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Adsorption ice makers for fishing boats driven by the exhaust heat from diesel engine: choice of adsorption pair
Energy Conversion and Management 45 (2004) 2043–2057 www.elsevier.com/locate/enconman
Adsorption ice makers for fishing boats driven by the exhaust heat from diesel engine: choice of adsorption pair L.W. Wang, R.Z. Wang *, J.Y. Wu, K. Wang, S.G. Wang Institute of Refrigeration and Cryogenics, Shanghai Jiao Tong University, Shanghai 200030, PR China Received 1 August 2003; accepted 26 October 2003
Abstract Different adsorbents, such as physical, chemical and composite adsorbents are analyzed. Three types of working pairs, activated carbon–methanol, chemical adsorbent-ammonia and composite adsorbentammonia can be used for adsorption ice makers on fishing boats. The advantages, disadvantages and performances of three types of adsorption ice makers, with activated carbon–methanol, CaCl2 –NH3 and compound adsorbent (made by CaCl2 and activated carbon)-NH3 as adsorption pairs, are compared at the condition of two bed systems. The activated carbon–methanol ice maker shows the advantage of reliable safety, and the composite adsorption ice maker shows the best adsorption performance. The cooling power of 20.32 kW can be obtained from the composite adsorption ice maker when the volume for each adsorber is 0.288 m3 , which is about 10 times that of the physical adsorption ice maker and 1.38 times that of the chemical adsorption ice maker. 2003 Elsevier Ltd. All rights reserved. Keywords: Adsorption; Ice maker; Fishing boat; Activated carbon–methanol; Calcium chloride–ammonia; Composite adsorbent
1. Introduction Because of sustainable development requirements, efficient refrigerators driven by low grade thermal energy from different sources have received much more attention in recent years [1]. Adsorption refrigerators are benign to the environment due to their use of water, methanol and ammonia as refrigerants. In comparison with absorption systems, an adsorption system has no problems such as coolant pollution, crystallization and fractionation, while in comparison with
*
Corresponding author. Tel.: +86-21-6293-3838; fax: +86-21-6293-3250. E-mail address: [email protected] (R.Z. Wang).
0196-8904/$ - see front matter 2003 Elsevier Ltd. All rights reserved. doi:10.1016/j.enconman.2003.10.021
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vapor compression refrigerating systems, an adsorption system has the advantages of simple control, low initial investment and circulating expenditure and less noise [2]. Nowadays, the fishing is mainly divided into fishing on adjacent seas and pelagic seas. The adsorption ice making systems are mainly designed for the fishing boats on adjacent seas. Most fishing boats on adjacent seas are middle type and mini-type, and the powers of most Diesel engines are between 136 and 441 kW. The energy of a Diesel engine is 30–35% exhausted into the air [2], which can be applied to drive an adsorptive ice maker. There are several advantages of the adsorption refrigerators on fishing boats such as less influences of vibration, less noise, compact structure, simple control, etc.
2. Appropriate adsorbents for ice makers on fishing boats Adsorption can be classified as physical adsorption due to a physical process caused by Van der Waals forces or chemical adsorption in which a chemical process is involved. Correspondingly, there are mainly three types of adsorbents, physical adsorbents that mainly include activated carbon, zeolite and silica gel, chemical adsorbents such as calcium chloride, metal hydrides and complex salts, and composite adsorbents that mainly are obtained by the combination of chemical adsorbents and a porous matrix. 2.1. Physical adsorbents Studies on physical adsorption working pairs mainly focus on activated carbon–methanol, activated carbon–ammonia, zeolite–water and silica gel–water, and the researches on adsorption systems focus mainly on systems driven by solar energy. A type of adsorption ice maker with a solar energy collector of 6 m2 and activated carbon (AC35) of 130 kg designed by Pons and Grenier could produce 5.3–5.6 kg ice/m2 under the radiant intensity of 19–22 MJ [3]. The adsorption system designed by Leite is also powered by solar energy and uses activated carbon–methanol as the working pair. The optimal COP of a TIM (transparent insulation material) cover system is 0.155 between March and December [4]. A solar and gas solid sorption machine that uses the active carbon fiber Busofit as a sorbent bed and ammonia as a working fluid was designed and studied by Vasiliev et al. [5]. This adsorption machine has a very short (12 min) non-intermittent cycle and a high solar COP near 0.3 with 1.2 m2 solar collection surface and solar collector efficiency being near 0.7. Critoph and Vogel [6] and Meunier [7] studied the performance of activated carbon–methanol, zeolite–water and other working pairs, and the results show that activated carbon–methanol is an ideal working pair for solar energy because of its high COP and low generation temperature. A two bed adsorption chiller using silica gel–water as working pair is researched by Yonezawa et al. [8,9], and it has already been successfully commercialized in Japan. Researches on zeolite–water adsorption cycle operated by periodic reversal forced convection show that the COP of the heat pump is as high as 1.7, and the COP of refrigeration is up to 0.9, corresponding to the SCP of 125 W/kg [10]. Shanghai Jiao Tong University has obtained many achievements on the adsorption system powered by solar energy or waste heat and using activated carbon–methanol as a working pair, including a continuous heat regenerative adsorption refrigerator using a spiral plate heat exchanger as adsorber [11] and an
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energy efficient hybrid system of solar powered water heater and adsorption ice maker [12]. The zeolite–water adsorption refrigerator with 140 kg adsorbent designed by Shanghai Jiao Tong University has already been installed and tested on train locomotive cabin air conditioning and the cooling power obtained from tests is about 3.1 kW [13]. 2.2. Chemical and composite adsorbents The main advantage of chemical adsorption is the high performance of refrigeration or heat pump. For example, a calcium oxide/carbon dioxide reactivity in a packed bed reactor is studied by Kato. Results showed that the reactor was capable of storing heat at 900 C by decarbonation of calcium carbonate and generating up to 997 C by carbonation of calcium oxide. The amount of stored heat in the reactor was 800–900 kJ/kg [14]. The main disadvantages of chemical adsorption are studied. Many researches show that the phenomena of swelling, disintegration and agglomeration of adsorbent are the main problems for its application. Researches also show that the addition of a porous matrix in the chemical adsorbent could improve the adsorption performance. The performance of CaCl2 confined to a mesoporous host matrix MCM-41 has been studied by Tokarev. Results show that the composite material based on CaCl2 as an impregnated salt and MCM-41 as a host matrix is able to absorb up to 0.75 g of H2 O/g of dry sorbent. That high absorptivity can ensure high values of the energy storage capacity (2.1 kJ/g) [15]. Five different compounds, pure MnCl2 , simple mixture of ex-PAN T300 carbon fibers and MnCl2 (mixture 1), simple mixture of ex-pitch FT700 carbon fibers and MnCl2 (mixture 2), impregnated ex-pitch FT500 carbon fibers with MnCl2 (ICF) and intercalation of MnCl2 into ex-pitch graphitized carbon fibers P120 (GFIC), are studied by Dellero. The best of all kinetic results are obtained with GFIC and IFC. These two compounds not only provide a very fast reaction but also give a complete reaction by avoiding the agglomeration phenomenon [16,17]. Iloeje has also controlled the problem of agglomeration of CaCl2 by setting 20% CaSO4 in the adsorbent [18]. 2.3. Appropriate working pairs for adsorption ice makers on fishing boats There are mainly two types of working pairs, zeolite–water and CaCl2 –NH3 , that have already been researched to be used in adsorption refrigerators on fishing boats. The feasibilities of a zeolite–water adsorption refrigerator on fishing boats that has 400 kg zeolite per unit tube have been studied by Han and Zhu [19–21]. They obtained the cooling power of 92 kJ and cooled the water from 24 to 2 C by a single adsorption tube unit when the cycle time is 3 h. Qi et al. studied the feasibility of an adsorption ice maker driven by the exhaust gas of the Diesel engine on fishing boats that uses CaCl2 –NH3 as working pair [22]. Considering the conditions of ice making, there are three types of appropriate adsorption working pairs, activated carbon–methanol, chemical adsorbent-ammonia and composite adsorbent-ammonia that can be used on fishing boats. The activated carbon–methanol system is safer than the chemical and composite adsorbent-ammonia system, while the chemical and composite adsorbent-ammonia system has a higher performance of refrigeration than the activated carbon– methanol system.
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3. Activated carbon–methanol adsorption ice maker 3.1. Adsorption ice making system Fig. 1 shows the schematic of an activated carbon–methanol adsorption ice maker. The corresponding developed system is shown in Fig. 2. Two adsorbers are designed in the system, and heat and mass recovery will proceed between the two beds because the heat and mass recovery process will improve the adsorption performance and economize the energy consumed [23]. There are one evaporator and one condenser in the two bed system. The water that cools the adsorber is coming from the sea directly or from the water tank or cooling water tower during the laboratory tests. The heat source is the exhaust gases from the diesel engine (in the experiments, we use an oil burner to simulate the diesel engine). In order to accumulate the heat from the exhaust gases, a
Fig. 1. Diagram of activated carbon–methanol adsorption system.
Fig. 2. The integrated adsorption ice maker prototype.
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hot water storage vessel (boiler) is used at the exit of the exhaust gas–water heat exchanger. The refrigeration effect of the adsorption prototype is transported to the flake ice maker to make ice, in which the methanol is pumped through the heat exchanger inside the flake ice maker. The heat transfer medium of the two adsorbers are connected to the hot water storage vessel and cooling water tank, respectively, corresponding to the switch of desorption and adsorption. The working processes are controlled by valves and pumps. For example, the electro-magnetic valves 0a, 0b, a, b, 0d, 0c, c, d, electro-pneumatic valves 1, 2, 3, 4 and pump 3 are shut off when heat recovery proceeds. The electro-pneumatic valves 1 and 3 should be open when the mass recovery proceeds. The electro-magnetic valves 0a, 0c, b, d, e, f, electro-pneumatic valves 2, 3 and pump 2 are shut off when adsorber 1 is desorbing and, at the same time, adsorber 2 is adsorbing. The electro-magnetic valves 0b, 0d, a, c, e, f, electro-pneumatic valves 1, 4 and pump 2 are shut off when adsorber 1 is adsorbing and, at the same time, adsorber 2 is desorbing. This prototype photograph is shown in Fig. 2. The length, width and height of this prototype are about 2.5, 1.5 and 2 m, respectively, when the adsorption refrigeration system, flake ice maker and waste heat recovery boiler are incorporated into a single unit. 3.2. Adsorber description Solidified activated carbon is chosen as the adsorbent because of its higher density, better adsorption performance and better heat transfer performance [24] compared with granular activated carbon. The cross-section structure of each adsorber is shown in Fig. 3. There are three blocks of solidified activated carbon at the section of the adsorber, and two fins of 0.15 thicknesses are kept to both sides of one block of solidified activated carbon that has the thickness of 10 mm. The heat transfer area of the fins is 25 m2 . Each adsorber contains activated carbon of 60 kg. 3.3. Performance Experiments are operated at the environmental temperature of 27 C, and mass and heat recovery proceeds between the two beds at the switch time. The arrangements and results are
Fig. 3. The cross-section of adsorber.
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Table 1 Arrangements and results of experiments with heat and mass recovery (two bed operation) No.
Cycle time (min)
Desorption and adsorption time (min)
Recovery time of mass (min)
Recovery time of heat (min)
Average evaporating temperature (C)
Ice productivity (kg/ h)
Cooling power (kW)
Average heating power (kW)
COP
SCP1 (W/kg)
SCP2 (W/kg)
1 2 3 4 5 6
36 46 56 66 76 86
15 20 25 30 35 40
1 1 1 1 1 1
2 2 2 2 2 2
)5.41 )9.47 )10.23 )10.31 )10.97 )10.53
12.00 13.50 15.00 15.25 15.43 15.38
1.51 1.72 1.92 1.96 1.99 1.98
15.21 17.62 16.03 15.63 17.23 19.60
0.088 0.097 0.120 0.125 0.115 0.101
30.2 33.1 36.1 35.9 36.5 35.5
12.6 14.4 16.1 16.3 16.8 16.5
shown in Table 1, where SCP1 is the specific cooling power per kilogram adsorbent in the adsorption time, and SCP2 is the specific cooling power per kilogram adsorbent in the whole cycle time. The optimal SCP1 and COP (coefficient of performance) obtained are 36.5 W/kg and 0.125, respectively. The optimal ice productivity is 15.43 kg/h and 370 kg/day, while the cycle time is 76 min, and the evaporating temperature is about )11 C.
4. Adsorption system and adsorbent performance testing unit for chemical and composite adsorption ice maker 4.1. Adsorption system description The key point for the design of chemical or composite adsorbent-ammonia ice makers is to ensure the safety in the cabin. In the adsorption system shown in Fig. 4, all the components in which ammonia flows are designed to be installed on the deck. Only the flake ice maker is installed in the cabin. The thermal fluid used in the system is heat conducting oil, which is cooled by the sea water at the time of adsorption and heated by the exhaust gas at the time of desorption. An oil tank is used at the exit of the oil–gas heat exchanger to accumulate the heat from the exhaust gases. Two vertical type adsorbers are designed in the system, and heat and mass recovery will proceed between two beds. The working processes of this ice maker are similar to those of the activated carbon–methanol system (Fig. 1). 4.2. Adsorber design for chemical and composite adsorption ice maker The section for the adsorber designed for the chemical and composite adsorption ice maker is circular (Fig. 5). Heat conducting oil is used as the thermal fluid instead of water because the optimal desorption temperature of the adsorbent is as high as 140 C. Expansion space must be provided between the adsorbent and fins because the swelling of CaCl2 is serious at the time of
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Fig. 4. Diagram of chemical and composite adsorption ice maker.
Fig. 5. Section of adsorber for chemical and composite adsorption ice maker.
adsorption. Thus, the adsorber will be set upright in the system to avoid the adsorbent falling off from the fins and to ensure the expansion space of the adsorbent. The key point for the design of chemical and composite adsorber is to choose an appropriate expansion space for the adsorbent. Thus, the adsorption performance testing unit is designed and set up to test the performance of adsorbents with different expansion spaces. 4.3. Adsorption performance testing unit for chemical and composite adsorption ice maker The adsorption performance testing unit (shown in Fig. 6) consists mainly of two parts, adsorber and condenser (evaporator), which, respectively, is put in the oil tank and ethanol jacket
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Fig. 6. Adsorbent testing unit.
considering the operation modes of desorption and adsorption. The adsorber is cooled by the water circuit heat exchanger at the time of adsorption and heated by the electric heater at the time of desorption. The temperature of the evaporator/condenser is controlled by the thermostatic bath, which can control the temperature from )40 to 90 C. The adsorption quantity and desorption quantity are measured by the magneto-strictive displacement sensor inserted into the ammonia container, whose measuring error is 0.05%. Two adsorbers (Fig. 7) are designed for the testing unit. The adsorber with edge fold is designed for the granular chemical and composite adsorbent, and the adsorber without edge fold is designed for the solidified composite adsorbent. Each adsorber is divided into two half parts that are connected by flange seals. The spaces are separated by the fins, and the distance between two fins
Fig. 7. Structure of adsorber for granular adsorbent (a) and solidified adsorbent (b).
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is 12 mm. Various samples are tested in order to find an optimal ratio between the expansion space and adsorbent volume. In this research, the thickness of 2 mm CaCl2 with the ratio of 5:1 is named sample 1, the thickness of 3 mm CaCl2 with the ratio of 3:1 is named sample 2, the thickness of 4 mm CaCl2 with the ratio of 2:1 is named sample 3, the thickness of 5 mm CaCl2 with the ratio of 1.4:1 is named sample 4, the thickness of 6 mm simple composite adsorbent that is combined by CaCl2 and activated carbon is named sample 5, and the thickness of 9 mm solidified adsorbent that is also combined by CaCl2 and activated carbon is named sample 6.
5. Performance prediction of chemical adsorption ice maker 5.1. Choice of layout of chemical adsorbent One important rule for the choice of layout for chemical adsorbent is the stability of performance. Repeated experiments at the condition of 40 C adsorption temperature and 0 C evaporating temperature are performed first in order to test the stability of the adsorption performance. Results are shown in Fig. 8. The adsorption performance of sample 2 is similar to that of sample 1, and they are much different from those of samples 3 and 4. The performances of samples 3 and 4 are much more stable than those of samples 1 and 2, and the performance of sample 4 is worse than that of sample 3 because of the deterioration of mass transfer that was caused by limited expansion space. After repeated tests, the expansion and agglomeration phenomena of the adsorbent for samples 1 and 3 are compared (Fig. 9) in order to find the reason for the different performance stability. There are no phenomena of agglomeration for sample 1 because there was enough expansion space, although the swelling of the adsorbent is as much as six to seven times. The agglomeration is serious in sample 3 after adsorption. An important conclusion is obtained from the experiments: moderate agglomeration is beneficial to the stability of adsorption performance, although it will lead to bad mass transfer performance, when the evaporating temperature is too low. One possible explanation for this phenomenon is the shielded factor (the ratio of repulsive force to attractive force for each anion) of a complex compound that is increasing with the number of ammonias in the ammoniate. For example, the shielded factor is 0.96 for tetra-ammoniate, while it is 1.66 for hexad-ammoniate. The NH3 is easier to fill around Ca2þ for sample 1 than sample 3 because of the larger pore space under the condition of larger expansion space, and then, the 1.15
Sample 1
x /(kg/kg)
1.10
Sample 2
Sample 3
1.05 1.00 0.95 0.90
Sample 4
0.85 0
3
6
9
12
15
Test number
Fig. 8. Adsorption performance in the repeated experiments.
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Fig. 9. (a) Sample 1 and (b) sample 3 after adsorption.
influence of shield factor on sample 1 will be larger because it has a larger concentration of NH3 around Ca2þ . Then the octo-ammoniate will be more difficult to be formed in sample 1 than sample 3. Then, there are two appropriate layouts of adsorbent for the chemical adsorption ice maker considering the stability of adsorption performance, samples 3 and 4. The performance of sample 3 is better than that of sample 4, while the filling quantity of sample 4 in the adsorber is larger than that of sample 3 assuming the volumes of adsorbers for these two types of samples are the same. 5.2. Performance prediction of chemical adsorption ice maker The performances of samples 3 and 4 are tested at the evaporating temperature of )15 C and condensing temperature of 25 C. The cooling power is computed by the adsorption isobars. The adsorption cooling power per kilogram is defined as Qm (kJ/kg). The formula for Qm is Dxht ; ð1Þ m where Dx (kg/kg) is the adsorption quantity at the fixed saturated temperature, ht (kJ/kg) is the latent heat of vaporization at that saturated temperature and m is the mass of adsorbent. The volume cooling density (Qs , kJ/m3 ) is defined as Qm ¼
Dxht ; ð2Þ V where V (m3 ) is the total volume for the adsorbent and its expansion space. Qm and Qs are compared in Fig. 10. Qm and Qs of sample 4 are all worse than those of sample 3 because its mass transfer performance is influenced by the narrow expansion space for the adsorbent. The highest Qm is 889 kJ/kg and the highest Qs is 1.845 · 105 kJ/m3 . Qs is more important than Qm for ice makers on fishing boats because the space on fishing boats is limited and the mass of adsorbent takes up only a small proportion compared with the mass of the whole system. The valid volume for the adsorbent and mass transfer space for the two physical adsorbers shown in Fig. 2 is 0.288 m3 . Assuming the valid volume for the adsorbent and expansion space in chemical adsorbers is the same as that of the physical adsorbers, then the cooling power of one cycle can be obtained by the experimental results in Fig. 10, and it is as high as 0.531 · 105 kJ. Qs ¼
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4.0 3.5
Q m for sample 3 3.0 2.5 2.0
400
1.5
5
600
3
Q m for sample 4 Q s for sample 3 Q s for sample 4
Q s/(×10 kJ/m )
Q m/(kJ/kg)
800
1.0 200 0.5 0
0.0 20
25
30
35
40
45
50
55
60
65
70
o
Adsorption temperature T / C
Fig. 10. Qm and Qs vs. adsorption temperature at the evaporating temperature of )15 C per cycle.
Assuming the cycle time of the chemical adsorption ice maker is 60 min, the cooling power of the chemical adsorption ice maker is as high as 14.75 kW, which is much higher than the cooling power of the activated carbon–methanol adsorption ice maker. This estimation is based on the waste heat being enough for the ice maker.
6. Performance prediction of composite adsorption ice maker 6.1. Choice of layout of composite adsorbent Fig. 10 shows that the performance of sample 4 is much lower than that of sample 3 because it is influenced by the bad mass transfer performance. The mass transfer performance of the chemical adsorbent can be improved by the addition of a porous medium, such as activated carbon. A simple composite adsorbent (sample 5) is mixed by the ratio of 4:1 between the mass of CaCl2 and the mass of activated carbon, and the thickness of the adsorbent on the fins is 6 mm. Repeated experiments at the condition of 40 C adsorption temperature and 0 C evaporating temperature are performed, and the results are shown in Fig. 11. There is great performance deterioration for sample 5. The adsorption quantity of the seventh time is about 20.4% less than the adsorption quantity of the first time. The adsorber is checked in order to find the reason for the deterioration. The adsorbent after adsorption is shown in Fig. 12. The phenomenon of agglomeration of sample 5 is much different from that of sample 3, although the mass of CaCl2 is the same in the adsorbers. There is no serious phenomenon of agglomeration in sample 5. It means that the moderate agglomeration did not occur in sample 5, and thus, the performance deterioration exists in the experiments. In order to obtain the moderate agglomeration in the compound adsorbent after adsorption, the compound adsorbent is solidified and tested.
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x /(kg/kg)
0.9
adsorption quantity for CaCl2 in composite adsorbent
0.8 0.7
adsorption quantity of composite adsorbent
0.6 0.5 0
1
2
3
4
5
6
7
8
Test number
Fig. 11. Performance deterioration.
Fig. 12. Adsorbent after adsorption.
Fig. 13. Solidified compound adsorbent (sample 6).
Sample 6 is made by CaCl2 , activated carbon and cement. The ratio between the mass of CaCl2 and activated carbon is also 4:1, and the cement and water are used to bind these two types of adsorbent. The solidified adsorbent is shown in Fig. 13. The thickness of sample 6 is about 9 mm, and the filling quantity of CaCl2 in the adsorber is about 16% added compared with sample 4. There is no phenomenon of performance deterioration in the repeated experiments. The adsorption quantity became stable at about 0.796 kg/kg after the second repeated test.
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3.5
1000
3.0
600
2.0
5
Q m/(kJ/kg)
3
2.5
Q s /(×10 kJ/m )
Qm Qm Qs
800
for sample 6 for CaCl2 in sample 6
1.5
400
1.0 200 0.5 0
0.0 20
25
30
35
40
45
50
55
60
65
70
o
Adsorption temperature T / C
Fig. 14. Qm and Qs at the evaporating temperature of )15 C for solidified composite adsorbent per cycle.
The appropriate composite adsorbent for ice makers is the solidified composite adsorbent considering the stability of adsorption performance. 6.2. Performance prediction of composite adsorption ice maker The performance of sample 6 is tested at the evaporating temperature of )15 C and condensing temperature of 25 C, Qm and Qs are shown in Fig. 14. The Qm of CaCl2 in sample 6 is similar to that of sample 3 (Fig. 10), and the Qs of sample 6 is much better than that of sample 3 (Fig. 10). The highest Qm is 844 kJ/kg for the CaCl2 in sample 6, and the highest Qs is 2.54 · 105 kJ/m3 which is enhanced about 38% compared with the Qs of sample 3. Assuming the valid volume for the adsorbent and expansion space in composite adsorbers is also 0.288 m3 , the cooling power of one cycle obtained from Fig. 14 is 0.57912 · 105 kJ, and then, the cooling power of the adsorption ice maker is as high as 20.32 kW when the cycle time is 60 min, which is much higher than the cooling power for both the physical adsorption ice maker and the chemical adsorption ice maker.
7. Conclusions The performance of a physical adsorption ice maker is tested, and the performance of chemical and composite adsorption ice makers are predicted from the performance of the adsorbent. The several conclusions obtained are as follows. (1) Considering the condition of ice making, there are three type of adsorption ice makers that can be used on fishing boats, physical, chemical and composite adsorption ice makers. A physical adsorption ice maker is safer than chemical and composite adsorption ice makers for the reason of less noxious refrigerant. Thus, it is easier to be installed on the ship. Contrarily, the design and
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installation of chemical and composite adsorption ice makers must consider the question of safety on the boat, and all the components in which ammonia flows must be installed outside the cabin. (2) The adsorption performance of an activated carbon–methanol system with two adsorbers and with 60 kg adsorbent per bed is tested. The optimal SCP1 (specific cooling power per kilogram adsorbent in adsorption time) and COP obtained are 36.5 W/kg and 0.125, respectively. (3) The performance stability of chemical adsorbents with different expansion spaces are studied. Two samples of CaCl2 (CaCl2 with the ratio between expansion space and volume of adsorbent of 2:1 and 1.4:1) show stable performance and can be used in the ice makers. Best performance was obtained from sample 3, which has the Qm (the adsorption cooling power per kilogram adsorbent) of 889 kJ/kg and Qs (the volume cooling density) of 1.845 · 105 kJ/m3 per cycle. (4) The performance stability of composite adsorbents is studied, and the solidified compound adsorbent (combined with CaCl2 and activated carbon) shows stable performance and can be used in the ice makers. Qs obtained from the solidified compound adsorbent is as high as 2.54 · 105 kJ/m3 , which is enhanced about 38% compared with Qs of sample 3. (5) The cooling power of the chemical adsorption ice maker and the cooling power of the composite adsorption ice maker are computed from the Qs assuming these two types of ice makers have the same valid volume for adsorbent and expansion space with that of the activated carbon– methanol ice maker. Best results are obtained from the composite adsorption ice maker that had the cooling power of 20.32 kW when the cycle time is 60 min, which is about 10 times that of the physical adsorption ice maker and 1.38 times that of the chemical adsorption ice maker.
Acknowledgements This work was supported by the State Key Fundamental Research Program under contract no. G2000026309, National Science Fund for Distinguished Young Scholars of China under contract no. 50225621, Shanghai Shuguang Training Program for the Talents, the Teaching and Research Award Program for Outstanding Young Teachers in Higher Education Institutions of MOE, PR China. The authors thank Mr. Y.X. Xu and Mr. Y.K. Sun for help to install the experimental setup.
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[7] Meunier F. Theoretical performance of solid adsorbent cascading cycles using the zeolite–water and active carbon– methanol pairs: four cases studies. Heat Recov Syst 1988;6(6):491–8. [8] Yonezawa Y, Ohnishi T, Okumura S, et al. Method of operating adsorption refrigerators. US patent no. 5024064, 1991. [9] Yonezawa Y, Matsushita M, Oku K, et al. Adsorption refrigeration system. US patent no. 4881376, 1989. [10] Lai H-M. An enhanced adsorption cycle operated by periodic reversal forced convection. Appl Therm Eng 2000;20:595–617. [11] Wang RZ, Wu JY, Xu YX. Experiments on a continuous heat regenerative adsorption refrigerator using spiral plate heat exchanger as adsorbers. Appl Therm Eng 1998;18:13–23. [12] Wang RZ, Li M, Xu YX, Wu JY. An energy efficient hybrid system of solar powered water heater and adsorption ice maker. Sol Energy 2000;68(2):189–95. [13] Lu YZ, Wang RZ, Zhang M, Jiangzhou S. Adsorption cold storage system with zeolite–water working pair used for locomotive air conditioning. Energy Convers Manage 2003;44(10):1733–43. [14] Kato Y, Yamada M, Kanie T, Yoshizawa Y. Calcium oxide/carbon dioxide reactivity in a packed bed reactor of a chemical heat pump for high-temperature gas reactors. Nucl Eng Des 2001;210:1–8. [15] Tokarev M, Gordeeva L, Romannikov V, Glaznev I, Aristov Y. New composite sorbent CaCl2 in mesopores for sorption cooling/heating. Int J Therm Sci 2002;41:470–4. [16] Dellero T, Sarmeo D, Touzain Ph. A chemical heat pump using carbon fibers as additive. Part I: enhancement of thermal conduction. Appl Therm Eng 1999;19:991–1000. [17] Dellero T, Touzain Ph. A chemical heat pump using carbon fibers as additive. Part II: study of constraint parameters. Appl Therm Eng 1999;19:1001–11. [18] Iloeje OC, Ndili AN, Enibe SO. Computer simulation of a CaCl2 solid-adsorption solar refrigerator. Energy 1995;20(11):1141–51. [19] Han B, Zhu R, Wu Y. A study on zeolite molecular sieve adsorption refrigeration using exhaust heat from Diesel engine. In: Proc Int Conf Energy Saving Refrig (17–19 September 1986), XiÕan Jiaotong University, China, 1986. p. 104–9. [20] Zhu R, Han B, Lin M, Yu Y. Experimental investigation on an adsorption system for producing chilled water. Rev Int Froid 1992;15(1):31–4. [21] Zhu RQ et al. Experimental investigation on an adsorption system for producing chilled water. Int J Refrig 1992;15(1):31–4. [22] Qi ZH, Tang GF, Li DY, et al. System analysis and performance research of an adsorption refrigerator driven by waste heat. Fluid Mach 2002;30(5):45–8 [in Chinese]. [23] Wang RZ. Performance improvement of adsorption cooling by heat and mass recovery operation. Int J Refrig 2001;24:602–11. [24] Wang LW, Wu JY, Wang RZ, et al. Influences of heat and mass transfer characteristics on the performance of adsorption ice maker driven by waste heat. In: Proc Int Sorption Heat Pump Conf Õ2002 (ISHPCÕ2002), Shanghai, 2002. p. 650–4.
Adsorption ice makers for fishing boats driven by the exhaust heat from diesel engine: choice of adsorption pair L.W. Wang, R.Z. Wang *, J.Y. Wu, K. Wang, S.G. Wang Institute of Refrigeration and Cryogenics, Shanghai Jiao Tong University, Shanghai 200030, PR China Received 1 August 2003; accepted 26 October 2003
Abstract Different adsorbents, such as physical, chemical and composite adsorbents are analyzed. Three types of working pairs, activated carbon–methanol, chemical adsorbent-ammonia and composite adsorbentammonia can be used for adsorption ice makers on fishing boats. The advantages, disadvantages and performances of three types of adsorption ice makers, with activated carbon–methanol, CaCl2 –NH3 and compound adsorbent (made by CaCl2 and activated carbon)-NH3 as adsorption pairs, are compared at the condition of two bed systems. The activated carbon–methanol ice maker shows the advantage of reliable safety, and the composite adsorption ice maker shows the best adsorption performance. The cooling power of 20.32 kW can be obtained from the composite adsorption ice maker when the volume for each adsorber is 0.288 m3 , which is about 10 times that of the physical adsorption ice maker and 1.38 times that of the chemical adsorption ice maker. 2003 Elsevier Ltd. All rights reserved. Keywords: Adsorption; Ice maker; Fishing boat; Activated carbon–methanol; Calcium chloride–ammonia; Composite adsorbent
1. Introduction Because of sustainable development requirements, efficient refrigerators driven by low grade thermal energy from different sources have received much more attention in recent years [1]. Adsorption refrigerators are benign to the environment due to their use of water, methanol and ammonia as refrigerants. In comparison with absorption systems, an adsorption system has no problems such as coolant pollution, crystallization and fractionation, while in comparison with
*
Corresponding author. Tel.: +86-21-6293-3838; fax: +86-21-6293-3250. E-mail address: [email protected] (R.Z. Wang).
0196-8904/$ - see front matter 2003 Elsevier Ltd. All rights reserved. doi:10.1016/j.enconman.2003.10.021
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vapor compression refrigerating systems, an adsorption system has the advantages of simple control, low initial investment and circulating expenditure and less noise [2]. Nowadays, the fishing is mainly divided into fishing on adjacent seas and pelagic seas. The adsorption ice making systems are mainly designed for the fishing boats on adjacent seas. Most fishing boats on adjacent seas are middle type and mini-type, and the powers of most Diesel engines are between 136 and 441 kW. The energy of a Diesel engine is 30–35% exhausted into the air [2], which can be applied to drive an adsorptive ice maker. There are several advantages of the adsorption refrigerators on fishing boats such as less influences of vibration, less noise, compact structure, simple control, etc.
2. Appropriate adsorbents for ice makers on fishing boats Adsorption can be classified as physical adsorption due to a physical process caused by Van der Waals forces or chemical adsorption in which a chemical process is involved. Correspondingly, there are mainly three types of adsorbents, physical adsorbents that mainly include activated carbon, zeolite and silica gel, chemical adsorbents such as calcium chloride, metal hydrides and complex salts, and composite adsorbents that mainly are obtained by the combination of chemical adsorbents and a porous matrix. 2.1. Physical adsorbents Studies on physical adsorption working pairs mainly focus on activated carbon–methanol, activated carbon–ammonia, zeolite–water and silica gel–water, and the researches on adsorption systems focus mainly on systems driven by solar energy. A type of adsorption ice maker with a solar energy collector of 6 m2 and activated carbon (AC35) of 130 kg designed by Pons and Grenier could produce 5.3–5.6 kg ice/m2 under the radiant intensity of 19–22 MJ [3]. The adsorption system designed by Leite is also powered by solar energy and uses activated carbon–methanol as the working pair. The optimal COP of a TIM (transparent insulation material) cover system is 0.155 between March and December [4]. A solar and gas solid sorption machine that uses the active carbon fiber Busofit as a sorbent bed and ammonia as a working fluid was designed and studied by Vasiliev et al. [5]. This adsorption machine has a very short (12 min) non-intermittent cycle and a high solar COP near 0.3 with 1.2 m2 solar collection surface and solar collector efficiency being near 0.7. Critoph and Vogel [6] and Meunier [7] studied the performance of activated carbon–methanol, zeolite–water and other working pairs, and the results show that activated carbon–methanol is an ideal working pair for solar energy because of its high COP and low generation temperature. A two bed adsorption chiller using silica gel–water as working pair is researched by Yonezawa et al. [8,9], and it has already been successfully commercialized in Japan. Researches on zeolite–water adsorption cycle operated by periodic reversal forced convection show that the COP of the heat pump is as high as 1.7, and the COP of refrigeration is up to 0.9, corresponding to the SCP of 125 W/kg [10]. Shanghai Jiao Tong University has obtained many achievements on the adsorption system powered by solar energy or waste heat and using activated carbon–methanol as a working pair, including a continuous heat regenerative adsorption refrigerator using a spiral plate heat exchanger as adsorber [11] and an
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energy efficient hybrid system of solar powered water heater and adsorption ice maker [12]. The zeolite–water adsorption refrigerator with 140 kg adsorbent designed by Shanghai Jiao Tong University has already been installed and tested on train locomotive cabin air conditioning and the cooling power obtained from tests is about 3.1 kW [13]. 2.2. Chemical and composite adsorbents The main advantage of chemical adsorption is the high performance of refrigeration or heat pump. For example, a calcium oxide/carbon dioxide reactivity in a packed bed reactor is studied by Kato. Results showed that the reactor was capable of storing heat at 900 C by decarbonation of calcium carbonate and generating up to 997 C by carbonation of calcium oxide. The amount of stored heat in the reactor was 800–900 kJ/kg [14]. The main disadvantages of chemical adsorption are studied. Many researches show that the phenomena of swelling, disintegration and agglomeration of adsorbent are the main problems for its application. Researches also show that the addition of a porous matrix in the chemical adsorbent could improve the adsorption performance. The performance of CaCl2 confined to a mesoporous host matrix MCM-41 has been studied by Tokarev. Results show that the composite material based on CaCl2 as an impregnated salt and MCM-41 as a host matrix is able to absorb up to 0.75 g of H2 O/g of dry sorbent. That high absorptivity can ensure high values of the energy storage capacity (2.1 kJ/g) [15]. Five different compounds, pure MnCl2 , simple mixture of ex-PAN T300 carbon fibers and MnCl2 (mixture 1), simple mixture of ex-pitch FT700 carbon fibers and MnCl2 (mixture 2), impregnated ex-pitch FT500 carbon fibers with MnCl2 (ICF) and intercalation of MnCl2 into ex-pitch graphitized carbon fibers P120 (GFIC), are studied by Dellero. The best of all kinetic results are obtained with GFIC and IFC. These two compounds not only provide a very fast reaction but also give a complete reaction by avoiding the agglomeration phenomenon [16,17]. Iloeje has also controlled the problem of agglomeration of CaCl2 by setting 20% CaSO4 in the adsorbent [18]. 2.3. Appropriate working pairs for adsorption ice makers on fishing boats There are mainly two types of working pairs, zeolite–water and CaCl2 –NH3 , that have already been researched to be used in adsorption refrigerators on fishing boats. The feasibilities of a zeolite–water adsorption refrigerator on fishing boats that has 400 kg zeolite per unit tube have been studied by Han and Zhu [19–21]. They obtained the cooling power of 92 kJ and cooled the water from 24 to 2 C by a single adsorption tube unit when the cycle time is 3 h. Qi et al. studied the feasibility of an adsorption ice maker driven by the exhaust gas of the Diesel engine on fishing boats that uses CaCl2 –NH3 as working pair [22]. Considering the conditions of ice making, there are three types of appropriate adsorption working pairs, activated carbon–methanol, chemical adsorbent-ammonia and composite adsorbent-ammonia that can be used on fishing boats. The activated carbon–methanol system is safer than the chemical and composite adsorbent-ammonia system, while the chemical and composite adsorbent-ammonia system has a higher performance of refrigeration than the activated carbon– methanol system.
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3. Activated carbon–methanol adsorption ice maker 3.1. Adsorption ice making system Fig. 1 shows the schematic of an activated carbon–methanol adsorption ice maker. The corresponding developed system is shown in Fig. 2. Two adsorbers are designed in the system, and heat and mass recovery will proceed between the two beds because the heat and mass recovery process will improve the adsorption performance and economize the energy consumed [23]. There are one evaporator and one condenser in the two bed system. The water that cools the adsorber is coming from the sea directly or from the water tank or cooling water tower during the laboratory tests. The heat source is the exhaust gases from the diesel engine (in the experiments, we use an oil burner to simulate the diesel engine). In order to accumulate the heat from the exhaust gases, a
Fig. 1. Diagram of activated carbon–methanol adsorption system.
Fig. 2. The integrated adsorption ice maker prototype.
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hot water storage vessel (boiler) is used at the exit of the exhaust gas–water heat exchanger. The refrigeration effect of the adsorption prototype is transported to the flake ice maker to make ice, in which the methanol is pumped through the heat exchanger inside the flake ice maker. The heat transfer medium of the two adsorbers are connected to the hot water storage vessel and cooling water tank, respectively, corresponding to the switch of desorption and adsorption. The working processes are controlled by valves and pumps. For example, the electro-magnetic valves 0a, 0b, a, b, 0d, 0c, c, d, electro-pneumatic valves 1, 2, 3, 4 and pump 3 are shut off when heat recovery proceeds. The electro-pneumatic valves 1 and 3 should be open when the mass recovery proceeds. The electro-magnetic valves 0a, 0c, b, d, e, f, electro-pneumatic valves 2, 3 and pump 2 are shut off when adsorber 1 is desorbing and, at the same time, adsorber 2 is adsorbing. The electro-magnetic valves 0b, 0d, a, c, e, f, electro-pneumatic valves 1, 4 and pump 2 are shut off when adsorber 1 is adsorbing and, at the same time, adsorber 2 is desorbing. This prototype photograph is shown in Fig. 2. The length, width and height of this prototype are about 2.5, 1.5 and 2 m, respectively, when the adsorption refrigeration system, flake ice maker and waste heat recovery boiler are incorporated into a single unit. 3.2. Adsorber description Solidified activated carbon is chosen as the adsorbent because of its higher density, better adsorption performance and better heat transfer performance [24] compared with granular activated carbon. The cross-section structure of each adsorber is shown in Fig. 3. There are three blocks of solidified activated carbon at the section of the adsorber, and two fins of 0.15 thicknesses are kept to both sides of one block of solidified activated carbon that has the thickness of 10 mm. The heat transfer area of the fins is 25 m2 . Each adsorber contains activated carbon of 60 kg. 3.3. Performance Experiments are operated at the environmental temperature of 27 C, and mass and heat recovery proceeds between the two beds at the switch time. The arrangements and results are
Fig. 3. The cross-section of adsorber.
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Table 1 Arrangements and results of experiments with heat and mass recovery (two bed operation) No.
Cycle time (min)
Desorption and adsorption time (min)
Recovery time of mass (min)
Recovery time of heat (min)
Average evaporating temperature (C)
Ice productivity (kg/ h)
Cooling power (kW)
Average heating power (kW)
COP
SCP1 (W/kg)
SCP2 (W/kg)
1 2 3 4 5 6
36 46 56 66 76 86
15 20 25 30 35 40
1 1 1 1 1 1
2 2 2 2 2 2
)5.41 )9.47 )10.23 )10.31 )10.97 )10.53
12.00 13.50 15.00 15.25 15.43 15.38
1.51 1.72 1.92 1.96 1.99 1.98
15.21 17.62 16.03 15.63 17.23 19.60
0.088 0.097 0.120 0.125 0.115 0.101
30.2 33.1 36.1 35.9 36.5 35.5
12.6 14.4 16.1 16.3 16.8 16.5
shown in Table 1, where SCP1 is the specific cooling power per kilogram adsorbent in the adsorption time, and SCP2 is the specific cooling power per kilogram adsorbent in the whole cycle time. The optimal SCP1 and COP (coefficient of performance) obtained are 36.5 W/kg and 0.125, respectively. The optimal ice productivity is 15.43 kg/h and 370 kg/day, while the cycle time is 76 min, and the evaporating temperature is about )11 C.
4. Adsorption system and adsorbent performance testing unit for chemical and composite adsorption ice maker 4.1. Adsorption system description The key point for the design of chemical or composite adsorbent-ammonia ice makers is to ensure the safety in the cabin. In the adsorption system shown in Fig. 4, all the components in which ammonia flows are designed to be installed on the deck. Only the flake ice maker is installed in the cabin. The thermal fluid used in the system is heat conducting oil, which is cooled by the sea water at the time of adsorption and heated by the exhaust gas at the time of desorption. An oil tank is used at the exit of the oil–gas heat exchanger to accumulate the heat from the exhaust gases. Two vertical type adsorbers are designed in the system, and heat and mass recovery will proceed between two beds. The working processes of this ice maker are similar to those of the activated carbon–methanol system (Fig. 1). 4.2. Adsorber design for chemical and composite adsorption ice maker The section for the adsorber designed for the chemical and composite adsorption ice maker is circular (Fig. 5). Heat conducting oil is used as the thermal fluid instead of water because the optimal desorption temperature of the adsorbent is as high as 140 C. Expansion space must be provided between the adsorbent and fins because the swelling of CaCl2 is serious at the time of
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Fig. 4. Diagram of chemical and composite adsorption ice maker.
Fig. 5. Section of adsorber for chemical and composite adsorption ice maker.
adsorption. Thus, the adsorber will be set upright in the system to avoid the adsorbent falling off from the fins and to ensure the expansion space of the adsorbent. The key point for the design of chemical and composite adsorber is to choose an appropriate expansion space for the adsorbent. Thus, the adsorption performance testing unit is designed and set up to test the performance of adsorbents with different expansion spaces. 4.3. Adsorption performance testing unit for chemical and composite adsorption ice maker The adsorption performance testing unit (shown in Fig. 6) consists mainly of two parts, adsorber and condenser (evaporator), which, respectively, is put in the oil tank and ethanol jacket
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Fig. 6. Adsorbent testing unit.
considering the operation modes of desorption and adsorption. The adsorber is cooled by the water circuit heat exchanger at the time of adsorption and heated by the electric heater at the time of desorption. The temperature of the evaporator/condenser is controlled by the thermostatic bath, which can control the temperature from )40 to 90 C. The adsorption quantity and desorption quantity are measured by the magneto-strictive displacement sensor inserted into the ammonia container, whose measuring error is 0.05%. Two adsorbers (Fig. 7) are designed for the testing unit. The adsorber with edge fold is designed for the granular chemical and composite adsorbent, and the adsorber without edge fold is designed for the solidified composite adsorbent. Each adsorber is divided into two half parts that are connected by flange seals. The spaces are separated by the fins, and the distance between two fins
Fig. 7. Structure of adsorber for granular adsorbent (a) and solidified adsorbent (b).
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is 12 mm. Various samples are tested in order to find an optimal ratio between the expansion space and adsorbent volume. In this research, the thickness of 2 mm CaCl2 with the ratio of 5:1 is named sample 1, the thickness of 3 mm CaCl2 with the ratio of 3:1 is named sample 2, the thickness of 4 mm CaCl2 with the ratio of 2:1 is named sample 3, the thickness of 5 mm CaCl2 with the ratio of 1.4:1 is named sample 4, the thickness of 6 mm simple composite adsorbent that is combined by CaCl2 and activated carbon is named sample 5, and the thickness of 9 mm solidified adsorbent that is also combined by CaCl2 and activated carbon is named sample 6.
5. Performance prediction of chemical adsorption ice maker 5.1. Choice of layout of chemical adsorbent One important rule for the choice of layout for chemical adsorbent is the stability of performance. Repeated experiments at the condition of 40 C adsorption temperature and 0 C evaporating temperature are performed first in order to test the stability of the adsorption performance. Results are shown in Fig. 8. The adsorption performance of sample 2 is similar to that of sample 1, and they are much different from those of samples 3 and 4. The performances of samples 3 and 4 are much more stable than those of samples 1 and 2, and the performance of sample 4 is worse than that of sample 3 because of the deterioration of mass transfer that was caused by limited expansion space. After repeated tests, the expansion and agglomeration phenomena of the adsorbent for samples 1 and 3 are compared (Fig. 9) in order to find the reason for the different performance stability. There are no phenomena of agglomeration for sample 1 because there was enough expansion space, although the swelling of the adsorbent is as much as six to seven times. The agglomeration is serious in sample 3 after adsorption. An important conclusion is obtained from the experiments: moderate agglomeration is beneficial to the stability of adsorption performance, although it will lead to bad mass transfer performance, when the evaporating temperature is too low. One possible explanation for this phenomenon is the shielded factor (the ratio of repulsive force to attractive force for each anion) of a complex compound that is increasing with the number of ammonias in the ammoniate. For example, the shielded factor is 0.96 for tetra-ammoniate, while it is 1.66 for hexad-ammoniate. The NH3 is easier to fill around Ca2þ for sample 1 than sample 3 because of the larger pore space under the condition of larger expansion space, and then, the 1.15
Sample 1
x /(kg/kg)
1.10
Sample 2
Sample 3
1.05 1.00 0.95 0.90
Sample 4
0.85 0
3
6
9
12
15
Test number
Fig. 8. Adsorption performance in the repeated experiments.
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Fig. 9. (a) Sample 1 and (b) sample 3 after adsorption.
influence of shield factor on sample 1 will be larger because it has a larger concentration of NH3 around Ca2þ . Then the octo-ammoniate will be more difficult to be formed in sample 1 than sample 3. Then, there are two appropriate layouts of adsorbent for the chemical adsorption ice maker considering the stability of adsorption performance, samples 3 and 4. The performance of sample 3 is better than that of sample 4, while the filling quantity of sample 4 in the adsorber is larger than that of sample 3 assuming the volumes of adsorbers for these two types of samples are the same. 5.2. Performance prediction of chemical adsorption ice maker The performances of samples 3 and 4 are tested at the evaporating temperature of )15 C and condensing temperature of 25 C. The cooling power is computed by the adsorption isobars. The adsorption cooling power per kilogram is defined as Qm (kJ/kg). The formula for Qm is Dxht ; ð1Þ m where Dx (kg/kg) is the adsorption quantity at the fixed saturated temperature, ht (kJ/kg) is the latent heat of vaporization at that saturated temperature and m is the mass of adsorbent. The volume cooling density (Qs , kJ/m3 ) is defined as Qm ¼
Dxht ; ð2Þ V where V (m3 ) is the total volume for the adsorbent and its expansion space. Qm and Qs are compared in Fig. 10. Qm and Qs of sample 4 are all worse than those of sample 3 because its mass transfer performance is influenced by the narrow expansion space for the adsorbent. The highest Qm is 889 kJ/kg and the highest Qs is 1.845 · 105 kJ/m3 . Qs is more important than Qm for ice makers on fishing boats because the space on fishing boats is limited and the mass of adsorbent takes up only a small proportion compared with the mass of the whole system. The valid volume for the adsorbent and mass transfer space for the two physical adsorbers shown in Fig. 2 is 0.288 m3 . Assuming the valid volume for the adsorbent and expansion space in chemical adsorbers is the same as that of the physical adsorbers, then the cooling power of one cycle can be obtained by the experimental results in Fig. 10, and it is as high as 0.531 · 105 kJ. Qs ¼
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4.0 3.5
Q m for sample 3 3.0 2.5 2.0
400
1.5
5
600
3
Q m for sample 4 Q s for sample 3 Q s for sample 4
Q s/(×10 kJ/m )
Q m/(kJ/kg)
800
1.0 200 0.5 0
0.0 20
25
30
35
40
45
50
55
60
65
70
o
Adsorption temperature T / C
Fig. 10. Qm and Qs vs. adsorption temperature at the evaporating temperature of )15 C per cycle.
Assuming the cycle time of the chemical adsorption ice maker is 60 min, the cooling power of the chemical adsorption ice maker is as high as 14.75 kW, which is much higher than the cooling power of the activated carbon–methanol adsorption ice maker. This estimation is based on the waste heat being enough for the ice maker.
6. Performance prediction of composite adsorption ice maker 6.1. Choice of layout of composite adsorbent Fig. 10 shows that the performance of sample 4 is much lower than that of sample 3 because it is influenced by the bad mass transfer performance. The mass transfer performance of the chemical adsorbent can be improved by the addition of a porous medium, such as activated carbon. A simple composite adsorbent (sample 5) is mixed by the ratio of 4:1 between the mass of CaCl2 and the mass of activated carbon, and the thickness of the adsorbent on the fins is 6 mm. Repeated experiments at the condition of 40 C adsorption temperature and 0 C evaporating temperature are performed, and the results are shown in Fig. 11. There is great performance deterioration for sample 5. The adsorption quantity of the seventh time is about 20.4% less than the adsorption quantity of the first time. The adsorber is checked in order to find the reason for the deterioration. The adsorbent after adsorption is shown in Fig. 12. The phenomenon of agglomeration of sample 5 is much different from that of sample 3, although the mass of CaCl2 is the same in the adsorbers. There is no serious phenomenon of agglomeration in sample 5. It means that the moderate agglomeration did not occur in sample 5, and thus, the performance deterioration exists in the experiments. In order to obtain the moderate agglomeration in the compound adsorbent after adsorption, the compound adsorbent is solidified and tested.
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x /(kg/kg)
0.9
adsorption quantity for CaCl2 in composite adsorbent
0.8 0.7
adsorption quantity of composite adsorbent
0.6 0.5 0
1
2
3
4
5
6
7
8
Test number
Fig. 11. Performance deterioration.
Fig. 12. Adsorbent after adsorption.
Fig. 13. Solidified compound adsorbent (sample 6).
Sample 6 is made by CaCl2 , activated carbon and cement. The ratio between the mass of CaCl2 and activated carbon is also 4:1, and the cement and water are used to bind these two types of adsorbent. The solidified adsorbent is shown in Fig. 13. The thickness of sample 6 is about 9 mm, and the filling quantity of CaCl2 in the adsorber is about 16% added compared with sample 4. There is no phenomenon of performance deterioration in the repeated experiments. The adsorption quantity became stable at about 0.796 kg/kg after the second repeated test.
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3.5
1000
3.0
600
2.0
5
Q m/(kJ/kg)
3
2.5
Q s /(×10 kJ/m )
Qm Qm Qs
800
for sample 6 for CaCl2 in sample 6
1.5
400
1.0 200 0.5 0
0.0 20
25
30
35
40
45
50
55
60
65
70
o
Adsorption temperature T / C
Fig. 14. Qm and Qs at the evaporating temperature of )15 C for solidified composite adsorbent per cycle.
The appropriate composite adsorbent for ice makers is the solidified composite adsorbent considering the stability of adsorption performance. 6.2. Performance prediction of composite adsorption ice maker The performance of sample 6 is tested at the evaporating temperature of )15 C and condensing temperature of 25 C, Qm and Qs are shown in Fig. 14. The Qm of CaCl2 in sample 6 is similar to that of sample 3 (Fig. 10), and the Qs of sample 6 is much better than that of sample 3 (Fig. 10). The highest Qm is 844 kJ/kg for the CaCl2 in sample 6, and the highest Qs is 2.54 · 105 kJ/m3 which is enhanced about 38% compared with the Qs of sample 3. Assuming the valid volume for the adsorbent and expansion space in composite adsorbers is also 0.288 m3 , the cooling power of one cycle obtained from Fig. 14 is 0.57912 · 105 kJ, and then, the cooling power of the adsorption ice maker is as high as 20.32 kW when the cycle time is 60 min, which is much higher than the cooling power for both the physical adsorption ice maker and the chemical adsorption ice maker.
7. Conclusions The performance of a physical adsorption ice maker is tested, and the performance of chemical and composite adsorption ice makers are predicted from the performance of the adsorbent. The several conclusions obtained are as follows. (1) Considering the condition of ice making, there are three type of adsorption ice makers that can be used on fishing boats, physical, chemical and composite adsorption ice makers. A physical adsorption ice maker is safer than chemical and composite adsorption ice makers for the reason of less noxious refrigerant. Thus, it is easier to be installed on the ship. Contrarily, the design and
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installation of chemical and composite adsorption ice makers must consider the question of safety on the boat, and all the components in which ammonia flows must be installed outside the cabin. (2) The adsorption performance of an activated carbon–methanol system with two adsorbers and with 60 kg adsorbent per bed is tested. The optimal SCP1 (specific cooling power per kilogram adsorbent in adsorption time) and COP obtained are 36.5 W/kg and 0.125, respectively. (3) The performance stability of chemical adsorbents with different expansion spaces are studied. Two samples of CaCl2 (CaCl2 with the ratio between expansion space and volume of adsorbent of 2:1 and 1.4:1) show stable performance and can be used in the ice makers. Best performance was obtained from sample 3, which has the Qm (the adsorption cooling power per kilogram adsorbent) of 889 kJ/kg and Qs (the volume cooling density) of 1.845 · 105 kJ/m3 per cycle. (4) The performance stability of composite adsorbents is studied, and the solidified compound adsorbent (combined with CaCl2 and activated carbon) shows stable performance and can be used in the ice makers. Qs obtained from the solidified compound adsorbent is as high as 2.54 · 105 kJ/m3 , which is enhanced about 38% compared with Qs of sample 3. (5) The cooling power of the chemical adsorption ice maker and the cooling power of the composite adsorption ice maker are computed from the Qs assuming these two types of ice makers have the same valid volume for adsorbent and expansion space with that of the activated carbon– methanol ice maker. Best results are obtained from the composite adsorption ice maker that had the cooling power of 20.32 kW when the cycle time is 60 min, which is about 10 times that of the physical adsorption ice maker and 1.38 times that of the chemical adsorption ice maker.
Acknowledgements This work was supported by the State Key Fundamental Research Program under contract no. G2000026309, National Science Fund for Distinguished Young Scholars of China under contract no. 50225621, Shanghai Shuguang Training Program for the Talents, the Teaching and Research Award Program for Outstanding Young Teachers in Higher Education Institutions of MOE, PR China. The authors thank Mr. Y.X. Xu and Mr. Y.K. Sun for help to install the experimental setup.
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