Supercritical heat transfer and heat exchanger design for organic rankine application

Abstract

Papers presented to the 11th International Conference on Heat Transfer, Fluid Mechanics and Thermodynamics, South Africa, 20-23 July 2015.

Supercritical Organic Rankine Cycle is introduced as promising technology for low grade waste heat recovery. The advantage of this supercritical cycle is a better thermal match between the heat source and the working fluid temperature profiles in the supercritical heat exchanger. Heat exchangers have a vital part of this cycle, in manner of cycle efficiency and for the economic feasibility of one ORC installation. There are many challenges in the design process of these components suitable to operate at relatively high pressure and temperature. The reason for that is strong properties variation of the organic working fluid at supercritical state. Since, the value of the heat transfer coefficient depends on these thermophysical properties of the working fluid, it is important to study and understand the behavior of the properties of the fluid going from subcritical to supercritical state. The supercritical heat exchanger was first modeled in EES software, and then designed and built. For the designing procedure of the heat exchanger several different correlations available from literature were selected. These correlations have been previously validated at supercritical state, but for working fluids such as H2O, CO2 and refrigerants blends R404a and R410a. However, the diameters tested and the applications differ from this research. In order to ensure proper operation of the heat exchanger, this component was oversized by 20%. From the first set of measurements it can be concluded that the size of this component can be lowered by 10% and will result in reduced cost of an ORC installation.