applied
sciences
Article
Experimental Evaluation of a Paraffin as Phase
Change Material for Thermal Energy Storage in
Laboratory Equipment and in a Shell-and-Tube
Heat Exchanger
Jaume Gasia 1 , Laia Miró 1 , Alvaro de Gracia 2 , Camila Barreneche 1,3 and Luisa F. Cabeza 1, *
1
2
3
*
GREA Innovació Concurrent, Universitat de Lleida, Edifici CREA, Pere de Cabrera s/n, 25001 Lleida, Spain;
[email protected] (J.G.); [email protected] (L.M.); [email protected] (C.B.)
Departament d’Enginyeria Mecanica, Universitat Rovira i Virgili, Av. Paisos Catalans 26,
43007 Tarragona, Spain; [email protected]
Department of Materials Science and Metallurgical Engineering, Universitat de Barcelona,
Martí i Franqués 1-11, Barcelona, Spain
Correspondence: [email protected]; Tel.: +34-973003576
Academic Editor: Mohammed Mehdi Farid
Received: 2 March 2016; Accepted: 8 April 2016; Published: 18 April 2016
Abstract: The thermal behavior of a commercial paraffin with a melting temperature of 58 ˝ C is
analyzed as a phase change material (PCM) candidate for industrial waste heat recovery and domestic
hot water applications. A full and complete characterization of this PCM is performed based on
two different approaches: a laboratory characterization (mass range of milligrams) and an analysis
in a pilot plant (mass range of kilograms). In the laboratory characterization, its thermal and cycling
stability, its health hazard as well as its phase change thermal range, enthalpy and specific heat are
analyzed using a differential scanning calorimeter, thermogravimetric analysis, thermocycling and
infrared spectroscopy. Laboratory analyses showed its suitability up to 80 ˝ C and for 1200 cycles.
In the pilot plant analysis, its thermal behavior was analyzed in a shell-and-tube heat exchanger
under different heat transfer fluid mass flow rates in terms of temperature, power and energy rates.
Results from the pilot plant analysis allowed understanding the different methods of heat transfer in
real charging and discharging processes as well as the influence of the geometry of the tank on the
energy transferred and required time for charging and discharging processes.
Keywords: thermal energy storage; latent heat; phase change material; paraffin; heat exchanger;
shell-and-tube; laboratory; pilot plant
1. Introduction
According to the International Energy Agency [1], current trends in energy supply and use are
economically, environmentally and socially unsustainable since energy-related emissions of carbon
dioxide will be doubled by 2050 and fossil energy demand will increase over the security of supplies.
Hence, researchers are forced to go deeper into finding materials and systems to make sure the presence
of energy storage in the domestic sector, transport and industry to support energy security and climate
change goals. Among the different energy storage technologies, thermal energy storage (TES) arises as
a key technology to reduce the mismatch between energy demand and supply on a daily, weekly and
even seasonal basis, increasing the potential of implementation of renewable energies and reducing
the energy peak demand.
There are three technologies of TES: sensible, latent and thermochemical heat storage. The storage
of energy within the sensible heat storage technology is linked to the rise of temperature of the TES
Appl. Sci. 2016, 6, 112; doi:10.3390/app6040112
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Appl. Sci. 2016, 6, 112
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material, which can be either in liquid or solid state. In latent heat storage technologies, the thermal
energy is stored when the TES material, also called phase change material (PCM), undergoes phase
change processes. Finally, in thermochemical heat storage technologies, the energy is stored after a
reversible chemical reaction between two substances.
Latent heat TES has been deeply studied during the past decades and it has been found to be
nowadays more attractive than sensible heat storage despite the fact that, in terms of cost, sensible
TES materials are cheaper than PCM because of their high storage densities and narrower operating
temperatures [2], and more attractive than thermochemical heat storage because of their maturity.
One of the most studied PCMs is paraffin [3], which is an organic PCM consisting of a mixture of
straight chain n-alkanes CH3 –(CH2 )–CH3 . However, due to the high cost of manufacturing, only
technical grade paraffins (mixtures of many hydrocarbons) are used for latent heat TES purposes [4].
Several studies have already reported and demonstrated the desirable properties of paraffins, which
make this organic PCM suitable for TES [5]. From the thermal point of view, paraffins are known
to have high thermal energy storage capacity, relatively constant melting temperatures close to the
assumed phase change temperature, negligible subcooling and low vapor pressure in the melting
process. Furthermore, paraffin is chemically inert and stable; it does not show phase segregation,
it is non-corrosive to the container material, and it is commercially available. There are also some
undesirable properties that limit the potential capabilities of this material. Such major drawbacks
are: large volume changes during phase transition, compatibility problems with plastic containers,
possibility of flammability, and low thermal conductivity. With some modifications in the wax and
in the storage unit, all the undesirable effects mentioned before can be partially addressed and even
eliminated [6].
Several experimental studies concerning the melting and solidification processes of different
technical grade paraffin have been conducted, reporting their desirable properties. Trp [7] analyzed
the behavior of paraffin RT-30 in a 48 L shell-and-tube heat exchanger during both melting and
solidification processes using water as heat transfer fluid (HTF) at a single constant mass flow rate,
observing that phase change occurred non-isothermally but within the melting zone. In a similar
shell-and-tube heat exchanger, but under different mass flow rates and inlet HTF (water) temperatures,
Rathod and Banerjee [8] studied the thermal behavior of a paraffin wax with a melting temperature
of 60 ˝ C, observing that inlet temperature had a higher effect on the heat fraction during the PCM
melting than the mass flow rate. Moreover, Akgün et al. [9] studied melting and solidification processes
of different commercial paraffins (P42–44, P46–48, P56–58), which were experimentally tested at
laboratory (5 mg) and at higher scale (2.2 kg) in a shell-and-tube heat exchanger with an inclination
angle of 5 ˝ C for various inlet temperatures and mass flow rates of the HTF (water), and obtained
similar conclusions as in the previous study. Furthermore, Jesumathy et al. [10,11] evaluated the
thermal behavior of 0.7 kg of commercial paraffin in a shell-and-tube heat exchanger, placed both
horizontally and vertically, under the influence of different HTF (water) mass flow rates and inlet
temperatures during the melting and solidification processes. Results showed no subcooling of the
paraffin and demonstrated that the modification of the HTF operating conditions has a higher influence
during melting than during solidification. Medrano et al. [12] evaluated the temperature profiles and
the power exchanged of commercial paraffin RT-35 in four different heat exchangers (0.35–1.1 kg of
PCM) during melting and solidification processes working at two different volumetric flow rates and
two different HTF (water) inlet temperatures. They observed that the increase of both the thermal
gradient between HTF and the PCM, and the mass flow rate resulted in a decrease of the phase change
time; therefore, an increase of the average heat transfer rate during charging and discharging processes.
He and Setterwall [13] investigated the behavior of paraffin RT-5 for cold storage at laboratory scale
(5 mg) and at a higher scale (100 mL) and the associated cost for its implementation at higher scale.
Results showed that paraffin had congruent melting, no subcooling and good stability.
The aim of the present study is to perform a complete laboratory analysis of paraffin RT58 at both
laboratory and pilot scale (around 15 mg and 108 kg, respectively). The laboratory characterization
Appl. Sci. 2016, 6, 112
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includes the thermal and cycling stability, health hazard, differential scanning calorimetry (DSC),
thermogravimetric analysis (TGA), and infrared (FT-IR) spectroscopy. In the pilot plant characterization
scale, the thermal behavior of 108 kg of paraffin was analyzed in a shell-and-tube heat exchanger, with
different HTF flow rates, in terms of temperature profiles, power exchanged and energy stored.
2. Materials Description
The selected PCM in the present study is the commercial paraffin RT58, commercialized by
Rubitherm GmbH (Berlin, Germany). Table 1 shows the main thermo-physical properties of the
material according to the manufacturer.
Table 1. Thermo-physical properties of RT58 according to the manufacturer.
Properties
Units
Values
Melting area
(˝ C)
53–59
Congealing area
(˝ C)
59–53
Heat storage capacity ˘ 7.5% (combination of latent and
sensible heat in a temperature range of 50 ˝ C to 65 ˝ C)
(kJ¨ kg´1 )
160
(Wh¨ kg´1 )
48
Specific heat capacity
˝ C)
(kJ¨ (kg¨ K)
´1 )
2
(kg¨ L´1 )
0.88
(kg¨ L´1 )
0.77
Heat conductivity (solid and liquid)
(W¨ (m¨ K)´1 )
0.2
Volume expansion
(%)
12.5
Flash point (PCM)
(˝ C)
>200
Max. operation temperature
(˝ C)
80
Density solid (at 15
Density liquid (at 80
˝ C)
Regarding the possible applications of RT58 and taking into account its melting thermal range,
the most suitable applications are domestic hot water (DHW) [14] and low temperature industrial
waste heat (IWH) recovery [15].
3. Laboratory Analyses
The laboratory methodology proposed by Miró et al. [16] to select a suitable PCM for a TES
application is used. In previous studies, the selection of a PCM was based only on the phase change
thermal range, the enthalpy and the specific heat. In this new methodology, a combination of thermal
stability, health hazard and cycling stability is presented to complement previous analyses.
Differential scanning calorimeter (DSC) analyses were carried out to measure the phase change
thermal range and the enthalpy of fusion along the range of temperatures in which the experimentation
at pilot plant was performed. The equipment used to carry out the analysis was a DSC-822e
commercialized by Mettler Toledo (Greifensee, Switzerland). A dynamic method analysis based
on earlier studies [17] was performed. A sample of 12.8 mg sealed in a 40 µL aluminium crucible was
heated under an inert atmosphere of N2 up from 20 ˝ C to 75 ˝ C at a heating rate of 0.5 ˝ C/min and
cooled back at the same range of temperatures and heating rate. The results obtained are compared
and discussed against the ones provided by the manufacturer.
The thermal stability of the paraffin is analyzed using a thermogravimetric analysis (TGA).
It measures weight changes in a material as function of temperature (or time) under a controlled
atmosphere. The TGA was performed with a Simultaneous SDTQ600 from TA Instruments
(New Castle, PA, USA) under nitrogen flow. The heating rate applied was 10 ˝ C/min between
25 ˝ C and 600 ˝ C to a 30 mg sample size.
Appl. Sci. 2016, 6, 112
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The cycling stability was analyzed to detect possible changes in the thermo-physical properties
of the material along a certain amount of freezing/melting cycles. In the present study, the studied
thermo-physical properties studied were the latent heat and the melting temperature. First, a cycling
test (1200 cycles) was performed using a thermal cycler from Termociclador GENE Q Hangzhou
Appl. Sci. 2016, 6, 112
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BioerTechnology (Hangzhou, China). Three samples have been gathered each time after 500, 1000, and
˝ C and
1200
samples
from
an Eppendorf.
Thesolidification
temperatureand
program
for one cycle
was
min at
45 applied
and 1.5
min at
70 °C
to ensure full
full melting
during
the1.5
cycle.
The
˝
1.5
min atrate
70 isC the
to ensure
fullaround
solidification
and full
melting
the cycle.
Thecycled
appliedsamples
heating was
rate
heating
fastest:
20 °C/min.
Second,
anduring
evaluation
of the
˝
is the fastest:
around
20 C/min.
Second,
an evaluationused
of thefor
cycled
samples
was performed using
performed
using
the same
apparatus
and methodology
the DSC
analysis.
the same
apparatus
and
methodology
used
for
the
DSC
analysis.
Finally, the infrared (FT-IR) spectroscopy was carried out to study the chemical stability of the
Finally,
infrared
(FT-IR)
was carried
outis to
study
stability
of
material
afterthe
a certain
amount
of spectroscopy
cycles. Its working
principle
based
onthe
thechemical
measurement
of the
the material
aftervibration
a certainunder
amount
of cycles.
working
principle
based on the
measurement
molecules
bonds
infrared
light. Its
FT-IR
spectroscopy
wasis performed
using
a Spectrum
of
the
molecules
bonds
vibration
under
infrared
light.
FT-IR
spectroscopy
was
performed
using a
Two from Perkin Elmer (Waltham, MA, USA) and supported by a Dynascan interferometer
Spectrum
Two
from
Perkin
Elmer
(Waltham,
MA,
USA)
and
supported
by
a
Dynascan
interferometer
(Perkin Elmer, Waltham, MA, USA) and OpticsGuard (Perkin Elmer, Waltham, MA, USA). This
(Perkin
Waltham,
MA,between
USA) and
Elmer,
MA, can
USA).
equipmentElmer,
performs
IR spectra
8300 OpticsGuard
and 350 cm−1, (Perkin
and liquids
andWaltham,
solid samples
be
´1 , and liquids and solid samples
This
equipment
performs
IR
spectra
between
8300
and
350
cm
measured.
can be measured.
4. Pilot Plant Analyses
4. Pilot Plant Analyses
4.1. Experimental Setup
4.1. Experimental Setup
The facility used to experimentally test the PCM at a real scale was designed and built at the
The facility used to experimentally test the PCM at a real scale was designed and built at the
University of Lleida, and it is integrated by three parts: (1) the heating system, which consists of a 24 kWe
University of Lleida, and it is integrated by three parts: (1) the heating system, which consists of a
electrical boiler that heats up the HTF acting as the heating energy source during a charging process in a
24 kWe electrical boiler that heats up the HTF acting as the heating energy source during a charging
real installation; (2) the cooling system, which is based on a 20 kWth air-HTF heat exchanger that
process in a real installation; (2) the cooling system, which is based on a 20 kWth air-HTF heat exchanger
simulates the energy consumption by the user during a discharging process in a real facility; and (3) the
that simulates the energy consumption by the user during a discharging process in a real facility; and
storage system, which consists of a stainless steel storage tank based on the shell-and-tube heat
(3) the storage system, which consists of a stainless steel storage tank based on the shell-and-tube
exchanger concept. The tank stores the thermal energy during the charging process and releases it
heat exchanger concept. The tank stores the thermal energy during the charging process and releases
during the discharging process. Figure 1 shows an overview of the pilot plant facility. All three systems
it during the discharging process. Figure 1 shows an overview of the pilot plant facility. All three
are linked through a piping system that distributes the HTF: Syltherm 800 (Dow Heat Transfer Fluids,
systems are linked through a piping system that distributes the HTF: Syltherm 800 (Dow Heat Transfer
Midland, TX, USA). Moreover, rock wool and Foamglass (Pittsburgh Corning, Pittsburgh, PA, USA) are
Fluids, Midland, TX, USA). Moreover, rock wool and Foamglass (Pittsburgh Corning, Pittsburgh, PA,
used as insulation to minimize the heat losses to the surroundings.
USA) are used as insulation to minimize the heat losses to the surroundings.
1. Overview
of plant
the pilot
plant
facility
available
at the
University
of Lleida
and used in
FigureFigure
1. Overview
of the pilot
facility
available
at the
University
of Lleida
and used
in the experimentation.
the experimentation.
To carry out an accurate analysis of the PCM thermal behavior and to get a precise map of
temperatures
charging
and discharging
thirty-oneand
temperature
sensors map
PT-100
To carry during
out an the
accurate
analysis
of the PCMprocesses,
thermal behavior
to get a precise
of
with
an
accuracy
of
±0.1
°C
were
installed
inside
the
storage
tank
at
different
positions
(Figure
2a).
temperatures during the charging and discharging processes, thirty-one temperature sensors PT-100
Nineteen of these probes were located in the main part (from TPCM.1 to TPCM.15), six probes were
located in the corner part close to the U bend (from Tc.1 to Tc.6) and six probes were located at the
central part (from Tin.1 to Tin.6). Each temperature sensor was associated to a control volume, which is
defined as the theoretical volume of PCM in which the value of the temperature sensor could be stated
as representative. Finally, two temperature sensors PT-100 were installed at the inlet and outlet of the
HTF tubes bundle to measure the inlet and outlet HTF temperature. All the temperature sensors and
Appl. Sci. 2016, 6, 112
5 of 12
with an accuracy of ˘0.1 ˝ C were installed inside the storage tank at different positions (Figure 2a).
Nineteen of these probes were located in the main part (from TPCM.1 to TPCM.15 ), six probes were
located in the corner part close to the U bend (from Tc.1 to Tc.6 ) and six probes were located at the
central part (from Tin.1 to Tin.6 ). Each temperature sensor was associated to a control volume, which is
defined as the theoretical volume of PCM in which the value of the temperature sensor could be stated
as representative. Finally, two temperature sensors PT-100 were installed at the inlet and outlet of the
HTF tubes bundle to measure the inlet and outlet HTF temperature. All the temperature sensors and
HTF flow meters are connected to a data acquisition system, which controls, measures and records the
Appl.
Sci. 2016, 6,at112
5 of 12
information
a time interval of 30 s. Concerning to the distribution of the PCM inside the storage
tank (Figure 2b), the biggest amount of material (78%) is located within the tubes bundle along the
(Figure
2b),ofthe
amount
of material
(78%) is 22%
located
within the
tubes
bundle
along
thecentral
main part
main part
thebiggest
storage
tank while
the remaining
is located
in the
corners
and
in the
part
of
of the
thestorage
tank. tank while the remaining 22% is located in the corners and in the central part of the tank.
(a)
(b)
Figure
2. Storage
Storage system:
system:(a)
(a)temperature
temperaturesensors
sensors
location
at three
different
levels,
highlighted
in
Figure 2.
location
at three
different
levels,
highlighted
in grey
grey
(lower),
black
(middle)
and
dotted
(upper);
(b)
distribution
of
the
phase
change
material
(PCM)
(lower), black (middle) and dotted (upper); (b) distribution of the phase change material (PCM) inside
inside
the storage
tank colored
blue (corner
part),
red part)
(mainand
part)
and(central
green (central
the storage
tank colored
in bluein(corner
part), red
(main
green
part). part).
4.2.
4.2. Methodology
Methodology
Different
experimentscomprising
comprisingcharging
charging
and
discharging
processes
at a thermal
Different experiments
and
discharging
processes
at a thermal
rangerange
of 48 ˝of
C
˝
48
°C
and
68
°C
and
at
different
constant
HTF
mass
flow
rates
were
performed.
A
narrow
and 68 C and at different constant HTF mass flow rates were performed. A narrow temperature
temperature
around
the temperature
PCM melting
temperature
is selected
study the
PCMinbehavior,
range aroundrange
the PCM
melting
is selected
to study
the PCMto
behavior,
mostly
the latent
mostly
in the
latent
phase.tested
The three
tested
flow rates
were 500
kg/h,
considered
as
phase. The
three
different
massdifferent
flow rates
weremass
500 kg/h,
considered
as low
flow,
1500 kg/h,
low
flow,
1500
kg/h,
considered
as
mid
flow
and
2500
kg/h,
considered
as
high
flow.
Despite
having
considered as mid flow and 2500 kg/h, considered as high flow. Despite having a maximum mass
aflow
maximum
masskg/h,
flow only
rate of
2500 kg/h,
onlycould
a laminar
regimebecause
could be
because of
rate of 2500
a laminar
regime
be achieved
ofachieved
the high viscosity
of the
the
high
viscosity
of
the
HTF
and
the
limitations
of
the
pilot
plant
facility.
HTF and the limitations of the pilot plant facility.
Before
Before starting
starting the
the charging
charging process,
process, aa warming
warming process
process was
was required
required to
to make
make the
the PCM
PCM be
be
uniform
uniform and
and homogeneous
homogeneous at
at the
the initial
initial temperature
temperature of
of charging.
charging. Once
Once the
the PCM
PCM reached
reached the
the initial
initial
˝ C), the
temperature
temperature conditions
conditions for
for charging
charging (48
(48 °C),
the HTF
HTF was
was heated
heated up
up outside
outside the
the tank
tank up
up to
to the
the inlet
inlet
˝
HTF
temperature
(68
°C)
and
afterwards
the
charging
process
started.
The
charging
process
HTF temperature (68 C) and afterwards the charging process started. The charging process was
was
considered
considered to
to be
be finished
finished once
once the
the difference
difference between
between the
the inlet
inlet and
and outlet
outlet HTF
HTF temperature
temperature was
was less
less
˝
˝ C.
than
2
°C
and
the
weighted
average
temperature
of
the
middle
temperature
sensors
reached
68
than 2 C and the weighted average temperature of the middle temperature sensors reached 68 °C.
Then, a reverse process took place to carry out the discharging process (PCM initial temperature of
68 °C and HTF inlet temperature of 48 °C).
5. Results and Discussion
Appl. Sci. 2016, 6, 112
6 of 12
Then, a reverse process took place to carry out the discharging process (PCM initial temperature of
68 ˝ C and HTF inlet temperature of 48 ˝ C).
5. Results and Discussion
5.1. Laboratory Analyses
The first parameter analyzed by the authors was the health hazard from the RT58 material safety
Appl.
Sci. 2016,
6, 112
6 of 12
data
sheet.
According
to standards 1272/2008 and 1999/45/EG, it represents no hazard.
Results from the DSC analysis (Figure 3) showed that the phase change temperature range
within˝ the melting temperature range and results
120.1 kJ/kg far from the value presented by the
is 50–61
C with a peak temperature at 58.3 ˝ C. Since
RT58 is technical grade paraffin, the fact of
manufacturer (Table 1). The reason might lie in the use of different techniques and methodology to
observing
temperature
range instead of a sharp melting point in the melting is coherent. The
Appl. Sci.a2016,
6, 112
6 ofphase
12
determine it [17].
change enthalpy for the melting temperature range is found, from the DSC analysis, by numerical
within the
melting
temperature
range and
results
120.1
kJ/kg
far kJ/kg
from the
by the
integration
within
the melting
temperature
range
and
results
120.1
farvalue
from presented
the value presented
manufacturer (Table 1). The reason might lie in the use of different techniques and methodology to
by the manufacturer (Table 1). The reason might lie in the use of different techniques and methodology
determine it [17].
to determine it [17].
Figure 3. Differential scanning calorimeter (DSC) analysis of RT58.
Figure 4a shows the TGA of RT58. It can be seen that the sample degrades in one step. The
paraffin degradation Figure
starts at Differential
200 °C andscanning
it is finished
at 350(DSC)
°C, which allows
for the conclusion that
calorimeter
RT58.
Figure 3. 3.
Differential scanning
calorimeter
(DSC)analysis
analysisofof
RT58.
usage of RT58 should be avoided above 200 °C. Since the temperature range of the potential
applications
the present
experimentation
is far
fromthe
thesample
beginning
of the in
degradation,
no
Figure and
4a shows
the TGA
of RT58. It can
be away
seen that
degrades
one step. The
Figure 4a shows the TGA of RT58. It can be seen that the sample degrades in one step. The paraffin
problem
this
matter
was
expected
during
the
present
experimentation.
If
this
result
is
paraffin regarding
degradation
starts
at
200
°C
and
it
is
finished
at
350
°C,
which
allows
for
the
conclusion
that
˝ C and it is finished at 350 ˝ C, which allows for the conclusion that usage of
degradation
starts
at 200
compared
against
the thermo-physical
properties
presented
by
the manufacturer
(Table
1), the
usage of RT58
should
be avoided
above
200
°C.
Since
the
temperature
range
of
the
potential
˝ C. Since the temperature range of the potential applications and
RT58
shouldresults
be avoided
above
200
obtained
match
with the
manufacturer
data
> 200 °C).ofHowever,
it has to no
be
applications
and
the present
experimentation
is farsheet
away(Flash
frompoint
the beginning
the degradation,
the noticed
present
experimentation
is
far
away
from
the
beginning
of
the
degradation,
no
problem
regarding
that
the
maximum
operational
temperature
of
RT58
is
80
°C
due
to
its
vapor
pressure.
Hence,
problem regarding this matter was expected during the present experimentation. If this result is
thisan
matter
wasagainst
expected
the
present
experimentation.
If this
is compared
against
appropriate
control
system
should
beproperties
used
to avoid
the operation
of the
PCM(Table
above
compared
the during
thermo-physical
presented
by
the result
manufacturer
1),these
the the
thermo-physical
properties
presented
by
the
manufacturer
(Table
1),
the
obtained
results
match
thermal
conditions.
obtained results match with the manufacturer data sheet (Flash point > 200 °C). However, it has to bewith
Figure
shows
the FT-IR
spectrograms
an RT58
sample
after
500,
1000
and
1200
the manufacturer
data
sheet
(Flash
pointtemperature
> 200of˝ C).
However,
it before
hasdue
to and
be
noticed
that
the
maximum
noticed
that4bthe
maximum
operational
of RT58
is 80
°C
to its
vapor
pressure.
Hence,
˝
cycles
andtemperature
the FT-IR
characteristic
peaks
of
this
of materials.
Results
show
RT58
presents
an appropriate
control
should
used
to vapor
avoid
the
operation
of
the
PCM
above
these
operational
ofsystem
RT58 is
80 Cbe
due
tokind
its
pressure.
Hence,
anthat
appropriate
control
non-chemical
sincethe
they
show equal
FT-IR
characteristic
peaks
with conditions.
similar profiles.
thermal
conditions.
system
should
bedegradation
used to avoid
operation
of the
PCM
above these
thermal
Figure 4b shows the FT-IR spectrograms of an RT58 sample before and after 500, 1000 and 1200
cycles and the FT-IR characteristic peaks of this kind of materials. Results show that RT58 presents
non-chemical degradation since they show equal FT-IR characteristic peaks with similar profiles.
(a)
(b)
Figure
4. RT58
laboratoryanalyses
analysesresults:
results: (a)
(a) Thermogravimetric
(b)(b)
Infrared
(FT-IR)
Figure
4. RT58
laboratory
ThermogravimetricanalysisTGA;
analysisTGA;
Infrared
(FT-IR)
spectroscopy before and after 500, 1000 and 1200 cycles.
spectroscopy before and(a)
after 500, 1000 and 1200 cycles.
(b)
Furthermore,
after
500, 1000
and
cycles were taken
to study
evolution
of the
Figure 4. RT58samples
laboratory
analyses
results:
(a) 1200
Thermogravimetric
analysisTGA;
(b)the
Infrared
(FT-IR)
before andand
afterlatent
500, 1000
cycles. material. Results are presented in Table 2,
phase spectroscopy
change temperature
heatand
of 1200
the studied
and show that after 1200 cycles, the variation on the phase change temperature is about 1.7% and the
Furthermore,
samples
500,
1000meaning
and 1200that
cycles
wereRT58
takenpresents
to studyno
thedegradation
evolution ofafter
the
variation
on the latent
heat isafter
about
2.7%,
paraffin
phase
change
and
latent
heat ofpattern
the studied
presented
in Table
2,
1200
cycles.
Thetemperature
lack of a clear
and
consistent
in thematerial.
enthalpyResults
valuesare
along
the cycles
may be
been widely seen in various studies involving different materials [18].
Table 2. Thermo-physical properties of RT58 before and after 500, 1000 and 1200 cycles.
Property
Phase change Temperature
Appl. Sci. 2016, 6, 112
Latent heat
Units
(°C)
(J/g)
Initial 0 Cycles
58.3
204.2
500 Cycles
58.8
255.5
1000 Cycles
58.6
242.9
1200 Cycles
57.3
209.9
7 of 12
5.2. Pilot
Plant
Figure
4bAnalyses
shows the FT-IR spectrograms of an RT58 sample before and after 500, 1000 and 1200
cycles and the FT-IR characteristic peaks of this kind of materials. Results show that RT58 presents
5.2.1.
Temperature
Evolution
non-chemical
degradation
since they show equal FT-IR characteristic peaks with similar profiles.
Furthermore,
samples
after 500,evolution
1000 and of
1200
were
to study
evolution
of the
the
Figure 5 shows the temperature
thecycles
HTF at
the taken
inlet and
outletthe
of the
tank and
phase
change
temperature
and
latent
heat
of
the
studied
material.
Results
are
presented
in
Table
2,
temperature profiles of the mid-height control volumes of the PCM (Figure 2a) during the charging
and discharging
show that after
1200 cycles,
the variation
theofphase
and
processes
at a HTF
mass flowon
rate
1500 change
kg/h. temperature is about 1.7% and the
variation
on
the
latent
heat
is
about
2.7%,
meaning
that
paraffin
presents no
after
During the charging process (Figure 5a), the PCM located atRT58
the beginning
of degradation
the tubes bundle
1200controlled
cycles. Theby
lack
a clear and consistent
pattern
in thehigher
enthalpy
values along
theless
cycles
may
be
and
theoftemperature
sensor TPCM.2
achieved
temperatures
with
time,
since
due
to
the
DSC
error
itself
along
with
differences
between
the
DSC
samples
and
cycling
conditions
the energy supplied by the HTF was received during the first stage, and, therefore, started and
(mass, pan,
orprocesses
the presence
of humidity
thePCM
DSC).
This kind
of the
behavior
has
finished
the sample
meltingpreparation,
and charging
before
the rest ofinthe
located
along
tank. The
been widely
in end
various
studies
different
materials
PCM
locatedseen
at the
of the
tube involving
bundle and
controlled
by the[18].
temperature sensor TPCM.5 had the
opposite behavior for analogous reason. From the first seven minutes (0.12 h), the PCM started to
Table 2. Thermo-physical properties of RT58 before and after 500, 1000 and 1200 cycles.
melt at a temperature of 50 °C; after 100 min (1.67 h) of experimentation, when the PCM reached
59 °C, the melting process was finished, and after 209 min (3.5 h), the charging process was
Property
Units
Initial 0 Cycles
500 Cycles
1000 Cycles 1200 Cycles
completed. During the discharging
process (Figure 5b), it can be seen that the solidification started
˝
Phase change Temperature
( C)
58.3
58.8
58.6
57.3
15 min (0.25 h) after starting the process, when the PCM reached 60 °C, and the PCM was fully
255.5
242.9
209.9 was
solidifiedLatent
after heat
190 min (3.16 (J/g)
h), when the204.2
PCM reached 50
°C. The discharging
process
terminated after 296 min (4.93 h). Comparing the two processes, it was observed that the discharge
5.2. Pilot
Analyses
was
42% Plant
slower
than the charge. The main reasons for these differences in time lie in the presence of
natural convection during the melting process, which enhanced the charging process and the
5.2.1. Temperature Evolution
creation of a solid layer around the tubes bundle when the PCM started to solidify, which hindered
the discharging
process.
Furthermore,
the upper
value
of the
Figure 5 shows
the temperature
evolution
of the
HTFofatthe
the melting
inlet andtemperature
outlet of the range
tank and
the
sample
analyzed
at pilot
scale lie
is 1–2volumes
°C lower
than
the (Figure
value obtained
from
the DSC
temperature
profiles
of theplant
mid-height
control
of the
PCM
2a) during
the charging
analysis,
while the
lower value
was the
same
both
and discharging
processes
at a HTF
mass
flowinrate
ofcases.
1500 kg/h.
(a)
(b)
Figure
transfer
fluid
(HTF)
temperature
profile
at an
massmass
flow flow
rate of
1500
Figure 5.
5. PCM
PCMand
andheat
heat
transfer
fluid
(HTF)
temperature
profile
at HTF
an HTF
rate
of
kg/h:
charging
processprocess
and (b)and
discharging
process.process.
Shaded area
indicates
the melting
the
1500 (a)
kg/h:
(a) charging
(b) discharging
Shaded
area indicates
the and
melting
solidification
temperature
range. range.
and the solidification
temperature
Figure 6 provides the comparison between the three evaluated mass flow rates (500, 1500 and
During the charging process (Figure 5a), the PCM located at the beginning of the tubes bundle and
2500 kg/h), showing the temperature profiles of two representative PCM temperature sensors (TPCM.2
controlled by the temperature sensor TPCM.2 achieved higher temperatures with less time, since the
and TPCM11) during a charging and a discharging process. Notice that in both processes, the higher
energy supplied by the HTF was received during the first stage, and, therefore, started and finished the
the mass flow rates were, the faster the PCM reached the desired temperatures, demonstrating the
melting and charging processes before the rest of the PCM located along the tank. The PCM located at
the end of the tube bundle and controlled by the temperature sensor TPCM.5 had the opposite behavior
for analogous reason. From the first seven minutes (0.12 h), the PCM started to melt at a temperature
of 50 ˝ C; after 100 min (1.67 h) of experimentation, when the PCM reached 59 ˝ C, the melting process
was finished, and after 209 min (3.5 h), the charging process was completed. During the discharging
process (Figure 5b), it can be seen that the solidification started 15 min (0.25 h) after starting the process,
when the PCM reached 60 ˝ C, and the PCM was fully solidified after 190 min (3.16 h), when the PCM
Appl. Sci. 2016, 6, 112
8 of 12
reached 50 ˝ C. The discharging process was terminated after 296 min (4.93 h). Comparing the two
processes, it was observed that the discharge was 42% slower than the charge. The main reasons for
these differences in time lie in the presence of natural convection during the melting process, which
enhanced the charging process and the creation of a solid layer around the tubes bundle when the
PCM started to solidify, which hindered the discharging process. Furthermore, the upper value of
the melting temperature range of the sample analyzed at pilot plant scale lie is 1–2 ˝ C lower than the
value obtained from the DSC analysis, while the lower value was the same in both cases.
Figure 6 provides the comparison between the three evaluated mass flow rates (500, 1500
and 2500 kg/h), showing the temperature profiles of two representative PCM temperature sensors
(TPCM.2
and 6,
TPCM11
) during a charging and a discharging process. Notice that in both processes,
Appl.
Sci. 2016,
112
8 ofthe
12
higher the mass flow rates were, the faster the PCM reached the desired temperatures, demonstrating
influence
of the
mass
flow
raterate
onon
thethe
PCM
heating
rate.
the influence
of the
mass
flow
PCM
heating
rate.The
Thereason
reasonlies
liesininthe
thefact
fact that
that the
the heat
heat
transfer
fromthe
theHTF
HTF
to the
of is
PCM
is dominated
by the convection
mechanism.
transfer from
to the
firstfirst
layerlayer
of PCM
dominated
by the convection
mechanism.
Although
Although
the heat
transfer coefficient
remainsinconstant
in theevaluated
different flow
evaluated
flow rates
(fully
the heat transfer
coefficient
remains constant
the different
rates (fully
developed
developed
internal
laminar
flow), the temperature
difference
between
thePCM
HTF isand
PCM is that
the
internal laminar
flow),
the temperature
difference between
the HTF
and the
thethe
parameter
parameter
that
determines
the
influence
of
the
flow
rate
on
the
heat
transfer
rate.
determines the influence of the flow rate on the heat transfer rate.
(a)
(b)
Figure
6. Comparison
Comparisonofofthe
thetemperature
temperature
profiles
of two
representative
temperature
sensors
Figure 6.
profiles
of two
representative
PCMPCM
temperature
sensors
with
with
three
different
mass
flow
rates.
(a)
charging
process
and
(b)
discharging
process.
Shaded
area
three different mass flow rates. (a) charging process and (b) discharging process. Shaded area indicates
indicates
theand
melting
and the solidification
temperature
range.
the melting
the solidification
temperature
range.
5.2.2.
5.2.2. Power
Power Profile
Profile
Figure
shows the
the power
power released/absorbed
released/absorbed by
average
Figure 77 shows
by the
the HTF
HTF and
and the
the PCM
PCM and
and HTF
HTF average
temperature
profiles
during
the
charging
and
discharging
processes
at
an
HTF
mass
flow
temperature profiles during the charging and discharging processes at an HTF mass flow rate
rate of
of
1500
kg/h.
At
the
beginning
of
both
processes,
higher
values
of
power
were
achieved
because
the
1500 kg/h. At the beginning of both processes, higher values of power were achieved because the heat
heat mainly
was mainly
transferred
to the metallic
tubes bundle,
which
has
a highconductivity,
thermal conductivity,
was
transferred
to the metallic
tubes bundle,
which has
a high
thermal
and at the
and
at
the
same
time
the
heat
transfer
between
the
HTF
and
the
PCM
located
around
the HTF
pipes
same time the heat transfer between the HTF and the PCM located around the HTF pipes
was
at a
was
at
a
maximum
because
the
PCM
thermal
resistance
was
low
and
the
thermal
gradient
between
maximum because the PCM thermal resistance was low and the thermal gradient between the HTF and
the
HTF was
and at
thea maximum.
PCM was at
a maximum.
Immediately
after,started
when the
the phase
PCM change,
started the
the power
phase
the PCM
Immediately
after,
when the PCM
change,
thedecreased
power drastically
due
the increase
of the PCMThe
thermal
drastically
due to thedecreased
increase of
thetoPCM
thermal resistance.
reasonresistance.
lies in the The
fact
reason
lies
in
the
fact
that
the
PCM
melting
front
moved
away
from
the
HTF
as
the
phase
change
that the PCM melting front moved away from the HTF as the phase change continued and therefore
continued
anddistance
thereforebetween
increased
distance
between
the HTF and
the PCM
melting.
Focused
on
increased the
thethe
HTF
and the
PCM melting.
Focused
on the
charging
process
the
charging
(Figure
a liquid
ring was
created
around
HTF pipes,
and, therefore,
the
(Figure
7a), aprocess
liquid ring
was7a),
created
around
the HTF
pipes,
and,the
therefore,
the natural
convection
natural
convection
became
the
predominant
heat
transfer
mechanism.
As
the
process
continued,
the
became the predominant heat transfer mechanism. As the process continued, the liquid fraction of
liquid
fraction ofand,
PCM
increased and,
the thermal
conductivity
associated,
until allwas
the
PCM increased
consequently,
theconsequently,
thermal conductivity
associated,
until all
the heat transfer
heat
transfer
was the
aimed
to increase
thePCM
temperature
thecorners
PCM located
the corners
andtank.
the
aimed
to increase
temperature
of the
located atofthe
and the at
central
part of the
central
part
of
the
tank.
During
the
discharging
process
(Figure
7b),
a
more
sharp
decrease
could
be
During the discharging process (Figure 7b), a more sharp decrease could be observed. The reason lies
observed.
The
reason
lies
in
the
fact
that
a
solid
PCM
ring
was
created
around
the
HTF
pipes,
which
in the fact that a solid PCM ring was created around the HTF pipes, which limited the heat transfer
limited
heat conductivity
transfer since
the thermal
conductivity
became
the dominant heat transfer
since thethe
thermal
became
the dominant
heat transfer
mechanism.
mechanism.
Figure 8 shows the comparison between the three evaluated mass flow rates (500, 1500 and
2500 kg/h) of the power released/recovered by/from the HTF during a charging and a discharging
process. It can be observed that at the beginning of both processes, the higher the mass flow rate, the
higher the power as previously detailed. However, as the processes continued, the thermal gradient
Appl. Sci. 2016, 6, 112
Appl. Sci. 2016, 6, 112
9 of 12
9 of 12
Appl. Sci. 2016, 6, 112
9 of 12
(a)
(b)
Figure
Figure 7.
7. HTF
HTF power
power profiles
profiles and
and average
average PCM
PCM and
and HTF
HTF temperature
temperatureprofiles
profilesat
at an
an HTF
HTF mass
mass flow
flow
rate
of
1500
kg/h:
(a)
charging
process
and
(b)
discharging
process.
rate of 1500 kg/h: (a) charging process and (b) discharging process.
(a)comparison between the three evaluated mass (b)
Figure 8 shows the
flow rates (500, 1500 and
2500Figure
kg/h) 7.ofHTF
the power
powerprofiles
released/recovered
by/from
thetemperature
HTF during
a charging
andmass
a discharging
and average PCM
and HTF
profiles
at an HTF
flow
process.
It can
observed
that at
the beginning
of both processes,
rate of
1500be
kg/h:
(a) charging
process
and (b) discharging
process. the higher the mass flow rate, the
higher the power as previously detailed. However, as the processes continued, the thermal gradient
between HTF and PCM decreased and the influence of the mass flow did not become as relevant.
(a)
(b)
Figure 8. Comparison of the HTF power profiles with three different mass flow rates. (a) charging
process and (b) discharging process.
5.2.3. Energy Profile
(a)
(b)
Because
of the non-linear
evolution
of the energy
accumulated/released
byrates.
the PCM
during full
Figure
flow
Figure 8.
8. Comparison
Comparison of
of the
the HTF
HTF power
power profiles
profiles with
with three
three different
different mass
mass flow
rates. (a)
(a) charging
charging
charging
andand
discharging
processes,
process
(b)
process.and in order to extrapolate the results for other processes which
process and
(b) discharging
discharging process.
are more likely to happen in a real installation, such as partial charging and discharging processes,
5.2.3.
Energy
an
important
parameter to be evaluated is the energy rate. This parameter is a ratio between the
5.2.3.
Energy Profile
Profile
energy accumulated along the specific process and the final energy that the system has accumulated
Because of
the
evolution
of the
the energy
energy accumulated/released
accumulated/released by
the
during
full
the non-linear
non-linear
evolution
of
the PCM
PCM
during
full
at theBecause
momentofwhen
the process
is considered
being finished.
Hence, this rateby
always
achieves
100%
charging
and
discharging
processes,
and
in
order
to
extrapolate
the
results
for
other
processes
which
charging
discharging
and in order
to extrapolate
the results
for other
processes
at
the endand
of the
process. processes,
This information
is useful
to understand
how the
geometry
of thewhich
tank
are
more
likely to
to happen
happenin
inaareal
realinstallation,
installation,such
suchasaspartial
partial
charging
and
discharging
processes,
are
more
likely
charging
and
discharging
processes,
an
influences the charging and discharging processes.
an important parameter to be evaluated is the energy rate. This parameter is a ratio between the
important
parameter
to
be
evaluated
is
the
energy
rate.
This
parameter
is
a
ratio
between
the
energy
Figure 9 shows the energy rate, bearing in mind the distribution of the PCM inside the tank
energy
accumulated
along
the specific
process
theenergy
final energy
that
the system
has accumulated
accumulated
alongthe
the
specific
process
and theand
final
that
system
the
(Figure 2b), during
charging
and discharging
processes
for
anthe
HTF
mass has
flowaccumulated
rate of 1500 at
kg/h.
at the moment when the process is considered being finished. Hence, this rate always achieves 100%
moment
when
the
process
is
considered
being
finished.
Hence,
this
rate
always
achieves
100%
at
the
It can be observed that the PCM located at the main part accumulates/releases the energy faster than
at the end of the process. This information is useful to understand how the geometry of the tank
endother
of theparts
process.
Thisitinformation
is useful
understand
theasgeometry
of direct
the tank
influences
the
because
exchanges the
energytowith
the HTFhow
easier
they are in
contact
with
influences the charging and discharging processes.
the
charging
and
discharging
processes.
the tubes bundle, while the PCM located at the corners takes much more time to start
Figure 9 shows the energy rate, bearing in mind the distribution of the PCM inside the tank
Figure 9 shows
the energy
bearing
in mind
the distribution
ofof
the
the tank
storing/releasing
the energy.
Thisrate,
situation
shows
a non-optimized
design
thePCM
heat inside
exchanger
and,
(Figure 2b), during the charging and discharging processes for an HTF mass flow rate of 1500 kg/h.
(Figure
during the
the heat
charging
andthe
discharging
processes
for an HTF
mass
flow ratethe
of 1500
in
order 2b),
to enhance
transfer,
corners should
be avoided.
In both
processes,
PCMkg/h.
took
It can be observed that the PCM located at the main part accumulates/releases the energy faster than
It can be50%
observed
the PCM
located
at the main part
accumulates/releases
theisenergy
than
around
of the that
process
time to
accumulate/release
90%
of the energy, which
a goodfaster
reference
the other parts because it exchanges the energy with the HTF easier as they are in direct contact with
the
other
parts
because
it
exchanges
the
energy
with
the
HTF
easier
as
they
are
in
direct
contact
with
for partial load processes such as the ones driven by intermittent renewable energies because it can
the tubes bundle, while the PCM located at the corners takes much more time to start
the tubes
bundle,
the PCMoflocated
at the corners
takes muchprocess,
more time
start
lead
on one
hand while
to a redesign
the charging
and discharging
andtoon
thestoring/releasing
other hand to a
storing/releasing the energy. This situation shows a non-optimized design of the heat exchanger and,
redesign of the storage tank.
in order to enhance the heat transfer, the corners should be avoided. In both processes, the PCM took
around 50% of the process time to accumulate/release 90% of the energy, which is a good reference
for partial load processes such as the ones driven by intermittent renewable energies because it can
lead on one hand to a redesign of the charging and discharging process, and on the other hand to a
redesign of the storage tank.
Appl. Sci. 2016, 6, 112
10 of 12
Appl. Sci. 2016, 6, 112
10 of 12
the energy. This situation shows a non-optimized design of the heat exchanger and, in order to enhance
In the
presentthe
study,
if compared
to avoided.
the processes
at a mass
flow rate
of 500took
kg/h,around
the process
the heat
transfer,
corners
should be
In both
processes,
the PCM
50% at
of
athe
mass
flow
rate
of
2500
kg/h
was
62%
faster
during
the
discharge
and
124%
faster
during
the
charge,
process time to accumulate/release 90% of the energy, which is a good reference for partial load
while
the such
process
at aones
mass
flowby
rate
of 1500 kg/h
was 29%
faster because
during the
discharge
and hand
60%
processes
as the
driven
intermittent
renewable
energies
it can
lead on one
faster
during ofthe
times
faster. The process,
differences
between
chargingofand
to a redesign
thecharge
charging
and discharging
and observed
on the other
hand tothe
a redesign
the
discharging
storage tank.are mainly due to the dominant heat transfer mechanisms—while, during the charging
process, it is mainly the convection, and during the discharging process it is the conduction.
(a)
(b)
Figure
Figure 9.
9. Evolution
Evolution of
of the
the PCM
PCM energy
energy rate
rate taking
taking into
into account
account the
the central,
central, main
main and
and corner
corner parts
parts of
of
the
tank
as
well
as
for
the
whole
tank
at
an
HTF
mass
flow
rate
of
1500
kg/h:
(a)
charging
process
and
the tank as well as for the whole tank at an HTF mass flow rate of 1500 kg/h: (a) charging process and
(b)
process
(b) discharging
discharging process.
6. Conclusions
In the present study, if compared to the processes at a mass flow rate of 500 kg/h, the process
A
fullflow
characterization
of a was
commercial
paraffin
a meltingand
temperature
58 °Cthe
is
at a mass
rate of 2500 kg/h
62% faster
during with
the discharge
124% fasterofduring
performed
from
different
approaches:
(massthe
range
of milligrams)
charge, while
the two
process
at a mass
flow rate aoflaboratory
1500 kg/h characterization
was 29% faster during
discharge
and 60%
and
pilot plant
characterization
range of kilograms).
The analyzed
PCM and
is selected
as a
fastera during
the charge
times faster.(mass
The differences
observed between
the charging
discharging
candidate
industrial
waste heat
recovery
domestic hot waterduring
applications.
are mainlyfor
due
to the dominant
heat
transferand
mechanisms—while,
the charging process, it is
In the
laboratory characterization,
the thermalprocess
and cycling
of the paraffin, its health
mainly
the convection,
and during the discharging
it is thestability
conduction.
hazards as well as its phase change thermal range, enthalpy and specific heat are analyzed using a
6. Conclusions
differential
scanning calorimeter, thermogravimetric analysis, thermocycling and infrared
spectroscopy.
Results showed
health hazards,
phase change
temperature
of 50–
A full characterization
of a no
commercial
paraffin with
with aa melting
temperature
of 58 ˝ Crange
is performed
−1. Thermal and cycling stability tests showed that this paraffin is
61
°C
and
enthalpy
of
120.11
kJ·kg
from two different approaches: a laboratory characterization (mass range of milligrams) and a pilot
suitable
for its use up to(mass
80 °Crange
and presents
no degradation
after 1200
plant characterization
of kilograms).
The analyzed
PCMcycles.
is selected as a candidate for
At
the
pilot
plant
facility,
the
characterization
of
108
kg of PCM was analyzed in a
industrial waste heat recovery and domestic hot water applications.
shell-and-tube
heat exchanger
under different
HTF mass
rates,
in terms
temperature
profiles,
In the laboratory
characterization,
the thermal
andflow
cycling
stability
ofofthe
paraffin, its
health
power
released/absorbed
and
energy
rates
during
charging
and
discharging
processes.
Comparing
hazards as well as its phase change thermal range, enthalpy and specific heat are analyzed using a
the
temperature
profiles
of the two
processes, it was
observed
that the discharge
showed
a slower
differential
scanning
calorimeter,
thermogravimetric
analysis,
thermocycling
and infrared
spectroscopy.
response
than the
theapresence
of natural
convection
during
the ˝melting
process
Results showed
nocharge
health because
hazards,of
with
phase change
temperature
range
of 50–61
C and enthalpy
´
1
and
the
creation
of
a
solid
layer
around
the
tube
bundle
when
the
solidification
process
is
of 120.11 kJ¨ kg . Thermal and cycling stability tests showed that this paraffin is suitable forstarted.
its use
The
of presents
the charging
and discharging
process
was also affected by the different HTF mass
up toduration
80 ˝ C and
no degradation
after 1200
cycles.
flow At
rates.
the power
released/absorbed,
at the
beginning
both processes,
the higher
the Regarding
pilot plant facility,
the characterization
of 108
kg of
PCM wasofanalyzed
in a shell-and-tube
the
mass
flow
rate,
the
higher
the
power.
However,
as
the
processes
continued,
this
temperature
heat exchanger under different HTF mass flow rates, in terms of temperature profiles, power
gradient
decreased and
influence
of the mass
flowand
did discharging
not become processes.
as relevant.Comparing
Finally, it was
released/absorbed
and the
energy
rates during
charging
the
showed
that
the
energy
rate
is
an
important
parameter
to
be
tested
because
of
the
non-linearity
temperature profiles of the two processes, it was observed that the discharge showed a slower response
evolution
of the because
energy accumulated/released
by the
PCM, which
alsothe
indicates
optimal
than the charge
of the presence of natural
convection
during
meltinghow
process
and the
the
design
tank
is. around
This last
parameter
willwhen
be studied
deeply by the
authors
in further
creationofofaaTES
solid
layer
the
tube bundle
the solidification
process
is started.
The work.
duration
of the charging and discharging process was also affected by the different HTF mass flow rates.
Acknowledgments: The work is partially funded by the Spanish government (ULLE10-4E-1305,
Regarding the power released/absorbed, at the beginning of both processes, the higher the mass
ENE2015-64117-C5-1-R and ENE2015-64117-C5-3-R). This project has received funding from the European
flow rate, the higher the power. However, as the processes continued, this temperature gradient
Commission
Seventh
Framework
Programme
(FP/2007–2013)
under
Grant
agreement
N°PIRSES-GA-2013-610692 (INNOSTORAGE) and No ENER/FP7/295983 (MERITS), and from the European
Union’s Horizon 2020 Research and Innovation Programme under Grant agreement No 657466 (INPATH-TES).
The authors would like to thank the Catalan Government for the quality accreditation given to their research
Appl. Sci. 2016, 6, 112
11 of 12
decreased and the influence of the mass flow did not become as relevant. Finally, it was showed that
the energy rate is an important parameter to be tested because of the non-linearity evolution of the
energy accumulated/released by the PCM, which also indicates how optimal the design of a TES tank
is. This last parameter will be studied deeply by the authors in further work.
Acknowledgments: The work is partially funded by the Spanish government (ULLE10-4E-1305, ENE201564117-C5-1-R and ENE2015-64117-C5-3-R). This project has received funding from the European Commission
Seventh Framework Programme (FP/2007–2013) under Grant agreement N˝ PIRSES-GA-2013-610692
(INNOSTORAGE) and No ENER/FP7/295983 (MERITS), and from the European Union’s Horizon 2020 Research
and Innovation Programme under Grant agreement No 657466 (INPATH-TES). The authors would like to
thank the Catalan Government for the quality accreditation given to their research groups GREA (2014 SGR
123) and DIOPMA (2014 SGR 1543). Jaume Gasia would like to thank the Departament d’Universitats,
Recerca i Societat de la Informació de la Generalitat de Catalunya for his research fellowship (2016FI_B
00047). Laia Miró would like to thank the Spanish Government for her research fellowship (BES-2012-051861).
Alvaro de Gracia and Camila Barreneche would like to thank the Ministerio de Economia y Competitividad de
España for Grant Juan de la Cierva, FJCI-2014-19940 and FJCI-2014-22886, respectively.
Author Contributions: Jaume Gasia, Laia Miró and Luisa F. Cabeza conceived and designed the study;
Jaume Gasia performed the experiments at pilot plant scale and Camila Barreneche performed the experiments at
the laboratory scale; all the coauthors collaborated on the interpretation of the results and on the preparation of
the manuscript.
Conflicts of Interest: The authors declare no conflict of interest.
References
1.
2.
3.
4.
5.
6.
7.
8.
9.
10.
11.
12.
13.
14.
International Energy Agency. Technology Roadmap: Energy Storage. Available online: https://www.iea.
org/publications/freepublications/publication/technology-roadmap-energy-storage-.html (accessed on 13
May 2015).
Mehling, H; Cabeza, L.F. Heat and Cold Storage with PCM: An. up to Date Introduction into Basics and Applications,
1st ed.; Springer Science & Business Media: Berlin, Germany, 2008; pp. 11–57.
Zalba, B.; Marı́n, J.M.; Cabeza, L.F.; Mehling, H. Review on thermal energy storage with phase change:
Materials, heat transfer analysis and applications. Appl. Therm. Eng. 2003, 23, 251–283.
Sharma, A.; Tyagi, V.V.; Chen, C.R.; Buddhi, D. Review on thermal energy storage with phase change
materials and applications. Renew. Sustain. Energy Rev. 2009, 13, 318–345. [CrossRef]
Farid, M.M.; Khudhair, A.M.; Razack, S.A.K.; Al-Hallaj, S. A review on phase change energy storage:
Materials and applications. Energy Convers. Manag. 2004, 45, 1597–1615. [CrossRef]
Hasnain, S.M. Review on sustainable thermal energy storage technologies, Part I: Heat storage materials and
techniques. Energy Convers. Manag. 1998, 39, 1127–1138. [CrossRef]
Trp, A. An experimental and numerical investigation of heat transfer during technical grade paraffin melting
and solidification in a shell-and-tube latent thermal energy storage unit. Sol. Energy 2005, 79, 648–660.
[CrossRef]
Rathod, M.K.; Banerjee, J. Experimental Investigations on Latent Heat Storage Unit using Paraffin Wax as
Phase Change Material. Exp. Heat Transf. 2014, 27, 40–55. [CrossRef]
Akgün, M.; Aydın, O.; Kaygusuz, K. Thermal energy storage performance of paraffin in a novel tube-in-shell
system. Appl. Therm. Eng. 2008, 28, 405–413. [CrossRef]
Jesumathy, S.P.; Udayakumar, M.; Suresh, S. Heat transfer characteristics in latent heat storage system using
paraffin wax. J. Mech. Sci. Technol. 2012, 26, 959–965. [CrossRef]
Jesumathy, S.P.; Udayakumar, M.; Suresh, S.; Jegadheeswaran, S. An experimental study on heat transfer
characteristics of paraffin wax in horizontal double pipe heat latent heat storage unit. J. Taiwan Inst. Chem. Eng.
2014, 45, 1298–1306. [CrossRef]
Medrano, M.; Yilmaz, M.O.; Nogués, M.; Martorell, I.; Roca, J.; Cabeza, L.F. Experimental evaluation of
commercial heat exchangers for use as PCM thermal storage systems. Appl. Energy 2009, 86, 2047–2055.
[CrossRef]
He, B.; Setterwall, F. Technical grade paraffin waxes as phase change materials for cool thermal storage and
cool storage systems capital cost estimation. Energy Convers. Manag. 2002, 43, 1709–1723. [CrossRef]
Kousksou, T.; Bruel, P.; Cherreau, G.; Leoussoff, V.; El Rhafiki, T. PCM storage for solar DHW: From an
unfulfilled promise to a real benefit. Sol. Energy 2011, 85. [CrossRef]
Appl. Sci. 2016, 6, 112
15.
16.
17.
18.
12 of 12
Brückner, S.; Liu, S.; Miró, L.; Radspieler, M.; Cabeza, L.F.; Lävemann, E. Industrial waste heat recovery
technologies: An economic analysis of heat transformation technologies. Appl. Energy 2015, 151, 157–167.
[CrossRef]
Miró, L.; Barreneche, C.; Ferrer, G.; Solé, A.; Martorell, I.; Cabeza, L.F. Health hazard, cycling and thermal
stability as key parameters when selecting a suitable Phase Change Material (PCM). Thermochim. Acta 2016,
627–629, 39–47. [CrossRef]
Barreneche, C.; Solé, A.; Miró, L.; Martorell, I.; Fernández, A.I.; Cabeza, L.F. Study on differential scanning
calorimetry analysis with two operation modes and organic and inorganic phase change material (PCM).
Thermochim. Acta 2013, 553, 23–26. [CrossRef]
Ferrer, G.; Solé, A.; Barreneche, C.; Martorell, I.; Cabeza, L.F. Review on the methodology used in thermal
stability characterization of phase change materials. Renew. Sustain. Energy Rev. 2015, 50, 665–685. [CrossRef]
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