跨临界CO_2引射制冷系统控制性能

本文建立了两种控制器(单通道最优控制器(SCOC)和多变量线性二次高斯控制器(LQG))以改善跨临界CO_2引射制冷系统的运行效率。首先建立了SCOC,通过在线调节喷嘴喉部面积,搜索系统最优的气冷器压力;其次针对SCOC作用下制冷量不可控的缺点,设计了LQG以实现系统制冷量可调。将两种控制器分别应用于实验系统中,结果表明:SCOC能够驱使系统不断接近系统的最优气冷器压力,给定工况下获得最大制热系数COPh为3.15,但导致系统制冷量的不可控。在LQG的作用下,气冷器压力、系统制冷量得到独立控制,显示了很好的参数跟随性,然而LQG无法保证系统的稳态运行效率。研究指出两种控制器各有优缺点,若实现满足系统负荷需求的同时保持系统最高的运行效率,需要设计结合两种算法特点的新型控制器。     

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。     

CO_2汽车空调系统排气压力优化研究

CO_2是一种环保制冷剂,也是未来汽车空调制冷剂的可行方案之一。经过研究发现,带回热器的跨临界CO_2汽车空调系统在制冷运行时,系统存在最优的排气压力,在此排气压力下,系统具有相同工况条件下的最优能效比COP。本文通过分析和计算,进一步确定了系统的最优压力和气冷器出口的温度线性相关。而气冷器的出口温度受制于环境温度,所以系统的最优排气压力取决于环境温度。CO_2热泵空调系统可以依据此建立系统的最优COP控制策略,保证系统的高能效运行,从而减少空调系统能耗。     

两级蒸发对跨临界CO_2引射制冷系统影响的实验研究

两级蒸发引射制冷循环中通过二级蒸发器不仅能调节引射器出口干度还能提高系统效率。通过改变第二蒸发器冷冻水流量对两级蒸发引射制冷系统进行实验研究,并与改变引射器面积比的调控效果进行比较。结果表明:在实验工况范围内,气冷器压力、第一蒸发器压力和压缩机流量都随第二蒸发器冷冻水流量的增加而增大;而且引射器面积比越大,气冷器压力越高而蒸发器压力和压缩机流量越低。同时,系统引射系数随第二蒸发器冷冻水流量的增加而降低,而制冷量和COP则升高,尤其是在小引射系数下,系统制冷量和COP提高的更为明显。本研究为引射循环提供了另外一种良好的调控思路。     

两级节流中间完全冷却CO2跨临界双级压缩制冷循环特性研究

CO2是零ODP、低GWP的天然制冷剂,在冷库制冷系统中应用前景广阔。本文针对用于低温冷库的两级节流中间完全冷却CO2跨临界双级压缩制冷循环（DTCC循环）建立数学模型,通过计算不同工况,分析蒸发温度、压缩机等熵效率、气冷器出口温度、排气压力以及回热循环方式对DTCC循环制冷系数的影响规律;给出DTCC循环的最优排气压力和最佳中间压力的计算式。研究表明：在蒸发温度-30~10 ℃、气冷器出口温度30~45 ℃范围内,DTCC循环的最优排气压力约比相同工况下的单级跨临界制冷循环的最优排气压力低0.3 MPa;低压级排气采用预冷气冷器、在高压级气冷器出口设置回热器均可有效改善DTCC循环的制冷系数。     

高温CO_2热泵套管式气冷器的仿真与优化设计

以Petrov换热关联式为基础,建立了CO_2气冷器的数值仿真模型,并根据实验数据对模型进行了修正,并通过联立拟合的压缩机模型,分析了CO_2系统在不同工况下,排气压力、气冷器单管长度和并联管程数对其性能的影响:给定气冷器布局,最优排气压力随供水温度上升而升高;给定工况下,气冷器CO_2出口温度随着排气压力升高而降低,而热水流量和制热量都会增加;不同工况下,当排气压力低于最优压力时,管长的增加对系统COP值增大的影响非常显著;在供水温度低于70℃时,并联管程数的增加使系统的COP值增大,但高于70℃时,并联管程数多的系统COP值反而更低。     

CO2热泵系统气冷器的热力学性能分析

针对CO₂热泵系统螺旋套管式气冷器,基于MATLAB建立了仿真模型,采用单因素分析方法,研究进水温度、CO₂压力和质量流量对气冷器换热量、■耗散、■损失、■效率以及出水温度的影响。经实验验证,在进水温度为24.5～35.0℃、CO₂压力为8.4～10.7 MPa、CO₂质量流量为0.032 6～0.047 6 kg/s工况下,气冷器模型制热量与实验数据相比总体误差在±10%以内。模拟结果表明：相比进水温度和CO₂质量流量,CO₂压力对气冷器性能的影响更为显著,且存在最优压力。在进水温度为20℃、CO₂进口温度为90℃工况下,当CO₂压力为10 MPa时气冷器■效率最高,当CO₂压力为11 MPa时气冷器换热量最大;当进水温度低于20℃时,CO₂压力为10.5 MPa时出水温度最高。     

基于多种群遗传算法的主动座椅LQG控制器优化

针对LQG控制器权重系数依靠经验选取的不足,提出基于多种群遗传算法的主动座椅悬架LQG控制器权重系数的多目标优化设计方法。在建立6自由度主动座椅悬架模型的基础上,利用最优控制理论设计LQG控制器;以LQG控制器权重系数为变量,以各性能指标为优化目标,建立基于权重系数的多目标优化函数;利用MPGA算法的全局搜索能力,对LQG控制器权重系数进行寻优,实现对主动座椅悬架最优控制器的优化。仿真结果表明:经MPGA算法优化权重系数后,适应度函数值比GA算法优化时降低0.9左右,验证了此优化方法的有效性。     

四种跨临界CO2压缩式热泵系统的热力性能研究

CO2是具有很大潜力的天然替代工质之一,CO2跨临界循环放热过程中具有较大温度滑移,与水侧温升过程相匹配,因此适合用于热泵热水器系统。国内外学者提出了许多提高跨临界CO2循环效率的方法,其中包括引入回热器、喷射器等设备,从不同角度对比分析在常规跨临界CO2热泵系统中引入回热器、喷射器后系统的性能变化。本文在前人工作的基础上,建立相关热力学计算模型,并进一步对四种不同形式的跨临界CO2热泵系统（常规跨临界CO2热泵系统（TCHS）、带回热器的跨临界CO2热泵系统（TCHSI）、带喷射器的跨临界CO2热泵系统（TCHSE）及带喷射器与回热器的跨临界CO2热泵系统（TCHSEI））的性能进行研究,对比分析排气压力一定的情况下四种循环的热力性能;从最优排气压力的角度出发,分析对比不同系统中气冷器出口温度变化对系统最优排气压力和制热系数的影响,以及喷射器等熵效率对系统性能的影响。以上研究为CO2压缩式热泵系统的实用化进展奠定良好的理论基础。     

跨临界二氧化碳热泵热水系统气冷器的仿真分析

建立了跨临界二氧化碳热泵热水系统中的气体冷却器模型，对管内二氧化碳和水侧的流动和传热进行了数值仿真；并运用该模型分析了系统运行时各有关参数对换热器性能的影响，并结合最优排气压力，研究使系统高效运行的方法，为气冷器的优化设计提供了基础。     

Performance optimization of a transcritical CO2 refrigeration system using a controlled ejector

An on-line optimal quasi cascade controller for an ejector with variable nozzle throat area is proposed to improve the operating performance of the transcritical CO₂ ejector refrigeration system. The optimal gas cooler pressure is tracked in real time by the controller including a tracker and a predictor. Using the system dynamic model, the dynamic responses of the system performance and ejector efficiency under variable nozzle throat area are first analyzed. Then the parameters of the tracker and predictor are determined by simulation respectively which exhibits a good dynamic characteristic with an acceptable settling time. Besides, the controller presents a good robustness under variable compressor speeds and mass flow rate of cooling water. Furthermore, the system performance is actually increased to the maximal value by the controller even at the variable operating conditions. Finally, the optimal controller is verified by experiments to be an effective way to improve the system performance automatically.     

翅片管式CO2气体冷却器模拟与优化

为了研究CO2在翅片管式气体冷却器内的流动特性,建立了稳态分布参数模型,并进行了实验验证。结果表明:CO2侧换热系数受入口压力和质量流量的影响较大,但入口温度对其影响很小。换热量随着入口压力的变化有一个最大值;且随着流量的增大,最大换热量所对应的入口压力值逐渐增大。压降和换热量均随入口温度的增加而线性增加。适当增加管程数,采用较小管径的气冷器性能更高。     

Thermodynamic analysis of a novel ejector-cascade refrigeration cycles for freezing process applications and air-conditioning

Waste heat from the gas cooler is a form of free energy, which can be utilized to drive an ejector cooling cycle. This paper presents a new CO₂ ejector-cascade refrigeration cycle. The effects of important parameters on the thermodynamic performance of the new cycle are theoretically investigated based on energetic and exergetic analyses. Furthermore, the performance comparison of the proposed cycle and conventional cycle is carried out. The theoretical study shows that the new cycle exhibits a reasonable value of COP (coefficient of performance) and system second law efficiency. For the same cooling capacity, the improvements of the maximum COP and second law efficiency could reach 37% and 12%, respectively, over those of the conventional cascade cycle under the given operating conditions and at the optimum gas cooler pressure.     

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.     

Characterization of a thermoelectric/Joule–Thomson hybrid microcooler

Micromachined Joule–Thomson (JT) coolers are attractive for cooling small electronic devices. However, microcoolers operated with pure gases, such as nitrogen gas require high pressures of about 9 MPa to achieve reasonable cooling powers. Such high pressures severely add complexity to the development of compressors. To overcome this disadvantage, we combined a JT microcooler with a thermoelectric (TE) pre-cooler to deliver an equivalent cooling power with a lower pressure or, alternatively, a higher cooling power when operating with the same pressure. This hybrid microcooler was operated with nitrogen gas as the working fluid at a low pressure of 0.6 MPa. The cooling power of the microcooler at 101 K operating with a fixed high pressure of 8.8 MPa increased from 21 to 60 mW when the precooling temperature was reduced by the thermoelectric cooler from 295 to 250 K. These tests were simulated using a dynamic numerical model and the accuracy of the model was verified through the comparison between experimental and simulation results. Based on the model, we found the high pressure of the microcooler can be reduced from 8.8 to 5.5 MPa by lowering the precooling temperature from 295 to 250 K. Moreover, the effect of TE cooler position on the performance of the hybrid microcooler was evaluated through simulation analysis.     

Thermodynamic analysis and optimization of a novel two-stage transcritical N2O cycle

Thermodynamic (energy and exergy) analyses and optimization studies of two-stage transcritical N₂O and CO₂ cycles, incorporating compressor intercooling, are presented based on cycle simulation employing simultaneous optimization of intercooler pressure and gas cooler pressure. Further, performance comparisons with the basic single-stage cycles are also presented. The N₂O cycle exhibits higher cooling COP, lower optimum gas cooler pressure and discharge temperature and higher second law efficiency as compared to an equivalent CO₂ cycle. However, two-stage compression with intercooling yields lesser COP improvement for N₂O compared to CO₂. Based on the cycle simulations, correlations of optimum gas cooler pressure and inter-stage pressure in terms of gas cooler exit temperature and evaporator temperature are obtained. This is expected to be of help as a guideline in optimal design and operation of such systems.     

System dynamic model and temperature control of a thermoelectric coolerModèle dynamique d'un système et régulation de la température d'un refroidisseur thermoélectrique

A linear dynamic model of the thermoelectric cooler including the heat sink and the cooling-load heat exchanger was derived using small-signal linearization method. It shows that the dynamic model of a thermoelectric cooler has two poles and one zero. The linear dynamic model is shown to vary with operating conditions. A linear feedback system is designed for the cold-end temperature control of a thermoelectric cooler using the average linear dynamic model of the thermoelectric cooler and a PDF controller structure. The step response tests show that the controller has a very satisfactory performance. Some tests under variable cooling load and ambient temperature are also performed to examine the disturbance-rejection property of the controller. Experimental results show that the cold-end temperature can be maintained at the fixed value within ±0.1°C irrespective of the variations of the cooling load and the ambient conditions.     

A steady and quasi-steady state analysis on the CO2 hybrid ground-coupled heat pumping system

This article contains the steady and quasi-steady state analysis on a CO₂ hybrid ground-coupled heat pumping system for warm climates. The hybrid system uses a combination of ambient air and ground boreholes as a heat sink for the cooling mode, while only the ground boreholes are used as a heat source in the heating mode. The steady state analysis suggests that the optimal control strategy of gas cooler pressure for a CO₂ hybrid transcritical cycle is based on the optimal cooling COP value and the ratio of heat rejected to ambient air. This optimal control strategy is important for decreasing the annual ground thermal imbalance performance of ground boreholes. In addition, the quasi-steady state model of a CO₂ hybrid ground-coupled heat pumping system is constructed for the hourly simulation with different boundary conditions. Simulation results show the details of the system operating characteristics both for heating and cooling modes and the COP values with different operating and design conditions are presented.     

便携式热电制冷器制冷性能优化的试验分析

设计、组装一台便携式热电制冷器并对其性能进行试验研究,结果显示,200 mL的水在33 min内降温17.0℃,折合制冷量7.3 W,制冷器容器的高度方向上存在较大温差,且水温降低后密度增大而下沉,使水的自然对流换热过程受到抑制,这2个因素的综合作用使制冷片冷热端温差增大,制冷量减小,工况恶化。为优化该制冷器的制冷性能,在制冷片冷端增设重力式热管(充注R134a)并进行试验研究,结果表明,1 L的水在75 min内温度降低12℃,折合制冷量9.3 W,比优化前增大了27.4%。表明重力式热管的加入能够改善制冷器内水的对流换热情况,增大换热面积,减小竖直方向上的传热温差。     

Performance evaluation of a CO2 heat pump system for fuel cell vehicles considering the heat exchanger arrangements

A novel CO₂ heat pump system was provided for use in fuel cell vehicles, when considering the heat exchanger arrangements. This cycle which had an inverter-controlled, electricity-driven compressor was applied to the automotive heat pump system for both cooling and heating. The cooling and heating loops consisted of a semi-hermetic compressor, supercritical pressure microchannel heat exchangers (a gas cooler and a cabin heater), a microchannel evaporator, an internal heat exchanger, an expansion valve and an accumulator. The performance characteristics of the CO₂ heat pump system for fuel cell vehicles were analyzed by experiments. Results for steady and transient state performance were provided for various operating conditions. Furthermore, experiments to examine the arrangements of a radiator and an outdoor heat exchanger were carried out by changing their positions for both cooling and heating conditions. The arrangements of the radiator and the outdoor heat exchanger were tested to quantify cooling/heating effectiveness and mutual interference. The improvement of heating capacity and coefficient of performance (COP) of the CO₂ heat pump system was up to 54% and 22%, respectively, when using preheated air through the radiator instead of cold ambient air. However, the cooling capacity quite decreased by 40–60% and the COP fairly decreased by 43–65%, for the new radiator-front arrangement.