Ka Chung Chan

Improved Thermal Conductivity of 13X/CaCl2 Composite Adsorbent by CNT Embedment

Adsorption cooling systems utilize the principle of adsorption to generate cooling effect. Composite adsorbents synthesized from zeolite 13X and CaCl2 have previously been shown to have a high adsorption capacity and high adsorption rate with lower desorption temperature where the adsorption capacity and adsorption rate are 420% and 122% of zeolite 13X under the same condition respectively. This results in more compact design and a lower temperature waste-heat source can be used. The system performance is, however, limited by the low thermal conductivity of the 13X/CaCl2 composite adsorbent which is common for many adsorbents. Due to the low thermal conductivity of the adsorbent, poor heat transfer and slow temperature change in the adsorbent bed lead to longer time for the adsorbent to achieve the adsorption/desorption temperature. This directly reduces the adsorption/desorption rate of the adsorbate on the adsorbent, such as water on zeolite, and results in lower system coefficient of performance (COP) and specific cooling power (SCP). It was proposed that embedding carbon nanotube (CNT) into the 13X/CaCl2 composite absorbents can increase the thermal conductivity of the adsorbent bed to improve the system performance. Thus, the properties of the multi-wall CNT (MWCNT) embedded zeolite 13X/CaCl2 composite adsorbents were investigated to find out the optimized composition for the cooling system. The material properties of the MWCNT embedded zeolite 13X/CaCl2 composite adsorbent were measured. The thermal conductivities of the MWCNT embedded 13X/CaCl2 composite adsorbents were predicted by developing a new theoretical model modified based on area contact model. The performance of the adsorption cooling system using zeolite 13X and MWCNT embedded composite adsorbent were studied numerically. It is found that the COP and SCP are improved by 3.6 and 26 times respectively. This results in a much more compact and energy efficient cooling system.Copyright

Heat and Mass Transfer Characteristics of a Zeolite 13X/CaCl2 Composite Adsorbent in Adsorption Cooling Systems

The performance of the adsorption cooling system using the zeolite 13X/CaCl2 composite adsorbent was studied using a numerical simulation. The novel zeolite 13X/CaCl2 composite adsorbent with superior adsorption properties was developed in previous studies [11]. It has high equilibrium water uptake of 0.404 g/g between 25 o C and 100 o C under 870Pa. The system specific cooling power (SCP) and coefficient of performance (COP) were successfully predicted for different operation parameters. The simulated COP with the composite adsorbent is 0.76, which is 81% higher than a system using pure zeolite 13X under desorption temperature of 75 o C. The SCP is also increased by 34% to 18.4 W/kg. The actual COP can be up to 0.56 compared to 0.2 for zeolite 13X-water systems, an increase of 180%. It is predicted that an adsorption cooling system using the composite adsorbent could be powered by a low grade thermal energy source, like solar energy or waste heat, using the temperature range of 75 o C to 100 o C. The performance of the adsorber with different design parameters was also studied in the present numerical simulation. Adsorbents with smaller porosity can have higher thermal conductivity and may result in better system performance. The zeolite bed thickness should be limited to 10mm to reduce the thermal response time of the adsorber. Addition of high thermal conductivity materials, for example carbon nanotube, can also improve the performance of the adsorber. Multi-adsorber tube connected in parallel can be employed to provide large heat transfer surface and maintain a large SCP and COP. The desorption temperature also showed a large effect on the system performance.

Development of New Zeolite 13X/CACL2 composite adsorbent for air-conditioning application

Composite adsorbents synthesized from zeolite 13X and CaCl2 were investigated for applications in solar adsorption systems. The effect of Ca-ion-exchange on the adsorption properties of zeolite 13X was studied. Sodium ions in the zeolite structure were replaced by calcium ions by ion exchange. It was found that the Ca-ion-exchange process decreased the specific surface areas of the Ca-ion-exchanged zeolites while the total pore volumes were increased. The optimized Ca-ion-exchange condition existed when soaking zeolite 13X in 46wt% CaCl2 solution for 36 hours. The increase in the total pore volume is good for further impregnating the zeolite with CaCl2 . A large difference in equilibrium water uptake, 0.404g/g, between 25°C and 100°C under 870Pa was recorded for the 13X/CaCl2 composite adsorbent impregnated in 40wt% CaCl2 solution. This was 295% of that of zeolite 13X under the same condition. The 13X/CaCl2 composite adsorbent showed a high potential in replacing vapor compression chillers in producing chilled water for central air-conditioning systems.Copyright

Performance investigation of nano-structured composite surfaces for use in adsorption cooling systems with a mass recovery cycle

With an increase of the heat transfer coefficient and condensation rate in a condenser, a lower pressure can be achieved in a desorber, which leads to a dryer adsorber for the next adsorption phase and a better cooling performance in an adsorption cooling system. This study aims to experimentally investigate the condensation rate of different nanostructured surfaces and improve the cooling performance of an adsorption cooling system by coating a superhydrophobic–zeolite 13X adsorbent composite surface in the condenser. An experiment was designed and built to investigate the condensation rate of various nanostructured surfaces on a copper plate. The results show that a water collection rate (condensation rate) of the superhydrophobic–zeolite 13X adsorbent composite surface of 49.3 g/m2 min is achieved, which shows an enhancement of about 50% compared to that of the copper surface. A mathematic model is developed to estimate the cooling performance of the adsorption cooling system utilizing the composite surface and a mass recovery cycle. The simulation results show that a specific cooling power (SCP) of 231.4 W/kg and a coefficient of performance (COP) of 0.317 are determined, which shows an improvement of 25.0% and 7.8%, respectively, compared to that of the system without coating the nanostructured composite surface.

Simulation Study of the Heat and Mass Recovery on the Performance of Adsorption Cooling Systems

In this study, simulation was conducted to investigate the effect of mass recovery, heat recovery, pre-heating and pre-cooling time on the system performance of a double-bed adsorption cooling system. Pressures of different system components were considered in the simulation. The adsorbent-adsorbate pair used was silica-gel and water. The heating and cooling temperatures were selected to be 85°C and 27°C respectively. Both the adsorption and desorption phase times were set at 15 minutes. The coefficient of performance (COP) and specific cooling power (SCP) were used to quantify the performance of the system. From the simulation, the basic cycle provided COP and SCP of 0.20 and 40.9W/kg respectively. By conducting heat recovery for 120 seconds, the system COP was largely increased by 99% to 0.40 compared to the basic cycle. The SCP was also increased to 42.3W/kg. Mass recovery, however, did not have too much effect on the system performance. The COP and SCP only increased by 4.5% and 3.9% respectively when conducting mass recovery for 4.7 seconds. For conducting heat and mass recovery, the COP and SCP were increased to 0.36 and 44.68W/kg, respectively. Pre-heating and pre-cooling can also be beneficial in improving both COP and SCP. The COP and SCP were increased by 14.5% and 10.1% respectively, to 0.23 and 45.0W/kg by conducting pre-heating and pre-cooling for 50.3 seconds. The combinations of these processes were also studied. It is suggested heat and mass recovery then pre-heating and pre-cooling should be conducted to improve COP and SCP. The improvements showed 31.2% for COP, increasing to 0.27, and 11.9% for SCP, increasing to 45.7W/kg.Copyright

Calcium Ion-Exchanged Zeolite 13X: Properties Measurement and Potential Usage in Solar Adsorption Cooling Systems

Ca-ion exchanged zeolites were synthesized from zeolite 13X and calcium chloride in this study. XRF, TGA and BET were used to measure the adsorption related properties of the Ca-ion-exchanged zeolites. The influence of the synthesis conditions of the zeolites and the potential usage of them in solar adsorption cooling systems were investigated. 0.27g/g of difference in equilibrium water uptake between two operation conditions was recorded, which has 17.8% improvement compared with pure zeolite 13X. Ideal Coefficients of Performance (COP) and Specific Cooling Power (SCP) for an adsorption cooling system using the Ca-ion-exchanged zeolites were also estimated. The increase of ideal COP was not much, 2.4%, but the improvement of the actual COP was predicted to be up to 70% by reducing the large unwanted energy loss generally experienced in actual systems. The ideal SCP was also found to be 504 W/kg, increased by 22.2%. This suggests that this Ca-ion-exchanged zeolite can be used in solar adsorption cooling system with much better performance than zeolite 13X.