AN EXPERIMENTAL STUDY AND SIMULATIONS OF PHASE CHANGE MATERIALS IN AN OFFICE THERMAL ENVIRONMENT

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1 2009/3 PAGES RECEIVED ACCEPTED R. PONECHAL AN EXPERIMENTAL STUDY AND SIMULATIONS OF PHASE CHANGE MATERIALS IN AN OFFICE THERMAL ENVIRONMENT Radoslav Ponechal Address: Faculty of Civil Engineering, Slovak University of Technology, Bratislava, Slovakia, Research field: latent heat and indoor thermal environment ABSTRACT KEY WORDS This paper provides an outline of how we can simulate phase change materials (PCM) more accurately. Experiments were conducted in order to verify the phase change material model in the Esp-r simulation program. At first, the component with the phase change material was evaluated in a climate chamber. The temperatures and the heat flow were measured on the surface of a polycarbonate slab within Rubitherm RT 20. Next, an appropriate test room was chosen, and the phase change material was applied to the construction of the ceiling. The indoor climate parameters of the room were monitored and measured as well as the outdoor parameters before the facade was added. The measured values were applied to refine the simple computation models. phase change simulation thermal comfort effective heat capacity 1. INTRODUCTION The latent heat storage capacity in phase change materials can be used for storing or releasing energy within a small temperature interval. It can be used for increasing the stability of light circumference building constructions. There are some simulations of PCM in phases of basic research around the world. The results are promising, but an evaluation with measurements in real conditions is rare. Ostry (2005) measured and simulated a room with phase change material, and he achieved a good degree of congruence in the temperatures, but the method of simulating the phase change material was not completely shown. 2. SIMULATING PHASE CHANGE MATERIAL IN THE ESP-R PROGRAME 2.1 Mathematical model The heat transfer processes in phase change material structures are complex, especially when the chemical compound is in a transition stage. During the phase change process, the encapsulated phase change material can exist in three states: solid, liquid and mushy (two-phase). Additionally, the thermal properties of the matrix of a construction material are different from the constituent properties. To simplify the mathematical model, the following assumptions were made: SLOVAK UNIVERSITY OF TECHNOLOGY

2 1. The encapsulated phase change materials are treated as a body of uniformly equivalent physical and thermal properties, principally specific and latent heat, density and thermal conductivity. 2. The heat transfer process across the phase change material is considered to be one-dimensional. The differential equations of transient heat conduction with variable thermo-physical properties can be written by defining the effective heat capacity c eff (Heim, 2003), (1), where θ is the temperature, τ time, λ conductivity and ρ density. Based on measurements with a Differential Scanning Calorimeter, the function of effective heat capacity can be assessed from the following equation: (2), where φ is the heat flow acquired from the Differential Scanning Calorimeter, m MFZ the amount of phase change material used in the calorimeter and c ps the specific heat capacity. 2.2 Effective heat capacity Effective heat capacity is a highly non-linear function of temperature. The shape of the effective heat capacity function depends significantly on the heating and cooling rates. From the results of calorimeter measurements (Arkar, 2005), it was found that during solidification, the peak temperature shifts towards higher temperatures at lower cooling rates, and the peak becomes narrower and higher as well, which shows that the greatest part of the latent heat evolves in a narrow temperature range. Also, the shift of the peak temperatures for both processes becomes smaller at lower heating/cooling rates. For a representation of effective heat capacity, approximate values are often used in the models describing experimentally obtained values for the latent heat of the melting or solidification of the PCM under investigation. In the ESP-r program it can be approximated as a constant or linear function. Those two functions are used in worldwide research. Gauche (2000) used a constant function. More preferable is a linear function (Strohbehn 2002), but it still only approximated the function of heat capacity from a differential scanning calorimeter. 3. EXPERIMENT IN A CLIMATE CHAMBER The first experiment with a climate chamber answered the question of how accurate a simulation is with constant and linear functions of effective heat capacity. 3.1 Test description During the experiment the element of a polycarbonate slab was filled with commercially available Rubitherm RT 20 organic phase change material. Organic phase change materials are chemically and thermally stable; they are not corrosive, and they show a negligible undercooling effect. Their main disadvantages are their inflammability and low thermal conductivity. The dimensions of the element were 1.2 x 1.5 m and consisted of a 10 mm thick polycarbonate slab with Rubitherm RT 20 and 200 mm insulation. The element was situated between two chambers. The air temperature in the first chamber was regulated, and it raised and fell within a wide temperature range. One temperature wave takes exactly one day. The second chamber behind the insulation has no regulatory function. 3.2 Results of measurement Fig. 1 Constant function of heat capacity cp from the research of Gauche [2]. The temperatures and the heat flow were measured on the surface of the polycarbonate slab with the PCM. It was found that the process of storing and realising energy occurs over a longer time and temperature interval than in the measurement with a differential scanning calorimeter. The shape of the heat transfer curve by melting and solidification was quite different. The process of energy storage was speedy, while the process of realising the energy was gradual and without any evident peak. AN EXPERIMENTAL STUDY AND SIMULATIONS OF PHASE CHANGE MATERIALS IN... 25

3 Fig. 2 Heat transfer curve measured on PCM surface in a climate chamber. The effective heat capacity function was carried out from the measured values using equation 2. The shape of the effective heat capacity curve is closer to the curve from a differential scanning calorimeter with a temperature rate of 1.0 K/min. In fact, the temperature of the polycarbonate slab with RT 20 raised only with a rate of 0.1 K/min. or less. The difference probably relates to the one dimensional heat flow in this experiment. Fig. 4 Surface temperature on a polycarbonate slab with phase change material from measurement in a climate chamber and from simulation in the ESP-r program. Table 1 Statistical data of modelling accuracy from a comparison between the measurement and simulation of PCM with the linear and constant function of effective heat capacity. function of effective average deflection heat capacity surface temperature heat flow linear 0.5 K 9 W constant 0.7 K 14 W The standard deviation was assessed (table 1). On the one hand, the linear function of the effective heat capacity gives more accurate simulation results in the heat flow and surface temperature of the phase change material. On the other hand, it only makes a small amount of progress compared to the constant function. It would be interesting to determine the non-linear function of the effective heat capacity in the ESP-r program because of the accuracy. 4. IN SITU EXPERIMENT Fig. 3 Effective heat capacity curves from measuring the difference in scanning calorimeter (DSC) and in a climate chamber. 3.3 Comparison between measurement and simulation Data from measuring the PCM in the climate chamber was used for the simulation in order to make the same boundary conditions. The simulation was carried out with the approximate linear and constant functions of the effective heat capacity. The simulated and measured surface temperatures and heat fluxes were compared. In the second experiment, in addition to the accuracy of the heat transfer model in the phase change material, the influence of the phase change material on the indoor thermal environment of the office room was also tested. For this purpose a south oriented office room in a high-rise building of the Slovak University of Technology was chosen. 4.1 Conditions in the test room The same organic phase change material was applied to the ceiling. The surface area of the ceiling was 100 m 2, and there was 120 kg of 26 AN EXPERIMENTAL STUDY AND SIMULATIONS OF PHASE CHANGE MATERIALS IN...

4 Fig. 5 Test room with organic phase change material applied to the ceiling. Rubitherm RT20. Double-glazing was used to encapsulate the phase change material in a liquid state. There was a 100 mm ventilated air gap between the ceiling and the concrete ceiling slab. The facade was south oriented with mechanical ventilation. The external heat sources from the solar radiation through the window were cut with blinds. All the relevant ambient parameters such as the ambient temperature, global solar radiation, wind direction and speed were measured. There were more than twenty temperature sensors in the test room. The potential of the phase change material was tested in 3 stages: constant air change with and without blinds and only night time ventilation with blinds. The measurement started on and ended Results of the measurement The influence of the phase change material on the thermal behaviour of a room and also the influence of the indoor climate on the Fig. 6 Measured temperatures on a PCM ceiling in the test room. Fig. 7 Picture from infrared camera senses the surface temperature in a corner of the test room. process of melting and solidification were scanned. It was found that the surface temperature on a ceiling close to a window was quite different from the surface temperature in another ceiling area. Surface temperatures near a window rise and fall more quickly because of the radiation from a too hot or too cold window surface. The phase change material did not change its state completely in the time measured. The process of solidification started in the rear of the office and always near the corner. The surface temperatures on the ceiling did not go below 22 C the whole time. Some infrared images were taken to depict the small difference in temperature. As shown in fig. 7, the ceiling surface temperature near the wall was different than in the middle of the room. This was because of the different rate of heat transfer in the corner. The air motion for convection and the view factor for radiation is low there. 4.3 Results of the simulation The boundary conditions measured in the test room were used to simulate the internal thermal environment in the ESP-r program. One model with and one without the phase change material were created and compared. A very good degree of congruence was achieved between the modelling and simulation due to the use of a mass flow model. It was found from the thermal comfort assessment that by using 120 kg RT 20 in the office room, the percentage of dissatisfied (PPD) people was reduced about 1.5 %. For better results it would be necessary to use a higher quantity of phase change material or improve its efficiency. AN EXPERIMENTAL STUDY AND SIMULATIONS OF PHASE CHANGE MATERIALS IN... 27

5 Fig. 8 Air temperatures from the measurement and simulation of the test room with and without phase change material (PCM). 5. CONCLUSION This paper has shown the results of a measurement and simulation focused on a better knowledge of heat transfers with phase change materials to improve the accuracy of simulations. The measurements and simulation in an office showed that phase change materials reduce high room temperatures by storing heat. But the process of heat storage and realization is quite affected by relatively small heat fluxes. It is necessary in the simulation of phase change material to pay close attention to the creation of the model and especially: use the approximate linear function of the effective heat capacity obtained from the differential scanning calorimeter or, even better, from the climate chamber, divide the modelling surface of the phase change material among Fig. 9 Evaluation of thermal comfort (predicted mean vote PMV and predicted percentage of dissatisfied PPD) from results of test room simulation with and without phase change material (PCM). enough of the elements to catch the differences in heat transfer near rear corners and windows, use computational fluid dynamics modelling to evaluate the air flow and heat transfer coefficient on each surface of the modelling space. Phase change materials have a lot of limitations in simple passive applications. That should be a reason for the good application of building services in phase change systems. ACKNOWLEDGEMENT This work was financed by the VEGA grant agency as Research Project No. 1/3324/06: Building simulations in the Slovak climate. 28 AN EXPERIMENTAL STUDY AND SIMULATIONS OF PHASE CHANGE MATERIALS IN...

6 REFERENCES [1] Ostrý, M. (2005) Vpliv tepelně akumulačních vlastností materiálu s fázovou změnou na vnitřní mikroklima, dissertation. Brno, Vysoké učení technické v Brně, Fakulta stavební, Ústav pozemného stavitelství, 2005 (in Czech) [2] Heim, D. Clarke, J. A. (2003) Numerical modelling and thermal simulation of phase change materials with ESP-r. In: Building Simulation th International conference, Eindhoven, The Netherlands, p [3] Arkar, C. Medved, S. (2005) The influence of the thermal properties of a PCM on the accuracy of a numerical model of packed bed latent heat storage with spheres. In: Advanced thermal energy storage through phase change materials and chemical reactions : Workshop. Kizkalesi, Mersin, Turkey: IEA ECES, [4] Gauché, P. Weiran, X. (2000) Modelling phase change material in electronics using CFD A Case Study. In: International conference on high-density interconnect and systems packaging, 2000, Denver, USA [5] Strohbehn J. Harman, Ch. (2002) Investigation of phasechange thermal storage in passive solar design for lightconstruction building in the southeastern climate region. In: A research program to promote energy conservation and the use of renewable energy, 2002, Duke University, Durham, NC USA AN EXPERIMENTAL STUDY AND SIMULATIONS OF PHASE CHANGE MATERIALS IN... 29