Generator Absorber Heat Exchanger Cycle Research Articles

Energy, economic, and environmental evaluation of a GAX-based cross-type absorption-resorption heat pump

Thermally driven absorption heat pump is regarded as one of the most promising methods to efficiently utilize low-grade heat, reduce energy consumption and greenhouse gas emissions. However, there are still limitations when using it at low ambient temperature in cold regions, and its application is limited by their low system performance. In this study, we proposed a novel Generator-absorber heat exchange (GAX) based NH3/H2O cross-type absorption-resorption heat pump driven by waste flue gas, which amalgamates the absorption-resorption and GAX cycles, yielding a dual advantage of diminished heat source temperature and operating pressure requirements. Additionally, it substantively enhances the system's performance by capitalizing on the cross-type solution cycle integration. A mathematical model has been developed and the results have been compared and validated with those reported in the literature. The proposed system is comprehensively studied from the view of energy, exergy, economic and environmental (4E) analysis to determine its advantages and disadvantages. The results reveal that the proposed system achieves a maximum COP of 2.742 under specific operating conditions, which is 82.8% higher than the conventional absorption heat pump (AHP), and 30.57% higher than the conventional GAX-AHP. Besides, the addition of a compressor can significantly enhance the performance of the system at lower driving heat source temperatures. At a PH/PL pair of 1.7/0.47 MPa, the static payback period is 4.113 years, with the heating capacity of 88.51 kW and the supply water temperature of 54.6 °C, showcasing favorable system performance. Furthermore, the total atmospheric pollutant emissions could be reduced by 30,998 kg/year compared to the conventional heating method.

Global sensitivity analysis of a generator-absorber heat exchange (GAX) system’s thermal performance with a hybrid energy source: An approach using artificial intelligence models

• Temperature, heat and flow measurements of an experimental GAX cycle. • Evaluation of artificial intelligence (AI) techniques for multivariate modeling. • Artificial neural networks represent the GAX system with the best accuracy. • AI model coupled to the PAWN method for a global sensitivity analysis of GAX cycle. • COP is affected the most by the temperature measured at the generator inlet. Generator-absorber heat exchange (GAX) systems represent a promising alternative to substitute environmentally harmful refrigeration devices based on conventional vapor compression, as long as a proper analysis of thermal performance and the complex interactions of heat transfer that occur into GAX cycle is taken in consideration. In this research, a cooling process based on a GAX system that uses ammonia-water working fluid and a hybrid source (natural gas-solar) is studied to analyze the variables that affect the system’s thermal performance. The work’s novelty is the hybridization between artificial intelligence (AI) modeling and the global sensitivity analysis (GSA) developed with the PAWN method. Experimental data was obtained from a system with a cooling capacity of 10.5 kW (3 Ton), designed to work at heat source temperatures of 200 °C. The measured variables were the temperatures at generator, heat at evaporator, and working fluid volumetric flow. Three AI techniques (artificial neural networks, genetic programming, and support vector machines) were evaluated for modeling the thermodynamic cycle. Results obtained from the PAWN method applied to the artificial neural network, since it was the best AI model, indicates that the operational parameters with a greater impact in the system’s performance are the inlet temperature at the generator (30.7 %) and the heat measured at the evaporator for NH 3 (27.4 %), for the first output COP N H 3 . For the second output COP H 2 O , the inlet temperature at the generator (32.5 %) and the and heat measured at the evaporator for H 2 O (26.7 %), have a greater impact for such output. The proposed IA-GSA methodology contributes to the development of operational decision-making related to instrumentation, operation performance, and corrective and/or preventive maintenance actions of GAX systems. The developed thermal performance model has potential for implementation in embedded systems (smart sensors) as a critical element in control and optimization strategies to improve the performance of these cycles.

Seasonal performance assessment of three alternative gas-driven absorption heat pump cycles

When designing a gas driven heat pump, the selection of the cycle impacts both on complexity and efficiency. Among absorption cycles, the nominal efficiency usually increases with complexity. However, heat pumps are required to operate over a wide range of conditions, of different temperatures and capacity. In this research, three alternative cycles, i.e single-effect (SE), generator-absorber heat-exchange (GAX) and vapor-exchange (VX) for a gas-driven air-source ammonia-water absorption heat pump are compared, considering small-scale space heating as target application. For each cycle, both full-load under various temperature conditions and seasonal performances are predicted by means of numerical simulations. Small capacity appliances are usually equipped with fixed geometry restrictors, meaning that the solution mass flow rate is driven by the pressure difference across the associated restrictor valve. This feature is included in the modeling assumption as it affects the temperature of the generator and, ultimately, the performance at the various conditions. At full load, the VX cycle resulted to achieve the highest Gas Utilization Efficiency (GUE), followed by the GAX and SE. The differences among the cycles are higher at low thermal lift conditions, where the GAX effect is achieved in both the GAX and VX cycles, while are limited at high thermal lift. Looking at the seasonal performances, calculated on the basis of the method prescribed in the European standard EN 12309, the differences among the cycles became much narrower, with the Seasonal GUE ranging between 1.427 (SE) and 1.493 (VX). The small differences in terms of seasonal performances are explained considering that, in the prescribed working conditions, the GAX effect is only partially exploited.

Thermo-economic analysis of a novel hybrid multigeneration system based on an integrated triple effect refrigeration system for production of power and refrigeration

Multigeneration systems provide substantial benefits in terms of improving efficiency and economics as well as environmental benefits. In this study, a multigeneration system is proposed for generating power and triple effect refrigeration at different evaporation temperatures. The system hybridizes a 25 MW recuperative Brayton cycle with an organic Rankine cycle (ORC) and an integrated refrigeration system that combines a branched generator–absorber heat exchange (GAX) absorption refrigeration system with a dual-evaporator cascade carbon dioxide-ammonia compression refrigeration system. The exhaust gas from the Brayton cycle is divided into two streams: one is supplied to the generator of the GAX cycle and the remaining is supplied to the ORC for additional power generation. Results show that additional power generation by ORC increases thermal efficiency from 0.394 to 0.510 and reduces the levelized cost of electricity from $75.5/MWh to $64.4/MWh. The integration of GAX cycle and cascade cycle is beneficial from both thermodynamic and economic viewpoints. Due to their integration, the coefficient of performance of the cascade cycle increases substantially from 2.37 to 4.69. Also, the levelized cost of cooling for the production of triple effect refrigeration decreases from $0.08/ton-hr to $0.059/ton-hr. Environmental analysis shows that the development of multigeneration system can lead to avoiding 24,185 tons of CO2 per year. The proposed multigeneration system brings a possibility to efficiently recover the waste energy for additional power generation and refrigeration production at different temperatures. Waste energy recovery by the ORC and GAX cycle as well as the integration of the GAX cycle with a cascade refrigeration cycle provide good potential for cleaner production of all products with improved efficiency, economics and environmental benefit.

Experimental investigation on NH3–H2O generator-absorber heat exchange (GAX) absorption heat pump

Compared with conventional fuel-direct-burning heating systems, heat-driven absorption heat pump has high efficiency. To further improve the heating performance, generator-absorber heat exchange (GAX) absorption heat pump was developed, but its experimental research was not enough. The parameters tested were usually just COP and heating capacity in the existing literature, the variation of some internal parameters were rarely measured and analyzed. Besides, the latent heat exchange between the generator and the absorber could not continue normally under high thermal lift (TCON-TEVA) and low thermal thrust (TGEN-TABS), instead, more sensible heat is transferred, the temperature overlap decreases or even disappears, and the circulation process of GAX cycle approaches to that of single-effect cycle. This phenomenon is named “cycle degradation”, and is crucial to understand the working principle of GAX for optimal design and control. The cycle degradation was only simulated under low evaporation temperature, but has not been analyzed with experimental data, the influence rules of other external operational parameters on the phenomenon are also not clear. Therefore, a prototype of GAX absorption heat pump is built up, many internal and external parameters are measured and used to investigate the cycle degradation and overall performance under different testing conditions. During the experiment, the COP is in the range of 1.185–1.506, the cycle degradation occurs when the “lifting factor (Eq [18])” is lower than 2.3. Moreover, the performance improves with the increase of driving temperature or low-grade heat source temperature, and deteriorates due to the rise of supply water temperature.

Thermodynamic analysis of GAX and hybrid GAX aqua-ammonia vapor absorption refrigeration systems

A thorough analysis of aqua-ammonia generator-absorber-heat exchanger (GAX) and hybrid GAX (HGAX) absorption refrigeration cycles based on energy and exergy has been carried out in this communication. The coefficient of performance (COP) and exergetic efficiencies are calculated at various operating conditions to study the effect of generator temperature, condenser temperature and evaporator temperature on them. The influence of generator temperature on exergetic efficiency is more pronounced than on COP. The effects of degassing range and approach temperature on first and second law efficiency are also examined. It is observed that the increase in approach temperature from 0 °C to 14 °C causes decrease in COP of GAX cycle by 30% and of HGAX cycles by 40%–45%. Desorber and absorber together accounts for highest exergy destruction.

Design and Construction of an Air Cooled Ammonia Absorber

This paper presents the design criteria, methodology. and results of the basic and detailed engineering for a descending film ammonia absorber using air cooled finned tubes, which is part of an advanced absorption cooling system (solar generator absorber heat exchange cycle). The design consists in determining all the construction parameters for the air cooled ammonia absorption unit, starting with the operating conditions defined by a thermodynamical simulation of the process considering both physical and operational design restrictions. The chosen option was based on a comparison between the advantages and disadvantages of each possible array, type, and geometry. After performing the operational simulation, thermal and mechanical designs, and the consistency analysis, it was found that an absorption unit using 29 5/8 NPT 14 (BWG) steel carbon ASTM A-179 tubes, with pure SB-234 aluminum fins was the best option. The tubes are arranged in an equilateral triangle fashion, with crossed air flow cooling.

Performance analysis of ammonia absorption GAX cycle for combined cooling and hot water supply modes

An ammonia Generator–Absorber heat eXchange (GAX) absorption cycle with combined cooling and hot water supply modes is developed in this study. This paper proposes new multi-modes GAX cycles which function in three different modes (case 1, case 2 and case 3) of cooling and hot water supply with one hardware (ammonia/water GAX absorption heat pump), and finds the best cycle for performance improvement by the parametric analysis. The key parameters are the outlet temperature of hot water and the split ratio of the solution. It is found that the COP c values for case 1, case 2 and case 3 are 60%, 42% and 87% of COP c for case 0, respectively, which is the standard cooling mode for the conventional GAX cycle. From the viewpoints of hot water supply, case 1 gives the best performance. However, during the summer season when the cooling mode is the primary purpose rather than the hot water supply, case 3 is the most desirable cycle. The split ratio of the solution should be carefully determined depending on the primary application of the modified GAX cycle; cooling or hot water supply applications. It is also recommended that the optimum design values of UA SCA and UA HCA for case 3 should be less than those for case 1.

α– h Diagram and principle of exergy coupling of GAX cycle

Ammonia absorption chiller systems of a single-stage cycle and a Generator Absorber heat exchanger cycle (GAX) were simulated and studied. At heat source temperatures of T H = 120 °C, T M = 25 °C and T L = 5 °C, the coefficient of performances of the two cycles are 0.589 and 0.776, the GAX cycle is higher 31.8% than the single-stage cycle. And the exergy efficiencies of the two cycles are 15.4% and 27.4%, the GAX cycle is higher up to 77.9%. This paper proposes a new method that adopts the energy quality factor α as a evaluation criterion and also uses the α– h diagram as a thermodynamic analysis tool graphically, and a concept that divides absorption cycle to a heat pump subcycle and a heat engine subcycle. By means of the α– h diagram, the thermodynamic frameworks of the two cycles were illustrated. The comparison analysis indicates that the improvement of cycle performance depends on its thermodynamic perfectibility. In fact, the exergy demand of heat pump subcycle in the GAX cycle is as the same as that of the single-stage cycle, however, the energy cascading use and the exergy coupling framework of the heat engine subcycle in GAX cycle is retrofitted, so that the exergy consumption is reduced and the increased benefit is obtained from the overall cycle.

Development of a slug flow absorber working with ammonia-water mixture: part I—flow characterization and experimental investigation

This study deals with an experimental investigation for a counter-current slug flow absorber, working with ammonia–water mixture, for significantly low solution flow rate conditions that are required for operating as the GAX (generator absorber heat exchanger) cycle. It is confirmed that the slug flow absorber operates well at the low solution flow rate conditions. From visualization results of the flow pattern, frost flow just after the gas inlet, followed by slug flow with well-shaped Taylor bubble, is observed, while dry patch on the tube wall are not observed. The liquid film at the slug flow region has smooth gas–liquid interface structure without apparent wavy motion. The local heat transfer rate is measured by varying main parameters, namely, ammonia gas flow rate, solution flow rate, ammonia concentration of inlet solution and coolant inlet conditions. The heat transfer rate while absorption is taking place is higher than that after absorption has ended. The absorption length is greatly influenced by varying main parameters, due to flow conditions and thermal conditions.

Methodology for the energy analysis of an air cooled GAX absorption heat pump operated by natural gas and solar energy

In this work a methodological analysis and energy evaluation of an air cooled absorption system, with generator–absorber heat exchange (GAX), and operated by a hybrid natural gas–solar energy source is presented. Given the characteristic non-linearity of the resulting system of equations, the methodology proposed envisages a calculation sequence for the external currents and an iterative procedure for the internal currents. The system studied intents to be an alternative for space conditioning in the residential sector, the unit was designed with a capacity of 10.6 kW (3 Ton) of cooling and uses as working fluid ammonia–water. Giving priority to internal energy integration, an arrangement is proposed for the GAX cycle that allows 19% of solar contribution at full load, being greater at partial loads. In spite of using ambient air up to 40 °C with a relative humidity of 24% as cooling source, a COP value of 0.86 for cooling and 1.86 for heating was calculated, with an internal energy integration of 16.9 kW, 37% more than the energy that is supplied to the generator.

Performance of a Residential-Sized GAX Absorption Chiller

This research effort involved experimentally testing an advanced-cycle, ammonia-water absorption chiller with a cooling capacity of 17.6 kW (5 refrigeration tons (RT)). The system was a generator-absorber heat exchange (GAX) cycle and was sized for residential and light commercial use, where very little absorption equipment is currently used. The components of the cycle were assembled with instrumentation, including flow meters, pressure transducers, and thermocouples. The findings of the research were cycle cooling load and coefficient of performance (COP), as well as many component heat duties and working fluid state points throughout the cycle. The COP of the chiller at essentially full load was measured at 0.68. A simulation of the GAX cycle was performed with a computer program that predicted the heat duties of each component and the cooling load of the cycle. The simulation of the GAX cycle and experimental testing compared closely. Existing market research shows that significant business opportunities exist for a GAX heat pump or chiller with a cooling COP of 0.70 or greater. The work performed in this study consisted of testing a GAX cycle with a COP that approached the target value of 0.70 and identified improvements that must be made to reach the target COP value.

An advanced GAX cycle for waste heat recovery: WGAX cycle

The objectives of this paper are to develop an advanced generator absorber heat exchanger cycle (WGAX) to reduce the generator exit temperature as low as possible using waste heat sources, and to compare it with the standard GAX cycle (SGAX). This paper performed parametric analysis to study the effects of the waste heat source temperature ( T w) and the outlet temperature of a gas fired desorber (GFD), T g, on the cycle performance. Three different WGAX cycles (type A, type B and type C) were compared from the viewpoint of performance improvement. It was found that the effect of the waste heat source temperature ( T w) on coefficient of performance (COP)s was negligible for a given GFD outlet temperature in the WGAX cycles. The GFD outlet temperature could be reduced down to 172°C with a higher COP β of WGAX cycle than the COP of the SGAX cycle. Therefore, the corrosion problem in the SGAX cycle at a higher T g than 200°C will be solved by adopting the WGAX cycles with a comparable COP. Type A had a merit from the viewpoint of the GAX effect while Type B had a merit from the viewpoint of exergy loss effect. In the WGAX cycles developed in this study, the GAX effect was dominant for a lower temperature than 181°C while the effect of exergy loss was dominant for a higher temperature than 181°C. It is found that the solution heated desorber should be placed below the GAXD to improve the cycle performance in the WGAX cycles. It is strongly recommended that there be a subcooling effect in the weak solution for cycle performance enhancement in the WGAX systems.

Comparative simulation and investigation of ammonia-water: absorption cycles for heat pump applications

Several recent programs in absorption research have focused on technology for domestic heating and cooling utilizing natural gas. In residential and small commercial size applications, ammonia-water cycles offer the possibility of a gas-fired heat pump for both winter heating and summer cooling, at better year-round COPS than currently available by various alternatives. Several cycles have been considered for this purpose, ranging from the simplest single effect to the GAX (Generator-Absorber heat eXchange) with its different variations. Detailed calculations of ammonia-water systems are rather difficult, particularly in complex cycles such as the GAX. This may be the reason that relatively few simulation studies have been published to date that give more than design point performance. The objective of the present study has been a comprehensive investigation of various ammonia-water cycles, with operating conditions and different design parameters varied over a wide range to compare their performance. To this end, a modular simulation program (ABSIM) was employed, which makes it possible to simulate absorption cycles in varying configurations. The cycles investigated increase in complexity from the basic single-effect cycle, through the same cycle with an added precooler, through an added solution-cooled absorber and solution-heated generator, to the GAX and branched GAX cycles with different types of branching. Each cycle was formed on the basis of the previous one by adding one or two components at each stage, resulting in increasing complexity rewarded by improved performance. This kind of investigation enabled determination at each stage of the influence of the added components on the cycle. The results of the investigation show cooling CON ranging from about 0.5 for the simplest single-effect cycle to about 1.0 for the GAX cycle.

Performance evaluation of a generator-absorber heat-exchange heat pump

A generator-absorber heat-exchange (GAX) heat pump was modeled in this study using OSU-ABSIM, a modular steady-state simulation program. While the thermodynamics of GAX cycles have been investigated in the past, a comprehensive implementation-related performance evaluation of this cycle is not available. Therefore, in the present study, the complete absorption system was analyzed for the heating and cooling modes, including air-to-hydronic heat exchangers and a natural gas-fired generator. Several significant variables that affect cycle performance were systematically investigated. The effect of ambient temperature on system COP was determined for the cooling and heating modes. The system cooling COP at the rating point was maximized by varying the heat-exchanger UA values. The decrease in GAX overlap at low ambients, and the corresponding transformation to the absorber heat-exchange cycle was also modeled; and the implications on system COP were investigated. Furthermore, the role of an additional solution-solution heat exchanger at the low ambients in enhancing COP was quantified. The results of this study can be used to determine the merits of this cycle versus other absorption heat-pump alternatives. The comprehensive investigation of performance (including off-design conditions) allows the evaluation of year-round performance, which can be used in conjunction with climate data to establish the applicability to different geographic locations.

Polybranched regenerative GAX cooling cycles

After a short characterization of today's advanced absorption cooling systems, of multi-effect and generator-absorber heat exchange (GAX) type, this work introduces polybranched regenerative GAX cycles (PBR(GAX)). They combine the advantages of four cycles: GAX, branched GAX, regenerative GAX, and those with rectification heat recovery. The general case of n-stage PBR, working with volatile or non-volatile absorbents, is thermodynamically modelled. The model is applied to cycles with one to three stages working with the NH 3 H 2 O LiBr combination, selected according to a special criterion, which has in view to maximize the GAX effect. The thermal match in the GAX device is verified using our own complete algorithm. The results highlight the net superiority of the cooling COP of PBR relative to any other published GAX or multi-effect cycles. Compared with those with vapour exchange, operated with NH 3 H 2 O , one of the best GAX cycles till now, a three-stage PBR has 1.25–1.9 times higher COP values and represents 71–82% of the Carnot cooling efficiency for lifts varying between 68 and 47°C. The PBR cycles are capable of simultaneously delivering cold and warm water, at 50–70°C. Compared with a double-effect cycle, provided with rectification heat recovery, a two-stage PBR is constructively simpler and thermally with about 40% more performance.