EVALUATION OF EVAPORATOR PERFORMANCE FOR SOLAR ADSORPTION COOLING SYSTEM
Muhammad Azman Ramli1, Fauziah Jerai1*, Muhammad Fairuz Bin Remeli1, Nor Afifah Yahaya1
1Fakulti Kejuruteraan Mekanikal,, Universiti Teknologi MARA, Shah Alam, Malaysia
*Corresponding Author: fauziahjerai@uitm.edu.my Accepted: 1 December 2019 | Published: 30 December 2019
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Abstract: Due to the increasing of climate changes, the need for renewable energy sources is constantly increasing. The effect of climate changes can be solved by using environmental air-cooling system which is solar adsorption cooling system. In this context, water was selected as the working fluid. Adsorption systems can be activated by a heat source by using solar energy. Combination of water which is safe substance as working fluid and solar energy which clean and renewable energy will represent long term and friendly solution to this climate changes. There are few things that need to be measured in order to improve performance cooling system. The performance of evaporator is one of the major factors that affect the system's efficiency. In order to improve the system, the evaporator should be managed to give a maximum amount of evaporated refrigerant for the silica gel to adsorb.
Further, an adsorption cooling system that used silica gel-water pair can be driven by utilizes solar energy or other low-grade waste. The result shows that temperature of the heat source is one of the most influential parameters for adsorption cooling system. The heat source for the system was set as low 50℃ as the current solar panel only can provide maximum to 60℃. The results show that the current heat exchangers also suitable to generate cooling power at 50℃ and 60℃.
Keywords: Adsorption cooling system, adsorber, coefficient of performance, Specific cooling Power, Cooling Capacity
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1. Introduction
Solar energy is by far the largest world’s permanent energy and also the most cleans energy.
The amount of solar radiation intercepted by the Earth’s surface is much higher than the annual global energy use which is this low-grade energy not fully utilized and has been wasted (Sharafian et al., 2016). Recently, many promising technologies have been designed and developed to harness the solar energy to give more benefit to humanity. These technologies contribute to protect the environment and save energy since some part of waste heat are used to produce useful refrigeration (Xia ZZ et al. 2008).
The solar cooling system is one of the alternative technologies that utilize renewable energy and use adsorption technologies. Adsorption cooling technologies become more popular and attractive field of research since the conventional vapor compression refrigeration system contribute to environmental pollution (Sabir HM et al.2002). Because of the problem, solar adsorption cooling systems are a good alternative since they operate with environmental- friendly refrigerant that is readily available and natural, free from CFCs.
Like in a vapor compression system, the adsorption refrigeration system also consists of a condenser, an expansion valve, and an evaporator. The evaporator and condenser work the opposite of each other. In the evaporator the refrigerant which is water is evaporate when chilled water is circulated inside the component whilst in condenser the refrigerant will turn into liquid again when the cool water is supplied to the component. Some research shows that the material of heat exchanger that used in evaporator also affects the performance that contributes to the efficiency of the system. However, the cooling performance indices of the adsorption cooling system mainly depends on the adsorbent and adsorbate that used in the system (H.B. Ma et al. 2008). That is why in the previous researches, they focusing on adsorbent effects to the system.
This current study focusing improvement of adsorption cooling system that can be applied to the system which investigated the effect of the chilled water into the evaporator. To run the system, a condenser, an adsorber, and an evaporator assembled to the system. Meanwhile, silica gel-water pair is chosen as an adsorbent-adsorbate pair.
2.1 Low Pressure Evaporator
Previous studies have reported that the falling film evaporation and capillary-assisted evaporation has been used as the evaporator design in the adsorption cooling system. Li, W.
et al., had investigated falling film evaporator by measured the average heat transfer coefficients of water falling film on five types enhanced tubes, with plain tubes as the benchmark. The study concluded that the tubes with enhanced outer and inner surfaces were required to achieve a high heat transfer flux (Li, W. et al., 2013). Other researchers also had confirmed that tubes with enhanced inner surfaces provided better heat transfer performance (Lanzerath, F. et al., 2016). However, the falling film evaporator founds to be impractical to be installed in a light-duty vehicle A/C system considering its parasitic power consumption and liquid spray equipment in the system.
Recent studies by Cheppudira Thimmaiah, P., et al. (2016) have reported a few findings regarding the design of the evaporator consisted of capillary-assisted tubes. The tubes with continuous parallel fins on their outer surfaces had significantly higher heat transfer rate and heat transfer coefficients relative to plain tubes, as shown in Figure 1. The findings also include that the highest transfer rate can be achieve if the refrigerant height in the evaporator is less than the tube diameter. The interior volume above the enhanced tubes of the capillary- assisted evaporator also did not have a significant effect on evaporator performance. They also reported that the fin with the highest density and height provided the highest external heat transfer coefficient.
Figure 1: Capillary-assisted evaporator tubes; (a) side view and (b) cross-sectional view (Cheppudira Thimmaiah, P., et al., 2016)
The design of the evaporator was decided from the previous studies results and analysis by Cheppudira Thimmaiah P., et al. (2016) as included in the literature review. However, dimensions and shaped used are minimized due to this project design is needed to be portable. The evaporator heat exchanger was design as shown in Fig 2. The heat exchanger had four-pass arrangement with a total tube length of 1.43m.
The evaporator was fabricated by using Carbon Steel-50 with chroming as show in Fig. 3.
The thread is made in each hole to assemble each component to evaporator such as pressure gauge, thermocouple and pipe connector. For the current application, this evaporator is used in the system to allow the refrigerant to evaporate from liquid to gas while adsorbing heat in the process.
2.2 Experimental Set Up
The experimental main component is composed of an adsorber, an evaporator and a condenser. These components are connected with Polyvinyl Chloride (PVC) tubes and a set of fittings and the flow are controlled by valves. To reduce heat loss to surrounding, the PVC tubes are insulated with rubber insulators. Hot water tank and chilled water tank are connected to adsorber and evaporator respectively. Water is used as the liquid refrigerant and the temperature of the components are kept constant by water circulator. Flow meters are used to measure flow rate of hot water, cooling water and chilled water. The parameters were gathered by data loggers from the measuring sensors and the measuring points of temperature and flow rate are detailed in the schematic diagram, Fig. 4. Figure 5 show the experimental component located on the trolley.
Table 1 shows the experimental conditions that applied in the experiments. Basically, this experiment consists of four processes to achieve one complete cycle which is pre-heating, desorption, pre-cooling, and adsorption. To run one complete cycle it took 28 minutes, 2 minutes for pre-heating, 2 minutes for pre-cooling, 12 minutes for adsorption and 12 minutes
Fig. 2: Evaporator tube
Fig. 3: Evaporator fabrication
process occur. Referring to Figure 4, at first, the adsorber is pressurizes using hot water during the pre-heating process to completely remove liquid refrigerant in the adsorbent. Then valve 1 is open to allow the desorption process. The refrigerant will be condensed in the condenser by cooling water that circulated through the condenser. Then the refrigerant changes phase to water and accumulated in the evaporator. The liquid refrigerant then evaporated by chilled water that supplied to the evaporator. The chilled water removes heat to the refrigerant due to low pressure in the evaporator and continue circulates to collect heat. The heat that gained from the chilled water allows the evaporation process to occur. The adsorber is depressurized first using cooling water during the pre-cooling process. Then the valve that connects evaporator and adsorber is open to allow the adsorption process to occur. The evaporated refrigerant is adsorbed by the adsorbent. The process is ended when the evaporator is disconnected from the adsorber. The experiment is repeated four times for each condition to get the average value.
Fig. 4: Schematic Diagram Adsorption Cooling System
In order to maximize the performance of the adsorption cooling systems there are few parameters of the condition had been done. Evaporator that can give maximum amount of evaporated gas for the adsorbent to adsorb is key to optimum performance of adsorption cooling. One of the parameters that being considered in this study is hot water temperature variations which referring to Table 1, 50℃ and 60℃ simulate the hot water temperature that can be produced by the solar panel shown in Fig. 7.
Figure 1 : Experimental Set up Fig. 6: Solar panel
Table 1: Experimental Condition
Parameter Value
Heat source water temperature 50°C,60°C,70°C, 75°C, 80°C
Cooled water temperature 30°C
Evaporator inlet temperature 15°C, 20°C, 25°C Heat source flow rate 2L/min
Condenser flow rate 4L/min
Chilled water flow rate 4L/min Pre-heating/Pre-cooling time 2 minutes Desorption/Adsorption time 12 minutes
3. Data Analysis
The governing equation uses for this experiment are summarized in the Table 2. Mass flow rate, ̇ [kg/s] can be determine from equation (1).The adsorption heat 𝑄𝑎𝑑𝑠 [J] during the adsorption process calculated by equation 2), the Δt [s] represent the duration taken to complete the cycle which is for this experiment it take 840 s to complete. The evaporation heat and desorption heat also calculated by using equation (3) and equation 4) respectively.
The performance indices such as coefficient of performance, COP can be defined by using equation 5). The cooling capacity, 𝑄̇ and specific cooling power, SCP also can be defined as equation in (6) and equation (7), where [kg] represent mass of adsorbent which is silica gel. For this experiment 0.1kg of silica gel was packed into adsorber bed.
Table 2: Governing equation
Proces Governing equation
Mass flow rate ̇ ρ ̇̇
Adsorption heat
𝑄𝑎𝑑𝑠 ∑ ̇ ( )
Evaporation heat
𝑄 𝑎 ∑ ̇
Desorption heat
𝑄𝑑 𝑠 ∑ ̇ ( )
COP
𝑄 𝑎
𝑄𝑑 𝑠 Cooling capacity
𝑄̇ 𝑄 𝑎
Specific cooling capacity ̇
4. Results and Discussion
Fig. 7 shows the pattern of adsorption and desorption process at condition 80˚C hot water and 15˚C chilled water for four cycle whilst Fig. 8 shows the reduction temperature profile for 1 complete cycle at condition hot water temperature, is ℃ and chilled water temperature, is ℃. By referring to adsorbers’ temperature curve, it shows that the temperature increases rapidly during the pre-heating process. In the desorption process, temperatures remain constant at around 80℃ until the end of the process.
Fig. 10, Fig. 11 and Fig. 12 show the coefficient of performance (COP), Specific cooling power (SCP) and cooling capacity (𝑄 ) by referring to equation (5) – (7). The data that extracted from the experiment into graph is at 20°C chilled water.
Figure 7: Temperature Profile for Four Cycle (80°C-15°C) 0
20 40 60 80 100
1 15 29 43 57 71 85 99 113
Temperatute [°C]
Cycle Time (min)
Cycle1 Cycle 2 Cycle 3 Cycle 4
Fig. 8: Reduction Temperature Profile at First Cycle (80°C-15°C)
Fig. 9: Coefficient of Performance 0
20 40 60 80 100
1 3 5 7 9 11 13 15 17 19 21 23 25 27 29
Temperature (°C)
Cycle Time (min)
(a) Pre-heating (b) Desorption (c) Pre-cooling (d) Adsorption
0 0.2 0.4 0.6 0.8
50 60 70 75 80
COP
Hot water temperature (°C) Coefficient of Performance
At 15°C Chilled water
At 20°C Chilled water
At 25°C Chilled water
0 100 200 300 400 500
50 60 70 75 80
SCP (J/kg)
Hot water temperature (°C) Specific Cooling Capacity (SCP)
At 15°C Chilled water
At 20°C Chilled water
At 25°C Chilled water (a) (b) (c) (d)
Fig. 11: Cooling Capacity
Fig. 9 shows the effect of the hot water source temperature and the chilled water on the COP of the adsorption cooling system. The value of the COP is varies depending on the temperature of both water sources. As for this experiment, the COP increase with the increase of the hot water source. This is because the desorption of the water vapor is promoted by the increasing of the hot water source and this will encourage the adsorbent which is silica gel to adsorb more water vapor during the adsorption process. This experiment also shows that the COP increasing with the increasing of chilled water temperature. This is because the refrigerant is more easily evaporate at the higher temperature. That’s mean, the evaporator can produce more water vapor for the adsorbent to adsorb at 25˚C compared to other chilled water temperature. As we can see, the COP value at heat source temperature at 50°C and chilled water at 15°C the adsorption and desorption process does not occur due to a low temperature of hot water and chilled water supply to the experiment. Based on the past study, the ideal temperature for hot water for this low-grade heat adsorption cooling system is in between 55-67°C (Wang, D.C., 2007)
Specific cooling power (SCP) indicate the cooling power of the adsorption system for one kilogram adsorbent. Based on the experimental result in Fig, 10, the highest SCP is obtained when 75˚C hot water and 25 ˚C is used which is 454 W/kg. In order to obtain higher cooling capacity of the system, the amount of the silica gel need to be increase. But, large amount of silica gel will lead to produce bulky adsorption system that will make the system less attractive and difficult to be commercialize.
Fig. 11 shows the cooling capacity of the system which the higher value recorded is 35.65W.
The value is low because the amount of the adsorbent which is silica gel that used in the system only 100g. The cooling capacity can be increase when the amount of the adsorbent increase respectively.
As a conclusion, the objective of the project to find the coefficient of performance (COP) of the adsorption system had successfully achieved where the highest COP is 0.67 at condition 80˚C hot water temperature and 25˚C chilled water temperature. This is because the
0 10 20 30 40 50
50 60 70 75 80
Cooling Capcity, 𝑸 ̇(W)
Hot water Temperature (°C) Cooling Capacity (𝑸 ̇)
At 15°C Chilled water
At 20°C Chilled water
At 25°C Chilled water
evaporator can effectively evaporating refrigerant at higher supplied chilled water temperature. At the same time, the adsorbent can adsorb more refrigerant in the adsorber. The experimental results indicated that the various supplied heat source temperature have a significant effect on the overall system performance. The highest specific cooling power that obtained from this experiment is 454 W/kg which is at highest supplied temperature for both hot water and chilled water. The same goes for the cooling capacity, the highest value also obtained at the similar condition as SCP.
The main findings of this study are summarized below:
The increment in temperature of supplied heat water and chilled water enhances the cooling performance of the adsorption system.
The amount of evaporated gas from the evaporator is depending on the increment of temperature supplied chilled water.
The measure of COP and Specific Cooling Capacity at low temperature (50 and 60°C) amid desorption and adsorption process which the hot water supply from solar panel energy and this current heat exchanger and experimental setup are suitable with lower solar heat source 50℃ to 60℃.
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