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Energy Procedia 61 ( 2014 ) 664 – 669

The 6th International Conference on Applied Energy – ICAE2014 Polyols as phase change materials for low-grade excess heat storage Saman Nimali Gunasekaraa*, Ruijun Pana, Justin Ningwei Chiua, and Viktoria Martina

aEnergy Technology- HPT, Royal Institute of Technology (KTH), Brinellvägen 68, 100 44 Stockholm, Sweden. Abstract Polyols are an emerging phase change materials (PCM) category for thermal energy storage (TES), with moderate phase change temperatures and considerable enthalpies. These can be employed in systems for harvesting surplus energy from, industrial and power generation processes. However, knowledge on the properties of polyols in relation to desirable PCM properties is presently sparse and rather inconsistent. This work summarizes a literature review on polyols as PCM for TES. In addition, preliminary T-History characterization of some selected polyols was done. This study is expected to be an initiation of an extensive polyol phase equilibrium evaluation.

© 2014© 2014 The GunasekaraAuthors. Published S.N., byPan Elsevier R., Chiu Ltd. NW. This J.,is anand open Martin access V. article Published under bythe ElsevierCC BY-NC-ND Ltd. license (http://creativecommons.org/licenses/by-nc-nd/3.0/Selection and/or peer-review under responsibility). of ICAE Peer-review under responsibility of the Organizing Committee of ICAE2014 Keywords: Phase change materials (PCM); Polyols; T-history method; Material properties

Nomenclature °C Degrees Celsius kJ/kg Kilojoule per kilogram PCM Phase change material PEG Polyethylene Glycol TES Thermal energy storage T-History Temperature History W/(m∙K) Watt per meter, kelvin 1. Introduction One important task for reaching a sustainable energy system is to realize resource efficiency. In this context, the utilization of surplus heat from industrial processes, including large scale power generation is important. This is done internally within a plant through process integration. To a smaller extent, the heat is utilized externally by e.g. connection to a district heating system [1], [2], [3]. However, the full potential of converting excess heat into a resource cannot be realized due to the mismatch in time, and location, between source and demand. Thermal energy storage using phase change materials (PCMs) is one effective solution to explore for managing this mismatch, now emerging with considerable momentum [4], [5], e.g., for, district heating [6] and heat exchange within industries [7]. In this work, polyols (poly /sugar alcohols) are evaluated as one particular material category of PCM interest, owing to their moderate phase change temperatures of around 0-200 °C [8] and higher mass-specific and volume-specific enthalpies than other organic PCMs [8]. Furthermore, many polyols are non-toxic [9] and safe

* Corresponding author. Tel.: Office: +46 (0)8 7907476, Mobile: +46 (0)736523339; E-mail address: [email protected].

1876-6102 © 2014 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/3.0/). Peer-review under responsibility of the Organizing Committee of ICAE2014 doi: 10.1016/j.egypro.2014.11.938 Saman Nimali Gunasekara et al. / Energy Procedia 61 ( 2014 ) 664 – 669 665

[10], [11], some even being common sugar substitutes; e.g. , , [11]. Their material compatibility is similar to paraffins [9], hence compatible with metals. In pure grades, polyols are comparatively expensive. Nevertheless, being produced via hydrogenation [8], [11], [12], they have a potential for large-scale production, which could be expected to reduce their prices. The overall objective of this paper is to present polyols in light of the desired material behavior needed in making use of these, pure and as blends as PCMs for storage of low grade excess heat in industrial applications. Literature findings and preliminary thermal characterizations of some selected polyols are used as input for the analysis. 2. Polyols as phase change materials for thermal energy storage PCMs including polyols, have great potential in many TES applications e.g., solar [13], buildings [14], vehicles [9], and more [15], [16]. A growing PCM interest in polyols is evident at present e.g., patents on pure and blends of polyols [17] and polyol-salt blends [10] as PCM TES, plus literature on evaluation of mixtures/composites: -fatty acid esters [18], Erythritol-porous materials [19], [20] and Polyethylene glycol-sugar [21]. However, the TES design data for polyol PCM still have gaps to be bridged, including for polyol blends. Table 1 compiles some thermal properties of pure polyols of PCM interest, from literature. Evidently many of these polyols properties, are in ranges, some inconsistent and, some with gaps particularly on thermal conductivity and fusion enthalpy. Table 1. Thermal properties of some polyols of PCM interest for TES, as found in literature Polyol Melting Temp. Freezin Heat of, fusion/* Thermal conductivity (W/(m·K)) References (˚C) g Temp. melting (kJ/kg) (**: metastable) (˚C) (Glycerin) CH2OH.CHOH.CH2OH (C3H8O3) ~18 ~18 ~199 ~0.14/ 0.28 (at 25°C) [9], [22], [23], [24], [25] Erythritol C4H6(OH)4, (C4H10O4) 117-118 315-344 ~0.73(S),0.33 (L) [9] ~118 ~315/~340/~355 ~0.73(S,20°C), ~0.33 (L,140°C) [26], [27], [20], [16], [28] 118-121/119 339/332-340 ~0.73(S,20°C), ~0.33 (L,140°C) [29]/ [30] Teritol 120 ~340 [8], [22], [31], [32], [13] D- (C4H10O4) 88.5 - 90 [22] L-Threitol HOCH2[CH(OH)]2 88.5 - 90 [22] D,L- Threitol CH2OH [33], [34] 69 -70 [22]

1-3 Butylene Glycol (1,3-Butanediol, C4H10O2) 10 [22] Diethylene glycol (C4H10O3) -10/-10 to -7 247 [14], [23] (C5H12O5) 100/ 102/ 104 250/ /247 [30]/ [22]/ [35] ** Xylitol C5H7 (OH)5, (C5H12O5) 61 – 61.5 [22] ~93/ 93.5 ~93-94.5 232±1/ ~246 [28], [35] 232–263/258-270/ [9], [13], [30], [31], [36] 94/ 94-95.5 ~263/ ~240 Penitol ~96/ 95-97 263, 221/~237 [8], [37]/ [36] ~219 [37], [38] Arabinitol () 90 230 [30] D-Arabitol (C5H12O5) [33] 101/ 103 ~256 [35]/ [22] L-Arabitol 102 -103 [22] Pentaerythritol (Tetramethylo1methane) 185-245 [22] (C5H12O4) 186-187 287-298 [30] Allitol 155 [22] Dulcitol 189 [22] 93/~90-92** [22] (D-Sorbitol, 95–~98/ 95±1.2 110-185/ 165±1 [9]/ [28] D-Glucitol) 96-101 196-217 [30] C6H8 (OH)6 97/~(97–98)/ 185 [8], [13], [22], [31] ~98 ~(97-99) ~171/~166 [36] L-Glucitol 89-91 [22] D,L- Glucitol 135 - 137 [22] D-Mannitol (C H O ) ~(163-165)/ ~270/~273 [36] Hexitol 6 14 6 C6H8 (OH)6 [33], [34] ~(164-166) 165-168 294-341 [9] ~166/ 166-168 ~335/ ~316 [18], [22]/ [13] ~167 117 316, *~261 [8], [13], [39] L- Mannitol 162 - 163 [22] D,L- Mannitol 168 [22] D-Talitol 88 -89 [22] L-Talitol 87 -88 [22] D,L-Talitol 95-96 [22] D- 73.5 [22] L-Iditol ~(76-77) [22]

Catechol (1,2-benzenediol) (C6H6O2) ~104 207 [31], [40] Trimethylopropane (TMP, 2,2-Dihydroxymethyl1-1- 57 - 59 59 [22] ) C2H5C(CH2OH)3 (C6H14O3) Triethylene glycol(C6H14O4) -7 247 [14], [23] C6H8 (OH)6 (Dulcitol) (C6H14O6) 167-185 350-413 [30] 188/ 188-189 351/ ~352 [8]/ [9], [13] Heptitol Glycero-gulo-heptitol 129 [22] D-glycero-D-ido-heptitol 129 [22] (C H O ) Perseitol 7 16 7 187 [22] Volemitol 153 [22] Octitol D-Erythro-D-galacto-octitol (C8H18O8) 169-170 [22] Thymol (2-Isopropyl-5-méthylphénol,C10H14O [34] 51.5 115 [40] 666 Saman Nimali Gunasekara et al. / Energy Procedia 61 ( 2014 ) 664 – 669

1-Decanol CH3(CH2)9OH (C10H22O) 11/ 6 205/ 206 [41]/ [9] 1- (C12H26O) 26 200 [14], [16], [31] 17.5-~23 ~189 [31], [42] (C12H24O11) 145-152 173 [9], [30], [32] Lactitol monohydrate (C12H24O11.H2O) 94-105 [30] Lactitol (C12H24O11) 146-152 135–149 [9], [30], [32] Palatinitol (C12H24O11 [33]) 145 170 [30] 1-Tetradecanol (C14H30O) 38 205 [16] Cetyl CH3(CH2)15OH, (C16H34O) ~49 141 [40] 400 8 ~100 ~0.19 (L, at ~39°C, 70°C) [14], [16], [31] 600 17-22 127 ~0.19 (L, at ~39°C, ~67°C ) [8], [14], [31] 20-25 146 ~0.19 (L, at ~39°C, ~67°C ) [14], [40] 22/~23 ~127 ~0.19 (L, at ~39°C, ~67°C ) [14], [16], [43], [44]/ [22] 800 28.5 ~14 *~137 [45] 1000 ~33/35-40 ~30/~38 *~173/154 [45]/ [8], [22], [21] Polyethylene Glycol (PEG) 1450 45 [22] 3000 52-56 [8] 4000 ~58 ~36 *~187 [45] 4500 ~56 [22] 6000 55-60/ ~58/ 66 190/ ~180/ 176 [8], [16]/ [44]/ [21] 10000 55-60/~61 /49.5 /*~187/181 [8]/ [45]/ [21] In addition to melting, freezing enthalpies (in kJ/kg) were available in literature for D-Mannitol: ~214 [39] and for PEG types 800, 1000, 4000 and 10000 of around: 130, 154, 152 and 142 [45] respectively. 1,2,4-Butanetriol (C4H10O4) is said to supercool [22], but with no temperature/ enthalpy indications. A comprehensive phase change understanding of polyols also is crucial in TES systems design using polyols PCM. Many polyols undergo some subcooling [8], [9], exhibit 10-15% volume expansion during melting [9], and decompose under excessive temperatures [46], [10]. For subcooling, some mitigation measures were reported; e.g. [47], [48]. Interestingly, phase transition with subcooling (with irreversible recalescence) could be less irreversible than a stable equilibrium, if the thermal conductivity ratio of solid and liquid phases is sufficiently below unity [49]. This is expected only with Biot numbers of intermediate values [49], and suggests better thermal cyclability. Many polyols are amorphous when anhydrous; undergoing glass transition. But they are plasticized by water, then becoming crystalline and thus solidify [50] (e.g. amorphous Xylitol and Sorbitol [28]). Glass transition is a state change, undesirable for a PCM because the expected sharp phase change is replaced by a continuous cooling over a wide temperature range. Glass transition temperature of polyols reduces with increasing water content and molecular weight [28]. Carbohydrates with high ratios of melting to glass transition temperature readily crystallize and have high enthalpy of fusion [28], 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 [51] although much details are unavailable on polyol- polyol solubility. 3. Preliminary T-history characterization of selected polyols Preliminary T-History characterization similar to [52] was done for selected polyols. The temperature histories of 99% pure Erythritol, Xylitol and synthesis grade PEG 10,000 were obtained, using two identical polyol samples and a reference (thermal Si oil) in horizontally placed glass test-tubes inside stone-wool insulated boxes, thermally cycled in an ACS Hygross 1200 climate chamber. Thermal sensors were of T-type thermocouples. Erythritol, Xylitol and PEG 10,000 exhibited considerable subcooling, probable glass transition after the first melting, and very little subcooling, respectively. These are briefly discussed with temperature histories in the Appendix for, Erythritol: Figure 1, Xylitol: Figure 2, Figure 3, and PEG: Figure 4, Figure 5, respectively. Glass transition was deduced due to an apparent smooth cooling curve replacing the expected phase change plateau, and a transparent liquid getting thicker with cooling, and ultimately becoming a rigid, transparent, glass-like material at the end of the cooling. This is not merely subcooling, judging by the appearance and thickness increase of the material, and the extended cooling range. 4. Discussion Various phase and state change behaviors like subcooling and glass transition differ even among pure polyols. For instance, Erythritol is often not reported with, and was not exhibiting glass transition, while Xylitol exhibits probable glass transition. Glass transition could possibly be related with the high hygroscopicity of Xylitol, whereas Erythritol is not hygroscopic. In contrast, PEG 10,000 exhibits proper phase change, and has negligible subcooling. Comparison of the chemical structure of PEG 10,000 to the others’ may provide a better understanding on these differences. 5. Concluding Remarks Saman Nimali Gunasekara et al. / Energy Procedia 61 ( 2014 ) 664 – 669 667

The literature survey shows that there are gaps, and even some inconsistencies on thermo-physical property data of polyols as PCM for TES design. Preliminary T-History based characterization of some selected polyols has been conducted in this work. A detailed analysis of polyols on subcooling, glass transition and like behaviors also is crucial to evaluate their suitability as PCM. To consider polyol blends as PCM, miscibility of polyols should be evaluated, followed by thermal property characterization and phase equilibrium evaluation of these blends. 6. Acknowledgements The authors extend their gratitude to the Swedish Energy Agency for funding this project. 7. References

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8. Biography

All the authors are from Energy Technology (EGI), KTH- Royal Institute of Technology. S. N. Gunasekara is a PhD candidate, with M.Sc. Sustainable Technology. R. Pan is a Research Engineer, with M.Sc. Sustainable Energy Engineering. Dr. J. NW. Chiu is a Research Associate. Dr. V. Martin is an Associate Professor.

9. Appendix

T-History, Pure Erythritol 140

130

120 C) 110 104,28 100 S1 90 S2 84,42 80 Temperature, ( ° 70

60 900 1100 1300 1500 Time (minutes)

Figure 1. Temperature-History for pure Erythritol: exhibiting subcooling during freezing. (S1 and S2 are two samples of Erythritol subjected to same conditions). Here a subcooling of around 20 °C was observed.

T-History, pure Xylitol, first heating cycle 120

100 93,3726 C)

° 91,0261 80

60 S1

40 S2 Temperature ( 20

0 0 400 800 1200 1600 Time (min)

Figure 2. Temperature-History of pure Xylitol: first heating cycle where phase change (melting) is observed around between 91-93 °C, which are rather similar in range to the literature-based expected melting temperatures. (S1 and S2 are two samples of Xylitol subjected to same conditions)

Figure 3 shows the temperature-history of pure Xylitol, heating and cooling cycles after the first heating/cooling cycle. During cooling, the material appeared as a transparent glass like material, with increasing viscosity (less flowability) with decreasing temperature, which was rigid when below around -10 °C. These characteristics are attributed as probable glass-transition. Also, 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.

Saman Nimali Gunasekara et al. / Energy Procedia 61 ( 2014 ) 664 – 669 669

T-History pure Xylitol, after the first heating and melting cycle 120

100

C) ° 80

60 S1

40 S2

20 Temperature (

0 0 1000 2000 3000 Time (Minutes)

Figure 3. Temperature-History of pure Xylitol, heating and cooling cycles after the first heating/cooling cycle where no phase change occurs within the temperature range of 120 to ~2 °C. (S1 and S2 are two samples of Xylitol subjected to same conditions)

T-History, pure PEG 10,000: heating 80 75 70 67,90

65 C)

° 60 60,35 55 50 45 S1 40 S2 35 Temperature ( 30 25 20 1750 1950 2150 2350 Time (Min)

Figure 4. Temperature- History of pure PEG 10,000 during heating. (S1 and S2 are two samples of PEG 10,000 subjected to same conditions). Exhibits melting at a temperature range of around 60-68 °C, a value rather close to the expected literature-based melting temperatures.

T-History, pure PEG 10,000: cooling 80 75 70 65

C) 60 ° 55 50 S1 45 S2 40 35 30 Temperature ( 25 20 2600 2800 3000 3200 Time (Min)

Figure 5 Temperature-History of pure PEG 10,000, for cooling. Freezing occurs with only very little subcooling (of around 3 °C, circled in red), around the expected melting temperature.