VALIDATION OF A TRNSYS SIMULATION MODEL FOR PCM ENERGY STORAGES AND PCM WALL CONSTRUCTION ELEMENTS

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1 VALIDATION OF A TRNSYS SIMULATION MODEL FOR PCM ENERGY STORAGES AND PCM WALL CONSTRUCTION ELEMENTS H. Schranzhofer, P. Puschnig, A. Heinz, and W. Streicher Institute of Thermal Engineering, University of Technology Graz Inffeldgasse 25/B, A-81 Graz, Austria Tel.: ( ) , Fax: ( ) w.streicher@tugraz.at 1. INTRODUCTION The idea to use phase change materials (PCM) for the purpose of energy storages 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 large latent heats 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 application of PCMs in wall layers, such as plasters or gypsum boards, or in the ceiling construction adds thermal mass to the building, thereby avoiding unpleasant temperature variations of the inside room temperature, or leading to a considerable reduction of room temperature peaks during summer (Schossig et al., 3 and Haussmann et al., 4). In the current paper we present a simulation model for a PCM storage integrated into the simulation environment Trnsys 16. It is capable of treating micro-encapsulated PCMs (so called PCM-slurries) (Jahns, 3 and Gschwander et al., 4) as well as modules of PCM materials of various shapes (cylinders, spheres, plates). In addition, we display a PCM wall model for Trnsys 16 that can be coupled to the multi-zone building type 56 of Trnsys 16. For both simulation models, the PCM storage model (Type 2) and the PCM wall layer model (Type 241), some exemplary simulation results are presented. 2. TYPE 2 PCM STORAGE MODEL Type 2 is a storage model for Trnsys 16 which is capable of treating microencapsulated PCM-slurries as storage medium as well as storage integrated modules of PCM materials of various shapes (cylinders, spheres, plates). Since the thermal properties of a phase change material (PCM) change more or less rapidly during a phase change, and in particular the latent heat of the phase change must be accounted for, the modeling of the thermal processes during a phase change requires a more general theoretical approach. For the current storage model we make use of the so called enthalpy approach (Voller, 199), in which the enthalpy density h has to be a continuous and invertible function of the temperature T. ( ), ( ) h= h T T = T h 1 The thermal modeling of the PCM storage tank has been implemented as a multi-node storage model into the simulation environment Trnsys 16. The storage volume is divided into N horizontal segments (nodes) each characterized by the enthalpy h i and the mass m i of the storage fluid in the node i. The energy balance equation for each storage node leads to the time evolution of the enthalpy h i and via the unique enthalpy / temperature relation also to the temperature T i of a given node: dhi ( dp) ( hx) ( aux) ( cond ) ( loss) ( modules) mi = Q i 2 dt

2 Here, m i is the mass of node i, while the terms on the right hand side of the above equation denote the energy change of node i due to charging / discharging via direct direct double port connections (dp), via internal heat exchangers (hx), energy input due to built-in electrical heater (aux), thermal conduction to neighboring nodes (cond), losses to the ambient (loss), and finally energy exchange between the storage fluid and storage integrated PCM modules (modules). A schematic representation of the node structure of the storage model including built-in cylindrical PCM modules can be seen in Figure 1. i = N m, h, T PCM PCM PCM ik ik ik h, T i i m i k = 1 k = n r PCM UA i i = 2 i = 1 Figure 1: Schematic representation of the node structure of the storage model including built-in cylindrical PCM modules. The present storage model ( Type 2 ) allows for the inclusion of up to 5 double port connections, where each double port can be either a direct inlet / outlet or an internal heat exchanger. The storage fluid can be either water or a PCM slurry. In the latter case, all relevant temperature dependent thermal properties (enthalpy, viscosity, thermal conductivity) must be supplied as an external text file. In addition it is also possible to include storage tank integrated PCM modules of defined geometry. In the current version of the storage model it is possible to handle three different shapes of the PCM modules: cylinders, spheres, and / or plates. For each geometry the heat conduction inside the PCM modules is simulated using a 2-dimensional node structure appropriate for the symmetry of the problem and the heat transfer between the storage fluid and the PCM modules is accounted for by considering free as well as forced convection. 3. VALIDATION OF THE MODEL For validation of the model experimental data from different experiments done at our department were used (Heinz, Streicher, 6). a) Power [kw] TRNSYS (Type2) EXP losses Time [min] T inlet m dot T [ C] / Mass flow rate [kg/h] b) temperature [ C] layer 1 (top) T = 7[ C ] inlet V = 1 [ l / h ] water: trnsys exp PCM (surface): trnsys exp PCM (center): trnsys exp Figure 2: Comparison of a charging process with seven Paraffin modules in a water storage; a.) power curve at given inlet temperature (T inlet ) and mass flow (m dot ). b.) Temperature of the PCM modules and the water at the top of the storage

3 The first experiments with a water and a PCM slurry storage showed already very good results (Puschnig et al., 5). For further validations of the model concerning the use of PCM modules in the water tank, experiments with cylindrical modules were carried out. The first PCM container material was polypropylen (PP) filled with paraffin as PCM. Figure 2 shows the results of a charging process with this setup (the insert in Figure 2b shows schematically the water tank with the seven PCM modules and the position of temperature evaluation). The correlation between experiment and simulation is already very satisfactory. The disadvantages of Paraffin are the low energy density and the circumstance that low cost Paraffin melts not at a constant temperature because it consists of hydrocarbons with different chain lengths that melt at different temperatures. To overcome these problems other PCMs were taken into account and the decision was to use sodium acetate trihydrate (SA) for further experiments. Again cylindrical PP-containers were used. A comparison between simulation and measurement for a discharging process is shown in Figure 3. a) power [kw] TRNSYS (Type2) EXP losses T inlet m dot T [ C] / Mass flow rate [kg/h] b) temperature [ C] layer 4 (bottom) water: trnsys exp PCM (surface): trnsys exp PCM (center): trnsys exp Figure 3: Comparison of a discharging process with seven SA modules in a water storage; a.) power curve at given inlet temperature (T inlet ) and mass flow (m dot ). b.) Temperature of the PCM modules and the water at the top of the storage As salt hydrates have the attribute of not crystallizing at the melting temperature but at temperatures that could be much lower. So the simulation model had to be improved to meet this circumstance. Two different datafiles for the enthalpy curve are used: one with the so called subcooling process and another one without it. To simulate the subcooling the algorithm switches from the enthalpy curve with subcooling to that one without subcooling when the lowest temperature of subcooling is reached. The consequence is that the model needs two different material data files for the PCM: one enthalpy curve for heating and a second one for cooling. Additionally the correct switching temperatures between those two files are necessary. Using the correct parameters and data files the results for the discharge power with time (shown in Figure 3a) and also the temperature sequence explicitly showing the occurrence of subcooling meet our demands (Figure 3b). a) Power [kw] TRNSYS (Type2) EXP losses 8 1 Time [min] T inlet m dot 8 T [ C] / Mass flow rate [kg/h] b) temperature [ C] layer 4 (bottom) water: trnsys exp PCM (surface): trnsys exp PCM (center): trnsys exp 8 1 Figure 4: Comparison of a charging process with seven SA + graphite modules in a water storage; a.) power curve at given inlet temperature (T inlet ) and mass flow (m dot ). b.) Temperature of the PCM modules and the water at the top of the storage

4 With the latest experiments the problem of low thermal conductivity was solved. For this the PCM graphite compound from the German Company SGL Carbon (a mixture of SA and graphite) was used which was encapsulated into stainless steel tubes. 4. TYPE 241 PCM WALL MODEL The use of PCMs for thermal storage in buildings was one of the first applications studied. An interesting possibility in building applications is the integration of PCMs into porous construction materials, such as plasters or concrete, to increase the thermal mass and thereby reduce the magnitude of daily temperature variations (Schossig et al., 3, Haussmann et al., 4 and Ibáñez et al., 5). In order to optimize the influence of the PCM on the thermal behavior of the building it is hence desirable to perform thermal building simulations including the effect of PCM wall layers. In this section we give a description of a novel approach to integrate PCM wall layers into the building simulation software Trnsys 16 and present some exemplary simulation results. We note that an alternative but somewhat simplified approach to simulate PCM wall layers within the framework of active layers within Trnsys 15 has also been recently reported (Ibáñez et al., 5). Wall construction: 2.5 [cm] external plaster / 38 [cm] brick / 1.5 [cm] PCM plaster room with PCM plaster Ambient air External wall room Type 56 q s,2 q s,1 direct contact zone T s,2 T s,1 Type 56 air zone TRNSYS Model Type 241 Figure 5: Coupling of the PCM wall model (Type 241) with the multi-zone building model of Trnsys 16 (Type 56). Figure 5 schematically displays the coupling of the novel PCM wall type (Type 241) to the multi-zone building model of Trnsys 16 (Type 56). It is necessary to split the wall construction, on to which the PCM layer should be attached, by introducing a so called direct contact zone. Such a direct contact zone is characterized by a vanishingly small air volume as well as by negligible heat transfer resistances. In the present example the wall construction (2.5 [cm] external plaster / 38 [cm] brick layer / 1.5 [cm] PCM plaster) has been split at the center of the brick layer. On the one side of the direct contact zone one half of the wall construction of type external is located while the remaining part of the brick layer constitutes a boundary wall where the boundary temperature T s,2 is defined as input value to Type 56. The actual air zone is bordered by a mass less layer with negligible heat resistance of type boundary wall with the temperature as input to T s,1 Type 56. The calculation of the thermal conductivity / heat storage inside the PCM plaster layer is carried out by the novel Type 241 using a finite difference approach. Type 241 takes the heat currents q s,1 and q s,2 entering both sides of the PCM layer as input values (outputs from Type 56) and in turn yields the surface temperatures T s,1 and T s,2 as output quantities which serve as input values for Type 56. In order to demonstrate the performance of the novel PCM wall model for Trnsys 16 (Type 241 ) we have simulated the thermal behavior of a reference room with 1.5 [cm] PCM plaster and for comparison with 1.5 [cm] normal gypsum plaster. The resulting operative room temperatures during a three week period (August, Graz ) are shown in Figure 6.

5 The reference room is a typical [m 2 ] living room with south and west oriented external walls with a window area of 3.75 [m 2 ]. During the day we have assumed a hygienic air change of.5 [h -1 ], while during the nights an air change of 6 [h -1 ] has been used in order to permit an efficient cooling of the PCM plaster below its melting point. The thermal properties of the normal gypsum plaster have been set to λ =.6 [W/mK], cp = 1 [J/kgK], ρ = 1 [kg/m 3 ], while the thermal properties of the PCM plaster have been taken from (Hill, 4) according to the material properties of the MAXIT clima plaster with a PCM content of %. The melting temperature range lies between 23 und 26 C with a heat of fusion of 18 [kj/kg], and a mass density of ρ = 13 [kg/m 3 ]. Operative room temperature [ C] cm gypsum plaster 1.5 cm PCM plaster Hour of year Figure 6: Simulation results for the operative room temperature of a reference room with 1.5 [cm] gypsum plaster (dashed lines) and 1.5 [cm] PCM plaster (full lines) on 38 [cm] brick layer. The melting range of the PCM is assumed to be between 23 und 26 C. For the specific heat capacity outside the melting range and the thermal conductivity the same values as for the normal gypsum plaster have been taken into account. In the thermal building simulation, the reference room has been either equipped with a 1.5 [cm] PCM plaster or a 1.5 [cm] gypsum plaster layer on the 38 [cm] brick layer on each of the four walls as well as on the ceiling. The comparison between the resulting operative room temperatures for the PCM and the gypsum layers, respectively, clearly demonstrates the influence of the PCM on the resulting room temperatures which are up to 1.5 C lower compared to the reference room with gypsum plaster. As soon as the PCM, however, is completely melted (T > 26 C), the PCM plaster no longer provides any advantage over the normal gypsum plaster. In this respect, we would like to emphasize the importance of an efficient cooling of the PCM below its melting temperature during the night, which is an essential prerequisite for the operation of the PCM layer. For this simulation can be a useful device in order to optimize the PCM layer thickness for a given air change rate during the night. 5. CONCLUSION Data from several different experiments was used to validate a new developed PCM storage model (Type 2) for Trnsys 16. The comparison between simulation and measurements given in this work show very good correlation. This makes us sure that the model is reliable and the approximations describe the real circumstances in the storage tank in a satisfying way. Further tests and development concerning reliability and usage in complex Trnsys simulations are planed.

6 Due to a lack of appropriate experimental data the PCM wall model (Type 241) has not been validated in that extended manner until now. But a comparative simulation of the operative room temperature for a reference room using gypsum and PCM plaster shows the expected results of reduced peak temperatures with the use of PCM plaster. ACKNOWLEDGMENTS The European Commission is thanked for funding the work undertaken as part of the project PAMELA ENK6- CT1-57. 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 N-GL. IEA SHC; Task Solarthermische Anlagen mit fortschrittlicher Speichertechnologie für Niedrigenergiegebäude Proj. Nr We also want to thank the industrial partners BASF and SGL Carbon for their support and the supply with materials. REFERENCES Gschwander, S., Schossig, P., Henning, H.-M. (4). Mikroverkapselte Phasenwechselmaterialien in Fluiden zur Erhöhung der Wärmekapazität. 14. Symposium Thermische Solarenergie OTTI Technologie-Kolleg, Staffelstein, Haussmann, T., Schossig, P., Henning, H.-M. (4). Latentmaterialien in Baustoffen. 14. Symposium Thermische Solarenergie OTTI Technologie-Kolleg, Staffelstein, Heinz, A., Streicher, W. (6). Application of Phase Change Materials and PCM slurries for thermal energy storage. 1 th International Conference on Thermal Energy Storage, Stockton. Hill, M. (4). Innenputz mit Latentwärmespeicher - maxit clima. ZAE-Symposium, Garching. Ibáñez, M., Lazaro, A., Zalba, B., Cabeza, L. F. (5). An approach to the simulation of PCMs in building applications using TRNSYS. Applied Thermal Enginieering, Volume 25, Jahns, E. (3). Microencapsulated phase change slurries. Proceeding of the Phase Change Material and Slurry Scientific Conference & Buisiness Forum, Yverdon-les-Bains, Puschnig, P., Heinz, A., Streicher, W. (5). Trnsys simulation model for an energy storage for PCM slurries and/or PCM modules. 2 nd Conference on Phase Change Material & Slurry, Yverdon, Schossig, P., Henning, H.-M., Raicu, A., Haussmann, T. (3). Mikroverkapselte Phasenwechselmaterialien in Baustoffen. 13. Symposium Thermische Solarenergie OTTI Technologie-Kolleg, Staffelstein, Voller, V. R. (199). Fast implicit finite-difference method for the analysis of phase change problems. Numerical Heat Transfer 17B, 155.

VALIDATION OF A TRNSYS SIMULATION MODEL FOR PCM ENERGY STORAGES AND PCM WALL CONSTRUCTION ELEMENTS

VALIDATION OF A TRNSYS SIMULATION MODEL FOR PCM ENERGY STORAGES AND PCM WALL CONSTRUCTION ELEMENTS H. Schranzhofer, P. Puschnig, A. Heinz, and Wolfgang Streicher, email: w.streicher@tugraz.at, internet:http://www.iwt.tugraz.at