Equivalent Cycle and Optimization of Auto-Cascade Absorption Refrigeration Systems

Auto-cascade absorption refrigeration(ACAR) systems are a class of new cycles that can achieve low refrigeration temperatures by utilizing low-quality thermal energy. In this study, the equivalent thermodynamic processes of a reversible ACAR system are established, and illustrated in a T-s diagram. The formula of the coefficient of performance for the reversible ACAR system is derived from the first and second thermodynamic laws. And then, the equivalent cycle of an irreversible ACAR system is established. The irreversible ACAR system is optimized by minimizing entropy generation of the thermodynamic processes. As a result, the optimum distribution ratio of heat fluxes at cascade process, which is defined as a ratio of heat fluxes between a condensing reservoir and cascade reservoir, and the optimum cascade temperature are obtained. Finally, its coefficient of performance and thermodynamic perfect degree are determined with minimum entropy generation.

Multiobjective-Multicriterion Decision Making for Selecting Design Scheme of an Absorption Refrigeration System

In this paper, first, a biobjective thermodynamics model of minimizing comprehensive quantity of boiler vapour consumption and maximizing thermodynamics efficiency of an absorption refrigeration system is established, solved by a multiobjective optimization method to generate a non-inferior scheme set. Then, applying fuzzy multicriterion decision method, a best compromise scheme is selected by taking account of five factors-investment, operation cost, total water consumption, compactness of apparatus and difficulty of operation and maintenance. In addition, other complex factors are considered simultaneously which can hardly be treated by classical methods. The decision thought and method in this paper is of certain universal significance to problems of scheme optimization.

Comparative Study on Two-Stage Absorption Refrigeration Systems with Different Working Pairs

The objective of this paper is to present a simulation study on the two-stage absorption refrigeration systems of 2.5 kW capacity using LiBr-H2O,NH3-H2O and R124-DMAC as working pairs.Under the design condition that the generating,absorbing,evaporating and condensing temperatures are 75℃,45℃,5℃and 40℃,respectively,the high and low pressure side solution circulation ratios and the coefficient of performance(COP)for the systems are calculated.Then the influences of medium,generating,absorbing,evaporating and condensing temperatures on system performances are analyzed.The results show that under the design condition,the COP of the LiBr-H2Osystem can reach 0.49,superior to those of the NH3-H2O and R124-DMAC systems,which are 0.32 and 0.31,respectively.Furthermore,the medium temperature for higher COP lies in an interval of 64-67℃for the LiBr-H2O.NH3-H2O and R124-DMAC systems.High generating temperature and low absorbing temperature can decrease the high and low pressure side solution circulation ratios,and can also increase the COP.High evaporating temperature can decrease the low pressure side solution circulation ratio and increase the COP.Low condensing temperature can decrease the high pressure side solution circulation ratio and increase the COP.

Aqueous ammonia solution cooling absorption refrigeration driven by fishing boat diesel exhaust heat

A solution cooling absorption（SCA）approach is proposed to modify the aqueous ammonia absorption refrigerat-ion cycle using the strong solution from the absorber to cool the forepart of the absorption in the cycle for reclaiming some portion of absorption heat.As a consequence of raised temperature at the inlet,the strong solution partially boils at the outlet of the solution heat exchanger,and diminishes the thermal heat consumption of the heat source.The calculation results show that the coefficient of performance（COP）of this modified cycle is about 28.3% higher than that of the traditional cycle under typical conditions;while the required heat transfer area of the total heat exchangers of the cycle is somewhat less than that of the traditional one.The capacity of refrigeration with the new absorption cycle is more than doubled in contrast to the adsorption scheme with an identical configuration.It is sufficient to supply a fishing boat the chilling capacity for preservation of fishing products with the modified cycle chiller driven by its diesel engine exhaust.

CFD Analysis of the Influence of Ionic Liquids on the Performances of a Refrigeration System

The falling film of an ionic liquid([EMIM][DMP]+H_(2)O)and its effect on a refrigeration system are numerically simulated in the framework of a Volume of Fluid(VOF)method(as available in the ANSYS Fluent computational platform).The properties of the liquid film and the wall shear stress(WSS)are compared with those obtained for a potassium bromide solution.Different working conditions are considered.It is noted that the ionic liquid demonstrates a better absorption capability,with a coefficient of performance(COP)of 0.55.It is proved that the[EMIM][DMP]+H_(2)O ionic liquid working substance is superior to the potassium bromide solution in terms of heat and mass transfer.

Performance Optimization of a Phosphoric Acid Fuel Cell/Absorption Refrigerator Hybrid System

A hybrid system that consists of a phosphoric acid fuel cell(PAFC),an absorption refrigerator and a refrigeration-space is proposed.The four-heat-source absorption refrigerator,which is driven by the waste heat produced from PAFC,provides cooling for a refrigeration-space.A numerical model is set up to analyze both the steady-state performance and transient performance considering the influences of the electrochemical and thermodynamic irreversibilities.Expressions of the equivalent power output and efficiency of the hybrid system are determined.Moreover,the transient behavior of cold-space temperature is performed and the time to reach a prescribed cold-space temperature is displayed.Thus,the operation regions of the current are optimized at different operating conditions.The results showthat in an appropriate current range,the overall power output and efficiencies of the hybrid system are enhanced.

Mass transfer enhancement for LiBr solution using ultrasonic wave

The methods were studied to improve the cooling performance of the absorption refrigeration system(ARS) driven by low-grade solar energy with ultrasonic wave, while the mechanism of ultrasonic wave strengthening boiling mass transfer in LiB r solution was also analyzed with experiment. The experimental results indicate that, under the driving heat source of 60–100 oC and the ultrasonic power of 20–60 W, the mass flux of cryogen water in Li Br solution is higher after the application of ultrasonic wave than auxiliary heating with electric rod of the same power, so the ultrasonic application effectively enhances the heat utilization efficiency. The distance H from ultrasonic transducer to vapor/liquid interface significantly affects mass transfer enhancement, so an optimal Hopt corresponding to certain ultrasonic power is beneficial to reaching the best strengthening effect for ultrasonic mass transfer. When the ultrasonic power increases, the mass transfer obviously speeds up in the cryogen water; however, as the power increases to a certain extent, the flux reaches a plateau without obvious increment. Moreover, the ultrasound-enhanced mass transfer technology can reduce the minimum temperature of driving heat source required by ARS and promote the application of solar energy during absorption refrigeration.

Measurement and Prediction of Vapor Pressure for H20 ＋ CHaOH] C2HsOH ＋ [BMIM][DBP] Ternary Working Fluids

The ionic liquid, 1-butyl-3-methylimidazolium dibutylphosphate ([BMIM][DBP]) was prepared and the vapor pressures of three set of binary solutions H2O(1)/CH3OH(1)/C2H5OH(1) + [BMIM][DBP](2) were measured at different temperature and in the ILs mole fraction range from 0.1 to 0.6 with a static equilibrium apparatus. The measured vapor pressures were correlated with Non-Random Two Liquid (NRTL) activity coefficient model and the average relative deviations (ARD) between experimental and correlated vapor pressures for these binary solutions were 3.19%, 2.42% and 2.95%, respectively. Then, the vapor pressures of two set of ternary solutions H2O(1) + CH3OH(2)/C2H5OH(2) + [BMIM][DBP](3) were measured with an inclined boiling apparatus and further predicted with NRTL activity coefficient model based on the binary interaction parameters coming from fitting the vapor pressures of the binary solutions. The results indicated that the ternary solutions containing [BMIM][DBP] were shown a strong negative deviation from Raoult's Law when the mole fraction of [BMIM][DBP] was larger than 0.2, which meant that ternary solutions could absorb the refrigerant vapors at the same or below solution temperature. Meanwhile, the average relative deviations between experimental and predicted vapor pressures for ternary solutions were 2.92% and 3.06%, respectively. Consequently, the NRTL active coefficient model used for non-electrolyte solutions was still valid for predicting vapor-liquid equilibrium of binary or ternary solutions containing ILs.

Multi-objective evolutionary optimization and thermodynamics performance assessment of a novel time-dependent solar Li-Br absorption refrigeration cycle

This research paper aims to perform dynamics analysis,3E assessment including energy,exergy,exergoeconomic,and the multiobjective evolutionary optimization on a novel solar Li-Br absorption refrigeration cycle.The research is time-dependent,owing to solar radiation variability during different timelines.Theoretically,all the necessary thermodynamic,energy,and exergy equations are applied initially.This is followed by the thermoeconomic analysis,which takes place after defining the designing variables during the thermoeconomic optimization process and is presented together with the economic relations of the system and its thermoeconomic characteristics.Furthermore,the sensitivity analysis is undertaken,the source of system inefficiency is determined,the multi-objective evolutionary optimization of the whole system is carried out,and the optimal values are compared with the primary stage.Engineering Equation Solver(EES)software has been used to accomplish comprehensive analyses.As part of the validation process,the results of the research are compared with those published previously and are found to be relatively consistent.

Performance Analysis and Evaluation of a Supercritical CO2 Rankine Cycle Coupled with an Absorption Refrigeration Cycle

The utilization of sensible waste heat such as flue gas and industrial surplus heat is essential for energy saving. Supercritical CO2 power generation cycle is a promising way to be used in this field. In this paper, a new supercritical CO2 Rankine cycle coupled with an absorption refrigeration cycle is proposed, which consists of a reheating supercritical CO2 cycle, a mixed-effect Li Br-H2O absorption refrigeration cycle and solar subsystem including evacuated-tube collector and a hot water storage tank. The system has four variants according to the presence or absence of solar subsystem and net cooling energy output. The thermodynamic model of the proposed system was established and its performance was evaluated. The proposed system is able to realize cascade utilization of flue gas waste heat and efficient conversion of solar energy. It has much higher thermodynamic efficiency than the reference system(i.e., the conventional supercritical CO2 Brayton cycle). Taking combined power and cooling system driven by flue gas waste heat and solar energy as an example, its thermal efficiency and exergy efficiency are 20.37% and 54.18% respectively, compared with the 14.74% and 35.96% of the reference system. Energy Utilization Diagrams(EUD) are implemented to investigate the irreversible losses and variation of the exergy destruction in the energy conversion process. Parametric analysis in two key parameters is conducted to provide guidance for the system optimal design.

Thermodynamic Performances of[mmim]DMP/Methanol Absorption Refrigeration

In order to study the theoretical cycle characteristic of [mmim]DMP （1-methyl-3-methylimidazolium dimethyl- phosphate）/methanol absorption refrigeration, the modified UNIFAC group contribution model and the Wilson model are established through correlating the experimental vapor pressure data of [mmim]DMP/methanol at T= 280~370 K and methanol mole fraction x= 0.529-0.965. Thermodynamic performances of absorption refrigera- tion utilizing [mmim]DMP/methanol, LiBr/H20 and H20/NH3 are investigated and compared with each other under the same operating conditions. From the results, some conclusions are obtained as follows： 1） the circula- tion ratio of the [mmim]DMP/methanol absorption refrigeration is higher than that of the LiBr/H2O absorption refrigeration, but still can be acceptable and tolerable. 2） The COP of the [mmim]DMP/methanol absorption refrigeration is smaller than that of the LiBr/H2O absorption refrigeration, while it is higher than that of the H2O/NH3 absorption refrigeration under most operating conditions. 3） The [mmim]DMP/methanol absorption refrigeration are still available with high COP when the heat source temperature is too high to drive LiBr/H2O absorption refrigeration.

Introducing a proper hydrogen liquefaction concept for using wasted heat of thermal power plants-case study: Parand gas power plant

A hydrogen liquefaction concept with an innovative configuration and a capacity of 4 kg·s^(-1)(345.6 t·d^(-1))is developed.The concept involves an ammonia absorption refrigeration system for the pre-cooling of hydrogen and MR streams from 25℃ to-30℃.The ammonia absorption refrigeration system is fed by exhaust gases of the Pa rand gas power plant that are normally dissipated to the environment with a temperature of 546℃.The simulation is performed by Aspen HYSYS V9.0,using two separate equations of state for simulating hydrogen and MR streams to gain more accurate results especially for ortho-para conversion.Results show that conversion enthalpy estimated by Aspen HYSYS,fits very well to the experimental data.Determining the important independent variables and composition of MRs are done using trial and error procedure,a functional and straightforward method for complicated systems.The minimum temperature limit in the cooling section is lowered,and an ortho-para converter is implemented in this section.The proposed concept performs well from energy aspects and leads to COP and SEC equal to 0.271 and 4.54 kW·h·kg^(-1),respectively.The main advantage of this study is in the low SEC,eliminating the losses of the distribution network,and improving the ability of the hydrogen liquefaction for energy storage in off-peak times.

Performance Analysis of an Integrated Supercritical CO_(2) Recompression/Absorption Refrigeration/Kalina Cycle Driven by Medium-Temperature Waste Heat

A novel power and cooling cogeneration system which combines a supercritical CO_(2) recompression cycle(SCRC), an ammonia-water absorption refrigeration cycle(AARC) and a Kalina cycle(KC) is proposed and investigated for the recovery of medium-temperature waste heat. The system is based on energy cascade utilization, and the waste heat can be fully converted through the simultaneous operation of the three sub-cycles. A steady-state mathematical model is built for further performance study of the proposed system. When the exhaust temperature is 505℃, it is shown that under designed conditions the thermal efficiency and exergy efficiency reach 30.74% and 61.55%, respectively. The exergy analysis results show that the main exergy destruction is concentrated in the heat recovery vapor generator(HRVG). Parametric study shows that the compressor inlet pressure, the SCRC pressure ratio, the main compressor and the turbine I inlet temperature, and the AARC generator pressure have significant effects on thermodynamic and economic performance of the combined system. The findings in this study could provide guidance for system design to achieve an efficient utilization of medium-temperature waste heat(e.g., exhaust heat from gas turbine, high-temperature fuel cells and internal combustion engine).

Integration of solid-oxide fuel cells and absorption refrigeration for efficient combined cooling,heat and power production

Combined cooling,heating and power(CCHP)systems are characterized by a substantially higher energy-utilization efficiency compared to standalone systems.In this study,an integrated system comprising a solid-oxide fuel cell(SOFC),hot-water storage tank(HWST)and absorption refrigeration(AR)cycle is considered.The SOFC model was developed in Aspen Plus®.It was used to determine the thermodynamic properties of the exhaust gas that was then used to provide heat for the HWST and to drive the AR cycle.Thermodynamic models for the AR cycles were developed in Engineering Equation Solver,considering LiBr-H2O and NH3-H2O as working fluids.The sensitivity analysis of a number of SOFC output parameters has been carried out.The most optimal case was characterized with the coefficient of performance(COP)and CCHP efficiency of 0.806 and 85.2%for the LiBr-H2O system,and 0.649 and 83.6%for the NH3-H2O system,respectively.Under such optimal operating conditions,the SOFC was characterized by the net electrical efficiency of 57.5%and the net power output of 123.66 kW.Data from the optimal solution were used to perform the thermodynamic study and sensitivity analysis to assess the influence of different absorption cycle operating conditions and to identify possible applications for the considered integrated systems.

Theoretical and Experimental Study of a Cylindro-Parabolic Solar Collector

This article outlines the theoretical and experimental performance studies of a cylindro-parabolic solar collector. The theoretical study consists on the establishment, through mass and energy balances, of a mathematical model to control the exiting temperature of the heating fluid as well as the temperatures of the absorber and the glass. The experimental level investigates the influence of the solar absorber tube diameter on the performances of the driving device. Several experiments were made in order to know the possibility to reach temperatures being able to ensure for example the ammonia vaporization in the generator of a solar absorption refrigeration system. These experiments were carried out under various operating and climatic conditions. The results are presented and discussed.

Analysis of an electricity-cooling cogeneration system for waste heat recovery of gaseous fuel engines

Waste heat recovery(WHR)is one of the most useful ways to improve the efficiency of internal combustion engines,and an electricity-cooling cogeneration system(ECCS)based on Rankin-absorption refrigeration combined cycle for the WHR of gaseous fuel engines is proposed in the paper.This system can avoid wasting the heat in condenser so that the efficiency of the whole WHR system improves,but the condensing temperature of Rankin cycle(RC)must increase in order to use absorption refrigeration system,which leads to the decrease of RC output power.Therefore,the relationship between the profit of absorption refrigeration system and the loss of RC in this combined system is the mainly studied content in the paper.Because the energy quality of cooling and electricity are different,cooling power in absorption refrigeration is converted to corresponding electrical power consumed by electric cooling system,which is defined as equivalent electrical power.With this method,the effects of some important operation parameters on the performance of the ECCS are researched,and the equivalent efficiency,exergy efficiency and primary energy rate are compared in the paper.

Flow hydrogen absorption of LaFe_(10.9)Co_(0.8)Si_(1.3) compound under constant low hydrogen gas pressure

The hydrogen absorption of the LaFe（10.9）-Co（0.8）Si（1.3） compound under constant 1.01 × 10-5 Pa H2 gas in a flow hydrogen atmosphere was studied. The effects of hydrogen absorption on structure, Curie temperature, phase transition and magnetic property were investigated by X-ray diffraction（XRD）, differential scanning calorimeter（DSC） and superconducting quantum interference device,respectively. The hydrides of LaFe（10.9）Co（0.8）Si（1.3） crystallize into NaZn（13）-type structural phase after hydrogen absorption at temperature from 548 to 623 K. Lower hydrogen absorption temperature is of no advantage for pure 1：13 phase formation in a flow H2 atmosphere. The Curie temperature（TC） of LaFe（10.9）Co（0.8）Si（1.3） compound increases by70 K or more after hydrogen absorption. For LaFe（10.9）-Co（0.8）Si（1.3）H（1.8） compound, the maximum magnetic entropy change and the relative cooling power under a magnetic field change of 0-2 T are 6.1 J·kg^-1·K^-1 and 170 J·kg^-1,respectively. Large refrigerant capacity, low hysteresis loss and wide temperature span of magnetic entropy change peak make it a competitive practical candidate for magnetic refrigerant.

Modelling and Thermodynamic Analysis of a Hot-Cold Conversion Pipe Using R134a-DMF-He as the Working Pair

Based on the concept of a diffusion absorption system,a hot-cold conversion pipe utilizing 1,1,1,2-tetrafluoroethane(R134 a)-dimethylformamide(DMF)-helium(He)as the working pair is presented with the aim of cooling output by recovering the low-grade waste heat.The model of the hot-cold conversion pipe is established,in which a heat pipe is used to transfer the waste heat as the heat input.The equations of the thermodynamic properties of the working pair are established by equation of state method(EOS).The model of the hot-cold conversion pipe is built based on the mass,species and energy balance equations of each component.The direct conversion of heat to cold is achieved by the desorption,absorption,condensation and diffusion evaporation processes of R134 a.The hot-cold conversion pipe is cooled by natural convection,which can be enhanced by chimney effect.The thermodynamic analysis is carried out to analyze the effect of the boundary conditions,i.e.the heat source temperature,the refrigeration temperature,and the environmental temperature,on the system performance.This paper provides a theoretical basis for actual application of the hot-cold conversion pipe in waste heat recovery field.