Theoretical analysis of LiBr/H2O absorption refrigeration systems

A computational model is developed for the parametric investigation of single‐effect and series flow double‐effect LiBr/H₂O absorption refrigeration systems. The effects of generator, absorber, condenser, evaporator and dead state temperatures are examined on the performance of these systems. The parameters computed are coefficient of performance (COP), exergy destruction rates, thermal exergy loss rates, irreversibility and exergetic efficiency. The results indicate that COP and exergetic efficiency of both the systems increase with increase in the generator temperature. There exist different optimum values of generator temperature for maximum COP and maximum exergetic efficiency. The optimum generator temperature is lower corresponding to maximum exergetic efficiency as compared to optimum generator temperature corresponding to maximum COP. The effect of increase in absorber, condenser and evaporator temperatures is to decrease the exergetic efficiency of both the systems. The irreversibility is highest in absorber in both systems. Copyright © 2009 John Wiley & Sons, Ltd.     

Thermodynamic performance of absorption-compression hybrid refrigeration cycles based on lithium nitrate+1-butyl-3-methylimidazolium nitrate/water working fluid

In this paper, LiNO₃ (lithium nitrate)-[BMIM]NO₃ (1-butyl-3-methylimidazolium nitrate)/H₂O is presented for absorption-compression hybrid refrigeration. The thermophysical properties and corrosion of this ternary system were measured. The performance of this hybrid system using LiNO₃-[BMIM]NO₃/H₂O was studied and compared with those based on LiBr (lithium bromide)/H₂O, LiNO₃/H₂O, and LiNO₃-[MMIM][DMP] (1,3-dimethylimidazolium dimethylphosphate)/H₂O. Results showed that the hybrid refrigeration cycles can work at much lower generator and evaporator temperatures in comparison with the pure absorption cycle. Generator temperatures of both hybrid cycles based on this working fluid were lower than those using the other working fluids. At different refrigeration temperatures, the generator temperatures of both hybrid cycles using this presented working fluid were reduced about 7 and 10 K, respectively, in comparison with those based on LiBr/H₂O. Compared with other working fluids, both single- and double-effect hybrid systems using this new working fluid achieved a larger coefficient of performance around 0.88 and 1.6, respectively. Additionally, the exergetic efficiency for both hybrid cycles is also about 10% larger than that using LiBr/H₂O. And this working fluid shows an excellent thermal stability below 598.32 K and acceptable high-temperature anti-corrosion to metallic materials. Moreover, the hybrid cycle using LiNO₃-[BMIM]NO₃/H₂O of 80 kW has a shorter payback period of 5.51 years when utilizing solar energy as the driven heat source. Therefore, LiNO₃-[BMIM]NO₃/H₂O shows great advantages in the absorption-compression hybrid cycles.     

Cycle analysis of air-cooled absorption chiller using a new working solution

A cycle analysis was achieved to predict the characteristics by comprehensive modelling and simulation of an air-cooled, double-effect absorption system using the new H₂O/LiBr+HO(CH₂)₃OH solution. The simulation results showed that the new working fluid may provide the crystallisation limit 8% higher than the conventional H₂O/LiBr solution. With a crystallisation margin of 3 wt%, the optimal solution distribution ratio was found to be in the range of 37–39%. Variation of cooling air inlet temperature had a sensitive effect on the cooling coefficient of performance (COP) and corrosion problem. The simulation of heat exchangers with UA value revealed that the absorber and evaporator were relatively important for an air-cooled system compared with the condenser and the low temperature generator. The effects of cooling air flow rate, circulation weak solution flow rate and chilled water inlet temperature were also examined. The new working fluid may provide a COP approximately 3% higher than the conventional H₂O/LiBr solution in normal conditions of circulation weak solution.     

Simulation of the compressor-assisted triple-effect H2O/LiBr absorption cooling cycles

The construction of a triple-effect absorption cooling machine using the lithium bromide-based working fluid is strongly limited by the corrosion problem caused by the high generator temperature. In this study four compressor-assisted H₂O/LiBr cooling cycles were suggested to solve the problem by lowering the generator temperature of the basic theoretical triple-effect cycle. Each cycle includes one compressor at a different state point to elevate the pressure of the refrigerant vapor up to a useful condensation temperature. Cycle simulations were carried out to investigate both a basic triple-effect cycle and four compressor-assisted cycles. All types of compressor-assisted cycles were found to be operable with a significantly lowered generator temperature. The temperature decrements increase with elevated compression ratios. This means that, if a part of energy input is changed from heat to mechanical energy, the machine can be operated in a favorable region of generator temperature not to cause corrosion problems. In order to obtain 40 K of generator temperature decrement (from 475.95 K) for all cycles, 3–5% of cooling capacity equivalent mechanical energies were required for operating the compressor. A great advantage of the investigated triple-effect cycles is that the conventionally used H₂O/LiBr solution can be used as a working fluid without the danger of corrosion or without integrating multiple solution circuits.     

Comparative study of a cascade cycle for simultaneous refrigeration and heating operating with ammonia,R134a,butane, propane,and CO2 as working fluids

In this paper, a cascade system for simultaneous refrigeration and heating is simulated with different working fluids. Ammonia, R134a, butane and propane are evaluated in the low-temperature (LT) cycle and carbon dioxide (CO₂) is used in the high cycle. The effects of the thermodynamic parameters on the cascade system are evaluated with the aim of finding the best working fluid performance and optimum design parameters. Coefficients of performance (COP) and exergetic efficiencies were estimated for each one of the cycles and for the entire system. The behaviour of these parameters is presented as a function of the internal heat exchanger effectiveness and main operating system temperatures. The results showed that the cascade system using butane in the LT cycle increased the COP up to 7.3% in comparison with those obtained with NH₃–CO₂. On the other hand, the cascade systems operating with the mixtures R134a–CO₂ and propane-CO₂ presented similar results reaching COPs up to 5% higher than those obtained with the NH₃–CO₂ system.     

A review and new approach to minimize the cost of solar assisted absorption cooling system

Solar energy is an alternative energy source for cooling systems where electricity is demand or expensive. Many solar assisted cooling systems have been installed in different countries for domestic purpose. Many researches are going on to achieve economical and efficient thermal systems when compared with conventional systems. This paper reviews the past efforts of solar assisted-single effect vapour absorption cooling system using LiBr–H₂O mixture for residential buildings. Solar assisted single-effect absorption cooling systems were capable of working in the driving temperature range of 70–100 °C. In this system LiBr–H₂O are the major working pairs and has a higher COP than any other working fluids. Besides the review of the past theoretical and experimental investigations of solar single effect absorption cooling systems, some new ideas were introduced to minimize the capital and operational cost, to reduce heat loss from generator and thus to increase COP to get effective cooling.     

Thermodynamic performances of [mmim]DMP/Methanol absorption refrigeration

In order to study the theoretical cycle characteristic of [mmim]DMP(1-methyl-3-methylimidazolium dimethylphosphate)/methanol absorption refrigeration,the modified UNIFAC group contribution model and the Wilson model are established through correlating the experimental vapor pressure data of [mmim]DMP/methanol at T=280～370K and methanol mole fraction x=0.529～0.965.Thermodynamic performances of absorption refrigeration utilizing [mmim]DMP/methanol,LiBr/H2O and H2O/NH3 are investigated and compared with each other under the same operating conditions.From the results,some conclusions are obtained as follows:1)the circulation ratio of the [mmim]DMP /methanol absorption refrigeration is higher than that of the LiBr/H2O absorption refrigeration,but still can be acceptable and tolerable.2)The COP of the [mmim]DMP/methanol absorption refrigeration is smaller than that of the LiBr/H2O absorption refrigeration,while it is higher than that of the H2O/NH3 absorption refrigeration under most operating conditions.3)The[mmim]DMP/methanol absorption refrigeration are still available with high COP when the heat source temperature is too high to drive LiBr/H2O absorption refrigeration.     

Development and comparison of advanced cascading cycles coupling a solid/gas thermochemical process and a liquid/gas absorption process

Currently marketed double-effect absorption machines attain coefficients of performance (COP) of the order of 1.2 and, therefore, do not enable standard vapour compression air-conditioning systems to compete. The improvement of the COP requires increasing the high driving temperature level of the system in order to make possible additional stages and further refrigeration effects. But at high temperatures, working couples currently used in absorption systems (H₂O/LiBr, NH₃/H₂O) pose corrosion problems for exchangers or decomposition of the working fluid. The implementation at these high temperatures of a solid/gas thermochemical reaction system enables bypassing these restrictions. The coupling of a chemical reaction process thermally cascaded with a liquid/gas absorption process enables leading to triple-effect machines, indeed quadruple effect, the COP of which are from 30% to 60% higher than commercialised double-effect absorption chillers. Numerous coupling configurations are presented in this paper. A method of evaluation of the COP of the global machine is also developed. A comparison of these different configurations is carried out through value criteria characterising the quality of the obtained coupling. In this way, a first selection of combinations of interest can be performed. As part of a Franco–German cooperation, a triple-effect machine based on this approach is currently being realised. This new concept of coupling must lead to a new generation of thermal machines which will be capable in the near future of competing with vapor compression machines by the complementary use of the potentialities appropriate to each of the sorption processes.     

Efficiency of Cascade Refrigeration Cycle Using C3H8, N2O,and N2

This article presents a new natural gas liquefaction cycle that utilizes propane (C₃H₈), nitrogen monoxide (N₂O), and nitrogen gas (N₂) cycles. A liquefaction cycle with staged compression was designed and simulated using HYSYS software for improving cycle efficiency. This included a cascade cycle with a three-stage compression consisting of a C₃H₈, an N₂O, and an N₂ cycle. The new C₃H₈, N₂O, and N₂ liquefaction cycle is compared to the optimized cascade cycle using propane, ethylene, and methane. The compressor work, specific energy, and coefficient of performance (COP) of the cascade cycles were compared and analyzed. The COP of the new cascade cycle that utilizes three-stage compression process 3 is 25% higher than that of basic single-stage process 1. Also, the new liquefaction cycle requires less specific power for the same amount of liquefied natural gas (LNG) produced by optimized cascade cycle. For example, the specific power and COP of the new cascade cycle are respectively up to 16% less and 14% higher than those of the optimized cascade cycle under the same conditions of feed gas composition and liquefaction rate.     

Experimental study of operating characteristics of compression/absorption high-temperature hybrid heat pump using waste heat

This research describes the development of a compression/absorption hybrid heat pump system that utilizes a mixture of NH₃ and H₂O as a working fluid. The heat pump cycle is based on a hybrid combination of vapor compression cycle and absorption cycle. The system consists of major components of two-stage compressors, absorbers, and a desorber. There are also auxiliary parts like a desuperheater, solution heat exchangers, a solution pump, a rectifier, and a liquid/vapor separator to support stable operation of the heat pump. This compression/absorption hybrid heat pump provides many advantages of performance over conventional vapor compression heat pumps including a large temperature glide, an improved temperature lift, a flexible operating range, and greater capacity control. These benefits are optimized by changing the composition of the mixture. In this study, the effect of the composition on the operating characteristics of the compression/absorption hybrid heat pump was experimentally observed.     

Performance of one and a half-effect absorption cooling cycle of H2O/LiBr system

The performances of half-effect, single-effect and double-effect H₂O/LiBr absorption cooling cycles were analyzed, and it was found that there is an obvious blank for generation temperature between the maximum generation temperature of the single-effect cycle and the minimum generation temperature of the double-effect cycle. It was proposed that the one and a half-effect (1.5-effect) cycle can fill up the blank perfectly. The state of the art in the 1.5-effect cycles was reviewed and analyzed, and two new configurations of 1.5-effect cycles were proposed. Three configurations of 1.5-effect cycles, which are suitable for H₂O/LiBr as working fluids, were selected to be analyzed in detail. The 1.5-effect cycle shows the optimum performance at the foregoing blank of generation temperature. For example, under the conditions of evaporation temperature t_E is 5 °C, and condensation temperature t_C is 42 °C, and absorption temperature t_A is 37 °C, the optimum range of generation temperature t_G for the 1.5-effect cycle is from 110 °C to 140 °C. The coefficient of performance of the 1.5-effect cycle is about 1.0, which is more than 30% higher than that of the single-effect cycle at the same condition. The effects of the efficiency of solution heat exchanger, the generation temperature, the absorption temperature (or the condensation temperature) and the evaporation temperature on the performances of the three configurations of 1.5-effect cycle were analyzed. It was shown that the configuration II, which is composed with a high-temperature single-effect subcycle and a low-temperature half-effect subcycle, has the highest coefficient of performance and the best operational flexibility. Among the four parameters analyzed, the performances of 1.5-effect cycles are most sensitive to the change of absorption temperature (or condensation temperature), and then to the change of generation temperature.     

Modeling and performance analysis of an absorption chiller with a microchannel membrane‐based absorber using LiBr‐H2O,LiCl‐H2O,and LiNO3‐NH3

In order to develop compact absorption refrigeration cycles driven by low heat sources, the simulated performance of a microchannel absorber of 5‐cm length and 9.5 cm³ in volume provided with a porous membrane is presented for 3 different solution‐refrigerant pairs: LiBr‐H₂O, LiCl‐H₂O, and LiNO₃‐NH₃. The high absorption rates calculated for the 3 solutions lead to large cooling effect to absorber volume ratios: 625 kW/m³ for the LiNO₃‐NH₃, 552 kW/m³ for the LiBr‐H₂O, and 318 kW/m³ for the LiCl‐H₂O solutions given the studied geometry. The performance of a complete absorption system is also analyzed varying the solution concentration, condensation temperature, and desorption temperature. The LiNO₃‐NH₃ and the LiBr‐H₂O solutions provide the largest cooling effects. The LiNO₃‐NH₃ can work at a lower temperature of the heating source, in comparison with the one needed in a LiBr‐H₂O system. The lowest cooling effect and coefficient of performance are found for the LiCl‐H₂O solution, but this mixture allows the use of lower temperature heating sources (below 70°C). These results can be used for the selection of the most suitable solution for a given cooling duty, depending on the available heat source and condensation temperature.     

Performance characteristics of refrigeration cycle with parallel compression economization

Thermodynamic analyses and economizer pressure optimizations of ammonia, propane and isobutane‐based refrigeration cycles with parallel compression economization are presented in this article. Energetic and exergetic performance comparisons with transcritical CO₂ cycle are presented as well. Results show that the optimum economizer mass fraction as well as COP improvement increase with increase in cycle temperature lift. The expression for optimum economizer pressure has been developed. Study shows that the performance improvements using parallel compression economization are strongly dependent on the refrigerant properties as well as the operating conditions. Using parallel compression economization, carbon dioxide yields maximum COP improvement of 31.9% followed by propane (29.8%), isobutane (27.2%) and ammonia (11.3%) for studies ranges. In spite of higher COP improvement, the cooling COP as well as second low efficiency for carbon dioxide is still significantly lower than that for others. Component‐wise irreversibility distributions show the similar trends for all refrigerants except CO₂. Employing parallel compression economization in refrigeration cycle not only improves the cooling COP but also increase the compactness of evaporator. Copyright © 2010 John Wiley & Sons, Ltd.     

The performance of a triple pressure level absorption cycle (TPLAC) with working fluids based on the absorbent DMEU and the refrigerants R22, R32, R124, R125, R134a and R152a

This paper compares the performance of a single-stage triple pressure level (TPL) absorption cycle with different refrigerant–absorbent pairs. Four HFC refrigerants namely: R32, R125, R134a and R152a which are alternative to HCFC, such as R22 and R124, in combination with the absorbent dimethylethylenurea (DMEU) were considered. The highest coefficient of performance (COP) and the lowest circulation ratio (f), were found as a function of the generator temperature for a given evaporating and cooling water temperatures. The sensitivity of the COP and f for evaporator and cooling water temperatures changes at the maximum COP for the best three working fluids were also examined. It was obtained that the preferable pair is R124–DMEU and among working fluids based on HFC the preferable pair is the R125–DMEU.     

Operating design conditions of a solar-powered vapor absorption cooling system with an absorber plate having different profiles: An analytical study

An effort has been devoted to analyze the collector performance parameters of a solar-assisted LiBr/H₂O vapor absorption cooling system with a flat-plate collector consisting of an absorber plate of different profiles. The effect of the collector fluid inlet temperature on the performance of solar collector, vapor absorption cycle, vapor absorption system and refrigerating efficiency has been studied for a wide range of design variables. A comparative study has also been made among the performance parameters of an absorber plate of different shapes with the variation of collector fluid inlet temperature. From the result, it can be highlighted that, at a particular collector fluid inlet temperature, the performances of a vapor absorption system attain a maximum value. Finally, an optimum collector fluid inlet temperature is determined by satisfying the minimization of volume of an absorber plate without affecting the cooling rate in the evaporator.     

Experimental investigations of a small-scale solar-assisted absorption cooling system

The present study deals with a small-scale solar-assisted absorption cooling system having a cooling capacity of 3.52 kW and was investigated experimentally under the climatic conditions of Taxila, Pakistan. Initially, a mathematical model was developed for LiBr/H₂O vapor absorption system alongside flat-plate solar thermal collectors to achieve the required operating temperature range of 75°C. Following this, a parametric analysis of the whole system was performed, including various design and climate parameters, such as the working temperatures of the generator, evaporator, condenser, absorber, mass flow rate, and coefficient of performance (COP) of the system. An experimental setup was coupled with solar collectors and instruments to get hot water using solar energy and measurements of main parameters for real-time performance assessment. From the results obtained, it was revealed that the maximum average COP of the system achieved was 0.70, and the maximum outlet temperature from solar thermal collectors was 75°C. A sensitivity analysis was performed to validate the potential of the absorption machine in the seasonal cooling demand. An economic valuation was accomplished based on the current cost of conventional cooling systems. It was established that the solar cooling system is economical only when shared with domestic water heating.     

Pumping characteristics of a thermosyphon applied for absorption refrigerators with working pair of LiBr/water

A thermosyphon with a separate heating chamber was taken into consideration as a model device for a high-temperature generator in a small capacity absorption refrigerator with LiBr/H₂O solution. Quantitative measurements of operating characteristics of the thermosyphon were made as functions of pipe diameter, pumping height and heating power. It was found that vapor production rate is mainly affected by the heating power, and the flow rate of pumped liquid is limited by the pumping height. It was also observed that, as the pipe diameter increases, dependency of liquid flow rate on the pumping height becomes stronger. For a LiBr/H₂O solution system, ratio of vapor production rate to liquid flow rate was found to be well correlated as a function of the power input. Present experimental data are expected to provide useful information for determination of design parameters of a thermosyphon.     

Transcritical CO2 refrigerator and sub‐critical R134a refrigerator: A comparison of the experimental results

This paper describes experiments comparing a commercial available R134a refrigeration plant subjected to a cold store and a prototype R744 (carbon dioxide) system working as a classical ‘split‐systems’ to cool air in residential applications in a transcritical cycle. Both plants are able to develope a refrigeration power equal to 3000 W. The R744 system utilizes aluminium heat exchangers, a semi‐hermetic compressor, a back‐pressure valve and a thermostatic expansion valve. The R134a refrigeration plant operates using a semi‐hermetic reciprocating compressor, an air condenser followed by a liquid receiver, a manifold with two expansion valves, a thermostatic one and a manual one mounted in parallel, and an air cooling evaporator inside the cold store. System performances are compared for two evaporation temperatures varying the temperature of the external air running over the gas‐cooler and over the condenser. The refrigeration load in the cold store is simulated by means of some electrical resistances, whereas the air evaporator of the R744 plant is placed in a very large ambient. The results of the comparison are discussed in terms of temperature of the refrigerants at the compressor discharge line, of refrigerants mass flow rate and of coefficient of performance (COP). The performances measured in terms of COPs show a decrease with respect to the R134a plant working at the same external and internal conditions. Further improvements regarding the components of the cycle are necessary to use in a large‐scale ‘split‐systems’ working with the carbon dioxide. Copyright © 2009 John Wiley & Sons, Ltd.     

Organic Brayton Cycles with solid sorption thermal compression for low grade heat utilization

While organic Rankine cycles have been widely used for power generation using low grade thermal energy, Brayton cycles have not been considered feasible because the work required to compress the gas nearly compensates the turbine work output. However, if the low grade energy can be used for thermal compression of the working fluid, it may be possible to gainfully operate the Brayton cycle. With this in mind, a solid sorption based Brayton cycle is proposed in this paper. R134a, CO₂, R507a, propane, R32 and R410a with activated carbon as sorbent, were considered in this proof-of-concept study due to the ready availability of adsorption data. Even though the thermal efficiency is low (<8%), the proposed scheme could add an option for distributed power generation using solar or waste heat. It is found that if irreversibilities in turbine and thermal compression are considered R32 gives a better performance than CO₂ and R410a.     

Performance analysis and optimization of a trigeneration process consisting of a proton-conducting solid oxide fuel cell and a LiBr absorption chiller

In this work, the trigeneration system, consisting of a proton-conducting solid oxide fuel cell (SOFC–H⁺) and a single-stage LiBr absorption chiller, was proposed. The SOFC–H⁺ and single-stage LiBr absorption chiller models were developed through Aspen Plus V10. From the sensitivity analysis, the results show that increases in temperature and fuel utilization can improve the performance of the SOFC–H⁺. Conversely, the air to fuel (A/F) molar ratio and pressure negatively affect the electrical efficiency and overall system efficiency. In the case of the absorption chiller, the coefficient of performance was increased and made stable according to a constant value when the generator temperature was increased from 90 to 100 °C. When the optimization was performed, it was found that the SOFC–H⁺ should be operated at 700 °C and 10 bar with fuel utilization of 0.8 and A/F molar ratio of 2 to achieve a maximum overall efficiency of 93.34%. For the energy and exergy analysis, a combined heat and power SOFC–H⁺ was found to have the highest energy and exergy efficiencies, followed by the trigeneration process. This indicates that the integration of the SOFC–H⁺ and LiBr absorption chiller is possible to efficiently produce electricity, heating and cooling.