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.

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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

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enthalpy [kJ/kg]

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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.

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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.

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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

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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].

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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)

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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.

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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.

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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.

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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.

REFERENCES Bony, J., Ibanez, M., Puschnig, P., Citherlet, S., Cabeza, L., Heinz, A., (2005), Three different approaches to simulate PCM bulk elements in a solar storage tank, Phase Change Material and Slurry: Scientific Conference and Business Forum, 15.-17. Juni 2005, Yverdon les Bains, Schweiz, S. 99 - 107 Cabeza, L., (2005), Storage Techniques with Phase Change Materials, Thermal energy storage for solar and low energy buildings, State of the art by the IEA Solar Heating and Cooling Task 32, June 2005, Seite 77-105 Cristopia, (2006), Information from the homepage of Cristopia Energy systems: www.cristopia.com Egolf, P.W., Sari,O., Brulhart, J., Gendre, F., Ata-Caesar,D., Vuarnoz,V, (2004), Physical Behaviour of Phase Change Material Slurries. Deliverable D6, Workpackage 3, EU project ENK6-CT-2001-00507 (PAMELA) Jahns, E., (2004), Mikroverkapselte PCM: Herstellung, Eigenschaften, Anwendungen; ZAE-Symposium 2004, Wärme- und Kältespeicherung mit Phasenwechselmaterialien (PCM), München, 4. - 5. März 04 Marín José M., Zalba Belen, Cabeza Luisa F., Mehling Harald (2002), Determination of enthalpy-temperature curves of phase change materials with the temperature-history method: improvement to temperature dependent properties Öttinger, O., (2004), PCM/Graphitverbund-Produkte für Hochleistungswärmespeicher, ZAE Symposium, Wärmeund Kältespeicherung mit Phasenwechselmaterialien (PCM), München, 4. - 5. März 04 Schossig, P., Henning, H.-M., Raicu, A., Haussmann, T., (2003), Mikroverkapselte Phasenwechselmaterialien in Baustoffen, 13. Symposium Thermische Solarenergie OTTI, Technologie-Kolleg, Staffelstein, S. 489-494. Schranzhofer, H., Puschnig, P., Heinz, A., Streicher, W., (2006), Validation of a TRNSYS simulation model for PCM energy storages and PCM wall construction elements, Ecostock 2006 - Tenth International Conference on Thermal Energy Storage, May 31 – June 2, 2006, USA Stritih, U., (2003), Heat transfer enhancement in latent heat thermal storage system for buildings, Energy and Buildings 35, 1097–1104.