NUMERICAL STUDY OF A THREE-BED (UNEQUAL BED) ADSORPTION CHILLER WITH MASS RECOVERY. (BUET), Dhaka, Bangladesh.

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1 Published by European entre for Research rag and Development UK (.eajournals.org) NUMERIAL SUDY OF A HREE-BED (UNEQUAL BED) ADSORPION HILLER WIH MASS REOVERY Gulshan Khatun 1 and Md. Zafar Iqbal Khan 2 1 Department of Electrical and Electronic Engeerg, Eastern University, Dhaka, Bangladesh. 2 Department of Mathematics, Bangladesh University of Engeerg and echnology (BUE), Dhaka, Bangladesh. ABSRA: In this paper, the performance of a three-bed (unequal bed) adsorption er ith mass recovery has been numerically studied. he mass recovery scheme is used to improve the coolg effect and a F-free-based sorption er driven by the lo-grade aste heat or any reneable energy source can be developed for the next generation of refrigeration. Silica gel/ater is taken as adsorbent/adsorbate pair for the present er. he three-bed adsorption er comprises ith three adsorber/desorber heat exchanger, one porator and one enser. In the present numerical solution, the heat source temperature variation is taken from 50 0 to 90 0 along ith coolant let temperature at 30 0 and the ed ater let temperature at In the ne strategy, mass recovery process occurs all beds here the configuration of Hex1 and Hex2 are identical, but the configuration of Hex3 is taken as half of Hex1 or Hex2. A cycle simulation computer program is constructed to analyze the fluence of operatg itions (hot and coolg ater temperature) on OP (coefficient of performance), (coolg capacity) and ed ater let temperature. KEY WORDS: Reneable Energy Sources, Silica Gel-Water, Mass Recovery, Adsorption hiller, oolg apacity and oefficient of Performance. INRODUION Over the past fe decades there have been considerable efforts to use adsorption (solid/vapor) for coolg and heat pump applications, but tensified efforts ere itiated only sce the imposition of ternational restrictions on the production and utilization of Fs and HFs. he severity of the ozone layer destruction problem due to Fs and HFs has been callg for rapid developments environment friendly air itiong technologies. Most of the advanced cycles adsorption refrigeration/heat pump are proposed to achieve high oefficient of Performance (OP) and/or oolg apacity () values. Fe cycles, hoever, are proposed to utilize relatively lo temperature heat source. Saha et al. [1] proposed to-stage er here the drivg heat source temperature as validated experimentally. A to-stage silica gel-ater adsorption refrigeration cycle can exploit the heat source of temperature around 60 0 ith the coolg source at Khan et al. [2] studied the performance vestigation on mass recovery three-bed adsorption cycle. Later, Khan et al. [3] proposed and vestigated numerically the advanced three-bed adsorption er employg mass recovery scheme. Saha et al. [4] studied aste heat driven dual-mode, multi-stage, multibed regenerative adsorption system. A novel adsorption er, namely, hree - bed adsorption er is also vestigated by Saha et al. [5] and shon that aste heat recovery efficiency of the three-bed system is ab 35% higher than that of the to-bed system. 13

2 Published by European entre for Research rag and Development UK (.eajournals.org) o improve the coefficient of performance, Shelton et al. [6] proposed a thermal ave regenerative adsorption heat pump system. Wang [7] shoed that mass recovery process is very effective for the high poratg pressure lift as ell as for the lo regeneratg temperature. Alam et al. [8] analyzed four-bed mass recovery cycle ith silica gel/ater pair employg a ne strategy to improve the coolg effect. Recently, Saha et al.[9] analyzed a dual-mode, multi-bed adsorption er to improve the heat recovery efficiency. he performance of the adsorption refrigeration cycle can be enhanced by applyg a mass recovery cycle to the adsorption cycle. he advanced mass recovery cycle as also applied to a three-bed cycle. Recently, Khan et al. [10] studied experimentally on a three-bed adsorption er and reported that it provided better OP values for heat source temperature. he Primary objective of the study is to determe the numerical result of a three-bed (unequal) adsorption er ith mass recovery. A cycle simulation computer program is constructed to analyze the fluence of operatg itions (hot and coolg ater temperature) on OP (oefficient of Performance), (oolg apacity) and ed ater let temperature. Workg Prciple of the Mass Recovery hiller he schematic diagram and time allocation of the proposed three-bed mass recovery er are shon Figure 1 and able 1, respectively. he three-bed mass recovery er comprises ith three sorption elements (adsorber/desorber heat exchangers), a enser, an porator, and metalic tubes for hot, coolg and ed ater flos as shon Figure 1. oolg ater Q losed Opened V 2 ondenser V 5 V 7 V: valve V 4 V 11 Hot ater V 10 oolg ater Q des Hot ater Hex1 V 9 Hex3 Evaporator V 3 V 8 Hex2 Q ads V 1 Q hilled ater V 6 Vapor Refrigerant Evaporator apillary ube Figure 1: Schematic of hree Bed hiller With Mass Recovery Operational strategy of the proposed er is shon able1. In proposed design, mass recovery process occurs all bed. o complete a full cycle for the proposed system, the er needs 20 modes, namely A, B,, D, E, F, G, H, I, J, K, L, M, N,O, P, Q, R, S and as can be seen from able 1. 14

3 Published by European entre for Research rag and Development UK (.eajournals.org) able 1: Operational Strategy of hree Bed hiller With Mass Recovery Mo de He x1 He x2 He x3 A B DE F G H I J K L M NO P Q R S Desorption Mass recovery ith heatg Pre-heatg Adsorption Mass recovery ith coolg Pre-coolg In mode A, Hex1 and Hex3 ork as desorber. he desorption-ensation process takes place at enser pressure (P). he desorber (Hex1, Hex3) is heated up to temperature (des) by heat put Qdes, provided by the drivg heat sources. he resultg refrigerant is cooled don by temperature () the enser by the coolg ater, hich removes ensation heat, Q. Hex2 orks as adsorber mode A. In the adsorption- poration process, refrigerant (ater) porator is porated at poration temperature,, and seized heat, Q from ed ater. he porated vapor is adsorbed by adsorbent (silica gel), at hich coolg ater removes the adsorption heat, Qads. Mode B is the pre-coolg process for Hex3. In pre-coolg process, Hex3 is isolated from porator, ensed or any other beds. oolg ater is supplied to the bed for short time (30s) this period. Hex1 orks as desorber and Hex2 orks as adsorber mode B also. Mode is the adsorption process for Hex3, Hex2 and desorption process for Hex1. In mode D, Hex3 (at the end position of adsorption-poration process) and Hex1 (at the end position of desorption-ensation process) are connected ith each other contug coolg ater and hot ater, respectively that can be classified as to-bed mass recovery process. his time Hex3 is isolated from porated and Hex1 is isolated from ensed. Here mass recovery occurs only bed to bed. In this mode Hex2 orks as adsorber. When the concentration levels of both beds Hex1 and Hex3 reach nearly equilibrium levels, then arm up process ill start, called mode E (pre-heatg or pre-coolg). In mode E, Hex2 and Hex3 are heated up by hot ater, and Hex1 is cooled don by coolg ater. When the pressure of Hex2 and Hex3 are nearly equal to the pressure of enser then Hex2 and Hex3 are connected to enser. When the pressure of Hex1 is nearly equal to the pressure of porator then Hex1 is connected to porator. In mode F, Hex2 and Hex3 ork as desorber and Hex1 orks as adsorber. Mode G is the pre-coolg process for Hex3. In this mode, Hex2 orks as desorber and Hex1 orks as adsorber. Mode H is the adsorptionporation process for Hex1 and Hex3. Hex2 orks as desorber this mode. In mode I, Hex3 (at the end position of adsorption-poration process) and Hex2 (at the end position of 15

4 Published by European entre for Research rag and Development UK (.eajournals.org) desorption-ensation process) are connected ith each other contug coolg ater and hot ater, respectively that can be classified as to-bed mass recovery process. When the concentration levels of both beds Hex3 and Hex2 reach nearly equilibrium levels, then arm up process ill start, called mode J (pre-heatg or pre-coolg). Hex1 orks as adsorber this mode. Mode J is the pre-heatg/pre-coolg process for all bed. In this period, Hex1 and Hex3 are heated up by hot ater; Hex2 is cooled don by coolg ater. Modes K, L and M are same as modes A, B and respectively. In mode K, L and M Hex1 and Hex3 ork as desorber and Hex2 orks as adsorber. he mode N is same as mode D. In these modes, Hex2 (at the end position of adsorption-poration process) and Hex1 (at the end position of desorption-ensation process) are connected ith each other contug coolg ater and hot ater respectively. In this mode Hex3 orks as adsorber. When the concentration levels of both beds Hex1 and Hex2 reach nearly equilibrium levels, then arm up process ill start, called mode O (pre-heatg or pre-coolg). he mode O is same as mode E. Modes P, Q and R are same as modes F, G and H respectively. In mode P, Q and R, Hex2 and Hex3 ork as desorber and Hex1 orks as adsorber. he mode S is same as mode I. In mode S, Hex1 (at the end position of adsorption-poration process) and Hex2 (at the end position of desorption-ensation process) are connected ith each other contug coolg ater and hot ater, respectively that can be classified as to-bed mass recovery process. When the concentration levels of both beds Hex1 and Hex2 reach nearly equilibrium levels, then arm up process ill start, called mode (pre-heatg or pre-coolg). Hex3 orks as adsorber this mode. Mode is the pre-heatg/pre-coolg process for all bed. In this period, Hex1 and Hex3 are heated up by hot ater; Hex2 is cooled don by coolg ater. Mode is the last process for all beds, after this mode, all beds ill return to its itial position (Mode A). hat s hy to complete one cycle, it needs 20 modes. MAHEMAIAL FORMULAION he heat transfer and energy balance equations for the adsorbent bed can be described as follos:,, U exp m A (1) d W...(2) s ps q W k pcu W f pal W Q s st dq W m s 1,, v dq here, is either 0 or 1 dependg hether the adsorbent bed is orkg as desorber or adsorber and is either 1 or 0 dependg on hether the bed is connected ith porator or another bed. he heat transfer and energy balance equations for porator can be expressed as: 16

5 Published by European entre for Research rag and Development UK (.eajournals.org) U,, exp m A p, (3) d W s m W (4), p, W, p, dq des, dq LWs ads he heat transfer and energy balance equations for enser can be ritten as:, (5) c, U A exp mc d W LW c, s dq des W W s, p, des p, dq des m c c, c, (6) he mass balance for the refrigerant can be expressed as: dw (7), dqdes Ws dq ads here, the subscripts des- and -ads stand for the vapor flo from desorber to enser and porator to adsorber, respectively. 17

6 Published by European entre for Research rag and Development UK (.eajournals.org) Measurement of the System Performance he performance of a three-bed adsorption er ith mass recovery is maly characterized by coolg capacity () and coefficient of performance (OP) and can be measured by the follog equations: oolg apacity () = m tcycle 0, t cycle, oefficient of Performance (OP) = m m hot tcycle 0 tcycle 0, hot, chil, hot, RESULS AND DISUSSION In the present analysis, a cycle simulation computer program is developed to predict the performance of the three-bed (unequal bed) er ith mass recovery. he systems of differential equations (1)-(7) are solved by fite difference approximation ith a time step 1 sec. In the numerical solution of the differential equations, successive substitutions of the nely calculated values ere used, ith the iterative loop repeatg the calculations until the convergence test is satisfied. he convergence factor for all parameters of the present study ill be taken as he base le parameters and standard operatg itions for the er operation are listed able. 2 and able. 3, respectively. 18

7 Published by European entre for Research rag and Development UK (.eajournals.org) able 2: Basele Parameters Values Adopted Simulation Symbol Value Unit A 1.45 m 2 A m 2 Acon m 2 ps 924 J/kg.K 4.18E+3 J/kg.K p, 4.20E+3 J/kg.K Dso 2.54E-4 m 2 /s Ea 2.33E+3 J/kg L 2.50E+6 J/kg Qst 2.80E+6 J/kg R 4.62E+2 J/kg.K Rp 0.35E-3 m Uads 1380 W/m 2. K Udes 1540 W/m 2. K U 3550 W/m 2. K U 4070 W/m 2. K Ws 14 kg Wc 5 kg p,cu 386 J/kg.K p,al 905 J/kg.K Wk kg Wf 5.33 kg W, 25 kg 19

8 [kw] International Journal of Mathematics and Statistics Studies Published by European entre for Research rag and Development UK (.eajournals.org) able 3: Standard Operatg ondition emperature [º] Flo rate (kg/s) Hot ater 50 ~ oolg ater [=0.2(ads)+0.34()] hilled ater ycle ime 2100s=(950 ads/ des+40 mr+30ph+30pc) s 2 Ads/des = adsorption/desorption, mr = mass recovery, ph/pc = pre-heat/pre-cool Effect of Drivg Heat Source emperature on and OP Fig. 2 and Fig. 3 sho heat source temperature variation on and OP, respectively. It is seen that for three-bed mass recovery er creases ith the crease of heat source temperature from 50 0 to 90 0 ith a coolg ater let temperature of his is because the amount of refrigerant circulated creases, due to creased refrigerant desorption ith higher drivg source temperature. he is improved due to the mass recovery process. he mass recovery process generates more desorption heat and that is transferred from the desorber through desorbed vapor. So, the lo heat source temperature ( ), proposed er gives better performance. he optimum OP value is for hot ater let temperature at 65 0 along ith the coolant and ed ater let temperature are at 30 0 and 14 0, respectively. he delivered ed ater temperature is 8 0 for this operation ition Heat Source emperature[ ] Figure 2: he effect of heat source temperature on 20

9 [kw] OP[-] International Journal of Mathematics and Statistics Studies Published by European entre for Research rag and Development UK (.eajournals.org) Figure 3: he effect of heat source temperature on OP Effect of oolg Source emperature On and OP Fig. 4 and Fig. 5 sho the effect of coolg ater let temperatures on and OP, respectively. In the present simulation, coolg ater mass flo rate to adsorber is taken as 0.2 kg/s, hile for the enser the coolant mass flo rate is taken as 0.34 kg/s. he creases steadily as the coolg ater let temperature is loered from 40 to his is due to the fact that loer adsorption temperatures result larger amounts of refrigerant beg adsorbed and desorbed durg each cycle. he simulated OP values also creases ith loer coolg ater let temperature. For the three bed er the OP value reaches ith 65 0 drivg source temperature combation ith a coolant let temperature of oolg ater let temperature[º] Figure 4: he effect of coolg ater let temperature on 21

10 [kw] OP[-] OP[-] International Journal of Mathematics and Statistics Studies Published by European entre for Research rag and Development UK (.eajournals.org) oolg ater let temperature[º] Figure 5: he effect of coolg ater let temperature on OP Effect of ycle ime on And OP and OP variations ith adsorption/desorption cycle time are depicted Fig. 6. he sensible heatg/coolg time is kept constant 30s. he highest values are obtaed for cycle time beteen 1800s and 2400 s. When cycle times are shorter than 900s, there is not enough time for adsorption or desorption, so decreases abruptly. On the other hand, hen cycle times are greater than 2400s, decreases gradually as the adsorbent approaches to its equilibrium ition. From the same Figure, it can also be observed that OP creases uniformly ith longer cycle time Mass recovery =40 hot =65 0 cool =30 0 ed = ycle ime[s] OP Figure 6: ycle time effect on and OP 22

11 hilled ater let temperature [ ] International Journal of Mathematics and Statistics Studies Published by European entre for Research rag and Development UK (.eajournals.org) Effect of Drivg Heat Source emperature on hilled Water let emperature he effect of heat source temperature on average ed ater let temperature is depicted Fig.7. he ed ater temperature level needs to be considered accordg to demand side requirement. Mass flo rate of ed ater can control the let temperature of ed ater. From Fig.7, it is seen that the cyclic average ed ater let temperature of the proposed cycle decreases ith the crease of the drivg heat source temperature. Lo ed ater let temperature is expected from real mache Heat Source emperature [ ] Figure 7: he effect of heat source temperature on ed ater let temperature ONLUSION A novel three-bed er (unequal bed) ith mass recovery scheme is proposed and the performances are luated by numerical technique. here is an creasg need for energy efficiency and requirement for the system driven ith lo temperature heat source. he follog concludg remarks can be dran from the present analysis: he ma feature of the proposed er is the ability to be driven by relatively lo temperature heat source. he er can utilize the fluctuated heat source temperature beteen 50 0 to 90 0 to produce effective coolg along ith a coolant let at oolg capacity of the proposed er is creased as heat source temperature is creased from 50 0 to 90 0 and coolg ater let temperature is decreased from 40 0 to he optimum OP value (0.6003) is obtaed for hot ater let temperature at 65 0 combation ith the coolant and ed ater let temperatures are 30 0 and 14 0, respectively. he delivered ed ater temperature is obtaed at

12 Published by European entre for Research rag and Development UK (.eajournals.org) Adsorption/desorption cycle time is very sensitive to the heat source temperature. he highest values are obtaed for cycle time beteen 1800s and 2400 s the present study. REFERENES [1] Saha BB, Akisaa A, Kashiagi. Solar/aste heat driven to-stage adsorption er : he Prototype. Reneable Energy 2001;23(4):pp [2] Khan MZI, Saha BB, Alam KA, Akisaa A, Kashiagi. Performance vestigation on mass recovery three-bed adsorption cycle. International onference onmechanical Engeerg 2005; pp [3] Khan MZI, Sultana S, Akisaa A, Kashiagi. Numerical simulation of advanced adsorption refrigeration er ith mass recovery. Journal of Naval Architecture and Mare Engeerg 2006;3(2):pp [4] Saha BB, Koyama S, Kashiagi, Akisaa A, Ng K, hua H. Waste heat driven dual-mode, multi-stage, multi-bed regenerative adsorption system. International Journal of Refrigeration 2003;26:pp [5] Saha BB, Koyama S, Lee JB, Kuahara K, Alam KA, Hamamoto Y, Akisaa A, Kashiagi. Performance luation of a lo-temperature aste heat driven multibed adsorption er. International Journal of Multiphase Flo 2003;29:pp [6] Shelton SV, Wepfer JW, Miles DJ. Ramp ave analysis of the solid/vapor heat pump. ASME Journal Energy Resources technology 1990;112:pp [7] Wang RZ. Performance improvement of adsorption coolg by heat and mass recovery operation. International Journal of Refrigeration 2001;24:pp [8] Alam KA, Akahira A, Hamamoto Y, Akisaa A, Kashiagi. A four-bed mass recovery adsorption refrigeration cycle driven by lo temperature aste/reneable heat source. Reneable Energy 2004;29:pp [9] Saha BB, Koyama S, Ng K, Hamamoto Y, Akisaa A, Kashiagi. Study on a dual-mode, multi-stage, multi-bed regenerative adsorption er. Reneable Energy 2006;31(13):pp [10] Khan MZI, Saha BB, Akisaa A. Experimental study on a three-bed adsorption er. International Journal of Air-onditiong and Refrigeration. 2011;19(4):pp