Composites salt inside porous matrix for adsorption heat transformation: a current state-of-the-art and new trends

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1 Composites salt inside porous matrix for adsorption heat transformation: a current state-of-the-art and new trends... L. G. Gordeeva and Yu. I. Aristov * Boreskov Institute of Catalysis, Ac. Lavrentiev av. 5, Novosibirsk , Russia... Abstract Adsorption heat transformation (AHT) is one of the challenging technical approaches for supporting the world community initiatives to alleviate or reverse the gravity of the problems arising from CO 2 emissions and global warming. The key tool for enhancement of the AHT efficiency and power is a harmonization of adsorbent properties with working conditions of the AHT cycles. It can be realized by means of target-oriented designing the adsorbent specified for a particular AHT cycle. Twocomponent composites salt in porous matrix (CSPMs) offer new opportunities for nano-tailoring their sorption properties by varying the salt chemical nature and content, porous structure of the host matrix and synthesis conditions. CSPMs have been recognized as promising solid sorbents for various AHT cycles, namely adsorption chilling, desiccant cooling, heat storage and regeneration of heat and moisture in ventilation systems. In this review, we survey a current state-of-the-art and new trends in developing efficient CSPMs for various AHT cycles. *Corresponding author. aristov@catalysis.ru Keywords: adsorption heat transformation; harmonization of adsorbent and the AHT cycle; composite sorbents salt in porous matrix ; target-oriented designing Received 2 March 2012; revised 25 April 2012; accepted 30 April INTRODUCTION Having realized the gravity of the problems arising from CO 2 emissions and global warming, the world community has taken initiatives to alleviate or reverse the situation (Montreal and Kyoto protocols). Fulfilment of these initiatives requires the replacement of fossil fuels with renewable energy sources and rational use of heat in industry, transport and dwellings (re-use of exhaust heat, energy storage, etc.). These new heat sources have lower temperature potential that opens a niche for applying adsorption technologies for energy transformation and storage. One of the challenging technical approaches to support these efforts is an adsorptive heat transformation (AHT) driven by low-temperature energy sources. AHT has recently been attracting the increasing attention because it uses environmentally benign working fluids and has a great potential in fossil fuel saving. However, the current state of the art in the field of AHT cannot be considered as wholly satisfactory, in particular, because of poor properties of adsorbent materials [1 3]. In our opinion, the future progress in AHT will be possible only if innovative adsorbents with advanced properties are used. The composites salt inside porous matrix (CSPM) have been recognized as the promising materials for AHT due to their enhanced sorption capacity to common working fluids (water, methanol and ammonia) [4 7]. They are two-component systems: one component is a host matrix and the other one is an inorganic salt placed inside the matrix pores. The salt S reacts with a sorbate vapor V (water, methanol/ethanol and ammonia) that results in the formation of a complex S.NV (hydrate, methanolate etc.) of the salt and sorbate molecules according to the reaction S þ NV ¼ S NV: Further sorption leads to the complex dissolution and transformation to the salt sorbate solution [7 10]. The confinement of the salt into the matrix pores can change fundamentally the equilibrium sorbent sorbate. Particularly, a bi-variant sorption equilibrium typical of conventional ð1þ International Journal of Low-Carbon Technologies 2012, 7, # The Author Published by Oxford University Press. All rights reserved. For Permissions, please journals.permissions@oup.com doi: /ijlct/cts050 Advance Access Publication 22 June

2 Composites salt inside porous matrix for adsorption heat transformation Figure 2. Clapeyron diagram of a 3 T AHT cycle. (Reprinted from [128], with permission from Elsevier.) Figure 1 Percentage of publications on CSPMs by years. adsorbents transforms to a mono-variant equilibrium between the salt and the vapor. The role of the porous matrix is also of importance. It helps in dispersing the salt particles, affects the phase composition and the sorption properties of the salt and provides heat and mass transport to the salt particles located inside the matrix grains. CSPMs are originated from common porous adsorbents. The modification of the conventional adsorbents by the salt was suggested in the beginning of twentieth century to increase their water adsorption capacity [11]. However, just few publications relating to the properties of CSPM materials had been available before 1990 [12, 13]. In the new century, the number of papers relating to the study of the CSPM s properties and applications, including AHT, has been increasing continuously (Figure 1). Nowadays the adsorption properties of various CSPMs are being studied intensively. CSPMs are applied to numerous AHT processes, in particular, to adsorption refrigeration [1 3, 5, 14 22], desiccant cooling [23, 24], heat storage [25 29] and regeneration of heat and moisture in ventilation systems [30, 31]. Despite a large number of publications describing CSPMs and their applications, the data are somewhat disembodied, often disagreeing and sometimes contradictory. This paper is aimed to systematize the latest publications on CSPMs and to outline new trends in synthesis, characterization and application of CSPMs in AHT. 2 HARMONIZATION OF ADSORBENT AND WORKING CONDITIONS OF THE AHT CYCLE An adsorbent is a key element of an AHT unit (heat pump, chiller, amplifier), and harmonization of its properties with the cycle working conditions could allow significant enhancing the AHT performance. A common way to select the best adsorbent for a given cycle is based on screening the properties of available adsorbents to choose one which meets the cycle demands (see below) better than others. The screening is usually performed among already existing conventional adsorbents developed for applications different from AHT, mainly silica gels, zeolites and activated carbons. This approach is very laborintensive and time-consuming and, in general, it results in an intermediate solution rather than the ultimate one. Recently, an alternative way has been proposed that is a target-oriented design (or tailoring) of novel adsorbents adapted to particular AHT cycle [32]. The main idea of this approach is that, for each AHT cycle, there is an optimal adsorbent, the properties of which could allow perfect performance of this cycle. Preparation of the adsorbent in the frame of this approach is divided into two parts: (i) formulating the demands of the particular application to the required adsorbent properties and (ii) synthesis of the adsorbent, which properties precisely or nearly fit these demands. The demand formulating is based on thermodynamic analysis of the cycle efficiency [32]: the adsorbent promising for a basic 3T AHT cycle should exchange a large amount of adsorbate Dw between rich (1 2) and week (3 4) isosters of the cycle (Figure 2). The rich isoster corresponds to the uptake w max and is characterized by two pairs of temperature and pressure (T 1, P e ) and (T 2, P c ), while the week one (w min ) by (T HS, P c ) and (T 4, P e ). Thus, one should seek for adsorbent which allows maximizing the difference Dw ¼ w max w min ¼ w(t 1, P e ) w(t 4, P e ) ¼ w(t 2, P c ) 2 w(t 3, P c ). From the thermodynamic point of view, the efficiency equal to the Carnot efficiency can be, in principle, obtained in chemical heat transformers [33]. This results from a mono-variant equilibrium of a gas solid chemical reaction. In adsorption heat transformers, the equilibrium is bi-variant, that leads to inevitable degradation of the efficiency due to the entropy production caused by external thermal coupling [34]. When the rich and weak sorption isosters are confluent, the cycle is considered as degenerated case: the stages of isobaric sorption 4-1 and desorption 2 3 (Figure 2) occur immediately and International Journal of Low-Carbon Technologies 2012, 7,

3 L.G. Gordeeva and Y.I. Aristov different adsorbents. However, for tailoring the adsorbent with predetermined properties the comprehensive knowledge on how different factors affect the CSPM properties is necessary. Below we describe the methods of CSPM preparation and outline the main issues affecting CSPM s properties. 3 TARGET-ORIENTED DESIGN OF CSPMS Figure 3. Degenerated AHT cycle characterized by a single adsorption/ desorption isoster (a); corresponding step-like (b) and S-shaped (c) isobars of adsorption (P ¼ P e ) and desorption (P ¼ P c ). completely at T c and T HS (Figure 3a), respectively. Such a degenerated cycle is the most profitable for AHT as follows from [33, 34]. The degenerated cycle can be realized with an adsorbent characterized by mono-variant equilibrium: the material has to sorb a large amount of adsorbate in a stepwise manner directly at T c and P e and completely releases it at T HS and P c (Figure 3b). Hence, in theory, the adsorbent optimal for AHT should have step-wise sorption isobars (Figure 3b). For real adsorbents, more realistic is an S-shaped adsorption isobar (Figure 3c) instead of strictly step-wise one. Such sorption equilibrium corresponds to isotherms of types V and VI in the IUPAC classification [35]. Position of the step (or the steep uptake rise) has to correlate with the cycle boundary temperatures as shown in Figure 3. Realization of the second task the goal-seeking design of the adsorbent which properties fit the formulated demands needs a number of tools for intent modification of the equilibrium adsorbent adsorbate. For single-component adsorbents, only two main parameters, which affect the adsorption equilibrium, are available for such modification, namely their chemical nature and the porous structure [36]. Two-component sorbents, like CSPMs, provide more opportunities for varying their sorption equilibrium. For instance, just simple examination of the composite components, the salt and the matrix, allows preparation of a huge number of 3.1 Synthesis of CSPMs The most common method of CSPM synthesis is an impregnation of the matrix with an aqueous salt solution. The whole procedure involves preliminary drying of the matrix to remove adsorbed water, impregnation of the grains with the salt solution, filtration and final drying of the wet composite. Two main modifications of this method are commonly used. The former is a dry (or incipient wetness) impregnation [37], when the volume V s of the salt solution equals the pore volume V p of the matrix. For this reason, the solution is located only inside the pores and the filtration stage is omitted [4 9, 38 40]. The liquid phase soaks quite fast into the matrix grains due to capillary forces, though the salt ions often penetrate rather slowly to the grain center due to successive adsorption on and desorption from the matrix surface. Thus, to reach uniform distribution of the salt in the grains, the wet composite has to be kept for several hours. When the matrix grains are immersed into the salt solution, which volume exceeds the pore volume of the matrix, V s. V p, the method is called a wet impregnation [37]. After equilibration for several hours, the excess solution has to be driven off through vacuum desiccator or by filtration [14, 41 43]. Thus, only a part of the dissolved salt enters into the pores and the rest of the salt solution is discarded. If the matrix grains are large (D gr. 2 3 mm), the impregnation can be carried out under evacuation to avoid the crush of the matrix grains due to disjoining action of capillary forces [44]. During drying of the wet composite, the solvate (water) evaporates and the salt phase precipitates on the matrix surface. The distribution of the salt inside matrix grains is affected by the salt adsorption on the matrix surface, the salt concentration in the solution, its viscosity and the drying scenario. Generally, the typical active salts for CSPMs are halides, nitrates, sulfates of alkali and alkali-earth metal that weakly adsorb on the surface of the matrixes (silica gel, carbons etc.). For this reason, the salt trends to form inside the pores crystalline or amorphous particles, which size is restricted by the pore walls [37]. At the dry impregnation, the formation of the salt particles on the external surface of the grain was not observed [45]. When using the wet impregnation method, a portion of the solution remains on the external surface of the grains after filtrating the excess of the salt solution. Therefore, during the composite drying, the salt can partially precipitate on the outside of the grains, forming large crystals. To remove them 290 International Journal of Low-Carbon Technologies 2012, 7,

4 Composites salt inside porous matrix for adsorption heat transformation from the grain surface, Gong et al. [44] suggested the additional treatment of the composite in a special temperature/humidity camera. The salt on the external surface of the composite adsorbent absorbs water and transforms to the salt solution, then falls down through the strainer. The composite adsorbent is taken out when there is no salt solution formed any more, and then dried. To summarize, since the filtration stage is omitted the dry impregnation method is easier in realization. Moreover, no salt solution exhaust is produced as the salt completely precipitates inside the grain pores. Nevertheless, when preparing large amount of CSPMs, special procedure and equipment are required to ensure a uniform distribution of the salt solution through the matrix. When the pore diameter d av. 6 8 nm, the formation of crystalline phase of the salt particles occurs inside the grains. However, adsorption/desorption of the salt cation and electrostatic interaction between the ions of the salt and the charged surface centers can result in the formation of an X-ray amorphous phase of the salt as a thin layer or small clusters on the internal matrix surface [9, 39, 46]. Besides the dry and wet impregnations, a sole gel method was used for CSPM preparation [47 49]. This method gives the CSPMs with aero-gel structure that possess an extremely large pore volume and sorption capacity. However, due to high value of the necessary equipment and reagents, this method is not widespread. 3.2 Effect of the active salt on adsorption properties of CSPMs Effect of the salt confinement into the host matrix on the sorption properties of the composite has been studied by a number of authors. Adsorption capacity of the composites is superior to that of both the untreated matrices and common adsorbents of the basic working fluids (water, methanol, ethanol and ammonia) (Figure 4). Aristov et al. [4] demonstrated that the water sorption capacity of a CaCl 2 /silica composite (SWS-1L) reaches 0.6 g/g which exceeds five to six times the adsorption capacity of the initial silica. A similar effect was observed for LiBr/silica composites [8]. In [50, 51], a sharp increase in the ethanol sorption on alumina and silica gel was observed due to their modification by various inorganic salts. Sharonov et al. [7] and Bandosz et al. [40] observed 4 70 times rise of ammonia sorption after impregnation of porous matrix (activated carbon, expanded vermiculite and alumina) with calcium, copper and zinc chlorides. The methanol sorption capacity of the LiCl/silica and LiBr/silica composites reached 0.75 g/g that was five times higher than that for the input silica gel [45]. Salt content of the composite is an important parameter affecting the sorption capacity of CSPMs profoundly [14, 52]. Oliveira et al. showed [53] that the rise of methanol sorption capacity of the composites LiCl (13.7%)/activated carbon (AC) and LiCl(21.3%)/AC was, respectively, 56 and 86% regarding Figure 4. Isotherms of ammonia (1 3) and methanol (4 6) sorption on vermiculite (1), KSK silica gel (4), and CSPMs:BaCl 2 (41 wt.%)/vermiculite (2), CaCl 2 (63 wt.%)/vermiculite (3), LiBr (29 wt.%)/sio 2 (5), LiCl (31 wt.%)/sio 2 (6). (Reprinted from [109], with permission from Pleiades Publishing, Inc.) the initial AC. Fortier et al. [39] and Petit et al. [9] studied the ammonia sorption on AC-based composites, modified by different amount of Cu, Ni and Zn chlorides. The linear growth of sorption at increasing salt loading broke down at 3.5 mmol (or C s ¼ 32 wt.%) of ZnCl 2 /g of AC. The authors explained this behavior by the fact that at the salt content lower than 3.5 mmol/ g, a salt monolayer formed on the AC surface. At C s. 3.5 mmol/g, relatively large aggregates of the salt particles formed which blocked the pore space and reduced the sorption. A similar dependence was detected for water sorption on the composite CaCl 2 /microporous silica [14]. The sorption capacity rose with the increase in salt content up to C s ¼ 31 wt.% and then remained near constant. On the contrast, the continuous augmentation of water sorption on CaCl 2 /AC composite was observed up to C s ¼ 70 wt.% [54] and on composites CaCl 2 / silica up to C s ¼ 51 wt.% [52]. It was shown in [46] that the salt content affects not only the sorption capacity, but the type of the sorption equilibrium as well. At large salt content (C s wt.%), a crystalline phase of the salt forms, and water sorption occurs through the formation of the salt crystalline hydrates. The variance of such systems equals 1 and the sorption isobars are step-wise, which are preferable for AHT transformation (see Section 2). At small salt content, an X-ray amorphous phase of the salt forms. The sorption isobars of such composites are smooth curves typical of a bi-variant system. While the salt content is mainly responsible for the sorption capacity of the composite, the position of the step on the sorption isotherm predominantly depends on the chemical nature of the salt (Figure 4). Running ahead, the step position is affected by the porous structure of the host matrix as well (see Section 3.3). Actually, the equilibrium pressure of reaction (1) is determined by the Van t Hoff equation lnp ¼ DG W =RT: ð2þ International Journal of Low-Carbon Technologies 2012, 7,

5 L.G. Gordeeva and Y.I. Aristov Consequently, the change in the standard Gibbs energy (DG o ) of reaction (1) and temperature (T) determine the pressure at which a complex S.NV forms. Effect of the chemical nature of the confined salt on the sorption properties of the composites was studied in [42, 46]. Xin et al. [42] assumed that the increase in sorption capacity of the composites MeCl n /mesoporous silica (Me ¼ Li, K, Na, Ca, Mg, Zn) depends on the ionic radius of the cation. In our opinion, this dependence should be examined in more detail, because the sorption at just one value of the relative humidity (RH) ¼ 30% was compared. Probably, the salt content of the composite and the cation charge have to be taken into account as well. The conditions at impregnation stage affect the sorption properties of CSPMs [43]. The authors demonstrated that the increase in salt solution temperature and impregnation duration raises the sorption capacity of CaCl 2 /silica composites. Probably, the decrease in the viscosity of salt solution at increasing temperature results in larger volume of the salt solution soaked in the silica grains at fixed time. Consequently, the salt content of the composites increases and gives augmentation of the sorption capacity. The ph of the salt solution and the drying temperature affect the phase composition of the confined salt and sorption equilibrium with water and methanol vapor as well [46]. Thus, the chemical nature and content of the confined salt, the conditions during the composite preparation have a strong impact on the sorption properties of CSPMs. The intelligent selection of these parameters has to be performed for intent designing the CSPMs with requisite properties meeting the demands of particular AHT cycle. 3.3 Effect of the host matrix: pore-size effect Despite the salt is the primary sorbing component of a CSPM, the role of the host matrix is of importance as well. First, the matrix is the component that prevents the salt particles agglomeration. The matrix provides the vapor transport to the salt particles and transfer of the heat released or consumed during reaction (1) that promotes faster sorption/desorption [10, 55 58]. Besides this transport function, the matrix strongly affects the sorption equilibrium of the salt with sorbates. The isobars of water sorption on the composites Ca(NO 3 ) 2 /mesoporous silica [59] and methanol sorption of the LiCl/mesoporous silica [45] (Figure 5) deviate from the lines calculated as a linear combination of the sorption isobars of the salt and the matrix, taken with proper weight coefficients equal to their relative content. That means the sorption equilibrium salt sorbate changes considerably due to the salt confinement to the matrix pores. The dramatic change of the salt sorption properties was observed for CaCl 2 /SBA-15 composites [60]. The dispersed salt forms a dihydrate CaCl 2.2H 2 O at the relative pressure of water vapor times lower than the bulk one. It was shown that the water sorption equilibrium of CaCl 2 confined to the pores of a meso-structured silicate SBA-15 with a variable pore size appeared to depend on the SBA pore diameter (8.1 and 11.8 nm) [61]. The sorption isotherms have two segments with a steep increase in the uptake, corresponding to the formation of CaCl 2.2H 2 O and transformation of this hydrate to an aqueous solution of CaCl 2 (Figure 6). Interestingly, the pressure at which CaCl 2.2H 2 O hydration occurs is lower in smaller pores, namely and kpa at 508C. The decrease in the hydration pressure is caused by the fact that particles of the confined salt are smaller in narrow pores and, hence, sorb water easier. Figure 5. Temperature-invariant curves of methanol sorption (solid symbols) and desorption (open symbols) for LiCl (31 wt.%)/sio 2 composite (1), bulk LiCl (2 and 3), silica gel (4) and curve simulated as a linear superposition of methanol sorption on bulk LiCl and the silica gel (5). (Reprinted from [45], with permission from Elsevier.) Figure 6. Isotherms of vapor sorption by CaCl 2 /SBA composites, SWS-1L (CaCl 2 /SiO 2 (KSK)) and pure SBA (11.8 nm). T ¼ 508C. (Reprinted from [61], with permission from Elsevier). 292 International Journal of Low-Carbon Technologies 2012, 7,

6 Composites salt inside porous matrix for adsorption heat transformation This effect can be used for tailoring the composites specialized for different AHT cycles. The stronger water binding by the salt confined to smaller pores results in the appropriate diminution of the evaporator temperatures T e at fixed adsorption temperature T a. On the base of these data, the temperature, which can be obtained in an evaporator during hydration of CaCl 2 /SBA composites, can be estimated as T e ¼ 7 8 and 9 118C for the composites with smaller and larger pores, respectively (the sorption heat is rejected to a thermostat at T a ¼ 508C). It is worth noting that SWS-1L based on CaCl 2 inside the silica gel KSK pores (d av ¼ 15 nm) allows T e ¼ C at the same conditions [61]. If the evaporator temperature is fixed at T e ¼ 58C, the reduction of pore size in the row KSK (15 nm). SBA (11.8 nm). SBA (8.1 nm) results in the increase in the heat rejection temperature T a ¼ 37.5, 42, 448C (Figure 7). More strong water bounding by the salt confined to smaller pores results in the appropriate enhancement of desorption temperature: 79.0, 88.5, 94.58C. The salt solution dispersed in the silica mesopores of 8 15 nm size absorbs water identically to the bulk solution (Figure 6) [4, 8, 60]. On the contrary, when the salt solution is confined into narrower pores of 3 8 nm, its sorption ability decreases compared with the bulk one [62]. It is probably caused by the distortion of solvation sphere of the salt ions as a result of the interaction with the pore walls. 3.4 Composites based on binary salt systems We have demonstrated above that two-component CSPMs give much more opportunities to manage their sorption properties than conventional one-component adsorbents. Recently, threecomponent composites based on the binary salt systems (S1 þ S2)/porous matrix have been proposed for even more accurate tuning the adsorption properties to fit requirements of a particular AHT cycle [63, 64]. We synthesized the binary systems (LiCl þ LiBr), (CaCl 2 þ CaBr 2 ) and (BaCl 2 þ BaBr 2 ) inside silica mesopores. The study of their phase composition at different molar ratio of the salts content n MeCln/MeBrn, as well as their adsorption equilibrium with water, methanol and ammonia demonstrated the following features. Experimental sorption isobars of the binary salt composites (MeCl n þ MeBr n )/SiO 2 (n ¼ 1 and 2) do not coincides with the theoretical curves calculated as a sum of the sorption isobars of the single-salt composites MeCl n /SiO 2 and MeBr n /SiO 2 taken with the proper weight coefficients [63]. That means the sorption equilibrium of the binary salt systems changes when compared with the mechanical mixture of the single-salt composites. The reason of this alteration is likely to be the formation of the salts solid solution inside the silica gel pores, which was detected by X-ray diffraction. The addition of MeCl n to MeBr n results in the formation of solid solution SS Br that is accompanied by the decrease in the spacing parameters of the crystalline lattice. That probably hinders the incorporation of the sorbed molecules into the lattice and leads to the increase in the equilibrium pressure P*(T) at fixed temperature T of the transition SS Br þ NV ¼ SS Br NV The composites based on binary salt systems (MeCl n þ MeBr n )/SiO 2 sorb vapor at the pressure (or temperature) intermediate between those for single-salt composites MeCl n /SiO 2 and MeBr n /SiO 2 (Figure 8). Thus, varying the relative salts content in the composite, the adsorbents with the required sorption properties can be prepared. This effect was used for intent designing the composites based on (BaCl 2 þ BaBr 2 ) inside the vermiculite pores which ð3þ Figure 7. AHT working cycles plotted for SWS-1L (open circle), CaCl 2 / SBA(11.8 nm) (filled triangle) and CaCl 2 /SBA(8.1 nm) (filled circle). (Reprinted from [61], with permission from Elsevier.) Figure 8. Isotherms of ammonia sorption on the input silica gel (1) and the composites (BaCl 2 þ BaBr 2 )/SiO 2 with various n BaCl2 =n BaBr2 ¼ 0:1 (2), 3:1 (3), 1:1 (4), 1:3 (5), and 1:0 (6). T ¼ 312 K. (Reprinted from [64], with permission from Elsevier.) International Journal of Low-Carbon Technologies 2012, 7,

7 L.G. Gordeeva and Y.I. Aristov were specialized for two AHT cycles, namely for air conditioning (ACon) (T c ¼ T a ¼ C, T e ¼ 108C) and ice making (IM) (T c ¼ T a ¼ 308C, T e ¼ 258C) driven by heat at T d ¼ C [65, 66]. First, the requirements to the adsorbents optimal for these particular cycles were formulated in terms of temperature of the salt ammonia complex formation: T*(P e ) ¼ C atp e ¼ 5.8 bar for ACon and T*(P e ) 408C at P e ¼ 3.5 bar for IM. Then, the molar ratios n Cl=Br ¼ n BaCl2 =n BaBr2 ¼ 3=2 and 1/1.2 were selected as optimal for ACon and IM cycles, respectively. Appropriate composites (BaCl 2 þ BaBr 2 (1/1.2))/vermiculite and (BaCl 2 þ BaBr 2 (3/2))/vermiculite were synthesized, and the dynamics and equilibrium of ammonia sorption were studied. The data obtained demonstrated that the actual temperatures of the ammonia complex formation for composites (BaCl 2 þ BaBr 2 (1/ 1.2))/vermiculite and (BaCl 2 þ BaBr 2 (3/2))/vermiculite were near equal to those for the optimal adsorbents. Thus, the confinement of binary salt system led to proper shift of the equilibrium temperature of the ammonia complex formation. As the result of such intent design, considerable accelerating of the sorption dynamics was observed [65, 66]. The specific cooling power (SCP) of the ACon cycle on the base of (BaCl 2 þ BaBr 2 (3/2))/vermiculite was estimated as W/kg that exceeds considerably the SCP for the BaCl 2 /vermiculite composite ( W/kg). The specific rate of ice production for (BaCl 2 þ BaBr 2 (1/1.2))/vermiculite was evaluated as 2 kg/(kg h) that is much higher than that for physical adsorbents ( kg/(kg h)). It is worth noting that the BaCl 2 /vermiculite does not sorb ammonia under conditions of the IM cycle at all. Thus, composites (BaCl 2 þ BaBr 2 (1/1.2))/vermiculite and (BaCl 2 þ BaBr 2 (3/2))/vermiculite can be considered optimal for the IM and ACon cycles, respectively. All in all, a large number of tools are available for intent modification of the sorption equilibrium of CSPMs to design the adsorbent with pre-required properties for particular AHT cycle. Among them there are variations in the chemical nature of the active salt, its content, conditions of the synthesis, porous structure of the matrix and confinement of the binary salt systems. All these tools allow practical realization of the considered approach of intent designing the optimal adsorbent for particular cycle that opens wide opportunities for tailoring new composites adapted for any AHT cycle. So far, a plenty of CSPMs have been synthesized and tested in various lab-scale prototypes and real AHT machines that is considered in the next section. 4 APPLICATIONS OF CSPMS 4.1 Adsorption cooling The common working pairs for adsorption cooling cycles are silica gel water, zeolites water, AC ammonia and AC methanol [2, 67, 68]. Alongside with them, chemo-sorbents are used, for instance, metal chlorides with ammonia [69 72], water [73] and methanol [74]. Chemo-sorbents are characterized by a large sorption capacity in the cycle. However, poor heat and mass transfer through the layers of the bulk salt and the salt sorbate complex restricts the dynamic efficiency of the cycle [10, 55, 56, 75]. To overcome that, composite materials, like compressed blocks of expanded graphite and an active salt [76, 77], composites based on activated carbon Busofit and salts [78, 79], and SWS-1L (mesoporous silica impregnated with CaCl 2 )[4, 5] were suggested for adsorption chilling. CSPMs based on activated carbon impregnated with CaCl 2 and LiCl were studied by Wang et al. In[80], comparison of the performances of three working pairs, namely AC methanol, chemo-sorbent CaCl 2 ammonia and composite CaCl 2 / AC ammonia in an adsorption ice maker was performed. The unit with the pair CaCl 2 /AC ammonia demonstrated the highest cooling power (20.3 kw). It is superior to that obtained with AC methanol and CaCl 2 ammonia by 10 and 1.4 times. This is due to high sorption capacity of the composite when compared with the pure AC and enhanced mass and heat transport in the composite in comparison with bulk CaCl 2 [81]. In the consolidated composite sorbents BaCl 2 /expanded graphite [82] and SaCl 2 /expanded graphite [19], the salt provides the high ammonia sorption capacity (0.61 kg/kg), and the matrix (expanded graphite) prevents agglomeration of the salt and raises the heat transfer. When water is a refrigerant, composites based on CaCl 2 and LiCl are often used [16, 18]. Theoretical [15] and experimental [16] study of the sorption and refrigerating performances of the composite CaCl 2 /microporous silica gel (d av ¼ nm) and those for the host silica was carried out by Daou et al. The cooling power with the composite was demonstrated to exceed times that with the host silica gel, and the coefficient of performance (COP) was higher by 25%. Moreover, the composite adsorbent can be regenerated at lower temperature. Recently, the composite sorbent of methanol LiCl/mesoporous silica gel (d av ¼ 15 nm) has been intently prepared for air conditioning cycle [6]. The composite exchanges 0.7 g/g under the following conditions of the cycle, T e ¼ 78C, T c ¼ 358C, T a ¼ 308C, T d ¼ C, and allows the theoretical COP as large as The composite sorbent LiCl/silica gel (d av ¼ 8 nm) was tested in an adsorption chiller with heat and mass recovery [83, 84]. The average COP ¼ 0.41, the cooling capacity of 5 kw and the SCP ¼ kw/kg were obtained at T d ¼ 84.88C, T e ¼ 158C and T c ¼ 308C, the heating/cooling time 680 s, the mass recovery time of 40 s and the heat recovery time of 20 s (Figure 9). The COP and SCP were improved by 24 and 15% when compared with the same chiller filled with the input silica gel and water as working pair. The temperature sufficient for the composite regeneration (T ¼ C) was lower than for the silica gel water chiller (85 908C). That means that the LiCl/silica methanol working pair is more preferable for chillers driven by low temperature heat sources, like solar energy. A chiller employing LiCl/AC methanol pair was studied in [53]. Though the sorption ability of AC was improved by International Journal of Low-Carbon Technologies 2012, 7,

8 Composites salt inside porous matrix for adsorption heat transformation Figure 9. The cooling capacity and COP with the variation of hot temperature inlet (Reprinted from [84].) 81% due to inserting LiCl, the sorption/desorption hysteresis was observed, which hindered the utilization of this adsorbent at low heat source temperature. When the heat source temperature T d was.1158c, T e ¼ C and T c ¼ 308C, the composite sorbent had the specific cooling capacity (SCC) by 11 31% higher than that obtained with the untreated AC. The COP values for both adsorbents were practically the same. An important advantage of composite sorbents is the possibility to vary their sorption properties by a number of tools, e.g. the chemical nature of the salt, in order to fit the particular cycle. Thus, Freni et al. demonstrated [85] that the composite adsorbent SWS-8L (Ca(NO 3 ) 2 /mesoporous silica) required lower regeneration temperature when compared with SWS-1L (CaCl 2 /mesoporous silica). For this reason, it is more appropriate for cooling cycles driven by low temperature heat. The modeling results demonstrated that it provided the operation of the cooling cycle driven by heat source of temperature T d ¼ S with the theoretical cooling COP ¼ 0.4 (at T e ¼ 108S and T c ¼ 408S). For comparison, the theoretical COP of the unit using SWS-1L approached to 0 at these conditions (Figure 10). The experimental cooling COP ¼ 0.18 and the SCP ¼ 190 W/kg were obtained at the heat source temperature as low as 778S, demonstrating that this sorbent can be efficiently driven by low-temperature heat. Composite SWS-9V (LiNO 3 /vermiculite) allows further lowering the heat source temperature. Sapienza et al. [86] showed that it exchanged 0.4 g H 2 O/g in an exceptionally narrow temperature range: C (adsorption at 12.6 mbar) and C (desorption at 56.2 mbar). The chiller utilizing SWS-9V can be driven by a low-grade thermal source T d, 708C providing quite efficient operation of air-conditioning cycle with COP ¼ 0.59 and SCP ¼ 96 W/kg for loose grains configuration. The working pair NaBr/expanded graphite ammonia allows an efficient realization of the cooling cycle driven by heat source of T d ¼ 658S at T c ¼ 308S and T e ¼ 5 158S [87]. The pair LiCl/expanded graphite ammonia was used in the IM cycle under the working conditions T e ¼ S, T c ¼ Figure 10. Cooling COP vs. desorption temperature calculated for SWS-8L, SWS-1L and a commercial silica gel. (Reprinted from [85], with permission from Elsevier.) S and T d ¼ S [88]. The highest average SCP during the sorption phase was 117 W per kg of the composite. The calculated theoretical COP under different cycle conditions was nearly constant and equal to Moreover, the new composite sorbent showed higher SCC, compared with the AC methanol pair. It allowed the production of kg of ice per 1 kg of the sorbent per cycle that is 2 5 times more than the SCC of the pair AC methanol under similar conditions. The other pairs tested in adsorption chillers are: BaCl 2 / vermiculite ammonia [89], CoCl 2 /AC ammonia [56], LiCl/ attapulgite water [18], LiBr/silica gel water [17], CaCl 2 / expanded graphite water [20] and CaCl 2 /ACF water [21]. 4.2 Desiccant cooling Typical adsorbents employed in open-cycle descant cooling systems are microporous silica gel, molecular sieve and alumina [90 92]. Nowadays, CSPMs are used in desiccant cooling systems along with the conventional adsorbents. Jia et al. [93] developed a novel composite desiccant material based on silica gel and LiCl and fabricated a honeycombed rotary wheel with this composite material. Experimental comparison between the two honeycombed desiccant wheels, namely a conventional one using silica gel and a new one fabricated with the composite LiCl/silica gel, was made in [94]. Moisture removal capacity of the composite desiccant wheel, on average, was larger than that of the traditional silica gel wheel by 20 50% (Figure 11). Highly hygroscopic LiCl embedded in the pores of silica gel improved its moisture adsorption capacity. Also encouraging was that the new desiccant wheel can be driven by a lower regeneration temperature for acquiring the same amount of moisture removal (Figure 11). These results demonstrated that the composite desiccant wheel has good potential for dehumidification applications. In [95], a desiccant rotary wheel based on silica gel (silica/ CP), CaCl 2 (CaCl 2 /CP) and composite CaCl 2 /silica (CaCl 2 / silica/cp) applied to a corrugated paper (CP) were compared for their abilities to remove moisture from wet air. Moisture International Journal of Low-Carbon Technologies 2012, 7,

9 L.G. Gordeeva and Y.I. Aristov Figure 11. Impacts of regeneration temperature on moisture removal capacity and DCOP. (Reprinted from [94], with permission from Elsevier.) capacity of the composites ranges as CaCl 2 /CP. CaCl 2 /silica/ CP. silica/cp. However, despite higher hygroscopic ability of CaCl 2 /CP, the CaCl 2 / silica/cp reaches the equilibrium sorption much faster. Moreover, the composite CaCl 2 /silica/cp has a longer lifespan than the CaCl 2 /CP, probably due to solidification effect of the silica gel. Adsorption/desorption characteristics of a packed porous bed based on CaCl 2 inserted into clay particles was studied for desiccant cooling in [96]. Testing the composite through multiple sorption and desorption cycles demonstrated a good performance though small masses of desiccant were lost due to surface disintegration of the desiccant spheres. 4.3 Low-temperature heat storage The key parameters that determine the efficiency of adsorption heat storage systems are high storage density, high temperature lift, low charging temperature and high thermal conductivity. Among different systems, chemical reactions between salts (Na 2 S, MgSO 4 etc.) and water vapor giving hydrates formation are characterized by high sorption capacity and, consequently, high storage density [97]. However, the corrosive activity of the salts, their low thermal conductivity and, consequently, low specific power of the storage unit hinder their practical application [98, 99]. The composite sorbents presenting intermediate behavior between solid adsorbents, salts hydrates and liquid absorbents ensure better efficiency. Combination of high storage density of the salts and good mass transfer of the porous matrix allows the design of advanced materials and devices with high COP and specific power. Jänchen et al. performed a comparative study of heat accumulating ability of low silica X zeolites, AlPO-5, AlPO-18, SAPO-34, mesoporous silica gel and attapulgite impregnated with hygroscopic salts [25, 29, 100]. It was demonstrated that the zeolite-based materials (zeolites of NaA-, NaY-, NaX-type and their ion exchanged forms) were characterized by the volume storage density of up to 576 kj/dm 3 (810 kj/kg) and provided a high temperature lift after charging at temperature of 453 K. The impregnated mesoporous materials although ensured a lower temperature lift; however, they offered a larger storage density 839 kj/dm 3 (864 kj/kg). Another advantage of these composites was lower charging temperature of 393 K [29]. Because of the low differential molar heats of sorption, the composite CaCl 2 /attapulgitte demonstrated lower energy density than zeolite LiLSX (1184 kj/kg) and SAPO-34 (997 kj/ kg) at the adsorption temperature T a ¼ 313 L and the relative humidity (RH) ¼ 50%, but it required the lowest temperature for water desorption T d, 400 K [100]. Thus, all the systems studied were promising for adsorptive heat storage for hot tap water supply and space heating of single family dwellings [100]. Attapulgite granulate impregnated with a mixture of MgSO 4 and MgCl 2 hydrates was investigated for suitability as a heat storage material in [101]. It was shown that the partial substitution of MgSO 4 by the MgCl 2 resulted in higher sorption heat. It increases for a larger amount of MgCl 2 due to its higher affinity to water vapor. This larger heat release was accompanied with higher temperature lift. The energy density of the composite containing a mixture of 20 wt.% MgSO 4 and 80 wt.% MgCl 2 was 1590 kj/kg at T a ¼ 308C and RH ¼ 85%. The desorption temperature of 1308C was sufficient for the composite regeneration that makes the new material promising for solar thermal energy storage. The salt content of CaCl 2 /silica gel composites was shown to be an important factor affecting the heat storage capacity [43]. The composite sorbents for thermal energy storage were extensively studied by Zhu et al [27, 28, 102]. The composite sorbent prepared by impregnating of silica gel with a 30 wt.% CaCl 2 solution showed the highest and stable storage capacity of 1020 kj/kg at the relatively low charging temperature of 908C. The specific heat storage capacity increased noticeably while the COP decreased as the regeneration temperature increased from 80 to 1008C [27]. The sorption capacity of the composite sorbent was stable (950 kj/kg) through over 500 repeated sorption/desorption cycles [28]. The water sorption on CaCl 2 -containing materials as heat storage media was studied in [26]. Amount of the water sorbed on CaCl 2 -containing 296 International Journal of Low-Carbon Technologies 2012, 7,

10 Composites salt inside porous matrix for adsorption heat transformation materials at higher humidity was larger than that on zeolites. The heat storage capacity of composite (33 wt.% CaCl 2 )/FSM16 (1200 kj/dm 3 ) was 3.5 times as high as a Na form Y-zeolite at P ¼ 18.7 Torr (24.6 mbar). Thus, a comparison of the composite sorbent with the other heat storage materials revealed that it provided great potential to reduce a storage system size and to obtain a high density of low-grade heat storage. As reported in [86], a new composite sorbent LiNO 3 /vermiculite (SWS-9V) was specifically developed to operate at low regeneration temperature (,65 708C) and demonstrated a remarkable heat storage capacity of 900 kj/kg. Thus, the main advantages of the composites for heat storage are as follows: (i) low charging temperature (solar energy absorbed by flat receivers can be stored), (ii) high storage capacity and (iii) possibility to controllably modify its sorption properties to match conditions of particular heat storage cycles. The main shortcoming is a relatively low temperature lift. It can be enhanced by applying salts with higher affinity to water vapor at the expense of appropriate increase in the charging temperature. The wide variability of the sorption properties of the composites allows for reasonable values of both temperature lift and charging temperature. 4.4 Heat and moisture regeneration in ventilation systems For countries with a cold climate (typical of Russia, Canada and the North Europe), the difference between indoor and outdoor temperatures can reach 608C or even more in winter, that leads to enormous heat losses and freezing of moisture at the system exit. As a result, common heat recovery units integrated in ventilation systems may not be capable to work at these conditions. Moreover, such systems are not able to manage the indoor humidity, which dramatically reduces in winter season that greatly disbalances the indoor heat comfort. Thus, to fill these three main gaps in the current technique, the following actions should be performed: efficient exchange of heat between the exhaust and supply air fluxes to reduce heat losses, reasonable drying of the exhaust air to avoid ice formation at the system exit, moisturizing the supplied air to provide indoor conditions of human thermal comfort. The so-called VENTIREG is a new approach for regenerating heat and moisture in ventilation systems in cold climates, which resolves the obstacles mentioned above [31, 103, 104]. The main principal of VENTIREG is as follows. To exchange the sensible heat between the inlet (fresh) and outlet (exhausted) air fluxes, a granulated layer 1 of heat storing material (HSM) is placed closer to the unit exit (Figure 12). Before this layer (closer to the room side), a layer 2 of water adsorbing material is located. It serves as a water buffer. The unit is intermittent and operates in two modes: Figure 12. Scheme of the regeneration process: the temperature and moisture profiles at various times t 1, t 2, t 3. (Reprinted from [31], with permission from Elsevier.) Outflow mode: a warm and humid indoor air is blown by an extract fan through the relatively dry adsorbent, which captures and retains the indoor moisture. Dried and warm air enters the layer 1 and heats it up. After that, the air flux switches; Inflow mode: a dry and cold outdoor air is blown by a supply fan through the warm layer 1 and is heated up to the temperature close to that in the room T in, thus recovers the stored heat. Passing through the layer of the humid adsorbent, warm and dry air causes the retained water to be desorbed and come back to the room, thus, maintaining the indoor moisture balance. Because of the finite heat capacity of layers 1 and 2, temperature T of incoming air is slowly decreasing, and the air flux switches when T in 2 T reaches a predetermined value DT 0, and so on. To study and optimize the heat and moisture recovery, three experimental units with the air flux up to 25 (I), 40 (II) and 135 (III) m 3 /h were built and tested. Both common (silica, alumina) and novel CSPM (alumina impregnated with CaCl 2 ) adsorbents were used as buffers of water. New composite adsorbent CaCl 2 (18 wt.%)/alumina was found to demonstrate better performance than common commercial adsorbents, like silica and alumina. Owing to higher adsorption capacity of this composite, the adsorbent loading is less that leads to lower hydrodynamic resistance of the unit. This could allow the using of cheap blade-type fans instead of centrifugal ones and give a reduction in the electricity consumption. Unit III consumes for air blowing W of electric power and gives the heating load of W, that corresponds to COP ¼ The suggested VENTIREG unit exchanges stale, contaminated room air with fresh outdoor air, recovering up to 95% of heat and 70 90% of moisture from the exhaust air and prevents the formation of ice at the unit exit. For countries with hot and humid climate, where indoor air conditioning and dehumidification is necessary, the opposite mode of heat and moisture recovery in ventilation systems was applied [ ]. In this process, a wheel coated by a desiccant (such as silica gel) exchanges heat and moisture between International Journal of Low-Carbon Technologies 2012, 7,

11 L.G. Gordeeva and Y.I. Aristov the outside fresh and humid air and the inside exhaust and dry air. The moisture is transferred from the fresh air to the exhaust air, with cyclic moisture adsorption and desorption by the desiccants. However, besides moisture, volatile organic compounds (VOCs) could be adsorbed by the desiccant as well. The adsorbed VOCs would then be released again to the fresh air resulting in deterioration of indoor air by these harmful gases. In [108], a novel high-selective desiccant MA NH 2 SiO 2 which can adsorb more water vapor and less VOCs is developed for rotary wheel-type heat and moisture recovery system. This novel desiccant was prepared by aminefunctionalization of the silica gel surface, with subsequent selfassembly with malic acid. Thus, the novel composite is similar to CSPMs considered above, but comprises organic salt in a porous matrix. Owing to abundant hydrophilic groups on the MA NH 2 SiO 2 surfaces the adsorption capacity of MA NH 2 SiO 2 for hydrophobic toluene and acetaldehyde as the most prevalent air pollutants was reduced considerably. On the contrary, adsorption capacity for water vapor was comparable with the initial SiO 2. That results in the selectivity of moisture/ toluene and moisture/acetaldehyde as improved in 3.6 and 2.3 times, respectively. Thus modification of silica gel allows for the development of novel desiccant material, promising for the future use in rotary wheels for heat and moisture recovery with improved indoor air quality. 5 PROBLEMS AND OUTLOOKS Starting from the mid-1990, the number of papers relating to the CSPM s synthesis, properties and applications, including AHT, has been increasing continuously. Moreover, several CSPMs have been tested for various practical applications, including gas drying [109, 110], maintaining relative humidity [111], heat insulation [112], regeneration of heat and moisture in ventilation systems [31], CO 2 abatement [113] and adsorptive transformation of heat [84, 85, 114]. This examination revealed pitfalls, obstacles and potential troubles that could impede an actual reduction of these new composites to practice. The most worrying could be a leakage of the salt solution formed during sorption out of the matrix pores, which can cause a corrosion of metal parts of AHT unit and emission of non-condensable gas as reported in [115]. There are several ways to avoid, or at least lessen, this shortcoming: (a) A proper selection of the salt content in the CSPM. At the uptake w fil (g/g) corresponding to a complete pore filling, the concentration of the salt aqueous solution formed inside the pores is equal to the concentration C imp (%) of the impregnating solution. This solution is in equilibrium with water vapor at the relative pressure h imp. The function C imp (h) can be found in literature for many aqueous salt solutions (see, for instance, [116, 117]). It is evidently that until the relative pressure over the adsorbent does not exceed h imp, the salt solution remains located inside the pores. Thus, for proper selection of the concentration C imp, one has, first, to calculate the maximal value of relative pressure reached in the cycle h max ¼ P o (T e )/P o (T a )as it corresponds to the final adsorption state ( point 1 in Figure 2). Then, the maximal concentration C imp (h max )of the impregnating solution can be determined from the literature dependence C imp (h). For the sake of safety, we would recommend to select the actual concentration of the impregnating solution lower than C imp by a factor of This analysis allows understanding why the authors of [115] met the mentioned problems when tested a unit for thermal energy storage. They performed the water adsorption at the three following conditions: (i) T a ¼ 308C, T e ¼ 208C; (ii) T a ¼ 308C, T e ¼ 158C and (iii) T a ¼ 358C, T e ¼ 158C. Corresponding relative pressures of water vapor were h ¼ 0.55, 0.40 and 0.30, respectively. The tested composite CaCl 2 /silica gel was synthesized by dry impregnation of silica with a saturated solution of the salt as described in [4]. The relative pressure over this solution at T ¼ C equals 0.22; therefore, the maximal relative pressure during the adsorption stage has to be lower than Instead, it was much higher for all the adsorption runs applied in [115]. To easily avoid such troubles, it is recommended to make this simple analysis before CSPMs usage or discuss safe operation conditions with a producer. (b) An anchoring the salt cations to the matrix surface. Attempts to do so have been performed by means of enhanced adsorption of the metal cations on the pore surface at increased ph of the impregnating solution [46]. However, a side effect of the cation adsorption is that mono-variant sorption equilibrium, optimal for AHT cycles, gradually transforms to bi-variant [46]. Thus, an encouraging task for further CSPM development is to anchor the salt to the matrix surface but keep the monovariant sorption equilibrium. (c) To cover the external surface of CSPM grains with a hydrophobic coating which is penetrable for water vapor, however complicates or prevents leakage of an aqueous salt solution. It should act similar to a waterproof/breathable fabric Gore-Tex w [ products/fabrics/goretex/goretex_clothing.html]. The solution leakage can also be avoided by proper organization of the AHT process rather than by improving the adsorbent: the amount of working fluid in an evaporator should be restricted in such a way that the outflow is not possible even if the whole of fluid from the evaporator is sorbed by CSPM. Another issue that needs more testing is a hydrothermal stability of CSPMs in multiple cooling/heating cycles in the vapor surrounding. The CSPM testing in the AHT cycles (up to 100 cycles [114, 118]) did not revealed any degradation of the material composition and sorption ability; only slight destruction of the silica-based composites was observed. Nevertheless, 298 International Journal of Low-Carbon Technologies 2012, 7,