机械过冷跨临界CO_2热泵供暖系统性能分析

采用辅助的蒸气压缩循环进行过冷,可改善传统跨临界CO_2热泵系统用于冬季供暖性能。本文通过构建机械过冷跨临界CO_2热泵系统的热力模型,分析了机械过冷跨临界CO_2热泵系统供暖工况下的运行特性,结果表明:机械过冷CO_2热泵系统存在最大COP,对应最优排气压力和过冷度,标准工况下比常规CO_2系统能效提高15.9%。该系统可有效解决常规CO_2热泵回水温度过高导致COP迅速衰减的问题,当回水温度由40℃升至50℃时,常规系统COP下降16.9%,而机械过冷热泵系统COP仅下降8.4%。通过改进可有效降低CO_2压缩机的排气压力和温度,且供水温度越低排气压力降低效果越显著。机械过冷循环工质的选取会影响系统整体性能,选取的11种过冷循环工质中能效最高的为R717,最低的为R1234yf。在低环境温度工况下性能的提升更加明显,通过配置小型常规工质蒸气压缩循环即可实现CO_2热泵系统性能显著改进,经济性优势明显。     

采用非共沸混合工质机械过冷的跨临界CO_2制冷循环性能分析

本文提出了非共沸混合工质机械过冷跨临界CO_2制冷循环。在最优排气压力和最优过冷度下循环取得最大COP。最大COP、最优排气压力和过冷度与混合制冷剂的温度滑移密切相关。当选取合理温度滑移的混合工质作为机械过冷循环的制冷剂时,可明显提升CO_2制冷循环能效,降低排气压力。与基本CO_2制冷循环相比,在蒸发温度为-40℃、环境温度为35℃时,采用R32/R152a(40/60)循环总COP可提升46.53%,CO_2排气压力可降低2.758 MPa。总COP的提升程度受混合制冷剂的温度滑移影响显著,推荐机械过冷循环使用温度滑移合理的混合制冷剂。在温暖和炎热的气候地区及冷冻冷藏等低温应用领域,采用非共沸混合制冷剂机械过冷跨临界CO_2制冷循环整体性能的提升更加显著。     

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。     

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。     

机械过冷CO_2跨临界制冷循环性能理论分析

采用蒸气压缩制冷循环(辅助循环)对CO_2跨临界制冷循环气体冷却器出口的CO_2流体进行冷却,可减小节流不可逆损失,提高循环性能。本文对机械过冷CO_2跨临界制冷循环进行热力学循环分析,结果表明:当在最优排气压力和最优过冷度两个参数条件下,循环存在最大COP。环境温度越高、蒸发温度越低,采用机械过冷方法使循环性能提升越显著,相对传统CO_2制冷循环,通过辅助循环可显著提高循环COP,降低CO_2排气压力和温度。相对CO_2压缩机,辅助循环压缩机的功耗较少。分析了辅助循环中采用11种不同制冷剂的性能,可得除R41外,其它10种工质对循环整体COP的提升程度差异不明显。综上所述,机械过冷CO_2跨临界制冷循环更适用于环境温度较高、蒸发温度较低的场合。     

N_2O跨临界双级压缩带膨胀机制冷循环

将天然工质N_2O用于跨临界循环,建立了相应的理论模型,比较了CO_2和N_2O用于跨临界两级压缩膨胀制冷循环的性能.结果表明:N_2O用于跨临界两级蒸气压缩膨胀制冷循环中的综合性能要优于CO_2.在所选定的工况范围内,N_2O系统的C_(cop)值(性能系数)比CO_2最多提高9.6%,当气冷器出口温度越低、蒸发温度越高时,N_2O系统的C_(cop)值增加越明显;N_2O系统的最优高压压力远低于CO_2,在气体冷却器出口温度为40 ℃时,最优高压侧排气压力最多降低了16.2%;N_2O系统在排气温度、单位质量制冷量方面也较CO_2具有优势.最后提出通过降低气体冷却器出口温度来提高跨临界带膨胀机制冷循环性能和降低最优高压侧排气压力的观点.     

跨临界CO_2热泵系统最优排气压力的极值搜索控制

ESC控制策略可以自动搜索性能指标最佳时系统所对应未知的或缓慢变化的系统输入,实际为一种基于梯度信号调制解调的动态搜索方法。本文针对空气源跨临界CO_2热泵热水器系统,选取压缩机排气压力设定值作为ESC控制器的输入,采用COP作为系统性能的输出指标,即极值搜索控制的反馈信号。通过搭建空气源跨临界CO_2热泵热水器的动态模型仿真平台,针对恒定工况,变环境温度条件和实时温度条件分别进行了模拟仿真。仿真结果表明,ESC可以搜索恒定或者系统边界变化时最佳的系统输入值,ESC控制的最优排气压力稳态误差在1.0%以内,与基于模型的控制结果相比,最优排气压力的控制偏差也在5%以内,验证了ESC控制策略的可行性和准确性。     

家用CO_2热泵热水器系统试验研究

为了提高CO_2热泵热水器的性能系数,笔者所在单位设计了一台制热量为4.5 kW的CO_2空气源热泵热水器,试验研究了系统最优高压压力以及节流元件对系统的影响,试验表明,膨胀阀开度越小,系统效率越高,但过热度也会越高,综合考虑150步为最佳开度,名义工况下的最优高压压力为9.5 MPa,对应的制热系数为2.7。在热泵出水温度60~80℃范围内,平均提高1℃温度,系统能效比降低1.3%。寒冷工况下,仍能达到65℃的出水温度,说明了CO_2热泵在低温环境下的运行优势。     

跨临界CO2循环制冷系统的实验研究

本文通过搭建跨临界CO2循环制冷系统实验台,研究在冷却水出口温度为55℃的时候,系统的COP随高压侧压力的变化趋势。研究结果表明:在一定的蒸发温度下,随着高压侧压力的升高,系统的性能系数呈现出先升高后降低的趋势,且系统的COP存在一个最大值。因此,在某一蒸发温度下,存在一个最优高压侧压力使系统的性能系数达到最大值。     

吸气喷液对涡旋压缩机及系统性能的影响

为了研究吸气喷液冷却对涡旋压缩机及制冷系统性能的影响,本文搭建了吸气喷液回路实验台,并建立了含有泄漏和换热损失的数学模型。研究了不同喷液流量和不同压比下,压缩机排气温度和系统制冷量以及COP的变化情况。数值计算结果和实验结果吻合程度较高,在低喷液量下,误差在2.5%左右;在高喷液量下,误差在10%左右。结果显示:排气温度随喷液量增大而降低,COP随喷液量有微小提升后降低。喷液量较小时,可使排气温度降低14℃,而系统COP微小提升0.6%;喷液使得排气温度降低42℃时,可保证系统COP降低小于5%;高压比下,吸气喷液系统COP存在最佳值,且压比越高COP降低幅度越小,吸气喷液可以有效的用于降低压缩机排气温度。     

Thermodynamic performance assessment of carbon dioxide blends with low-global warming potential (GWP) working fluids for a heat pump water heater

Blends of CO₂ with ten low-global warming potential (GWP) working fluids are evaluated for use in a heat pump water heater. The effects that the discharge pressure, component ratio, hot-water outlet temperature and chilled water inlet temperature have on the coefficient of performance (COP) of heat pump are analyzed when the pinch point of the heat exchange is considered. It is found that temperature glide of zeotropic mixture has a good thermal match with the temperature change of water as two pinch points appear in the gas cooler/condenser or evaporator. The good thermal match in the heat exchangers promotes the system COP. Addition of low-GWP working fluids to pure CO₂ can reduce the high-side pressure. The results show that CO₂/R41 and CO₂/R32 are suitable candidates for heat pump water heaters because of their high COP and low high-side pressure in comparison with those of a pure CO₂ cycle.     

以毛细管为节流装置的CO_2小型热泵热水器实验研究

介绍以毛细管为节流装置跨临界CO2热泵热水试验系统。实验研究了不同气冷器进水温度下系统的COP及其变化、气冷器沿管长水温温升梯度变化。分析了不同环境温度下制冷剂充注量对系统高压侧压力的影响。可以得出:以毛细管做节流装置也可以得到较高的COP;气冷器水温在高温CO2进口段温升幅度最大;系统对环境温度的变化很敏感,环境温度较低时要想得到合适的高压侧压力,制冷剂的充注量要比环境温度高时多。     

Theoretical and experimental analysis of a CO2 heat pump for domestic hot water

This paper describes the development of a CO₂ air/water heat pump for the production of tap hot water in a residential building. The basic design consists of a single-stage piston compressor, a coaxial type gas cooler, an electronic expansion valve, a finned tube evaporator and a low pressure receiver. The heat pump is combined with a storage tank designed to maintain internal water stratification.The gas cooler pressure optimisation in the case of fixed water delivery temperature was theoretically analysed.A new control method for the upper cycle pressure was developed to maximise the COP of the heat pump, while the water mass flow was adjusted to maintain the set water temperature at the gas cooler exit.Before commissioning, the heat pump was factory tested to verify its energy performance and to validate the high pressure control logic.     

Pinch point analysis and design considerations of CO2 gas cooler for heat pump water heaters

Due to strong nonlinear variation of supercritical CO₂ specific heat capacity with temperature, pinch point would occur in water-cooled CO₂ gas cooler, which has great impacts on the heat transfer characteristics of gas cooler and overall system performance. Pinch point analysis was conducted for CO₂ gas cooler in the present study. The effects of refrigerant pressure, mass flow ratio (m_w/m_c), inlet water temperature and heat transfer area on pinch point location, approach temperature difference and heat transfer rate were analyzed in detail. Based on the analysis of pinch point location in CO₂ gas cooler, the critical flow ratios were proposed to effectively control the approach temperature difference. Furthermore, the actual conductance of gas cooler was calculated and compared with that estimated by LMTD method. The results showed that CO₂ gas cooler may be undersized by as much as a factor of 30–60% for different pressures if LMTD method is used. However, the UA value evaluated by LMTD method also may be overestimated under high refrigerant pressures when the approach temperature difference tends to be zero. Results of the present study are helpful to practical designs of CO₂ gas cooler and heat pump water heaters.     

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.     

带有分凝器的小型自复叠制冷循环性能分析

为了提高小型自复叠制冷装置中CO2和丙烷的分离效率,在循环中增设了分凝器。分析了CO2浓度、冷凝器出口蒸汽干度及分凝器高压侧出口温度对带有分凝器的自复叠制冷循环性能的影响。结果表明,当CO2质量浓度从0.22上升到0.31时,实验和理论计算的蒸发温度分别升高了6.8 K和5.7 K,相应的COP分别提高了68.3%和24.9%。当冷凝器出口蒸汽干度从0.5增加到0.9时,蒸发温度先降低后升高,在干度等于0.7时达到了211.37 K的最低值;COP陡降了98.8%。此外,分凝器高压侧出口温度升高8.21 K导致蒸发温度升高了1.41 K,COP增加了21.3%。     

A comparison of refrigerants in a two-stage ejector-expansion transcritical refrigeration cycle based on exergoeconomic and environmental analysis

A comprehensive energy, exergy, exergoeconomic and environmental comparison between carbon dioxide, ethane and nitrous oxide as the refrigerants of a two-stage ejector-expansion transcritical refrigeration cycle is carried out. All of the obtained results are attained by optimizing COP subject to gas cooler and intercooler pressures in different values of gas cooler and evaporator temperatures. It is observed that the compressors operating pressure and temperature levels in the cycle for ethane are lower than other refrigerants, which leads to higher system safety and lifetime. Furthermore, the highest COP and exergy efficiency in a wide range of gas cooler temperature belongs to the ethane. The nitrous oxide refrigerant has the lowest product unit cost, which is about 4.2% lower than that of the ethane refrigerant with the highest product unit cost. Therefore, ethane is the most preferable refrigerant from energy and exergy aspects and nitrous oxide is suitable based on exergoeconomic viewpoint.     

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.     

非标准状况下直燃型溴化锂机组动态特性的实验研究

根据现有检测手段对非标准工况下溴化锂机组进行了现场测试,分析了非标准工况下冷媒水进出口温度、冷却水进口温度、冷媒水流量、燃气流量的变化对制冷量和COP的影响.结果表明非标准工况下COP值与标准工况下的COP值相比,减少的最大幅度达到了29%,机组的启停次数、燃烧器的控制对系统的COP都有着较大的影响.