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Experimental study on an adsorption chiller employing lithium chloride in silica gel and methanol
i n t e r n a t i o n a l j o u r n a l o f r e f r i g e r a t i o n 3 5 ( 2 0 1 2 ) 1 9 5 0 e1 9 5 7
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Experimental study on an adsorption chiller employing lithium chloride in silica gel and methanol L.X. Gong, R.Z. Wang*, Z.Z. Xia, Z.S. Lu Institute of Refrigeration and Cryogenics, Shanghai Jiao Tong University, 800 Dong Chuan Road, Shanghai 200240, China
article info
abstract
Article history:
A novel composite adsorbent e methanol adsorption chiller was proposed and manufac-
Received 14 September 2011
tured. It was filled by the adsorbent composed by Lithium Chloride and silica gel. Methanol
Received in revised form
was used as adsorbate and refrigerant. Experiment results showed that compared with
18 June 2012
silica gel-water chiller, SCP (specific cooling power) and COP (coefficient of performance) of
Accepted 30 June 2012
this novel chiller were improved by 16.3% and 24.2% separately when the temperatures of
Available online 20 July 2012
hot water inlet, cooling water inlet and chilled water outlet were 85 C, 30 C and 15 C. ª 2012 Elsevier Ltd and IIR. All rights reserved.
Keywords: Adsorption system Performance Lithium chloride Silica gel Methanol
Etude expe´rimentale sur un refroidisseur a` adsorption utilisant le chlorure de lithium dans du gel de silice et du me´thanol Mots cle´s : Syste`me a` adsorption ; Performance ; Chlorure de lithium ; Gel de silice ; Me´thanol
1.
Introduction
Now energy consumption is one of the most popular topics. People are facing more challenges to balance sustainable development and environmental protection. Adsorption technology, which is a clean tech attracts more and more researchers. It employs friendly refrigerants (water, methanol, etc.) instead of the ozone depleting substances such as CFCs and HCFCs. In addition, the adsorption chiller can be
driven by industry waste heat or solar energy with the temperature of 55e90 C. As a result, this tech cuts a part of the greenhouse gas emitted from coal plants. A family of adsorption working pairs has been studied (Aristov et al., 2002; Restuccia et al., 2004; Bauer et al., 2009; Veselovskaya and Tokarev, 2011). The typical working pairs are silica gelewater, zeoliteewater, activated carbone ammonia, activated carbonemethanol, activated carbon fibereammonia, inorganic salt-ammonia, etc. The utilizations
* Corresponding author. Tel.: þ86 021 021 34206548; fax: þ86 021 34206309. E-mail address: [email protected] (R.Z. Wang). 0140-7007/$ e see front matter ª 2012 Elsevier Ltd and IIR. All rights reserved. http://dx.doi.org/10.1016/j.ijrefrig.2012.06.013
i n t e r n a t i o n a l j o u r n a l o f r e f r i g e r a t i o n 3 5 ( 2 0 1 2 ) 1 9 5 0 e1 9 5 7
Nomenclature Cw L M _ m Dm qe qh t1 t2 t3 T
1
1
specific heat, kJ kg K latent heat, kJ kg1 mass of adsorbent material, kg mass flow rate, kg h1 cycle adsorption capacity, kg cooling capacity, kW heating power, kW adsorption/desorption time, s mass recovery time, s heat recovery time, s temperature, K
Subscripts a adsorbent
of water and methanol as refrigerants in adsorption system make it possible to use the heat source below 100 C (Saha et al., 2007). However, typical water/methanol working pairs do not have ideal characteristics. When water is used as an adsorbate, the low working pressure increases the risk of leakage problems and implies severe mass transfer limitation through the component of the machine through the adsorbent bed itself (Gordeeva et al., 2009). Methanol has a higher operation pressure. However the sorption capacity is rather low and usually does not exceed 0.15e0.4 g g1 (Gordeeva et al., 2008). And these working pairs do not guarantee adequate performance when desorption temperature is lower than 90 C (Freni et al., 2012). Some composite adsorbents have been proposed. Novel adsorbents with metal materials have been developed to enhance heat and mass transfer of adsorption material (Hu et al., 2009). The composites with hydrophilic salt largely improve the adsorption capacity (Aristov et al., 2002; Gordeeva et al., 2008). However, it is quite possible that those additives bring some new problems, for instance, corrosion, which hinders the real application in industry. As a result, a new way to produce the composite adsorbent which employing Lithium Chloride in silica gel has been proposed by the researchers in SJTU (Shanghai Jiao Tong University). The salt on the surface is removed and the characteristics of the new composite adsorbent have been studied (Gong et al., 2010a). Now a chiller with this kind of adsorbent has been manufactured and tested. Usually the adsorption working pair for conventional adsorption chiller is wateresilica gel. However the working pressure of water is very low. As a result, the mass transfer in the adsorbent bed is limited. Furthermore, small amount of air or gas, which are leaked into the machine or emerged by some unknown reactions, weaken the performance of the chiller significantly (Gordeeva et al., 2008). Methanol is used as an alternative adsorbate in this chiller. Its high working pressure enhances the stability performance of the chiller; also it has a lower freezing temperature. So tentative experiments for cold-storage or ice making are allowed to do. But the latent heat of evaporation of methanol is lower than that of water. It can only be compensated by the improved adsorption capacity of the composite adsorbent. The theoretical adsorption refrigeration cycle of working pair
ad c chilled c-m cooling cyc hot i m o r s s-w w
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adsorbate composite adsorbent chilled water composite adsorbent-methanol cooling water cycle hot water inlet methanol outlet refrigerant silica gel silica gel-water water
(composite adsorbent-methanol) has been studied (Gong et al., 2010b). Though there are many research papers about the composite adsorbents (Aristov et al., 2002; Wang et al., 2009) and the chillers filled with typical working pairs (silica gelewater, activated carbonemethanol, etc.) (Anyanwu and Ezekwe, 2003; Freni et al., 2012 and Wang et al., 2005), there are few papers about the experimental study on the chiller filled by composite adsorbent. This paper presents the design and the experimental study of a chiller using the composite adsorbent and methanol. The objects of this paper are (1) analyzing the dynamic performances of the novel chiller. It is compared with that of silica gel-water adsorption chiller in similar structure. The result gives an experimental verification for real application of the composite adsorbent; (2) using methanol as refrigerant and heat pipe fluid. The working pressure is higher than that of water so that the system leakage risk can be reduced. As a result, the performance of the chiller and mass transfer performances should be improved; (3) finding an optimal operation strategy for the chiller, including the proper cycle time, mass recovery time and the water inlet temperatures. How these factors affect the chiller’s performance is analyzed; (4) testing the chiller in both air conditioning condition and cold-storage condition.
2.
System description
The schematic diagram of the composite adsorbent-methanol chiller is shown in Fig. 1a. The chiller is composed of three chambers. Both chamber 1 and chamber 2 contain one adsorber, one condenser, one evaporator. Chamber 3 is a heatpipe evaporator. By using the loop heat pipe in evaporator (shown in Fig. 1b), the vacuum valves are not used inside the chiller, so that the reliability of the chiller could be enhanced. This heat pipe evaporator is similar to the evaporator described before (Wang et al., 2005). Its working principle will be explained in the following section. Six three way valves (V1eV6), one mass recovery valve (V7) and one check valve (V8) are adopted outside the chiller in this system. The photographs of the adsorber, condenser and evaporators are shown respectively in Fig. 1c and d.
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Fig. 1 e Schematic diagrams and photographs of the chiller.
The test system is shown in Fig. 2. It mainly consists of a chilled water tank, a cooling tower and a boiler. Cooling capacity is balanced by a heating rod in chilled water tank instead of indoor air. To set the temperature of the chilled water inlet at specific value, the heating power of the heating rod is adjusted by voltage regulation. Cooling tower is adopted
cooling tower F
flow meter
T temperature sensor
water valve
water pump
tap water (makeup lines)
T F
boiler
T
chiller
T T
T F
T
chilled water tank Fig. 2 e Schematic diagram of the test system.
to reject heat of condensation and adsorption to the atmosphere. The cooling water temperature can be adjusted by fan control of the cooling tower. Unlike normal central air condition system, the heat needed to be rejected in adsorption cooling system changes remarkably in the cycle. It contributes the fluctuation of the cooling water temperature. The boiler supplies the heat energy. However, the control mode of the test system in the lab is simple, so in this paper the heating power is constant and there is regularly inlet temperature fluctuation of hot water. The average temperature of hot water inlet can be set in the boiler. A data logger (Agilent 34970A) and temperature sensors (PT 100, 4ewire system, accuracy standard of A) are used to measure the temperatures. Flow-meters (accuracy: 0.5%) are used to measure the rate of flow. The chiller is controlled by SIMENS LOGO controller.
3.
Working principle
The working cycle is made up of adsorption/desorption process, mass recovery process and heat recovery process. The operating states of the valves are shown in Fig. 3. The working processes are described in the following:
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Fig. 3 e The states of valves in operation. The working principle in (1)e(6) steps are explained below. The red line stands for hot water. The blue line stands for cooling water. The pink line stands for the mixed water. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
(1) Adsorber 1 works in adsorption process while adsorber 2 is in desorption process
(2) Mass recovery process In this process, valve V7 opens, the right chamber connects to the left one. Vapors flow from desorption chamber to adsorption chamber. The large pressure difference of these two chambers contributes to re-desorption and re-adsorption. At the end of this step, V7 closes.
4.
Experiment and discussion
4.1.
Temperatures profiles of the working fluids
Fig. 4 shows the temperature histories of the inlets and outlets of hot water, cooling water, and chilled water: (1) Hot water: The volume flow rate of chilled water is 2 m3 h1. An average hot water inlet temperature Ti,hot can set by the boiler. At the
90
1 2
80
Temperature/ oC
Hot water flows into adsorber 2. Adsorber 2 is heated and the methanol in adsorber 2 is desorbed. As the adsorber 2, condenser 2 and evaporator 2 are in the same chamber and they are all connected, methanol vapor is condensed in condenser 2 and evaporator 2. The condensate drips into evaporator 2 at last. Cooling water flows though condenser 2 firstly and then flows into adsorber 1. It carries away the heat of condensation of condenser 2 and cools down the adsorber 1 to adsorption temperature. On the right, evaporator 2 is heated by condensates of the methanol desorbed from adsorber 2. So the right heat-pipe evaporator does not work. On the left, adsorber 1 adsorbs methanol vapor, as a result, it evaporates outside the tubes in evaporator 1 and cools down the tubes. Meanwhile, the vapors, which are from the heat-pipe evaporator flow into the cold tubes at evaporator 1 and condense inside the cold tubes, flowing back to the heat-pipe evaporator at last. The evaporation of the methanol in the heat-pipe evaporator results in the production of cooling capacity.
(5) By rotating three way valves V2 and V3, the heat recovery proceeds and the valves are ready for the next cycle. (6) By rotating three-way valves V5 and V6, a half cycle is finished. The adsorber 2 begins adsorption process while adsorber 1 works in desorption process. The flowing steps are similar to the steps above.
70 60
t1 t3
50 40
t2
3 4 5 6
30 20 10
(3)e(5) Heat recovery process (3) By rotating three way valves V4 and V5, hot water is bypassed. It doesn’t flow into the chiller which means that no heat is used in this step. (4) By rotating three way valves V1 and V6, hot water flows into the adsorber 1 while cool water flows into adsorber 2. The adsorber 1 is pre-heated which means that less heat is used to heat up the bed in next half cycle. The adsorber 2 is pre-cooled down which is good for next half cycle. COP is improved.
0
500
1000
1500
2000
Time /s Fig. 4 e Experimentally measured fluids inlet and outlet temperatures from adsorber, condenser and evaporator (Ti,hot [ 85 C, Ti,chilled [ 20 C, Ti,cooling [ 30 C, t1 [ 360 s, t2 [ 60 s, t3 [ 20 s) 1-hot water inlet 2-hot water outlet 3cooling water outlet 4-cooling water inlet 5-chilled water inlet 6echilled water outlet. t1: heating/cooling time; t2: mass recovery time; t3: heat recovery time.
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beginning of the cycle, the hot water flows into the desorber which is in adsorption process in last cycle, To,hot decreases sharply because the metal and adsorbent need to be heated up. After a short time, To,hot increases. In mass recovery process, though the water in boiler is heated, To,hot decreases because of the re-desorption. Then, in heat recovery process, the hot water is bypassed and Ti,hot and To,hot are the same.
temperatures every 5 s. As a result, those parameters of the chillers in Table 1 are calculated and averaged as following: PN s¼1
N
PN (2) Chilled water:
s¼1
qh ¼
Ti,chilled keeps constant (cooling capacity is balanced by a heating rod). The volume flow rate of chilled water is 0.7 m3 h1 (It is 0.8 m3 h1 in silica gel-water chiller which is mentioned in Table 1.). At the beginning of the cycle, To,chilled decreases sharply. The reason is that the adsorption rate is very large at the beginning. As time goes on, the cooling capacity becomes smaller due to the decreasing adsorption rate. In mass recovery process, the variation trend of To,chilled depends on many factors. It will be analyzed in detail in the following section.
The performance of the composite adsorbent-methanol chiller is compared with that of the silica gel-water chiller in similar constructions (Wang et al., 2005) and using the same test system. The temperatures of water inlet are almost the same. It should be noted that in order to be impregnated by salt, the composite adsorbent should use the silica gel with large pore size and volume. As a result, the density of the composite adsorbent (600 kg m3) is much smaller than that of silica gel (880 kg m3) used before. The mass of the composite adsorbent is about 40 kg in this chiller while it is 60 kg if using pure silica gel. Also the fact that the latent of evaporation of methanol is 1102 kJ kg1 while that of water is 2258 kJ kg1 should be noted. Table 1 shows the performance of composite-methanol chiller and silica gel-water chiller. The data logger gets
_ hot Cw Tsi;hot Tso;hot m
(2)
N qe M
(3)
COP ¼
qe qh
(4)
Wheres is the sequence number and N is the number of the data obtained in one cycle.qe also can be estimated by this equation:
(3) Cooling water:
4.2. Performances comparison with that of the silica gelwater chiller
(1)
SCP ¼
qe ¼
The average temperature of cooling water inlet Ti,cooling is set at 30 C. It fluctuates slightly (2e3 C) due to the simple control of the fan in cooling tower. The variation trend of To,cooling depends on the heat needed to be rejected from condenser and desorber. It increases sharply at the beginning of the cycle and decreases with the declining adsorption rate. It increases again at the end of cycle due to re-condensation and re-adsorption in mass recovery process.
qse
s ¼ 1; 2; 3:::; N _ chilled Cw Tsi;chilled Tso;chilled qse ¼ m
qe ¼
Ma Dmad Lr tcyc
(5)
Where Lr is the latent heat of refrigerant.tcyc is the cycle time. By combing Eq. (1) with Eq. (5), Eq. (6) is obtained. The cycle adsorption capacity can be derived by Eq. (6). Dmad ¼
qe tcyc Mc Lm
(6)
For the chiller with composites-methanol: Dmcm ¼ qe tcyc 4:99 760 ¼ ¼ 0:172kg=kg Mc Lm 20 1102 qe tcyc For the chiller with silica gel-water: Dmsw ¼ ¼ Ms Lw 6:44 760 ¼ 0:072kg=kg 30 2258 Dmcm The mass ratio ¼ 2:39. It means that adsorption Dmsw capacity of composite adsorbent-methanol is 1.39 time larger than silica gel-water. What’s more, though the cooling capacity of the new chiller is 22.5% smaller than the old one. SCP is improved by 16.3%, and COP is increased by 24.2%. These improvements give an experimental verification for real application of the composite adsorbent and their advantages compared with silica gel.
4.3.
Operation performance with different cycle time
It is well known that cycle time is one of the main influential parameters on both cooling capacity and COP. Optimal cycle time is dependent on the requirements of COP and the cooling capacity (Alam et al., 2003; Miyazaki et al., 2009). By Boelman, the cycle time is maximized at a certain cycle time, while COP
Table 1 e The performances of composite adsorbent-methanol adsorption chiller and silica-water chiller (t1 [ 680 s, t2 [ 60 s, t3 [ 20 s). Working pairs Composite adsorbent/ methanol Silica gel/water
Ti,chilled ( C)
To,chilled ( C)
Ti,hot ( C)
To,hot ( C)
To,cooling ( C)
Ti,cooling ( C)
Cooling capacity (kW)
COP
SCP (kW kg1)
21.3
15.3
84.8
79.6
34.2
29.8
4.99
0.41
0.250
21.9
15.0
85.6
77.4
34.9
29.4
6.44
0.33
0.215
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7
cooling capacity
0.5
0.2 0.1
2 1 300
400
500
600
700
800
0.35 0.30
5
0.25 0.20
4
0.15 3
0.0
0
50
100
150
200
250
0.10
300
t2/ s
t1/ s
increases monotonically with cycle time at least until 1300 s (Boelman et al., 1995). Saha and Chua have proved the results by investigating the mathematical model (Saha et al., 1995; Chua et al., 1999). 1) Effect of heating/cooling time t1 The similar conclusion has been got in this experiment. Fig. 5 shows that the cooling capacity decreases firstly and then increases, after it reaches the maximum when t1 ¼ 680 s, it decreases again while COP keeps increasing until t1 ¼ 720 s. Here the cooling capacity is referred to qe . The variation trend of qse is very complicated, for that there is cooling capacity transport delay from the evaporator to heat-pipe evaporator and the heat-pipe evaporator to the heat-transfer medium. Generally, longer cycle time results in larger qse as Fig. 6 shows, but qe may decrease as qse is relatively small after
Fig. 7 e The cooling capacity and COP with the variation of mass recovery time (Ti,hot [ 85 C, Ti,chilled [ 20 C, Ti,cooling [ 30 C, t1 [ 680 s, t3 [ 20 s).
7
Cooling capacity /kW
Fig. 5 e The cooling capacity and COP with the variation of heating/cooling time (Ti,hot [ 85 C, Ti,chilled [ 20 C, Ti,cooling [ 30 C, t2 [ 60 s, t3 [ 20 s).
0.5
cooling capacity
6
COP
0.4
5 0.3
4
COP
3
0.40
6
COP
COP
4
Cooling capacity /kW
0.4 5
0.3
0.45
COP
COP
6
Cooling capacity / kW
0.50
7
cooling capacity
0.2
3
0.1
2 1 55
60
65
70
75
80
85
0.0 90
o
Temperature/ C Fig. 8 e The cooling capacity and COP with the variation of hot temperature inlet (Ti,chilled [ 20 C, Ti,cooling [ 30 C, t1 [ 680 s, t2 [ 60 s, t3 [ 20 s).
10 t1=680 s, t2=60 s, t3=20 s t1=360 s, t2=60 s, t3=20 s
2.0
T o,chilled
1.5
8
T i,chilled
1.0
7
o
To,chilled / C
Cooling capacity/kW
9
6 5
mass recovery
4
mass recovery
3
0.5 0.0 -0.5 -1.0 -1.5 -2.0
2
0
1 0
200
400
600
800
1000
Time /s Fig. 6 e The variation of cooling capacity with different heating/cooling time (Ti,hot [ 85 C, Ti,chilled [ 20 C, Ti,cooling [ 30 C).
200
400
600
800
1000
1200
1400
1600
Time /s Fig. 9 e Experimentally measured fluids inlet and outlet temperatures from evaporator (Ti,hot [ 85 C, Ti,chilled [ L0.8 C, Ti,cooling [ 29 C, t1 [ 680 s, t2 [ 60 s, t3 [ 20 s).
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Table 2 e The performances of the novel chiller at different evaporating temperature (t1 [ 680 s, t2 [ 60 s, t4 [ 20 s). Ti,chilled ( C) a
0.8 6.4b 15.3b
To,chilled ( C)
Ti,hot ( C)
To,hot ( C)
To,cooling ( C)
Ti,cooling ( C)
Cooling capacity (kW)
COP
1.0 11.0 21.3
85.7 84.3 84.8
82.5 79.2 79.6
29.0 29.2 34.2
33.1 33.6 29.8
1.26 3.78 4.99
0.16 0.32 0.41
a Using antifreeze as chilled fluid. Cp ¼ 3.6 kJ kg1 K1. b Using water as chilled fluid. Cp ¼ 4.2 kJ kg1 K1.
some time. In practice, the adsorption/desorption and condensation/evaporation reacted incompletely. The longer desorption time can make the methanol being desorbed more adequately, which means that in the next cycle, the “drier” adsorbent bed has a larger adsorption capacity, as the concentration difference of the methanol inside and outside of the adsorber is larger. In this experiment, it is entirely possible that the adsorption/desorption process nearly completely finished before 720 s. So the highest cooling capacity can be got when heating/cooling time is between 680 s and 720 s t1 should be longer if a higher COP is needed. The largest COP can be obtained when t1 is 720 s and 800 s. Optimal cycle time is dependent on the requirements of COP or the cooling capacity. It is in the range of 680 se720 s 2) Effect of mass recovery times The mass recovery process affects the chiller’s performance from three aspects: 1) the adsorption bed adsorbs the vapor from the desorption bed instead of the evaporator, so the evaporation in the heat pipe evaporator is weakened and thus the cooling capacity is seriously decreased; 2) The hot vapor flows from adsorber 2 to adsorber 1 because of the pressure difference in mass recovery process. Most of the hot vapor will be condensed in the condenser. However when vapor velocity is too fast and according to the connection of the condenser 1 and evaporator 1, it is entirely possible that a small part of the vapor will be condensed in evaporator 1. These vapors will heat evaporator 1. As a result, the higher temperature of the evaporator results intensified evaporation which increases the cooling capacity; 3) According to Wang et al. (2005), long mass recovery time causes a full desorption and then its refrigeration capacity increases. It is very complicated to decide which one is the leading factor in this stage. Fig. 7 shows that the mass recovery improves the cooling capacity and COP. However, when t2 is larger than 60 s, the cooling capacity and COP almost remain the same. So in this experiment, 60 s is enough for the mass recovery.
4.4. inlet
Operation performance with different hot water
Fig. 8 shows the variations of cooling capacity and COP with the increasing of hot water temperature. It demonstrates that the performance of the chiller is affected by Ti,hot. The suitable temperature is about 80 C. The machine can work even when the heat source is below 60 C, though the performance deteriorates with the decreasing Ti,hot.
4.5. Performance of a tentative experiment at different evaporation temperature In the prior research, the silica gel-water chiller was usually used for air conditioning. The reference (Maggio et al., 2009) described the simulation of a solid sorption ice-maker based on the composite adsorbent “lithium chloride in silica gel” and methanol. It was discontinuous and used solar energy as heating source. The simulation result showed that a maximum daily ice production of 0.56 kg m2 can be obtained. In this experiment, the antifreeze is used as working fluid in heat-pipe evaporator when the evaporating temperature is too low. Fig. 9 shows the variation of the chilled water inlet and outlet temperatures when the evaporating temperature is at 0 C. To,chilled rises sharply when the half cycle is ended. The reason is that at the low evaporating temperature, the adsorption capacity is small. So a lot of hot methanol vapor stay in the dead volume of the chamber. When the process of adsorber changes from adsorption to desorption, many refrigerant vapor would be condensed and heat the evaporator. A large part of cooling capacity is lost to cool it down. The lost accounts the majority of the cooling capacity especially when the cooling capacity is small at low evaporating temperature condition. Table 2 shows that cooling capacity and COP are reduced when the evaporating temperature declines. As a result, the cooling method such as radiant cooling which can use the chiller water in higher temperature will have a better performance.
5.
Conclusions
A novel composite adsorbent-methanol chiller has been tested. Based on the experiments, these conclusions are achieved: (1) The new composite adsorbent, which is dealt with to eliminate the corrosion effect, can be applied in the chiller and the adsorption performance is improved. It enables the usage of methanol in the adsorption chiller. When the temperatures of hot water inlet, chilled water outlet, cooling water inlet are 84.8 C, 15.3 C, 29.8 C, heating/ cooling time is 680 s, mass recovery time is 60 s and heating recovery time is 20 s, the average COP is 0.41. The cooling capacity is 4.99 kW. (2) Compared with the similar chiller filling with silica gel and water, COP is improved by 24.2%. Though the cooling capacity is decreased due to the low latent heat of evaporation of methanol and the smaller filling amount of
i n t e r n a t i o n a l j o u r n a l o f r e f r i g e r a t i o n 3 5 ( 2 0 1 2 ) 1 9 5 0 e1 9 5 7
adsorbent material, SCP is increased as much as 16.3%. The cycle adsorption capacity is increased by 138.1%. (3) For this new chiller, the best heating/cooling time is 680 s to get the highest cooling capacity, or it can be set at 720 s to get the highest COP. The mass recovery improves the cooling capacity and COP. The appropriate time is 60 s. (4) The best temperature of heat source is about 80e85 C. It is lower than that of silica gel-water chiller (85e90 C), which means that it can work better when it is combined with solar energy. (5) The performance is not ideal at low evaporating temperature. Cooling capacity and COP are deteriorated with the deceasing evaporating temperature while the cooling water inlet and hot water inlet remain at 30 C and 80 C. The reason is possibly attributed to the construction with continuous cooling heat-pipe evaporator. So the chiller with this design needs reconstruction if it is operated in this condition. It is better to be used in the radiation cooling system.
Acknowledgments This work was supported by the Key project of the Natural Science Foundation of China under the contract No.51020105010.
references
Alam, K.C.A., Kang, Y.T., Saha, B.B., Akisawa, A., Kashiwagi, T., 2003. A novel approach to determine optimum switching frequency of a conventional adsorption chiller. Energy 28 (10), 1021e1037. Anyanwu, E.E., Ezekwe, C.I., 2003. Design, construction and test run of a solid adsorption solar refrigerator using activated carbon/methanol as adsorbent/adsorbate pair. Energy Convers. Manage 44, 2879e2892. Aristov, Yu. I., Restuccia, G., Cacciola, G., Parmon, V.N., 2002. A family of new working materials for solid sorption air conditioning systems. Appl. Therm. Eng. 22, 191e204. Bauer, J., Herrmann, R., Mittelbach, W., Schwieger, W., 2009. Zeolite/aluminum composite adsorbents for application in adsorption refrigeration. Int. J. Energy.Res. 33, 1233e1249. Boelman, B.C., Saha, B.B., Kashiwagi, T., 1995. Experimental investigation of a silica gelewater adsorption refrigeration cycledthe influence of operating conditions on cooling output and COP. ASHRAE Trans. Res. 101 (2), 358e366.
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Chua, H.T., Ng, K.C., Malek, A., Kashiwagi, T., Akisawa, A., Saha, B.B., 1999. Modeling the performance of two-bed, silica gelewater adsorption chillers. Int. J. Refrigeration 22, 194e204. Freni, A., Sapienza, A., Glaznev, I.S., Aristov, Yu.I., Restuccia, G., 2012. Experimental testing of a lab-scale adsorption chiller using a novel selective water sorbent “silica modified by calcium nitrate”. Int. J. Refrigeration 35 (3), 518e524. Gong, L.X., Wang, R.Z., Xia, Z.Z., Chen, J.J., 2010a. Adsorption equilibrium of water on a composite adsorbent employing lithium chloride in silica gel. J. Chem. Eng. Data 55, 2920e2923. Gong, L.X., Wang, R.Z., 2010b. Preparation and study the characteristic of a new composite adsorbent employing lithium chloride in silica gel. In: Innovative Materials for Processes in Energy Systems, pp. 33e38. Gordeeva, L.G., Freni, A., Krieger, T.A., Restuccia, G., Aristov, Yu.I., 2008. Composites lithium halides in silica gel pores: methanol sorption equilibrium. Microporous Mesoporous Mater. 112, 254e261. Gordeeva, L.G., Freni, A., Aristov, Yu. I., Restuccia, G., 2009. Composite sorbent of methanol “lithium chloride in mesoporous silica gel” for adsorption cooling machine: performance and stability evaluation. Ind. Eng. Chem. Pes. 48, 6197e6202. Hu, P., Yao, J.J., Chen, Z.S., 2009. Analysis for composite zeolite/ foam aluminumewater mass recovery adsorption refrigeration system driven by engine exhaust heat. Energy Convers. Manage. 50, 255e261. Maggio, G., Gordeeva, L.G., Freni, A., Aristov, Yu.I., Santori, G., Polonara, F., Restuccia, G., 2009. Simulation of a solid sorption ice-maker based on the novel composite sorbent “lithium chloride in silica gel pores”. Appl. Therm. Eng. 29, 1714e1720. Miyazaki, T., Akisawa, A., Saha, B.B., El-Sharkawy, I.I., Chakraborty, A., 2009. A new cycle time allocation for enhancing the performance of two-bed adsorption chillers. Int. J. Refrigeration 32, 846e853. Restuccia, G., Freni, A., Vasta, S., Aristov, Yu.I., 2004. Selective water sorbent for solid sorption chiller: experimental result and modeling. Int. J. Refrigeration 27, 284e293. Saha, B.B., Boelman, E.C., Kashiwagi, T., 1995. Computer simulation of silica gelewater adsorptionrefrigeration cycledthe influence of operating conditions on cooling output and COP. ASHRAE Trans. Res. 101 (2), 348e357. Saha, B.B., El-Sharkawy, I.I., Chakraborty, A., Koyama, S., 2007. Study on an activated carbon fiber-ethanol adsorption chiller: part I e system description and modeling. Int. J. Refrigeration 30, 87e95. Veselovskaya, J.V., Tokarev, M.M., 2011. Novel ammonia sorbents “porous matrix modified by active salt” for adsorptive heat transformation: 4. Dynamics of quasi-isobaric ammonia sorption and desorption on BaCl2/vermiculite. Appl. Therm. Eng. 31, 566e572. Wang, D.C., Wu, J.Y., Xia, Z.Z., 2005. Study of a novel silica gelewater adsorption chiller. Part II. Experimental study. Int. J. Refrigeration 28 (7), 1084e1091. Wang, L.W., Wang, R.Z., Oliveira, R.G., 2009. A review on adsorption working pairs for refrigeration. Renew Sust Energy Rev. 13 (3), 518e534.
Available online at www.sciencedirect.com
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journal homepage: www.elsevier.com/locate/ijrefrig
Experimental study on an adsorption chiller employing lithium chloride in silica gel and methanol L.X. Gong, R.Z. Wang*, Z.Z. Xia, Z.S. Lu Institute of Refrigeration and Cryogenics, Shanghai Jiao Tong University, 800 Dong Chuan Road, Shanghai 200240, China
article info
abstract
Article history:
A novel composite adsorbent e methanol adsorption chiller was proposed and manufac-
Received 14 September 2011
tured. It was filled by the adsorbent composed by Lithium Chloride and silica gel. Methanol
Received in revised form
was used as adsorbate and refrigerant. Experiment results showed that compared with
18 June 2012
silica gel-water chiller, SCP (specific cooling power) and COP (coefficient of performance) of
Accepted 30 June 2012
this novel chiller were improved by 16.3% and 24.2% separately when the temperatures of
Available online 20 July 2012
hot water inlet, cooling water inlet and chilled water outlet were 85 C, 30 C and 15 C. ª 2012 Elsevier Ltd and IIR. All rights reserved.
Keywords: Adsorption system Performance Lithium chloride Silica gel Methanol
Etude expe´rimentale sur un refroidisseur a` adsorption utilisant le chlorure de lithium dans du gel de silice et du me´thanol Mots cle´s : Syste`me a` adsorption ; Performance ; Chlorure de lithium ; Gel de silice ; Me´thanol
1.
Introduction
Now energy consumption is one of the most popular topics. People are facing more challenges to balance sustainable development and environmental protection. Adsorption technology, which is a clean tech attracts more and more researchers. It employs friendly refrigerants (water, methanol, etc.) instead of the ozone depleting substances such as CFCs and HCFCs. In addition, the adsorption chiller can be
driven by industry waste heat or solar energy with the temperature of 55e90 C. As a result, this tech cuts a part of the greenhouse gas emitted from coal plants. A family of adsorption working pairs has been studied (Aristov et al., 2002; Restuccia et al., 2004; Bauer et al., 2009; Veselovskaya and Tokarev, 2011). The typical working pairs are silica gelewater, zeoliteewater, activated carbone ammonia, activated carbonemethanol, activated carbon fibereammonia, inorganic salt-ammonia, etc. The utilizations
* Corresponding author. Tel.: þ86 021 021 34206548; fax: þ86 021 34206309. E-mail address: [email protected] (R.Z. Wang). 0140-7007/$ e see front matter ª 2012 Elsevier Ltd and IIR. All rights reserved. http://dx.doi.org/10.1016/j.ijrefrig.2012.06.013
i n t e r n a t i o n a l j o u r n a l o f r e f r i g e r a t i o n 3 5 ( 2 0 1 2 ) 1 9 5 0 e1 9 5 7
Nomenclature Cw L M _ m Dm qe qh t1 t2 t3 T
1
1
specific heat, kJ kg K latent heat, kJ kg1 mass of adsorbent material, kg mass flow rate, kg h1 cycle adsorption capacity, kg cooling capacity, kW heating power, kW adsorption/desorption time, s mass recovery time, s heat recovery time, s temperature, K
Subscripts a adsorbent
of water and methanol as refrigerants in adsorption system make it possible to use the heat source below 100 C (Saha et al., 2007). However, typical water/methanol working pairs do not have ideal characteristics. When water is used as an adsorbate, the low working pressure increases the risk of leakage problems and implies severe mass transfer limitation through the component of the machine through the adsorbent bed itself (Gordeeva et al., 2009). Methanol has a higher operation pressure. However the sorption capacity is rather low and usually does not exceed 0.15e0.4 g g1 (Gordeeva et al., 2008). And these working pairs do not guarantee adequate performance when desorption temperature is lower than 90 C (Freni et al., 2012). Some composite adsorbents have been proposed. Novel adsorbents with metal materials have been developed to enhance heat and mass transfer of adsorption material (Hu et al., 2009). The composites with hydrophilic salt largely improve the adsorption capacity (Aristov et al., 2002; Gordeeva et al., 2008). However, it is quite possible that those additives bring some new problems, for instance, corrosion, which hinders the real application in industry. As a result, a new way to produce the composite adsorbent which employing Lithium Chloride in silica gel has been proposed by the researchers in SJTU (Shanghai Jiao Tong University). The salt on the surface is removed and the characteristics of the new composite adsorbent have been studied (Gong et al., 2010a). Now a chiller with this kind of adsorbent has been manufactured and tested. Usually the adsorption working pair for conventional adsorption chiller is wateresilica gel. However the working pressure of water is very low. As a result, the mass transfer in the adsorbent bed is limited. Furthermore, small amount of air or gas, which are leaked into the machine or emerged by some unknown reactions, weaken the performance of the chiller significantly (Gordeeva et al., 2008). Methanol is used as an alternative adsorbate in this chiller. Its high working pressure enhances the stability performance of the chiller; also it has a lower freezing temperature. So tentative experiments for cold-storage or ice making are allowed to do. But the latent heat of evaporation of methanol is lower than that of water. It can only be compensated by the improved adsorption capacity of the composite adsorbent. The theoretical adsorption refrigeration cycle of working pair
ad c chilled c-m cooling cyc hot i m o r s s-w w
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adsorbate composite adsorbent chilled water composite adsorbent-methanol cooling water cycle hot water inlet methanol outlet refrigerant silica gel silica gel-water water
(composite adsorbent-methanol) has been studied (Gong et al., 2010b). Though there are many research papers about the composite adsorbents (Aristov et al., 2002; Wang et al., 2009) and the chillers filled with typical working pairs (silica gelewater, activated carbonemethanol, etc.) (Anyanwu and Ezekwe, 2003; Freni et al., 2012 and Wang et al., 2005), there are few papers about the experimental study on the chiller filled by composite adsorbent. This paper presents the design and the experimental study of a chiller using the composite adsorbent and methanol. The objects of this paper are (1) analyzing the dynamic performances of the novel chiller. It is compared with that of silica gel-water adsorption chiller in similar structure. The result gives an experimental verification for real application of the composite adsorbent; (2) using methanol as refrigerant and heat pipe fluid. The working pressure is higher than that of water so that the system leakage risk can be reduced. As a result, the performance of the chiller and mass transfer performances should be improved; (3) finding an optimal operation strategy for the chiller, including the proper cycle time, mass recovery time and the water inlet temperatures. How these factors affect the chiller’s performance is analyzed; (4) testing the chiller in both air conditioning condition and cold-storage condition.
2.
System description
The schematic diagram of the composite adsorbent-methanol chiller is shown in Fig. 1a. The chiller is composed of three chambers. Both chamber 1 and chamber 2 contain one adsorber, one condenser, one evaporator. Chamber 3 is a heatpipe evaporator. By using the loop heat pipe in evaporator (shown in Fig. 1b), the vacuum valves are not used inside the chiller, so that the reliability of the chiller could be enhanced. This heat pipe evaporator is similar to the evaporator described before (Wang et al., 2005). Its working principle will be explained in the following section. Six three way valves (V1eV6), one mass recovery valve (V7) and one check valve (V8) are adopted outside the chiller in this system. The photographs of the adsorber, condenser and evaporators are shown respectively in Fig. 1c and d.
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Fig. 1 e Schematic diagrams and photographs of the chiller.
The test system is shown in Fig. 2. It mainly consists of a chilled water tank, a cooling tower and a boiler. Cooling capacity is balanced by a heating rod in chilled water tank instead of indoor air. To set the temperature of the chilled water inlet at specific value, the heating power of the heating rod is adjusted by voltage regulation. Cooling tower is adopted
cooling tower F
flow meter
T temperature sensor
water valve
water pump
tap water (makeup lines)
T F
boiler
T
chiller
T T
T F
T
chilled water tank Fig. 2 e Schematic diagram of the test system.
to reject heat of condensation and adsorption to the atmosphere. The cooling water temperature can be adjusted by fan control of the cooling tower. Unlike normal central air condition system, the heat needed to be rejected in adsorption cooling system changes remarkably in the cycle. It contributes the fluctuation of the cooling water temperature. The boiler supplies the heat energy. However, the control mode of the test system in the lab is simple, so in this paper the heating power is constant and there is regularly inlet temperature fluctuation of hot water. The average temperature of hot water inlet can be set in the boiler. A data logger (Agilent 34970A) and temperature sensors (PT 100, 4ewire system, accuracy standard of A) are used to measure the temperatures. Flow-meters (accuracy: 0.5%) are used to measure the rate of flow. The chiller is controlled by SIMENS LOGO controller.
3.
Working principle
The working cycle is made up of adsorption/desorption process, mass recovery process and heat recovery process. The operating states of the valves are shown in Fig. 3. The working processes are described in the following:
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Fig. 3 e The states of valves in operation. The working principle in (1)e(6) steps are explained below. The red line stands for hot water. The blue line stands for cooling water. The pink line stands for the mixed water. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
(1) Adsorber 1 works in adsorption process while adsorber 2 is in desorption process
(2) Mass recovery process In this process, valve V7 opens, the right chamber connects to the left one. Vapors flow from desorption chamber to adsorption chamber. The large pressure difference of these two chambers contributes to re-desorption and re-adsorption. At the end of this step, V7 closes.
4.
Experiment and discussion
4.1.
Temperatures profiles of the working fluids
Fig. 4 shows the temperature histories of the inlets and outlets of hot water, cooling water, and chilled water: (1) Hot water: The volume flow rate of chilled water is 2 m3 h1. An average hot water inlet temperature Ti,hot can set by the boiler. At the
90
1 2
80
Temperature/ oC
Hot water flows into adsorber 2. Adsorber 2 is heated and the methanol in adsorber 2 is desorbed. As the adsorber 2, condenser 2 and evaporator 2 are in the same chamber and they are all connected, methanol vapor is condensed in condenser 2 and evaporator 2. The condensate drips into evaporator 2 at last. Cooling water flows though condenser 2 firstly and then flows into adsorber 1. It carries away the heat of condensation of condenser 2 and cools down the adsorber 1 to adsorption temperature. On the right, evaporator 2 is heated by condensates of the methanol desorbed from adsorber 2. So the right heat-pipe evaporator does not work. On the left, adsorber 1 adsorbs methanol vapor, as a result, it evaporates outside the tubes in evaporator 1 and cools down the tubes. Meanwhile, the vapors, which are from the heat-pipe evaporator flow into the cold tubes at evaporator 1 and condense inside the cold tubes, flowing back to the heat-pipe evaporator at last. The evaporation of the methanol in the heat-pipe evaporator results in the production of cooling capacity.
(5) By rotating three way valves V2 and V3, the heat recovery proceeds and the valves are ready for the next cycle. (6) By rotating three-way valves V5 and V6, a half cycle is finished. The adsorber 2 begins adsorption process while adsorber 1 works in desorption process. The flowing steps are similar to the steps above.
70 60
t1 t3
50 40
t2
3 4 5 6
30 20 10
(3)e(5) Heat recovery process (3) By rotating three way valves V4 and V5, hot water is bypassed. It doesn’t flow into the chiller which means that no heat is used in this step. (4) By rotating three way valves V1 and V6, hot water flows into the adsorber 1 while cool water flows into adsorber 2. The adsorber 1 is pre-heated which means that less heat is used to heat up the bed in next half cycle. The adsorber 2 is pre-cooled down which is good for next half cycle. COP is improved.
0
500
1000
1500
2000
Time /s Fig. 4 e Experimentally measured fluids inlet and outlet temperatures from adsorber, condenser and evaporator (Ti,hot [ 85 C, Ti,chilled [ 20 C, Ti,cooling [ 30 C, t1 [ 360 s, t2 [ 60 s, t3 [ 20 s) 1-hot water inlet 2-hot water outlet 3cooling water outlet 4-cooling water inlet 5-chilled water inlet 6echilled water outlet. t1: heating/cooling time; t2: mass recovery time; t3: heat recovery time.
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beginning of the cycle, the hot water flows into the desorber which is in adsorption process in last cycle, To,hot decreases sharply because the metal and adsorbent need to be heated up. After a short time, To,hot increases. In mass recovery process, though the water in boiler is heated, To,hot decreases because of the re-desorption. Then, in heat recovery process, the hot water is bypassed and Ti,hot and To,hot are the same.
temperatures every 5 s. As a result, those parameters of the chillers in Table 1 are calculated and averaged as following: PN s¼1
N
PN (2) Chilled water:
s¼1
qh ¼
Ti,chilled keeps constant (cooling capacity is balanced by a heating rod). The volume flow rate of chilled water is 0.7 m3 h1 (It is 0.8 m3 h1 in silica gel-water chiller which is mentioned in Table 1.). At the beginning of the cycle, To,chilled decreases sharply. The reason is that the adsorption rate is very large at the beginning. As time goes on, the cooling capacity becomes smaller due to the decreasing adsorption rate. In mass recovery process, the variation trend of To,chilled depends on many factors. It will be analyzed in detail in the following section.
The performance of the composite adsorbent-methanol chiller is compared with that of the silica gel-water chiller in similar constructions (Wang et al., 2005) and using the same test system. The temperatures of water inlet are almost the same. It should be noted that in order to be impregnated by salt, the composite adsorbent should use the silica gel with large pore size and volume. As a result, the density of the composite adsorbent (600 kg m3) is much smaller than that of silica gel (880 kg m3) used before. The mass of the composite adsorbent is about 40 kg in this chiller while it is 60 kg if using pure silica gel. Also the fact that the latent of evaporation of methanol is 1102 kJ kg1 while that of water is 2258 kJ kg1 should be noted. Table 1 shows the performance of composite-methanol chiller and silica gel-water chiller. The data logger gets
_ hot Cw Tsi;hot Tso;hot m
(2)
N qe M
(3)
COP ¼
qe qh
(4)
Wheres is the sequence number and N is the number of the data obtained in one cycle.qe also can be estimated by this equation:
(3) Cooling water:
4.2. Performances comparison with that of the silica gelwater chiller
(1)
SCP ¼
qe ¼
The average temperature of cooling water inlet Ti,cooling is set at 30 C. It fluctuates slightly (2e3 C) due to the simple control of the fan in cooling tower. The variation trend of To,cooling depends on the heat needed to be rejected from condenser and desorber. It increases sharply at the beginning of the cycle and decreases with the declining adsorption rate. It increases again at the end of cycle due to re-condensation and re-adsorption in mass recovery process.
qse
s ¼ 1; 2; 3:::; N _ chilled Cw Tsi;chilled Tso;chilled qse ¼ m
qe ¼
Ma Dmad Lr tcyc
(5)
Where Lr is the latent heat of refrigerant.tcyc is the cycle time. By combing Eq. (1) with Eq. (5), Eq. (6) is obtained. The cycle adsorption capacity can be derived by Eq. (6). Dmad ¼
qe tcyc Mc Lm
(6)
For the chiller with composites-methanol: Dmcm ¼ qe tcyc 4:99 760 ¼ ¼ 0:172kg=kg Mc Lm 20 1102 qe tcyc For the chiller with silica gel-water: Dmsw ¼ ¼ Ms Lw 6:44 760 ¼ 0:072kg=kg 30 2258 Dmcm The mass ratio ¼ 2:39. It means that adsorption Dmsw capacity of composite adsorbent-methanol is 1.39 time larger than silica gel-water. What’s more, though the cooling capacity of the new chiller is 22.5% smaller than the old one. SCP is improved by 16.3%, and COP is increased by 24.2%. These improvements give an experimental verification for real application of the composite adsorbent and their advantages compared with silica gel.
4.3.
Operation performance with different cycle time
It is well known that cycle time is one of the main influential parameters on both cooling capacity and COP. Optimal cycle time is dependent on the requirements of COP and the cooling capacity (Alam et al., 2003; Miyazaki et al., 2009). By Boelman, the cycle time is maximized at a certain cycle time, while COP
Table 1 e The performances of composite adsorbent-methanol adsorption chiller and silica-water chiller (t1 [ 680 s, t2 [ 60 s, t3 [ 20 s). Working pairs Composite adsorbent/ methanol Silica gel/water
Ti,chilled ( C)
To,chilled ( C)
Ti,hot ( C)
To,hot ( C)
To,cooling ( C)
Ti,cooling ( C)
Cooling capacity (kW)
COP
SCP (kW kg1)
21.3
15.3
84.8
79.6
34.2
29.8
4.99
0.41
0.250
21.9
15.0
85.6
77.4
34.9
29.4
6.44
0.33
0.215
1955
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7
cooling capacity
0.5
0.2 0.1
2 1 300
400
500
600
700
800
0.35 0.30
5
0.25 0.20
4
0.15 3
0.0
0
50
100
150
200
250
0.10
300
t2/ s
t1/ s
increases monotonically with cycle time at least until 1300 s (Boelman et al., 1995). Saha and Chua have proved the results by investigating the mathematical model (Saha et al., 1995; Chua et al., 1999). 1) Effect of heating/cooling time t1 The similar conclusion has been got in this experiment. Fig. 5 shows that the cooling capacity decreases firstly and then increases, after it reaches the maximum when t1 ¼ 680 s, it decreases again while COP keeps increasing until t1 ¼ 720 s. Here the cooling capacity is referred to qe . The variation trend of qse is very complicated, for that there is cooling capacity transport delay from the evaporator to heat-pipe evaporator and the heat-pipe evaporator to the heat-transfer medium. Generally, longer cycle time results in larger qse as Fig. 6 shows, but qe may decrease as qse is relatively small after
Fig. 7 e The cooling capacity and COP with the variation of mass recovery time (Ti,hot [ 85 C, Ti,chilled [ 20 C, Ti,cooling [ 30 C, t1 [ 680 s, t3 [ 20 s).
7
Cooling capacity /kW
Fig. 5 e The cooling capacity and COP with the variation of heating/cooling time (Ti,hot [ 85 C, Ti,chilled [ 20 C, Ti,cooling [ 30 C, t2 [ 60 s, t3 [ 20 s).
0.5
cooling capacity
6
COP
0.4
5 0.3
4
COP
3
0.40
6
COP
COP
4
Cooling capacity /kW
0.4 5
0.3
0.45
COP
COP
6
Cooling capacity / kW
0.50
7
cooling capacity
0.2
3
0.1
2 1 55
60
65
70
75
80
85
0.0 90
o
Temperature/ C Fig. 8 e The cooling capacity and COP with the variation of hot temperature inlet (Ti,chilled [ 20 C, Ti,cooling [ 30 C, t1 [ 680 s, t2 [ 60 s, t3 [ 20 s).
10 t1=680 s, t2=60 s, t3=20 s t1=360 s, t2=60 s, t3=20 s
2.0
T o,chilled
1.5
8
T i,chilled
1.0
7
o
To,chilled / C
Cooling capacity/kW
9
6 5
mass recovery
4
mass recovery
3
0.5 0.0 -0.5 -1.0 -1.5 -2.0
2
0
1 0
200
400
600
800
1000
Time /s Fig. 6 e The variation of cooling capacity with different heating/cooling time (Ti,hot [ 85 C, Ti,chilled [ 20 C, Ti,cooling [ 30 C).
200
400
600
800
1000
1200
1400
1600
Time /s Fig. 9 e Experimentally measured fluids inlet and outlet temperatures from evaporator (Ti,hot [ 85 C, Ti,chilled [ L0.8 C, Ti,cooling [ 29 C, t1 [ 680 s, t2 [ 60 s, t3 [ 20 s).
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Table 2 e The performances of the novel chiller at different evaporating temperature (t1 [ 680 s, t2 [ 60 s, t4 [ 20 s). Ti,chilled ( C) a
0.8 6.4b 15.3b
To,chilled ( C)
Ti,hot ( C)
To,hot ( C)
To,cooling ( C)
Ti,cooling ( C)
Cooling capacity (kW)
COP
1.0 11.0 21.3
85.7 84.3 84.8
82.5 79.2 79.6
29.0 29.2 34.2
33.1 33.6 29.8
1.26 3.78 4.99
0.16 0.32 0.41
a Using antifreeze as chilled fluid. Cp ¼ 3.6 kJ kg1 K1. b Using water as chilled fluid. Cp ¼ 4.2 kJ kg1 K1.
some time. In practice, the adsorption/desorption and condensation/evaporation reacted incompletely. The longer desorption time can make the methanol being desorbed more adequately, which means that in the next cycle, the “drier” adsorbent bed has a larger adsorption capacity, as the concentration difference of the methanol inside and outside of the adsorber is larger. In this experiment, it is entirely possible that the adsorption/desorption process nearly completely finished before 720 s. So the highest cooling capacity can be got when heating/cooling time is between 680 s and 720 s t1 should be longer if a higher COP is needed. The largest COP can be obtained when t1 is 720 s and 800 s. Optimal cycle time is dependent on the requirements of COP or the cooling capacity. It is in the range of 680 se720 s 2) Effect of mass recovery times The mass recovery process affects the chiller’s performance from three aspects: 1) the adsorption bed adsorbs the vapor from the desorption bed instead of the evaporator, so the evaporation in the heat pipe evaporator is weakened and thus the cooling capacity is seriously decreased; 2) The hot vapor flows from adsorber 2 to adsorber 1 because of the pressure difference in mass recovery process. Most of the hot vapor will be condensed in the condenser. However when vapor velocity is too fast and according to the connection of the condenser 1 and evaporator 1, it is entirely possible that a small part of the vapor will be condensed in evaporator 1. These vapors will heat evaporator 1. As a result, the higher temperature of the evaporator results intensified evaporation which increases the cooling capacity; 3) According to Wang et al. (2005), long mass recovery time causes a full desorption and then its refrigeration capacity increases. It is very complicated to decide which one is the leading factor in this stage. Fig. 7 shows that the mass recovery improves the cooling capacity and COP. However, when t2 is larger than 60 s, the cooling capacity and COP almost remain the same. So in this experiment, 60 s is enough for the mass recovery.
4.4. inlet
Operation performance with different hot water
Fig. 8 shows the variations of cooling capacity and COP with the increasing of hot water temperature. It demonstrates that the performance of the chiller is affected by Ti,hot. The suitable temperature is about 80 C. The machine can work even when the heat source is below 60 C, though the performance deteriorates with the decreasing Ti,hot.
4.5. Performance of a tentative experiment at different evaporation temperature In the prior research, the silica gel-water chiller was usually used for air conditioning. The reference (Maggio et al., 2009) described the simulation of a solid sorption ice-maker based on the composite adsorbent “lithium chloride in silica gel” and methanol. It was discontinuous and used solar energy as heating source. The simulation result showed that a maximum daily ice production of 0.56 kg m2 can be obtained. In this experiment, the antifreeze is used as working fluid in heat-pipe evaporator when the evaporating temperature is too low. Fig. 9 shows the variation of the chilled water inlet and outlet temperatures when the evaporating temperature is at 0 C. To,chilled rises sharply when the half cycle is ended. The reason is that at the low evaporating temperature, the adsorption capacity is small. So a lot of hot methanol vapor stay in the dead volume of the chamber. When the process of adsorber changes from adsorption to desorption, many refrigerant vapor would be condensed and heat the evaporator. A large part of cooling capacity is lost to cool it down. The lost accounts the majority of the cooling capacity especially when the cooling capacity is small at low evaporating temperature condition. Table 2 shows that cooling capacity and COP are reduced when the evaporating temperature declines. As a result, the cooling method such as radiant cooling which can use the chiller water in higher temperature will have a better performance.
5.
Conclusions
A novel composite adsorbent-methanol chiller has been tested. Based on the experiments, these conclusions are achieved: (1) The new composite adsorbent, which is dealt with to eliminate the corrosion effect, can be applied in the chiller and the adsorption performance is improved. It enables the usage of methanol in the adsorption chiller. When the temperatures of hot water inlet, chilled water outlet, cooling water inlet are 84.8 C, 15.3 C, 29.8 C, heating/ cooling time is 680 s, mass recovery time is 60 s and heating recovery time is 20 s, the average COP is 0.41. The cooling capacity is 4.99 kW. (2) Compared with the similar chiller filling with silica gel and water, COP is improved by 24.2%. Though the cooling capacity is decreased due to the low latent heat of evaporation of methanol and the smaller filling amount of
i n t e r n a t i o n a l j o u r n a l o f r e f r i g e r a t i o n 3 5 ( 2 0 1 2 ) 1 9 5 0 e1 9 5 7
adsorbent material, SCP is increased as much as 16.3%. The cycle adsorption capacity is increased by 138.1%. (3) For this new chiller, the best heating/cooling time is 680 s to get the highest cooling capacity, or it can be set at 720 s to get the highest COP. The mass recovery improves the cooling capacity and COP. The appropriate time is 60 s. (4) The best temperature of heat source is about 80e85 C. It is lower than that of silica gel-water chiller (85e90 C), which means that it can work better when it is combined with solar energy. (5) The performance is not ideal at low evaporating temperature. Cooling capacity and COP are deteriorated with the deceasing evaporating temperature while the cooling water inlet and hot water inlet remain at 30 C and 80 C. The reason is possibly attributed to the construction with continuous cooling heat-pipe evaporator. So the chiller with this design needs reconstruction if it is operated in this condition. It is better to be used in the radiation cooling system.
Acknowledgments This work was supported by the Key project of the Natural Science Foundation of China under the contract No.51020105010.
references
Alam, K.C.A., Kang, Y.T., Saha, B.B., Akisawa, A., Kashiwagi, T., 2003. A novel approach to determine optimum switching frequency of a conventional adsorption chiller. Energy 28 (10), 1021e1037. Anyanwu, E.E., Ezekwe, C.I., 2003. Design, construction and test run of a solid adsorption solar refrigerator using activated carbon/methanol as adsorbent/adsorbate pair. Energy Convers. Manage 44, 2879e2892. Aristov, Yu. I., Restuccia, G., Cacciola, G., Parmon, V.N., 2002. A family of new working materials for solid sorption air conditioning systems. Appl. Therm. Eng. 22, 191e204. Bauer, J., Herrmann, R., Mittelbach, W., Schwieger, W., 2009. Zeolite/aluminum composite adsorbents for application in adsorption refrigeration. Int. J. Energy.Res. 33, 1233e1249. Boelman, B.C., Saha, B.B., Kashiwagi, T., 1995. Experimental investigation of a silica gelewater adsorption refrigeration cycledthe influence of operating conditions on cooling output and COP. ASHRAE Trans. Res. 101 (2), 358e366.
1957
Chua, H.T., Ng, K.C., Malek, A., Kashiwagi, T., Akisawa, A., Saha, B.B., 1999. Modeling the performance of two-bed, silica gelewater adsorption chillers. Int. J. Refrigeration 22, 194e204. Freni, A., Sapienza, A., Glaznev, I.S., Aristov, Yu.I., Restuccia, G., 2012. Experimental testing of a lab-scale adsorption chiller using a novel selective water sorbent “silica modified by calcium nitrate”. Int. J. Refrigeration 35 (3), 518e524. Gong, L.X., Wang, R.Z., Xia, Z.Z., Chen, J.J., 2010a. Adsorption equilibrium of water on a composite adsorbent employing lithium chloride in silica gel. J. Chem. Eng. Data 55, 2920e2923. Gong, L.X., Wang, R.Z., 2010b. Preparation and study the characteristic of a new composite adsorbent employing lithium chloride in silica gel. In: Innovative Materials for Processes in Energy Systems, pp. 33e38. Gordeeva, L.G., Freni, A., Krieger, T.A., Restuccia, G., Aristov, Yu.I., 2008. Composites lithium halides in silica gel pores: methanol sorption equilibrium. Microporous Mesoporous Mater. 112, 254e261. Gordeeva, L.G., Freni, A., Aristov, Yu. I., Restuccia, G., 2009. Composite sorbent of methanol “lithium chloride in mesoporous silica gel” for adsorption cooling machine: performance and stability evaluation. Ind. Eng. Chem. Pes. 48, 6197e6202. Hu, P., Yao, J.J., Chen, Z.S., 2009. Analysis for composite zeolite/ foam aluminumewater mass recovery adsorption refrigeration system driven by engine exhaust heat. Energy Convers. Manage. 50, 255e261. Maggio, G., Gordeeva, L.G., Freni, A., Aristov, Yu.I., Santori, G., Polonara, F., Restuccia, G., 2009. Simulation of a solid sorption ice-maker based on the novel composite sorbent “lithium chloride in silica gel pores”. Appl. Therm. Eng. 29, 1714e1720. Miyazaki, T., Akisawa, A., Saha, B.B., El-Sharkawy, I.I., Chakraborty, A., 2009. A new cycle time allocation for enhancing the performance of two-bed adsorption chillers. Int. J. Refrigeration 32, 846e853. Restuccia, G., Freni, A., Vasta, S., Aristov, Yu.I., 2004. Selective water sorbent for solid sorption chiller: experimental result and modeling. Int. J. Refrigeration 27, 284e293. Saha, B.B., Boelman, E.C., Kashiwagi, T., 1995. Computer simulation of silica gelewater adsorptionrefrigeration cycledthe influence of operating conditions on cooling output and COP. ASHRAE Trans. Res. 101 (2), 348e357. Saha, B.B., El-Sharkawy, I.I., Chakraborty, A., Koyama, S., 2007. Study on an activated carbon fiber-ethanol adsorption chiller: part I e system description and modeling. Int. J. Refrigeration 30, 87e95. Veselovskaya, J.V., Tokarev, M.M., 2011. Novel ammonia sorbents “porous matrix modified by active salt” for adsorptive heat transformation: 4. Dynamics of quasi-isobaric ammonia sorption and desorption on BaCl2/vermiculite. Appl. Therm. Eng. 31, 566e572. Wang, D.C., Wu, J.Y., Xia, Z.Z., 2005. Study of a novel silica gelewater adsorption chiller. Part II. Experimental study. Int. J. Refrigeration 28 (7), 1084e1091. Wang, L.W., Wang, R.Z., Oliveira, R.G., 2009. A review on adsorption working pairs for refrigeration. Renew Sust Energy Rev. 13 (3), 518e534.