3.1. Textural Properties
Figure 1 shows the ER-SEM images of EVM, EVMLiCl10, and EVMLiCl20. As seen in
Figure 1a, the pores in EVM are formed by the stacking of numerous layers, and irregular shapes are presented. In
Figure 1b,c, the small white spots represent the aggregates of LiCl crystals. It is shown that LiCl crystals distributed separately on the surface of the EVM layers. The regularity of distribution is influenced by morphology and roughness of the layers. Typically, with larger salt content, more salt crystals are observed on EVMLiCl20.
Figure 1d is 25 times magnified image of the red circle in
Figure 1c, which represents the aggregation state of LiCl crystals adhered to the surface of pores inside EVM. It can be seen that the LiCl crystals array orderly on the surface of the layer.
Pore structure parameters of EVM and EVM/LiCl composite sorbents are shown in
Table 2. The pore volume of pure EVM is 4.7626 cm
3/g. When impregnating LiCl into pores of EVM, a decreased pore volume is induced by three major cases: (1) part of open pores is blocked by LiCl crystals; (2) newly closed pores are created by the stacking of LiCl crystals; (3) other parts of the volume are occupied by LiCl crystals. As seen in
Table 2, the pore volume which is related to the EVM mass of composite sorbents is almost constant, slightly ranging from 2.6 to 2.9 cm
3/g, which is 0.55–0.62 times smaller than that of pure EVM. As the volume of LiCl crystals is lower than 0.3 cm
3/g, this decrease of pore volume is mainly ascribed to reasons (1) and (2) described above. Corresponding to the decrease of pore volume, the pore area oppositely increases from 2.623 to 3.121 m
2/g. Pore diameter lies in the range of 7300–8000 nm, and its connection/relation to salt content is weak.
3.2. Sorption Kinetics
Figure 2 shows the sorption kinetics of pure EVM, bulk LiCl crystals, and EVM/LiCl sorbents at 30 °C and 60% RH. Physical sorption rate of EVM is extremely fast, but the equilibrium value is only 0.04 g/g, which is ascribed to its macrospore structure. Capillary condensation of water vapor hardly happens in pores whose size are larger than 50 nm, thus the sorption performance of EVM is similar to that on ordinary flat, solid surface. For composite sorbents, mass transfer gets weaker with accumulating salt confined to pores in EVM, which requires more time to reach sorption equilibrium. Ninety-five percent of total water uptake of EVMLiCl5, EVMLiCl10, and EVMLiCl20 is reached in 90 min, 180 min, and 270 min, respectively. With enlarged salt content, the total water uptake increases from 0.49 g/g to 1.38 g/g. The sorption rate of pure LiCl is evidently lower than that of composite sorbents, and the sorption equilibrium is not reached until 690 min. This tremendous difference in equilibrium times verifies that sorption kinetics of LiCl is strengthened, which is attributed to the dispersed LiCl crystals in the pore structure of EVM. This porous structure in EVM provides vapor transport paths as well as heat transfer paths to LiCl crystals, and as a consequence sorption performance is promoted.
Due to the very poor sorption capacity of EVM, the vast majority of sorption process is caused by the reaction between LiCl and water vapor. The DRH of LiCl is only 11% at 30 °C, which guarantees the successful hydration reaction between LiCl and water vapor as well as liquid–gas absorption of LiCl solution where LiCl is finally dissolved in the solution at sorption equilibrium phase. According to the mechanics of salt solution absorption, absorption equilibrium is reached when saturated water vapor above aqueous solution of LiCl equals water vapor pressure of ambient air. Equilibrium mass concentration of LiCl solution is 23.7% at 30 °C and 60% RH based on the fitting formula summarized by Conde et al. [
24], and the mass concentration is decided by temperature and water vapor pressure (which can be calculated from temperature and related humidity of moist air).
To evaluate the influence of EVM on absorption equilibrium of LiCl solution at 30 °C and 60% RH, mass concentration and volume of the LiCl solution formed in EVM pores are calculated by formulas below (assuming the sorption of EVM is not affected by LiCl and is related to its mass coefficients):
where
w is mass concentration of LiCl solution in EVM pores,
η is salt content of composite sorbents,
xcs and
xEVM are corresponding equilibrium water uptake of composite sorbents and raw EVM respectively (obtained from
Figure 2),
vs is the volume of LiCl solution, and
ρs is the density of LiCl solution. The value of
ρs is calculated by the simulation formula given by Conde et al. [
24], and the value is in the function of temperature and mass concentration of LiCl solution.
Calculation results are shown in
Figure 3 and
Figure 4. As seen from
Figure 3, the mass concentration of LiCl solution generated in sorption process reveals a tenuous decreasing tendency with the increase of salt content in composite sorbents. While in the same condition, pure LiCl crystals without the restriction of a matrix achieve a 1.2 times higher mass concentration at equilibrium phase. This result demonstrates that sorption kinetics of LiCl crystals is enhanced when confined to host matrix pores.
Figure 4 shows the comparison of LiCl solution volume and related pore volume of three composite sorbents. Volume of formed LiCl solution increases from 0.5153 to 1.5220 cm
3/g with the increased salt content. EVMLiCl20 has a salt content threshold that can prevent solution leakage. However, it is noticed that solution volume of EVMLiCl20 is lower than total pore volume, which testifies that not all of the pore volume in EVM can be utilized. This can be explained from two aspects: on one hand, part of pore volume is blocked by salt solution and cannot be used as a result. On the other hand, the ability to hold solution is too weak to retain the solution close to the outside surface of EVM; this is due to the macrospores structures of EVM. In a word, the practical salt solution volume that EVM can hold is lower than its total pore volume.
3.3. Thermal Characterizations Measurement
Results of STA measurements of EVM, LiCl, and EVM/LiCl composite sorbents are shown in
Figure 5. The changes of mass and heat flow of sorbents as a function of temperature in the desorption process were simultaneously recorded, shown by the TGA curve (the blue line) and the DSC curve (the rose line), respectively.
For EVM, the original water uptake is only 0.037 g/g. As seen in TGA curve, EVM loses almost all the adsorbed water before 100 °C, during which a weight fluctuation appears at about 77.0 °C. Accordingly, three endothermic peaks appear in DSC curve before 100 °C. An unexpected exothermic peak happens at 176.5 °C. Repeated STA tests were conducted considering the complex chemical composition and individual differences of EVM. Results indicate some general characterizations: the desorption process mainly happens before 100 °C, and can be divided into two or three phases. An exothermic peak emerges after 100 °C and is possibly initiated by the complex adherence force between adsorbed water and EVM.
In the TGA curve of LiCl·H
2O, two plateaus are observed, which means the formation of LiCl·H
2O and LiCl, respectively, according to weight calculation based on molar mass of LiCl and H
2O. The first plateau indicates that LiCl·H
2O deliquesces during the very short process of both weight measurement and transfer to STA instrument. Due to the highly hydroscopic nature of LiCl·H
2O, its deliquescence can hardly be avoided. The red dotted line in the TGA curve represents the formation of LiCl·H
2O, namely the beginning of the dehydration reaction of LiCl·H
2O. Based on analysis of TGA/DSC curves, the process of free water loss has no direct connection with the dehydration reaction of LiCl·H
2O. The dehydration reaction of LiCl·H
2O is shown as follows, where the reaction heat is calculated based on the endothermic peak related to the desorption reaction:
For all the EVM/LiCl sorbents, total water uptake acquired from TGA curve is almost the same as the result of sorption kinetics despite the tiny difference of water uptake caused by individual differences of composite sorbents. Accordingly, similar subtle difference in mass concentration between LiCl solution and the sorption kinetics results also exists. An obvious weight plateau is observed in the final stage of TGA curve, which means the formation of LiCl. Water uptake related to the formation of LiCl·H
2O is calculated by following formula:
where
xLiCl is chemical sorption capacity of LiCl, 0.42 g/g LiCl.
The formation of LiCl·H2O is showed on TGA curves by a red dotted line. Ignoring the negligible part of EVM, the desorption process of EVM/LiCl composite sorbents is divided into two processes:
Only one desorption peak occurs in the DSC curve during the whole desorption process, identifying no distinct boundary between the desorption processes above. However, a turning point appears in the temperature representing the end of process 1, after which the slope of the DSC curve has an apparent change. Combining the mass calculation and this slope difference, processes 1 and 2 are demonstrated as different desorption stages. As a reversible process to the desorption process, the sorption of EVM/LiCl sorbents is composed of three stages: water adsorption of EVM, water adsorption of LiCl crystal, and liquid–gas absorption of LiCl solution. As soon as the dry EVM/LiCl sorbents get in contact with water vapor, EVM adsorbs a small amount of water vapor and reaches sorption equilibrium quickly. Meanwhile, LiCl in the pores gradually adsorbs water vapor and LiCl·H2O is generated through a hydration reaction. After that, LiCl·H2O crystal continues absorbing water, and LiCl solution is formed in consequence.
For EVM/LiCl sorbents, starting temperature of desorption peaks is almost the same, while the width of peaks increases with enhanced salt content. This phenomenon declares that the obstacle of mass transfer and heat conductivity is strengthened when a larger quantity of salt is embedded into EVM. Peak value of LiCl·H2O is 101.3 °C, which is only slightly higher than 100 °C. Considering that the practical reaction temperature is lower than peak value, since the STA test is conducted at a rising temperature, the dehydration reaction of LiCl·H2O is believed to happen below 100 °C. Peak values of EVMLiCl5, EVMLiCl10, and EVMLiCl20 are 64.7 °C, 74.7 °C, and 90.0 °C, respectively, which are entirely below 100 °C. In general, LiCl can be finally formed in EVM/LiCl sorbents when employing low-temperature heat below 100 °C the as desorption energy resource.
3.4. Theoretical Evaluation of Sorption Performance
To analyze sorption performance and energy storage potential for a TES system, further theoretical calculation and analyses are implemented. Water uptake of EVM/LiCl composite sorbents is divided into three portions: physical adsorption of EVM (
xad,EVM), chemical adsorption of LiCl crystal (
xad,LiCl), and liquid-gas absorption of LiCl solution (
xab,LiCl). Total water uptake
xcs is obtained from the TGA curve,
xad,LiCl is calculated through Equation (3), and
xad,EVM and
xab,LiCl are calculated using the following equations:
Figure 6 shows the sorption performances of EVM, LiCl·H
2O, and EVM/LiCl sorbents. When taking the hydration reaction alone into consideration, the total water uptake of LiCl is 0.42 g/g, which is smaller than those of all the composite sorbents. The value of
xcs increases from 0.50 g/g to 1.41 g/g with increasing salt content. EVMLiCl20 reaches the largest water uptake of 1.41 g/g, which is 3.4 times larger than that of LiCl·H
2O. For EVM/LiCl sorbents, values of
xad,LiCl and
xab,LiCl increase with the enhanced salt concentration. Adsorption of EVM is responsible for 1.9%–6.8% of total water sorption, while water adsorbed by LiCl contributes only 9.8%–10.2%. 83.0%–88.3% of the total water uptake is introduced by liquid-gas absorption of LiCl solution. Moreover, the ratio of
xab,LiCl/
xad,LiCl increases from 8.2 to 9.0 with increasing salt content, which indicates that larger pore volume of EVM contributes to LiCl solution preservation.
3.5. Theoretical Evaluation of Energy Storage Density
Mass energy storage density (
qm) can be approximately assesses by desorption heat obtained from DSC curves. Sorption heat in three parts of the whole sorption process (
qad,EVM,
qad,LiCl, and
qab,LiCl) is analyzed, whose calculation equations are listed as follows:
where
qm,LiCl is hydration reaction heat gained from the DSC curve in
Figure 5b,
qm,EVM is adsorption heat of EVM obtained from the DSC curve in
Figure 5a, and
qm,LiCl is total sorption heat of composite sorbents acquired from
Figure 5c–e.
Figure 7 shows the mass energy storage density of EVM, LiCl·H
2O, and EVM/LiCl sorbents.
qm,EVM is merely 0.02 kWh/kg, while that of LiCl·H
2O is 0.42 kWh/kg. Mass energy storage density increases with greater salt content, and those of EVMLiCl10 and EVMLiCl20 exceed that of LiCl·H
2O. EVMLiCl20 can reach a mass energy storage density of 1.21 kWh/kg, which is 2.9 times higher than that of LiCl·H
2O.
qad,EVM and
qab,LiCl also increase with greater salt content. Absorption heat contributes 77.4%–87.5% of mass energy storage density, while adsorption heat of LiCl contributes 11.3%–16.1%. Compared with the sorption process without solution absorption, the addition of the absorption process extensively improves
qm,cs by 2.42–5.97 times, which is more evident with larger salt content.
Volume energy storage density (
qv) is another key parameter for the realization of a compact TES system. The calculation equation of
qv is as follows:
where
ρ is bulk density of sorbent listed in
Table 1.
Calculation results are shown in
Figure 8. Volume energy storage density of EVM is only 2.96 kWh/m
3. The theoretical volume energy storage density of LiCl·H
2O can reach 745.94 kWh/m
3. However, this high theoretical value is usually confined by serious mass and heat transfer restrictions. Moreover, the reliability and repeatability are further hindered due to the swelling and agglomerate phenomenon of salt. Surprisingly, EVM/LiCl composite sorbents suffer no such problems. The value of
qv of EVM/LiCl sorbents increases with greater salt concentration, and the largest value of 167.04 kWh/m
3 is acquired by EVMLiCl20.
Sorption heat is originated from the interaction force between sorbent and water vapor, and a common relation is usually followed. Reaction enthalpy of whole desorption process (Δ
hr,w) is a vital parameter. For composite sorbents, sorption reaction enthalpy is composed of three parts: physical adsorption enthalpy of EVM (Δ
had,EVM), chemical adsorption enthalpy of LiCl (Δ
had,LiCl), and liquid–gas absorption of LiCl solution (Δ
hab,LiCl). Δ
had,EVM and Δ
had,LiCl share the same value with Δ
hr,w of pure EVM and pure LiCl, accordingly. The value of Δ
hab,LiCl is influenced by salt content and pore structure of composite sorbents. They are calculated using the following equations:
Figure 9 shows the calculation results. Generally, the desorption heat for losing one mole water ranges from 38 to 64 kJ, which can be a rough calculation criterion to assess desorption heat on the basis of water uptake. Among all the sorbents, EVM has the lowest Δ
hr,w of 38.90 kJ/mol while LiCl·H
2O has the largest Δ
hr,w of 63.95 kJ/mol, and the value of Δ
hr,w for EVM/LiCl sorbents is modest. Thus, the chemical sorption process is demonstrated to release more heat than the physical sorption process. For EVM/LiCl sorbents, the value of Δ
hr,w increases with increased salt content, and the differences between composite sorbents lay in different values of Δ
hab,LiCl. As mentioned above, mass concentration of LiCl solution held in EVM pores is almost the same for three composite sorbents, but the value of Δ
hab,salt increases from 37.59 to 55.19 kJ/mol with increasing salt content. For composite sorbents, LiCl solution is confined in numerous pores. Adhesive forces between LiCl solution and the surface of pores, mass transfer resistance, and the concentration heat of LiCl are three forces influencing Δ
hab,salt. The former two kinds of forces increase with increased salt content, which eventually contributes to the enhancement of Δ
hab,salt.