water chemical heat pump

Applied Thermal Engineering 91 (2015) 377e386

Contents lists available at ScienceDirect

Applied Thermal Engineering journal homepage: www.elsevier.com/locate/apthermeng

Research paper

Energy density enhancement of chemical heat storage material for magnesium oxide/water chemical heat pump Odtsetseg Myagmarjav a, **, Massimiliano Zamengo b, Junichi Ryu c, Yukitaka Kato a, * a

Research Laboratory for Nuclear Reactors, Tokyo Institute of Technology, 2-12-1-N1-22 Ookayama, Meguro-ku, Tokyo 152-8550, Japan Department of Organic and Polymeric Materials, Graduate School of Engineering, Tokyo Institute of Technology, 2-12-1-S8-29 Ookayama, Meguro-ku, Tokyo 152-8550, Japan c Department of Urban Environment Systems, Graduate School of Engineering, Chiba University, 1-33 Yayoi-cho, Inage-ku, Chiba 263-8522, Japan b

h i g h l i g h t s
A new chemical heat storage material, donated as EML, was developed.
EML composite made from pure Mg(OH)2, expanded graphite and lithium bromide.
EML tablet was demonstrated by compressing the EML composite.
Compression force did not degrade the conversion in dehydration and hydration.
EML tablet demonstrated superior heat storage and output performances.

a r t i c l e i n f o

a b s t r a c t

Article history: Received 22 July 2015 Accepted 4 August 2015 Available online 13 August 2015

A novel candidate chemical heat storage material having higher reaction performance and higher thermal conductivity used for magnesium oxide/water chemical heat pump was developed in this study. The material, called EML, was obtained by mixing pure Mg(OH)2 with expanded graphite (EG) and lithium bromide (LiBr), which offer higher thermal conductivity and reactivity, respectively. With the aim to achieve a high energy density, the EML composite was compressed into figure of the EML tablet (f7.1 mm  thickness 3.5 mm). The compression force did not degrade the reaction conversion, and furthermore it enabled us to achieve best heat storage and output performances. The EML tablet could  store heat of 815.4 MJ m3 tab at 300 C within 120 min, which corresponded to almost 4.4 times higher the heat output of the EML composite, and therefore, the EML tablet is the solution which releases more heat in a shorter time. A relatively larger volumetric gross heat output was also recorded for the EML tablet, which was greater than one attained for the EML composite at certain temperatures. As a consequence, it is expected that the EML tablet could respond more quickly to sudden demand of heat from users. It was concluded that the EML tablet demonstrated superior performances. © 2015 Elsevier Ltd. All rights reserved.

Keywords: Thermochemical energy storage Magnesium hydroxide Expanded graphite Lithium bromide Volumetric heat storage/output performances

1. Introduction Thermal energy storage is an important part of the overall solution to serious environmental problems; especially global warming and natural resource depletion, and to achieve sustainable development without sacrificing economic growth. Thermal

* Corresponding author. ** Corresponding author. Present address: IS Process R&D Group, Nuclear Hydrogen and Heat Application Research Center, Japan Atomic Energy Agency (JAEA), 4002 Narita-cho, Oarai-machi, Ibaraki 311-1393, Japan. E-mail address: [email protected] (Y. Kato). http://dx.doi.org/10.1016/j.applthermaleng.2015.08.008 1359-4311/© 2015 Elsevier Ltd. All rights reserved.

energy storage is a technology which designed to accumulate energy when production exceeds demand and to make it available at the user's request. There are three kinds of thermal energy storage systems, namely: 1) sensible heat storage that is based on storing thermal energy by temperature change of liquid or solid storage medium (e.g. water, sand, molten salts, rocks), with water being the cheapest option; 2) latent heat storage using phase change materials (e.g. between a solid state and liquid state); and 3) thermochemical energy storage using chemical reactions for storing and releasing thermal energy. Among these thermal energy storages, the thermochemical energy storage is considered as the most promising thermal energy storage candidate because of higher energy density

378

O. Myagmarjav et al. / Applied Thermal Engineering 91 (2015) 377e386

of the occurring reversible chemical reactions, which can be even higher than what is usually encountered for the other thermal storage processes: sensible and latent heat storages [1e3]. At medium temperature range between 300 and 400  C a magnesium oxide/water (MgO/H2O) chemical heat pump is one of candidate thermochemical energy storages, and it has been studied in this study. The MgO/H2O chemical heat pump uses a reversible chemical reaction between MgO and H2O for thermal energy storage and transfer, and it is based on the following equilibria [4]:

MgOðsÞ þ H2 OðgÞ4MgðOHÞ2 ðsÞ H2 OðgÞ4H2 OðlÞ

DHr+ ¼ 81:0 kJ mol1

DHs+ ¼ 41:0 kJ mol1

(1)

(2)

The forward reaction in Eq. (1), hydration, is exothermic and corresponds to the heat output operation of the heat pump system. The backward reaction in Eq. (1), dehydration, is endothermic and corresponds to the heat storage operation. Based on Mg(OH)2 dehydration and MgO hydration, respectively, heat can be stored at around 300e350  C and re-utilized at temperatures between 110 and 150  C. Even though pure Mg(OH)2 and MgO are considered as promising pure materials for the thermochemical energy storage in terms of its low cost, stability, non-toxicity, the MgO/H2O chemical heat pump is still difficult to be deployed in a practical application due to two issues involved in properties of pure materials such as heat transfer property and reaction performance. The limitation of heat transfer is because of the low thermal conductivity of pure Mg(OH)2 and MgO. For instance, a packed bed of manufactured pure Mg(OH)2 pellets (f2 mm  10 mm of average length) provided a thermal conductivity around 0.15 W m1 K1 [5]. For enhancement of thermal conductivity, expanded graphite (EG) as a high thermal conductive material is a good choice to be mixed with pure Mg(OH)2 powder, which not only increases the thermal conductivity of the pure materials by a factor of 5e10, but also creates a kind of carrier structure that avoids the agglomeration of particles of pure materials [6e9]. Thereby, a mixture of Mg(OH)2/EG has been demonstrated and the thermal conductivity of the Mg(OH)2/EG mixture was measured after forming a slab (20 mm  45.2 mm  110 mm). It has been reported that the thermal conductivity of the Mg(OH)2/EG slab with mass ratio of Mg(OH)2:EG ¼ 8:1 resulted in 1.2 W m1 K1, which was more than 8 times larger than the thermal conductivity of manufactural Mg(OH)2 pellets (0.15 W m1 K1), and moreover, a continuous tight contact between the storage composite and the wall of the heat exchanger was obtained [10]. Even though first issue involved in the heat transfer properties could be solved by introducing of EG, reaction performance of pure materials for relevant to another issue for MgO/H2O chemical heat pump is remained; since EG is a carbon based material. Several investigations have been reported regarding the improvement of reaction performance of pure material for the MgO/H2O chemical heat pump [11e13], because it is essential that the heat storage material be developed. From these studies, it has been well established that the reaction performance, and in particular the dehydration and hydration reactivities could be enhanced by surface modification via high hygroscopic compounds such as LiCl, NaCl, LiOH, NaOH [11e13]. These hygroscopic compounds act to accelerate the release of the H2O product during dehydration and to attract H2O from the surroundings during hydration, thus increasing the reactivity. The hygroscopic property is determined by the value of enthalpy changes during dissolution into water (DHsol) and thereby, in case a compound having a large negative value of DHsol can be a candidate for reactivity enhancer. For candidate hygroscopic compounds such as LiCl, NaCl, LiOH,

NaOH an order of DHsol values is NaOH (44.52 kJ mol1) < LiCl (37.03 kJ mol1) < LiOH (23.56 kJ mol1) < NaCl (3.88 kJ mol1). Even though metal hydroxides have large negative values of DHsol, they are not suggested to use as the reactivity enhancer due to their strong basic property [13]. Ishitobi et al. [13,14] developed new chemical heat storage material, which was proposed to use for MgO/H2O chemical heat pump, by mixing LiCl to pure Mg(OH)2; since LiCl showed large negative value of DHsol so far. The mixture of LiCl/Mg(OH)2 demonstrated the higher reactivity due to high hygroscopic property of LiCl and moreover, LiCl/Mg(OH)2 had a high heat output density. However, the limitation of heat transfer was still remained for the mixture of LiCl/Mg(OH)2. With the aim to achieve a high reaction performance and high thermal conductivity at same time, a novel candidate chemical heat storage material, named EML, was developed. It was obtained by mixing pure Mg(OH)2 with EG and lithium bromide (LiBr), which offer higher thermal conductivity and reactivity, respectively. A potential of the EML composite, and the effects of LiBr and EG on dehydration and hydration reactivity were kinetically investigated based on thermogravimetic analysis in our former studies [15e18]. It was experimentally demonstrated that LiBr was successfully employed as reactivity enhancer because of its strong hygroscopic property which proven by its DHsol value of 48.80 kJ mol1, which was largest negative value among high hygroscopic compounds reported, and particularly, it was negatively greater than that recorded for LiCl (37.03 kJ mol1). The contribution of LiBr into the EML composite also results in good adhesion between Mg(OH)2 particles and the EG surface. For further studies of determining the optimal mixing ratios of LiBr and EG in the EML composite, the experiments were carried out at five different mixing molar ratios of LiBr-to-Mg(OH)2, a: 0.300, 0.100, 0.050, 0.010, 0.005, and seven different mixing mass ratios of EG-to-Mg(OH)2, w: 0.50, 0.67, 0.75, 0.80, 0.83, 0.86 and 0.88 from which the kinetic parameters, i.e., the reaction rate constants and activation energies, were obtained for both reactions. The investigation revealed a of 0.10 (Mg(OH)2:LiBr ¼ 10:1) and w of 0.83 (Mg(OH)2:EG ¼ 5:1) were the optimal mixing molar and mass ratios based on the evaluation of kinetic parameters [17,18]. The objective of this study is to achieve higher energy density for the chemical heat storage material used for MgO/H2O chemical heat pump, and thereby, the EML tablet (f7.1 mm  thickness 3.5 mm) was demonstrated by compressing the EML composite with optimal mixing ratios of a ¼ 0.10 and w ¼ 0.83. Dehydration and hydration behavior of the EML tablet (a ¼ 0.10 and w ¼ 0.83) was investigated experimentally based on thermogravimetric method, and strength points for tablet formation were discussed comparing with the results of the composite. The thermal conductivity measurement was carried out by a quick thermal conductivity meter in order to quantify the enhancement in thermal conductivity, which achieved via presence of EG in the EML. Then, tablet volumetric heat storage and output performances of the EML tablet were evaluated based on the results of dehydration and hydration, respectively. 2. Experimental section 2.1. Sample preparation Firstly, the EML composite was prepared by comprising pure Mg(OH)2 powder (0.07 mm and 99.9%), lithium bromide monohydrate (LiBr$H2O, 99.5%), which provided by Wako Pure Chemical Industries, Ltd., and EG. For preparation of EG, raw-expandable graphite (SS-3, Air Water Inc.) was heated at 700  C for 10 min in an electric muffle furnace. A simple and high-yield impregnation method was used to prepare the EML composite as follows:

O. Myagmarjav et al. / Applied Thermal Engineering 91 (2015) 377e386

379

a) A specified amount of LiBr$H2O was mixed with 200 mL of ethanol; b) Pure Mg(OH)2 powder was added to the LiBr solution; c) The mixture was sonicated for ~20 min to generate a homogenous solution with sufficient particle dispersion; d) The flask containing the sonicated solution was charged with EG and soaked; e) The excess ethanol in the solution was gradually removed by evaporation under reduced pressure for ~120 min; f) The remaining wet product was dried at 120  C in an oven overnight. The composite obtained via the method described above was named EML and it was initially in the hydroxide form. Two mixing ratios were defined in preparing the EML composite since three kinds of materials mixed: 1. The molar mixing ratio a, defined as follows:

a¼

amount of LiBr ½mol amount of MgðOHÞ2 ½mol

Fig. 1. A mold set used for tablet preparation.

(3) (TG), evaporator, water reservoir and argon (Ar) cylinder; they are linked by tubes. Four different types of experiments were performed:

2. The mass mixing ratio w, defined as follows:

w¼

amount of MgðOHÞ2 ½g amounts of MgðOHÞ2 and EG ½g

(4)

For further studies of determining the optimal mixing ratios of LiBr and EG in the EML composite, the experiments were carried out at five different mixing molar ratios of LiBr-to-Mg(OH)2, a: 0.300, 0.100, 0.050, 0.010, 0.005, and seven different mixing mass ratios of EG-to-Mg(OH)2, w: 0.50, 0.67, 0.75, 0.80, 0.83, 0.86 and 0.88 from which the kinetic parameters, i.e., the reaction rate constants and activation energies, were investigated for both reactions in our former studies [16e18]. The investigation revealed a of 0.10 (Mg(OH)2:LiBr ¼ 10:1) and w of 0.83 (Mg(OH)2:EG ¼ 5:1) were the optimal mixing molar and mass ratios based on the evaluation of kinetic parameters [17,18]. Thereby the EML composite (a ¼ 0.10 and w ¼ 0.83) was suggested as the desired material. It was noted that the a ¼ 0.0, w ¼ 1.0 indicates pure Mg(OH)2 which was used as a standard reference sample. Secondly, the EML tablet was produced by compressing the EML composite using a stainless steel mold set as shown in Fig. 1 in order to achieve a high energy density. Simply, preparation procedure of the tablet is as follows: initially, the mold cavity was completely filled with the EML composite (a ¼ 0.10, w ¼ 0.83) and compressed. Via this way the EML tablet (a ¼ 0.10, w ¼ 0.83) was formed in a diameter of 7.1 mm and a thickness of 3.5 mm. This size was attributed to TG cell size having diameter of 7.5 mm and length of 10 mm. Fig. 2 shows some examples of the EML tablets (a ¼ 0.10, w ¼ 0.83) used in the TG experiments. In following discussions one before compressing is abbreviated as “composite” and compressed one is identified as “tablet” hereafter in order to clarify these each other.







Single-cycle experiment Dehydration experiment Hydration experiment Cyclic operation experiment.

Single-cycle experiment consisted of a primary drying process, and dehydration, hydration, and secondary drying processes. In the primary drying process, during the initial 60 min of the experiment, the sample was preheated at 110  C under a purged Ar flow of 100 mL min1 to remove the physically adsorbed water. When the temperature was raised from 110  C to dehydration temperature of 300  C at a heating rate of 20  C min1, dehydration proceeded for 120 min. Then, the temperature was decreased to hydration temperature of 110  C at 20  C min1 also. Here, MgO hydration was performed at a reaction water vapor pressure of 57.8 kPa, achieved by mixing water supplied by a micro feeder at 37 mL-water min1 and Ar as a carrier gas at 35 mL min1. After terminating the vapor

2.2. Experimental procedure for thermogravimetric analysis The dehydration and hydration reactivities of the samples were measured by the thermogravimetric method using a thermobalance (TG-9600; Ulvac Shinku-Riko Inc.) under Ar atmosphere. The apparatus designed to make thermogravimetric analysis from sample mass change consists of main four parts: a thermobalance

Fig. 2. The EML tablets used for TG experiments.

380

O. Myagmarjav et al. / Applied Thermal Engineering 91 (2015) 377e386

supply (140 min), the sample was again kept at 110  C under Ar purge gas at 100 mL min1 for 30 min to remove the physical water from the sample during the second round of drying. A temperature profile for single-cycle experiment is presented in Fig. 3. Each measurement was conducted more than three times by using new samples to determine reproducibility. The mean values obtained from the individual measurements were hereafter employed as the measured results. In the dehydration experiments, the samples were initially dried at 110  C under 100 mL min1 of purged Ar for 60 min. Then, the temperature was raised from 110  C to dehydration temperatures of 200, 220, 240, 270 and 300  C, respectively at a heating rate of 20  C min1. The temperatures were maintained for 300 min. In the hydration experiments, the procedure used was the same as that used in the single-cycle experiment, except that the hydration temperature of 110  C was changed to 130, 150, 170, and 200  C. In the cyclic operation, the procedure was also the same as that of the single-cycle experiment, except for the time required for each process, i.e., primary drying (120 min), dehydration (60 min), hydration (80 min), and secondary drying (30 min). One cyclic operation consists of dehydration and hydration. Ten runs of this cyclic operation were examined in this study. 2.3. Measured values Reacted mole fraction: During the experiment, the sample mass changed because of the movement of water vapor during both dehydration and hydration (Eq. (1)). Using the TG method, this mass change in the sample was continuously measured as a function of temperature and time. Thus, the reacted mole fraction, x [%], was calculated using the following equation:

x¼

1þ

DmH2 O $MMgðOHÞ2 mMgðOHÞ2 $MH2 O

!

DHr+ Dxd $w$rtab $ MMgðOHÞ2 100

(6)

qh;v ¼

DHr+ Dxh $w$rtab $ MMgðOHÞ2 100

(7)

zh;vmean ¼

qh;v th

where, td and th are dehydration and hydration time. The density of the tablet, rtab [kg m3], is defined as the ratio between the mass of the tablet, mtab, and the volume of the tablet, Vtab:

rtab ¼

mtab Vtab

(9)

3.1. Dehydration and hydration behavior of EML tablet

hydration

dehydration

In following discussions, the EML material before compressing is abbreviated as “composite” and compressed one is identified as “tablet” hereafter in order to distinguish these each other. The changes in the reacted mole fraction resulting from the dehydration and hydration processes for the EML (a ¼ 0.10, w ¼ 0.83) tablet are presented in Fig. 5. Additionally, results of the EML (a ¼ 0.10, w ¼ 0.83) composite obtained at identical experimental condition with the tablet are also plotted in this figure in order to exhibit the effects of compression on reactivity in dehydration and hydration.

350

160

o

T = 300 C d

EML composite ( α = 0.10, w = 0.83)

140

3

x

100

150

H2O vapor supply

100

o

T = 110 C h

50

P = 57.8 kPa s

0

50

100 150 200 250 300 350

Measurement time, t [min] Fig. 3. Temperature profile for single-cycle experiment.

300

x

120

200

0

(8)

(5)

350

250

qd;v ¼

3. Results and discussion

$100

where DmH2 O [g] is the mass change in the sample caused by the reaction, mMgðOHÞ2 [g] is the initial mass of Mg(OH)2 charged in the sample, and MMgðOHÞ2 and MH2 O [g mol1] are the molecular masses of Mg(OH)2 and H2O, respectively. The reacted mole fraction of Mg(OH)2 at beginning and after dehydration are denoted as x1 and x2, final the reacted mole

300

fraction of MgO at the point of water supply termination is denoted as x3, and that of Mg(OH)2 after the drying operation of 30 min is denoted as x4. The conversion of dehydration, Dxd [%], is defined as Dxd ¼ x1  x2, the conversion of hydration, Dxh [%], is defined as Dxh ¼ x4  x2, and the apparent change in x caused by sorption, Dxs [%], is defined as Dxs ¼ x3  x4, respectively. These definitions are presented in Fig. 4, which shows example of one cycle experiment for the EML composite (a ¼ 0.10, w ¼ 0.83). Volumetric Heat Storage and Output Performance of Tablet: The amount of stored energy per unit volume is the key indicator for the quantity of heat storage. Therefore, tablet volumetric heat storage capacity, qd,v [MJ m3 tab], tablet volumetric gross heat output 3 qh,v [MJ m3 tab], volumetric mean heat output rate zh,v-mean [kW mtab] are expressed per unit volume of the tablet as follows:

1

80

Δx

d

60

Δx

s

x

Δx

150

h

100

x

0

20 0

200

4

40

x

2

0

250

50 100 150 200 250 300 350

50 0

Measurement time [min] Fig. 4. Definitions of conversion in dehydration (Dxd) and hydration (Dxh).

O. Myagmarjav et al. / Applied Thermal Engineering 91 (2015) 377e386

Td = 300 oC, Th = 110 oC, Ps = 57.8 kPa

140

EML com. ( = 0.1, w = 0.83)

120

350 300

100

250

80

200

60

EML tab. ( = 0.1, w = 0.83)

40 20

150 100 50

0 0

50 100 150 200 250 300 350 Measurement time, t [min]

0

Fig. 5. Dehydration and hydration behavior of the EML tablet and composite (a ¼ 0.10, w ¼ 0.83).

Herein, the open symbol represents data of tablet whereas the solid one indicates data of composite. As it can be seen from a comparison plot, dehydration and hydration proceeded slightly slowly for the tablet in comparison to the composite at initial part of each process. It might be because of enough water diffusion for the composite, which allows accelerating the dehydration and hydration rate. Despite that, the tablet showed higher conversions in dehydration (Dxd) and hydration (Dxh), which were estimated at over 120 min of dehydration and 140 min of hydration, respectively. Differences of Dxd and Dxh values resulted as 9.8 and 4.5%, respectively. As a result, the compression force did not degrade the reaction conversion in the dehydration and hydration. After completing TG experiment, surfaces of the EML tablet were characterized by optical microscope and scanning electron microscope (SEM), respectively. As illustrated in Fig. 6(a) and (b), which presents the microscope images of the top surfaces of the EML tablet before and after the TG experiment, no significant difference on the top surface of fresh and used tablets was appeared. This result was proven by finding no obvious change on the outer surfaces of the used tablet as shown in Fig. 7(a) and (b), the microscope images taken for outer surfaces both fresh and used tablets. As a consequence, further characterizations on the surface of the tablets had done by SEM. Figs. 8 and 9(a) and (b) show images of SEM at magnifications of 50 and 1k taken on the top surfaces of the EML tablet before and after the experiment. This time some small cracks were appeared (Figs. 8 and 9(b)), particularly at high magnification of 1k which

381

indicated by arrow in the figure. Conversely, continues layer was resulted for the fresh tablet. Those cracks presumed to be produced due to movement of water vapor during the dehydration and hydration. As a result, generation of cracks is believed to pass the water vapor smoothly, thus reaction conversion was enhanced. To further investigate dehydration of the EML tablet, the dehydration measurements were conducted at five different temperatures of 200, 220, 240, 270 and 300  C, respectively, from which the mole reacted fractions of dehydration were determined. A comparison plot of mole reacted fraction of dehydration at 200, 220, 240, 270 and 300  C for the EML (a ¼ 0.10, w ¼ 0.83) tablet is presented in Fig. 10, which also includes the data recorded for the EML composite at identical experimental condition. It was assumed that dehydration at 0 min corresponded to the time at which the sample possessed 90% of reacted fraction due to the effect of removal of physically adsorbed water during the initial dehydration period, as revealed from the dehydration kinetic calculation. For both tablet and composite it was similarly appeared that the reacted mole fraction of dehydration increased with increasing reaction temperature due to higher temperatures producing the larger dehydration reactivity. Comparing to the composite, the dehydration occurred slowly for the tablet. It resulted that a longer time was needed for the tablet to attain same conversion with the composite, and however, it was depended on dehydration temperature. For instance, it is required about 20 min to obtain similar xd value at higher dehydration temperature of 300  C. When the temperature decreased from 300 to 270  C, a time to achieve same xd value was extended till 70 min or more. It suggests that the tablet would be suitable for prompt storage of heat at around 300  C; since the faster rate of dehydration can be translated in reducing the time for the heat storage. Subsequently, the effects of temperature on the hydration reactivity of the tablet were investigated. For this purpose, hydration experiments for the tablet were carried out at 110, 150, 170 and 200  C after dehydration at 300  C for 120 min. Fig. 11 compares the reacted mole fraction of hydration for the EML (a ¼ 0.10, w ¼ 0.83) tablet and composite at 110, 150, 170 and 200  C. As it is illustrated, the tablet showed almost similar hydration reactivity to the composite at all temperature studied entire of the process. This result confirms that the compression force did not degrade the hydration reactivity, especially at higher hydration temperature. It is a significant result to exhibit a merit of the tablet production. Moreover, it could be seen that the reacted mole fraction of hydration increased with decreasing temperature and showed a maximum at 110  C for both tablet and composite, indicating that the hydration was enhanced at lower temperatures. Moreover, the hydration reactivities could be maintained at temperature of 200  C. It was concluded that the EML tablet is able to store and transfer heat at same temperature of 200  C. This observation is important in regard to put the EML tablet in the commercial usage.

Fig. 6. Microscope images of the top surface of the EML tablet (a ¼ 0.10, w ¼ 0.83) before (a) and after (b) experiment (Td ¼ 300  C, Th ¼ 110  C, and Ps ¼ 57.8 kPa).

382

O. Myagmarjav et al. / Applied Thermal Engineering 91 (2015) 377e386

Fig. 7. Microscope images of the outer surface of the EML tablet (a ¼ 0.10, w ¼ 0.83) before (a) and after (b) experiment (Td ¼ 300  C, Th ¼ 110  C, and Ps ¼ 57.8 kPa).

Fig. 8. SEM images the top surface of the EML tablet (a ¼ 0.10, w ¼ 0.83) before (a) and after (b) experiment (Td ¼ 300  C, Th ¼ 110  C, and Ps ¼ 57.8 kPa) at magnification of 50.

Fig. 9. SEM images the top surface of the EML tablet (a ¼ 0.10, w ¼ 0.83) before (a) and after (b) experiment (Td ¼ 300  C, Th ¼ 110  C, and Ps ¼ 57.8 kPa) at magnification of 1k.

Figs. 12 and 13 show the Arrhenius plots of dehydration and hydration of the EML (a ¼ 0.10, w ¼ 0.83) tablet and composite, respectively based on the data presented in Figs. 10 and 11 applying a first-order reaction model and an ash diffusion control of shrinking core model, respectively for the kinetics analyses for dehydration and hydration. It was demonstrated in our earlier studies that those models showed good agreement with the experimental data attained for the EML composite [15e18]. Moreover, the basis of these studies showed that the first order reaction models and shrinking core model are the best simple representation for the majority of reacting MgOeH2O system [19e22]. It was proven also here by obtaining straight-lines with high correlation coefficients values of 0.958e0.998 from the Arrhenius plots of dehydration and hydration; notably, those models are satisfactory for explaining the dehydration and hydration of the composite as well the tablet. The activation energy for dehydration, Ea-d, determined from the slope of the Arrhenius plot for the tablet was 125.4 kJ mol1, which was greater than the value 117.8 kJ mol1 calculated for the EML composite. It might be due to poor vapor diffusivity in the tablet. Conversely, activation energy in hydration,

Ea-h, was calculated as 54.6 kJ mol1 for the tablet, whilst it was equal to 64.2 kJ mol1 for the composite. A minus sign is placed before the Ea-h value because it calculated from overall reaction rate constants; since it consists of chemical reaction and adsorption reaction. For the adsorption reaction, the rate tends to decrease with increasing temperature and thereby, the activation energy is always a negative [4].

3.2. Cyclic ability of EML tablet A cyclic ability of the tablet was investigated. The dehydrationehydration profiles of the EML (a ¼ 0.10, w ¼ 0.83) tablet over the ten cycles are shown in Fig. 14, from which Dxh values were determined each cycle. It was found that the value of Dxh was as low as ~97.7% in the first cycle. From the 2nd cycle it increased progressively between the 1st and the10th cycles, and it became nearly constant at ~100%. This result showed that the EML tablet was able to withstand repetitive cyclic reactions without significant failures.

O. Myagmarjav et al. / Applied Thermal Engineering 91 (2015) 377e386

-2

EML ( = 0.10, w = 0.83) composite tablet

com

y = 18.471 - 14.17x R= 0.99804

tab

y = 19.796 - 15.09x R= 0.99849

-4

100

-6

o

T = 200 C

80

383

h

"composite"

-8 60

-10

o

240 C

40 20

-14 o

300 C

0

"tablet"

-12

o

270 C

0

1.5 1.6 1.7 1.8 1.9

20

40

60

80

100

2

2.1 2.2 2.3

-1

1000/T [K ] d

Dehydration time, t [min] d

Fig. 10. Comparison of mole reacted fraction of dehydration of the EML tablet and composite (a ¼ 0.10, w ¼ 0.83) at 200, 240, 270, and 300  C.

Fig. 12. Arrhenius plots of dehydration for the EML tablet and composite (a ¼ 0.10, w ¼ 0.83).

-6 EML ( = 0.10, w = 0.83) p-110 composite tablet tab-110

120

com

y = -28.909 + 7.7168x R= 0.98091

tab

y = -26.045 + 6.5713x R= 0.95849

-8 "tablet"

o

T = 110 C

-10

h

100

o

150 C

"composite"

80

-12

60 170 C

2

20 0

-14

o

40

2.2

o

40

80

2.6

2.8

-1

200 C

0

2.4

1000/T [K ] 120

160

Hydration time, t [min] h

Fig. 11. Comparison mole reacted fraction of hydration of the EML tablet and composite (a ¼ 0.10, w ¼ 0.83) at 110, 150, 170 and 200  C.

h

Fig. 13. Arrhenius plots of hydration for the EML tablet and composite (a ¼ 0.10, w ¼ 0.83).

350

140

EML tablet ( = 0.10, w = 0.83)

120 3.3. Thermal conductivity of EML slab Unique points promoting the EML tablet to be a novel chemical heat storage material are the presence of EG which provides larger thermal conductivity, and compression form. It has been well established that utilization of EG gives a high thermal conductivity [5e9]. In order to quantify the enhancement in thermal conductivity, achieved via the presence of EG in the EML, the thermal conductivity measurement was carried out by a quick thermal conductivity meter (QTM-500, Kyoto Electronics). To do that, the EML slab has been produced in size of 110 mm in length, 20 mm in width and different sizes of thickness: 32.6, 35.8, 39.7, and 45.2 mm. By changing thickness of the slab, it was a possible to

300

100

250

80

200

60

150

40

100

20

50

0 0

500

1000

1500

2000

0

Measurement time, t [min] Fig. 14. Dehydration and hydration profiles of the EML tablet and composite (a ¼ 0.10, w ¼ 0.83) during ten repetitive reaction cycles.

384

O. Myagmarjav et al. / Applied Thermal Engineering 91 (2015) 377e386

characterize the slab in function of density; since other two sides (length and width) are constant. In this way the mass of the slab was unchanged and its volume decreased due to change of thickness. This measurement method was reported in previous work by Zamengo et al. [5,10]. Four different densities of the EML slab were examined: 0.60, 0.69, 0.77 and 0.83 g cm3 and corresponding thicknesses of 45.2, 39.7, 35.8, and 32.6 mm. It must be noted that the thermal conductivity was measured in two directions because the EML slab showed anisotropic properties. Thereby, the ways to measure thermal conductivity were in the direction parallel and perpendicular to the direction of compression. Then the measurements of thermal conductivity were measured on the 4 faces of the slabs. The measurements were repeated 5 times on every face. A comparison of the thermal conductivities of the EML slabs which measured in directions parallel and perpendicular to the direction of compression, respectively, is displayed in Fig. 15. It is evident that the thermal conductivity measured in the direction perpendicular to the direction of compression was relatively greater than that to the direction of parallel. This effect is due to the plastic deformation of EG, which flattens and forms horizontal layers. Critoph et al. [23] observed similar behavior for an activated carbon-expanded graphite composite. Fig. 16 shows a comparison of thermal conductivities of the EML slab (measured in the directions parallel and perpendicular to the direction of compression of the slab) at certain density of 0.83 g cm3. Additionally, data of pure Mg(OH)2 slab is also presented in this figure. It was noticed that the value of the thermal conductivity of the EML slab in the direction perpendicular to the direction of compression was 1.91 W m1 K1 at density of 0.833 g cm3. This value is 6.8 times larger than the corresponding value for the slab made of compressed Mg(OH)2 having density of 1.0 g cm3 (0.28 W m1 K1). Moreover, this value was greater than the one obtained by Zamengo et al. [10] (1.2 W m1 K1) resulted for the Mg(OH)2/EG slab (20 mm  45.2 mm  110 mm) with mass ratio of Mg(OH)2:EG ¼ 8:1. Thus, it was concluded that the EML slab demonstrated a high thermal conductivity. 3.4. Volumetric heat storage and output performances of EML tablet

2

1.91

1.5 1.15

1

0.5 0.28

0 EML slab

( )

Mg(OH) 2 slab

EML slab (II)

-3

-3

( = 0.83 g cm )

( = 0.83 g cm )

( = 1.0 g cm -3)

Fig. 16. Comparison of thermal conductivities of the EML slab (a ¼ 0.10, w ¼ 0.83) measured in the parallel and perpendicular direction to compression, and Mg(OH)2 slab measured in the perp. direction to compression.

volumetric heat storage and output performances of the EML tablet were evaluated, respectively. The amount of stored energy per unit volume of the tablet is the key indicator for the quantity of heat storage. Therefore, the volumetric heat storage capacity, qd,v 3 [MJ m3 tab], and the volumetric gross heat output, qh,v [MJ mtab], 3 volumetric mean heat output zh,v-mean [kW mtab] expressed per unit volume of the EML tablet were calculated using Eqs. (6)e(8) based on data of the dehydration and hydration given in Figs. 10 and 11, respectively. Fig. 17 presents a comparison of qd,v values of the EML (a ¼ 0.10, w ¼ 0.83) tablet having a density of 0.73 g cm3, and composite having a density of 0.18 g cm3 evaluated at Td ¼ 240 and 300  C, respectively based on data of the dehydration given in Fig. 10.

The EML tablet was demonstrated by compressing the EML composite in order to achieve a high energy density, therefore

EML ( = 0.10, w = 0.83) ( II ) measure in the direction parallel to direction of compression ( ) measure in the direction perpendicular to direction of compression

2

tablet composite

1000 800

o

Mg(OH) tab. @300 C

1.6

2

600

( )

o

T = 300 C d

1.2

400

0.8

200

( II )

0

0.4 0.5

o

240 C

0.6 Density,

0.7

0.8

0.9

-3

[g cm ]

Fig. 15. A comparison of thermal conductivities of the EML slab (a ¼ 0.10, w ¼ 0.83) measured in the parallel and perpendicular direction to compression.

0

20

40

60

80

100

120

Time, t [min] Fig. 17. Volumetric heat storage capacities of the EML (a ¼ 0.10, w ¼ 0.83) tablet having 0.73 g cm3 and composite having 0.18 g cm3 at Td ¼ 240, 300  C and Mg(OH)2 tablet having 0.73 g cm3 at 300  C.

O. Myagmarjav et al. / Applied Thermal Engineering 91 (2015) 377e386

Additionally, the Mg(OH)2 tablet with a density of 0.73 g cm3 was produced and its qd,v value estimated at 300  C was also inserted in this figure. It was found that the best heat storage performance was given by the EML tablet as it could store heat of 815.4 MJ m3 tab at 300  C within 120 min, which corresponded to almost 4.4 times higher the heat stored by the EML composite at identical temperature and period. In case of pure Mg(OH)2 tablet at 300  C within 120 min it could store 531.4 MJ m3 tab, which was indeed 2.8 times larger with respect to the EML composite. In addition, faster rate recorded for the EML tablet as evidenced by the steeper slope translated in reducing the time for the heat storage. For instance, within 30 min the EML tablet was able to store highest thermal energy of 727.8 MJ m3 tab with fast rate. Consequently, this result revealed a strength point of the EML tablet having superior heat storage performance. Now an attention is turned to compare volumetric heat output capacities, qh,v, of the EML (a ¼ 0.10, w ¼ 0.83) tablet having 0.73 g cm3 and composite having 0.18 g cm3, which evaluated based on data of the hydration given in Fig. 11. The results obtained at Th ¼ 110 and 170  C are plotted in Fig. 18, which also including data of the Mg(OH)2 tablet with 0.73 g cm3 at 110  C to exhibit the strength point of the tablet on output performance. A relatively larger volumetric gross heat output was also recorded by the EML  tablet, and it was equal to 898.3 MJ m3 tab at 110 C during 100 min operation. At identical condition, indeed the EML composite and pure Mg(OH)2 tablet had just qh,v values of 224.1 and 335.8 MJ m3 tab, respectively. As a result, the tablet is the solution which releases a higher amount of heat in comparison with the composite. Fig. 19 presents volumetric mean heat output rate, zh,v-mean, of the EML tablet (a ¼ 0.10, w ¼ 0.83) evaluated at 110, 150, 170 and 200  C and pure Mg(OH)2 tablet at 110  C based on results from the hydration experiment as indicated in Fig. 11. The peak value of zh,vmean calculated for the EML tablet decreased progressively for the Th of 110, 150, 170 and 200  C, in the same order. As consequence, a largest zh,v-mean value was obtained at 110  C, whilst lowest the value was recorded at 200  C. Furthermore, the zh,v-mean value attained for the EML tablet even at 200  C better than pure Mg(OH)2 tablet at 110  C at initial period of hydration. Based on these results, the EML tablet is the solution which releases more heat in a shorter time in comparison with the Mg(OH)2 tablet. It

EML (α = 0.10, w = 0.83)

1000 tablet composite

800 o

T = 110 C d

600

o

Mg(OH) tab. @110 C 2

o

170 C

400 200 0

0

20

40

60

80

100

Time, t [min] Fig. 18. Volumetric heat output capacities of the EML (a ¼ 0.10, w ¼ 0.83) tablet having 0.73 g cm3 and composite having 0.18 g cm3 at Th ¼ 110, 170  C and Mg(OH)2 tablet having 0.73 g cm3 at 110  C.

385

1200

1200 T = 110 C

qh-110 qh-150 150 C qh-170 170 C qh-200 qh-pure 110

1000

1000

200 C

800

800

110 C (pure)

600

600

400

400

200

200

0

0

20

40

60

80

100

120

0 140

Time, t [min] Fig. 19. Volumetric mean heat output rate and capacities of the EML (a ¼ 0.10, w ¼ 0.83) tablet having 0.73 g cm3 at Th ¼ 110, 150, 170, 200  C and pure Mg(OH)2 tablet having 0.73 g cm3 at 110  C under Ps ¼ 57.8 kPa and Td ¼ 300  C.

would respond more quickly to sudden demand of heat from users. For explaining this effect, the qh,v-values were also plotted. Specifically, the qh,v value at 110  C after 140 min for the EML tablet was 994.3 MJ m3 tab which was corresponded to 2.9 times the heat output  of pure Mg(OH)2 tablet (343.3 MJ m3 tab) at 110 C. Indeed, the qh,v value at 170  C was 538.4 MJ m3 , which was still larger than pure tab Mg(OH)2 tablet. It was concluded that the EML tablet demonstrated superior performance with respect to the EML composite and pure Mg(OH)2 tablet. 4. Conclusions With the aim to achieve a high energy density, the EML tablet (f7.1 mm  thickness 3.5 mm) was demonstrated by compressing the EML composite with optimal mixing ratios of a ¼ 0.10 and w ¼ 0.83. Results exhibited that compression force did not degrade the reactivity in dehydration and hydration. Moreover, the EML composite demonstrated to be withstanding ten repetitive cyclic reactions without significant failures. The value of the thermal conductivity of the EML slab (20 mm  32.6  110 mm) with a density of 0.833 g cm3 in the direction perpendicular to the direction of compression was 1.91 W m1 K1. This value was 6.8 times larger than the corresponding value for the slab made of compressed Mg(OH)2 having a density of 1.0 g cm3 (0.28 W m1 K1), and it was 10 times larger than the thermal conductivity of manufactured Mg(OH)2 pellets (0.15 W m1 K1). The best heat storage performance was given by the EML tablet  as it could store heat of 815.4 MJ m3 tab at 300 C within 120 min, which corresponded to almost 4.4 times higher the heat output of the EML composite at identical temperature and period. Even pure Mg(OH)2 tablet at 300  C within 120 min it could store 531.4 MJ m3 tab, which was indeed 2.8 times larger with respect to the EML composite. Furthermore, a higher heat output perfor mance of 994.3 MJ m3 tab at 110 C after 140 min of operation resulted for the EML tablet. Indeed, it was corresponded to 2.9 times the  heat output of pure Mg(OH)2 tablet (343.3 MJ m3 tab) at 110 C. It was concluded that the EML tablet demonstrated superior performance with respect to the EML composite and pure Mg(OH)2 tablet.

386

O. Myagmarjav et al. / Applied Thermal Engineering 91 (2015) 377e386

Acknowledgements This study was executed with the financial support of the Grantin-Aid for Scientific Research (B) #24360404 from the Ministry of Education, Culture, Sports, Science and Technology (MEXT), Japan. The authors greatly thank to this foundation. References [1] A. Fernandez, M. Martínez, M. Segarra, L. Martorell, L. Cabeza, Selection of materials with potential in sensible thermal energy storage, Sol. Energy Mater. Sol. Cells 94 (2010) 1723e1729. [2] P. Tatsidjodoung, N.L. Pieres, L. Luo, A review of potential materials for thermal energy storage in building applications, Renew. Sustain. Energy Rev. 18 (2013) 327e349. [3] A. Gil, M. Medrano, I. Martorell, A. Lazaro, P. Dolado, B. Zalba, State of the art on high temperature thermal energy storage for power generation: Part 1 e concepts, materials and metallization, Renew. Sustain. Energy Rev. 14 (2010) 31e55. [4] Y. Kato, N. Yamashita, K. Kobayashi, Y. Yoshizawa, Kinetic study of the hydration of magnesium oxide for a chemical heat pump, Appl. Therm. Eng. 16 (1996) 853e862. [5] M. Zamengo, J. Ryu, Y. Kato, Thermochemical performance of magnesium hydroxide-expanded graphite pellets for chemical heat pump, Appl. Therm. Eng. 64 (2014) 339e347. [6] K. Fujioka, K. Hatanaka, Y. Hirata, Composite reactants of calcium chloride combined with functional carbon materials for chemical heat pumps, Appl. Therm. Eng. 28 (2008) 304e310. [7] R. Olives, S. Mauran, A highly conductive porous medium for solidegas reactions: effect of the dispersed phase on the thermal tortousity, Transp. Porous Media 43 (2001) 377e394. [8] S. Mauran, P. Prades, F. L'Haridon, Heat and mass transfer in consolidated reacting beds for thermochemical systems, Heat Recovery Syst. CHP 13 (1993) 315e319. [9] M. Bonnissel, L. Luo, D. Tondeur, Compacted exfoliated natural graphite as heat conduction medium, Carbon 39 (2001) 2151e2161. [10] M. Zamengo, J. Ryu, Y. Kato, Composite block of magnesium hydroxideexpanded graphite for chemical heat storage and heat pump, Appl. Therm. Eng. 69 (2014) 29e38. [11] J. Ryu, N. Hirao, R. Takahashi, Y. Kato, Dehydration behavior of metalmodified-added magnesium hydroxide as chemical heat storage Media, Chem. Lett. 37 (2008) 1140e1141. [12] J. Ryu, R. Takahashi, N. Hirao, Y. Kato, Effect of transition metal mixing on reactivities of magnesium oxide for chemical heat pump, JCEJ 40 (2007) 1281e1286. [13] H. Ishitobi, K. Uruma, M. Takeuchi, J. Ryu, Y. Kato, Dehydration and hydration behavior of metal-salt-modified materials for chemical heat pumps, Appl. Therm. Eng. 50 (2011) 1639e1644. [14] H. Ishitobi, N. Hirao, J. Ryu, Y. Kato, Evaluation of heat output densities of lithium chloride-modified magnesium hydroxide for thermochemical energy storage, Ind. Eng. Chem. 52 (2013) 5321e5325. [15] O. Myagmarjav, J. Ryu, Y. Kato, Lithium bromide-mediated reaction performance enhancement of a chemical heat-storage material for magnesium oxide/water chemical heat pumps, Appl. Therm. Eng. 63 (2014) 170e176. [16] O. Myagmarjav, J. Ryu, Y. Kato, Kinetic analysis of the effects of mixing mole ratios of LiBr-to-Mg(OH)2 on dehydration and hydration, JCEJ 47 (2014) 595e601.

[17] O. Myagmarjav, J. Ryu, Y. Kato, Dehydration kinetic study of a chemical heat storage material with lithium bromide for a magnesium oxide/water chemical heat pump, Prog. Nucl. Energy 82 (2015) 153e158. [18] O. Myagmarjav, J. Ryu, Y. Kato, Waste heat recovery from iron production by using magnesium oxide/water chemical heat pump as thermal energy storage, ISIJ Int. 55 (2015) 464e472. [19] R.S. Gordon, W.D. Kingery, Thermal decomposition of Brucite: II, kinetics of decomposition in vacuum, J. Am. Ceram. Soc. 50 (1967) 8e14. [20] G.L. Smithson, N.N. Bakhshi, The kinetics and mechanism of the hydration of magnesium oxide in a batch reactor, Chem. Eng. 47 (1969) 508e513. [21] A. Kitamura, K. Onizuka, K. Tanaka, Hydration characteristics of magnesia, Taikabutsu Overseas 16 (1969) 3e11. [22] S.T. Kim, J. Ryu, Y. Kato, Reactivity enhancement of chemical materials used in packed bed reactor of chemical heat pump, Prog. Nucl. Energy 53 (2011) 1027e1033. [23] L.W. Wang, Z. Tamainot-Telto, R. Thorpe, R.E. Critoph, S.J. Metcalf, R.Z. Wang, Study of thermal conductivity, permeability, and adsorption performance of consolidated composite activated carbon adsorbent for refrigeration, Renew. Energy 36 (2011) 2062e2066.

Nomenclature Ea-d: activation energy for dehydration [kJ mol1] Ea-h: activation energy for hydration [kJ mol1] MMgðOHÞ2 : molecular mass of Mg(OH)2 [kg mol1] MH2 O : molecular mass of H2O [kg mol1] mMgðOHÞ2 : initial mass of Mg(OH)2 charged in the sample [kg] mtab: mass of tablet [kg] Ps: pressure of water vapor [kPa] qd,v: volumetric heat storage capacity [MJ m3 tab] qh,v: volumetric heat output capacity [MJ m3 tab] R: gas constant [¼8.314 kJ mol1 K1)] Rl: correlation coefficient [e] Td: temperature for dehydration [K] Th: temperature for hydration [K] t: measurement time [s] td: dehydration time [s] th: hydration time [s] Vtab: volume of tablet [m3] w: mixing mass ratio [e] x: reacted mole fraction [%] xd: reacted mole fraction in dehydration [%] xh: reacted mole fraction in hydration [%] x1: reacted mole fraction at point of dehydration start [%] x2: reacted mole fraction after dehydration [%] x3: reacted mole fraction at the point of water supply termination [%] x4: reacted mole fraction after the drying operation of 30 min [%] zh,v-mean: volumetric mean heat output rate [MJ m3 tab] a: mixing molar ratio [e] r: density [kg m3]  DHr : enthalpy change of reaction [kJ mol1]  DHs : enthalpy change of sorption [kJ mol1] DHsol: enthalpy changes during dissolution into water [kJ mol1] DmH2 O : mass change in the sample caused by reaction [kg] Dxd: conversion of dehydration [%] Dxh: conversion of hydration [%] Dxs: apparent change in the reacted mole fraction by sorption [%]