79 research outputs found
Thermoelectric cooling devices: thermodynamic modelling and their application in adsorption cooling cycles
Ph.DDOCTOR OF PHILOSOPH
Modelling and Simulation of the Dynamics of Cross Vane Expander-Compressor Unit for Vapour Compression Cycle
The Cross Vane mechanism was recently invented and employed in the design and development of an expander-compressor unit, named Cross Vane expander-compressor or CVEC in short. This device replaces the compressor and expansion valve of a conventional vapour compression cycle, and improves the energy efficiency of the system. Theoretical models which include geometrical, thermodynamics, mass flow, as well as mechanical loss models are developed to characterize the performance of CVEC. Numerical simulation was performed, and the results show that the device is capable of reducing the energy consumption and the peak power requirement of the refrigeration system. Energy saving and peak power reduction of up to 18.0% and 3.8% respectively is achievable with the introduction of CVEC. In addition, due to its unique rotating cylinder design the mechanical losses in CVEC are relatively low. Its mechanical efficiency is calculated to be up to 96.5%
Introduction of the Novel Cross Vane Expander-Compressor Unit for Vapour Compression Cycle
A new type of expander-compressor system called Cross Vane expander-compressor (CVEC) was introduced. This device replaces the expansion valve and compressor in conventional refrigeration systems to reduce the energy consumption and improve the coefficient of performance (COP) of the system. As opposed to many expander-compressor systems in the literature, whereby a separated expander is coupled to a separated compressor of the same or different mechanism, the novel Cross Vane mechanism performs both the function of expander and compressor in a single mechanism. Therefore, one machine is built rather than two as required in the former system. This has significant advantages on the manufacturing process and mechanical efficiency. In this paper, the design and working principle of CVEC are presented and the comparison between the novel CVEC and various types of rotary expander-compressor systems are discussed
Thermodynamic trends for the adsorption of non polar gases on activated carbons employing a new adsorption isotherm modelling
In this paper, the author reports a thermodynamic framework for understanding the surface – energy and the surface – structural interaction factors of activated carbons with various non polar adsorbate molecules. For better understanding, the author employs adsorption uptakes data of activated carbons and some non-polar gases such as H2, Ar, N2, CO2, O2 and CH4 to calculate the enthalpy and entropy of adsorption in pressure-temperature-uptake coordinate systems. The RMS errors are calculated with respect to the proposed model and the experimental data. The minimum RMSEs are found as the model fits well with the experimental data. From theoretical observations, the heterogeneity factors (m) are obtained 1 for microporous and 2 for mesoporous activated carbons, and the interactions of non-polar gases on activated carbons are found to be more sensitive to the adsorbent pore geometry and the adsorbate size. It is also established that the enthalpy and entropy of adsorbent – adsorbate system are closely related to the kinetic diameter of adsorbate molecules, and the pore size equivalent to adsorbate kinetic diameter is the key to store more adsorbate at low pressures. For example, the pore width of activated carbon is roughly maintained 3.8 Å for more methane storage and 3.3 Å for more CO2 captures.NRF (Natl Research Foundation, S’pore)MOE (Min. of Education, S’pore
Experimental investigation for water adsorption characteristics on functionalized MIL-125 (Ti) MOFs: enhanced water transfer and kinetics for heat transformation systems
Employing adsorption technology, metal organic frameworks (MOFs) are found promising to generate cooling, heating, and desalinated water with zero or negligible carbon footprint. In this article, we present various functional groups namely hydroxyl (-OH), amino (-NH2), nitro (-NO2), bromo (-Br) and pyridine (-C5H5N) assisted MIL-125 (Ti) MOFs to control water adsorption/ desorption capacity under the working domain of adsorption heat transformation (A-HT) process. The MOFs are at first synthesized, and later these are characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), thermo-gravimetric analyzer (TGA) and nitrogen (N2) adsorption techniques. The water adsorption isotherms and kinetics are measured by gravimetric methods. Using isotherms data in pressure-temperature coordinate system, the isosteric adsorption-heat (Qst) is evaluated. The experimental results show that the functional group not only influences the hydrophobic and hydrophilic behaviors of MIL-125 (Ti) MOFs but also controls the water transfer per adsorption-desorption cycle with variable uptake/offtake rates (kinetics). Employing isotherms and kinetics data of functionalized (MOFs + water) systems plus the thermodynamic modeling of A-HT cycle, the performance parameters such as cooling, heating and desalination capacities are estimated for various cycle times and heat inputs. The mass and heat transfer processes involved in each component of A-HT system are presented. It is found that OH-MIL-125 (Ti) provides 3 times faster kinetics as compared with the parent MIL-125 (Ti) MOFs. Hence the cooling/heating and water production of A-HT are improved significantly with the addition of functional group, and NH2-MIL-125 (Ti) shows the best cooling and water production performances.Ministry of Education (MOE)The authors acknowledge the financing support from Ministry of Education (MOE), Singapore (grant reference no. RG 98/17)
Adsorption characteristics of methyl-functional ligand MOF-801 and water systems: adsorption chiller modelling and performances
The use of environmentally friendly adsorption assisted thermal storage and chiller is hindered by their bulky sizes and poor performances, which are mainly due to the limited characteristics of solid adsorbents and their poor interactions with water. Therefore, this article presents the parent and CH3 ligand MOF-801 (Zr) adsorbents and their synthesization procedures with surface characterization. Later, the water uptakes on these two MOFs are measured under dynamic and equilibrium conditions for the temperatures of 25–80 °C. Employing experimentally confirmed water uptake and kinetics data of MOFs + water systems, the energetic performances of an adsorption chiller are calculated in terms of the specific cooling power (SCP) and the coefficient of performance (COP). Hence, the methyl functional group enhances the thermal stability of the parent MOF-801 structures with water uptake/offtake rates i.e. kinetics. Under chiller operating conditions, the SCP and the COP of CH3 ligand MOF-801 (Zr) are found 40% and 30% higher as compared with the parent MOF-801(Zr). Methyl-functional ligand in MOF-801 (Zr) shows promising results for cooling application.Ministry of Education (MOE)The authors acknowledge the financing support from Ministry of Education, Singapore (grant reference no. RG 98/17)
Tailoring zirconium-based metal organic frameworks for enhancing hydrophilic/hydrophobic characteristics: simulation and experimental investigation
This article focuses on Grand Canonical Monte Carlo (GCMC) simulation to tailor zirconium-based metal organic frameworks (MOFs) such as UiO-66 (Zr) and MOF-801(Zr). The hydrophilic functional group such as amine (–NH2), hydroxyl (–OH) and pyridine (-C5H5N) and the hydrophobic functional group such as methyl (–CH3) are studied by GCMC simulation. The water adsorption on these functionalized MOFs is calculated within a wide temperature range (25–80 °C) and pressures up to the saturated condition. Based on simulation, the functional MOFs are synthesized and characterized. Water adsorption isotherms are measured experimentally for a wide range of pressures and temperatures. The water adsorption phenomena captured by GCMC simulation are compared with experimental data within acceptable error ranges. The trends of isosteric heat of adsorption (Qst) are also evaluated by GCMC simulation techniques, and these results are compared with Qst calculated by isotherms data and Clausius-Clapeyron equation. The present findings show that the GCMC simulation can be applied to design and develop functionalized MOFs for characterizing and controlling both the hydrophobic/hydrophilic behaviors (especially at low pressure region) with water adsorption. Based on GCMC simulation, the suitable MOFs are designed and optimized for various heat transformation applications.Ministry of Education (MOE)The authors acknowledge the financing support from Ministry of Education (MOE), Singapore (grant reference no. RG 98/17)
Evaluation on the performances of adsorption desalination employing functionalized metal-organic frameworks (MOFs)
This study focuses on a comprehensive study of MOFs (metal-organic frameworks) plus water systems for adsorption desalination applications. The paper first deals with the modification of parent MOFs such as aluminium fumarate, MOF-801 (Zr) and UiO-66 (Zr) employing MOFs functionalization, alkali ion doping and impregnation of zeolites, and secondly, we focus on their characterizations for understanding both structural and thermal stabilities. Later, water adsorption on these parents and modified MOFs are investigated for a wide range of pressure (0 34 m3 /tonne of MOFs/day) and 5 %Li-Al-Fum shows minimum entropy generation.Ministry of Education (MOE)The authors acknowledge the financing supporting from Ministry of Education (MOE), Singapore (grant reference no. RG53/21)
Evaluation of defect induced surface heterogeneity in Metal-Organic Framework materials with alkali dopants employing adsorption isotherm modelling
In this article, an assessment of surface structural heterogeneity in porous metal organic framework (MOF) structure has been demonstrated by employing the methane and carbon-dioxide adsorption isotherms data. The virgin MIL-101-(Cr) MOF was synthesized by the hydrothermal method and defects were induced in the MOF structure by doping with various alkali (K, Na, Li) cations. The synthesized MOFs were characterized by XRD, SEM, EDX and BET measurement techniques. In order to understand the defect induced surface heterogeneity by alkali cation dopants, the surface energy distributions for CH4 and CO2 adsorptions on MOFs were measured by Dubinin – Astakhov model equation. The surface heterogeneity is mainly controlled by the limiting uptakes of adsorbates, the polarizability of adsorbates and the adsorbate-adsorbent interaction energy
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