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Experimental investigation on the optimal performance of Zeolite–water adsorption chiller
Applied Energy 102 (2013) 582–590
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Applied Energy journal homepage: www.elsevier.com/locate/apenergy
Experimental investigation on the optimal performance of Zeolite–water adsorption chiller Aung Myat a,⇑, Ng Kim Choon a, Kyaw Thu b, Young-Deuk Kim b a b
Mechanical Engineering Department, National University of Singapore, 9 Engineering Drive 1, Singapore 117576, Singapore King Abdullah University of Science and Technology, Water Desalination and Reuse Center, Thuwal 23955-6900, Saudi Arabia
h i g h l i g h t s " Experimental analysis on performance of Zeolite adsorption chiller. " To optimize the performance of the adsorption cooling system. " To investigate effect of cycle time and switching time on the performance of system.
a r t i c l e
i n f o
Article history: Received 5 October 2011 Received in revised form 25 July 2012 Accepted 7 August 2012 Available online 7 September 2012 Keywords: Adsorption Waste heat Zeolite Environmentally friendly
a b s t r a c t This paper presents the performance testing of Zeolite adsorption cooling system driven by low grade waste heat source extracted from prime mover’s exhaust, power plant’s exhaust and the solar energy. The adsorbent FAM Z01 is used as an adsorbent in the adsorption chiller facility. Owing to its large equilibrium pore volume, it has the high affinity for the water vapor adsorbate. The key advantages of the Zeolite adsorption cooling system are: (i) it has no moving parts rendering less maintenance, (ii) the energy efficient means of cooling by the adsorption process with a low temperature heat source, (iii) the use of vapor pipes are replaced by self actuating vapor valves rendering smaller footprint area and (iv) it is environmental friendly with low carbon footprint. The experimental investigations were carried out for Zeolite adsorption chiller at different key operating conditions namely (i) heat source temperature, (ii) the cycle time and (iii) the heat recovery time. It is investigated that performance of coefficient (COP) of this system could be as high as 0.48 while the waste heat source temperature is applicable as low as 55 °C. Crown Copyright Ó 2012 Published by Elsevier Ltd. All rights reserved.
1. Introduction Cooling or heating produced in conventional vapor compression cooling systems uses ozone-depleting refrigerants such as chlorofluorocarbons (CFCs). Despite the development of less ozone damaging alternatives such as the hydro chlorofluorocarbons (HCFCs) and the hydro fluorocarbons (HFCs), they still contribute to the undesirable effect of global warming. Furthermore, such cooling systems utilize electricity and fossil fuels as their driving sources. As cooling demand for climate control and refrigeration soars in the future, it will lead to increasing energy consumption. This intrinsically leads to a faster depletion of known fossil fuel reserves, more carbon dioxide emissions and a higher peak electricity demand. Such environmental issues have intensified research efforts on the development of environmentally benign refrigerants and energy saving cooling technologies and renewed interest in cooling applications. ⇑ Corresponding author. Tel.: +65 6796 7370. E-mail address: [email protected] (A. Myat).
One form of environmentally friendly alternatives for vapor compression cooling is the sorption cooling systems. In the past few decades, adsorption processes have been used exclusively for gas separation and purification purposes. It is only recently that this phenomenon has been exploited to produce cooling and heating. Promising technologies include heat driven liquid absorption cooling system [1,2] and solid sorption heat pumps [2]. As they can be driven by waste heat or solar energy and uses environmentally benign chemicals, both their ODP (ozone depletion potential) and GWP (global warming potential) are virtually zero [3]. Absorption cycles which use lithium bromide–water (LiBr–H2O) or water–ammonia (H2O–NH3) pairs encounter many operational limitations. For example, thermally driven single-stage LiBr–H2O absorption chiller utilizes relatively expensive corrosion resistant alloy steel due to the corrosive nature of the LiBr–H2O. Due to the risk of crystallization, the lowest temperature that can be used for generator the said type of chiller, without crystallization, is 61 °C in combination with a coolant inlet temperature at 32 °C [3]. Hence, the applications of absorption cycles are limited to high and medium temperature heat sources. Furthermore, a solution
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A. Myat et al. / Applied Energy 102 (2013) 582–590
pump has to be employed for the circulation of the LiBr solution from absorber to generator. The crystallization of salt solution and higher cooling water in temperature are major concerns for the liquid sorption chiller. Thus, our main focus is on the adsorption (solid adsorbent) chiller. It has potential to apply very low grade waste heat and less maintenance. It could also avoid the potential crystallization issue which is normally a big issue with the liquid sorption chiller. The use of low grade waste heat available from power plant is an important contemporary issue. Adsorption cycles which use silica gel–water [4,5], activated carbon–methanol [6,7] and Zeolite– water [8–11] adsorbent–refrigerant pairs have a distinct advantage as they can be driven by heat of relatively low, near environmental temperature such that waste heat below 80 °C can be recovered [12]. Besides, these systems are non-corrosive and required almost no moving parts rendering less maintenance. The adsorption chiller has many advantages, fewer moving parts, simple construction, being able to be driven by waste heat as low as 60 °C. Therefore, the adsorption devices can be economically and technologically competitive with the abundant supply of waste heat. Silica gel adsorption chillers that use water vapor as the refrigerants are one of the most widespread adsorption chillers around as they can be driven by low grade heat source temperature about 80 °C. However, the output cooling power per unit volume of such chiller is much smaller than absorption heat pump, resulting in bulky and space consuming silica gel adsorption chillers. The large sizing is one of the reasons why the adoption of such chillers is not as widespread. Recently, various heat and mass transfer models have been proposed to study the thermal performance in terms of the coefficient of performance (COP) and specific cooling power (SCP) of adsorption cooling systems [12–18]. Nevertheless, the effects of operating conditions on the thermal performance of such systems especially those pertaining to operating temperature effects are scarcely reported in the literature. The effect of operating conditions on the adsorption cooling cycle based on thermodynamic analyses has been numerically investigated by a number of researchers [19–24]. Only a few literatures could be referred to the Zeolite–water pair adsorption chiller [8–11]. Adsorption properties such as equilibrium uptake and heat of adsorption were experimentally investigated and compared with the different adsorption pair such silica gel water and carbon methanol [8]. It was reported that Zeolite–water pair is the least dependency on the evaporator and condenser pressure as compared to silica gel– water and carbon–methanol adsorption pair and it has the lowest regeneration temperature among those adsorbent–adsorbate pairs. Performance of a natural Zeolite–water adsorption chiller was experimentally investigated at various evaporation temperatures [9]. It was reported that mean COP is achieved about 0.25. The experimental investigation on the performance of Zeolite– water adsorption chiller at different operational parameters had yet to be reported. Thus, the motivation of this paper is to conduct the experiment and investigate the performance of Zeolite–water adsorption chiller at assorted operation conditions; namely (i) heat source temperature, (ii) cycle time and (iii) switching time and heat recovery time.
2. System description The adsorption cooling (AD) cycle is one of the environmentally friendly methods to produce the cooling by utilizing the low grade waste heat source typically 55–85 °C. The AD cycle comprises of three major components namely (1) the evaporator, (2) the condenser and (3) regenerative adsorber and desorber beds where the adsorbent is placed. Fig. 1 shows the schematic layout of AD cycle.
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2.1. Description of the Zeolite water adsorption chiller In this context, the authors would like to introduce the compactness of the waste heat driven Zeolite–water adsorption chiller. In order to make the chiller size to be compact, there are some possible solution such as (i) enhanced the surface area of the adsorbent so as to reduce the foot print area of the chiller and (ii) introduced the self actuation facility to eliminate the vapor pipes connecting to the adsorber–evaporator and desorber–condenser. In addition, introducing such a self actuating valve facility could also eliminate the valve sequence controlled for the opening and closing for the vapor flow to adsorber and condenser, respectively. 2.1.1. Adsorption properties (equilibrium uptake) of Zeolite FAM Z01 For the enhancement of adsorbent, Functional Adsorbent Material Z01 (FAM Z01) is employed and it is an AFI type molecular sieve. It has a one-dimensional structure with 0.73 nm windows and its framework consists of AlO4, PO4 and FeO4 tetrahedrons, where Al and P atoms are partially substituted by Fe atoms. The advantage of FAM Z01 over silica-gel is that regeneration can be performed at the temperature range of 50–80 °C [25]. It is also important to investigate the adsorption properties such as equilibrium uptake of water vapor at assorted temperatures [26]. The isotherm characteristic of FAM Z01 is illustrated in Fig. A.1. In Fig. A.1, relative pressure on the horizontal axis is defined as the ratio of vapor pressure of the reactors bed, which is denoted as Pads for adsorber bed and Pdes for desorber bed, to the saturation pressure at the given temperature. As shown in Fig. A.1, at an adsorption temperature of 25 °C (298 K) and its corresponding saturation pressure (Psat) is found to be 3.17 kPa whilst adsorption pressure, Pads, is approximately 0.8 kPa. This gives a relative pressure (Pads/Psat) of 0.25. By locating a point on the adsorption isotherm with corresponding adsorption temperature of 298 K and relative pressure of 0.25, it is found that the uptake of adsorbate, q is about 0.19 kg-H2O/kg of dry mass of FAM Z01, at a desorption temperature of 60 °C (333 K) and its corresponding saturation pressure (Psat) is found to be 19.92 kPa whilst desorption pressure, Pdes, is approximately 3.73 kPa. Thus, relative pressure (Pdes/Psat) is about 0.18. Similarly, the equilibrium uptake, q of 0.02 kg-H2O/kg of dry mass of FAM Z01 is found at the corresponding desorption temperature of 333 K and relative pressure of 0.18. Thus, the average adsorption capacity Dq is thus 0.17 kg-H2O/kg of dry mass of FAM Z01 at adsorption and desorption temperature of 298 K and 333 K, respectively. As compared to silica gel–water equilibrium uptake at 25 °C (298 K) and 60 °C (333 k), Dq is found to be about 0.045 [27] and FAM Z01 has a significantly higher equilibrium uptake which leads to significantly reduce in size of the chiller. Thermo physical properties of FAM Z01 and Type A silica gel are furnished in Table 1. 2.1.2. Self actuating vapor valves (valve less design) of Zeolite–water adsorption chiller The ‘‘valve less’’ design is such that the connections between the sorption reactors and the condenser/evaporator are replaced by counter-weight ‘‘valves’’ instead of butterfly valves found in typical adsorption chiller. It works as simple lever in such a way that the opening or closing of the valves is dependent on the pressure difference in the adjacent chambers. Due to the low density of the refrigerant vapor, typical gas valves require large piping connections to reduce the pressure drop as vapor pass through. This contributes significantly to the footprint area of the adsorption chiller. The ‘‘valve less’’ design adopted in this research eradicates the problem of pressure drop and can reduce the footprint area significantly. The positions of the valves connecting the sorption reactors with the evaporator or condenser are determined by the pressure difference in the adjacent
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Fig. 1. The schematic lay out diagram of Zeolite adsorption chiller.
Fig. A.1. Adsorption and desorption isotherms of FAM Z01.
Table 1 Thermal properties of FAM Z01 and Type A silica gel. Properties Bulk density Effective thermal conductivity Differential heat of adsorption (H2O, 298 K) Specific heat
FAM Z01
A type silica gel
(kg/dm3) (W/mK) kJ/mol
0.6–0.7 0.113 (303 K) 56
0.6–0.7 0.12 (303 K) 52
kJ/kg K
0.805 (303 K)
0.92
Fig. A.2. The operation of self actuating valve or valve less facility equipped with Zeolite water adsorption chiller. (a) Close position P1 > P2 (X), (b) close to open P2 > P1 and (c) open position P2 > P1 (O).
chambers as shown in Fig. A.2. The pressure of the sorption chamber varies as heat is added or removed from the chamber to enable the adsorption or desorption processes to take place. The opening and closing of the valve is critical in determining whether
Fig. 2. Pictorial view of the Zeolite coated heat exchanger with coated thickness of 1–2.5 mm adsorbent layer.
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Fig. 3. The pictorial view of the Zeolite adsorption chiller test facility.
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adsorbent coated on the fins surface of the exchanger coil is shown in Fig. 2. A continuous vapor uptake is achieved when the vapor valve is opened. Owing to nature of the adsorption process, the heat is released during the vapor uptake and the adsorption process would continue to a preset cycle time. In general, the cycle time in the adsorption process is one of the parameters to control the cycle performance. The saturated FAM-Z01 (performed adsorption process previous half cycle) could be now regenerated by introducing a low grade waste heat source typically 50–85 °C and this is known as desorption process. The waste heat source or solar energy which is supplied to the saturated reactor beds to regenerate the water vapor. With the vapor valve set to open position, the regenerated water vapor flows to the cooler surfaces of a water cooled tubing and it condenses on the bundle of tubes and is sent through the U-tube to the evaporator where the cooling effect is produced. The condensation process is accompanied by the rejection of latent heat of condensation into the cooling water passing inside the tubing in the condenser. The AD cycle operates in batch-type process to achieve the useful effect using two reactor beds, cycling one of reactor beds as adsorption mode whilst the other is in desorption mode during the first half cycle of operation. The roles of the reactor beds are reversed in next half cycle. Prior to switching their roles, pre-cooling of desorber bed and preheating of adsorber beds need to be performed for a short period which is known as a switching process. The pictorial view of Zeolite adsorption test facility is illustrated in Fig. 3. The structure of Zeolite FAM Z01 is shown in Fig. 4a. Scanning Electron Micrograph (SEM) picture of FAM Z01 is presented in Fig. 4b. 2.3. Operation of Zeolite chiller running at switching mode
Fig. 4a. Structure of FAM Z01 Zeolite.
evaporation and condensation takes place in the respective chambers. By incorporating such a valve less facility to the waste heat driven adsorption chiller, the size of the chiller could be reduced significantly.
2.2. Operation of Zeolite chiller running at batch operated cycle As shown in Fig. 1, the condensate from the condenser is depressurized through the U-tube and sent to the evaporator where external heat to sustain the evaporation at sub atmospheric pressure, from chilled water flowing inside the tubing, is supplied. The evaporated water vapor is adsorbed by the adsorbent (Zeolite type FAM Z01) which is coated on the fins surface of heat exchanger housed inside the reactor beds. The evaporation process is enhanced by a spray system using the full-cone type nozzles. The
To improve efficiency, the residual heat contained in the adsorbent bed at the end of the desorption cycle is recovered during switching mode. The modes of operation during switching process are illustrated in Fig. 5. During Normal A mode, ADS 1 (Bed 1) is undergoing the desorption in the first half of the cycle as shown in Fig. 5 diagram 1. At the end of the first half cycle, ADS 1 remains hot with residual hot water in its finned tube heat exchanger and there is significant amount of heat contained in its adsorbent bed. If cooling water is directed straight into ADS 1 (Bed 1), much of this useful energy in ADS 1 (Bed 1) will be removed and dumped as waste heat as the cooling water carries it and releases it into the ambient in the cooling tower reservoir. To recover this heat, at the end of the first half of the cycle, the hot water supply to ADS 1 is directed to the bypass channel as shown in diagram (2). In diagrams (3)–(4), cooling water is then directed into ADS 1 and then to ADS 2 to push the residual hot water from ADS 1 (Bed 1) to ADS 2 (Bed 2). This is to pre heat the adsorbent bed in ADS 2
Fig. 4b. Scanning Electron Micrograph (SEM) pictures of FAM Z01 Zeolite.
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(1)
(4)
(2)
(5)
(3)
(6)
Fig. 5. Schematic diagrams to show the heat recovery processes during switching mode.
(Bed 2) to prepare it for desorption in the second half of the cycle. Thus, heat removal from the hot water supplied to ADS 2 during the second half can be reduced (diagram (5)), allowing for better
performance (COP) with same amount of cooling obtained. Meanwhile, ADS 1 undergoes pre-cooling to enhance the adsorption process at next cycle.
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3. Results and discussion This section describes the detailed of the experiment of Zeolite adsorption chiller. The experiments were performed for a nominal capacity of 3 Rton (10.5 kW) adsorption cooling system. The hot water source temperatures are varied from 60 °C to 85 °C whilst the cooling water and the chilled water temperatures are kept at around 29 °C ± 1° and 12 °C ± 1. The cycle time is varied from 100 to 700 s while switching time is set at 15 s. Fig. 6a dictates the temperature profiles of the outlet of heat transfer fluid operating under batch mode. It is found out that cycle average hot water outlet temperature is about 75 °C while adsorber cooling water outlet water temperature is about 33 °C. During the beginning of the adsorption process or desorption process, it is observed that the temperature gradient of all the heat transfer fluid is higher compared to the ending of the operation time. Figs. 6a and 6b also shows that chilled water outlet temperature can be achieved as low as 7 °C while the inlet temperature is set to be about 12 °C. During the first switching process in Figs. 6a and 6b, ADS 1 outlet temperature increases steeply as hot water is ‘‘push’’ from ADS 2 to ADS 1 to prepare it for desorption. It is observed that there is a significant temperature drop right after switching. This is because there is a sharp increase in the adsorbent bed temperature which causes the water molecules in the adsorbent to expand. This led to a very high rate of desorption due to the large pressure difference between the bed and the surrounding. During the first switching process, ADS 2 (Bed 2) outlet temperature undergoes a sharp decrease because cooling water is supplied to cool down the previously warm adsorbent bed. Around 300 s, it is observed that the outlet temperatures of the heat transfer fluids of ADS 1 and ADS 2 become close to the inlet temperature of each heat transfer fluids. While the beds are near their equilibrium state at that particular temperature and pressure, the sorption rates of reactor beds are significantly reduced. The pressure profiles of key components in the Zeolite adsorption cycle are illustrated in Fig. 7. The experiments were carried out at assorted half cycle time at different hot water inlet temperature to observe the sensitivity of the optimal performance of the system. The effect of hot water inlet temperature and the half cycle time on the key performance parameters namely (i) Coefficient of performance (COP), (ii) cooling capacity and (iii) heat input to the system are shown in Fig. 8. Average cooling capacity generally decreases with longer cycle time as shown in Fig. 8b. For the longer cycles, cooling capacity decreases because adsorption has become less intense after some time and gradually decreases as the adsorbent approaches its equilibrium conditions. The highest cooling capacity value was obtained at a half cycle time of 100 s for the higher heat source temperatures (70 °C and 80 °C). However, it is obtained at 200 s
Fig. 6b. Details temperature profiles of heat transfer fluids and bed during switching period.
Fig. 7a. Pressure–time history of Zeolite adsorption chiller.
Fig. 7b. Details pressure profiles of reactor beds, evaporator and condenser during switching period.
Fig. 6a. The performance of Zeolite adsorption chiller showing the outlet heat transfer fluid temperatures.
or 300 s for lower heat source temperatures (55 °C, 60 °C and 65 °C) instead of 100 s. This is because for the shorter cycles (evident from lower heat source temperatures), there is not enough time for adsorption to occur satisfactorily and the lower driving temperature is insufficient for desorption to take place. Therefore, cooling capacity decreases abruptly. For the higher heat source temperature, it is observed that cooling capacities would also drop for cycle times shorter than 100 s. As seen in Fig. 8, the consumption of driving heat source decreases with longer cycle time. This is because the reactor bed undergoing desorption mode is approaching to its equilibrium desorption that leads to minimal
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Fig. 9. The temperature–time histories of reactors outlet heat transfer fluids and chilled water outlet at different half cycle time.
Fig. 8a–c. The effect of half cycle time on the cycle average heat input, cooling capacity and coefficient of performance (COP).
requirement heat supply. For the initial part of the desorption cycle, large heat input is required owing to its considerably high desorption rates. Thus, it is occurred that there is the significantly high temperature gradient between the adsorbent bed and the hot water outlet temperature. Hence, short cycle times often require large average heat input. Fig. 8c shows the effect of the cycle times on the COP. Generally, the measured COP increases monotonically with time. This occurs because of the lower consumption of driving heat with longer cycles. It can be observed that both the average cooling capacity and the average heat input decrease with longer cycle time while COP increases with longer cycle time. It is also noted from Fig. 8c that a high COP of 0.48 is achieved when the heat source temperature is about 55 °C. Owings to its S-shaped isotherm nature (as shown in Fig. A.1) of FAM Z01, which is functionalized mainly for low temperature regeneration processes, the
equilibrium uptake (Dq) of adsorption–desorption (at 303 K and 333 K) is significantly higher than that of (303 K and 348 K) and this results that COP of chiller is high at lower waste heat temperature at the same cycle time. However, it is also observed that chiller COP is quite low at lower heat source temperature when it is running at a shorter cycle time (100 s). This is due to the fact that regeneration time of adsorbent is not enough at lower heat source (55 °C). However, such an even shorter cycle time (100 s) could be applied for the chiller running at higher heat source temperature (80 °C) because Kinetics or regeneration rate is faster at higher heat source temperature. Moreover, the cooling capacity and heat supplied to the system is decreased with a decrease in heat source temperature. Thus, COP is found to be higher at lower driving heat source temperature. The effect of cycle time on the outlet temperature of the heat transfer fluids is clearly demonstrated in Fig. 9. The longer the cycle time is, the smaller the temperature difference between inlet and outlet of heat transfer fluids will be. Thus, cooling capacity and heat input to the system is significantly reduced with the longer cycle time. The effect of heat source temperature on the key parameters such heat input, cooling capacity and the COP of the system is illustrated in Fig. 10. Generally, cooling capacity increases as the driving heat source temperature increases for the same cycle time as shown in Fig. 10b. With higher driving heat source temperatures, more desorption takes place at the same cycle time. Therefore, more heat is required to supply to reactor bed. However, for the half cycle time of 500 s, cooling capacity decreases when the heat source temperature is increased from 75 °C to 80 °C. For a cycle time of 500 s, a temperature of 80 °C may be excessive for the desorption process such that a temperature of 75 °C is sufficient to achieve the maximum amount of circulated refrigerant. Since the rate of uptake of adsorbent is compromised at higher
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Table 2 The optimal operation conditions for each of the waste heat activated temperature. Condition Temperature Half cycle Heat input Cooling capacity Average COP
°C s kW kW
1
2
3
4
5
55 515 12.92 5.71 0.442
60 515 16.8 7.78 0.463
65 515 18.2 8.728 0.480
70 415 23.15 10.58 0.457
80 415 26.36 11.52 0.437
Fig. 11. COP against heat recovery cycle time for a half cycle time of 400 s at various driving heat source temperature.
Fig. 10. The effect of heat source temperature on the heat input, cooling capacity and COP of Zeolite adsorption cooling system.
temperatures, a temperature of 80 °C may have been too high for cooling to occur satisfactorily during the adsorption process. With the lower rate of adsorption at the higher temperature of 80 °C, average cooling capacity naturally decreases. In fact for cooling capacity to be optimum at a half cycle time of 500 s, a temperature of 75 °C would have been optimal such that maximum amount of circulated refrigerant can be achieved and that the rates of adsorption are not compromised as much. For the same cycle time, heat input increases with higher heat source temperature as seen in Fig. 10a. At higher temperatures for the same cycle time, the temperature gradient between the adsorbent bed and driving heat source is higher. Hence, more heat can be transferred to the adsorbent bed, leading to higher heat inputs. Since both cooling capacity and heat input increases with increasing driving temperature, changes in COP is dependent upon the percentage increases of both cooling capacity and heat input respectively as shown in Fig. 10c.
The maximum COP is obtained at 70 °C and 65 °C with corresponding cycle times of 100–300 s and the higher cycle times 400–500 s respectively. The most optimal set of operating conditions is one which has a high cooling capacity and low heat input, thus resulting a high COP value. Such low grade temperature heat is considered to be of low value because it is often the by-product of power plant or industrial processes. Hence, in this case, more consideration would be given to the amount of cooling capacity provided. The optimal conditions (highest COP) with the performance indicators for each of the driving temperatures are furnished in Table 2. An optimal data set would be Data Set 4 where 10.6 kW of cooling is produced. It produces about 21% more cooling than Data Set 3 by using 24% more heat input. It is important to note that adsorption chillers in this context are supposed to utilize waste heat or solar heat which are of lower temperature range as compared to heat generated by burning fossil fuels. Hence, in consideration for the optimal operating conditions, more emphasis should be placed on the amount of cooling effect produced. It is appropriate as long as the additional heat input to produce the additional cooling is not excessively high. An optimal allocation of heat recovery time allows the system to recover residual heat in the adsorbent bed that previously underwent desorption and allows the other adsorbent bed to pre heat appropriately. However, excessive heat recovery time can actually abates the pre-heating of the adsorbent bed because the residual heat gained can be carried away by the cooling water flow in the excess heat recovery time. For the same cycle time, it is observed from Fig. 11 that optimal COP is achieved for a certain heat recovery time for each driving heat source temperature. For the higher driving heat source temperatures (70 °C and 80 °C), a heat recovery time of 12.5 s will give a peak COP value while for the lower driving heat source temperatures (55 °C, 60 °C and 65 °C), a recovery time of 15 s will give a peak COP value. As compared to the higher driving heat source temperature, the lower driving heat temperature needs longer heat recovery time to adsorb or desorb the refrigerant vapor from the adsorbent bed, which is the reason for the longer heat recovery cycle time. It is interesting
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to note that the COP for the heat recovery time of 10 s is smaller than that for 15 s for all of the optimal operating conditions in the graph. This is because with shorter heat recovery time, there is insufficient cooling of the adsorbent bed that underwent desorption previously. Since the adsorption capacity is compromised at a higher temperature, the adsorbent bed, being slightly warmer at the start of the adsorption process, will not be able to adsorb as much at the beginning though the cooling process later on will balance off the effect. This results in lower cooling capacity for shorter heat recovery time, and hence a smaller COP. 4. Conclusion The experimental investigation on the performance of Zeolite adsorption cooling system has been successfully carried out. The experiments were carried out at assorted hot water inlet temperatures ranging from 55 °C to 80 °C while cooling water and chilled water inlet temperature were maintained at 29.5 °C and 12 °C, respectively. The intensive investigation on the all of the key parameters such as half cycle, heat source temperature and heat recovery time had been carried out. For each of the above mentioned parameters, optimal operation condition was found out and tabulated in Table 2. Since Zeolite adsorption cooling system is a batched operated system, the cycle time was fixed at 300 s whilst the switching time was set at 15 s. It is found out that the Zeolite adsorption cooling system could be driven with a very low grade waste heat as low as 55 °C. However, the optimal COP of the system was achieved when the activation waste heat temperature is 65 °C. Such a very low waste heat driven system could help not only to recover the energy from the exhaust gas emanating from the industries but also to reduce CO2 emission to the environment. Thus, such a waste heat activated system is a promising technology to maintain the environment clean and green. Acknowledgements The authors gratefully express the gratitude to Agency of Science, Technology and Research (ASTAR) for their generous financial support for the project (Grant Number R265-000-287-305). Appendix A. Supplementary material Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.apenergy.2012. 08.005. References [1] Berlitz T, Satzger P, Summerer F, Ziegler F, Alefeld G. A contribution to the economic perspectives of absorption chillers. Int J Refrig 1999;22:67–76. [2] Pons M, Meunier F, Cacciola G, Critoph EE, Groll M, Puigjaner L, et al. Thermodynamic based comparison of sorption systems for cooling and heat pumping. Int J Refrig 1999;22:5–7.
[3] 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. [4] Huilong Luo, Ruzhu Wang, Yanjun Dai. The effects of operation parameter on the performance of a solar-powered adsorption chiller. Appl. Energy 2010;87(10):3018–22. http://dx.doi.org/10.1016/j.apenergy.2010.03.013. ISSN:0306-2619. [5] Rezk Ahmed RM, Al-Dadah Raya K. Physical and operating conditions effects on silica gel/water adsorption chiller performance. Appl Energy 2012;89(1): 142–9. [6] Zhao YL, Hu Eric, Blazewicz Antoni. A non-uniform pressure and transient boundary condition based dynamic modeling of the adsorption process of an adsorption refrigeration tube. Appl Energy 2012;90(1):280–7. [7] Louajari Mohamed, Mimet Abdelaziz, Ouammi Ahmed. Study of the effect of finned tube adsorber on the performance of solar driven adsorption cooling machine using activated carbon–ammonia pair. Appl Energy 2011;88(3):690–8. [8] Solmus Ismail, Yamali Cemil, Kaftanoglu Bilgin, Baker Derek, Çaglar Ahmet. Adsorption properties of a natural zeolite–water pair for use in adsorption cooling cycles. Appl Energy 2010;87(6):2062–7. [9] Solmus Ismail, Kaftanoglu Bilgin, Yamali Cemil, Baker Derek. Experimental investigation of a natural zeolite–water adsorption cooling unit. Appl Energy 2011;88(11):4206–13. [10] Karamanis D, Vardoulakis E. Application of zeolitic materials prepared from fly ash to water vapor adsorption for solar cooling. Appl Energy 2012. [11] Akhtar Farid, Andersson Linnéa, Keshavarzi Neda, Bergström Lennart. Colloidal processing and CO2 capture performance of sacrificially templated zeolite monoliths. Appl Energy 2012. [12] Saha BB, Kashiwagi T. Experimental investigation of an advanced adsorption refrigeration cycle. ASHRAE Trans Res 1997:50–7. [13] Ben Amar N, Sun LM, Meunier F. Numerical analysis of adsorptive temperature wave refrigerative heat pump. Appl Therm Eng 1996;16:405–18. [14] Alam KCA, Saha BB, Kang YT, Akisawa A, Kashiwagi T. Heat exchanger design effect on the system performance of silica gel adsorption refrigeration systems. Int J Heat Mass Transfer 2000;43:4419–31. [15] Zhang LZ. A three-dimensional non-equilibrium model for an intermittent adsorption cooling system. Sol Energy 2000;69:27–35. [16] Miltkau T, Dawoud B. Dynamic modeling of the combined heat and mass transfer during the adsorption/desorption of water vapor into/from a zeolite layer of an adsorption heat pump. Int J Therm Sci 2002;41:753–62. [17] Marletta L, Maggio G, Freni A, Ingrasciotta M, Restuccia G. A non-uniform temperature non-uniform pressure dynamic model of heat and mass transfer in compact adsorbent beds. Int J Heat Mass Transfer 2002;45:3321–30. [18] Chua HT, Ng KC, Wang W, Yap C, Wang XL. Transient modelling of a two-bed silica gel–water adsorption chiller. Int J Heat Mass Transfer 2004;47:659–69. [19] Leong KC, Liu Y. Numerical modeling of combined heat and mass transfer in the adsorbent bed of a zeolite/water cooling system. Appl Therm Eng 2004;47:4761–70. [20] Turner L. Improvement of activated charcoal–ammonia adsorption heat pumping/refrigeration cycles investigation of porosity and heat/mass transfer characteristics. Ph.D. thesis. University of Warwick; 1992. [21] Luo L, Feidt M. Thermodynamics of adsorption cycles: a theoretical study. Heat Trans Eng 1992;13:19–31. [22] Cacciola G, Restuccia G. Reversible adsorption heat pump: a thermodynamic mode. Int J Refrig 1995;18:100–6. [23] Saha BB, Boelman E, Kashiwagi T. Computer simulation of a silica gel–water adsorption refrigeration cycle-the influence of operating conditions on cooling output and COP. ASHRAE Trans: Res 1995;101(2):348–57. [24] Critoph RE, Metcalf SJ. Specific cooling power intensification limits in ammonia carbon adsorption refrigeration systems. Appl Therm Eng 2004;24:661–78. [25] Mitsubishi Plastics. Inc. AQSOAÒ Product Brochure. [26] Kakiuchi H, Iwade M, Shimooka S, Ooshima K, Yamazaki M, Takewaki T. Novel zeolite adsorbents and their application for AHP and desiccant system. Mitsubishi Chemical Group Science and Technology Research Center. [27] Chua HT, Ng KC, Chakraborty Anutosh, Oo MNay, Othman Mohamed A. Adsorption characteristics of silica gel + water systems. J Chem Eng Data 2002;47:1177–81.
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Applied Energy journal homepage: www.elsevier.com/locate/apenergy
Experimental investigation on the optimal performance of Zeolite–water adsorption chiller Aung Myat a,⇑, Ng Kim Choon a, Kyaw Thu b, Young-Deuk Kim b a b
Mechanical Engineering Department, National University of Singapore, 9 Engineering Drive 1, Singapore 117576, Singapore King Abdullah University of Science and Technology, Water Desalination and Reuse Center, Thuwal 23955-6900, Saudi Arabia
h i g h l i g h t s " Experimental analysis on performance of Zeolite adsorption chiller. " To optimize the performance of the adsorption cooling system. " To investigate effect of cycle time and switching time on the performance of system.
a r t i c l e
i n f o
Article history: Received 5 October 2011 Received in revised form 25 July 2012 Accepted 7 August 2012 Available online 7 September 2012 Keywords: Adsorption Waste heat Zeolite Environmentally friendly
a b s t r a c t This paper presents the performance testing of Zeolite adsorption cooling system driven by low grade waste heat source extracted from prime mover’s exhaust, power plant’s exhaust and the solar energy. The adsorbent FAM Z01 is used as an adsorbent in the adsorption chiller facility. Owing to its large equilibrium pore volume, it has the high affinity for the water vapor adsorbate. The key advantages of the Zeolite adsorption cooling system are: (i) it has no moving parts rendering less maintenance, (ii) the energy efficient means of cooling by the adsorption process with a low temperature heat source, (iii) the use of vapor pipes are replaced by self actuating vapor valves rendering smaller footprint area and (iv) it is environmental friendly with low carbon footprint. The experimental investigations were carried out for Zeolite adsorption chiller at different key operating conditions namely (i) heat source temperature, (ii) the cycle time and (iii) the heat recovery time. It is investigated that performance of coefficient (COP) of this system could be as high as 0.48 while the waste heat source temperature is applicable as low as 55 °C. Crown Copyright Ó 2012 Published by Elsevier Ltd. All rights reserved.
1. Introduction Cooling or heating produced in conventional vapor compression cooling systems uses ozone-depleting refrigerants such as chlorofluorocarbons (CFCs). Despite the development of less ozone damaging alternatives such as the hydro chlorofluorocarbons (HCFCs) and the hydro fluorocarbons (HFCs), they still contribute to the undesirable effect of global warming. Furthermore, such cooling systems utilize electricity and fossil fuels as their driving sources. As cooling demand for climate control and refrigeration soars in the future, it will lead to increasing energy consumption. This intrinsically leads to a faster depletion of known fossil fuel reserves, more carbon dioxide emissions and a higher peak electricity demand. Such environmental issues have intensified research efforts on the development of environmentally benign refrigerants and energy saving cooling technologies and renewed interest in cooling applications. ⇑ Corresponding author. Tel.: +65 6796 7370. E-mail address: [email protected] (A. Myat).
One form of environmentally friendly alternatives for vapor compression cooling is the sorption cooling systems. In the past few decades, adsorption processes have been used exclusively for gas separation and purification purposes. It is only recently that this phenomenon has been exploited to produce cooling and heating. Promising technologies include heat driven liquid absorption cooling system [1,2] and solid sorption heat pumps [2]. As they can be driven by waste heat or solar energy and uses environmentally benign chemicals, both their ODP (ozone depletion potential) and GWP (global warming potential) are virtually zero [3]. Absorption cycles which use lithium bromide–water (LiBr–H2O) or water–ammonia (H2O–NH3) pairs encounter many operational limitations. For example, thermally driven single-stage LiBr–H2O absorption chiller utilizes relatively expensive corrosion resistant alloy steel due to the corrosive nature of the LiBr–H2O. Due to the risk of crystallization, the lowest temperature that can be used for generator the said type of chiller, without crystallization, is 61 °C in combination with a coolant inlet temperature at 32 °C [3]. Hence, the applications of absorption cycles are limited to high and medium temperature heat sources. Furthermore, a solution
0306-2619/$ - see front matter Crown Copyright Ó 2012 Published by Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.apenergy.2012.08.005
A. Myat et al. / Applied Energy 102 (2013) 582–590
pump has to be employed for the circulation of the LiBr solution from absorber to generator. The crystallization of salt solution and higher cooling water in temperature are major concerns for the liquid sorption chiller. Thus, our main focus is on the adsorption (solid adsorbent) chiller. It has potential to apply very low grade waste heat and less maintenance. It could also avoid the potential crystallization issue which is normally a big issue with the liquid sorption chiller. The use of low grade waste heat available from power plant is an important contemporary issue. Adsorption cycles which use silica gel–water [4,5], activated carbon–methanol [6,7] and Zeolite– water [8–11] adsorbent–refrigerant pairs have a distinct advantage as they can be driven by heat of relatively low, near environmental temperature such that waste heat below 80 °C can be recovered [12]. Besides, these systems are non-corrosive and required almost no moving parts rendering less maintenance. The adsorption chiller has many advantages, fewer moving parts, simple construction, being able to be driven by waste heat as low as 60 °C. Therefore, the adsorption devices can be economically and technologically competitive with the abundant supply of waste heat. Silica gel adsorption chillers that use water vapor as the refrigerants are one of the most widespread adsorption chillers around as they can be driven by low grade heat source temperature about 80 °C. However, the output cooling power per unit volume of such chiller is much smaller than absorption heat pump, resulting in bulky and space consuming silica gel adsorption chillers. The large sizing is one of the reasons why the adoption of such chillers is not as widespread. Recently, various heat and mass transfer models have been proposed to study the thermal performance in terms of the coefficient of performance (COP) and specific cooling power (SCP) of adsorption cooling systems [12–18]. Nevertheless, the effects of operating conditions on the thermal performance of such systems especially those pertaining to operating temperature effects are scarcely reported in the literature. The effect of operating conditions on the adsorption cooling cycle based on thermodynamic analyses has been numerically investigated by a number of researchers [19–24]. Only a few literatures could be referred to the Zeolite–water pair adsorption chiller [8–11]. Adsorption properties such as equilibrium uptake and heat of adsorption were experimentally investigated and compared with the different adsorption pair such silica gel water and carbon methanol [8]. It was reported that Zeolite–water pair is the least dependency on the evaporator and condenser pressure as compared to silica gel– water and carbon–methanol adsorption pair and it has the lowest regeneration temperature among those adsorbent–adsorbate pairs. Performance of a natural Zeolite–water adsorption chiller was experimentally investigated at various evaporation temperatures [9]. It was reported that mean COP is achieved about 0.25. The experimental investigation on the performance of Zeolite– water adsorption chiller at different operational parameters had yet to be reported. Thus, the motivation of this paper is to conduct the experiment and investigate the performance of Zeolite–water adsorption chiller at assorted operation conditions; namely (i) heat source temperature, (ii) cycle time and (iii) switching time and heat recovery time.
2. System description The adsorption cooling (AD) cycle is one of the environmentally friendly methods to produce the cooling by utilizing the low grade waste heat source typically 55–85 °C. The AD cycle comprises of three major components namely (1) the evaporator, (2) the condenser and (3) regenerative adsorber and desorber beds where the adsorbent is placed. Fig. 1 shows the schematic layout of AD cycle.
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2.1. Description of the Zeolite water adsorption chiller In this context, the authors would like to introduce the compactness of the waste heat driven Zeolite–water adsorption chiller. In order to make the chiller size to be compact, there are some possible solution such as (i) enhanced the surface area of the adsorbent so as to reduce the foot print area of the chiller and (ii) introduced the self actuation facility to eliminate the vapor pipes connecting to the adsorber–evaporator and desorber–condenser. In addition, introducing such a self actuating valve facility could also eliminate the valve sequence controlled for the opening and closing for the vapor flow to adsorber and condenser, respectively. 2.1.1. Adsorption properties (equilibrium uptake) of Zeolite FAM Z01 For the enhancement of adsorbent, Functional Adsorbent Material Z01 (FAM Z01) is employed and it is an AFI type molecular sieve. It has a one-dimensional structure with 0.73 nm windows and its framework consists of AlO4, PO4 and FeO4 tetrahedrons, where Al and P atoms are partially substituted by Fe atoms. The advantage of FAM Z01 over silica-gel is that regeneration can be performed at the temperature range of 50–80 °C [25]. It is also important to investigate the adsorption properties such as equilibrium uptake of water vapor at assorted temperatures [26]. The isotherm characteristic of FAM Z01 is illustrated in Fig. A.1. In Fig. A.1, relative pressure on the horizontal axis is defined as the ratio of vapor pressure of the reactors bed, which is denoted as Pads for adsorber bed and Pdes for desorber bed, to the saturation pressure at the given temperature. As shown in Fig. A.1, at an adsorption temperature of 25 °C (298 K) and its corresponding saturation pressure (Psat) is found to be 3.17 kPa whilst adsorption pressure, Pads, is approximately 0.8 kPa. This gives a relative pressure (Pads/Psat) of 0.25. By locating a point on the adsorption isotherm with corresponding adsorption temperature of 298 K and relative pressure of 0.25, it is found that the uptake of adsorbate, q is about 0.19 kg-H2O/kg of dry mass of FAM Z01, at a desorption temperature of 60 °C (333 K) and its corresponding saturation pressure (Psat) is found to be 19.92 kPa whilst desorption pressure, Pdes, is approximately 3.73 kPa. Thus, relative pressure (Pdes/Psat) is about 0.18. Similarly, the equilibrium uptake, q of 0.02 kg-H2O/kg of dry mass of FAM Z01 is found at the corresponding desorption temperature of 333 K and relative pressure of 0.18. Thus, the average adsorption capacity Dq is thus 0.17 kg-H2O/kg of dry mass of FAM Z01 at adsorption and desorption temperature of 298 K and 333 K, respectively. As compared to silica gel–water equilibrium uptake at 25 °C (298 K) and 60 °C (333 k), Dq is found to be about 0.045 [27] and FAM Z01 has a significantly higher equilibrium uptake which leads to significantly reduce in size of the chiller. Thermo physical properties of FAM Z01 and Type A silica gel are furnished in Table 1. 2.1.2. Self actuating vapor valves (valve less design) of Zeolite–water adsorption chiller The ‘‘valve less’’ design is such that the connections between the sorption reactors and the condenser/evaporator are replaced by counter-weight ‘‘valves’’ instead of butterfly valves found in typical adsorption chiller. It works as simple lever in such a way that the opening or closing of the valves is dependent on the pressure difference in the adjacent chambers. Due to the low density of the refrigerant vapor, typical gas valves require large piping connections to reduce the pressure drop as vapor pass through. This contributes significantly to the footprint area of the adsorption chiller. The ‘‘valve less’’ design adopted in this research eradicates the problem of pressure drop and can reduce the footprint area significantly. The positions of the valves connecting the sorption reactors with the evaporator or condenser are determined by the pressure difference in the adjacent
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Fig. 1. The schematic lay out diagram of Zeolite adsorption chiller.
Fig. A.1. Adsorption and desorption isotherms of FAM Z01.
Table 1 Thermal properties of FAM Z01 and Type A silica gel. Properties Bulk density Effective thermal conductivity Differential heat of adsorption (H2O, 298 K) Specific heat
FAM Z01
A type silica gel
(kg/dm3) (W/mK) kJ/mol
0.6–0.7 0.113 (303 K) 56
0.6–0.7 0.12 (303 K) 52
kJ/kg K
0.805 (303 K)
0.92
Fig. A.2. The operation of self actuating valve or valve less facility equipped with Zeolite water adsorption chiller. (a) Close position P1 > P2 (X), (b) close to open P2 > P1 and (c) open position P2 > P1 (O).
chambers as shown in Fig. A.2. The pressure of the sorption chamber varies as heat is added or removed from the chamber to enable the adsorption or desorption processes to take place. The opening and closing of the valve is critical in determining whether
Fig. 2. Pictorial view of the Zeolite coated heat exchanger with coated thickness of 1–2.5 mm adsorbent layer.
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Fig. 3. The pictorial view of the Zeolite adsorption chiller test facility.
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adsorbent coated on the fins surface of the exchanger coil is shown in Fig. 2. A continuous vapor uptake is achieved when the vapor valve is opened. Owing to nature of the adsorption process, the heat is released during the vapor uptake and the adsorption process would continue to a preset cycle time. In general, the cycle time in the adsorption process is one of the parameters to control the cycle performance. The saturated FAM-Z01 (performed adsorption process previous half cycle) could be now regenerated by introducing a low grade waste heat source typically 50–85 °C and this is known as desorption process. The waste heat source or solar energy which is supplied to the saturated reactor beds to regenerate the water vapor. With the vapor valve set to open position, the regenerated water vapor flows to the cooler surfaces of a water cooled tubing and it condenses on the bundle of tubes and is sent through the U-tube to the evaporator where the cooling effect is produced. The condensation process is accompanied by the rejection of latent heat of condensation into the cooling water passing inside the tubing in the condenser. The AD cycle operates in batch-type process to achieve the useful effect using two reactor beds, cycling one of reactor beds as adsorption mode whilst the other is in desorption mode during the first half cycle of operation. The roles of the reactor beds are reversed in next half cycle. Prior to switching their roles, pre-cooling of desorber bed and preheating of adsorber beds need to be performed for a short period which is known as a switching process. The pictorial view of Zeolite adsorption test facility is illustrated in Fig. 3. The structure of Zeolite FAM Z01 is shown in Fig. 4a. Scanning Electron Micrograph (SEM) picture of FAM Z01 is presented in Fig. 4b. 2.3. Operation of Zeolite chiller running at switching mode
Fig. 4a. Structure of FAM Z01 Zeolite.
evaporation and condensation takes place in the respective chambers. By incorporating such a valve less facility to the waste heat driven adsorption chiller, the size of the chiller could be reduced significantly.
2.2. Operation of Zeolite chiller running at batch operated cycle As shown in Fig. 1, the condensate from the condenser is depressurized through the U-tube and sent to the evaporator where external heat to sustain the evaporation at sub atmospheric pressure, from chilled water flowing inside the tubing, is supplied. The evaporated water vapor is adsorbed by the adsorbent (Zeolite type FAM Z01) which is coated on the fins surface of heat exchanger housed inside the reactor beds. The evaporation process is enhanced by a spray system using the full-cone type nozzles. The
To improve efficiency, the residual heat contained in the adsorbent bed at the end of the desorption cycle is recovered during switching mode. The modes of operation during switching process are illustrated in Fig. 5. During Normal A mode, ADS 1 (Bed 1) is undergoing the desorption in the first half of the cycle as shown in Fig. 5 diagram 1. At the end of the first half cycle, ADS 1 remains hot with residual hot water in its finned tube heat exchanger and there is significant amount of heat contained in its adsorbent bed. If cooling water is directed straight into ADS 1 (Bed 1), much of this useful energy in ADS 1 (Bed 1) will be removed and dumped as waste heat as the cooling water carries it and releases it into the ambient in the cooling tower reservoir. To recover this heat, at the end of the first half of the cycle, the hot water supply to ADS 1 is directed to the bypass channel as shown in diagram (2). In diagrams (3)–(4), cooling water is then directed into ADS 1 and then to ADS 2 to push the residual hot water from ADS 1 (Bed 1) to ADS 2 (Bed 2). This is to pre heat the adsorbent bed in ADS 2
Fig. 4b. Scanning Electron Micrograph (SEM) pictures of FAM Z01 Zeolite.
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(1)
(4)
(2)
(5)
(3)
(6)
Fig. 5. Schematic diagrams to show the heat recovery processes during switching mode.
(Bed 2) to prepare it for desorption in the second half of the cycle. Thus, heat removal from the hot water supplied to ADS 2 during the second half can be reduced (diagram (5)), allowing for better
performance (COP) with same amount of cooling obtained. Meanwhile, ADS 1 undergoes pre-cooling to enhance the adsorption process at next cycle.
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3. Results and discussion This section describes the detailed of the experiment of Zeolite adsorption chiller. The experiments were performed for a nominal capacity of 3 Rton (10.5 kW) adsorption cooling system. The hot water source temperatures are varied from 60 °C to 85 °C whilst the cooling water and the chilled water temperatures are kept at around 29 °C ± 1° and 12 °C ± 1. The cycle time is varied from 100 to 700 s while switching time is set at 15 s. Fig. 6a dictates the temperature profiles of the outlet of heat transfer fluid operating under batch mode. It is found out that cycle average hot water outlet temperature is about 75 °C while adsorber cooling water outlet water temperature is about 33 °C. During the beginning of the adsorption process or desorption process, it is observed that the temperature gradient of all the heat transfer fluid is higher compared to the ending of the operation time. Figs. 6a and 6b also shows that chilled water outlet temperature can be achieved as low as 7 °C while the inlet temperature is set to be about 12 °C. During the first switching process in Figs. 6a and 6b, ADS 1 outlet temperature increases steeply as hot water is ‘‘push’’ from ADS 2 to ADS 1 to prepare it for desorption. It is observed that there is a significant temperature drop right after switching. This is because there is a sharp increase in the adsorbent bed temperature which causes the water molecules in the adsorbent to expand. This led to a very high rate of desorption due to the large pressure difference between the bed and the surrounding. During the first switching process, ADS 2 (Bed 2) outlet temperature undergoes a sharp decrease because cooling water is supplied to cool down the previously warm adsorbent bed. Around 300 s, it is observed that the outlet temperatures of the heat transfer fluids of ADS 1 and ADS 2 become close to the inlet temperature of each heat transfer fluids. While the beds are near their equilibrium state at that particular temperature and pressure, the sorption rates of reactor beds are significantly reduced. The pressure profiles of key components in the Zeolite adsorption cycle are illustrated in Fig. 7. The experiments were carried out at assorted half cycle time at different hot water inlet temperature to observe the sensitivity of the optimal performance of the system. The effect of hot water inlet temperature and the half cycle time on the key performance parameters namely (i) Coefficient of performance (COP), (ii) cooling capacity and (iii) heat input to the system are shown in Fig. 8. Average cooling capacity generally decreases with longer cycle time as shown in Fig. 8b. For the longer cycles, cooling capacity decreases because adsorption has become less intense after some time and gradually decreases as the adsorbent approaches its equilibrium conditions. The highest cooling capacity value was obtained at a half cycle time of 100 s for the higher heat source temperatures (70 °C and 80 °C). However, it is obtained at 200 s
Fig. 6b. Details temperature profiles of heat transfer fluids and bed during switching period.
Fig. 7a. Pressure–time history of Zeolite adsorption chiller.
Fig. 7b. Details pressure profiles of reactor beds, evaporator and condenser during switching period.
Fig. 6a. The performance of Zeolite adsorption chiller showing the outlet heat transfer fluid temperatures.
or 300 s for lower heat source temperatures (55 °C, 60 °C and 65 °C) instead of 100 s. This is because for the shorter cycles (evident from lower heat source temperatures), there is not enough time for adsorption to occur satisfactorily and the lower driving temperature is insufficient for desorption to take place. Therefore, cooling capacity decreases abruptly. For the higher heat source temperature, it is observed that cooling capacities would also drop for cycle times shorter than 100 s. As seen in Fig. 8, the consumption of driving heat source decreases with longer cycle time. This is because the reactor bed undergoing desorption mode is approaching to its equilibrium desorption that leads to minimal
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Fig. 9. The temperature–time histories of reactors outlet heat transfer fluids and chilled water outlet at different half cycle time.
Fig. 8a–c. The effect of half cycle time on the cycle average heat input, cooling capacity and coefficient of performance (COP).
requirement heat supply. For the initial part of the desorption cycle, large heat input is required owing to its considerably high desorption rates. Thus, it is occurred that there is the significantly high temperature gradient between the adsorbent bed and the hot water outlet temperature. Hence, short cycle times often require large average heat input. Fig. 8c shows the effect of the cycle times on the COP. Generally, the measured COP increases monotonically with time. This occurs because of the lower consumption of driving heat with longer cycles. It can be observed that both the average cooling capacity and the average heat input decrease with longer cycle time while COP increases with longer cycle time. It is also noted from Fig. 8c that a high COP of 0.48 is achieved when the heat source temperature is about 55 °C. Owings to its S-shaped isotherm nature (as shown in Fig. A.1) of FAM Z01, which is functionalized mainly for low temperature regeneration processes, the
equilibrium uptake (Dq) of adsorption–desorption (at 303 K and 333 K) is significantly higher than that of (303 K and 348 K) and this results that COP of chiller is high at lower waste heat temperature at the same cycle time. However, it is also observed that chiller COP is quite low at lower heat source temperature when it is running at a shorter cycle time (100 s). This is due to the fact that regeneration time of adsorbent is not enough at lower heat source (55 °C). However, such an even shorter cycle time (100 s) could be applied for the chiller running at higher heat source temperature (80 °C) because Kinetics or regeneration rate is faster at higher heat source temperature. Moreover, the cooling capacity and heat supplied to the system is decreased with a decrease in heat source temperature. Thus, COP is found to be higher at lower driving heat source temperature. The effect of cycle time on the outlet temperature of the heat transfer fluids is clearly demonstrated in Fig. 9. The longer the cycle time is, the smaller the temperature difference between inlet and outlet of heat transfer fluids will be. Thus, cooling capacity and heat input to the system is significantly reduced with the longer cycle time. The effect of heat source temperature on the key parameters such heat input, cooling capacity and the COP of the system is illustrated in Fig. 10. Generally, cooling capacity increases as the driving heat source temperature increases for the same cycle time as shown in Fig. 10b. With higher driving heat source temperatures, more desorption takes place at the same cycle time. Therefore, more heat is required to supply to reactor bed. However, for the half cycle time of 500 s, cooling capacity decreases when the heat source temperature is increased from 75 °C to 80 °C. For a cycle time of 500 s, a temperature of 80 °C may be excessive for the desorption process such that a temperature of 75 °C is sufficient to achieve the maximum amount of circulated refrigerant. Since the rate of uptake of adsorbent is compromised at higher
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Table 2 The optimal operation conditions for each of the waste heat activated temperature. Condition Temperature Half cycle Heat input Cooling capacity Average COP
°C s kW kW
1
2
3
4
5
55 515 12.92 5.71 0.442
60 515 16.8 7.78 0.463
65 515 18.2 8.728 0.480
70 415 23.15 10.58 0.457
80 415 26.36 11.52 0.437
Fig. 11. COP against heat recovery cycle time for a half cycle time of 400 s at various driving heat source temperature.
Fig. 10. The effect of heat source temperature on the heat input, cooling capacity and COP of Zeolite adsorption cooling system.
temperatures, a temperature of 80 °C may have been too high for cooling to occur satisfactorily during the adsorption process. With the lower rate of adsorption at the higher temperature of 80 °C, average cooling capacity naturally decreases. In fact for cooling capacity to be optimum at a half cycle time of 500 s, a temperature of 75 °C would have been optimal such that maximum amount of circulated refrigerant can be achieved and that the rates of adsorption are not compromised as much. For the same cycle time, heat input increases with higher heat source temperature as seen in Fig. 10a. At higher temperatures for the same cycle time, the temperature gradient between the adsorbent bed and driving heat source is higher. Hence, more heat can be transferred to the adsorbent bed, leading to higher heat inputs. Since both cooling capacity and heat input increases with increasing driving temperature, changes in COP is dependent upon the percentage increases of both cooling capacity and heat input respectively as shown in Fig. 10c.
The maximum COP is obtained at 70 °C and 65 °C with corresponding cycle times of 100–300 s and the higher cycle times 400–500 s respectively. The most optimal set of operating conditions is one which has a high cooling capacity and low heat input, thus resulting a high COP value. Such low grade temperature heat is considered to be of low value because it is often the by-product of power plant or industrial processes. Hence, in this case, more consideration would be given to the amount of cooling capacity provided. The optimal conditions (highest COP) with the performance indicators for each of the driving temperatures are furnished in Table 2. An optimal data set would be Data Set 4 where 10.6 kW of cooling is produced. It produces about 21% more cooling than Data Set 3 by using 24% more heat input. It is important to note that adsorption chillers in this context are supposed to utilize waste heat or solar heat which are of lower temperature range as compared to heat generated by burning fossil fuels. Hence, in consideration for the optimal operating conditions, more emphasis should be placed on the amount of cooling effect produced. It is appropriate as long as the additional heat input to produce the additional cooling is not excessively high. An optimal allocation of heat recovery time allows the system to recover residual heat in the adsorbent bed that previously underwent desorption and allows the other adsorbent bed to pre heat appropriately. However, excessive heat recovery time can actually abates the pre-heating of the adsorbent bed because the residual heat gained can be carried away by the cooling water flow in the excess heat recovery time. For the same cycle time, it is observed from Fig. 11 that optimal COP is achieved for a certain heat recovery time for each driving heat source temperature. For the higher driving heat source temperatures (70 °C and 80 °C), a heat recovery time of 12.5 s will give a peak COP value while for the lower driving heat source temperatures (55 °C, 60 °C and 65 °C), a recovery time of 15 s will give a peak COP value. As compared to the higher driving heat source temperature, the lower driving heat temperature needs longer heat recovery time to adsorb or desorb the refrigerant vapor from the adsorbent bed, which is the reason for the longer heat recovery cycle time. It is interesting
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to note that the COP for the heat recovery time of 10 s is smaller than that for 15 s for all of the optimal operating conditions in the graph. This is because with shorter heat recovery time, there is insufficient cooling of the adsorbent bed that underwent desorption previously. Since the adsorption capacity is compromised at a higher temperature, the adsorbent bed, being slightly warmer at the start of the adsorption process, will not be able to adsorb as much at the beginning though the cooling process later on will balance off the effect. This results in lower cooling capacity for shorter heat recovery time, and hence a smaller COP. 4. Conclusion The experimental investigation on the performance of Zeolite adsorption cooling system has been successfully carried out. The experiments were carried out at assorted hot water inlet temperatures ranging from 55 °C to 80 °C while cooling water and chilled water inlet temperature were maintained at 29.5 °C and 12 °C, respectively. The intensive investigation on the all of the key parameters such as half cycle, heat source temperature and heat recovery time had been carried out. For each of the above mentioned parameters, optimal operation condition was found out and tabulated in Table 2. Since Zeolite adsorption cooling system is a batched operated system, the cycle time was fixed at 300 s whilst the switching time was set at 15 s. It is found out that the Zeolite adsorption cooling system could be driven with a very low grade waste heat as low as 55 °C. However, the optimal COP of the system was achieved when the activation waste heat temperature is 65 °C. Such a very low waste heat driven system could help not only to recover the energy from the exhaust gas emanating from the industries but also to reduce CO2 emission to the environment. Thus, such a waste heat activated system is a promising technology to maintain the environment clean and green. Acknowledgements The authors gratefully express the gratitude to Agency of Science, Technology and Research (ASTAR) for their generous financial support for the project (Grant Number R265-000-287-305). Appendix A. Supplementary material Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.apenergy.2012. 08.005. References [1] Berlitz T, Satzger P, Summerer F, Ziegler F, Alefeld G. A contribution to the economic perspectives of absorption chillers. Int J Refrig 1999;22:67–76. [2] Pons M, Meunier F, Cacciola G, Critoph EE, Groll M, Puigjaner L, et al. Thermodynamic based comparison of sorption systems for cooling and heat pumping. Int J Refrig 1999;22:5–7.
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