基于引射器的双温蒸发CO2热泵系统性能分析

采用高效的热泵系统替代常规锅炉是实现“双碳”目标的有效措施。本文提出采用引射器实现双温蒸发的CO₂热泵系统,以实现余热梯级利用和高效制热。通过窄点分析法,建立了基本CO₂系统、CO₂引射器系统、双温蒸发CO₂引射系统的热力学模型,发现双温蒸发系统存在最优排气压力及最大COP。基于最优工况进行分析,结果表明：在热泵热水器名义工况下,双蒸发器系统COP最高达4.84,比基本CO₂热泵系统提高了9.88%。双温蒸发过程可显著降低吸热过程中的换热不可逆损失,双温蒸发系统蒸发器侧的不可逆性能指数为1.51,比基本CO₂和带有引射器的常规CO₂系统降低了24.50%。     

CO2制冷压缩机性能检测装置与试验研究

根据CO₂制冷压缩机的运行特点,设计了一套适用于跨临界循环的压缩机性能检测装置,该装置适用于制冷量为14 kW⁷0 kW的冷冻冷藏及热泵系统用CO₂压缩机的测试,试验方法为制冷剂气体流量计法和跨临界气体冷却器法;在此基础上开展了CO₂制冷压缩机的性能测试,结果表明,该压缩机的主侧制冷量、辅侧制冷量及主辅侧平均值的测量结果重复性偏差均在±1%以内。     

两段式喷嘴几何参数对引射制冷系统性能的影响

用引射器代替膨胀阀可以提高制冷系统COP,而影响引射制冷系统性能的重要因素是引射器性能。对使用不同喉部直径喷嘴的引射制冷系统性能进行实验研究,分析引射器两段式喷嘴第一和第二喉部直径对引射器和系统性能的影响。实验结果表明,在冷凝温度40℃、蒸发温度-10℃工况下,随着第一喉部直径增大,引射比先升高后降低,制冷量和COP均降低,喉部直径为1. 8 mm时系统性能最佳;随着第二喉部直径增大,引射比先升高后降低,制冷量和COP均先降低后升高,喉部直径为1. 4 mm时系统性能最佳。     

跨临界CO2两相流引射制冷系统性能实验研究

对跨临界CO2两相流引射制冷系统性能进行了实验,分析了工况及引射器几何参数对系统性能的影响,结果表明:在实验工况范围内,跨临界CO2两相流引射制冷系统制冷量和COP随气体冷却器压力的升高而升高,随气体冷却器出口温度的升高而降低。对于使用不同喉部直径喷嘴的系统,在相同工况下,引射器喷嘴喉部直径较大的系统的性能较好。对于使用不同直径混合室的系统,随着气体冷却器压力的升高,使用小直径混合室的系统COP变化较大;当气体冷却器压力较低时,使用大直径混合室的系统COP较高,而当气体冷却器压力较高时,使用小混合室直径的系统性能较好。在相同工况下,与传统跨临界CO2循环进行比较,两相流引射制冷循环系统COP最大可提高14%。     

跨临界CO2两相流引射制冷系统性能实验研究

对跨临界CO2两相流引射制冷系统性能进行了实验,分析了工况及引射器几何参数对系统性能的影响,结果表明:在实验工况范围内,跨临界CO2两相流引射制冷系统制冷量和COP随气体冷却器压力的升高而升高,随气体冷却器出口温度的升高而降低.对于使用不同喉部直径喷嘴的系统,在相同工况下,引射器喷嘴喉部直径较大的系统的性能较好.对于使用不同直径混合室的系统,随着气体冷却器压力的升高,使用小直径混合室的系统COP变化较大;当气体冷却器压力较低时,使用大直径混合室的系统COP较高,而当气体冷却器压力较高时,使用小混合室直径的系统性能较好.在相同工况下,与传统跨临界CO2循环进行比较,两相流引射制冷循环系统COP最大可提高14%.     

多联引射器双温制冷系统的模拟研究

建立了多联引射器跨临界CO2双温制冷系统集总参数模型,并采用Matlab调用Refprop软件进行编程。对不同工况条件下的系统性能进行了模拟研究,分析了气冷器出口参数对系统性能的影响。模拟结果表明:在相同工况下,受气冷器出口温度的影响,系统制冷量及COP呈先增加后逐渐减少趋势,且气冷器出口压力越高,系统制冷量及COP峰值所对应的气冷器出口温度也越高;在相同工况下,受气冷器出口压力的影响,系统制冷量呈逐渐增加趋势,且在较高气冷器出口温度下,系统COP随气冷器出口压力的升高呈先升高后降低趋势,系统存在最佳气冷器出口压力,此时COP取得最大值。     

两段式喷嘴引射器及其引射制冷系统性能实验研究

对采用两段式喷嘴引射器的两相流引射制冷系统进行了实验研究,并将两段式喷嘴的引射比及其系统COP分别与拉法尔喷嘴引射器的引射比及其系统COP进行了比较。实验结果表明：在冷凝/蒸发温度为45 ℃/1 ℃工况下,使用不同几何尺寸两段式喷嘴引射器的引射比均大于拉法尔喷嘴引射器的引射比,最大提高了约18%;使用两段式喷嘴引射器的制冷系统COP大于使用拉法尔喷嘴引射器的制冷系统COP,最大提高了约12%;在蒸发温度为1 ℃条件下,两段式喷嘴引射器及拉法尔喷嘴引射器的引射比均在冷凝温度为45 ℃时达到最大值,而在冷凝温度为50 ℃条件下,两种引射器的引射比均在蒸发温度为3 ℃时达到最大值。     

集成引射器与机械过冷的CO2冷热联供系统性能分析

为提高CO₂系统用于建筑全年空间供热供冷的性能，本文提出集成引射器与机械过冷的跨临界CO₂冷热联供系统(EJ-DMS)。通过构建系统的热力学模型，以性能系数(COP)为目标函数，采用遗传算法对排气压力和过冷度进行优化，并对系统应用于5个典型城市的能耗、全年性能系数(COP_ann)进行场景分析。结果表明：EJ-DMS相比常规机械过冷系统、常规引射系统，COP在制热和制冷模式下分别提高10.90%、5.58%和8.99%、18.12%,COP_ann分别提高7.95%和5.98%。EJ-DMS相比常规引射系统在制热和制冷模式下排气压力分别降低0.47 MPa和0.77 MPa。此外，EJ-DMS系统在广州和哈尔滨运行时的COP_ann提升率最大，表明其更适合环境温度较高或较低的地区，如夏热冬暖和严寒地区。本文可为CO₂冷热联供系统的构建和优化提供理论参考。     

送风温度对车用跨临界CO2制冷系统影响的仿真研究

为研究送风温度对实际车用跨临界CO₂制冷系统综合性能的影响,借助GT-Suite仿真软件,建立了单级跨临界CO₂制冷系统的仿真模型。基于设计的三种工况,在风量设置上限的情况下对比了不同送风温度下系统的性能,提出了有效COP_eff的概念并对此进行研究。结果表明：在其他工况相同的条件下,提高送风温度可以提高系统的COP、有效COP_eff以及带风机功耗的有效COP_{eff, b};在低冷负荷工况下,考虑系统风机功耗后的综合性能COP_b存在最优值为3.819,即系统存在对应的最优送风温度,但当负荷增大至一定水平时,最优送风温度不再存在。     

蒸汽压缩/喷射制冷系统喷射器设计及节能分析

蒸汽压缩/喷射制冷系统是一种有效的节能系统,可以减少节流膨胀损失,降低压缩机压力比,提高制冷系统效率。选择5种计算工况对蒸汽压缩/喷射制冷系统进行计算,研究喷射器结构与蒸发温度和冷凝温度的变化规律,并与普通蒸汽压缩系统对比,从制冷量、压缩机耗功、性能系数三个角度分析新系统的节能效果。计算结果表明蒸汽压缩/喷射制冷系统在低温工况条件下节能效果最优,制冷量最大可提高29%,压缩机耗功最大可降低65%,COP值最大可提高63%。     

Numerical simulation and experimental validation of vapour compression refrigeration systems. Special emphasis on CO2 trans-critical cycles

A numerical and experimental comparative study of a carbon dioxide trans-critical refrigerating system and a conventional sub-critical refrigerating cycle is presented. Attention is focussed not only on the whole refrigeration cycle, but also on the behaviour of the hermetic reciprocating compressors used in these systems. The comparative cases presented have been specially designed for small cooling capacity units with an evaporation temperature around 0 °C. A detailed numerical simulation model for hermetic reciprocating compressors performance, widely validated under conventional fluid refrigerants, has been extended to numerically obtain the CO₂ compressor prototypes behaviour. Two CO₂ compressor prototypes working with CO₂ have been experimentally tested in a specific unit, specially designed and built to analyse high-pressure single stage vapour compression trans-critical refrigerating equipments. This set-up has allowed validating a detailed numerical simulation code for the thermal and fluid-dynamic behaviour of single stage vapour compression refrigeration system working with CO₂ as fluid refrigerant. The numerical results and the experimental data obtained to validate compressors, heat exchangers and whole cycle behaviour have shown a really good agreement. Finally, the numerical and experimental comparison between the carbon dioxide system and the sub-critical conventional cycle has shown the possibility of CO₂ as fluid refrigerant under the studied working conditions.     

Theoretical evaluation of trans-critical CO2 systems in supermarket refrigeration. Part II: System modifications and comparisons of different solutions

The performance of CO₂ refrigeration systems strongly depends on the operating conditions. The specific characteristics of low critical temperature and high operating pressure limit its applications and imply the implementation of different control strategies. This study compares the performance of different CO₂ system solutions for supermarket refrigeration with R404A system. Some possible modifications and improvements on the CO₂ system have been investigated. The COP of the investigated CO₂ system solution can be improved by about 3–7% along the ambient temperature range of 10–40 °C. The annual energy consumption calculations in three different climates; cold, moderate and hot, show that the centralized trans-critical CO₂ system is good solution for cold climates whereas the NH₃–CO₂ cascade system has the lowest energy consumption in hot climates. Both systems proved to be good alternatives to R404A DX system for supermarket refrigeration.     

用于低温复叠式制冷的CO2螺杆式压缩机组的性能实验

CO2在冷冻冷藏系统中适宜作为低温级制冷剂与其他制冷剂组成复叠式制冷循环。建立采用螺杆式压缩机组的NH3/CO2复叠式制冷实验系统,对低温级的CO2螺杆式压缩机组进行性能测试,并对主要技术参数进行分析,给出机组制冷量、轴功率、容积效率和绝热效率等在不同工况下的变化关系。在相同工况下CO2制冷机组的制冷量约是同型号氨机组的7.5～10.5倍,且在蒸发温度越低时差值越大。对NH3/CO2复叠式制冷机组和NH3单机双级压缩制冷机组的性能系数进行比较,前者在蒸发温度低于-40℃时性能系数更高。     

Multi-ejector R744 booster refrigerating plant and air conditioning system integration – A theoretical evaluation of energy benefits for supermarket applications

The multi-ejector rack is the most promising technology to push the so-called “CO₂ equator” further south and improve the global energy efficiency of R744 supermarket refrigeration systems.This paper theoretically compares the energy consumption of a CO₂ refrigerating plant equipped with a multi-ejector unit with that of a R404A direct expansion system (DXS), of a conventional CO₂ booster configuration and of two CO₂ solutions using parallel compression. The energy benefits related to the adoption of low temperature (LT) overfed display cabinets were also assessed. Furthermore, various scenarios involving different sizes of the supermarket, integration and capacity of the air conditioning (AC) system and efficiency of the parallel compressors were investigated. The evaluations were carried out by considering different locations in Southern Europe. The results showed that, as a function of the selected boundary conditions, energy savings ranging from 15.6% to 27.3% could be accomplished with the multi-ejector concept over DXS.     

Theoretical evaluation of trans-critical CO2 systems in supermarket refrigeration. Part I: Modeling, simulation and optimization of two system solutions

Using CO₂ trans-critical system solutions in supermarket refrigeration is gaining interest with several installations already running in different European countries. Using a computer simulation model, this study investigates the performance of two main system solutions: centralized with accumulation tank at the medium temperature level and parallel with two separate circuits for low and medium temperature levels. Both system solutions are presented and the simulation model is described in details. Calculations have been performed to design the systems and optimize their performances where basic layout and size of each solution have been defined. For ambient temperature range of 10–40 °C, the reference centralized system solution shows higher COP of about 4–21% than the reference parallel solution. Using two-stage compression in the centralized system solution instead of single stage will result in total COP which is about 5–22% higher than that of the reference centralized system and 13–17% higher than that of the improved two-stage parallel system. The two-stage centralized system solution gives the highest COP for the selected ambient temperature range.     

Experimental investigation of a CO2 automotive air conditioner

In this study, a CO₂ automotive air conditioner prototype was designed and constructed. The compressor was of swash plate design; the gas cooler and evaporator were made of fin-tubes; a manual expansion valve and an internal heat exchanger accumulator were used. The lubricant, the CO₂ charge, the evaporator outlet pressure, the compressor speed, the air inlet temperature and flow rate of the gas cooler and the air flow rate of the evaporator were varied and the performance of the prototype was experimentally investigated in detail. The cooling capacity, compressor power consumption, CO₂ mass flow rate, and COP value were analyzed. The experimental results showed that the CO₂ system performance was greatly affected by different lubricants; the CO₂ system performance was sensitive to the mass charge; the high side pressure affected the system performance greatly and a high side pressure controller was needed.     

Simulated performance and cofluid dependence of a CO2-cofluid refrigeration cycle with wet compression

Recent experiments demonstrate the viability of a low-pressure CO₂-cofluid compression refrigeration cycle in which CO₂ and a non-volatile cofluid are circulated in tandem and co-compressed in a compliant scroll compressor. This work explores the theoretical performance limitations of such a cycle operating under environmental conditions representative of automotive air conditioning and studies the dependence of this performance on the properties of the CO₂-cofluid mixture. The vapor–liquid equilibrium and thermodynamic properties of the mixture are described using a previously reported activity-coefficient model. A coupled system of physically based equations that allows for consideration of both ideal and real hardware components is used to represent the system hardware and its interaction with the environment. The system efficiency is analyzed in terms of entropy generation rates in the various hardware components; entropy generation in the internal heat exchanger—a component required to achieve sufficiently low cooling temperatures—strongly influences overall system efficiency. The vapor pressure of the CO₂-cofluid mixture and the heat of solution of CO₂ in cofluid have large and somewhat independent contributions to the system performance: lower saturation pressure lowers the optimal operating pressures at fixed CO₂ loading, while increasingly negative heat of solution contributes to higher specific refrigeration capacity and efficiency.

Résumé

Recent experiments demonstrate the viability of a low-pressure CO₂-cofluid compression refrigeration cycle in which CO₂ and a non-volatile cofluid are circulated in tandem and co-compressed in a compliant scroll compressor. This work explores the theoretical performance limitations of such a cycle operating under environmental conditions representative of automotive air conditioning and studies the dependence of this performance on the properties of the CO₂-cofluid mixture. The vapor–liquid equilibrium and thermodynamic properties of the mixture are described using a previously reported activity-coefficient model. A coupled system of physically based equations that allows for consideration of both ideal and real hardware components is used to represent the system hardware and its interaction with the environment. The system efficiency is analyzed in terms of entropy generation rates in the various hardware components; entropy generation in the internal heat exchanger—a component required to achieve sufficiently low cooling temperatures—strongly influences overall system efficiency. The vapor pressure of the CO₂-cofluid mixture and the heat of solution of CO₂ in cofluid have large and somewhat independent contributions to the system performance: lower saturation pressure lowers the optimal operating pressures at fixed CO₂ loading, while increasingly negative heat of solution contributes to higher specific refrigeration capacity and efficiency.     

全新风家用空气制冷系统实验台性能测试

为解决空气制冷技术对气源的依赖问题,同时简化系统回收膨胀功,本文以高速电机驱动的无油气浮轴承压缩-膨胀一体机为核心部件,搭建了采用开式逆增压循环的全新风家用空气制冷系统实验台,可实现膨胀机、压缩机进出口的温度、压力、流量测量,并可与焓差室对接,获得制冷系统的制冷量和送风参数。在标准空调工况下进行了多个转速下的性能测试,额定转速为38 000 r/min时,制冷量为1.6 kW。送风温度随转速升高而降低,制冷量随转速升高而增大,基于该特点,空气制冷系统有直接送风和送风温度可调的优势,使空气制冷技术在新风空调领域的应用成为可能。     

制冷系统湿压缩时吸气干度控制方法的实验研究

不完全湿压缩能大幅度降低压缩机排气温度,然而该应用的最大难点是如何控制实时压缩机吸气干度在合适的范围内。本文提出了假拟饱和等熵压缩排气温度控制压缩机吸气该干度的方法,理论分析了在AHRI(空调供暖制冷协会)空调和低温制冷两种典型工况下,R22、R32、R134a和R410A四种制冷剂作为冷媒时,应用该方法控制压缩机吸气带液时系统性能的变化,并通过R32实验验证该结论的正确性。结果表明:利用假拟饱和等熵压缩排气温度可以将压缩机吸气状态控制在少量湿蒸气的状态;在T-s图上具有钟型饱和线形状的R32制冷剂,利用假拟饱和等熵压缩所控制的制冷系统,当吸气干度在0.96~1时,制冷量和COP均能达到最大值。     

Study of capillary tubes in a transcritical CO2 refrigeration system

Capillary tubes have been used in refrigeration systems for many years, but not with a transcritical CO₂ system. In this article, the effects of capillary tubes in a transcritical CO₂ refrigeration system have been investigated experimentally and theoretically. Different types of capillary tubes with different lengths (0.5–4 m) and diameters (1–2 mm) have been tested. The result of this work is a static model, which is used in the further work to make a simulation model (static) of a complete refrigeration system. The model is based on Friedel's and Colebrook's pressure drop correlations.

The behaviour of an adiabatic capillary tube in a refrigeration cycle has been investigated theoretically. The conclusion is that the COP of a system with capillary tubes generally is better than when a fixed high pressure is used, but not as good as when variable optimal high pressure is used. Capillary tubes are especially interesting in applications where the evaporation pressure is constant and the temperature out of the gas cooler varies no more than ±10 K from the design condition. The reduction in COP is more significant at low temperatures out of the gas cooler.