Experimental investigation of wallboard containing phase change material: Data for validation of numerical modeling
Summary (4 min read)
- Nowadays, thermal energy storage systems are essential for reducing dependency on fossil fuels and then contributing to a more efficient environmentally benign energy use .
- As demand in thermal comfort of buildings rises increasingly, the energy consumption is correspondingly increasing.
- As the temperature increases, the material changes phase from solid to liquid.
- Even if these conditions are more realistic, all of the weather parameters are not known (or measured).
- From a numerical point of view, having experiments with and without PCM is useful to verify the basic modeling.
2 Description of the test cells MICROBAT
- Two identical test cells are used to investigate the effects of the PCM wallboards.
- The design of the test cells is presented part 2.1.
- The phase change material tested is described part 2.2.
- The part 2.3 deals with the measurement systems and the probes, the experimental protocol is given part 2.4.
2.1 Design of the cells
- Two identical test cells are used in the study.
- Each test cell is a cubical enclosure with an internal dimension of 0.50m .
- This face is called the active face because of its low thermal inertia and low thermal resistance.
- It enhances the heat transfer between the exterior and the interior of MICROBAT.
- The witness box has five normal faces whereas the PCM box contains three PCM faces which are the back face, the right face and the left face.
2.2 Phase change material tested
- The product tested, ENERGAINr has been achieved by the Dupont de Nemours Society and is constituted of 60% of microencapsulated paraffin within a copolymer.
- This rate corresponds to 3◦C/h which is an average heating rate in a light weight building during summer season and when solar gains are maximum.
- The freezing curve (cooling from 34◦C to 1◦C) and the melting curve (heating from 1◦C to 34◦C), also known as Two curves are presented.
- The physical properties of the composite PCM are: ⋆ peak melting temperature = 22.3◦C, ⋆ peak freezing temperature = 17.8◦C, ⋆ specific heat at the melting peak = 13.4Jg−1K−1, ⋆ specific heat at the freezing peak = 12.9Jg−1K−1, ⋆ solid specific heat = 2.4Jg−1K−1.
- This curve is typical of the hysteresis loop.
2.3 Instrumentation and measurements
- All of the temperatures are measured using Pt100 sensors with a calculated resolution of ±0.25◦C.
- In each test cell the set of probes is: • the internal face temperature, measured at the center of the face, with a probe included in the wall surface, • the active face external temperature, measured at the center of the face, • the air temperature, measured at the center of the cubical air volume.
- The time step chosen between two series of measurement is 2mn and the duration of each test is about three days.
2.4 Experimental protocol
- The two MICROBAT test cells are placed into a climatic chamber.
- The climatic chamber temperature can vary between −10◦C and 40◦C as a function of time.
- The active face allows to impose the same external temperature for each test cell.
- Two types of external temperature evolutions are investigated: • a temperature step, heating and cooling, • a sinusoidal temperature evolution.
- Concerning the heating/cooling temperature step, three cases are tested depending on the slope (SL) i.e. the time needed to reach the constant temperature value: 1hour, 2hours or 3hours.
3 Temperature step
- The first experiment concerns the temperature steps (heating and cooling) and the present part deals with the results of the cases tested.
- The part 3.1 deals with general considerations concerning the measurements.
- The air temperatures is analyzed part 3.2 and the internal face temperature part 3.3.
3.1 General considerations
- The box without PCM is strongly insulated but with a relatively low thermal inertia.
- The figure 7 shows the measured temperatures for the internal faces and air in the test cell without PCM for the case SL = 1hour.
- Except for the active face, the other faces temperatures and the air temperature are nearly identical.
- The temperature gradient in the box does not affect the heat transfer, and particularly the convection heat transfer.
- This conclusion is valid for all the cases tested and then only air temperature is presented in the following paragraphs of the article.
3.2.1 Heating step
- The air temperature measured for the three kinds of heating steps and for the two test cells is presented in figure 8.
- The air temperature for the box without PCM is close to the exterior temperature with a little time lag due to the low thermal inertia of the box.
- The air temperature evolution of the witness box is nearly exponential.
- 21◦C, the air temperature is increasing linearly with a slope SL1 , (2) for 21◦C . θ(t) .
- The table 3 summarizes the τ values for the heating step cases.
3.2.2 Cooling step
- The air temperature measured for the three kinds of cooling steps and for the two test cells is presented in figure 9.
- For the box with PCM, the temperature evolution is composed of three stages: (1) for θ(t) & 19◦C, the air temperature is decreasing linearly with a slope SLc, (2) for 15◦C . θ(t) .
- This flat part of the curve lasts about 1h.
- The PCM composite is composed of microencapsulated paraffin spheres included in a copolymer matrix.
- As the solidification proceeds, the melt volume decreases with a simultaneous decrease in the magnitude of natural convection within the melt and the process is therefore much longer .
3.3 Test cell with PCM: temperature evolution
- The phase change material included in the wallboards stores/releases energy transferred from the air volume mainly by convection.
- Then, the wall surfaces temperatures are an important feature in the phase change phenomenon.
- The figures 10 and 11 present the internal faces temperature , for the cell with PCM, and for the heating and cooling steps.
- For all the cases tested, the three PCM walls have the same surface temperature.
- For all of the cases tested, the temperature difference between the six faces of the cell never exceeds 5◦C.
- Of course, the presence of PCM allows to delay the box air temperature increase or decrease (as long as the temperature varies within the phase change range).
- The analysis of the time lag defined paragraph 3.2.1 shows that the more the thermal excitation is rapid, the more the PCM is efficient.
- In buildings, rapid thermal excitation can be solar spot or internal load.
- For the PCM used in wallboards, hysteresis exists and has clearly been exhibited in paragraph 2.2.
- This phenomenon has been few studied in literature and never been taken into account in numerical modeling as well as in experimental analysis.
4 External sinusoidal temperature evolution
- The response of the MICROBAT to an external temperature step allows to characterize the time lag due to PCM wallboards.
- In order to characterize the phase difference and the decrement factor due to PCM, an external sinusoidal temperature evolution is used, case which is closer to the real building configuration.
- The phase difference ζ is defined as the time difference between outdoor temperature maximum and indoor temperature maximum.
- The decrement factor f is defined as the ratio between indoor temperature amplitude and outdoor temperature amplitude.
- For the walls with PCM, the temperature curve has two breaks of slope: one at about 19◦C and one at about 22◦C.
5 Numerical modeling
- In the previous section, experimental data are described so that in the present section numerical modeling can be held.
- Then, the numerical results are presented in paragraph 5.2.
- The paragraph 5.3 is a discussion concerning the numerical modeling.
5.1 Presentation of the numerical modeling
- The first modeling step consists in decomposing the test cell (or the building) in elementary objects: air zones, walls,...
- In order to solve numerically the problem, a finite-difference method is used: the continuous information contained in the exact solution of the differential equation are replaced by discrete temperature values.
- Only the longwave radiation exchanges are considered in the present modeling.
- The internal exchanges occur between the internal surfaces of the walls.
- The software MATLAB is used for all the simulations.
5.2 Numerical results
- The experimental data are compared with the numerical results obtained from their modeling.
- The figure 14 shows the comparisons between experiments and modeling for the heating step.
- The figure (d) shows that the freezing specific heat curve is not adapted for the heating step modeling.
- The modelings (a), (b) and (c) use the equivalent specific heat obtained with the data of the figure 4 freezing curve.
- For the three cases, the numerical data are in quite good agreement with experiment, excepted for the flat part of the curve around 19◦C.
5.3 Discussion concerning the numerical modeling
- The main problem of the PCM modeling is the way to introduce the phase change.
- The equivalent specific heat has been tested.
- Unfortunately, the paraffin used is not an eutectic mixture, so hysteresis occurs during the phase change.
- When simulating heating step or cooling step separately, the numerical data are in good agreement with experiments only if the corresponding specific heat curve is used, e.g. melting curve and cooling curve.
- The hysteresis phenomenon must the be taken into account correctly in order to predict the PCM composite thermal behavior.
6 Conclusions and outlook
- The objective of this article is first, providing reliable experimental data that can be used for the validation of numerical modeling and then, studying some features related to the use of phase change material wallboard.
- The external temperature is the thermal excitation; a heating/cooling step and a sinusoidal evolution are tested.
- The effects of PCM wallboard are to cause time lag between indoor and outdoor temperature evolutions and to reduce the external temperature amplitude in the cell.
- The effect of hysteresis phenomenon has been clearly exhibited with the experimental data: the melting process arises at a temperature higher than for the solidification process.
- Further investigations are needed to have a better numerical description of this special phase change feature.
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Cites background or methods from "Experimental investigation of wallb..."
...[60,61] Copolymer Paraffin wax 60%  CSM panel with brick RT27; SP 25 A 8 --- Gypsum board MPCM 28D 23%, 30% 40%  Aluminium Paraffin A22; Paraffin A26 --- Honeycomb panel Mixture of Tetradecane and Octadecane --- Concrete (Regular block; Autoclaved block) Butyl stearate (Autoclaved block) Unicere 55 (Autoclaved block) Unicere 55 (Regular block) 5....
...Kuznik and Virgone  also tested two identical test cells under two kinds of external temperature evolutions, heating and cooling steps with various slopes and sinusoidal temperature evolution with 24h period....
"Experimental investigation of wallb..." refers background in this paper
...Nowadays, thermal energy storage systems are essential for reducing dependency on fossil fuels and then contributing to a more efficient environmentally benign energy use ....
...00m 2 outdoor  paraffin / gypsum 2....
"Experimental investigation of wallb..." refers background in this paper
...PCM composite Size Number of cells Conditions  butyl stearate / gypsum 2....
...00m 2 outdoor  paraffin / polyethylene 3....
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