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Performance evaluation of a zeolite–water adsorption chiller with entropy analysis of thermodynamic insight
Applied Energy 130 (2014) 702–711
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Applied Energy journal homepage: www.elsevier.com/locate/apenergy
Performance evaluation of a zeolite–water adsorption chiller with entropy analysis of thermodynamic insight Ang Li, Azhar Bin Ismail, Kyaw Thu, Kim Choon Ng ⇑, Wai Soong Loh Department of Mechanical Engineering, National University of Singapore, 9 Engineering Drive 1, Singapore 117576, Singapore
h i g h l i g h t s An adsorption chiller using novel adsorbent Zeolite FAM Z01 is presented. Chiller’s highest COP was achieved at low grade heat source of 65 °C. Second law equations were developed to study the chiller’s irreversibilities. Entropy generation against heat source temperature and cycle time is presented. ‘Specific entropy generation’ is used to reveal the trend of the chiller’s COP.
a r t i c l e
i n f o
Article history: Received 9 November 2013 Received in revised form 11 January 2014 Accepted 25 January 2014 Available online 26 February 2014 Keywords: Adsorption chiller Zeolite–water pair Entropy analysis Specific entropy generation
a b s t r a c t This paper presents an environment-friendly adsorption chiller using Zeolite FAM Z01–water pair as opposed to the conventional silica gel and water pair. The adsorbent, zeolite, is thinly coated onto the surfaces of fin-tube heat exchanger for faster rates of heat and mass transfer. Another feature of the adsorption chiller is the use of a lever-countered weighted valve which can be open or closed by the pressure difference between the reactors and the condenser or evaporator. Experiments are conducted to evaluate the performance of zeolite-based chiller in terms of total heat input, cooling capacity, and coefficient of performance (COP) with respect to heat source temperature and adsorption/desorption cycle time where an optimal operational zone can be determined: (i) hot water inlet temperatures range from 65 °C to 85 °C, (ii) adsorption/desorption cycle times of 200–300 s at optimum cooling and COP, Entropy analyses have been conducted to understand the irreversibility contributed by both the desorption and adsorption beds at assorted hot water inlet temperatures and cycle time. Ó 2014 Elsevier Ltd. All rights reserved.
1. Introduction In recent decades, increasing cooling demand in the industrial and residential sectors aggravates energy consumption leading to a corresponding deterioration of environment from higher fossil fuel utilization. Cooling by the conventional vapor compression chillers consumes much electricity at 0.8–1.2 kW h/Rtons. Improvements to energy efficiency of key chiller components have been reported to have reached their asymptotic peaks, and huge investment is needed for only marginal improvements of chiller’s energy efficiency. An alternative method to improving energy efficiency is to focus on the development of thermally-driven cycles that can be powered by low temperature waste heat which is available in abundance from exhaust of industrial processes or from ⇑ Corresponding author. Tel.: +65 65162214; fax: +65 65161459. E-mail address: [email protected] (K.C. Ng). http://dx.doi.org/10.1016/j.apenergy.2014.01.086 0306-2619/Ó 2014 Elsevier Ltd. All rights reserved.
renewable energy sources such as solar or geothermal heat [1]. Concerning such issues, scientists and engineers have studied green alternative solutions, and amongst such approaches, the adsorption cycle has been mooted as one of the most attractive technologies [2,3]. Being low-grade waste heat driven, maintenance free and environment benign are main advantages of adsorption chillers [4–8]. A proper designed such system operates in the heat source temperature as low as 55 °C. No major moving parts present in the chiller structure, requesting only minimum maintenance. High durability of adsorbent material added as a plus for its long lasting performance. Furthermore, the green nature of the technology comes from the utilization of non-ozone-depleting refrigerants and waste heat by which no additional CO2 is emitted. Extensive work has been conducted to study the adsorption cooling systems. The system can be constructed by implementing various adsorbent–adsorbate pairs, for example, silica gel–water [9], zeolite–water [10–12], activated carbon–ammonia [13] or
A. Li et al. / Applied Energy 130 (2014) 702–711
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Nomenclature Abbreviations ADS adsorption bed Cond condenser COP coefficient of performance DES desorption bed Evap evaporator FAM Z01 Functional Adsorbent Material, Zeolite Z01
t tcycle Sgen U Ws h d
time (s) cycle time (s) entropy generation (kJ/K) overall heat transfer coefficient (kW/m2 K) power input of spray pump (kW) operation indicator (–) operation indicator (–)
Symbols A Cp Dso Ea M _ m P QE QH Qst R Rp q q T
Subscript abe ads c chi cw des e f g hw hx i o zl
adsorbed phase of adsorbate adsorption bed condenser chilled water cooling water desorption bed evaporator liquid phase gaseous phase hot water heat exchanger inlet outlet zeolite
area (m2) specific heat capacity (kJ/kg K) pre-exponential factor for surface diffusion (m2/s) activation energy (kJ/kmol) mass (kg) mass flow-rate (kg/s) pressure (Pa) cooling capacity (kW) heat input (kW) heat of adsorption (kW/kg) universal gas constant (kJ/kmol K) average radius of adsorbant particles (m) adsorbate uptake (kg/kg of adsorbent) equilibrium adsorbate uptake (kg/kg of adsorbent) temperature (K)
R134a [14], in which the latter substances play the role of refrigerant. A multistage design from Saha et al. [15] effectively brought down the minimum heat source temperature to near ambient from a single stage type silica gel–water adsorption chiller. Saha and coworkers experimentally tested a three stage [16] and later a two stage chiller [17], and achieved 50 °C heat source temperature from the former, and 55 °C from the latter with 30 °C cooling water in both situations. Saha et al. [18], Wang et al. [19], Alam et al. [20], and Ng et al. [21] investigated operational strategies of multi-bed adsorption systems. Chen et al. [22] tested an adsorption chiller that eliminates the vacuum valves in the structure. Shahzad et al. [23] and Thu et al. [24] creatively hybridized adsorption chiller technology with multi-effect desalination (MED), which break through the conventional MED lower temperature limit to below ambient condition. Simulation and numerical studies [25–28] on the adsorption refrigeration systems at the same time helped to establish theoretical frame work, and explore the behavior of such machine in extreme as well as optimal conditions. Given the research activities conducted in the adsorption cooling field, Choudhury et al. [29] comprehensively summarized the development of this technology from three aspects, i.e., thermal energy harvesting, heat and mass transfer enhancement, and lastly advanced cycles and stages. However, the vast application of this green technology is bottlenecked by low coefficient of performance (COP) and relatively larger foot-print. The existing adsorption chillers operate well below the theoretical Carnot limit. Meunier et al. [30] explained the reason through thermodynamic second law analysis. Meunier and coauthors suggested that in the conditions of isothermal heat reservoirs and ideal transfer, a temperature gap exists between the reactor sorption cycles and heat reservoirs, which is responsible for the thermal irreversibilities. Even infinite cascades of sorption reactors yield no more than 68% of the ideal efficiency [31]. Chua et al. [32] pointed out that the largest portion of entropy is generated by sorption heat transfer. Ng [33] has suggested that the COP of such thermally driven adsorption systems is normally lower than 1. On the other hand, the size of the chiller is bounded by
the characteristics of the adsorbent–adsorbate pair. In order to achieve desired cooling capacity, the amount of evaporated refrigerant must be adsorbed simultaneously by the solids. This requires large amount of adsorbent and sufficient void space to be introduced into reactors. This paper presents an adsorption chiller using a novel adsorbent, FAM Z01 Zeolite (composition: FexAlyPzO2nH2O, x = 0.02– 0.10, y = 0.35–0.5, z = 0.4–0.6, n = 0–1), with water as adsorbate. The chiller is equipped with adsorbent coated reactor heat exchangers and lever mechanism valves. The performance of the system is evaluated experimentally with respect to various parameters, namely, adsorption/desorption cycle time and hot water inlet temperature. A further investigation of the thermodynamic insight of the chiller is carried out by entropy analysis.
2. System characteristics 2.1. Adsorbent–adsorbate pair The adsorbent introduced in the current work, namely Functional Adsorbent Material (FAM Z01), is a novel aluminophosphate based water sorption zeolite developed by Kakiuchi et al.[34]. The material has a one-dimensional AFI type molecular sieve structure with a molecular window of diameter 0.73 nm. Only molecules smaller than this dimension, such as water molecules, are permitted free migration though the window. FAM Z01 owns S-shape water vapor sorption isotherms with excellent durability over 200,000 adsorption–desorption cycles and little hysteresis behavior. Sorption takes place dramatically in a narrow range of relative pressure (defined as the ratio of sorption pressure to the saturation pressure of vapor at the same temperature), while out of which the amount of vapor uptake possesses little changes with respect to the relative pressure [35]. This translates to a much larger adsorption capacity over conventional silica gel at normal chiller operation conditions, and further results in a compact chiller design. On the other hand, owning big latent heat, being absolutely
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environmental friendly and inexpensive, water has its intrinsic advantages to be a refrigerant. 2.2. Operation description The present adsorption chiller comprises of four major components, including two reactor beds, a condenser and an evaporator, as shown in Fig. 1. The zeolite is constructed stationary onto heat exchangers of both beds. Due to a lack of mobility of these solid adsorbent, a batch-wise operation is designed for the chiller to achieve continuous cooling, in which the reactors alternate their roles to perform adsorption and desorption. Adsorption bed (ADS) and desorption bed (DES) are named on the respective reactor depending on its ongoing function. A complete cooling cycle consists of two adsorption/desorption periods and two successive switching periods. In the former one, the adsorption bed interacts only with the evaporator. A spontaneous mass transport takes place as water molecules move from gaseous state to a lower energy and more thermodynamically stable adsorbed phase [36,37]. As a result, heat of adsorption is released. To maintain this process, the temperature of the zeolite must remain low. This is achieved by flushing cooling water into the ADS heat exchanger to carry away the rejected heat. In the evaporator, vapor transport evacuates the pressure. Boiling of refrigerant is then triggered through which chilling effect is produced. Simultaneously, DES reactor that has been saturated with adsorbed phase water is coupled with the condenser. Heat source is supplied to desorb the attached water molecular from DES and drive it to the condenser. The vapor condenses to liquid phase and flows back to the evaporator via a U-tube. Cooling water from the ADS outlet is directed to the condenser for removal of the latent heat of condensation. Upon completion of sorption process, switching period starts, in which the two reactor beds swap the role by changing the hot water and cooling water flow paths. The connections between both reactor beds to the condenser or the evaporator are shut and no mass transport occurs. Throughout the operation the refrigerant water circulates among the components at sub-atmospheric pressures.
exchanger fins of the reactor beds by using a special organic binder, instead of being packed among heat exchanger fins by metal meshes in the conventional method. The motivation of using the coating method is to promote heat transfer. Poor contact among these metal surface and solids that is exposed in the conventional method causes very big thermal resistance between thermal source and adsorbent, resulting in a large swing of the chiller performance due to adsorbent packing density [38]. By introducing binder, the new method well addresses this bottleneck. Fig. 2a and b show microscopic photos of pure FAM Z01 and FAM Z01 with organic binder, respectively, taken by Field Emission Scanning Electron Microscopy (FESEM) method. Clearly, the binder material seals up the space between the fins and the adsorbent solid, as well as among the adsorbent itself. The temperature difference of adsorbent and the hot or cooling water is extensively reduced. Furthermore, the mesh material is eliminated in the reactor. This considerably diminishes the thermal mass of the bed as well as heat loss to these materials. These improvements contribute to a higher overall heat transfer efficiency and a shorter cycle time. The thermophysical properties of FAM Z01 with and without binder are tabulated in Table 1.
2.3. Structural innovation There are two innovative features found in the design of the current reactor beds. First of all, the zeolite is coated onto the heat
Fig. 1. Schematic presentation of the zeolite–water adsorption chiller in adsorption/desorption period (ADS: adsorption bed; DES: desorption bed).
Fig. 2. Field Emission Scanning Electron Microscopic (FESEM) photos of FAM Z01 zeolite without (a) and with (b) organic binder.
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A. Li et al. / Applied Energy 130 (2014) 702–711 Table 1 Thermophysical properties of FAM Z01 with and without organic binder. Adsorbent
BET surface area (m2/g)
Micropore volume (cm3/g)
Average pore diameter (Å)
FAM Z01 FAM Z01 with organic binder
147.3 41.15
0.071 0.035
19.29 34.24
Secondly, the system utilizes a ‘‘valve-less’’ mechanism between the reactors and the evaporator or condenser instead of butterfly valves used in typical adsorption chillers. This mechanism is basically a counter-weight lever whose opening and closing are triggered by pressure difference of adjacent chambers. For instance, in the adsorption/desorption period, the pressure in the adsorption bed is lower than evaporator due to sufficient cooling. The pressure difference creates lifting force that effectively pushes the lever up and starts the mass transport. In the case of saturation or swapping to desorption by flushing in the hot water, pressure of the bed will increase and shut the connection to the evaporator. The same principle is implemented between the DES bed and the condenser. Because of the low density of the refrigerant vapor, the jointed components require large cross-section pipes to reduce pressure drop. The ‘‘valve less’’ design saves the space taken up by conventional pipes and valves, and further eliminates the valve control panel and accessories. Hence, the structure of the adsorption chiller system is greatly simplified. 3. Governing equations The irreversibilities involved in the phase change of the refrigerant, its interaction with the solid adsorbent as well as the exterior cooling and heating source during the thermodynamic process result in the generation of extra entropy [39]. Moreover, temperature variation of the adsorbent and heat exchanger materials cannot be simply neglected as a significant amount of heat is reserved or emitted from them due to batch-wise operation, especially at the beginning of the adsorption/desorption cycle time. The analysis below provides the entropy calculation for the adsorption/ desorption cycle times. Assuming the component to be analyzed a block box, the equations are written in a way that considering the inlet and outlet conditions, as well as the initial and final states of the substance inside the component. Heat loss to the surrounding is omitted due to the fact that the major components are well insulated, and the hot object is wrapped by heater tapes. During the adsorption process, by choosing the control volume to be inside the boundary of the reactor bed, the instantaneous entropy generation of the ADS bed can be written as: ads
dSgen ½ðMCpÞhx;ads þ ðMCpÞzl þ Mzl qads Cpabe dT ads ¼ T ads dt dt dqads ½sg ðT ads ; Pe Þ sg ðT e Þ þ dMzl dt dqads Q st;ads ðT ads ; Pe Þ _ o si Þcw;ads þ ½mðs M zl dt T ads
the evaporator. In the beginning of each adsorption/desorption period, a backward flow of refrigerant vapor may take place because of pressure swing desorption, which, however, has negligible influence on chiller operation [40]. The introduction of the operation indicator helps to tackle out the backward desorption. d equals to 1 for normal adsorption and 0 otherwise. Comparably, by considering the same control volume as the adsorption bed, desorption bed entropy generation is described below: des
dSgen ½ðMCpÞhx;des þ ðMCpÞzl þ Mzl qdes Cpabe dT des ¼ T des dt dt dqdes ½sg ðT des ; Pc Þ sg ðT c Þ ð1 hÞM zl dt dq Q st;des ðT des ; Pc Þ _ i so Þhw Mzl des ½mðs T des dt
ð2Þ
The terms of the above equation carry similar meaning with Eq. (1). The Greek alphabet h is another operation indicator for the communication of desorption bed with the condenser, and it is set to 1 for desorption and 0 for adsorption. In the evaporator, entropy generation is expressed as below, in which the first term on the right hand side denotes entropy variation of the internal substance; the second term implies entropy gain from the external cooling load; and last but not least, the last two terms count for refrigerant water transporting into or out of the control volume. e
dSgen ½ðMCpÞhx;e þ ðMCpÞf ;e dT e _ i so Þchi ¼ ½mðs Te dt dt dqdes dqads sf ðT c Þ þ ½ð1 dÞsg ðP e ; T ads Þ þ dsg ðT e ÞM zl þ hM zl dt dt ð3Þ For the condenser, an assumption is made that all refrigerant vapor is condensed to liquid. The following equation can be applied. c
dSgen ½ðMCpÞhx;c þ ðMCpÞf ;c dT c _ o si Þcw;c ¼ þ ½mðs dt dt T c dqdes dqdes sf ðT c Þ þ hsg ðT des ; Pc Þ þ ð1 hÞsg ðT c Þ M zl hMzl dt dt ð4Þ The total entropy generation during adsorption/desorption period is therefore the sum of that in each of the major components, and expressed as
ð1Þ
where the first term of the right hand side of the equation denotes the entropy change due to temperature variation of heat exchanger materials, zeolite adsorbent and the adsorbed phase refrigerant water; the next term gives super heating of water vapor incoming from evaporation; a third term presents the phase change of vapor from gaseous to adsorbed phase calculated from the heat of adsorption released in the process; and the last term is contributed by the cooling water flashing through the heat exchanger tubes. The sum of the terms causes instantaneous entropy generation for the ADS bed, given on the left hand side of the equation. The Greek alphabet d is an operation indicator for the interaction of adsorption bed with
ads
des
c
e
dSgen dSgen dSgen dSgen dSgen ¼ þ þ þ dt dt dt dt dt
ð5Þ
The Linear Driving Force (LDF) model is used to predict kinetics of adsorption and desorption [41]. It describes that the rate of uptake of vapor is proportional to the difference between the equilibrium uptake, q and the instantaneous uptake q.
Ea dqðtÞ 15Dso exp RT ¼ ½q qðtÞ dt R2p
ð6Þ
Here Dso is a pre-exponential factor for surface diffusion, Ea denotes the activation energy of the adsorption system, and Rp is the
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average radius of the adsorbent particles. The isotherms for the equilibrium uptake q is given by Kakiuchi et al. [34]. The temperature of both adsorption and desorption bed computed by UA-LMTD method is given below:
ð7Þ
The zeolite–water adsorption chiller is evaluated by its cooling capacity QE as a cycle average and the COP of the system. Both parameters are defined as follows:
COP ¼
Z
ðT i T o Þchi dt
ð9Þ
Z
t cycle
0
ðT i T o Þhw dt
ð10Þ
The Carnot COP of a heat driven chiller has set a theoretical limit on the efficacy of such machines. Regarding the respective heat sources to be the thermal reservoirs, it can be expressed as:
1 ¼ COPcarnot
40
Adsorption (ADS)
30
10
T i;cw T o;chi T o;chi
T i;hw T i;hw T i;cw
0
ð8Þ
QE QH þ WS
_ ðmCpÞ hw t cycle
Desorption (DES)
50
0
where WS denotes the work input from a spray pump in the evaporator. QH is the total heat input to the system and calculated from the hot water behavior.
QH ¼
Cooling Water Outlet
20
tcycle
0
Chilled Water Outlet
60
ð11Þ
4. Results and discussion The study of the zeolite adsorption chiller is carried out experimentally to investigate its operational behavior and the influence of key parameters to its performance. The behavior is reflected by the outlet temperature of the thermal source of the respective components and the pressure inside their chamber. Inlet temperature of hot water is one variable changing from 55 °C to 95 °C. The chilled water outlet temperature is maintained at 12 °C ± 2 °C expect that in the 55 °C heat source condition 18 °C ± 1 °C is observed due to the chiller limitation. The other key parameter to be studied is the adsorption/desorption cycle time. A range between 100 s and 400 s is tested. Other operation parameters are listed in Table 2. 4.1. Temperature and pressure profile A direct result from the batch-wise operation of the chiller is the cyclic manner of the temperature and pressure profile. Fig. 3 displays the reactor beds’ temperature history during a typical cycle at the hot water inlet temperature of 65 °C and the adsorption/desorption cycle time of 250 s. The reflected parameters are the outlet temperature of the thermal source at the relevant Table 2 Operation conditions of zeolite–water adsorption chiller. Parameter
Value
Inlet temperature of hot water Inlet temperature of cooling water Outlet temperature of chilled water Flow-rate of hot water Flow-rate of cooling water Flow-rate of chilled water Adsorption/desorption cycle time Switching period
55–95 °C 27 °C ± 1 °C 12 °C ± 2 °C 3.8 m3/h 8.2 m3/h 2.3 m3/h 100–400 s 22 s
100
200
300
400
500
600
700
Time [s] Fig. 3. Temperature profile of the zeolite–water adsorption chiller at hot water temperature of 65 °C and adsorption/desorption cycle time of 250 s.
components. One can observe, in an adsorption/desorption period, a rapid change of the values is taking place in the beginning, whereas the profile tends to be steady afterwards. As illustrated in the first adsorption/desorption cycle time, for example, bed 1 starts desorption at an intermediate temperature. This is due to the fact that at the end of switching from cooling to heating source, the bed still remains at a relatively cool state. The large temperature difference between the bed and the heat source drives a faster heat transfer than any other part of the period. The adsorption for bed 2 can be explained in a similar way. In contrast, mass transport plays a major role in the variation of chilled water and cooling water outlet temperature. The initiated adsorption and desorption trigger massive molecular transfer from the evaporator and to the condenser, leading to an accelerated cooling production and heat rejection at the early stage of adsorption/desorption period. With the ongoing sorption process in the later stage, both beds are moving towards saturation, flattening the respective temperature profiles. In addition, the temperature profiles in the switching period are determined by the flow pattern of thermal sources. The temporal pressure profiles of the four chambers at the same period as above are shown in Fig. 4. They reflect clearly the interactions of the components. During the adsorption/desorption period, the pressure of the adsorption (or desorption) bed is equalized with the evaporator (or condenser) after the valve in between is open. However, at the start of the switching period, a sudden change in the heat source also alters the pressure of the reactor beds, causing their pressure departs from the condenser or evaporator. As a result, lever valves are shut and stop mass transport in
Bed 1
Bed 2
Evaporator
Condenser
A Complete Cooling Cycle 50
Adsorption / Desorption SwitchingPeriod Period
40
Pressure [Torr]
_ ðmCpÞ chi QE ¼ t cycle
Bed 2 Outlet
A CompleteCooling Cycle Adsorption/ Desorption Switching Period Period
70
Temperature [ºC]
T des=ads ¼ T i;hw=cw;ads þ ðT o T i Þhw=cw;ads " !#1 ðUAÞdes=ads 1 exp _ ðmCpÞ hw=cw;ads
Bed 1 Outlet
80
Desorption (DES 30
20 Adsorption (ADS) 10
0 0
100
200
300
400
500
600
700
Time [s] Fig. 4. Pressure profile of the zeolite–water adsorption chiller at hot water temperature of 65 °C and adsorption/desorption cycle time of 250 s.
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4.3. Influence of adsorption/desorption cycle time A further investation of the role of adsorption/desorption cycle time is provided in this subsection. Fig. 6a shows its influence on the total heat input. Generally, increasing adsorption/desorption cycle time helps to ease the demand of thermal energy. The influence is stronger on the shorter periods and high hot water temperature since trend lines are steeper. A major cause to this phenomenon is due to the water mixing. At the moment the beds swaps the function, an amount of cooling water is mixed into the hot water stream. Part of the heat input is inevitably lost because of the invasion of the low temperature source. If shorter adsorption/desorption cycle time is used, more frequent switching takes place, which may translate to increasing heat loss during the role swapping period and hence growing energy consumption. Compared to the heat input, the cooling capacity in Fig. 6b at elongated adsorption/desorption cycle time is not monotonous. It increases till 200 s and then starts droping, while after 300 s
Heat Input, QH [kW]
This subsection describes the effect of the hot water inlet temperature to the performance of the adsorption chiller. Evaluation parameters are presented in terms of total heat input, cooling capacity as well as the COP in various adsorption/desorption cycle times. In Fig. 5a, the demand to the total heat input increases with assorted hot water temperature. The trend tends to be smaller at higher end, which is more clear when the adsorption/desorption cycle time is long. Moreover, the input at 95 °C is approximately double of that at lower end. On the contrary, cooling capacity is not following the trend. As shown in Fig. 5b, the cooling production experiences a peak as the hot water temperature increases. At the adsorption/desorption cycle time of 100–200 s, the maximum is obtained at around 85 °C with the highest recored production of 12.13 kW at the latter. In the rest of the adsorption/desorption cycle times, the cooling effect is caped at 65–75 °C. It is observed that peak cooling capacity shifts towards to the left as the adsorption/desorption cycle time lasts longer. In addition, a significant reduction is realized at 55 °C. It shows that regeneration of the bed is not sufficiently conducted at this temperature level. Furthermore, a spray pump located in the evaporator has a constant power consumption of 0.6 kW. With the presence of the pump power, heat input and cooling capacity, the chiller’s COP can be computed. The experiment indicates that the maximum COP is observed at hot water inlet temperature of around 65 °C for all adsorption/desorption cycle times, as illustrated in Fig. 5c. The highest COP value is 0.44 when the adsorption/desorption cycle time is set to 400 s. The hot water temperature determines the desorption process and hence the cooling production of the chiller. In a given adsorption/desorption cycle time, the quality of bed regeneration has a direct relation with the bed thermal status. The isotherm theory of adsorption indicates that the amount of equilibrium uptake of vapor reduces when the temperature raises, meaning that a higher heat source temperature gives a better regeneration condition [42]. However, hotter heat source leads to higher desorption and condensation temperature, and hence the pressure. As the equilibrium uptake tends to increase with the pressure, the regeneration of the DES bed is inversely affected. This dilemma may explain the variation of the cooling capacity at different hot water temperature.
100 s
150 s
200 s
250 s
300 s
350 s
400 s
50
40
30
20
10 50
60
70
80
90
100
Temperature of Inlet Hot Water, Ti,hw [ºC]
(b)
100 s
150 s
200 s
250 s
300 s
350 s
400 s
13 12
Cooling Capacity, QE [kW]
4.2. Influence of hot water temperature
(a)
11 10 9 8 7 6 5 4 3 50
60
70
80
90
100
Temperature of Inlet Hot Water, Ti,hw [ºC]
(c) 0.45 Coefficient of Performance [-]
this period. Besides, pre-heating the ADS reactor increases its pressure, and vice versa for DES bed. This also indicates that the ‘valveless’ valves have fulfilled the functions of traditional electrical or pneumatic activated butterfly valves.
100 s
150 s
200 s
250 s
300 s
350 s
400 s
0.4 0.35 0.3 0.25 0.2 0.15 50
60
70
80
90
100
Temperature of Inlet Hot Water, Ti,hw [ºC] Fig. 5. Influence of hot water inlet temperature to (a) total heat input, (b) cooling capacity, and (c) COP of the adsorption chiller at various adsorption/desorption cycle times.
becomes stable. At 55 °C hot water temperature the cooling production at all adsorption/desorption cycle time is exceptionally lower than other temperature, indicating that the chiller has reached its lower limit. In general, however, the influence of adsorption/desorption cycle time to the cooling production is insignificant as compared to the heat input. As expected, the dominance of the heat input leads to an increase of the COP with respect to the duration of the adsorption/desorption period (See Fig. 6c). The adsorption/desorption cycle time affects both reactors. For desorption, a more thorough regeneration can be achieved when the sorption duration is longer. This is beneficial to the next cycle’s cooling production as a better regeneration condition is able to provide greater adsorption driving force to the evaporated refrigerant molecules, especially in the beginning period of the process.
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(a)
55 C
65 C
75 C
85 C
95 C
250
300
350
Heat Input, Q H [kW]
50
40
30
20
10 50
100
150
200
400
450
Adsorption / Desorption Cycle Time [s]
(b)
55 C
65 C
75 C
85 C
250
300
95 C
13
Cooling Capacity, Q E [kW]
12 11 10 9 8 7 6 5 4 3 50
100
150
200
350
400
450
Adsorption / Desorption Cycle Time [s]
Coefficient of Performance [-]
(c) 0.45
55 C
65 C
75 C
150
200
250
85 C
95 C
0.4 0.35 0.3 0.25 0.2 0.15 50
100
300
350
400
450
Adsorption / Desorption Cycle Time [s] Fig. 6. Influence of adsorption/desorption cycle time to (a) total heat input, (b) cooling capacity, and (c) COP of the adsorption chiller at various hot water inlet temperatures.
However, adsorption rate reduces extensively when the adsorbate uptake is approaching to saturation. Long exposure to the evaporator eventually leads to diminishing cooling rate, especially at the end of the adsorption/desorption period. This may explain the relative insensitivity of the cooling capacity to the various adsorption/ desorption cycle time. 4.4. Entropy analysis A computational result of the entropy generation on the adsorption/desorption period for the zeolite–water adsorption chiller is presented below. From two aspects, i.e. the inlet hot water temperature and the adsorption/desorption cycle time, the irreversibility of the chiller major components is analyzed. A term named as
specific entropy generation, which is defined as the cyclic average entropy generation per unit power of cooling capacity, is used to reveal the behavior of the chiller. Values of some parameters used in the calculation are shown in Table 3. The sensible heat of the heat exchanger material, zeolite and adsorbed phase is computed from energy balance. Water property functions are provided by Wagner et al. [43]. The rest are taken from experimental data. Fig. 7 displays the temporal entropy generation rate of each major component and the total value in a typical adsorption/desorption period in the conditions of the hot water inlet temperature 65 °C and adsorption/desorption cycle time 200 s. The cooling water inlet and chilled water outlet are 26.5 °C and 11 °C, respectively. As illustrated, a significant amount of entropy is produced by both reactor beds in the beginning of the adsorption/desorption period. The degree of magnitude is one order greater than that of the evaporator and condenser. However, the entropy generation of both beds decreases rapidly as time proceeds. After 70 s the profiles of both beds join to the evaporator and condenser whose entropy production is more consistent throughout the period. In the initial part of the adsorption/desorption period, two phenomena is observed. Firstly, there exist a large temperature difference between the reactors and the respective thermal sources. Secondly, a strong driving force of mass transport is also present. These result in a large expected entropy generation. As the beds approach the thermal source temperature and adsorbed phase saturation, irreversibility is also diminished. In the overall process, the cyclic average of total entropy production is 5.39 W/K, in which the DES bed encounters the most irreversibility, followed by the ADS bed. The smallest entropy generation is observed in the condenser. Cyclic average entropy generation rate of every component in the adsorption / desorption period with respect to the temperature of inlet hot water is illustrated in Fig. 8. The adsorption/desorption cycle time is fixed at 200 s. It is shown that production of entropy escalates linearly to the heat source temperature for all components in which the desorption bed is influenced most severely. This is reasonable since the DES bed endures the less efficient heat transfer efficiency. Furthermore, increasing the hot water temperature at the same time enlarges the temperature disparity between adsorption and desorption state. This results in a growing gap of temperature between the DES bed contents and the heat source in the initial stage of the adsorption/desorption period. Since the starting part is responsible for provoking tremendous amount of dissipation, the cyclic average of the entropy generation in the DES bed is more profoundly influenced. On the other hand, with the help of sufficient pre-cooling and more effective heat transfer, the ADS bed acts more resistively to the irreversibility as the hot water temperature increases. The evaporator and condenser are impacted through indirect ways by mass transport. A further investigation of contribution of each component to the total entropy generation at 200 s adsorption/desorption cycle time and various heat source temperatures is represented in Fig. 9. Clearly, the DES bed is responsible for the biggest portion of dissipation, approximately 50–70% of the total, among four components. Both reactors courts for 80% in average of entropy
Table 3 Values of parameters used in the computation of entropy. Parameter
Value
Pre-exponential surface diffusion factor, Dso Activation energy, Ea Average radius of zeolite particles, Rp Heat of adsorption, Qst Mass of the FAM Z01 zeolite, Mzl ADS bed heat transfer coefficient, (UA)ads DES bed heat transfer coefficient, (UA)des
2.54 104 m2/s 4.55 104 kJ/kmol 1.0 104 m 3110 kJ/kg [34] 12 kg 5.13 kW/K 3.13 kW/K
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A. Li et al. / Applied Energy 130 (2014) 702–711 ADS
DES
Evap
Total
Cond
ADS
25
1
Evap
Cond
20
0.5
80%
Percentage Contribution
Entropy Generation Rate, dSgen /dt [W/K]
DES
100%
30
0
15
160
170
180
10
60%
40%
20% 5 0% 55
0 0
50
100
150
200
65
75
85
95
Temperature of Inlet Hot Water, Ti,hw [ºC]
Time [s] Fig. 7. Temporal entropy generation rate of chiller major components in a typical adsorption/desorption period (the result is computed from the experimental data in the conditions of hot water inlet temperature 65 °C and adsorption/desorption cycle time 200 s).
ADS
DES
Evap
Cond
Fig. 9. Percentage of entropy generation of the chiller major components in the adsorption/desorption period at 200 s adsorption/desorption cycle time and increasing temperature of inlet hot water.
Specific Entropy Generation
Total
COP
COP Carnot
1.6
10
8
6
4
2
1.4 3 1.2 2.5 1 2 0.8 1.5
0.6 0.4
1
0.2
0.5
0
0 50
0 50
60
70
80
90
60
Fig. 8. Cyclic average entropy generation rate of the chiller major components in the adsorption/desorption period at 200 s adsorption/desorption cycle time and increasing temperature of inlet hot water.
70
80
90
100
Temperature of Inlet Hot Water, Ti,hw [ºC]
100
Temperature of Inlet Hot Water, Ti,hw [ºC]
Coefficient of Performance, Carnot [-]
Specific Entropy Generation [W/K-kW] & Coefficient of Performance [-]
Entropy Generation Rate, dSgen /dt [W/K]
3.5 12
Fig. 10. Specific entropy generation, COP and Carnot COP at 200 s adsorption/ desorption cycle time and assorted temperature of inlet hot water.
ADS
DES
Evap
Cond
Total
250
300
350
generation, implying that improvement should be made to address high irreversibility of the reactors. Entropy generation is a direct measurement of inefficiency of a system. It reveals how far away the system is to the thermodynamic limit, or in the case of adsorption chiller, the Carnot COP. Fig. 10 illustrates specific entropy generation, COP and Carnot COP of the adsorption chiller at the tested hot water temperature range. Although increasing heat source temperature brings up the theoretical Carnot limit of the chiller, the chiller’s real COP is following an opposite trend after 65 °C. This is, however, not unpredictable as specific entropy generation rate is accelerated after this temperature, leading to growing gap between the chiller’s efficiency to the upper limit. Figs. 11–13 analyzes the cyclic average entropy generation rate with respect to the duration of adsorption/desorption period. The result is computed based on 65 °C inlet hot water experimental data. In Fig. 11, the average generation rate of both reactor beds decreases with increasing duration of adsorption/desorption period, whereas that of the evaporator and the condenser experiences only little change. The total average entropy production is thus
Entropy Generation Rate, dS gen /dt [W/K]
7
6
5
4
3
2
1
0 50
100
150
200
400
450
Adsorption / Desorption Cycle Time [s] Fig. 11. Cyclic average entropy generation rate of the chiller major components in the adsorption/desorption period at 65 °C inlet hot water and various adsorption/ desorption cycle times.
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A. Li et al. / Applied Energy 130 (2014) 702–711 ADS
DES
Evap
Entropy generation of the adsorption chiller is presented for several key parameters, namely (i) the adsorption/desorption cycle time, (ii) the hot water inlet temperature. The experiments suggest that the cyclic average entropy generation rate increases with increasing hot water temperature and reduces with decreasing cycle time. It is found that, amongst all components in the plant, the desorption bed contributes to the biggest portion of irreversibilities. This is followed by the adsorption bed, the evaporator and the condenser unit. On a specific entropy generation basis, defined as entropy generation per unit mass of useful effect, it demonstrates the optimized efficiency of the adsorption chiller, i.e., the highest COP, occurs when the specific entropy generation is smallest.
Cond
100%
Percentage Contribution
80%
60%
40%
20%
0% 100
150
200
250
300
350
400
Adsorption / Desorption Cycle Time [s] Fig. 12. Percentage of entropy generation of the chiller major components in the adsorption/desorption period at 65 °C inlet hot water and various adsorption/ desorption cycle times.
Specific Entropy Generation
COP
COP Carnot 2.5
0.8
2
0.6
1.5
0.4
1
0.2
0.5
0
Coefficient of Performance, Carnot [-]
Specific Entropy Generation [W/K-kW] & Coefficient of Performance [-]
1
0 50
150
250
350
450
Adsorption / Desorption Cycle Time [s] Fig. 13. Specific entropy generation, COP and Carnot COP at 65 °C inlet hot water and assorted adsorption/desorption cycle time.
decreased. In terms of percentage contribution, as shown in Fig. 12, the DES bed also declines as adsorption/desorption cycle time is extended. On the contrary, the contribution of the rest of the components is amplified. In addition, it is noted that the percentage contribution of entropy generation of every major component tends to be more evenly distributed at longer sorption duration. In Fig. 13, the Carnot COP of the chiller is fairly a constant regardless of adsorption/desorption cycle time, due to a maintained thermal reservior temperatures. Moreover, it is clearly indicated that at the same thermal reservior temperature, minimum specific entropy generation leads to the maximum COP of the adsorption chiller. 5. Conclusion An adsorption chiller using FAM Z01 zeolite–water as the adsorbent–adsorbate pair is presented. Experimental evaluation indicates that the cooling capacity increases with heat input over the supplied hot water temperatures and a peak in cooling capacity occurs at 65–85 °C for a wide range of cycle time. Optimum hot water temperature for most efficient operation is 65 °C. However, the cooling capacity reduces at longer sorption duration due to diminishing uptake potential of adsorbent. Our experiments indicated an optimum adsorption/desorption cycle time for the tube-fin heat exchanger design is between 200 s and 300 s.
References [1] Li A, Loh WS, Ng KC. Transient modeling of a lithium bromide-water absorption chiller. Appl Mech Mater 2013;388:83–90. [2] Critoph R, Zhong Y. Review of trends in solid sorption refrigeration and heat pumping technology. Proc Inst Mech Eng Part E J Process Mech Eng 2005;219:285–300. [3] Meunier F. Solid sorption heat powered cycles for cooling and heat pumping applications. Appl Therm Eng 1998;18:715–29. [4] Ülkü S. Solar adsorption heat pumps, solar energy utilization: fundamentals and applications. The Netherlands: Martinus Nijkoff Publishers; 1987. [5] Meunier F. Adsorptive cooling: a clean technology. Clean Technol Environ Policy 2001;3:8–20. [6] Ulku A, Mobedi M. Adsorption in energy storage. NATO ASI Series. Series E Appl Sci 1989;167:487–507. [7] Wang X, Ng KC. Experimental investigation of an adsorption desalination plant using low-temperature waste heat. Appl Therm Eng 2005;25:2780–9. [8] Chua H, Ng K, Malek A, Kashiwagi T, Akisawa A, Saha B. Multi-bed regenerative adsorption chiller—improving the utilization of waste heat and reducing the chilled water outlet temperature fluctuation. Int J Refrig 2001;24:124–36. [9] Ng KC, Thu K, Chakraborty A, Saha BB, Chun WG. Solar-assisted dual-effect adsorption cycle for the production of cooling effect and potable water. Int J Low-Carbon Technol 2009;4:61–7. [10] Wang DC, Xia ZZ, Wu JY. Design and performance prediction of a novel zeolite– water adsorption air conditioner. Energy Convers Manage 2006;47:590–610. _ Kaftanog˘lu B, Yamalı C, Baker D. Experimental investigation of a [11] Solmusß I, natural zeolite–water adsorption cooling unit. Appl Energy 2011;88:4206–13. _ Yamalı C, Kaftanog˘lu B, Baker D, Çag˘lar A. Adsorption properties of a [12] Solmusß I, natural zeolite–water pair for use in adsorption cooling cycles. Appl Energy 2010;87:2062–7. [13] Vasiliev LL, Mishkinis DA, Antukh AA, Vasiliev Jr LL. Solar–gas solid sorption heat pump. Appl Therm Eng 2001;21:573–83. [14] Loh WS, Saha BB, Chakraborty A, Ng KC, Chun WG. Performance analysis of waste heat driven pressurized adsorption chiller. J Therm Sci Technol 2010;5:252–65. [15] Saha BB, Boelman EC, Kashiwagi T. Computational analysis of an advanced adsorption–refrigeration cycle. Energy 1995;20:983–94. [16] Saha BB, Kashiwagi T. Experimental investigation of an advanced adsorption refrigeration cycle. ASHRAE Trans 1997;133:50–7. [17] Saha BB, Akisawa A, Kashiwagi T. Solar/waste heat driven two-stage adsorption chiller: the prototype. Renew Energy 2001;23:93–101. [18] Saha BB, Koyama S, Kashiwagi T, Akisawa A, Ng KC, Chua HT. Waste heat driven dual-mode, multi-stage, multi-bed regenerative adsorption system. Int J Refrig 2003;26:749–57. [19] Wang X, Chua HT, Ng KC. Experimental investigation of silica gel–water adsorption chillers with and without a passive heat recovery scheme. Int J Refrig 2005;28:756–65. [20] Alam KCA, Akahira A, Hamamoto Y, Akisawa A, Kashiwagi T. A four-bed mass recovery adsorption refrigeration cycle driven by low temperature waste/ renewable heat source. Renew Energy 2004;29:1461–75. [21] Ng KC, Thu K, Saha BB, Chakraborty A. Study on a waste heat-driven adsorption cooling cum desalination cycle. Int J Refrig 2012;35:685–93. [22] Chen CJ, Wang RZ, Xia ZZ, Kiplagat JK, Lu ZS. Study on a compact silica gel– water adsorption chiller without vacuum valves: design and experimental study. Appl Energy 2010;87:2673–81. [23] Shahzad MW, Ng KC, Thu K, Chun WG. Multi effect desalination and adsorption desalination (MEDAD) cycle. In: International symposium on innovative materials for processes in energy systems 2013, Fukuoka, Japan; 2013. p. 115–119. [24] Thu K, Kim Y-D, Amy G, Chun WG, Ng KC. A hybrid multi-effect distillation and adsorption cycle. Appl Energy 2013;104:810–21. [25] Chua HT, Ng KC, Malek A, Kashiwagi T, Akisawa A, Saha BB. Modeling the performance of two-bed, silica gel–water adsorption chillers. Int J Refrig 1999;22:194–204. [26] Rezk ARM, Al-Dadah RK. Physical and operating conditions effects on silica gel/ water adsorption chiller performance. Appl Energy 2012;89:142–9.
A. Li et al. / Applied Energy 130 (2014) 702–711 [27] Zhao Y, Hu E, Blazewicz A. Dynamic modelling of an activated carbon– methanol adsorption refrigeration tube with considerations of interfacial convection and transient pressure process. Appl Energy 2012;95:276–84. [28] Chua HT, Ng KC, Wang W, Yap C, Wang XL. Transient modeling of a two-bed silica gel–water adsorption chiller. Int J Heat Mass Transfer 2004;47:659–69. [29] Choudhury B, Saha BB, Chatterjee PK, Sarkar JP. An overview of developments in adsorption refrigeration systems towards a sustainable way of cooling. Appl Energy 2013;104:554–67. [30] Meunier F, Poyelle F, LeVan MD. Second-law analysis of adsorptive refrigeration cycles: the role of thermal coupling entropy production. Appl Therm Eng 1997;17:43–55. [31] Meunier F. Second law analysis of a solid adsorption heat pump operating on reversible cascade cycles: application to the zeolite–water pair. J Heat Recov Syst 1985;5:133–41. [32] Chua HT, Ng KC, Malek A, Kashiwagi T, Akisawa A, Saha BB. Entropy generation analysis of two-bed, silica gel–water, non-regenerative adsorption chillers. J Phys D Appl Phys 1998;31:1471–7. [33] Ng KC. Thermodynamic tools for chiller diagnostics and optimization. Heat Transfer Eng Dec 2004;25:1–4. [34] Kakiuchi H, Iwade M, Shimooka S, Ooshima K, Yamazaki M, Takewaki T. Novel zeolite adsorbents and their application for AHP and desiccant system, presented at the IEA-Annex 17 meeting, Beijing; 2004.
711
[35] Kakiuchi H, Shimooka S, Iwade M, Oshima K, Yamazaki M, Terada S, et al. Novel water vapor adsorbent FAM-Z01 and its applicability to an adsorption heat pump. Kagaku Kogaku Ronbunshu 2005;31:361–4. [36] Radha AV, Bomati-Miguel O, Ushakov SV, Navrotsky A, Tartaj P. Surface enthalpy, enthalpy of water adsorption, and phase stability in nanocrystalline monoclinic zirconia. J Am Ceram Soc 2009;92:133–40. [37] Zhang H, Penn RL, Hamers RJ, Banfield JF. Enhanced adsorption of molecules on surfaces of nanocrystalline particles. J Phys Chem B 1999;103:4656–62. [38] Yanagi H, Ino N. Heat and mass transfer characteristics in consolidated silica gel–water adsorption-cooling system, presented at the ASME ASIA’97; 1997. [39] Ismail AB, Li A, Thu K, Ng KC, Chun W. On the thermodynamics of refrigerant + heterogeneous solid surfaces adsorption. Langmuir 2013;29:14494–502. [40] Thu K, Kim Y-D, Myat A, Chun WG, Ng KC. Entropy generation analysis of an adsorption cooling cycle. Int J Heat Mass Transfer 2013;60:143–55. [41] Ismail A, Loh WS, Thu K, Ng KC. A study on the kinetics of propane-activated carbon: theory and experiments. Appl Mech Mater 2013;388:76–82. [42] Loh WS, Ismail AB, Xi B, Ng KC, Chun WG. Adsorption isotherms and isosteric enthalpy of adsorption for assorted refrigerants on activated carbons. J Chem Eng Data 2012;57:2766–73. [43] Wagner W, Cooper JR, Dittmann A, Kijima J, Kretzschmar HJ, Kruse A, et al. The IAPWS industrial formulation 1997 for the thermodynamic properties of water and steam. J Eng Gas Turbines Power 2000;122:150–80.
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Applied Energy journal homepage: www.elsevier.com/locate/apenergy
Performance evaluation of a zeolite–water adsorption chiller with entropy analysis of thermodynamic insight Ang Li, Azhar Bin Ismail, Kyaw Thu, Kim Choon Ng ⇑, Wai Soong Loh Department of Mechanical Engineering, National University of Singapore, 9 Engineering Drive 1, Singapore 117576, Singapore
h i g h l i g h t s An adsorption chiller using novel adsorbent Zeolite FAM Z01 is presented. Chiller’s highest COP was achieved at low grade heat source of 65 °C. Second law equations were developed to study the chiller’s irreversibilities. Entropy generation against heat source temperature and cycle time is presented. ‘Specific entropy generation’ is used to reveal the trend of the chiller’s COP.
a r t i c l e
i n f o
Article history: Received 9 November 2013 Received in revised form 11 January 2014 Accepted 25 January 2014 Available online 26 February 2014 Keywords: Adsorption chiller Zeolite–water pair Entropy analysis Specific entropy generation
a b s t r a c t This paper presents an environment-friendly adsorption chiller using Zeolite FAM Z01–water pair as opposed to the conventional silica gel and water pair. The adsorbent, zeolite, is thinly coated onto the surfaces of fin-tube heat exchanger for faster rates of heat and mass transfer. Another feature of the adsorption chiller is the use of a lever-countered weighted valve which can be open or closed by the pressure difference between the reactors and the condenser or evaporator. Experiments are conducted to evaluate the performance of zeolite-based chiller in terms of total heat input, cooling capacity, and coefficient of performance (COP) with respect to heat source temperature and adsorption/desorption cycle time where an optimal operational zone can be determined: (i) hot water inlet temperatures range from 65 °C to 85 °C, (ii) adsorption/desorption cycle times of 200–300 s at optimum cooling and COP, Entropy analyses have been conducted to understand the irreversibility contributed by both the desorption and adsorption beds at assorted hot water inlet temperatures and cycle time. Ó 2014 Elsevier Ltd. All rights reserved.
1. Introduction In recent decades, increasing cooling demand in the industrial and residential sectors aggravates energy consumption leading to a corresponding deterioration of environment from higher fossil fuel utilization. Cooling by the conventional vapor compression chillers consumes much electricity at 0.8–1.2 kW h/Rtons. Improvements to energy efficiency of key chiller components have been reported to have reached their asymptotic peaks, and huge investment is needed for only marginal improvements of chiller’s energy efficiency. An alternative method to improving energy efficiency is to focus on the development of thermally-driven cycles that can be powered by low temperature waste heat which is available in abundance from exhaust of industrial processes or from ⇑ Corresponding author. Tel.: +65 65162214; fax: +65 65161459. E-mail address: [email protected] (K.C. Ng). http://dx.doi.org/10.1016/j.apenergy.2014.01.086 0306-2619/Ó 2014 Elsevier Ltd. All rights reserved.
renewable energy sources such as solar or geothermal heat [1]. Concerning such issues, scientists and engineers have studied green alternative solutions, and amongst such approaches, the adsorption cycle has been mooted as one of the most attractive technologies [2,3]. Being low-grade waste heat driven, maintenance free and environment benign are main advantages of adsorption chillers [4–8]. A proper designed such system operates in the heat source temperature as low as 55 °C. No major moving parts present in the chiller structure, requesting only minimum maintenance. High durability of adsorbent material added as a plus for its long lasting performance. Furthermore, the green nature of the technology comes from the utilization of non-ozone-depleting refrigerants and waste heat by which no additional CO2 is emitted. Extensive work has been conducted to study the adsorption cooling systems. The system can be constructed by implementing various adsorbent–adsorbate pairs, for example, silica gel–water [9], zeolite–water [10–12], activated carbon–ammonia [13] or
A. Li et al. / Applied Energy 130 (2014) 702–711
703
Nomenclature Abbreviations ADS adsorption bed Cond condenser COP coefficient of performance DES desorption bed Evap evaporator FAM Z01 Functional Adsorbent Material, Zeolite Z01
t tcycle Sgen U Ws h d
time (s) cycle time (s) entropy generation (kJ/K) overall heat transfer coefficient (kW/m2 K) power input of spray pump (kW) operation indicator (–) operation indicator (–)
Symbols A Cp Dso Ea M _ m P QE QH Qst R Rp q q T
Subscript abe ads c chi cw des e f g hw hx i o zl
adsorbed phase of adsorbate adsorption bed condenser chilled water cooling water desorption bed evaporator liquid phase gaseous phase hot water heat exchanger inlet outlet zeolite
area (m2) specific heat capacity (kJ/kg K) pre-exponential factor for surface diffusion (m2/s) activation energy (kJ/kmol) mass (kg) mass flow-rate (kg/s) pressure (Pa) cooling capacity (kW) heat input (kW) heat of adsorption (kW/kg) universal gas constant (kJ/kmol K) average radius of adsorbant particles (m) adsorbate uptake (kg/kg of adsorbent) equilibrium adsorbate uptake (kg/kg of adsorbent) temperature (K)
R134a [14], in which the latter substances play the role of refrigerant. A multistage design from Saha et al. [15] effectively brought down the minimum heat source temperature to near ambient from a single stage type silica gel–water adsorption chiller. Saha and coworkers experimentally tested a three stage [16] and later a two stage chiller [17], and achieved 50 °C heat source temperature from the former, and 55 °C from the latter with 30 °C cooling water in both situations. Saha et al. [18], Wang et al. [19], Alam et al. [20], and Ng et al. [21] investigated operational strategies of multi-bed adsorption systems. Chen et al. [22] tested an adsorption chiller that eliminates the vacuum valves in the structure. Shahzad et al. [23] and Thu et al. [24] creatively hybridized adsorption chiller technology with multi-effect desalination (MED), which break through the conventional MED lower temperature limit to below ambient condition. Simulation and numerical studies [25–28] on the adsorption refrigeration systems at the same time helped to establish theoretical frame work, and explore the behavior of such machine in extreme as well as optimal conditions. Given the research activities conducted in the adsorption cooling field, Choudhury et al. [29] comprehensively summarized the development of this technology from three aspects, i.e., thermal energy harvesting, heat and mass transfer enhancement, and lastly advanced cycles and stages. However, the vast application of this green technology is bottlenecked by low coefficient of performance (COP) and relatively larger foot-print. The existing adsorption chillers operate well below the theoretical Carnot limit. Meunier et al. [30] explained the reason through thermodynamic second law analysis. Meunier and coauthors suggested that in the conditions of isothermal heat reservoirs and ideal transfer, a temperature gap exists between the reactor sorption cycles and heat reservoirs, which is responsible for the thermal irreversibilities. Even infinite cascades of sorption reactors yield no more than 68% of the ideal efficiency [31]. Chua et al. [32] pointed out that the largest portion of entropy is generated by sorption heat transfer. Ng [33] has suggested that the COP of such thermally driven adsorption systems is normally lower than 1. On the other hand, the size of the chiller is bounded by
the characteristics of the adsorbent–adsorbate pair. In order to achieve desired cooling capacity, the amount of evaporated refrigerant must be adsorbed simultaneously by the solids. This requires large amount of adsorbent and sufficient void space to be introduced into reactors. This paper presents an adsorption chiller using a novel adsorbent, FAM Z01 Zeolite (composition: FexAlyPzO2nH2O, x = 0.02– 0.10, y = 0.35–0.5, z = 0.4–0.6, n = 0–1), with water as adsorbate. The chiller is equipped with adsorbent coated reactor heat exchangers and lever mechanism valves. The performance of the system is evaluated experimentally with respect to various parameters, namely, adsorption/desorption cycle time and hot water inlet temperature. A further investigation of the thermodynamic insight of the chiller is carried out by entropy analysis.
2. System characteristics 2.1. Adsorbent–adsorbate pair The adsorbent introduced in the current work, namely Functional Adsorbent Material (FAM Z01), is a novel aluminophosphate based water sorption zeolite developed by Kakiuchi et al.[34]. The material has a one-dimensional AFI type molecular sieve structure with a molecular window of diameter 0.73 nm. Only molecules smaller than this dimension, such as water molecules, are permitted free migration though the window. FAM Z01 owns S-shape water vapor sorption isotherms with excellent durability over 200,000 adsorption–desorption cycles and little hysteresis behavior. Sorption takes place dramatically in a narrow range of relative pressure (defined as the ratio of sorption pressure to the saturation pressure of vapor at the same temperature), while out of which the amount of vapor uptake possesses little changes with respect to the relative pressure [35]. This translates to a much larger adsorption capacity over conventional silica gel at normal chiller operation conditions, and further results in a compact chiller design. On the other hand, owning big latent heat, being absolutely
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environmental friendly and inexpensive, water has its intrinsic advantages to be a refrigerant. 2.2. Operation description The present adsorption chiller comprises of four major components, including two reactor beds, a condenser and an evaporator, as shown in Fig. 1. The zeolite is constructed stationary onto heat exchangers of both beds. Due to a lack of mobility of these solid adsorbent, a batch-wise operation is designed for the chiller to achieve continuous cooling, in which the reactors alternate their roles to perform adsorption and desorption. Adsorption bed (ADS) and desorption bed (DES) are named on the respective reactor depending on its ongoing function. A complete cooling cycle consists of two adsorption/desorption periods and two successive switching periods. In the former one, the adsorption bed interacts only with the evaporator. A spontaneous mass transport takes place as water molecules move from gaseous state to a lower energy and more thermodynamically stable adsorbed phase [36,37]. As a result, heat of adsorption is released. To maintain this process, the temperature of the zeolite must remain low. This is achieved by flushing cooling water into the ADS heat exchanger to carry away the rejected heat. In the evaporator, vapor transport evacuates the pressure. Boiling of refrigerant is then triggered through which chilling effect is produced. Simultaneously, DES reactor that has been saturated with adsorbed phase water is coupled with the condenser. Heat source is supplied to desorb the attached water molecular from DES and drive it to the condenser. The vapor condenses to liquid phase and flows back to the evaporator via a U-tube. Cooling water from the ADS outlet is directed to the condenser for removal of the latent heat of condensation. Upon completion of sorption process, switching period starts, in which the two reactor beds swap the role by changing the hot water and cooling water flow paths. The connections between both reactor beds to the condenser or the evaporator are shut and no mass transport occurs. Throughout the operation the refrigerant water circulates among the components at sub-atmospheric pressures.
exchanger fins of the reactor beds by using a special organic binder, instead of being packed among heat exchanger fins by metal meshes in the conventional method. The motivation of using the coating method is to promote heat transfer. Poor contact among these metal surface and solids that is exposed in the conventional method causes very big thermal resistance between thermal source and adsorbent, resulting in a large swing of the chiller performance due to adsorbent packing density [38]. By introducing binder, the new method well addresses this bottleneck. Fig. 2a and b show microscopic photos of pure FAM Z01 and FAM Z01 with organic binder, respectively, taken by Field Emission Scanning Electron Microscopy (FESEM) method. Clearly, the binder material seals up the space between the fins and the adsorbent solid, as well as among the adsorbent itself. The temperature difference of adsorbent and the hot or cooling water is extensively reduced. Furthermore, the mesh material is eliminated in the reactor. This considerably diminishes the thermal mass of the bed as well as heat loss to these materials. These improvements contribute to a higher overall heat transfer efficiency and a shorter cycle time. The thermophysical properties of FAM Z01 with and without binder are tabulated in Table 1.
2.3. Structural innovation There are two innovative features found in the design of the current reactor beds. First of all, the zeolite is coated onto the heat
Fig. 1. Schematic presentation of the zeolite–water adsorption chiller in adsorption/desorption period (ADS: adsorption bed; DES: desorption bed).
Fig. 2. Field Emission Scanning Electron Microscopic (FESEM) photos of FAM Z01 zeolite without (a) and with (b) organic binder.
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A. Li et al. / Applied Energy 130 (2014) 702–711 Table 1 Thermophysical properties of FAM Z01 with and without organic binder. Adsorbent
BET surface area (m2/g)
Micropore volume (cm3/g)
Average pore diameter (Å)
FAM Z01 FAM Z01 with organic binder
147.3 41.15
0.071 0.035
19.29 34.24
Secondly, the system utilizes a ‘‘valve-less’’ mechanism between the reactors and the evaporator or condenser instead of butterfly valves used in typical adsorption chillers. This mechanism is basically a counter-weight lever whose opening and closing are triggered by pressure difference of adjacent chambers. For instance, in the adsorption/desorption period, the pressure in the adsorption bed is lower than evaporator due to sufficient cooling. The pressure difference creates lifting force that effectively pushes the lever up and starts the mass transport. In the case of saturation or swapping to desorption by flushing in the hot water, pressure of the bed will increase and shut the connection to the evaporator. The same principle is implemented between the DES bed and the condenser. Because of the low density of the refrigerant vapor, the jointed components require large cross-section pipes to reduce pressure drop. The ‘‘valve less’’ design saves the space taken up by conventional pipes and valves, and further eliminates the valve control panel and accessories. Hence, the structure of the adsorption chiller system is greatly simplified. 3. Governing equations The irreversibilities involved in the phase change of the refrigerant, its interaction with the solid adsorbent as well as the exterior cooling and heating source during the thermodynamic process result in the generation of extra entropy [39]. Moreover, temperature variation of the adsorbent and heat exchanger materials cannot be simply neglected as a significant amount of heat is reserved or emitted from them due to batch-wise operation, especially at the beginning of the adsorption/desorption cycle time. The analysis below provides the entropy calculation for the adsorption/ desorption cycle times. Assuming the component to be analyzed a block box, the equations are written in a way that considering the inlet and outlet conditions, as well as the initial and final states of the substance inside the component. Heat loss to the surrounding is omitted due to the fact that the major components are well insulated, and the hot object is wrapped by heater tapes. During the adsorption process, by choosing the control volume to be inside the boundary of the reactor bed, the instantaneous entropy generation of the ADS bed can be written as: ads
dSgen ½ðMCpÞhx;ads þ ðMCpÞzl þ Mzl qads Cpabe dT ads ¼ T ads dt dt dqads ½sg ðT ads ; Pe Þ sg ðT e Þ þ dMzl dt dqads Q st;ads ðT ads ; Pe Þ _ o si Þcw;ads þ ½mðs M zl dt T ads
the evaporator. In the beginning of each adsorption/desorption period, a backward flow of refrigerant vapor may take place because of pressure swing desorption, which, however, has negligible influence on chiller operation [40]. The introduction of the operation indicator helps to tackle out the backward desorption. d equals to 1 for normal adsorption and 0 otherwise. Comparably, by considering the same control volume as the adsorption bed, desorption bed entropy generation is described below: des
dSgen ½ðMCpÞhx;des þ ðMCpÞzl þ Mzl qdes Cpabe dT des ¼ T des dt dt dqdes ½sg ðT des ; Pc Þ sg ðT c Þ ð1 hÞM zl dt dq Q st;des ðT des ; Pc Þ _ i so Þhw Mzl des ½mðs T des dt
ð2Þ
The terms of the above equation carry similar meaning with Eq. (1). The Greek alphabet h is another operation indicator for the communication of desorption bed with the condenser, and it is set to 1 for desorption and 0 for adsorption. In the evaporator, entropy generation is expressed as below, in which the first term on the right hand side denotes entropy variation of the internal substance; the second term implies entropy gain from the external cooling load; and last but not least, the last two terms count for refrigerant water transporting into or out of the control volume. e
dSgen ½ðMCpÞhx;e þ ðMCpÞf ;e dT e _ i so Þchi ¼ ½mðs Te dt dt dqdes dqads sf ðT c Þ þ ½ð1 dÞsg ðP e ; T ads Þ þ dsg ðT e ÞM zl þ hM zl dt dt ð3Þ For the condenser, an assumption is made that all refrigerant vapor is condensed to liquid. The following equation can be applied. c
dSgen ½ðMCpÞhx;c þ ðMCpÞf ;c dT c _ o si Þcw;c ¼ þ ½mðs dt dt T c dqdes dqdes sf ðT c Þ þ hsg ðT des ; Pc Þ þ ð1 hÞsg ðT c Þ M zl hMzl dt dt ð4Þ The total entropy generation during adsorption/desorption period is therefore the sum of that in each of the major components, and expressed as
ð1Þ
where the first term of the right hand side of the equation denotes the entropy change due to temperature variation of heat exchanger materials, zeolite adsorbent and the adsorbed phase refrigerant water; the next term gives super heating of water vapor incoming from evaporation; a third term presents the phase change of vapor from gaseous to adsorbed phase calculated from the heat of adsorption released in the process; and the last term is contributed by the cooling water flashing through the heat exchanger tubes. The sum of the terms causes instantaneous entropy generation for the ADS bed, given on the left hand side of the equation. The Greek alphabet d is an operation indicator for the interaction of adsorption bed with
ads
des
c
e
dSgen dSgen dSgen dSgen dSgen ¼ þ þ þ dt dt dt dt dt
ð5Þ
The Linear Driving Force (LDF) model is used to predict kinetics of adsorption and desorption [41]. It describes that the rate of uptake of vapor is proportional to the difference between the equilibrium uptake, q and the instantaneous uptake q.
Ea dqðtÞ 15Dso exp RT ¼ ½q qðtÞ dt R2p
ð6Þ
Here Dso is a pre-exponential factor for surface diffusion, Ea denotes the activation energy of the adsorption system, and Rp is the
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average radius of the adsorbent particles. The isotherms for the equilibrium uptake q is given by Kakiuchi et al. [34]. The temperature of both adsorption and desorption bed computed by UA-LMTD method is given below:
ð7Þ
The zeolite–water adsorption chiller is evaluated by its cooling capacity QE as a cycle average and the COP of the system. Both parameters are defined as follows:
COP ¼
Z
ðT i T o Þchi dt
ð9Þ
Z
t cycle
0
ðT i T o Þhw dt
ð10Þ
The Carnot COP of a heat driven chiller has set a theoretical limit on the efficacy of such machines. Regarding the respective heat sources to be the thermal reservoirs, it can be expressed as:
1 ¼ COPcarnot
40
Adsorption (ADS)
30
10
T i;cw T o;chi T o;chi
T i;hw T i;hw T i;cw
0
ð8Þ
QE QH þ WS
_ ðmCpÞ hw t cycle
Desorption (DES)
50
0
where WS denotes the work input from a spray pump in the evaporator. QH is the total heat input to the system and calculated from the hot water behavior.
QH ¼
Cooling Water Outlet
20
tcycle
0
Chilled Water Outlet
60
ð11Þ
4. Results and discussion The study of the zeolite adsorption chiller is carried out experimentally to investigate its operational behavior and the influence of key parameters to its performance. The behavior is reflected by the outlet temperature of the thermal source of the respective components and the pressure inside their chamber. Inlet temperature of hot water is one variable changing from 55 °C to 95 °C. The chilled water outlet temperature is maintained at 12 °C ± 2 °C expect that in the 55 °C heat source condition 18 °C ± 1 °C is observed due to the chiller limitation. The other key parameter to be studied is the adsorption/desorption cycle time. A range between 100 s and 400 s is tested. Other operation parameters are listed in Table 2. 4.1. Temperature and pressure profile A direct result from the batch-wise operation of the chiller is the cyclic manner of the temperature and pressure profile. Fig. 3 displays the reactor beds’ temperature history during a typical cycle at the hot water inlet temperature of 65 °C and the adsorption/desorption cycle time of 250 s. The reflected parameters are the outlet temperature of the thermal source at the relevant Table 2 Operation conditions of zeolite–water adsorption chiller. Parameter
Value
Inlet temperature of hot water Inlet temperature of cooling water Outlet temperature of chilled water Flow-rate of hot water Flow-rate of cooling water Flow-rate of chilled water Adsorption/desorption cycle time Switching period
55–95 °C 27 °C ± 1 °C 12 °C ± 2 °C 3.8 m3/h 8.2 m3/h 2.3 m3/h 100–400 s 22 s
100
200
300
400
500
600
700
Time [s] Fig. 3. Temperature profile of the zeolite–water adsorption chiller at hot water temperature of 65 °C and adsorption/desorption cycle time of 250 s.
components. One can observe, in an adsorption/desorption period, a rapid change of the values is taking place in the beginning, whereas the profile tends to be steady afterwards. As illustrated in the first adsorption/desorption cycle time, for example, bed 1 starts desorption at an intermediate temperature. This is due to the fact that at the end of switching from cooling to heating source, the bed still remains at a relatively cool state. The large temperature difference between the bed and the heat source drives a faster heat transfer than any other part of the period. The adsorption for bed 2 can be explained in a similar way. In contrast, mass transport plays a major role in the variation of chilled water and cooling water outlet temperature. The initiated adsorption and desorption trigger massive molecular transfer from the evaporator and to the condenser, leading to an accelerated cooling production and heat rejection at the early stage of adsorption/desorption period. With the ongoing sorption process in the later stage, both beds are moving towards saturation, flattening the respective temperature profiles. In addition, the temperature profiles in the switching period are determined by the flow pattern of thermal sources. The temporal pressure profiles of the four chambers at the same period as above are shown in Fig. 4. They reflect clearly the interactions of the components. During the adsorption/desorption period, the pressure of the adsorption (or desorption) bed is equalized with the evaporator (or condenser) after the valve in between is open. However, at the start of the switching period, a sudden change in the heat source also alters the pressure of the reactor beds, causing their pressure departs from the condenser or evaporator. As a result, lever valves are shut and stop mass transport in
Bed 1
Bed 2
Evaporator
Condenser
A Complete Cooling Cycle 50
Adsorption / Desorption SwitchingPeriod Period
40
Pressure [Torr]
_ ðmCpÞ chi QE ¼ t cycle
Bed 2 Outlet
A CompleteCooling Cycle Adsorption/ Desorption Switching Period Period
70
Temperature [ºC]
T des=ads ¼ T i;hw=cw;ads þ ðT o T i Þhw=cw;ads " !#1 ðUAÞdes=ads 1 exp _ ðmCpÞ hw=cw;ads
Bed 1 Outlet
80
Desorption (DES 30
20 Adsorption (ADS) 10
0 0
100
200
300
400
500
600
700
Time [s] Fig. 4. Pressure profile of the zeolite–water adsorption chiller at hot water temperature of 65 °C and adsorption/desorption cycle time of 250 s.
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4.3. Influence of adsorption/desorption cycle time A further investation of the role of adsorption/desorption cycle time is provided in this subsection. Fig. 6a shows its influence on the total heat input. Generally, increasing adsorption/desorption cycle time helps to ease the demand of thermal energy. The influence is stronger on the shorter periods and high hot water temperature since trend lines are steeper. A major cause to this phenomenon is due to the water mixing. At the moment the beds swaps the function, an amount of cooling water is mixed into the hot water stream. Part of the heat input is inevitably lost because of the invasion of the low temperature source. If shorter adsorption/desorption cycle time is used, more frequent switching takes place, which may translate to increasing heat loss during the role swapping period and hence growing energy consumption. Compared to the heat input, the cooling capacity in Fig. 6b at elongated adsorption/desorption cycle time is not monotonous. It increases till 200 s and then starts droping, while after 300 s
Heat Input, QH [kW]
This subsection describes the effect of the hot water inlet temperature to the performance of the adsorption chiller. Evaluation parameters are presented in terms of total heat input, cooling capacity as well as the COP in various adsorption/desorption cycle times. In Fig. 5a, the demand to the total heat input increases with assorted hot water temperature. The trend tends to be smaller at higher end, which is more clear when the adsorption/desorption cycle time is long. Moreover, the input at 95 °C is approximately double of that at lower end. On the contrary, cooling capacity is not following the trend. As shown in Fig. 5b, the cooling production experiences a peak as the hot water temperature increases. At the adsorption/desorption cycle time of 100–200 s, the maximum is obtained at around 85 °C with the highest recored production of 12.13 kW at the latter. In the rest of the adsorption/desorption cycle times, the cooling effect is caped at 65–75 °C. It is observed that peak cooling capacity shifts towards to the left as the adsorption/desorption cycle time lasts longer. In addition, a significant reduction is realized at 55 °C. It shows that regeneration of the bed is not sufficiently conducted at this temperature level. Furthermore, a spray pump located in the evaporator has a constant power consumption of 0.6 kW. With the presence of the pump power, heat input and cooling capacity, the chiller’s COP can be computed. The experiment indicates that the maximum COP is observed at hot water inlet temperature of around 65 °C for all adsorption/desorption cycle times, as illustrated in Fig. 5c. The highest COP value is 0.44 when the adsorption/desorption cycle time is set to 400 s. The hot water temperature determines the desorption process and hence the cooling production of the chiller. In a given adsorption/desorption cycle time, the quality of bed regeneration has a direct relation with the bed thermal status. The isotherm theory of adsorption indicates that the amount of equilibrium uptake of vapor reduces when the temperature raises, meaning that a higher heat source temperature gives a better regeneration condition [42]. However, hotter heat source leads to higher desorption and condensation temperature, and hence the pressure. As the equilibrium uptake tends to increase with the pressure, the regeneration of the DES bed is inversely affected. This dilemma may explain the variation of the cooling capacity at different hot water temperature.
100 s
150 s
200 s
250 s
300 s
350 s
400 s
50
40
30
20
10 50
60
70
80
90
100
Temperature of Inlet Hot Water, Ti,hw [ºC]
(b)
100 s
150 s
200 s
250 s
300 s
350 s
400 s
13 12
Cooling Capacity, QE [kW]
4.2. Influence of hot water temperature
(a)
11 10 9 8 7 6 5 4 3 50
60
70
80
90
100
Temperature of Inlet Hot Water, Ti,hw [ºC]
(c) 0.45 Coefficient of Performance [-]
this period. Besides, pre-heating the ADS reactor increases its pressure, and vice versa for DES bed. This also indicates that the ‘valveless’ valves have fulfilled the functions of traditional electrical or pneumatic activated butterfly valves.
100 s
150 s
200 s
250 s
300 s
350 s
400 s
0.4 0.35 0.3 0.25 0.2 0.15 50
60
70
80
90
100
Temperature of Inlet Hot Water, Ti,hw [ºC] Fig. 5. Influence of hot water inlet temperature to (a) total heat input, (b) cooling capacity, and (c) COP of the adsorption chiller at various adsorption/desorption cycle times.
becomes stable. At 55 °C hot water temperature the cooling production at all adsorption/desorption cycle time is exceptionally lower than other temperature, indicating that the chiller has reached its lower limit. In general, however, the influence of adsorption/desorption cycle time to the cooling production is insignificant as compared to the heat input. As expected, the dominance of the heat input leads to an increase of the COP with respect to the duration of the adsorption/desorption period (See Fig. 6c). The adsorption/desorption cycle time affects both reactors. For desorption, a more thorough regeneration can be achieved when the sorption duration is longer. This is beneficial to the next cycle’s cooling production as a better regeneration condition is able to provide greater adsorption driving force to the evaporated refrigerant molecules, especially in the beginning period of the process.
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(a)
55 C
65 C
75 C
85 C
95 C
250
300
350
Heat Input, Q H [kW]
50
40
30
20
10 50
100
150
200
400
450
Adsorption / Desorption Cycle Time [s]
(b)
55 C
65 C
75 C
85 C
250
300
95 C
13
Cooling Capacity, Q E [kW]
12 11 10 9 8 7 6 5 4 3 50
100
150
200
350
400
450
Adsorption / Desorption Cycle Time [s]
Coefficient of Performance [-]
(c) 0.45
55 C
65 C
75 C
150
200
250
85 C
95 C
0.4 0.35 0.3 0.25 0.2 0.15 50
100
300
350
400
450
Adsorption / Desorption Cycle Time [s] Fig. 6. Influence of adsorption/desorption cycle time to (a) total heat input, (b) cooling capacity, and (c) COP of the adsorption chiller at various hot water inlet temperatures.
However, adsorption rate reduces extensively when the adsorbate uptake is approaching to saturation. Long exposure to the evaporator eventually leads to diminishing cooling rate, especially at the end of the adsorption/desorption period. This may explain the relative insensitivity of the cooling capacity to the various adsorption/ desorption cycle time. 4.4. Entropy analysis A computational result of the entropy generation on the adsorption/desorption period for the zeolite–water adsorption chiller is presented below. From two aspects, i.e. the inlet hot water temperature and the adsorption/desorption cycle time, the irreversibility of the chiller major components is analyzed. A term named as
specific entropy generation, which is defined as the cyclic average entropy generation per unit power of cooling capacity, is used to reveal the behavior of the chiller. Values of some parameters used in the calculation are shown in Table 3. The sensible heat of the heat exchanger material, zeolite and adsorbed phase is computed from energy balance. Water property functions are provided by Wagner et al. [43]. The rest are taken from experimental data. Fig. 7 displays the temporal entropy generation rate of each major component and the total value in a typical adsorption/desorption period in the conditions of the hot water inlet temperature 65 °C and adsorption/desorption cycle time 200 s. The cooling water inlet and chilled water outlet are 26.5 °C and 11 °C, respectively. As illustrated, a significant amount of entropy is produced by both reactor beds in the beginning of the adsorption/desorption period. The degree of magnitude is one order greater than that of the evaporator and condenser. However, the entropy generation of both beds decreases rapidly as time proceeds. After 70 s the profiles of both beds join to the evaporator and condenser whose entropy production is more consistent throughout the period. In the initial part of the adsorption/desorption period, two phenomena is observed. Firstly, there exist a large temperature difference between the reactors and the respective thermal sources. Secondly, a strong driving force of mass transport is also present. These result in a large expected entropy generation. As the beds approach the thermal source temperature and adsorbed phase saturation, irreversibility is also diminished. In the overall process, the cyclic average of total entropy production is 5.39 W/K, in which the DES bed encounters the most irreversibility, followed by the ADS bed. The smallest entropy generation is observed in the condenser. Cyclic average entropy generation rate of every component in the adsorption / desorption period with respect to the temperature of inlet hot water is illustrated in Fig. 8. The adsorption/desorption cycle time is fixed at 200 s. It is shown that production of entropy escalates linearly to the heat source temperature for all components in which the desorption bed is influenced most severely. This is reasonable since the DES bed endures the less efficient heat transfer efficiency. Furthermore, increasing the hot water temperature at the same time enlarges the temperature disparity between adsorption and desorption state. This results in a growing gap of temperature between the DES bed contents and the heat source in the initial stage of the adsorption/desorption period. Since the starting part is responsible for provoking tremendous amount of dissipation, the cyclic average of the entropy generation in the DES bed is more profoundly influenced. On the other hand, with the help of sufficient pre-cooling and more effective heat transfer, the ADS bed acts more resistively to the irreversibility as the hot water temperature increases. The evaporator and condenser are impacted through indirect ways by mass transport. A further investigation of contribution of each component to the total entropy generation at 200 s adsorption/desorption cycle time and various heat source temperatures is represented in Fig. 9. Clearly, the DES bed is responsible for the biggest portion of dissipation, approximately 50–70% of the total, among four components. Both reactors courts for 80% in average of entropy
Table 3 Values of parameters used in the computation of entropy. Parameter
Value
Pre-exponential surface diffusion factor, Dso Activation energy, Ea Average radius of zeolite particles, Rp Heat of adsorption, Qst Mass of the FAM Z01 zeolite, Mzl ADS bed heat transfer coefficient, (UA)ads DES bed heat transfer coefficient, (UA)des
2.54 104 m2/s 4.55 104 kJ/kmol 1.0 104 m 3110 kJ/kg [34] 12 kg 5.13 kW/K 3.13 kW/K
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A. Li et al. / Applied Energy 130 (2014) 702–711 ADS
DES
Evap
Total
Cond
ADS
25
1
Evap
Cond
20
0.5
80%
Percentage Contribution
Entropy Generation Rate, dSgen /dt [W/K]
DES
100%
30
0
15
160
170
180
10
60%
40%
20% 5 0% 55
0 0
50
100
150
200
65
75
85
95
Temperature of Inlet Hot Water, Ti,hw [ºC]
Time [s] Fig. 7. Temporal entropy generation rate of chiller major components in a typical adsorption/desorption period (the result is computed from the experimental data in the conditions of hot water inlet temperature 65 °C and adsorption/desorption cycle time 200 s).
ADS
DES
Evap
Cond
Fig. 9. Percentage of entropy generation of the chiller major components in the adsorption/desorption period at 200 s adsorption/desorption cycle time and increasing temperature of inlet hot water.
Specific Entropy Generation
Total
COP
COP Carnot
1.6
10
8
6
4
2
1.4 3 1.2 2.5 1 2 0.8 1.5
0.6 0.4
1
0.2
0.5
0
0 50
0 50
60
70
80
90
60
Fig. 8. Cyclic average entropy generation rate of the chiller major components in the adsorption/desorption period at 200 s adsorption/desorption cycle time and increasing temperature of inlet hot water.
70
80
90
100
Temperature of Inlet Hot Water, Ti,hw [ºC]
100
Temperature of Inlet Hot Water, Ti,hw [ºC]
Coefficient of Performance, Carnot [-]
Specific Entropy Generation [W/K-kW] & Coefficient of Performance [-]
Entropy Generation Rate, dSgen /dt [W/K]
3.5 12
Fig. 10. Specific entropy generation, COP and Carnot COP at 200 s adsorption/ desorption cycle time and assorted temperature of inlet hot water.
ADS
DES
Evap
Cond
Total
250
300
350
generation, implying that improvement should be made to address high irreversibility of the reactors. Entropy generation is a direct measurement of inefficiency of a system. It reveals how far away the system is to the thermodynamic limit, or in the case of adsorption chiller, the Carnot COP. Fig. 10 illustrates specific entropy generation, COP and Carnot COP of the adsorption chiller at the tested hot water temperature range. Although increasing heat source temperature brings up the theoretical Carnot limit of the chiller, the chiller’s real COP is following an opposite trend after 65 °C. This is, however, not unpredictable as specific entropy generation rate is accelerated after this temperature, leading to growing gap between the chiller’s efficiency to the upper limit. Figs. 11–13 analyzes the cyclic average entropy generation rate with respect to the duration of adsorption/desorption period. The result is computed based on 65 °C inlet hot water experimental data. In Fig. 11, the average generation rate of both reactor beds decreases with increasing duration of adsorption/desorption period, whereas that of the evaporator and the condenser experiences only little change. The total average entropy production is thus
Entropy Generation Rate, dS gen /dt [W/K]
7
6
5
4
3
2
1
0 50
100
150
200
400
450
Adsorption / Desorption Cycle Time [s] Fig. 11. Cyclic average entropy generation rate of the chiller major components in the adsorption/desorption period at 65 °C inlet hot water and various adsorption/ desorption cycle times.
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A. Li et al. / Applied Energy 130 (2014) 702–711 ADS
DES
Evap
Entropy generation of the adsorption chiller is presented for several key parameters, namely (i) the adsorption/desorption cycle time, (ii) the hot water inlet temperature. The experiments suggest that the cyclic average entropy generation rate increases with increasing hot water temperature and reduces with decreasing cycle time. It is found that, amongst all components in the plant, the desorption bed contributes to the biggest portion of irreversibilities. This is followed by the adsorption bed, the evaporator and the condenser unit. On a specific entropy generation basis, defined as entropy generation per unit mass of useful effect, it demonstrates the optimized efficiency of the adsorption chiller, i.e., the highest COP, occurs when the specific entropy generation is smallest.
Cond
100%
Percentage Contribution
80%
60%
40%
20%
0% 100
150
200
250
300
350
400
Adsorption / Desorption Cycle Time [s] Fig. 12. Percentage of entropy generation of the chiller major components in the adsorption/desorption period at 65 °C inlet hot water and various adsorption/ desorption cycle times.
Specific Entropy Generation
COP
COP Carnot 2.5
0.8
2
0.6
1.5
0.4
1
0.2
0.5
0
Coefficient of Performance, Carnot [-]
Specific Entropy Generation [W/K-kW] & Coefficient of Performance [-]
1
0 50
150
250
350
450
Adsorption / Desorption Cycle Time [s] Fig. 13. Specific entropy generation, COP and Carnot COP at 65 °C inlet hot water and assorted adsorption/desorption cycle time.
decreased. In terms of percentage contribution, as shown in Fig. 12, the DES bed also declines as adsorption/desorption cycle time is extended. On the contrary, the contribution of the rest of the components is amplified. In addition, it is noted that the percentage contribution of entropy generation of every major component tends to be more evenly distributed at longer sorption duration. In Fig. 13, the Carnot COP of the chiller is fairly a constant regardless of adsorption/desorption cycle time, due to a maintained thermal reservior temperatures. Moreover, it is clearly indicated that at the same thermal reservior temperature, minimum specific entropy generation leads to the maximum COP of the adsorption chiller. 5. Conclusion An adsorption chiller using FAM Z01 zeolite–water as the adsorbent–adsorbate pair is presented. Experimental evaluation indicates that the cooling capacity increases with heat input over the supplied hot water temperatures and a peak in cooling capacity occurs at 65–85 °C for a wide range of cycle time. Optimum hot water temperature for most efficient operation is 65 °C. However, the cooling capacity reduces at longer sorption duration due to diminishing uptake potential of adsorbent. Our experiments indicated an optimum adsorption/desorption cycle time for the tube-fin heat exchanger design is between 200 s and 300 s.
References [1] Li A, Loh WS, Ng KC. Transient modeling of a lithium bromide-water absorption chiller. Appl Mech Mater 2013;388:83–90. [2] Critoph R, Zhong Y. Review of trends in solid sorption refrigeration and heat pumping technology. Proc Inst Mech Eng Part E J Process Mech Eng 2005;219:285–300. [3] Meunier F. Solid sorption heat powered cycles for cooling and heat pumping applications. Appl Therm Eng 1998;18:715–29. [4] Ülkü S. Solar adsorption heat pumps, solar energy utilization: fundamentals and applications. The Netherlands: Martinus Nijkoff Publishers; 1987. [5] Meunier F. Adsorptive cooling: a clean technology. Clean Technol Environ Policy 2001;3:8–20. [6] Ulku A, Mobedi M. Adsorption in energy storage. NATO ASI Series. Series E Appl Sci 1989;167:487–507. [7] Wang X, Ng KC. Experimental investigation of an adsorption desalination plant using low-temperature waste heat. Appl Therm Eng 2005;25:2780–9. [8] Chua H, Ng K, Malek A, Kashiwagi T, Akisawa A, Saha B. Multi-bed regenerative adsorption chiller—improving the utilization of waste heat and reducing the chilled water outlet temperature fluctuation. Int J Refrig 2001;24:124–36. [9] Ng KC, Thu K, Chakraborty A, Saha BB, Chun WG. Solar-assisted dual-effect adsorption cycle for the production of cooling effect and potable water. Int J Low-Carbon Technol 2009;4:61–7. [10] Wang DC, Xia ZZ, Wu JY. Design and performance prediction of a novel zeolite– water adsorption air conditioner. Energy Convers Manage 2006;47:590–610. _ Kaftanog˘lu B, Yamalı C, Baker D. Experimental investigation of a [11] Solmusß I, natural zeolite–water adsorption cooling unit. Appl Energy 2011;88:4206–13. _ Yamalı C, Kaftanog˘lu B, Baker D, Çag˘lar A. Adsorption properties of a [12] Solmusß I, natural zeolite–water pair for use in adsorption cooling cycles. Appl Energy 2010;87:2062–7. [13] Vasiliev LL, Mishkinis DA, Antukh AA, Vasiliev Jr LL. Solar–gas solid sorption heat pump. Appl Therm Eng 2001;21:573–83. [14] Loh WS, Saha BB, Chakraborty A, Ng KC, Chun WG. Performance analysis of waste heat driven pressurized adsorption chiller. J Therm Sci Technol 2010;5:252–65. [15] Saha BB, Boelman EC, Kashiwagi T. Computational analysis of an advanced adsorption–refrigeration cycle. Energy 1995;20:983–94. [16] Saha BB, Kashiwagi T. Experimental investigation of an advanced adsorption refrigeration cycle. ASHRAE Trans 1997;133:50–7. [17] Saha BB, Akisawa A, Kashiwagi T. Solar/waste heat driven two-stage adsorption chiller: the prototype. Renew Energy 2001;23:93–101. [18] Saha BB, Koyama S, Kashiwagi T, Akisawa A, Ng KC, Chua HT. Waste heat driven dual-mode, multi-stage, multi-bed regenerative adsorption system. Int J Refrig 2003;26:749–57. [19] Wang X, Chua HT, Ng KC. Experimental investigation of silica gel–water adsorption chillers with and without a passive heat recovery scheme. Int J Refrig 2005;28:756–65. [20] Alam KCA, Akahira A, Hamamoto Y, Akisawa A, Kashiwagi T. A four-bed mass recovery adsorption refrigeration cycle driven by low temperature waste/ renewable heat source. Renew Energy 2004;29:1461–75. [21] Ng KC, Thu K, Saha BB, Chakraborty A. Study on a waste heat-driven adsorption cooling cum desalination cycle. Int J Refrig 2012;35:685–93. [22] Chen CJ, Wang RZ, Xia ZZ, Kiplagat JK, Lu ZS. Study on a compact silica gel– water adsorption chiller without vacuum valves: design and experimental study. Appl Energy 2010;87:2673–81. [23] Shahzad MW, Ng KC, Thu K, Chun WG. Multi effect desalination and adsorption desalination (MEDAD) cycle. In: International symposium on innovative materials for processes in energy systems 2013, Fukuoka, Japan; 2013. p. 115–119. [24] Thu K, Kim Y-D, Amy G, Chun WG, Ng KC. A hybrid multi-effect distillation and adsorption cycle. Appl Energy 2013;104:810–21. [25] Chua HT, Ng KC, Malek A, Kashiwagi T, Akisawa A, Saha BB. Modeling the performance of two-bed, silica gel–water adsorption chillers. Int J Refrig 1999;22:194–204. [26] Rezk ARM, Al-Dadah RK. Physical and operating conditions effects on silica gel/ water adsorption chiller performance. Appl Energy 2012;89:142–9.
A. Li et al. / Applied Energy 130 (2014) 702–711 [27] Zhao Y, Hu E, Blazewicz A. Dynamic modelling of an activated carbon– methanol adsorption refrigeration tube with considerations of interfacial convection and transient pressure process. Appl Energy 2012;95:276–84. [28] Chua HT, Ng KC, Wang W, Yap C, Wang XL. Transient modeling of a two-bed silica gel–water adsorption chiller. Int J Heat Mass Transfer 2004;47:659–69. [29] Choudhury B, Saha BB, Chatterjee PK, Sarkar JP. An overview of developments in adsorption refrigeration systems towards a sustainable way of cooling. Appl Energy 2013;104:554–67. [30] Meunier F, Poyelle F, LeVan MD. Second-law analysis of adsorptive refrigeration cycles: the role of thermal coupling entropy production. Appl Therm Eng 1997;17:43–55. [31] Meunier F. Second law analysis of a solid adsorption heat pump operating on reversible cascade cycles: application to the zeolite–water pair. J Heat Recov Syst 1985;5:133–41. [32] Chua HT, Ng KC, Malek A, Kashiwagi T, Akisawa A, Saha BB. Entropy generation analysis of two-bed, silica gel–water, non-regenerative adsorption chillers. J Phys D Appl Phys 1998;31:1471–7. [33] Ng KC. Thermodynamic tools for chiller diagnostics and optimization. Heat Transfer Eng Dec 2004;25:1–4. [34] Kakiuchi H, Iwade M, Shimooka S, Ooshima K, Yamazaki M, Takewaki T. Novel zeolite adsorbents and their application for AHP and desiccant system, presented at the IEA-Annex 17 meeting, Beijing; 2004.
711
[35] Kakiuchi H, Shimooka S, Iwade M, Oshima K, Yamazaki M, Terada S, et al. Novel water vapor adsorbent FAM-Z01 and its applicability to an adsorption heat pump. Kagaku Kogaku Ronbunshu 2005;31:361–4. [36] Radha AV, Bomati-Miguel O, Ushakov SV, Navrotsky A, Tartaj P. Surface enthalpy, enthalpy of water adsorption, and phase stability in nanocrystalline monoclinic zirconia. J Am Ceram Soc 2009;92:133–40. [37] Zhang H, Penn RL, Hamers RJ, Banfield JF. Enhanced adsorption of molecules on surfaces of nanocrystalline particles. J Phys Chem B 1999;103:4656–62. [38] Yanagi H, Ino N. Heat and mass transfer characteristics in consolidated silica gel–water adsorption-cooling system, presented at the ASME ASIA’97; 1997. [39] Ismail AB, Li A, Thu K, Ng KC, Chun W. On the thermodynamics of refrigerant + heterogeneous solid surfaces adsorption. Langmuir 2013;29:14494–502. [40] Thu K, Kim Y-D, Myat A, Chun WG, Ng KC. Entropy generation analysis of an adsorption cooling cycle. Int J Heat Mass Transfer 2013;60:143–55. [41] Ismail A, Loh WS, Thu K, Ng KC. A study on the kinetics of propane-activated carbon: theory and experiments. Appl Mech Mater 2013;388:76–82. [42] Loh WS, Ismail AB, Xi B, Ng KC, Chun WG. Adsorption isotherms and isosteric enthalpy of adsorption for assorted refrigerants on activated carbons. J Chem Eng Data 2012;57:2766–73. [43] Wagner W, Cooper JR, Dittmann A, Kijima J, Kretzschmar HJ, Kruse A, et al. The IAPWS industrial formulation 1997 for the thermodynamic properties of water and steam. J Eng Gas Turbines Power 2000;122:150–80.