Effect of operating temperatures on the coefficient of performance of active carbon-methanol systems

Experimental and predicted results of an active carbon (AC35)-methanol pair as a function of operating temperatures are discussed.Experimental COP's higher than 0.5 are obtained when the evaporating temperature lift is less than 25°C. The COP is seen to be very sensitive to the evaporating and adsorbing temperatures. The regenerating temperature is of the order of 100°C which makes that pair a good candidate for solar cooling applications.A comparison between the results obtained with that pair and predicted performances of three other pairs (NH₃H₂O, ZeoliteH₂O and another active carbon-methanol pair) shows that:

• 1.(1) two adsorber cycles give higher COP's than the NH₃H₂H₂O continuous cycle;
• 2.(2) intermittent adsorptive cycles give higher COP's than the NH₃H₂O continuous cycle as long as the evaporating temperature lift is less than 50°C;
• 3.(3) among the solid adsorbents, the active carbon-methanol pairs seem to be the most promising for cooling applications. The choice of the active carbon depends on the evaporating temperature lift: AC35 is very well adapted for temperature lifts larger than 30°C; ACLH could be better adapted to smaller evaporating temperature lifts, as suggested by Passos et al.

     

Thermodynamic cycle,correlations and nomograph for NH3NaSCN absorption refrigeration systems

The detailed thermodynamic cycle of the NH₃NaSCN absorption refrigeration unit is presented, based on the thermodynamic properties of the working media. Correlations are developed, which express the coefficient of performance and the cooling capacity in terms of the required evaporation temperature, T_ev, and the available ambient temperature, T_amb. A nomograph is also presented, which shows in a compact form the behaviour of the NH₃NaSCN system and allows direct estimation of its main characteristics. It is concluded that if (T_amb − T_ev) varies from 0 to 40°C, the theoretical coefficient of performance decreases linearly from 95 to 77%. For the same range of (T_amb − T_ev) the theoretical cooling capacity varies from 1150 to 1300 kJ/kg NH₃ if T_ev varies from 0 to −15°C. Under the conditions examined, for T_amb − T_ev > 23°C, the coefficient of performance of the NH₃NaSCN system becomes higher than that of the NH₃LiNO₃ system. The observed increase reached 4% at T_amb − T_ev = 40°C.     

Cl35 NQR study of the structural phase transition in (CH3NH3)HgCl3

The Cl³⁵ nuclear quadrupole resonance spectra of (CH₃NH₃)HgCl₃ have been measured between -150°C and + 100°C. The spectra clearly show that a structural phase transition of first order takes place around T_c? 60°C. The transition may be related to a disordering of the CH₃NH₃ groups which are reorienting both above and below T_c. The positive temperature coefficient of the Cl NQR frequency, dv/dT may be also explained by the CH₃NH₃ motion.     

EPR of cadmium ferric voltaite above the ferrimagnetic transition

A single isotropic EPR line of Fe³⁺ in synthetic cadmium ferric voltaite, (NH₄)₂Cd₅Fe₃Al(SO₄)₁₂ · 18H₂O, was observed in a wide temperature range from 295 to 1.57°K. The ferrimagnetic transition temperature of CdFe voltaite was determined to be ～ 0.7°K using the temperature dependence of the g-factor and the line width. The cubic crystal field parameter, a, for Fe³⁺ in CdFe voltaite is extracted from the EPR line width measurement using the exchange-narrowed line width model of Anderson and Weiss. The parameter a for Fe³⁺ in CdFe voltaite at 4.2°K is 157 × 10^-4cm^-1 which is consistent with the corresponding values for Fe³⁺ in other cubic structures.     

Conceptual thermodynamic design of a dual mode absorption cycle with an auxiliary heat exchanger

This paper presents a conceptual thermodynamic design and feasibility study of a dual mode absorption cycle with auxiliary heat exchanger. The system consists of two single stage absorption cycles connected through the condenser of the first stage with the generator of the second stage. The purpose of the auxiliary heat exchanger is to maintain the condenser (1) temperature sufficiently high to operate the generator (2). A case study, using a single fluid pair H₂OLiBr in both the stages and two fluid pairs (H₂OLiBr at first stage and NH₃LINO₃ at second stage) has been considered. It is found that the COP for the two pair system in comparison to the single pair system. However, the former has the advantage of providing simultaneous refrigeration and air-conditioning.     

Performance prediction of an absorption heat pump for utilization in industry

Recently, an absorption heat pump has been put on the market for industrial utilization. It is a H₂O-LiBr absorption machine which heats up water to 90°C with a cold source at 40°C. The capacity and COP of the machine have been studied by a computer program as a function of cold source temperature and heated fluid temperature. Also, a comparison with a compression heat pump is reported.     

Synthesis of superconducting compounds by thermolysis of volatile hydrides and organometallic compounds on glowing wires

The synthesis of high temperature superconducting phases in the NbGe, NbSn, VSi, VGe, VSn, NbC and MoC systems is described by method consisting in the thermolysis of volatile hydrides or organometallic compounds on resistively heated wires. For face-centred cubic NbC a higher transition temperature than previously reported was obtained. The A15 phase boundary of NbGe is extended towards the stoichiometric 3:1 composition, affording samples of Nb₃ Ge with a T_c onset of 15.8°K.     

Estimation of solar collector area and thermal energy requirement in absorber heat recovery cycle

The present communication includes a thermodynamic evaluation of the solar collector area required and thermal energy needed for a fully solar-dependent vapour absorption system. A comparative study of the absorber heat recovery cycle and the basic absorption cycle has been made in terms of percentage decrease in solar collector area, amount of heat rejection and thermal energy required in Watt thermal equivalent per 1 kW of cooling. Three refrigerant absorbent combinations, namely H₂OLiBr, NH₃H₂O and NH₃LiNO₃ are used for detailed analysis of the absorber heat recovery cycle for solar cooling purposes.     

P−T phase diagram for ferromagnetic semiconductor CdCr2Se42

The P?T phase diagram associated with the ferromagnetic semiconductor CdCr₂Se₄ was determined between 600 and 964°C for the first time from the P_{Se2(or Cd)} ? T diagram for the stability of the compound disclosed by annealing experiments under controlled Se₂ and Cd vapor pressures, and from the phase equilibria in the CdSeCrSeSe system examined by usual annealing experiments. The following characteristic features are also clarified. (1) The maximum temperature for the stability of CdCr₂Se₄ is about 900°C. (2) CdCr₂Se₄ dissolves into Se melt and is in equilibrium with the Se-rich melt which has a CdSe content in excess of CdSe·Cr₂Se₃. The solubility decreases rapidly as temperature falls from 900 to 860°C. (3) Cr₂Se₃ is the only compound in the CrSe system in equilibrium with CdCr₂Se₄ above about 860°C. Both Cr₃Se₄ and Cr₂Se₃ exist below this temperature. (4) The P_Se2 range for the stability of CdCr₂Se₄ is estimated as a measure of the concentration ranges of native defects due to nonstoichiometry. It increases as temperature decreases and is about 5 × 10⁹ at 600°C.     

Thermodynamic analysis and design data for a double-effect absorption heat pump system using four working pairs

Performance analysis of a double-effect absorption heat pump system has been done for water-four working pairs (or mixture) by computer simulation. The coefficient of performance and mass flow ratio are investigated to compare these aqueous solutions [waterLiCl, waterLiBrLiBr waterLiClCaCl₂Zn(NO₃)₂] which was developed for only cooling, with conventional waterLiBr solution, based on mass, material and heat balance equations for each part.From this analysis, it is found that the performances of the new aqueous solutions are better than that of LiBrwater solution not only in cooling systems, but also in heating systems, although the operating temperature ranges of these new aqueous solutions are very narrow in heating. Theoretical thermodynamic performance data can be used and are given here by design data.     

Microwave spectrum of dimethyldichlorosilane

The microwave spectrum of dimethyldichlorosilane has been observed and the rotational constants and centrifugal distortion constants have been determined for ³⁵Cl₂ and ³⁵Cl³⁷Cl species. From these constants, the molecular structure is determined as $\text{r(}\text{SiCl}\text{) = 2.055 ± 0.003}\text{A}\text{?}$, $\text{r(}\text{SiC}\text{) = 1.845 ± 0.005}\text{A}\text{?}$, ∠ClSiCl = 107.2 ± 0.3°, ∠CSiC = 114.7 ± 0.3°. An analysis of the ³⁵Cl₂ quadrupole splittings leads to quadrupole coupling constants of χ_aa = ?19.6 ± 0.3 MHz, χ_bb = ?3.7 ± 1.4 MHz, χ_cc = 23.3 ± 1.4 MHz, χ_bond = ?38.0 ± 1.6 MHz, and η_bond = 0.22 ± 0.08.     

Absorption panel cooling using a TFE-NMP mixture

A panel system for cooling buildings is proposed, working with an absorption-refrigeration unit, the evaporator of which is the panel itself. Trifluoroethanol (TFE) and N-methylpyrrolidinone (NMP) are proposed as refrigerant and absorbent, respectively. Use of the TFE-NMP mixture seems to offer advantages over the usual H₂OLiBr or NH₃H₂O mixtures, and suits well the modest refrigeration temperatures needed in panel cooling. Analytical expressions are presented for the properties of the TFE-NMP mixture and a calculation procedure for NH₃H₂O units is used after suitable modifications to simulate the operation of the TFE-NMP absorption-refrigeration unit. Under the conditions considered, the lowest evaporation temperature varies from 3 to 16°C, and the theoretical coefficient of performance from 60% to 68.4%, with its maximum value obtained at 9°C. Under the above conditions, for panel cooling power 50–150 W/m², a refrigerant mass flow rate through the panel from 400 g/m² h to 1600 g/m² h is needed with a driving thermal power from 80 W/m² to 220 W/m².     

Performance of a Single-Family Heat Pump at Different Working Conditions Using Small Quantity of Propane as Refrigerant

The performance of a domestic heat pump that uses a low quantity of propane as refrigerant has been experimentally investigated. The heat pump consists of two minichannel aluminium heat exchangers, a scroll compressor, and an electronic expansion valve. It was charged with the minimum amount of refrigerant propane required for the stable operation of the heat pump without permitting refrigerant vapor into the expansion valve at incoming heat source fluid temperature to the evaporator of +10°C. The inlet temperature of the heat source fluid passing through the evaporator was varied from +10°C to ?10°C while holding the condensing temperature constant at 35°C, 40°C, 50°C, and 60°C, respectively. The minimum refrigerant charges required at above-tested condensing temperatures were found to decrease when the condensing temperature increased and were recorded as 230 g, 224 g, 215 g, and 205 g, respectively. The results confirm that a heat pump with 5 kW capacity can be designed with less than 200 g charge of refrigerant propane in the system. Due to the high solubility of propane in compressor lubrication oil, the amount of refrigerant which may escape rapidly in case of accident or leakage is less than 150 g.     

Laser-induced Raman scattering in calcium titanate

The Raman spectrum of polycrystalline calcium titanate prepared by a liquid mix technique and heated to 800°C has been recorded at room temperature using an argon-ion laser as exciter. The observed spectrum was interpreted on the basis of factor-group C_2V. Not all of the Raman active modes predicted by factor group analysis were observed and this could be due to: over-lapping of bands, or very low polarizabilities of some of the modes or masking of the weak bands by intense bands. The band at 639 cm^?1 is tentatively assigned to the TiO symmetric stretching vibration (γ₁) and the bands at 495 and 471 cm^?1 to torsional modes. The bands in the region 180–340 cm^?1 are assigned to the OTiO bending modes and the 155 cm^?1 band to the Ca(TiO₃) lattice mode. The observed Raman bands are compared with the available infrared absorption data and, as expected, some coincidences in frequencies are seen for this compound with a noncentrosymmetric structure.     

Optically stimulated heating using Nd3+ doped NaYF4 colloidal near infrared nanophosphors

Although efficient in heat generation, gold nanoparticles dedicated for photostimulated localized hyperthermia treatment (LHT) lack luminescent properties suitable for detection in heterogeneous and autofluorescent tissue. Here, we study and report the use of bifunctional luminescent neodymium (Nd³⁺) ions doped ??-NaYF₄ colloidal nanoparticles as potential nanoheaters suitable for LHT. Up to 35°C (0.8°C/mW@514.5?nm) temperature rise in ??0.5?ml colloidal 25%Nd³⁺:NaYF₄ solution was achieved in comparison to around a 4°C rise for the undoped colloidal NaYF₄. The maximum temperature (T _max?) was linearly proportional to the concentration of Nd³⁺ dopant. The time required to elevate temperature to 1/e×T _max? varied from 100 to 135 seconds. The proposed approach gives premises to the construction of multi-functional therapeutic agents detectable by means of fluorescence molecular imaging.     

Determination of A0 for CH335Cl and CH337Cl from the ν4 infrared and Raman bands

The ν₄ infrared and Raman bands of CH₃Cl were analyzed simultaneously. A direct fit yielded a complete set of constants for CH₃³⁵Cl, including A₀ = 5.20530 ± 0.00010 cm^?1 and D_K = (8.85 ± 0.13) × 10^?5cm^?1. For CH₃³⁷Cl an incomplete set of constants was obtained from the infrared band, and A₀ = 5.2182 ± 0.0010 cm^?1 was estimated by curve fitting of the Raman spectrum. The resulting equilibrium structure is $\text{r(}\text{CH) = 1.0854 ± 0.0005}\text{A}\text{?}$, $\text{r(}\text{CCl) = 1.7760 ± 0.0003}\text{A}\text{?}$, and <(HCH) = 110°.35 ± 0°.05.     

Microwave spectrum,centrifugal distortion,substitution structure,and dipole moment of sulfine,CH2SO

The microwave spectrum of the reactive species sulfine (CH₂SO) has been studied. Assignments of 86 transitions of the ground vibrational state normal isotopic species, with J up to 60, have allowed a thorough centrifugal distortion analysis. With planarity implied by the I_c-I_a-I_b value of 0.1333 amu $\text{A}\text{?}{}^2$, spectral assignments of seven other isotopic modifications have resulted in the following substitution bond lengths and angles: CH_syn = 1.085 Å, CH_anti = 1.077 Å, CS = 1.610 Å, SO = 1.469 Å, ?HCH = 121.86^°, ?SCH_syn = 122.51^°, ?SCH_anti = 115.63^°, and ?CSO = 122.51^°. From Stark effect measurements of the normal and d₂ species, the dipole moment has been determined to be 2.994 D, oriented 25.50° relative to the SO bond and 9.61° relative to the normal species “a” axis. At an initial pressure of 30 mTorr in a clean brass waveguide, the lifetime of sulfine at 25°C is ～30 min.     

X-ray investigation of the transition I→II of NH4I at temperatures between 22° and −163°C

The lattice parameters of modifications I and II of NH₄I at temperatures between 22° and ?39°C were determined at twelve different temperatures by using the same Debye-Scherrer method as in the case of NH₄Br[1]. At the assumed transition temperature, ?16°C, the molar volume was found to increase by 16.96% with increasing temperature. By using a low temperatureX-ray diffractometer, the half time (50% mod. I and 50% mod. II) of the transition I→II of NH₄I was determined at different temperatures between ?23° and ?163 C° for four different particle sizes. It was found that, closely below the transition temperature, the needed supercooling increased with decreasing particle size.     

Kinetic and thermodynamic considerations for oxygen absorption/desorption using cobalt oxide

Cobalt oxide absorbs oxygen as the high-temperature rock-salt CoO structure is transformed into the lower-temperature cubic Co₃O₄ spinel. The absorption and desorption processes are reversible, which makes the material a potential candidate for use in temperature and/or pressure swing absorption modes for the production of oxygen. TGA/DTA experiments were conducted to quantify the kinetics and thermodynamics of the reaction. Over the temperature range investigated, both absorption and desorption were highly dependent on the thermodynamic driving force with faster kinetics occurring as ΔG became more negative. The reaction kinetics were primarily controlled by heat transfer and the thermodynamic driving force. Kinetic data suggest that cobalt oxide of 1–5 μm particle size can be oxidized or reduced at temperatures 40° above or below 890 °C in air, in less than 10 min at a bed thickness of 10 mm. A large change in enthalpy (ΔH ≈ 195 kJ/mol) as Co₃O₄ is converted to CoO affects heat transfer and the economy of producing oxygen by this method.     

Investigation of the temperature dependence for the spectra of supercooled water in the middle infrared

The temperature dependence of the bending ν₂, combination ν₂ + ν _L , and stretching (ν₁, ν₃, 2ν₂) absorption bands in the infrared spectra of supercooled water with a temperature-change step Δt from 2 to 2.5°C was studied using an advanced infrared Fourier spectrometer. It was found that the frequency of the maximum of the stretching absorption band (2700–3700 cm^?1) decreases with the reduction of the water temperature from ?0.5 to ?5.0°C. The frequency of the maximum of the combination absorption band (2130 cm^?1) increases with the reduction of the water temperature in a range from ?3.0 to ?5.0°C. The frequency of the maximum of the absorption band of bending oscillation (1640 cm^?1) is invariable with a reduction of the water temperature from ?0.5 to ?5.0°C.