A Review on Performance Improvement of Adsorber Bed by Effective Heating and Cooling

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1 A Review on Performance Improvement of Adsorber Bed by Effective Heating and Cooling 1 Ameya C. Lohokare, 2 V. N. Kapatkar 1,2 Sinhgad College of Engineering 1 ameya.c.lohokare@gmail.com, 2 vnkapatkar.scoe@sinhgad.edu Abstract Adsorption refrigeration system is used due to the need of waste heat recovery and due to use of harmful refrigerants in available refrigeration systems which are cause of global warming and ozone layer depletion. This system can work on low grade energy source like waste heat from engine exhaust, boilers or solar energy. It has applications in automobile air conditioning, ice making and water chillers. In Adsorption refrigeration, adsorber bed acts as compressor. The main drawback of adsorber is its low thermal conductivity. The objective of the study is to understand different designs of an adsorber beds and adsorption refrigeration systems which will have effective heating and cooling during desorption and adsorption. Index Terms Adsorption, Adsorber Bed, Desorption, SCP (Specific Cooling Power) I. INTRODUCTION Adsorption refrigeration technology has gained interests of researchers because the problem of global warming is reached to an unbearable extent. Vapor compression refrigeration cycles (VCRC) are the most popular type of refrigeration systems in which different refrigerants such as chlorofluorocarbons (CFCs), hydro chlorofluorocarbons (HCFCs), and hydro fluorocarbons (HFCs) are used. Ozone depletion and global warming resulting from such refrigerants are direct environmental impacts of VCRCs. The adsorption refrigeration system has several advantages compared to the absorption refrigeration system like Wide range of operating temperatures, No crystallization issue, Suitability for application where serious vibration occurs [1]. Adsorption is the general phenomenon based on a physical or chemical reaction process, resulting from the interaction between a solid (adsorbent) and a gas/liquid (refrigerant). Adsorption can be divided into physical adsorption, which is caused by the Van der Waals forces and chemical adsorption, in which a chemical reaction is involved. It is an exothermic process due to the gas liquid phase change. The energy liberated depends on the nature of the adsorbent adsorbate pair and is called the isosteric heat. The atoms, molecules or ions in a liquid are diffused to the surface of a solid in a liquid phase adsorption. They are bonded with the solid surface or are held by weak intermolecular forces [2]. adsorbent and silicate have more affinity for oil and gases than that for water and these substances are termed hydrophobic. Adsorption is the term for the enrichment of gaseous or dissolved substances on the boundary surface of a solid. The active centres on their surface adsorbents are where the binding forces between the individual atoms of the solid structure are not completely saturated. At these centres, an adsorption of foreign molecules takes place. To understand the adsorptive solvent recovery, some fundamentals of adsorption and desorption must be considered, as shown in Fig. 1 [2]. Fig. 1 Fundamentals of adsorption and desorption [2]. The design of adsorber bed is critical task in adsorption refrigeration. Adsorbents used for it has low thermal conductivity. Various mathematical models are developed by different experimentations. It is found that adsorption and desorption are functions bed temperature. Many researchers are trying to improve bed design and its heat transfer performance characteristics by composite adsorbents. Different nano-fluids are under study as an refrigerant for such beds. Use of different types of fins is also important way of improving the bed performance. It is possible to improve SCP by Improving thermal conductivity of working pairs by using new synthetic materials such as carbon nanotubes embedded in zeolite, Increasing effective surface area of adsorber bed to increase the heat transfer rate between the exhaust gas (waste heat) and the adsorbent particles, Reducing cycle time by increasing adsorbate diffusion rate within the Adsorbents like silica gel, zeolite and porous or active alumina have special affinity with polar substances like water. These adsorbents are termed hydrophilic. Nonpolar adsorbents such as activated carbon, polymer 109

2 adsorbent particles during the adsorption/desorption processes. II. LITERATURE REVIEW Sharafian and Bahrami (2014) [1] studied different adsorber bed designs for vehicle air conditioning. To investigate the available studies in the open literature, desired SCP of 350 W/kg dry adsorbent and adsorber bed to adsorbent mass ratio of less than one are calculated for a 1-ton-of-refrigeration, 2-adsorberbed, silica gel water ACS. They proposed the finned tube adsorber bed design, and after optimization fin spacing and fin height and enhancing thermal conductivity of adsorbent material by adding metal wool inside the finned tube adsorber bed are proposed as the practical solutions to increase heat and mass transfer rates within the adsorber bed. T.H.C Yeo et al. (2012) [2] reviewed the adsorption airconditioning technology using modified activated carbon. One of the difficulties which prevent the improvement of the adsorption refrigeration technology using activated carbon is the use of the readily available commercial activated carbon without prior treatment, which has resulted in relatively lower performance. The activated carbon treated by oxidation had higher adsorption property on Cr (VI) than that of the raw activated carbon. Oxidative treatment of activated carbon was very favourable for enhancing uptakes of metal ions while thermal treatment of activated carbon was generally favourable for enhancing adsorption of organic compounds from aqueous solutions. In ASHRAE Journal 2011, Kai Wang et al. [3] explained adsorption refrigeration with different working pairs for applications like ice-making chilled water and air conditioning. Physical, chemical and composite adsorbents are elaborated X.H. Li et al. (2015) [4] studied and reviewed the various methods to improve the performance of adsorber bed and adsorption system. Performance of adsorption system is dependent on its size and shape. Increasing the heat transfer area can effectively reduce the general transfer resistance of the bed. Popular methods to enlarge the transfer area include the plate-finned bed, the spiral plate bed, and the pin-fin bed. Incorporating the heat pipe into the adsorption bed causes the cycling time to decline and the coefficient of performance (COP) to improve. It is been studied that to improve the heat transfer performance is possible by reducing the contact resistance between the wall and the adsorbent and Reducing the thermal resistance of adsorbent itself. Performance of system can be improved by incorporating advancements in adsorption cycles. Wang R. Z. et al (2010) [5] studied the use of low grade thermal energy for adsorption refrigeration. Different methods like Heat Pipe technology and use of extended surfaces are reviewed. Two ways of shortening the cycle time are suggested in which one is to intensify the heat transfer performance, and another is to improve the mass transfer as the kinetics in the sorption bed influences the adsorption or desorption For ammonia refrigerant usually working at high pressure, the mass transfer for an activated carbon bed is not very critical. However, if metal chlorides are used as the adsorbent, agglomeration of the salt occurs in the process of adsorption, which affects negatively the mass transfer performance Hamid Niazmand et al. (2012) [6] developed A transient three-dimensional numerical scheme considering both inter-particle and intra-particle mass transfer resistances to examine the performance of silica gel/ water adsorption chiller. Bed geometrical specifications, like different shapes of fins, fin height, fin spacing, were studied and results were plotted. A design procedure is proposed to configure the adsorbent bed specifications for a given application assuming that SCP and COP are specified parameters based on practical considerations. It is shown that modelling part of the bed including just three fins can well predict the performance of entire bed with 50 fins. It was found that relatively large SCP can be obtained only by annular fins with smaller fin spacing of 3 mm. While for the same working conditions if higher COP is desired, annular fins are not superior to the square fins, although, heat exchangers with square fins are more cost effective. Zhang and Wang (1997) [7] performed numerical study on dynamic performance of an adsorption cooling system for automobile heat recovery. It is studied the effects of various operating temperature and overall heat transfer coefficient on system performance. It is found that improving the overall heat transfer coefficient is most effective way to obtain increased SCP. Huang Hongyu et al. (2014) [8] investigated effect of particle diameter on effective thermal conductivity The heat transfer coefficient of the refrigerant and the void rate of the adsorbent layer can also affect the effective thermal conductivity of adsorbents. The performance of mass transfer in the adsorber is better when pressure drop decreases. Pressure drop decreases with increasing permeability. The permeability of the adsorbent layer can be improved with increasing adsorbent diameter. Effective thermal conductivity can remain stable without 110

3 obvious changes at adsorbent diameters of 305, 390, 513 and 605 μm. Effective thermal conductivity obviously decreases at an adsorbent diameter of 700 μm. Permeability increases with increasing adsorbent diameter and void ratio. The permeability at an adsorbent diameter of 700 μm is obviously higher than that at other adsorbent diameters. Output power initially increases and then decreases with increasing adsorbent diameter under different cycle time conditions. Output power increases with decreasing cycle time under similar diameter conditions. III. BASIC ADSORPTION REFRIGERATION CYCLE A basic adsorption cycle consists of four steps as shown in fig 2. heating and pressurization, desorption and condensation, cooling and depressurization, and adsorption and evaporation. In the first step, the adsorber is heated by a heat source at a temperature of T H. The pressure of the adsorber increases from the evaporating pressure up to the condensing pressure while the adsorber temperature increases. This step is equivalent to the compression in the vaporcompression cycle. In the second step, the adsorber continues receiving heat and its temperature keeps increasing, which results in the desorption (or generation) of refrigerant vapor from adsorbent in the adsorber. This desorbed vapor is liquefied in the condenser and the condensing heat is released to the first heat sink at a temperature of T C. This step is equivalent to condensation in the vapor-compression cycle. At the beginning of the third step, the adsorber is disconnected from the condenser. Then, it is cooled by heat transfer fluid at the second heat sink temperature of T M. The pressure of the adsorber decreases from the condensing pressure down to the evaporating pressure due to the decrease in the adsorber temperature. This step is equivalent to the expansion in the vaporcompression cycle. In the last step, the adsorber keeps releasing heat while being connected to the evaporator. The adsorber temperature continues decreasing, which results in the adsorption of refrigerant vapor from the evaporator by adsorbent, producing the desired refrigeration effect. This step is equivalent to the evaporation in the vapor-compression cycle [3]. Fig. 2 Basic adsorption refrigeration system A. Heating and pressurization. B. Desorption and condensation. C. Cooling and depressurization. D. Adsorption and evaporation. [3] The basic adsorption refrigeration cycle is an intermittent system and the cooling output is not continuous. A minimum of two adsorbers are required to obtain a continuous cooling effect (when the first adsorber is in the adsorption phase, the second adsorber is in desorption phase). These adsorbers will sequentially execute the adsorption-desorption process [3]. IV. TYPES OF ADSORBER BEDS Different types of adsorber beds are used in adsorption refrigeration systems. Some of them are reviewed here. A. Plate-Finned Bed These structures improve the heat transfer of the bed due to the fin enlarges the specific transfer area per unit volume and shorten the heat transfer path. In addition to the good heat transfer effect, the plate-finned bed also has the advantage of compact structure and less heat loss. However, the large heat capacity ratio of the metal fin and the tube over the adsorption material will impose a non-ignorable negative effect to the COP and the specific cooling power of the system. The results showed that the performance of the machine with fins was higher than the one without fins. The maximal 111

4 temperature in the adsorber with fins attained 97 C, while for the one without fins it reached 77 C only. The COP was increased from to correspondingly [4]. B. Spiral Plate Bed As a type of compact adsorption bed, the spiral plate bed is of efficient heat transfer rate and relatively small volume. The sandwich structure results in the uniform temperature difference between the thermal fluid and the adsorbent material, and then generates high heat flux. With the high efficiency of performance, the bed will not demand very high temperature to drive as the ordinary bed does. This is highly consistent with the tendency of full use of the low level energy such as the industrial waste heat. Meanwhile, the spiral plate bed also has the advantage of simple structure, low manufacture cost, and easy to machine. However, for such bed the working temperature and the pressure cannot be too high. Sealing and repairing the bed body is relatively difficult. More efforts are also needed to fill the adsorbent into the bed [4]. C. Porous Bed Such type of bed includes many connected holes and tunnels to facilitate the adsorbate vapour to flow through, and then is able to reduce the mass transfer resistance. The porous bed has the good feature of high heat and mass transfer, the low thermal contact resistance with the metallic sealing wall, and the low specific heat capacity. The processing technology of the porous bed is complex and the cost is high. With much more pores and more specific surface area than usual materials, the new material can trap three to four times of water vapour than that of the silica gel. As a result, substituting the silica gel with such nano-structured material will reduce the size of the cooling system by 75%. Furthermore, the binding energy of the material to the water molecule is not so strong that the needed heat for the material to desorb will be less consumed. All these features promote the adsorption cooling system to perform efficiently [4]. D. Pin-Fin Bed Comparatively, the expanded area of the pin-fin bed is more than that of the plate-fin bed. The enlarged area reduces the overall transfer resistance and improves the uniformity of the temperature field. The heat transfer enhancement mechanism of the pin-fin bed is making use of the rich secondary extended surface of the acupuncture to enhance the heat transfer. Compared with the plate-fin at the same mass weight, the surface area of the pin-fin is increased by 22.36%. Therefore, the pinfin bed is more compact, and less additional cooling/heating load of the bed is resulted. In addition, as the adsorbent is directly filled into the acupuncture space, the density of the adsorbent for a given volume is also improved at the same time of the heat transfer enhancement. On the other hand, matching the thermal resistance on both sides of the wall is significant to keep the heat transfer in consistency. Therefore, the channel for the heating/ cooling fluid on the back of the wall should also be taken into account of the enhancement [4]. E. Effect of Fins in Adsorption Fig. 4 compares the time variations of the averaged adsorbed amount for beds with annular and square fins. Desorption process occurs faster for bed with annular fins as compared to square fins with identical length scale, which is consistent with the bed averaged temperature variations in Fig. 3 [6]. Fig. 3 Comparing the time variations of averaged bed temperature [6] Fig. 4 Comparing the time variations of the averaged adsorbed amount for annular and square finned beds [6] Fig. 5 The variations of COP as a function of fin spacing for different fin heights for both annular and square beds of identical length scale [6] 112

5 Fig. 6 The variations of SCP as a function of fin spacing for different fin heights for both annular and square beds of identical length scale [6] The COP increases as fin height increases at a given fin spacing, since the amount of adsorbent material is directly related to the produced cooling energy. Fig. 5 shows that COP is less sensitive to the fin spacing as compared to the fin height. This can be attributed to the fact that the fins are very thin and increasing the fin spacing slightly increases the total mass of adsorbent material and decreases the energy needed to heat and cool the fins. Similarly, the system SCP is expected to decrease as the fin height increases since both the cycle time and the solid adsorbent mass increase as shown in fig. 6. On the other hand, the increase in solid adsorbent mass is accompanied by an increase in the cooling power of the system that has a positive effect on the SCP [6]. F. Effect of Adsorber Bed Regeneration Temperature Fig. 7 Influence of the T g on system performance: T c = 45 C; T e = 10 C; T ad = 50 C; T hfi =450 C; T cfi =35 C [7] Fig. 8 Influence of the UA on system performance: T c = 45 C; T e = 10 C; T hfi =450 C; T cfi =35 C. W max =0.23 W min =0.02 [7] The influence of the maximum regenerating temperature T g on system coefficients are shown in Fig. 7. It is seen that the COP, SCP and the Coefficient of waste heat cooling (WCOP) are increased rapidly, reaches a maximum value and then is decreased gradually with an increase in T g, while the coefficient of waste heat recovery (WCOE) drops continuously. Increasing the T g results in more refrigerant desorbed, but it is also leads to more sensible heat losses from the adsorber. And the step of increase becomes smaller when the T g goes beyond the optimum value, and the sensible heat losses become substantial. The effects of the overall heat transfer coeffiecient UA on system performance are analyzed and shown in Fig. 8. The SCP increases abruptly, and then gradually reaches a stable value with an increase in UA. In this particular case, for UA between 1 and 50 kwm -3 K -1, the SCP rises from 50 to 370 W/kg; for UA beyond 50 kwm -3 K l, the SCP becomes stable, and there would seem to be little merit in achieving a higher SCP by an further increase in UA. This is called turning point the threshold, and the value of UA at this point the threshold value. Increasing the overall heat transfer coefficient is highly effective in the ranges below the threshold value, which in this case is 50 kwm -3 K -1 [7]. V. EXPERIMENTAL SET UP Fig. 9 shows the experiemental set up to test adsorber beds for their performance. Table 1 and Table 2 show the component list and specifications respectively. Table 3 gives dimensions of adsorber beds. Fig. 10 and Fig. 11 show the front and side cut section views of adsorber bed. This adsorber bed is cylindrical in structure. At its centre there is a stainless steel pipe carrying the hot fluid. This pipe has radially outward extended small steel pipe surfaces. These are provided in order to get more heat transfer surface for hot fluid. This will help for better temperature distribution in bed. Also cylindrical cooling water flow pipes are provided for cooling of adsorber bed for adsorption phase. In between outer casing and inner pipes cylinder is filled with 113

6 activated carbon (charcoal) forming a porous bed. R134a will be passed through this activated carbon bed. Fig. 10 Cut Section Front View of Adsorber Bed Fig. 9 Schematic of Experimental Set up Table 1 Component List Part No. Description 1:1-A 1-B Adsorber Beds 2 Condenser 3 Thermostatic Expansion Valve 4 Evaporator 5 Radiator 6: 6-A, 6-B, 6- Pressure Gauges C, 6-D 7: 7-A, 7-B, 7- Flow Control Valves (Solenoid C, 7-D, 7-E, 7- F, 7-G Valve) 8 Hot Fluid Supply Table 2 Component Specifications Components Condenser Evaporator Thermostatic Expansion Valve Pipes Hot fluid (Source Temperature Range) Cold Water Temperature Refrigerant Adsorbent Material Specifications 390 x 290 x 16 Capacity: 8.5 kw 240 x 240 x 58 Capacity: 3 kw Capacity: 1 TR Liquid Pipes: Dia. 8mm Suction Pipes: Dia. 16mm Discharge Pipes: Dia. 12mm Material: Stainless Steel 100 C to 200 C Ambient (30 C) R134a Activated Carbon (Charcoal) Fig. 11 Cut Section Side View of Adsorber Bed Table 3 Adsorber Bed Design Specifications Adsorber Bed Parameter Hot Fluid Pipe Dia: Outer Stainless Steel Casing Dia: Cold Water Dia: Extended Pipe Dia : Extended Pipe Length: Adsorber Bed Length: Dimensions 30mm 80mm 8mm 8mm 20mm 450mm This adsorber design would give better heating and cooling effect compared to one without extended surface without change in its outer dimension. VI. CONCLUSION Adsorption refrigeration system despite having low COP, is useful means of waste heat recovery. This system has scope in numerous applications and can be effectively implemented. Performance of this system can be improved by proper selection of working pair and also by controlling factors like operating temperatures and overall heat transfer coefficients. Selection of working pair is dependent on available heat source, temperature range and type of application. 114

7 Use of multiple adsorber beds can increase COP of adsorption refrigeration. It is also found that heat and mass recovery increase COP and SCP of system. Adsorber bed acts as a compressor so adsorber bed design is an important issue as it has low heat transfer rate. Relatively large SCP can be obtained only by annular fins with smaller fin spacing. While for the same working conditions if higher COP is desired, annular fins are not superior to the square fins, although, heat exchangers with square fins are more cost effective. COP is less affected due to the fin spacing as compared to the fin height. Increase in fin height requires the heat to be transferred through longer distances; the cycle time increases. More studies may still be needed on the related topics such as the space-occupying, the costly manufacturing, the costly operation, the long-time of the investment recovery, and the reliability of the system. ACKNOWLEDGEMENT I offer my sincere thanks to Mr. Dwijendra Mani, AGM, Tata Motors for technical support. I am thankful to Prof. Dr. Y. P Reddy, and Prof. Dr. S. D. Lokhande, Principal for their support. REFERENCES [1] Amir Sharafian, Majid Bahrami, Assessment of adsorber bed designs in waste heat driven adsorption cooling systems for vehicle air conditioning and refrigeration, Renewable and Sustainable Energy Reviews 30,pp , [2] T.H.C. Yeo, I.A.W. Tan, M.O. Abdullah, Development of adsorption air-conditioning technology using modified activated carbon A review, Renewable and Sustainable Energy Reviews 16, pp , [3] Kai Wang, Edward A. Vineyard, New opportunities for solar adsorption refrigeration, ASHRAE Journal, pp 14-24, [4] X.H. Li, X.H. Hou, X. Zhang, Z.X. Yuan, A review on development of adsorption cooling Novel beds and advanced cycles, Energy Conversion and Management 94, pp , [5] Wang R.Z., Xia Z.Z., Wang L.W., Lu Z.S., Li S.L.,Li T.X., Wu J.Y.He S, Heat transfer design in adsorption refrigeration systems for efficient use of low grade thermal energy, Proceedings of the 14th International Heat Transfer Conference, pp 1-15, [6] Hamid Niazmand, Hoda Talebian, Mehdi Mahdavikhah, Bed geometrical specifications effects on the performance of silica/water adsorption chillers, International journal of refrigeration 35, pp , [7] Li Zhi Zang, Ling Wang, Performance estimation of an adsorption cooling system for automobile waste heat recovery, Applied Thermal Engineering Vol. 17 No. 12, pp , [8] Huang Hongyu, HE Zhaohong, Yuan Haoran, Kobayashi Noriyuki, Zhao Dandan, Kubota Mitsuhiro, Guo Huafang, Effect of Adsorbent Diameter on the Performance of Adsorption Refrigeration, Chinese Journal of Chemical Engineering, 22(5), pp ,