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Mg(OH)2 chemical heat pumps
Applied Energy 162 (2016) 31–39
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Efficiency improvement of heat storage materials for MgO/H2O/Mg(OH)2 chemical heat pumps E. Mastronardo a,⇑, L. Bonaccorsi a, Y. Kato b, E. Piperopoulos a, C. Milone a a b
Department of Electronic Engineering, Chemistry and Industrial Engineering, University of Messina, Messina, Italy Research Laboratory for Nuclear Reactors, Tokyo Institute of Technology, Tokyo, Japan
h i g h l i g h t s A new approach was used to develop a hybrid material (DP-MG) for chemical heat pump. Mg(OH)2 was finely dispersed on exfoliated graphite by deposition–precipitation route. DP-MG sample showed enhanced stability and efficiency to chemical heat pump cycles.
a r t i c l e
i n f o
Article history: Received 16 February 2015 Received in revised form 7 October 2015 Accepted 8 October 2015 Available online 11 November 2015 Keywords: Chemical heat pump Magnesium hydroxide Exfoliated graphite Deposition–precipitation
a b s t r a c t MgO/H2O/Mg(OH)2 chemical heat storage of waste energy from industrial processes is a promising technology in view of a more efficient use and saving of primary energy sources. A new approach was used to develop a hybrid heat storage material made of magnesium hydroxide (Mg(OH)2) and exfoliated graphite (which is used to improve the heat transfer with its high thermal conductivity). Mg(OH)2 nanoplatelets were directly grown on graphite surface via a deposition–precipitation method to increase the compatibility between the two materials. The material thus obtained, named DP-MG, was experimentally tested to determine its heat storage and output capacities. An improvement of the material efficiency was obtained with a higher storage capacity at lower reaction temperature and a higher heat output rate. Ó 2015 Elsevier Ltd. All rights reserved.
1. Introduction A significant amount of energy is inevitably wasted in the form of heat as by-product of industrial processes and oil and gas processing thus reducing their overall efficiency. Hence, in light of the 2020 EU climate and energy package and of the carbon tax, thermal energy storage of waste heat is a key issue for industries to make their processes more efficient and to reduce fuel consumption [1–4]. A promising technology for the recovery of waste heat are chemical heat pumps (CHPs) that are based on a reversible chemical reaction for heat storage and reuse on demand [5–7]. Different kind of substances can be used in a CHP system involving reversible gas–gas reactions (NH3/N2/H2, SO3/O2/SO2, CH3OH/H2/ CO, cyclohexane/benzene/hydrogen) [8–11], liquid–gas reactions (isopropanol/acetone/hydrogen) [12,13] and solid–gas reactions (BaO2/O2/BaO [14], ZnO/O2/Zn [15], PbO/CO2/PbCO3 [16], CaO/ CO2/CaCO3 [17], CaO/H2O/Ca(OH)2 [18,19]). The choice of storage material depends on the temperature range in which the storage ⇑ Corresponding author. E-mail address: [email protected] (E. Mastronardo). http://dx.doi.org/10.1016/j.apenergy.2015.10.066 0306-2619/Ó 2015 Elsevier Ltd. All rights reserved.
system will to operate. Specifically, this study focuses on the solid–gas MgO/H2O/Mg(OH)2 CHP which operates in the temperature range 200–400 °C and whose feasibility has already been demonstrated by Kato and co-workers [20]:
MgðOHÞ2 ðsÞ $ MgOðsÞ þ H2 OðgÞ DH0 ¼ 81 kJ=mol
ð1Þ
Dehydration, being endothermic, represents the energy storage step while the exothermic hydration of MgO releases the thermal energy when required (Fig. 1). The water vapour produced during the dehydration reaction is condensed in a reservoir and then reused in vapour phase for the hydration reaction closing the pump cycle. The main requirements that a storage system has to satisfy are (i) high energy density (per-unit mass or per-unit volume) of the storage material, (ii) good heat transfer through the storage medium, (iii) mechanical and chemical stability of the storage material, (iv) high durability to a large number of charging/discharging cycles, (v) low thermal losses. The MgO/H2O/Mg(OH)2 CHP complies with these requirements. However, the low thermal conductivity of the reagents (Mg(OH)2 and MgO) may lower the performance of the CHP. Moreover, the thermal decomposition of
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Nomenclature EG exfoliated graphite I001, I101, I110 intensity reflections respectively of [0 0 1], [1 0 1] and [1 1 0] planes min initial sample mass (g) mist instantaneous mass (g) MMg(OH)2 molecular weight of Mg(OH)2 (g mol1) molecular weight of MgO (g mol1) MMgO pHPZC point of zero charge Qr released heat per initial mass unit of Mg(OH)2 (kJ kg1 Mg(OH)2) heat output rate (kW kg1 qr Mg(OH)2) Qs stored heat per initial mass unit of Mg(OH)2 (kJ kg1 Mg(OH)2) QVr released heat per unit volume (kJ cm3) QVs stored heat per unit volume (kJ cm3) specific surface area (m2 g1) SBET Td dehydration temperature (°C) Th hydration temperature (°C) hydration time (min) th Tonset onset temperature of dehydration reaction (°C)
Mg(OH)2 is always accompanied by the sintering of the newly formed MgO product, which leads to its grain growth and to the loss of pore volume thus hindering MgO rehydration. It has been already observed by Zamengo et al. [21] that a physical mixture of magnesium hydroxide and expanded graphite can improve the poor thermal conductivity of pure Mg(OH)2. Nevertheless, because of the weak interaction between the inorganic (Mg(OH)2) and organic (expanded graphite) phase, the hybrid material exhibits a poor durability to repetitive reactions. Kim et al. [22] and Myagmarjav et al. [23] introduced a hygroscopic salt in the mixture of Mg(OH)2 and graphite, respectively calcium chloride (CaCl2) and lithium bromide (LiBr), which should promote the adhesion of Mg(OH)2 on the graphite surface and enhance the reactivity of the heat storage material. However, these salts are well known to be highly corrosive, especially the combination of CaCl2 with expanded graphite [24,25]. The state of the art demonstrates that a more efficient heat storage material for MgO/H2O/Mg(OH)2 CHP needs to be developed for the industrial application of such a technology. This study examines a novel synthesis route for the deposition of Mg(OH)2 on graphite surface to enhance the compatibility between the two phases, thus improving the stability of the composite under operating conditions and avoiding MgO sintering. A deposition–precipi tation method is used for the direct growth in aqueous medium of Mg(OH)2 crystals on exfoliated graphite surface (consisting in thin layers split along the c axis). The compatibility between the active phase (Mg(OH)2) and exfoliated graphite is improved by the electrostatic interaction promoted by the different point of zero charge (pHPZC) of the materials. The performance of the resulting hybrid heat storage material was evaluated by thermogravimetric analysis which simulates the CHP cycle and compared with that of a physical mixture of Mg(OH)2 and exfoliated graphite.
2. Experimental section 2.1. Deposition–precipitation For the deposition–precipitation (DP) method the following raw materials were used: magnesium nitrate hexahydrate (Mg(NO3)26H2O, 99% Sigma Aldrich) as magnesium source, ammonia solution (NH4OH, 30 wt.% Carlo Erba) as precipitating
tr w
total reaction time (min) mass ratio between EG and Mg(OH)2
Greek symbols b reacted fraction (%) bdf reacted fraction at the end of the dehydration treatment (%) bid reacted fraction at the beginning of the dehydration treatment (%) bd reacted fraction of Mg(OH)2 after dehydration (%) final reacted fraction of MgO at the point of water supbh ply termination (%) Dmreal instantaneous real mass change (%) Dmth theoretical mass change due to the dehydration of Mg (OH)2 normalized to the total amount present in the sample (%) Dbd dehydration conversion (%) Dbh hydration conversion (%) qc density of the composite (g cm3)
agent and exfoliated graphite (TIMREX C-THERM 002 TIMCAL Ltd. for briefness named EG). The DP procedure was carried out as follows: under magnetic stirring 50 ml of Mg(NO3)26H2O solution were gradually added (2.5 ml/min) through a peristaltic pump to 150 ml of NH4OH solution containing a specified amount of EG (weight ratio Mg(OH)2/EG = 1). The final solution (pH 11.5) was aged at ambient conditions for 24 h, then it was vacuum filtered (0.22 lm) and the collected solid was washed with deionized water and dried in a vacuum oven at 50 °C overnight. The Mg2+ ions concentration in solution was chosen on the base of the desired Mg(OH)2 deposited amount: the initial moles of Mg2+ in the solution are those that theoretically should be precipitated in case of complete precipitation according to the stoichiometric reaction:
MgðNO3 Þ2 þ 2NH4 OH ! MgðOHÞ2 þ 2NH4 NO3
ð2Þ
The molar ratio Mg(NO3)26H2O:NH4OH on the final solution was 1:100. 2.2. Impregnation A physical mixture of Mg(OH)2 and EG (weight ratio Mg(OH)2/ EG = 1) was prepared according to an impregnation method reported in literature [22]: a specific amount of Mg(OH)2 (obtained by the precipitation reaction reported in Section 2.1) was soaked in 200 ml of ethanol and sonicated for 30 min. The same amount of EG was charged in the flask and soaked with the solution. The excess ethanol was eliminated by evaporation in about 1 h and the remaining product was oven-dried at 120 °C overnight. 2.3. Thermogravimetric analysis A thermogravimetric analysis (TGA) was performed (TG-9600, Ulvac Shinku-Riko Inc.) to evaluate the behavior of the heat storage materials obtained to cyclic dehydration/hydration reactions representative of the CHP operation cycle. In this experiment the sample was first dried at 110 °C in inert atmosphere (Ar 100 ml/min) for 60 min to remove the physically adsorbed water and then subjected to the dehydration/hydration cycles. In a single cycle the temperature was increased by 10 °C/min to the desired dehydration temperature (Td = 350 °C) and Mg(OH)2 dehydration reaction proceeded over 120 min. After the complete dehydration reaction,
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Fig. 1. Schematic diagram of a MgO/H2O/Mg(OH)2 chemical heat pump cycle.
the temperature was decreased to the hydration temperature (Th = 110 °C–10 °C/min). The water vapour necessary for the MgO hydration reaction was supplied by a micro-feeder at 37 ll/min and mixed with 35 ml/min Ar as carrier gas under a vapour pressure of 57.8 kPa. After the hydration reaction proceeded over 120 min, the water vapour supply was stopped and the sample was kept at 110 °C for 10 min in inert atmosphere (100 ml/min Ar) to remove physically adsorbed water from the sample. This process was repeated for each cycle. A 3 cycles experiment was performed on the samples obtained by DP and impregnation (respectively named DP-MG and I-MG) and on pure Mg(OH)2 (obtained by the precipitation reaction reported in Section 2.1.) to determine the effect of the new synthesis method and the role of EG on the hybrid materials’ performance. The analyzed samples are reported in Table 1. In order to normalize the mass change of the sample that is continuously measured as a function of temperature and time to the initial mass of Mg(OH)2 present in the sample, the reacted fraction (b ½%) can be defined by Eq. (3):
Dmreal 100 b ½% ¼ 1 Dmth
ð3Þ
where Dmreal ½% is the instantaneous real mass change and Dmth ½% is the theoretical mass change due to the dehydration of 1 mol Mg(OH)2 normalized to the total amount present in the sample, respectively expressed by Eqs. (4) and (5):
Dmreal ½% ¼
min minst 100 min
Dmth ½% ¼
MMgðOHÞ2 MMgO 100 w MMgðOHÞ2
ð4Þ
Table 1 Analysed samples. Samples code
Synthesis method
Mg(OH)2 content (wt.%)
Mg(OH)2 I-MG DP-MG
Precipitation Impregnation Deposition–precipitation
100 50 50
Dbd ½% ¼ bid bdf
ð6Þ
Dbh ½% ¼ bh bdf
ð7Þ
where bid ½% and bdf ½% are respectively the reacted fraction at the beginning and at the end of the dehydration treatment. While, bd ½% is the reacted fraction of Mg(OH)2 after dehydration and bh ½% the final reacted fraction of MgO at the point of water supply termination. The stored/released heat per initial mass unit of Mg(OH)2 (Qs/r [kJ/kgMg(OH)2]) can be calculated using Eq. (8):
h i Q s=r kJ=kgMgðOHÞ2 ¼
DH 0 M MgðOHÞ2
Dbd
ð8Þ
being DH0 [kJ/mol] the enthalpy of reaction. The heat output rate (qr [kW/kgMg(OH)2]) is calculated as follows:
h i Q qr kW=kgMgðOHÞ2 ¼ r th
ð9Þ
being th [min] the hydration time.
ð5Þ
where min ½g and minst ½g are respectively the initial sample mass and the instantaneous mass during TG analysis. While, MMgðOHÞ2 ½g=mol and MMgO ½g=mol are respectively the molecular weight of Mg(OH)2 and MgO, and w is the mass ratio between EG and Mg(OH)2. In case of 100% wt. of Mg(OH)2 present in the sample Dmth ¼ 30:89%. The dehydration and hydration conversions (Dbd=h ½%) can be calculated respectively by Eqs. (6) and (7):
2.4. Morphological characterization of storage materials The hybrid materials obtained via the procedure described above were analyzed by means of Scanning Electron Microscopy (JSM-5600-LV JEOL), X-ray Diffraction (D8 Advance Bruker) and BET analysis (Chemisorb 2750 Micromeritics) to define the morphology and crystal structure of samples. Moreover, the samples were characterized after cyclic experiments in order to evaluate the morphological and crystallographic changes occurred.
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Table 2 Dehydration (Dbd) and hydration (Dbh) conversions of a 3 cycles experiment. Samples code
Mg(OH)2 I-MG DP-MG
1st Cycle
2nd Cycle
3rd Cycle
Dbd (%)
Dbh (%)
Dbd (%)
Dbh (%)
Dbd (%)
Dbh (%)
100 101a 109.42a
58.27 66.23 87.54
58.05 67.45 88.31
52.51 60.44 85.60
51.93 59.24 83.85
49.89 57.76 84.62
a The fact that the 1st dehydration reaction conversion of both hybrid materials exceeds 100 means that the real weight change (Dmreal ) is more than the theoretical one (Dmth ) due to 1 wt.% decomposition of the carbonaceous phase (as inferred by the TG profile of mere EG showed in Fig. A1 of Appendix A).
Fig. 2. 2nd dehydration/hydration cycle of a 3 cycles TG experiment.
3. Results 3.1. Thermogravimetric behavior of storage materials The results of a 3 cycles experiment in terms of dehydration and hydration conversions (Dbd/h) for each cycle performed on the hybrid materials and on pure Mg(OH)2 are listed in Table 2. From the reported values it can be observed for each cycle that both hybrid materials have higher reaction conversions than pure Mg (OH)2 and, as a more important result, the DP-MG sample shows higher reaction conversions than I-MG sample. As exemplification of a single cycle experiment, the 2nd cycle profile of the samples mentioned above is illustrated in Fig. 2 where reacted fraction (b [%]) is plotted against reaction time (t [min]). The specimen profile curves show that the hybrid materials have a higher hydration rate. Fig. 3 shows the first 60 min of DP-MG sample single dehydration profiles. A significant change in the thermogravimetric behavior occurs after the 1st dehydration reaction: compared to the 1st dehydration profile (black curve), the beginning of both the 2nd (red curve1) and 3rd (blue curve) dehydration profiles is shifted toward lower temperature. Moreover, after the 1st dehydration the slope of the following reaction profiles increases indicating a higher reaction rate, while the dehydration conversion decreases. This behavior was observed even in pure Mg(OH)2 and I-MG, whose profiles are reported respectively in Fig. A2a and b in Appendix A. 1 For interpretation of color in Fig. 3, the reader is referred to the web version of this article.
The single dehydration reaction conditions, expressed as onset temperature of dehydration reaction (Tonset [°C]) and total reaction time (tr [min]), are reported in Table 3 while the stored heat capacity (Qs [kJ/kgMg(OH)2]) and the heat output rate (qr [kW/kgMg(OH)2]) are reported respectively in Figs. 4 and 5. A lower Tonset is observed in all the samples after each cycle, also reaction time (tr) decreases from 30 to 20 min (15 min in case of pure Mg(OH)2). Consequently, and as shown in Fig. 4, after the 1st dehydration/hydration cycle most part of the heat is stored in the first 30 min from the beginning of the dehydration reaction, while during the 1st dehydration reaction heat is mostly stored during the first 60 min. The DP-MG sample shows the highest stored heat values (with an average stored heat Qs 1200 kJ/kgMg(OH)2) being the sample with the highest dehydration conversion. Moreover, it can respond immediately in case of energy demand since it is able to release most part of stored heat in about 10 min as shown in Fig. 5. 3.2. Morphological and structural modification of the heat storage materials after cyclic experiments Fig. 6 shows the SEM images of the samples I-MG and DP-MG before and after a 3 cycles experiment. The post-cycles specimens obtained by impregnation and DP route have been respectively indicated with the codes CE/I-MG and CE/DP-MG. From the comparison of the I-MG and DP-MG samples (Fig. 6a and b), it emerges that the precipitated Mg(OH)2 in both hybrid materials has a platelike morphology, typical of brucite nanoplates due to its hexagonal structure [26]. Sample DP-MG presents a thin overcoating of EG
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Fig. 3. First 60 min of the single dehydration reactions of DP-MG sample.
Table 3 Single dehydration reaction conditions: initial temperature of dehydration reaction (Tonset) and total reaction time (tr). Sample code
Dehydration reactions 1st
Mg(OH)2 I-MG DP-MG
2nd
3rd
Tonset (°C)
tr (min)
Tonset (°C)
tr (min)
Tonset (°C)
tr (min)
275 285 290
30 30 30
225 225 250
15 20 20
210 225 240
15 20 20
Fig. 4. Comparison of the stored heat capacity (Qs [kJ/kgMg(OH)2]) during each cycle.
surface with Mg(OH)2 nanoplatelets in the range 40–80 nm, while Mg(OH)2 in sample I-MG has a more packed aspect with larger particle dimensions (200–300 nm). This morphology can explain also the specific surface area (SBET [m2/g]) values obtained by BET analysis reported in Table 4. Indeed, the SBET of I-MG sample (38 m2/g) that is close to the value of EG (25 m2/g) indicates that the Mg(OH)2 particles cling compactly to the EG surface; while the higher SBET of the DP-MG hybrid material (50 m2/g) is probably due to the smaller Mg
(OH)2 particle dimensions and to the better dispersion on EG surface. Hence, with the DP method a better dispersion of Mg(OH)2 particles is obtained avoiding the coalescence of small nanoparticles that naturally tend to aggregate to decrease their surface energy. Fig. 6c shows that after only 3 cycles experiment Mg(OH)2 particles have detached from the EG surface in the I-MG sample and, as shown in the miniature in Fig. 6c, the thicker layers of Mg(OH)2 are damaged with the formation of some cracks. On the contrary,
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Fig. 5. Heat output rate (qr [kW/kgMg(OH)2]) of pure Mg(OH)2 (black line), I-MG (blue line) and DP-MG (red line) samples during the 1st hydration reaction. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
the morphology of DP-MG sample (Fig. 6d) is almost unchanged after the dehydration/hydration cyclic experiment: the particle size is not increased and Mg(OH)2 is still attached on EG surface. In both XRD patterns of the hybrid materials as reported in Fig. 7a and b (black lines) peaks relative to highly crystalline brucite (2-theta: 18.5°, 32.5°, 38°, 58.5°, 62°, 68°, 72°) and graphite (2-theta: 26.5°, 42.5°, 45°, 51°, 55°, 78°) can be identified in agreement with standard data, respectively JCPDS 7-0239 and JCPDS 250284. No peaks arising from impurities were detected. Brucite has a hexagonal crystal structure in which each Mg ion is hoctaedrally coordinated by O atoms. The Mg ions lie in plans with the OH ions above and below them [27]. The Mg(OH)2 intensity ratios of reflections [0 0 1] to [1 0 1] and [1 1 0] for all samples (I001/I101, I001/I110) are listed in the following Table 2. XRD patterns of both samples are comparable and Mg(OH)2 particles are preferentially oriented to the (0 0 1) direction. The comparison of XRD patterns of both hybrid materials before (black lines) and after (red lines) the cyclic experiment (reported in Fig. 7a and b) reveal a change in Mg(OH)2 crystal lattice: the peaks relative to brucite are broadened and with lower intensity. Besides, the intensity ratios of the samples (listed in Table 5), more significantly in DP-MG, are increased after cyclic experiment. This means that the number of reflections from the [0 0 1] plans have increased, presumably due to an exfoliation phenomenon in the Mg(OH)2 nanoparticles. Moreover, due to the incomplete hydration reactions (clearly visible from TG analysis) the presence of MgO (2-theta: 42.8°, 62°, 78°) cannot be excluded being the diffraction lines overlapped with those of EG and Mg(OH)2. 3.3. Durability to repeated cyclic experiments The durability of the DP-MG sample was evaluated with a 9 cycles experiment, the profile of which is shown in Fig. 8a. The performance of DP-MG sample is stable with an average dehydration/ hydration conversion of around 83% (Fig. 8b).
3.4. Pellet Due to the presence of EG, the DP-MG sample is easily moldable, thus the sample powder was compressed as pellet and its performance compared with that of the loose powder. A disk of diameter and height respectively of 7.3 and 1.5 mm and density equal to 0.7 g/cm3 was prepared and tested with a 3 cycles experiment by TGA. The experiment was performed using the same amount of loose powder and pellet, that is different volumes being the density of loose powder (0.15 g/cm3) about 4.6 times lower than that of pellet. Since an increase in density could have hindered the vapour flow through the pellet during the dehydration and hydration reactions, a lower performance of the sample was expected. Instead, from the comparison of the TG profiles, DPMG powder and pellet showed a very similar trend (Fig. 9) with an average stored/released heat capacity for both samples equal to 1200 kJ/kgMg(OH)2. This result can be explained considering that in case of pellet two opposing effects act on the vapour flow: on the one hand, the reduction of permeability hinders the vapour flow; on the contrary, the decrease of the bulk material thickness favors the vapour flow. As a result from the combination of these effects, the pellet showed unaltered thermogravimetric behavior compared to that of loose powder. The stored/released heat per unit volume (QVs/r [kJ/cm3]) of the composite can be evaluated by Eq. (10):
Q Vs=r ½kJ=cm3 / Q s=r qc
ð10Þ
being Qs/r [kJ/kgMg(OH)2] the stored/released heat per initial mass unit of Mg(OH)2 (calculated by Eq. (8)) and qc [g/cm3] the density of the composite. Since the density of the loose powder is lower than that of pellet and having both samples the same stored/ released heat per initial mass unit of Mg(OH)2, the stored/released heat per unit volume of the composite as pellet is higher compared to the loose powder.
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Fig. 6. SEM images of samples obtained by impregnation and deposition–precipitation route before and after cyclic experiment: (a) I-MG, (c) DP-MG, (d) CE/I-MG and (e) CE/ DP-MG samples. Table 5 Comparison of the samples XRD intensity ratios before and after cyclic experiment. Table 4 Measures of specific surface area (SBET [m2/g]) on the EG and the hybrid materials. Sample code
EG
I-MG
DP-MG
SBET (m2/g)
25
38
50
Sample code
I-MG CE/I-MG DP-MG CE/DP-MG
Intensity ratios I001/I101
I001/I110
2.07 3.74 1.45 2.96
6.23 8.6 4.8 9.6
Fig. 7. XRD pattern of samples (a) I-MG and (b) DP-MG before (black line) and after (red line) the cyclic experiment. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
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Fig. 8. (a) Dehydration/hydration cycles and (b) dehydration (Dbd) and hydration (Dbh) conversions of a 9 cycles experiment performed on DP-MG sample.
Fig. 9. Comparison of a 3 cycles experiment performed on powder (black line) and pellet (orange line) of DP-MG sample. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
4. Discussion The better results of hybrid materials (in terms of reaction conversions and rate) with respect to pure Mg(OH)2 could be attributed to the presence of the graphitic support which hinders MgO sintering, which in turn has a negative impact not only on the thermodynamic equilibrium but also on the reaction kinetics. Furthermore, the higher reaction conversions of DP-MG compared to I-MG can be explained in light of the different morphology of the two hybrid materials. In fact, not only the thicker Mg(OH)2 layers deposited on EG surface in I-MG sample seem to be detrimental for water vapour diffusion but also the smaller Mg(OH)2 particle size of the DP-MG sample can positively affect the reaction conversions [28]. It can be argued that during the dehydration reaction the water vapour released by the deeper Mg(OH)2 particles which must flow through the coating tends to generate cracks (as noted by SEM
images of I-MG sample in Fig. 6c) after each cycle causing a detachment of the Mg(OH)2 layer from the carbonaceous surface. A thick Mg(OH)2 layer is detrimental even for the hydration reaction; indeed during hydration the surface of the MgO particles is progressively covered by the Mg(OH)2 formed. As a result, the water vapour flow is hindered and the deeper MgO layers cannot be easily reached and hence reconverted in Mg(OH)2. This reduces the overall reaction rate and conversion. The high durability to repeated reactions of the DP-MG sample can probably be explained with reference to the stronger electrostatic interaction between the two components, EG and Mg(OH)2. In facts, in the precipitation conditions the solution pH (11.5) is lower than the pHPZC of Mg(OH)2 (12), but higher than pHPZC of EG (6). Therefore, the surface of Mg(OH)2 is positively charged, while the EG surface is negatively charged. This electrostatic interaction prevents the detachment of Mg(OH)2 particles from the EG surface.
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The different thermogravimetric behavior after the 1st dehydration reaction (lower onset temperature of dehydration reaction and decrease of dehydration reaction time) observed in all the samples (reported in Section 3.2.) is likely correlated to the modification of Mg(OH)2 crystal structure as inferred by XRD analysis. In Fig. 7 it is indeed evidenced a variation of the Mg(OH)2 crystal size after the 1st dehydration reaction, which in turn causes an increase of exposed active material. On the basis of the obtained results, in accordance with the hydration mechanism of polycrystalline MgO proposed by Kitamura et al. [29], it can be argued that hydration begins on the grain boundaries near the surface of polycrystalline MgO, which causes grain boundary separation as a consequence of lattice expansion (the volume expansion for the transformation of MgO to Mg(OH)2 is calculated at around 163.5%). The bulk material is therefore separated into finer particles or aggregates dispersed over the graphitic support, which could explain the modifications of XRD spectra.
5. Conclusions A novel synthesis route was used to develop a hybrid storage material for MgO/H2O/Mg(OH)2 CHPs made of Mg(OH)2 nanoplatelets deposited on the graphite surface. The sample, named DP-MG, was prepared through a deposition–precipitation method and its storage and output capacities evaluated. The evaluation demonstrated that the direct growth of Mg(OH)2 on graphite surface in aqueous medium avoided the natural particles coalescence and improved the compatibility between the inorganic (Mg(OH)2) and the organic (EG) phases. The morphology resulting from this new synthesis approach, with a thin layer of Mg(OH)2 nanoparticles finely dispersed on EG surface, led to the improvement of the hybrid system’s efficiency. Compared to the sample named I-MG, obtained by a simple physical mixture of Mg(OH)2 and EG, the DP-MG showed the highest stored heat values, with an average stored heat of about 1200 kJ/kgMg(OH)2. It can also respond immediately to energy demand due to the high heat output rate, releasing most part of stored heat in 10 min. A decrease of the onset temperature of dehydration reaction and total reaction time after the first dehydration/hydration cycle was also observed, which allows for faster, lower-temperature storage. Further analysis showed a good durability to repeated reactions of the DP-MG sample, which is also easily pelletizable without losses in performance. A feasibility study for a large scale MgO/H2O/Mg(OH)2 CHP is planned to be carried out, which will aim to evaluate the kinetic behavior of the hybrid storage material.
Acknowledgements Financial support by University of Messina for this study is acknowledged. The authors would like to thank the Tokyo Institute of Technology for the provided collaboration.
Appendix A. Supplementary material Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.apenergy.2015. 10.066.
39
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Applied Energy journal homepage: www.elsevier.com/locate/apenergy
Efficiency improvement of heat storage materials for MgO/H2O/Mg(OH)2 chemical heat pumps E. Mastronardo a,⇑, L. Bonaccorsi a, Y. Kato b, E. Piperopoulos a, C. Milone a a b
Department of Electronic Engineering, Chemistry and Industrial Engineering, University of Messina, Messina, Italy Research Laboratory for Nuclear Reactors, Tokyo Institute of Technology, Tokyo, Japan
h i g h l i g h t s A new approach was used to develop a hybrid material (DP-MG) for chemical heat pump. Mg(OH)2 was finely dispersed on exfoliated graphite by deposition–precipitation route. DP-MG sample showed enhanced stability and efficiency to chemical heat pump cycles.
a r t i c l e
i n f o
Article history: Received 16 February 2015 Received in revised form 7 October 2015 Accepted 8 October 2015 Available online 11 November 2015 Keywords: Chemical heat pump Magnesium hydroxide Exfoliated graphite Deposition–precipitation
a b s t r a c t MgO/H2O/Mg(OH)2 chemical heat storage of waste energy from industrial processes is a promising technology in view of a more efficient use and saving of primary energy sources. A new approach was used to develop a hybrid heat storage material made of magnesium hydroxide (Mg(OH)2) and exfoliated graphite (which is used to improve the heat transfer with its high thermal conductivity). Mg(OH)2 nanoplatelets were directly grown on graphite surface via a deposition–precipitation method to increase the compatibility between the two materials. The material thus obtained, named DP-MG, was experimentally tested to determine its heat storage and output capacities. An improvement of the material efficiency was obtained with a higher storage capacity at lower reaction temperature and a higher heat output rate. Ó 2015 Elsevier Ltd. All rights reserved.
1. Introduction A significant amount of energy is inevitably wasted in the form of heat as by-product of industrial processes and oil and gas processing thus reducing their overall efficiency. Hence, in light of the 2020 EU climate and energy package and of the carbon tax, thermal energy storage of waste heat is a key issue for industries to make their processes more efficient and to reduce fuel consumption [1–4]. A promising technology for the recovery of waste heat are chemical heat pumps (CHPs) that are based on a reversible chemical reaction for heat storage and reuse on demand [5–7]. Different kind of substances can be used in a CHP system involving reversible gas–gas reactions (NH3/N2/H2, SO3/O2/SO2, CH3OH/H2/ CO, cyclohexane/benzene/hydrogen) [8–11], liquid–gas reactions (isopropanol/acetone/hydrogen) [12,13] and solid–gas reactions (BaO2/O2/BaO [14], ZnO/O2/Zn [15], PbO/CO2/PbCO3 [16], CaO/ CO2/CaCO3 [17], CaO/H2O/Ca(OH)2 [18,19]). The choice of storage material depends on the temperature range in which the storage ⇑ Corresponding author. E-mail address: [email protected] (E. Mastronardo). http://dx.doi.org/10.1016/j.apenergy.2015.10.066 0306-2619/Ó 2015 Elsevier Ltd. All rights reserved.
system will to operate. Specifically, this study focuses on the solid–gas MgO/H2O/Mg(OH)2 CHP which operates in the temperature range 200–400 °C and whose feasibility has already been demonstrated by Kato and co-workers [20]:
MgðOHÞ2 ðsÞ $ MgOðsÞ þ H2 OðgÞ DH0 ¼ 81 kJ=mol
ð1Þ
Dehydration, being endothermic, represents the energy storage step while the exothermic hydration of MgO releases the thermal energy when required (Fig. 1). The water vapour produced during the dehydration reaction is condensed in a reservoir and then reused in vapour phase for the hydration reaction closing the pump cycle. The main requirements that a storage system has to satisfy are (i) high energy density (per-unit mass or per-unit volume) of the storage material, (ii) good heat transfer through the storage medium, (iii) mechanical and chemical stability of the storage material, (iv) high durability to a large number of charging/discharging cycles, (v) low thermal losses. The MgO/H2O/Mg(OH)2 CHP complies with these requirements. However, the low thermal conductivity of the reagents (Mg(OH)2 and MgO) may lower the performance of the CHP. Moreover, the thermal decomposition of
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Nomenclature EG exfoliated graphite I001, I101, I110 intensity reflections respectively of [0 0 1], [1 0 1] and [1 1 0] planes min initial sample mass (g) mist instantaneous mass (g) MMg(OH)2 molecular weight of Mg(OH)2 (g mol1) molecular weight of MgO (g mol1) MMgO pHPZC point of zero charge Qr released heat per initial mass unit of Mg(OH)2 (kJ kg1 Mg(OH)2) heat output rate (kW kg1 qr Mg(OH)2) Qs stored heat per initial mass unit of Mg(OH)2 (kJ kg1 Mg(OH)2) QVr released heat per unit volume (kJ cm3) QVs stored heat per unit volume (kJ cm3) specific surface area (m2 g1) SBET Td dehydration temperature (°C) Th hydration temperature (°C) hydration time (min) th Tonset onset temperature of dehydration reaction (°C)
Mg(OH)2 is always accompanied by the sintering of the newly formed MgO product, which leads to its grain growth and to the loss of pore volume thus hindering MgO rehydration. It has been already observed by Zamengo et al. [21] that a physical mixture of magnesium hydroxide and expanded graphite can improve the poor thermal conductivity of pure Mg(OH)2. Nevertheless, because of the weak interaction between the inorganic (Mg(OH)2) and organic (expanded graphite) phase, the hybrid material exhibits a poor durability to repetitive reactions. Kim et al. [22] and Myagmarjav et al. [23] introduced a hygroscopic salt in the mixture of Mg(OH)2 and graphite, respectively calcium chloride (CaCl2) and lithium bromide (LiBr), which should promote the adhesion of Mg(OH)2 on the graphite surface and enhance the reactivity of the heat storage material. However, these salts are well known to be highly corrosive, especially the combination of CaCl2 with expanded graphite [24,25]. The state of the art demonstrates that a more efficient heat storage material for MgO/H2O/Mg(OH)2 CHP needs to be developed for the industrial application of such a technology. This study examines a novel synthesis route for the deposition of Mg(OH)2 on graphite surface to enhance the compatibility between the two phases, thus improving the stability of the composite under operating conditions and avoiding MgO sintering. A deposition–precipi tation method is used for the direct growth in aqueous medium of Mg(OH)2 crystals on exfoliated graphite surface (consisting in thin layers split along the c axis). The compatibility between the active phase (Mg(OH)2) and exfoliated graphite is improved by the electrostatic interaction promoted by the different point of zero charge (pHPZC) of the materials. The performance of the resulting hybrid heat storage material was evaluated by thermogravimetric analysis which simulates the CHP cycle and compared with that of a physical mixture of Mg(OH)2 and exfoliated graphite.
2. Experimental section 2.1. Deposition–precipitation For the deposition–precipitation (DP) method the following raw materials were used: magnesium nitrate hexahydrate (Mg(NO3)26H2O, 99% Sigma Aldrich) as magnesium source, ammonia solution (NH4OH, 30 wt.% Carlo Erba) as precipitating
tr w
total reaction time (min) mass ratio between EG and Mg(OH)2
Greek symbols b reacted fraction (%) bdf reacted fraction at the end of the dehydration treatment (%) bid reacted fraction at the beginning of the dehydration treatment (%) bd reacted fraction of Mg(OH)2 after dehydration (%) final reacted fraction of MgO at the point of water supbh ply termination (%) Dmreal instantaneous real mass change (%) Dmth theoretical mass change due to the dehydration of Mg (OH)2 normalized to the total amount present in the sample (%) Dbd dehydration conversion (%) Dbh hydration conversion (%) qc density of the composite (g cm3)
agent and exfoliated graphite (TIMREX C-THERM 002 TIMCAL Ltd. for briefness named EG). The DP procedure was carried out as follows: under magnetic stirring 50 ml of Mg(NO3)26H2O solution were gradually added (2.5 ml/min) through a peristaltic pump to 150 ml of NH4OH solution containing a specified amount of EG (weight ratio Mg(OH)2/EG = 1). The final solution (pH 11.5) was aged at ambient conditions for 24 h, then it was vacuum filtered (0.22 lm) and the collected solid was washed with deionized water and dried in a vacuum oven at 50 °C overnight. The Mg2+ ions concentration in solution was chosen on the base of the desired Mg(OH)2 deposited amount: the initial moles of Mg2+ in the solution are those that theoretically should be precipitated in case of complete precipitation according to the stoichiometric reaction:
MgðNO3 Þ2 þ 2NH4 OH ! MgðOHÞ2 þ 2NH4 NO3
ð2Þ
The molar ratio Mg(NO3)26H2O:NH4OH on the final solution was 1:100. 2.2. Impregnation A physical mixture of Mg(OH)2 and EG (weight ratio Mg(OH)2/ EG = 1) was prepared according to an impregnation method reported in literature [22]: a specific amount of Mg(OH)2 (obtained by the precipitation reaction reported in Section 2.1) was soaked in 200 ml of ethanol and sonicated for 30 min. The same amount of EG was charged in the flask and soaked with the solution. The excess ethanol was eliminated by evaporation in about 1 h and the remaining product was oven-dried at 120 °C overnight. 2.3. Thermogravimetric analysis A thermogravimetric analysis (TGA) was performed (TG-9600, Ulvac Shinku-Riko Inc.) to evaluate the behavior of the heat storage materials obtained to cyclic dehydration/hydration reactions representative of the CHP operation cycle. In this experiment the sample was first dried at 110 °C in inert atmosphere (Ar 100 ml/min) for 60 min to remove the physically adsorbed water and then subjected to the dehydration/hydration cycles. In a single cycle the temperature was increased by 10 °C/min to the desired dehydration temperature (Td = 350 °C) and Mg(OH)2 dehydration reaction proceeded over 120 min. After the complete dehydration reaction,
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Fig. 1. Schematic diagram of a MgO/H2O/Mg(OH)2 chemical heat pump cycle.
the temperature was decreased to the hydration temperature (Th = 110 °C–10 °C/min). The water vapour necessary for the MgO hydration reaction was supplied by a micro-feeder at 37 ll/min and mixed with 35 ml/min Ar as carrier gas under a vapour pressure of 57.8 kPa. After the hydration reaction proceeded over 120 min, the water vapour supply was stopped and the sample was kept at 110 °C for 10 min in inert atmosphere (100 ml/min Ar) to remove physically adsorbed water from the sample. This process was repeated for each cycle. A 3 cycles experiment was performed on the samples obtained by DP and impregnation (respectively named DP-MG and I-MG) and on pure Mg(OH)2 (obtained by the precipitation reaction reported in Section 2.1.) to determine the effect of the new synthesis method and the role of EG on the hybrid materials’ performance. The analyzed samples are reported in Table 1. In order to normalize the mass change of the sample that is continuously measured as a function of temperature and time to the initial mass of Mg(OH)2 present in the sample, the reacted fraction (b ½%) can be defined by Eq. (3):
Dmreal 100 b ½% ¼ 1 Dmth
ð3Þ
where Dmreal ½% is the instantaneous real mass change and Dmth ½% is the theoretical mass change due to the dehydration of 1 mol Mg(OH)2 normalized to the total amount present in the sample, respectively expressed by Eqs. (4) and (5):
Dmreal ½% ¼
min minst 100 min
Dmth ½% ¼
MMgðOHÞ2 MMgO 100 w MMgðOHÞ2
ð4Þ
Table 1 Analysed samples. Samples code
Synthesis method
Mg(OH)2 content (wt.%)
Mg(OH)2 I-MG DP-MG
Precipitation Impregnation Deposition–precipitation
100 50 50
Dbd ½% ¼ bid bdf
ð6Þ
Dbh ½% ¼ bh bdf
ð7Þ
where bid ½% and bdf ½% are respectively the reacted fraction at the beginning and at the end of the dehydration treatment. While, bd ½% is the reacted fraction of Mg(OH)2 after dehydration and bh ½% the final reacted fraction of MgO at the point of water supply termination. The stored/released heat per initial mass unit of Mg(OH)2 (Qs/r [kJ/kgMg(OH)2]) can be calculated using Eq. (8):
h i Q s=r kJ=kgMgðOHÞ2 ¼
DH 0 M MgðOHÞ2
Dbd
ð8Þ
being DH0 [kJ/mol] the enthalpy of reaction. The heat output rate (qr [kW/kgMg(OH)2]) is calculated as follows:
h i Q qr kW=kgMgðOHÞ2 ¼ r th
ð9Þ
being th [min] the hydration time.
ð5Þ
where min ½g and minst ½g are respectively the initial sample mass and the instantaneous mass during TG analysis. While, MMgðOHÞ2 ½g=mol and MMgO ½g=mol are respectively the molecular weight of Mg(OH)2 and MgO, and w is the mass ratio between EG and Mg(OH)2. In case of 100% wt. of Mg(OH)2 present in the sample Dmth ¼ 30:89%. The dehydration and hydration conversions (Dbd=h ½%) can be calculated respectively by Eqs. (6) and (7):
2.4. Morphological characterization of storage materials The hybrid materials obtained via the procedure described above were analyzed by means of Scanning Electron Microscopy (JSM-5600-LV JEOL), X-ray Diffraction (D8 Advance Bruker) and BET analysis (Chemisorb 2750 Micromeritics) to define the morphology and crystal structure of samples. Moreover, the samples were characterized after cyclic experiments in order to evaluate the morphological and crystallographic changes occurred.
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Table 2 Dehydration (Dbd) and hydration (Dbh) conversions of a 3 cycles experiment. Samples code
Mg(OH)2 I-MG DP-MG
1st Cycle
2nd Cycle
3rd Cycle
Dbd (%)
Dbh (%)
Dbd (%)
Dbh (%)
Dbd (%)
Dbh (%)
100 101a 109.42a
58.27 66.23 87.54
58.05 67.45 88.31
52.51 60.44 85.60
51.93 59.24 83.85
49.89 57.76 84.62
a The fact that the 1st dehydration reaction conversion of both hybrid materials exceeds 100 means that the real weight change (Dmreal ) is more than the theoretical one (Dmth ) due to 1 wt.% decomposition of the carbonaceous phase (as inferred by the TG profile of mere EG showed in Fig. A1 of Appendix A).
Fig. 2. 2nd dehydration/hydration cycle of a 3 cycles TG experiment.
3. Results 3.1. Thermogravimetric behavior of storage materials The results of a 3 cycles experiment in terms of dehydration and hydration conversions (Dbd/h) for each cycle performed on the hybrid materials and on pure Mg(OH)2 are listed in Table 2. From the reported values it can be observed for each cycle that both hybrid materials have higher reaction conversions than pure Mg (OH)2 and, as a more important result, the DP-MG sample shows higher reaction conversions than I-MG sample. As exemplification of a single cycle experiment, the 2nd cycle profile of the samples mentioned above is illustrated in Fig. 2 where reacted fraction (b [%]) is plotted against reaction time (t [min]). The specimen profile curves show that the hybrid materials have a higher hydration rate. Fig. 3 shows the first 60 min of DP-MG sample single dehydration profiles. A significant change in the thermogravimetric behavior occurs after the 1st dehydration reaction: compared to the 1st dehydration profile (black curve), the beginning of both the 2nd (red curve1) and 3rd (blue curve) dehydration profiles is shifted toward lower temperature. Moreover, after the 1st dehydration the slope of the following reaction profiles increases indicating a higher reaction rate, while the dehydration conversion decreases. This behavior was observed even in pure Mg(OH)2 and I-MG, whose profiles are reported respectively in Fig. A2a and b in Appendix A. 1 For interpretation of color in Fig. 3, the reader is referred to the web version of this article.
The single dehydration reaction conditions, expressed as onset temperature of dehydration reaction (Tonset [°C]) and total reaction time (tr [min]), are reported in Table 3 while the stored heat capacity (Qs [kJ/kgMg(OH)2]) and the heat output rate (qr [kW/kgMg(OH)2]) are reported respectively in Figs. 4 and 5. A lower Tonset is observed in all the samples after each cycle, also reaction time (tr) decreases from 30 to 20 min (15 min in case of pure Mg(OH)2). Consequently, and as shown in Fig. 4, after the 1st dehydration/hydration cycle most part of the heat is stored in the first 30 min from the beginning of the dehydration reaction, while during the 1st dehydration reaction heat is mostly stored during the first 60 min. The DP-MG sample shows the highest stored heat values (with an average stored heat Qs 1200 kJ/kgMg(OH)2) being the sample with the highest dehydration conversion. Moreover, it can respond immediately in case of energy demand since it is able to release most part of stored heat in about 10 min as shown in Fig. 5. 3.2. Morphological and structural modification of the heat storage materials after cyclic experiments Fig. 6 shows the SEM images of the samples I-MG and DP-MG before and after a 3 cycles experiment. The post-cycles specimens obtained by impregnation and DP route have been respectively indicated with the codes CE/I-MG and CE/DP-MG. From the comparison of the I-MG and DP-MG samples (Fig. 6a and b), it emerges that the precipitated Mg(OH)2 in both hybrid materials has a platelike morphology, typical of brucite nanoplates due to its hexagonal structure [26]. Sample DP-MG presents a thin overcoating of EG
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Fig. 3. First 60 min of the single dehydration reactions of DP-MG sample.
Table 3 Single dehydration reaction conditions: initial temperature of dehydration reaction (Tonset) and total reaction time (tr). Sample code
Dehydration reactions 1st
Mg(OH)2 I-MG DP-MG
2nd
3rd
Tonset (°C)
tr (min)
Tonset (°C)
tr (min)
Tonset (°C)
tr (min)
275 285 290
30 30 30
225 225 250
15 20 20
210 225 240
15 20 20
Fig. 4. Comparison of the stored heat capacity (Qs [kJ/kgMg(OH)2]) during each cycle.
surface with Mg(OH)2 nanoplatelets in the range 40–80 nm, while Mg(OH)2 in sample I-MG has a more packed aspect with larger particle dimensions (200–300 nm). This morphology can explain also the specific surface area (SBET [m2/g]) values obtained by BET analysis reported in Table 4. Indeed, the SBET of I-MG sample (38 m2/g) that is close to the value of EG (25 m2/g) indicates that the Mg(OH)2 particles cling compactly to the EG surface; while the higher SBET of the DP-MG hybrid material (50 m2/g) is probably due to the smaller Mg
(OH)2 particle dimensions and to the better dispersion on EG surface. Hence, with the DP method a better dispersion of Mg(OH)2 particles is obtained avoiding the coalescence of small nanoparticles that naturally tend to aggregate to decrease their surface energy. Fig. 6c shows that after only 3 cycles experiment Mg(OH)2 particles have detached from the EG surface in the I-MG sample and, as shown in the miniature in Fig. 6c, the thicker layers of Mg(OH)2 are damaged with the formation of some cracks. On the contrary,
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Fig. 5. Heat output rate (qr [kW/kgMg(OH)2]) of pure Mg(OH)2 (black line), I-MG (blue line) and DP-MG (red line) samples during the 1st hydration reaction. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
the morphology of DP-MG sample (Fig. 6d) is almost unchanged after the dehydration/hydration cyclic experiment: the particle size is not increased and Mg(OH)2 is still attached on EG surface. In both XRD patterns of the hybrid materials as reported in Fig. 7a and b (black lines) peaks relative to highly crystalline brucite (2-theta: 18.5°, 32.5°, 38°, 58.5°, 62°, 68°, 72°) and graphite (2-theta: 26.5°, 42.5°, 45°, 51°, 55°, 78°) can be identified in agreement with standard data, respectively JCPDS 7-0239 and JCPDS 250284. No peaks arising from impurities were detected. Brucite has a hexagonal crystal structure in which each Mg ion is hoctaedrally coordinated by O atoms. The Mg ions lie in plans with the OH ions above and below them [27]. The Mg(OH)2 intensity ratios of reflections [0 0 1] to [1 0 1] and [1 1 0] for all samples (I001/I101, I001/I110) are listed in the following Table 2. XRD patterns of both samples are comparable and Mg(OH)2 particles are preferentially oriented to the (0 0 1) direction. The comparison of XRD patterns of both hybrid materials before (black lines) and after (red lines) the cyclic experiment (reported in Fig. 7a and b) reveal a change in Mg(OH)2 crystal lattice: the peaks relative to brucite are broadened and with lower intensity. Besides, the intensity ratios of the samples (listed in Table 5), more significantly in DP-MG, are increased after cyclic experiment. This means that the number of reflections from the [0 0 1] plans have increased, presumably due to an exfoliation phenomenon in the Mg(OH)2 nanoparticles. Moreover, due to the incomplete hydration reactions (clearly visible from TG analysis) the presence of MgO (2-theta: 42.8°, 62°, 78°) cannot be excluded being the diffraction lines overlapped with those of EG and Mg(OH)2. 3.3. Durability to repeated cyclic experiments The durability of the DP-MG sample was evaluated with a 9 cycles experiment, the profile of which is shown in Fig. 8a. The performance of DP-MG sample is stable with an average dehydration/ hydration conversion of around 83% (Fig. 8b).
3.4. Pellet Due to the presence of EG, the DP-MG sample is easily moldable, thus the sample powder was compressed as pellet and its performance compared with that of the loose powder. A disk of diameter and height respectively of 7.3 and 1.5 mm and density equal to 0.7 g/cm3 was prepared and tested with a 3 cycles experiment by TGA. The experiment was performed using the same amount of loose powder and pellet, that is different volumes being the density of loose powder (0.15 g/cm3) about 4.6 times lower than that of pellet. Since an increase in density could have hindered the vapour flow through the pellet during the dehydration and hydration reactions, a lower performance of the sample was expected. Instead, from the comparison of the TG profiles, DPMG powder and pellet showed a very similar trend (Fig. 9) with an average stored/released heat capacity for both samples equal to 1200 kJ/kgMg(OH)2. This result can be explained considering that in case of pellet two opposing effects act on the vapour flow: on the one hand, the reduction of permeability hinders the vapour flow; on the contrary, the decrease of the bulk material thickness favors the vapour flow. As a result from the combination of these effects, the pellet showed unaltered thermogravimetric behavior compared to that of loose powder. The stored/released heat per unit volume (QVs/r [kJ/cm3]) of the composite can be evaluated by Eq. (10):
Q Vs=r ½kJ=cm3 / Q s=r qc
ð10Þ
being Qs/r [kJ/kgMg(OH)2] the stored/released heat per initial mass unit of Mg(OH)2 (calculated by Eq. (8)) and qc [g/cm3] the density of the composite. Since the density of the loose powder is lower than that of pellet and having both samples the same stored/ released heat per initial mass unit of Mg(OH)2, the stored/released heat per unit volume of the composite as pellet is higher compared to the loose powder.
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Fig. 6. SEM images of samples obtained by impregnation and deposition–precipitation route before and after cyclic experiment: (a) I-MG, (c) DP-MG, (d) CE/I-MG and (e) CE/ DP-MG samples. Table 5 Comparison of the samples XRD intensity ratios before and after cyclic experiment. Table 4 Measures of specific surface area (SBET [m2/g]) on the EG and the hybrid materials. Sample code
EG
I-MG
DP-MG
SBET (m2/g)
25
38
50
Sample code
I-MG CE/I-MG DP-MG CE/DP-MG
Intensity ratios I001/I101
I001/I110
2.07 3.74 1.45 2.96
6.23 8.6 4.8 9.6
Fig. 7. XRD pattern of samples (a) I-MG and (b) DP-MG before (black line) and after (red line) the cyclic experiment. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
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Fig. 8. (a) Dehydration/hydration cycles and (b) dehydration (Dbd) and hydration (Dbh) conversions of a 9 cycles experiment performed on DP-MG sample.
Fig. 9. Comparison of a 3 cycles experiment performed on powder (black line) and pellet (orange line) of DP-MG sample. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
4. Discussion The better results of hybrid materials (in terms of reaction conversions and rate) with respect to pure Mg(OH)2 could be attributed to the presence of the graphitic support which hinders MgO sintering, which in turn has a negative impact not only on the thermodynamic equilibrium but also on the reaction kinetics. Furthermore, the higher reaction conversions of DP-MG compared to I-MG can be explained in light of the different morphology of the two hybrid materials. In fact, not only the thicker Mg(OH)2 layers deposited on EG surface in I-MG sample seem to be detrimental for water vapour diffusion but also the smaller Mg(OH)2 particle size of the DP-MG sample can positively affect the reaction conversions [28]. It can be argued that during the dehydration reaction the water vapour released by the deeper Mg(OH)2 particles which must flow through the coating tends to generate cracks (as noted by SEM
images of I-MG sample in Fig. 6c) after each cycle causing a detachment of the Mg(OH)2 layer from the carbonaceous surface. A thick Mg(OH)2 layer is detrimental even for the hydration reaction; indeed during hydration the surface of the MgO particles is progressively covered by the Mg(OH)2 formed. As a result, the water vapour flow is hindered and the deeper MgO layers cannot be easily reached and hence reconverted in Mg(OH)2. This reduces the overall reaction rate and conversion. The high durability to repeated reactions of the DP-MG sample can probably be explained with reference to the stronger electrostatic interaction between the two components, EG and Mg(OH)2. In facts, in the precipitation conditions the solution pH (11.5) is lower than the pHPZC of Mg(OH)2 (12), but higher than pHPZC of EG (6). Therefore, the surface of Mg(OH)2 is positively charged, while the EG surface is negatively charged. This electrostatic interaction prevents the detachment of Mg(OH)2 particles from the EG surface.
E. Mastronardo et al. / Applied Energy 162 (2016) 31–39
The different thermogravimetric behavior after the 1st dehydration reaction (lower onset temperature of dehydration reaction and decrease of dehydration reaction time) observed in all the samples (reported in Section 3.2.) is likely correlated to the modification of Mg(OH)2 crystal structure as inferred by XRD analysis. In Fig. 7 it is indeed evidenced a variation of the Mg(OH)2 crystal size after the 1st dehydration reaction, which in turn causes an increase of exposed active material. On the basis of the obtained results, in accordance with the hydration mechanism of polycrystalline MgO proposed by Kitamura et al. [29], it can be argued that hydration begins on the grain boundaries near the surface of polycrystalline MgO, which causes grain boundary separation as a consequence of lattice expansion (the volume expansion for the transformation of MgO to Mg(OH)2 is calculated at around 163.5%). The bulk material is therefore separated into finer particles or aggregates dispersed over the graphitic support, which could explain the modifications of XRD spectra.
5. Conclusions A novel synthesis route was used to develop a hybrid storage material for MgO/H2O/Mg(OH)2 CHPs made of Mg(OH)2 nanoplatelets deposited on the graphite surface. The sample, named DP-MG, was prepared through a deposition–precipitation method and its storage and output capacities evaluated. The evaluation demonstrated that the direct growth of Mg(OH)2 on graphite surface in aqueous medium avoided the natural particles coalescence and improved the compatibility between the inorganic (Mg(OH)2) and the organic (EG) phases. The morphology resulting from this new synthesis approach, with a thin layer of Mg(OH)2 nanoparticles finely dispersed on EG surface, led to the improvement of the hybrid system’s efficiency. Compared to the sample named I-MG, obtained by a simple physical mixture of Mg(OH)2 and EG, the DP-MG showed the highest stored heat values, with an average stored heat of about 1200 kJ/kgMg(OH)2. It can also respond immediately to energy demand due to the high heat output rate, releasing most part of stored heat in 10 min. A decrease of the onset temperature of dehydration reaction and total reaction time after the first dehydration/hydration cycle was also observed, which allows for faster, lower-temperature storage. Further analysis showed a good durability to repeated reactions of the DP-MG sample, which is also easily pelletizable without losses in performance. A feasibility study for a large scale MgO/H2O/Mg(OH)2 CHP is planned to be carried out, which will aim to evaluate the kinetic behavior of the hybrid storage material.
Acknowledgements Financial support by University of Messina for this study is acknowledged. The authors would like to thank the Tokyo Institute of Technology for the provided collaboration.
Appendix A. Supplementary material Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.apenergy.2015. 10.066.
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