ORC与非恒温热源换热匹配对工质筛选及系统性能的影响

发布时间：2019-06-21 15:30

【摘要】：有机朗肯循环(ORC)是回收低品位工业余热促进节能减排的重要技术手段,过去十年间大量学者对循环本身进行研究,取得了丰富的成果。但是目前该技术尚未大范围推广,还有若干关键问题比如如何减小ORC机组与热源换热中的不可逆损失、如何在考虑换热匹配的影响下选择合适的工质、如何评价热源对机组主要部件蒸发器及膨胀机之间(火用)损分布及匹配的影响等亟待解决。对于低温非恒温热源驱动的动力系统,热源条件及其与循环的相互作用显然对系统性能有重要影响。本文的目的正是研究此类热源对工质筛选及循环性能的影响。非恒温热源驱动的ORC,蒸发器中的(火用)损情况对机组性能具有重要影响,但是目前换热过程的匹配程度与不可逆损失之间还没有明确的量化关系,因此本文尝试建立两者间的联系,从而为通过控制换热路径实现控制不可逆损失提供理论基础。有机工质的定压比热容在临界区发生突变,在质量流量一定时,T-Q图上的换热过程线斜率在临界区附近有明显变化,这为利用变比热控制换热路径提供了可能。本文第二章首先通过推导换热过程积分温差及无量纲(火用)温度的表达式建立了换热路径与换热不可逆损失之间的联系,证明了积分温差与不可逆损失之间的拟线性关系,将对不可逆损失的衡量转化为对积分温差的衡量。之后提出利用工质比热变化改善换热匹配的模型。对于固定两侧流体出入口温度的换热器,计算了流体变比热对换热积分温差的影响,提出可使换热过程不可逆损失最小的比热组合,对于减小ORC机组与热源换热中的不可逆损失有一定指导意义。目前对于亚临界循环的最佳工质尚无统一结论,对于热源的影响及关键物性的影响讨论不足。本文以增大输出功及减小积分温差为目标,为亚临界及跨临界循环建立工质筛选准则。第二章数学推导时没有采用实际的工质物性及压力等参数,而第三、四章根据热力学定律建立模型,以REFPROP 9.0数据库中的实际物性为基础进行热力学计算。首先研究有机工质在100~300°C热源条件下,亚临界饱和蒸汽基本循环中的表现,以输出功和换热器面积为筛选指标,同时考虑毒性、可燃性、环境影响等因素。计算发现,临界温度Tc低于热源入口温度Tgas,in的工质,不同的Tc对输出功影响明显,Tc高于Tgas,in的工质,不同的Tc对输出功的影响不明显。能够提供最大输出功的工质,是T-S图上包络线与热源线相切或接近相切的工质,其临界温度低于热源入口温度15~25K。其它物性对循环热效率的影响规律较分散,仅考虑输出功时临界温度可以作为工质筛选准则。第四章利用约束热源出入口温度的热力学模型,计算工质在非恒温热源驱动的跨临界ORC中的表现,分析蒸发器内窄点温差及工质物性对循环性能的影响。结果表明,Tc低于烟气出口温度的工质,及Tc高于0.88倍烟气入口温度的工质,临界温度是循环效率的主要影响因素;Tc在上述范围之间的工质,干湿性对循环效率影响显著,湿工质效率明显高于干工质。所有循环中,该临界温度范围内的湿工质热效率最高。Tc高于0.88倍烟气入口温度的工质,窄点温差可能出现在蒸发过程中或蒸发器出口,从热力性能角度看,窄点出现在蒸发过程中的循环明显优于窄点出现在蒸发器出口的循环。改变热源入口及出口温度不会影响上述结论。以上二至四章主要关注了热源对蒸发器的影响,没有考虑蒸发器与膨胀机之间的联系。热源影响蒸发器内的(火用)损,而蒸发器与膨胀机间又存在(火用)损匹配问题。近期有文献报道显示积分温差会影响膨胀机效率,并由此影响机组整体效率。为了反映热源对蒸发器及膨胀机之间(火用)损分布及匹配的影响,提出了将膨胀机效率假设为无量纲积分温差的函数的热力学模型,用数值模拟的方式复现以R123为工质采用涡旋膨胀机的ORC实验过程,在给定蒸发器面积及换热量、热源入口温度等条件下,计算热力学循环中的各状态点参数及输出功、热效率、(火用)效率等系统性能,并对采用积分温差决定膨胀机效率的模型及以膨胀机效率为定值的模型结果进行比较,分析了存在使系统性能最佳的无量纲积分温差的原因,反映出非恒温热源与ORC的换热匹配不仅影响蒸发器(火用)损,也会影响膨胀机的性能,将膨胀机效率视为无量纲积分温差的函数的模型比将其作为定值的模型计算结果更符合实际。
[Abstract]:The organic Rankine cycle (ORC) is an important technical means for recovering low-grade industrial waste heat to promote energy-saving and emission reduction. However, the technology has not been widely promoted at present, and there are several key problems such as how to reduce the irreversible loss of the ORC unit and the heat source heat exchange, and how to select the proper working medium under the influence of the heat exchange matching. How to evaluate the influence of heat source on the distribution and matching of the main component evaporator and expander of the unit. The dynamic system, the heat source condition and the interaction with the circulation of the low-temperature non-constant-temperature heat source obviously have an important influence on the performance of the system. The purpose of this paper is to study the effect of such a heat source on the selection and circulation of the working medium. In the case of the ORC driven by the non-constant temperature heat source, the damage of the (fire) in the evaporator has an important influence on the performance of the unit, but there is no definite quantitative relation between the matching degree of the heat exchange process and the irreversible loss, so this paper attempts to establish the relationship between the two. So as to provide a theoretical basis for controlling the irreversible loss by controlling the heat exchange path. The specific heat capacity of the organic working medium is changed in the critical region, and when the mass flow is constant, the slope of the heat transfer peak line on the T-Q graph is obviously changed near the critical region, which provides a possibility to control the heat exchange path by using the variable heat of the variable heat. In the second chapter, the relationship between the heat transfer path and the irreversible loss of heat transfer is established by deriving the integral temperature difference of the heat transfer process and the dimensionless (exergy) temperature, and the quasi-linear relationship between the integral temperature difference and the irreversible loss is proved. The measurement of the irreversible loss is converted into a measure of the integral temperature difference. And then a model for improving the heat exchange matching by utilizing the specific heat change of the working medium is proposed. The influence of the specific heat of the fluid on the temperature difference of the heat exchange integral is calculated for the heat exchanger which is used for fixing the temperature of the fluid inlet and outlet on both sides, and the specific heat combination which can minimize the irreversible loss of the heat exchange process is proposed, which is of great significance for reducing the irreversible loss in the heat exchange between the ORC unit and the heat source. At present, there is no unified conclusion on the optimal working medium of the subcritical cycle, and the influence of the heat source and the influence of the key physical properties is not enough. In order to increase the output power and reduce the integral temperature difference, the working medium selection criteria are established for subcritical and transcritical cycles. In the second chapter, the parameters such as the physical properties and the pressure of the working medium are not used in the mathematical derivation, and the third and the fourth chapter establish the model according to the law of thermodynamics, and the thermodynamic calculation is carried out based on the actual physical properties in the REFPROP 9.0 database. First, the performance of the organic working medium under the condition of 100-300 掳 C heat source and the basic cycle of the sub-critical saturated steam is studied, and the output work and the heat exchanger area are selected as the screening index, and the factors such as toxicity, flammability and environmental impact are considered. It is found that the critical temperature Tc is lower than that of the heat source inlet temperature Tgas, in, the influence of different Tc on the output work is obvious, Tc is higher than that of Tgas and in, and the effect of different Tc on the output work is not obvious. The working medium capable of providing the maximum output work is a working medium which is tangent to or close to the heat source line on the T-S graph, and the critical temperature of the working medium is lower than that of the heat source inlet temperature of 15 to 25K. The influence of other physical properties on the circulating thermal efficiency is more dispersed, and only the critical temperature of the output power can be used as the working medium screening criterion. In the fourth chapter, by using the thermodynamic model of the temperature of the inlet and outlet of the constrained heat source, the performance of the working medium in the transcritical ORC driven by the non-constant temperature heat source is calculated, and the influence of the temperature difference of the narrow point and the physical properties of the working medium on the circulation performance of the evaporator is analyzed. The results show that Tc is lower than the working medium of the flue gas outlet temperature and the working medium with Tc higher than 0.88 times the inlet temperature of the flue gas, the critical temperature is the main influence factor of the cycle efficiency; the working medium between the above ranges, the dry-wet property has significant influence on the circulation efficiency, and the efficiency of the wet working medium is obviously higher than that of the dry working medium. In all cycles, the thermal efficiency of the wet working medium in the critical temperature range is the highest. A working medium with Tc higher than 0.88 times the inlet temperature of the flue gas, the temperature difference of the narrow point may appear in the evaporation process or the outlet of the evaporator, and the circulation of the narrow point in the evaporation process is obviously better than the circulation of the narrow point in the outlet of the evaporator from the viewpoint of thermal performance. Changing the heat source inlet and outlet temperature does not affect the above conclusion. The above two and four chapters mainly focus on the influence of the heat source on the evaporator, and the relationship between the evaporator and the expander is not taken into account. The heat source affects the (fire) loss in the evaporator, and there is a (fire) loss matching problem between the evaporator and the expander. Recent literature reports show that the integral temperature difference will affect the efficiency of the expander and thus affect the overall efficiency of the unit. In order to reflect the influence of the heat source on the distribution and matching of the loss of heat between the evaporator and the expander, the thermodynamic model of the function of increasing the efficiency of the expander to the dimensionless integral temperature difference is put forward, and the ORC experimental process of using the R123 as the working medium by the numerical simulation is repeated. in that condition of a given evaporator area and heat exchange rate, a heat source inlet temperature and the like, the parameters of each state point in the thermodynamic cycle and the system performance such as output work, thermal efficiency, exergy efficiency and the like are calculated, The model for determining the efficiency of the expander and the model result of the expander efficiency are compared, and the reason of the non-dimensional integral temperature difference which is the best performance of the system is analyzed, and the heat exchange matching between the non-constant temperature heat source and the ORC not only influences the damage of the evaporator (fire), The performance of the expander is also affected, and the model ratio of the expansion machine efficiency as a function of the dimensionless integral temperature difference is more practical than the model calculation result of the fixed value.
【学位授予单位】：华北电力大学(北京)
【学位级别】：博士
【学位授予年份】：2017
【分类号】：TK115

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