EXPERIMENTAL INVESTIGATION OF AGGLOMERATE MARBLES CONTAINING
PHASE CHANGE MATERIALS
I. Mandilaras, M. Founti
National Technical University of Athens
School of Mechanical Engineering
9, Heroon Polytechniou, Polytechnioupoli Zografou
Athens 15780, Greece
[email protected]
ABSTRACT
The work investigates the thermal response of a new composite building material based on
natural stone wastes and Phase Change Materials (PCM). In order to be able to measure the
transient thermal response of the new products under various temperature conditions, a
specialized device has been developed at NTUA and implemented for the study of the nonlinear
heat-transfer characteristics associated with phase-change of the PCM. Thermal performance
analysis and evaluation for three different agglomerate marble samples with different percentages
of PCM in their mass, have been undertaken. Comparative analysis of the results shows the
ability of the new materials to stabilize their temperature near the melting range of the PCM, and
potentially reduce heat losses of a building.
1.
INTRODUCTION
Thermal analysis of conventional building materials requires determination of specific heat and
thermal conductivity. These two thermal properties along with the density of the material can
fully characterize and predict its performance. Incorporation of a Phase Change Material (PCM)
in a building material necessitates a different approach in thermal characterization. Additional
properties have to be measured such as melting/solidification range of the PCM and enthalpy
change during phase transition. Furthermore, comparative analysis including measurements of
specific heat capacity and heat-flux/energy is required [1,2]. Current methods for thermal
conductivity measurements can be applied for the measurement of materials containing PCMs as
long as the temperature of the sample during the measuring procedure remains outside of the
phase change region. However, specific heat capacity cannot be measured with conventional
transient methods. The only applicable method is Differential Scanning Calorimetry, which
requires expensive equipment and a very small quantity (in the order of mg) of the material that
in many cases is not representative of the original sample [3]. For the needs of inexpensive and
reliable measurements a prototype experimental set up was specifically developed for the thermal
characterization of building materials containing PCM. The application of the new experimental
apparatus for the investigation of an agglomerate marble containing PCM provides the focus for
this paper.
2.
EXPERIMENTAL SET-UP
An experimental setup (Fig. 1,2) has been developed for the measurement of heat capacity,
thermal mass and thermal energy conservation of building materials containing PCMs. The
device applies variable thermal loads at the two sides of a flat-surface material sample, while
measuring its thermal response. Two thermo-regulated aluminum plates that support and hold the
sample are used to impose thermal loads. Thermoelectric elements with a control unit are capable
of applying any constant temperature or temperature profile on the sample facing sides of the
aluminum plates. Thermal response and heat flux on the two surfaces of the sample are
continuously recorded via a data acquisition system. A thin insulating layer of Neopren between
the sample and the device simulates air-wall convection heat transfer. An insulation of 5 mm with
thermal conductivity of k = 0.065 W/m·k corresponds to convection heat transfer coefficient h =
13 W/m2·k which is a typical value for wall-air heat transfer cases. In addition, this flexible layer
assures perfect contact between samples with rough surface and the device and allows the sensors
to be placed on the two surfaces of the sample.
Flexible
interface
Water out
Aluminium
Plates
Sample
Water in
Figure 1. Sample holder
The test rig is pictured in figs. 1-2 and can be divided into four main components:
•
•
•
•
The frame that holds the plates and the sample (Fig. 1)
Thermoelectric heating/cooling devices
The temperature control unit
The data acquisition system
Control
Unit
Thermoelectric
device 1
PC
DAQ
Sample
Holder
Thermoelectric
device 2
Figure 2. Schematic representation of the experimental set up
The aluminium plates are rectangular in shape (200x200 mm2) and they can move along a
horizontal slide allowing specimen insertion. The dimensions were chosen to ensure that there
exists a central section suitable for one-dimensional heat transfer. To achieve the uniform onedimensional temperature field required for the calculations, the lateral surfaces of the specimen
and the device are covered with insulating material of 60 mm thickness and thermal conductivity
of approximately k=0.03W/mK. The temperature control of the surfaces is achieved with the
recirculation of water into four independent channels in each plate.
.
Heating/cooling of the water is performed by two 500 W thermoelectric type devices. The
devices are capable of extended use at temperatures from 5oC to 75oC. Up to 10 L/min of water
are supplied to the plates via two 220 V centrifugal pumps.
The control unit is capable of stabilizing the temperature of the plates within the range of
±0.05oC. It comprises of two temperature controllers and two power supply units 500 W each. It
is connected to the PC with RS232 interface.
For the data acquisition (Fig. 2) an Agilent DAQ/Switch unit is used. The unit collects the signals
from 12 sensors attached on the plates and the sample to be measured. Temperatures are
measured with NTC thermistors. The probes are very small with 0.46mm diameter, to ensure fast
time response of 200 ms and accuracy of ±0.2oC at 25oC. Heat flux is measured with two thin
(0.18mm) heat flux sensors with sensitivity 2.06 μV/W/m2. A PC running LabView software
collects and records all the data from the control and the DAQ unit.
Main sources of systematic error in the system are associated with:
• Sensors’ accuracy
• DAQ unit accuracy
• Control Unit accuracy
• Deviation from the one-dimensional heat transfer characteristics through the sample
Maximum systematic errors are expected in the measurement of the heat capacity, and for the
specific configuration they have been estimated to be of the order of ± 10% of the measured
value.
3.
AGGLOMERATE MARBLE
The material examined is a new type of agglomerate marble in the form of tile. It consists of
wastes of the Greek semi-white dolomitic marble “Thassos Crystallina” and PCM in microencapsulated form. Chips and powder from the marble are mixed with the PCM and bounded
with polyesteric resin. The samples have been developed by GeoAnalysis S.A [4].
The PCM used is the BASF Micronal 5001 DS, in powder form (micro-encapsulated paraffin).
The melting temperature of the material is 26oC and the specific heat capacity is approximately
110 kJ/kg. The value of the density varies from 250 to 350 kg/m3. The product has the form of
white microcapsules with average size less than 150μm. It is composed of a mixture of paraffin
encapsulated in highly cross-linked polymethyl methacrylate microcapsules. The paraffin wax
absorbs or releases heat as it changes from solid to liquid or vice versa. The release and
absorption of heat in a narrow temperature range tends to stabilize the temperature of the material
near the phase change region.
Three agglomerate marble mixes were prepared (Fig. 3). The reference mix does not contain any
PCM in its mass. In the other two mixes, 10% and 20% of the marble mass was substituted by
PCM. The composition of the mixes is shown in table 1. Thermal conductivity (measured by hot
wire transient method at 30oC) and density of the three samples is presented in table 2.
(a)
(b)
(c)
Figure 3. Agglomerate marble samples with (a) 0%, (b) 10% and (c) 20% PCM
10% PCM
20% PCM
per marble mass
per marble mass
Marble Chips
4.0 kg
4.0 kg
3.5kg
Marble powder
1.0 kg
0.5 kg
0.5kg
PCM
0.5 kg
1.0 kg
Resin
1.0 kg
1.5 kg
2.5 kg
Table 1. Mixture composition of the samples
Reference
Reference
10% PCM
per marble mass
20% PCM
per marble mass
Thermal Conductivity
1.54
0.73
0.44
[W/mK]
Density [kg/m3]
2128.6
1945.7
1635.7
Table 2. Thermal conductivity and density of the samples
4.
THERMAL ANALYSIS
The first part of the thermal analysis includes measurements of specific heat capacity and thermal
mass of the samples. The second part is an experimental attempt to estimate and compare the
performance of the materials in terms of energy saving assuming that the material constitutes a
wall of a room with constant inner temperature.
4.1
Specific heat capacity/thermal mass
For the specific heat capacity measurements three samples of the three different mixes were
prepared at the appropriate dimensions, 200mm x 200mm x 15.5mm. The samples were
introduced in the sample holder of the device at a temperature of 10 °C and were heated up to 36
°C. The temperature of the two plates during the heating process was set 4oC higher that the
sample temperature. This corresponds to a constant heating rate of approximately 61 W/m2. The
temperature of the samples and the heat flux from the device to the samples were recorded.
Temperature and heat flux measurements allow the calculation of the heat capacity and thermal
mass of the samples (Figures 4 and 5). Assuming one-dimensional heat transfer and according to
the lump capacitance method (Bi<0.1) we have:
Aq
⎛ dT ⎞
m⎜ ⎟
⎝ dt ⎠
(1)
M th = m c p
(6)
cp =
where Cp is the heat capacity of the sample, Mth the thermal mass, A the heat exchange area of
the sample, q the heat flux per square meter, m the mass of the sample, T the temperature of the
sample and, t the time.
Figures 4 and 5 present the measured specific heat capacity and thermal mass for the three
samples versus temperature. In both Figures, the effect of increasing the percentage of PCM in
the mixture is apparent in the melting temperature range of the PCM (25 °C – 30 °C).
Comparison of figures 4 and 5 indicates that, as expected, increasing the amount of PCM in the
mixture increases significantly the maximum value of specific heat capacity (up to 4 times for the
20% PCM content). However, the thermal mass does not exhibit the same trend. The 20% PCM
mixture has more thermal mass than the 10% mixture inside the melting range of the PCM but
less outside of this region. This is the consequence of the decreasing density of the sample with
the increasing PCM content. As a result, there is a limitation on the PCM percentage in the
material depending mostly on the temperature range that the material is going to be used. For the
temperature range from 16oC to 31oC the mean thermal mass for 10% and 20% p/m PCM content
has the same value. The use of higher PCM percentage in this range will reduce the mean thermal
mass of the sample. Moreover, the use of a 20% PCM mixture in a wider temperature range will
have a negative effect regarding the mean thermal mass in comparison to the 10% mixture.
5000
Specific Heat Capacity [J/kgK]
4500
Reference
10% PCM
20% PCM
4000
3500
3000
2500
2000
1500
1000
500
0
10
15
20
25
30
35
o
Temperature [ C]
Figure 4. Specific heat capacity
9000
8000
Reference
10% PCM
20% PCM
Thermal mass [J/K]
7000
6000
5000
4000
3000
2000
1000
0
10
15
20
25
30
35
o
Temperature [ C]
Figure 5. Thermal mass
4.2
Energy saving aspects
Potential energy savings with the agglomerate marbles can be estimated by measuring heat losses
through the samples when they are assumed to be part of a wall. In this case the device is used to
simulate indoor and outdoor temperatures and appropriate temperature profiles are imposed on
the two sides of the sample. The outdoor temperature is assumed to have a sinusoidal variation
from 21 °C to 31 °C for 48 hours (resembling temperature variations in a South European
country), while the indoor temperature is set stable at a level of 23.5 °C. Temperatures and heat
fluxes on both surfaces of the same samples as the ones used for the specific heat capacity
measurements are recorded.
Integration of the measured heat flux (Figure 6) on the inner side of the sample provides a
measure of the total heat losses towards the indoor environment. The heat flux measurements of
Figure 6 demonstrate an up to 18% variation in the measured maximum and minimum peak
values for the sample with 20% PCM content. The calculated energy (Figure 7) corresponds to
the energy required by an air-conditioning system to maintain the indoor temperature constant at
26oC. Savings up to 20% can be expected as a result of the inclusion of 20% p/m PCM in the
mix.
30
Reference
10% PCM
20% PCM
Heat Flux [W/m 2]
20
10
0
0
5
10
15
20
25
30
35
40
45
-10
-20
-30
Time [h]
Figure 6. Heat flux on the side of the sample corresponding to the indoor wall surface
900
800
Reference
10% PCM
20% PCM
Energy [Wh/m2]
700
600
500
400
300
200
100
0
0
4
8
12
16
20
24
28
32
36
40
44
48
Time [h]
Figure 7. Energy required for maintaining indoor temperature stable at 26 oC
The measured total “energy consumption” under the above conditions in 48 hours for the
reference, 10% and 20% mixtures are 885 Wh/m2, 774 Wh/m2 and 705 Wh/m2 respectively. As it
can be seen the PCM mixes improve significantly the thermal performance of agglomerate
marble in terms of energy saving. This is not only due to the increased thermal mass but also due
to the improvement in thermal insulation (conductivity reduction).
5.
CONCLUSIONS
The thermal performance of a new composite building material based on natural stone wastes and
PCM was presented in this paper. For the needs of the experimental investigation, a simple to use
apparatus for measuring thermal properties of samples containing PCMs was designed and
constructed. Three samples with different percentages of PCM were investigated. Experiments
showed a significant increase of the specific heat capacity of the samples containing PCM in the
temperature region within the phase change. The thermal mass of the sample was also increased
inside the melting region but decreased outside of the region as a result of the decrease in the
density of samples containing PCM. This fact reveals a limitation in the increase of the PCM in
the mass of the mixture, which depends on the temperature range at which the material is used.
Finally, the energy saving potential with this new material was examined. Experiments showed
the capability of the material to reduce energy requirements of a building by reducing heat losses
from the building envelope.
6.
ACKNOWLEDGMENTS
The authors would like to acknowledge the financial support of the European Commission
though the I-Stone project, FP6-NMP-IP-515762-2 and the MESSIB project, FP7-NMP2-LA2008-211624.
7.
REFERENCES
1. M. Founti, I. Mandilaras, K. Laskaridis, M. Patronis, M.D. Romero-Sánchez, A.M. López-Buendía, MultiFunctional Building Products Based On Natural Stone And PCMs With Stabilised Thermal And Dynamic Load, 7th
IIR Conference on Phase Change Materials and Slurries for Refrigeration and Air Conditioning, Paris, France, 2006.
2. M.D. Romero-Sánchez, M. Founti, C. Guillem-López, A.M. López-Buendía, Thermal Energy Storage in Natural
Stone Treated with PCMs, 11th International Conference on Thermal Energy Storage-EffStock, 2009
3. H. Mehling, H.-P. Ebert, P. Shossig, Development of standards for materials testing and quality control of PCM,
7th IIR Conference on Phase Change Materials and Slurries for Refrigeration and Air Conditioning, Paris, France,
2006.
4. GeoAnalysis S.A, http: //www.geoanalysis.gr