1、4669 Thermal Analysis of Solar-Powered Continuous Adsorption Ai r-Co n d it i o n i n g Sy s t e m Yong Li K. Sumathy, Ph.D. N.D. Kaushika, Ph.D. ABSTRACT A simple lumpedparameter model is established to inves- tigate the performance of a solar-powered adsorption air- conditioning system driven by f
2、lat-type solar collectors. The dynamic performance of a continuous adsorption cycle using a double adsorber along with heat recovery is measured in terms of the temperature histories, gross solar coeficient of performance, and specijic cooling power Also, the influences of some important design and
3、operational parameters on the performance of the system are studied. The study shows that the adsorbent mass and lumped capacitance have a signijkant effect on the system performance as well as on the system size. Simulation results indicate that the effect of the overall heat transfer coeficient is
4、 not predominant if the cycle duration is longer Also, there exists an optimum time to initiate the heating of the adsorbent bed and the number of cycles that can be performed in a day S operation. INTRODUCTION Adsorption refrigeration systems present the advantage of being absolutely benign for the
5、 environment: zero ozone depletion potential (ODP) as well as zero global warning potential (GWP). Also, refrigeration is an attractive applica- tion of solar energy because the supply of sunshine and the need for refrigeration reach maximum levels in the same season. In the last two decades, variou
6、s solar-powered refrig- eration systems have been studied. Compared with the exist- ing absorption systems, adsorption systems can be built on a small scale and can be operated with no moving parts, which means that the rectifier or solution pump are not needed. Also, there exists no corrosion probl
7、em in adsorption systems (Li and Sumathy 2000). The performance of a solar sorption refrigerator has been studied by Critoph and Tamainot-telto (1997) experimentally for three different configurations of its collector cover: with single glazing, with double glazing, and with single glazing plus tran
8、sparent insulation (TIM). Similarly, Li and Sumathy (1 999) have carried out tests on a solid adsorption ice-making system with activated carbon and methanol as the working pair. It has been reported that by using a simple flat-plate collector with an exposed area of 0.92 m2, it was possible to prod
9、uce ice of about 4-5 kg/day. Boubakri et al. (2000) proposed a model to simulate the operating performance of an adsorptive solar-powered ice maker that had been validated experimentally. This global model could also estimate the limits of ice production by means of adsorptive collector- condenser t
10、echnology; the daily ice production (DIP) could reach about 1 1.5 kg per m2 of collector, and the corresponding COP was about 19%. Recently, a new hybrid system of solar- powered water heater and adsorption ice maker was proposed by Wang et al. (2000). In their system, the adsorber of the adsorption
11、 ice maker is placed in a water bath that is powered directly by a vacuum solar collector. Various solar-powered heating systems using collectors of the flat plate type, vacuum tube type, heat pipe vacuum tube, etc., have been commercialized. In a developing country such as China, solar water heater
12、s have been marketed for about 1 billion yuan per year (Wang et al. 2000). They are usually used during the spring, autumn, and winter seasons. But in summer, heating requirement reduces and cooling requirement increases. In general, for any solar-operated system, collectors have been the most expen
13、sive component, and, hence, solar systems should be utilized for different applications suitable to different seasons. An attempt is made in this study to utilize Yong Li is a research assistant and K. Sumathy is an assistant professor in the Department of Mechanical Engineering, University of Hong
14、Kong. N.D. Kaushika is a professor in the Centre for Energy Studies, Indian Institute of Technology, Haw Khas, New Delhi, India. 02004 ASHRAE. 33 the collectors used in the water-heating system (during winter) as the heat source to energize an adsorption refrigeratiodair- conditioning system (during
15、 summer) with relatively low investment. The literature shows that adsorption systems have mostly been intermittent and used only for ice-making applications. For applications such as air conditioning, when the tempera- ture requirement is only around 6C to SOC, two or more adsorption beds can be us
16、ed to produce cooling effect contin- uously. Two-beds systems using different heat sources have been studied intensively in recent years. Saha et al. (1995) studied the use of adsorption cycles driven by waste heat of near ambient temperature. Sami and Tribes (1 996) have devel- oped a lumped parame
17、ter model to predict the dynamic perfor- mance of such adsorption cycles with single andor double adsorber but with electrical resistance as heat source. However, very few reports have addressed the performance of a solar-powered continuous adsorption system (Vasiliev et al. 2001). In this paper, a
18、lumped model has been established to investigate the performance of a solar-powered adsorption air- conditioning system driven by a simple flat-plate collector with double-glazed cover. The temperature profile of the heat storage tank and two adsorbers has been analyzed, and the influence of some im
19、portant desigdoperational parameters on the performance of the system is reported in this study. SYSTEM DESCRIPTION Figure 1 shows the schematic diagram of a continuous adsorption air-conditioning system considered in this study. The main focus here is to modify the existing solar water heater for u
20、se as an air-conditioning system in summer. The collectors and the storage tank of the “solar water heater” are used as the heat source for heating the adsorbent beds. The modified system consists of the following components: (1) a flat-plate solar collector, (2) two tanks to store hot and cold wate
21、r, respectively, (3) two adsorbers (A and B), (4) a condenser, and (5) an evaporator. As mentioned before, the most expensive component in this system is the collector. Since the system considered in this study utilizes an existing solar water heater (solar collector + water storage tank), rela- tiv
22、ely less investment is made on the other components, such as the adsorber, condenser, and evaporator. A flat-plate solar collector is chosen for its low cost and wide use. Also, with solar energy being the heat source, the adsorption system has the advantage of low maintenance as well as low operati
23、onal costs. However, the COP of an adsorption system is low compared to a vapor compression system; also, the size as well as weight of the system is mainly higher, but this is not a major constraint in a developing country such as China. TO begin with, solar energy gained through a collector is acc
24、umulated in Tank 1, and when water reaches the required temperature, Tank 1 is opened to Adsober A to heat and desorb the refrigerant (adsorbate) from the adsorbent (desorption phase). The refrigerant vapor is, in turn, cooled down in the condenser, and then passed to the evaporator, wherein it agai
25、n gets evaporated at low pressure, thereby providing cooling to the space to be cooled. During the same period, Adsorber B adsorbs the refrigerant vapor leaving the evaporator. Cooling water from Tank 2 removes the heat of adsorption and conden- sation. Hence, the operation of the system follows a p
26、eriodic succession of cycles. That is, at any time of operation when Adsorber A is in the desorption process (heating period), Adsorber B will be in the adsorption process (cooling period). These periods are separated by an isosteric heating period in which the bed is closed and heated and an isoste
27、ric cooling period in which the bed is closed and cooled. In order to increase the efficiency of the system at the end of each desorp- tion phase, the adsorbers (A and B) are connected with each other to recover heat. As a consequence, it is possible to attain a continuous production of cooling (i.e
28、., 50% to 90%, depend- ing on cycle time). The entire system operation can be grouped into four time-periods, Le., (1) time to heat the water in the tank to the required temperature, (2) time in which adsorber A/B is in the desorption phase, (3) heat recovery phase, and (4) time in which adsorber Ai
29、B is in the adsorption phase. This opera- tional scheme is referred to as the base case, and the respective time periods are shown in Figure 2. MATHEMATICAL MODEL In order to study the evolution of the heat transfer process in the adsorbent bed, which is the heart of the system, and to identify the
30、parameters that influence the system performance, a mathematka1 model has been developed. To simplify the model, the following assumptions are made: Temperature and pressure in the components are uni- form. Hot water Cooling water - - - Refrigerant - . . - . . - Heat recovery water Solar collector I
31、 I / Figure 1 Diagram of the solar-powered continuous air- conditioning system. 34 ASH RAE Transactions: Research Period 1 Period 2 Period 3 6:00-10:00 1O:OO-13:OO 13:OO-13:25 Figure 2 Base case operational scheme. Period 4 13:25-16:OO Thermodynamic equilibrium exists in the adsorber at any given ti
32、me. Mass transfer resistance in the adsorbent bed is negligi- ble. Specific heat and density of dry adsorbent and water are constant. Cooling ability of the condenser is unlimited, and tem- perature of evaporation is constant. Based on the assumptions stated above, a lumped param- eter model has bee
33、n developed. The governing equations for the heat transfer process are derived by considering energy balance on Tank 1 and adsorbers. To simpliQ the equations, the heat capacity of the solar collector, connecting pipes, tank i, and water are lumped together and referred to as lumped capacitance (W).
34、 The radiation intensity on a horizontal plane is assumed to vary sinusoidally from sunrise to sunset according to where I, is the maximum intensity of solar radiation occur- ring at solar noon, L is the length of the day, and Ois the differ- ence between the time of the day (at a given instant) and
35、 the sunrise time in hours. The total intensity of radiation (et) is obtained from The overall heat-loss coefficient U, of a double-glazed collector with a bottom loss coefficient of 0.81 W/m2K, corre- sponding to weather conditions at Hong Kong (Sumathy 1999), is UL = 5.5 + 0.024tp . (3) The rate a
36、t which energy is received by the collector is balanced by the following: (I) increase in internal energy of lumped capacitance, (2) heat loss from the solar collector, and (3) heat transferred to the adsorbent bed. Hence, F Wang et al. 1997) and are summarized in Table 1. In Hong Kong, the ambient
37、temperature varies between 25C and 30C in summer. Hence, the following operational condition has been chosen to study the performance of the air- conditioning system and is presented in Table 2. The time evolutions of adsorbers A and B, as well as the hot water in Tank 1, are presented in Figure 3.
38、To begin with, the hot water temperature increases slowly because of the low intensity of radiation during early hours of the day. As the day progresses, with the increase in intensity of radiation, the temperature increases rapidly. When the water temperature in the Tank 1 reaches Point 1, it is co
39、nnected to adsorber A and, hence, a slight dip in the temperature is observed. Beyond that, the temperature of the hot water along with the bed increases. At Point 2, yet another dip is noticed, which indicates that Tank 1 is connected to Adsorber B. Beyond 13:OO hours, the increase in temperature i
40、s very slow, which is due to the fact 36 ASHRAE Transactions: Research 8 10 12 14 16 Time (hour) 0.08 0.04 Figure 3 Temporal history of the system for the base case. - - - - - - Coolingproduction COPsolar that not only does the solar radiation intensity decrease, but also the losses from the collect
41、or are high because of higher temperature in the system. It can also be seen that when Tank 1 is connected to Adsorber A (Point i), there exists a large temperature difference between the hot water and the adsorber, which enhances the heating process and, within a short period of time (about 25 minu
42、tes), the adsorber could reach close to hot water temperature. Figure 4 shows the variation in the adsorption cooling system performance with starting time (time at the start of the first cycle). The first cycle begins when Tank 1 is connected to Adsorber A to initiate desorption. It is quite eviden
43、t from this figure that the performance varies with change in starting time. With other values unchanged, if the starting time is delayed, the performance increases due to the fact that the water gets heated more, which would, in turn, increase the desorption temperature. For any cooling system, it
44、is always preferred to have a higher desorption temperature but lower adsorption temperature. However, if the starting time is delayed beyond noon, the performance reduces because of two reasons: the heat loss becomes significant, and the system could only perform a few cycles in the operating perio
45、d. It can be seen from Figure 5 that the SCP is more sensitive and decreases with increase in adsorbent mass. Similarly, the COPsolar and cooling production increases as long as the adsorbent mass is less than 60 kg. The increase in adsorbent mass indicates more methanol being adsorbed initially. He
46、nce, during the desorption phase, more methanol vapor can be desorbed, which affects more cooling and thereby results in high COPsola, On the other hand, if the mass of adsorbent is increased more than 60 kg, COPsola, as well as cooling production, decreases. This is because, with the given heat inp
47、ut, only the bed could be heated and is not sufficient enough to desorb the required amount of methanol. For the given -m -8 $ -0 -6 00 C n -4 $ e- L . I SCP 42 fi 8 I,.,o 9 10 11 12 13 14 Desorption Starting Time (hour) Figure4 Influence of the starting time on the system performance. O ., scp - CO
48、Psolar - - - - Coolingproduction 0.16 i J 11 IO ta cn C o O -8 0 08 -7 - b, B -6 3 *. - -5 0 -9 = m 0.12 - 8 n O 0 0.08 - - % / 0.04 - 9% . * I I I I I , L / h. 25 50 75 1 O04 O adsorbent mass (kg) Figure 5 Variation in the COP and SCP with the adsorbent mass. system, maximum COPsolar of O. 14 is ac
49、hieved for an adsor- bent mass of 60 kg. In the above discussed case, the lumped capacitance is assumed to be 5 x lo5 J/K. It is essential to know whether this parameter affects the performance of the system or not. The analysis carried out for different values of lumped capacitance revealed the following. As the lumped capacitance increases, the system performance decreases, as shown in Figure 6. It is obvious that with low lumped capacitance, more heat can be utilized to drive the adsorption system. For the given modified adsorption hybrid syst
