DISCUSSION OF ADSORPTION REFRIGERATION APPLIED ON HIGHER TEMPERATURE WASTE HEAT SITUATION W. Wang and R.Z. Wang Institute of Refrigeration & Cryogenics, Shanghai Jiao Tong University, Shanghai, 200030, P.R.China
ABSTRACT There is abundant waste heat in the fume exhausted from diesel and gasoline engine, which is a potential candidate to drive a adsorption refrigeration system. Generally, the temperature of fume is high enough, but its convective heat transfer coefficient to adsorber and the thermal conductivity of adsorbent are low. The capability of heat transfer should be taken into account in designing an adsorber for refrigeration. However, the structure and operating parameters, related to heat transfer, are connected each other, their optimum values for the performance of refrigeration system should be elected according to their relation and operation condition. This paper is try to provide some reference consideration in design an adsorption refrigeration system.
KEYWORDS Adsorption, Refrigeration, Heat Transfer
1. INRODUCTION Adsorption refrigeration is usually driven by lowgrade heat source, and is beneficial to be applied on situation of solar energy utilization and waste heat recovery. By now, there are many researches reporting their works on it [1,2]. The temperatures of heat source from solar energy and waste heat recovery almost are lower than 200 oC. For driving a system of adsorption refrigeration, the exhausted gas from engine is also a potential candidate of heat source. Generally, the temperature of waste gas exhausted from the engine muffler is about from 350 oC to 450 oC, it is high enough to drive an adsorption refrigeration system. However, too high temperature would bring some additional problems, for example: making some adsorbates to decompose (for example: methanol, ethanol), and reducing the adsorption capacity of cycle. On the occasion of recovering waste heat from engine, there are several feasible working pairs, such as: zeolite –water and activated carbon – ammonia, etc. Zeolite -water is a pair with higher COP, but its system works at vacuum pressure, so high credibility is required in this system. The pair of activated carbon ammonia is better than the pair of zeolite - water on the credibility for its working pressure higher than the atmosphere. The character of adsorption refrigeration about active carbon – ammonia had been researched much such as the work by R.E. Critoph etc.[3,5]. Based them, this paper mainly discuss the system of activated carbon - ammonia applied on heat recovery of higher temperature such as fume exhausted from engine. For a system of adsorption refrigeration working on high temperature area, below problems should be considered in general: selection of adsorption pairs according to characters of waste gas, enhancement of heat transfer character between heat sources (including heating and cooling usage) and adsorbent, fluctuant working state of engine, compact construct and size for this system, etc.. Above all, one of the most important W. Wang,
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requirements is that the capacities of cooling and heating adsorbent bed should match to guarantee high COP cycle’s adsorption character. This paper show some discussion based on the simulation of a adsorption refrigeration system, and try to uncover some relations among COP, volume of heat exchanger, temperature of heat source, conductivity of adsorbent, cycle time, heat capacity of heat exchanger, et al.
2. MODEL FOR SIMULATION Supposing that an adsorption refrigeration system adopts a group of finned tube served as the adsorbent bed, and the adsorbent is filled into the annular space of the tubes, the adsorption pair is activated carbon 208C and ammonia. The base tube for bed is of diameter of 51mm, and thickness of 2mm. For continuous cooling supply, this system employs two adsorbent beds. Therefore, one bed can keep adsorption for cool supplying while another bed is being regenerated. Figure 1 shows the structure of a finned tube for adsorbent bed in the simulation.
Activated carbon
Finned tube
Figure1. scheme of adsorbent bed For simplifying, the heat transfer in the adsorbent is considered as one-dimension problem of heat conduction with inner heat source (adsorption or
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desorption heat) in a columnar coordination. Its control equation is written as: ∂T 1 ∂T ∂ 2T (1) ρC p = λ eff ( + ) + ρq sorp r ∂r ∂r 2 ∂τ In Equ.1, the heat source comes from the reacting heat of adsorption or desorption, which is calculated out of equation of Clausius- Clapeyron, Eq. 2. In which, Ts is saturated temperature corresponding to the pressure during adsorption and desorption, presents time, R is the gas constant of ammonia, A is a constant from experiment which is listed in table 1. T dx (2) q sorp = RA T s dτ The adsorption character of activated carbon with ammonia is adopted as D-R equation, and those coefficients are same as previous reference [5].
T − 1) 2 ] Ts
(3)
In which, the parameter B and x0 are shown on Table 1. Tl is the temperature of cooling fluid. 2 is the convective coefficient between cooling fluid and the wall of adsorber. Th is the temperature of heating fluid, its convective coefficient is 1. The temperature of evaporation is Te and The temperature of condensation is Tc. Due to the adsorbent bed is the type of finned tube, compared to naked tube, its surface for heat exchanging is increased into 2.3 times, and steel volume 1.4 times. In below calculations and discussion, the design and working parameters are shown in table 1 unless specially illustrating. Table1 parameters used in below calculation 0.3 Te(oC) 10 λ eff (J/M oC)
ρ (kg/M3)
700 Tl(oC) hw(W/M2 oC) 50 Th(oC) X0 0.247 B 40 A α 1(W/m2 oC) 70 Tc(oC) α 2(W/m2 oC) Cycle time(minutes) Thickness of adsorbent (mm)
50 350 -10.29 2823.4 45 20 5
3. SIMULATION AND DISCUSSION Generally, the performance of an refrigeration system can be presented as,
COP =
Qcool Qheat
adsorption (4)
Where Qcool is the cooling supplying out of evaporation of refrigerant, Qheat is heat quantity for heating adsorbent bed and regenerating the adsorbent. In order to analysis the effect of steel heat capacity of presents data adsorbent bed, in below figures, the considering steel heat capacity in calculating Qheat, and presents data without considering steel heat capacity.
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3.1 Thermal conductance of adsorbent In fact, The thermal conductivity of adsorbent is variational during the working cycle because of nonsteady temperature distribution and adsorbed rate. For convenience to simulation here, it is simplified as a constant, and neglected its variation with temperature and adsorption rate. On one hand, it also includes adsorbate effects of heat conduction and convective heat transfer in adsorbent. 0.30
0.20 COP
x = x 0 exp[ B (
Furthermore, for application, the volume and weight of adsorbent bed is an important parameter in design. So, in below analysis, the length of adsorbent bed (structure as above) for 1kW cooling power, that is, the tube length required of an adsorbent bed for 1kW cooling power, is proposed as a parameter to present the size of adsorbent bed.
0.10
0.00 0.0
0.5
1.0
1.5
2.0
Thermal conductivity (W/mC)
Figure 2. The effect of thermal conductivity on the COP of adsorption refrigeration In fact, it has been identified that the main shortcoming of adsorption refrigeration results from low value of conductivity of adsorbent. By now, many methods are proposed, such as: synthesizing adsorbent with steel or other high conductivity materials, consolidating adsorbent on the wall of adsorber, compressing adsorbent, et al. It is usually believed that adsorbent bed filled with pure activated carbon and methanol is working at total thermal conductivity from 0.1 to 0.6W/m oC, while consolidated activated carbon with porous metallic foams (Ni, Cu) can reach conductivity of about 2~9 W/ m oC. In general, enhancing conductivity is certainly beneficial to adsorption refrigeration system, but it would bring extra cost also. Is it worthy to strengthen the adsorbent conductivity limitless? Figure2 shows the thermal conductivity's effect on COP, and Figure 3 is its effect on volume of adsorbent bed. At present, improving conductivity of adsorbent usually induces reducing adsorption capacity per unit weight of adsorbent.
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19 length for 1kW cooling power(m)
Length for 1kW cooling power(m)
30 25 20 15 10 0.0
0.5
1.0
1.5
13
10
0
2.0
Thermal conductivity(W/mC)
Figure 3. The effect of thermal conductivity on the size of adsorbent bed According to figure 2 and 3, in this simulation condition, improving conductivity is helpful to increasing COP, and decreasing volume of adsorbent bed. However, the apparent improvement to COP is only in the range with low thermal conductivity. When the thermal conductivity is big enough, it is not sensitive to COP and volume of adsorbent bed. 3.2 Thickness of adsorbent 0.3
COP .
16
0.2 0.1
3
6
9
Thickness of adsorbent (mm)
Figure 5 the relation between size of adsorbent bed and the thickness of adsorbent Figure 4 and 5 show results on the cycle time of 20 minutes. Its optimum thickness is about 4 mm, where the COP of refrigeration cycle is bigger, but volume of bed required is smaller. Besides, It can be predicted that the optimum thickness is increased with raising cycle time. 3.3 Cycle time Cycle time is an important operation parameter, but must be determined in designing adsorption refrigeration system. It involves cooling performance coefficient, cooling power in a cycle because it brings about directly the bed's heat transfer character and the adsorption quantity in a cycle. Present discussions and experiments indicate that growth of cycle time enables COP to be increased if leakage of heat is neglected, but there is an optimum cycle time corresponding to maximum cooling power. 0.25
0.0 0
3
6
9
0.20
Figure 4. The relation between COP and the thickness of adsorbent Thickness of adsorbent should be confirmed in design the adsorbent bed. Since it is an important role in heat transfer process, its value is related to cycle time, adsorbed quantity in a cycle, and volume of adsorbent bed directly. In normal situation, the adsorbent bed, heated, or cooled alternatively, is working at no steady heat transfer character all the time. If it were too thick, some of the adsorbent volume would be invalid as a heat accumulator, this is not beneficial to improve the adsorption rate in a cycle. Otherwise, if it was too thin, the most heat exchanging quantity is used for raising and reducing the temperature of steel adsorber wall, and a part of time for heating and cooling the bed is of low effect since the variation of adsorbate quantity is very small. Accordingly, there should be an optimum thickness of adsorbent corresponding to a certain cycle time.
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COP
Thickness of adsorbent (mm)
0.15 0.10 0.05 0.00 0
100
200
300
400
500
Cycle time (minute)
Figure 6. Relation between COP and cycle time Figure 6 and figure 7 show the cycle effects on COP and volume of adsorbent bed. To such adsorbent bed, while cycle time is littler than 60 minutes, the COP rises with cycle time; while cycle time is greater than 60 minutes, the COP is nearly a constant. Therefore, From figure7, cycle time had better stay in the range from 15~40 minutes for reducing volume of adsorbent bed.
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length for 1kW cooling power(m)
200 150 100 50 0 0
100
200 300 Cycle time (minutes)
400
Figure 7. Relation between the size of adsorbent bed and cycle time 3.4 Temperature of fume 0.3
COP
0.2 0.1 0.0 100
250
400
550 o
Temperature of waste gas ( C)
Figure 8 Relation between COP and temperature of waste gas The fume exhausted from engine is used to regenerate the adsorbate from the adsorbent, therefore, higher temperature is absolutely helpful to desorption process. However, in another half cycle of adsorption refrigeration, the regenerated adsorbent bed should be cooled for adsorption, moreover, the lower the temperature of adsorbent bed, the more beneficial to adsorption. So, just only enhancing fume temperature is not certainly to induce satisfied COP and cooling power. Results of simulation here verify this point. In figure 8 and figure 9, waste gas is supposed varying from 150 oC to 550 oC, the fume temperature for optimum COP is of about 250 oC, and its value for the little size of bed is of about 300 oC. According to figure8 and figure9, the performance of refrigeration system would be reduced sharply if temperature of waste gas was lower than 200 oC. It should be noted that the optimum value of temperature would be involved to convective coefficient outside the tube wall also, however it is simplified as a constant in simulations.
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30
20
10 100
500
250
400
550
Temperature of waste gas (C) Figure 9 Relation between size of adsorbent bed and fume temperature
3.5 Convective coefficient of fume with the wall of tube In designing of adsorbent bed, the outside of bed for waste gas should be arranged carefully to meet the demand of heat transfer and flowing resistance. On usual condition of heat recovery, the gas flowing speed is confined since the flowing resistance of additional heat exchanger is limited strictly. On other hand, the requirement of heat exchanging relates to high gas speed in general. Figure10 indicates overall COP (taking into account the steel capacity of bed) is not sensitive to outside convective coefficient, however, there is an optimum convective coefficient for compact volume of adsorbent bed in figure 11. Generally, the optimum convective coefficient is involved with several factors in heat transfer process. In this simulation, corresponding to higher temperature of waste gas (350 o C), the optimum one is just 30W/m2 oC.
0.3 COP
Length for 1kW cooling power(m)
250
0.2 0.1 0.0 0
20
40
60
80
Convective coefficient of fume (W/m2 oC)
Figure 10 Relation between COP and convective coefficient of fume
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16 length for 1 kW cooling power(m)
15 14 13 12 0
20 40 60 2o Convective coefficient of fume (W/m C)
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Figure 11 Relation between size of adsorbent bed and convective coefficient of waste gas 3.6 Steel heat capacity of adsorber Shell of adsorbent bed is usually made of steel. Although it is not of contribution to adsorption directly, but it is indispensable to the structure and heat transfer of adsorber. However, during system working, the steel shell need spend some heat load in whatever adsorption or desorption process, thus, the performance of refrigeration COP is lower than the COP without considering the effect of steel heat capacity. According to the figures about COP above, the difference is large. 0.25
Length for 1kW cooling power(m)
250 200 150 100 50 0 0
100
200 300 Cycle time (minutes)
400
500
Figure 13 The relation between size of adsorber and cycle (when the adsorber is of naked tube) However the steel capacity is a remarkable problem. According to all the calculation of COP above, the performance considering steel capacity is much lower than the one without considering steel capacity. If all the values of COP, cooling power, adsorber volume and adsorber weight are thought as designing objectives, it should be possible to find some design that satisfies the objectives and employs lower steel quantity through careful selection of heat exchanger and check the character of thermodynamics and heat transfer.
4. CONCLUSION
0.20 COP
0.15 0.10 0.05 0.00 0
100
200 300 400 Cycle time (minutes)
500
Figure 12 The relation between COP and cycle time (when the adsorber is of naked tube) Fins, as a method to enhancing heat transfer, are often added into heat exchanger to augment the heat exchanging surface area, especially while heat exchanging with gas. Increasing heat transfer area palliates the requirement to the convective heat transfer coefficient, which is often confined by flowing resistance. However, Adding fins augments the steel weight of absorber also, so it would require more heat load for its steel heat capacity. In simulation in figure12 and 13, the adsorber is modified, and the tube for adsorber is naked without fins. Comparing them with Figure 6 and 7, the value of COP with naked tube adsorber rise slightly, but the volume (length for adsorber) rise slightly yet. This illustrates that the reduction of heat capacity of steel shell induces the reduction of heating or cooling quantity to adsorber and adsorbent also.
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Adsorbent bed is a key part in the adsorption refrigeration system. In design on adsorbent bed, there are many factors should be considered synthetically. Although thermal conductivity is a bottleneck problem in adsorption heat pump system, higher thermal conductivity is certainly beneficial to improve performance of adsorber. But, there are several factors effecting heat transfer process also, such as: cycle time, adsorbent thickness, temperature of heat source, et al.. It is properly not economical to enhance thermal conductivity sightlessly. The performance of adsorption refrigeration is not sure to be improved by enhancing the temperature of waste gas, there is an optimum value for a certain system and operating condition. Besides, cycle time and thickness of adsorbent are related closed each other and other components about the system, and exists optimum matching groups also. Therefore, once the limitations and objectives are determined, the residual structure and operation parameters usually rely on each other and exist optimum values, which should be checked carefully in design and operation. Acknowledgement This work is supported by the State Fundamental Research Program (Contract No. G2000026309). The support from UTRCC is also appreciated.
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References [1] Srivastava N.C., Eames I.W., A review of adsorbents and adsorbates in solid-vapor adsorption heat pump systems, Applied thermal engineering, 18,pp707-714,1998 [2] Pons M., Meunier F., Caccola G., Critoph R.E., Groll M., Puigjaner L., Spinner B.and Ziegler F., Thermodynamic based comparison of sorption systems for cooling and heat pumping, Int. J. of Refrigeration, 22, pp5-17,(1999) [3] Critoph R. E., Performance limitations of adsorption cycles for solar cooling, Solar Energy, 14(1), pp21-31,(1988). [4] Restuccia G. And Cacciola G., Performances of adsorption systems for ambient heating and air conditioning, Int. J. of Refrigeration, 22, pp1826,(1999) [5] Critoph R.E. and Turner L.H., Performance of ammonia- activated carbon and ammonia zeolite heat pump adsorption cycles, Proceedings of conference pompe a chaleur chiminique de haut performance, Perpignan, Sept.pp202-211,(1988) [6] Suzuki M., Application of Adsorption Cooling Systems to Automobiles, Heat Recovery Systems & CHP, 13(4), pp335-340, (1993)
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Nomenclature A Constant B Constant COP Coefficient of performance C Specific heat (J kg-1 oC -1) h Coefficient of heat exchanging Q Heat quanity (J) R Gas constant (N kg-1 oC -1) T Temperature (oC) x Adsorption rate α Convective coefficient (Wm-2 oC -1) ρ Density (kg/m3)
λ τ
Thermal conductivity (Wm-1 oC -1) Time (s) Subscripts 1 Fume for heating 2 Air for cooling c Condensation cool Cooling output e Evaporation l Cooling fluid h Heating fluid heat Heating adsorbent bed for desorption p Constant pressure s Saturation w Wall of adsorner
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