基于当量热阻的ORC工质选择方法研究

A Method of Selecting Working Fluid and Operating Conditions at Once in an ORC(Organic Rankine Cycle)

根据有机朗肯循环(ORC)蒸发器中夹点出现的位置,将其换热过程分为夹点出现在预热起始点(PPP)、夹点出现在工质汽化起始点(VPP)、以及夹点同时出现在预热起始点和汽化起始点(PVPP)三种情况,定义吸热工质和热源流体的热容流率之比为热容比ε,经分析发现PPP,PVPP,VPP三种换热过程分别对应ε>1,ε=1,ε<1的情况,即可用ε判断夹点出现的位置.讨论夹点温差一定时,热源温度和蒸发温度对夹点位置的影响,发现随着热源温度和蒸发温度的升高,蒸发器的换热过程逐渐由VPP(ε<1)变化到PVPP(ε=1)再变化为PPP(ε>1).使用基于(火积)耗散率定义的当量热阻来度量换热过程的不可逆性,对比分析三种换热过程对热回收量和当量热阻的影响情况,发现换热过程为PVPP(ε=1)时蒸发器热回收量最大、当量热阻最小.最后对于热源条件确定的蒸发器,以ε=1为目标函数,同时对循环工质和运行参数进行优化。

Organic Rankine Cycle （ORC） can use low and medium temperature heat source to generate electricity, thus it has received widespread attentions and researches. According to the position of the pinch point in evaporator, the process of heat transfer can be divided into VPP （Vaporization Pinch Point）, PVPP（Preheating ＆ Vaporization Pinch Point） and PPP （Preheating Pinch Point）. ε is defined as the ratio of heat capacity flow rate between working fluid and heat source fluid. It is demonstrated that e can estimate the position of pinch point. According to the variation of ε with heat source temperature and evaporation temperature, it can be summarized that with heat source temperature and evaporation temperature increase, VPP progress change to PVPP then to PPP. Entransy dissipation-based equivalent thermal resistance is used to describe the irreversible destruction of the evaporator in ORC system, then compare the effects of position of pinch point on heat recovery and thermal resistance, ε= 1 is considered as the best operation condition. Using the conclusion above, a numerical algorithm is set up to select working fluid and operating conditions at once when the heat source temperature is decided.