High performance storage composite for the enhancement of solar domestic hot water systems Part 1: Storage material investigation

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1 Available online at Solar Energy 85 (2011) High performance storage composite for the enhancement of solar domestic hot water systems Part 1: Storage material investigation D. Haillot a,b,, V. Goetz a,x.py a, M. Benabdelkarim b a CNRS-PROMES: PRocesses, Materials and Solar Energy Laboratory, Université de Perpignan Via Domitia, Rambla de la Thermodynamique, Tecnosud, Perpignan, France b Saunier Duval Eau Chaude Chauffage Industrie, 17 rue de la Petite Baratte, BP Nantes Cedex 03, France Received 18 November 2009; received in revised form 6 January 2011; accepted 22 February 2011 Communicated by: Associate Editor Halime Paksoy Abstract This work aims to evaluate the performance of a solar domestic hot water (SDHW) system including a latent storage material. The originality of our approach consists to place a composite made of compressed expanded natural graphite (CENG) and phase change material (PCM) directly inside a flat plate solar collector in order to replace the traditional copper-based solar absorber. According to this target, the study is composed of two steps: the composites preparation and characterization; and the analysis of the system to achieve optimal integration of the material in the process. The present paper is focused on the selection of the most promising composite to implement in the solar collector. In order to reach this objective, several composites based on CENG and various storage materials (paraffin, stearic acid, sodium acetate trihydrate and pentaglycerin) have been elaborated and characterized. The synthesis of all these measurements allowed us to select three composites whose characteristics match their integration into a solar thermal collector. Ó 2011 Elsevier Ltd. All rights reserved. Keywords: Thermal energy storage; Phase change materials; Solar domestic hot water system 1. Introduction Solar energy is an important alternative energy source for the present and the future. In the industrial market of solar domestic hot water (SDHW) systems, flat plate collector is the most popular technology due to its low cost, its reliability and simplicity. Nevertheless, a large storage tank is needed for such a system, sometimes too large in small houses. The integration of phase change materials (PCM) at the top of the water storage tank has been Corresponding author. Address: CNRS-PROMES: PRocesses, Materials and Solar Energy Laboratory, Université de Perpignan Via Domitia, Rambla de la Thermodynamique, Tecnosud, Perpignan, France. Tel.: ; fax: addresses: didier.haillot@gmail.com (D. Haillot), goetz@ univ-perp.fr (V. Goetz). already proposed several times as a high energy density storage media to reduce the tank volume and to enhance the system efficiency (Mehling et al., 2003; Cabeza et al., 2006; Ibanez et al., 2006). An evaluation of the actual contribution of such system based on a yearly balance has been recently proposed concluding that the addition of PCM inside a storage tank does not increase the overall system efficiency (Talmatsky and Kribus, 2008). The new approach developed in the present paper is to use the PCM directly inside the solar collector, as solar absorber. Such approach could present several advantages like the limitation of the maximum collector temperature under stagnation (which is responsible for solar collector life time reduction) and the reduction or even the avoidance of the storage tank volume. These advantages will be developed in a next paper devoted to the system analysis X/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi: /j.solener

2 1022 D. Haillot et al. / Solar Energy 85 (2011) Nomenclature Cp calorific capacity, J kg 1 K 1 L mass enthalpy, J kg 1 p total hemispheric reflectance T temperature, K Greek symbols a absorptivity k thermal conductivity, W m 1 K 1 q density, kg m 3 Subscript Axi axial CENG compressed expanded natural graphite F fusion Max maximum Rad radial S solidification Sub subcooling Tr transition The first part of this study, presented here, is devoted to storage materials. The proposed insertion of the composite as solar absorber would lead him to a double functionality: collection/conversion of the solar flux and storage of the thermal energy. These objectives reveal two major issues: Available raw PCMs present too low thermal conductivities. This leads to a severe limitation in discharge power of the stored energy and therefore must be improved (Tey et al., 2002). The storage material must collect the maximum amount of the available solar flux and so it should have a high absorptivity in the solar spectra. The different intensification techniques concerning the thermal transfer between the storage material and a heat transfer fluid are based upon either the increase of the exchange surface between these two elements (by macro (Cristopia, 2009) or by micro-encapsulation (Basf, 2009)) or by dispersion of a high thermal conductivity material within the storage material. This conductive material can be a metal under different shapes such as ring (Velraj et al., 1997), fins (Velraj et al., 1999; Stritih, 2004) or foam (Hafner and Schwarzer, 1999). Another kind of thermal binder is graphite because of its excellent thermal diffusivity. Carbon fiber (Fukai et al., 2000) or compressed expanded natural graphite (CENG) additions have been already tested. According to its particular efficiency, this latter technique has been chosen to reach the first objective, previously defined. Such PCM/CENG composites have been already published in the particular cases of paraffins (Py et al., 2001) for low temperature thermal storage (below 100 C) and inorganic salts (Pincemin et al., 2008) for higher temperature range (300 C). They have been proved to present high energetic properties with a thermal conductivity ranging from 5 to 70 W m 1 K 1 (Pincemin et al., 2008; Py et al., 2001, 2006). Moreover, the combination of CENG and latent storage materials leads to black materials which could potentially fulfill the second issues. These previous studies devoted to limiting amount of PCMs will be extended to almost all type of latent heat materials with a transition temperature ranging between 40 and 80 C. The main objective is to identify materials with optimal features for our application. On the basis of a literature review, the most promising latent heat materials will be first selected. Corresponding composites, based on CENG, will then be elaborated and characterized. The thermal conductivity, which represents the material capability to transfer the heat flux, will be systematically measured. The possibility of developing a composite that combines the two functionalities (solar energy collection/conversion and thermal storage) will be examined by measuring its absorptivity in the solar spectrum. The thermal storage capacity of the composite will be quantified by calorimetric measurements. Compiling all these measurements, the most promising composites will be selected to be tested by thermal cycling to assess their lifetime. This will allow us to select the composite whose characteristics match their integration into a solar thermal collector. 2. Composite elaboration 2.1. Latent heat material selection Many latent heat storage materials are available. Most common are the PCM, extensively described in the open literature. Solid solid transition materials (SSTM), although less known, have also been considered. Each material has its own properties leading to specific advantages and associated drawbacks. On the basis of available literature reviews (Abhat, 1983; Farid et al., 2004; Hasnain, 1998; Janz, 1967; Kenisarin and Mahkamov, 2007; Lane, 1983; Mayer, 1987; Zalba et al., 2003) more than a hundred latent storage materials within a transition temperature range comprised between 40 and 80 C had been listed (Haillot, 2009). We have then defined criteria for selecting latent heat storage material corresponding to our application. These criteria take into account properties of each storage material: Thermo physical properties (a transition temperature (T tr ) as nearest as possible from 65 C, a latent heat (L) of at least 150 J g 1, a density (q) and a calorific capacity (C p ) as higher as possible). Kinetic properties (subcooling lower than 10 K).

3 D. Haillot et al. / Solar Energy 85 (2011) Chemical properties (stability, low toxicity). Economic properties (low cost, inferior to 10 kg 1 for a purchase at laboratory scale). Four raw storage materials have been selected by applying the criteria described above: Two organic PCMs, a paraffin and the stearic acid. One inorganic PCM, the sodium acetate trihydrate. And one SSTM, the pentaglycerin Elaboration route Composites elaboration route will not be detailed in this paper as it has been done previously by several authors (Haillot et al., 2008; Pincemin et al., 2008; Py et al., 2001). We can only note that CENG/PCM organic: CENG/RT65 paraffin (Fig. 1a), CENG/stearic acid; and CENG/pentaglycerin composites have been elaborated by the warm impregnation technique while we have used the so-called compression technique to elaborate CENG/ sodium acetate trihydrate composites. These two techniques lead to an anisotropic composite structure inherited from the spatial rearrangement of the graphen layers under axial compression (Fig. 1b). The bulk materials were respectively provided by SGL Ò, Rubitherm Ò and Alfa-aesar Ò for the expanded natural graphite, the RT65 paraffin, and the others storage materials. 3. Composite characterization 3.1. Thermal conductivity (k) Thermal conductivity measurements have been performed at ambient temperature under steady state conditions on macroscopic samples (cubes 25 mm in side) to characterize heterogeneous composite materials. The corresponding experimental set up has been previously presented (Olives, 1999). When measuring conductivities Fig. 1. Composite elaboration: (a) CENG/RT65 paraffin composite (q CENG = 100 kg m 3 ) and (b) composite anisotropic structure. greater than 10 W m 1 K 1 the measurement accuracy is about 10%. The uncertainties may reach 25% when the conductivity of the sample analyzed is of the order of a few W m 1 K 1. In Fig. 2 the obtained thermal conductivity measurements performed on the elaborated composites are presented. As both elaboration routes induce anisotropic thermal conductivities inherited from the compression step of the graphite matrix, the radial (Fig. 2a) and axial (Fig. 2b) thermal conductivities have been distinguished. Measurements are presented as a function of the bulk graphite matrix density, q CENG, corresponding to the relative amount of CENG in the composite volume. Concerning composites obtained by warm impregnation, whatever the involved storage material, RT65 paraffin, stearic acid or pentaglycerin, the radial thermal conductivity increases exponentially with bulk graphite matrix density. The obtained values range from 5 to more than 40 W m 1 K 1 for bulk graphite matrix density ranging from 50 to 320 kg m 3. This gives rise to an increase by a factor of of the thermal conductivity compared to the single PCM (with an average thermal conductivity equals to 0.2 W m 1 K 1 ). Similar measurements performed on CENG/sodium acetate trihydrate composites show that, for a bulk graphite matrix density ranging from 50 to 320 kg m 3 its thermal conductivity ranges respectively from 5 to 30 W m 1 K 1. These results can be compared to the corresponding values obtained for the graphite matrix alone at the same bulk graphite density. It is then found that in the case of composites elaborated by warm impregnation, and for bulk graphite matrix density below 170 kg m 3, the presence of storage material does not yield to a significant decrease of the conductivity of the corresponding composite. Above this value, the conductivity of the composite is lower than the matrix alone. However the measured values are notably high. As an example for a bulk graphite matrix density equal to 250 kg m 3 the thermal conductivity of the composite is 38 W m 1 K 1. In the case of composites elaborated by cold compression, orientation of graphenes layers perpendicular to the direction of compression is disturbed by the presence of particles of, resulting in a reduction of the conductivity. Thermal conductivity of about 20 W m 1 K 1 is obtained for a bulk graphite matrix density equal to 320 kg m 3. This corresponds to a decrease of 60% compared to the conductivity of the matrix, which is equal to 50 W m 1 K 1 for a same CENG density. Globally, for a same density of graphite, thermal conductivity measurements of composites obtained by compression are two times lower than those obtained for composites made by impregnation. Composite anisotropy is underlined by lower values of the axial conductivity (Fig. 2b) between 2 and 8 W m 1 K 1 for bulk graphite matrix density from 50 to 320 kg m 3.

4 1024 D. Haillot et al. / Solar Energy 85 (2011) Fig. 2. Radial (a) and axial (b) elaborated composite thermal conductivity versus bulk graphite matrix density for (+) CENG matrix, (s) CENG/RT65 paraffin, (d) CENG/stearic acid, (e) CENG/ pentaglycerin and (h) CENG/sodium acetate trihydrate composites Absorptivity (a) Absorptivity measurements have been performed at ambient temperature using a reflectometer ELAN Ò EL510. During the measurement, a flash is sent by a light source to the sample, the reflected light is then integrated by a totally reflecting sphere and analysed by a photo detector. Spectrum of the incident light provided by a Xenon Lamp is closed to solar irradiation and the direction of the light is normal to the sample. The ratio between the light reflected by the sample and the flux emitted by the source is the total hemispheric reflectance (p). Considering that the thickness of our samples (25 mm) prevents any transmission of the light beam, the absorptivity has been deduced by the following equation. a ¼ 1 p ð1þ The accuracy of reflectance measurements is about 5%. CENG matrix and composites absorptivity measurements according to the bulk graphite matrix density are presented in Fig. 3. All the samples present absorptivity values higher than The best results were obtained for CENG/RT65 paraffin composites with absorptivity close to 0.9. The reasons why the absorptivity is higher with PCM are yet still not clear. This particular point of interest will be the focus of another more fundamental paper. No significant difference between values obtained on the radial (Fig. 3a) and the axial (Fig. 3b) direction were observed. Applied on pure CENG matrix, these measurements show an increase in absorptivity of by impregnation with RT65 paraffin. Similar observations can be done on the composites CENG/stearic acid and CENG/pentaglycerin. However, in this case, the increase of the absorptivity compared to pure CENG matrix is only equal on average to CENG/sodium acetate trihydrate composites present the lowest absorptivity with measured values varying from 0.8 to All these values tend to demonstrate that these materials are adapted for a direct absorption of the solar irradiations Storage capacity (L, T Tr ) It has been previously showed that the presence of graphite does not lead to a modification of the intrinsic storage characteristics of PCM (Pincemin, 2007). As a consequence, the storage capacity per mass unit of the composites is simply determined by applying a correction factor corresponding to the mass proportions of PCM and graphite according to the composition of the material. Fig. 3. Radial (a) and axial (b) elaborated composite absorptivity versus bulk graphite matrix density for (+) CENG matrix, (s) CENG/RT65 paraffin, (d) CENG/stearic acid, (e) CENG/pentaglycerin and (h) CENG/sodium acetate trihydrate composites.

5 D. Haillot et al. / Solar Energy 85 (2011) Comparison of the storage capacity has been therefore done on pure storage material. Measurements have been carried out using a Setaram Ò C80 calorimeter with a temperature ranging from 40 to 120 C. The upper temperature of 120 C was chosen to validate the use of these materials in a solar thermal collector in operating conditions, taking into account the possible rise of temperature under stagnation encountered when the collector is not circulated by the heat transfer fluid. Paraffin, stearic acid and pentaglycerin thermograms obtained by calorimetry are presented in Fig. 4a. From analysis of these thermograms, the transition characteristics of these three storage materials have been determined and are gathered in Table 1a. The melting temperature (defined here as the temperature corresponding to the maximum value of the fusion peak) of RT65 paraffin is 65 C and the related enthalpy is equal to 167 kj kg 1. These results are consistent with data from the supplier, indicating an enthalpy of 170 kj kg 1 (Rubitherm, 2009). The observed difference in temperature between the beginning and the end of the melting transition is equal to 14 C. This large transition temperature range is due to the composition of the paraffin which is a mixture of alkanes and not a real eutectic. Concerning the stearic acid, we measured a melting temperature of 72 C and an enthalpy of 218 kj kg 1. The melting peak width is 6 C lower than that observed in the case of the paraffin. The transition, when heating the pentaglycerin, starting at 80 C, ends at 89 C and its maximum peak is located at 86 C. The associated enthalpy is equal to 152 kj kg 1. During the cooling of the pentaglycerin, the transition begins at 69 C, its maximum is located at 65 C and ends at 62 C. The associated enthalpy is identical to that measured during heating. There is potentially a tendency of these materials to subcooling, with a value of degree of subcooling of 11 C. The degree of subcooling DT Sub is defined by Eq. (2) and the corresponding value reported in Table 1 for every experimented PCM. When negative, this criterion is regarded as zero. DT Sub ¼ T Fusion Start T Solidification Start T Start is defined as the temperature from which begins the fusion/solidification peak. For these three storage materials, beyond 80 C no effect other than thermal heat capacity has been recorded on the obtained thermograms. To conclude about those results, congruent melting, low subcooling and stability up to 120 C suggest that these materials can be inserted in flat plate collectors. Tests of stability during thermal cycling have been performed on these materials to determine their ð2þ Fig. 4. Storage material calorimetric thermal analysis: (a) RT65 paraffin ( ), stearic acid ( ) and pentaglycerin (...); (b) sodium acetate trihydrate ( ), temperature operating conditions (...). Table 1 Thermal properties of RT65 paraffin, stearic acid and pentaglycerin. Fusion Solidification T Start ( C) T Max ( C) T End ( C) L F (kj kg 1 ) T Start ( C) T Max ( C) T End ( C) L S (kj kg 1 ) DT Sub ( C) (a) First thermal cycle RT65 Paraffin Stearic acid Pentaglycerin (b) After 650 fusion solidification cycles RT65 Paraffin Stearic acid Pentaglycerin

6 1026 D. Haillot et al. / Solar Energy 85 (2011) lifetime and thus confirm this hypothesis. The corresponding results are presented in the next paragraph. The inorganic PCM, sodium acetate trihydrate was also analyzed and the thermogram corresponding to the calorimetric analysis is presented in Fig. 4b. During the first rise in temperature a peak is observed at 57 C related to its phase change. When the sample temperature exceeds 100 C, a signal corresponding to the latent heat of vaporization of water is recorded. When the temperature decreases and during the following thermal cycle, no signal corresponding to a latent enthalpy is measured. In conclusion, the analysis of the sodium acetate trihydrate reveals a change in the chemical composition of the hydrated salt which is not compatible with its integration into a solar thermal collector. The following stability tests will not be undertaken for this material Life time A specific experimental device and protocol have been established to quantify the influence of cumulative thermal cycles on the storage capacity of the studied PCM/SSTM. Storage materials have been placed in individual stainless steel cells (5 cm in internal diameter and 10 cm in height) surrounded by an electrical heating device. The cooling of the cells was performed with fans blowing ambient air. Thermocouples, placed on both the cell and the sample, were used to record the temperature of the storage materials during the successive thermal treatments. An acquisition card linked to a computer and a Labview Ò interface controlled the heating device and performed automatic thermal cycles. For the three storage materials, thermal cycles were performed between 40 and 95 C with a heating rate of 2 C min 1. At cycle intervals defined by the user, samples were withdrawn from the material and analyzed by calorimetry. The weighing of the whole tested material was done at each sampling. Main characteristics of the studied storage material after 650 cycles are presented in Table 1b. RT65 paraffin characteristics after 650 cycles are almost similar to those measured during the first cycle. We note only a slight shift of the maximum melting temperature of 1 C. The same experiment concerning stearic acid shows that the melting peak of this material thermally cycled 650 times takes place with a thermal shift of 3 less than that of the original sample (Fig. 5). The same shift is recorded during the solidification of this material. Indeed the temperature peak is in this case observed at 62 C while the peak temperature of solidification of non-cycled sample is equal to 65. Considering the storage capacity of the cycled material, a 10% decrease of the stearic acid latent heat is observed. Concerning the pentaglycerin, the only difference observed between the two samples (after one and 650 cycles) is a shift of the peak solidification of 4 C which reduces consequently the degree of subcooling. Moreover, the solid solid transition calculated enthalpies are identical. This result confirms the thermal stability of the pentaglycerin. 4. Conclusion Fig. 5. Stearic acid calorimetric thermal analysis comparison between initial ( ) and after 650 cycles ( ). Among the numerous PCM/SSTM present in the literature and listed for our particular application (the integration in a flat plate solar thermal collector), only four were selected as good candidates according to the considered criteria. Corresponding composites, made of those selected PCM and CENG, have been elaborated and characterized. The desired properties of those composites are linked to their particular application as simultaneous solar absorber and thermal storage media of high thermal conductivity (allowing high charge and discharge rates). The corresponding obtained values of the concerned thermal Table 2 Summary of all measurements on elaborated composites. Storage material Thermal conductivity (W m 1 K 1 ) 50 < q CENG < 300 (kg m 3 ) Compression Sodium acetate trihydrate Absorptivity Initial latent heat (kj kg 1 ) L F (T Max/F ) L S (T Max/S ) Over cooling/ dehydration Latent heat after 650 cycles (kj kg 1 ) L F (T Max/F ) Radial: 5 20, Axial: (58) 0 Dehydration > 100 C Non available L S (T Max/S ) Impregnation RT65 Paraffin Radial: 5 40, Axial: Maximal with 167 (65) 159 (59) No 166 (66) 159 (59) Stearic acid paraffin 218 (72) 207 (65) No 201 (69) 187 (62) Pentaglycerin 152 (86) 152 (65) DT Sub = (86) 152 (69)

7 D. Haillot et al. / Solar Energy 85 (2011) properties needed for further studies are summarized in Table 2. According to the obtained characteristics of the CENG/ sodium acetate trihydrate composite, this particular one has been considered as being finally inappropriate for the integration in a solar collector. The storage capacity of this latent heat storage material under thermal cycling reveals a change in the composition of the PCM when it is heated to a temperature above 100 C. This property, due to a partial departure of the initial water content, is not compatible with the needed thermal stability characteristic of the material required for our solar thermal application. Initial limitations related to the low intrinsic thermal conductivity of the RT65 paraffin, stearic acid and pentaglycerin have been resolved by means of the development of composites based on CENG. Moreover these composites present also high absorptivity of the solar flux, thus addressing the second issue. Finally, the thermal cycling tests have demonstrated the stability of the RT65 paraffin and pentaglycerin after 650 cycles. This number of cycle represents about 2 years of operation. Strictly, this not long enough regarding the life cycle expected for a thermal flat collector. Nevertheless, the fact that almost no change is detected during several hundreds of cycles, allows confidence regarding the stability of the composite. The same test applied to stearic acid shows a decrease of 10% of the storage capacity of this material after 650 cycles. In conclusion, we demonstrated that CENG/RT65 paraffin and CENG/pentaglycerin composites present all the properties that were evaluated in this paper for their efficient use as solar absorber and also to store the corresponding produced thermal energy. An other important property, surface emissivity, will be treated in a future publication. The use of CENG/stearic acid composite is conditioned by thermal stability testing beyond 650 cycles. Acknowledgements This project was financially supported by the French Government through a grant from the ANRT and both financially and technically by the Saunier Duval society. The authors wish to express a very special acknowledgement to those two partners. References Abhat, A., Low temperature latent heat thermal energy storage: heat storage materials. Solar Energy 30, Basf, < Cabeza, L., Ibanez, M., Solé, C., Roca, J., Nogués, M., Experimentation with a water tank including a PCM module. Solar Energy Materials and Solar Cells 90, Cristopia, < Farid, M., Kudhair, A., Razack, S., Al- Hallaj, S., A review on phase change energy storage: materials and applications. Energy Conversion and Management 45, Fukai, J., Kanou, M., Kodama, Y., Miyatake, O., Thermal conductivity enhancement of energy storage media using carbon fibers. Energy Conversion and Management 41, Hafner, B., Schwarzer, K., Improvement of the heat transfer in a phase change material storage. 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