Absorption chiller crystallization control strategies for integrated cooling heating and power systems

The concept of an air-cooled absorption chiller system is attractive because the cooling tower and the associated installation and maintenance issues can be avoided. However, crystallization of the LiBr–H₂O solution then becomes the main challenge in the operation of the chiller, since the air-cooled absorber tends to operate at a higher temperature and concentration level than the water-cooled absorber due to the relative heat transfer characteristics of the coolant. This leads to crystallization of the working fluid. The paper focuses on the crystallization issues and control strategies in LiBr–H₂O air-cooled absorption chillers. As a result a novel application opportunity is proposed for the integration of absorption chillers into cooling, heating and power (CHP) systems. This new methodology allows for air cooler operation while avoiding crystallization.     

公共建筑风冷冷水机组的节能策略研究

通过建立机组模型,对变冷凝温度控制和冷凝器变速风扇对风冷式冷水机组性能系数（COP）的改善和节能潜力进行了研究。当这两种技术同时应用时,机组COP能提高达51.8%。利用所建立机组模型,对某典型公共建筑的节能潜力进行分析,机组年度能耗下降了7.7%,节能效果显著。     

Combined effects of fluid selection and flow condensation on ejector operation in an ejector-based chiller

This work investigates the optimization of the coefficient of performance of an ejector-based chiller through changes in the two-phase flow characteristics inside the ejector using wet R134a and dry R245fa fluids. Reducing the superheat at the motive nozzle inlet results in a 12–13% increase in COP with a 14–16 K drop in driving waste heat temperature. The roles of momentum transfer, heat transfer, and two-phase flow on performance are delineated. The change in COP appears to be a combination of the choice of fluid and the effect of phase change on momentum transfer effectiveness. Larger degrees of condensation reduce momentum transfer effectiveness; however, energy savings from reduced motive superheating compensates for the effect of condensation, and causes a net increase in COP. It is recommended that ejector-based chillers be operated such that the motive nozzle inlet is near saturation, and dry fluids like R245fa are used to improve performance.     

State-of-the-art review on crystallization control technologies for water/LiBr absorption heat pumps

The key technical barrier to using water/lithium bromide (LiBr) as the working fluid in air-cooled absorption chillers and absorption heat-pump systems is the risk of crystallization when the absorber temperature rises at fixed evaporating pressure. This article reviews various crystallization control technologies available to resolve this problem: chemical inhibitors, heat and mass transfer enhancement methods, thermodynamic cycle modifications, and absorption system-control strategies. Other approaches, such as boosting absorber pressure and J-tube technology, are reviewed as well. This review can help guide future efforts to develop water/LiBr air-cooled absorption chillers and absorption heat-pump systems.     

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.     

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.     

Experimental boiling heat transfer coefficients in the high temperature generator of a double effect absorption machine for the lithium bromide/water mixture

The aim of this work is to determine the boiling heat transfer coefficients in the high temperature desorber (HTD) of an air-cooled double effect lithium bromide/water absorption prototype. The HTD is a plate heat exchanger (PHE) with thermal oil on one side, and a lithium bromide solution on the other side. Several experiments were performed with this PHE while the prototype was working with an outdoor dry bulb temperature around 42 °C and condensation temperature around 55 °C. The registered data allowed to calculate the global heat transfer coefficient and the heat transfer coefficient for the LiBr/water mixture in forced convective boiling. The pressure drop produced by the boiling of the refrigerant has been calculated as well. It has been verified that the largest part of the heat supplied in the generator is required for desorbing the refrigerant (except for the maximum solution mass flow), while the sensible heat varies from 10% to 50% of the total heat supplied.     

Synthesis of silver chromate nanoparticles: Parameter optimization using Taguchi design

The Taguchi method of experimental design is very well suited to improving the production process of synthetic nanoparticles. The current application of the Taguchi method was successful in optimizing the experimental parameters affect on synthesis procedure of silver chromate nanoparticles. Ultrafine silver chromate particles were synthesied by precipitation method using addition of silver ion solution to the chromate reagent. The effect of reaction conditions such as: silver and chromate concentrations, flow rate of reagent addition and temperature on the particle size of synthesized silver chromate particles were investigated. The effect of these factors on the diameter of silver chromate particles were quantitavely evaluated by the analysis of variance (ANOVA). The results showed that silver chromate particles can be synthesized by controlling silver concentration, flow rate and temperature. Finally, the optimum conditions for synthesis of silver chromate particles by this simple and fast method were proposed. The results of ANOVA showed that 0.001 mol/l silver ion concentration, 40 ml/min flow rate for addition of silver reagent to the chromate solution and 0°C temperature are optimum conditions for producing silver chromate particles with 100 ± 33 nm width. On the other hand, the Ag₂CrO₄ nano-superstructures were synthesized by electrosynthesis method. The results showed that Ag₂CrO₄ nanoparticles synthesized by this method have 75 nm average diameter.     

Multi-objective optimization of a stirling-type pulse tube refrigerator

A genetic algorithm (GA) optimization method which is coupled to a one-dimensional finite volume method is proposed and implemented as a computer program for the modeling and optimization of a stirling-type pulse tube refrigerator (PTR). The multi-objective optimization procedure is applied to provide the optimization design parameters which are charge pressure, operating frequency, and temperature of after-cooler as well as swept volume of compressor. The procedure is selected to obtain the maximum coefficient of performance (COP) and the minimum cooling temperature (T_cold) as two objective functions. In order to validate the simulation code, the results were compared with the results of other models and experiments. The results showed a reasonably well agreement between simulation output and experimental data. The results of optimal designs are a set of multiple optimum solutions, called Pareto optimal solutions. Moreover, the closed form relations between two objectives are derived for Pareto optimal solutions of pulse tube refrigerator. Finally, a sensitivity analysis of the variation of each design parameter on both objective functions was carried out as well and the results are presented. As a result, the COP is more sensitive than T_cold in the optimum design points. The frequency of refrigerator is the most sensitive factor which affects the COP even with little changes.     

R404A直接接触凝结制冷循环的性能分析

提出R404A直接接触凝结换热的制冷循环,分析R404A直接接触凝结制冷循环的热力性能,并与常规双级压缩制冷循环的性能进行对比。得出结论:在一定的冷凝温度、蒸发温度和过冷液体的过冷度下,直接接触凝结制冷循环存在最佳的饱和液体温度,并在此最佳的饱和液体温度下,获得最优的性能和最小的冷凝热负荷,随着过冷液体的过冷度增大和蒸发温度升高,直接接触凝结制冷循环的性能系数增加、冷凝热负荷减少,获得最优性能的最佳饱和液体温度值提高。过冷液体的过冷度为25℃时,直接接触凝结制冷循环的最佳性能系数较双级压缩制冷循环的最佳性能系数提高6.2%。直接接触凝结制冷循环的最小冷凝热负荷较双级压缩制冷循环的最小冷凝热负荷减小1.8%。     

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.     

NH3／CO2复叠式制冷系统的仿真与实验研究

运用集总参数法建立了NH3／CO2复叠式制冷系统的稳态仿真数学模型,计算和分析了低温循环冷凝温度对系统COP值和质量流量比的影响;最优冷凝温度随蒸发温度的变化：蒸发温度和冷凝温度对系统COP值的影响,以及不同冷凝蒸发传热温差下系统COP值的变化关系,比较了某工况下仿真结果与实验测试数据,验证了仿真模型正确性,对进一步深入研究NH3／CO2复叠式制冷系统具有一定的指导作用。     

基于吸气喷液的制冷系统热力性能理论研究

吸气喷液是降低压缩机排气温度的有效手段,通过建立制冷系统热力循环计算模型,研究基于吸气喷液的制冷系统热力学状态,以制冷剂R410A为工质,分析在不同工况下热力性能随喷液流量比例的变化趋势。计算结果表明,当喷液流量比例增加到5%时,排气温度平均降低幅度为9℃,功率、制冷量和COP值分别平均下降0.4%、0.6%和0.3%;若蒸发温度增加,功率呈先上升后下降的趋势,蒸发温度每增加5℃,排气温度平均降低幅度为4.5℃,制冷量、COP值分别平均增加17.6%、16.9%;冷凝温度每降低5℃,排气温度平均降低幅度为8℃,功率平均下降11.3%,制冷量、COP值分别平均增加6%、17.9%。     

R404A与CO2复叠式制冷系统的理论分析

本文针对所建立的新型R404A／CO2复叠式制冷设备进行了理论研究,该系统可提供零下40℃以下的低温环境。根据R404A和CO2的物性特征及复叠式循环流程,通过数值模拟寻找一定工况下CO2低温级的最佳冷凝温度及二者的最佳质量流量比,分析冷凝蒸发器的工作温度、CO2侧蒸发温度、R404侧的冷凝温度等对R404A／CO2复叠式系统COP的影响。结果表明,为了提高循环效率并保证循环的安全运行,应尽可能地升高低温段蒸发温度、降低高温段冷凝温度,缩小冷凝蒸发器的传热温差,环保工质R404A和CO2的复叠式制冷系统在低温制冷条件下有良好的发展前景。     

非共沸工质非完全冷凝实验现象及判据的验证

针对非共沸工质可能出现的非完全冷凝现象，首先针对两种非共沸工质进行了实验研究，得到制热量和COP随冷凝器水流量逐渐增加的变化规律，证明非完全冷凝现象存在的可能性和普遍性；然后根据已经提出的此现象是否发生的理论判据（极限流量判别式）进行计算，并用实验测得极限流量对其进行验证；最后，分析了实验与理论计算的差异和原因，并提出完善非完全冷凝现象理论判据的改进方向，即考虑工质在冷凝器中的压降、工质和换热流体间的实际温差和实际换热过程中的各种外力。     

Numerical investigation on semi-GAX NH3–H2O absorption cycles

Double-lift absorption cycles represent a suitable solution for air-cooled thermally driven cooling applications. Among the several existing double-lift configurations, semi-GAX cycles are known as the most promising in terms of efficiency. These cycles incorporate the GAX effect in a pressure staged cycle, by means of a split on the solution leaving the low pressure absorber. Two configurations of the semi-GAX cycle have been proposed in the past, the semi-GAX 1 and the semi-GAX 2. The former achieves the GAX effect between the intermediate and the high pressure levels, the latter between the low and the intermediate. Within this paper, the semi-GAX cycles are numerically investigated at operating conditions suitable for a low temperature driven (e.g., by flat plate solar collectors) air conditioning application. The peculiarities of the two cycles are described and the factors affecting their performances are underlined. The COP resulted to be strongly influenced by the split ratio, which determines the intermediate pressure and the possibility to achieve the GAX effect. If the split ratio is optimized to achieve the maximum COP, the COP is higher for semi-GAX 2 for air temperatures below 27 °C and for semi-GAX 1 above. In both cases, the maximum air temperature which allows a circulation ratio below 15 is 40 °C, with chilled water at 7/12 °C and driving temperature of 90 °C.     

双模风冷模块化冷水机组节能性分析

针对目前常见风冷冷水机组节能空间有限的问题,提出采用机械制冷和自然冷却模块化设计的双模风冷冷水机组。以北方某地区一冷水系统为例,分析双模风冷冷水机组的节能性,为北方需要常年制冷的大型计算机或大功率发热设备冬季运行提供比较适用的方案。     

一种制冷机组能效定量评价方法

在冷水机组比选过程中,通常会优先选择能效高、变工况性能好的产品,但冷水机组的性能对价格的影响,难以定量评价.通过对逐时负荷和气象参数的研究和变工况条件下COP值的计算,得出不同品牌机组年运行费用差异,并折算成价格因素,将机组性能量化,用此方法,可方便不同品牌的冷水机组间进行比选.     

Modeling and simulation of the transcritical CO2 heat pump system

In this study, a CO₂ transcritical cycle model without imposing any excessive constraints such as fixed discharge pressure and suction pressure is developed. The detailed geometrical variation of the gas cooler and the evaporator have been taken into account. The model is validated with the experimental measurements. Parametric influences on the CO₂ system with regard to the effect of dry bulb temperature, relative humidity, inlet water temperature, compressor speed, and the capillary tube length are reported. The COP increases with the dry bulb temperature or the inlet relative humidity of the evaporator. Despite the refrigerant mass flowrate may be increased with the inlet water temperature, the COP declines considerably with it. Increasing the compressor speed leads to a higher heating capacity and to a much lower COP. Unlike those of the conventional sub-critical refrigerant, the COP of the transcritical CO₂ cycle does not reveal a maximum value against the capillary tube length.     

水冷式冷水机组冷凝温度控制法研究

论述了在集中空调冷却水变流量系统中冷凝温度控制法的特点;分析了冷却水变流量对离心式制冷机性能系数的影响;结合实例说明虽然冷却水变流量对于冷水机组制冷性能的负面影响不可忽略,但这一负面影响较对于水泵节能的正面影响要小,采用冷凝温度控制法对于空调系统综合节能是有益的.