Synthesis of carbon-doped TiO2 nanoparticles using CO2 decomposition by thermal plasma

This study examined the synthesis of carbon-doped titanium dioxide using TiCl₄ and CO₂ as titanium and carbon sources, respectively, by thermal plasma at atmospheric pressure. The effect of the CO₂ gas flow rate on the preparation of TiO₂ was investigated. The results showed that the decomposition rate of CO₂ was 90% at a CO₂ gas flow rate of 1 L/min. When TiCl₄ was added to produce TiO₂, the decomposition rate of CO₂ reached 95% at a CO₂ gas flow rate of 1 L/min. The resulting powders contained mixed anatase and rutile phases with particle sizes ranging from 20 to 50 nm. The carbon in the CO₂ acted as a dopant to produce the carbon-doped TiO₂. The prepared samples were mainly characterized by X-ray diffraction, X-ray photoelectron spectroscopy, specific surface area measurements and ultraviolet-visible spectroscopy.     

Heat and mass transfer enhancement of binary nanofluids for H2O/LiBr falling film absorption process

The objectives of this study are to measure the vapor absorption rate and heat transfer rate for falling film flow of binary nanofluids, and to compare the enhancement of heat transfer and mass transfer under the same conditions of nanofluids. The key parameters are the base fluid concentration of LiBr, the concentration of nanoparticles in weight %, and nanoparticle constituents. The binary nanofluids are H₂O/LiBr solution with nanoparticles of Fe and Carbon nanotubes (CNT) with the concentrations of 0.0, 0.01 and 0.1 wt %. The vapor absorption rate increases with increasing the solution mass flow rate and the concentration of Fe and CNT nanoparticles. It is found that the mass transfer enhancement is much more significant than the heat transfer enhancement in the binary nanofluids with Fe and CNT. It is also found that the mass transfer enhancement from the CNT nanoparticles becomes higher than that from the Fe nanoparticles. Therefore, the CNT is a better candidate than Fe nanoparticles for absorption performance enhancement in H₂O/LiBr absorption system.     

Photoreduction of CO2 by TiO2 nanocomposites synthesized through reactive direct current magnetron sputter deposition

The photoreduction of CO₂ into methane provides a carbon-neutral energy alternative to fossil fuels, but its feasibility requires improvements in the photo-efficiency of materials tailored to this reaction. We hypothesize that mixed phase TiO₂ nano-materials with high interfacial densities are extremely active photocatalysts well suited to solar fuel production by reducing CO₂ to methane and shifting to visible light response. Mixed phase TiO₂ films were synthesized by direct current (DC) magnetron sputtering and characterized by X-ray diffraction (XRD), atomic force microscope (AFM), scanning electron microscope (SEM) and transmission electron microscope (TEM). Bundles of anatase-rutile nano-columns having high densities of two kinds of interfaces (those among the bundles and those between the columns) are fabricated. Films sputtered at a low deposition angle showed the highest methane yield, compared to TiO₂ fabricated under other sputtering conditions and commercial standard Degussa P25 under UV irradiation. The yield of methane could be significantly increased (~ 12% CO₂ conversion) by increasing the CO₂ to water ratio and temperature (< 100 °C) as a combined effect. These films also displayed a light response strongly shifted into the visible range. This is explained by the creation of non-stoichiometric titania films having unique features that we can potentially tailor to the solar energy applications.     

CO2 and H2 detection with a CHx/porous silicon-based sensor

There has been great interest in the last years in gas sensors based on porous silicon (PS). Recently, a gas sensing device based on a hydrocarbon CH_x/porous silicon structure has been fabricated. The porous samples were coated with hydrocarbon groups deposited in a methane argon plasma. We have experimentally demonstrated that the structure can be used for detecting a low concentration of ethylene, ethane and propane gases [Gabouze N, Belhousse S, Cheraga H. Phy State Solidi (C), in press].In this paper, the CH_x/PS/Si structure has been used as a sensing material to detect CO₂ and H₂ gases. The sensitivity of the devices, response time and impedance response to different gas exposures (CO₂, H₂) have been investigated.The results show that current-voltage and impedance-voltage characteristics are modified by the gas reactivity on the PS/CH_x surface and the sensor shows a rapid and reversible response to low concentrations of the gases studied at room temperature.     

Oil free turbo-compressors for CO2 refrigeration applications

Feasibility of replacing oil-lubricated compressing equipment in CO₂ based refrigeration systems with oil-free turbo-machinery is assessed. Presented concept enables efficient compression for systems ranging from 0.1 to 5 MW of cooling capacity, provided that the operating pressures are low, i.e. 30/10 bar. Performance of the systems with higher operating pressures, i.e. 77/30 bar is penalized in wide range of capacities due to the excessive windage losses, especially pronounced in the systems with cooling capacities lower than 1 MW. In some cases, possibility of using longer motor should be analyzed. This may require special strategies for rotordynamic issues or driving each impeller with separately mounted motor. It is observed that optimal specific speed of the compressor stage does not always result in optimal overall performance. The trade-off between aerodynamic efficiency and non-stage losses must be found.     

Efficiencies of transcritical CO2 cycles with and without an expansion turbine: Rendement de cycles transcritiques au CO2 avec et sans turbine d'expansion

With the discovery that fluorocarbons may have a profoundly detrimental effect on the earth's atmosphere, it has become necessary to find a suitable replacement for a fluorocarbon-based refrigeration cycle. Such a replacement must perform comparably to current refrigerants, be economically feasible, and significantly reduce the possibility of a negative environmental impact compared with current refrigerants. A review of the literature on the first vapor compression refrigeration cycles indicate that carbon dioxide was used as the working fluid prior to and after the turn of the century. Moreover, recent literature has suggested that carbon dioxide will work for certain air conditioning applications. Initial investigation of the heat transfer properties of carbon dioxide and subsequent modeling of a transcritical carbon dioxide vapor compression cycle suggest potential performance comparable to that of existing refrigeration systems. This paper will present a detailed literature review of carbon dioxide's role in refrigeration cycles and what has been investigated so far to find a suitable application for carbon dioxide as a refrigerant. In addition, two thermodynamic models for transcritical carbon dioxide cycles with and without an expansion turbine have been developed. Both models contain an internal suction-to-supercritical line heat exchange. The efficiencies of the transcritical carbon dioxide cycles will be compared with the ones of conventional HCFC-22 cycles. A second law analysis on both carbon dioxide and HCFC-22 cycles will be included.La découverte de l'effet nocif que peuvent avoir les fluorocarbons sur l'atmosphère terrestre a exigé de trouver des remplaçements aux cycles frigorifiques fondés sur eux. De tels remplaçants doivent avoir un rendement comparable, être économique acceptables et réduire nettement le risque d'impact négatif sur l'environnement.La littérature relative aux premiers cycles à compression de vapeur montre que le dioxyde de carbone fur utilisé à cette fin à la fin du XIXe et au début du XXe siècle. En outre des publications récentes suggèrent que le dioxyde de carbone à certaines applications en conditionnemnt d'air. Un examen préalable des propriétés de transfert de chaleur du dioxyde de carbone puis la modélisation d'un cycle transcritique avec ce fluide suggèrent que l'on peut obtenir ainsi des performances comparables à celles des systèmes frigorifiques actuels. Cet article présente une revue bibliographique détaillée sur le dioxyde de carbone dans les cycles frigorifiques et l'état des recherches effectuées pour trouver une utilisation possible de dioxyde de carbone comme frigorigène. Deux modèles thermodynamiques pour des cycles transcritiques au dioxyde de carbone avec et sans turbine d'expansion ont été réalisés. Ces deux modèles comprennent un échange de chaleur interne entre admission et ligne supercritique. Leurs rendements sone comparés avec ceux de cycles conventionnels, au R22. Une analyse comparée selon la 2e loi y est jointe.     

The effect of chemical surfactants on the absorption performance during NH3/H2O bubble absorption process

The objectives of this paper are to visualize the bubble behavior by shadow graphic method, to examine the effect of surfactants on the bubble type absorption, and to find the optimal conditions to design highly effective compact absorber for NH₃/H₂O absorption system. The initial concentrations of NH₃/H₂O solution and the kinds and the concentrations of surfactants are considered as key parameters. By measuring the absorption rate for each condition, two effects of the addition of surfactants, the Marangoni and the barrier effect, are compared with each other. The results show that the addition of surfactant enhances the absorption performance up to 4.81 times. The experimental correlations of the effective absorption ratio for each surfactant, 2-ethyl-1-hexanol, n-octanol, and 2-octanol, are suggested within ±15, ±10, and ±10%, respectively.     

Catalytic reduction of low-concentration CO2 with water by Pt/Co@NC

The reduction of low-concentration carbon dioxide with water to organic fuels is still a huge challenge. In this study, we successfully designed the partially oxidized cobalt nanoparticles coated by the nitrogen-doped carbon layer (Co@NC) of 2–8?nm via a facile method and then interspersed with different amount of Pt nanoparticles. The Co@NC decorated with 1?wt% Pt exhibits the best ability for CO₂ reduction to CH₄ and a CH₄ production rate of 14.4?μmol·g^?1·h^?1 is achieved. It is worth noting that the system is carried out under low-concentration CO₂ (400?ppm) circumstance without any sacrificial agent, which could be meaningful to the design of catalysts for atmospheric CO₂ reduction.     

Experimental evaluation of a cascade refrigeration system prototype with CO2 and NH3 for freezing process applications

A prototype of a cascade refrigeration system using NH₃ and CO₂ as refrigerants has been designed and built. The prototype is used to supply a 9 kW refrigeration capacity horizontal plate freezer at an evaporating temperature of −50 °C as design conditions. The prototype includes a specific control system and a data acq*uisition system. The experimental evaluation started with the real conditions within the design operating parameters. Subsequently, several tests were performed fixing four CO₂ evaporating temperatures (−50, −45, −40 and −35 °C). At each one of the evaporating temperatures evaluated, the CO₂ condensing temperature was varied from −17.5 to −7.5 °C and an experimental optimum value of CO₂ condensing temperature was determined. The discussions on the experimental results include the influence of the operating parameters on the cascade system’s performance. In addition, the experimental results are compared with two common double stage refrigeration systems using NH₃ as refrigerant.     

Thermodynamic analysis and experimental investigation of a CO2 household heat pump dryer

Carbon dioxide is regarded as an optimal working fluid for heat pump dryers. The transcritical cycle well fits the closed-loop drying process which requires dehumidification and re-heating according to high temperature lift of the air stream.In this paper, the transcritical CO₂ cycle is compared with a sub-critical R134a cycle. The theoretical analysis is based on fixed temperature approach values at the heat exchangers. The study considers optimal high pressure for the transcritical cycle and optimal refrigerant subcooling for the sub-critical cycle. The theoretical analysis investigates the energy performance of the thermodynamic cycle as a function of the temperature and mass flow rate of the drying air. The optimisation of the operating conditions for CO₂ involves lower air temperature than in the case of R134a; this conditions can be satisfied by a suitable design of the appliance, whose thermal balance is achieved when the dissipated heat corresponds to the work spent by the compressor and the fan; the air temperature is a floating variable that adjusts its value to comply with the thermal balance. Experimental results, conducted on a prototype, give a positive assessment for CO₂ as working fluid for heat pump dryers: a negligible decrease in the electric power consumption, with a limited (+9%) increase in the cycle time, is shown in comparison with the reference R134a heat pump dryer.     

Design-theoretical study of cascade CO2 sub-critical mechanical compression/butane ejector cooling cycle

In this paper an innovative micro-trigeneration system composed of a cogeneration system and a cascade refrigeration cycle is proposed. The cogeneration system is a combined heat and power system for electricity generation and heat production. The cascade refrigeration cycle is the combination of a CO₂ mechanical compression refrigerating machine (MCRM), powered by generated electricity, and an ejector cooling machine (ECM), driven by waste heat and using refrigerant R600. Effect of the cycle operating conditions on ejector and ejector cycle performances is studied. Optimal geometry of the ejector and performance characteristics of ECM are determined at wide range of the operating conditions. The paper also describes a theoretical analysis of the CO₂ sub-critical cycle and shows the effect of the MCRM evaporating temperature on the cascade system performance. The obtained data provide necessary information to design a small-scale cascade system with cooling capacity of 10 kW for application in micro-trigeneration systems.     

Experimental study on the improvement of CO2 air conditioning system performance using an ejector

Several preceding researches have evidenced that the transcritical air conditioning system using CO₂ as a refrigerant has an inherent inefficiency resulting in degraded steady-state system performance of a CO₂ air conditioning system compared with that of a conventional air conditioning system. As a practical improvement, two-phase ejector was considered in place of expansion device in this study. The two-phase ejector for CO₂ air conditioning system was designed and developed considering the non-equilibrium state for evaluating the sonic velocity and the critical mass flux. The experiments of performance with respect to variation of ejector geometry such as the motive nozzle throat diameter, mixing section diameter and the distance between motive nozzle and diffuser were carried out. There exist optimum design parameters in each test. Experiments showed that the coefficient of performance of the system using an ejector was about 15% higher than that of the conventional system.     

Visualization of bubble behavior and bubble diameter correlation for NH3–H2O bubble absorption

The objectives of this paper are to visualize the bubble behavior for an ammonia–water absorption process, and to study the effect of key parameters on ammonia–water bubble absorption performance. The orifice diameter, orifice number, liquid concentration and vapor velocity are considered as the key parameters. The departing bubbles tend to be spherical for surface tension dominant flow, and the bubbles tend to be hemispherical for inertial force dominant flow. A transition vapor Reynolds number is observed at a balance condition of internal absorption potential (by the concentration difference) and external absorption potential (by the vapor inlet mass flow rate). As the liquid concentration increases, the transition Reynolds number and the initial bubble diameter increase. The initial bubble diameter increases with an increase of the orifice diameter while it is not significantly affected by the number of orifices. Residence time of bubbles increases with an increase in the initial bubble diameter and the liquid concentration. This study presents a correlation of initial bubble diameter with ±20% error band. The correlation can be used to calculate the interfacial area in the design of ammonia-water bubble absorber.     

Theoretical and experimental analysis of a CO2 heat pump for domestic hot water

This paper describes the development of a CO₂ air/water heat pump for the production of tap hot water in a residential building. The basic design consists of a single-stage piston compressor, a coaxial type gas cooler, an electronic expansion valve, a finned tube evaporator and a low pressure receiver. The heat pump is combined with a storage tank designed to maintain internal water stratification.The gas cooler pressure optimisation in the case of fixed water delivery temperature was theoretically analysed.A new control method for the upper cycle pressure was developed to maximise the COP of the heat pump, while the water mass flow was adjusted to maintain the set water temperature at the gas cooler exit.Before commissioning, the heat pump was factory tested to verify its energy performance and to validate the high pressure control logic.     

Condensation heat transfer and pressure drop characteristics of CO2 in a microchannel

The condensation heat transfer coefficient and pressure drop of CO₂ in a multiport microchannel with a hydraulic diameter of 1.5 mm was investigated with variation of the mass flux from 400 to 1000 kgm⁻²s⁻¹ and of the condensation temperature from −5 to 5 °C. The heat transfer coefficient and pressure drop increased with the decrease of condensation temperature and the increase of mass flux. However, the rate of increase of the heat transfer coefficient was retarded by these changes. The gradient of the pressure drop with respect to vapor quality is significant with the increase of mass flux. The existing models for heat transfer coefficient overpredicted the experimental data, and the deviation increased at high vapor quality and at high heat transfer coefficient. The smallest mean deviation of ±51.8% was found by the Thome et al. model. For the pressure drop, the Mishima and Hibiki model showed mean deviation of 29.1%.     

CO2 and propane blends: Experiments and assessment of predictive methods for flow boiling in horizontal tubes

The CO₂ and propane blends are an interesting alternative to solve technical and safety issues related to the use of pure CO₂ or pure hydrocarbons. These mixtures of pure fluids are environmentally friendly and have a large glide, that affects remarkably heat transfer.In this paper a review of works and predictive methods on flow boiling of wide-boiling mixtures is first presented. Experiments during flow boiling in a smooth horizontal tube with an internal diameter equal to 6.00 mm of CO₂ and propane mixtures (with 83.2/16.8% and 70.0/30.0% in mass concentrations) are reported. The experiments are related to the following operating ranges: mass fluxes from 200 to 350 kg m⁻² s⁻¹, heat fluxes from 10 to 20.2 kW m⁻², temperatures of the mixture from 6.9 to 14.0 °C in the whole range of vapor qualities.An assessment of predictive methods based on the present and independent databases is reported.     

A correlation-free on-line optimal control method of heat rejection pressures in CO2 transcritical systems

This paper proposes a novel correlation-free on-line optimal control method for CO₂ transcritical refrigeration systems. It uses the on-line correction formula to track the optimal pressure set point. As a critical advantage against the existing empirical correlations of the heat rejection pressure, it is independent of the cycle, the system specifications, and the operating conditions. Dynamic numerical simulation demonstrates how to apply the new method to a basic CO₂ transcritical refrigeration system. The results show that the proposed method can well track the optimal pressures and is robust to resist the sampling noise.     

Zn0.4(CuGa)0.3Ga2S4/CdS光催化材料的制备及其CO2还原性能

利用光催化技术将CO2转化为燃料有望解决能源危机和温室效应.Zn1–2x(CuGa)xGa2S4具有可见光响应及较高的导带电势,从热力学角度上看是较为理想的CO2还原材料,但是其光催化CO2还原活性仍然较低,亟待从动力学角度提高其活性.本研究采用Zn0.4(CuGa)0.3Ga2S4与不同比例的CdS纳米颗粒复合,制备...     

The effect of oil-droplet on bubble absorption performance in binary nanoemulsions

The objectives of this paper are to study the absorption characteristics of NH₃ bubbles in binary nanoemulsions and to quantify the effect of oil-droplet on the bubble absorption performance. C₁₂E₄ and Tween20 are used as the surfactants and n-decane oil is added into NH₃/H₂O solution to make the binary nanoemulsion. The initial concentration of NH₃/H₂O solution and the concentration of oil are considered as the key parameters. The absorption rates are calculated by measuring the inlet and outlet mass flow rates per a given time period. In addition, the droplet size in the binary nanoemulsion is measured by the particle size analyzer (ELS-Z, OTSUKA ELECTRONICS). It is found that the effective absorption ratio for 2.0 vol% oil and 14.3 wt% NH₃/H₂O becomes 17% higher than that for the base fluid.