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The market share of adsorption refrigeration systems and heat pumps is still relatively low compared to traditional compression systems.Despite the huge advantage of being powered by cheap heat (rather than expensive electrical work), the implementation of systems based on adsorption principles is still limited to a few specific applications.The main disadvantage that needs to be addressed is the reduction in specific power due to the low thermal conductivity and low stability of the adsorbent.Current state-of-the-art commercial adsorption cooling systems rely on adsorbers based on coated fin heat exchangers to optimize cooling power.It is a well-known result that a reduction in coating thickness leads to a reduction in mass transfer impedance, and an increase in the surface area to volume ratio of the conductive structure increases power without reducing efficiency.The metal fibers used in this work can provide specific surface areas in the range of 2500–50,000 m2/m3.Three methods for producing very thin but stable hydrate coatings on metal surfaces, including metal fibers, for the production of coatings demonstrate for the first time a heat exchanger with high specific power.A surface treatment based on aluminum anodization is chosen to create a stronger bond between the coating and the substrate.The microstructure of the resulting surface was analyzed by scanning electron microscopy.To verify the presence of the desired species, attenuated total reflection-Fourier transform infrared spectroscopy and energy dispersive X-ray spectroscopy were used in the analysis.Their ability to form hydrates was verified by simultaneous thermogravimetric analysis (TGA)/differential thermogravimetric analysis (DTG).Poor quality over 0.07 g(water)/g(composite) was detected in the MgSO4 coating, showing signs of dehydration at temperatures around 60 °C and reproducible after rehydration.Positive results were also obtained with SrCl2 and ZnSO4, with a mass difference of about 0.02 g/g below 100 °C.Hydroxyethyl cellulose was chosen as an additive to increase the stability and adhesion of the coating.The adsorption properties of the products were evaluated by simultaneous TGA-DTG, while their adhesion was characterized by a procedure based on the tests described in ISO2409.The consistency and adhesion of the CaCl2 coating is greatly improved while maintaining its adsorption capacity, with a mass difference of about 0.1 g/g at temperatures below 100 °C.Furthermore, MgSO4 retains the ability to form hydrates, showing a mass difference of over 0.04 g/g below 100 °C.Finally, coated metal fibers are investigated.The results show that the effective thermal conductivity of the fiber structure coated with Al2(SO4)3 can be as high as 4.7 times compared to the pure Al2(SO4)3 bulk.The coverage of the coatings pursued was visually investigated and the internal structure was assessed by microscopic imaging of cross-sections.An Al2(SO4)3 coating of about 50 µm was produced, but in general the process needs to be optimized to achieve a more uniform distribution.
Adsorption systems have attracted a lot of attention over the past few decades as they represent an environmentally friendly alternative to traditional compression heat pumps or refrigeration systems.With rising comfort standards and global average temperatures, adsorption systems have the potential to reduce reliance on fossil fuels in the near future.Furthermore, any improvements in adsorption refrigeration or heat pumps can be transferred to thermal energy storage, which constitutes an additional increase in the capacity for efficient utilization of primary energy.The main advantage of adsorption heat pumps and refrigeration systems is that they can be handled with low mass heat.This makes them suitable for low temperature sources such as solar energy or waste heat.Regarding energy storage applications, adsorption has the advantage of higher energy density and lower energy dissipation compared to sensible or latent heat storage.
Adsorption heat pumps and refrigeration systems follow a similar thermodynamic cycle to their vapor compression counterparts.The main difference is the replacement of compressor components with adsorbers.The element is capable of adsorbing low pressure vapor refrigerant when maintained at moderate temperatures, evaporating a greater amount of refrigerant even when the liquid is cold.It must be ensured that the adsorber is continuously cooled to eliminate the adsorption enthalpy (exotherm).The adsorber is regenerated at high temperature, forcing the vapor refrigerant to desorb.Heating must be continued to provide the desorption enthalpy (endothermic).Since adsorption events are characterized by changes in temperature, high thermal conductivity is required for high specific power.However, low thermal conductivity is by far the main disadvantage in most applications.
The main challenge for conductivity is to increase its average value while ensuring that a transport path that allows the flow of adsorption/desorption vapors is maintained.Two approaches are commonly used to achieve this: composite and coated heat exchangers.The most popular and successful composite materials are those using carbon-based additives, namely expanded graphite, activated carbon or carbon fibers.Oliveira et al. 2 impregnated calcium chloride in expanded graphite powder to produce an adsorber with specific cooling power (SCP) up to 306 W/kg and coefficient of performance (COP) up to 0.46.Zajaczkowski et al. 3 proposed a combination of expanded graphite, carbon fiber, and calcium chloride with a total conductivity of 15 W/mK.Jian et al4 tested the composites with sulfuric acid-treated expanded natural graphite (ENG-TSA) as the substrate in a two-stage adsorption refrigeration cycle.The model predicted a COP between 0.215 and 0.285 and an SCP between 161.4 and 260.74 W/kg.
What constitutes by far the most viable solution is a coated heat exchanger.The mechanisms for coating these heat exchangers can be divided into two categories: direct synthesis and adhesives.The most successful method is direct synthesis, which involves the formation of adsorbent materials directly on the surface of heat exchangers from the corresponding reactants.Sotech5 has patented a method for the synthesis of a coated zeolite for use in one of the cooler series commercialized by Fahrenheit GmbH.Schnabel et al6 tested the performance of two zeolites coated on stainless steel.However, this method only works with specific adsorbents, making coating with adhesives an interesting alternative.Binders are passive species that are selected to maintain sorbent adhesion and/or mass transfer, but do not play any role in the adsorption process or conductivity increase.Freni et al. 7 coated aluminum heat exchangers with zeolite AQSOA-Z02 stabilized by a clay-based binder.Calabrese et al.8 studied the preparation of zeolite coatings with polymeric binders.Ammann et al. 9 proposed a method to fabricate porous zeolite coatings from magnetic mixtures of polyvinyl alcohol.Alumina (alumina) is also used as a binder 10 in the adsorber.To the best of our knowledge, cellulose and hydroxyethyl cellulose are only used in combination with physical adsorbents11,12.Sometimes the adhesive is not used for the paint, but is used to create the structure 13 on its own.Combining alginate-derived polymer matrices with several salt hydrates forms flexible structures of composite beads that prevent leakage during deliquescence and allow adequate mass transfer.Clays such as bentonite and attapulgite have been used as binders for the preparation of composites15,16,17.Ethyl cellulose has been used to microencapsulate calcium chloride18 or sodium sulfide19.
Composites with porous metal structures can be divided between additive and coated heat exchangers.The high specific surface area nature of these structures is an advantage.This results in a larger contact surface between the adsorbent and metal without adding inert mass, which reduces the overall efficiency of the refrigeration cycle.Lang et al. 20 improved the overall conductivity of a zeolite adsorber with an aluminum honeycomb structure.Gillerminot et al. 21 enhanced the thermal conductivity of NaX zeolite beds with copper and nickel foams.Although composites are used as phase change materials (PCMs), the conclusions of Li et al. 22 and Zhao et al. 23 are also interesting for chemisorption.They compared the performance of expanded graphite and metal foam and concluded that the latter is preferred only if corrosion is not an issue.Palomba et al. recently compared other metallic porous structures24.Metal salts embedded in metal foams were studied by van der Pal et al. 25 .All previous examples correspond to dense beds of particulate adsorbents.Metal porous structures are hardly used for coating adsorbers, which is a more optimized solution.An example of binding to zeolites can be found in the publication of Wittstadt et al. 26 , but no attempt has been found to bind hydrate salts, despite their higher energy density 27 .
Therefore, three methods for preparing adsorbent coatings will be investigated in this paper: (1) binder coating, (2) direct reaction, and (3) surface treatment.Hydroxyethylcellulose was the binder of choice in this work because of the previously reported stability and good adhesion of the coating in combination with physical adsorbents.The method was initially investigated for flat coatings and then applied to metal fiber structures.Preliminary analyses of the potential for chemical reactions to produce adsorbent coatings have been reported previously.Previous experience is now transferred to the coating of metal fiber structures.The surface treatment chosen for this work is a method based on aluminum anodization.Aluminum anodization has been successfully combined with metal salts for aesthetic purposes29.In these cases, very stable and corrosion-resistant coatings can be produced.However, they cannot perform any adsorption or desorption process.In this work, a variant of this approach is presented that allows the transfer of mass while benefiting from the adhesive properties of the original process.To our knowledge, none of the methods described here have been studied before.They constitute a very interesting new technology, as they allow the formation of hydrated adsorbent coatings, which have several advantages over the often studied physical adsorbents.
Stamped aluminum plates used as substrates for these experiments were provided by ALINVEST Břidličná, Czech Republic.They contain 98.11% aluminum, 1.3622% iron, 0.3618% manganese and traces of copper, magnesium, silicon, titanium, zinc, chromium and nickel.
The materials chosen for the preparation of composites are chosen based on their thermodynamic properties, more specifically, they are chosen based on the amount of water they can adsorb/desorb at temperatures below 120°C.
Magnesium sulfate (MgSO4) is one of the most interesting and studied salt hydrates30,31,32,33,34,35,36,37,38,39,40,41.The thermodynamic properties were systematically measured and proved to be suitable for applications in the fields of adsorption refrigeration, heat pump and energy storage.Dry magnesium sulfate CAS-Nr. 7487-88-9 99% was used (Grüssing GmbH, Filsum, Niedersachsen, Germany).
Calcium chloride (CaCl2) (H319) is another well-studied salt because its hydrate has interesting thermodynamic properties41,42,43,44.Calcium chloride hexahydrate CAS-Nr.7774-34-7 97% employed (Grüssing, GmbH, Filsum, Niedersachsen, Germany).
Zinc sulfate (ZnSO4) (H302, H318, H410) and its hydrates have thermodynamic properties suitable for low-temperature adsorption processes45,46.Zinc sulfate heptahydrate CAS-Nr.7733-02-0 99.5% was used (Grüssing GmbH, Filsum, Niedersachsen, Germany).
Strontium chloride (SrCl2) (H318) also has interesting thermodynamic properties4,45,47, although it is often combined with ammonia for adsorption heat pump or energy storage studies.Strontium chloride hexahydrate CAS-Nr. 10, 476-85-4 99.0–102.0% was used in the synthesis (Sigma Aldrich, Saint Louis, MO, USA).
Copper sulfate (CuSO4) (H302, H315, H319, H410) is not one of the hydrates frequently found in the professional literature, although its thermodynamic properties are interesting for applications at low temperatures48,49.Copper sulfate CAS-Nr. 7758-99-8 99% was used in the synthesis (Sigma Aldrich, Saint Louis, MO, USA).
Magnesium chloride (MgCl2) is one of the salt hydrates that has recently received more attention in the field of thermal energy storage50,51.Magnesium chloride hexahydrate CAS-Nr.7791-18-6 pure pharmaceutical grade was used for experiments (Applichem GmbH., Darmstadt, Germany).
As mentioned above, hydroxyethyl cellulose was chosen because of positive results in similar applications.The material used in our synthesis is hydroxyethylcellulose CAS-Nr 9004-62-0 (Sigma Aldrich, Saint Louis, MO, USA).
Metal fibers are made from short wires joined together by compression and sintering, a process known as crucible melt extraction (CME)52.This means that their thermal conductivity depends not only on the bulk conductivity of the metals used in fabrication and the porosity of the final structure, but also on the quality of the bonds between the filaments.Fibers are not isotropic and tend to distribute in a certain direction during production, which makes the thermal conductivity in the transverse direction much lower.
Water absorption properties were investigated using a simultaneous thermogravimetric analysis (TGA)/differential thermogravimetric analysis (DTG) vacuum-sealed apparatus (Netzsch TG 209 F1 Libra).Measurements were carried out in a flowing nitrogen atmosphere with a flow rate of 10 ml/min and a temperature range of 25 to 150 °C in alumina crucibles.The heating rate was 1 °C/min, the sample mass was varied from 10 to 20 mg, and the resolution was 0.1 µg.It must be noted in this work that the mass difference per unit surface has a large uncertainty.The samples used in TGA-DTG are very small and unevenly cut, which makes their area determination imprecise.These values ​​can only be extrapolated to a larger area if large deviations are taken into account.
Attenuated total reflection-Fourier transform infrared (ATR-FTIR) spectra were acquired in a Bruker Vertex 80 v FTIR spectrometer (Bruker Optik GmbH, Leipzig, Germany) using a platinum ATR accessory (Bruker Optik GmbH, Germany).The spectra of clean, dry diamond crystals were measured directly in vacuum before using the samples as background for experimental measurements.The samples were measured in vacuum using a spectral resolution of 2 cm-1 and an average number of scans of 32.Wavenumbers range from 8000 to 500 cm-1.Spectral analysis was done using OPUS software.
SEM analysis was performed with a DSM 982 Gemini from Zeiss at accelerating voltages of 2 and 5 kV.Energy dispersive X-ray spectroscopy (EDX) was provided with the help of a Thermo Fischer System 7 with a Peltier cooled silicon drift detector (SSD).
The preparation of metal plates was carried out according to a procedure similar to that described in 53.First immerse the plate in 50% vol sulfuric acid.15 minutes.Then, they were introduced into 1 M sodium hydroxide for about 10 seconds.The samples were then washed with copious amounts of distilled water and then soaked in distilled water for 30 minutes.After surface pretreatment, the samples were immersed in a 3% wt saturated solution.HEC and target salt.Finally, take them out and dry at 60°C.
The method of anodizing strengthens and strengthens the naturally occurring oxide layer on the passive metal.The aluminum panels were anodized with sulfuric acid in a fortified condition and subsequently sealed in hot water.Anodization followed an initial etching treatment with 1 mol/l NaOH (600 s) followed by neutralization in 1 mol/l HNO3 (60 s).The electrolyte solution is a mixture of 2.3 M H2SO4, 0.01 M Al2(SO4)3 and 1 M MgSO4 + 7H2O.Anodization was carried out at (40 ± 1) °C, 30 mA/cm2 for 1200 seconds.The sealing process was carried out in different saturated salt solutions as described in Materials (MgSO4, CaCl2, ZnSO4, SrCl2, CuSO4, MgCl2).The sample is boiled in it for 1800 seconds.
Three different composite production methods were investigated: adhesive coating, direct reaction and surface treatment.The advantages and disadvantages of each preparation method are systematically analyzed and addressed.Direct observation, nanoimaging, and chemical/elemental analysis were used to evaluate the results.
Anodizing was chosen as a method of surface conversion treatment to increase the adhesion of salt hydrates.This surface treatment creates a porous structure of alumina (alumina) directly on the aluminum surface.Traditionally, this method consists of two steps: a first step produces a porous alumina structure, while a second step produces an aluminum hydroxide coating that seals the pores.In the following, two methods are proposed to lock the salt without blocking access to the gas phase.The first consists of a honeycomb system using small alumina tubes (Al2O3) produced in the first step to retain the crystals of the adsorbent and increase its adhesion to metal surfaces.The resulting honeycombs are approximately 50 nm in diameter and 200 nm in length (Fig. 1a).As mentioned before, these cavities are usually closed in the second step by a thin layer of boehmite, Al2O(OH)2, maintained by the boiling process of the alumina tube.In the second method, this sealing process is modified so that the salt crystals are trapped in a uniformly covering layer of boehmite (Al2O(OH)), which is not used for sealing in this case.The second step is carried out in a saturated solution of the corresponding salt.The described patterns show sizes in the 50–100 nm range and appear as splattered droplets (Fig. 1b).The surface produced as a result of the sealing process exhibits an explicit spatial structure with enhanced contact area.This surface pattern, along with their numerous bonding configurations, is ideal for carrying and retaining salt crystals.Both described structures appear to be indeed porous and exhibit small cavities, which appear to be well suited to allow retention of salt hydrates and adsorption of vapour by salt during operation of the adsorber.However, EDX elemental analysis of these surfaces can detect trace amounts of magnesium and sulfur on the boehmite surface, which are not detectable in the case of the alumina surface.
ATR-FTIR of the sample confirmed that the element was magnesium sulfate (see Figure 2b).In the spectrum, characteristic peaks of sulfate ion at 610-680 and 1080-1130 cm-1 and characteristic peaks of lattice water at 1600-1700 cm-1 and 3200-3800 cm-1 can be seen (see Fig. 2a, C).The presence of magnesium ions hardly changes the spectrum54.
(a) EDX of boehmite-coated aluminum plate coated with MgSO4, (b) ATR-FTIR spectra of boehmite and MgSO4 coatings, (c) ATR-FTIR spectra of pure MgSO4.
The maintenance of adsorption performance was verified by TGA.Figure 3b shows the desorption peak at ca.60°C.This peak does not correspond to the temperature of the two peaks observed in the TGA of pure salt (Fig. 3a).The repeatability of the adsorption-desorption cycle was evaluated and the same curve was observed after placing the samples in a humid atmosphere (Fig. 3c).The differences observed in the second desorption step may be the result of dehydration under flowing atmosphere, as this often results in incomplete dehydration.These values ​​correspond to approximately 17.9 g/m2 during the first dehydration and 10.3 g/m2 during the second dehydration.
Comparison of TGA analysis of boehmite and MgSO4: TGA analysis of pure MgSO4 (a), mixture (b) and after rehydration (c).
The same method was carried out with calcium chloride as adsorbent.The results are summarized in Figure 4.Visual inspection of the surface revealed slight changes in metallic glow.The coat is barely visible.SEM confirmed the presence of small crystals uniformly distributed on the surface.However, TGA did not show any dehydration below 150 °C.This may be due to the fact that the proportion of salt is too small compared to the total mass of the substrate to be detected by TGA.
The results of the copper sulfate coating surface treated by anodization are shown in Figure 5.In this case, the expected insertion of CuSO4 into the Al oxide structure did not occur.Instead, loose needles are observed as they are typically used for copper hydroxide Cu(OH)2, used with typical turquoise dyes.
Anodized surface treatments were also tested in combination with strontium chloride.The results showed an uneven coating (see Figure 6a).To determine if the salt covered the entire surface, EDX analysis was performed.The curve for the point in the grey area (point 1 in Fig. 6b) shows few strontium counts and many aluminium counts.This is evidence of low strontium content in the measured area, which in turn is evidence of low strontium chloride coverage.Conversely, the white areas have high strontium counts and low aluminum counts (points 2-6 in Figure 6b).EDX analysis of the white area shows darker points (points 2 and 4 in Fig. 6b), low in chlorine and high in sulfur.This could mean the formation of strontium sulfate.Brighter dots return high counts of chlorine and low counts of sulfur (dots 3, 5, and 6 in Figure 6b).It can be explained that the main part of the white coating is composed of the expected strontium chloride.The TGA of the sample confirmed the interpretation of the analysis, with a peak at the characteristic temperature of pure strontium chloride (Fig. 6c).Their small magnitude can be justified by a small proportion of salt compared to the mass of the metal support.The desorption mass determined in the experiment corresponds to an amount of 7.3 g/m2 given per unit of adsorber area at a temperature of 150 °C.
Zinc sulfate coatings treated with eloxal were also tested.The macroscopic appearance of the coating is a very thin and uniform layer (Fig. 7a).However, SEM revealed a surface area covered by small crystals separated by blank areas (Fig. 7b).The TGA of the coating and substrate was compared with that of the pure salt (Fig. 7c).The pure salt exhibits a single asymmetric peak at 59.1 °C.The coated aluminum showed two small peaks at 55.5 °C and 61.3 °C, indicating the presence of zinc sulfate hydrate.The mass difference detected in the experiment corresponds to 10.9 g/m2 with a dehydration temperature of 150 °C.
As in the previous application53, hydroxyethyl cellulose was used as a binder to increase the adhesion and stability of the sorbent coating.The compatibility of materials and the effect on adsorption performance were evaluated by TGA.The analysis is performed with reference to the total mass, ie the sample includes the metal plate used as the coating substrate.Adhesion is tested by a test based on the cross-cut test defined in the ISO2409 specification (cannot meet specification separation between cuts as a function of thickness and specification width).
Coating the panels with calcium chloride (CaCl2) (see Figure 8a) resulted in a non-uniform distribution, which was not observed in the pure aluminium coating used for the cross-cut test.Compared with the results for pure CaCl2, TGA (Fig. 8b) shows two characteristic peaks shifted towards lower temperatures of 40 and 20 °C, respectively.The cross-cut test does not allow a fair comparison because the sample of pure CaCl2 (the sample on the right in Fig. 8c) is a powdery deposit, resulting in the removal of the uppermost particles.The results from HEC presented a very thin and uniform coating with satisfactory adhesion.The mass difference represented in Fig. 8b corresponds to 51.3 g/m2 per unit area of ​​the adsorber at a temperature of 150 °C.
Positive results regarding adhesion and uniformity were also obtained with magnesium sulfate (MgSO4) (see Figure 9).Analysis of the coating desorption process showed a single peak at ca.60°C.This temperature corresponds to the main desorption step observed during the dehydration of pure salts, presenting another step at 44 °C.It corresponds to the transition from hexahydrate to pentahydrate and is not observed in the case of coatings with binders.Cross-section tests show improved distribution and adhesion compared to coatings obtained with pure salt.The mass difference observed in TGA-DTC corresponds to 18.4 g/m2 given per unit adsorber area at a temperature of 150 °C.
Due to its surface irregularities, strontium chloride (SrCl2) exhibits an uneven coating on the fins (Fig. 10a).However, the results during the cross-cut test resulted in a uniform distribution with highly improved adhesion (Fig. 10c).The TGA analysis showed very little mass difference, which must be due to the lower proportion of salt compared to the metal support.However, the steps in the curve demonstrate the existence of a dehydration process, although the peak is related to the temperature obtained in the pure salt characterization.The peaks at 110°C and 70.2°C observed in Figure 10b were also detected in the analysis of pure salt.However, the major dehydration step observed in pure salt at 50 °C was not reflected in the curves using the binder.In contrast, the mixture with the binder showed two peaks at 20.2 °C and 94.1 °C, which were not measured with pure salt (Fig. 10b).At a temperature of 150 °C, the observed mass difference corresponds to 7.2 g/m2 per unit adsorber area.
The combination of HEC and zinc sulfate (ZnSO4) did not provide acceptable results (Figure 11).TGA analysis of the coated metal did not reveal any dehydration process.Although the distribution and adhesion of the coating have improved, its properties are still far from optimal.
The simplest method to coat metal fibers with a thin and uniform layer is wet impregnation (Fig. 12a), which involves preparing the target salt and impregnating the metal fibers in an aqueous solution.
Two main challenges are faced during wet impregnation preparation.On the one hand, the surface tension of the salt solution hinders the proper incorporation of the liquid into the porous structure.Crystallization on the outer surface (Fig. 12d) and air bubbles trapped within the structure (Fig. 12c) can only be reduced by lowering the surface tension, pre-wetting the sample with distilled water.Forced dissolution into the sample by evacuating the air inside or by creating a flow of solution in the structure are other effective ways to ensure complete filling of the structure.
The second challenge faced during preparation was the “skinning” of some of the salt (see Figure 12b).This phenomenon is characterized by the formation of a dry coating on the dissolving surface, which stops the convective-driven drying and begins the diffusion-driven process.The second mechanism is much slower than the former.The result is a high temperature required for a reasonable drying duration, which increases the risk of bubble formation inside the sample.This problem is solved by implementing an alternative crystallization method that is not based on concentration changes (evaporation) but on temperature changes (as in the example of MgSO4 in Figure 13).
Schematic illustration of the crystallization process by cooling and separation of the solid and liquid phases with MgSO4.
Using this method, saturated solutions of salts can be prepared at room temperature or higher (HT).In the first case, crystallization was forced by lowering the temperature below room temperature.In the second case, crystallization occurred by allowing the sample to cool at room temperature (LT).The result is a mixture of crystals (B) and dissolved (A), the liquid portion of which is removed with compressed air.This approach not only avoids the skinning of these hydrates, but also reduces the time required to prepare other composites.However, removing the liquid with compressed air causes additional salt to crystallize, resulting in a thicker coating.
Another method that can be used to coat metal surfaces involves directly generating target salts through chemical reactions.Coated heat exchangers produced by the reaction of acids on the metal surfaces of the fins and tubes have several advantages, as reported in our previous research.Transferring this method to fibers resulted in very poor results due to the gases produced in the reaction.The pressure of the hydrogen gas bubbles builds up inside the probe and displaces as the product is expelled (Figure 14a).
The coating has been altered through a chemical reaction to better control the thickness and distribution of the coating.The method involves forcing an acid mist current through the sample (Figure 14b).This is expected to produce a uniform coating by reacting with the metal of the substrate.The results were satisfactory, but the process was too slow to constitute an efficient method (Fig. 14c).Shorter reaction times can be achieved by localized heating.
In order to overcome the disadvantages of the above-mentioned methods, a coating method based on the use of adhesives has been studied.HEC was selected based on the results presented in the previous section.All samples were prepared at 3% wt.The binder is mixed with salt.The fibers were pretreated following the same procedure as for the fins, i.e. soaking in 50% vol for 15 minutes.Sulfuric acid, then soak in sodium hydroxide for 20 seconds, wash in distilled water, and finally soak in distilled water for 30 minutes.In this case, an extra step was added before impregnation.Immerse the sample briefly in the diluted solution of the target salt and dry at approximately 60 °C.The process is designed to modify the metal surface, creating seed sites that improve the distribution of the coating in the final step.The fiber structure exhibits one side where the filaments are thinner and tightly packed and the opposite side where the filaments are thicker and more poorly distributed.This is the result 52 of the manufacturing process.
The results for calcium chloride (CaCl2) are summarized and illustrated in the images in Table 1.Good coverage after the sowing process.Even those filaments that had no visible crystals on their surfaces had reduced metallic reflections, suggesting a change in the finish.However, after impregnating the samples with an aqueous mixture of CaCl2 and HEC and drying them at around 60 °C, the coatings were concentrated at the intersections of the structures.This is an effect caused by the surface tension of the solution.After soaking, the liquid remains within the sample due to its surface tension.This mainly occurs at the intersection of structures.The best side of the sample has some holes blocked by salt.The mass increased by 0.06 g/cm3 after coating.
Coating with magnesium sulfate (MgSO4) yielded higher amounts of salt per unit volume (Table 2).In this case, the measured increment is 0.09 g/cm3.The seeding process has resulted in extensive coverage of the samples.After the coating process, the salt blocks large areas of the thin side of the sample.In addition, some areas of the matte are blocked, but some porosity is preserved.In this case, salt formation is readily observed at the intersection of the structures, confirming that the coating process is primarily driven by the surface tension of the liquid rather than by the interaction between the salt and the metal substrate.
The results for the combination of strontium chloride (SrCl2) and HEC exhibited similar properties to the previous examples (Table 3).In this case, the thinner side of the sample is almost completely blocked.Only some pores can be seen, which were created during drying as steam escaped from the sample.The pattern observed on the matte side is very similar to the previous case, the area is blocked by the salt and the fibers are completely uncoated.
To evaluate the positive effect of the fiber structure on the thermal performance of the heat exchanger, the effective thermal conductivity of the coated fiber structure was determined and compared with the pure coating material.Thermal conductivity was measured according to ASTM D 5470-2017 using the flat panel device shown in Figure 15a, applying a reference material with known thermal conductivity.Compared to other transient measurement methods, this principle is beneficial for the porous materials applied in the current study, as the measurements are performed at steady state and with sufficient sample size (30 × 30 mm2 footprint, approximately 15 mm height).Samples have been prepared for the pure coating material (reference) and the coated fiber structure for measurements in the fiber direction and perpendicular to the fiber direction to evaluate the effect of anisotropic thermal conductivity.The samples were ground on the surface (grain P320) to minimize the effect of rough surfaces due to sample preparation, which are not representative of the structure inside the sample.