Theoretical evaluation of trans-critical CO2 systems in supermarket refrigeration. Part II: System modifications and comparisons of different solutions

The performance of CO₂ refrigeration systems strongly depends on the operating conditions. The specific characteristics of low critical temperature and high operating pressure limit its applications and imply the implementation of different control strategies. This study compares the performance of different CO₂ system solutions for supermarket refrigeration with R404A system. Some possible modifications and improvements on the CO₂ system have been investigated. The COP of the investigated CO₂ system solution can be improved by about 3–7% along the ambient temperature range of 10–40 °C. The annual energy consumption calculations in three different climates; cold, moderate and hot, show that the centralized trans-critical CO₂ system is good solution for cold climates whereas the NH₃–CO₂ cascade system has the lowest energy consumption in hot climates. Both systems proved to be good alternatives to R404A DX system for supermarket refrigeration.     

Optimized transcritical CO2 heat pumps: Performance comparison of capillary tubes against expansion valves

A capillary tube based CO₂ heat pump is unique because of the transcritical nature of the system. The transcritical cycle has two independent parameters, pressure and temperature, unlike the subcritical cycle. In the present study, a steady state simulation model has been developed to evaluate the performance of a capillary tube based transcritical CO₂ heat pump system for simultaneous heating and cooling at 73 °C and 4 °C, respectively against optimized expansion valve systems. Capillary tubes of various configurations having diameters of 1.4, 1.5 and 1.6 mm along with internal surface roughness of 0.001–0.003 mm have been tested to obtain the optimum design and operating conditions. Subcritical and supercritical thermodynamic and transport properties of CO₂ are calculated employing a precision in-house property code.

It is observed that the capillary tube system is quite flexible in response to changes in ambient temperature, almost behaving to offer an optimal pressure control. System performance is marginally better with a capillary tube at higher gas cooler exit temperature. Capillary tube length turns out to be the critical parameter that influences system optimum conditions. A novel nomogram has been developed that can be employed as a guideline to select the optimum capillary tube.     

Residential CO2 heat pump system for combined space heating and hot water heating

A theoretical and experimental study has been carried out for a residential brine-to-water CO₂ heat pump system for combined space heating and hot water heating. A 6.5 kW prototype heat pump unit was constructed and extensively tested in order to document the performance and to study component and system behaviour over a wide range of operating conditions. The CO₂ heat pump was equipped with a unique counter-flow tripartite gas cooler for preheating of domestic hot water (DHW), low-temperature space heating and reheating of DHW.

The CO₂ heat pump was tested in three different modes: space heating only, DHW heating only and simultaneous space heating and DHW heating. The heat pump unit gave off heat to a floor heating system at supply/return temperatures of 33/28, 35/30 or 40/35 °C, and the set-point temperature for the DHW was 60, 70 or 80 °C. Most tests were carried out at an evaporation temperature of −5 °C, and the average city water temperature was 6.5 °C. The experimental results proved that a brine-to-water CO₂ heat pump system may achieve the same or higher seasonal performance factor (SPF) than the most energy efficient state-of-the-art brine-to-water heat pump systems as long as: (1) the heating demand for hot water production constitutes at least 25% of the total annual heating demand of the residence, (2) the return temperature in the space heating system is about 30 °C or lower, (3) the city water temperature is about 10 °C or lower and (4) the exergy losses in the DHW tank are small.     

Study of capillary tubes in a transcritical CO2 refrigeration system

Capillary tubes have been used in refrigeration systems for many years, but not with a transcritical CO₂ system. In this article, the effects of capillary tubes in a transcritical CO₂ refrigeration system have been investigated experimentally and theoretically. Different types of capillary tubes with different lengths (0.5–4 m) and diameters (1–2 mm) have been tested. The result of this work is a static model, which is used in the further work to make a simulation model (static) of a complete refrigeration system. The model is based on Friedel's and Colebrook's pressure drop correlations.

The behaviour of an adiabatic capillary tube in a refrigeration cycle has been investigated theoretically. The conclusion is that the COP of a system with capillary tubes generally is better than when a fixed high pressure is used, but not as good as when variable optimal high pressure is used. Capillary tubes are especially interesting in applications where the evaporation pressure is constant and the temperature out of the gas cooler varies no more than ±10 K from the design condition. The reduction in COP is more significant at low temperatures out of the gas cooler.     

Effect of negatively charged species on the growth behavior of silicon films in hot wire chemical vapor deposition

The deposition behavior of silicon in hot wire chemical vapor deposition was investigated, focusing on the generation of negatively charged species in the gas phase using a gas mixture of 20% SiH₄ and 80% H₂ at a 450 °C substrate temperature under a working pressure of 66.7 Pa. A negative current of 6–21 µA/cm² was measured on the substrate at all processing conditions, and its absolute value increased with increasing wire temperature in the range of 1400 °C–1900 °C. The surface roughness of the films deposited on the silicon wafers increased with increasing wire temperature in the range of 1510 °C–1800 °C. The film growth rate on the positively biased substrates (+ 100 V, + 200 V) was higher than that on the neutral (0 V) and negatively biased substrates (− 100 V, − 200 V, − 300 V). These results indicate that the negatively charged species are generated in the gas phase and contribute to deposition. The surface roughness evolved during deposition was attributed to the electrostatic interaction between these negatively charged species and the negatively charged growing surface.     

Pyroelectric properties of BST/PLT composite thick films with addition of xPbO–(1 − x)B2O3 glass

The Ba_xSr_1−xTiO₃ (BST)/Pb_1−xLa_xTiO₃ (PLT) composite thick films (20 μm) with 12 mol% amount of xPbO–(1 − x)B₂O₃ glass additives (x = 0.2, 0.35, 0.5, 0.65 and 0.8) have been prepared by screen-printing the paste onto the alumina substrates with silver bottom electrode. X-ray diffraction (XRD), scanning electron microscope (SEM) and an impedance analyzer and an electrometer were used to analyze the phase structures, morphologies and dielectric and pyroelectric properties of the composite thick films, respectively. The wetting and infiltration of the liquid phase on the particles results in the densification of the composite thick films sintered at 750 °C. Nice porous structure formed in the composite thick films with xPbO–(1 − x)B₂O₃ glass as the PbO content (x) is 0.5 ≥ x ≥ 0.35, while dense structure formed in these thick films as the PbO content (x) is 0.8 ≥ x ≥ 0.65. The volatilization of the PbO in PLT and the interdiffusion between the PLT and the glass lead to the reduction of the c-axis of the PLT phase. The operating temperature range of our composite thick films is 0–200 °C. At room temperature (20 °C), the BST/PLT composite thick films with 0.35PbO–0.65B₂O₃ glass additives provided low heat capacity and good pyroelectric figure-of-merit because of their porous structure. The pyroelectric coefficient and figure-of-merit F_D are 364 μC/(m² K) and 14.3 μPa^−1/2, respectively. These good pyroelectric properties as well as being able to produce low-cost devices make this kind of thick films a promising candidate for high-performance pyroelectric applications.     

Synthesis, phase transformation and dielectric properties of sol–gel derived Bi2Ti2O7 ceramics

Sol–gel derived Bi₂Ti₂O₇ ceramic powders have been prepared from methoxyethoxides of bismuth and titanium (molar ratio of Ti/Bi = 1.23 and water/alkoxides = 1.31). The Bi₂Ti₂O₇ phase was stable at a low temperature (700 °C), but it then transformed into mixed phases of Bi₄Ti₃O₁₂ and Bi₂Ti₄O₁₁ at 850–1150 °C. The single phase of Bi₂Ti₂O₇ reoccurred at 1200 °C. Dielectric properties and ferroelectric behavior of samples sintered at 1150 and 1200 °C were examined. Under frequency of 1 MHz, samples sintered at 1150 and 1200 °C had a dielectric constant of 101.3 and 104.2, and a loss tangent of 0.0193 and 0.0145, respectively. Only the sample sintered at 1150 °C showed ferroelectric behavior, where remanent polarization is 3.77 μC cm⁻² and coercive field is 24 kV cm⁻¹. Thus, the Bi₂Ti₂O₇ did not exhibit ferroelectricity, but the mixed phase of Bi₄Ti₃O₁₂ and Bi₂Ti₄O₁₁ did.     

Flow pattern and heat transfer characteristics during flow boiling of CO2 in a horizontal micro fin tube and comparison with smooth tube data

Flow pattern observations and measurements of the heat transfer in a helical grooved micro fin tube are presented and compared with results for a smooth tube. The micro fin tube used (OD of 9.52 mm) was a Wieland Cuprofin EDX tube with 60 fins (height 0.25 mm) and a helix angle of 18°. The flow pattern observations at 39.7 bar (T_s=+5 °C, p_r=0.54) and 26.4 bar (T_s=−10 °C, p_r=0.36) show a wide range of the annular flow region. The transition from slug to annular flow does not occur, as expected, at constant vapour quality for all mass fluxes but there is an interdependence between transition vapour quality and mass flux. For the heat transfer in the micro fin tube, measurements at 39.7 bar are presented for heat fluxes up to 120 kW m⁻², mass fluxes between 75 and 250 kg m⁻² s⁻¹ and vapour qualities between 0.1 and 0.9.     

A simple and an effective method for the fabrication of densified glass-ceramics of low temperature co-fired ceramics

This study reports a new, simple and effective pre-calcined method for fabrication BaO–TiO₂–B₂O₃–SiO₂ low temperature co-fired ceramics (LTCC) at a sintering temperature below 900 °C, and with dielectric losses (tan δ) lower than 2 × 10⁻³. The research results have shown that the addition of 2–5 wt% Al₂O₃ could easily eliminate the porosity of the glass-ceramics because of the excellent wetting behavior between alumina and the BaO–B₂O₃–SiO₂ glass liquid phase in the low temperature co-fired ceramic system.     

Influence of Mn doping on microstructure and DC-accelerated aging behaviors of ZnO–V2O5-based varistors

The microstructure, electrical properties, dielectric characteristics, and DC-accelerated aging behavior of the ZnO–V₂O₅–MnO₂ system sintered were investigated for MnO₂ content of 0.0–2.0 mol% by sintering at 900 °C. For all samples, the microstructure of the ZnO–V₂O₅–MnO₂ system consisted of mainly ZnO grain and secondary phase Zn₃(VO₄)₂. The incorporation of MnO₂ to the ZnO–V₂O₅ system was found to restrict the abnormal grain growth of ZnO. The nonlinear properties and stability against DC-accelerated aging stress improved with the increase of MnO₂ content. The ZnO–V₂O₅–MnO₂ system added with MnO₂ content of 2.0 mol% exhibited not only a high nonlinearity, in which the nonlinear coefficient is 27.2 and the leakage current density is 0.17 mA/cm², but also a good stability, in which %ΔE_1 mA = −0.6%, %Δ = −26.1%, and %Δtan δ = +22% for DC-accelerated aging stress of 0.85E_1 mA/85 °C/24 h.     

Destruction of carbon disulfide in aqueous solutions by sonochemical oxidation

Carbon disulfide (CS₂) is toxic to animals and aquatic organisms, and can also decompose to carbonyl sulfide (OCS) and hydrogen sulfide (H₂S) in aqueous environment. The kinetics of the sonochemical degradation of aqueous CS₂ was studied in a batch reactor at 20 kHz and 20 °C, and the effects of process parameters (e.g. concentration, ultrasonic intensity, irradiating gas) investigated. The concentrations of unbuffered CS₂ solutions used were (6.4–7.0)×10⁻⁴, 10.5×10⁻⁴ and (13.2–13.6)×10⁻⁴ M and the intensities were varied from 14 to 50 W. The reaction rate was found to be zero-order and the rate constant for the degradation at 20 °C and 14W in air was 21.1 μM/min using the largest initial concentration range studied. At the same initial concentration range but at 50 W (39.47 W/m²) the degradation rate of CS₂ was 46.7 μM/min, more than two times that at 14 W (11.04 W/m²). The rate of CS₂ sonochemical degradation in the presence of the different gases was in the order of He>air≥N₂O>Ar; the rate with helium was found to be about three times that of argon. The formation of sulfate (SO₄²⁻) as reaction product with air as the irradiating gas was enhanced in the presence of hydrogen peroxide (H₂O₂) and inhibited in the presence of 1-butanol. The sonochemical oxidation of CS₂ may prove to be an efficient and environmentally benign way for the removal of this hazardous pollutant from natural water and wastewater.     

A torsion–effusion study on the sublimation of Cu2Te

The sublimation of Cu₂Te was studied and its vapor pressure was measured by the torsion method. In the covered temperature range, the temperature dependence of the total vapor pressure can be expressed by the following equation: log p (kPa)=(6.17±0.20)−(11 680±300)(K/T) (1084–1234 K). The sublimation process of Cu₂Te is incongruent, and Te(g) and Te₂(g) being the only gaseous species present in the vapor, the partial sublimation reactions were studied: Cu₂Te(s)→2Cu(s)+Te(g), 2Cu₂Te(s)→4Cu(s)+Te₂(g). The standard sublimation enthalpies of these reactions, Δ_rH⁰(298)=254±10 and 248±8 kJ, respectively, were derived by the second- and third-law treatment of the pressure data. The heat of formation of Cu₂Te, Δ_fH⁰(298)=−43±6 kJmol⁻¹, was also derived.     

Sintering behavior and microwave dielectric properties of Bi3(Nb1−xTax)O7 solid solutions

Solid solutions of Bi₃(Nb_1−xTa_x)O₇ (x = 0.0, 0.3, 0.7, 1) were synthesized using solid state reaction method and their microwave dielectric properties were first reported. Pure phase of fluorite-type could be obtained after calcined at 700 °C (2 h)⁻¹ between 0 ≤ x ≤ 1 and Bi₃(Nb_1−xTa_x)O₇ ceramics could be well densified below 990 °C. As x increased from 0.0 to 1.0, saturated density of Bi₃(Nb_1−xTa_x)O₇ ceramics increased from 8.2 to 9.1 g cm⁻³, microwave permittivity decreased from 95 to 65 while Q_f values increasing from 230 to 560 GHz. Substitution of Ta for Nb modified temperature coefficient of resonant frequency τ_f from −113 ppm °C⁻¹ of Bi₃NbO₇ to −70 ppm °C⁻¹ of Bi₃TaO₇. Microwave permittivity, Q_f values and τ_f values were found to correlate strongly with the structure parameters of fluorite solid solutions and the correlation between them was discussed in detail. Considering the low densified temperature and good microwave dielectric proprieties, solid solutions of Bi₃(Nb_1−xTa_x)O₇ ceramics could be a good candidate for low temperature co-fired ceramics application.     

Theoretical evaluation of trans-critical CO2 systems in supermarket refrigeration. Part I: Modeling, simulation and optimization of two system solutions

Using CO₂ trans-critical system solutions in supermarket refrigeration is gaining interest with several installations already running in different European countries. Using a computer simulation model, this study investigates the performance of two main system solutions: centralized with accumulation tank at the medium temperature level and parallel with two separate circuits for low and medium temperature levels. Both system solutions are presented and the simulation model is described in details. Calculations have been performed to design the systems and optimize their performances where basic layout and size of each solution have been defined. For ambient temperature range of 10–40 °C, the reference centralized system solution shows higher COP of about 4–21% than the reference parallel solution. Using two-stage compression in the centralized system solution instead of single stage will result in total COP which is about 5–22% higher than that of the reference centralized system and 13–17% higher than that of the improved two-stage parallel system. The two-stage centralized system solution gives the highest COP for the selected ambient temperature range.     

Growth of lithium doped silver niobate single crystals and their piezoelectric properties

Lithium doped silver niobate (Ag_1−xLi_xNbO₃, 0 < x < 0.1) is one of the candidate materials for lead-free piezoelectric materials. In this study, Ag_1−xLi_xNbO₃ single crystals were successfully grown by a slow cooling method. Crystal structure was assigned to perovskite-type orthorhombic (monoclinic) phase. Dielectric properties were measured as a function of temperature. As a result, with increasing lithium contents, the phase transition at around 60 °C was shifted to lower temperature while the phase transition at around 400 °C was shifted to higher temperature. On the basis of these peak shifts, the lithium contents in Ag_1−xLi_xNbO₃ single crystals were determined. Moreover, P–E hysteresis measurement revealed that pure silver niobate crystal was weak ferroelectrics with P_r of 0.095 μC/cm² while Ag_0.9Li_0.1NbO₃ (ALN10) crystal was normal ferroelectrics with P_r of 10.68 μC/cm². About this ALN10 crystal, polling treatment was performed and finally piezoelectric properties were measured. As a result, high electromechanical coupling coefficient k₃₁ over 70% was observed.     

Optical absorption and emission properties of Er doped mixed alkali borosilicate glasses

This paper presents the optical absorption and luminescence properties of Er³⁺ doped mixed alkali borosilicate glasses: 59.5SiO₂ · 20B₂O₃ · xLi₂O · (20 − x)Na₂O · 0.5Er₂O₃ and 59.5SiO₂ · 20B₂O₃ · xLi₂O · (20 − x)K₂O · 0.5Er₂O₃, with x = 0, 4, 8, 12, 16 and 20 mol%. The variations of Judd–Ofelt intensity parameters (Ω₂, Ω₄, and Ω₆), hypersensitive transition intensities, total radiative transition probability (A_T), radiative lifetimes (τ_R), integrated absorption cross-sections (Σ) and stimulated emission cross-sections (σ_p) as a function of x are discussed in detail. The changes in Ω₂ and intensities of hypersensitive transitions are attributed to optical basicity changes in the host glass matrix, which leads to variations in the covalency of the Er–O bond. The luminescence properties are reported for certain transitions, and the emission cross-section is high at x = 8–12 in the case of lithium sodium glass, whereas in lithium potassium glass it is high at x = 8.     

Effect of silver addition on the properties of combustion synthesized nanocrystalline LiCoO2

Nanocrystalline (50 nm) LiCoO₂ powders containing 0–10 mol% of Ag have been prepared by combustion synthesis using citrate–nitrate combustion route. Thermal analyses show a sharp decomposition of the gel at 177 °C for pristine LiCoO₂. With addition of silver, the decomposition becomes sluggish and it completes only above 430 °C. X-ray powder diffraction analyses show an increase in lattice parameter, c, with increasing Ag content suggesting the occupation of Ag within LiCoO₂ interlayer spacings. Transmission electron microscopy indicates diffusion of Ag into LiCoO₂ grains. It has been observed that adding 1.0 mol% silver increases the room temperature electrical conductivity by more than two orders of magnitude (1.5 × 10⁻³ S cm⁻¹). Galvanostatic charge–discharge profiles of coin cells fabricated with the synthesized powders show a two-fold enhancement in the discharge capacity for 1.0 mol% Ag-added LiCoO₂ cathode (140 mAh g⁻¹) compared to that for pristine LiCoO₂ (70 mAh g⁻¹).     

Separation of CO2 and CH4 through two types of polyimide membrane

Two asymmetric polyimide membranes that have a thermal stability up to 550 K and good resistance to solvents were prepared. Some membrane-forming conditions such as coagulation solution, evaporation and immersion time were examined. Regarding the coagulation solution, a water–methanol mixture gives good membranes in eleven solutions tested. The permeabilities of two asymmetric polyimide membranes for CO₂ and CH₄ pure gases were determined at 25°C and at applied pressures up to 0.25 MPa. The prepared polyimide membranes, 3,3′,4,4′-biphenyltetracarboxylic dianhydride (BPDA)-4,4′-diaminodiphenyl ether (ODA) and 1,2,3,4-butanetetracarboxylic dianhydride (BDA)-4,4′-diaminodiphenyl ether (ODA) exhibited higher selectivity and permeability for CO₂/CH₄.     

Chemical vapour deposition of praseodymium oxide films on silicon: influence of temperature and oxygen pressure

Metal-organic chemical vapour deposition (MOCVD) of various phases in PrO_x system has been studied in relation with deposition temperature (450–750 °C) and oxygen partial pressure (0.027–100 Pa or 0.2–750 mTorr). Depositions were carried out by pulsed liquid injection MOCVD using Pr(thd)₃ (thd = 2,2,6,6-tetramethyl-3,5-heptanedionate) precursor dissolved in toluene or monoglyme. By varying deposition temperature and oxygen partial pressure amorphous films or various crystalline PrO_x phases (Pr₂O₃, Pr₇O₁₂, Pr₆O₁₁) and their mixtures can be grown. The pure crystalline Pr₂O₃ phase grows only in a narrow range of partial oxygen pressure and temperature, while high oxygen pressure (40–100 Pa) always leads to the most stable Pr₆O₁₁ phase. The influence of annealing under vacuum at 750 °C on film phase composition was also studied. Near 90% step coverage conformity was achieved for PrO_x films on structured silicon substrates with aspect ratio 1:10. In air degradation of Pr₂O₃ films with transformation to Pr(OH)₃ was observed in contrast to Pr₆O₁₁ films.     

Boiling heat transfer of R-22, R-134a, and CO2 in horizontal smooth minichannels

This study examined convective boiling heat transfer in horizontal minichannels using R-22, R-134a, and CO₂. The local heat transfer coefficients were obtained for heat fluxes ranging from 10 to 40 kW m⁻², mass fluxes ranging from 200 to 600 kg m⁻² s⁻¹, a saturation temperature of 10 °C, and quality up to 1.0. The test section was made of stainless steel tubes with inner diameters of 1.5 mm and 3.0 mm, and a length of 2000 mm. The section was heated uniformly by applying an electric current to the tubes directly. Nucleate boiling heat transfer was the main contribution, particularly at the low quality region. An increasing and decreasing heat transfer coefficient occurred at the lower vapor quality with increasing heat flux and mass flux. The mean heat transfer coefficient ratio of R-22:R-134a:CO₂ was approximately 1.0:0.8:2.0. Laminar flow was observed in the minichannels. A new boiling heat transfer coefficient correlation based on the superposition model for refrigerants in minichannels was developed with a mean deviation of 11.21%.