A Theoretical Criterion for Evaluating the Thermodynamic Effectiveness of Regenerators in Organic Rankine Cycle Systems

Organic Rankine Cycle (ORC) is a promising technology for converting low or medium temperature heat into power. In order to improve the thermodynamic efficiency of ORC systems, adding a regenerator to the system has been widely discussed. However, this needs additional pipes and devices and they will produce additional pressure drop. Considering both the contributions of the regenerator and the power losses caused by additional pressure drop, a theoretical criterion for evaluating the thermodynamic effectiveness of adding a regenerator was proposed by means of mathematic and thermodynamic equations. The core of the criterion is a new concept named Critical Pressure Drop produced by the Regenerator (CPDR). A negative value of CPDR indicates that adding a regenerator will not contribute to the thermodynamic efficiency of ORC systems in any case. An effective regenerator needs a positive value of CPDR, and requires that the value of additional pressure drop produced by introducing the regenerator should be smaller than CPDR. The influences of evaporation temperature, condensation temperature, expander efficiency, regenerator effectiveness and working fluids on CPDR were also examined. It was found that lower evaporation temperature or using working fluids with higher critical temperature tends to reduce the value of CPDR.