Comparitive analysis of an automotive air conditioning systems operating with CO2 and R134a

This paper evaluates performance merits of CO₂ and R134a automotive air conditioning systems using semi-theoretical cycle models. The R134a system had a current-production configuration, which consisted of a compressor, condenser, expansion device, and evaporator. The CO₂ system was additionally equipped with a liquid-line/suction-line heat exchanger. Using these two systems, an effort was made to derive an equitable comparison of performance; the components in both systems were equivalent and differences in thermodynamic and transport properties were accounted for in the simulations. The analysis showed R134a having a better COP than CO₂ with the COP disparity being dependent on compressor speed (system capacity) and ambient temperature. For a compressor speed of 1000 RPM, the COP of CO₂ was lower by 21% at 32.2°C and by 34% at 48.9°C. At higher speeds and ambient temperatures, the COP disparity was even greater. The entropy generation calculations indicated that the large entropy generation in the gas cooler was the primary cause for the lower performance of CO₂.     

The natural fluid nitrous oxide—an option as substitute for low temperature synthetic refrigerants

A theoretical investigation was performed concerning the coefficient of performance (COP) of cascade refrigerating systems using N₂O as refrigerant for the low temperature cascade stage and various natural refrigerants like ammonia, propane, propene, carbon dioxide and nitrous oxide itself for the high temperature stage. The basis of the comparison was a conventional R23/R134a-cascade refrigerating system for heat rejection temperatures of +55, +35 and +25 °C for air cooling, cooling tower water cooling and city water cooling, respectively. It can be stated that such an application of N₂O at the primary stage and ammonia or hydrocarbons as refrigerants at the secondary stage in refrigerating systems achieves similar COP-values compared to the R23/R134a-cascade refrigerating system, whereas CO₂ and N₂O in a transcritical cycle in general perform worse.An application of N₂O in a two-stage compression cycle with interstage injection and city water cooling at low and high interstage temperatures has a nearly equal COP as a conventional R23/R134a-cascade refrigerating system and is an interesting alternative for small laboratory refrigerating systems.     

CO2 and R410A flow boiling heat transfer, pressure drop, and flow pattern at low temperatures in a horizontal smooth tube

Flow boiling heat transfer coefficient, pressure drop, and flow pattern are investigated in the horizontal smooth tube of 6.1 mm inner diameter for CO₂, R410A, and R22. Flow boiling heat transfer coefficients are measured at the constant wall temperature conditions, while pressure drop measurement and flow visualization are carried out at adiabatic conditions. This research is performed at evaporation temperatures of −15 and −30 °C, mass flux from 100 to 400 kg m⁻² s⁻¹, and heat flux from 5 to 15 kW m⁻² for vapor qualities ranging from 0.1 to 0.8. The measured R410A heat transfer coefficients are compared to other published data. The comparison of heat transfer coefficients for CO₂, R410A, and R22 is presented at various heat fluxes, mass fluxes, and evaporation temperatures. The difference of coefficients for each refrigerant is explained with the Gungor and Winterton [K.E. Gungor, R.H.S. Winterton, A general correlation for flow boiling in tubes and annuli, Int. J. Heat Mass Transfer 29 (1986) 351–358] correlation based on the thermophysical properties of refrigerants. The Wattelet et al. [J.P. Wattelet, J.C. Chato, B.R. Christoffersen, J.A. Gaibel, M. Ponchner, P.J. Kenny, R.L. Shimon, T.C. Villaneuva, N.L. Rhines, K.A. Sweeney, D.G. Allen, T.T. Heshberger, Heat Transfer Flow Regimes of Refrigerants in a Horizontal-tube Evaporator, ACRC TR-55, University of Illinois at Urbana-Champaign, 1994], and Gungor and Winterton [K.E. Gungor, R.H.S. Winterton, A general correlation for flow boiling in tubes and annuli, Int. J. Heat Mass Transfer 29 (1986) 351–358] correlations give the best agreement with the measured heat transfer coefficients for CO₂ and R410A. Pressure drop for CO₂, R410A, and R22 at various mass fluxes, evaporation temperatures and qualities is presented in this paper. The Müller-Steinhagen and Heck [H. Müller-Steinhagen, K. Heck, A simple friction pressure drop correlation for two-phase flow in pipes, Chem. Eng. Process. 20 (1986) 297–308], and Friedel [L. Friedel, Improved friction pressure correlations for horizontal and vertical two-phase pipe flow, in: The European Two-Phase Flow Group Meeting, Ispra, Italy, 1979 (paper E2)] correlation can predict most of the measured pressure drop within the range of ±30%. The relation between pressure drop and properties for each refrigerant is described by applying the Müller-Steinhagen and Heck correlation. The observed two-phase flow patterns for CO₂ and R410A are presented and compared with flow pattern maps. Most of the flow patterns can be determined by the Weisman et al. [J. Weisman, D. Duncan, J. Gibson, T. Crawford, Effect of fluid properties and pipe diameter on two-phase flow patterns in horizontal lines, Int. J. Multiphase Flow 5 (1979) 437–462] flow pattern map.     

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.     

Cooling performance of a variable speed CO2 cycle with an electronic expansion valve and internal heat exchanger

The cooling performance of a CO₂ cycle must be improved to develop a competitive air-conditioning system with the conventional air-conditioners using HFCs. In this study, the cooling performance of a variable speed CO₂ cycle was measured and analyzed by varying the refrigerant charge amount, compressor frequency, EEV opening, and length of an internal heat exchanger (IHX). The basic CO₂ system without the IHX showed the maximum cooling COP of 2.1 at the compressor discharge pressure of 9.2 MPa and the optimum normalized charge of 0.282. The cooling COP decreased with the increase of compressor frequency at all normalized charges. The optimum EEV opening increased with compressor frequency. Simultaneous control of EEV opening and compressor frequency allowed optimum control of the compressor discharge pressure. The optimal compressor discharge pressure of the modified CO₂ cycle with the IHX was reduced by 0.5 MPa. The IHX increased the cooling capacity and COP of the CO₂ cycle by 6.2–11.9% and 7.1–9.1%, respectively, at the tested compressor frequencies from 40 to 60 Hz.     

(0 0 6)-oriented α-Al2O3 films prepared in CO2-H2 atmosphere by laser chemical vapor deposition using a diode laser

(0 0 6)-oriented α-Al₂O₃ films were prepared by laser chemical vapor deposition (LCVD) using aluminum acetylacetonate (Al(acac)₃) in CO₂-H₂ atmosphere. The effects of the CO₂ mole fraction (FCO₂) and laser power (P_L) on the crystal phase, microstructure, and deposition rate (R_dep) were investigated. α- and γ-Al₂O₃ mixture films were prepared at P_L = 90 W (deposition temperature of 818 K), whereas (0 0 6)-oriented single-phase α-Al₂O₃ films were obtained at P_L = 110 W (863 K). The texture coefficient and the grain size of the (0 0 6)-oriented films increased with increasing FCO₂. The orientation of the α-Al₂O₃ films changed from (0 0 6) to (1 0 4) to (0 1 2) with increasing P_L (T_dep). The R_dep of the (0 0 6)-oriented α-Al₂O₃ films increased with increasing FCO₂.     

Performance of a finned-tube evaporator optimized for different refrigerants and its effect on system efficiency

This paper presents a comparable evaluation of R600a (isobutane), R290 (propane), R134a, R22, R410A, and R32 in an optimized finned-tube evaporator, and analyzes the impact of evaporator effects on the system coefficient of performance (COP). The study relied on a detailed evaporator model derived from NIST's EVAP-COND simulation package and used the ISHED1 scheme employing a non-Darwinian learnable evolution model for circuitry optimization. In the process, 4500 circuitry designs were generated and evaluated for each refrigerant. The obtained evaporator optimization results were incorporated in a conventional analysis of the vapor compression cycle. For a theoretical cycle analysis without accounting for evaporator effects, the COP spread for the studied refrigerants was as high as 11.7%. For cycle simulations including evaporator effects, the COP of R290 was better than that of R22 by up to 3.5%, while the remaining refrigerants performed approximately within a 2% COP band of the R22 baseline for the two condensing temperatures considered.     

Thermal conductivity of gaseous fluorocarbon refrigerants R 12, R 13, R 22, and R 23, under pressure

The thermal conductivity of four gaseous fluorocarbon refrigerants has been measured by a vertical coaxial cylinder apparatus on a relative basis. The fluorocarbon refrigerants used and the ranges of temperature and pressure covered are as follows: R 12 (Dichlorodifluoromethane CCl₂F₂): 298.15–393.15 K, 0.1–4.28 MPa R 13 (Chlorotrifluoromethane CClF₃): 283.15–373.15 K, 0.1–6.96 MPa R 22 (Chlorodifluoromethane CHClF₂): 298.15–393.15 K, 0.1–5.76 MPa R 23 (Trifluoromethane CHF₃): 283.15–373.15 K, 0.1–6.96 MPaThe apparatus was calibrated using Ar, N₂, and CO₂ as the standard gases. The uncertainty of the experimental data is estimated to be within 2%, except in the critical region. The behavior of the thermal conductivity for these fluorocarbons is quite similar; thermal conductivity increases with increasing pressure. The temperature coefficient of thermal conductivity at constant pressure, (/T) _p , is positive at low pressures and becomes negative at high pressures. Therefore, the thermal conductivity isotherms of each refrigerant intersect each other in a specific range of pressure. A steep enhancement of thermal conductivity is observed near the critical point. The experimental results are statistically analyzed and the thermal conductivities are expressed as functions of temperature and pressure and of temperature and density.     

Flow condensation heat transfer coefficients of R22, R134a, R407C, and R410A inside plain and microfin tubes

Flow condensation heat transfer coefficients (HTCs) of R22, R134a, R407C, and R410A inside horizontal plain and microfin tubes of 9.52 mm outside diameter and 1 m length were measured at the condensation temperature of 40 °C with mass fluxes of 100, 200, and 300 kg m⁻² s⁻¹ and a heat flux of 7.7–7.9 kW m⁻². For a plain tube, HTCs of R134a and R410A were similar to those of R22 while HTCs of R407C are 11–15% lower than those of R22. For a microfin tube, HTCs of R134a were similar to those of R22 while HTCs of R407C and R410A were 23–53% and 10–21% lower than those of R22. For a plain tube, our correlation agreed well with the present data for all refrigerants exhibiting a mean deviation of 11.6%. Finally, HTCs of a microfin tube were 2–3 times higher than those of a plain tube and the heat transfer enhancement factor decreased as the mass flux increased for all refrigerants tested.     

Microstructure and mechanical properties of ZrB2-SiC composites

A ZrB₂-based composite containing 20 vol.% nanosized SiC particles (ZSN) was fabricated at 1900 °C for 30 min under a uniaxed load of 30 MPa by hot-pressing. The microstructure and mechanical properties of the composite were investigated. It was shown that the grain growth of ZrB₂ matrix was effectively suppressed by submicrosized SiC particles located along the grain boundaries. In addition, the mechanical properties of ZSN composite were strongly improved by incorporating the nanosized SiC particles into a ZrB₂ matrix, especially for flexural strength (925 ± 28 MPa) and fracture toughness (6.4 ± 0.3 MPa•m^1/2), which was much higher than that of monolithic ZrB₂ and ZrB₂-based composite with microsized SiC particles, respectively. The formation of intragranular nanostructures plays an important role in the strengthening and toughening of ZrB₂ ceramic.     

Spray pyrolysis deposition of indium sulphide thin films

In₂S₃ thin films were grown by the chemical spray pyrolysis (CSP) method using the pneumatic spray set-up and compressed air as a carrier gas. Aqueous solutions containing InCl₃ and SC(NH₂)₂ at a molar ratio of In/S = 1/3 and 1/6 were deposited onto preheated glass sheets at substrate temperatures T_s = 205-410 °C. The obtained films were characterised by X-ray diffraction (XRD), scanning electron microscopy (SEM,) optical transmission spectra, X-ray photoelectron spectroscopy (XPS) and energy dispersive spectroscopy (EDS). According to XRD, thin films deposited at T_s = 205-365 °C were composed of the (0 0 12) orientated tetragonal β-In₂S₃ phase independent of the In/S ratio in the spray solution. Depositions performed at T_s = 410 °C led to the formation of the In₂O₃ phase, preferably when the 1/3 solution was sprayed. Post-deposition annealing in air indicated that oxidation of the sulphide phase has a minor role in the formation of In₂O₃ at temperatures up to 450 °C. In₂S₃ films grown at T_s below 365 °C exhibited transparency over 70% in the visible spectral region and E_g of 2.90-2.96 eV for direct and 2.15-2.30 eV for indirect transitions, respectively. Film thickness and chlorine content decreased with increasing deposition temperatures. The XPS study revealed that the In/S ratio in the spray solution had a significant influence on the content of oxygen (Me-O, BE = 530.0 eV) in the In₂S₃ films deposited in the temperature range of 205-365 °C. Both XPS and EDS studies confirmed that oxygen content in the films deposited using the solution with the In/S ratio of 1/6 was substantially lower than in the films deposited with the In/S ratio of 1/3.     

Performance analysis of desiccant dehumidification systems driven by low-grade heat source

If a desiccant dehumidification system can be driven by a heat source whose temperature is below 50 °C, exhaust heat from devices such as fuel cells or air conditioners can be used as its heat source, thereby saving energy. Therefore, in this study, we used a previously validated simulation model to determine the minimum heat source temperature for driving a desiccant dehumidification system. We considered four desiccant dehumidification systems that can be driven by waste heat—conventional desiccant-type systems (wheel type and batch type with only desiccant), a system with a precooler, double-stage-type systems (a type with two desiccant wheels and a four-partition desiccant wheel type), and a batch-type system with an internal heat exchanger. We found that among these systems, the last system can be driven by the lowest heated air temperature—approximately 33 °C—which is considerably lower than that of the conventional system.     

Silica materials recovered from photonic industrial waste powder: its extraction, modification, characterization and application

This study explored the possibility of recovering waste powder from photonic industry into two useful resources, sodium fluoride (NaF) and the silica precursor solution. An alkali fusion process was utilized to effectively separate silicate supernatant and the sediment. The obtained sediment contains purified NaF (>90%), which provides further reuse possibility since NaF is widely applied in chemical industry. The supernatant is a valuable silicate source for synthesizing mesoporous silica material such as MCM-41. The MCM-41 produced from the photonic waste powder (PWP), namely MCM-41(PWP), possessed high specific surface areas (1082 m²/g), narrow pore size distributions (2.95 nm) and large pore volumes (0.99 cm³/g). The amine-modified MCM-41(PWP) was further applied as an adsorbent for the capture of CO₂ greenhouse gas. Breakthrough experiments demonstrated that the tetraethylenepentamine (TEPA) functionalized MCM-41(PWP) exhibited an adsorption capacity (82 mg CO₂/g adsorbent) of only slightly less than that of the TEPA/MCM-41 manufactured from pure chemical (97 mg CO₂/g adsorbent), and its capacity is higher than that of TEPA/ZSM-5 zeolite (43 mg CO₂/g adsorbent). The results revealed both the high potential of resource recovery from the photonic solid waste and the cost-effective application of waste-derived mesoporous adsorbent for environmental protection.     

Determination of photo-catalytic activity of un-doped and Mn-doped TiO2 anatase powders on acetaldehyde under UV and visible light

Titanium dioxide (TiO₂) photocatalytic powder materials doped with various levels of manganese (Mn) were synthesized to be used as additives to wall painting in combating indoor and outdoor air pollution. The heterogeneous photocatalytic degradation of gaseous acetaldehyde (CH₃CHO) on Mn-TiO₂ surfaces under ultraviolet and visible (UV/Vis) irradiation was investigated, by employing the Photochemical Static Reactor coupled with Fourier-Transformed Infrared spectroscopy (PSR/FTIR) technique. Experiments were performed by exposing acetaldehyde (~ 400 Pa) and synthetic air mixtures (~ 1.01 × 10⁵ Pa total pressure) on un-doped TiO₂ and doped with various levels of Mn (0.1-33% mole percentage) under UV and visible irradiation at room temperature. Photoactivation was initiated using either UV or visible light sources with known emission spectra. Initially, the photo-activity of CH₃CHO under the above light sources, and the physical adsorption of CH₃CHO on Mn-TiO₂ samples in the absence of light were determined prior to the photocatalytic experiments. The photocatalytic loss of CH₃CHO on un-doped TiO₂ and Mn-TiO₂ samples in the absence and presence of UV or visible irradiation was measured over a long time period (≈ 60 min), to evaluate their relative photocatalytic activity. The gaseous photocatalytic end products were also determined using absorption FTIR spectroscopy. Carbon dioxide (CO₂) was identified as the main photocatalysis product. It was found that 0.1% Mn-TiO₂ samples resulted in the highest photocatalytic loss of CH₃CHO under visible irradiation. This efficiency was drastically diminished at higher levels of Mn doping (1-33%). The CO₂ yields were the highest for 0.1% Mn-TiO₂ samples under UV irradiation, in agreement with the observed highest CH₃CHO decomposition rates. It was demonstrated that low-level (0.1%) doping of TiO₂ with Mn results in a significant increase of their photocatalytic activity in the visible range, compared to un-doped TiO₂. This elevated activity is lost at high doping levels (1-33%). Finally, the photocatalytic degradation mechanism of CH₃CHO on 0.1% Mn-TiO₂ surfaces under visible irradiation leading to low CO₂ yields is different than that under UV irradiation resulting to high CO₂ yields.     

Synthesis and mechanical properties of high-purity Cr2AlC ceramic

High-purity and dense Cr₂AlC has been successfully fabricated by hot-pressing, using Cr, Al and graphite as raw materials. Delamination, kink bands, monolamellar kink, transgranular crack and transgranular fracture of bulk Cr₂AlC are found during the room-temperature test. The density, Vickers hardness, flexural strength, Young's modulus, compressive strength and fracture toughness of the Cr₂AlC are 5.17 g/cm³, 4.9 GPa, 469 ± 27 MPa, 282 GPa, 949 ± 22 MPa and 6.22 ± 0.26 MPa m^1/2, respectively. The strength of Cr₂AlC could be greatly improved by second phase of Cr₇C₃. And the slipping of basal planes and slip system cold be hindered by Cr₇C₃, thus resulting in a lower toughness.     

Effect of pressure on the electrodeposition of nanocrystalline Ni-C in supercritical CO2 fluid

Electrodeposition of Ni in a Watt's bath at different applied pressure, and in the presence of CO₂ fluid was investigated. The reduction of carbon and its alloying into the Ni deposit was focused. The current efficiency of electrodeposition and the carbon content in the Ni deposit were found to vary with the applied pressure. The crystal structure of the resulting Ni-C film was characterized by performing X-ray diffraction. The composition of the deposit was analyzed using X-ray photoelectron spectroscopy. Transmission electron microscopy was employed for microstructure analysis. The results showed that nanocrystalline Ni-C deposit could be obtained. The grain size of the Ni-C film varied from 14 to 43 nm, depending on the deposition pressure and carbon content. A significant increase in microhardness from 450 to 720 Hv could be obtained for the Ni film electrodeposited from a bath of 15 MPa supercritical CO₂ fluid. In 1 M HCl solution, a higher open circuit potential and a lower anodic current density were found when the carbon content in the Ni deposit was increased.     

Mathematical modeling and thermodynamic investigation of the use of two-phase ejectors for work recovery and liquid recirculation in refrigeration cycles

This paper presents the results of a numerical investigation on the performance of ejector cycles in which the work recovered is used to recirculate liquid through the evaporator. The ejector recirculation cycle, in which the ejector is only used to recirculate liquid and improve evaporator performance, and the standard ejector cycle, in which the ejector can be used to both recirculate liquid and directly unload the compressor, are investigated. The analysis uses a microchannel evaporator and refrigerants R134a, R410A, and CO₂. It is seen that fluids that have large throttling loss but gain little benefit from liquid recirculation (CO₂) should use the ejector to directly unload the compressor, while fluids that have lower throttling loss but gain significant benefit from liquid recirculation (R134a) should use the ejector to improve evaporator performance through liquid recirculation. It is also seen that the ejector recirculation cycle is better suited for ejector off-design operation.     

Diesel emission control system using combined process of nonthermal plasma and exhaust gas components' recirculation

A NO_x aftertreatment system, using nonthermal plasma (NTP) reduction and exhaust gas components' recirculation, is investigated. A pilot-scale system is applied to a stationary diesel engine. In this system, NO_x is first removed by adsorption, and subsequently, the adsorbent is regenerated by thermal desorption. NO_x desorbed is reduced by using nitrogen NTP. Moreover, NO_x, CO₂, and water vapor recirculated into the engine intake reduce NO_x. In this study, approximately 57% of the NO_x of the exhaust (NO_x: 240-325 ppm, flow rate = 300 NL/min) can be continuously treated for 58 h. A system energy efficiency of 120 g (NO₂)/kWh is obtained.     

Preparation and properties of nano-SiC strengthening Al2O3 composite ceramics

The processing and mechanical behaviors of Al₂O₃-xwt.%SiC (x = 1, 2, 5, ASx) nano-composites prepared by the in situ synthesis of SiC from polycarbosilane (PCS) were investigated. The composites were densified by hot pressing. The microstructure and mechanical properties of the sintered composites were analyzed. The results showed that a fully dense structure was obtained when a few nano-SiC were doped and that the fracture toughness and strength were highly improved compared with those of monolithic Al₂O₃. The fracture toughness reached 5.1 MPa m^1/2 in AS2 composite. The maximum flexural strength was 516 MPa obtained in AS1 composite.     

The influence of Na2CO3 content and Ni concentration on the physicochemical properties of nanometer Y-substituted nickel hydroxide

Nanometer Y-substituted nickel hydroxide was prepared by supersonic co-precipitation method with Na₂CO₃ as a buffer and NiCl₂ as a nickel source. The crystal structure, morphology, particle size distribution and electrochemical performance affected by the buffer (Na₂CO₃) content and Ni²⁺ concentration are characterized. The results indicate most of the samples are co-existence with α and β phases and the proportion of α-Ni(OH)₂ increases with the increase of Na₂CO₃, but decreases with the increase of Ni²⁺ concentration. The primary particles of samples are nanometer particles and the shape of primary particles transform from acicular to quasi-spherical with increasing Na₂CO₃ content, but converse process for the increase of Ni²⁺ concentration. The average particle size decreases initially and then increases. Complex electrodes were prepared by mixing 8 wt.% nickel hydroxides with commercial micro-size spherical nickel. The discharge capacities of samples increase initially and then decrease with increasing Na₂CO₃ content or decreasing Ni²⁺ concentration. When Na₂CO₃ content is 0.08 g and Ni²⁺ concentration is 0.2 mol/L, the sample has better electrochemical performance, such as larger discharge capacity (316.3 mAh/g at 0.2 C rate), lower charge voltage and higher discharge plateau, than those of other samples.