Review of advanced absorption cycles: performance improvement and temperature lift enhancement

The objective of this study is to propose and evaluate advanced absorption cycles for the coefficient of performance (COP) improvement and temperature lift enhancement applications. The characteristics of each cycle are assessed from the viewpoints of the ideal cycle COP and its applications. The advanced cycles for the COP improvement are categorized according to their heat recovery method: condensation heat recovery, absorption heat recovery, and condensation/absorption heat recovery. In H₂O–LiBr systems, the number of effects and the number of stages can be improved by adding a third or a fourth component to the solution pairs. The performance of NH₃–H₂O systems can be improved by internal heat recovery due to their thermal characteristics such as temperature gliding. NH₃–H₂O cycles can be combined with adsorption cycles and power generation cycles for waste heat utilization, performance improvement, panel heating and low temperature applications. The H₂O–LiBr cycle is better from the high COP viewpoints for the evaporation temperature over 0°C while the NH₃–H₂O cycle is better from the viewpoint of low temperature applications. This study suggests that the cycle performance would be significantly improved by combining the advanced H₂O–LiBr and NH₃–H₂O cycles.     

Performance research of self regenerated absorption heat transformer cycle using TFE-NMP as working fluids

A heat transformer is proposed in order to upgrade low-temperature-level energy to a higher level and to recover more energy in low-temperature-level waste heat. It is difficult to achieve both purposes at the same time using a conventional heat transformer cycle and classical working pairs, such as H₂O–LiBr and HN₃–H₂O. The new organic working pair, 2,2,2-trifluoroethanol (TFE)-N-methylpyrolidone (NMP), has some advantages compared with H₂O–LiBr and NH₃–H₂O. One of the most important features is the wide working range as a result of the absence of crystallization, the low working pressure, the low freezing temperature of the refrigerant and the good thermal stability of the mixtures at high temperatures. Meanwhile, it has some negative features like NH₃–H₂O. For example, there is a lower boiling temperature difference between TFE and NMP, so a rectifier is needed in refrigeration and heat pump systems. Because TFE–NMP has a wide working range and does not cause crystallization, it can be used as the working pair in the self regenerated absorption heat transformer (SRAHT) cycle. In fact, the SRAHT cycle is the generator–absorber heat exchanger (GAX) cycle applied in a heat transformer cycle. In this paper, the SRAHT cycle and its flow diagram are shown and the computing models of the SRAHT cycle are presented. Thermal calculations of the SRAHT cycle under summer and winter season conditions have been worked out. From the results of the thermal calculations, it can be found that there is a larger temperature drop when the waste hot water flows through the generator and the evaporator in the SRAHT cycle but the heating temperature can be kept the same. That means more energy in the waste heat source can be recovered by the SRAHT cycle.     

Experimental investigation on the performance of NH3/CO2 cascade refrigeration system with twin-screw compressor

This paper describes the experiment carried out to analyze the performance of a refrigeration system in cascade with ammonia and carbon dioxide as working fluids. The effect of operation parameters, such as the evaporating temperature of the low temperature cycle, the condensing temperature of low temperature cycle, temperature difference in cascade heat exchanger and superheat degree, on the system performance was investigated. Performance of the cascade system with NH₃/CO₂ was compared with that of two-stage NH₃ system and single-stage NH₃ system with or without economizer. It was found that the COP of the cascade system is the best among all the systems, when the evaporating temperature is below −40 °C. Also, the cascade system performance is greatly affected by evaporating temperature, condensing temperature of low temperature cycle, temperature difference in cascade heat exchanger and is only slightly sensitive to superheat degree. All the experimental results indicate that the NH₃/CO₂ cascade system is very competitive in low temperature applications.     

Optimization of generator temperatures in two-stage dual-fluid absorption cycles operated by biogas

Generator temperatures in ammonia absorption systems at subfreezing evaporator conditions have been optimized to use the minimum volume of biogas required to operate two-stage dual-fluid cycles. In this dual-fluid cycle, a LiBr---H₂O absorption system is used at the first stage with ammonia absorption systems at the second stage. Three different refrigerant-absorbent combinations (NH₃---H₂O, NH₃---NaSCN, NH₃---LiNO₃) were selected for this study. The absorber at the second stage is cooled by the low temperature water-refrigerant from the evaporator at the first stage. Lowering the absorber temperature reduces the heat input to the generator, which lowers the generating temperature and improves the performance of the absorption systems at the second stage. The optimum generator temperatures and performance coefficients of the systems at the first and second stages and the overall two-stage dual-fluid cycles are presented graphically. A comparative study between the three ammonia systems used in the two-stage dual-fluid cycle has been carried out.     

Analysis of a hybrid compression—absorption cycle using lithium bromide and water as the working fluid

Hybrid refrigeration cycles which combine a mechanical compressor and an absorption cycle in such a way that they share a single evaporator were analysed. The motivation for the investigation of hybrid cycles was the need to more efficiently utilize the output of an internal combustion engine. The hybrid cycles make efficient use of both the work and the heat output of an engine. Performance calculations are reported for a promising cycle which utilizes LiBr-H₂O as the working fluid. For this working fluid, the refrigerant is water. Owing to the potential sensitivity of the absorption cycle components to oil contamination, the cycle was analysed assuming an oil-free steam compressor (screw design). Although oil-free steam compressors are available, they are used only sparingly in the industry. The capital cost for such a compressor is very high and the isentropic efficiency of the available units is low. This combination of high cost and low performance results in poor economics for the hybrid cycle based on the available technology. However, the cycle has significant potential from a thermodynamic viewpoint and it provides an incentive for compressor manufacturers to refine the oil-free steam compression technology.     

General method for the evaluation of complex heat pump cycles

Whenever the fractional temperature lift ΔT/T_c of a heat pump is 0.15, simple cycles with one-stage throttling exhibit unsatisfactory energy performance. The adoption of multi-stage throttling, both in non-regenerative and regenerative cycles, is the most direct way of improving the cycle coefficient of performance (COP). The performance of these complex cycles is found to be a function of the molecular complexity of the working fluid, the reduced evaporation temperature, the fractional temperature lift and the number of stages of throttling. Furthermore, complex cycles are shown to be equivalent to a combination of simple cycles and their performance may be directly inferred by this route. Such calculations show that for a given fractional temperature lift an optimum molecular complexity (between that of R12 and n-butane) exists. Fluids with simpler molecules exhibit excessive vapour superheating during compression, and those with more complex molecules have excessive throttling losses. Also, with complex cycles, regeneration should be applied only to the cycle at the lowest temperature in order to improve the cycle COP and to prevent condensation during compression. As a general trend, however, complex cycles suffer a significant loss in performance compared to optimized simple cycles due to the adverse area of the two-phase diagram in which they work.     

Solar absorption system for air conditioning

This Paper presents a new continous operating solar desiccant absorption system in which CaCl₂-H₂O is used as the absorbent. The flat-plate solar collector is utilized as the desorber where water from the solution is evaporated to ambient air in passing over the collector above the solution film. The plant is equipped with a latent heat accumulation system which is extremely compact in size and very efficient. The coefficient of performance—solar collector efficiency product, indicating the grade of solar energy utilized, is estimated as being the highest of all presently known systems.     

Heat transfer enhancement by Marangoni convection in the NH3–H2O absorption process

The objectives of this paper are to quantify the effect of Marangini convection on the absorption performance for the ammonia–water absorption process, and to visualize Marangoni convection that is induced by adding a heat transfer additive, n-octanol. A real-time single-wavelength holographic interferometer is used for the visualization using a He–Ne gas laser. The interface temperature is always the highest due to the absorption heat release near the interface. It was found that the thermal boundary layer (TBL) increased faster than the diffusion boundary layer (DBL), and the DBL thickness increased by adding the heat transfer additive. At 5 s after absorption started, the DBL thickness for 5 mass% NH₃ without and with the heat transfer additive was 3.0 and 4.5 mm, respectively. Marangoni convection was observed near the interface only in the cases with heat transfer additive. The Marangoni convection was very strong just after the absorption started and it weakened as time elapsed. It was concluded that the absorption performance could be improved by increasing the absorption driving potential (x_vb−x_vi) and by increasing the heat transfer additive concentration. The absorption heat transfer was enhanced as high as 3.0–4.6 times by adding the heat transfer additive that generated Marangoni convection.     

Etude comparative de la performance des machines frigorifiques à absorption utilisant de l'energie thermique à très faible valeur exergétique

This study deals with cooling systems driven with low grade heat energy delivered by heat sources whose temperature do not exceed 100°C. Since we cannot neglect the irreversibilities associated to heat depreciation, we consider an approach temperature of 10°C for all the heat exchangers. So the temperature of the generators is <90°C to fulfill the approach conditions. For a heat sink temperature of 30°C and an evaporating temperature of −10°C, only two kinds of cycles are concerned: the cycles with staged absorption and desorption (CADE) and the cascade systems (CCAR) in which a series of two cycles is used, the evaporator of the former cooling the absorber of the later.The cycles with staged absorption and desorption are described. They permit a decrease of generators' temperature down to a level no system among those previously offered can run.To point out the importance of the CADE, we compare them to cascade systems. This comparative study shows that the CADE yields better performances. The gap of performance between the two systems increases with the decrease of the heat sink temperature and it is maximal for lower temperature at generators. The comparative study uses five criteria: the coefficient of performance, , and the operating area in a (T_fr, T_g) plane. LiNO₃/NH₃ and H₂O/NH₃ are used as working fluids of the CADE while LiBr/H₂O + LiNO₃/NH₃ and LiBr/H₂O + H₂O/NH₃ are used for the CCAR.The CADE are very suitable for the use of low grade heat energy (70°C ? T_sc ? 100°C) in refrigeration (T_ev ? −10°C) with sink temperature as high as 30°C and a realistic approach of 10°C for all the heat exchangers.     

Simulation of ternary ammonia–water–salt absorption refrigeration cyclesSimulation des cycles frigorifiques à absorption à ammoniac / eau / sel

This paper proposes a new working fluid for refrigeration cycles utilizing low temperature heat sources. The proposed working fluid consists of the ammonia–water working fluid mixture and a salt. The salt is used to aid the removal of ammonia from the liquid solution. This effect is a manifestation of the well known “salting-out” effect. While the addition of salt improves the generator performance, it also has a detrimental effect on the absorber. The overall effects on the performance of three absorption cycles using the NH₃–H₂O–NaOH working fluid have been investigated using computer simulations. The results indicated that salting out can lower the generator operating temperature while simultaneously improving the cycle performance. Furthermore, limiting the salt to the generator suggests potential for further improvement in cycle performance.     

Working fluids for an absorption system based on R124 (2-chloro-1,1,1,2,-tetrafluoroethane) and organic absorbents

The possibility of using R124 (2-chloro-1,1,1,2,-tetrafluoroethane, CHClFCF₃) and organic absorbents as working fluids in absorption heat pumps was investigated. Various classes of organic compounds, all commercially available, were tested as absorbents for possible combination with R124; the absorbents included DMAC (N′, N′-dimethylacetamide, C₄H₉NO), NMP (N-methyl-2-pyrrolidone, C₅H₉NO), MCL (N-methyl ε-caprolactam, C₇H₁₃NO), DMEU (dimethylethylene urea, C₅H₁₀N₂O), and DMETEG (dimethylether tetraethyleneglycol, C₁₀H₂₂O₅). To evaluate the performance of a candidate refrigerant-absorbent pair in a refrigeration or heat pump cycle, the thermophysical properties of the pure components and the mixture and the equilibrium and transport properties have to be determined, either from experimental data or by prediction methods. The thermal stability of the refrigerant-absorbent must also be tested. A method for the calculation of the concentration in the liquid and gas phases and the excess thermodynamic properties of the mixture as a function of the system temperature and pressure based on our experimental setup is described. On the basis of vapor-liquid equilibrium measurements, density and viscosity measurements and thermostability testing, enthalpy-concentration diagrams were constructed. The performance characteristics of the investigated working fluids in terms of the coefficient of performance (COP) and the circulation ratio (f) were calculated for a single-stage absorption cycle. In terms of overall performance (COP, f and stability) R124-DMAC was found to be the superior combination, followed by R124-NMP, R124-DMEU and R124-MCL (the three pairs for which stability problems were found at high temperatures), and finally by R124-DMETEG.     

CO2-heat pump water heater: characteristics, system design and experimental results

CO₂ is one of the few non-toxic and non-flammable working fluids that do not contribute to ozone depletion or global warming, if leaked to the atmosphere. Tap water heating is one promising application for a trans-critical CO₂ process. The temperature glide at heat rejection contributes to a very good temperature adaptation when heating tap water, which inherits a large temperature glide. This, together with efficient compression and good heat transfer characteristics of CO₂, makes it possible to design very efficient systems. A heating-COP of 4.3 is achieved for the prototype when heating tap water from 9°C to 60°C, at an evaporation temperature of 0°C. The results lead to a seasonal performance factor of about 4 for an Oslo climate, using ambient air as heat source. Thus, the primary energy consumption can be reduced with more than 75% compared with electrical or gas fired systems. Another significant advantage of this system, compared with conventional heat pump water heaters, is that hot water with temperatures up to 90°C can be produced without operational difficulties.     

Amélioration de la performance des machines frigorifiques á absorption par l'utilisation de cycles à absorption et désorption étagés

This study deals with staged absorption and desorption cooling systems which increase the performance of absorption cycles that are driven by only low-grade energy, particularly when the working fluids are NH₃H₂0. Instead of working with only one absorber, these systems use a cascade of absorbers composed by one operating at the evaporator pressure, followed by a series of absorbers operating at staged pressures P_j, between P_ev and P_c In the same way, a cascade of generators is used for desorption. For the same operating parameters for other equipment and the same COP, the systems that we propose permit the generators to run at temperatures below those of all other systems offered up to now and using the same working fluids. When T_ev = −10°C, T_a = T_c = 30°C, the temperature of the generators can be as low as 65°C while the COP of the system is 0.258 and the COP_ex 0.317. By increasing the temperature of generators to 85°C while maintaining the other parameters at the same values, COP becomes 0.374 and the COP,, 0.336. These results improve the performance of absorption systems using only low-grade energy (T < 100°C). Particularly, they are better than the performance of two-stage absorption systems which consist of two single-stage absorption cycles coupled with each other through the evaporator of the first cycle and the absorber of the second cycle. With the same operating parameters indicated above for our system at the evaporator, the condenser, and the absorber, these coupled cycles need temperatures at generators of 80 and 100°C, whereas they give a COP of only 0.270     

The effect of nano-particles on the bubble absorption performance in a binary nanofluid

The objectives of this paper are to examine the effect of nano particles on the bubble type absorption by experiment and to find the optimal conditions to design highly effective compact absorber for NH₃/H₂O absorption system. The initial concentrations of NH₃/H₂O solution and the kinds and the concentrations of nano particles are considered as key parameters. The results show that the addition of nano particles enhances the absorption performance up to 3.21 times. Moreover, the absorption rate increases with increasing concentration of nano particles and the nano particles are more effective for lower absorption potential solution. The potential enhancement mechanism for binary nanofluid is suggested. The experimental correlations of the effective absorption ratio for each nano particles, Cu, CuO, and Al₂O₃, are suggested within ±10% error-band.     

An environmentally friendly GAX cycle for panel heating: PGAX cycle

The objectives of this paper are to develop an environmentally friendly GAX cycle using NH₃–H₂O for panel heating applications (PGAX), and compare it to a single effect cycle for panel heating applications (PSE cycle). The PGAX cycle can be operated in three different modes with just one hardware — cooling, space heating and panel heating applications. The total COP of the PGAX cycle is higher than that of the PSE cycle due to the internal heat recovery in the GAX component. The UA ratio has more significant effect on the total COP of the PGAX cycle than that of the PSE cycle. The panel heating COP is more significantly affected by the absorber UA variation than the space heating COP. There should be optimum ratios of absorber UAs to provide the highest total COP for a given split ratio of the coolant mass flow rate in the PGAX cycle. The results from the parametric analysis of UA ratio can be used to obtain the best UA combination of the absorbers for given space heating and panel heating capacities. This paper provides the optimum UA values of the absorbers for a given split ratio of the coolant mass flow rate.     

Efficiency of the indicated process of CO2-compressors

The compressor of a refrigerant compression process is the component with the major influence on the efficiency and reliability of the entire system. Due to the fluid properties of carbon dioxide (CO₂), the pressure ratio of the refrigeration process with CO₂ as the working fluid is, in relation to common refrigeration processes, rather low while the pressure difference is extremely high. From experimental and theoretical considerations it becomes obvious that at these conditions a high volumetric and energetic efficiency of the compressor may be achieved if its design is appropriate. In this paper, the effects on the efficiency of the indicated compression process of a CO₂-compressor are discussed and evaluated and a promising design concept for an efficient CO₂-compressor is derived.     

Experimental studies on air-cooled NH3-H2O based modified gax absorption cooling system

An experimental investigation on the performance of an air-cooled modified generator absorber heat exchange (GAX) absorption cooling system has been carried out and presented in this paper. The conventional system is modified by incorporating high pressure GAX, low pressure GAX, a solution cooler and an additional solution heat exchanger to reduce the heat input to the system. The system is designed for a cooling capacity of 10.5 kW using ammonia-water (NH₃-H₂O) as the working fluid. The performance of the system in terms of the circulation ratio, internal heat recovery and coefficient of performance (COP) has been obtained. The system is capable of producing a low evaporator temperature of −5 °C, at a sink temperature of 35 °C, under no load conditions. The results indicate that at a generator and evaporator temperature of 120 °C and 2 °C respectively, the system delivers a maximum cooling capacity of about 9.5 kW with a fuel and total COP of 0.61 and 0.57 respectively.     

Cooling performance of several CO2/propane mixtures and glide matching with secondary heat transfer fluid

This paper presents the cooling performance of several CO₂/propane mixtures measured in air-conditioning test rig at several conditions. The discharge pressure of CO₂/propane mixtures is reduced with increasing mole fraction of propane and their reduced values coincide approximately with the circulation concentrations of propane. Since propane is the refrigerant having a higher refrigerating effect and a much lower vapor density than CO₂, adding propane to CO₂ improves the system efficiency and reduces the cooling capacity. The temperature glide effect of CO₂/propane mixtures on the cooling performance was analyzed based on the experimental data. To utilize the temperature glide effect successfully, a sufficient heat exchange area is required, and the temperature gradient of refrigerant must be similar to that of secondary heat transfer fluid. It is better the temperature change of refrigerant can prevent pinching with that of the secondary heat transfer fluid.     

Absorption performance enhancement by nano-particles and chemical surfactants in binary nanofluids

The objectives of this paper are to visualize the bubble behavior during the NH₃/H₂O absorption process with chemical surfactant and nano-particles and to study the effect of nano-particles and surfactants on the absorption characteristics. Binary nanofluid which means binary mixture with nano-sized particles is tested to apply nanofluid to the absorption system. Cu, CuO and Al₂O₃ nano-particles are added into NH₃/H₂O solution to make the binary nanofluids, and 2-ethyl-1-hexanol, n-octanol and 2-octanol are used as the surfactants. The concentration of ammonia in the basefluid, that of nano-particles in the nanofluid, and that of surfactants in the nanofluid are considered as the key parameters. The results show that the addition of surfactants and nano-particles improves the absorption performance up to 5.32 times. It can be concluded that the addition of both surfactants and nano-particles enhances significantly the absorption performance during the ammonia bubble absorption process.     

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.