Synthesis and simultaneous optimization of multi-heat source multi-pressure evaporation organic Rankine cycle with mixed working fluid

The organic Rankine cycle (ORC)'s performance relies on heat source properties, cycle structures, and working fluids. Influences of these factors, however, are often interdependent, making it difficult to determine the optimal design of ORC systems. To address this issue, we propose a mixed-integer nonlinear programming model for the multi-pressure evaporation ORCs driven by multiple heat sources that not only optimizes operating conditions, but also identifies the best working fluid composition and cycle structure. Results demonstrate that the proposed model is accurate and effective. The dual-pressure evaporation ORC driven by multi-heat source shows a 10.87% rise in the net power output (NPO) compared to the one with single pressure evaporation. In general, 0.9/0.1 npentane/n-heptane is the optimal mixed working fluid generating 2.71-12.11% more power than the pure working fluids for the numerical studies. Dual-pressure evaporation ORC is found to be a balanced design for the multi-heat source driven ORC in terms of its NPO and cycle complexity.