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Thermal characterization of phase change materials based on linear low-density polyethylene, paraffin wax and expanded graphite
Renewable Energy 88 (2016) 372e382
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Renewable Energy journal homepage: www.elsevier.com/locate/renene
Thermal characterization of phase change materials based on linear low-density polyethylene, paraffin wax and expanded graphite Patrik Sobolciak a, *, Mustapha Karkri c, **, Mariam A. Al-Maadeed a, d, Igor Krupa b a
Center for Advanced Materials, Qatar University, 2713, Doha, Qatar QAPCO Polymer Chair, Center for Advanced Materials, Qatar University, P.O. Box 2713, Doha, Qatar c Universit e Paris-Est, CERTES, 61 avenue du G en eral de Gaulle, 94010, Cr eteil, France d Materials Science and Technology Program, Qatar University, P.O. Box 2713, Doha, Qatar b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 24 October 2014 Received in revised form 23 October 2015 Accepted 20 November 2015 Available online xxx
Thermal characterization of Phase Change Materials (PCMs) based on linear low-density polyethylene (LLDPE), paraffin wax (W) and expanded graphite (EG) is reported in this paper. Investigated PCMs showed high potential for application in energy storage systems. The latent heat, Lm, sensible heat Qsens, and the ability of the prepared PCMs to store and release thermal energy were investigated using specific home-made equipment based on the transient guarded hot plane method (TGHPT). The sensible heat of PCM containing 40 wt.% of paraffin wax was investigated in the temperature range 25e35 C, they exhibited a drop in Qsens from 31 to 24 J/g depending on the concentration of EG. A similar decrease in sensible heat with increased loading of EG was observed for PCMs containing 50 wt.% of EG. The storage and release of thermal energy during phase change which is associated with the latent heat of the materials were investigated within the temperature range 20e50 C. PCMs containing 40 wt.% of paraffin wax exhibited latent heat of 36 J/g, whereas the latent heat of PCMs containing 50 wt.% of paraffin wax was 49 J/g. The addition of EG decreased the time needed to melt and solidify PCMs due to increase in thermal conductivity of PCMs with increase in EG content. This behavior was confirmed by the thermal conductivity measurements, where thermal conductivity increased from 0.252 for sample without EG to 1.329 W/m C for PCM containing 15 wt.% of EG. The reproducibility of storage and release of thermal energy by PCMs was demonstrated by subjecting them to repeated heating and cooling cycles (over 150 cycles). © 2015 Elsevier Ltd. All rights reserved.
Keywords: Phase change material Paraffin wax Expanded graphite Thermal conductivity Thermal energy storage Solar energy
1. Introduction Thermal-energy storage systems are crucial for reducing dependency on fossil fuels and minimizing CO2 emissions. The Kyoto Summit secured a commitment from most countries to establish a global program for carbon dioxide (CO2) emissions reduction. According to the World Business Council for Sustainable Development, buildings account for up to 40% of global energy use [1]. Therefore, there is a critical need to reduce CO2 emissions from buildings. A portion of this reduction can be achieved by using building materials that are energy efficient.
* Corresponding author. ** Corresponding author. E-mail addresses: [email protected] (P. Sobolciak), [email protected] (M. Karkri). http://dx.doi.org/10.1016/j.renene.2015.11.056 0960-1481/© 2015 Elsevier Ltd. All rights reserved.
Thermal energy storage can be accomplished either by using sensible heat storage or latent heat storage components. Sensible heat storage components have been used by builders for centuries to store and release thermal energy passively, but a much larger volume of material is required to store the same amount of energy in comparison to latent heat storage systems [2]. Latent heat storage is more attractive than sensible heat storage because of its high storage density with smaller temperature fluctuations [3,4]. PCMs, can undergo phase changes (usually solid to liquid changes) at relatively low temperatures while absorbing or releasing high amounts of energy [5]. During the last four decades, many PCMs, with different phase transitions (e.g., solideliquid, solidesolid) and a wide range of transition temperatures, have been designed and studied extensively [6,7]. A suitable phase change temperature and a large melting enthalpy are two crucial requirements of PCMs.
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Various inorganic and organic substances have been used in the production of PCMs, the most common of which include various inorganic salts (e.g., polyhydric alcohols) and their eutectics, fatty acids, and n-alkanes. Inorganic salts (salts hydrates) were used as PCMs in the past, but they faced issues such as, such as supercooling, corrosion, relatively high volume and chemical instability, hence making them an unfavorable choice. Whereas, metallic compounds have limited use as PCMs for commercial purpose due to their weight and cost [8]. The most promising materials used as PCMs for lowtemperature applications (below 100 C) are paraffin waxes due to their high latent heat of fusion, negligible super-cooling, low vapor pressure in the melt and chemical inertness [9]. As for practical applications, another important issue has to be taken into account, namely a suppression of flow (leaching), an issue faced by PCMs that undergo solideliquid transition. There are a many strategies for avoiding this problem. First, paraffin waxes are kept in tanks of various shapes and volume that are incorporated into the building according to the specific needs. Second, porous materials can be impregnated by paraffin waxes; in this case capillary forces suppress leaching. Third, paraffin waxes can be encapsulated within a polymeric shell to form microcapsules. These microcapsules are frequently used in the textile industry (an impregnation of the surface of fabrics or direct incorporation of microcapsules into fibers during spinning) and in buildings. The most widely known example of such materials is MICROCONAL from BASF, which is frequently blended with plaster or concrete for designing of heat protective blocks. Lastly, paraffin waxes can be directly blended with polymeric matrices to avoid leaching and to retain a compact shape even after the paraffin wax has melted. Presently, paraffin waxes blended with appropriate polymers appear to be the best candidates for preparation of smart PCMs for various applications, such as thermal storage of solar energy, thermal protection of electronic devices, thermal protection of food and medical goods, passive storage in bioclimatic buildings, use of off-peak rates and reduction of installed power and thermal comfort in vehicles [10e12]. Polyethylenes appear to be the most suitable polymer for blending with paraffin waxes due to their chemical and structural similarities with paraffin waxes. Krupa et al. [13] investigated lowdensity polyethylene (LDPE) blends with soft and hard paraffin waxes. The blends were able to absorb large amounts of heat energy due to melting of paraffin wax, whereas the LDPE matrix kept the material in a compact shape on the macroscopic level. The importance of the structural similarity of was reported in a study that dealt with PCMs based on isotactic polypropylene (PP) blended with soft and hard Fischer-Tropsch paraffin [14]. The PP matrix held the material in a compact shape during the transition from solid to liquid. However, a much lower content of paraffin wax was incorporated due to strong phase separation of components, which is caused by the differing structure of crystallites. In the case of polyethylene, the zigzag structure of crystallites is much more favorable for mutual compatibility with paraffin waxes than the helical crystalline structure of polypropylene despite the fact that paraffin waxes, polyethylene and polypropylene are practically chemically identical. In addition, paraffin waxes can co-crystallize with polyethylene as discussed in a prior publication by the authors. The results obtained from solution crystallization confirmed a strong indication of co-crystallization in the case of LLDPE on one hand, and practically no miscibility in the crystalline regions of LDPE and oxidized Fischer-Tropsch paraffin wax on the other [15]. The fact that chemical and structural differences lead to strong phase separation of components was demonstrated in the work focused on PCM created from paraffin wax and polyamide [16]. Other polymer matrices, such as styreneeethyleneebutylenee
373
styrene (SEBS), styreneeisopreneestyrene (SIS), and styreneeethyleneepropyleneestyrene (SEPS), have been examined [17,18]. Mentioned studies described a shape-stabilized PCM with a melting temperature of 56e58 C. The results showed that the composites can retain their shape even when paraffin wax is in the liquid state, and no paraffin wax leakage was observed during thermal performance testing. Thermosetting resins (epoxy resin cured with amino-based hardener) were also tested for applications where blending with paraffin waxes at room temperature is required, so that low transition-temperature PCMs can remain in the solid phase during composite processing [19]. A Major drawback of polymer/paraffin wax PCM's is their low thermal conductivity. The thermal conductivity of paraffin waxes is approximately 0.2 W/m C, and the thermal conductivity of polymers varies from 0.15 W/m C for amorphous polymers such as polystyrene or polymethylmethacrylate, to the 0.5 W/m C for highly crystalline polymers such as high density polyethylene [20]. Thermal conductivity of polymers can be improved by blending them with inorganic materials [21,22], such as graphite and exfoliated graphite [23e25], metals [26e28], boron nitride [29], various metal-coated fillers [30] as well as carbon nanotubes [31]. EG is particularly useful for tailoring the thermal conductivity of materials (including PCMs) due to its inherently high thermal conductivity, favorable shape and high aspect ratio [30e34]. Recently, Trigui et al. [34] developed a new method and constructed a reliable device for testing the long-term performance of PCMs in large sized samples. They described the first store built with the composite material developed to improve different properties of PCM such as latent heat, storage and release energy, heat capacity and thermal conductivity. This unique method for thermal investigation of PCMs was used for resin epoxy/spherical paraffin wax [35] as well as for LDPE/paraffin wax composites at different concentrations of paraffin wax [36]. The objective of these investigations was to study the thermal conductivity of these PCMs and the amount of energy exchanged during the variation of thermodynamic state of the samples when the boundary temperatures vary [37]. Thermophysical properties obtained through this method for PCMs are crucial in studying and simulating their behavior. In this study, the thermal properties and the storage and release of energy in PCMs based on LLDPE, W (melting point approximately 42 C) and EG are reported. The storage and the release of heat energy were determined using the Transient Guarded Hot Plate Technique (TGHPT), which allows investigation of the thermal properties of large samples [36]. In comparison with results obtained from Differential Scanning Calorimetry (DSC) good reproducibility of the measurements was observed. . Since prepared PCMs have a melting point of around 42 C, they can be feasible for outdoor use in regions where heat accumulates at a higher temperature [38]. Furthermore, it can be useful for heat protection of solar collectors, batteries etc … [39]. 2. Experimental 2.1. Materials Linear low density of polyethylene (MFI ¼ 1 g/10 min, QAPCO, Qatar), paraffin wax (Grade RT42, Rubitherm Technologies, Germany) and expanded graphite (GFG200, SGL Carbon, Germany) having an average size of 200 mm were used for the PCMs preparation. 2.2. Composite preparation LLDPE powder was mixed with paraffin wax using a Brabender instrument (Plasticorder PLE 331, Germany) at 140 C.
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Subsequently, the blends were hot pressed (Fontijne TP 50, The Netherlands) at 140 C for 5 min. The composition of samples investigated in this study and their specific density (r) are summarized in Table 1. 2.3. Characterization 2.3.1. Scanning electron microscopy The fracture surfaces of the samples were investigated using a Nova Nano SEM 450 Scanning Electron Microscope. Brittle fracture of the samples was achieved in liquid nitrogen. 2.3.2. Transient Guarded Hot Plate Technique (TGHPT) Differential Scanning Calorimetry is widely used for characterization of thermal properties of PCMs as the simplest way to estimate the key parameters such as melting and crystallization temperatures, the specific heat of melting and crystallization or the specific heat capacity [32,33]. However, DSC gives information about thermal properties in micro-sized samples that can be significantly influenced by local heterogeneities. For investigation of PCMs in real conditions it is necessary to design an experimental device to obtain information about the thermal properties of large samples. TGHPT method for measurement of thermal conductivity has been described previously [34,35]. In brief, determination of the overall thermophysical properties of PCMs over several cycles (melt and solidify) requires the design of a genuine experimental device (Fig. 1). The proposed test bench for the parallelepiped-shaped composite provides temperature and heat flux measurements at the material's borders. The sample is located between two horizontal aluminum exchanger plates. Thermo-regulated baths connected to the plates allow a fine regulation of the injected fluid temperature with a precision of approximately 0.1 C. Heat flux sensors and thermocouples (type T) are placed on each side of the composite. The set-up is held in place using a tightened pneumatic jack. The thickness of the flux-meters is approximately 0.2 mm and their sensitivity is approximately 202 mV/W/m2 for a sensor having an active surface area of 400 cm2. The sensors are connected to a Labview program adapted to measure temperature fluctuations and heat flux exchanged during fusion and solidification. Experimental data is recorded with regular and adjustable time steps (6 s). The lateral side faces are insulated by polyethylene expanded foam (PE) which reduces multidimensional heat transfer to a 1D problem. In this work, the temperature varied between 20 C and 50 C. 2.3.3. Thermophysical characterization of PCMs Thermophysical properties measurements such as thermal
Table 1 Composition of prepared samples and their densities (r) with standard deviations (SD). Notation x/y/z means LLDPE/W/EG w/w/w ratio. Samples
S0 S1 S2 S3 S4 S5 S6 S7 S8
LLDPE/W/EG
Thickness
r measured
SD
w/w/w
mm
g/cm3
±
100/0/0 60/40/0 50/50/0 55/40/5 50/40/10 45/40/15 45/50/5 40/50/10 35/50/15
4.39 4.36 4.35 4.24 4.37 4.39 4.37 4.31 4.29
0.922 0.899 0.898 0.933 0.961 0.989 0.926 0.965 0.987
0.002 0.003 0.005 0.002 0.003 0.001 0.002 0.002 0.002
conductivity and thermal diffusivity were obtained using the periodic temperature ramp method (Fig. 2) by means of a home-made device called DICO [30]. The method is based on the use of a minor temperature modulation in a parallelepiped-shaped sample (45 mm 45 mm 5 mm) and allows obtaining all of these thermophysical parameters in only one measurement with their corresponding statistical confidence bounds [40e42]. The composite sample is fixed between two metallic plates. The use of conductive grease (lg ¼ 1 W/m C) ensures a good thermal exchange between the different plates and the sample. The front side of the first metallic plate is heated periodically using a sum of five sinusoidal signals and the temperature is measured with thermocouples placed inside both front and rear metallic plates. The thermophysical parameters of the sample are identified by comparison of the experimental and theoretical heat transfer functions. The system under study is modeled with one-dimensional quadrupole theory. The experimental heat transfer function H is calculated at each excitation frequency as the ratio between the Fouriertransform temperatures of the front and rear plates. A parameter estimation technique is then applied to simultaneously estimate thermal conductivity l and diffusivity a. The identification of the set of thermophysical parameters is a non-linear optimization problem that is solved iteratively: starting with sufficiently accurate initial guesses for the unknown parameters, we successively refine the estimates by using the LevenbergeMarquardt method [43]. 2.3.4. Differential scanning calorimetry The differential scanning calorimetry measurements were carried out employing a Perkin Elmer DSC 8500 differential scanning calorimeter. Samples were accurately weighed and subsequently placed in aluminum pans. The sample weight ranged from 3 to 15 mg. In case of measuring the specific heat capacity, the sample was cooled to 18 C at a rate of 2 C/min, and held at this temperature for 3 min, following which it was heated to 60 C, and held at it for 3 min. Calibration was done to sapphire standard. Nitrogen gas was passed through the instrument at a flow rate of 20 ml/min. DSC software was used for calculation of cp values. All experiments were repeated at least three times and average values are presented. In case of heating/cooling measurement cycling, temperature interval from 0 to 60 C/min at rate 10 C/min was employed. 3. Results and discussion 3.1. Morphology SEM images illustrated in Fig. 3 clearly show separate fractions of LLDPE and W as well as morphology of EG of PCM with the following compositions of LLDPE/W/EG: 60/40/0 w/w/w, 55/40/ 5 w/w/w and 45/40/15 w/w/w, respectively. No indication of miscibility between LLDPE and W on the microscopic level was observed. This immiscibility is a requirement for PCMs to have well-distinguished melting points. 3.2. Thermal conductivity measurements Thermal conductivity (leff) and thermal diffusivity (a) of PCMs have been summarized in Table 2. It was observed that thermal conductivity and diffusivity of PCMs without EG decreased with increased wax content. Thermal conductivity decreased from 0.369 W/m C for pure LLDPE to 0.252 for PCM containing 40 wt.% of paraffin wax and 0.269 W/m C for PCM containing 50 wt.% of W at T ¼ 20 C. This happens due to lower thermal conductivity of paraffin wax compared to that of LLDPE.
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375
Fig. 1. Experimental set-up of Transient Guarded Hot Plate Technique.
Conditioning modules
T rear Second metallic plate Sample First metallic plate T front
Microcomputer with Multifunction Analog/Digital
Thermoelectric cooler
I/O card Power amplifier
Turbomolecular pump Rough pump Fig. 2. Schematic representation of the experimental setup.
Fig. 3. SEM images of PCMs with composition of LLDPE/W/EG (w/w/w) a) 60/40/0 b) 55/40/5 and c) 45/40/15.
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Table 2 The thermal conductivity and thermal diffusivity of PCMs. Sample
S0 S1 S2 S3 S4 S5 S6 S7 S8
W
EG
leff
Dleff
a 107
Da 107
Ileff
wt.%
wt.%
W/m C
W/m C
m2/s
m2/s
%
0 40 50 40 40 40 50 50 50
e e e 5 10 15 5 10 15
0.369 0.252 0.269 0.502 0.988 1.329 0.483 0.974 1.278
0.018 0.006 0.013 0.012 0.012 0.037 0.013 0.031 0.057
2.713 1.214 1.283 1.877 3.881 4.966 1.856 3.716 4.422
0.536 0.063 0.099 0.076 0.119 0.368 0.067 0.292 0.499
e 32 27 36 168 260 31 164 246
EG particles were added into the LLDPE/W blends for an improvement of the thermal conductivity of PCMs (maintaining 40 and 50 wt.% of the W in the final PCMs composition). The thermophysical properties and associated uncertainties of the LLDPE/ W/EG composites have been summarized in Table 2. The addition of graphite significantly improved the thermal conductivity and thermal diffusivity of the materials. In both cases, a non-linear increase in thermal conductivity with increasing graphite mass fraction was observed. The thermal conductivity intensification Ileff, obtained by the addition of graphite, was calculated using Equation (1): Ileff ¼ (leff lm)/lm
(1)
where, leff is the effective conductivity of measured PCMs and lm is the measured conductivity of matrix (LLDPE). The intensification of
a/a*m e 32 28 55 220 309 44 189 244
materials consisting of 40 wt.% wax increased from 36% to 260% if filled with EG in the range from 5 to 15 wt.% (Table 2). Similar behavior was observed for materials consisting of 50 wt.% wax with an intensification maximum of 246%. This result is attributed to the high thermal conductivity of EG Ref. [44]. 3.3. The specific heat capacity in liquid and solid state The method used to measure sensible and specific heat of the composites consists of simultaneously measuring the heat flux 41 and 42 and temperatures T1 and T2 on the two faces of the sample (T1 and T2 are the two thermocouples integrated in the flux meters). Initially, both the exchangers are maintained at a constant temperature until a constant density flux on the two faces of the sample is reached, then the temperature of the exchangers is increased. Between these two isothermal states, the sample stores a
Fig. 4. Heat flux and temperature evolution of PCMs at solid state (from 25 to 35 C) based on LLDPE/W/EG (w/w/w) a) 55/40/5, b) 45/50/5, c) 40/50/10, d) 45/40/15.
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377
Fig. 5. Heat flux and temperature evolution of PCMs at liquid state (from 43 to 53 C) based on LLDPE/W/EG (w/w/w) a) 55/40/5, b) 45/50/5, c) 40/50/10 and d) 45/40/15.
quantity of energy Qsens which represents the internal variation of energy of the system. Stored sensible heat is given by Equation (2)
Qsens ¼
1 r$e
Ztf
Df$dt ¼ cm $ Tf Ti
(2)
ti
where cp is specific heat (J/g C), Dɸ is the difference of heat flows measured with each step of time of acquisition dt, r (g/cm3) is the density of the sample, and e (cm) is the thickness of the sample. Figs. 4 and 5 show an example of heat storage in the samples for solid and liquid states, respectively. The temperatures measured by the heat flux meters on the lower (T1) and the upper (T2) faces of the sample evolve in an asymptotic way to the set point. The measured heat flux on both sides of the sample also evolves very quickly at the beginning of recording and then goes to zero, which corresponds to a new state of balance obtained at the end of the test. This confirms that lateral thermal losses are negligible. Fig. 4 presents variation of the heat storage capacity of selected PCMs in the solid phase for a temperature range varying from 25 C to 34 C while Fig. 5 presents similar results for the liquid phase from 43 C to 53 C. The results for this phase only were also obtained but the behavior despite additional natural convection heat transfer is somewhat similar to the curves presented in Fig. 4. For the liquid and solid phases, the temperatures evolve in an asymptotic manner with respect to the set point. The flow also evolves very quickly at the beginning of the recording and then to a constant value which corresponds to a new state of balance obtained at the end of the test. The quantities of heat stored and heat capacities for solid and
liquid states are given in Table 3. These values will be helpful to determine the apparent latent heat of these materials. The cp of PCMs based on LLDPE, W and EG was also investigated by DSC at a heating rate 2 C/min. Fig. 6a illustrates the temperature dependence on cp of LLDPE, W and EG from 20 to 60 C. In case of LLDPE and EG, there is no significant change of cp within entire investigated temperature interval due to no transition change in the entire temperature interval. DSC-curve of paraffin wax shows two inflection points. The first minor inflection point at 25 C corresponds to a solidesolid transition of paraffin wax. Another, major inflection point in the vicinity of the melting point of paraffin wax
Table 3 Average specific heat capacity, cp, determined by TGHPT and DSC at a heating rate 2 C/min with standard deviation (SD). Sample TGHPT Average Qsens (SD) (J/g)
Average cp (J/g C)
Solid Liquid Temperature 25 25e35 C 43e53 C e35 C
Temperature 43 e53 C
TGHPT DSC Error % TGHPT DSC Error % S1 S2 S3 S4 S5 S6 S7 S8
31 31 27 28 24 28 26 22
(2) (1) (1) (1) (2) (1) (2) (3)
27 30 32 28 32 30 29 30
(2) (1) (1) (1) (2) (1) (2) (2)
3.1 3.1 2.7 2.8 2.4 2.8 2.6 2.2
3.0 3.2 2.7 2.8 2.7 2.8 3.1 2.7
3 3 0 0 13 0 19 23
2.7 3.0 3.2 2.8 3.2 3.0 2.9 3.0
3.3 3.3 3.1 3.1 3.0 2.9 2. 3.4
18 9 3 15 7 3 21 12
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Fig. 6. Average cp of a) LLDPE, W and EG and b) various LLDPE/W content of PCMs, at temperature range from 20 to 60 C at a heating rate 2 C/min.
Fig. 7. Average cp of a) PCMs with 40 wt.% of W and various concentration of EG and b) PCMs with 50 wt.% of W and various concentration of EG at temperature range from 20 to 60 C at a heating rate 2 C/min.
belongs to the solideliquid transition. The ability to store and release thermal energy increased with increasing W content within PCMs. The cp of PCMs with a ratio of LLDPE/W w/w 70/30, 60/40 and 50/50 is illustrated in Fig. 6b. The dramatic influence of cp was observed in vicinity of the melting range of paraffin wax. Fig. 7a illustrates the average cp for PCMs with 40 wt.% of W and varying content of EG, and Fig. 7b shows PCMs with 50 wt.% of W and varying content of EG. Ability to store thermal energy was more pronounced in the case of PCMs containing 50 wt.% of W. This is predominantly caused by the solideliquid transition of W. The cp of PCMs in the solid and liquid state measured by TGHPT setup are compared to DSC measurements in Table 3, where Qsens obtained from TGHPT measurements are listed in the second and third column. Main phase transition of W is solideliquid transition which took place around 42 C. Therefore, temperature range from 25 to 35 C was chosen for determination Qsens in the solid state of W in order to eliminate solideliquid transition effect. Similarly, Qsens in liquid state of W was evaluated in temperature range from 43 to 53 C. Subsequently, cp values were calculated according to Equation (2) Slight decrease in cp values with increasing EG content is expected, however, the standard deviations of the cp measurements by either TGHPT (fall into 15%) or DSC (fall into 10%) making it difficult to compare individual samples. It is worth noting that the error varied between 0% and 23% (Table 3), which indicates that the TGHPT method is suitable for prediction of the cp of PCMs.
3.4. Energy storage and release Storage and release of energy were measured for all PCMs by a procedure similar to the one used for determining the sensible heat. A temperature range of 20e50 C was chosen to obtain information about heat storage capacity from the solid to the liquid phase of W. At the beginning of each experiment the samples were brought to initial isothermal temperature (Tinit ¼ 20 C). Subsequently, the sample was heated by modifying the temperature set point of the thermo-regulated bath. The material will thus evolve from Tinit to Tend. Between these two permanent steady states, the material stores energy. The flux meters measure the heat fluxes exchanged at the borders of the sample. Fig. 8 presents the heat storage and release capacity for pure LLDPE and PCMs with 40 and 50 wt.% of W. Fig. 8a illustrates the storage and release ability of pure LLDPE. Operating in a temperature range of 20e50 C ensures that no heat transition arises due to the high melting point of pure LLDPE. Two inflection points, were observed for PCMs containing 40 and 50 wt.% of W (Fig. 8b and c and 9) which belonged to solidesolid or solideliquid transition of W. Heat storage capacities of PCMs containing 40 and 50 wt.% of W and various content of EG are shown in Fig. 9. Minor inflection points, approximately at 28 C for all PCMs, which are lower than melting point of waxes (42 C), indicated solidesolid transition. The second inflection point (approximately 42 C) indicates the ability to store substantial amount of energy due to the melting of W inside the PCM.
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379
Fig. 8. Heat flux and temperature evolution from solid to liquid for a) LLDPE and different composition of LLDPE/W (w/w) PCMs b) 50/50 and c) 60/40, for cyclic variation of temperature from 20 to 48 C.
The reproducibility of storage and release of heat for selected PCMs by repeated heating and cooling process has been demonstrated (Figs. 8c and 9aec). Addition of EG led to a decreasing of the transition time of heat for all samples, which is caused due to the high conductivity of added EG in comparison with LLDPE and W. The comparison of the cumulative heat flux (ɸ1ɸ2) for PCMs containing 50 wt.% of W and 0 or 5 wt.% of EG is illustrated in Fig. 10. This figure shows that the addition of 5 wt.% of EG led to decreased storage and release time for thermal heat of PCMs. Table 4 summarizes the time needed to reach the temperature equilibrium state for various PCMs. It needs to be noticed that calculation was normalized in order to suppress small differences in the specimen's thicknesses. As can be seen, the time needed to reach the temperature equilibrium state decreases with increasing EG content for both types of PCMs, 40 as well as 50 wt.% of W. This feature confirmed that the addition of EG enhanced the heat transfer through the samples due to significant increasing of thermal conductivity and diffusivity of PCMs. Importantly, Lm, which is the energy absorbed or released by a thermodynamic system during isothermal process [45], has been calculated by Equation (3).
Qtotal ¼ Qsens þ Lm
Qtotal ¼
1 e$r
Zend Df dt ¼ cp ðTend Tinit Þ init
Qsensible ¼ cp $DT Lm ¼ Qtotal cp ðsolidÞ$DTðsolidÞ þ cp ðliquidÞ$DTðliquidÞ
J g (3)
where cp(solid) and cp(liquid) are the average solid state and the liquid state cp of the material, DT(solid) and DT(liquid) are the temperature variations for the material in solid phase and in liquid phase and Lm is the latent heat of melting. Notably, Lm increases with increasing W content. This behavior is understandable because the ability to store and release thermal energy is attributed to the paraffin wax content. The increase in EG did not significantly affect Lm of PCMs. Table 5 summarizes the values of the total heat calculated as the average values of the second heating and cooling cycles of the samples, sensible heat in the solid and liquid states of W and the Lm of PCMs with varying content of LLDPE, W and EG during temperature evolution from approximately 20 to 50 C by TGHPT. Qsens is calculated according to Equation (3) based on average cp values in solid state or liquid state paraffin wax with respect to exact experimental initial and final temperature. Although melting temperature of W is around 42 C, Qsens for solid state is much higher than for liquid state of W, because Qsens in the solid state was evaluated in the extended temperature range compare to Qsens in the liquid state of W. Total heat as well as sensible heat in the solid and liquid state was calculated according to Equation (2). Apparently, these values
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are quite different especially for sample S6, S7 and S8 (113, 119 and 103 J/g). Hence, it need to be mentioned that temperature interval chosen for these experiments were generally from 20 to 50 C. However, depending on the surrounding conditions during experiments, temperature variation around 2 C was observed and therefore simple comparison of the Qtotal as well as Qsens heat in solid and liquid state is not possible. It needs to be pointed out, that temperature variation of individual measurements was taken into account during the calculations. It can be noted that the latent heat of PCMs with 40 wt.% EG varied from 35 to 37 J/g. Whereas, PCMs with 50 wt.% EG exhibited latent heat from 48 to 51 J/g. The amount of latent heat of PCMs increased with increasing W content. This behavior is reasonable, because the Lm investigated in the temperature range from 20 to 50 C should be proportional to W content. One of the important parameters of PCMs in view of their application in the building industry is the repeatability in the
storage and release of thermal energy over a long time. These parameters were investigated by DSC. Fig. 11 demonstrated the ability to store and release thermal energy over 150 heating and cooling cycles. Differences between first and subsequent cycles can be explained by the thermal history of samples due to the mode of preparation. If the thermal history of the sample is suppressed during the first heating/cooling cycle, a nearly identical heat evolution has been observed during the next 150 cycles. Recently, reproducibility of thermal as well as mechanical behavior of PCMs has been studied by DSC and DMA, with different heating rates and slenderer number of thermal cycles [46]. 4. Conclusions The thermal behavior of phase change materials based on linear low density polyethylene, paraffin wax (melting point of 42 C) and expanded graphite has been studied. A significant increase in thermal conductivity and thermal diffusivity was achieved by
Fig. 9. Heat flux and temperature evolution from solid to liquid for different composition of LLDPE/W/EG (w/w/w) blends a) 55/40/5, b) 45/50/5, c) 50/40/10, d) 40/50/10 e) 45/40/ 15 and f) 35/50/15 for cyclic variation of temperature from 20 to 48 C.
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Fig. 10. Comparison of cumulated heat flux (ɸ1eɸ2) of PCMs with 50 wt.% of W and various content of EG.
Table 4 Time needed to reach the temperature equilibrium state of various PCMs. EG (wt.%)
0 5 10 15
Melting
Solidification
40 wt.% of W
50 wt.% of W
40 wt.% of W
50 wt.% of W
Time (h)
Time (h)
Time (h)
Time (h)
0.625 0.615 0.613 0.595
0.633 0.620 0.617 0.608
0.705 0.690 0.682 0.673
0.712 0.702 0.690 0.684
Table 5 Total heat, Qtotal, sensible heat Qsens in the solid and liquid state of W and latent heat, Lm, of PCM with various content of LLDPE, W and EG. Sample
S0 S1 S2 S3 S4 S5 S6 S7 S8
LLDPE/W/EG (w/w/w)
100/0/0 60/40 50/50 55/40/5 50/40/10 45/40/15 45/50/5 40/50/10 35/50/15
Average Qtotal (SD)
Qsens (Solid)
Qsens (Liquid)
Lm
J/g
J/g
J/g
J/g
e 53 53 46 46 40 49 47 39
e 17 12 15 13 14 15 14 13
e 36 49 37 35 37 49 48 51
55 106 114 98 94 91 113 119 103
(1) (2) (1) (1) (2) (2) (2) (2) (3)
381
Fig. 11. Heat flux evolution of PCMs with the composition of LLDPE/W/EG w/w/w of 35/50/15 during cyclic variation of temperature from 0 to 60 C temperature dependence.
temperature range from 43 to 53 C did not show a significant trend. The comparison of specific heat capacities determined by DSC and TGHPT exhibited good consistency. The primary finding is that the storage and release of heat energy during phase change is associated with the latent heat. PCMs consisting of 40 wt.% paraffin wax exhibited latent heat of 36 J/g, whereas the latent heat of PCMs consisting of 50 wt.% paraffin wax was 49 J/g. Moreover, the reproducibility of store and release thermal energy during the lifetime of PCMs is an important parameter during real use. This behavior was investigated by DSC. An almost identical heat evolution has been observed during 150 heating and cooling cycles. Acknowledgment
adding 10 and 15 wt.% of EG. DSC was to measure the specific heat capacity. A dramatic increase in the specific heat capacity was observed in the vicinity of the melting point of paraffin wax while the materials remain in a compact, solid phase due to the higher melting point of LLDPE. The sensible heat and the ability of PCMs to store and release thermal energy were investigated by specific home-made equipment based on the transient hot guarded plane method. The sensible heat of paraffin wax in the solid state was investigated in the temperature interval (25e35 C). For PCMs containing 40 wt.% of paraffin wax sensible heat decreased with increasing EG content, from 31 J/g in case of PCMs without EG to 24 J/g for PCMs with 15 wt.% of EG. This result was expected due to the lower value of the specific capacity of graphite, which is approximately 0.75 J/g C within the investigated temperature interval. A similar decrease in sensible heat with increasing EG has been observed for PCMs with 50 wt.% EG from 31 to 22 J/g. The sensible heat investigated in the
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Contents lists available at ScienceDirect
Renewable Energy journal homepage: www.elsevier.com/locate/renene
Thermal characterization of phase change materials based on linear low-density polyethylene, paraffin wax and expanded graphite Patrik Sobolciak a, *, Mustapha Karkri c, **, Mariam A. Al-Maadeed a, d, Igor Krupa b a
Center for Advanced Materials, Qatar University, 2713, Doha, Qatar QAPCO Polymer Chair, Center for Advanced Materials, Qatar University, P.O. Box 2713, Doha, Qatar c Universit e Paris-Est, CERTES, 61 avenue du G en eral de Gaulle, 94010, Cr eteil, France d Materials Science and Technology Program, Qatar University, P.O. Box 2713, Doha, Qatar b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 24 October 2014 Received in revised form 23 October 2015 Accepted 20 November 2015 Available online xxx
Thermal characterization of Phase Change Materials (PCMs) based on linear low-density polyethylene (LLDPE), paraffin wax (W) and expanded graphite (EG) is reported in this paper. Investigated PCMs showed high potential for application in energy storage systems. The latent heat, Lm, sensible heat Qsens, and the ability of the prepared PCMs to store and release thermal energy were investigated using specific home-made equipment based on the transient guarded hot plane method (TGHPT). The sensible heat of PCM containing 40 wt.% of paraffin wax was investigated in the temperature range 25e35 C, they exhibited a drop in Qsens from 31 to 24 J/g depending on the concentration of EG. A similar decrease in sensible heat with increased loading of EG was observed for PCMs containing 50 wt.% of EG. The storage and release of thermal energy during phase change which is associated with the latent heat of the materials were investigated within the temperature range 20e50 C. PCMs containing 40 wt.% of paraffin wax exhibited latent heat of 36 J/g, whereas the latent heat of PCMs containing 50 wt.% of paraffin wax was 49 J/g. The addition of EG decreased the time needed to melt and solidify PCMs due to increase in thermal conductivity of PCMs with increase in EG content. This behavior was confirmed by the thermal conductivity measurements, where thermal conductivity increased from 0.252 for sample without EG to 1.329 W/m C for PCM containing 15 wt.% of EG. The reproducibility of storage and release of thermal energy by PCMs was demonstrated by subjecting them to repeated heating and cooling cycles (over 150 cycles). © 2015 Elsevier Ltd. All rights reserved.
Keywords: Phase change material Paraffin wax Expanded graphite Thermal conductivity Thermal energy storage Solar energy
1. Introduction Thermal-energy storage systems are crucial for reducing dependency on fossil fuels and minimizing CO2 emissions. The Kyoto Summit secured a commitment from most countries to establish a global program for carbon dioxide (CO2) emissions reduction. According to the World Business Council for Sustainable Development, buildings account for up to 40% of global energy use [1]. Therefore, there is a critical need to reduce CO2 emissions from buildings. A portion of this reduction can be achieved by using building materials that are energy efficient.
* Corresponding author. ** Corresponding author. E-mail addresses: [email protected] (P. Sobolciak), [email protected] (M. Karkri). http://dx.doi.org/10.1016/j.renene.2015.11.056 0960-1481/© 2015 Elsevier Ltd. All rights reserved.
Thermal energy storage can be accomplished either by using sensible heat storage or latent heat storage components. Sensible heat storage components have been used by builders for centuries to store and release thermal energy passively, but a much larger volume of material is required to store the same amount of energy in comparison to latent heat storage systems [2]. Latent heat storage is more attractive than sensible heat storage because of its high storage density with smaller temperature fluctuations [3,4]. PCMs, can undergo phase changes (usually solid to liquid changes) at relatively low temperatures while absorbing or releasing high amounts of energy [5]. During the last four decades, many PCMs, with different phase transitions (e.g., solideliquid, solidesolid) and a wide range of transition temperatures, have been designed and studied extensively [6,7]. A suitable phase change temperature and a large melting enthalpy are two crucial requirements of PCMs.
P. Sobolciak et al. / Renewable Energy 88 (2016) 372e382
Various inorganic and organic substances have been used in the production of PCMs, the most common of which include various inorganic salts (e.g., polyhydric alcohols) and their eutectics, fatty acids, and n-alkanes. Inorganic salts (salts hydrates) were used as PCMs in the past, but they faced issues such as, such as supercooling, corrosion, relatively high volume and chemical instability, hence making them an unfavorable choice. Whereas, metallic compounds have limited use as PCMs for commercial purpose due to their weight and cost [8]. The most promising materials used as PCMs for lowtemperature applications (below 100 C) are paraffin waxes due to their high latent heat of fusion, negligible super-cooling, low vapor pressure in the melt and chemical inertness [9]. As for practical applications, another important issue has to be taken into account, namely a suppression of flow (leaching), an issue faced by PCMs that undergo solideliquid transition. There are a many strategies for avoiding this problem. First, paraffin waxes are kept in tanks of various shapes and volume that are incorporated into the building according to the specific needs. Second, porous materials can be impregnated by paraffin waxes; in this case capillary forces suppress leaching. Third, paraffin waxes can be encapsulated within a polymeric shell to form microcapsules. These microcapsules are frequently used in the textile industry (an impregnation of the surface of fabrics or direct incorporation of microcapsules into fibers during spinning) and in buildings. The most widely known example of such materials is MICROCONAL from BASF, which is frequently blended with plaster or concrete for designing of heat protective blocks. Lastly, paraffin waxes can be directly blended with polymeric matrices to avoid leaching and to retain a compact shape even after the paraffin wax has melted. Presently, paraffin waxes blended with appropriate polymers appear to be the best candidates for preparation of smart PCMs for various applications, such as thermal storage of solar energy, thermal protection of electronic devices, thermal protection of food and medical goods, passive storage in bioclimatic buildings, use of off-peak rates and reduction of installed power and thermal comfort in vehicles [10e12]. Polyethylenes appear to be the most suitable polymer for blending with paraffin waxes due to their chemical and structural similarities with paraffin waxes. Krupa et al. [13] investigated lowdensity polyethylene (LDPE) blends with soft and hard paraffin waxes. The blends were able to absorb large amounts of heat energy due to melting of paraffin wax, whereas the LDPE matrix kept the material in a compact shape on the macroscopic level. The importance of the structural similarity of was reported in a study that dealt with PCMs based on isotactic polypropylene (PP) blended with soft and hard Fischer-Tropsch paraffin [14]. The PP matrix held the material in a compact shape during the transition from solid to liquid. However, a much lower content of paraffin wax was incorporated due to strong phase separation of components, which is caused by the differing structure of crystallites. In the case of polyethylene, the zigzag structure of crystallites is much more favorable for mutual compatibility with paraffin waxes than the helical crystalline structure of polypropylene despite the fact that paraffin waxes, polyethylene and polypropylene are practically chemically identical. In addition, paraffin waxes can co-crystallize with polyethylene as discussed in a prior publication by the authors. The results obtained from solution crystallization confirmed a strong indication of co-crystallization in the case of LLDPE on one hand, and practically no miscibility in the crystalline regions of LDPE and oxidized Fischer-Tropsch paraffin wax on the other [15]. The fact that chemical and structural differences lead to strong phase separation of components was demonstrated in the work focused on PCM created from paraffin wax and polyamide [16]. Other polymer matrices, such as styreneeethyleneebutylenee
373
styrene (SEBS), styreneeisopreneestyrene (SIS), and styreneeethyleneepropyleneestyrene (SEPS), have been examined [17,18]. Mentioned studies described a shape-stabilized PCM with a melting temperature of 56e58 C. The results showed that the composites can retain their shape even when paraffin wax is in the liquid state, and no paraffin wax leakage was observed during thermal performance testing. Thermosetting resins (epoxy resin cured with amino-based hardener) were also tested for applications where blending with paraffin waxes at room temperature is required, so that low transition-temperature PCMs can remain in the solid phase during composite processing [19]. A Major drawback of polymer/paraffin wax PCM's is their low thermal conductivity. The thermal conductivity of paraffin waxes is approximately 0.2 W/m C, and the thermal conductivity of polymers varies from 0.15 W/m C for amorphous polymers such as polystyrene or polymethylmethacrylate, to the 0.5 W/m C for highly crystalline polymers such as high density polyethylene [20]. Thermal conductivity of polymers can be improved by blending them with inorganic materials [21,22], such as graphite and exfoliated graphite [23e25], metals [26e28], boron nitride [29], various metal-coated fillers [30] as well as carbon nanotubes [31]. EG is particularly useful for tailoring the thermal conductivity of materials (including PCMs) due to its inherently high thermal conductivity, favorable shape and high aspect ratio [30e34]. Recently, Trigui et al. [34] developed a new method and constructed a reliable device for testing the long-term performance of PCMs in large sized samples. They described the first store built with the composite material developed to improve different properties of PCM such as latent heat, storage and release energy, heat capacity and thermal conductivity. This unique method for thermal investigation of PCMs was used for resin epoxy/spherical paraffin wax [35] as well as for LDPE/paraffin wax composites at different concentrations of paraffin wax [36]. The objective of these investigations was to study the thermal conductivity of these PCMs and the amount of energy exchanged during the variation of thermodynamic state of the samples when the boundary temperatures vary [37]. Thermophysical properties obtained through this method for PCMs are crucial in studying and simulating their behavior. In this study, the thermal properties and the storage and release of energy in PCMs based on LLDPE, W (melting point approximately 42 C) and EG are reported. The storage and the release of heat energy were determined using the Transient Guarded Hot Plate Technique (TGHPT), which allows investigation of the thermal properties of large samples [36]. In comparison with results obtained from Differential Scanning Calorimetry (DSC) good reproducibility of the measurements was observed. . Since prepared PCMs have a melting point of around 42 C, they can be feasible for outdoor use in regions where heat accumulates at a higher temperature [38]. Furthermore, it can be useful for heat protection of solar collectors, batteries etc … [39]. 2. Experimental 2.1. Materials Linear low density of polyethylene (MFI ¼ 1 g/10 min, QAPCO, Qatar), paraffin wax (Grade RT42, Rubitherm Technologies, Germany) and expanded graphite (GFG200, SGL Carbon, Germany) having an average size of 200 mm were used for the PCMs preparation. 2.2. Composite preparation LLDPE powder was mixed with paraffin wax using a Brabender instrument (Plasticorder PLE 331, Germany) at 140 C.
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Subsequently, the blends were hot pressed (Fontijne TP 50, The Netherlands) at 140 C for 5 min. The composition of samples investigated in this study and their specific density (r) are summarized in Table 1. 2.3. Characterization 2.3.1. Scanning electron microscopy The fracture surfaces of the samples were investigated using a Nova Nano SEM 450 Scanning Electron Microscope. Brittle fracture of the samples was achieved in liquid nitrogen. 2.3.2. Transient Guarded Hot Plate Technique (TGHPT) Differential Scanning Calorimetry is widely used for characterization of thermal properties of PCMs as the simplest way to estimate the key parameters such as melting and crystallization temperatures, the specific heat of melting and crystallization or the specific heat capacity [32,33]. However, DSC gives information about thermal properties in micro-sized samples that can be significantly influenced by local heterogeneities. For investigation of PCMs in real conditions it is necessary to design an experimental device to obtain information about the thermal properties of large samples. TGHPT method for measurement of thermal conductivity has been described previously [34,35]. In brief, determination of the overall thermophysical properties of PCMs over several cycles (melt and solidify) requires the design of a genuine experimental device (Fig. 1). The proposed test bench for the parallelepiped-shaped composite provides temperature and heat flux measurements at the material's borders. The sample is located between two horizontal aluminum exchanger plates. Thermo-regulated baths connected to the plates allow a fine regulation of the injected fluid temperature with a precision of approximately 0.1 C. Heat flux sensors and thermocouples (type T) are placed on each side of the composite. The set-up is held in place using a tightened pneumatic jack. The thickness of the flux-meters is approximately 0.2 mm and their sensitivity is approximately 202 mV/W/m2 for a sensor having an active surface area of 400 cm2. The sensors are connected to a Labview program adapted to measure temperature fluctuations and heat flux exchanged during fusion and solidification. Experimental data is recorded with regular and adjustable time steps (6 s). The lateral side faces are insulated by polyethylene expanded foam (PE) which reduces multidimensional heat transfer to a 1D problem. In this work, the temperature varied between 20 C and 50 C. 2.3.3. Thermophysical characterization of PCMs Thermophysical properties measurements such as thermal
Table 1 Composition of prepared samples and their densities (r) with standard deviations (SD). Notation x/y/z means LLDPE/W/EG w/w/w ratio. Samples
S0 S1 S2 S3 S4 S5 S6 S7 S8
LLDPE/W/EG
Thickness
r measured
SD
w/w/w
mm
g/cm3
±
100/0/0 60/40/0 50/50/0 55/40/5 50/40/10 45/40/15 45/50/5 40/50/10 35/50/15
4.39 4.36 4.35 4.24 4.37 4.39 4.37 4.31 4.29
0.922 0.899 0.898 0.933 0.961 0.989 0.926 0.965 0.987
0.002 0.003 0.005 0.002 0.003 0.001 0.002 0.002 0.002
conductivity and thermal diffusivity were obtained using the periodic temperature ramp method (Fig. 2) by means of a home-made device called DICO [30]. The method is based on the use of a minor temperature modulation in a parallelepiped-shaped sample (45 mm 45 mm 5 mm) and allows obtaining all of these thermophysical parameters in only one measurement with their corresponding statistical confidence bounds [40e42]. The composite sample is fixed between two metallic plates. The use of conductive grease (lg ¼ 1 W/m C) ensures a good thermal exchange between the different plates and the sample. The front side of the first metallic plate is heated periodically using a sum of five sinusoidal signals and the temperature is measured with thermocouples placed inside both front and rear metallic plates. The thermophysical parameters of the sample are identified by comparison of the experimental and theoretical heat transfer functions. The system under study is modeled with one-dimensional quadrupole theory. The experimental heat transfer function H is calculated at each excitation frequency as the ratio between the Fouriertransform temperatures of the front and rear plates. A parameter estimation technique is then applied to simultaneously estimate thermal conductivity l and diffusivity a. The identification of the set of thermophysical parameters is a non-linear optimization problem that is solved iteratively: starting with sufficiently accurate initial guesses for the unknown parameters, we successively refine the estimates by using the LevenbergeMarquardt method [43]. 2.3.4. Differential scanning calorimetry The differential scanning calorimetry measurements were carried out employing a Perkin Elmer DSC 8500 differential scanning calorimeter. Samples were accurately weighed and subsequently placed in aluminum pans. The sample weight ranged from 3 to 15 mg. In case of measuring the specific heat capacity, the sample was cooled to 18 C at a rate of 2 C/min, and held at this temperature for 3 min, following which it was heated to 60 C, and held at it for 3 min. Calibration was done to sapphire standard. Nitrogen gas was passed through the instrument at a flow rate of 20 ml/min. DSC software was used for calculation of cp values. All experiments were repeated at least three times and average values are presented. In case of heating/cooling measurement cycling, temperature interval from 0 to 60 C/min at rate 10 C/min was employed. 3. Results and discussion 3.1. Morphology SEM images illustrated in Fig. 3 clearly show separate fractions of LLDPE and W as well as morphology of EG of PCM with the following compositions of LLDPE/W/EG: 60/40/0 w/w/w, 55/40/ 5 w/w/w and 45/40/15 w/w/w, respectively. No indication of miscibility between LLDPE and W on the microscopic level was observed. This immiscibility is a requirement for PCMs to have well-distinguished melting points. 3.2. Thermal conductivity measurements Thermal conductivity (leff) and thermal diffusivity (a) of PCMs have been summarized in Table 2. It was observed that thermal conductivity and diffusivity of PCMs without EG decreased with increased wax content. Thermal conductivity decreased from 0.369 W/m C for pure LLDPE to 0.252 for PCM containing 40 wt.% of paraffin wax and 0.269 W/m C for PCM containing 50 wt.% of W at T ¼ 20 C. This happens due to lower thermal conductivity of paraffin wax compared to that of LLDPE.
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375
Fig. 1. Experimental set-up of Transient Guarded Hot Plate Technique.
Conditioning modules
T rear Second metallic plate Sample First metallic plate T front
Microcomputer with Multifunction Analog/Digital
Thermoelectric cooler
I/O card Power amplifier
Turbomolecular pump Rough pump Fig. 2. Schematic representation of the experimental setup.
Fig. 3. SEM images of PCMs with composition of LLDPE/W/EG (w/w/w) a) 60/40/0 b) 55/40/5 and c) 45/40/15.
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Table 2 The thermal conductivity and thermal diffusivity of PCMs. Sample
S0 S1 S2 S3 S4 S5 S6 S7 S8
W
EG
leff
Dleff
a 107
Da 107
Ileff
wt.%
wt.%
W/m C
W/m C
m2/s
m2/s
%
0 40 50 40 40 40 50 50 50
e e e 5 10 15 5 10 15
0.369 0.252 0.269 0.502 0.988 1.329 0.483 0.974 1.278
0.018 0.006 0.013 0.012 0.012 0.037 0.013 0.031 0.057
2.713 1.214 1.283 1.877 3.881 4.966 1.856 3.716 4.422
0.536 0.063 0.099 0.076 0.119 0.368 0.067 0.292 0.499
e 32 27 36 168 260 31 164 246
EG particles were added into the LLDPE/W blends for an improvement of the thermal conductivity of PCMs (maintaining 40 and 50 wt.% of the W in the final PCMs composition). The thermophysical properties and associated uncertainties of the LLDPE/ W/EG composites have been summarized in Table 2. The addition of graphite significantly improved the thermal conductivity and thermal diffusivity of the materials. In both cases, a non-linear increase in thermal conductivity with increasing graphite mass fraction was observed. The thermal conductivity intensification Ileff, obtained by the addition of graphite, was calculated using Equation (1): Ileff ¼ (leff lm)/lm
(1)
where, leff is the effective conductivity of measured PCMs and lm is the measured conductivity of matrix (LLDPE). The intensification of
a/a*m e 32 28 55 220 309 44 189 244
materials consisting of 40 wt.% wax increased from 36% to 260% if filled with EG in the range from 5 to 15 wt.% (Table 2). Similar behavior was observed for materials consisting of 50 wt.% wax with an intensification maximum of 246%. This result is attributed to the high thermal conductivity of EG Ref. [44]. 3.3. The specific heat capacity in liquid and solid state The method used to measure sensible and specific heat of the composites consists of simultaneously measuring the heat flux 41 and 42 and temperatures T1 and T2 on the two faces of the sample (T1 and T2 are the two thermocouples integrated in the flux meters). Initially, both the exchangers are maintained at a constant temperature until a constant density flux on the two faces of the sample is reached, then the temperature of the exchangers is increased. Between these two isothermal states, the sample stores a
Fig. 4. Heat flux and temperature evolution of PCMs at solid state (from 25 to 35 C) based on LLDPE/W/EG (w/w/w) a) 55/40/5, b) 45/50/5, c) 40/50/10, d) 45/40/15.
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377
Fig. 5. Heat flux and temperature evolution of PCMs at liquid state (from 43 to 53 C) based on LLDPE/W/EG (w/w/w) a) 55/40/5, b) 45/50/5, c) 40/50/10 and d) 45/40/15.
quantity of energy Qsens which represents the internal variation of energy of the system. Stored sensible heat is given by Equation (2)
Qsens ¼
1 r$e
Ztf
Df$dt ¼ cm $ Tf Ti
(2)
ti
where cp is specific heat (J/g C), Dɸ is the difference of heat flows measured with each step of time of acquisition dt, r (g/cm3) is the density of the sample, and e (cm) is the thickness of the sample. Figs. 4 and 5 show an example of heat storage in the samples for solid and liquid states, respectively. The temperatures measured by the heat flux meters on the lower (T1) and the upper (T2) faces of the sample evolve in an asymptotic way to the set point. The measured heat flux on both sides of the sample also evolves very quickly at the beginning of recording and then goes to zero, which corresponds to a new state of balance obtained at the end of the test. This confirms that lateral thermal losses are negligible. Fig. 4 presents variation of the heat storage capacity of selected PCMs in the solid phase for a temperature range varying from 25 C to 34 C while Fig. 5 presents similar results for the liquid phase from 43 C to 53 C. The results for this phase only were also obtained but the behavior despite additional natural convection heat transfer is somewhat similar to the curves presented in Fig. 4. For the liquid and solid phases, the temperatures evolve in an asymptotic manner with respect to the set point. The flow also evolves very quickly at the beginning of the recording and then to a constant value which corresponds to a new state of balance obtained at the end of the test. The quantities of heat stored and heat capacities for solid and
liquid states are given in Table 3. These values will be helpful to determine the apparent latent heat of these materials. The cp of PCMs based on LLDPE, W and EG was also investigated by DSC at a heating rate 2 C/min. Fig. 6a illustrates the temperature dependence on cp of LLDPE, W and EG from 20 to 60 C. In case of LLDPE and EG, there is no significant change of cp within entire investigated temperature interval due to no transition change in the entire temperature interval. DSC-curve of paraffin wax shows two inflection points. The first minor inflection point at 25 C corresponds to a solidesolid transition of paraffin wax. Another, major inflection point in the vicinity of the melting point of paraffin wax
Table 3 Average specific heat capacity, cp, determined by TGHPT and DSC at a heating rate 2 C/min with standard deviation (SD). Sample TGHPT Average Qsens (SD) (J/g)
Average cp (J/g C)
Solid Liquid Temperature 25 25e35 C 43e53 C e35 C
Temperature 43 e53 C
TGHPT DSC Error % TGHPT DSC Error % S1 S2 S3 S4 S5 S6 S7 S8
31 31 27 28 24 28 26 22
(2) (1) (1) (1) (2) (1) (2) (3)
27 30 32 28 32 30 29 30
(2) (1) (1) (1) (2) (1) (2) (2)
3.1 3.1 2.7 2.8 2.4 2.8 2.6 2.2
3.0 3.2 2.7 2.8 2.7 2.8 3.1 2.7
3 3 0 0 13 0 19 23
2.7 3.0 3.2 2.8 3.2 3.0 2.9 3.0
3.3 3.3 3.1 3.1 3.0 2.9 2. 3.4
18 9 3 15 7 3 21 12
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Fig. 6. Average cp of a) LLDPE, W and EG and b) various LLDPE/W content of PCMs, at temperature range from 20 to 60 C at a heating rate 2 C/min.
Fig. 7. Average cp of a) PCMs with 40 wt.% of W and various concentration of EG and b) PCMs with 50 wt.% of W and various concentration of EG at temperature range from 20 to 60 C at a heating rate 2 C/min.
belongs to the solideliquid transition. The ability to store and release thermal energy increased with increasing W content within PCMs. The cp of PCMs with a ratio of LLDPE/W w/w 70/30, 60/40 and 50/50 is illustrated in Fig. 6b. The dramatic influence of cp was observed in vicinity of the melting range of paraffin wax. Fig. 7a illustrates the average cp for PCMs with 40 wt.% of W and varying content of EG, and Fig. 7b shows PCMs with 50 wt.% of W and varying content of EG. Ability to store thermal energy was more pronounced in the case of PCMs containing 50 wt.% of W. This is predominantly caused by the solideliquid transition of W. The cp of PCMs in the solid and liquid state measured by TGHPT setup are compared to DSC measurements in Table 3, where Qsens obtained from TGHPT measurements are listed in the second and third column. Main phase transition of W is solideliquid transition which took place around 42 C. Therefore, temperature range from 25 to 35 C was chosen for determination Qsens in the solid state of W in order to eliminate solideliquid transition effect. Similarly, Qsens in liquid state of W was evaluated in temperature range from 43 to 53 C. Subsequently, cp values were calculated according to Equation (2) Slight decrease in cp values with increasing EG content is expected, however, the standard deviations of the cp measurements by either TGHPT (fall into 15%) or DSC (fall into 10%) making it difficult to compare individual samples. It is worth noting that the error varied between 0% and 23% (Table 3), which indicates that the TGHPT method is suitable for prediction of the cp of PCMs.
3.4. Energy storage and release Storage and release of energy were measured for all PCMs by a procedure similar to the one used for determining the sensible heat. A temperature range of 20e50 C was chosen to obtain information about heat storage capacity from the solid to the liquid phase of W. At the beginning of each experiment the samples were brought to initial isothermal temperature (Tinit ¼ 20 C). Subsequently, the sample was heated by modifying the temperature set point of the thermo-regulated bath. The material will thus evolve from Tinit to Tend. Between these two permanent steady states, the material stores energy. The flux meters measure the heat fluxes exchanged at the borders of the sample. Fig. 8 presents the heat storage and release capacity for pure LLDPE and PCMs with 40 and 50 wt.% of W. Fig. 8a illustrates the storage and release ability of pure LLDPE. Operating in a temperature range of 20e50 C ensures that no heat transition arises due to the high melting point of pure LLDPE. Two inflection points, were observed for PCMs containing 40 and 50 wt.% of W (Fig. 8b and c and 9) which belonged to solidesolid or solideliquid transition of W. Heat storage capacities of PCMs containing 40 and 50 wt.% of W and various content of EG are shown in Fig. 9. Minor inflection points, approximately at 28 C for all PCMs, which are lower than melting point of waxes (42 C), indicated solidesolid transition. The second inflection point (approximately 42 C) indicates the ability to store substantial amount of energy due to the melting of W inside the PCM.
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379
Fig. 8. Heat flux and temperature evolution from solid to liquid for a) LLDPE and different composition of LLDPE/W (w/w) PCMs b) 50/50 and c) 60/40, for cyclic variation of temperature from 20 to 48 C.
The reproducibility of storage and release of heat for selected PCMs by repeated heating and cooling process has been demonstrated (Figs. 8c and 9aec). Addition of EG led to a decreasing of the transition time of heat for all samples, which is caused due to the high conductivity of added EG in comparison with LLDPE and W. The comparison of the cumulative heat flux (ɸ1ɸ2) for PCMs containing 50 wt.% of W and 0 or 5 wt.% of EG is illustrated in Fig. 10. This figure shows that the addition of 5 wt.% of EG led to decreased storage and release time for thermal heat of PCMs. Table 4 summarizes the time needed to reach the temperature equilibrium state for various PCMs. It needs to be noticed that calculation was normalized in order to suppress small differences in the specimen's thicknesses. As can be seen, the time needed to reach the temperature equilibrium state decreases with increasing EG content for both types of PCMs, 40 as well as 50 wt.% of W. This feature confirmed that the addition of EG enhanced the heat transfer through the samples due to significant increasing of thermal conductivity and diffusivity of PCMs. Importantly, Lm, which is the energy absorbed or released by a thermodynamic system during isothermal process [45], has been calculated by Equation (3).
Qtotal ¼ Qsens þ Lm
Qtotal ¼
1 e$r
Zend Df dt ¼ cp ðTend Tinit Þ init
Qsensible ¼ cp $DT Lm ¼ Qtotal cp ðsolidÞ$DTðsolidÞ þ cp ðliquidÞ$DTðliquidÞ
J g (3)
where cp(solid) and cp(liquid) are the average solid state and the liquid state cp of the material, DT(solid) and DT(liquid) are the temperature variations for the material in solid phase and in liquid phase and Lm is the latent heat of melting. Notably, Lm increases with increasing W content. This behavior is understandable because the ability to store and release thermal energy is attributed to the paraffin wax content. The increase in EG did not significantly affect Lm of PCMs. Table 5 summarizes the values of the total heat calculated as the average values of the second heating and cooling cycles of the samples, sensible heat in the solid and liquid states of W and the Lm of PCMs with varying content of LLDPE, W and EG during temperature evolution from approximately 20 to 50 C by TGHPT. Qsens is calculated according to Equation (3) based on average cp values in solid state or liquid state paraffin wax with respect to exact experimental initial and final temperature. Although melting temperature of W is around 42 C, Qsens for solid state is much higher than for liquid state of W, because Qsens in the solid state was evaluated in the extended temperature range compare to Qsens in the liquid state of W. Total heat as well as sensible heat in the solid and liquid state was calculated according to Equation (2). Apparently, these values
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are quite different especially for sample S6, S7 and S8 (113, 119 and 103 J/g). Hence, it need to be mentioned that temperature interval chosen for these experiments were generally from 20 to 50 C. However, depending on the surrounding conditions during experiments, temperature variation around 2 C was observed and therefore simple comparison of the Qtotal as well as Qsens heat in solid and liquid state is not possible. It needs to be pointed out, that temperature variation of individual measurements was taken into account during the calculations. It can be noted that the latent heat of PCMs with 40 wt.% EG varied from 35 to 37 J/g. Whereas, PCMs with 50 wt.% EG exhibited latent heat from 48 to 51 J/g. The amount of latent heat of PCMs increased with increasing W content. This behavior is reasonable, because the Lm investigated in the temperature range from 20 to 50 C should be proportional to W content. One of the important parameters of PCMs in view of their application in the building industry is the repeatability in the
storage and release of thermal energy over a long time. These parameters were investigated by DSC. Fig. 11 demonstrated the ability to store and release thermal energy over 150 heating and cooling cycles. Differences between first and subsequent cycles can be explained by the thermal history of samples due to the mode of preparation. If the thermal history of the sample is suppressed during the first heating/cooling cycle, a nearly identical heat evolution has been observed during the next 150 cycles. Recently, reproducibility of thermal as well as mechanical behavior of PCMs has been studied by DSC and DMA, with different heating rates and slenderer number of thermal cycles [46]. 4. Conclusions The thermal behavior of phase change materials based on linear low density polyethylene, paraffin wax (melting point of 42 C) and expanded graphite has been studied. A significant increase in thermal conductivity and thermal diffusivity was achieved by
Fig. 9. Heat flux and temperature evolution from solid to liquid for different composition of LLDPE/W/EG (w/w/w) blends a) 55/40/5, b) 45/50/5, c) 50/40/10, d) 40/50/10 e) 45/40/ 15 and f) 35/50/15 for cyclic variation of temperature from 20 to 48 C.
P. Sobolciak et al. / Renewable Energy 88 (2016) 372e382
Fig. 10. Comparison of cumulated heat flux (ɸ1eɸ2) of PCMs with 50 wt.% of W and various content of EG.
Table 4 Time needed to reach the temperature equilibrium state of various PCMs. EG (wt.%)
0 5 10 15
Melting
Solidification
40 wt.% of W
50 wt.% of W
40 wt.% of W
50 wt.% of W
Time (h)
Time (h)
Time (h)
Time (h)
0.625 0.615 0.613 0.595
0.633 0.620 0.617 0.608
0.705 0.690 0.682 0.673
0.712 0.702 0.690 0.684
Table 5 Total heat, Qtotal, sensible heat Qsens in the solid and liquid state of W and latent heat, Lm, of PCM with various content of LLDPE, W and EG. Sample
S0 S1 S2 S3 S4 S5 S6 S7 S8
LLDPE/W/EG (w/w/w)
100/0/0 60/40 50/50 55/40/5 50/40/10 45/40/15 45/50/5 40/50/10 35/50/15
Average Qtotal (SD)
Qsens (Solid)
Qsens (Liquid)
Lm
J/g
J/g
J/g
J/g
e 53 53 46 46 40 49 47 39
e 17 12 15 13 14 15 14 13
e 36 49 37 35 37 49 48 51
55 106 114 98 94 91 113 119 103
(1) (2) (1) (1) (2) (2) (2) (2) (3)
381
Fig. 11. Heat flux evolution of PCMs with the composition of LLDPE/W/EG w/w/w of 35/50/15 during cyclic variation of temperature from 0 to 60 C temperature dependence.
temperature range from 43 to 53 C did not show a significant trend. The comparison of specific heat capacities determined by DSC and TGHPT exhibited good consistency. The primary finding is that the storage and release of heat energy during phase change is associated with the latent heat. PCMs consisting of 40 wt.% paraffin wax exhibited latent heat of 36 J/g, whereas the latent heat of PCMs consisting of 50 wt.% paraffin wax was 49 J/g. Moreover, the reproducibility of store and release thermal energy during the lifetime of PCMs is an important parameter during real use. This behavior was investigated by DSC. An almost identical heat evolution has been observed during 150 heating and cooling cycles. Acknowledgment
adding 10 and 15 wt.% of EG. DSC was to measure the specific heat capacity. A dramatic increase in the specific heat capacity was observed in the vicinity of the melting point of paraffin wax while the materials remain in a compact, solid phase due to the higher melting point of LLDPE. The sensible heat and the ability of PCMs to store and release thermal energy were investigated by specific home-made equipment based on the transient hot guarded plane method. The sensible heat of paraffin wax in the solid state was investigated in the temperature interval (25e35 C). For PCMs containing 40 wt.% of paraffin wax sensible heat decreased with increasing EG content, from 31 J/g in case of PCMs without EG to 24 J/g for PCMs with 15 wt.% of EG. This result was expected due to the lower value of the specific capacity of graphite, which is approximately 0.75 J/g C within the investigated temperature interval. A similar decrease in sensible heat with increasing EG has been observed for PCMs with 50 wt.% EG from 31 to 22 J/g. The sensible heat investigated in the
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