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energies Article

The Influence of Operating Parameters on Adsorption/Desorption Characteristics and Performance of the Fluidised Desiccant Cooler Zbigniew Rogala *, Piotr Kolasinski ´

ID

and Przemysław Błasiak

ID

Department of Thermodynamics, Theory of Machines and Thermal Systems, Wroclaw University of Science and ˙ Wyspianskiego Technology, Wybrzeze ´ 27, 50-370 Wroclaw, Poland; [email protected] (P.K.); [email protected] (P.B.) * Correspondence: [email protected]; Tel.: +48-320-27-92





Received: 21 May 2018; Accepted: 14 June 2018; Published: 19 June 2018

Abstract: This paper concerns the issue of the proper selection of the operating parameters of the fluidised desiccant cooler. Despite the fact that fluidised desiccant cooling technology is being reported in the literature as an efficient way to provide cooling for the purposes of air-conditioning, the improper control of its operation can lead to a significantly worse performance than expected. The objective of the presented theoretical study is to provide guidelines on the proper selection of such operating parameters of a fluidized desiccant cooler, such as superficial air velocity, desiccant particle diameter, bed switching time, and desiccant filling height. The influence of the chosen operating parameters on the performance of fluidised desiccant cooling technology is investigated based on their impact on electric and thermal coefficients of performance (COP) and specific cooling power (SCP). Moreover, the influence of the outlet air temperature, humidity, and desiccant water uptake on the adsorption/desorption characteristics was investigated, contributing to better understanding of sorption processes. Keywords: fluidisation; adsorption; modelling; silica gel-water; optimization

1. Introduction Alternative cooling technologies, such as fluidised desiccant coolers, are nowadays of great interest. Thanks to its advantages, fluidised desiccant cooling is especially promising. Unlike adsorption cooling [1,2], it does not require low pressure maintenance, which greatly affects investment costs. On the other hand, when compared to fixed beds systems, it provides increased adsorption kinetics and a lower air pressure drop [3]. This technology is at the early stage of development, however, a few prototypes of fluidised desiccant dehumidifiers (fluidised beds) have been already developed. The prototype of a low-cost self-circulating fluidised desiccant dehumidifier was presented by Chen et al. [4]. The next generation of this prototype was presented by Chiang et al. [5]. The maximum values of the electric coefficients of performance (COP) and the specific cooling power (SCP) of these prototypes were reported as 0.37 and 287 W/kg, respectively. According to the theoretical study by Rogala et al. [6], the performance of a fluidized desiccant cooler FDC can be much better (reaching an electric COP of about 10 and a SCP of about 500 W/kg). This lower than expected performance probably results from the improper selection of operating parameters, such as desiccant particle diameter, desiccant filling height, superficial air velocity, and bed switching time. A better understanding of the adsorption mechanism in the fluidised bed and the relation between the above mentioned operating parameters and FDC performance would greatly contribute to the development of FDC technology. This paper presents a modelling analysis of the influence of operating parameters

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on the performance of FDC, as well as on air desiccation (adsorption) and desiccant regeneration (desorption). The modelling was conducted using the model that was developed by the authors, which was comprehensively presented in the work of Rogala et al. [7]. The operating parameters that were analyzed are the desiccant particle diameter, bed switching time (tsw ), superficial air velocities during adsorption (Uads ) and desorption (Udes ), and desiccant filling height (Hd ). The performance of the FDC should be assessed and compared to the performance of the reference technologies, such as adsorption cooling [3] and vapor-compression cooling [8,9]. The adsorption chillers are reported to feature a thermal COP ranging from 0.5 to 0.6 [10] (while driven by similar heating and cooling temperatures to the FDC). Such systems do not utilize electric power as a primary energy source. However, according to Chorowski et al. [2], some electric power is consumed by the cooling tower and pumping systems. The maximum reported SCP of the adsorption chillers (while driven by similar heating and cooling temperatures to the FDC) does not exceed 300 W/kg [11]. On the other hand, vapor-compression chillers are supplied only with the electric power, which is spent on running the compressor and the secondary systems (e.g., condenser fans). Vapor-compression chillers are reported to feature electric COP up to 3 [8,9]. A FDC is primarily supplied with thermal power and, when it is properly designed and operated, it should achieve a thermal COP similar to the adsorption chillers. Furthermore, the electric power is needed to run the fans of the FDC. A FDC should consume significantly less electricity than a vapor-compression chiller, as electric power is its secondary energy source. Therefore, the electric COP of a FDC should be 2–3 times higher. The primary aim of this study is to assess whether the abovementioned performance can be theoretically be reached and to indicate the proper ranges of the operating parameters. The fulfillment of these requirements will make the FDC competitive to the reference technologies. 2. Modelling The scheme of the FDC is presented in Figure 1. The operating principle of FDC can be described on the basis of this figure. The system consists of two fluidised beds (3) filled with a desiccant (e.g., silica gel). The beds are periodically supplied with hot or cold air. The fan (4) blows the air through the air cooler (5). The cooled air enters the fluidised bed (3). Because of the adsorption, the air is dehumidified and warmed up. Then, the dehumidified air is cooled again in the air cooler (2), and finally, it is humidified in the humidifier (1), which results in an air temperature drop. Simultaneously, the desiccant contained in the second fluidised bed is regenerated. The fan (8) blows the air through the recovery heat exchanger (7) and the air heater (6). Then, the hot air enters the fluidised bed, where, as a result of the high temperature of the air, the desorption of water from the desiccant proceeds. At last, the regenerated air flows through the recovery heat exchanger (7). The modelling of the above described FDC was carried out using our original, experimentally validated model, which is comprehensively described by Rogala et al. [7]. The modelling is based on energy and conservation equations, which are used to calculate the temperature and humidity of the air at the outlet of the fluidised bed and the temperature of the desiccant. The model provides full information about the temperatures and water contents of both the air and desiccant at any time of the sorption process. This data can be used to estimate the performance of the FDC, which is described by thermal COP, electric COP, and SCP, as follows: .

COPth =

Q0 .

Qh

(1)

.

COPel =

Q0 .

Nel

(2)

.

Q0 SCP = Md

(3)

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Q SCP  0 Md

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Q00 isisgenerated The generatedas asaaresult resultof ofthe thewater water evaporation. evaporation. Assuming Assuming that The cooling cooling power power Q that the the air air humidity at the inlet and the outlet of the FDC does not change, the cooling power can be estimated based on the change of the desiccant water uptake within the switching time, as follows: .

w牋 H ev M d . ∆w ∆H ev Md QQ 0 = 0 tswtsw

(4) (4)

where Δw is the change of the desiccant water uptake within the switching time tsw. On the other where ∆w is the change of the desiccant water uptake within the switching time tsw . On the other hand, . Q can be estimated based on the mean air temperature change during hand, the heating power the heating power Qh can be hestimated based on the mean air temperature change during regeneration meanafter before after thebed, desorption bed, as follows: regeneration ∆Tmean beforeΔT and theand desorption as follows: . QQ.h =∆T Tmean cgcmm g,des mean g g,des h

(5) (5)

Figure 1.1.The of the desiccant cooler (FDC). 2—air cooler, 3—fluidized Figure Thescheme scheme of fluidized the fluidized desiccant cooler1—humidifier, (FDC). 1—humidifier, 2—air cooler, beds, 4—electric fan, 5—air cooler, 6—air heater, 7—recovery heat exchanger, and 8—electric fan. 3—fluidized beds, 4—electric fan, 5—air cooler, 6—air heater, 7—recovery heat exchanger, and 8—electric fan.

The electric power is consumed by electric fans and is expressed as follows: The electric power is consumed .by electricfans and is expressed as follows:  N el = ∆pA Ug,ads + Ug,des

N el  pA U g ,ads  U g ,des 

where the pressure drop that is generated in the fluidised bed is equal to [3], as follows: where the pressure drop that is generated in the fluidised bed is equal to [3], as follows: ∆p = Hd (1 − ε)(ρp − ρg ) g

p  H d (1   )( p  g ) g

(6) (6)

(7) (7)

Combining Equations (1)–(7), the performance of FDC can be expressed as follows: Combining Equations (1)–(7), the performance of FDC can be expressed as follows: ∆wρ ∆Hev H COPth= w d dH ev H d d COP ρ ∆Tmean Udes cg tsw th  g g Tmean U des cg tsw

(8) (8)

∆w∆Hev (Uads + Udes ) gtsw

(9)

COPel =

SCP =

∆w∆Hev tsw

(10)

COPel 

ev

(U ads  U des ) gtsw

SCP  Energies 2018, 11, 1597

wH ev tsw

(9)

(10) 4 of 16

The change of the desiccant water uptake Δw within the given time tsw and the mean air temperature change ΔTdesiccant mean are provided by the modeltime wastsw implemented in air the The change of the water uptake ∆w model. within The the given and the mean simulation procedure, presented in Figure 2, model. and was the programming language temperature change ∆Tmean are provided by the Thedeveloped model wasin implemented in the simulation (PYTHON).presented The simulation procedure the modeling of programming the influence oflanguage the chosen operating procedure, in Figure 2, and allows was developed in the (PYTHON). parameters. Based on theallows operating parameters that were chosen to be varied throughout the The simulation procedure the modeling of the influence of the chosen operating parameters. simulation, meshgrid is generated. input data the sorption model. Based on the aoperating parameters that The weremeshgrid chosen to provides be varied the throughout thefor simulation, a meshgrid modelling of meshgrid the sorption processes the fluidised bedsorption is performed forThe every single node of isThe generated. The provides the in input data for the model. modelling of the the meshgrid. The model provides information thenode sorption or the sorption processes in the fluidised bed iseither performed for everyon single of the characteristics meshgrid. The model parameters that are required for performance evaluation, Equations (8)–(10), as the change of for the provides either information on the sorption characteristics or the parameters that are required water uptakeevaluation, Δw and mean air temperature onchange the desorbing fluidised bed ΔT . The results performance Equations (8)–(10),drop as the of the water uptake ∆wmean and mean air that were obtained during a single simulation step ∆T aremean stored in results separatethat directories. In some of the temperature drop on the desorbing fluidised bed . The were obtained during the additional optimization procedure was In implemented. When the simulations are asimulations, single simulation step are stored in separate directories. some of the simulations, the additional completed, graphical results are generated.When A complete simulationare consists of thousands single optimization procedure was implemented. the simulations completed, graphicalofresults simulations. The betweenofhours even a few days, are generated. A presented complete simulation simulation procedure consists oftook thousands singletosimulations. The depending presented on the simulation type. simulation procedure took between hours to even a few days, depending on the simulation type.

Figure Figure2.2. Description Descriptionof ofthe thesimulation simulationprocedure. procedure.

Theperformance performance of ofthe theFDC FDCand andthe theadsorption/desorption adsorption/desorption characteristics characteristics were were modelled modelled for for The the selected range of the operating parameters. The analyzed ranges of these parameters are the selected range of the operating parameters. The analyzed ranges of these parameters are reported reported Table 1. The temperatures of the desorption, adsorption,and desorption, and inlet were air humidity were in Table 1.in The temperatures of the adsorption, inlet air humidity kept constant ◦ ◦ kept constant during the modelling and were assumed to be 25 °C, 60 °C, and 12 g/kg, respectively. during the modelling and were assumed to be 25 C, 60 C, and 12 g/kg, respectively.

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Table 1. Operating parameters used in analyzes. Energies 2018, 11, 1597

Operating Parameter

dp, mm

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tsw, s

Desiccant particle diameter (see Figure 4) 1, 3, 5 3000 2 2 Desiccant particle diameter (see Figure 5) 1, 3, 5 2000 2 2 Table 1. Operating parameters used in analyzes. Desiccant particle diameter (see Figure 6) 1–5 optimum 2 2 Switching time (see Figure 7) 1, 3, 200–1500 2 U des , m/s 2 Operating Parameter dp , mm tsw5, s U ads , m/s Desiccant particle diameter (see(see Figure 4) 1, 3, 5 2 2 Switching time Figure 8) 13000 200, 350, 650 2 2 Desiccant particle diameter (see Figure 5) 1, 3, 5 2000 2 2 Superficial air velocity during adsorption (see Figure 9) 1 2000 2, 4, 6 Desiccant particle diameter (see Figure 6) 1–5 optimum 2 2 time (see Figuredesorption 7) 3, 5 200–1500 2 2, 4, 6 SuperficialSwitching air velocity during (see Figure1,10) 1 1500 2 Switching time (see Figure 8) 1 200, 350, 650 2 2 Superficial air velocities (seeFigure Figure 12000 optimum 1–6 - 1–6 Superficial air velocity during adsorption (see 9) 11) 1 2, 4, 6 Superficial air velocity during desorption (see Figure 10) 1 1500 2, 4, 6 Desiccant filling height (see Figure 12) 1 3000 2 Superficial air velocities (see Figure 11) 1 optimum 1–6 1–6 Desiccant filling(see height 13000 optimum2 2 Desiccant filling height Figure(see 12) Figure 6) 1 Desiccant fillingand height (see Figurefilling 6) 1 optimum Superficial air velocities desiccant height (see Figure 14) 1 optimum2 2–6 2 Superficial air velocities and desiccant filling height (see Figure 14)

1

optimum

2–6

2

5 5 5 4 H d , cm 54 5 4 5 44 4 44 4 1, 5, 10 4 1,1,5,5, 1010 1, 5,1–6 10 1–6

3. Results and Discussion 3. Results and Discussion 3.1. The Effect of Desiccant Particle Diameter 3.1. The Effect of Desiccant Particle Diameter At first, the effect of the desiccant particle diameter on the performance of the FDC was At first, the effect of the desiccant particle diameter on the performance of the FDC was investigated. In the FDC, the desiccant was simply poured into the fluidised bed. Therefore, some of investigated. In the FDC, the desiccant was simply poured into the fluidised bed. Therefore, some of the limitations that were related to the adsorption cooling systems did not occur (i.e., the contact the limitations that were related to the adsorption cooling systems did not occur (i.e., the contact resistance and the inter-particle mass transfer resistance) [12,13]. On the other hand, the desiccant resistance and the inter-particle mass transfer resistance) [12,13]. On the other hand, the desiccant particle diameter influenced the minimum [4] and the maximum [14] fluidisation velocities, which is particle diameter influenced the minimum [4] and the maximum [14] fluidisation velocities, which is visualized in Figure 3. For the desiccant particle with a diameter of 5 mm, the difference between the visualized in Figure 3. For the desiccant particle with a diameter of 5 mm, the difference between the minimum and the maximum fluidisation velocities was significant. The smaller the desiccant minimum and the maximum fluidisation velocities was significant. The smaller the desiccant particle particle was, the more narrow the regime of fluidisation was. For the desiccant particle with a was, the more narrow the regime of fluidisation was. For the desiccant particle with a diameter of diameter of 1 mm, fluidisation was possible for the superficial air velocities ranging between 1 and 6 1 mm, fluidisation was possible for the superficial air velocities ranging between 1 and 6 m/s. For the m/s. For the smaller desiccant particles, which were not discussed in this paper, the fluidisation smaller desiccant particles, which were not discussed in this paper, the fluidisation regime may have regime may have been even more limited. been even more limited.

Figure Minimum and and maximum maximum fluidization fluidization velocities velocities with with respect respect to to the Figure 3.3. Minimum the diameter diameter of of the the fluidized particles. fluidized particles.

The The influence influence of of the the desiccant desiccant particle particle diameter diameter on on the the adsorption adsorption characteristics characteristics is is presented presented in in Figure diameters of 1of mm, 3 mm,3and 5 mm investigated. Other operating Figure 4.4.Particle Particle diameters 1 mm, mm, andwere 5 mm were investigated. Otherparameters operating parameters were set were set according toaccording Table 1. to Table 1.

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Figure 4. The influence of the desiccantparticle particlediameter diameter on characteristics: (a) Outlet air air Figure 4. The influence of the desiccant onadsorption adsorption characteristics: (a) Outlet temperature; (b) outlet humidity; and(c) (c)desiccant desiccant water water uptake. temperature; (b) outlet air air humidity; and uptake.

The desiccant particle diameter significantly affects the kinetics of adsorption. Because of the

The particle significantly kinetics of adsorption. Because of the verydesiccant good contact of eachdiameter particle with the passingaffects air, thethe intraparticle resistance became the main very good contact of each particle with the passing air, the intraparticle resistance became the limitation of the heat and mass transfer. The particles with the diameter of 1mm increased the watermain limitation ofmuch the heat and mass transfer. with the(see, diameter 1mm increased the water uptake faster than those with a 3The mmparticles or 5 mm diameter Figureof 4c). A higher adsorption uptake much faster than those a 3 mm or 5air mm diameter (see, higher rate resulted in better initial with desiccation of the (see, Figure 4b), butFigure on the 4c). otherAhand, theadsorption outlet air humidity returned much faster to the baseair value. tobut Figure 4a, the adsorption tended rate resulted in better initial desiccation of the (see,According Figure 4b), on the other hand, the outlet air to start at higher temperatures in the case of the smaller desiccant particle diameters. The application humidity returned much faster to the base value. According to Figure 4a, the adsorption tended to of smaller particles could therefore notof only kinetics of the adsorption, butThe enabled the start at higher temperatures in the case theenhance smallerthe desiccant particle diameters. application utilization of higher temperature heat sinks. of smaller particles could therefore not only enhance the kinetics of the adsorption, but enabled the The positive influence of the decreased desiccant particle diameter can also be seen in Figure 5, utilization of higher temperature heat sinks. which presents desorption characteristics of the FDC, namely the air temperature at the outlet (see The positive of the decreased desiccant particle also be seen Figure 5, Figure 5a), theinfluence air humidity at the outlet (see Figure 5b), and thediameter desiccant can water uptake (see in Figure which5c). presents desorption characteristics of the FDC, namely the air temperature at the outlet The start of the desorption was shifted towards a lower temperature and, in the case of desiccant (see Figure humidity at the (seewas Figure 5b), and desiccant uptake particles5a), withthe the air diameter of 1 mm, the outlet desorption established at 40the °C (which waswater indicated by the sudden change in Figure 5a). This meant that a decrease of the desiccant (see Figure 5c). The start of of line the steepness desorption was shifted towards a lower temperature and, in the particle diameter enabled the the usediameter of lower of temperature heat sources and expandedatthe case of desiccant particles with 1 mm, the desorption wasthus established 40 ◦ C utilization of the low potential energy. The kinetics of desorption were enhanced with smaller (which was indicated by the sudden change of line steepness in Figure 5a). This meant that a decrease which resulted in a much greater humidity at the outlet at the beginning the thus of theparticles, desiccant particle diameter enabled theairuse of lower temperature heat sourcesofand desorption (see Figure 5b), and a shorter time was needed to reach the equilibrium water uptake (see expanded the utilization of the low potential energy. The kinetics of desorption were enhanced Figure 5c). The reduction of the desiccant particle diameter resulted in the reduced time of the with smaller particles, which resulted in a much greater air humidity at the outlet at the beginning of complete regeneration (see Figure 5c) and could probably greatly increase the SCP without the loss the desorption in the COP.(see Figure 5b), and a shorter time was needed to reach the equilibrium water uptake (see Figure 5c). The reduction of the desiccant particle diameter resulted in the reduced time of the complete regeneration (see Figure 5c) and could probably greatly increase the SCP without the loss in the COP.

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Figure 5. 5. The The influence influence of of the the desiccant desiccant particle particle diameter diameter on on desorption desorption characteristics: characteristics: (a) (a) Outlet Outlet air air Figure temperature; (b) (b) outlet outlet air air humidity; humidity; and and(c) (c)desiccant desiccantwater wateruptake. uptake. temperature;

Such aa conclusion bebe legitimised by the results that that are presented in Figure 6, which Such conclusionseemed seemedtoto legitimised by the results are presented in Figure 6, presented the influence of a desiccant particle diameter on thermal COP, electric COP, and SCP, which presented the influence of a desiccant particle diameter on thermal COP, electric COP, and SCP, which were were obtained obtained for for the the optimum optimum switching switching time. time. The The performance performance of of the the FDC FDC was was analyzed analyzed for for which desiccantparticle particlediameters diameters ranging from 1 to mm to 5The mm. The operating parameters were set desiccant ranging from 1 mm 5 mm. operating parameters were set according according to Table 1. Each simulation was run with its specific optimum switching time (i.e.,time the to Table 1. Each simulation was run with its specific optimum switching time (i.e., the switching switching time for which the electric COP and SCP reached maximum values). The optimum for which the electric COP and SCP reached maximum values). The optimum switching time enabled switching time enabled the maximization the SCP. Decreasing thefrom desiccant the maximization of the SCP. Decreasing the of desiccant particle diameter 5 mm particle to 1 mmdiameter not only from 5 mm to 1 mm not only nearly doubled the SCP (see Figure 6b) and the electric COP the (seeCOP Figure nearly doubled the SCP (see Figure 6b) and the electric COP (see Figure 6c), but it increased as 6c), but it increased the COP as well. Simultaneously, the shortening of the optimum switching time well. Simultaneously, the shortening of the optimum switching time by more than half was observed by more than halfThe wasdecrease observed 6d).particle The decrease of the desiccant particle diameter had (see Figure 6d). of(see the Figure desiccant diameter had a positive influence on both a positive influence on both the adsorption/desorption characteristics and the performance of the the adsorption/desorption characteristics and the performance of the FDC. It resulted in improved FDC. It resulted in improved sorption kinetics, optimum switching time, and better FDC sorption kinetics, shorter optimum switching time, shorter and better FDC performance. The desiccant particle performance. The desiccant particle diameter seemed to have a crucial influence on the performance diameter seemed to have a crucial influence on the performance of the FDC, especially the SCP and of the FDC, theof SCP electricparticle COP. The changefrom of the desiccant particle from electric COP.especially The change theand desiccant diameter 5mm to 1mm could diameter nearly double 5mm to 1mm could nearly double the unchanged SCP and electric COP for aOn nearly unchanged COP. the SCP and electric COP for a nearly thermal COP. the other hand, thermal the desiccant On the other hand, the desiccant particle diameter affected the fluidisation regime that was particle diameter affected the fluidisation regime that was constricted by the minimum and maximum constricted by the minimum and maximum fluidisation velocities. In the case of very small desiccant fluidisation velocities. In the case of very small desiccant particles, fluidisation could be hard to particles, and fluidisation maintain control. could be hard to maintain and control.

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Figure6.6.The Theinfluence influenceofofthe the desiccant particle diameter (a) thermal (b) electric (c) Figure desiccant particle diameter on: on: (a) thermal COP;COP; (b) electric COP;COP; (c) SCP; Figure 6. The influence of the desiccant particle diameter on: (a) thermal COP; (b) electric COP; (c) SCP; (d) optimum switching time. (d) optimum switching time. SCP; (d) optimum switching time.

3.2. The The Effect of Switching Switching Time 3.2. of Time 3.2.Effect The Effect of Switching Time The influence influence ofthe the switching time on the performance ofFDC FDC was investigated inthe therange range of The of switching time on of FDC was investigated in The influence of the switching time onthe theperformance performance of was investigated in the range of of 200 s to 1500 s. The results of the simulations are presented in Figure 7. 200 s to 1500 s. The results of the simulations are presented in Figure 7. 200 s to 1500 s. The results of the simulations are presented in Figure 7.

Figure 7. The influence switchingtime timeon: on: (a) (a) thermal thermal COP; COP; (c) SCP. Figure 7. The influence of of switching COP;(b) (b)electric electric COP; (c) SCP.

Figure 7. The influence of switching time on: (a) thermal COP; (b) electric COP; (c) SCP.

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Simulations particles with with aa diameter diameterof of11mm, mm,33mm mmand and5 Simulations were were carried carried out for the desiccant particles 5mm. mm.ForFor each particle diameter, a certain switching time (called optimum switching time), each particle diameter, at a at certain switching time (called optimum switching time), electric electric COP (see, 7b) Figure and SCP (see, 7c) Figure 7c)maximum reach maximum The smaller desiccant COP (see, Figure and7b) SCP (see, Figure reach values. values. The smaller desiccant particle particle diameter is, the the shorter the optimum switching timethe is. other On the other hand, COP thermal COP diameter is, the shorter optimum switching time is. On hand, thermal increases increases with the increase in the switching time. This results from the phenomena reported in with the increase in the switching time. This results from the phenomena reported in [7]. Firstly, [7]. the Firstly, the cooling/heating the the bedair with the air streamon is the spent on theofchange of cooling/heating provided toprovided the bed to with stream is spent change desiccant desiccant temperature and then on adsorption/desorption (see,Too Figure Too short temperature and then on adsorption/desorption (see, Figure 8a). short8a). switching timeswitching can cause time can cause undesired deterioration in FDC performance. The influence switchingtime time on undesired deterioration in FDC performance. The influence of of switching on adsorption/desorption adsorption/desorption characteristics is presented in Figure Figure 8. 8.

Figure Figure8.8. The Theinfluence influenceofofswitching switchingtime timeon onadsorption/desorption adsorption/desorption characteristics: characteristics: (a) (a) Outlet Outlet air air temperature; (b) Outlet air humidity; and (c) Desiccant water uptake. temperature; (b) Outlet air humidity; and (c) Desiccant water uptake.

This figure figure presents presents the the variation variation of of the the air airtemperature temperature and and humidity humidity at at the the outlet, outlet, and and the the This desiccantwater wateruptake uptakeof ofaasingle singlebed bedagainst againsttime. time.The Thesecond secondbed bedalways alwaysoperated operatedin inan anopposite opposite desiccant mode.This Thissimulation simulationwas wasperformed performedfor forthe the desiccant particle with a diameter 1 mm. According mode. desiccant particle with a diameter of 1ofmm. According to to Figure for such operating parameters, the optimum switching was s. The Figure 7b,c,7b,c, for such operating parameters, the optimum switching time wastime about 350about s. The 350 operation operation of the the FDC under the switching time ofthan 200 optimum), s (shorter than (optimum), of the FDC under switching time of 200 s (shorter 350 s optimum), (optimum), 350 and s650 s (longer and optimum) 650 s (longer optimum) was modelled. The switching time affected the change of water than was than modelled. The switching time affected the change of water uptake, which appeared uptake, the which appeared between the desorption and A adsorption Figuretime 8c). had A change of between desorption and adsorption (see Figure 8c). change of (see switching a greater switching time had a (maximum) greater impact on than the desorption adsorption(minimum) (maximum) uptake thanbedesorption impact on the adsorption uptake uptake. It could seen that (minimum) It could be seen that better the desiccant greater the initial the better theuptake. desiccant was desorbed, thethe greater the initial drop was of airdesorbed, humiditythe was (see Figure 8b). ◦ C, which meant drop of200 air s,humidity was (see Figure 8b). Within the air at thethat outlet not reach Within the air temperature at the outlet did200 nots,reach 60temperature thedid heat source 60 °C, meant that the heat source not properly Increasing theair switching time at to was notwhich properly utilized. Increasing thewas switching time toutilized. 350 s resulted in the temperature ◦ 350outlet s resulted the to air60temperature at the being close to 60 °C (see desiccant Figure 8a), as well as a the beingin close C (see Figure 8a),outlet as well as a lowered minimum water uptake lowered desiccant water uptake (see Figure 8c). This a better initial (see Figureminimum 8c). This finally caused a better initial desiccation of air (seefinally Figure caused 8b). A further increase desiccation of airtime (see(up Figure 8b). increase inimprove the switching time (up to not in the switching to 650 s) A didfurther not significantly the desorption, but650 on s) thedid other significantly improve the desorption, but on the other hand, it enhanced the adsorption. Increasing the switching time mostly led to the increase of the thermal COP. The presented analyses showed

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hand, it enhanced the adsorption. Increasing the switching time mostly led to the increase of the thermal COP. The presented analyses showed that the switching time should not be shorter than Energies 2018, 11, x FOR PEER REVIEW of 16 optimum. In order to maximize the electric COP and SCP, the FDC should operate with the10optimum switching time. Operating the FDC with a switching time longer than the optimum would increase that the switching time should not be shorter than optimum. In order to maximize the electric COP the thermal COP, electric COP SCP. Moreover, as a result of the and SCP, the but FDClower shouldthe operate with theand optimum switching time. Operating thepoor FDC adsorption with a kinetics, the application of multi-bed systemswould could be considered, for example, (two beds switching time longer than the optimum increase the thermal COP, buta 3-bed lower FDC the electric adsorbing and SCP. one desorbing sameoftime). Thisadsorption solution would lead the further COP and Moreover, at as the a result the poor kinetics, theto application of increase multi-bedof the systems be considered, example, a 3-bedapplied FDC (two adsorbing and one desorbing at thermal COP.could The 3-bed systems for were successfully in beds the adsorption cooling [10]. the same time). This solution would lead to the further increase of the thermal COP. The 3-bed

3.3. The Effectwere of Superficial Airapplied Velocityin the adsorption cooling [10]. systems successfully The influence of a superficial air velocity on the performance of a FDC and its sorption 3.3. The Effect of Superficial Air Velocity characteristics was investigated. Because of the different adsorption and desorption kinetics, The influence of a superficial air velocity on the performance of a FDC and its sorption the influence of the superficial air velocity during these processes was investigated separately. characteristics was investigated. Because of the different adsorption and desorption kinetics, the The modelling was performed for the desiccant particle with a diameter of 1 mm, desiccant filling influence of the superficial air velocity during these processes was investigated separately. The heightmodelling of 0.04 m, withinfor the of superficial velocities 1 to 6 m/s (see Table 1). wasand performed therange desiccant particle withair a diameter of 1from mm, desiccant filling height The optimum switching time was applied to all of the simulations. The influence of the superficial of 0.04 m, and within the range of superficial air velocities from 1 to 6 m/s (see Table 1). The air velocity onswitching the adsorption characteristics is the presented in Figure 9. Superficial air velocities optimum time was applied to all of simulations. The influence of the superficial air of the6adsorption is presented in Figure 9. Superficial air velocities of 2the m/s, 4 2 m/s,velocity 4 m/s,on and m/s werecharacteristics analyzed. The increase of superficial air velocity increased kinetics and 6 m/s(see were analyzed. of superficial air velocity kinetics the of them/s, adsorption Figure 9c). The Theincrease decrease of the superficial air increased velocity the enabled theofincrease Figure 9c). TheFigure decrease of but the on superficial air hand, velocityitenabled thethe increase of the rate of theadsorption initial air(see desiccation (see 9b), the other lowered adsorption initial air desiccation (see Figure 9b), but on the other hand, it lowered the adsorption rate (see (see Figure 9c). The increase of the superficial air velocity increased the kinetics, but it was not Figure 9c). The increase of the superficial air velocity increased the kinetics, but it was not enough to enough to desiccate a much larger volume of passing air (see Figure 9b). The change of superficial desiccate a much larger volume of passing air (see Figure 9b). The change of superficial air velocity air velocity had little influence on the temperature at which the adsorption started (see Figure 9a). had little influence on the temperature at which the adsorption started (see Figure 9a). Furthermore, Furthermore, an increase of superficial airenabled velocitythe enabled theof reaching of the adsorption an increase of superficial air velocity reaching the adsorption equilibriumequilibrium faster, faster,which which probably resulted in shortening the shortening the optimum switching time, which probably resulted in the of the of optimum switching time, which could havecould led tohave led tothe thegeneral general improvement of FDC the FDC performance. improvement of the performance.

Figure 9. The influence of the superficial air velocity on adsorption characteristics: (a) Outlet air Figure 9. The influence of the superficial air velocity on adsorption characteristics: (a) Outlet air temperature; (b) Outlet air humidity; (c)(c)Desiccant temperature; (b) Outlet air humidity; Desiccantwater water uptake. uptake.

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Similar effects effectscould couldbe beobserved observedduring duringthe the desorption (see Figure higher superficial Similar desorption (see Figure 10).10). TheThe higher superficial air air velocity increased the desorption kinetics (see Figure 10c), however it decreased initial velocity increased the desorption kinetics (see Figure 10c), however it decreased the initialthe increase increase the air humidity at the outlet (see, Figure 10b).the Ateffect last, of thethe effect of the superficial air of the airof humidity at the outlet (see, Figure 10b). At last, superficial air velocities velocities on the FDC performance were investigated. The results are presented in Figure 11. The on the FDC performance were investigated. The results are presented in Figure 11. The electric COP electric COP11b) (see,was Figure waswhen the highest when both of the air superficial airwere velocities were similar. (see, Figure the 11b) highest both of the superficial velocities similar. However, However, it remained high in the case of the higher superficial air velocity during the adsorption. it remained high in the case of the higher superficial air velocity during the adsorption. This might This resulted might have resulted from of a difference of the adsorption and desorption kinetics.thermal The highest have from a difference the adsorption and desorption kinetics. The highest COP thermal COP (see Figure 11a) was achieved with a high superficial air velocity during the adsorption (see Figure 11a) was achieved with a high superficial air velocity during the adsorption and was low and was low during AsinitFigures can be9seen in Figures 9 and 10, the desorption proceeded during desorption. As desorption. it can be seen and 10, the desorption proceeded significantly faster significantly faster than the adsorption. higher during superficial air velocity during the adsorption than the adsorption. The higher superficial The air velocity the adsorption promoted the adsorption promoted adsorption todesorption, proceed faster, the kinetics, desorption, due to better kinetics, proceeded to proceed the faster, while the due while to better proceeded fast enough under lower fast enough under lower superficial air velocities. The differences of the adsorption and desorption superficial air velocities. The differences of the adsorption and desorption kinetics could be mitigated kinetics could be mitigated byand a change of switching time andsystems, the application systems, by a change of switching time the application of multibed but also of bymultibed setting a different but also by setting a different superficial air velocity during the adsorption and desorption. superficial air velocity during the adsorption and desorption.

Figure Figure 10. 10. The influence of superficial superficial air air velocity velocity on on desorption desorption characteristics: characteristics: (a) Outlet Outlet air air temperature; (b) Outlet air humidity; (c) Desiccant water uptake. temperature; (b) Outlet air humidity; (c) Desiccant water uptake.

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Figure Figure11. 11.The Theinfluence influenceof ofsuperficial superficialair airvelocity velocityon: on:(a) (a)thermal thermalCOP; COP;(b) (b)electric electricCOP; COP;(c) (c)SCP. SCP.

3.4. 3.4.The TheEffect EffectofofDesiccant DesiccantFilling FillingHeight Height The operating parameter waswas the desiccant fillingfilling height.height. The desiccant filling height Thelast lastanalyzed analyzed operating parameter the desiccant The desiccant filling was equivalent to the mass ofmass the desiccant that wasthat poured into theinto fluidised bed. Its bed. effectItsoneffect the height was equivalent to the of the desiccant was poured the fluidised adsorption/desorption characteristics was investigated with the desiccant filling height ranging from on the adsorption/desorption characteristics was investigated with the desiccant filling height 1ranging cm to 10from cm. 1The operating parameters were set according to Table 1. Figure 12 presents cm other to 10 cm. The other operating parameters were set according to Table 1. Figurethe 12 influence desiccantoffilling height onfilling the adsorption The higher filling height was presents of thethe influence the desiccant height oncharacteristics. the adsorption characteristics. The higher equivalent to more poured into the bed. It resulted deeper air desiccation (see 12b). filling height wasdesiccant equivalent to more desiccant poured in into the bed. It resulted inFigure deeper air The increase in the12b). desiccant filling height led to the decrease of the initial outlet humidity. desiccation (see Figure Moreover, the effectin of the desiccation lasted much longer. trend could beinitial observed in the case of The increase desiccant filling height led A tosimilar the decrease of the outlet humidity. the desorption Figure 13). More lasted desiccant required more time totrend get fully adsorbed (see Figure Moreover, the(see effect of desiccation much longer. A similar could be observed in the12c). case The more desiccant(see that Figure underwent the more cooling that needed to be provided. In (see the of the desorption 13). adsorption, More desiccant required more time to get fully adsorbed case of the FDC, power was by the air stream and it was proportional to the Figure 12c). The cooling more desiccant thatprovided underwent adsorption, the more cooling that needed toair be superficial velocity. In the case of the small desiccant filling height, the amount of cooling that was provided. In the case of the FDC, cooling power was provided by the air stream and it was provided withto thethe air air stream was enough to run adsorption. the other hand,height, when the proportional superficial velocity. In the the quick case of the smallOn desiccant filling the filling height was high, amount of cooling wasair notstream enough to run the quick adsorption. amount of cooling thatthe was provided with the was enough to run the quick Adsorption adsorption. was hindered because of the cooling that the wasamount provided with thewas air not stream. Nottoonly On the other hand, when theinsufficient filling height was high, of cooling enough run were properadsorption. parameters Adsorption to achieve high (e.g., small desiccant particle the quick wasadsorption hindered kinetics because required of the insufficient cooling that was size), but also, be provided, proportionally to achieve the amount the desiccant that provided withenough the air cooling stream. had Not to only were proper parameters to highofadsorption kinetics required (e.g., small desiccant particle size), but also, enough cooling had to be provided, proportionally to the amount of the desiccant that was poured into the fluidised bed. Due to the

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limitations superficial air velocity, desiccant filling heightsair were recommended. Similar was poured of into the fluidised bed. Duesmaller to the limitations of superficial velocity, smaller desiccant conclusions drawn from Figure presentingcould the influence offrom the desiccant height filling heightscould werebe recommended. Similar14, conclusions be drawn Figure 14,filling presenting and superficial air velocities during the adsorption and desorption (both velocities were equal) on the influence of the desiccant filling height and superficial air velocities during the adsorption and the performance of the FDC. The operating parameters that were used during this simulation are desorption (both velocities were equal) on the performance of the FDC. The operating parameters that givenused in Table Thesimulation electric COP decreased with the electric increaseCOP of the desiccant height.ofIt were during1.this are given in Table 1. The decreased withfilling the increase dropped from 30 to less than 8 when the filling height was increased from 1 cm to 6 cm. Increasing, the desiccant filling height. It dropped from 30 to less than 8 when the filling height was increased from and Increasing, thermal COP inthermal contradiction. The thermal COP wasThe thethermal highestCOP for the 1the cmSCP to 6 cm. the stayed SCP and COP stayed in contradiction. washigh the desiccant filling height and low velocities. On the other hand, the SCP increased for the small highest for the high desiccant filling height and low velocities. On the other hand, the SCP increased desiccant filling heightsfilling and heights high superficial results of the presented for the small desiccant and highvelocities. superficialThe velocities. The results of thesimulations presented showed theshowed significant influence of the desiccant filling height the on performance of theofFDC, simulations the significant influence of the desiccant fillingon height the performance the especially on the electric COP. FDC, especially on the electric COP.

Figure 12. The influence of desiccant filling height on adsorption characteristics: (a) Outlet air Figure 12. The influence of desiccant filling height on adsorption characteristics: (a) Outlet air temperature; (b) Outlet air humidity; (c) Desiccant water uptake. temperature; (b) Outlet air humidity; (c) Desiccant water uptake.

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Figure 13. The desiccant filling height on desorption desorptioncharacteristics: characteristics: air Figure 13. influence of of desiccant desorption characteristics: (a) Outlet Figure 13. The influence desiccantfilling filling height height on on (a) Outlet air air temperature; (b) Outlet Outlet air humidity; humidity; and (c) Desiccant wateruptake. uptake. temperature; (b) air and uptake. temperature; (b) Outlet air humidity; and(c) (c)Desiccant Desiccant water water

Figure 14. The of desiccant filling height andand superficial airairvelocities Figure 14. influence The influence of desiccant filling height superficial velocitiesduring duringadsorption adsorption and desorption (both velocities are equal)are onequal) the performance of the FDC: (a) thermal (b) COP; electric and desorption (both velocities on the performance of the FDC: (a) COP; thermal (b)COP; electric COP; (c) SCP. (c) SCP. Figure 14. The influence of desiccant filling height and superficial air velocities during adsorption and desorption (both velocities are equal) on the performance of the FDC: (a) thermal COP; (b) electric COP; (c) SCP.

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4. Conclusions The influence of operating parameters on the adsorption/desorption characteristics and the performance of the fluidised desiccant cooler was investigated by means of a simulation procedure based on our theoretical modelling. A comprehensive study on the influence of the desiccant particle diameter, the switching time, the superficial air velocities, and the desiccant filling height on the FDC performance was carried out. Based on the obtained results, the following conclusions can be drawn:

•

•

•

•

A decrease of the desiccant particle diameter improves the performance of FDC, mainly by reducing the intraparticle mass transfer resistance and thereby enhancing the sorption kinetics. Nevertheless, the particle diameter affects the minimum and the maximum fluidisation velocity, which puts significant limitations on FDC operation. In the case of desiccant particle diameters smaller than 1 mm, fluidisation maintenance can be problematic because of the very narrow fluidisation regime. A FDC should operate with a optimum switching time that maximizes the electric COP and SCP. On the other hand, increasing the switching time above the optimum value will result in an increased thermal COP, but at the expense of the electric COP and SCP. Operating the FDC with the switching time below optimum is not recommended, as it deteriorates the performance of the FDC. Optimum electric COP can be achieved with similar superficial air velocities. The increase of the superficial air velocity during the adsorption over desorption allows for the mitigation of the differences of the adsorption and desorption kinetics, and can be applied as an alternative method for increasing COP in multibed systems, for example. The cooling power, which is provided with the air stream, is proportional to the superficial air velocity. In turn, the superficial air velocity is limited by the minimum and the maximum fluidisation velocities. The amount of desiccant in the fluidised bed has to be matched to the provided cooling power. Therefore, decreasing the desiccant filling height results in the significant increase of SCP and electric COP. The desiccant filling height has a significant impact on the performance of a FDC, especially on the electric COP. A desiccant filling height of below 4 cm has been recommended. This essentially affects the scalability of the FDC systems.

As a result of poor kinetics, the adsorption proceeds much slower than desorption. Therefore, the application of a multi-bed FDC should be considered. In the case of a FDC, an alternative solution is to increase the superficial air velocity during the adsorption over desorption. Both solutions will result in better adsorption of the desiccant and thus an improved thermal COP. Author Contributions: Z.R. conceived the model, preformed the modelling, and wrote the paper; P.K. supervised the research and revised the paper; and P.B. revised the paper. Funding: This research was supported by the Dean of the Faculty of Mechanical and Power Engineering of Wrocław University of Science and Technology. Research grant for young scientists, no. 0402/0164/17. Conflicts of Interest: The authors declare no conflict of interest.

Nomenclature cg COPel COPth A g Hd . mg,ads . mg,des Md . N el

Specific heat of air J/(kg K) electric COP thermal COP cross section area of the fluidised bed, m2 gravitational acceleration m/s2 desiccant filling height m air mass flow during adsorption, kg/s air mass flow during desorption, kg/s desiccant mass in the bed kg electric power W

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Q0

cooling power W

Qh SCP tsw Uads Udes ∆Hev ∆p ∆Tmean ∆w ε ρd ρg ρp

heating power W specific cooling power W/kg switching time s superficial air velocity during adsorption m/s superficial air velocity during desorption m/s heat of evaporation J/kg pressure drop on fluidised bed Pa mean air temperature drop during regeneration K change of desiccant water uptake kg/kg void fraction desiccant bulk density kg/m3 air density kg/m3 desiccant particle density kg/m3

.

References 1. 2. 3. 4. 5. 6.

7. 8.

9. 10. 11. 12.

13. 14.

Wang, D.; Zhang, J.; Tian, X.; Liu, D.; Sumathy, K. Progress in silica gel-water adsorption refrigeration technology. Renew. Sustain. Energy Rev. 2014, 30, 85–104. [CrossRef] Chorowski, M.; Rogala, Z.; Pyrka, P. System options for cooling of buildings making use of district heating heat. Int. J. Refrig. 2016, 70, 183–195. [CrossRef] Chen, C.H.; Schmid, G.; Chan, C.T.; Chiang, Y.C.; Chen, S.L. Application of silica gel fluidised bed for air-conditioning systems. Appl. Therm. Eng. 2015, 89, 229–238. [CrossRef] Chen, C.H.; Ma, S.S.; Wu, P.H.; Chiang, Y.C.; Chen, S.L. Adsorption and desorption of silica gel circulating fluidized beds for air conditioning systems. Appl. Energy 2015, 155, 708–718. [CrossRef] Chiang, Y.C.; Chen, C.H.; Chiang, Y.C.; Chen, S.L. Circulating inclined fluidized beds with application for desiccant dehumidification systems. Appl. Energy 2016, 175, 199–211. [CrossRef] Rogala, Z.; Kolasinski, ´ P.; Gnutek, Z. Effect of operating conditions on performance of silica gel-water air-fluidised desiccant cooler. In Proceedings of the International Conference on Advances in Energy Systems and Environmental Engineering (ASEE17), Wrocław, Poland, 2–5 July 2017; Volume 22. Rogala, Z.; Gnutek, Z.; Kolasinski, ´ P. Modelling and experimental analyzes on air-fluidised silica gel-water adsorption and desorption. Appl. Therm. Eng. 2017, 127, 950–962. [CrossRef] Nunes, T.K.; Vargas, J.V.C.; Ordonez, J.C.; Shah, D.; Martinho, L.C.S. Modeling, simulation and optimization of a vapor compression refrigeration system dynamic and steady state response. Appl. Energy 2015, 158, 540–555. [CrossRef] Ibrahim, N.I.; Al-farayedhi, A.A.; Gandhidasan, P. Experimental investigation of a vapor compression system with condenser air pre-cooling by condensate. Appl. Therm. Eng. 2017, 110, 1255–1263. [CrossRef] Chorowski, M.; Pyrka, P. Modelling and experimental investigation of an adsorption chiller using low-temperature heat from cogeneration. Energy 2015, 92, 221–229. [CrossRef] Rogala, Z. Adsorption chiller using flat-tube adsorbers—Performance assessment and optimization. Appl. Therm. Eng. 2017, 121, 431–442. [CrossRef] Brancato, V.; Gordeeva, L.; Sapienza, A.; Freni, A.; Frazzica, A. Dynamics study of ethanol adsorption on microporous activated carbon for adsorptive cooling applications. Appl. Therm. Eng. 2016, 105, 28–38. [CrossRef] Santamaria, S.; Sapienza, A.; Frazzica, A.; Freni, A.; Girnik, I.S.; Aristov, Y.I. Water adsorption dynamics on representative pieces of real adsorbers for adsorptive chillers. Appl. Energy 2014, 134, 11–19. [CrossRef] Wen-Ching, Y. Handbook of Fluidization and Fluid-Particle Systems; Taylor & Francis Group LLC: New York, NY, USA, 2003. © 2018 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/).