Preparation and Performance Study of CaCl₂ Composite Adsorbent Based on Rock Wool Board Suitable for Continuous Heat Storage/Release of Trombe Wall

Abstract

As a passive solar design technology, the Trombe wall can improve buildings’ energy efficiency and thermal comfort. However, the traditional Trombe wall heating efficiency is low and cannot meet the needs of continuous night heating of the building. To solve these problems, a new type of sheet-like composite adsorbent is proposed in this study, prepared from calcium chloride supported by a rock wool board, a high-porosity building material. The high adaptability of rock wool board to the building wall makes it possible for the composite adsorbent to be directly applied to the Trombe wall. The results show that the macroporous structure of the rock wool board provides a wealth of space for loading hydrated salts. The smaller the density and thickness, the more calcium chloride the rock wool board can carry, speeding up the absorption/deportation process. The rock wool slab-based calcium chloride composite adsorbent has a maximum adsorption capacity of 51% and a heat storage density of about 838 J/g. Achieving the desorbed balance within 8 h and applying it to the Trombe wall is expected to attain continuous heating of buildings and has significant potential in building energy conservation.

1. Introduction

The construction industry is the main energy consumer, accounting for 36% of the world’s total energy consumption [1] and half of the world’s electricity consumption. Approximately 1/3 of the world’s carbon dioxide emissions are produced by this [2]. Therefore, countries have actively responded to the international community’s “dual carbon” goals [3,4], committed to reducing carbon emissions and promoting the country’s green and low-carbon transformation. The International Energy Agency (IEA) predicts that the share of renewables in the power sector will increase from 29% in 2022 to 42% in 2028 [5]. Making the most of renewable energy is essential to reduce energy and electricity consumption, and it has become a key strategy to achieve our carbon neutrality goals [6]. Solar energy is one of the most promising renewable energy sources and can be combined with buildings to provide the energy needed for heating and domestic hot water in homes [7]. French scientist Felix Trombe was the first to propose a new research topic, which has attracted wide attention because of its simple structure, good thermal performance, and low operating cost [8,9]. It is an effective means to reduce building energy consumption and improve solar energy efficiency. However, the Trombe wall could not meet the demand for continuous heating at night and was less thermally efficient. Excess heat generated during the day can be stored using heat storage technology and Trombe walls for use during peak demand at night, reducing the building’s energy consumption and improving thermal comfort by increasing its heat storage capacity.

According to the different principles of heat storage, heat storage technologies can be divided into three categories: apparent heat storage, phase change heat storage, and thermochemical heat storage [10]. The energy density of apparent heat storage is relatively low, and there may be greater heat loss during storage and release. The phase change material has a high energy density and a small temperature change during the phase change process, which is an ideal material to improve the thermal efficiency of the Trombe wall [11,12]. Studies by Onishi et al. [13] have shown that using phase change materials in Trombe walls can reduce the energy consumption of building operations. Different phase change materials and operating conditions affect the performance of Trombe walls. Zhu et al. [14] optimized the key influencing factors of the phase transition Trombe wall by coupling and found that, compared with the traditional Trombe wall, the energy consumption of the optimized phase transition Trombe wall was reduced by 13.52%. The application of phase change materials in building energy conservation has gradually matured, including the combination of building envelope [15,16,17] and air conditioning systems [18,19]. However, the phase change material is prone to leakage in the molten state, and the thermal conductivity is low [20]. Thermochemical heat storage uses reversible chemical reactions to realize energy conversion and decomposition products to realize the storage of thermal energy and chemical energy [21,22]. The storage cycle is long and the energy density is high (about 1000–2000 J/g [10]). Among them, the thermochemical adsorption and heat storage based on hydrated salt are heated by solar collector panels to realize the dehydration and heat storage of hydrated salt. The exothermic temperature can also meet the heating needs of buildings. At the same time, the cost is low. It is a promising way of heat storage in buildings [23,24,25]. For the first time, Li et al. [26] proposed to apply hydrated salt heat storage materials to Trombe walls, prepared a composite adsorbent (LiO₂C1@EG), and pressed it into a porous block with abrasive tools as a heat storage wall. During most of the day, the average air outlet temperature is above 30 °C, which is suitable for space heating; at night, the heat energy stored in the porous wall is released to continuously provide heat to the building through wet air. Compared with other Trombe walls based on apparent or latent heat, this Trombe wall has a higher light and heat collection efficiency and energy storage density. Zeng et al. [7] constructed a new type of passive solar heating system, based on the concept of adsorption and heat storage in a Trombe wall, which operates both day and night without any power consumption. When the Trombe wall has an area of 30 m² and a wall thickness of 16 mm, its heat storage density is expected to reach 7.2 × 10⁵–1.08 × 10⁶ J/g, which is expected to meet the day and night heating needs of ordinary buildings with a space of 100 m².

However, there are some disadvantages when using pure hydrated salts as a single material, such as the expansion of the salt volume due to the hydration/dehydration process due to changes in the crystal structure, which may limit the transfer of heat and mass [27]. Excessive adsorption can lead to the dripping of salt solutions, making it difficult to use in industry. To overcome this problem, researchers often use porous materials combined with hydrated salts to make composite adsorbents [28,29]. The hydrated salt liquefaction and the resulting salt solution can be limited by the pores of the matrix to prevent leakage; therefore, the absorption of the solution is promoted, thereby increasing the total adsorption capacity of the adsorbent [30]. Liu et al. [31] prepared up to 50 wt% MgSO₄-based composite adsorbents using mesoporous silica nanoparticles with different porous structures. The results show that with the increase in the pore size and pore volume of the matrix, the adsorption capacity and dehydration rate of the composite adsorbent increase. Wang et al. [28] used volcanic stone as a matrix and immersed it in 36.50% wt% saturated MgCl₂ and 54.00 wt% saturated CaCl₂ solutions to prepare composite adsorbents, which were called composite MgCl₂ and composite CaCl₂, respectively. Compared with zeolite MgCl₂ the water absorption of composite MgCl₂ and CaCl₂ increased by 0.15 g/g and 0.03 g/g, respectively. The thermochemical energy storage densities were 642 J/g and 983 J/g, respectively. It can be seen that the current research mainly focuses on the preparation of granular and powdery composite adsorbents, which are not easy to form or maintain shape when processed into building materials and cannot be directly combined with Trombe walls.

Currently, most of the research focuses on preparing granular and powdery composite adsorbents. Still, they have difficulties forming and retaining shape when processed into building materials, which limits their direct use in combination with passive solar building technologies such as Trombe walls. A summary of relevant study results is presented in

Table 1. Summary of powdered/granular composite adsorbents.

Composite Adsorbents	Matrix	Salt	Salt Content (wt%)	Water Uptake (g/g)	Energy Storage Density (J/g)	Ref.
Powdered	Activated carbon	MgSO4	30	0.52	1324	[34]
	Attapulgite	LiCl	30	0.49	1140	[35]
	Silica	CaCl2	13.84	0.27	746	[36]
Granular	Vermiculite	CaCl2	56	1.40	364	[37]
	Volcanic	CaCl2	29.13	0.25	983	[28]
	Bead activated carbon	MgSO4	7.6	0.37	920	[38]

. To overcome this limitation, based on the existing literature, a new type of sheet-like composite adsorbent was proposed for application in building materials. The composite adsorbent is prepared by taking rock wool board as a porous matrix and loading CaCl₂. Rock wool board (hereinafter referred to as YMB) as a building material is widely used in the insulation system of walls, roofs, and other enclosure structures [32]. The internal structure of the rock wool board is composed of fine fibers, forming a porous and interconnected network structure [33], making it easy to composite with various materials. The composite adsorbent can avoid the problems of leakage and agglomeration encountered when using pure salt, and at the same time benefit from the adaptability of the matrix itself as a building material, which can be directly applied to building walls. In this study, the influence of the thickness and density of the matrix material on the absorption/exothermic properties of the composite adsorbent was analyzed, and the energy density of the composite adsorbent was calculated. On this basis, it is envisaged that the YMB-CaCl₂ composite adsorbent will be combined with the Trombe wall, and the characteristics of the material in the process of depletion/adsorption can absorb and release heat, which is expected to regulate the indoor temperature and improve indoor thermal comfort, to provide a reference for long-period and high-density heat storage technology.

2. Experiment

2.1. Synthesis of Composite Adsorbent

The anhydrous calcium chloride (CaCl₂) provided by Aladdin Biochemical Technology Co., Ltd., Shanghai, China, has a purity of >99.9%, a density of 1.086 g/cm³, a melting point of 782 °C, and a boiling point of 1600 °C. The porous matrix rock wool board (YMB) is provided by Shanghai Jiyuan Thermal Insulation Materials Co., Ltd., Shanghai, China, and has a thermal conductivity of 0.04 W/(m·K). The preparation of YMB-CaCl₂ composite adsorbent is shown in Figure 1. Firstly, samples with a thickness of 10 mm, 15 mm, 20 mm, and 30 mm, and a length and height of 100 mm are prepared from the rock wool of different densities. The composite adsorbent was prepared by the impregnation method, as shown in Figure 1. The YMB was impregnated in the 20 wt% CaCl₂ solution, and the solid–liquid mixture was separated by filtration and then dried in an oven at 160 °C. The synthetic composite adsorbent takes a 10 mm thick 80 kg/m³ sample as an example, the number was YMB-CaCl₂(10), YMB represented the rock wool board, 80 represented its density, and the number in parentheses was the thickness of the sample, and so on.

During the experiment, it was found that the actual density of the rock wool slab is sometimes inconsistent with its marked density. There may be local density unevenness inside the rock wool board, which may be due to fiber laying, resin distribution, or unevenness in the curing process. To ensure the accuracy of the study, this study uses the actual measured density as the reference, then uniformly names and calculates the actual density, and limits the density range of the sample to 60 ± 10 kg/m³ and 90 ± 10 kg/m³ in two intervals. The synthetic composite adsorbent takes a 10 mm thick 55 kg/m³ sample as an example, numbered YMB55-CaCl₂(10); YMB represents the rock wool board, 55 represents its density, the number in brackets is the thickness of the substrate, and so on. At the same time, the matrix materials YMB-61 kg/m³ and YMB-96 kg/m³ representing the two density intervals were prepared for comparative analysis of subsequent characterization results. The parameters of each composite adsorbent are shown in

Table 2. Sample parameter table.

Samples	Actual Density (kg/m3)	Thickness (mm)	Salt Content (%)
YMB-61 kg/m3	61	10	0
YMB-96 kg/m3	96	10	0
YMB55-CaCl2(10)	55	10	51
YMB74-CaCl2(10)	74	10	33
YMB97-CaCl2(10)	97	10	32
YMB120-CaCl2(10)	120	10	14
YMB55-CaCl2(15)	55	15	42
YMB77-CaCl2(15)	77	15	23
YMB100-CaCl2(15)	100	15	19
YMB68-CaCl2(20)	68	20	26
YMB71-CaCl2(20)	71	20	22
YMB95-CaCl2(20)	95	20	14
YMB65-CaCl2(30)	65	30	21
YMB95-CaCl2(30)	95	30	15
YMB116-CaCl2(30)	116	30	14

.

2.2. Characterisation of Porous Matrix and Salt Composites

(1)

Scanning electron microscope test

The high-energy electron beam of a scanning electron microscope (JSM 7800F, Japan Electronics Co., Ltd., Amagasaki, Japan) was used to scan the prepared composite heat storage materials, and the microstructure and morphological characteristics of the composite adsorbent were observed by imaging the interaction between light and matter.

(2)

FTIR

Samples were analyzed in the 500 cm⁻¹–4000 cm⁻¹ spectrum range using a Fourier transform infrared spectrometer (Thermo Feini Colliers IS50, Thermo Fisher Scientific in Waltham, MA, USA) to ascertain the substance’s molecular structure and chemical composition. The chemical composition and structural characteristics of the sample, as well as its qualities and uses, can be further explored by analyzing the position and strength of the absorption peaks in the spectra.

(3)

TGA/DSC

The content of each component in the samples was analyzed, and the samples were tested using a thermogravimetry/differential thermal analyzer (TA-SDT Q600, TA Instruments in New Castle, DE, USA). In this experiment, the samples were calcined at room temperature to 80 °C at a heating rate of 10 °C/min in an air atmosphere, and then the mass fraction of each component was measured, and its heat absorption and release within the test temperature range were analyzed.

(4)

Pore analysis

The mercury compression method was used to assess the porosity of the samples to determine their specific surface area and pore size. The samples need to be pretreated by grinding, drying, and weighing them before the test, and then they need to be measured using a mercury pressure meter (MicroActive Autopore V9620, Micromeritics Company in Norcross, GA, USA). Using the non-wettability of mercury, mercury is pressed into the pores of the sample, and the size and distribution of the pores are calculated by measuring the volume and pressure of mercury.

(5)

The sorption/desorption performance testing

The performance of the composite adsorbents was tested by spreading the dried composite adsorbent in a glass dish, recording the initial mass m1 of the dried composite adsorbents, and exposing it to air (temperature ±5 °C, humidity ±5% RH). The weight of the composite adsorbents was measured with a balance every 1 h (the accuracy of the balance is ±0.01 g), and the mass change during weighing is negligible due to the short weighing time (about 30 s). The weighed weight is recorded as m_t′ and continues to stand still in room air. Repeat the above steps to determine the adsorption capacity of the materials. Then, the water content of the composite adsorbent at time t of the adsorption process is:

$${\mathrm{D}}_1={\displaystyle \frac{{{\mathrm{m}}_{\mathrm{t}}}^{\prime }- {\mathrm{m}}_1}{{\mathrm{m}}_1}},$$

(1)

where D₁ is the water content of the composite adsorbent during adsorption; m_t′ is the mass of the composite adsorbent during the adsorption process at time t; m₁ is the initial mass of the composite adsorbent after drying.

For the desorption process, the adsorbed composite adsorbents were placed in an electrothermal constant temperature drying oven to determine the dehydration capacity of the materials, and the measurement steps were the same as the drying process. Equation (2) defines the water content of the composite adsorbent over time.

$${\mathrm{D}}_2={\displaystyle \frac{{\mathrm{m}}_{\mathrm{t}}″- {\mathrm{m}}_1}{{\mathrm{m}}_1}} \times 100\%,$$

(2)

where D₂ is the water content of the composite adsorbent during desorption; m_t″ is the mass at time t of the composite adsorbent during the desorption process at time t.

3. Results and Discussion

3.1. Physicochemical Characterization

As can be seen from Table 2, the salt content of YMB55-CaCl₂(10) and YMB97-CaCl₂(10) is the highest in the two density ranges, respectively. Therefore, in the subsequent performance characterization, YMB55-CaCl₂(10), YMB97-CaCl₂(10), and the corresponding matrix materials YMB-61 kg/m³ and YMB-96 kg/m³ were mainly carried out. The results of FTIR spectroscopy of the four samples are shown in Figure 2a. The main components of the rock wool slab are basalt (SiO₂) and phenolic resin binder. The peaks of the matrix material YMB-61 kg/m³ at 708 cm⁻¹ and 968 cm⁻¹ belong to the bending vibration of the Si-O functional group and the symmetrical tensile vibration of the Si-O functional group, respectively. YMB97-CaCl₂(10) is the same. The composite adsorbent takes the spectrum of YMB55-CaCl₂(10) as an example. The peaks at 1627 cm⁻¹ and 3450 cm⁻¹ are related to the vibration of the OH functional group in the calcium chloride crystallized water [39]. The diffraction peaks of CaCl₂·6H₂O can be observed in the spectra of the two composite adsorbents, and no special new peaks appear. In summary, the CaCl₂ in the composite adsorbent is physically combined with the rock wool board, and no new substances are produced during the preparation of the composite adsorbent.

Figure 2b shows the XRD spectra of YMB55-CaCl₂(10), YMB97-CaCl₂(10), and the corresponding matrix materials YMB-61 kg/m³ and YMB-96 kg/m³. There are no obvious sharp peaks in the YMB-61 kg/m³ and YMB-96 kg/m³ maps. This phenomenon shows that the rock wool slab is mainly composed of amorphous substances. Amorphous substances lack a long-range ordered crystal structure; therefore, they do not exhibit typical diffraction peaks in the XRD spectrum. For the YMB-CaCl₂(10) composite adsorbent, the XRD spectrum is very similar to the matrix material, and the characteristic peaks of hydrated salts cannot be observed. This may be because the hydrated salts are very evenly dispersed in the rock wool matrix in the form of fine particles. When the hydrated salt particles are small enough in size and evenly distributed, their crystal structure may not produce additional significant characteristic peaks in the XRD map. Combined with the FTIR test results, it can be seen that the hydrated salt is indeed filled in the rock wool slab, indicating that although the characteristic peaks of the hydrated salt cannot be directly observed in the XRD map, we can confirm the existence of the hydrated salt through other analytical techniques.

YMB-61 kg/m³ was tested by thermogravimetric method and differential scanning calorimetry (DSC). The results are shown in Figure 2c. The weightlessness stage is not obvious, and the total mass loss rate of YMB-61 kg/m³ is about 1.4%. The main component of YMB that can be decomposed within 300 °C is only phenolic resin. Li et al. [40] divided the pyrolysis of phenolic resin into three stages, the first stage of which was due to the evaporation of the remaining water in the resin after curing and the further condensation of the resin, the quality loss rate was 12.64%. The temperature range of this study is in the first stage. It is roughly calculated that the phenolic resin in YMB-61 kg/m³ accounts for about 10.7% of the matrix, and the phenolic resin in YMB-96 kg/m³ accounts for about 9.2% of the matrix. It can be seen that as the temperature rises, the sample dehydrates. At about 50 °C, an endothermic peak is produced, which may be the peak of binding water in the sample. There is a wide endothermic peak at about 250 °C. The initial decomposition temperature of a general phenolic resin is about 220 °C. At this time, a large amount of CO₂ has been released, and the weightlessness rate of the thermogravimetric curve is accelerated.

Micron-scale pores need to be characterized by mercury intrusion. Figure 2d shows that the pore size distribution of the two samples is similar, with pore sizes around 60 μm. The results indicated that the salt filling of the pores of the composite adsorbent in the adsorption reaction was limited, and the composite adsorbent still retained the original macroporous structure of the rock wool board. Sample pore parameters are shown in

Table 3. Pore structure parameters of the samples.

Sample	Average Pore Size (μm)	Pore Volume (cm3·g−1)	Porosity (%)
YMB-61 kg/m3	62.54	5.18	87.52
YMB-96 kg/m3	60.78	5.62	91.28
YMB55-CaCl2(10)	60.75	4.04	88.74
YMB97-CaCl2(10)	67.63	5.13	88.54

. It can be seen that the addition of hydrated salts reduces the pore volume and pore size of macropores under the condition of similar densities, which is consistent with the conclusion of Wang et al. [28].

3.2. Structural Characteristics

The surface morphology of the two substrates and their composite adsorbents is shown in Figure 3 and Figure 4. As can be seen in Figure 3a–c and Figure 4a–c, YMB has a significant fibrous structure, which increases the pore space. It can not only provide a complex and continuous path structure to enhance the thermal conductivity of the matrix but also constrain the composite adsorbent, as the liquid hydrolysis of the composite adsorbent provides the structural basis. Figure 3d–f and Figure 4d–f show the distribution of CaCl₂ on the surface of YMB fibers in YMB55-CaCl₂(10) and YMB97-CaCl₂(10), respectively. The white covering represents the accumulated CaCl₂. It can be seen that CaCl₂ is wrapped on the surface of the fiber. As the salt content increases, a larger white covering is formed on the fiber of the YMB-CaCl₂(10) composite adsorbent, indicating that more CaCl₂ is attached to the surface of the fiber structure. Compared with the rock wool/calcium chloride hexahydrate-based insulation board prepared by Yang et al. [41], the SEM image in this paper shows similar fiber structure and hydrated salt distribution characteristics. The hydrated salt is also filled and attached to the rock wool fiber lap; there is a wall-climbing adhesion on the fiber tube wall, and a large area of filling is formed between the fiber pores. At the same time, the fiber structure of the rock wool board remains intact after carrying the hydrated salt.

Taking YMB-61 kg/m³ and YMB55-CaCl₂(10) as examples, the elemental distributions of YMB-61 kg/m³ and YMB55-CaCl₂(10) were analyzed using EDX mapping. The main components of rock wool board are dolomite and basalt; dolomite is rich in calcium, magnesium, and other elements, and basalt is rich in silicon, aluminum, and other components. This is confirmed by the fact that Figure 5a shows that Si and Ca elements are concentrated in the YMB-61 kg/m³ fibers. In addition, small amounts of Cl are scattered everywhere, which may be impurities incorporated during processing. In Figure 5b, the Ca and Cl elements in YMB55-CaCl₂(10) are dense and widely distributed between the fibers and fiber pores, indicating that the hydrate salts are successfully loaded into the fiber structure.

3.3. Salt Content Analysis

The salt content of the composite adsorbent is calculated using Equation (4). Based on the results of the calculations, Figure 6 is drawn.

$$\mathrm{m}\%={\displaystyle \frac{{\mathrm{m}}_1-{\mathrm{m}}_0}{{\mathrm{m}}_0}} \times 100\%,$$

(3)

where m₀ is the mass of the substrate before impregnation, g; m₁ is the mass of the composite adsorbent after drying, g.

Figure 6 clearly shows a significant correlation between the salt content of the composite adsorbent and its density. Theoretically, the porosity of a porous matrix of a certain density is certain, and the higher the density, the smaller the porosity, and the salt content decreases. This is mainly due to the weakening of the filling and diffusion ability of hydrate salts inside the material, and it is difficult for composite adsorbents with high density to achieve complete hydration simultaneously. Under the condition that the thickness is the same as 10 mm when the matrix density increases from 55 kg/m³ to 77 kg/m³, the salt content of the composite adsorbent decreases from 51% to 33%.

It can be seen from Figure 7 that the salt content decreases with the increase in the thickness of the substrate. This is because the salt of the composite adsorbent is mainly concentrated on the surface of the substrate. When the density of the composite adsorbent is certain, according to the calculation formula of the salt content, it can be determined that the greater the thickness of the substrate, the greater its overall mass, which means that in the compound adsorbent per unit mass, the relative content of salt will decrease.

3.4. Analysis of Adsorption/Desorption Properties of Composite Adsorbents

3.4.1. Adsorption Performance

In chemical heat storage systems, the vapor adsorption rate and water content of composite adsorbents have a great influence on their heat storage performance. The adsorption experiment was carried out under the conditions of 28 °C-75%RH (temperature ±5 °C, humidity ±5%RH) and indoor environment. The adsorption curves of different composite adsorbents are shown in Figure 8. With the increase in salt content, the water content of each composite adsorbent increased. The water content of the eight composite adsorbents increased rapidly in the initial 5 h, and then the rate decreased gradually until equilibrium was reached at 25 h. Because rock wool board pressed into pieces may increase mass transfer resistance, water vapor cannot quickly and completely contact and react with all salt particles; therefore, the composite adsorbent water absorption rate is low. With the increase in salt content in the composite adsorbent, the water absorption increased, showing a higher adsorption capacity. Specifically, YMB55-CaCl₂(10) exhibited the best adsorption capacity, with an equilibrium adsorption capacity of up to 0.6 g/g.

3.4.2. Desorption Performance

Figure 9 shows the desorption curves of different composite adsorbents at 80 °C. The water content of the eight composite adsorbents decreased rapidly in the first 1 h, decreased slowly in 1–4 h, and remained stable after 4 h. In the two density intervals, the decreasing rates of the water content of the composite adsorbents in the first 1 h were as follows: YMB-CaCl₂(10), YMB-CaCl₂(15), YMB-CaCl₂(20), and YMB-CaCl₂(30), indicating that the desorption rate is faster with the increase in salt content. When desorption equilibrium is reached, the final water content gradually decreases with the decrease in salt content, but none of them can be reduced to 0 because the desorption temperature of 80 °C cannot completely dehydrate the calcium chloride hydrate salt [42]. The desorption equilibrium of the four heat storage materials can be reached in 8 h, which meets the needs of heat storage during the day and heat release at night.

3.5. Cycle Performance of Composite Adsorbent

The thermochemical heat storage of hydrated salts is achieved using the adsorbent for water vapor absorption/desorption cycles; therefore, the cycling stability of the material is an important indicator. The test method was as follows: the adsorption process was carried out indoors at a temperature of 30 ± 2 °C, a relative humidity of 70 ± 5%RH, and an adsorption time of 2 h. This is followed by a desorption process by placing the adsorbent in a blast drying oven at 80 °C for 1 h and performing water vapor desorption. Repeat the adsorption step after desorption. In this way, 10 adsorption-desorption cycles were performed to measure the change in water absorption of the material during each absorption/desorption process, and the results are shown in Figure 10. The water absorption of the eight composite adsorbents fluctuated slightly, confirming the stable adsorption capacity and long-term operational stability of the sample.

3.6. The Energy Density of the Composite Adsorbent

The adsorbent was desorbed by the thermogravimetric method, and the results are shown in Figure 11. As can be seen from the figure, the curves of the two composite adsorbents are similar. In the range of room temperature to 200 °C, each of the two adsorbents has two weightlessness stages representing the desorbed of water, corresponding to two endothermic peaks. The first weightlessness stage occurs at 45 °C, lower than the desorbed temperature of SP/Ca, DT/Ca, and EP/Ca prepared by Wei et al. (80 °C) [43]. Rock wool board may have better heat conduction properties, allowing calcium chloride to be released at lower temperatures. When the temperature reaches 140 °C, the second weightlessness stage of the adsorbent is completed and the curve reaches equilibrium.

As can be seen from the heat flow curve in Figure 11, the two adsorbents have two endothermic peaks at 45 °C and 140 °C, which are consistent with the two weightlessness stages of the thermogravimetric curve. The dried sample’s energy density and dehydration amount were calculated as shown in

Table 4. The energy density of composite adsorbent.

Composite Adsorbents	T1 (°C)	T2 (°C)	$∆{\mathbf{H}}_1$ (J/g)	$∆{\mathbf{H}}_2$ (J/g)	$∆\mathbf{H}$
YMB55-CaCl2(10)	46	140	82	756	838
YMB97-CaCl2(10)	45	140	32	222	254

. The heat of the second endothermic peak is significantly higher than that of the first, indicating that the second dehydration stage is the main endothermic process. The energy density of YMB55-CaCl₂(10) is 838 J/g, which is higher than the heat of dehydration (254 J/g) of YMB97-CaCl₂(10), indicating that the energy density is positively correlated with the salt content of the sample. Therefore, YMB55-CaCl₂(10), which has a higher salt content under the same conditions, has greater application potential.

4. Application of Composite Adsorbents in Trombe Walls

The YMB-CaCl₂ composite adsorbent has efficient heat storage properties, and its matrix is used as a building material, making it directly applicable to building envelopes. In addition, YMB-CaCl₂ composite materials support large-scale production, which is essential to meet the large demand for efficient heat storage materials in the construction industry. By applying the composite adsorbent to the south wall of the building, a thermochemically adsorbed Trombe wall can be constructed. Using the principle of thermochemical adsorption, solar energy is absorbed during the day and stored as heat energy, and this heat energy is released at night or when heat is needed to meet the heating needs of the building. The initial cost of a rock wool board is higher than that of traditional materials, and the overall cost of a 100 mm thick rock wool board is about 120 yuan/m²; the cost of CaCl₂ is about 0.1 euro/kg. However, in the long run, composite adsorbents can reduce the operating costs of buildings and reduce the use of air conditioners. They not only improve the utilization efficiency of solar energy but also reduce the dependence on traditional heating systems and significantly reduce the energy consumption of buildings. After a comprehensive comparison, this kind of Trombe wall based on thermochemical adsorption is more cost-effective than energy storage systems based on traditional phase change materials and concrete.

In winter, during the day (Figure 12a), the upper and lower vents of the wall are opened. The solar radiation shines through the glass cover on the outer surface of the heat storage layer to increase the wall temperature. The interlayer air is heated from the upper vent into the room, and the cold air flows into the air interlayer from the lower vent, forming a convective heat cycle. At the same time, the composite material in the thermal storage layer absorbs and stores solar radiation heat through the glass cover. When the air temperature of the interlayer is lower than the adsorption temperature of the composite material in the thermal storage layer, the material releases heat and reduces the temperature fluctuation of the interlayer air. When the indoor temperature is higher than the desorption temperature of the composite material in the thermal storage layer, the material stores heat; on the contrary, it releases heat and improves indoor thermal comfort. At night (Figure 12b), when the temperature of the interlayer and the indoor air is too low, the composite material releases heat and the interlayer air flows into the room by heating for night heating.

In this study, a model room (130 mm × 100 mm × 120 mm) composed of the acrylic board was built with a Trombe wall (air layer 30 mm × 100 mm × 120 mm). The thermal insulation and YMB55-CaCl₂(10) were installed sequentially on the Trombe wall. Two vents, an upper vent and a lower vent with dimensions of 10 mm × 10 mm, were installed on the Trombe wall. Thermocouples, humidity sensors and anemometers were installed near the vents. Meanwhile, thermocouples and humidity sensors were installed in the center of the room. A humidifier was added to increase the humidity in the room. The exothermic behavior of the Trombe wall in winter was evaluated by measuring the temperature (T_center) and humidity (RH_center) at the center of the room and the velocity (V_upper, V_lower), temperature (T_upper, T_lower) and humidity (RH_upper, RH_lower) at the air vents. The room model and test points are shown in Figure 13.

Figure 14 shows the temperature and humidity curves of the experimental device and the external environment. The humidity curve remained relatively stable throughout the process, with the humidity in the unit’s center slightly higher than the upper and lower vents. The temperature change curve rises first and then falls, the temperature fluctuation in the device gradually slows down, the adsorption reaction of the material ends completely at 300 min, and the temperature of the three measuring points is the same. During the process, the temperature of the measurement point in the device was about 3 °C higher than that of the ambient temperature outside the device, indicating that YMB55-CaCl₂(10) could effectively improve indoor thermal comfort. In their study, Duan et al. [44] integrated a mixture of 55% capric acid and 45% lauric acid as a phase change material with the Trombe wall, and the indoor air could be heated by 0.82–1.88 °C at 225 W/m² solar radiation intensity. In contrast, the integration of YMB55-CaCl₂(10) further improves the thermal performance of the Trombe wall, significantly improving indoor thermal comfort.

The wind speed at the upper and lower vents of the experimental device was tested to study the distribution law of wind speed. The test results are shown in

Table 5. Wind speed at the vent of the experimental device.

Times (h)	Wind Speed at Upper Vent (m/s)	Wind Speed at Lower Vent (m/s)
1	0.45	0.20
2	0.40	0.25
3	0.40	0.35
4	0.35	0.25
5	0.20	0.15
6	0.30	0.15
7	0.35	0.20

. It can be seen that the wind speed at the vent shows large random fluctuations, and there is no obvious pattern. However, the presence of wind speed at each vent confirms that the air in the device is in a state of continuous flow. This shows that the passive solar thermochemical adsorption heat collector and storage wall can promote indoor air flow and achieve natural ventilation.

5. Conclusions

To solve the shortcomings of low thermal efficiency and inability to continuously heat traditional Trombe walls, we prepared a new type of sheet composite adsorbent by using the building material rock wool board as the carrier of CaCl₂ and proposed the idea of directly combining the Trombe wall with the sheet composite adsorbent. To study the thermal properties of this hypothesis, an experimental analysis of a new Trombe wall based on thermochemical adsorption was carried out. The main conclusions are as follows:

• The smaller the density and thickness of the matrix, the higher the salt content of the composite adsorbent. Among the eight samples, the YMB55-CaCl₂(10) composite adsorbent has the highest salt content, up to 51%. The macroporous structure of the rock wool board is conducive to the loading of CaCl₂ and the mass transfer of water vapor. During the experiment, the composite adsorbent had no solution leakage and had good structural stability.
• The high salt content is conducive to improving the adsorption capacity. Under the conditions of 18 ℃-70%RH (temperature ±5 ℃, humidity ±5%RH), the adsorption capacity of YMB55-CaCl₂(10) is 0.6 g/g, and the water absorption performance is optimal. The TG-DSC test shows that the heat storage density of YMB55-CaCl₂(10) is 838 J/g.
• The water absorption performance of the composite adsorbent was evaluated in the repeated adsorption/desorption cycle, and the results showed that the composite adsorbent had relatively stable adsorption performance.
• The thermal performance test of the Trombe wall experimental device combined with YMB55-CaCl₂(10) was carried out. The experimental results showed that the temperature in the experimental device increased by about 3 ℃, and the indoor airflow was promoted to achieve natural ventilation.