Polyols as phase change materials for surplus thermal energy storage

Polyols as phase change materials for surplus thermal energy storage

Applied Energy xxx (2015) xxx–xxx Contents lists available at ScienceDirect Applied Energy journal homepage: www.elsevier.com/locate/apenergy Polyo...

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Applied Energy xxx (2015) xxx–xxx

Contents lists available at ScienceDirect

Applied Energy journal homepage: www.elsevier.com/locate/apenergy

Polyols as phase change materials for surplus thermal energy storage q Saman Nimali Gunasekara ⇑, Ruijun Pan 1, Justin Ningwei Chiu, Viktoria Martin Department of Energy Technology, Royal Institute of Technology (KTH), Brinellvägen 68, 100 44 Stockholm, Sweden

h i g h l i g h t s  Polyols are attractive PCM candidates for storing surplus low-temperature heat.  Their melting temperatures and enthalpies are around 15 to 245 °C and 100–413 kJ/kg.  Complex behaviors like glass transition and thermal degradation were reported.  T-history experimental evaluations of PEG 10,000, Erythritol, and Xylitol were done.  Large subcooling, thermally activated change and hysteresis were observed for some.

a r t i c l e

i n f o

Article history: Received 31 October 2014 Received in revised form 16 February 2015 Accepted 13 March 2015 Available online xxxx Keywords: Phase change materials (PCM) Polyols T-history method Material properties Glass transition Thermally activated change

a b s t r a c t Storing low-temperature surplus thermal energy from industries, power plants, and the like, using phase change materials (PCM) is an effective alternative in alleviating the use of fossil based thermal energy provision. Polyols; of some also known as sugar alcohols, are an emerging PCM category for thermal energy storage (TES). A review on polyols as PCM for TES shows that polyols have phase change temperatures in the range of 15 to 245 °C, and considerable phase change enthalpies of 100–413 kJ/kg. However, the knowledge on the thermo-physical properties of polyols as desirable PCM for TES design is presently sparse and rather inconsistent. Moreover, the phase change and state change behaviors of polyols need to be better-understood in order to use these as PCM; e.g. the state change glass transition which many polyols at pure state are found to undergo. In this work preliminary material property characterization with the use of Temperature-History method of some selected polyols, Erythritol, Xylitol and Polyethylene glycol (PEG) 10,000 were done. Complex behaviors were observed for some of the polyols. These are: two different melting temperatures, 118.5–120 °C and 106–108 °C at different cycles and an average subcooling 18.5 °C of for Erythritol, probable glass-transition between 0 and 113 °C for Xylitol, as well as a thermally activated change that is likely an oxidation, after three to five heating/cooling cycles for Xylitol and Erythritol. PEG 10,000 had negligible subcooling, no glass-transition nor thermally activated oxidation. However a hysteresis of around 10 °C was observed for PEG 10,000. Therefore these materials require detailed studies to further evaluate their PCM-suitability. This study is expected to be an initiation of an upcoming extensive polyol-blends phase equilibrium evaluation. Ó 2015 Elsevier Ltd. All rights reserved.

1. Introduction The growing energy demands and the escalation of the use of fossil fuels to fulfill these demands have given rise to q This article is based on a short proceedings paper in Energy Procedia Volume 161 (2014). It has been substantially modified and extended, and has been subject to the normal peer review and revision process of the journal. This paper is included in the Special Issue of ICAE2014 edited by Prof. J Yan, Prof. DJ Lee, Prof. SK Chou, and Prof. U Desideri. ⇑ Corresponding author. Tel.: +46 (0)8 7907476 (O), +46 (0)736523339 (mobile). E-mail address: [email protected] (S.N. Gunasekara). 1 Present address: Department of Chemistry, Ångström Laboratory, Uppsala University, Box 538 75121 Uppsala, Sweden.

environmental burdens globally. To alleviate these burdens, a sustainable energy system is a crucial aspect, for which realizing resource efficiency is one important task. In this context, the utilization of surplus heat from industrial processes, including large scale power generation is imperative. This surplus heat utilization is done internally within a plant through process integration. To a smaller extent, this is utilized externally by e.g. connection to a district heating system [1–3]. However, the full potential of converting surplus thermal energy into a resource cannot be realized due to the mismatch in time and location, between the source and demand. Thermal energy storage using phase change materials (PCM) is one effective solution to explore to manage this mismatch, now emerging with considerable

http://dx.doi.org/10.1016/j.apenergy.2015.03.064 0306-2619/Ó 2015 Elsevier Ltd. All rights reserved.

Please cite this article in press as: Gunasekara SN et al. Polyols as phase change materials for surplus thermal energy storage. Appl Energy (2015), http:// dx.doi.org/10.1016/j.apenergy.2015.03.064


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Nomenclature A Cp dt dT h H k lmtd m Q Q_ t T U

area (m2) specific heat (J/kg K) time step (s) temperature difference (°C or K) specific enthalpy (J/kg) enthalpy (J) thermal conductivity (W/(m K)) log mean temperature difference (°C or K) mass (kg) heat (J) heat transfer rate (W) time (s) temperature (°C or K) heat transfer coefficient (W/(m2 K))

Abbreviations FT-IR Fourier transform infrared spectroscopy NMR nuclear magnetic resonance

momentum [4,5], e.g., for, district heating [6] and heat exchange within industries [7]. In this work, polyols (also known as polyhydric alcohols [8] or poly alcohols) are evaluated as one particular material category of PCM interest. It is mainly owing to their moderate phase change temperatures of generally around 0–200 °C2 [9] and higher massspecific and volume-specific enthalpies in comparison to other organic PCM [9]. Furthermore, many polyols are non-toxic [10] and generally safe for use [11,12], some even being common sugar substitutes; e.g. Erythritol, Xylitol, Lactitol [12]. Particularly, Erythritol and Xylitol are mainly used today as sugar substitutes for low calorie foods and beverages, and for diabetics [13,14]. These are also used in some pharmaceutical and cosmetics applications [13,14], where in comparison PEGs are used the most [15]. Nevertheless, except for Glycerine (Glycerol), polyols in general are not recommended for therapeutic applications [8]. Their material compatibility is said to be similar to paraffins [10], hence supposedly compatible with metals. However, some polyols could undergo autoxidation at higher temperatures thus producing various acidic products [16] that could corrode metals. In pure grades, polyols are usually rather expensive, compared to organics like paraffins and fatty acids [17–19]. Nevertheless, being produced via carbohydrates hydrogenation [9,12,20], they have a potential for large-scale production, which could be expected to reduce their prices. More specifically, these can be produced via the hydration of epoxides, hydrolysis of natural fats, hydrogenation of dicarboxylic acids and dicarboxylic acid derivatives, and the aldol additional reaction [8]. In addition, polyols are of renewable origin being plants-based compared to paraffins (are of fossil-origin), and are more compatible with metals compared to fatty acids. The overall objective of this paper is to present polyols in light of the desired material behavior needed in making use of these, in pure forms and as blends as PCM for storage of low grade excess thermal energy in industrial applications. Literature findings and preliminary thermal property characterizations of some selected polyols are used as input for the analysis.

2 More specifically between 15 and 245 °C as the literature survey yielded later in this work.

PCM phase change materials PEG Polyethylene glycol SEM scanning electron microscopy TEM transmission electron microscopy TES thermal energy storage T-history temperature history XRD X-ray diffraction Subscripts and superscripts amb,box ambience inside the experimental set-up container Cu copper n time step n ou Outside (the experimental set-up container) PCM phase change material SS_TT stainless steel test-tube Tot total

2. Methodology The thermal and physical properties of polyols were collected as available from various literature, with or without being focused on PCM TES design. For the experimental evaluations, three polyols: Erythritol, Xylitol and Polyethylene glycol (PEG) 10,000 were chosen, based on several factors. These have attractive melting temperatures and enthalpies (as reported in literature) for moderate temperature heat storage applications like district heating. Also, these are safe for handling, and are renewable materials. Apart from the general compatibility details mentioned for polyols, their specific compatibilities are however unknown. In this paper, the cost of material was not used as a selection criteria as particularly Erythritol is presently expensive. It is however believed that upon large-scale bulk production on the world-wide market, polyols, if shown feasible from a material point of view, are likely to obtain a reasonable price for use in PCM-based TES systems. The selected polyols, meso-Erythritol and Xylitol each of 99% purity ([21]), and PEG 10,000 of synthesis-grade ([22]) were purchased from VWR international AB, Sweden [23]. The experimental evaluations of these were done using the temperature-history (Thistory) method.

2.1. T-history method The T-history method used was based on Chiu and Martin [24] with some procedural modifications. The temperature histories of 99% pure (meso-)Erythritol, Xylitol and synthesis grade PEG 10,000 were obtained, using two identical polyol samples in stainless-steel (SS) armed test-tubes each, and a reference; a solid copper block of the same geometry as the test-tubes. The two polyol samples are identified as S1 and S2, for each material. The two test-tubes and the solid copper block were insulated with identical cylinders and end-covers made of HT-Armaflex of thickness 19 mm. These insulated stainless steel test-tubes with molten polyol samples poured in and the thermal sensors placed-in, and the insulated solid copper block with thermal sensors attached, were placed horizontally inside experimental set-up container. This test set-up with the containment was then kept inside an ACS Hygross 1200 climate chamber and thermally cycled, between

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three heating and cooling cycles respectively. The thermal sensors used were T-type thermocouples, used with calibration. The mass of the samples used; with two samples each from a single polyol type, were 51.7 g and 53.39 g of PEG 10,000, 62.84 g and 62.97 g of Erythritol, and 66.91 g and 65.83 g for Xylitol. The reference used was a solid copper block with an identical geometry to that of the test-tubes used. The type of copper used for the solid block construction was oxygen-free (OFHC) type for the PEG 10,000 and Erythritol characterizations, while for Xylitol, this was the electrolytic copper (Cu-ETP).3 The specific heat of these coppers are known, and thus these reference blocks respectively were used to measure the total thermal energy stored over the time period considered, c.f. Chiu and Martin [24]. The stainless steel of the test-tubes was SS 316. The specific heats and the thermal conductivities of the OFHC copper, Cu-ETP and SS 316 used are given in Table 1. The T-history method is based on the ‘lumped capacitance method’. This entails maintaining a uniform temperature profile in the samples over the transient process. This is verified by maintaining a small Biot number; as expressed in Eq. (1), which is maintained well below 1 [24]. The identical HT-Armaflex insulations are used to obtain such a small Biot number. By further placing these inside an experimental set-up container, the convective heat transfer effects from possible air-currents inside the climate chamber are restricted.

Ur Bi ¼ << 1 2k

obtained (Eq. (7)). Since the specific heat Cp, temperature and the mass of the SS test-tubes are also known, the respective heat gain by each test-tube can also be calculated, as in Eq. (9). By deducting this heat gained by the test-tube form the total heat gain, the heat gained by the PCM can be determined (Eq. (10)). Thus the enthalpy of the PCM can be back-calculated, using Eq. (11) [24].

Q_ Cu ¼ mCu  dHCu =dt ¼ mCu  C P;Cu  dT Cu =dt


Q_ Cu ¼ k1  lmtdCu




h   i n1 n n T n1 PCM  T Box;amb  T PCM  T Box;amb  i ¼ h n1 n n ln ðT n1 PCM  T Box;amb Þ= T PCM  T Box;amb

Material type

Specific Heat kJ/(kg K)

Thermal conductivity W/(m K)



OFHC copper ETP copper

0.3935 0.3894

391.1 389.1

[25] [26]

SS 316


At 20 °C Average between 20–100 °C Between 0– 100 °C/⁄At 100 °C







lmtdTot ¼ lmtdSS


 Q_ PCM þ Q_ SS Q_ Cu Q_ Tot UA¼ ¼ ¼ lmtdCu lmtdTot lmtdPCM Q_ Tot ¼ Q_ SS Q_ SS



_ SS ¼m





_ SS ¼ k1  lmtdPCM  m


hPCM ¼

¼ lmtdPCM

ð8Þ ð9Þ




 dT SS


Q_ PCM  dt mPCM




m  dhPCM ðTÞ ¼





dhðTÞ C P ðTÞ  dT ¼ dt dt Q PCM ¼



þ Q_ PCM ¼ k1  lmtdPCM

Q_ PCM ¼ k1  lmtdPCM  Q_ SS


The overall heat accumulation by the reference copper block over the sampling time was determined using the known specific heat, and mass of it, as in Eq. (2). Also, the total heat gained by the copper reference block was accounted by using a curve-fit for the heat versus the logarithmic mean temperature difference (lmtd) [24], determining the overall heat transfer coefficient k1 using Eq. (3). The lmtd is as defined [24], as in Eqs. (4) and (5) for the reference and PCM. The temperature of the PCM sample and the SS test-tubes are assumed to be equal, and therefore the lmtd for the PCM and the SS test-tubes, and these as a whole (lmtdTot) are also the same (c.f. Eq. (6)). For the identical geometries of the reference as the PCM samples in the SS test-tubes, the overall heat transfer coefficients for the reference and the samples are the same: k1 [24] (c.f. Eqs. (7) and (8)). Back-calculating, the total heat gained by the PCM and the SS test tube together (Q_ Tot ) can be

h   i n1 n n T n1 Cu  T Box;amb  T Cu  T Box;amb   i ¼ h n1 n n ln T n1 Cu  T Box;amb = T Cu  T Box;amb

Table 1 Thermal properties of the reference and the test-tube materials used in the T-history method.

ð12Þ Z


m  C P;PCM ðTÞ  dT



Then, similar to Chiu and Martin [24], the specific heat Cp of the PCM (polyol) was determined using the enthalpy obtained, according to the expression in Eq. (12). Also, the overall thermal energy storage capacity of the PCM (polyols in this case) over the temperature range T1–T2 can be determined employing Eq. (13) [24]. 3. Polyols as phase change materials for thermal energy storage PCM including polyols, have great potential in many TES applications e.g., solar energy storage [28], buildings thermal comfort applications [29], vehicles components thermal buffering [10], cold thermal applications [30], and other numerous applications [31– 33]. A growing PCM interest in polyols is evident at present with for instance, patents on pure and blends of polyols for TES [34] and polyol mixtures with some salts [11], plus literature on the evaluation of various mixtures/composites: Mannitol–fatty acid esters [35], Erythritol–porous materials [36,37] and Polyethylene glycol–sugar [16]. However, there are numerous TES design data, particularly thermal properties, unknown for polyol PCM including for polyol blends. 3.1. Polyols thermal properties of PCM–TES interest


3 The two type of coppers were used to compare the effects of oxidation of the outer surface of the copper blocks when subject to these moderately large temperatures while being in contact with the HT Armaflex insulation.

Table 2 compiles some thermal properties of pure polyols of PCM interest, from literature. In addition to the melting enthalpy, the freezing enthalpies were available in literature for certain polyols, e.g. (in kJ/kg), D-Mannitol: around 214 [38] and for PEG types 800, 1000, 4000 and 10,000 of around: 130, 154, 152 and 142 [39] respectively. 1,2,4-Butanetriol (C4H10O4) and PEG 200 are said to supercool [40,8] but their melting-freezing temperatures or fusion enthalpies were not found.

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Melting Temp.  (C) (⁄⁄: metastable)

Freezing Temp. (C)

Heat of, fusion/⁄ melting (kJ/kg)

Thermal conductivity (W/(mK))

Glycol (Ethylene glycol, monoethylene glycol, 1,2-ethanediol) CH2OHCH2OH (C2H6O2) Glycerol (Glycerin) CH2OHCHOHCH2OH (C3H8O3) Tetritol Erythritol C4H6(OH)4, (C4H10O4)

14.9 18 104⁄⁄/102–112⁄⁄

18 /314


0.14/0.28 (at 25 °C)

[8] [10,40,42–44] [45]/[46]

0.73(S),0.33 (L)


0.73(S,20 °C), 0.33 (L,140 °C) 0.73(S,20 °C), 0.33 (L,140 °C)



D-Threitol L-Threitol

(C4H10O4) HOCH2[CH(OH)]2CH2OH [54,55]


1–3 Butylene Glycol (1,3-Butanediol, C4H10O2)10 Diethylene glycol (C4H10O3) Pentitol Ribitol (C5H12O5) Xylitol C5H7(OH)5, (C5H12O5)


370, 352 ± 12/ 315–344 315/340/355



120 88.5–90



[40] [40] 10


94/94–95.5 96/95–97 90 101/103



Pentaerythritol (Tetramethylo1methane) (C5H12O4)

185–245 186–187

Allitol Dulcitol Sorbitol (D-Sorbitol, D-Glucitol)

C6H8 (OH)6

(C6H14O6) [54,55]

247 250//247

232 ± 1/246 232–263/258–270/ 263/240 263, 221/237 219 230 256

L-Glucitol D,L-




C6H8 (OH)6

165–168 166/166–168 167

[40] [29,42] [51]/[40]/[56] [40]

[49,56] [10,28,51,52,57] [9,58]/[57] [58,59] [51] [56]/[40] [40]

155 189 93/90–92⁄⁄ 95–98/95 ± 1.2 96–101 97/(97–98)/98 (97–99) 89–91


[9,40,52,53,28] [40]


10/10 to 7 100/102/104 61–61.5⁄⁄

(C5H12O5) [54]



93/93.5 93–94.5

Arabinitol (arabitol)



[40] [51]

110–185/165 ± 1 196–217

[40] [40] [40] [10]/[49] [51]

185 171/166

[9,28,40,52] [57] [40] [40]



294–341 335/316 316, ⁄261

[57] [10] [35,40]/[28] [9,28,38]
























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Table 2 Thermal properties of some polyols of PCM interest for TES, as found in literature.

Table 2 (continued) Melting Temp.  (C) (⁄⁄: metastable)

Catechol (1,2-benzenediol) (C6H6O2) Trimethylopropane (TMP, 2,2-Dihydroxymethyl1–1-Butanol) C2H5C(CH2OH)3 (C6H14O3) Triethylene glycol(C6H14O4) Galactitol C6H8 (OH)6 (Dulcitol) (C6H14O6)

104 57–59 7 167–185 188/188–189 129 129





Perseitol Volemitol Octitol

D-Erythro-D-galacto-octitol (C8H18O8) Thymol (2-Isopropyl-5-methylphenol),C10H14O [55] 1-Decanol CH3(CH2)9OH (C10H22O) 1-Dodecanol (C12H26O)

Heat of, fusion/⁄ melting (kJ/kg)

Thermal conductivity (W/(mK))



[52,31] [40] [29,42] [51] [9]/[10,28] [40] [40]

59 247 350–413 351/352

187 153 169–170

[40] [40] [40]

51.5 11/ 6

115 205/206

[31] [60]/[10]



145–152 94–105 146–152 145 38 49

173 135–149 170 205 141

[29,33,52]/ [52,61] [10,51,53] [51] [10,51,53] [51] [33] [31]









Maltitol (C12H24O11) Lactitol monohydrate (C12H24O11.H2O) Lactitol (C12H24O11) Palatinitol (C12H24O11 [54]) 1-Tetradecanol (C14H30O) Cetyl alcohol CH3(CH2)15OH, (C16H34O) Polyethylene Glycol (Polyglycol E, PEG)

Freezing Temp. (C)

800 1000 1450 3000 4000 4500 6000 10,000

28.5 33/35–40 52–56 58 55–60/58/66 55–60/61

14 30/38 45

⁄137 ⁄173/154

36 56



190/180/176 /⁄187/181

0.19 (L, at 39 °C, 70 °C) 0.19 (L, at 39 °C, 67 °C)

[29,33,52] [9,29,52]/ [29,31] [29,33,62,32]/ [40] [39] [39]/[9,40,16] [40] [9] [39] [40] [9,33]/[32]/[16] [9]/[39]/[16]

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As evident in Table 2, Erythritol, Xylitol and Sorbitol are identified with metastable states at 104 °C or 102–112 °C, 61–61.5 °C and 90–92 °C respectively, in addition to the stable melting points within the ranges according to the various sources: 112–120 °C, 93–97 °C and 93–101 °C respectively. A metastable state is a local minimum in the Gibbs free energy, however is not the lowest possible [41]; which corresponds to the stable equilibrium state. In overall, as evident from all the polyols found with thermal properties, a wide range of melting points of between 15 and 245 °C, and melting enthalpies between around 100 and 413 kJ/ kg (considering the lowest and the highest values reported) can be obtained. From the enthalpies available, several polyols; Erythritol, Pentaerythritol, D-Mannitol and Galactitol apparently have phase change enthalpies that are comparable even with the melting enthalpy of ice; 330 kJ/kg [9], at melting ranges of 117–120 °C, 185–245 °C, 163–168 °C, and 167–189 °C respectively. For quite a few of the polyols, the fusion enthalpies were not available in the literature, while for most of the polyols the thermal conductivities are also unaccounted in the open literature, all of which are important design parameters for a TES system design. In addition, volume expansion, viscosity and even specific toxicity details are also important design parameters that have still not been reported for most of these polyols found. Many of these also have data inconsistencies between various literature sources over the stated melting/freezing points and the enthalpies, with some very large variations observed for e.g., Pentaerythritol, Sorbitol, DMannitol, Galactitol, Maltitol, and PEG 6000.

fact can be considered as a subcooled liquid that is frozen in the glassy state [68]. Glass transition temperature of polyols reduces with the increasing water content and molecular weight [49]. Carbohydrates with high ratios of melting to glass transition temperature readily crystallize and have high enthalpy of fusion [49], which should also be true for polyols, for being carbohydrates. For most of these complex phase and state transitions of polyols, except for a few generalizations, comprehensive knowledge on pure components and blends of PCM TES design interest is not that available in open literature. Further, to study polyol blends, they should first-of-all be miscible in one another. Polyols are soluble in polar solvents (and insoluble in non-polar solvents) [69] although much details are unavailable on polyolpolyol solubility. 4. Results and discussion: T-history characterization of selected polyols The highlights from the thermal characterizations of the three selected polyols PEG 10,000, Erythritol, and Xylitol are detailed and discussed briefly in this section. Here as explained, S1 and S2 refer to the two samples of each of the polyol tested. Particularly in the temperature versus time (i.e., T-history) curves presented, the reference is indicated by R or R0 . A, in is the average temperature inside the experimental set-up containment while A, out is the temperature inside the climate chamber (outside the containment). 4.1. PEG 10,000

3.2. Phase and state change behaviors of Polyols A comprehensive phase change understanding of polyols is also crucial in TES systems design using polyols PCM. Many polyols undergo subcooling [9,10], exhibit 10–15% volume expansion during melting [10], and decompose under excessive temperatures [63,11]. For subcooling, some mitigation methods with chemical, mechanical and electrical approaches were reported that are claimed with some success, e.g. [64,65]. Interestingly, phase transition with subcooling (with irreversible recalescence4) could be less irreversible than a stable equilibrium, if the thermal conductivity ratio of solid and liquid phases is sufficiently below unity [66]. This is expected only with Biot numbers of intermediate values [66], and suggests better thermal cyclability. Polyols are aliphatic compounds that have two more hydroxyl groups per each molecule [8], where sugar alcohols are a sub-category of polyols [8]. The hydrogen bonding of these hydroxyl groups contribute to polyols common properties such like high boiling points, high viscosities and solubility in polar substances [8]. Many polyols are amorphous when anhydrous [67]. Amorphous materials undergo the state change glass transition if they are at a supercooled (subcooled) or supersaturated state [67,49], thus so would the amorphous polyols. However, these become crystalline with the addition of water and thus solidify, and this is referred to as being plasticized by water [67]. Some example polyols that are plasticized by water are amorphous Xylitol and Sorbitol [49]. Glass formation (or transition) is found common among the polyhydroxy susbtances [68] supposedly due to their strong hydrogen bonds, that likely prevent their crystallization [68]. Since polyols are also polyhydroxy substances, their hydrogen bonding possibly has a significant effect on the glass transition these exhibit. Glass transition is a state change that is undesirable for a PCM because the expected sharp phase change is replaced by a continuous cooling over a wide temperature range. A glassy material in 4 Recalescence is the temporary increase in temperature during the cooling of a metal due to a crystal structure change [75].

A sufficient amount of PEG 10,000 (to obtain two samples of the mentioned weight) was prepared by melting at 80 °C in an oven for about 5 h. This melt was then prepared into the two samples and subjected to three heating/cooling cycles in the T-history test, between temperature ramps of 80 and 35 °C (climate chamber temperature). Fig. 1 shows the sample and reference temperatures versus time upon, (a) heating, and (b) cooling. The melting points were found to be between around 60–66 °C, and the freezing points between around 51.5–55 °C, on average, as observable in Fig. 1, corresponding to the second heating/cooling cycle. The curves for S1 and S2 almost coincide as evident in Fig. 1. The other two melting and freezing cycles have displayed similar behavior to as in Fig. 1 and thus are not shown here. Comparing with the manufacturers’ data of a melting range between 61.3 and 62.9 °C [70], therefore a discrepancy exists in the samples’ observed melting range. A very little subcooling of around 1.7 °C was observed (on average). A moderate hysteresis of about 10 °C was evident between the melting and freezing. No thermally activated change was evident in PEG 10,000 after the three cycles. The enthalpy of fusion of PEG 10,000 is found to be 169 kJ/kg, considering the normalized average of the two samples, tested over three consecutive heating/cooling cycles. This is 8% lesser than the average of the enthalpy found in literature (within the range 181– 187 kJ/kg). The normalized average melting enthalpy was found to be 167 kJ/kg for the average melting range of 60–67.5 °C, and the normalized average freezing enthalpy was found to be 170 kJ/kg, within a freezing temperature range of 49–53.5 °C. The curves for the specific heat versus temperature of PEG 10,000 for the heating and cooling cycles conducted are shown in Fig. 2. The Cp is calculated over 0.5 °C temperature intervals using the normalized enthalpies obtained. The negative Cp observed (during freezing) is due to the subcooling of the material. The deviation observed in the enthalpy as compared to the literature may be due to the differences in the analyzed samples. To determine the reasons for the rather large hysteresis, further analysis of PEG 10,000 will be helpful, also including microstructural evaluations.

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Fig. 1. A selected Temperature-history of PEG 10,000 of the second (a) heating cycle, and (b) cooling cycle, with negligible subcooling (in (b)) and moderate hysteresis.

Fig. 2. The specific heat (Cp) curves for several selected melting and freezing cycles of PEG 10,000.

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4.2. Erythritol To conduct the characterization of Erythritol, observations from a previous pre-test were employed. In that, a significant subcooling of around 20 °C was evident before reaching the observed freezing plateau at around 104 °C (c.f. Fig. 3(b)). In addition, a thermally activated change of the samples; starting from a transparent liquid to a golden-brown was observed at the end of the melting-freezing processes (c.f. Fig. 3(a)). Also the manufacturer-stated melting range of 119–124 °C [71] was considered when determining the heating–cooling temperature margins for the current test. Therefore, for the current characterizations, the two samples of pure Erythritol (denoted as S1 and S2 later in the figures) were prepared by slow melting in an oven maintained at 127 °C for about 9 h. Then these, as shown in Fig. 4(a), were first subjected to heating/cooling ramps between 126–73 °C, and 128–74 °C (the climate chamber temperature outside the samples containment) for the first and second cycles. These temperatures were chosen to have

as less thermally activated change as possible, and to allow around 20 °C margin in cooling to accommodate subcooling. However, the melting of Erythritol; although was almost close to the manufacturers-stated range of around 119–124 °C, was not found to reach completion at this upper margin at the first two heating cycles, as evident in Fig. 4(a). This is assumed as most likely due to insufficient temperature gradient between the melting point and the ambient temperature. Thus no freezing plateau was observable in the first two consecutive cooling cycles. Thereafter, the preceding three heating/cooling cycles were set at a larger temperature margin of between 132 and 67 °C. The first complete melting was observed at the 3rd heating, between 118.5 and 120 °C as shown in Fig. 4(b). This is then followed by three freezing and melting cycles, shown in Figs. 4(b) and 5(a) and (b). Within these, freezing between around 106–105 °C with an average subcooling of around 23 °C for one of the two samples (S1) and 14 °C for the other sample (S2), and melting between around 106–108 °C for both samples are observed. The different starting

Fig. 3. (a) Phase change observations of Erythritol at a pre-test: the solid appears golden-brown, compared to the initial white crystals, while the glass test-tubes were found with cracks. (b) The temperature-history of Erythritol as observed at the pre-test, displaying subcooling of around 20 °C. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

Fig. 4. Erythritol T-history for the (a) 1st two heating/cooling cycles with incomplete melting, (b) 3rd heating/cooling and 4th heating cycles; with the first complete melting, and then a shift in the phase change to around 105–108 °C with considerable subcooling during freezing.

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Fig. 5. Erythritol T-history for the (a) 4th cooling and 5th heating cycles, (b) 5th cooling and 6th heating cycles; at each case with considerable subcooling and a modified phase change temperature between around 105–108 °C observable.

Fig. 6. Thermally activated change of Erythritol: initial transparent liquid has become brown, and solidifies into a brown solid compared to the initial white crystals. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

temperatures seen of the samples and the reference here are due to the fact that the reference was at ambient temperature and the samples at molten conditions (at higher temperatures) at the very beginning of the experiment. A noticeable difference in the degree of subcooling between the two samples appears during the first freezing, while it has subsided at the later cycles (c.f. Figs. 4(b) and 5). This difference between the two samples subcooling could be related to the differences in the availability of nucleation sites within the two samples. The subsided difference at the later cycles is possibly evidence of stabilization of the thermal behavior of the samples at later cycles, compared to the first freezing that occurred. The final material at the end of the experiments was found to have browned significantly as observable in Fig. 6, and appeared to be thicker (i.e., had less flowability) than the initial molten sample. Thus, it appears that Erythritol has thermally degraded or changed into this browned material, which could be an oxidation at elevated temperatures. The resultant material has a different

phase change temperature within around 105–108 °C, and a higher viscosity. Some mass (weight) changes of the samples were observed at the end of the cycles, and this was taken into consideration in the data analysis. These mass changes observed may also be related to the thermal and physical properties changes such like viscosity, volume expansion or even an increased vapor pressure of the final material due to the thermally activated change. The enthalpy of fusion of Erythritol found from the experimental results was found to be lower than that mentioned in literature. For the first melting, between the 117–122 °C, an average fusion enthalpy of 284 kJ/kg was obtained (considering the enthalpies of both samples), which is about 10–17% less than the literature values (314–344 kJ/kg) for the similar temperature ranges. Then for the second type of melting and the freezing observed, on average, the enthalpies of 255 kJ/kg, and 273 kJ/kg were obtained for the melting and freezing temperature ranges 105–111 °C and 100– 107 °C respectively. The overall fusion enthalpy thus obtained for this shifted phase change is 264 kJ/kg, and this is also 16% less than the literature value found (314 kJ/kg). The specific heat versus temperature curves of Erythritol for the heating and cooling cycles conducted are given in Fig. 7. The Cp is calculated over 1 °C temperature intervals using the normalized enthalpies obtained. Here again, the negative Cp observed (during freezing) is due to the subcooling of the material. The thermally activated change observed and the large degree of subcooling that has a significant effect on the Biot number of the set-up are suspected to have effect on these large enthalpy deviations. Therefore, for verifying the obtained enthalpy of fusion of Erythritol, further experimental verifications are expected to be done.

4.3. Xylitol A sufficient amount of Xylitol to obtain two samples of the desired weight was melted in an oven at 105 °C for about 5 h, and then prepared into the two samples. These two were characterized by the T-history method, subjected to a number of continuous heating/cooling cycles. Here also, the temperature ramps were decided based on a pre-test result conducted on Xylitol, and the manufacturer-stated melting point range 92– 96 °C [71] as was for Erythritol. At this pre-test, a first and the only

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Fig. 7. The specific heat (Cp) curves for the different melting and freezing cycles of Erythritol.

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Fig. 8. Temperature-history pre-test results of pure Xylitol: (a) the first heating cycle where phase change (melting) is observed around between 91 and 93 °C. (b) The heating and cooling cycles after the first heating and cooling cycles, with no phase change occurrence within a temperature range of 120 to around 2 °C. (S1 and S2 are two samples of Xylitol subjected to the same conditions).

Fig. 9. The temperature-history of Xylitol. After a first melting, continuous cooling-heating curves were observed for even lower temperature ramps than at 74 °C, i.e., for 20 °C and 0 °C (climate chamber temperature) respectively.

melting (which is rather similar to the literature-based expected melting temperatures) followed by probable glass-transition was observed (c.f. Fig. 8). The glass-transition was suspected to occur based on: the continuous heating and cooling curves observed for several consecutive heating cooling cycles even with extended temperature margins; and the material at the end of these cooling appearing/behaving like a rigid-transparent glass-like material (even below around 10 °C). Similar behavior was observed at the current T-history test as well, as shown in Fig. 9, with no apparent freezing or melting after the first melting, even when the low-temperature margin was lowered from 74 °C to 20 °C and then to 0 °C. The thickness (and therefore the viscosity) of the transparent melt was also observed to increase with decreasing temperature. This can be observed in Fig. 10(a), where the stainless-steel spatula remains almost vertical, stuck within the thick glassy Xylitol melt. This behavior is deduced to be glass-transition and not subcooling due to these changes such like the increase in thickness (less flowability), and the rigid, transparent glassy nature of the material at the end of each cooling. Once this glass transition was observed, there were no further identifiable phase change plateaus even during heating cycles.

Xylitol also was identified with some thermally activated change as indicated by a slight browning of the melt after the several thermal cycles it was subjected to, for the T-history measurements. This is seen in Fig. 10(b), where the two beakers contain the contents of the two test-tubes, browned after the four heating and three cooling cycles. The upper margin of the temperature of these cycles was 115 °C (chamber temperature). This indicates that Xylitol when subject to a temperature of about 10 °C above the expected melting point for an extended time period, the thermally activated change happens. The melting enthalpy obtained being merely 159 kJ/kg within the temperature range 88–96 °C as compared to the literaturebased enthalpies in the range 219–270 kJ/kg has a deviation of 35%. This large departure from the literature possibly is due to several factors; the glass transition process and its effects on the latent heat, and the thermally activated change. In addition, since the thermal conductivity of Xylitol was not found in literature, verifications on the Biot number were not done. Therefore, for better verification of the melting enthalpy of Xylitol also further experimental evaluations are expected to be done, including the thermal conductivity determination. The specific heat variations (calculated over 0.5 °C temperature intervals using the enthalpies

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Fig. 10. (a) Glassy Xylitol (b) the thermal decomposition of Xylitol after the thermal cycling for the T-history measurements, as indicated by the slight browning of the melt.

Fig. 11. The specific heat (Cp) curves for the first and only melting observed for Xylitol.

obtained) during the first and the only melting observed for Xylitol samples are also given in Fig. 11. 4.4. Discussion From the two different melting ranges identified for Erythritol, the lower range found between 105 and 108 °C, is comparable with the metastable state reported; 104 °C [45] and 102–112 °C [46]. Lopes Jesus et al. [45] have identified that Erythritol has two crystalline states at 104 °C and 117 °C. At 117 °C Erythritol undergoes a solid–solid phase transition, and at 104 °C if the cooling is rapid, transforms into an amorphous structure [45]. In addition, based on detailed microstructural evaluations, it was explained that the rapid cooling gives rise to an amorphous state while the slow cooling results in the stable state [45]. The metastable state was said to revert to the stable one if sufficient time is allowed [45]. Microstructural evaluations of the final browned material that is supposed as due to the thermally activated change in this work would therefore enable better comparison with the explanations by Lopes Jesus et al. [45]. These polyols microstructural evaluations could be on their chemical structural and covalent bonding details with e.g. Fourier transform infrared spectroscopy (FT-IR) or nuclear magnetic resonance (NMR), their degree of crystallinity and the particular crystalline structures by e.g. X-ray diffraction (XRD), or, even the morphological details by e.g. scanning electron microscopy (SEM) or transmission electron microscopy (TEM) [72].

Rathgeber et al. [46] have identified these two states to be polymorphs, also giving another similar example; D-Mannitol. This attribute of many polyols existing in two or more crystalline states; or in other words having polymorphs, was also verified by Solé et al. [73]. They have evaluated the cycling stability of several polyols as PCM. In that, Myo-inositol was identified with several polymorphs that could arise still with a rather acceptable stability of melting temperature and enthalpy, Galactitol with poor cycling stability with inconsistent melting temperature, and D-Mannitol to be undergoing degradation under oxygen-rich ambient conditions, and no degradation at conditions with no exposure to oxygen [73]. This behavior reported for D-Mannitol reflects the observations on Erythritol in this work, where the Erythritol was in continuous contact with a small amount of ambient air inside the test-tubes. By Solé et al. [73] also the degradation of D-Mannitol was observed with a browning, identified as an oxidation. Their suggestion of thermal cycling of these material in anoxic conditions may therefore be helpful in avoiding this degradation (thermally activated change as observed in this work) if it is an oxidation. In summary, various phase and state change behaviors; like subcooling and glass transition, differ even among pure polyols. For instance, Erythritol did not exhibit glass transition at the T-history characterizations done in this work. However, Lopes Jesus et al. [45] have identified an amorphous state of Erythritol giving rise to glassy material. Xylitol in contrast has exhibited a mostprobable glass transition behavior. This is verifiable with literature where polyols as being comprised of several hydroxyl groups, are identified as having increasing viscosity with decreasing temperatures, then forming subcooled (or supercooled) liquids and ultimately solidify in a glass-like state [8]. To understand this more prominent glass transition behavior in Xylitol compared to Erythriol, the comparison of the high hygroscopicity of Xylitol to the non-hygroscopic Erythritol [74] could be helpful. The thermally activated change of these two pure polyols Erythritol and Xylitol also needs to be understood very well, and hence the means to avoid it if these materials are to be used as PCM, which require long-term stability. The effect of oxygen in this degradation reaction also needs to be understood well. The design of the TES system within an oxygen-free inert ambience as per Solé et al. [73]’s suggestion would be one method of avoiding the degradation, if its influence can be confirmed. In contrast, PEG 10,000 exhibits proper phase change, and has negligible subcooling. Comparison of the chemical structures of these three materials is most-likely to offer better understanding on these complex and dissimilar behaviors.

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5. Concluding remarks As evident from the literature surveyed, the thermal and physical properties of polyols in the light of TES design as PCM have data gaps to be filled, for instance on the phase change enthalpy, thermal conductivity, volume expansion, viscosity and more, and even some inconsistencies within the data documented. However, evidently polyols have significant potential to be used as PCM candidates in extracting low-temperature thermal energy from industries and power plants. This is mainly owing to their rather moderate phase change temperatures found between around 15 to 245 °C, and the enthalpies in the range of 100–413 kJ/kg. Several polyols have phase change enthalpies that can compete with even ice, which indeed is an advantage. In overall polyols are generally an interesting material category even compared to other less expensive organics like paraffins and fatty acids. This is with the advantages of polyols over e.g. paraffins which are of non-renewable origin, and fatty acids which have more metal corrosion attributes. The T-history based characterization of some selected polyols conducted in this work indicates the complex phase and some state changes these exhibit in real applications. For instance, the significant subcooling observed in Erythritol, the thermally activated change observed in both Erythritol and Xylitol, and the glass-transition in Xylitol need to be understood well. A necessity exists in identifying means to eliminate these phenomena prior to selecting them as PCM. Subcooling; if large, is undesirable in a TES as then the controlling of the storage system is difficult, and the expected storage capacity is compromised. Glass-transition is also not preferable in a PCM as it replaces the expected sharp melting/ freezing and thus acting more like a sensible storage material, also therefore reducing the achievable thermal energy storage density. The thermally activated change observed in the materials, which appears to alter the initial materials’ thermal properties, is also detrimental for a PCM. This is since long-term stability of the PCM depends on the consistency of these various thermal properties used as design parameters, when used in a TES system. The number of thermal cycles in this current work are insufficient to draw conclusions on the three polyols’ thermal stability as PCM in a real TES system. However, preliminarily it is indicative that the glass transition of Xylitol will affect the functionality, and the thermally activated change of Erythritol could affect the durability, of a TES system, if not addressed properly. Still, both these polyols remain attractive for low-temperature applications for their melting temperatures and enthalpies. PEG 10,000 in contrast has a much less complex phase change, thus implying better durability in a real TES system. However when considering it as a PCM, the lower enthalpy of fusion it has and the hysteresis it displays need to be accounted. The solid–solid phase transitions of these polyols, their heattransfer rate dependence on the phase change where rapid rates giving rise to metastable states also need to be studied in detail, involving microstructural evaluations for better explanations of the behaviors. In addition, in-depth study of the thermally activated change observed in some of these materials, and the effect of factors like oxygen on this phenomena would assist its avoidance. Therefore, a comprehensive analysis of polyols on subcooling, glass transition, polymorphism and the thermally activated change also is crucial to evaluate their suitability as PCM, and the design of a TES system using them. This essentially also should include thermal cycling evaluations of these materials to determine the durability of the TES system. The other thermal and physical parameters essential for a TES system design such like the thermal conductivity, volume expansion, viscosity, and even the toxicity of these many polyols also need to be determined. To consider polyol


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Please cite this article in press as: Gunasekara SN et al. Polyols as phase change materials for surplus thermal energy storage. Appl Energy (2015), http:// dx.doi.org/10.1016/j.apenergy.2015.03.064