Heat cascade and heuristics to optimize ORC and identify the best internal configuration

Abstract

Most ORC design optimization methods require specifying beforehand its internal configuration, and in particular its heat exchange network (HEN). This strongly limits the search space and thus, the resulting ORC performance. On the contrary, the heat cascade approach, a method to ensure physically sound heat transfers in thermal systems, can be adapted to optimize ORCs without having to specify the HEN configuration and instead, reveal the best configuration afterward. This work presents the gap between the traditional and heat cascade methods. Most importantly, it introduces heuristics to determine the best HEN configuration of an ORC designed with the heat cascade. The method is demonstrated for subcritical and transcritical reheat ORCs, for which 6 and 10 different HEN types were identified, respectively. Two case studies show how to use the method and calculate the stream split ratios present in the HEN types. Optimization results with the proposed method over a wide range of external conditions indicate an increase of the specific work output of up to 16% for the subcritical reheat cycle and 20% for the transcritical reheat cycle, when compared to the traditional optimization method.