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Galactitol as phase change material for latent heat storage of solar cookers: Investigating thermal behavior in bulk cycling
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ScienceDirect Solar Energy 119 (2015) 415–421 www.elsevier.com/locate/solener
Galactitol as phase change material for latent heat storage of solar cookers: Investigating thermal behavior in bulk cycling Geoffrey John a, Andreas Ko¨nig-Haagen b, Cecil K. King’ondu a, Dieter Bru¨ggemann b, Lameck Nkhonjera a,⇑ a
Nelson Mandela African Institution of Science and Technology, Department of Materials and Energy Science and Engineering, P.O. Box 447, Arusha, Tanzania b Universita¨t Bayreuth, Zentrum fu¨r Energietechnik, Lehrstuhl fu¨r Technische Thermodynamik und Transportprozesse, Bayreuth, Germany Received 23 April 2015; received in revised form 30 June 2015; accepted 1 July 2015
Communicated by: Associate Editor Luisa F. Cabeza
Abstract Galactitol, in terms of its phase change enthalpy and temperature, is a promising phase change material (PCM) for medium temperature (150–200 °C) latent heat storage of solar cookers. This study aimed at determining the effect of upper cycle temperature on thermal behavior of galactitol in bulk thermal cycling. Three bulk samples were repetitively melted and frozen with each sample having fixed upper cycle temperature different from the others. Temperature histories of the samples were recorded whereas phase change enthalpies and specific heat capacities were obtained by differential scanning calorimetry. Thermal diffusivities of fresh galactitol within a range of 20–240 °C were determined by a flash diffusivity instrument. The results show that the upper cycle temperature has a great influence on the attainable number of melting and freezing cycles, the degree of subcooling, the rate of change of degree of subcooling as well as the phase change enthalpy and temperature. The upper cycle temperatures above but close to the melting temperature are favorable. The lowest upper cycle temperature was around 200 °C and yielded about 90 thermal cycles feasible for solar cooking at temperatures greater than 150 °C. Therefore, galactitol as a PCM in thermal energy storage of solar cookers that are thermally cycled at least once a day, can afford a lifespan of less than 100 days, which is far lower than lifespans of the other parts of the cooker system. Galactitol was thus found to be unstable and with a too short lifespan for practical application as PCM for medium temperature thermal energy storage purposes. Ó 2015 Elsevier Ltd. All rights reserved.
Keywords: Galactitol; Subcooling; Latent heat storage; Solar cookers
1. Introduction The state of the art of solar cookers, albeit utilizing a greener and sustainable energy source, requires further development for their performance to be comparable to conventional cooking stoves. To allow for off-sun cooking, solar cookers incorporated with sensible heat storage system for example in Esen (2004), Schwarzer and Vieira da ⇑ Corresponding author. Tel.: +255 272 555070; fax: +255 272 920016.
E-mail address: [email protected] (L. Nkhonjera). http://dx.doi.org/10.1016/j.solener.2015.07.003 0038-092X/Ó 2015 Elsevier Ltd. All rights reserved.
Silva (2003) or latent heat storage systems like in Mussard et al. (2013) have been proven to work. However, latent heat storage systems are preferred because of their high energy density. On the other hand, optimizations of the geometry and heat transfer characteristics of latent heat storage systems for various applications have been studied (Esen and Ayhan, 1996; Esen et al., 1998; Fukai et al., 2003; Hamada et al., 2003; Horbaniuc et al., 1999; Lacroix, 1993; Velraj et al., 1999; Zivkovic and Fujii, 2001). Nevertheless, it appears that very few out of the available phase change materials (PCM) (Agyenim
416
G. John et al. / Solar Energy 119 (2015) 415–421
et al., 2010; Cabeza et al., 2011; Farid et al., 2004; Zalba et al., 2003) have been explored for solar cooking application. Out of those PCM utilized in latent heat storage systems of solar cookers (Cuce and Cuce, 2013; Muthusivagami et al., 2010; Sharma et al., 2009), most have phase change temperatures (Tm) below 120 °C and thus the stored latent heat is released for cooking at temperatures below 120 °C. Cooking at these temperatures takes a long time and is inapplicable for frying where temperatures between 150 and 190 °C are involved (Choe and Min, 2007). Therefore, storing solar thermal energy in latent form at 150–200 °C and rapidly retrieving it for cooking could reduce cooking time and enable frying. According to Mehling and Cabeza (2008), nitrate salts and sugar alcohols are suitable PCM for latent heat storage in the 150–200 °C range. On the contrary, nitrate salts with Tm between 150 and 200 °C and phase change enthalpy (Dph) greater than 150 J/g are so hygroscopic that their technical application as PCM is difficult (Waschull et al., 2009). On the other hand, Waschull et al. (2009) list D-mannitol, galactitol and pentaerythritol as sugar alcohols with Tm within 150–200 °C and having substantial Dph for practical application. Nevertheless, D-mannitol has little practical application because of its high degree of subcooling (Waschull et al., 2009) and when operated above 175 °C its thermal behavior is negatively affected by polymorphism (Barreneche et al., 2013) whereas values of its Tm and Dph rapidly decrease with increasing thermal cycles (Sole´ et al., 2014). On the other hand, pentaerythritol, in contrast to galactitol, has lower Dph (Benson et al., 1986) and slow crystallization rate (Hu et al., 2014) but unlike galactitol, it has previously been employed in the heat exchanger for solar cooking (Bushnell and Sohi, 1992; Bushnell, 1988). Sole´ et al. (2014) investigated the cycling stability of galactitol at milligram level using differential scanning
calorimetry (DSC). However, DSC samples may not be representative of large quantities as they would be present in heat storage of solar cookers and, in addition, subcooling of PCM is strongest in small samples (Gu¨nther et al., 2009). Thus, the cycling stability and the degree of subcooling of galactitol in bulk cycling are not yet understood. Besides, the previous studies have not brought out the effect of upper cycle temperature on the cycling stability of galactitol. Therefore, the aim of this work is to investigate the thermal behavior of galactitol when cycled in bulk. Specifically the study seeks to investigate the effect of upper cycle temperature on cycling stability and the degree of subcooling, determine the degree of subcooling, and examine its thermal stability as well as the dependence of its heat capacity and thermal diffusivity on temperature. 2. Methodology Galactitol (CAS: 608-66-2) with 97% purity was purchased from Alfa Aesar. Galactitol samples were repetitively melted and frozen at different plate temperatures using a setup shown in Fig. 1, which was placed in a controlled airflow chamber. Three 30.0 g samples placed in a closed container with limited air were heated for 15 min by hotplate with plate temperature set at 275, 300 and 375 °C for samples 1, 2 and 3, respectively. Averaging the upper temperatures in each cycle over the number of thermal cycles gave the upper cycle temperature for each sample. However, to account for random errors that occurred in capturing the upper temperature in each cycle, the upper cycle temperature is reported as the mean ± standard deviation. After the heating cycle, each sample was then cooled for 45 min through a forced convection induced by an electric fan. A two-channel Voltcraft temperature data logger with K-type thermocouples monitored both the sample
4
1
2
3
5
6
7
Fig. 1. Experimental setup for the bulk cycling. (1) Galactitol sample, (2) temperature data logger, (3) hotplate, (4) electric fan, (5) thermocouple (K-type), (6) timer switch (fan), and (7) timer switch (heater).
G. John et al. / Solar Energy 119 (2015) 415–421
and ambient temperatures at intervals of five seconds. For each sample, temperatures were monitored at a position with (r1/2, t1/2) coordinates where r1/2 and t1/2 are half the radius of the sample container and half the thickness of the sample respectively. The programmed timer switches controlled the heating and cooling times. The Dph and Tm for a fresh sample and samples extracted at 20 cycles intervals were measured by a NETZSCH DSC200F3. The measurements were conducted in dynamic mode with a heating and cooling rate of 0.5 °C/min and samples of about 10.0 mg placed in aluminum crucible with pieced lid. On the other hand, thermal stability of galactitol was analyzed from mass loss data obtained from SEIKO TG/DTA 300 thermogravimetry in nitrogen atmosphere mode. The heating rate was 2 °C/min and temperatures were held constant for 2 h at 120 °C and for a subsequent 10.0 °C step until 220°C. The NETZSCH LFA447 Flash Diffusivity Instrument measured thermal diffusivity of galactitol at 20.0, 50.0, 100, 150, 170, 180, 190, 200, 210, 225, and 240 °C. At every temperature point, five measurements were taken and their average represented thermal diffusivity. Therefore, thermal diffusivities were presented with error bars when plotted and their in text values were reported as mean ± standard deviation. To take care of the uncertainties of the measuring instrument in reproducing the measured value, measurement errors are implicitly given in the number of significant figures of the reported measured value. The Voltcraft temperature data logger offered the least precision (0.005 °C) amongst all measuring instruments and thus 3 significant figures are used to report all in text values of the measured quantities. The systematic errors of the measurements were, however, minimized by calibrating the measuring instruments with the standards.
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3. Results and discussion This work gives results on the effect of upper cycle temperature (Tup) on the rate of change of the phase change enthalpy and temperature per cycle, the number of thermal cycles with noticeable phase change enthalpy and the degree of subcooling of galactitol when cycled in bulk. In addition, results on thermal stability, thermal diffusivity and heat capacity of galactitol as functions of temperature were obtained. Heating galactitol samples at the set plate temperatures of 275, 300 and 350 °C for 15 min afforded Tup of 203 ± 3.00, 230 ± 2.00 and 243 ± 3.00 °C, respectively, for the sample. Upon forced cooling for 45 min, each sample cooled to 50.2 ± 4.8 °C. As shown in Table 1, fresh galactitol is a white-grained powder that oxidized and changed structure upon repetitive heating and cooling in a limited amount of air. This is in agreement with the findings of Sole´ et al. (2014). However, as the upper cycle temperature and number of cycles increased, the color of a solidified sample changed from white to brown whereas its texture changed from rough and hard to smooth and flaccid. The samples cycled at Tup of 203 ± 3.00, 230 ± 2.00 and 243 ± 3.00 °C correspondingly appeared dark and flaccid after 220, 40 and 20 cycles. This indicates that the oxidation and the structural change of galactitol increased with both the upper cycle temperature (Tup) and the number of cycles. The phase change enthalpies and temperatures of the three samples when plotted against thermal cycles are shown in Fig. 2 from which it can be read that a fresh galactitol sample melted at 189 °C, which is within the 187–191 °C melting range reported in the data sheet of the manufacturer. Therefore, the Tup of 203 ± 3.00,
Table 1 Appearance of three galactitol samples cycled with upper cycle temperature (Tup) of 203, 230 and 243 °C per sample. Tup (°C)
Number of cycles 0
203
230
243
20
40
60
80
180
220
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Temperature/ oC
Enthalpy/ J.g
-1
400 melting (203 oC Tup)
200
solidification (203 oC T ) up
0
o
1
20
40
60
80
100
melting (230 C T ) up
120
solidification (230 oC T ) up
190
melting (243 oC Tup)
160 130
solidification(243 oC T ) up
100
1
20
40
60
80
100
120
Thermal cycles
Fig. 2. DSC phase change enthalpies and temperatures for samples taken at first and after every 20 cycles from three galactitol samples that were cycled with upper cycle temperature (Tup) of 203, 230 and 243 °C respectively.
Temperature / oC
260 240
T
220
T is the highest observed temperature during s the growing of crystals
200
b
is the temperature at which nucleation started
t is the time during which nucleation occured and thus d period durinch which latent heat was relaesed
180
T
160 T
140
s
degree of subcooling = Ts - Tb
b
120
t
d
100 80 60
0
5
10
15
20
25
30
35
40
time /min
Fig. 3. Temperature history showing subcooling and duration of crystal growth as a 30.0 g fresh galactitol sample cooled from molten to solid state.
230 ± 2.00 and 243 ± 3.00 °C ensured that each sample completely melted. Further, from Fig. 2, it was deduced that phase change enthalpies and temperatures decreased in value when the number of thermal cycles increased. However, Tup influenced the rate at which the values decreased. Both during melting and solidification, the rates of decrease of enthalpy and temperature per cycle were correspondingly slow, intermediate and fast for Tup of 203 ± 3.00, 230 ± 2.00 and 243 ± 3.00 °C. In addition, Tup had an effect on the number of thermal cycles with noticeable phase change enthalpies. The sample that was cycled at Tup of 203 ± 3.00 °C yielded the highest number of thermal cycles and that at Tup of 243 ± 3.00 °C yielded the least. It is therefore inferred that moving Tup closer to but above the melting temperature of galactitol reduces the rate of change of phase change enthalpies and temperatures per cycle and improves the number of thermal cycles and hence cycling stability. It can also be seen from Fig. 2 that at any thermal cycle, the values of melting enthalpy and temperature differ from the solidification counterparts. The difference could be attributed to material hysteresis and subcooling. The temperature history of a fresh bulk sample when cooled from 240 to around 60.0 °C showed that galactitol subcooled
with a degree of subcooling as high as 31.0 °C. The degree of subcooling is the difference between the highest observed temperature (Ts) during the growing of the crystals and the base temperature (Tb) to which the molten sample cooled and started nucleating as illustrated in Fig. 3. However, the subcooling degree found in this work is far lower than 72.0 °C reported in the work of Sarı et al. (2011). The discrepancy is probably due to different measurement methods. The latter was calculated from the difference between DSC melting and solidification temperatures, yet in DSC when the sample temperature reaches Tb the recorded temperature does not heat up to Ts rather it continues decreasing as per temperature program. As a result, the solidification temperature obtained by DSC is lower than the actual one. For instance, Fig. 3 shows that Ts obtained from temperature history of bulk galactitol sample is about 170 °C while for the DSC it is about 125 °C as shown in Fig. 2. In addition, Gu¨nther et al. (2009) hinted that the degree of subcooling of the milligram level samples encountered in DSC is stronger than that of a larger sample. This shows the advantage of measuring the degree of subcooling by probing temperatures of a bulk sample as it cools from molten to solid state. Furthermore, time td in Fig. 3 is the duration at which crystals were grown.
G. John et al. / Solar Energy 119 (2015) 415–421
419
T
s1
200
Ts2
o
Temperature/ C
Ts3 degree of subcooling
150
degree of subcooling degree of subcooling
100
1 2 3
Ts is the highest observed temperature during the growing of crystals in each cyle whereas subscripts 1, 2 and 3 stands for samples cycled to Tup of 203,
50
230 and 243 oC respectively.
0
0
50
100
150
200
250
300
Thermal cycles
Fig. 4. The variation of the highest observed temperature (Ts) during the growing of crystals as well as the degree of subcooling with the number of thermal cycles for samples that were cycled with upper cycle temperature (Tup) of 203, 230 and 243 °C. In each sample, the number of thermal cycles were terminated on the last cycle where noticeable td duration of the growing of crystals was observed.
The analysis of several cooling curves showed that td decreased with increasing thermal cycles to the point that it was negligible. Since td corresponds to the duration of crystal growth then in all cycles where td was negligible, the sample was considered to have a trivial number of crystals grown. The plot of the degree of subcooling (Ts Tb) against the number of thermal cycles in Fig. 4 depicts that as the number of thermal cycles increased from zero, the degree of subcooling increased to a peak value then decreased and approached zero in the cycles with negligible crystal growth. The rates of change of subcooling degree were however influenced by Tup such that the rates were correspondingly slow, intermediate and fast for Tup of 203 ± 3.00, 230 ± 2.00 and 243 ± 3.00 °C. Likewise, the degree of subcooling increased at any particular cycle as Tup moved from low to high values. This suggests that lowering Tup slightly improves the degree of subcooling. Fig. 4 also shows that Tup increased the rate of change of Ts after the first 7 and 30 cycles for samples heated to 243 ± 3.00 and 230 ± 2.00 °C respectively. This indicates that Tup had an influence on the rate of change of Ts per cycle only after some thermal cycles; the rate is slowed when Tup is lowered. In general, Ts decreased in value when the number of thermal cycles increased. The decreasing values of Ts between thermal cycles entail that in practical application the stored latent heat would be retrieved at different temperatures in each cooling cycle. Consequently, in medium temperature (150–200 °C) TES for solar cookers, the feasible cycles are the ones at which Ts is above 150 °C. From Fig. 4, samples heated to Tup of 203 ± 3.00, 230 ± 2.00, 243 ± 3.00 °C respectively yielded 89, 35 and 14 feasible cycles. This represents an 84% increase in number of feasible thermal cycles when Tup is around 15.0 °C as opposed to 55.0 °C above the melting temperature. Therefore, cycling galactitol at Tup above but close to the melting temperature prolongs the feasible thermal cycles. For the sample cycled with Tup of 203 ± 3.00 °C, its initial solidification enthalpy, melting temperature and
solidification temperature respectively dropped by 28, 3, and 13% after the 89 feasible cycles. This denotes that in terms of solidification enthalpy galactitol is not a stable PCM in TES for solar cookers. It is nonetheless more stable in relation to melting than solidification temperatures. On the other hand, degrees of subcooling for the 89 feasible cycles fell within 25.0–37.0 °C range, see Fig. 4. Taking the first cycle for example, the galactitol sample subcooled with a degree of 25.0 °C which when multiplied by mean specific heat capacity for the interval within which the sample subcooled gives 45 J/g. This heat raised temperature of 1 g of the sample from Tb to Ts. If the galactitol had not subcooled, this heat could be available for discharge at high temperature. From this, it is deduced that galactitol subcools to a degree that substantially reduces the amount of latent heat that would be available for discharge at high temperature and hence it would be less efficient if employed as PCM for TES. Thermal stability results show that galactitol is not stable at temperatures above 200 °C. Fig. 5 depicts that at temperatures up to 170 °C, the mass loss was negligible and between 180 and 190 °C (within which melting occurs) the mass loss was less than 1%. On the other hand, holding the galactitol sample at any temperature above 200 °C for 2 h resulted into a mass loss greater than 2%. This suggests that in order to preserve thermal integrity, galactitol should not be kept at temperatures above 200 °C for long hours. In this regard, cycling galactitol with Tup above melting temperature but less than 200 °C does not only improve cycling stability but also safeguards thermal stability. Concerning the physical properties of galactitol, Fig. 6 shows that thermal diffusivity decreased from 0.960 ± 0.009 mm2/s at 20 °C to 0.310 ± 0.004 mm2/s at 200 °C. Conversely, Fig. 6 shows that the heat capacity was almost steady before and after 180–190 °C melting range. Since thermal diffusivity is the ratio of thermal conductivity to the product of density and heat capacity, and considering that the heat capacity and density of solid galactitol slightly varies from room temperature to about
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G. John et al. / Solar Energy 119 (2015) 415–421 100.5 100 99.5
Mass %
99 98.5 98 97.5 97 96.5 96 20
40
60
80
100
120
140
160
180
200
220
Temperature/ oC
Fig. 5. Thermogravimetry mass percent of galactitol in nitrogen atmosphere.
Thermal diffussivity/ mm 2.s-1
Thermal diffussivity Heat capacity
0.8
120
0.6
90
0.4
60
0.2
30
Heat capacity/ J.(g.oC)-1
150
1
0
0 0
20
40
60
80
100
120
140
160
180
200
220
240
o
Temperature/ C
Fig. 6. Thermal diffusivity and heat capacity of galactitol as functions of temperature.
180 °C, then the decreasing thermal diffusivity is directly related to decreasing thermal conductivity. The decreasing thermal conductivity would therefore retard heat retrieval from the storage to cooking device if heat transfer enhancement is not employed. 4. Conclusions From the analysis of results obtained in this work, three conclusions are drawn. 1. The upper cycle temperature of bulk galactitol that is repetitively melted and frozen in a limited air environment has great influence on the rate of structural change. This rate is slowed when the upper cycle temperature is lowered. In turn, the structural change influences the number of feasible thermal cycles, the degree of subcooling and the rates at which the degree of subcooling, the phase change enthalpy and temperature change occur. The upper cycle temperatures above but close to the melting temperature are favorable. 2. Bulk galactitol has a degree of subcooling that falls within a 25.0–40.0 °C range and therefore substantially reduces the amount of latent heat discharged.
3. Galactitol is thermally stable at temperatures up to 200 °C. Its thermal diffusivity decreases drastically with temperature. The variation in heat capacity with temperature is small both before and after the melting temperature range but relatively higher in molten than in solid state. As part of ongoing research, a study on pentaerythritol as PCM in medium temperature (150–200 °C) thermal energy storage of solar cookers is underway. Acknowledgements The authors acknowledge the financial and material support from DAAD, University of Bayreuth and Nelson Mandela African Institution of Science and Technology (NM-AIST) as well as the technical support from Anja Lorenz, Bernd Gassenfeit, Theresa Mangartz and Michel Krannich. References Agyenim, F., Hewitt, N., Eames, P., Smyth, M., 2010. A review of materials, heat transfer and phase change problem formulation for latent heat thermal energy storage systems (LHTESS). Renew. Sustain. Energy Rev. 14, 615–628.
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ScienceDirect Solar Energy 119 (2015) 415–421 www.elsevier.com/locate/solener
Galactitol as phase change material for latent heat storage of solar cookers: Investigating thermal behavior in bulk cycling Geoffrey John a, Andreas Ko¨nig-Haagen b, Cecil K. King’ondu a, Dieter Bru¨ggemann b, Lameck Nkhonjera a,⇑ a
Nelson Mandela African Institution of Science and Technology, Department of Materials and Energy Science and Engineering, P.O. Box 447, Arusha, Tanzania b Universita¨t Bayreuth, Zentrum fu¨r Energietechnik, Lehrstuhl fu¨r Technische Thermodynamik und Transportprozesse, Bayreuth, Germany Received 23 April 2015; received in revised form 30 June 2015; accepted 1 July 2015
Communicated by: Associate Editor Luisa F. Cabeza
Abstract Galactitol, in terms of its phase change enthalpy and temperature, is a promising phase change material (PCM) for medium temperature (150–200 °C) latent heat storage of solar cookers. This study aimed at determining the effect of upper cycle temperature on thermal behavior of galactitol in bulk thermal cycling. Three bulk samples were repetitively melted and frozen with each sample having fixed upper cycle temperature different from the others. Temperature histories of the samples were recorded whereas phase change enthalpies and specific heat capacities were obtained by differential scanning calorimetry. Thermal diffusivities of fresh galactitol within a range of 20–240 °C were determined by a flash diffusivity instrument. The results show that the upper cycle temperature has a great influence on the attainable number of melting and freezing cycles, the degree of subcooling, the rate of change of degree of subcooling as well as the phase change enthalpy and temperature. The upper cycle temperatures above but close to the melting temperature are favorable. The lowest upper cycle temperature was around 200 °C and yielded about 90 thermal cycles feasible for solar cooking at temperatures greater than 150 °C. Therefore, galactitol as a PCM in thermal energy storage of solar cookers that are thermally cycled at least once a day, can afford a lifespan of less than 100 days, which is far lower than lifespans of the other parts of the cooker system. Galactitol was thus found to be unstable and with a too short lifespan for practical application as PCM for medium temperature thermal energy storage purposes. Ó 2015 Elsevier Ltd. All rights reserved.
Keywords: Galactitol; Subcooling; Latent heat storage; Solar cookers
1. Introduction The state of the art of solar cookers, albeit utilizing a greener and sustainable energy source, requires further development for their performance to be comparable to conventional cooking stoves. To allow for off-sun cooking, solar cookers incorporated with sensible heat storage system for example in Esen (2004), Schwarzer and Vieira da ⇑ Corresponding author. Tel.: +255 272 555070; fax: +255 272 920016.
E-mail address: [email protected] (L. Nkhonjera). http://dx.doi.org/10.1016/j.solener.2015.07.003 0038-092X/Ó 2015 Elsevier Ltd. All rights reserved.
Silva (2003) or latent heat storage systems like in Mussard et al. (2013) have been proven to work. However, latent heat storage systems are preferred because of their high energy density. On the other hand, optimizations of the geometry and heat transfer characteristics of latent heat storage systems for various applications have been studied (Esen and Ayhan, 1996; Esen et al., 1998; Fukai et al., 2003; Hamada et al., 2003; Horbaniuc et al., 1999; Lacroix, 1993; Velraj et al., 1999; Zivkovic and Fujii, 2001). Nevertheless, it appears that very few out of the available phase change materials (PCM) (Agyenim
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G. John et al. / Solar Energy 119 (2015) 415–421
et al., 2010; Cabeza et al., 2011; Farid et al., 2004; Zalba et al., 2003) have been explored for solar cooking application. Out of those PCM utilized in latent heat storage systems of solar cookers (Cuce and Cuce, 2013; Muthusivagami et al., 2010; Sharma et al., 2009), most have phase change temperatures (Tm) below 120 °C and thus the stored latent heat is released for cooking at temperatures below 120 °C. Cooking at these temperatures takes a long time and is inapplicable for frying where temperatures between 150 and 190 °C are involved (Choe and Min, 2007). Therefore, storing solar thermal energy in latent form at 150–200 °C and rapidly retrieving it for cooking could reduce cooking time and enable frying. According to Mehling and Cabeza (2008), nitrate salts and sugar alcohols are suitable PCM for latent heat storage in the 150–200 °C range. On the contrary, nitrate salts with Tm between 150 and 200 °C and phase change enthalpy (Dph) greater than 150 J/g are so hygroscopic that their technical application as PCM is difficult (Waschull et al., 2009). On the other hand, Waschull et al. (2009) list D-mannitol, galactitol and pentaerythritol as sugar alcohols with Tm within 150–200 °C and having substantial Dph for practical application. Nevertheless, D-mannitol has little practical application because of its high degree of subcooling (Waschull et al., 2009) and when operated above 175 °C its thermal behavior is negatively affected by polymorphism (Barreneche et al., 2013) whereas values of its Tm and Dph rapidly decrease with increasing thermal cycles (Sole´ et al., 2014). On the other hand, pentaerythritol, in contrast to galactitol, has lower Dph (Benson et al., 1986) and slow crystallization rate (Hu et al., 2014) but unlike galactitol, it has previously been employed in the heat exchanger for solar cooking (Bushnell and Sohi, 1992; Bushnell, 1988). Sole´ et al. (2014) investigated the cycling stability of galactitol at milligram level using differential scanning
calorimetry (DSC). However, DSC samples may not be representative of large quantities as they would be present in heat storage of solar cookers and, in addition, subcooling of PCM is strongest in small samples (Gu¨nther et al., 2009). Thus, the cycling stability and the degree of subcooling of galactitol in bulk cycling are not yet understood. Besides, the previous studies have not brought out the effect of upper cycle temperature on the cycling stability of galactitol. Therefore, the aim of this work is to investigate the thermal behavior of galactitol when cycled in bulk. Specifically the study seeks to investigate the effect of upper cycle temperature on cycling stability and the degree of subcooling, determine the degree of subcooling, and examine its thermal stability as well as the dependence of its heat capacity and thermal diffusivity on temperature. 2. Methodology Galactitol (CAS: 608-66-2) with 97% purity was purchased from Alfa Aesar. Galactitol samples were repetitively melted and frozen at different plate temperatures using a setup shown in Fig. 1, which was placed in a controlled airflow chamber. Three 30.0 g samples placed in a closed container with limited air were heated for 15 min by hotplate with plate temperature set at 275, 300 and 375 °C for samples 1, 2 and 3, respectively. Averaging the upper temperatures in each cycle over the number of thermal cycles gave the upper cycle temperature for each sample. However, to account for random errors that occurred in capturing the upper temperature in each cycle, the upper cycle temperature is reported as the mean ± standard deviation. After the heating cycle, each sample was then cooled for 45 min through a forced convection induced by an electric fan. A two-channel Voltcraft temperature data logger with K-type thermocouples monitored both the sample
4
1
2
3
5
6
7
Fig. 1. Experimental setup for the bulk cycling. (1) Galactitol sample, (2) temperature data logger, (3) hotplate, (4) electric fan, (5) thermocouple (K-type), (6) timer switch (fan), and (7) timer switch (heater).
G. John et al. / Solar Energy 119 (2015) 415–421
and ambient temperatures at intervals of five seconds. For each sample, temperatures were monitored at a position with (r1/2, t1/2) coordinates where r1/2 and t1/2 are half the radius of the sample container and half the thickness of the sample respectively. The programmed timer switches controlled the heating and cooling times. The Dph and Tm for a fresh sample and samples extracted at 20 cycles intervals were measured by a NETZSCH DSC200F3. The measurements were conducted in dynamic mode with a heating and cooling rate of 0.5 °C/min and samples of about 10.0 mg placed in aluminum crucible with pieced lid. On the other hand, thermal stability of galactitol was analyzed from mass loss data obtained from SEIKO TG/DTA 300 thermogravimetry in nitrogen atmosphere mode. The heating rate was 2 °C/min and temperatures were held constant for 2 h at 120 °C and for a subsequent 10.0 °C step until 220°C. The NETZSCH LFA447 Flash Diffusivity Instrument measured thermal diffusivity of galactitol at 20.0, 50.0, 100, 150, 170, 180, 190, 200, 210, 225, and 240 °C. At every temperature point, five measurements were taken and their average represented thermal diffusivity. Therefore, thermal diffusivities were presented with error bars when plotted and their in text values were reported as mean ± standard deviation. To take care of the uncertainties of the measuring instrument in reproducing the measured value, measurement errors are implicitly given in the number of significant figures of the reported measured value. The Voltcraft temperature data logger offered the least precision (0.005 °C) amongst all measuring instruments and thus 3 significant figures are used to report all in text values of the measured quantities. The systematic errors of the measurements were, however, minimized by calibrating the measuring instruments with the standards.
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3. Results and discussion This work gives results on the effect of upper cycle temperature (Tup) on the rate of change of the phase change enthalpy and temperature per cycle, the number of thermal cycles with noticeable phase change enthalpy and the degree of subcooling of galactitol when cycled in bulk. In addition, results on thermal stability, thermal diffusivity and heat capacity of galactitol as functions of temperature were obtained. Heating galactitol samples at the set plate temperatures of 275, 300 and 350 °C for 15 min afforded Tup of 203 ± 3.00, 230 ± 2.00 and 243 ± 3.00 °C, respectively, for the sample. Upon forced cooling for 45 min, each sample cooled to 50.2 ± 4.8 °C. As shown in Table 1, fresh galactitol is a white-grained powder that oxidized and changed structure upon repetitive heating and cooling in a limited amount of air. This is in agreement with the findings of Sole´ et al. (2014). However, as the upper cycle temperature and number of cycles increased, the color of a solidified sample changed from white to brown whereas its texture changed from rough and hard to smooth and flaccid. The samples cycled at Tup of 203 ± 3.00, 230 ± 2.00 and 243 ± 3.00 °C correspondingly appeared dark and flaccid after 220, 40 and 20 cycles. This indicates that the oxidation and the structural change of galactitol increased with both the upper cycle temperature (Tup) and the number of cycles. The phase change enthalpies and temperatures of the three samples when plotted against thermal cycles are shown in Fig. 2 from which it can be read that a fresh galactitol sample melted at 189 °C, which is within the 187–191 °C melting range reported in the data sheet of the manufacturer. Therefore, the Tup of 203 ± 3.00,
Table 1 Appearance of three galactitol samples cycled with upper cycle temperature (Tup) of 203, 230 and 243 °C per sample. Tup (°C)
Number of cycles 0
203
230
243
20
40
60
80
180
220
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Temperature/ oC
Enthalpy/ J.g
-1
400 melting (203 oC Tup)
200
solidification (203 oC T ) up
0
o
1
20
40
60
80
100
melting (230 C T ) up
120
solidification (230 oC T ) up
190
melting (243 oC Tup)
160 130
solidification(243 oC T ) up
100
1
20
40
60
80
100
120
Thermal cycles
Fig. 2. DSC phase change enthalpies and temperatures for samples taken at first and after every 20 cycles from three galactitol samples that were cycled with upper cycle temperature (Tup) of 203, 230 and 243 °C respectively.
Temperature / oC
260 240
T
220
T is the highest observed temperature during s the growing of crystals
200
b
is the temperature at which nucleation started
t is the time during which nucleation occured and thus d period durinch which latent heat was relaesed
180
T
160 T
140
s
degree of subcooling = Ts - Tb
b
120
t
d
100 80 60
0
5
10
15
20
25
30
35
40
time /min
Fig. 3. Temperature history showing subcooling and duration of crystal growth as a 30.0 g fresh galactitol sample cooled from molten to solid state.
230 ± 2.00 and 243 ± 3.00 °C ensured that each sample completely melted. Further, from Fig. 2, it was deduced that phase change enthalpies and temperatures decreased in value when the number of thermal cycles increased. However, Tup influenced the rate at which the values decreased. Both during melting and solidification, the rates of decrease of enthalpy and temperature per cycle were correspondingly slow, intermediate and fast for Tup of 203 ± 3.00, 230 ± 2.00 and 243 ± 3.00 °C. In addition, Tup had an effect on the number of thermal cycles with noticeable phase change enthalpies. The sample that was cycled at Tup of 203 ± 3.00 °C yielded the highest number of thermal cycles and that at Tup of 243 ± 3.00 °C yielded the least. It is therefore inferred that moving Tup closer to but above the melting temperature of galactitol reduces the rate of change of phase change enthalpies and temperatures per cycle and improves the number of thermal cycles and hence cycling stability. It can also be seen from Fig. 2 that at any thermal cycle, the values of melting enthalpy and temperature differ from the solidification counterparts. The difference could be attributed to material hysteresis and subcooling. The temperature history of a fresh bulk sample when cooled from 240 to around 60.0 °C showed that galactitol subcooled
with a degree of subcooling as high as 31.0 °C. The degree of subcooling is the difference between the highest observed temperature (Ts) during the growing of the crystals and the base temperature (Tb) to which the molten sample cooled and started nucleating as illustrated in Fig. 3. However, the subcooling degree found in this work is far lower than 72.0 °C reported in the work of Sarı et al. (2011). The discrepancy is probably due to different measurement methods. The latter was calculated from the difference between DSC melting and solidification temperatures, yet in DSC when the sample temperature reaches Tb the recorded temperature does not heat up to Ts rather it continues decreasing as per temperature program. As a result, the solidification temperature obtained by DSC is lower than the actual one. For instance, Fig. 3 shows that Ts obtained from temperature history of bulk galactitol sample is about 170 °C while for the DSC it is about 125 °C as shown in Fig. 2. In addition, Gu¨nther et al. (2009) hinted that the degree of subcooling of the milligram level samples encountered in DSC is stronger than that of a larger sample. This shows the advantage of measuring the degree of subcooling by probing temperatures of a bulk sample as it cools from molten to solid state. Furthermore, time td in Fig. 3 is the duration at which crystals were grown.
G. John et al. / Solar Energy 119 (2015) 415–421
419
T
s1
200
Ts2
o
Temperature/ C
Ts3 degree of subcooling
150
degree of subcooling degree of subcooling
100
1 2 3
Ts is the highest observed temperature during the growing of crystals in each cyle whereas subscripts 1, 2 and 3 stands for samples cycled to Tup of 203,
50
230 and 243 oC respectively.
0
0
50
100
150
200
250
300
Thermal cycles
Fig. 4. The variation of the highest observed temperature (Ts) during the growing of crystals as well as the degree of subcooling with the number of thermal cycles for samples that were cycled with upper cycle temperature (Tup) of 203, 230 and 243 °C. In each sample, the number of thermal cycles were terminated on the last cycle where noticeable td duration of the growing of crystals was observed.
The analysis of several cooling curves showed that td decreased with increasing thermal cycles to the point that it was negligible. Since td corresponds to the duration of crystal growth then in all cycles where td was negligible, the sample was considered to have a trivial number of crystals grown. The plot of the degree of subcooling (Ts Tb) against the number of thermal cycles in Fig. 4 depicts that as the number of thermal cycles increased from zero, the degree of subcooling increased to a peak value then decreased and approached zero in the cycles with negligible crystal growth. The rates of change of subcooling degree were however influenced by Tup such that the rates were correspondingly slow, intermediate and fast for Tup of 203 ± 3.00, 230 ± 2.00 and 243 ± 3.00 °C. Likewise, the degree of subcooling increased at any particular cycle as Tup moved from low to high values. This suggests that lowering Tup slightly improves the degree of subcooling. Fig. 4 also shows that Tup increased the rate of change of Ts after the first 7 and 30 cycles for samples heated to 243 ± 3.00 and 230 ± 2.00 °C respectively. This indicates that Tup had an influence on the rate of change of Ts per cycle only after some thermal cycles; the rate is slowed when Tup is lowered. In general, Ts decreased in value when the number of thermal cycles increased. The decreasing values of Ts between thermal cycles entail that in practical application the stored latent heat would be retrieved at different temperatures in each cooling cycle. Consequently, in medium temperature (150–200 °C) TES for solar cookers, the feasible cycles are the ones at which Ts is above 150 °C. From Fig. 4, samples heated to Tup of 203 ± 3.00, 230 ± 2.00, 243 ± 3.00 °C respectively yielded 89, 35 and 14 feasible cycles. This represents an 84% increase in number of feasible thermal cycles when Tup is around 15.0 °C as opposed to 55.0 °C above the melting temperature. Therefore, cycling galactitol at Tup above but close to the melting temperature prolongs the feasible thermal cycles. For the sample cycled with Tup of 203 ± 3.00 °C, its initial solidification enthalpy, melting temperature and
solidification temperature respectively dropped by 28, 3, and 13% after the 89 feasible cycles. This denotes that in terms of solidification enthalpy galactitol is not a stable PCM in TES for solar cookers. It is nonetheless more stable in relation to melting than solidification temperatures. On the other hand, degrees of subcooling for the 89 feasible cycles fell within 25.0–37.0 °C range, see Fig. 4. Taking the first cycle for example, the galactitol sample subcooled with a degree of 25.0 °C which when multiplied by mean specific heat capacity for the interval within which the sample subcooled gives 45 J/g. This heat raised temperature of 1 g of the sample from Tb to Ts. If the galactitol had not subcooled, this heat could be available for discharge at high temperature. From this, it is deduced that galactitol subcools to a degree that substantially reduces the amount of latent heat that would be available for discharge at high temperature and hence it would be less efficient if employed as PCM for TES. Thermal stability results show that galactitol is not stable at temperatures above 200 °C. Fig. 5 depicts that at temperatures up to 170 °C, the mass loss was negligible and between 180 and 190 °C (within which melting occurs) the mass loss was less than 1%. On the other hand, holding the galactitol sample at any temperature above 200 °C for 2 h resulted into a mass loss greater than 2%. This suggests that in order to preserve thermal integrity, galactitol should not be kept at temperatures above 200 °C for long hours. In this regard, cycling galactitol with Tup above melting temperature but less than 200 °C does not only improve cycling stability but also safeguards thermal stability. Concerning the physical properties of galactitol, Fig. 6 shows that thermal diffusivity decreased from 0.960 ± 0.009 mm2/s at 20 °C to 0.310 ± 0.004 mm2/s at 200 °C. Conversely, Fig. 6 shows that the heat capacity was almost steady before and after 180–190 °C melting range. Since thermal diffusivity is the ratio of thermal conductivity to the product of density and heat capacity, and considering that the heat capacity and density of solid galactitol slightly varies from room temperature to about
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G. John et al. / Solar Energy 119 (2015) 415–421 100.5 100 99.5
Mass %
99 98.5 98 97.5 97 96.5 96 20
40
60
80
100
120
140
160
180
200
220
Temperature/ oC
Fig. 5. Thermogravimetry mass percent of galactitol in nitrogen atmosphere.
Thermal diffussivity/ mm 2.s-1
Thermal diffussivity Heat capacity
0.8
120
0.6
90
0.4
60
0.2
30
Heat capacity/ J.(g.oC)-1
150
1
0
0 0
20
40
60
80
100
120
140
160
180
200
220
240
o
Temperature/ C
Fig. 6. Thermal diffusivity and heat capacity of galactitol as functions of temperature.
180 °C, then the decreasing thermal diffusivity is directly related to decreasing thermal conductivity. The decreasing thermal conductivity would therefore retard heat retrieval from the storage to cooking device if heat transfer enhancement is not employed. 4. Conclusions From the analysis of results obtained in this work, three conclusions are drawn. 1. The upper cycle temperature of bulk galactitol that is repetitively melted and frozen in a limited air environment has great influence on the rate of structural change. This rate is slowed when the upper cycle temperature is lowered. In turn, the structural change influences the number of feasible thermal cycles, the degree of subcooling and the rates at which the degree of subcooling, the phase change enthalpy and temperature change occur. The upper cycle temperatures above but close to the melting temperature are favorable. 2. Bulk galactitol has a degree of subcooling that falls within a 25.0–40.0 °C range and therefore substantially reduces the amount of latent heat discharged.
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