THERMOPHYSICAL PROPERTIES OF THE PHASE CHANGE MATERIAL MIXTURES PRELIMINARY STUDIES ON MACROMOLECULAR HYDROCARBONS EXAMPLE

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1 THEMOPHYSICAL POPETIES OF THE PHASE CHANGE MATEIAL MIXTUES PELIMINAY STUDIES ON MACOMOLECULA HYDOCABONS EXAMPLE E. KLUGMANN-ADZIEMSKA 1, P. WCISŁO 1, H. DENDA 1, M. YMS 1 1. Gdansk University of Technology, Faculty of Chemistry, Gdansk, Poland ABSTACT The aim of this work is a theoretical and experimental analysis of the macromolecular hydrocarbons mixtures composition and the impact on thermophysical parameters of the phase change materials (PCM) made from these mixtures. The analysis of the current state of knowledge extended by the author s own studies have been presented. Thermophysical characteristics of the hydrocarbons and their mixtures have been specified, in such a way, that on this basis description of the nature of the effects from individual fractions can be obtained, and the most important parameters characterizing the PCMs, such as the temperature peak of the phase transition or the heat of transition, can be set down. INTODUCTION One of the major tasks assigned to current knowledge of phase change materials (PCM) are both research for the new compounds and description of properties of the already known substances and mixtures. At the same time the requirements for these materials, such as high purity, heat capacity and durability, a narrow range of the phase transition temperature, low price, determine the intensity of activities in this field. Therefore there is a high demand for a description of existing mixtures (sometimes fairly well known) of materials that could be used as PCMs. This is an extremely important issue, from an economic, as well as technological and environmental points of view. Solid-liquid phase change materials during isothermal phase transitions absorb, store and release heat. The heat is stored at the time of solid to liquid transition, and released during the phase change from liquid to solid. This allows for the economic utilization of the waste heat, heat from the sun, surplus heat in passive constructions or just for efficient heat management. esearch on phase change materials have been undertaken many times before, but still there is a demand for both new materials and a new usage of existing materials. Among phase change materials can be divided into [Kenisarin M., 2011]: organic compounds (e.g.: waxes, paraffins, fatty acids, alcohols), inorganic compounds (hydrated salts) and eutectic mixtures. Based on analyzes and literature the need for a theoretical and an experimental examination of the mutual relationships between the various thermophysical parameters, such as: phase transition enthalpy or melting temperature, can be indicated, not only for the pure PCMs, but also their mixtures as a function of their composition. This applies in particular to macromolecular hydrocarbon mixtures, for which primary thermophysical properties could be well defined but only as an encyclopedic data very useful form application point of view, but with rather little use in research. Due to their ability to absorb, during isothermal phase transitions, store and then release heat, phase change materials (PCMs) are very useful substances in many applications: plates with PCM layer that keeps the meal warm or cups sustaining high temperatures of the drinks, used in food industry for a constant temperatures control, cardboard plates or bags filled with PCM or directly mixed with cement, used in floors and walls as an improvement in buildings energy efficiency [Lewandowski W., 2014], storing heat during engine operation, and recovering this energy when the engine starts, heat-receiving materials to prevent overheating of the devices [Höhne G., 2003], inserts or containers for the thermo-sensitive materials transport e.g.: blood, organs, drugs, groceries, sensitive electronics, chemicals etc., protection when carrying out exothermic chemical reactions in chemistry, sportswear materials, vests for firefighters, suits for astronauts protecting from them temperature fluctuations. Mehling and Cabeza [Cabeza L.F., 2011] have been expanded above division with reference to PCMs enthalpy and melting temperature levels. Dubovsky at al. [Dubovsky V., 2011] provides PCMs tests in terms of heat exchange. Xiao at al. [Xiao W., 2009] presents a possible application of phase change materials in construction utilities. Felix at al. [Felix A., 2008] presented new PCM technological innovations such as: Nr I-IV/2014 Polska Energetyka Słoneczna 39

2 Thermal storage of solar energy, Passive storage in bioclimatic building/architecture, Cooling: use of off-peak rates and reduction of installed power, icebank, Heating and sanitary hot water: using off-peak rate and adapting unloading curves, Thermal protection of food: transport, hotel trade, ice-cream, etc., Thermal protection of electronic devices (integrated in the appliance), Medical applications: transport of blood, operating tables, hot and cold therapies, Cooling of engines (electric and combustion), Thermal comfort in vehicles, Solar power plants. In Dirand at al. [Dirand M., 2002] paraffins of straight hydrocarbon chains analysis in a wide range of carbon atoms in the molecule has been conducted. In that paper the authors also considered two Broadhurst s models, describing the melting point of hydrocarbons as a function of number of carbon atoms in the molecule. This description allowed determining the relationship between the melting point and enthalpy of straight-chain alkanes in a wide range of carbon atoms in the molecule. According to data presented in [9] and other above papers, PCMs in the form of paraffins and waxes may find their application as heat storage. In the present paper, the analysis of the hydrocarbons and their mixtures has been conducted for describing termophisical properties of PCMs made of them. THEOETICAL CONSIDEATIONS: DSC DIAGAMS COMPOSITION Differential scanning calorimetry is a useful tool for detecting the phase change transitions. The result of a DSC experiment is a curve of heat flux versus temperature level or time. This curve can be used to calculate enthalpy of transitions H by integrating the peak corresponding to a given transition [Pungor E., 1995] or may be obtained from the definition of constant-pressure specific heat: C p δh = δ T p. (1) Mathematical models for enthalpy may be obtained by integrating expressions of specific heat with respect to temperature. esulting equation in practice is simplified into: H = k A, (2) where: k the so-called calorimetric constant it can be determined by analyzing a well-characterized sample with known enthalpies of transition, A the surface area under the curve which can be determined, for example, by graphic integration. For a pure PCM substance, a DSC diagram would show a single significant growth of the energy flow at points where a phase transition occurs. In the case of mixtures, the overall performance of the mixture is a function of the characteristics of its components. However, provided that the components are neutral to each other, they will react to temperature changes independently. The theoretical characteristic of such a mixture containing two exemplary substances was shown in Figure 1. Those were chosen particulary due to relatively distant temperature levels of their phase change, respectively t 1 and t 2. It is valid for all measuring systems which work lineary, that is to say the measured signal for two distinct pulse-like events in the sample must be the superposition of the two single functions from each individual event (Fig.1) [oduit B., 2008]. Inverting this issue, this means, that observing the characteristic growth we may infer qualitative composition of the mixture. Another condition is that all measured curves of various pulse-like events should have the same shape, in other words all these measured functions divided by their peak area must yield the same function, the socalled apparatus function α(t) called Green s function. If these conditions are fulfilled, the following is valid: 40 Polska Energetyka Słoneczna Nr I-IV/2014

3 Fig. 1. Theoretical DSC curve for a exemplary PCM substances φ m ( T) c [ φ ( T ) α( T T )] dt = φ ( T) α( T) = (3) where: φ measured signal heat flow rate, m φ heat flow rate developed in the sample, T temperature level, α apparatus function, c constant. This defines the so-called convolution product of two functions in the form of integral equation. The equation is valid for all DSCs which work in the above-described linear manner, irrespective of whether a certain approximate formula is explicitly known. The seamy side of this desmearing method also called deconvolution, is the rather ambitious mathematics required to solve integral equation (3) for the function of interest φ ( T). There are essentially two methods, the Fourier transform and the recursion method. Both require numerical calculations. The DSC trace shows the value of the total energy flow needed to change the temperature by a set value. Thermodynamically, it depends directly on the specific heat and mass of the individual components of the mixture. This means that by measurement of the total heat transported in the vicinity of specific points, the quantitative component mixture can also be estimated. EXPEIMENTAL SECTION Macromolecular hydrocarbons under considerations The In order to confirm (or not confirmed) a dependency defined by the equation (3) PCMs and their mixtures with different proportions of the individual components has been examined. All mixtures were analyzed by the TA Q20 DSC device with the compressor cooling unit, which allows operating in the wide temperature range between 90 to 450ºC. For individual mixture its theoretically predicted DSC diagram has been calculated. Then the curves obtained that way were compared with experimental data collected from the DSC device. To determine the presence (or absence) of dependencies between the composition of PCMs and their thermophysical parameters and to confront it with the results obtained by a particular test, a verification process of existing knowledge on the subject procedure should be performed. Samples were prepared in two steps. In the first step two selected higher hydrocarbons (mixtures of various higher hydrocarbons with a chain length from C 19 to C 45 ) were weighted and closing in measuring cells. In the second one source materials from the first step were mixture in proportions of 1:1, 1:3 and also closing in separate measuring cells. All prepared samples were analyzed by the DSC device in the temperature range of from about -50 to 90ºC. In such way reference samples and their mixtures compositions have been obtained. Outcome DSC diagrams were recalculated according to the composition of the sample in such way that the curves obtained for the pure substances and their mixtures can be compared on one graph. As an example of above mentioned procedure analysis of the macromolecular hydrocarbons has been taken into considerations. Those substances have quite well known properties that were promising in term of theoretical and experimental comparisons. In Table 1 most significant DSC data results for hydrocarbons and in Table 2 their mixture samples has been presented. According the fact, that most investigated substances has distinct hysteresis between heating and cooling of the samples, the overall analysis contains this data, but as more important only heating DSC diagrams has been investigated in subsequent analysis. Seven PCM samples from the pure substances and their mixtures mentioned in Table 1 have been chosen for further analysis and comparison with theoretical considerations. Nr I-IV/2014 Polska Energetyka Słoneczna 41

4 No. Mass [mg] Table 1. DSC results of preliminary tests of various higher hydrocarbons Heating/ Temperature Enthalpy H Cooling Melting Congealing Melting Crystalliza ate Area/Main Area [kj/kg] -tion [ºC/min] Peak [kj/kg] Heating/ Cooling Program Temperature range Sample No. Table 2. DSC results of selected higher hydrocarbons and their compositions DSC Mass [mg] Melting Area/Main Peak Temperature Congealing Area Melting [kj/kg] Enthalpy H Crystallization [kj/kg] Temperature range ESULTS DSC diagrams obtained during measurements have been compared with theoretically calculated functions representing superposition of the basic component mixtures form Table 1. Calculations has been conducted with specially designed computer software, according to theoretical considerations, and will be the subject of separate article. 42 Polska Energetyka Słoneczna Nr I-IV/2014

5 Fig. 2. Theoretical DSC curve for a exemplary PCM substances Nr I-IV/2014 Polska Energetyka Słoneczna 43

6 Fig. 3. Theoretical DSC curve for a exemplary PCM substances In Figure 2 and 3 all experimental and theoretical data for chosen examples of differently composition mixtures has been presented. Samples 9 and 10 are a mixture of test samples 1 and 2 in 1:1 and 1:3 ratio respectively. Sample 11 and 12 are a mixture of test samples 1 and 3 in 1:1 and 1:3 ratio respectively. Sample 13 and 14 are a mixture of test samples 1 and 4 in 1:1 and 1:3 ratio respectively. Sample 15 and 16 is a mixture of test samples 1 and 5 in a 1:1 and 1: 3 ratio respectively. Sample 17 and 18 is a mixture of test samples 1 and 6 in a 1:1 and 1: 3 ratio respectively. Sample 19 and 20 is a mixture of test samples 1 and 6 in a 1:1 and 1: 3 ratio respectively. As shown in the Figure 2 the correlation between the resulting from the measurement and the designated theoretical overlap of 80% (the standard differential individual values in the range of the graph is equal about 20%, hence known that the curves are consistent at about 80%). CONCLUSIONS Nearly 100 different samples with different compositions have been examined. DSC diagrams analysis confirmed a dependency in signals from not only the pure substances and their mixtures, but also between mixtures and their mixtures compositions. However, the correlation of those diagrams, with theoretical superposition functions, reaches only about 80%. Therefore it is necessary to continue this future analysis to determine the relevant correlating functions, which allows better matching between theoretical and experimental DSC diagrams. The analysis of the graphs shows that it is possible to predict with fairly good accuracy the theoretical shape of DSC diagrams of the mixtures made from substances with well known DSC diagrams and thus to evaluate the usefulness of the potential PCM mixtures, taking into account the probable properties of such product. EFEENCES Cabeza L.F., Castell A., Barreneche C., Gracia A., Fernández A.I., 2011, Materials used as PCM in thermal energy storage in buildings: a review, enewable and Sustainable Energy eviews, Vol. 15, pp Dirand M., Bouroukba M., Briard A.J., Chevallier V., Petitjean D., Corriou J.P., 2002, Temperatures and enthalpies of (solid + solid) and (solid + liquid) transitions of n-alkanes, Journal of Chemical Thermodynamics, Vol. 34, pp Dubovsky V., Ziskind G., Letan., 2011, Analytical model of a PCM-air heat exchanger, Applied Thermal Engineering, 31 No. 16, Felix A., Solanki S.C., Saini J.S., 2008, Heat transfer characteristics of thermal energy storage system using PCM capsules: A review, enewable and Sustainable Energy eviews, 12, Np. 9, pp Polska Energetyka Słoneczna Nr I-IV/2014

7 Höhne G., Hemminger W., Flammersheim H.J., 2003, Differential Scanning Calorimetry, Springer, Germany 2003 Kenisarin M., Mahkamov K., 2011, Solar energy storage using phase change materials, enewable Sustainable Energy evievs, 11, No. 9, pp , 2007 Lewandowski W., Lewandowska-Iwaniak W., 2014, The external walls of a passive building: A classification and description of their thermal and optical, Energy and Buildings, Vol. 69, pp Pungor E., 1995, A Practical Guide to Instrumental Analysis, Boca aton, Florida egin A.F., Solanki S.C., Saini J.S., 2008, Heat transfer characteristics of thermal energy storage system using PCM capsules: A review, enewable and Sustainable Energy eviews, 12, No. 9, pp oduit B., Xia L., Folly P., Berger B., Mathieu J., Sarbach A., Andres H., amin M., Vogelsanger B., Spitzer D., Moulard H., Dilhan D., 2008, The simulation of the thermal behavior of energetic materials based on DSC and HFC signals, Journal of Thermal Analysis and Calorimetry, Vol. 93, pp Xiao W., Wang X., Zhang Y., 2009, Analytical optimization of interior PCM for energy storage in a lightweight passive solar room, Applied Energy, 86, No. 10, pp Nr I-IV/2014 Polska Energetyka Słoneczna 45