Fluid Selection of Organic Rankine Cycle Using Decision Making Approach

An attempt has been made to rank and classify potential fluids for power production through organic rankine cycle (ORC) using technique for order preference by similarity to ideal solution (TOPSIS) method. In order to calculate subjective weights for the attributes under study, the modified digital logic (MDL) method has been used. It has been observed under two different case studies that R601 (pentane) shows promising results. These fluids are further classified using dendrogram, a hierarchical clustering technique. Finally Pearson's correlation coefficient is calculated for the attributes to find out the nature and degree of correlation between different attributes under study. 1. Introduction It is well known that grade of thermal energy is a function of temperature and thus its efficiency in general for power production is variable. However, cheap and easy availability of fuels and large scale economy of the infrastructure compared to alternate sources of energy make it inevitable to choose thermal over any other form of power. Nevertheless, thermal energy possesses variable irreversibility and any system designed to run on it may have a low efficiency, with most of the input heat being wasted at sink temperatures, occasionally going as high as 40% to 60%. Thus, methods must be employed to convert this enormous amount of waste heat energy into useful form. One of the ways of harvesting this energy is to make use of organic rankine cycle (ORC) to produce electricity. ORC is a method of using thermal rankine cycle with organic (low boiling point) fluids to generate electricity from low grade heat energy [1–4]. ORC generally works in the range of temperatures 80 to 350°C but this is not a fixed operating range. ORC is based on the fact that most of the primary thermal sinks used in industry or power production have temperatures that are substantially higher than ambient. These sinks can be used as boilers for the ORC to provide heat to the working fluid which can later be discharged at much lower temperatures. Thereby, it serves the double purpose of producing power and reducing the thermal pollution [5–10]. However, the efficiency of any thermal cycle, including ORC, largely depends upon the properties of the working fluid used [11–13]. Thus, fluid selection forms a critical part of the design process of an ORC based power plant. Fluid properties can largely affect the nature and number of auxiliaries (accessories) that are to be installed for smooth running of the power plant. For example dry fluids require a smaller size of condenser than their wet