Design and Operation Strategy of Micro Organic Rankine Cycle with Zeotropic Binary Mixtures for Low Grade Geothermal Energy

Abstract

In this study, we proposed a micro ORC with zeotropic mixture of less than 20 kW using low-grade geothermal heat energy and investigated the power generation performance of 3 kinds of zeotropic working fluids with different vapor-liquid equilibrium. Simulations were carried out for 3 cases with a constant heat source temperature of 373K and flow rates of 3000, 7000 and 15000 kg/h. In the flow rate of the heat source was set to 7,000 kg/h, only 4 cases generated of power. For n-butane / ethanol = 0.7 / 0.3 was used as the working fluid, the power generation was 16.1 kW with 5.83% of the thermal efficiency which was higher than Carnot efficiency (4.28%). In the flow rate of 3,000 kg/h of heat source, by using n-butane / 1-propanol = 0.4 / 0.6 as the zeotropic working fluid, 6.52 kW could be generated. From the analysis using T-H and T-s diagrams, it is found that the zeotropic working fluid can effectively function as a temperature glide with composition change for low-grade geothermal heat sources with large temperature drop, and thus can be a power generation.