reaction for CSP. The objective is to develop a new solution for
the heat storage with the following characteristics: Mid-term tolong-term heat storage; High storage density; High temperatures
(300 –550 °C), which are representative of the CSP plants with
cylinder-parabolic or CLFR technologies
Redox Materials-based
Structured Reactors/Heat
Exchangers for Thermo-Chemical Heat Storage
Systems in Concentrated
Solar Power Plants(RESTRUCTURE)FP7-ENERGY-2011-
1. Grant agreement:
283015 (2011 –2015)Thermo chemical storage (TCS) involves the exploitation of the
heat effects of reversible chemical reactions for the “storage ”of
solar heat. Among gas –solid reactions proposed for such an
approach the utilization of a pair of redox reactions involving
multivalent solid oxides has several inherent advantages that
make it attractive for large-scale deployment
Concentrated Solar Power
in Particles (CSP2)FP7-ENERGY-2011-
1. Grant agreement:282932 (2011 –2015)The CSP2 project puts forward an alternative heat transfer fluid
(HTF) for concentrating solar power (CSP) plants. The use ofdense gas-particle suspensions in tubes as HTF is proposed
Thermochemical Energy
Storage for ConcentratedSolar Power Plants
(TCSPOWER)FP7-ENERGY-2011-
1. Grant agreement:282889 (2011 –2015)The objective is to realise a new, ef ficient, reliable and economic
thermochemical energy storage (TCS) for CSP, with thecapability to contribute signi ficantly to further cost reduction of
regenerative electricity production. This will be achieved by
applying reversible gas –solid reactions. Dissociation of calcium
hydroxide is used for storing thermal energy in 450 and 550 °C
Optimization of a thermal
energy storage system withintegrated steam generator
(OPTS)FP7-ENERGY-2011-
1. Grant agreement:283138 (2011 –2014)OPTS project aims at developing a new TES system based on
single tank con figuration using stratifying Molten Salts as heat
storage medium at 550 °C, integrated with a Steam Generator
(SG), to provide ef ficient, reliable and economic energy storage
for the next generation of trough and tower plants
SAM.SSA /C0Sugar Alcohol
based Materials for
Seasonal StorageApplicationsFP7-ENERGY-2011-
2. Grant agreement:
296006 (2012 –2015)The project aims at developing new PCM for thermal energy
seasonal storage applications in the range of medium
temperatures. The generated materials provide: -Low cost,environmentally sound and safe solutions for seasonal storage
applications, -Easy adjustment of the melting point for optimal
“tuning ”to the required applications, -Energy
densities >200 kWh/m3 for compact storage4 A.I. Tudor et al.: Manufacturing Rev. 5, 4 (2018)

the heat transfer, and requiring large heat transfer area and
hence a higher cost. Considering the technical problems
encountered with the use of PCMs, micro-encapsulatedPCMs may be an optimal solution for TES.
In order to establish the economic impact of the project
in case of adaption of the developed micro-encapsulatedPCMs to a CSP plant, a balance between the additional
cost due to micro-encapsulation process and advantages
produced in terms of energy ef ficiency and cost reduction in
maintenance works, possible elimination of heat exchang-
ers and/or HTFs, reduction on the required amount of
PCMs, etc.
Macro ( >1 mm) and micro-encapsulation properties
(particle sizes of mm or nm) have been successfully
developed and patented for low temperature of PCMs
(organic and inorganic materials) using mainly poly-meric coatings [ 30–35]. In some literature papers and
even pilot TES installation s, macro-encapsulation of
inorganic salts was used. Zn, NaNO
3,M g C l 2and eutectic
mixtures having melting temperatures higher than
300 °C have been encapsulated according to patent
US2011/0259544 [ 36] in which cylinders of materials
base of Ni or carbon and stainless steel were useddimensions of mm to cm . US patent no. 0055661/2012
[37] refers to salt nitrates melted in metallic tubes which
are sealed for permanent isolation. Recently, anothermethod has been patented for encapsulating melted salts
(US 2015/0284616) [ 38]s u c ha sN a N O
3or KNO 3.T h i s
method is based on the coating of PCM pellets (27 nm)with a flexible polymer followed by a metallic coating by
non-electric and electrolyt ic processes (Ni, Cu, Zn, Zn-
Fe alloys, etc.).
For a maximum economic bene fit, the use of inorganic
shells (SiO 2, ZnO, graphite, etc.) and different oxides
and ceramics as shell materials for the micro-encapsula-tion of PCMs, which are not expensive raw materials,
may be developed based on sol-gel and hydrolysis, which
are simple and cost-effecti ve technologies, easy to be
scaled-up and with machinery and equipment which are
already developed for the food and pharmaceutical
industries [ 39].
Regarding costs, the use of PCMs for thermal
energy storage as latent heat materials is usually more
expensive that sensible hea t storage materials. Howev-
er, the storage ef ficiency of latent heat materials is
higher (75 –90% ef ficiency versus 50 –90%) for sensible
materials.
The economic viability of a TES depends heavily on
application and operation needs, including the number and
frequency of the storage cycles; the thermal cycling
stability of the PCM microcapsules (and therefore,reduction of work maintenance, replacement of energystorage materials, loss of storage capacity, etc.) are
necessary improvements in the stability of the storage
performance, which is associated with energy storagematerial properties. The importance of the R&D required
in the development of new energy storage systems can be
reflected in Table 4 .Table 3. Storage properties of inorganic high temperature PSMs.
Nr. Crt. Compound Melting temperature [ °C] Latent heat [KJ/Kg]
1 MgCl 2*6 H 2O 117 165 –168, 6
2 NaOH 64, 3 227, 6
3 LiNO 3 254 360
4 KNO 3 333 266
5 MgCl 2 714 452
6 NaCl 800 4927 NaCO
3 854 276
8 KF 857 452
9K 2CO 3 897 236
10 KOH 380 145
12 67%KNO 3+ 33%LiNO 3 133 170
12 54%KNO 3+ 46%NaNO 3 222 100
13 68%KCl + 31.9%ZnCl 2 235 198
Table 4. Typical parameters of TES.
TES Capacity (kWh/t) Power (MW) Ef ficiency (%) Storage period (h, d, m) Cost ( €/kWh)
Sensible 10 –50 0.001 –10 50 –90 d/m 0.1 –10
PCM 50 –150 0.001 –17 5 –90 h/m 10 –50
Chemical reactions 120 –250 0.01 –17 5 –100 h/d 8 –100A.I. Tudor et al.: Manufacturing Rev. 5, 4 (2018) 5

4 Green solvothermal /C0spray drying method
for PCMs micro-encapsulation in inorganicshell
4.1 Synthesis and characterization
A green chemical process for micro-encapsulation of
inorganic PCM salts such as KNO 3in ZnO shell was
proposed with the aim to obtain encapsulated micro-spheresto be used as PCMs systems working in high temperature
range, improve thermal stability and reduce TES corrosion
produced by the molten salts. In the first step the shell-
forming material zinc nitrate tetra-hydrate (Zn(NO
3)2/C24
H2O p.a. Merck) was dissolute in ethanol (p.a. grade) by
mechanical stirring at room temperature in a glass reactor.A calculated weight of PCM salt was than dispersed under
continuous stirring, according to a mass ratio ZnO: PCM in
the range 1:1 –1:5. A 3M KOH solution was then added
under stirring until a pH ∼9 was reached. The suspension
was than treated in a vertical autoclave endowed with
TEFLON liner (CORTEST Inc., USA) for 2 h at 200 –
250 °C, under external Ar pressure of 40 atm. The final
suspension was granulated in a spray dryer at a constant
feeding rate of 5 l suspension/hour with different nozzle sizes
(LabPlant, U.K.) and hot air at 100 °Ca sd r y i n ga g e n t .T h e
optimal spray drying conditions were selected based on the
measurement of their flowability using a Hall Flowmeter
funnel according to ASTM B213 –17. The as-obtained spray
dried micro-granulated powder was sieved and powders withsizes in the range 20 –50mm were characterized for
determination of the chemical composition, phase composi-
tion and distribution and thermal behavior. The K and Zncontents were analyzed by atomic absorption spectroscopy
(AAS ZEEnit 700, Karl Zeiss) and chemical titration
according to the Romanian Standards 3223/1 –92 and 1269/4–89 respectively. The experimental procedure is presented
in the schematic flowsheet from Figure 1 .
The phase composition was analyzed by XRD (Brucker
D8 Advance) with the help of DIFFRAC
+software by the
Brag-Brentano method in u/C0ucoupling mode, using the
CuK asource in the range 2 u= 4…74 °.
The thermal properties of the initial KNO 3salt and
KNO 3micro-encapsulated in ZnO shell were measured by
differential scanning calorimetry (DSC Netzsch 200 MayaF3) in the temperature range /C040…600 °C. Samples of 10 –
15 mg of powders have been weight using a four decimal
precision analytical balance. The samples were charged in
special Al crucibles preview with Al perforated lead toavoid material losses. DSC spectra were recorded in Ar flow
gas at a constant heating rate of 10 Kmin
/C01.
Phase distribution and the formation of shell structure
was analyzed by SEM /C0EDX method using a TESCAN
/C0Vega II LMU unit.
4.2 Results and discussion
Spray drying is a key step in encapsulating the KNO 3salt as
PCM phase in an inorganic ZnO shell with desired properties
for thermal energy storage. The flowability of KNO3 micro-
encapsulated in ZnO shell was selected to rapidly control the
encapsulation process based on the assumption that powders
with round shapes and homogeneous sizes have betterflowability properties. During spray-drying process the best
method that could be used to adjust the sizes and flowablility
of microcapsules was to use nozzles with different sizes. It
was observed that flowability of the microcapsules increases
with increasing nozzle size from 0.25 mm to 1 mm The
microcapsules obtained by spry drying using a 1 mm nozzle
size have the highest flow speed ( Fig. 2 ) and all micro-
capsules were further prepared in these conditions.
The solvothermal treatment temperature is the main
parameter that in fluences the phase composition of the
micro-encapsulated powder. According to the thermody-namic prediction done using the E-pH module of HSC 8.0
software for Zn-K-H-O system, it is expected that
increasing the solvothermal temperature from roomtemperature ( Fig. 3 ) to 250 °C(Fig. 4 ), the formation of
the metastable phase zinc hydroxi-nitrate with different
NO
3:H2O ratios is suppressed.
Fig. 2. Flowability of micro-encapsulated powders vs. spray
drier nozzle size.KNO 3Ethanol (p.a) Zn(NO 3)2*4H 2O
Mechanical stirring Mechanical stirring
PCM dispersion Zn (II) solution
Mixing and pH adjustment KNO 3solution
Solvothermal treatment
ZnO: PCM dispersion
Spray drying
KNO 3 microencapsulated in ZnO
Fig. 1. Experimental procedure.6 A.I. Tudor et al.: Manufacturing Rev. 5, 4 (2018)

The XRD patterns of two selected spray-dried powders
from KNO 3–ZnO system obtained at the lowest experi-
mental temperature (110 °C) and highest experimental
temperature (250 °C) con firm these prediction, as seen in
Figure 5 . The results are summarized in Table 5 .Figure 6 presents the SEM image of the spray dried
sample P2. It may be observed the morphology of thepowder consisting of core KNO
3irregular shape crystals
covered by transparent flattened ZnO crystals. The EDX
analysis from Figures 7 and8performed on the core and
Fig. 3. E-pH (Pourbaix) diagram of Zn-K-O-H system at room temperature.
Fig. 4. E-pH (Pourbaix) diagram of Zn-K-O-H system at 250 °C.A.I. Tudor et al.: Manufacturing Rev. 5, 4 (2018) 7

shell components respectively shows clearly the bi-phasic
composition with no inter-diffusion between the two
components.Thermal properties of the micro-encapsulated PCMs
are the main properties enabling the future application ofthese materials in thermal energy storage. It is important
to compare the phase transformation temperatures and
related transformation enthalpies of pure and micro-encapsulated PCM for future utilization in thermal energystorage systems.
Figures 9 –11present the DSC spectra of initial pure
KNO
3, KNO 3-ZnO-P2 and KNO 3-ZnO-P4 micro-encapsu-
lated samples. The calculated values of the melting and
crystallization temperatures and enthalpies are summa-
rized in Table 6 .Zinc Nitrate Hydroxide Hydrate
ZinciteNiter
KNO3_ZnO_P2KNO3_ZnO_P4Intensity (CPS)
010002000
2-Theta – Scale14 20 30 40 50 60 70(100)
(002)(101)
(103) (112)(110)
(020)(111)
(021)
(021)
(102)(112)
(220)(221)
(041)
(132) (113)
(102)(110)
(400) (-311) (020)(221)
P4P2
Fig. 5. XRD pattern of samples P2 and P4.
Table 5. Phase composition of KNO 3–ZnO microcapsules
with different weight ratios obtained at different sol-
vothermal synthesis temperatures.
Sample
codeKNO 3:
ZnO
ratioSynthesis
temperature, °CPhases
detectedWt.%
P1 1:2 110KNO 3 76.1
ZnO 20.2
Zn5(OH) 8
(NO3) 2(H2O) 23.6
P1 TT 1:3 150KNO 3 51.6
ZnO 14.4
Zn5(OH) 8
(NO3) 2(H2O) 233.9
P2 1:5 110KNO 3 63
ZnO 18
Zn5(OH) 8
(NO3) 2(H2O) 218
P3 1:5 200KNO 3 88.8
ZnO 11.2
P4 1:5 250KNO 3 79.5
ZnO 20.5
Fig. 6. SEM image of KNO 3-ZnO/C0P2 sample.8 A.I. Tudor et al.: Manufacturing Rev. 5, 4 (2018)

From the experimental results presented it may be
observed that micro-encapsulation of KNO 3in inorganic
ZnO shell does not affect the PCM behavior of the KNO 3
core material. The melting and crystallization temper-
atures are very similar for all samples. A decrease in the
transformation enthalpies could be observed in the micro-
encapsulated materials. The transformation enthalpies ofsample P4 are about 2 times higher than values for sampleP2; this may be explained by the presence of secondary zinc
nitrate hydroxide hydrate (Zn
5(OH) 8(NO3) 2(H2O) 2)
phase. It may be concluded that solvothermal synthesisof ZnO shell at temperatures >200 °C ensures the optimal
micro-encapsulation process for high temperature PCM
based on KNO
3salt. Further experiments are in course tostudy the durability of the microcapsules by performing
cyclic DSC measurements in conditions similar to real
TES.
5 Conclusions
–Development of green energy solutions and achieving the
20% energy savings by 2020 using energy storage systems
is one of the main objectives of the European Energy
Strategy 2020. Thermal energy systems developed usinglatent heat storage using PCMs have as main advantage
higher energy densities compared to traditional sensible
heat storage materials but are more expensive and their
Fig. 7. EDX of a core region. The chemical composition corresponds to K = 30.28 wt.% (15.09 at.%) and O = 69.72 wt.% (84.91 at.%).
Fig. 8. EDX of a shell region. The chemical composition corresponds to Zn = 39.03 wt.% (13.55 at.%) and O = 60.97 wt.% (86.45 at.%).A.I. Tudor et al.: Manufacturing Rev. 5, 4 (2018) 9

stability for a high number of cycles remain a barrier to
entry on the market. For PCMs working at higher
temperatures required for CSP stations, based mainly on
inorganic salts such as nitrates, corrosion and thermalstability became crucial for application. One method to
solve these problems is to encapsulate the inorganic PCM
phase in a shell materials to avoid contact with thestorage reactor, leading to reduced corrosion and betterthermal stability;
–a combined solvothermal synthesis followed by spray
drying process to micro-encapsulate KNO
3-based
P C Mi na ni n o r g a n i cZ n Os h e l lw a sf o rt h e first time
proposed. Solvothermal synthesis in ethanol at
temperatures up to 250 °Cl e a dt ot h ef o r m a t i o no fa bi-phasic system consisting only from the PCM and
shell without any interactions between them. Micro-
capsules with different ZnO:KNO3 ratios were devel-
oped by spray drying with hot air at 100 °Cu s i n ga
1 mm size nozzle;
–the thermal characteristics of these core/shell micro-
capsules were analyzed by DSC showing the best resultscorresponds to micro-capsules obtained in solvothermalconditions avoiding the pre sence of secondary hydroxy-
zincite phases, without affecting the transformation
temperatures and enthalpies of the micro-encapsulatedPCM. Works are in progress to study the thermal
stability for high number of cycles required for
application.
Fig. 10. DSC spectra of KNO 3-ZnO-P2 sample.
Fig. 9. DSC spectra of initial pure KNO 3.10 A.I. Tudor et al.: Manufacturing Rev. 5, 4 (2018)

We gratefully acknowledge the financial support received from
European Commission and Romanian Government-Management
Authority from Ministry of Research and Innovation, in the frame
of Competiveness Operational Programme, Action A1.1.4-E-
2015, project ID P_37_776, SMIS code 104730, Acronym
ENERHIGH.
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Cite this article as : Albert Ioan Tudor, Adrian Mihail Motoc, Cristina Florentina Ciobota, Dan. Nastase Ciobota, Radu Robert
Piticescu, Maria Dolores Romero-Sanchez, Solvothermal method as a green chemistry solution for micro-encapsulation of phase
change materials for high temperature thermal energy storage, Manufacturing Rev. 5, 4 (2018)12 A.I. Tudor et al.: Manufacturing Rev. 5, 4 (2018)