Multi-Effect Plants and Ionic Liquids for Improved Absorption Chillers

State-of-the-art absorption chillers using conventional working pairs still suffer from problems like crystallization, corrosiveness, and a relatively low efficiency. To improve this technology, different working pairs as well as plant designs are investigated using the simulation tool AspenPlus. The simulation is validated by comparing the results of single-effect absorption chillers using the current commercially applied working pairs water/lithium bromide and ammonia/water with literature data. To increase the efficiency, double-effect absorption chillers are implemented and analyzed. The performance of two kinds of double-effect cycles, series and parallel, is compared using the working pair water/lithium bromide. In addition, ionic liquids (ILs) are investigated as a sorbent in order to improve the technology. So far, ILs have not been implemented in AspenPlus yet. Therefore, a guideline for the implementation of ILs in AspenPlus is outlined and the accordant phase equilibria results are validated with literature data. Simulations of single-effect cycles using the ILs 1,3-dimethylimidazolium dimethylphosphate ([MMIM][DMP]) and 1-ethyl-3-methylimidazolium dimethylphosphate ([EMIM][DMP]) in combination with water as a refrigerant are performed and the results are compared to conventional working pairs. It is shown that by using ILs, similar or even higher coefficients of performance (COPs) can be achieved in comparison to conventional working pairs. Moreover, the findings reveal that the main benefit of using ILs as a sorbent consists in providing a broader operating range with respect to heat source temperature.     

Screening of novel water/ionic liquid working fluids for absorption thermal energy storage in cooling systems

Absorption thermal energy storage (ATES) is significant for renewable/waste energy utilization in buildings. The ATES systems using ionic liquids (ILs) are explored to avoid crystallization and enhance the performance. Property model and cycle model have been established with verified accuracies. Based on the preliminary screening, seven ILs are found feasible to be ATES working fluids, while four ILs ([DMIM][DMP], [EMIM][Ac], [EMIM][DEP], and [EMIM][EtSO₄]) have been selected for detailed comparisons. The coefficient of performance (COP) and energy storage density (ESD) of the ATES using different H₂O/ILs are compared with H₂O/LiBr. Results show that the operating temperatures of LiBr are constrained by crystallization, limiting the COPs and ESDs under higher generation temperatures and lower condensation temperatures. With varying T_g, [DMIM][DMP] yields higher COPs with T_g above 100°C and [EMIM][Ac] yields comparable ESDs (67.7 vs 67.1 kWh/m³) with T_g around 120°C, as compared with LiBr. The maximum COP is 0.745 for [DMIM][DMP]. With varying T_c, [DMIM][DMP] yields higher COPs with T_c below 38°C and [EMIM][Ac] yields higher ESDs with T_c below 33°C, as compared with LiBr. The maximum ESD is 87.5 kWh/m³ for [EMIM][Ac]. With varying T_e, [DMIM][DMP] yields higher COPs with T_e above 8°C, as compared with LiBr. The maximum ESD of ILs is 104.0 kWh/m³ for [EMIM][EtSO₄]. Comparing with the volume-based ESDs, the differences between ILs and LiBr are smaller for the mass-based ESDs. This work can provide suggestions for the selection of novel working fluids for ATES for performance and reliability enhancement.     

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.     

Thermodynamic properties and performance evaluation of [EMIM] [DMP]-H2O working pair for absorption cooling cycle

The combination of ionic liquid-refrigerant based [EMIM][DMP]-H₂O as an alternative working pair for single effect vapor absorption cycles (VACs) is assessed and optimized by using energy and exergy based performances. Thermodynamics properties of binary mixture of [EMIM] [DMP]-H₂O like Dühring's (P-T-x1) and h-T-X1 plots are computed from the activity coefficient based non-random two-liquid model (NRTL) model. Further modeling and simulation of VACs are accomplished in open source Scilab as mathematical programing software and used to ascertain the optimal generator temperature established on energetic and exergetic COP. Optimal results include an extensive range of temperatures like T_e from 2.5 to 15°C and T_a and T_c from 30 to 45°C. Simulation of the single effect VAC with SHE by using [EMIM][DMP]-H₂O mixture at T_e = 10°C, T_g = 100°C, T_a = 30°C, and T_c = 40°C were evaluated and compared with the 5 working fluids. Simulation outcomes depicted greater COP of 0.82 for [EMIM][DMP]-H₂O in comparison with NH₃-H₂O, EMISE-H₂O, [EMIM][BF4]-H₂O and nearly equal to LiBr-H₂O (COP = 0.83). In addition, the effect of T_g on the COP, ECOP f _, and composition are compared and optimized for the evaporation temperature range from 2.5 to 15°C, T_a/T_c from 30 to 45°C and cooling water (CW) flow in series and parallel. Additionally, the optimal T_g exhibited distinction based on energy and exergy analysis. Thus, it resulted in optimized performances of [EMIM] [DMP]-H₂O that can be suitable to replace corrosive aqueous LiBr in VACs.     

Comprehensive performance analysis and optimization of 1,3-dimethylimidazolylium dimethylphosphate-water binary mixture for a single effect absorption refrigeration system

The energy and exergy analyses of the absorption refrigeration system (ARS) using H₂O-[mmim][DMP] mixture were investigated for a wide range of temperature. The equilibrium Dühring (P-T-X_IL) and enthalpy (h-T-X_IL) of mixture were assessed using the excess Gibbs free non-random two liquid (NRTL) model for a temperature range of 20°C to 140°C and X_IL from 0.1 to 0.9. The performance validation of the ARS cycle showed a better coefficient of performance (COP) of 0.834 for H₂O-[mmim][DMP] in comparison to NH₃-H₂O, H₂O-LiBr, H₂O-[emim][DMP], and H₂O-[emim][BF4]. Further, ARS performances with various operating temperatures of the absorber (T_a), condenser (T_c), generator (T_g), and evaporator (T_e) were simulated and optimized for a maximum COP and exergetic COP (ECOP). The effects of T_g from 50°C to 150°C and T_a and T_c from 30°C to 50°C on COP and ECOP, the X_a, X_g, and circulation ratio (CR) of the ARS were evaluated and optimized for T_e from 5°C to 15°C. The optimization revealed that T_g needed to achieve a maximum COP which was more than that for a maximum ECOP. Therefore, this investigation provides criteria to select low grade heat source temperature. Most of the series flow of the cases of cooling water from the condenser to the absorber was found to be better than the absorber to the condenser.     

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.     

溴化锂吸收式制冷技术研究进展

回顾了近十年来有关溴化锂吸收式制冷技术的发展及主要研究成果。H₂O/LiBr作为一种广泛应用的吸收式制冷工质对,具有优良的热力学性能与环保特性,但存在结晶、腐蚀和循环性能低等问题。文章简述了醇类、盐混合物、离子液体及纳米颗粒等添加剂对H₂O/LiBr溶液传热传质、防结晶及防腐蚀等性能的提升;介绍了关键部件吸收器和发生器的理论及实验研究现状;回顾了吸收式制冷系统循环优化的研究成果。通过归纳分析,总结溴化锂吸收式制冷技术存在的一些问题及未来发展趋势,为后续的研究提供参考。     

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.     

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.     

A new power/cooling cogeneration system using R1234ze(E)/ionic liquid working fluid

A novel power/cooling system integrated with organic Rankine cycle and absorption-compression refrigeration cycle was proposed in order to realize the cascade utilization of low-grade energy. In the proposed system, R1234ze(E) (trans-1,3,3,3-tetrafluoropropene) is used as the working fluid for the organic Rankine cycle subsystem and the binary mixtures of R1234ze(E) with three ionic liquids [HMIM][BF₄], [EMIM][BF₄] and [OMIM][BF₄] are used as working fluid for absorption-compression refrigeration cycle subsystem due to their superior environmental protection property and physicochemical property. Moreover, in order to recover the heat of the exhaust gas from turbine in organic Rankine cycle subsystem, the exhaust gas is mixed with R1234ze(E)/ionic liquid solution directly in desorber, while the heat of refrigerant from desorber is recovered to reduce the heat load of condenser. The proposed system has much higher energy and exergy efficiency and lower heat load of condenser than reference system. Under specific conditions, increases of 0.24 and 0.07 in thermal efficiency and exergy efficiency of reference system can be achieved. The effect of distribution ratio, expansion ratio, heat source temperature, condensation temperature, generation temperature, evaporation temperature and compression ratio were analyzed for better design in actual application.     

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.     

Hydrogen storage in amine boranes: Ionic liquid supported thermal dehydrogenation of ethylene diamine bisborane

A variety of ionic liquids has been tested for its catalytic effect on the dehydrogenation of ethylene diamine bisborane (EDB). The catalytic activity of ionic liquids, such as 1-butyl-2,3-dimethylimidazolium chloride ([BMMIM]Cl), 1-butyl-2,3-dimethylimidazolium acetate ([BMMIM][OAc]), 1-butyl-3-methylimidazolium acetate ([BMIM][OAc]) and 1-butyl-3-methylimidazolium methylsulfonate ([BMIM][OMs]) is compared and the mixture [BMMIM]Cl/EDB was investigated. This system is able to deliver about 6.5 wt% of hydrogen at 140 °C competing with conventional hydrogen storage pressure tanks. The correlation between polarity of the ILs and hydrogen yield was investigated and the suitability for hydrogen storage systems is evaluated and discussed.     

Simple LiBr/Water absorption cycle limitations

The aim of this paper is to determine the optimal design temperatures and pressures of a simple LiBr/water absorption cycle. To achieve this goal, a computer program, based on the characteristics of lithium bromide water combinations, has been designed. The output results were the generator temperature operating range which is defined relative to the other component temperatures. The limitations of the cycle operation, based on the crystallization of lithium bromide at high generator temperatures, are also shown. The variation of the theoretical COP and the circulation ratio of the cycle are calculated.     

Absorption power cycles for low‐temperature heat sources using aqueous salt solutions as working fluids

There are many low‐temperature heat sources; however, current technologies for their utilization have a relatively low efficiency and high cost. The leading technology in the low‐temperature domain for heat‐to‐work conversion is the organic Rankine cycle (ORC). Absorption power cycles (APCs) are a second option. Nearly all currently known APCs, most importantly the Kalina cycle, use a water‐ammonia mixture as their working fluids. This paper offers a theoretical exploration of the possibility of utilizing aqueous solutions of three salts (lithium bromide, lithium chloride and calcium chloride), known mainly from absorption cooling, as working fluids for APCs. The cycles are compared with a typical steam Rankine cycle, a water‐ammonia APC, and (subcritical) ORCs with a range of working fluids explored. The analysis includes a parasitic load for heat rejection by a cooling tower or air‐cooled condenser. The absorption cycles exhibit better performance than all Rankine‐based cycles analysed in temperatures below 120°C. For the LiBr‐based APC, a detailed thermal design of the cycle is provided for 100°C water as a heat source and a sensitivity analysis is performed of the parameters controlling the main cycle. Mechanical design considerations should not pose a problem for small power units, especially in the case of expansion machines, which are often problematic in ORCs. The salt‐based APCs also carry environmental benefits, as the salts utilized in the working fluids are non‐toxic. Copyright © 2016 John Wiley & Sons, Ltd.     

Thermodynamic basis for the choice of working fluids for solar absorption cooling systems

Through the application of the first and second laws of thermodynamics upper and lower limits for the coefficient of performance (COP) of absorption cooling cycles are derived. These upper and lower limits, besides being dependent on the environmental temperatures of components of the cycle, are also dependent on the thermodynamic properties of refrigerants, absorbents, and their mixtures. With the use of these upper and lower limits of COP it is now possible to make a quantitative comparative study of different refrigerant-absorbent combinations. The technique developed is applied for the comparative evaluation of NH₃ + H₂O, NH₃ + NaSCN and H₂O + LiBr combinations which are the favorable candidates used in solar absorption cooling cycles.     

Cooling cogeneration cycle

The existed combined power and cooling cycle operates with ammonia–water mixture as working fluid having low cooling due to the vapor at the inlet of evaporator. It also demands high ammonia concentration at turbine inlet to get cooling and suitable only at low sink temperature (10–12°C). A new cooling cogeneration cycle has been proposed and solved to generate more cooling with adequate power generation from single source of heat with two options in working fluids i.e. ammonia–water mixture and LiBr–water mixture. The results show that an increase in cycle maximum temperature is only supporting the power but not the cooling. A suitable range for separator temperature has been developed and optimized to maximize the total output. From this study, the resulted specific power, specific cooling, cycle power efficiency, cycle coefficient of performance (COP) and cycle energy utilization factor (EUF), plant EUF, and specific area of solar collector are 0.008 kW/m², 0.11 kW/m², 2%, 0.28, 0.3, 0.13 and 8 m²/kW for ammonia–water cycle and 0.04 kW/m², 0.3 kW/m², 9.5%, 0.7, 0.8, 0.37 and 3 m²/kW for LiBr–water mixture plant respectively.     

Theoretical performances of various refrigerant–absorbent pairs in a vapor-absorption refrigeration cycle by the use of equations of state

The vapor-absorption refrigeration cycle is an old and well-established technique, particularly with ammonia/water and water/LiBr systems. New types of refrigerant–absorbent pairs are also being actively studied. Modeling the cycle performance requires thermodynamic properties, which have been largely based on empirical correlation equations fitted to a large amount of experimental data such as solubility at various temperatures, pressures, and compositions. In this report, we have demonstrated, for the first time, a thermodynamically consistent model based on the equations of state for refrigerant–absorbent mixtures. Various commonly known binary-pairs for the absorption cycle are used as examples. Cycle performances and some new insights on understanding the cycle process are shown.     

太阳能溴化锂吸收式制冷技术的研究进展

介绍了太阳能澳化锂吸收式制冷循环的工作原理和系统构成,具体阐述了该制冷循环的几种典型结构,包括单效、双效、两级以及三效涣化锂吸收式制冷循环,分析了各种制冷循环的优缺点以及目前研究进展;进一步讨论了太阳能澳化锂吸收式制冷机组的性能特点受冷媒水出口温度、冷却水进口温度、加热蒸汽温度、污垢系数及不凝性气体等诸多因素的影响;提出了太阳能溴化锂吸收式制冷技术现存问题,最后指出,随着科学技术的发展和绿色建筑的兴起,太阳能溴化锂吸收式制冷将会有非常大的发展前景。     

Behaviors of SO2 absorption in [BMIm][OAc] as an absorbent to recover SO2 in thermochemical processes to produce hydrogen

The solubility behavior of SO₂ in 1-butyl-3-methylimidazolium acetate ([BMIm][OAc]) was investigated. The solubility of SO₂ in [BMIm][OAc] was measured to be 0.6 in mole fraction at 50 °C in a stream of SO₂ gas. The desorption of SO₂ from [BMIm][OAc] was never completed, until the temperature was raised to 170 °C in a stream of N₂, indicating that the absorption of SO₂ is irreversible at the experimental condition. An in-situ IR study showed that acetate anion in [BMIm][OAc] transformed into acetic acid during the SO₂ absorption. After removing acetic acid at 170 °C by evacuation, the bands at 1210, 1045 and 850 cm⁻¹ appeared in IR spectra. The bands at 1210 and 1047 cm⁻¹ were assigned to S–O stretching mode of HOSO₂⁻ (an isomer of HSO₃⁻) and the band at 885 cm⁻¹ was assigned to the symmetric stretching mode of HO–S. FT-IR study suggested that acetic acid and most plausibly [BMIm][HOSO₂] were generated from the interactions of [BMIm][OAc] with SO₂ and adventitious water in feed gas and/or [BMIm][OAc] during absorption–desorption process. [BMIm][HOSO₂] recovered after removing acetic acid was found to be a new and reversible SO₂ absorbent with the SO₂ absorption capacity of 0.55 in mole fraction.     

Performance optimization of an irreversible four-heat-reservoir absorption refrigerator

On the basis of an endoreversible absorption refrigeration cycle model with linear phenomenological heat transfer law of Q∝Δ(T⁻¹), an irreversible four-heat-reservoir cycle model is built by taking account of the heat resistance, heat leak and irreversibilities due to the internal dissipation of the working fluid. The fundamental optimal relation between the coefficient of performance (COP) and the cooling load, the maximum COP and the corresponding cooling load, as well as the maximum cooling load and the corresponding COP of the cycle coupled to constant-temperature heat reservoirs are derived by using finite-time thermodynamics. The optimal distribution relation of the heat-transfer surface areas is also obtained. Moreover, the effects of the cycle parameters on the COP and the cooling load of the cycle are studied by detailed numerical examples. The results obtained herein are of importance to the optimal design and performance improvement of real absorption refrigerators.