Chapter 2 Literature review
2.3 Activated carbon based adsorption cooling systems
CHAPTER 2 LITERATURE REVIEW
Page | 30 The experiment was conducted in N2 atmosphere at temperatures ranging from 30 to 150°C. Table 2.5 shows the Cp of widely used ACs in the operating temperature of AHP cycle. It is observed that Cp varies between 0.7 to 1.2 J g-1 K-1 and KOH-H2-Maxsorb III shows highest Cp whereas KOH6-PR possess lowest Cp.
Table 2.5 Specific heat capacity of ACs [74].
Temperature [°C]
Specific heat capacity (Cp) [J g-1 K-1]
Maxsorb III H2-Maxsorb III KOH-H2-Maxsorb III
KOH6-PR
30 0.844 0.949 1.06 0.751
50 0.876 0.969 1.098 0.756
70 0.914 0.994 1.135 0.768
80 0.931 1.007 1.164 0.774
90 0.95 1.023 1.184 0.783
100 0.969 1.035 1.20 0.789
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Page | 31 respectively. The proposed pair seems to be attractive for solar adsorption cooling applications due to high adsorption uptake of ethanol onto Maxsorb III and low adsorption temperature of the pair, which is below 100ºC. El-Sharkawy et al. [76] investigated the adsorption equilibrium of Unitika activated carbon fiber (ACF) of types (A-20)/ethanol and (A-15)/ethanol pairs. Dubinin–Radushkevich (D–R) equation was used to correlate the measured data. They found ACF (A-20)/ethanol pair had higher adsorption capacity than ACF (A-15)/ethanol pair. They also observed that adsorption rate per unit volume of adsorber increased with increasing bed apparent density, which may lead to the design a compact adsorption system. Saha et al. [77,78] modelled a two bed adsorption cooling system using ACF/ethanol pair. This innovative system utilized effectively low-temperature waste heat low-temperature between 60 and 95ºC along with a heat sink at 30ºC.
Optimum cooling capacity values were obtained for a cycle time at 600 s with a fixed pre-heating or pre-cooling cycle time. Optimum switching time for the chiller was between 30 and 50 s. They found that COP reached about 0.6 with a cycle time of 600–700 s.
2.3.2 Activated carbon/methanol pair
Zhao et al. [79] presented a mechanical and experimental study on freeze proof solar powered adsorption cooling tube using activated carbon/methanol pair. It was found that the maximum adsorbent bed and evaporation temperature were 110ºC and −4ºC, respectively. The cooling capacity of the freeze proof solar cooling tube was about 87–99 kJ; and the COP was about 0.11. El-Sharkawy et al. [80] studied the adsorption of methanol onto carbon based adsorbents. The study presented the methanol adsorption isotherms onto two sample namely, Maxsorb III and Tsurumi activated charcoal. Dubinin Raduskevich (D-R) equation was employed to fit the measured data and to draw the pressure–
temperature–uptake (P-T-W) plot for both pairs. Results showed adsorption uptake of Maxsorb III/methanol pair was about 72% higher compared to activated charcoal/methanol pair. The maximum COP was 0.78 with Maxsorb III/methanol at desorption temperature of 90ºC. This study revealed that Maxsorb III/methanol pair was superior among other carbon-based pairs for both of air-conditioning and ice-making applications. Wang et al.
[81] proposed a hybrid system of activated carbon/methanol based solar powered water heater and refrigerator. It was competent of heating 60 kg of water to about 90ºC and producing 10 kg of ice per day with a 2 m2 solar collector. Luo et al. [82] established activated carbon/methanol adsorption pair for a solar adsorption ice maker application.
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Page | 32 The main feature was utilization of a water cooled condenser and removing all valves in the refrigerant circuit except the one that was necessary for refrigerant charging. Compared with an air-cooled condenser, the water-cooled condenser run more effectively. Results showed that the COP of the solar icemaker is about 0.083–0.127. The daily ice production varied within the range of 3.2–6.5 kg m-2. Pons et al. [83] accomplished an experiment with activated carbon and methanol based solar adsorption refrigeration system. The system was loaded with 130 kg of activated carbon and produced around 30–35 kg of ice on sunny days. The system used flat-plate collector having an exposed area of 6m2. This study reported that the system COP was about 0.12.
2.3.3 Activated carbon/carbon dioxide (CO2) pair
Saha et al. [17] studied adsorption isotherms of CO2 onto highly porous activated carbon fiber (ACF) of type A-20 and activated carbon powder of type Maxsorb III for wide temperatures and pressure range. Authors found that the Tóth and modified Dubinin-Astakhov (D-A) models were well matched with the experimental data within ±5% RMSD compared to Langmuir and D-A equations. It was found that the maximum uptake of ACF (A-20)/CO2 pair was 1 kg kg-1 whereas 1.7 kg of CO2 per kg of Maxsorb III, which was 1.7 times higher than that of A-20. The average heats of adsorption of CO2 in Maxsorb III and ACF (A-20) were found to be 20.37 and 19.23 kJ mol-1, respectively. Jribi et al. [84]
studied activated carbon (Maxsorb and ACF-A10)/CO2 based adsorption cooling cycles using pressure–temperature–uptake (P–T–W) diagram. The SCE and the COP of these two cooling systems were simulated for various temperatures. It was found that Maxsorb/CO2
couple showed higher cooling capacity and COP. The maximum COPs of Maxsorb/CO2
and ACF (A10)-CO2 based cooling systems were found 0.15 and 0.083, respectively. The main feature of this cooling cycle was to utilize low temperature waste heat or solar energy.
Jribi et al. [85] introduced a transient mathematical model for a 4-bed adsorption chiller using Maxsorb III/CO2 pair for different heating, cooling and chilled water inlet temperatures. For cooling water inlet temperature of 27°C, the optimal desorption pressure and the optimal adsorption/desorption time were found to be 7.9 MPa and 575 s, respectively. The system was able to produce 1.72 kW of cooling power at driving heat source temperature of 85ºC. The maximum COP was found to be 0.1. Shen et al. [86]
studied adsorption equilibria experimentally and kinetics of activated carbon in beads form/CO2 pair. The CO2 adsorption uptake was measured at various temperatures and
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Page | 33 pressures. The highest adsorption capacity was 0.0844 g g-1 at 30°C and 100 kPa. The isosteric heat was determined to be 23.17 kJ mol-1. Zhong et al. [87] investigated the performance of CO2 as a refrigerant in adsorption refrigeration systems using several types of carbon as adsorbents.
2.3.4 AC based consolidated composites
A fundamental problem with the activated carbon (AC) powder as an adsorbent used in adsorption cooling systems is their very low thermal conductivity. In order to complete a thermodynamic cycle, adsorbent must be heated during the desorption process and then cooled down in adsorption process and finally returns to ambient temperature. For completing each adsorption cycle, a specific quantity of heat is needed for pumping between two processes. In order to improve the performance of adsorption cooling systems, it must be physically small and therefore the time per adsorption cycle must be small. This, in turn, entails high rates of heat transfer in and out of the adsorber bed.
Therefore, using of the consolidated composite adsorbent with high thermal conductivity is one of the methods in order to increase adsorption system performance. Recently, many notable researchers intensely focused on consolidated composite adsorbents to enhance the heat transfer into adsorber/desorber bed. As a result, it improves the adsorption cooling system performance. Researchers have been achieved significant improvement of thermal conductivity of adsorbents using consolidated composite materials as adsorbents.
Following is some representative examples: Zhao et al. [88] studied on consolidated carbon for refrigeration application. Six samples of three types of adsorbents (consolidated AC with expanded natural graphite treated with sulfuric acid (ENG-TSA), consolidated AC with expanded natural graphite (ENG) and granular AC) with different density and different grain size were produced and compared. The making process of consolidated composite AC is shown in Fig. 2.5. The thermal conductivity was measured by the NETZSCH LFA 447 NanoFlash using the laser flash measuring method Results show that with the addition of ENG-TSA, the thermal conductivity of composite AC increases significantly. The thermal conductivity of consolidated AC with ENG-TSA is 7.45 W m-1 K-1 at the density of 400 kg m-3, which is 3 times as high as that of consolidated AC with ENG at the density of 400 kg m-3 and is 45 times higher than that of granular AC. The samples with AC of small grain size have much higher thermal conductivity. The permeability of consolidated AC with ENG-TSA is between 5.00 ×10-12 m2 and 1.00 × 10
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Page | 34
12 m2 when the density is ranged between 350 kg m-3 and 450 kg m-3. Cacciola et al. [89]
developed brick shaped adsorbent materials based on activated carbons and PTFE (poly-tetra-fluorethylene) as a binder. They reported that the adsorption capacity of methanol onto the developed adsorbents is higher than 40% and the thermal conductivity ranging from 0.13 to 0.20 W m-1 K-1. Jin et al. [70] measured thermal conductivity and permeability of granular activated carbon (AC), consolidated AC with a chemical binder and consolidated AC with expanded natural graphite. The authors reported that thermal conductivity of granular activated carbon and consolidated activated carbon with chemical binder are about 0.36 W m-1 K-1 and 0.4 W m-1 K-1, respectively. The thermal conductivity was 2.08 W m-1 K-1 to 2.61 W m-1 K-1, however, its permeability is lower than that of the granular AC and consolidated AC with a chemical binder.
Fig. 2.5 Manufacture processes of consolidated composite AC [88].
(a) drying process of granular ENG-TSA or ENG, (b) drying process of granular AC, (c) mixture of granular AC and water, (d) the composite adsorbent of AC, ENG-TSA or ENG, and water, (e) compressing process of composite adsorbent, (f) drying process of the consolidated adsorbent, (g) the consolidated composite adsorbent samples for test.
Tamainot-Telto et al. [90] measured the thermos-physical properties of two types of monolithic activated carbons. The authors reported that the thermal conductivity of the tested samples was 0.44 W m-1 K-1 and the adsorption capacity for ammonia was 0.36 kg kg-1. Wang et al. [91] developed composites which were combinations of activated carbon and expanded natural graphite. They found thermal conductivity and permeability were 2.47 W m-1 K-1 and 4.378×10-12 m2 s-1, respectively. Wang et al. [69] developed consolidated composite activated carbon (AC) with a host matrix of expanded natural graphite treated with sulfuric acid (ENG-TSA). Fig. 2.6 shows the photograph of developed adsorbents. The authors reported that the highest effective thermal conductivity
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Page | 35 of consolidated composite AC was 34.2 W m-1 K-1 which was 150 times higher than conventional granular activated carbon.
Fig. 2.6 Consolidated composite adsorbents, (a) density of 249 kgm-3 with cracks, (b) density of 388 kgm-3 without cracks, (c) density of 448 kgm-3 with cracks [69].
