470
J SCI IND RES VOL 66 JUNE 2007
Journal of Scientific & Industrial Research
Vol. 66, June 2007, pp. 470-476
Capric acid and palmitic acid eutectic mixture applied in building wallboard
for latent heat thermal energy storage
Ali Karaipekli and Ahmet Sari*
Department of Chemistry, Gaziosmanpasa University, 60240, Tokat, Turkey
Received 20 July 2006; accepted 16 January 2007
The paper aims: (1) preparation of the phase change gypsum wallboard as novel phase change wallboard (PCW)
incorporating with the eutectic mixture of capric acid (CA) and palmitic acid (PA) for latent heat thermal energy storage; (2)
determination of thermal properties and thermal reliability of prepared PCW using differential scanning calorimetry (DSC)
technique; and (3) estimation of thermal performance of PCW by preparing a simple test cell. Maximum CA/PA eutectic
mixture as phase change material absorbed in PCW was about 25 wt% of total weight. No leakage of the mixture from PCW
was observed after 5000 thermal cycling. The melting and freezing temperatures and latent heats of PCW were measured as
22.94 and 21.66°C, 42.54 and 42.18 J/g, respectively by DSC analysis. These properties make it functional as TES medium,
which can be applied to peak load shifting, improved use of waste heat and solar energy as well as more efficient operation of
heating and cooling equipment. In addition, PCW has good thermal reliability in terms of the changes in its thermal properties
after accelerated 1000, 2000, 3000, 4000, and 5000 thermal cycling. Use of such PCW can decrease indoor air fluctuation and
have the function of keeping warmth to improve indoor thermal comfort due to absorption of heat in conjunction with melting
of the eutectic mixture.
Keywords: Capric acid, Latent heat thermal energy storage, Palmitic acid, Phase change wallboard, Thermal properties,
Thermal reliability
IPC Code: E04B1/99
Introduction
Energy storage for heating and cooling of the
building/space is mainly based on sensible heat storage
materials. Another energy storage type used for the same
purposes is latent heat thermal energy storage (LHTES)
by using a phase change material (PCM). When
compared with the sensible heat storage materials, such
as water, sand or rocks, PCMs (melting temp., 20-32°C)
store more heat per unit volume and storage/release heat
at an almost constant temperature. PCMs have been
recommended and used for LHTES in conjunction with
both passive storage and active storage for heating and
cooling in buildings1-4.
Hadjieva et al5 investigated heat storage capacity
and structural stability at multiple thermal cycling of
sodium thiosulphate pentahydrate absorbed in the porous
concrete. Hawes et al6 investigated thermal performance
and estimation of the benefits from application of PCM
gypsum board in passive solar buildings in terms of
*Address for correspondence
E-mail: [email protected]
reduction of room overheating and energy savings.
Salyer & Sircar 7 studied most promising PCM
containment methods and applied to solite hollow core
building blocks. Kissock et al8 presented results of an
experimental study on thermal performance of
wallboards imbibed to 30 wt% with commercial paraffin
PCM in simple structures. Paraffin type-PCMs (paraffin
waxes, 1-dodecanol and 1-tetradecanol) have been
impregnated into porous building materials to create a
direct gain storage element8-11. Fatty acids, their esters
and mixtures are promising PCMs to obtain phase
change wallboard (PCW) for LHTES applications in
passive solar buildings because of their good thermophysical properties, thermal reliability12-16 and the
advantage of easily impregnation, or directly
incorporation into conventional building products17,18.
Shapiro19 showed several PCMs (mixtures of methylesters, methyl palmitate, methyl stearate, and capric
acid/lauric acid mixture) to be suitable PCMs for
introduction into gypsum wallboard for possible LHTES
applications. Feldman et al 20 measured thermal
properties of a PCM gypsum, which was made by
KARAIPEKLI & SARI : CAPRIC/PALMITIC EUTECTIC MIXTURE ON BUILDING FOR ENERGY STORAGE
471
Fig. 1— Pore size distribution of gypsum used in the study
soaking conventional gypsum board in liquid butyl
stearate. Neeper21 examined thermal dynamics of a
gypsum wallboard impregnated with fatty acids as PCMs
subjected to the diurnal variation of room temperature
but not directly illuminated by the Sun. Shilei et al22,23
studied PCW incorporated in the eutectic mixture of
capric acid and lauric acid and tested its impact on indoor
thermal environment in winter conditions. Gypsum,
being porous and light, cheap, conveniently and simply
constructed and insulator against sound and heat, is very
suitable for PCM encapsulation18,20,22,23.
The objectives of this paper are follows: (1)
Preparation of gypsum PCW as novel PCW by
incorporating with the capric acid (CA)/palmitic acid
(PA) eutectic mixture for LHTES purposes;
(2) Determination of thermal properties and thermal
reliability of the PCW by DSC analysis technique; and
(3) Estimation of thermal performance of PCW in a
simple test cell.
Materials and Methods
CA (98% pure) and PA (96% pure) were
supplied by Aldrich and Fluka Companies. Gypsum
material with Turkish origin was used as wallboard
material. Pore size (20-1000 nm) of gypsum was
determined (Fig. 1) using a mercury porosimeter
(Quantachrome Corporation, Poremaster 60 model).
Preparation of CA/PA Eutectic Mixture and Gypsum PCW
CA/PA (76.5:23.5 wt%) eutectic mixture was
prepared14-16. PCW samples (100 g) were formed by
mixing gypsum material in melted CA/PA eutectic
mixture for 1 h. The mixture (25%) was absorbed in
gypsum and no PCM seepage from PCW was observed
after 5000 melting/freezing cycles. Moreover, density
of PCW (1247 kg/m3) was less than that of gypsum PCW
(1452 kg/m3).
Determination of Thermal Properties by DSC Analysis
Thermal properties [melting temperature (Tm),
freezing temperature (Tf), latent heat of melting (∆Hm )
and latent heat of freezing (∆Hf)] of CA, PA, CA/PA
eutectic mixture and PCW were measured by a DSC
instrument (SETARAM DSC 131 model). A 5°C/min
heating rate was performed for all DSC measurements.
Liquid nitrogen was used to cool DSC samples below
room temperature. Tm and Tf values were taken as onset
temperatures by drawing lines at the points of maximum
slope of leading edges of the melting and freezing peak
and extrapolating base lines on the same side as leading
edge of the peaks. ∆Hm and ∆Hf values were determined
by numerical integration of the area under the melting
and freezing peak, respectively. All DSC measurements
were repeated three times. Standard deviations for phase
change temperature and latent heat respectively were
found to be: CA, ±0.13 °C, 0.40 J/g; PA, ±0.14 °C,
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J SCI IND RES VOL 66 JUNE 2007
Fig. 2— DSC curves of CA, PA compounds, and CA/PA eutectic mixture
± 0.44 J/g; CA/PA eutectic mixture, ±0.10 °C, ±0.41 J/
g; and PCW, ±0.11°C, ±0.43 J/g.
(Pt-RhPt) with an accuracy of ± 0.1°C. When lamp was
switch on, temperature variations at specified locals of
the test cells were monitored by using a data logger.
Thermal Cycling Test
Accelerated thermal cycling test14 was used to
determine thermal reliability of PCW as changes in its
thermal properties after repeated numbers of melt/freeze
cycles, 1000, 2000, 3000, 4000, and 5000.
Thermal Performance Test
To study thermal performance of PCW, six
identical PCWs were fabricated as mold (100 mm x 100
mm x 10 mm) by pouring the prepared slurry (gypsum
powder, water, and CA/PA eutectic mixture) into a
stainless steel mold. After vibrated for about 10 min,
molds were first kept at the room temperature for 24 h,
and then dried in a vacuum drier at 50°C before use. In
addition, six identical ordinary gypsum wallboards with
control purpose were fabricated using the same method.
After then, two simple test cells were separately
fabricated by using these molds. Two halogen tungsten
lamps (150 W) were used as heat source for each test
cell. Temperature variations at inner and outer surfaces
of front wall, and indoor centers of the both test cells
were simultaneously recorded using thermocouples
Results and Discussion
Thermal Properties of CA/PA Eutectic Mixture
Melting temperature of binary system (Fig. 2)
was found lower than those of the single acids (CA and
PA) and components of mixture melted simultaneously
at 21.85°C as the binary system reached eutectic ratio
(CA:PA, 76.5:23.5 wt %). CA/PA eutectic mixture had
a melting latent heat of 171.22 J/g and a freezing latent
heat of 173.16 J/g. Thermal properties of CA and PA
compounds and CA/PA eutectic mixture obtained from
DSC curves (Fig. 3) were given in Table 1.
Peippo et al17 measured eutectic ratio (CA:PA, 75.2:24.8
wt %), melting temperature (22.1°C) and latent heat of
melting (153 J/g) of the same eutectic mixture. It was
probably due to the impurity in single acids (3-4 wt %)
and the experimental errors through the DSC
analysis12,14-16,22.
Latent heat of eutectic mixture was high to be
compared to some PCMs such as salt hydrates and
polyalcohols1-3. In addition, CA/PA eutectic mixture can
be considered as a promising PCM to obtain PCW for
473
Heat flow, mW
KARAIPEKLI & SARI : CAPRIC/PALMITIC EUTECTIC MIXTURE ON BUILDING FOR ENERGY STORAGE
Temperature, oC
Fig. 3— Melting temperatures of CA/PA binary mixtures versus mass combination of the components
Table 1—Thermal properties of CA, PA, CA/PA eutectic mixture and gypsum PCW
PCM
CA
PA
CA/PA eutectic mixture
Phase change wallboard
Tm, °C
32.14
59.40
21.85
22.94
LHTES applications in passive solar buildings because
of its good thermal properties, chemical stability, being
non-corrosiveness and non-toxicity and the advantage
of easily impregnation, or directly incorporation into
conventional building materials. Although, initial cost
of this fatty acid mixture is higher than that of other
PCMs such as salt hydrates and paraffins, the installed
cost is competitive.
Thermal Properties and Thermal Reliability of PCW
Uncycled PCW melts at 22.94°C and solidifies
at 21.66°C (Fig. 4). Melting and freezing temperatures
of PCW are suitable for storing and releasing heat in the
human comfort zone. It has a latent heat of melting as
42.54 J/g and latent heat of freezing as 42.18 J/g
(Table 1). The latent heats of phase change of PCW are
as high as comparable with those of PCWs prepared by
impregnation of gypsum materials with different PCMs
(Table 2)11. After repeated 1000, 2000, 3000, 4000, and
∆Hm, J/g
156.40
218.53
171.22
42.54
Tf, °C
32.53
58.23
22.15
21.66
∆Hf, J/g
154.24
216.46
173.16
42.18
5000 thermal cycling, Tm value of PCW changed as
-0.21, 0.18, 0.46, 0.05, and -0.20°C, and its Tf value
changed as -0.30, 0.18, 0.35, 0.08, and -0.42°C,
respectively (Table 3). Thus, changes in Tm and Tf that
are irregular with increasing number of thermal cycling
are not in significant magnitude for solar passive heating
and cooling purposes in buildings. Therefore, PCW has
good thermal reliability in terms of changes in its melting
and freezing temperatures (Tm and Tf).
On the other hand, after repeated 1000, 2000,
3000, 4000 and 5000 thermal cycling, ∆Hm value of PCW
changed by 3.9%, 9.2%, 10.8%, 2.5%, and -3.2% and
its ∆Hf value changed by -1.3%, 3.0%, 9.2%, 1.7%, and
-3.7%, respectively (Table 3). The changes in ∆Hm and
∆Hf values of PCW are in the range of -3.7% to 20.3%
as thermal cycling number was raised from 1000 to 5000.
These results are in reasonable level for a PCW used for
LHTES applications in passive solar buildings18,19,22.
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J SCI IND RES VOL 66 JUNE 2007
Heat flow, mW
Phase change wallboard
Temperature, oC
o
Temperature, C
Fig. 4 — DSC curves of the PCW before and after thermal cycling
Fig. 5— Temperature variations at inner and outer surfaces of front wall of test cells
Table 2 — Thermal properties of some gypsum PCWs11
PCM
Tm, °C
Tf, °C
∆H, J/g
Capric-lauric acid +
fire reterdant
Butyl stearate
17
18
21
21
28
30
Propyl palmitate
19
16
40
Dodecanol
20
21
17
Thermal Performance of PCW
Optimum inner surface temperatures of the test
cells fabricated by ordinary gypsum wallboard (32°C)
and PCW (29°C) indicate a 3°C temperature difference
because some of heat loaded during the heating period
was absorbed by CA/PA eutectic mixture into PCW
(Fig. 5). Maximum indoor center temperature in the test
cell made of ordinary gypsum wallboard was about 27°C
475
Temperature, oC
KARAIPEKLI & SARI : CAPRIC/PALMITIC EUTECTIC MIXTURE ON BUILDING FOR ENERGY STORAGE
Fig. 6 — Temperature variations at indoor center of the test cells
Table 3 — Changes in thermal properties of PCW with respect
to thermal cycling number
Phase change wallboard
Number of
thermal cycling
Tm, °C
∆Hm, J/g
Tf, °C
∆Hf, J/g
0 (uncyled)
22.94
42.54
21.66
42.18
1000
22.73
44.22
21.36
41.65
2000
23.12
46.44
21.84
43.45
3000
23.40
47.12
22.01
46.05
4000
22.99
43.61
21.74
42.88
5000
22.74
41.16
21.24
40.63
as it was about 24°C in case of test cell made of PCW
(Fig. 6). Thermal performance of PCW in a simple cell
was good as compared with that of gypsum PCW
prepared using different PCMs10,22. Therefore, it is
suggested that the utilization of PCW in buildings can
increase human comfort by decreasing the amplitude of
internal air temperature swings and maintaining
temperature closer to human comfort temperature range
(16-25°C) to cut down on peak cooling loads and the
energy consumption in buildings.
Conclusions
CA/PA eutectic mixture is a promising PCM to
prepare a novel PCW for LHTES applications in passive
solar buildings due to its desirable thermal properties,
chemical stability, and good compatibility with gypsum
or gypsum wallboard as a conventional building material.
CA/PA eutectic mixture can be loaded into gypsum
wallboard in maximum proportion of 25% of total weight
and no eutectic PCM leakage from the prepared PCW is
observed after 5000 thermal cycling. Uncycled PCW
melts at 22.94°C and freezes at 21.66°C, which is
suitable for storing and releasing heat in the human
comfort zone. It has: latent heat of melting, 42.54 J/g;
and latent heat of freezing, 42.18 J/g. PCW has good
thermal reliability in terms of the changes in its thermal
properties with respect to a large number (5000) of
thermal cycling. The use of such PCW can decrease
indoor air fluctuation and have the function of keeping
warmth to improve indoor thermal comfort due to
absorption of heat in conjunction with melting of eutectic
mixture. Thus resultant indoor temperature will not rise
significantly above comfortable temperature despite
continuous heat gain. However, a further study is needed
to investigate large-scale thermal performance of a
building envelope in practical dimensions and to test
the fire resistance, mechanical properties and
compatibility with paints and wallpapers. An extensive
study can also be considered to determine compatibility
of CA/PA eutectic mixture with various porous building
materials (cement, concrete blocks, brick, and
plasterboard) and investigate the effect of such PCW on
the environment.
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J SCI IND RES VOL 66 JUNE 2007
Acknowledgement
Authors thank Dr Orhan UZUN for DSC
analyses and the staff of Central Laboratory in Middle
East Technical University for porosity measurement.
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