APPLICATION OF PHASE CHANGE MATERIALS AND PCM-SLURRIES
FOR THERMAL ENERGY STORAGE
A. Heinz, W. Streicher
Institute of Thermal Engineering
Graz University of Technology
8010 Graz, AUSTRIA
Tel: 011-43-316-873-7319
[email protected]
1.
INTRODUCTION
The idea to use phase change materials (PCM) for the purpose of storing thermal energy is to make use of the latent
heat of a phase change, usually between the solid and the liquid state. Since a phase change involves a large amount
of latent energy at small temperature changes, PCMs are used for temperature stabilization and for storing heat with
large energy densities in combination with rather small temperature changes.
The successful usage of PCMs is on one hand a question of a high energy storage density, but on the other hand it is
very important to be able to charge and discharge the energy storage with a thermal power, that is suitable for the
desired application. One major drawback of latent thermal energy storage is the low thermal conductivity of the
materials used as PCMs, which limits the power that can be extracted from the thermal energy storage.
In the work presented in this paper different ways of the integration of PCMs into a thermal energy storage were
investigated. Different PCM materials, with and without enhancement of the thermal conductivity, were used, and
their performance concerning the resulting charge/discharge power of a storage tank was tested experimentally.
2.
PCM MATERIALS AND THEIR CHARACTERISTICS
In our work different kinds of materials were used as PCM. In principal materials should fulfill different criteria in
order to be suitable to serve as a PCM.
•
•
•
•
•
•
•
•
Suitable melting temperature
High melting enthalpy per volume unit [kJ/m³]
High specific heat [kJ/(kg.K)]
Low volume change due to the phase change
High thermal conductivity
Cycling stability
Not flammable, not poisonous
Not corrosive
As one of the goals of latent energy storage is to achieve a high storage density in a relatively small volume, PCMs
should have a high melting enthalpy [kJ/kg] and a high density [kg/m³], i.e. a high volumetric melting enthalpy
[kJ/m³]. As shown in table 1, there are two main groups of PCMs.
Paraffins have an excellent stability concerning the thermal cycling, i.e. a very high number of phase changes can be
performed without a change of the material’s characteristics. On the other hand they are flammable and their
melting enthalpy and density is relatively low compared to salt hydrates. The problem with salt hydrates is their
corrosiveness and the cycling stability, which can often only be guaranteed if certain conditions are met. Another
disadvantage of salt hydrates is the so called subcooling. That means that the material does not crystallize at the
melting temperature but at a temperature that can be much lower. The subcooling can be reduced by adding so
called nucleators into the material.
Table 1: Advantages and disadvantages of PCMs [Cabeza, 2005)
organic (paraffins)
Advantages
• not corrosive
• chemically and thermally stable
• No or little subcooling
Disadvantages
• lower melting enthalpy
• lower density
• flammable
Inorganic (salt hydrates)
Advantages
• high melting enthalpy
• high density
Disadvantages
• subcooling
• corrosive
• cycling stability
800
enthalpy [kJ/kg]
enthalpy [kJ/kg]
As an example Figure 1 shows the energy that can be stored in different materials per mass and per volume unit as a
function of temperature. These properties were measured with the so called T-History method (Marin et al., 2002).
While the salt hydrate releases its latent heat at a certain temperature of about 58 [°C], the paraffin is melting in a
temperature range of about 20 [K]. This is because the paraffin consists of hydrocarbons with different chain
lengths that melt at different temperatures.
600
400
so
rih
et
ta t
e
ac
n
m
affi
par
diu
ra
yd
800
600
m
diu
so
te
r
te t
eta
e
at
et
ac
dr
hy
tr i
e
at
e
rat
yd
rih
t
te
eta
ac
m te
i
diu
so raph
+g
te
d ra
ihy
400
ac
ium
sod aphite
r
+g
ffin
para
200
200
ter
wa
ter
wa
0
0
0
20
40
60
temperature [°C]
80
100
0
20
40
60
80
100
temperature [°C]
Figure 1: specific enthalpy as a function of temperature for different materials, per mass unit [kJ/kg]
(left side) and per volume unit [kJ/dm³] (right side)
In applications with low temperature differences very high storage densities can be achieved using PCMs.
Concerning the storage density PCMs should be compared to water, which is the standard sensible storage medium,
that is broadly used. Figure 2 shows the improvement of the volumetric storage density for different materials
compared to water. With increasing temperature differences the advantage compared to water gets lower because of
the increasing influence of the sensible heat. Thus the importance of a high specific heat capacity becomes higher
for increasing temperature differences.
improvement of storage capacity [-]
8,0
Sodium Acetate Trihydrate
7,0
Sodium Acetate Trihydrate+ Graphite
Paraffin
6,0
5,0
4,0
3,0
2,0
1,0
50 - 60
50 - 65
50 - 70
50 - 75
50 - 80
50 - 85
50 - 90
temperature range [°C]
Figure 2: improvement of the volumetric storage
density (compared to water) for different materials
3.
Unfortunately PCM materials have a relatively low
thermal conductivity. In principle there are two ways of
solving this problem. On one hand the distances for heat
transfer by conduction in the PCM can be shortened.
This can be done by encapsulating the material into
relatively small capsules or by highly dispersed heat
exchangers with low distances between fins or pipes. On
the other hand the thermal conductivity can be enhanced
by embedding structures of materials with high
conductivity into the PCM. This is e.g. done by adding
graphite powder into the PCM, which not only increases
the thermal conductivity of the PCMs by a factor of 1020 (Öttinger, 2004), but also creates a kind of carrier
structure that inhibits the segregation of salt hydrates and
therefore improves their cycling stability. This kind of
PCMs with graphite compound are manufactured by the
German company “SGL Carbon”.
INTEGRATION OF PCMs INTO THERMAL ENERGY STORAGES
There are different possibilities to integrate PMCs into thermal energy storages, each of which has advantages and
disadvantages. The PCM can be encapsulated into modules, that are integrated into a tank, and that serve as heat
exchanger between the PCM and the surrounding heat transfer medium. Another possibility is to fill the tank
directly with the PCM and to charge and discharge it via a suitable heat exchanger. In the following each of these
possibilities is discussed in more detail.
Macroencapsulation
The PCM is encapsulated in e.g. cylindrical or spherical modules which are integrated into the storage tank. In order
to ensure a good heat exchange between the surrounding heat transfer medium (water or a mixture of water and
glycol in most of the cases) and the PCM, the modules should have a high ratio between surface area and volume,
i.e. a high heat transfer area per volume unit. This implies that the modules should in principle be as small as
possible, which is of course a matter of cost. The advantages of this kind of integration are the possibility of a
relatively simple integration of PCMs into an existing storage tank and the possibility to use PCMs with different
melting points in one tank.
Cylindrical PCM modules are e.g. manufactured by the French company “Cristopia”, with diameters of 78 to
98 [mm] and PCM melting temperatures of -33 to 27 [°C].
At the Institute of Thermal Engineering at Graz University of Technology a small experimental storage tank with
cylindrical PCM modules has been constructed. This tank is used to analyze the heat transfer processes between the
PCM and the surrounding water (convection), the heat conduction inside of PCM modules and the storage capacity
of different PCMs. Additionally the experimental data is used to validate simulation models for storage tanks with
integrated PCM modules, that are developed in the framework of the IEA SHC TASK 32 (Bony et al., 2005).
The tank is charged and discharged via one inlet and one outlet at the top and the bottom respectively. The PCM is
encapsulated in seven cylindrical modules with a diameter of 5 [cm], the resulting PCM volume fraction is about
30 %. So far three different PCM materials were used for the experiments:
•
•
•
Paraffin
Sodium Acetate Trihydrate (salt hydrate)
Sodium Acetate Trihydrate – graphite coumpound
75
3,0
T_return
T_flow
T_water
T_PCM_surface
T_PCM_center
discharge power
70
T_PCM_center
65
2,5
2,0
60
1,5
55
1,0
50
0,5
45
discharge power [kW]
temperature [°C]
The results of the measurements show, as expected, that the highest discharge powers can be achieved with the
sodium acetate trihydrate graphite compound. Figure 3 shows the evolution of the temperature with time in the
storage water, the PCM module surface and the center of the PCM module at one vertical position in the tank for a
discharging experiment with the sodium acetate trihydrate graphite compound inside of the modules. The tank was
discharged with a mass flow of 100 [kg/h] and a flow temperature of 50 [°C].
water
PCM modules
zz
z
z
zz
z
z
zz
z
z
zz
z
z
temperature
sensors
0,0
discharge power
T_water
T_PCM_surface
40
0
25
50
75
time [min]
100
-0,5
150
125
tank
Figure 3: discharging experiment with the sodium acetate trihydrate graphite compound inside of the modules,
evolution of water temperature, PCM module surface temperature and PCM center temperature with time (left side);
schematic experimental setup (right side)
discharge power [kW]
power from water
power from PCM
2,5
2,0
1,5
1,0
Sodium Acetate Trihydrate + Graphite
0,5
Sodium Acetate Trih
ydrate
0,0
Paraffin
-0,5
0
50
100
150
200
250
300
time [min]
Figure 4: Discharge power with different PCM materials for a
cooling experiment, mass flow 100 kg/h, flow temperature
50 °C, start temperature of the tank 70 °C
The evolution of the discharge power with
time for different PCM materials inside of the
modules is shown in Figure 4. At the
beginning of the experiment the discharge
powers are relatively high, which is a result of
the hot water being pushed out of the tank by
the cold water entering at the bottom. After
that the heat is discharged only from the PCM
modules. With the paraffin as well as with
sodium acetate trihydrate the discharge power
is quite low, due to the low thermal
conductivity of these materials. This results in
a very long discharge time and a limitation
concerning the possible applications. When
the sodium acetate trihydrate graphite
compound is used inside the modules, the
achievable discharge power is much higher,
due to the enhancement of the thermal
conductivity. In the graphs of the
measurements with sodium acetate trihydrate
(with and without graphite) a subcooling
effect can be observed, resulting in a local
minimum of the discharge power.
Microencapsulation, PCM slurries
Paraffins can also be microencapsulated with diameters of just a few μm (see Figure 5). Due to the small diameter
the ratio of surface area to volume is very high and the low thermal conductivity is not a problem. If these
microcapsules are dispersed in a fluid (mostly water), they form a pumpable slurry, that can be used as an energy
transport- and storage medium, as a so-called PCM slurry. A microencapsulation of salt hydrates is not possible
(Jahns, 2004).
At the Institute of Thermal Engineering a PCM slurry from BASF with a
melting point at about 60°C has been tested within the European project
PAMELA. The goal of the work was to develop and test suitable concepts
for storage tanks and heat exchangers for the use with PCM slurries and to
test the material concerning its usability in practical applications.
Figure 5:
(BASF)
PCM
Microcapsule
Because of the small diameter of the microcapsules the slurry can be
treated like a homogenous fluid. PCM slurries with concentrations of
microcapsules of up to 50 % (mass fraction) were tested. The storage
capacity of the slurry increases with increasing concentration of
microcapsules, but also the viscosity increases strongly (Egolf et al., 2004),
Slurries with a concentration of 40 % have been successfully pumped at
the Institute of Thermal Engineering, but because of the viscosity the
pressure losses are much higher and the heat transfer coefficients are far
lower than e.g. with water.
An experimental storage set-up with a volume of 200 liters was built up in order to analyze the energy storage
capacity and the heat transfer into and out of the storage tank. The slurry inside the tank was charged and
discharged via a typical spiral type internal heat exchanger (see Figure 6), movement inside of the tank was only
caused by natural convection. With internal heat exchangers the limiting factor for the heat transfer from the heat
exchanger fluid to the storage fluid is the natural convection from the heat exchanger surface to the storage fluid.
Therefore it was interesting to determine the heat transfer coefficient for natural convection, and especially to
investigate the effect of the relatively high viscosities of concentrations higher than 30 %. Therefore thermocouples
were mounted at different horizontal positions and at different vertical levels. For measuring the surface temperature
of the heat exchanger 4 thermocouples were soldered onto the heat exchanger pipe. In order to be able to determine
the power of the heat exchanger, the inlet- and outlet-temperatures and the mass flow through the heat exchanger
were measured.
heat transfer coefficient [W/(m².K)]
1200
water
1000
Slurry 20%
Slurry 30%
800
Slurry 40%
Slurry 50%
600
400
200
0
0
Figure 6: the internal heat
exchanger inside the tank
50
100
150
200
time [min]
Figure 7: evolution of the average heat transfer coefficient (natural convection)
with time for different storage fluids during charging the storage tank
In a series of measurements the tank was heated up and cooled down in a temperature range from 50 to 70 °C,
which is around the melting temperature range of the slurry. The flow temperature of the heat exchanger fluid was
kept constant at 70 °C for heating and 50 °C for cooling throughout all the experiments.
The heat transfer coefficient for natural convection was calculated from the measured data. Figure 7 shows the
evolution of the heat transfer coefficient with time during charging the tank filled with different storage fluids. The
heat transfer coefficient decreases with increasing charging of the storage tank due to the decreasing temperature
difference between the heat exchanger and the storage fluid. Because of the higher viscosities the heat transfer
coefficient also decreases with increasing concentration of microcapsules in the water. Even with the lowest used
concentration of 20 % the values of the heat transfer coefficient are much lower than those measured with water.
Side 1
Water
l
V = 400 dm³/h
h
Side 2
Water
Slurry 20%
Slurry 30%
Slurry 40%
overall heat transfer coefficient [W/(m².K)]
In a series of measurements the heat transfer coefficient of a flat plate heat exchanger was determined both for water
and for PCM slurries with different concentrations. The heat exchanger was operated in different modes concerning
the flow rates through the two fluid cycles (see Figure 8). The secondary side (cold side) was flown through by
water in the first series of measurements and by slurries with different concentrations in the following series, while
the primary side (hot side) was flown through by water in both test series. The measurements were carried out for
two different flow rates on the primary side and five flow rates on the secondary side. All measurements were
carried out under steady state conditions.
3500
3000
2500
2000
1500
1000
water
slurry 20%
slurry 30%
500
slurry 40%
0
0
100
200
300
400
500
600
flow rate secondary side [dm³/h]
Figure 8: experimental setup of the
flat plate heat exchanger
Figure 9: overall heat transfer coefficient of the heat exchanger for
different fluids on the secondary side (primary side is flown through by
water with 400 [dm³/h])
The results are shown in Figure 9. The convection heat transfer coefficient on the secondary side decreases with
increasing concentration of microcapsules, due to the higher viscosity and the lower thermal conductivity. This is
why the overall heat transfer coefficient is decreasing. With the lowest used concentration of 20 % the heat transfer
coefficient is about 30 % lower than with water, with a concentration of 40 % it decreases to about 40 % of the
values measured with water.
The pressure drop of the flat plate heat exchanger was measured for different concentrations of the slurry. The
pressure drop is rising with increasing concentrations of microcapsules in water due to the increasing viscosity.
With increasing concentration there is also an increasing dependence of the pressure drop on the temperature, which
is a result of the high dependence of the viscosity on the temperature (Egolf et al., 2004). Up to a concentration of
30 % the pressure drop is not much higher than that for water, therefore (for applications where the slurry is
pumped) this concentration should be a good compromise between storage capacity on the one hand and pressure
drop on the other hand.
PCM – heat exchanger-system
Another approach for the integration of PCMs into thermal storage systems is to fill a tank directly with the PCM
and to charge and discharge it via a suitable heat exchanger. In this case the effort of filling the PCM into a large
number of modules is not necessary and higher PCM volume fractions can be achieved.
For the heat transfer between the heat carrier fluid and the PCM for example air-to-water heat exchangers can be
used. These heat exchangers are used for heating and cooling of air in air conditioning and in industry. They have a
large number of thin fins, that are usually used to extend the heat exchanger surface because of the low convective
heat transfer coefficient on the air side, but they can also be used to enhance the heat transfer in a PCM (Stritih,
2003).
First measurements and calculations for a storage filled with sodium acetate trihydrate which is charged via a heat
exchanger like this have shown promising results. The PCM volume fraction can be higher than 80 %, which results
in storage densities that are a multiple of that for water.
The disadvantage of this kind of PCM integration is that the storage envelope and the heat exchanger have to be
geometrically adjusted to each other, in order to guarantee a proper heat transfer in all parts of the PCM. An
integration of PCM with different melting points is also more difficult than with encapsulated PCMs.
4.
CONCLUSION
A very important criterion concerning thermal energy storage with PCMs is the necessary discharge power of the
storage. For short term storage of thermal energy the discharge power normally has to be relatively high in
comparison to the storage volume. This requires certain conditions concerning PCM module sizes or the thermal
conductivity of the PCM material respectively. For high discharge powers a PCM-heat exchanger-system is
favorable. For storages that are used for long term storage the maximum necessary discharge power is typically
lower in comparison to the quite big storage volume. Here perhaps bigger modules with no enhancement of the
conductivity are sufficient. Further investigations are necessary to find suitable configurations concerning the
conductivity of the PCM, size and geometry of modules, PCMs with different melting points in one tank etc. This
will be done by means of a simulation model which was developed at the Institute of Thermal Engineering
(Schranzhofer et al, 2006).
ACKNOWLEDGMENTS
The European Commission is thanked for funding the work undertaken as part of the project PAMELA ENK6CT2001-00507.
The Austrian ministry BMVIT is thanked for the financing of the projects:
• „Fortschrittliche Wärmespeicher zur Erhöhung von solarem Deckungsgrad und Kesselnutzungsgrad sowie
Emissionsverringerung durch verringertes Takten, Projekt zum IEA-SHC Task 32“ Proj. Nr. 807807
• „N-GL. IEA SHC; Task Solarthermische Anlagen mit fortschrittlicher Speichertechnologie für
Niedrigenergiegebäude“ Proj. Nr. 805790
We also want to thank the industrial partners “BASF” and “SGL Carbon” for their support and the supply with
materials.
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