Ammonia-water Absorption Refrigeration Research Articles

Thermodynamical Analysis of the Distillation Column for Targeting Minimum Energy in Ammonia–Water Absorption Refrigeration Cycle

In this study, a comprehensive method to optimize distillation columns in ammonia–water absorption refrigeration, power, and cogeneration cycles is introduced. The novelty of this work lies in a comprehensive analysis of the distillation column's thermal performance and the introduction of a new design framework that maximizes heat integration. Key findings include the critical role of the solution heat exchanger, especially at lower generation and evaporation temperatures. Using a saturated liquid or two‐phase feed can achieve up to 80% efficiency at a feed composition of 0.3. Moreover, in the results, the substantial potential for improving the coefficient of performance (COP) through strategic design is highlighted. Notably, minimizing the pinch point temperature difference to 0 °C can boost the COP by 6%, while optimal feed placement can increase it by 9%. Incorporating feed preconditioning and side exchangers can lead to a 22% COP enhancement, indicating the significance of heat integration to the distillation column. In this study, a graphic guideline for selecting cycles that can achieve a minimum of 10% COP improvement without introducing unnecessary complexity is delivered and it suggests that distillation columns can be simplified by using a combination of separators, heaters, and coolers in certain cases.

Maximizing efficiency in solar ammonia–water absorption refrigeration cycles: Exergy analysis, concentration impact, and advanced optimization with GBRT machine learning and FHO optimizer

A detailed analysis of energy and exergy is conducted on a single-effect solar ammonia–water (NH3–H2O) absorption refrigeration cycle (ARC) using TRNSYS and EES software. Considering the physical and chemical exergies, the exergy destruction rate (ĖD) in each component of the system is calculated, highlighting its contribution to the overall ĖD. The study explores the effects of varying refrigerant mass flow rate (ṁᵣ) and ammonia concentration in strong and weak solutions (Xs and Xw) on key performance parameters, including coefficient of performance (COP), exergy efficiency (ĖD), and overall ĖD across a range of generator temperatures (Tg). In this study, a gradient boosting regression tree (GBRT) is employed as a supervised machine-learning technique for classification and regression problems, utilizing boosting to enhance conventional decision tree predictions. The Fire Hawk Optimizer (FHO) approach is also utilized to optimize performance parameters, maximizing COP and ηηEwhile minimizing Tg and ĖD. The GBRT models are developed using available experimental and simulation data, revealing relationships between variables (ṁᵣ, Xs, Xw, and Tg) and outcomes (COP, ĖD, and overall ĖD). The results revealed that the generator exhibits considerable ĖD regardless of operating conditions, underscoring its pivotal role in the ARC. It emerges as the primary ĖD contributor (50 %), followed by the evaporator (17 %) and the absorber (15 %). However, ĖD associated with the recooler, pump, and expansion valves is negligible in comparison. Optimization results reveal that, when minimizing Tg and ĖD, the highest COP and ĖD at Tg of 373.15 K reach 0.8081 and 0.46, respectively.

Theoretical and experimental study of the ammonia water absorption refrigeration cycle with side reboiling and side condensing

In the conventional ammonia-water absorption refrigeration cycle, the rectification heat is usually dissipated to the surrounding environment, and the temperature of the weak solution flows at the bottom of the distillation column is normally high. This paper presents an improved cycle implemented with side reboiling by utilizing the heat content of the weak solution, and side condensing by recovering the rectification heat based on the single-effect cycle. The effect of feed thermal conditions and split ratio on the thermal performance of the improved cycle has been explored. The effectiveness of the improved cycle with respect to the single-effect cycle under different operating conditions has been examined. A more conservative effectiveness is selected in this work. The theoretical study demonstrates that the thermal performance of the improved cycle can be as high as 15% more than that of the single-effect cycle, and the improved cycle is more efficient when the heating source temperature or temperature lift is higher. A prototype is developed and tested for experimental verification. The experimental effectiveness ranges from 1.9% to 4.4% based on the test results. The relationship between system performance and temperature lift shows a close tendency with the theoretical results.

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.

Thermodynamic Analysis of a New Combined Cooling and Power System Coupled by the Kalina Cycle and Ammonia–Water Absorption Refrigeration Cycle

In order to improve the utilization efficiency of low-temperature heat sources, a new combined cooling and power system using ammonia–water is proposed. The system combines Kalina cycle with absorption refrigeration cycle, in which the waste heat of the Kalina cycle serves as the heat source of the absorption refrigeration cycle. The steady-state mathematical model of system is established in detail first, and then the simulation results of design condition are obtained, which show that the thermal efficiency and exergy efficiency can reach 24.62% and 11.52%, respectively. Based on the system design condition, an exergy destruction analysis is conducted and shows that four heat exchangers and the turbine contribute most of the total exergy destruction. Finally, the effects of five key parameters on the system performance are examined, which reveal that within certain ranges, there is an optimal turbine inlet pressure that makes the exergy efficiency maximal. Increasing the ammonia–water temperature at the vapor generator outlet and the ammonia-weak solution temperature at the bottom outlet of the rectification column will reduce the thermal efficiency but raise the exergy efficiency. With the increase of rectification column pressure, both the thermal efficiency and exergy efficiency drop, while the evaporation pressure has an opposite effect on the system performance.

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.

A new combined cooling and power system based on ammonia-water absorption refrigeration cycle: Thermodynamic comparison and analysis

In this paper, a new combined cooling and power (CCP) system, based on ammonia-water absorption refrigeration cycle, is proposed to utilize low-temperature heat sources effectively. The system is characterized by that an ejector is introduced to lower the turbine back pressure by the evaporated ammonia vapor used for cooling, and is appropriate for the working scenarios needing much cooling capacity but bits of electricity. The system mathematical models are set up in detail, based on which the data of the system design working condition are obtained by numerical simulation. The results show that the net power output of the proposed system is 13% more than that of a similar system without ejector and moreover the system thermal efficiency could approach 45%. By performance comparison with a cascade-type CCP system, the proposed system shows the advantages of simple structure and good economy. Finally a thermodynamic parameter analysis is carried out to investigate the effects of five parameters on the system performance. The results show that there is an optimal rectification column pressure leading to the maximal net power output. The increases of rectification column pressure, turbine back pressure and evaporation temperature have negative effects on cooling capacity and thermal efficiency of the system, while the increase of absorber pressure has positive effects on the two indicators. Especially for the increase of ammonia-weak solution temperature of rectification column bottom, all the system performance experiences a process of increasing sharply, increasing slowly and decreasing slightly.

A Conceptual Design and Analysis of a Novel Trigeneration System Consisting of a Gas Turbine Power Cycle with Intercooling, Ammonia–Water Absorption Refrigeration, and Hot Water Production

In this study, the performance of a novel trigeneration system with a gas turbine prime mover, an ammonia–water refrigeration system, and a hot water generation system is investigated from thermodynamic and economic standpoints. The effects of various operating conditions on energy efficiency and the levelized cost of energy are investigated. The proposed system has a production capacity of 45.4 kW power, 14.07 kW cooling rate, and 16.32 kW heat rate. The efficiency of the gas turbine cycle is 49.7%, and it becomes 83.0% after the implementation of the trigeneration system. Through combined heating, cooling, and power generation, primary energy input and the CO2 emissions will be 49% lower compared to separate production. According to the exergy analysis, the combustion chamber is the main component where the greatest exergy destruction occurs. Sensitivity analysis revealed that an increase in the ambient temperature results in a decrease in the energy utilization factor and the net power output. The LCOE of the system is around 0.02 $/kWh, whereas the unit price of the local electricity from the grid is 0.09 $/kWh. The payback period of the absorption sub-cycle is between 4 months and 4 years, depending on the annual operation time of the chiller.

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.

Comparative analyses of a novel solar tower assisted multi-generation system with re-compression CO2 power cycle, thermoelectric generator, and hydrogen production unit

In the present paper, a new energy generation system is suggested for multiple outputs, including a hydrogen generation unit. The plant is powered by a solar tower and involves six different subsystems; supercritical carbon dioxide (sCO2) re-compression Brayton cycle, ammonia-water absorption refrigeration cycle, hydrogen generation, steam generation, drying process, and thermoelectric generator. The thermodynamic assessment of the multi-generation system is carried out for three different cities from Turkey, Iran, and Qatar. The energy and exergy efficiencies are calculated for base conditions to compare the different locations. The operating output parameters for the suggested system and simple re-compression Brayton system are compared. A parametric analysis is also done for investigating the influences of different system variables on plant performance. According to the results, Doha city is found to be more effective due to its geographical conditions. Moreover, based on the comparative study, the proposed cycles produce more power than the basic re-compression cycle with 64.59 kW, 47.33 kW, and 52.25 kW for Doha, Isparta, and Tehran, respectively. Additionally, the analyses revealed that in the term of energy efficiency, the suggested system has 32.29%, 32.28%, and 32.29% better performance than the simple cycle, and in terms of exergy efficiency, it has 4%, 4.8%, and 5% better performance than the simple cycle in Doha, Isparta, and Tehran, respectively.

Performance of a Hybrid TEG/Single Stage Ammonia-Water Absorption Refrigeration Cycle with a Combined Effect of Rectifier and Condensate Precooler

In this article, a single stage ammonia-water absorption refrigeration cycle integrated with thermoelectric power generators driven by evacuated tube solar collectors is presented and investigated. The working fluid is ammonia-water solution, and the hybrid system was modeled, and analyzed using EES software. The system coefficient of performance (COP) was found to increase with increasing the generator temperature. From the results obtained, when the cycle operated with 0.99 ammonia mass fraction, it was found that the best location of the thermoelectric generators is at the rectifier between the generator and the condenser where the value of COP exceeds 0.82 at temperature difference across the TEG of 60℃. The other locations, between condenser-evaporator and evaporator-absorber, show similar performance.

Compression-assisted absorption refrigeration using ocean thermal energy

Renewable ocean energy is regarded as an attractive candidate resource for the development of maritime cold chain systems. To meet the refrigeration requirement of seafood freezing and preservation storage, this study proposes two-type compression-assisted ammonia-water absorption refrigeration cycles using ocean thermal energy. The effects of intermediate pressure on the exergy efficiency and primary energy rate ratio are analyzed and discussed. Moreover, the role of the heat source and sink temperatures on the primary energy rate ratio was examined. The results indicated that the absorption refrigeration system with a compressor in the low-pressure stage has a higher exergy efficiency and a higher primary energy rate ratio than the corresponding compressor in the high-pressure stage. For a warm seawater temperature of 29 °C and cold seawater temperature of 8 °C, the low-pressure compression-assisted refrigeration cycle is the best solution with an exergy efficiency up to 0.284, and the optimal primary energy rate ratio equals 1.392. Furthermore, a higher primary energy rate ratio can be realized by increasing warm seawater temperature or reducing cold seawater temperature.

Exergy analysis of a hybrid thermoelectric generator/two-stage double-lift aqua-ammonia absorption cooling system

Nowadays, absorption refrigeration cycles using renewable energy or waste heat are very attractive. A hybrid thermoelectric generator/two-stage double-lift ammonia-water absorption refrigeration cycle powered by an evacuated tube solar collector was examined. A detailed thermal analysis was carried out by means of the engineering equation solver. The typical location to install a thermoelectric generator was found to be after the evaporator and desorber 2. The coefficient of performance improves from 0.2 to 0.5 when the temperature difference after desorber 2 is raised from 5°C to 19°C. Installing TEGs after desorber 1 and condenser improves cycle performance by 0.153 and 0.168, respectively.

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.