SAPO-34 based zeolite coatings for adsorption heat pumps

Energy 187 (2019) 115981

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SAPO-34 based zeolite coatings for adsorption heat pumps L. Calabrese a, c, *, L. Bonaccorsi b, P. Bruzzaniti a, E. Proverbio a, A. Freni c a

Department of Engineering, University of Messina, Contrada di Dio Sant'Agata, 98166, Messina, Italy Department of Civil Engineering, Energy, Environment and Materials, University Mediterranea of Reggio Calabria, Salita Melissari, 89124, Reggio Calabria, Italy c CNR-Institute of Chemistry of Organo Metallic Compounds (ICCOM), Via G. Moruzzi 1, I-56124, Pisa, Italy b

a r t i c l e i n f o

a b s t r a c t

Article history: Received 27 February 2019 Received in revised form 18 July 2019 Accepted 19 August 2019 Available online 20 August 2019

In this work, at first, an overview on SAPO-34 zeolite coating for adsorption heat pumps is presented, highlighting current quality standard and open technological issues (i.e. the need for good mechanical and hydrothermal stability, high aging stability, etc.). Afterwards, we present an improved formulation of adsorbent composite coatings on aluminum support, having improved thermal and mechanical properties, especially when subjected to an impulsive stress. Specifically, the coated samples were prepared by dip-coating method starting from a water suspension of SAPO-34 zeolite and a hybrid polymer binder. Adhesive and mechanical properties were evaluated by pull-off test confirming the good interaction between metal substrate, filler and matrix. Adsorption equilibrium of water vapor on the adsorbent coating was measured in the range T ¼ 30e150  C and partial pressure of moisture equal to 11 mbar. It was found that binder does not affect the water adsorption capacity and adsorption rate of the original SAPO-34 zeolite. © 2019 Published by Elsevier Ltd.

Keywords: SAPO-34 Zeolite Coating Adsorption heat pumps Composites

1. Introduction Several research activities on adsorption heat pumps development are focused on the optimization of the integration between heat exchanger and adsorbent material, to create the so called AdHex. Alumino-silicate zeolites (13X, 4A) and Microporous silica gel Microporous silica gel are the standard adsorbent of water widely used in adsorption heat pumps. However, recently the (silico)alumino-phosphate SAPO-34 has been considered as a more attractive solution, because this adsorbent can combine a moderate hydrophilicity with a high capacity of adsorption of water vapor [1]. Concerning the practical integration of the adsorbent into the heat exchanger, two different approaches are currently evaluated: embedding of the granular adsorbent inside an efficient heat exchanger [2] or coating the heat exchanger with the adsorbent material [3,4]. By using a loose grains adsorbent bed configuration, it is possible to reach higher volumetric adsorption capacity thanks to

* Corresponding author. Department of Engineering, University of Messina, Contrada di Dio Sant'Agata, 98166, Messina, Italy. E-mail address: [email protected] (L. Calabrese). https://doi.org/10.1016/j.energy.2019.115981 0360-5442/© 2019 Published by Elsevier Ltd.

the larger amount of sorbent material loaded inside the HEX. Moreover, granular zeolite adsorbent inside the heat exchanger induces in principle a low vapor transfer resistance and low manufacturing costs. However, if the grain size distribution is improperly chosen, then the mass transfer could be strongly limited [5,6]. Furthermore, a significant limit that restrict its use is the very low heat transfer efficiency due to a point contact between the grains and the surface of heat exchanger [7,8]. On this concern, Aristov et al. [9] investigated the influence of heat transfer surface, adsorbent mass ratio and the grain size on the dynamic behavior of loose grains adsorber configurations identifying two different regimes: lumped regime and grain sensitive regime. In the former regime, related to small grain sizes, the dynamic performances are influenced mainly by the heat transfer efficiency, while the mass transfer effect is negligible. On the other hand, the grain sensitive regime takes place for large adsorbent grain size and the kinetic is mainly affected by intra-grain diffusion resistance, while the heat transfer plays a minor role. Summarizing, the main limits linked to the use of loose grains configurations can be as in the following: (i) low efficiency of the secondary fins which limit the overall heat transfer coefficient.

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(ii) possible pressure drop along the packaged grains if grain size distribution is improperly chosen which limits the kinetic performance (iii) The use of a metallic net as packing system to preserve the confinement of the adsorbent grains inside the heat exchanger that could limit the mass transfer. In order to overcome these issues, several research developments were recently focused on the optimization of the adsorber in order to better integrate the heat exchanger module and adsorbent material [10e12]. Freni et al. [13] demonstrated, by using a mathematical model, that zeolite coated adsorbers for adsorption pumps show better performances, in terms of specific and volumetric powers, compared to other configurations, evidencing a specific and volumetric cooling power up to 3.8 kW/kg and 214 kW/m3 of the adHEX, respectively. In this concern, coated exchanger choice is related mainly to its good thermal contact at the coating/metal interface and to the cycle times reduction, becoming an effective approach able to optimize the thermal management inside the adsorber. The most promising coating methods are in-situ zeolite growth [14,15] and binder-based coating processes [16]. 1.1. Direct synthesis zeolite coatings Direct accretion of zeolite crystals on the metal surface allows very good adhesion. The in situ crystallization is a technique able to obtain a regular and homogenous zeolite coating deposition also on complex geometries, making it potentially suitable as coating deposition approach for HEX components. Among the different in situ crystallization approaches the hydrothermal synthesis is the most applied [17,18]. The hydrothermal synthesis process is based on the evolution, in an alkaline environment and in hydrothermal conditions, of amorphous silico-aluminate systems towards a zeolite type structure. The hydrothermal synthesis (a scheme of the process is reported in Fig. 1) is carried out by reacting, under continuous mixing, specific reagent ratios in a perfectly sealed autoclave in order to achieve autogenic pressure conditions. Alkaline silicate and aluminate solutions are commonly used as starting reagents. After mixing, the solution constituents start to gelate. Pressure and temperature are the main factors that stimulate the activation of the chemical reaction. Therefore, the hydrothermal synthesis process is performed in the sealed autoclave in order to complete the evolution in a zeolite type structure. Depending on synthesis configuration and metal alloy support specification, the properties and morphology of the zeolite coatings obtained by in situ crystallization may be different and, if suitably controlled, adapted to the final application conditions. In some application fields, for example in sensors or in anticorrosive surface coatings, the prerogative is to obtain very thin, regular and flawless

coatings [20,21]. On the other hand, for applications such as adsorption heat pumps or adsorption applications in general, multiple layers of the adsorbent material are required [3,22]. Therefore multiple or long time deposition steps are required to reach an acceptable zeolite layer thickness (~0.1 mm). To obtain thick zeolite layer is a relevant issue due to the risk to have defects, cracks or delaminated areas that have detrimental effect on hydrothermal stability of the coating at long time [23]. Moreover, the support nature and interface interaction, which are responsible for the bonding of the zeolite and the substrate, are significantly influenced by the chemical and physical conditions created during the synthesis process, which may limit homogeneous growth. Wittstadt et al. [24] developed a new composite material based on SAPO-34 coated aluminum fibers in order to achieve process intensification for adsorption chillers and heat pumps. Their results highlighted that the specific cooling power for the adsorption step per volume of the zeolite coated HEX exceeded 500 kW/m3 under specified conditions. Furthermore, Bonaccorsi et al. in Ref. [25] evaluated the growth of different SAPO (CHA) zeolites by microwave assisted hydrothermal synthesis on aluminum foamed supports for AHP applications. Adsorption tests demonstrated that the synthesized SAPO34 was more appropriate for the typical thermodynamic cycle for adsorption pumps. Chanda et al. [26] evidenced the relevant impact of several synthesis parameters on the formation of zeolite coating by direct synthesis and as such parameters influence the coating performance making it as a potentially attractive adsorbent material synthesis for the heat pump and heat storage applications. In zeolite coating obtained by direct synthesis, the main advantage is the heat transfer across the interface that is strongly increased due to the nearly perfect contact between the heat exchanger surface and the adsorbent material. However, for this last solution some issues nevertheless are still pending. The mass transfer resistance is substantially reduced due to the coating high density. Also in the recent AHP systems the transport dynamics phenomena in the adsorbers are still a pending point [27]. This behavior can be explained considering that at the moment the larger effort on this research field were focused to enhance the heat transport. Although, as recently suggested by Ammann et al. [28] this approach is not very suitable for adsorbers, because the sorption kinetic behavior of SAPO-34 composite coatings is hindered mainly by mass transfer phenomena rather than heat transfer one. Furthermore, a further drawback on coated HEX, is that the synthesis process is complex and expensive. The optimization of the hydrothermal synthesis process is characterized by several parameters that are all key points to create a coating with high homogeneity as well as good adsorption capabilities and durability for adsorption heat pump applicability [21]. Based on these considerations, hydrothermal synthesis of zeolite coatings is really

Fig. 1. Scheme of hydrothermal synthesis process [19].

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attractive, considering that this deposition strategy is able to simply obtain well-ordered and interconnected pure zeolite grain films. Although the limited process flexibility combined to process parameters and economic issues represents effective risks that need to be taken into account for its industrial applicability. 1.2. Composite zeolite coating The binder-based coating method is an alternate way to deposit a thin layer of adsorbent on the heat exchanger surface. The binderbased coating method offers the possibility to vary coating thickness in the range 0.1e1 mm by, e.g., controlling the viscosity of the liquid suspension and the dipping velocity. Earlier studies on the subject focused on the preparation of silica gel or zeolite-based coatings employing different inorganic or organic binders [29e31]. However, the resulting thermo-physical properties were not optimized and cycling stability was not proven. Looking at the recent developments in field, Okamoto et al. [32] presented a relevant study on a SAPO-34 (coded AQSOA Z02) based composite coating produced by Mitsubishi Plastics Inc., by using an organic binder. The deposition procedure allowed obtaining coating thickness of 0.3 mm in aluminum lamellas usually used for heat exchanger. Their results evidenced an effective increase of thermal conductivity of the coating set-up (0.36 W/m K) compared to zeolite powders one (0.113 W/m K). Recently, silane based coating was evaluated to obtain effective and adsorptive coatings with good mechanical performances and long durability in severe environmental conditions [33,34]. The approach is based on a dip coating procedure (a scheme of the process is reported in Fig. 2), where the metal component is dipped in a silane-zeolite alcohol solution followed by curing at 80  C for 24e48 h aimed to guarantee sufficient cross-linking reactions between silanol groups and metal interface and to be sure that the optimal curing of the coating is reached [35]. Full scale dip-coated adsorbers were experimentally tested in Ref. [36], showing encouraging results in terms of reduced adsorption cycle time and elevated specific power. Analogously, Bendix et al. [4] optimized power output and metal to adsorbent weight ratio on small scale and full scale adsorbers coated with increasing amounts of adsorbent. Freni et al. introduced a new coating composition, employing SAPO 34 powder as adsorbent and silane as binder [3] to obtain a coated adsorbent heat exchanger for adsorption chiller, evidencing the promising results of this technology. Utilization of a silane matrix in combination with zeolites as a coupling agent generates an interlayer with a good adhesion and homogeneity, able to provide a further barrier action, as protective layer. Indeed, the zeolite surface is covered by a large amount of

Fig. 3. Thermogravimetric microbalance dynamic vapor system.

silanol groups which guarantees a relatively high chemical reactivity and superficial interaction with several chemical compounds, including silanes [37]. Do et al. [38] evaluated the performances of six different ferroaluminophosphate (FAPO) zeolite coating with a 5.0 wt%, 10.0 wt %, and 15.0 wt% content of epoxy-based and silicone-based binders, respectively. The results evidenced that the adsorption performance of the adsorbent applied on metal substrate is able to offer effective sorbent performances without affecting the durability by adding 5.0 wt% epoxy binder. However experimental results reported in Ref. [28] evidence, on 95 wt% SAPO-34 composite coatings, that mass transfer during adsorption could be a rate-limiting factor during temperaturedriven water vapor sorption cycles. On this concern, more recently, Ammann et al. [39] to increase the rate of water sorption in SAPO-34 coatings designed a SAPO-34 coating with an hierarchical structure constituted by interspaced longitudinal pathways tailored to enhance vapor transport. Chanda et al. [40] performed sorption kinetic measurements in order to evaluate the heat and mass transfer properties of on reactive and spray coated systems. The results evidenced that high solid packing density of the coating play an effective role on its thermal conductivity, thus also ensuring adsorption heat pumps with high power density. Although, reversely, the mass transfer is negatively influenced by coating packing density, requiring the design of an optimum thickness of the coating layer (identifying a

Fig. 2. Scheme of dip-coating process.

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threshold at 135 mm of layer thickness) to allow suitable adsorption performance of the composite material. It is expected that, the new research activities are focused on the realization of a zeolite-based composite coating able to exalt the vapor mass transport and maintain the same adsorbent properties of the zeolite itself (thereby ensure the industrialization potential for the adsorption heat pumps). But that should not limit, at the same time, the typical mechanical stability and the durability characteristic of binder based composite coatings in hydrothermal aging conditions [41,42]. 1.3. Adsorbent coatings drawbacks and step forward Nevertheless, still some issues are present. The polymer used in the composite coating as matrix could occlude the zeolite porosity, increasing the vapor transfer resistance and thus reducing the efficiency of the adsorbent material. Depending on binder amount and characteristics a limitation of the mass or heat transfer can be highlighted [43]. Furthermore the zeolite based coatings are usually characterized by unreliable mechanical strength and brittle behavior that favors easy loss of zeolite particles. The low mechanical performances of the coating favors local delamination phenomena during repeated hydrothermal adsorption-desorption cycles. Degradation phenomena could also be stimulated by the significant difference in thermal expansion coefficients between composite coating and aluminum exchanger fins. A further drawback of this route is the low zeolite amount in the adsorber, due to the limited coating thickness, that induces low adsorbent density (typically 150e300 g/dm3). To overcome the previous issues, a new formulation based on a mixture of hybrid polymer binder with proper catalyst was employed to form an elasto-plastic composite coating. During the raw components mixing, the SAPO-34 powder (2 mm crystal dimension) was dispersed at different percentages (from 70 wt% up to 90 wt%). The experimental characterization of the new composite coating was carried out by pull-off, impact and adsorption tests. The novelty of this approach is based on the use of a polymeric matrix with an effective elasto-plastic behavior and high flexibility. That should be able to offer greater efficiency in terms of impulsive stress resistance or to internal stresses induced by mechanical or thermohygrometric cycles. This should offer valid assumptions to obtain a composite adsorbent material with potentially suitable durability and stability performances preserving the sorption capability of the zeolite based composite coating. 2. Experimental part 2.1. Sample preparation Commercial aluminum alloy 6061 rectangular strips, size 20 mm  50 mm, were cut, by using a laboratory circular saw with water cooling system, from a large aluminum sheet (thickness 0.5 mm). All samples were degreased in a diluted alkaline solution (0.1 N NaOH) for 60 s, washed in distilled water and finally treated with acetone. Afterwards, the composite hybrid zeolite coating was applied. In particular, the coating's preparation procedure involves the following steps: i) preparation of the composite slurry, by mixing zeolite and hybrid polymer constituents in a water/ethanol solution. The slurry homogenization was carried out at first in an ultrasonic bath for 15 min followed by magnetically stirring for 15 min before the dip coating procedure; ii) pre-treatment of the aluminum substrate.

iii) the coating deposition was obtained by dipping coating procedure (dipping speed 4 cm/min) after that the substrate was maintained into the composite slurry for 60 s. iv) final drying (open to air for 5 min) and curing (12 h at 80  C). The so obtained zeolite coatings are categorized in this paper with the code “SZ” followed by a number, which identifies the percentage of the zeolite added to the polymer matrix. For instance, the code “SZ-80” indicates the sample made with 80 wt% of SAPO34 zeolite filler. The morphology of the different zeolite coatings were checked by using a scanning electron microscopy (SEM-FIB Zeiss Cross Beam 540).

2.2. Mechanical tests Impact and pull-off tests were used to evaluate mechanical stability and adhesive properties of the coating/support composite. The flexibility of coatings was estimated by the drop-weight impact test in accordance to the experimental procedures defined in Refs. [16,42]. The drop weight impact test was performed on the coated sample by using a metallic sphere (diameter 24 mm, weight 56 g) initially suspended by an electromagnet and then freely fall down from different heights. In this way it was possible to quantify the impact energy transferred to the sample. The sample was placed on a proper support with a tilt angle of 45 , to avoid the rebound of the falling sphere over the sample. After impact, the specimen was carefully removed from the support for post-impact optical damage evaluation. The failure impact energy was identified for the damaged area with a diameter about 1.35 mm. Three replicas for each drop weight height were applied. Pull-off tests were carried out by using a DeFelsko PosiTest AT-M pull-off tester according to the experimental procedure reported in Refs. [33,42].

2.3. Adsorption tests The water vapor adsorption isobars of coated samples were measured by a thermogravimetric microbalance dynamic vapor system (Dynamic Vapor Sorption e DVS), according to Ref. [44] (see Fig. 3). First of all, the sample (about 0.5 cm3) was slowly heated up to 150  C (heating rate 1  C/min) and kept at this temperature for about 6 h under continuous evacuation (vacuum level: 104 Pa), in order to degas the sample and determine its dry weight. Subsequently, a valve connecting the evaporator containing liquid water (maintained at T ¼ 23  C) and the sample chamber was opened. The vapor flows through the system and the vapor (absolute) pressure was kept constant at the set value (P ¼ 11 mbar) by a butterfly valve automatically controlled and by the downstream vacuum pump. The system is controlled by a computer, which also regulates the sample temperature, following the defined temperatures steps (from 23  C to 150  C). At each temperature step, the pressure was kept constant until the sample weight equilibrium was reached. The water uptake was calculated as

    m p H2 O; Ts  m0 g ¼ w g m0

(1)

where m(pH2O,Ts) [g], represents the mass of the sample at given water vapor pressure and sample temperature, while m0 [g] is the dry mass of the sample.

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3. Results and discussion 3.1. Morphological analysis Fig. 4 shows, as reference, the top view and cross section micrographs at 300x and 400x magnification, respectively, of SZ-90 zeolite composite. The coating structure is homogeneous and almost compact. In addition, no delamination or cracked area can be identified. It can be highlighted that the coating surface was quite regular, although some large “hills” and “valleys” might be shown as identifiable by grayscale color gradients in Fig. 4a. However, this morphology is quite common in binder based composite coating and not is strictly related to a not optimal stratification of local zeolite domains or to limited sorption performances [45]. Furthermore, it is possible also to highlight very few small heterogeneities or defects, identifiable as a darker area in the SEM image, ascribed to local superficial voids. Although, these local defects are very local shape and they were not large enough to reach the under-lying metal substrate and so did not affect the adhesion or covering action of the coating itself. Moreover, the zeolite grains are interconnected each other and well packed in the polymer matrix indicating a good interaction between the adsorbent filler and the polymer binder providing a good adhesion and cohesion between the composite constituents. From Fig. 4b, realized applying a mechanical pull-off by a cutter (Stanley Cutter SM18) of the composite coating, it is also possible to evaluate that the coating thickness is quite homogeneous and compact. The thickness, obtained by single dip-coating stage, is about 200e250 mm. 3.2. Mechanical properties In Table 1, pull-off adhesion strength and drop weight impact energy (defined according to Ref. [42]) results are summarized for the composite coatings at varying zeolite content. For comparison, results from Refs. [16,46], related to adsorbent coatings prepared by Mitsubishi Plastic Incorporation (MPI) and silane-zeolite coating, respectively, are also reported. The pull-off test results evidence that the SZ composite coatings exhibit good adhesion with the aluminum substrate. Best results were observed for composite coating with lowest zeolite content (SZ-70). However, quite good adhesion strength can be observed for other composite compositions, showing also properties compatible to literature reference coatings, [16,46]. The filler content in the coating adversely affects the adhesion strength of the coating with the substrate, favoring premature fracture of the joint. As proposed by Kahraman et al.

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[47], this behavior can be explained considering that high filler content favors high stress level at the adhesive-metal substrate interface. Analyzing fracture surface, a progressive transition from cohesive to adhesive fracture mechanism can be observed from SZ-70 to SZ-90 samples. Increasing the content of zeolite, the adhesive resistance of the composite layer decreases. For SZ-90 samples (Fig. 5) inter-laminar crack starts and progressively evolves in the complete debonding areas at the layers interface. In this case, the energy necessary to start the crack within the coating is higher than at the layers interface, consequently the sample evidenced an adhesive failure mechanism. At lower content of zeolite the failure mechanism is related with the combination of crack formation and propagation within the coating bulk or at the polymer layer interface. Locally, the failure propagation occurs within the coating indicating that the strength of adhesion to the metal substrate surface is stronger than the strength in the coating bulk. The metal surface can promote both physical and chemical bond and consequently the necessary energy to induce a crack at the coating/ adherent substrate is higher that the adhesive bulk; then the failure could occur by a cohesive mode. Further information can be argued analyzing drop-weight impact damage energy results. Concerning reference literature results [16], good impact performances were observed for ZS2S3 samples. With regard to SZ coatings, further discussion is required in order to better interpret the results. Such coatings evidenced a significant impact performance improvement. The damaged area is always smaller than that of other coating formulations. This behavior can be ascribed to the intrinsically elasto-plastic mechanical behavior of the polymer used as matrix. This implies that at low energy levels all impact energy is absorbed by surface deformation without giving fracture or indentation phenomena. With the purpose to better clarify the impact behavior of the composite coatings, the damaged area at increasing impact energy for SZ-80 coating compared to literature reference was plotted in Fig. 6. The damage diameter increased by increasing impact energy. For reference samples exhibiting an impact energy under 40 mJ no damage effects on the coated surfaces were observed. For SZ-80 sample, a threshold impact energy at about 150 mJ can be identified. However, only above 200 mJ impact energy there is a clear local surface damage due to the drop weight impact (damaged area about 0.85 mm2). As the impact energy increases, however, there is a progressive increase in the damaged area dimension, which anyway remains relatively small. This behavior is related to the improved mechanical properties under impulsive stress of the SZ composite coatings formulation, which involves a localized wrinkling phenomenon in the contact area between the coating and the

Fig. 4. SEM images of a) top view and b) cross section of SZ-90 coating.

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Table 1 Pull-off strength and drop weight damage energy for SZ composite coatings at varying zeolite content. Comparison with literature composite coatings ([16,46]). SZ Coating

Pull-off strength [MPa] Damage energy [mJ]

Reference Composite coatings

SZ-70

SZ-80

SZ-90

ZS3-80 [42]

ZS2S3-80 [42]

ZS8-80 [42]

MPI [12]

1.30 ± 0.10 335 ± 16.7

1.09 ± 0.09 318 ± 15.9

0.89 ± 0.06 297 ± 14.8

1.12 ± 0.06 225 ± 11.1

1.35 ± 0.09 270 ± 13.8

1.41 ± 0.09 222 ± 11.3

0.85 220

drop-weight tool. This deformation state localizes the damage only in the impact region without causing the formation of delamination cracks, which are usually found in the thick coatings with brittle behavior.

3.3. Adsorption properties

Fig. 5. Pull-off failure surface for SZ-90 coating.

Fig. 6. Damage diameter at increasing impact energy for the composite coatings.

Fig. 7 shows the adsorption isobar at pH2O ¼ 11 mbar in the temperature range 23e150  C for all coating formulations. For comparison purpose, the isobar measured for the pure SAPO-34 powder at the same water vapor pressure level was presented. The water pressure of 11 mbar was specifically selected as it corresponds to evaporation temperature Tev of 7  C, which represents the typical temperature level for adsorption chiller application, to provide cooling effect. Water uptake curves are characterized by the typical S-shape adsorption trend. An abrupt increase of water uptake can be observed, at about 45  C, according with the typical SAPO-34 silico-alumino-phosphates behavior. Equilibrium isobars have been measured also in desorption mode (not reported here), increasing the temperature from room temperature up to 150  C, without evidence of significant hysteresis phenomenon. This aspect is relevant for adsorption cycles energy performances. As expected, adsorption capacity of the zeolite coating is lower than pure SAPO34 powder. The adsorption behavior is consistent with the consideration that the filled zeolite coatings are characterized by specific amount of adsorbent material (different quantities of SAPO-34 filler into the composite coating) and complementary content of inert polymer matrix, which acts as binder between zeolite grains. However, the maximum adsorption value for the composite coating was observed for SZ-90 batch, where a water uptake above 27.4 wt% is reached. Considering that the SAPO-34

Fig. 7. Water adsorption isobars at 11 mbar for zeolite composite coatings and pure zeolite powder (SAPO-34).

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Fig. 8. Normalized Water adsorption isobars at 11 mbar for zeolite coatings and pure SAPO-34 powder a) in the entire range 30e150  C b) detail in the temperature range 30e45  C.

evidenced a maximum water adsorption of 31.7 wt%, this confirms that almost all the zeolite filler loaded inside the composite coating has an active action on the adsorption performances of the coating. With the purpose to better highlight the adsorption efficiency of adsorbent filler used in the composite coating the adsorption curves were normalized respect to the effective zeolite content. In Fig. 8, the obtained adsorption curves of the zeolite foams are reported. The results are representative of a reference sample for each composite coating batch. Three replicas for each batch was performed evidencing all of them very similar results (discrepancy lower than 3%). Normalized water adsorption isobars at 11 mbar for zeolite coatings were obtained dividing the water uptake obtained for the composite coatings (reported in Fig. 7) for the zeolite content in the coating formation (e.g. 0.8 for SZ-80 batch). All composites evidenced an adsorption curve almost similar to the pure SAPO-34 one. This clarifies that the polymer binder is not hindering the adsorbent filler, and does not have a detrimental effect on the adsorption capacity of the SAPO-34 grains. As identifiable in the detail at low temperature in Fig. 8b, best results were observed for SZ-90 composite coating with adsorption performances almost similar to the pure zeolite observing a maximum zeolite water uptake of about 30.0%. Consequently, considering that the maximum water adsorption of zeolite SAPO-34 was about 31.7% it can be concluded that about 96% of the zeolite filled in the polymer matrix is able to allow sorption or desorption phenomenon. The polymer binder does not influence the adsorption efficiency of zeolite filler added in the composite coating as confirmed by good normalized adsorption performances observed also on composite coating at lower zeolite content (SZ-70 showed a zeolite adsorption efficiency of 94.3%). Hydrothermal stability and adsorption dynamics are important features of coatings for AHP. Therefore, an important aspect that will be investigated in future activities is to evaluate the performance stability in typical operating conditions of adsorption heat pumps. Durability of the composite coatings is an important issue in order to evaluate its potential applicability in AHP applications. Although, these preliminary results indicate that these composite adsorbent coatings could have good prospects in the optimization process of adsorbent beds for heat pump system. 4. Conclusions Based on a brief overview on SAPO-34 based zeolite coated

adsorber for adsorption heat pumps a new composite material was investigated, in order to well tailor the advantages related to the use of coating technologies: - possibility of easily coat complex heat exchanger geometries with an adsorbent layer, maintaining a uniform thickness, - tunable coating thickness changing the formulation parameters, typically between 0.1 and 0.5 mm, The new composite coating characteristic by hybrid polymer binder evidenced a good flexibility and temperature stability. Promising results were obtained on mechanical performances especially when subjected to an impulsive stress evidencing a very performing damage energy resistance compared to literature coatings. In addition, compatible adhesion strength was observed. Finally, adsorption characteristics of the composite material are comparable to the pure adsorbent, revealing the binder embed the adsorbent filler without affecting the maximum sorption capacity. Therefore, adsorption capabilities are in general preserved, although slight differences (about 4%) in the isobar at low temperature were observed. These preliminary promising results indicate these composite adsorbent coatings as potential alternative to conventional adsorbent materials and coatings in the adsorber optimization process for the heat pump system. References [1] Henninger SK, Ernst SJ, Gordeeva L, Bendix P, Frohlich D, Grekova AD, et al. New materials for adsorption heat transformation and storage. Renew Energy 2017;110:59e68. https://doi.org/10.1016/j.renene.2016.08.041. [2] Girnik IS, Aristov YI. Making adsorptive chillers more fast and efficient: the effect of bi-dispersed adsorbent bed. Appl Therm Eng 2016;106:254e6. https://doi.org/10.1016/j.applthermaleng.2016.06.016. [3] Freni A, Bonaccorsi L, Calabrese L, Caprì A, Frazzica A, Sapienza A. SAPO-34 coated adsorbent heat exchanger for adsorption chillers. Appl Therm Eng 2015;82:1e7. https://doi.org/10.1016/j.applthermaleng.2015.02.052. €llers M, Kummer H, Schnabel L, Henninger S, et al. [4] Bendix P, Füldner G, Mo Optimization of power density and metal-to-adsorbent weight ratio in coated adsorbers for adsorptive heat transformation applications. Appl Therm Eng 2017;124:83e90. https://doi.org/10.1016/j.applthermaleng.2017.05.165. [5] Santamaria S, Sapienza A, Frazzica A, Freni A, Girnik IS, Aristov YI. Water adsorption dynamics on representative pieces of real adsorbers for adsorptive chillers. Appl Energy 2014;134:11e9. https://doi.org/10.1016/ j.apenergy.2014.07.053. [6] Mitra S, Muttakin M, Thu K, Saha BB. Study on the influence of adsorbent particle size and heat exchanger aspect ratio on dynamic adsorption characteristics. Appl Therm Eng 2018;133:764e73. https://doi.org/10.1016/ J.APPLTHERMALENG.2018.01.015. [7] Aristov YI. Challenging offers of material science for adsorption heat

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