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Numerical investigation on semi-GAX NH3-H2O absorption cycles
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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.
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... • In the generator, a saturation condition is set for the liquid and vapor leaving the phase-separator and a minimum pinch of 5 • C is set at the heat exchanger between hot water and solution (Xu and Wang, 2018). • At the absorber, a fixed subcooling of 1 • C at the solution outlet and minimum temperature difference of 5 • C is imposed along the heat exchanger (Toppi et al. 2016). • In the condenser, a fixed subcooling of 1 • C is imposed at the refrigerant outlet and the minimum temperature difference of 3 • C is imposed between the refrigerant and the cooling water (Toppi et al. 2016). ...
... • At the absorber, a fixed subcooling of 1 • C at the solution outlet and minimum temperature difference of 5 • C is imposed along the heat exchanger (Toppi et al. 2016). • In the condenser, a fixed subcooling of 1 • C is imposed at the refrigerant outlet and the minimum temperature difference of 3 • C is imposed between the refrigerant and the cooling water (Toppi et al. 2016). ...
... However, the use of additional heat exchangers may lead a disadvantage of the system. Toppi et al. [13] presented a double-lift based semi-GAX cycle using the ammonia-water mixture. In that cycle, the GAX effect was achieved by reusing a part of energy released from the one device of the absorption system to produce the vapour. ...
Combined absorption cooling device and humidification-dehumidification desalination component is newly proposed to be an alternative system to substitute the existing individual conventional arrangement for the betterment of energy usage. Specifically, among many waste heat recovery technologies, the combination of cooling and humidification-dehumidification processes is an innovative thought to implement a new thermodynamic system for improving the overall system performance. To be successful implementation of this innovation concept, in this study, both the above systems are combined to produce simultaneously 150 kW cooling effect and 0.1 kg s⁻¹ freshwater production. This combined system also improves the effective use of waste heat. A parametric study of the proposed system is carried out to determine the optimum operational effect by varying different design constants based on the energy, exergy, and economic analyses. A multi-objective optimization model is also developed using the non-dominated storing genetic algorithm (NSGA-II) to obtain the minimum operating cost. In the optimization study, the total exergy efficiency is considered to be a maximization function and the total product cost is treated as a minimization objective. The results obtained from the exergy and economic studies reveal that the cooling assembly is the primary source for the irreversibility production with the exergy destruction rate of 17.0415 kW. Also, the exergoeconomic factor of humidifier is the lowest one (0.94%). From the multi-objective optimization, the total exergy efficiency and the total product cost are calculated as 0.5014 and 68.1680 $ GJ⁻¹, respectively. At the optimum operating condition, calculated values of the gained output ratio (GOR) and coefficient of performance (COP) are 2.0181 and 1.2474, respectively. Furthermore, the present combined system is compared with the traditional humidification-dehumidification desalination system. This comparison highlights that the GOR of the proposed system is about 140% higher than the traditional system. Therefore, there is no doubt to be arising in mind that the operation of the proposed system produces more beneficial thermal aspects in comparison to the existing system.
Absorption heat pumps (AHPs) can be thermally activated by low-grade heat sources for space heating. The efficiency can be greatly improved by adopting GAX-based internal heat recovery, which lifts the heat source temperature demand and deteriorates the cold climate condition adaptability, and thus immensely limits the widespread application. In this paper, a hybrid GAX-based absorption-compression heat pump (ACHP) system is proposed to expand the geographical scope of application in extremely cold climate conditions, to lift the heat supply temperature, and more importantly to utilize heat sources of lower temperatures. A mathematical model was developed and validated in comparison with experimental and theoretical results from the literature. Feasible generation-evaporation temperature (Tg-Te) conditions to enable GAX were presented in detail. Coefficient of performance (COP), primary energy ratio (PER), heat supply temperature, heating capacity, and heat duty of main components were investigated concerning different working conditions. The possible sensible and latent heat recovery ratio of the GAX was explored. The results show that when Pr increases, the feasible Tg-Te area moves towards the lower Tg and Te sides. The system can be utilized for building floor heating with PER above 1.116, COP higher than 1.614, temperature lift (ΔTlift) up to 81.85 °C even when ambient air temperature (Tamb) is down to −40.6 °C, and for fan coil heating when Tamb is above −40.0 °C with PER above 1.074, COP over 1.615 and ΔTlift up to 85.92 °C. The system can utilize heat sources down to 80 °C by adjusting the working pressure ratio (Pr), highlighting the possibility of integration with solar collectors for heating purposes.
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.
The trend to reduce CO2 emissions in cooling processes has made it possible to increase the alternatives for integrating solar energy with thermal equipment whose viability depends on its adaptation to polygeneration schemes. Despite the enormous potential offered by the industry for cooling and heating processes, solar cooling technologies (SCT) have been explored in a limited way in the industrial sector. This work discusses the potential applications of industrial SCTs and classifies hybrid polygeneration schemes based on supplying cold, heat, electricity, and desalination of water; summarizes the leading SCTs, and details the main indicators of polygeneration configurations in terms of reductions on primary energy consumption and payback times. To achieve an energy transition in refrigeration processes, the scenarios with the most significant potential are: the food manufacturing industry (water immersion and crystallization processes), the beverage industry (fermentation and storage processes), and the mining industry (underground air conditioning).
Heat pump is effective to recover ultra-low grade waste heat, and includes compression and absorption heat pumps. However, the driving source of these heat pumps are different. This makes the efficiency comparison unfair, and multi-criterion comparison is necessary. In this paper, the compression and absorption heat pumps are compared under the same condition, where 30 oC waste heat is recovered to provide 60 oC domestic heating supply. Analyses with coefficient of performance (COP), second law efficiency, exergy efficiency and exergy rate are carried out. Exergy-to-energy ratio of driving source, instead of temperature, is used to unify the evaluation of different driving sources. Results show that compression heat pump has higher COP but lower exergy efficiency, indicating more irreversible loss. This is followed by double effect, single effect and double lift absorption heat pumps. The high COP lead to effective recovery of exergy from waste heat, with higher exergy rate. However, the strong sensitivity of COP versus temperature lift in compression heat pump makes it more effective under small temperature lift, while absorption heat pumps are more effective under higher temperature lift. The multi-criterion comparisons provide both deeper understanding about heat pumps and useful framework for waste heat recovery analysis.
An exhaustive analysis of double evaporator ammonia water vapour absorption refrigeration system (DEVARS) based on energy analysis has been carried out in this communication. To carry out analysis, a programme is made in the engineering equation solver (EES) software. The coefficient of performance (COP) is calculated at various operating conditions to study the effect of variation of evaporators, generators, absorbers and condenser temperatures and generators pressure on the COP. It is observed that by using DEVARS instead of using simple VARS, there is an increase of 11.11% COP when the load on both the evaporators is equal, i.e. 10 TR, and there is an increase of 22.22% COP when load on high temperature evaporator is 20 TR, and low temperature evaporator operates at no load. It is also observed that the variation in the temperature of absorber-2 has more effect on the COP of the system than the variation in absorber-1 and condenser temperatures. The study shows that NH3–H2O DEVARS is more promising than simple VAR systems with single evaporator in a wide operating range of evaporator temperatures.
Due to rapid technology scaling in the nanometer regime accompanied by supply voltage reduction, high leakage currents such as subthreshold leakage, junction leakage, and gate leakage have become prominent sources of power consumption in CMOS VLSI circuits. Consequently, it is imperative to estimate and decrease leakage power at the nanometer regime. However, this unabated scaling makes the CMOS circuits more prone to process (P), voltage (V), and temperature (T) variations at nanometer technologies. This work discusses a comprehensive investigation of different circuit-level leakage power reduction techniques such as power gating (PG), drain gating (DG), LECTOR, and GALEOR and analyzes the impact of PVT variations on leakage power dissipation and delay using ISCAS C17 benchmark circuit.
An equation of state for ammonia-water mixtures is presented. Separate equations of state for liquid and gas phases are provided for pure ammonia and pure water. In the gas phase, the mixture is assumed to behave as an ideal solution, while in the liquid phase, the Gibbs excess energy is used to allow departure from ideal solution behavior. An existing correlation for the liquid Gibbs excess energy is modified to include experimental data at higher temperatures and pressures. The new correlation covers vapor-liquid equilibrium pressures of 0.2 to 110 bar and temperatures of 230 to 600 K. Comparisons with reported experimental data show good agreement.
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.
An air-cooled two-stage NH3-H2O absorption refrigeration system is proposed for potential application of residential small scale cooling system driven by solar heated hot water. It can reduce the initial fabrication and maintenance costs of both the solar collection system and the absorption chiller. An experimental prototype for 2 kW cooling capacity has been built to investigate the feasibility and performance of the proposed system. The experimental results indicate that the prototype operates smoothly and steadily. When the prototype is driven by 85 °C hot water with an evaporating temperature of 8 °C and ambient air temperature of 29 °C, its thermal COP and electric efficacy (ε) reach 0.21 and 5.1, respectively. COP stabilizes within the range of 0.18–0.25, and ε varies between 3.6 and 5.1 under air-conditioning conditions in summer, which are the right applications of air-cooled two-stage absorption systems. The study reveals the technical feasibility of the air-cooled two-stage NH3-H2O absorption system. It provides a way to develop low-cost small bulk solar absorption air-conditioning systems for residential applications.
This project attempted to find some basic performance data for an earth-sink air conditioning system. The system failed to perform adequately, for reasons which may relate more to the approaches used than to the viability of the basic idea. Some possible reasons for the lack of performance are discussed in detail in the body of the report.
An experimental investigation on the performance of a two-stage half effect vapour absorption cooling system has been carried out and presented. The prototype is designed for 1kW cooling capacity using HFC based working fluids (R134a as refrigerant and DMAC as absorbent). The performance of the system in terms of degassing range, coefficient of performance and second law efficiency has been obtained. The system is capable of producing evaporating temperature as low as −7°C with generator temperatures ranging from 55 to 75°C. The degassing range is 40% more in high absorber than in low absorber as the high absorber is operated at optimum intermediate pressure. The optimum generator temperature is found to be in the range of 65–70°C at which the coefficient of performance is 0.36.
A method is presented by which the coefficients of performance of advanced absorption cycles can be calculated very quickly if the efficiencies of single-stage heat pumps and heat transformers are known. The method provides detailed results for the heat input or output of the individual exchange units. These values can be used as a basis for comparison of different cycles, for numerical cycle calculations or for estimates of investment costs. Examples are given for heat pumps, refrigerators, heat transformers and heat pump transformers. Numerical results are presented for the working fluid pairs H2O/LiBr and NH3/H2O.RésuméA method is presented by which the coefficients of performance of advanced absorption cycles can be calculated very quickly if the efficiencies of single-stage heat pumps and heat transformers are known. The method provides detailed results for the heat input or output of the individual exchange units. These values can be used as a basis for comparison of different cycles, for numerical cycle calculations or for estimates of investment costs. Examples are given for heat pumps, refrigerators, heat transformers and heat pump transformers. Numerical results are presented for the working fluid pairs H2O/LiBr and NH3/H2O.
At present, much interest is being shown in absorption refrigeration cycles driven by low temperature heat sources, such as solar energy or low-grade waste-heat. Double-lift absorption cycles working with ammonia-water have been recommended for refrigeration applications which require cold at 0°C and which are activated by waste heat between 70 and 100°C. This paper discusses the potential of the organic fluid mixtures trifluoroethanol (TFE)-tetraethylenglycol dimethylether (TEGDME or E181) and methanol-TEGDME as working pairs in series flow and vapour exchange double-lift absorption cycles. The ammonia-water mixture was used for comparison purposes. The results show that the performances of these cycles improve significantly when they have the above mentioned organic fluid mixtures as working pairs. For example, the coefficient of performance of the vapour exchange cycle working with TFE-TEGDME is 15% higher than with ammonia-water. In this study, we used a modular software package, which we developed for the thermodynamic properties and cycles simulation of absorption systems.