Experimental study of two cascaded organic Rankine cycles with varying working fluids

Abstract

Organic Rankine cycles convert low-temperature heat from different sources, like solar, geothermal or biomass, into electricity and may thus help to meet the energy demand in an environmentally friendly way. While single ORC systems have been studied extensively, there are only very few experimental works on systems consisting of two cascaded organic Rankine cycles (two-ORC). In this work, an experimental study is carried out on the performance of a two-ORC system that consists of a high temperature (HT) cycle and a low temperature (LT) cycle. Each cycle is composed of the four significant components, i.e. expander, evaporator, condenser and pump, while the LT cycle is equipped with a throttle as expansion device. The HT cycle utilized heat from electrical heaters, while the LT cycle was driven by the waste heat from the HT cycle. The test rig utilizes Therminol 66 as a source that is heated up by electrical heaters with a power of 158 kW. Propane, butane, pentane and cyclopentane are chosen as working fluids for the present experiments. Parameter variations are carried out to study the thermodynamic characteristics of each cycle. The aim is to investigate the HT cycle performance considering turbine power output, thermal efficiency and exergy efficiency. The effect of the HT cycle on the LT cycle is examined by studying the heat transfer rate between the two cycles, characteristics of heat exchangers and pinch point temperature difference. A further goal is to explore the system performance under different conditions to maximize the exergetic utilization of the heat source. The results confirm that turbine power output and thermal efficiency increase with heat source temperature and turbine inlet pressure in the HT cycle. The maximum achieved thermal and exergy efficiencies are 5.5% and 20.2%, respectively, while the maximum turbine power output is 4.92 kW. Heat transfer measurements show that the maximum transferred heat flow from the HT cycle to the LT cycle is 23 kW when pentane is used as a working fluid. Temperature profiles and the pinch point temperature difference in the heat exchangers of both cycles are assessed under conditions where the highest turbine power is obtained. The experimental tests are promising and show that the two-ORC system is suitable to utilize heat sources in various temperature ranges