Review of advanced absorption cycles: performance improvement and temperature lift enhancement

The objective of this study is to propose and evaluate advanced absorption cycles for the coefficient of performance (COP) improvement and temperature lift enhancement applications. The characteristics of each cycle are assessed from the viewpoints of the ideal cycle COP and its applications. The advanced cycles for the COP improvement are categorized according to their heat recovery method: condensation heat recovery, absorption heat recovery, and condensation/absorption heat recovery. In H₂O–LiBr systems, the number of effects and the number of stages can be improved by adding a third or a fourth component to the solution pairs. The performance of NH₃–H₂O systems can be improved by internal heat recovery due to their thermal characteristics such as temperature gliding. NH₃–H₂O cycles can be combined with adsorption cycles and power generation cycles for waste heat utilization, performance improvement, panel heating and low temperature applications. The H₂O–LiBr cycle is better from the high COP viewpoints for the evaporation temperature over 0°C while the NH₃–H₂O cycle is better from the viewpoint of low temperature applications. This study suggests that the cycle performance would be significantly improved by combining the advanced H₂O–LiBr and NH₃–H₂O cycles.     

Experimental evaluation of the performances of a H2O–LiBr absorption refrigerator under different service conditions

The paper describes an experimental plant aimed at simulating and verifying the performances of a single-stage H₂O–LiBr absorption machine. The machine is water cooled and it is supplied by hot water produced by an electrical boiler; it is possible to simulate different service conditions by varying the temperatures and the flow rate of water in the external circuits. Measurement facilities allow to record in real time all the main operating parameters of internal and external circuits (temperatures, pressures and flow rates). The paper illustrates the characteristics of the machine and of the plant and the results of various experimental campaigns. In particular, the acquisitions on the plant have tested different service conditions by varying the flow rate and the temperature of the supplying hot water; the energy and energy performances of the plant are presented and compared with data from literature and from a simulation code developed for the plant.The results show that the absorption machine can work, with acceptable efficiency, with input temperatures of about 65–70 °C; this result is interesting for a future supply of the machine with solar energy.     

Numerical simulation of an advanced energy storage system using H2O–LiBr as working fluid, Part 2: System simulation and analysis

This paper is the second part of our study on the advanced energy storage system using H₂O–LiBr as working fluid. In the first part, the system working principle has been introduced, and the system dynamic models in the operation process have also been developed. Based on the previous research, this paper focuses on the numerical simulation to investigate the system dynamic characteristics and performances when it works to provide combined air-conditioning and hot water supplying for a hotel located near by Yangzi River in China. The system operation conditions were set as follows: the outdoor temperature was between 29 °C and 38 °C, the maximum air-conditioning load was 1450 kW, the total air-conditioning capacity was 19,890 kWh and the 50 °C hot water capacity for showering was 20 tons which needed heat about 721 kWh on a given day. Under these conditions, the system operation characteristics were simulated under the full- and partial-storage strategies. The simulation results predicted the dynamic characteristics and performances of the system, including the temperature and concentration of the working fluid, the mass and energy in the storage tanks, the compressor intake mass or volume flow rate, discharge pressure, compression ratio, power and consumption work, the heat loads of heat exchanger devices in the system and so on. The results also showed that the integrated coefficient of performances (COP_int) of the system were 3.09 and 3.26, respectively, under the two storage strategies while the isentropic efficiency of water vapor compressor was 0.6. The simulation results are very helpful for understanding and evaluating the system as well as for system design, operation and control, and device design or selection in detail.     

Performance research of self regenerated absorption heat transformer cycle using TFE-NMP as working fluids

A heat transformer is proposed in order to upgrade low-temperature-level energy to a higher level and to recover more energy in low-temperature-level waste heat. It is difficult to achieve both purposes at the same time using a conventional heat transformer cycle and classical working pairs, such as H₂O–LiBr and HN₃–H₂O. The new organic working pair, 2,2,2-trifluoroethanol (TFE)-N-methylpyrolidone (NMP), has some advantages compared with H₂O–LiBr and NH₃–H₂O. One of the most important features is the wide working range as a result of the absence of crystallization, the low working pressure, the low freezing temperature of the refrigerant and the good thermal stability of the mixtures at high temperatures. Meanwhile, it has some negative features like NH₃–H₂O. For example, there is a lower boiling temperature difference between TFE and NMP, so a rectifier is needed in refrigeration and heat pump systems. Because TFE–NMP has a wide working range and does not cause crystallization, it can be used as the working pair in the self regenerated absorption heat transformer (SRAHT) cycle. In fact, the SRAHT cycle is the generator–absorber heat exchanger (GAX) cycle applied in a heat transformer cycle. In this paper, the SRAHT cycle and its flow diagram are shown and the computing models of the SRAHT cycle are presented. Thermal calculations of the SRAHT cycle under summer and winter season conditions have been worked out. From the results of the thermal calculations, it can be found that there is a larger temperature drop when the waste hot water flows through the generator and the evaporator in the SRAHT cycle but the heating temperature can be kept the same. That means more energy in the waste heat source can be recovered by the SRAHT cycle.     

PVT measurements of water-ammonia refrigerant mixture in the critical and supercritical regions

The PVTx properties of H₂O + NH₃ mixture (0.2607 mole fraction of ammonia) have been measured in the near- and supercritical regions. Measurements were made along 40 liquid and vapor isochores in the range from 120.03 to 727.75 kg m⁻³ and at temperatures from 301 to 634 K and at pressures up to 28 MPa. Temperatures and densities at the liquid–gas phase transition curve, dew- and bubble-pressure points, and the critical parameters for the 0.7393 H₂O + 0.2607 NH₃ mixture were obtained using the quasi-static thermograms and isochoric (P–T) break-point techniques. The expanded uncertainty of the density, pressure, temperature measurements at the 95% confidence level with a coverage factor of k = 2 is estimated to be 0.06%, 0.02–0.09%, and 15 mK, respectively.     

Extended temperature–entropy (T–s) diagrams for aqueous lithium bromide absorption refrigeration cycles

Temperature–entropy (T–s) diagrams have the unique capability of being able to quantify processes in terms of both the first and second laws of thermodynamics. Although use of generalised T–s diagrams has been made to indicate or represent generalised absorption cycles, with the exception for NH₃/water systems, these diagrams have not been specifically tailored to scale to quantify LiBr/water systems. The main barrier for this is that the diagram needs to represent the necessary properties of both the refrigerant (water) and of the solution (LiBr/water). This paper describes the use of the T–s diagram of water extended with additional curves to represent real and ideal LiBr/water absorption cycles. An explanation is provided on several methods available, including details of the thermodynamic justification of the method that was used, to construct the extended diagrams. Finally, the extended T–s diagram is provided with the representation of a real single-effect LiBr/water absorption refrigeration cycle. This should prove to be a valuable tool for design and research engineers to study and optimise LiBr/water chillers.     

Measurements of Vapor–Liquid Equilibria in the Systems NH3–H2O–NaOH and NH3–H2O–KOH at Temperatures of 303 and 318 K and Pressures 0.1 MPa<p<1.3 MPa

A static method has been used to obtain vapor–liquid equilibrium data for the systems ammonia (NH₃)–water (H₂O)–potassium hydroxide (KOH) and ammonia–water–sodium hydroxide (NaOH) at temperatures of 303 and 318 K and pressures from 0.1 to 1.3 MPa. The salt concentration in the liquid phase was chosen in the range from 2 to 60 mass% salt in water. In both systems NH₃–H₂O–NaOH and NH₃–H₂O–KOH, solid–liquid–vapor equilibria were observed. In the NH₃–H₂O–KOH system, liquid–liquid–vapor equilibrium was observed at 318 K and 1.1 MPa but at yet unknown concentrations of the liquid phases.     

Modelling of the heat exchangers of a small capacity, hot water driven, air-cooled H2O–LiBr absorption cooling machine

General models for the design of the heat exchangers (absorber, generator, condenser and evaporator) of a prototype of an air-cooled absorption chiller of 2 kW for air-conditioning using the pair H₂O–LiBr have been developed. An absorption machine of such characteristics has been constructed to be used as a test facility for validating the results obtained from the mathematical models developed. The discrepancies considering the heat exchanged between numerical results and experimental data are under 15% in most cases for all these components except the condenser, where the discrepancies are higher. The conclusions reported will lead to: (i) future improvements of the mathematical simulation models and (ii) improvements in the experimental infrastructure.     

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.     

Supersonic two-phase flow of CO2 through converging–diverging nozzles for the ejector refrigeration cycle

CO₂ is environmentally friendly, safe and more suitable to ejector refrigeration cycle than to vapor compression cycle. Supersonic two-phase flow of CO₂ in the diverging sections of rectangular converging–diverging nozzles was investigated. The divergence angles with significant variation of decompression were 0.076°, 0.153°, 0.306° and 0.612°. This paper presents experimental decompression phenomena which can be used in designing nozzles and an assessment of Isentropic Homogeneous Equilibrium (IHE). Inlet conditions around 6–9 MPa, 20–37 °C were used to resemble ejector nozzles of coolers and heat pumps. For inlet temperature around 37 °C, throat decompression boiling from the saturated liquid line, supersonic decompression and IHE solution were obtained for the two large divergence angles. For divergence angles larger than 0.306°, decompression curves for inlet temperature above 35 °C approached IHE curves. For divergence angles smaller than 0.306° or for nozzles with inlet temperature below 35 °C, IHE had no solution.     

Simulation of ternary ammonia–water–salt absorption refrigeration cyclesSimulation des cycles frigorifiques à absorption à ammoniac / eau / sel

This paper proposes a new working fluid for refrigeration cycles utilizing low temperature heat sources. The proposed working fluid consists of the ammonia–water working fluid mixture and a salt. The salt is used to aid the removal of ammonia from the liquid solution. This effect is a manifestation of the well known “salting-out” effect. While the addition of salt improves the generator performance, it also has a detrimental effect on the absorber. The overall effects on the performance of three absorption cycles using the NH₃–H₂O–NaOH working fluid have been investigated using computer simulations. The results indicated that salting out can lower the generator operating temperature while simultaneously improving the cycle performance. Furthermore, limiting the salt to the generator suggests potential for further improvement in cycle performance.     

Thermodynamic properties for the quaternary system H2O + CH3OH + LiBr + ZnCl2

The thermodynamic properties (solubility, vapour pressure, density, viscosity, heat capacity and heat of mixing) of the H₂O + CH₃OH + LiBr + ZnCl₂ (9:1 H₂O:CH₃OH and 1:1 LiBr:ZnCl₂ by mass) system using H₂O + CH₃OH as the working media and LiBr + ZnCl₂ as the absorbents were measured. The solubility data were obtained in the temperature range from 270.35 to 389.55 K. The measurements of vapour pressure, density, viscosity and heat capacity were carried out at various temperatures and absorbent concentrations. The differential heat of dilution and differential heat of solution at 298.15 K were measured for solutionw with absorbent concentrations from 0 to 75.2 wt%. The integral heat of mixing data at 298.15 K were obtained from both sets of experimental data. The integral heats of mixing for this quaternary system showed exothermic behaviour. The vapour pressure data were correlated with an Antoine-type equation. An empirical formula for the heat capacity was obtained from experimental data. The experimental data for the basic thermodynamic properties of this quaternary system were compared with those of the basic H₂O + LiBr system.     

Thermodynamic properties of lithium bromide–water solutions at high temperatures

Emerging triple-effect LiBr–water absorption chillers operate at higher temperatures and pressures than traditional double-effect chillers. However, there is not enough data about thermodynamic properties of LiBr–water solutions at such high temperatures. Using recently measured data of vapor pressure and heat capacity, we have developed the equations which can calculate the vapor pressure, enthalpy and entropy of LiBr solutions at such high temperatures. The developed equations are valid from concentrations of 40–65 wt.% and also from temperatures of 40–210°C. These equations will be very helpful for the modeling and design of triple-effect LiBr–water chillers.     

Analyses of nano-crystalline LiCoVO4 prepared by solvothermal reaction

LiOH·H₂O, Co(NO₃)₂·6H₂O and NH₄VO₃ were used to prepare nano-crystalline LiCoVO₄ by 150 °C solvothermal reaction in isopropanol for 10–360 h and subsequent calcination at 300–500 °C for 6 h. XRD, TEM and selected area electron diffraction (SAED) revealed the presence of nano-crystalline LiCoVO₄ with inverse spinel structure. The V–O stretching vibration modes of VO₄ tetrahedrons were detected by FTIR over the range 617–835 cm^− 1 and by Raman spectrometer at 805.7 and 783.1 cm^− 1. Co, V and O were detected by EDX. TGA of solvothermal products shows weight loss due to the evaporation and decomposition processes at 40–648 °C.     

Optical, electrical and magnetic properties of Co3O4 nanocrystallites obtained by thermal decomposition of sol–gel derived oxalates

Nanocrystallites of tricobalt tetraoxide (Co₃O₄) have been synthesized by sol–gel process using cobalt acetate tetrahydrate, oxalic acid as precursors and ethanol as a solvent. The process comprises of gel formation, drying at 80 °C for 24 h to obtain cobalt oxalate dihydrate (α-CoC₂O₄·2H₂O) followed by calcination at or above 400 °C for 2 h in air. These results combined with thermal analysis have been used to determine the scheme of oxide formation. The room temperature optical absorption spectra exhibits blue shift in both (i) ligand to metal (p(O²⁻) → e_g(Co³⁺), 3.12 eV), and (ii) metal to metal charge transfer transitions (a) t_2g(Co³⁺) → t₂(Co²⁺), 1.77 eV, (b) t₂(Co²⁺) → e_g(Co³⁺), 0.95 eV together with the d–d transitions (0.853 and 0.56 eV) within the Co²⁺ tetrahedra. The temperature dependent ac electrical and dielectric properties of these nanocrystals have been studied in the frequency range 100 Hz to 15 MHz. There are two regimes distinguishing different temperature dependences of the conductivity (70–100 K and 200–300 K). The ac conductivity in both the temperature regions is explained in terms of nearest neighbor hopping (NNH) mechanism of electrons. The carrier concentration measured from the capacitance (C)–voltage (V) measurements is found to be 1.05 × 10¹⁶ m⁻³. The temperature dependent dc magnetic susceptibility curves under zero field cooled (ZFC) and field cooled (FC) conditions exhibit irreversibilities whose blocking temperature (T_B) is centered at 35 K. The observed Néel temperature (T_N  25 K) is significantly lower than the bulk Co₃O₄ value (T_N = 40 K) possibly due to the associate finite size effects.     

Experimental investigation on the performance of NH3/CO2 cascade refrigeration system with twin-screw compressor

This paper describes the experiment carried out to analyze the performance of a refrigeration system in cascade with ammonia and carbon dioxide as working fluids. The effect of operation parameters, such as the evaporating temperature of the low temperature cycle, the condensing temperature of low temperature cycle, temperature difference in cascade heat exchanger and superheat degree, on the system performance was investigated. Performance of the cascade system with NH₃/CO₂ was compared with that of two-stage NH₃ system and single-stage NH₃ system with or without economizer. It was found that the COP of the cascade system is the best among all the systems, when the evaporating temperature is below −40 °C. Also, the cascade system performance is greatly affected by evaporating temperature, condensing temperature of low temperature cycle, temperature difference in cascade heat exchanger and is only slightly sensitive to superheat degree. All the experimental results indicate that the NH₃/CO₂ cascade system is very competitive in low temperature applications.     

A low-temperature reactive sintering process for the 5Li2O–1Nb2O5–5TiO2 microwave dielectric ceramics

The B₂O₃-doped 5Li₂O–1Nb₂O₅–5TiO₂ composite microwave dielectric ceramics prepared by conventional and low-temperature single-step reactive sintering processes were investigated in the study. Without any calcinations involved, the Nb₂O₅ mixture of Li₂CO₃ and TiO₂ was pressed and sintered directly in the reactive sintering process. More uniform and finer grains could be obtained in the 5Li₂O–1Nb₂O₅–5TiO₂ ceramics by reactive sintering process, which could effectively save energy and manufacturing cost. And relatively good microwave dielectric properties of _r = 41, Q × f = 9885 GHz and τ_f = 43.6 ppm/°C could be obtained for the 1 wt.% B₂O₃-doped ceramics reactively sintered at 900 °C.     

The effect of micro-scale surface treatment on heat and mass transfer performance for a falling film H2O/LiBr absorber

The objectives of this paper are to develop a new method of wettability measurement, to study the effect of micro-scale surface treatment on the wettability across horizontal tubes and to apply it for numerical analysis of heat and mass transfer in a H₂O/LiBr falling film absorber. Three types of tubes with roughness are tested in a test rig. Inlet solution temperature (30–50 °C), concentration (55–62 wt.% of LiBr) and mass flow rate (0.74–2.71 kg/min) are considered as key parameters. Reynolds number ranged from 30 to 120 by controlling the inlet mass flow rate. The wettability on the roughened tubes was higher than that for the smooth tubes. The wettability decreased linearly along the vertical location but was proportional to the solution temperature and mass flow rate. The experimental correlations of the wettability for the smooth and the roughened tubes were developed with error bands of ±20 and ±10%, respectively. These are used for the heat and mass transfer analysis of absorbers with micro-scale hatched tubes.