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Low-temperature ammonia absorption refrigeration system based on the temperature difference uniformity principle: Optimization analysis
... Cold production machines are classified in the category of receiving machines, i.e., to operate them requires energy consumption, either in the form of free heat sources such as waste heat [23][24][25], geothermal energy [24], or solar energy [26], or in the form of mechanical energy [27] to drive compressors. In the case of a TCRS, minimizing energy consumption and improving performance consists of the search for optimal operating conditions which are intimately linked to the MTC. ...
This paper presents a comprehensive energy, exergy, and environmental analysis of a triple-stage cascade refrigeration system (TCRS) using four refrigerant blends: R1150/R170/R717, R1150/R170/R290, R1150/R170/R1270, and R1150/R170/RE170, aiming to achieve ultra-low temperatures. Both thermodynamic performance and environmental impact of the system are modeled considering the real gases behavior by coupling REFPROP to MATLAB. Several bio-inspired optimization algorithms including NRBO, Puma, QIO, JSOA, and HsO were tested, with HsO proving to be the most efficient in terms of convergence times and accuracy over 500 iterations. The model was validated against literature data, showing an error margin of less than 3% in COP, when condensing temperature was set at 40 °C and evaporating temperatures at − 120 °C and at − 80 °C. Further performance analysis revealed that system efficiency (COP) is inversely proportional to condensing temperature and proportional to evaporating temperature, with COP values ranging between 0.35 and 0.9. Increasing the temperature difference between condensing and evaporating stages affects significantly compressor discharge temperatures, particularly for ammonia, which can exceed 125 °C, leading to potential lubrication issues. Environmentally, the R1150/R170/RE170 blend demonstrated the lowest total equivalent warming impact (TEWI) at approximately 7.15 MTCO2, making it the recommended refrigerant blend due to its superior thermodynamic and environmental performance compared to the others.
Pinch point (PP) analysis is critical for integrating, designing, and improving the thermal performance of counter-flow heat exchangers (HEXs) in energy systems, particularly those utilizing binary mixtures such as ammonia-water. Traditional PP analysis methods struggle with the complexities of non-isothermal phase change behaviors, leading to inaccuracies and computational inefficiencies. This study proposes a novel and systematic approach for PP analysis that integrates mathematical and numerical techniques to determine the unknown outlet state of a target stream based on the known states and a predefined PP temperature difference that constrains the streams. The methodology employs polynomial functions of temperature-specific enthalpy relationships, transformed into dimensionless enthalpy-based expressions, enabling an iterative process to precisely identify the minimum temperature difference between streams. Verification against existing analytical and numerical methods demonstrates the superior accuracy and computational efficiency of the proposed approach. A comprehensive parametric study further reveals the impact of mass fraction, working pressure, polynomial degree, and numerical tolerance on PP characteristics. The method applies to a wide range of thermodynamic and industrial processes, such as the Kalina cycle, absorption refrigeration systems, and binary distillation, facilitating heat integration and energy recovery.
Although absorption technology is environment-friendly and energy-saving, the coefficient of performance (COP) of absorption systems is comparatively low. The absorber, a crucial component of these systems, is one of the important factors that influence the absorption system COP, and its efficiency significantly influences the system’s overall performance. Therefore, absorber performance needs to be improved to increase the COP. Generally, these four approaches are adopted to increase the absorber performance: enhancing heat and mass transfer by upgrading the structure, shape, and surface of the heat exchanger tubes, adding additives in the working solution, increasing absorption interface area by employing an atomizer and ultrasound inside the absorber, and applying a magnetic field around the absorber. This paper reviews the absorption enhancement by a magnetic field. The configurations of absorption experimental setups employing a magnetic field, the methods to build a magnetic field in absorption systems, and the influence of magnetic field intensity and its direction on absorption performance are all thoroughly reviewed. Insights about working solutions and nanoparticles employed in magnetized absorption systems are presented. Furthermore, the numerical modeling of absorption enhancement by an external magnetic field and the prospective research direction are also discussed. It is expected that this work will help in a more profound understanding of absorption enhancement by the magnetic field and provide a base for further improvements.
The objective of the present investigation work is the analysis of a solar-driven refrigeration unit with parabolic trough solar collectors. The refrigerator is a single-stage absorption machine with NH3/water working pair for producing refrigeration in the temperature range from -35°C up to 5°C. The system is studied energetically, exergetically and financially. The yearly analysis is performed for the weather data of Athens, Greece. The thermodynamic model was created in Engineering Equation Solver and it is validated with literature data, while the dynamic model was programmed in language Matlab. More specifically, the thermodynamic analysis is conducted by applying the energy rate balances and the mass flow rates balances in the system devices, while the dynamic analysis is performed by applying the energy balance in the storage tank of the solar field. According to the final results, there is an optimum generator temperature that maximizes the system performance and it is depended on the operating conditions. For refrigeration product at -20°C and heat rejection at 40°C, the yearly system coefficient of performance is 0.255, the yearly exergy efficiency 4.86% and the simple payback period is close to10 years.
This article is dedicated to the design, calculation and dimensioning of a small powered refrigeration system (132W) which produces ice bars (freezing) using solar thermal power, and resorts to an intermittent cycle absorption circuit with a water-ammonia mixture (H2O-NH3). The aim of this equipment is to minimize problems faced in places where there is no electric network to supply traditional refrigeration systems which preserve perishable products produced or stocked there, as well as drugs (vaccines), namely for specific regions of developing countries. The system developed can be divided into two parts. The intermittent cycle absorption refrigeration system uses a binary water-ammonia solution (H2O-NH3), where water is the absorber and the ammonia is the coolant and the thermal solar system. This is made up of CPC flat plate thermal collectors or vacuum tubes in which solar energy heats the water that circulates in the primary circuit. In the absorption circulation system, circulation occurs in a natural way due to the fluids affinity, and the temperature and pressure internal variations. This article shows the assumptions underlying the conception, calculation and dimensioning of the system’s construction.
Heat recovery of marine engine exhaust gas is an effective way of improving the onboard fuel economy and environmental compliance of fishing ships. Among such heat recovery techniques, the absorption refrigeration cycle shows potential as it can convert the exhaust thermal energy into refrigeration output and meet the onboard refrigeration requirement. However, the severe operating conditions on the shipboard poses a great challenge for its application. This paper presents an experimental investigation of an absorption refrigeration system for the heat recovery of marine engine exhaust gas. To overcome the adverse effect of the severe onboard condition on the rectification process of the absorption refrigeration system, a ternary ammonia–water–lithium bromide mixture is selected as the working fluid. A prototype of the absorption system is designed and an experimental investigation is conducted. Then, the performances of both the ternary ammonia–water–lithium bromide-based system and binary ammonia–water-based system are compared. The results show that the rectifier heat exchange area can be reduced by approximately 16% under the experimental working condition. Furthermore, the ternary system operates at a relatively lower pressure, with a refrigeration temperature of less than −15.0 °C, which is higher compared to the temperature of less than −23.6 °C associated with the binary system. Nevertheless, the ternary system achieves a remarkably higher cooling capacity. Moreover, by using the ternary ammonia–water–lithium bromide mixture, the heat loss of the prototype is reduced while the coefficient of performance and electric coefficient of performance are increased, indicating that the ternary system has a higher energy conversion efficiency.
Double-lift absorption cycles represent a suitable solution for air-cooled thermally driven cooling applications. Among the several existing double-lift configurations, semi-GAX cycles are known as the most promising in terms of efficiency. These cycles incorporate the GAX effect in a pressure staged cycle, by means of a split on the solution leaving the low pressure absorber. Two configurations of the semi-GAX cycle have been proposed in the past, the semi-GAX 1 and the semi-GAX 2. The former achieves the GAX effect between the intermediate and the high pressure levels, the latter between the low and the intermediate. Within this paper, the semi-GAX cycles are numerically investigated at operating conditions suitable for a low temperature driven (e.g., by flat plate solar collectors) air conditioning application. The peculiarities of the two cycles are described and the factors affecting their performances are underlined. The COP resulted to be strongly influenced by the split ratio, which determines the intermediate pressure and the possibility to achieve the GAX effect. If the split ratio is optimized to achieve the maximum COP, the COP is higher for semi-GAX 2 for air temperatures below 27 °C and for semi-GAX 1 above. In both cases, the maximum air temperature which allows a circulation ratio below 15 is 40 °C, with chilled water at 7/12 °C and driving temperature of 90 °C. © 2016 Elsevier Ltd and International Institute of Refrigeration. All rights reserved.
The experimental analysis of a gas-driven NH3-H2O heat pump whose cycle approaches the GAX concept is carried out. Full load operation is investigated by varying hot water temperatures and partial load operation is investigated by decreasing gas input down to 50% of the full load value. Numerical simulations bolster measurements accuracy and provide insight on the variation of cycle COP and Gas Utilization Efficiency (GUE), based on gross calorific value. A nearly constant GUE of about 1.5 is found for hot water temperatures lower than 50°C. The GUE steadily decreases above 50°C, reaching about 1.33 at 60°C. The COP varies more smoothly, from 1.73 at 45°C to 1.60 at 60°C. The GUE and COP reduction at 50% of the nominal gas input is 6.8% and 6.4%, respectively. Simulations suggest that performances at partial loads can improve if active control of solution mass flow rate is implemented. © 2016 Elsevier Ltd. and International Institute of Refrigeration. All rights reserved.
The prototype of an air-cooled double-lift NH3-H2O absorption chiller driven by hot water at low temperature is presented. The main objective of the study is to illustrate the experimental performances of the prototype under different operating conditions. A mathematical model of the cycle is developed, along with a procedure for the identification of otherwise difficult to measure data, with the purpose of providing the complete picture of the internal thermodynamic cycle. The combined experimental and numerical data allowed assessing the effects on the thermodynamic cycle with varying operating conditions. The unit operated steadily with chilled water inlet 12 °C, outlet 7 °C, air temperature between 22 °C and 38 °C, and hot water driving temperatures between 80 °C and 90 °C. The reference cooling capacity at air temperature of 30 °C is 2.5 kW, with thermal COP about 0.3 and electrical COP about 10.
A breadboard prototype of an absorption system for truck refrigeration using heat from the exhaust-gases was designed, built
and tested. Measured COP values of the unoptimized single-stage ammonia-water absorption cycle varied between 23 and 30%,
but system modeling shows that this can be improved to values considerably over 30%. Computer simulation for the system included
cycle analysis as well as component modeling, using a detailed two-fluid model for flow of the ammonia-water mixture in the
condenser and absorber. This detailed model was also validated using test data. In addition, the recoverable energy of the
exhaust gases was analyzed for representative truck-driving conditions for city traffic, mountain roads and flat roads. The
results show that the system is promising for long distance driving on flat roads.
Es wurde ein Prototyp einer abgasbetriebenen Absorptionskälteanlage für Transportkühlung ausgelegt, gebaut und getestet.
Das gemessene Wärmeverhältnis der noch nicht optimierten einstufigen Ammoniak-Wasser Absorptionskältemaschine (AKM) liegt
zwischen 23 und 30%. Simulationsrechnungen zeigen, daß das Wärmeverhältnis auf deutlich über 30% gesteigert werden kann. Die
Simulationsrechnungen wurden sowohl für den gesamten Kreisprozeß als auch für einzelne Komponenten durchgeführt. Insbesondere
wurde für den Verflüssiger und Absorber ein detailiertes Zweifluidmodell für die Strömung des Ammoniak-Wasser-Gemisches entwickelt
und mit Hilfe der Meßdaten validiert. Zusätzlich wurde für repräsentative Lkw Fahrten im Stadtverkehr und auf gebirgigen und
ebenen Außerortsstrecken die nutzbare Abgasenergie analysiert. Die Ergebnisse zeigen, daß eine abgasbetriebene AKM besonders
für Langstreckenfahrten auf ebener Strecke geeignet ist.
This work presents an experimental study of an ammonia–water absorption refrigeration system using the exhaust of an internal combustion engine as energy source. The exhaust gas energy availability and the impact of the absorption refrigeration system on engine performance, exhaust emissions, and power economy are evaluated. A production automotive engine was tested in a bench test dynamometer, with the absorption refrigeration system adapted to the exhaust pipe. The engine was tested for 25%, 50%, 75% and wide-open throttle valve. The refrigerator reached a steady state temperature between 4 and 13 °C about 3 h after system start up, depending on engine throttle valve opening. The calculated exhaust gas energy availability suggests the cooling capacity can be highly improved for a dedicated system. Exhaust hydrocarbon emissions were higher when the refrigeration system was installed in the engine exhaust, but carbon monoxide emissions were reduced, while carbon dioxide concentration remained practically unaltered.
In this study, the effect of heat exchangers on the system performance in an aqua–ammonia absorption refrigeration system has been investigated. For this purpose, the thermodynamic performance of the system is analysed, and the irreversibilities in the thermal processes have been determined for three different cases. These cases are: both of the heat exchangers are included in the system, only the refrigerant heat exchanger is included in the system and only the mixture heat exchanger is included in the system. It is assumed that the ammonia concentration at the rectifier outlet is independent of the other parameters, constant and equal to 0.999. The coefficient of performance, exergetic coefficient of performance, circulation ratio (f) and non-dimensional exergy loss of each component of the system are calculated separately. After these calculations, some graphics indicating the change of the variables with the system parameters have been plotted.
Aiming at the problems of low solar energy utilization rate, poor intermittent, low stability and poor energy saving effect in traditional solar absorption refrigeration system, a lithium bromide-water absorption refrigeration system driven by solar in Tai’an was taken as the research object. An improved solar absorption refrigeration system with phase change was presented. In typical day and whole refrigeration period, the traditional solar absorption refrigeration system with and without phase change were simulated by TRNSYS transient simulation software. Their collector efficiency, unit cooling capacity and COP were comparatively analyzed. The results showed that the collector efficiency of the improved solar absorption refrigeration system with phase change was 4.2% higher than that of the traditional solar absorption refrigeration system without phase change. The unit cooling capacity was 2.8% lower than that of the traditional system, which store the excess cooling capacity with the help of PCMs. The average COP of traditional absorption system and improved solar absorption refrigeration system were 0.737 and 0.679, respectively. This manuscript provides a theoretical basis for the development of solar phase change heat storage absorption refrigeration system.
This paper experimentally studies the influence of lithium bromide concentration on the coefficient of performance (COP) and other operating parameters in the high-pressure part of a ternary working fluid ammonia-water absorption refrigeration system. The lithium bromide concentrations in the ternary working fluid were 5%, 10%, 15%, and 20%. The lithium bromide concentration is the ratio of the mass of lithium bromide and the sum mass of lithium bromide and water. The generation temperature ranged from 100 °C to 130 °C, evaporation temperature was -10 °C, and the cooling water temperature was 28 °C. The experiment results show that 15% is the optimum lithium bromide concentration, and the highest COP is 0.408 when the generation temperature was 130 °C, which is 7.2% higher than AARS with binary working fluid. The COP improved because of the reduction of rectification cooling load, heat recovery, and operating pressure. The cooling water mass flows in the reflux condenser were reduced when applying lithium bromide in the working fluid, and the rectification cooling load decreases as low as 16% of the heating load in the generator. The heat recovery in the solution heat exchanger is too large after using ternary working fluid because the heat transfer coefficient of the ternary solution rises. Moreover, the generation pressure is related to the condensing temperature rather than lithium bromide concentration. Generally, AARS operates more efficiently, and the main components can be miniaturized with the application of ammonia-water-lithium bromide ternary working fluid.
Energy shortage and environmental pollution are among the most serious challenges facing mankind. The utilization of efficient and green solvents has great urgency, if true action is to be taken to alleviate these issues. The Deep Eutectic Solvents (DESs) were recently introduced as sustainable solvents with special characteristics. These novel solvents have very low vapor pressures and can be “designed” for desired properties through the engineered selection of appropriate hydrogen-bond donors and acceptors. Research is necessary to investigate the feasibility of applying DESs in various industrial applications, including absorption refrigeration systems. Since the coefficient of performance (COP) and the exergetic coefficient of performance (ECOP) of an absorption refrigeration system depend highly on the properties of the absorbent and refrigerant, selecting a proper DES with particular attention to its physical properties, can dramatically affect its efficiency. In this study, three DESs were investigated as potential absorbents, consisting of Reline (1 Choline chloride + 2 urea), Ethaline (1 Choline chloride + 2 ethylene glycol) and Glyceline (1 Choline chloride + 2 glycerol), with ammonia as the refrigerant. A modified SRK-NRTL model was used to estimate the physical and thermodynamic properties of the DESs and their mixtures with ammonia. The COPs and ECOPs of the absorption refrigeration cycles were calculated within wide ranges of absorber and regenerator temperatures. The results were compared to literature systems, including ammonia/water and ammonia/ionic liquids cycles. The performances based on energy and exergy analyses showed that ammonia/DES working fluids have the potential to be used in such cycles.
The experimental assessment of a new absorption cooling system cooled exclusively by air operating with the ammonia-lithium nitrate mixture is presented. The generator, the economizer and the evaporator are flat plate heat exchangers, while the condenser and absorber consist of serpentine of finned tubes. A pump is used to circulate the solution into the system, and a fan is used to circulate the air through the condenser and absorber. The effect of the generation and condensation temperatures, on the cooling power, evaporation temperatures and the coefficient of performance was analyzed. The system operated at generation temperatures between 80 °C and 100 °C, while condensation temperatures varied between 20 °C and 32 °C. External cooling powers of up to 3.4 kW were achieved with external coefficients of performance of up to 0.33. The cooling temperatures were as low as 2.6 °C. Due to the developed system is air-cooled; a cooling tower is not required.
Ammonia/water absorption–resorption refrigeration systems are an interesting technological alternative to conventional absorption systems thanks to their additional degree of freedom, which makes operation more flexible and can significantly reduce the typically high working pressure, so cheaper assembly materials can be used. This paper aims to review the theoretical and experimental works carried out so far on these systems, paying special attention to the experimental plants currently operating and recent progress in improving system control.
This paper reports how to determine the operational pressure range when the temperatures of the main components of the ammonia/water single-stage cycle are fixed and how the pressure range changes when these working temperatures are varied. As a result of the theoretical study of the base case selected, the cycle performance was evaluated without rectification after the generator in order to reduce the size and cost of the system without reducing the performance. Finally, it should be pointed out that the effectiveness of the resorption solution heat exchanger plays an important role in the performance and operating conditions of the cycle because the cycle cannot work below certain values of effectiveness.
The concept of diabatic distillation is applied to desorption and rectification in an ammonia–water absorption system. Results of a hydrodynamic investigation of novel components for these processes are used to develop a non-equilibrium model to predict heat and mass transfer performance. A modular modeling approach is proposed whereby physical consistency of various heat and mass transfer processes is achieved while reducing model complexity. A target vapor generation rate of 3 g s⁻¹ with an ammonia mass fraction of 0.9985 is specified. It is shown that highly compact, lightweight desorption and rectification components can be realized. Heat source temperature and mass flow rate control are used to optimize system performance, and performance predictions are made over wide range of operating conditions. These results are also used to obtain reduced-order heat and mass transfer performance measures that facilitate incorporation of system performance into system level models and dynamic models.
Absorption chillers constitute a valuable option for utilising solar energy. Specifically, when installed in tropical regions, this technology ideally matches the needs for refrigeration and air-conditioning because of the abundance of solar energy throughout the year. A single-double-effect absorption chiller combines the single and double-effect configurations to compensate for the unpredictable instantaneous availability of solar radiation and cooling load fluctuations. The operative performance of this system is strongly affected by internal parameters such as the absorber outlet solution flow rate and the solution distribution ratio, which connect the operability of the single and double-effect configurations. Therefore, these important parameters are currently used to maximise system performance while assuring its stability. This study discusses how the COP of a single-double-effect absorption chiller, for solar cooling applications in tropical areas, can be maximised (1.55 at full load, and up to 2.42 at 60% partial load) by manipulating those internal parameters. The simulation results were compared with the experimental data (field test data) and, by adopting the appropriate control method, showed an improvement of the system performance between 12 and 60% when compared to a corresponding double-effect configuration.
A standalone absorption chiller based on microchannel heat exchanger technology was developed with a cooling capacity of 7 kW. A review of recent advances in microscale heat and mass exchangers and vapor absorption technology guided this development. Thermodynamic and heat and mass transfer design procedures are discussed. A control system for standalone operation was developed. Experimental evaluation demonstrated the achievement of target cooling capacities. An overall system COP of 0.44, and an ammonia-water cycle COP of 0.51 were achieved. Reasons for differences between model predictions and actual performance are discussed. This development validates scalability and application of microscale heat exchanger technology for absorption heat pumps at residential-scale capacities. In contrast to conventional absorption systems, core heat and mass exchangers require only small portion of overall system volume and weight, which significantly improves the prospects of this technology for small capacity applications.
A hydrophobic microporous membrane contactor was proposed and studied as an absorber for an ammonia/water absorption cycle. The heat and mass transfer process in the adiabatic flat sheet membrane module was investigated experimentally and analytically. For the experimental study, a membrane absorber test bench was designed and built. Moreover, a unidimensional model was developed and validated with the experimental results. The influence of the temperature, concentration, flow rate and membrane characteristics on the absorption process was evaluated. For the conditions considered in this study, the results indicate that the absorption flux is clearly governed by the mass transfer in the liquid phase. The maximum absorption flux was 4.7 ·10⁻³kg/m²s with a solution flow rate of 45 kg/h. According to the model, the hydrophobic membrane should be as thin as possible and the pore diameter between 0.03 μm and 0.10 μm.
This paper presents the development and experimental study of an ammonia water absorption refrigeration prototype for waste heat utilization of diesel engine exhaust. Side cooling rectification and side heating generation are designed to achieve desirable heat matching for better internal heat recovery thus improving the system performance. An active open heat pipe method is applied for taking the exhaust heat to make the heat input stable. The condensation and absorption processes are combined in one unit and cooled by circulated precooled solution. Small diameter tube bundle heat exchangers with large specific surface area are employed for all components. Both the features make the system bulk small. The experimental results show that the operation of the system is reliable with a sharp variation of the exhaust condition. The prototype produces cooling capacity of 33.8 kW and the system thermal COP reaches 0.53 under the test conditions that the temperatures of the cooling water, secondary refrigerant and exhaust inlet are 26.1oC, -15.2oC and 567oC, respectively. The novel design of the prototype is proved to be valid and its concept can be extended to other applications.
This paper presents the results from an experimental evaluation of a small-scale, waste-heat driven ammonia-water absorption chiller. The cycle is thermally driven, using the waste heat from diesel generator exhaust to desorb the refrigerant solution. The absorber and condenser are directly-coupled to the ambient air. The remaining heat exchangers are packaged in a compact microchannel monolithic structure for enhancing heat transfer. The system is designed to deliver 2.71-kW of cooling at extreme ambient temperature of 51.7 °C at a coefficient of performance of 0.55. Experiments on a heat pump breadboard system are conducted at ambient conditions of 29.7–44.2 °C, with delivered cooling duties of 2.54–1.91 kW. System and individual component performance is analyzed and compared with cycle model predictions, and deviations are explained. The absorber and desorber were identified to be the limiting components in the system. Effects of variation in ambient temperature are studied to characterize the performance at off-design conditions.
A uniformity factor of temperature difference (UFTD) is proposed and set up to guide the optimization of Heat exchanger network (HEN). At first, the factor is presented to evaluate the whole enhancement of HEN by handling the logical mean temperature difference as two-dimensional discrete temperature field in system. Then, the factor is applied to different HENs, of which the comparison indicates that a more uniform discrete temperature field leads to a lower UFTD which correlated with a better whole enhancement to improve the optimization level of HEN. A novel stage-wise superstructure model where inner utility can be generated is presented for further analysis of correlation between UFTD and the efficiency of HEN, and more optimal HEN structures can be obtained as inner utility added. Inner utility appears to violate the thermodynamic law, but it makes the discrete temperature field more uniform and improves the heat transfer efficiency of the whole HEN, which brings much more profit than the side effect of inner utility. In sum, the UFTD can not only evaluate the optimization level of the HEN, but also be an optimization object to design new HEN with higher efficiency of energy utilization and lower total annual cost.
This paper presents the development and demonstration of a compact packaged prototype of a single-effect ammonia–water absorption cooling system with a nominal cooling capacity of 3.5 kW. The packaged unit encompasses a combustion module to supply thermal energy to the system where heat is exchanged between combusted gas and a heat transfer fluid. The heat transfer fluid provides heat to the desorber where refrigerant is generated. The heat and mass exchange components of the absorption system are integrated into two monolithic blocks. The sub-components of each monolith utilize microscale heat and mass transfer passages, which have higher heat and mass transfer coefficients compared to conventional designs, resulting in substantial reductions in the required size of the system. Semi-autonomous control algorithms are developed to enable stand-alone operation of the packaged unit. System performance is measured over a range of operating conditions and compared with predictions of the corresponding system model.
This study presents an experimental investigation of a solar thermal powered ammonia-water absorption refrigeration system. The focus of this study lies on the design of the components of the absorption chiller, the ice storages and the solar collector field as well as the integration of the data acquisition and control unit. An ammonia-water (NH3/H2O) absorption chiller was developed in the laboratory of the Institute of Thermodynamics & Thermal Engineering (ITW) at the University of Stuttgart (Germany). A demonstration plant was built in the laboratory of the CoRE-RE at King Fahd University of Petroleum & Minerals (KFUPM - Saudi Arabia). The whole system was tested successfully. The results of the experiments indicated a chiller coefficient of performance (COP) of 0.69 and a cooling capacity of 10.1 kW at 114/23/-2 (°C) representing the temperatures of the generator inlet, the condenser/absorber inlet and the evaporator outlet respectively. Even at 140/45/-4 (°C), the chiller was running with a cooling capacity of 4.5 kW and a COP of 0.42. © 2015 Elsevier Ltd and International Institute of Refrigeration. All rights reserved.
The two-stage configuration of a double-effect absorption cycle using ammonia/lithium nitrate as working fluid is studied by means of a thermodynamic model. The maximum pressure of this cycle configuration is the same as the single-effect cycle, up to 15.8bars, being an advantage over the double-effect conventional configuration with three pressure levels that exhibits much higher maximum pressure. The performance of the cycle and the limitation imposed by crystallization of the working fluid is determined for both adiabatic and diabatic absorber cycles. Both cycles offer similar COP; however the adiabatic variant shows a larger margin against crystallization. This cycle can produce cold for external inlet evaporator temperatures down to -10°C, but for this limit the crystallization could happen at high inlet generator temperatures. The maximum COP can be 1.25 for an external inlet generator temperature of 100°C. This cycle shows a better COP than a typical double effect cycle with in-parallel configuration for the range of the moderate temperatures under study and using the same working fluid. Comparisons with double effect cycles using H2O/LiBr and NH3/H2O as working fluids are also offered, highlighting the present configurations advantages regarding COP, evaporation and condensation temperatures as well as crystallization.
This work presents an experimental study of an important component of an ammonia-water absorption refrigeration cycle based on the falling film technology - the distiller, which is formed by two parts: the generator and the rectifier. It was carried out a parametrical study on the operation of the distiller by testing several parameters. The experimental results showed that the distilled ammonia vapor concentration always degraded as rectifier, generator, and concentrated solution temperatures increased and improved as concentrated solution mass flow rates and concentrated solution concentrations increased. The highest distilled ammonia vapor concentration obtained was of 0.9974. Photographic documentation of flow pattern over the generator tube bundle was also obtained resulting in both jet column and sheet patterns. Finally, a Nusselt number correlation for the ammonia-water falling film over the generator tube bundle was proposed based on the experimental results and compared with other authors' correlations. © 2015 Elsevier Ltd and International Institute of Refrigeration.
A dual-source powered mixed effect AC (absorption chiller) is applied in a CCHP (combined cooling heating and power) system. This CCHP system integrates both fossil fuel energy and solar energy together. Refrigeration performance of this mixed effect AC under both waste heat mode and solar mode are tested and compared. In waste heat mode, the AC is simultaneously powered by exhaust gas waste heat and hot water waste heat from an ICE (internal combustion engine). And five working conditions under different electric output of ICE are tested to study the refrigeration performance. In solar mode, the AC is powered by hot water from a solar thermal collector matrix, and its refrigeration performance is tested in a selected day to study the operation characteristics. Test results show that, the mixed effect AC can reach a COP (coefficient of performance) of 0.91 in waste heat mode. But its COP is only about 0.6 in solar mode for there is only hot water heating source. Comparison results show that, waste heat mode is not necessarily better than solar mode and vice versa. In terms of primary energy consumption, waste heat mode is worse than solar mode when the part load ratio of ICE is below 0.62.
Ammonia/LiNO3 and ammonia/NaSCN absorption refrigeration cycles are alternatives to ammonia/water cycles for refrigeration applications at temperatures below 0°C. They exhibit higher coefficient of performance (COP) values and do not require purification of the refrigerant vapor. Entropy data for such solutions calculated using their most recently published thermophysical property data; we study and compare the cycles thermodynamically. The simulation results are used to examine the influence of various operating parameters on performance and the possibility of crystallization in these cycles. It is shown that, for low generator temperatures, ammonia/LiNO3 cycles have better performance. For high generator temperatures, ammonia/NaSCN cycles have better performance, but the range of allowable generator temperatures is quite limited for this mixture.
This paper presents the performance of an ammonia-water absorption chiller. This single-stage 10 kW absorption chiller is cooled with a water-ethylene glycol solution. The required heat source is hot water between 75 degrees C and 85 degrees C. Different operating conditions can be imposed by varying temperatures and flow rates of secondary circuits and the flow of the rich solution. This equipment, designed for solar air conditioning applications, was tested under various operating conditions to assess its performance. This study shows that the performance of the absorption chiller decreases significantly with the evaporator temperature. This is due to a problem of partial evaporation in the evaporator when the absorption machine is operated outside its design specifications. Cooling capacity oscillations, caused by refrigerant expansion control, were also observed. Absorption chiller performance is also influenced by heat source temperature, cooling temperatures and flow of the rich solution.
In principle, absorption chillers of the ammonia-water type could work at temperatures well below the usual air-conditioning temperatures, arriving at the range 250–260 K, which can be useful for refrigeration applications. This possibility is studied for an air-cooled machine, comparing the results with the experimental data supplied by a manufacturer that recently commercialized such a refrigerator. The prediction is fair, and the study allows an insight into the internal parameters and into the possible behaviour for more severe conditions than those studied.RésuméEn principe, les refroidisseurs à absorption du type ammoniac-eau pourraient fonctionner à des températures bien inférieures à celles utilisées en conditionnement d'air, allant jusqu'á 250 ou même 260 K, ce qui rendrait ces machines intéressantes pour les applications frigorifiques. Cette possibilité est étudiée pour une machine refroidie à l'air, avec comparaison des résultats et des données expérimentales fournies par un fabricant ayant récemment commercialisé un tel réfrigérateur. Les données du fabricant se sont révelées justes et l'étude a été l'occasion de voir de près les paramètres internes de la machine, ainsi que le comportement possible sous des conditions plus sévères que celles des expériences déjà effectuées.
Practical experience in working with ammonia–water absorption systems shows that the ammonia purification process is a crucial issue in order to obtain an efficient and reliable system. In this paper, the detrimental effects of the residual water content in the vapour refrigerant are described and quantified based on the system design variables that determine the effectiveness of the purification process. The study has been performed considering a single stage system with a distillation column with complete condensation. The ammonia purification effectiveness of the column is analysed in terms of the efficiencies in the stripping and rectifying sections and the reflux ratio. By varying the efficiencies from 0 to 1, systems with neither the rectifying nor stripping section, with either the rectifying or stripping section, or with both sections can be considered. The impact of the ammonia purification process on the absorption system performance is studied based on the column efficiencies and reflux ratio; and its effects on refrigerant concentration, system COP, system pressures and main system mass flow rates and concentrations are analysed. When the highest efficiency rectifying sections are used a combination of generation temperature and reflux ratio which leads to optimum COP values is found. The analysis covers different operating conditions with air and water cooled systems from refrigeration to air conditioning applications by changing the evaporation temperature. The importance of rectification in each kind of application is evaluated.
Splitting the exergy destruction into endogenous/exogenous and unavoidable/avoidable parts represents a new development in the exergy analysis of energy conversion systems. This splitting improves the accuracy of exergy analysis, improves our understanding of the thermodynamic inefficiencies and facilitates the improvement of a system.An absorption refrigeration machine is used here as an application example. This refrigeration machine represents the most complex type of a refrigeration machine, in which the sum of physical and chemical exergy is used for each material stream.
In this paper, the mass transfer performance of a large-specific-area corrugated sheet structured packing for ammonia–water absorption refrigeration systems (AARS) is reported. An experimental facility was used to test the performance of the packing. Experimental results of the temperature, ammonia concentration and mass flow rate of the rectified vapour are presented and discussed for different operating conditions including reflux ratio values from 0.2 to 1. The volumetric vapour phase mass transfer coefficient is calculated from the measured data and compared with different correlations found in the literature. A new correlation is proposed which was fitted from the experimental data. Finally, a comparison is made between the actual packing height used in the experimental setup and the height required to obtain the same ammonia rectification in AARS with different packings previously tested by the authors.
This paper deals with the modelling of the thermodynamic properties of the water–ammonia refrigerant mixture. Three different approaches are formulated and compared. The first is an empirical approach based on a free enthalpy model of the mixture considered as the resultant of the properties of its pure components and of an excess term corresponding to the deviation to the ideal solution concept. Secondly, a semi-empirical approach based on the PATEL and TEJA cubic equation of state is considered. Finally, a theoretical approach formulated as PC-SAFT (perturbed chain statistical associating fluid theory) equation of state is treated. A comparison of these three methods proves the superiority of PC-SAFT in predicting and extrapolating the thermodynamic properties of the water–ammonia system up to very high temperatures and pressures.
In this paper, the GAX and GAX hybrid absorption refrigeration cycles are studied and compared from the viewpoint of both first and second law of thermodynamics. Exergy analyses were performed in order to calculate the total exergy destruction rate within the cycles and also reveal the contribution of different components to the destructions. In order to evaluate the efficiencies of the cycles at different working conditions, particularly from the viewpoint of the second law, parametric studies were also performed. It was found that in both cycles the generator temperature (Tgen) has more influence on the second law efficiency whereas, the coefficient of performance (COP) of the cycles are comparatively less affected by this temperature. An increase of about 75% in the second law efficiency of the GAX cycle was found as the generator temperature was varied from 400 to 440 K. With this variation of the generator temperature, the increase in the corresponding COP was around 5%. In addition, compared to that in the GAX cycle, the maximum value of exergetic efficiency in the GAX hybrid cycle occurs at a slightly higher value of Tgen.Research highlights► In this paper, the GAX and GAX hybrid absorption refrigeration cycles are studied and compared from the view point of both first and second law of thermodynamics. ► It was found that in both cycles the generator temperature (Tgen ) has more influence on the second law efficiency whereas, the coefficient of performance (COP) of the cycles are comparatively less affected by this temperature. ► In addition, compared to that in the GAX cycle, the maximum value of exergetic efficiency in the GAX hybrid cycle occurs at a slightly higher value of Tgen.
In many industrial processes there is a simultaneous need for electric power and refrigeration at low temperatures. Examples are in the food and chemical industries. Nowadays the increase in fuel prices and the ecological implications are giving an impulse to energy technologies that better exploit the primary energy source and integrated production of utilities should be considered when designing a new production plant. The number of so-called trigeneration systems installations (electric generator and absorption refrigeration plant) is increasing. If low temperature refrigeration is needed (from 0 to −40 °C), ammonia–water absorption refrigeration plants can be coupled to internal combustion engines or turbogenerators. A thermodynamic system study of trigeneration configurations using a commercial software integrated with specifically designed modules is presented. The study analyzes and compares heat recovery from the primary mover at different temperature levels. In the last section a simplified economic assessment that takes into account disparate prices in European countries compares conventional electric energy supply from the grid and optimized trigeneration plants in one test case (10 MW electric power, 7000 h/year).
A new correlation of equilibrium properties of ammonia-water mixtures is presented for use in the design and testing of absorption units, and especially for heat pumps. The temperature range has been extended to 500 K and the pressure range to 50 bar. The equations of state used are based on those of Schulz. Values of specific volume, vapour pressure, enthalpies and equilibrium constants for mixtures are compared with the best experimental data. The results are presented in the form of vapour pressure and enthalpy-concentration diagrams.
A new physical quantity, , has been identified as a basis for optimizing heat transfer processes in terms of the analogy between heat and electrical conduction. This quantity, which will be referred to as entransy, corresponds to the electric energy stored in a capacitor. Heat transfer analyses show that the entransy of an object describes its heat transfer ability, as the electrical energy in a capacitor describes its charge transfer ability. Entransy dissipation occurs during heat transfer processes as a measure of the heat transfer irreversibility. The concepts of entransy and entransy dissipation were used to develop the extremum principle of entransy dissipation for heat transfer optimization. For a fixed boundary heat flux, the conduction process is optimized when the entransy dissipation is minimized, while for a fixed boundary temperature the conduction is optimized when the entransy dissipation is maximized. An equivalent thermal resistance for multi-dimensional conduction problems is defined based on the entransy dissipation, so that the extremum principle of entransy dissipation can be related to the minimum thermal resistance principle to optimize conduction. For examples, the optimum thermal conductivity distribution was obtained based on the extremum principle of entransy dissipation for the volume-to-point conduction problem. The domain temperature is substantially reduced relative to the uniform conductivity case. Finally, a brief introduction on the application of the extremum principle of entransy dissipation to heat convection is also provided.