余热资源的能级及其与ORC工质的匹配

Energy level of residual heat resources and its matching property with ORC working medium

鉴于温度作为余热资源品位评价具有一定的局限性,提出了基于热力学函数e的余热能级评价指标——可用能势e和可用势能级ω,对具有显热、潜热和压力属性的余热资源(如烟气、水蒸气)进行定量评价.同时,e也是系统工质做功能力的体现,以此作为定量指标,有利于实现余热资源与系统工质的匹配,为工业余热有效利用提供指导.基于e分析了ORC系统中热源与工质的匹配特性及有机工质在蒸发器内做功能力提升途径.结果表明:可用能势准则可定量反映余热资源所含的显热、潜热及压力势能水平,能够对余热资源的能源品位和利用潜力进行综合评价;与夹点温差法相比,可用势分析法更能反映资源与工质的匹配特性,可作为选择ORC工质的一种参考.

Reducing energy consumption and improving energy utilization efficiency are the inevitable measures to solve the energy problems in China. It is of great significance to achieve the strategic objectives of energy reduction that industrial sector, as a major energy- consuming areas, improves its energy efficiency and waste heat recycling level, follows the principle of maching temperature requirement and cascaded utilization, evaluates reasonably and makes full use of the waste heat, especially the one in low temperature. Considering certain limitations of using temperature as the grade evaluation of residual heat resources, available potential e and available potential energy level ω were proposed to quantitatively appraise the residual heat resources（such as flue gas, water vapor） which with sensible heat, latent heat and pressure properties. This paper details physical significance of available potential and available potential energy level and derivation process of their formulas, and calculates the available potential and the available potential energy level of water vapor and industrial flue gas in different temperature and pressure conditions to quantitative analysis the energy level grade of these two kinds of waste heat resources. Meaning while, due to e can represent the working capacity of working medium in a system, taking it as a quantitative evaluation index, is a tentative way to match the residual heat resources and the working medium of the system, and can provide guidance for efficient utilization of residual heat resources. On the basis of e, this paper analyses the matching properties between heat source and working medium of the ORC（Organic Rankine Cycle） system and puts forward some approaches for the increase of working performance in the evaporator of the ORC system. The results show that the enthalpy and available potential of water vapor increase with the rising of temperature when pressure is constant and the available potential on the point of the isotherm increase with the increasing of pressure, and the change regularity of available potential level along with the change of temperature and pressure has the same trend with available potential; Besides, the composition of industrial flue gas has a great influence on its enthalpy and available potential, especially the content of water vapor. The available potential level of specified flue gas decreases with the increasing of temperature when pressure is constant, and the available potential level of different kinds of industrial flue gas those who have the same temperature decreases with the increasing of the content of water vapor, in other words, the higher content of water vapor of industrial flue gas lead to the lower proportion of available potential of enthalpy. In general, the available potential criteria could quantitative reflect the sensible heat, latent heat and pressure potential of residual heat resources and comprehensive evaluate the energy level and utilization potential of residual heat resources; comparing with the pinch analysis method, the available potential analysis method was more intuitive to assess the matching properties between resource and working medium of the ORC system, which could be an alternative reference for choosing working medium of an ORC system.