Optimum loading of aluminum additive particles in unconsolidated beds of finned flat-tube heat exchangers in an adsorption cooling system

https://doi.org/10.1016/j.applthermaleng.2021.117267Get rights and content

Highlights

  • Adsorption cooling system integrated with aluminum additive is proposed.

  • Simultaneous effects of fin height, particle size, and additives amount are studied.

  • Applying more additives leads to a continuous rise of specific cooling power.

  • Using additive particles may drop volumetric cooling power due to bed conditions.

  • Optimum particle size and loading quantity of additive particles are determined.

Abstract

In this study, the effects of employing aluminum additive particles in an unconsolidated adsorbent bed on the performance of an adsorption cooling system (ACS) are studied. A three-dimensional distributed-parameter model is developed to investigate whether there is an optimum additive volume fraction for different bed conditions of a finned flat-tube heat exchanger filled with silica gel SWS-1L. Moreover, a qualitative comparison between temporal water uptake patterns in the presence and absence of aluminum additive is made to study the adsorption dynamics. Considering identical diameters for adsorbent and additive particles, results show that 0.3 mm is the optimal particles diameter for the fixed fin depth of 20 mm, yet, different fin heights (3–20 mm) and additive volume fractions (0–25%). Furthermore, while increasing the additive volume fraction leads to a continuous increase and decrease in specific cooling power and coefficient of performance, respectively, it is revealed that the variations of average and volumetric cooling power (VCP) are non-monotonic depending on particle size and fin height. Consequently, for the beds with fin heights of 8 and 20 mm filled with optimal particle diameter, 15% and 20% are found as the optimum additive volume fractions in which the VCP based on a given adsorber heat exchanger total volume is maximized to 370.8 and 399.7 kW/m3, respectively.

Introduction

Attentions to the mitigation of global warming by finding sustainable solutions are increasing since its effects are more tangible these days. A large share of worldwide produced energy is consumed by refrigeration and cooling systems both in industry and domestic sectors. This huge amount of energy in the form of electricity or thermal energy is mostly supplied by burning fossil fuels, leading to the emission of greenhouse gases and increasing the average global temperature. Also, automotive air-conditioning systems in internal combustion engine vehicles are mostly of the type of vapor compression refrigeration systems in which compressors impose extra loads on engines by approximately 10% resulting in further fuel consumption and consequently increase in greenhouse gas emissions [1]. Therefore, one of the sustainable alternatives is adsorption cooling systems (ACS), offering numerous advantages. ACS are thermally-driven systems with the ability to utilize solar energy [2], waste heat [3], and even low-grade heat sources [4] because adsorbent materials employed in these systems have low regeneration temperatures. Adsorption cooling and air-conditioning systems offer several other considerable advantages such as enjoying environmentally friendly refrigerants and adsorbents, requiring little electricity, operating quietly without vibration, and lower maintenance cost [5].

Despite several benefits, some challenges hinder the widespread commercial use of ACS. These systems typically suffer from having low performance parameters, particularly specific cooling power (SCP), which is mainly due to relatively weak transport characteristics. In an ACS, porous adsorber beds constituted of adsorbent particles act as a thermal compressor that adsorbs refrigerant vapor by being cooled down and is regenerated by heating up. Therefore, enhancing the heat and mass transport properties of adsorber beds has been the most important researches on ACS, falling into two major categories: i) improving adsorber bed design, ii) developing desirable adsorbent material. In the first category, researchers have focused on the adsorber heat exchanger (AdHEx) design and optimizing the bed geometrical specifications. In this regard, several notable numerical studies have been carried out on investigating the performance of finless cylindrical adsorber beds [6] and AdHExs with different configurations of extended surfaces, namely rectangular plate fins [7], longitudinal fins [8], annular fins [9], trapezoidal fins [10], rectangular fins [11], star-shaped fins [12], and wire fins [13]. In addition, the performance of different types of finned-tube AdHExs in comparable configurations has been investigated in Refs. [14], [15]. On the other hand, as one of the main reasons for the relatively low SCP of ACS is originated from the poor thermal conductivity of adsorbent particles, in the second category of researches, it has been attempted to rise the total thermal conductivity of beds by different methods. These approaches are including: i) coating AdHEx surfaces with one thin layer of composite adsorbents by direct synthesis, spray coating, or dip coating methods, and ii) adding highly thermal conductive materials in the forms of particles, strips, fiber, and flakes in consolidated or unconsolidated beds.

The method of coating a thin layer of consolidated adsorbents, e.g., 0.1–1 mm, on the heat transfer surfaces of an AdHEx is an effective way to reduce the thermal resistance between adsorbent grains and AdHEx surfaces, yet imposing an extra inter-particle mass transfer resistance. For coating processes, appropriate binders, which typically have relatively high thermal conductivity, are required to establish continuous contact between adsorbent materials and AdHEx metallic surfaces [16]. In such cases, the uptake rate is higher than non-coated beds, leading to shorter cycle times and, therefore, larger SCP values. However, when it comes to design a commercial ACS with desirable capacity, the weight and volume of the AdHEx would be excessively large as a certain value of cooling capacity requires an adequate amount of adsorbent mass [17]. Moreover, it is evident that using a bulky AdHEx with large metal surface area results in more sensible heat loss and, therefore, a lower coefficient of performance (COP) [12]. To support this argument, Freni et al. [18] experimentally showed that the SCP of a full-scale finned flat-tube AdHEx with 0.1 mm uniform zeolite coating is 1.35 times (SCP = 675 W/kg) higher than that of the same AdHEx filled with loose grain adsorbents (SCP = 498 W/kg). However, the latter respectively produced 2.28 and 1.67 times higher volumetric cooling power (VCP) and COP (equal to 212 W/dm3 and 0.4) than those of the coated one. On the other hand, higher thicknesses may be considered for consolidated beds as another technique rather than coating a thin layer. However, in such cases, although the heat transfer characteristics are improved, consolidating results in a significant increase in the inter-particle mass transfer resistance within adsorbent beds because binder materials considerably occupy the voids between the adsorbent grains [16]. Bendix et al. [19] defined a fill factor for coated AdHExs and experimentally indicated that increasing adsorbent coating thickness up to a certain fill factor could lead to a higher COP and keeping VCP almost constant. However, a sharp drop was observed for the VCP by a further increase in the coating thickness. To address this drawback, Rezk et al. [7] suggested a hybrid adsorber bed in which the thermal contact resistance between AdHEx surfaces and adsorbent particles was eliminated by coating the first adsorbent layer to the AdHEx body, while the rest of the bed was unconsolidated, filling with loose grains. This technique resulted in an improvement in the chiller’s total cooling power and COP by 8.9% and 4.6%, respectively. Sapienza et al. [20] experimentally evaluated the VCP of a novel ACS using an aluminum finned flat-tube AdHEx with triangular louvered fins in three various scenarios, including the AdHEx only coated with AQSOA FAM Z02, filled with loose grains of AQSOA FAM Z02 without coating, and a hybrid configuration integrating of both methods. Their results on small-scale adsorbers showed that the first and second methods achieve the VCP of 178 and 300 kW/m3, respectively, while the VCP of the combination of these two methods with the similar adsorbent material reached 353 kW/m3.

Another method for improving the effective thermal conductivity of an adsorber bed is to employ metallic particles or any highly thermal conductive materials either in consolidated or unconsolidated adsorber beds. Chan et al. [21] synthesized the composite adsorbent material of zeolite 13X/CaCl2 consolidated with multi-walled carbon nanotube (MWCNT), which has a thermal conductivity of roughly 3000 W/m K. It was experimentally shown that the effective thermal conductivity of the composite adsorbent in the temperature range of 30–70 °C was increased from 0.08 to 0.13 W/m K for zeolite 13X/CaCl2 up to 0.27–0.8 W/m K for the beds consolidated with MWCNT. They evaluated the performance of different adsorbents with a finned flat-tube AdHEx using a distributed-parameter model. Applying the equal ad/desorption duration times of 300 s, they showed that the SCP of the adsorber containing MWCNT additives reached 1113.4 W/kg, which was a 6.5 and 1.22 times improvement in comparison with zeolite 13X and zeolite 13X/CaCl2, respectively. However, the COP decreased from 0.37 for zeolite 13X to 0.26 for the composite case with MWCNT additives. Fayazmanesh et al. [22] experimentally studied the use of 0–20 wt% graphite flakes (GF) as additives for the composite adsorbent of silica gel/CaCl2 and 12–15 wt% 40000 MW polyvinylpyrrolidone (PVP) as a binder. They found that adding 20 wt% of GF improved the effective thermal conductivity of the adsorbent bed from nearly 0.15 to 0.48 W/m K, though as expected, the new synthesized consolidated adsorbent had lower water uptake capacity than that of the silica gel/CaCl2. Bahrehmand et al. [23] experimentally investigated the effects of consolidating GF in silica gel/CaCl2 composite by coating the composite adsorbent on 1.8 mm graphite sheets and bolted to a lab-scale copper AdHEx. In a case with the cycle time of 5 min, they observed that using 0–20 wt% GF additives resulted in an increase in the adsorbent effective thermal diffusivity (from 0.23 to 1.38 mm2/s), and consequently leading to an increase in the SCP and COP by 65% (from 365 to 604 W/kg) and 17% (from 0.46 to 0.54), respectively. However, this enhancement deteriorated by increasing the cycle time and approaching the equilibrium conditions. In addition, they argued that the content of GF additives in a consolidated adsorbent bed should have an optimal value because employing more amount of additives means reducing the active amount of adsorbent material and increasing inter-particle mass transfer resistance. Bahrehmand and Bahrami [24], in another study, presented a transient 2D analytical model to predict the SCP and COP of a finned-tube sorption cooling system. They showed that the optimum amount of GF additives in the studied consolidated adsorbent depended on the adsorber bed geometrical and thermophysical characteristics as well as cycle time. It should be mentioned that the GF content in terms of weight percentage in the coated adsorbent material was not directly considered in their 2D model, and the inter-particle mass transfer resistance was neglected.

As previously mentioned, this is also feasible to use highly thermal conductive additives in beds packed with loose adsorbent grains that no binder is used, and the adsorber bed is called unconsolidated. In unconsolidated beds, in contrast to the consolidated ones, which have a stronger heat transfer through their beds, there is no such limitation of vapor transfer due to the reduction in free paths inside the compact layers, resulting in reducing the mass transfer resistance through the bed. Therefore, by adding highly thermal conductive additives to the unconsolidated beds, the heat transfer characteristics of adsorber beds are improved without imposing additional intra and inter-particle mass transfer resistances. It should be mentioned that based on the application and place where the ACS is supposed to be designed for, one of the consolidated or unconsolidated beds could be used. For example, for a large building, where high cooling capacity is needed and the bulkiness of the system does not matter, employing consolidated adsorbent beds, especially with a thin coated layer, is more justified. However, in automobile applications where the compactness and lightness of the system are crucial, adsorbent beds with greater thickness should be considered. In this case, using unconsolidated beds is more reasonable. Demir et al. [25] conducted an experimental work to examine the impacts of loading 0–15 wt% metallic additives such as copper, brass (40% zinc and 60% copper), stainless steel (AISI-304), and aluminum in the shape of spiral strips homogeneously packed within a cylindrical unconsolidated bed filled with 3–5 mm silica gel grains (manufactured by Merck Co). They observed that the addition of such additives significantly improved the bed heat transfer characteristics, and among the mentioned species, aluminum was more effective than the others. For instance, adding 15 wt% aluminum pieces with a range size of 1.0–2.8 mm improved the overall thermal conductivity from 0.106 for a pure silica gel bed to 0.363 W/m K. However, the impacts of the additive loading amount on the SCP and COP of the ACS were not studied. Rezk et al. [7] investigated employing four different additive particles, including 0–15 wt% (additive/adsorbent mass ratio) aluminum, copper, brass, and stainless steel in an unconsolidated adsorber bed of silica gel RD with a diameter of 0.32 mm and the thermal conductivity of 0.198 W/m K. Their results showed that using 15 wt% aluminum additives led to a 12.5% improvement for the chiller cooling capacity at the fin pitch ratio of 2, while at lower fin pitch ratios (less than1.2), using that amount of additives deteriorated the system performance. It should be noted that their AdHEx was copper plain tubes with aluminum rectangular plate fins, and they used an empirical lumped-body analytical model, which fails to take into account the effects of inter-particle mass transfer resistance. Askalany et al. [26] conducted an experimental work to evaluate the effects of utilizing 0–30 wt% metallic additives such as iron, copper, and aluminum (with dimensions of 5 ± 1, 2 ± 0.5, and 1 ± 0.1 mm in length, width, and thickness, respectively) on the overall thermal conductivity of a cylindrical adsorbent bed packed with 1–2 mm granular activated carbon. Using a lumped-body model, they predicted that employing 30 wt% aluminum led to a 50% decline in the cycle time and a 100% increase in the SCP (up to about 27 W/kg). Gediz Ilis et al. [12] performed a 2D numerical simulation based on a distributed-parameter model to study the effects of adsorbent grain size (0.4–4 mm) and geometrical parameters on the performance of an adsorber bed with star type fins packed with Fuji RD-type silica gel. They showed that the grain size of 0.4 mm was the best value to achieve the best SCP; however, they did not consider smaller grain sizes in their work. They also investigated the effects of adding different volume fractions (in the range of 0–60% concerning the total bed volume) of swarf type aluminum particles on the SCP. It was shown that adding 0–60% of aluminum particles led to a continuous increase in the SCP from 14.8 to nearly 50 W/kg. It should be noted that such a large improvement due to adding highly thermal conductive may be observed in AdHExs with weak heat transfer characteristics, i.e., low heat transfer surface area per unit mass of adsorbent. Prior to considering metallic additive particles (MAP) as an effective technique, it is more reasonable to employ an AdHEx with sufficient compactness and extensive heat transfer surfaces, such as finned flat-tube heat exchanger.

The literature survey shows that there have been limited studies, which investigated the effects of using MAP on the performance parameters of an ACS with unconsolidated packed adsorbent beds. Therefore, there is still room for further study of using such additive particles. Accordingly, the present study attempts to present a comprehensive picture of employing MAP in unconsolidated beds of ACS. To our best knowledge, it is for the first time that the simultaneous effects of adsorbent bed conditions in terms of fin geometry, particle diameter, and additive volume fraction are investigated to identify an optimum amount of aluminum additive particles loaded through adsorbent loose grains of composite SWS-1L (CaCl2 in mesoporous silica KSK) packed in a finned flat-tube AdHEx. In this regard, important designing parameters of the ACS, including the SCP, average cooling power (ACP), and VCP (based on a given AdHEx total volume and whole module volume) as well as the COP and energy consumption, are examined after applying additive particles with different diameters in adsorbent beds with different fin heights. To this end, a validated three-dimensional distributed-parameter model and requisite mathematical corrections for applying the metallic additive particles are developed to simulate the ACS performance. Using a comprehensive in-house code, qualitative and quantitative results are produced to represent different aspects of using metallic additive particles in ACS.

Section snippets

Mathematical modeling

Fig. 1(a-c) schematically display adsorber beds positioned within a chamber, the single unit of finned flat-tube AdHEx, and the numerical domain with 3D structured grids used in this study, respectively. According to Fig. 1(c), the AdHEx body is comprised of two major parts, namely channels and fins as the primary and secondary heat transfer surfaces, respectively, where the porous adsorbent loose grains and MAP are uniformly packed between them. The rectangular shape is considered for fins

Validation

The presented numerical scheme in the absence of MAP has already been validated in comparison with the experimental documentation. In this context, a commercial finned flat-tube heat exchanger (HEx1 in Ref. [48]) packed with composite SWS-1L loose grains was employed, and the water content in the adsorbent was experimentally measured using a Thermal Large Temperature Jump (T-LTJ) method. Utilizing the experimental conditions reported in Table 3, the numerical temporal variations of average

Results and discussions

To get a clear sense of the complex heat and mass transfers inside the porous bed, a qualitative comparison between temporal water uptake patterns in the presence and absence of MAP is made to study the adsorption dynamics. Next, the impacts of loading MAP with different volume fractions through the adsorbent loose grains on the SCP and COP are presented by considering the simultaneous effects of the fin height and particle size. Moreover, the existence of any possible optimum amount of MAP is

Conclusion

Employing metallic additive particles within unconsolidated adsorbent beds was recognized as an effective way to improve the heat transfer characteristics of adsorbent beds and the cooling power of ACS. In this regard, the loading of aluminum particles into the adsorbent beds of an ACS with finned flat-tube AdHEx and SWS-1L-water working pair were assessed to identify an optimum loading amount for the additives. Using a validated 3D distributed-parameter model, the performance of the ACS was

CRediT authorship contribution statement

Meysam Khatibi: Conceptualization, Methodology, Data curation, Investigation, Visualization, Formal analysis, Project administration, Writing - original draft, Writing - review & editing. Milad Mohammadzadeh Kowsari: Methodology, Software, Validation, Formal analysis, Writing - review & editing. Behzad Golparvar: Formal analysis, Writing - original draft, Writing - review & editing. Hamid Niazmand: Funding acquisition, Supervision, Writing - review & editing.

Declaration of Competing Interest

The author declare that there is no conflict of interest.

Acknowledgements

We hereby acknowledge that part of this computation was performed on the HPC center of the Ferdowsi University of Mashhad. The authors greatly appreciate the financial support provided by the Ferdowsi University of Mashhad under the grant number of 3/45477.

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