Thermal energy storage in phase change materials under nano-porous confinement

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1 Thermal energy storage in phase change materials under nano-porous confinement Diarmid Roberts for 2017 ESA-CDT mini-project with Dr Siddharth Patwardhan 1 BACKGROUND & INTRODUCTION In the introductory report for this project, the benefits of phase change materials (PCM) as thermal energy storage media were highlighted, specifically the high volumetric energy storage densities and the high exergy related to release at a constant temperature. The need for encapsulation to prevent segregation in salt hydrates, and general leakage was also discussed. A number of benefits of encapsulation of paraffin wax or salt hydrate PCMs in a nano-porous material were proposed: Table 1: postulated benefits of nano-porous confinement for PCM material systems Speculated Benefit Class Applicability Wax Salt-hydrate Reduce supercooling Technical? x Reduce phase segregation Technical n/a x Modify MP Technical n/a x Broaden T response Technical x x Reduce fire risk Practical x n/a Easy to handle / structural rigidity Practical x x The salt hydrate sodium thiosulphate pentahydrate (Na 2S 2O 3.5H 2O) was highlighted as an interesting material due to its low cost ($ /ton) and high heat of fusion (~332MJ/m 3 ). If it were possible to reduce the melting point of the material from ~49 C in the bulk by nano-porous confinement, then it was proposed that such a system would be ideal for heat storage with underfloor heating (at ~40 C) in order to reduce peak electrical demand. Paraffin wax is also attractive despite its lower heat of fusion, as it is chemically inert and likely more stable across multiple cycles. 1.1 REVIEW OF LITERATURE ON PCM IN POROUS MATERIALS Fundamental studies The changes in the properties of substances at very small domain sizes is interesting from both theoretical and practical standpoints. The deviation from the bulk melting point (T m ) that occurs in a pore of radius r for a substance with solid molar volume V m, heat of fusion H f, and solid liquid surface tension γ sl, is given by the Gibbs-Thompson equation. The formulation given below is taken from Christensen s review on confinement effects, where the derivation is well explained. [1] Equation 1: T = 2T mv m γ sl r H f

2 Confinement in well characterised porous structures has allowed researchers to probe the properties of finite systems. The Gibbs Thompson equation was used by Jackson and McKenna to estimate the solid-liquid surface tension for various organic compounds, and demonstrate that H f drops with pore size. [2] Furo et al. showed that the salt hydrate Zn(NO 3) 2.6H 2O was ideal for cryoporometry, due to a large ΔT compared to water. [3] Applied studies of PCM in porous materials There are many articles covering the incorporation of PCMs in building materials, and only a few particularly relevant examples may be mentioned here. Memon s extensive 2014 review of PCM in building walls is recommended. [4] This review reports several systems where porous materials similar to silica gel are used. Xu and Li impregnated diatomite with paraffin wax at 47 wt.%, to give a free flowing powder and then added this material to concrete (ref. 56 in Menon). DSC analysis showed that the melting point was unchanged from the bulk, likely because the modal pore diameter (~400Å) is too large (cf. Jackson and McKenna). They also highlighted a downside of adding PCM to walls a loss of compressive strength. Wang et al. made reference to melting point depression in their study of stearic acid in fumed silica, but only observed a 1 C decrease in melting point. No pore structure data were reported for the system (ref. 67 in Menon). Fumed silica is interesting because it is a low value by-product of silicon production. Slightly larger peak melting and freezing depressions (2 C and 4 C) were observed by Karaipekli and Sari for a capric/myristic acid eutectic in vermiculite, but again no porosity data were included (ref. 53 in Menon). Despite a wealth of information, it is difficult to make valid practical comparisons of energy storage densities, as data tend to be reported on a mass basis. Underfloor thermal energy storage is less studied, despite the efficient use of space (sacrificing room height rather than width/length) and the reduced requirement for structural strength. Farid and Kong found that encapsulating 75 mm nodules of CaCl 2.6H 2O in concrete resulted in a comfortable temperature all day despite only heating for 8h. [5] This system is a competitor with the proposed Na 2S 2O 3.5H 2O/silica gel system, as the thermal energy storage densities are similar, and the CaCL 2.6H 2O is only slightly more expensive. [6] The advantage of the proposed system is that higher temperatures are accessible (melting point of CaCl 2.6H 2O, is 30 C) which may be better suited to systems where a heat pump is used. 2 AIMS AND OBJECTIVES The introductory report outlined the following aims and objectives for the project: 1. Review literature in order to determine what work has already been done in this field (PCM + nano-porous confinement), and select appropriate materials for experimental study. 2. Demonstrate proof of concept of benefit of nano-porous confinement in two identified systems: Wax confined in silica o Are there any issues with cracking or extrusion of PCM from pores? o Does the melting point broaden as expected? o Is the heat of fusion altered? (Other than a mass dilution effect due to the host) Salt hydrate confined in silica o Does the melting point reduce as desired? o Does confinement prevent supercooling? o Is the heat of fusion altered?

3 o Is segregation behaviour altered? 3. Provide insight to the potential applications of this method in real-life systems 3 METHOD 3.1 EXPERIMENTAL Silica characterisation Two commercial silica gels were selected for study: SiliaFlash P60 (Silicycle), and Davisil Grade 643 (Grace). A water pickup test was used to measure the accessible pore volume (end point defined by clumping that could not be removed by shaking the sample, volume calculated from penultimate drop addition). The packed bed density of each silica was measured by filling a 5 ml grade A cylinder and tapping it on the bench until no further compaction occurred. Both the measurements were performed twice on silica that was previously dried in an oven at 120 C for 3 h. The results are given in Table 2 along with the specified properties. Table 2: physical properties of the commercial silica gels used in this work. Silica Spec. particle size range (μm) Spec. mean pore diam. (Å) Spec. pore vol. (ml/g) H 2O pore volume (ml/g) Tapped bed density (g/ml) SiliaFlash P Davisil PCM materials Two PCM materials were studied. A paraffin wax with nominal melting point C was obtained from Sigma Aldrich ( KG). Sodium thiosulphate pentahydrate was also sourced from Sigma Aldrich ( ). Both PCM materials have lower densities in the liquid form than the solid. A paraffin wax of similar melting point was reported in the literature to have solid density of 0.92 g/ml and a liquid density of 0.80 g/ml. [7] Na 2S 2O 3.5H 2O is reported to have a liquid density of 1.67 g/ml, and a solid density of g/ml. [6] Incipient wetness impregnation (IWI) for paraffin wax In this method, 1.00 g of silica and the desired mass of PCM in solid form were placed in a plastic vial. A stirrer bar was added then the sealed vial was submerged in a 70 C water bath on a hotplate. The samples were stirred for 2h. In the first 5 min it was necessary to remove the vial and tap it to break up clumps. Impregnating Davisil silica with a mass of wax such that molten volume = H 2O pore volume resulted in a free-flowing powder at both 70 C and room temperature (PW-150Å-1). Taking the same approach for the SiliaFlash P60 led to clumping of the material while hot, although the powder became freeflowing if shaken while cooling. For the Davisil silica a second sample was prepared with the mass of wax mass increased by 5% (PW-150Å-2). This exhibited clumping while hot. For SiliaFlash P60, a 10% reduction in added wax prevented clumping while hot (PW-60 Å -3).

4 3.1.4 Excess wetness impregnation (EWI) for Na 2S 2O 3.5H 2O For the Na 2S 2O 3.5H 2O system a molten PCM volume in excess of the water pore volume was targeted. The goal was to add enough PCM that an excess liquid layer would allow diffusion throughout the bed of particles allowing homogeneous uptake, but not so much that isolated pools of molten Na 2S 2O 3.5H 2O would form (causing sampling issues). Na 2S 2O 3.5H 2O and Davisil silica were placed in a small plastic vial and heated in an oven at 70 C. In the first 30 minutes the sample was removed periodically and shaken on the vortex mixer. Then the sample was left at 70 C overnight. When shaken on the vortex mixer, the sample coalesced into balls of 2-4mm diameter with a waxy translucent appearance. The sample was split into two vials. The first was seeded with a pristine Na 2S 2O 3.5H 2O crystal, leading to heat generation, and pulverisation of the balls. The sample was then crushed to improve homogeneity for DSC analysis (NaTS-150Å-6). Later it was observed that the unseeded sample had also gone off (NaTS-150 Å -7). NB: after impregnation of Na 2S 2O 3.5H 2O on silica there was a sulphurous smell upon opening the vial, which was not present in the precursor Summary of samples A summary of the composite samples prepared and analysed by DSC is given in Table 3 below. The % of H 2O pore volume is calculated assuming molten densities of 0.80 g/ml for paraffin wax, and 1.67 g/ml for Na 2S 2O 3.5H 2O. Table 3: compositions of composite PCM materials prepared. Sample Synthesis method Mass silica (g) Mass PCM (g) PW-60Å-1 IWI PW-60Å-3 IWI PW-150Å-1 IWI PW-150Å-2 IWI NaTS-150Å-6 NaTS-150Å-7 Molten PCM volume as % of H 2O pore volume EWI DSC analysis Samples were analysed using a MettlerToledo DSC apparatus mg of sample was added to a 40 μl aluminium pan. Samples were heated and cooled at a rate of 5 C/min, with cooling provided by a 50 ml/min flow of nitrogen cooled by liquid nitrogen. 1 For H f the Math/Integration function was used, with boundaries assessed by eye, and a linear baseline drawn between the start and end. 3.2 RESULTS AND DISCUSSION Paraffin wax system The paraffin wax shows a broad endothermic DSC profile with total heat release of approximately 181 J/g between 30 and 70 C (Figure 1). There are clearly two separate processes occurring: a minor event between C followed by a major event lasting until approximately 68 C. This has been shown for similar waxes to correspond to a solid-solid transition followed by melting. [8] Due to the time limitations an attempt was not made to deconvolute the peaks. There is also an offset of approximately 5 C between the peak of melting and the peak of freezing C/min was also tested for two samples and this confirmed there was no lag at 5 C/min.

5 Figure 1: DSC plot of 5 C/min melting/freezing cycle for paraffin wax and composite materials. Endothermic processes show as negative and exothermic as positive. Melting and freezing point depression is observed in the composite samples, and the depression is greater for the 60Å silica (~9 C) than the 150Å silica (~4 C). 2 DSC analyses of PW-150Å-2 and PW- 60Å-3 show small peaks at the same temperature as the bulk melting (and freezing). This indicates that clumping observed while preparing these samples is due to wax remaining outside the pore structure. The heat flow integrals for the samples with no extra-porous melting were normalised to wax content (using data from Table 3), and it was found that the heats of fusion are reduced by porous confinement to 117 J/g and 158 J/g in the 60Å and 150Å silicas respectively. This is consistent with the work of Jackson and McKenna. [2] Sample PW-150Å- was cycled ten times without change to the heat of fusion (158 J/g) Na 2S 2O 3.5H 2O system The DSC analysis of bulk Na 2S 2O 3.5H 2O shows onset of melting at 51 C, and a heat of fusion of 205 J/g (Figure 2), which is in agreement with results collated by Kenisarin and Mahkanov. [6] The melting process is not reversible, although there was an exothermic event between ~45 C and -10 C, with a total heat release of 23 J/g. Supercooling was observed in a sample of Na 2S 2O 3.5H 2O melted in a closed plastic vial then left to cool to room temperature. The liquid was stable over a period of 6 days. Dipping a glass pipette in the liquid led to precipitation of needle like crystals, as did adding a few granules of silica gel. These are likely crystals of NaS 2O 3.2H 2O. [9] Adding a crystal of Na 2S 2O 3.5H 2O to the supercooled liquid initiated rapid growth of large faceted crystals followed by a slower solidification of the remaining liquid. To determine whether seeding 2 These values are based on the peak heat flow, as it is difficult to determine the exact temperature of onset.

6 resets the system to solid Na 2S 2O 3.5H 2O, supercooled melt was placed in a DSC pan (using a plastic pipette) and a single crystal of Na 2S 2O 3.5H 2O added, causing rapid solidification. The DSC profile for this material is also shown in Figure 2. There appears to be inhomogeneity in the seeded sample, but the onset of melting is unchanged, and the heat evolved is 205 J/g (no meaningful difference from the 203 J/g for the pristine sample) Figure 2: DSC endothermic profile of pristine Na 2S 2O 3.5H 2O and the material formed when a crystal of pristine Na 2S 2O 3.5H 2O is added to the supercooled melt. The same approach was used to obtain a solidified sample of Na 2S 2O 3.5H 2O in Davisil silica (NaTS- 150Å-6, see section 3.2.4). The DSC profile of this material is shown in Figure 3. The appearance of an endotherm at reduced temperature implies that some material is incorporated in the porous host, and that this material was solid prior to analysis. The apparent melting point depression (15 C) is greater than that observed for the paraffin wax in the same silica. This may be due to a higher solidliquid surface tension for the former (see Equation 1). Measuring this parameter is difficult, as discussed by Jones. [10] One approach is to derive the value via Equation 1 for ΔT data observed for different pore sizes. Although it has not been possible to find literature data for paraffin wax and Na 2S 2O 3.5H 2O, Jackson and McKenna obtained values of 11.6 and 18.4 mn/m for cis and trans-decalin, whereas Furo et al. calculated values of 46nN/m and 49 mn/m for Zn(NO 3) 2.6H 2O and CaCl 2.6H 2O. [2] [3] The factor of approximately three agrees with the difference observed here. Assuming that the silica porosity is completely filled with liquid Na 2S 2O 3.5H 2O in the synthesis, then the heat of fusion for the extra-porous material is calculated as 190 J/g, which is only 7% out from the bulk value. 3 The heat of fusion for the Na 2S 2O 3.5H 2O believed to be confined in the pores is calculated to be 39 J/g a large reduction in the heat of fusion from the bulk. 3 Mean of triplicate analyses given. Standard deviations: 1.0 J/g for low temp peak, 3.5 J/g for high temp peak.

7 Figure 3: Endothermic DSC profiles for Na 2S 2O 3.5H 2O/silica composite NaTS-150A-6 and bulk material. In both cases a crystal of pristine Na 2S 2O 3.5H 2O was used to seed the supercooled melt. The appearance of a discrete peak raised the possibility of testing a hypothesised benefit of nanoporous encapsulation: the maintenance of a solid extra-porous phase that could re-seed the melted material within the porous structure. This was tested by taking another sample of DR-PW-150A-6 and heating it to only 45 C, where the extra-porous material should remain solid. The DSC profile for this test is shown in Figure 4. Figure 4: DSC profile for cycles between 25 and 65 C, and 25 and 45 C on seeded NaTS-150A-6. Despite the presence of un-melted material in the sample, there is no exotherm evident in the DSC profile, implying a rapid return to Na 2S 2O 3.5H 2O did not occur. Based on the observation in section that the composite material sample NaTS-150A-7 solidified without seeding, it would be useful to study the process over longer time periods Storage capacities Using the DSC data (J/g normalised to PCM content), and assuming the sample volume is defined by the silica bed densities reported in Table 2, it was possible to estimate the thermal energy storage

8 capacity per m 3 for the studied materials. These are shown in Table 4, alongside the bulk values (calculated from experimental heats of fusion and liquid densities from [6]). Table 4: thermal energy storage capacity per cubic meter for composite and bulk materials. Sample Thermal energy capacity (MJ/m 3 ) Bulk paraffin wax 145 PW-150Å-1 56 PW-60Å-3 32 Bulk Na 2S 2O 3.5H 2O 342 NaTS-150Å-6 28 Despite the volumetric thermal storage capacity of bulk Na 2S 2O 3.5H 2O being more than twice that of the paraffin wax, this is not realised in the composite due to the apparent reduction in ΔH f. Benchmark energy consumptions for different building types are reported in the CIBSE report Energy Benchmarks TM2046: [11] The fossil fuel consumption for a typical residence was given as 1.15 kwh/m 2.day. UKERC reported that 77.5% of domestic heat usage was for space heating, giving 0.89 kwh/m 2.day. [12] The thermal storage capacity of the paraffin wax composite material PW-150Å-1 is 15.6 kwh/m 3, so a thickness of 5.7 cm should be able to store the daily energy requirement. This seems practical, and manually compacting the material should increase the density. It is envisaged that the powder could be packed in around the piping of the underfloor system then sealed with resin. 4 DELIVERABLES In the introductory report a number of deliverables were outlined for this project: Literature review identifying work already done in this area, and justifying choice of PCM Experimental data demonstrating proof of concept for at least one system (described in Aims and Objectives) Discussion (with further literature review) of potential applications, including cost element The proof of concept has been partly demonstrated for the Na 2S 2O 3.5H 2O system: the melting point was depressed as intended, but the apparent heat of fusion was drastically reduced compared to the bulk and the absorbed heat was not released upon cooling. The system requires detailed study to understand the behaviour. In the paraffin wax system, the proof of concept has been displayed: there was a melting point depression, and a reduction in heat of fusion, but a useful thermal storage capacity remained, which was stable over ten cycles, implying the PCM is not ejected from the porosity. Although the melting point of the studied wax is too high, the above calculations show that the composite paraffin wax material is a feasible energy store for the underfloor heating application in energy terms. There has not been time to study the cost of the system in detail. The silica gel is likely the most expensive part of the system, and commercial data is required to quantify this.

9 5 CONCLUSIONS The experiments with paraffin wax and silica demonstrate that it is easy to prepare a composite PCM in which the operating temperature is reduced relative to the bulk. This composite appears stable over ten cycles, implying that there is no leakage of PCM in this timeframe. The 150Å pore size silica yields a higher thermal capacity on a volume basis. The Na 2S 2O 3.5H 2O system, which was initially attractive for the low cost and high thermal storage density of the PCM is not as practical in reality, as the heat of fusion appears to be reduced significantly due to confinement, and the thermal storage is not reliably reversible. 6 CHALLENGES & FURTHER WORK There are unanswered questions relating to the Na 2S 2O 3.5H 2O/silica composite material. The sulphurous odour following synthesis implies that a reaction is occurring between the silica surface and the salt hydrate. This may reduce the thermally useful fraction of PCM within the pores. Modifying the silica surface prior to impregnation or looking at a porous carbon material may provide illumination. XRD would also be useful to confirm whether the low temperature endothermic event shown in Figure 3 is really due to melting within pores. Testing the samples by alternate thermal methods, such as the cooling curves used by Stunic may yield more useful information on the crystallisation process. [13] Regarding the paraffin wax composite, further research is needed to understand the commercial product range in melting point terms. The broad melting range challenges the conception that a PCM will release heat at a particular temperature. It may be necessary to look at other organic materials. The thermal conductivity of the system is an important property that requires further investigation. There are also practical questions regarding the durability of the material. 7 REFERENCES [1] H. K. Christensen, Confinement effects on freezing and melting, Journal of Physics: Condensed Matter, vol. 13, p. 95, [2] L. C. Jackson and B. McKenna, The melting behaviour of organic materials confined in porous solids, J. Chem. Phys., vol. 93, no. 12, p. 9002, [3] I. Furo, O. Petrov and D. Vargas-Florencia, Inorganic salt hydrates as cryoporometric probe materials to obtain pore size distribution, J. Phys. Chem B, vol. 110, p. 3867, [4] S. A. Memon, Phase change materials integrated in building walls: a state of the art review, Renewable and sustainable energy reviews, vol. 31, p. 870, [5] M. Farid and W. J. Kong, Underfloor heating with latent heat storage, Proceedings of the Institution of Mechanical Engineers part A - Journal of Power and Energy, vol. 215, p. 601, 2001.

10 [6] M. Kenisarin and K. Mahkamov, Salt hydrates as latent heat storage materials: Thermophysical properties and costs, Solar Energy Materials & Solar Cells, pp , [7] N. Ukrainczyk, S. Kurajica and J. Šipušic, Chem. Biochem. Eng., vol. 24, no. 2, [8] A. S. Luyt and I. Krupa, Thermal behaviour of low and high molecular weight paraffin waxes used for designing phase hange materials, thermochimica acta, vol. 467, p. 117, [9] Z. Stunic, V. Djurickovic, S. Majstorovic and V. Buljan, Thermal Study of Multicycle Melting and Freezing of Sodium Thiosulphate Pentahydrate, J. Chem. Biotechnol., vol. 32, p. 393, [10] D. R. H. Jones, Review: The Free Energies of Solid Liquid Interfaces, Journal of Materials Science, vol. 9, p. 1, [11] CIBSE, TM46: 2008 Energy benchmarks, CIBSE, London, [12] P. Eames, D. Loveday, V. Haines and P. Romanos, The Future Role of Thermal Energy Storage in the UK Energy System: An Assessment of, UKERC, London, [13] Z. Stunic, V. Djurickovic and Z. Stunic, Thermal storage: Nucleation of melts of Inorganic Salt Hydrates, J. appl. Chem. Biotechnol., vol. 28, p. 761, [14] A. Sharma, V. V. Tyagi, C. R. Chen and D. Buddhi, Review on thermal energy storage with phase change materials and applications, Renewable and sustainable energy reviews, vol. 13, p. 318, 2009.