Ammonia-water Absorption Refrigeration System Research Articles

Utilization of Geothermal Fluid as a Heat Source for Absorption Refrigeration System for Food Preservation – A Case of Bwanda and Gwisho Hotsprings

Zambia has significant geothermal resources with over 86 hotsprings identified. Installation of a 250kW off-grid pilot binary-cycle power plant is currently underway at the Bwanda and Gwisho goethermal site. It is estimated that there is 90,000 herd of cattle in the villages around the pilot power plant. Little milk from these herds of cattle reaches the market due to scarce collection points with cooling facilities. This study assesses the applicability of utilizing exhaust geothermal fluid from a binary-cycle geothermal power plant for powering an absorption refrigeration system (ARS) for storing food, specifically milk. Two absorption systems; Lithium Bromide-water and the ammonia-water ARS are compared. The COPs, energy input and mass flowrates required to produce various cooling loads (40kW – 160kW) are compared. The results show that the exhaust geothermal fluid energy (504kW) is sufficient for powering both ARSs for storing milk to its suitable storage temperature of 4oC. The results also show that the Lithium Bromide-water system is more suitable for this application, because it produces a higher maximum COP (0.76) compared to the ammonia-water system’s maximum COP (0.64), it produces a higher maximum cooling capacity (380kW) compared to the ammonia-water systems maximum capacity (216kW). It requires lower input mass flowrate and pump work at all cooling capacities (40kW – 160kW). Therefore, a Lithium Bromide-water absorption chiller for storing the amounts of milk currently collected in the study area is designed. Optimized design specifications are provided.

Downsizing strategy for an air-cooled indirect-fired single-effect ammonia-water absorption chiller in part-load operation in hot climates

A modular mathematical model has been created to simulate an ammonia-water absorption refrigeration system indirectly fired and air-cooled. The model includes governing equations based on mass, species, and energy balances, implemented for the main components of the system. It accounts for both thermal and mass resistances in the transfer processes that occur in the system. The study evaluates the performance of the ROBUR® absorption refrigeration system, model ACF60-00 LB, operating under part-load conditions, driven by hot water temperatures ranging between 160 and 210 °C, while the ambient temperature remains up to 40 °C. This refrigeration system is characterised by including an extra valve that allows active control of the pressure levels of the system. The analysis focusses on the effect of its active control on the size of the system. The results show that increasing the pressure loss in this valve reduces the size of the air-cooled absorber to 37.3 % of its nominal size at an ambient temperature of 40 °C, while the reduction in refrigerant mass flow is 18.5 %, while the condenser size decreases 3.1 times. Evaporator, air-cooled absorber and condenser effectiveness are minimally affected. Additionally, contribution of condenser and evaporator to exergy destruction is balanced.

Energy and exergy analysis of a subfreezing evaporator environment ammonia-water absorption refrigeration cycle: Machine learning and parametric optimization

The coefficient of performance (COP) and exergy efficiency of a single and double-effect ammonia-water absorption refrigeration system powered by compound parabolic concentrating collectors were analyzed under various operating situations. A novel method was proposed using support vector machine regression and particle swarm optimization to identify optimal operating parameters. The optimal pressure-temperature conditions, including evaporator pressure (Pe), generator pressure (Pg), and absorber temperature (Ta) that maximize the COP and exergy efficiency while minimizing generator temperature (Tg) and evaporator temperature (Te), were investigated. The generator temperature was the main independent variable, ranging from 370 to 470 K. The findings demonstrated that the gain in COP and exergy efficiency caused by raising the generator temperature to more than 430 K is not cost-effective. The COP increased when the evaporator temperature increased along the investigated range of generator temperatures but yielded lower exergy efficiency in all cases. The exergy destruction rate in condenser, pump, recooler, reheater, and expansion valves is insignificant compared to other components. The generator has the highest exergy destruction rate regardless of operating conditions, making it the most crucial component of the absorption system. The optimization process findings showed that, at Pe = 2.8 bar, Pg = 14.5 bar, and Ta = 303.15 K, the maximum COP and exergy efficiency were 0.8483 and 0.3605, respectively, concerning the minimization of Tg and Te, which were 408 and 267 K, respectively. The model produced an acceptable performance with a high prediction accuracy (coefficient of determination > 0.99 and mean square error < 0.0064).

Simulation studies on GAX based absorption cooling and power cycle using a mixture of ammonia-water working fluid and aluminium oxide nano particles

The present work is focused on analyzing the influence of nano material aluminium oxide on the performance of generator absorber heat exchange ammonia-water absorption refrigeration system. Matlab code has been used to simulate the performance of the combined cycle. The effect of refrigeration, absorber and condenser temperatures on the combined cycle performance have been studied. The results of the analysis indicate that the performance of the combined cycle is higher by 15 to 20% when Al2O3 nano particles were added when compared to the same cycle without the addition of nano particles.

Experimental study on the influence of adding TiO2 nanoparticles on practical ammonia-water absorption refrigeration system-the generation and rectification processes

The efficiency of the rectification process determines whether the ammonia-water absorption refrigeration system (AARS) can operate efficiently. Therefore, this paper experimentally investigates the effects of added TiO2 nanoparticles on the generation and rectification processes. The stable TiO2 ammonia-water nanofluids are prepared through an own preparation process and are circulated in the solution loop of the AARS. During the experiments, the addition of TiO2 nanoparticles is adjusted to explore the relationship between the generation and rectification processes and the amount of nanoparticles added. The reliable experimental data show that under the same working conditions, the circulation factor can be effectively reduced by up to 8% and the deflation ratio can be effectively increased by up to 10% on account of the growth of heat and mass transfer after adding the TiO2 nanoparticles, which results in a maximum increase of around 14% in system’ COP. According to later research, the improvement of rectification efficiency can also be attributed to the hydrophilicity of TiO2 nanoparticles. Overall, this paper provides reliable and practical experimental data and a new direction for improving the generation and rectification processes in AARS to facilitate the application of the system.

Study on the ammonia-water two-stage evaporation absorption refrigeration system for the cooling process with temperature-distributed heat load

The absorption refrigeration system can recover low-grade heat and produce cooling capacity, which can realize low-carbon refrigeration. However, due to the constant-temperature evaporation, the conventional system fails to provide efficient cooling for temperature-distributed heat load with large temperature spans, including process cooling, mixture liquefaction and ice slurry making. In this paper, a modified ammonia-water absorption refrigeration system with two pairs of evaporation and absorption processes at different pressure levels is proposed, in order to enhance the temperature matching with the temperature-distributed heat load and promote the absorption heat recovery. A process model is developed and validated, and comparison and parametric studies are conducted to illustrate the advantages of the new system and optimize operating conditions. Results show that the proposed system is suitable for the cooling process with temperature-distributed heat load with large temperature span, and the exergy destruction during the cooling process with temperature-distributed heat load decreases by nearly 40%. Moreover, the system internal heat recovery is improved by harvesting absorption heat. As a consequence, the COP increases by 31% and reaches 0.75, when providing cooling capacity with temperature below -10 °C. It is hoped that this study raises concerns about the characteristics of temperature-distributed heat load and provides feasible approach for efficient cooling with large temperature span.

Performance Prediction for a Marine Diesel Engine Waste Heat Absorption Refrigeration System

The output of the absorption refrigeration system driven by exhaust gas is unstable and the efficiency is low. Therefore, it is necessary to keep the performance of absorption refrigeration systems in a stable state. This will help predict the dynamic parameters of the system and thus control the output of the system. This paper presents a machine-learning algorithm for predicting the key parameters of an ammonia–water absorption refrigeration system. Three new machine-learning algorithms, Elman, BP neural network (BPNN), and extreme learning machine (ELM), are tested to predict the system parameters. The key control parameters of the system are predicted according to the exhaust gas parameters, and the cooling system is adjusted according to the predicted values to achieve the goal of stable cooling output. After comparison, the ELM algorithm has a fast learning speed, good generalization performance, and small test set error sum, so it is selected as the final optimal prediction algorithm.

THERMODYNAMIC AND HEAT TRANSFER ANALYSIS OF COOLING TECHNOLOGIES: A COMPARATIVE STUDY

Refrigeration systems applications are broadly used in food and drug conservation and in air conditioning systems. Commercial buildings may demand as much as 80% of total electrical power just for powering the air-conditioning system based on conventional vapor compression refrigeration systems (VCRS), which contributes to reach peak demands on the electrical distribution network that could cause an unstable condition. Implementing absorption refrigeration systems (ARS) to produce cooling effects driven by thermal energy could decrease that power demand. Thermodynamic models of these systems can be found in the literature with a variety of working fluids and also integrated with other cycles such as power generation plants. However, a few previous analyses have a direct comparison between ARS and VCRS at the same operational conditions. Thus, the current study aims to simulate and compare two different refrigeration technologies: single stage ammonia-water absorption refrigeration system and vapor compression refrigeration system working with refrigerants R-134a and R-717. Thermodynamic simulation was carried out by evaluating heat transfer rates in the main devices, coefficients of performance, and specific areas of evaporator and condenser. As evaporator temperature decreases from 10°C to -20°C, ARS requires 16.9 kW or 67.5% more heat in generator and COP decreased from 0.601 to 0.359. Utilizing the same comparison parameter, VCRS needed 3.26-3.54 kW or 154-160% more compressor power, depending on refrigerant used, and COP decreased from 6.77 to 2.60 with R-134a and 7.07 to 2.79 using R-717 at the same condensation temperature (40°C). Compared to ARS, condenser specific area required for VCRS is smaller, evaporator is twofold smaller when using R-134a, and is equal when using R-717. Those results can justify the usage of ARS in facilities with high amount of waste heat, mainly on applications working with lower evaporator temperatures.

Feasibility study on ammonia water absorption refrigeration cycle without distillation column

The water content in the refrigerant may undermine the cooling effect in ammonia water absorption refrigeration systems. Distillation column is normally implemented. The use of distillation columns, however, may increase system complexity, especially in practical operation. In fact, the distillation column can be eliminated under particular operating conditions. This paper presented an ammonia water absorption refrigeration cycle without distillation column, which is replaced by two separators and one heater. The resulting vapor is nearly pure with an ammonia mass fraction of 0.998. Heat exchanger network analysis is carried out for searching all the potential heating-saving possibilities. The thermal performance was improved by the combination of solution recirculation and absorption heat recovery, and the results indicated that the thermal performance has been increased by 42.6% than that of the basic cycle at certain working condition. The proposed cycle has limited ranges of working conditions operated under small temperature lifts,and the payback period is 4.28years.

Performance improvements on energy and exergy basis for an ammonia-water absorption refrigeration system in a coffee industry

Ammonia-water absorption refrigeration systems have broad applications in industrial and commercial facilities, easily driven by any available waste heat of low-medium temperatures. The objective is to evaluate, identify and propose improvements in the existing coffee industry using refrigeration system with a double absorption stage and single ammonia generator. A complete energy and exergy modeling and analysis are applied - components and overall efficiencies, including physical and chemical irreversibilities. Thermal system modeling uses Engineering Equation Solver containing 34 streams and 22 components. The main findings indicate that coupled energy-exergy analysis proves the ability to identify poor performance components and processes. Therefore, allowing to propose feasible modifications in the real facility by improving the systeḿs efficiency on 1st and 2nd Thermodynamics laws basis, respectively β and ε from 0.259 and 0.204 to reaching 0.388 and 0.304. As a result of the irreversibility analysis, the ammonia generator, regenerative heat exchanger and the pressure reducing valve had the most improvement in their performances when applying the proposed modifications. Altogether, about 33% lower fuel consumption at the steam generator, once the steam demand significantly reduced to 1.66 kg.s−1 in comparison to original demand of 2.49 kg.s−1.

Numerical analysis of the enhancement of ammonia-water falling film absorption process by atomized equipment

As a typical absorption device, falling film absorber has a significant effect on the performance of the ammonia water absorption refrigeration system. In this paper, a novel absorber with atomizer installed at the bottom of the absorber is proposed. The atomizer is introduced to increase the effective mass transfer area between ammonia vapor and the solution. A theoretical model in established and simulation investigation is carried out to obtain the absorption enhancement by the atomizer. The result indicates that the enhancement is 10.5% with the 1 mm initial droplet diameter in the design point of this paper, and it can reach 34.2% when the initial droplet diameter is reduced to 0.2 mm. Furthermore, the optimization of enhancement effect by other droplet parameters has been analyzed.

An improved absorption refrigeration system for recovering two waste heat with different temperatures: Parametric analysis and comparative study

Heat capacity and temperature matching between the heat sources and energy system is of vital importance to improve energy efficiency, especially in occasions with multiple temperature-distributed heat sources. In this paper, an improved ammonia-water absorption refrigeration system with additional intermediate-pressure generator and absorber is proposed, aiming at recovering two kinds of heat sources with different grades. To match the heat capacity of two sources, solution distribution to the high- and intermediate-pressure generator can be adjusted. Moreover, heating areas of the two generators are extended from the reboilers to the stripping sections, in order to improve the heat utilization ratio. Simulation study shows that as the solution split ratio to the intermediate-pressure generator increases, the capacity for recovering low-grade heat sources is improved. The coefficient of performance of the proposed system is 5% higher than that of the modified vapor exchange system within a large range of the intermediate pressure. Under the baseline working condition with evaporation temperature of -15°C and condensation temperature of 31°C, a significant increment in exergy efficiency is achieved by the proposed system, which is 32.6% and 56.5% higher than that of the conventional single-effect and conventional vapor exchange system, respectively.

Experimental investigation on the effect of TiO2 nanoparticles on the performance of NH3 – H2O - LiBr absorption refrigeration system

The newly NH3 – H2O – LiBr working fluid can decrease the power wasted in the generator, thus improves the COP of the ammonia absorption refrigeration system. But the applying of NH3 – H2O – LiBr working fluid decreases the ammonia absorption efficiency, so the COP of the system is still unsatisfactory. To further enhance the COP of the NH3 – H2O - LiBr absorption refrigeration system, TiO2 nanoparticles are added to the NH3 – H2O - LiBr working fluid in this study. And the effects of TiO2 nanoparticles concentration on the performances of the generator, absorber, evaporator, condenser, and solution heat exchanger are tested in an ARS experimental system. The experimental results indicate that the addition of TiO2 nanoparticles increases the Pg, Pe, Pa, and Pc, meaning that the value of ammonia produced in the generator increases. Therefore, the COP and iCOP of the system are improved by adding TiO2 nanoparticles, and the COP is maximum improved by 18.96% compared with the ammonia-water absorption refrigeration system. When the xTiO2 is increased from 0.3% to 0.5%, the increase rate of the COP decreases because the newly added nanoparticles do not disperse well. So 0.3% is a better choice for the TiO2 nanoparticles concentration when comprehensively considering the dispersion stability of nanoparticles and the COP.

Thermal Analysis of a Forced Flow Diffusion Absorption Refrigeration System for Fishing-Boat Exhaust Waste Heat Utilization

Ammonia water absorption refrigeration systems are effective in utilizing fishing-boat exhaust waste heat for cryopreservation. However, the liquid level control and the use of a solution pump characterized by small flowrate and high-pressure head result in poor reliability in the traditional system. Besides, the system must necessarily be designed anti-swaying and anti-corrosion. This paper proposes a forced flow diffusion absorption refrigeration system, in which an inherently leak-free canned motor pump and an ejector are employed to provide the driving forces of the gas and liquid loops. The approximate single pressure operation allows for a simple passive liquid sealing control without throttling valves. The system adopts an integrated cooling strategy which allows the system to operate under swaying conditions, and the external seawater cooled heat exchanger avoids internal corrosion and leakage. The thermal analysis shows the system is valid to be operated under wide operating conditions, and the coupled gas and solution circulation ratios determined the performance of the novel system. There is an optimal ammonia mass fraction difference in the gas loop to obtain the optimal COP. The COP reaches 0.4 when the temperatures at the outlets of the generator, evaporator, absorber, and condenser are 160, −15, 35, and 35°C, respectively. The novel system provides a reliable absorption refrigeration system design for fishing-boat applications.

Experimental investigation of the effect of LiBr on the high-pressure part of a ternary working fluid ammonia 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.

Review on performance enhancement of solar absorption refrigeration system using various designs and phase change materials

Recent trend of increasing global warming throughout the world rising at very alarming rate. In leu of that, several efforts have been made by various researchers, among this solar refrigeration is one of the areas which required to be explored further. Conventional refrigerator consumes electricity which is mostly produced by power plant and most of the power plants in India running on coal, oil etc., which leads to rise of greenhouse gas production, that directly contribute to global warming. Several efforts have been made by researchers to enhance the performance of solar absorption refrigeration system. It has been found that the utilization of waste heat either from the marine ship engine or from the economizer can be used to run the ammonia water absorption refrigeration system. Few studies have been done on using phase change material to enhance the latent heat of fluid by various method which can be further used at night to supply it to generator of refrigeration system. The performance of vapour absorption refrigeration system has been enhanced significantly by using nanoparticles. Simulations studies gives interpretation of various design changes effect on the performance of solar refrigeration system and that can be used for further experimentation works. The effect of various design and operating parameters on the COP of refrigeration system have been reported by various researchers.

Theoretical Formulation of the Thermodynamic Properties of Ammonia-Water and Ammonia-Water-Lithium Bromide Solutions

System of Ammonia-Water-Lithium Bromide overcomes the disadvantage of Ammonia-Water absorption refrigeration system. Addition of lithium bromide reduces the formation of water vapour thus presents its entry into the condenser. A set of computational formulations of thermodynamic and Thermophysical properties of Ammonia-Water, Water-Lithium bromide and Ammonia-Water-Lithium Bromide solutions at different pressures and temperatures are presented in the paper. Obtained results are validated with the experimental data which were available in the literature. It is found to be in good agreement with those values of the literature..

In Silico Modeling of a Novel Refrigeration Process of the Ammonia–Water Falling-Film Absorption

Nowadays, the ammonia–water absorption refrigeration system (AARS) has attracted much attention due to its merits such as energy conservation and environment-friendliness. Meanwhile, growing studies have focused on the key equipment of ammonia–water absorber of AARS. Based on the thermodynamic and transfer property models that are specially determined in this study for the ammonia–water system, a calculational model of the ammonia–water falling-film absorber outside vertical tubes (AW-FFA-OVT) is established. The simulation of the ammonia absorption process is then carried out by the Fortran program in silico. The reliability of the physical models and the absorber model is verified by comparing with the experimental data from the literature. The mechanism of the proposed ammonia absorber is investigated by observing various key factors. The proposed model has already been successfully applied in the design and simulation of the key equipment in an actual AARS process.

Modeling and analysis of an ammonia–water absorption refrigeration system utilizing waste heat with large temperature span

Waste heat sources such as flue gas generate large temperature span during recovery process. In order to improve the temperature match with such waste heat sources, a novel absorption refrigeration system with continuous-temperature-changing generation process is proposed. Internal heat recovery process is introduced to both the generator and absorber, and a premixer is added to take the place of the solution heat exchanger in traditional system. A numerical model of the system is built, in which the continuous-temperature-changing generation process is simulated based on a stripping column with heat input at each stage. New parameters are defined and optimized to strengthen the internal heat recovery process, and thus to improve system efficiency. Simulation results show that the system can operate under a large waste heat temperature span of 67.5 K, almost twice of the conventional system. With the evaporation temperature, waste heat inlet temperature, and cooling water temperature at −15 °C, 150 °C and 20 °C, respectively, a maximum COP of 0.93 and exergy efficiency of 49.7% is acquired while the waste heat temperature span is 60 K, showing that the system can acquire larger utilization rate of waste heat with higher system efficiency.

Analysis of working parameters for an ammonia-water absorption refrigeration system powered by automotive exhaust gas

In the present work, an experimental investigation was performed to determine effective working parameters for an ammonia-water absorption refrigeration system powered by waste heat from the exhaust gases of a vehicular internal combustion engine. The automotive exhaust system was connected to the generator of a commercial absorption refrigerator originally intended to operate with the heat generated from an LPG burner. In an attempt to increase performance, a close-looped exhaust gas flow control system was designed and implemented that allowed the generator temperature to be maintained at pre-determined values. Thus, a series of tests were performed with varying generator temperatures (180, 200, 240, and 270 °C) while monitoring engine torque, speed, and temperature at different points of the system. Using this methodology, it was found that the system is significantly sensitive to generator temperatures, and satisfactory performance was only noted for the set value of 200 °C. Under this operating condition, after 240 min test runs, minimum temperatures of −12.5 and −0.6 °C were obtained, respectively, at the evaporator element and interior of the absorption refrigerator, while the maximum coefficient of performance (COP) registered was almost 0.05.