Constructal thermodynamic optimization for dual-pressure organic Rankine cycle in waste heat utilization system

Abstract

Dual-pressure organic Rankine cycle (DPORC) is a technology with great potential to solve energy problems. This paper performs constructal thermodynamic optimization (CTO) for the DPORC based on a combination of constructal theory and finite-time thermodynamics. The tube outside diameters of the heat exchangers, such as high- and low-temperature evaporators (HTE and LTE) and condenser, and the volume ratio of the high-pressure turbine are the design variables, the system net power output (POW) is the optimization function, and the total volume of the turbines and total heat transfer area (HTA) of the heat exchangers are the constraints. The influences of some parameters, such as the HTA ratios of the HTE and LTE, and the total mass flow rate (MFR) of the working medium on the CTO results are studied. The performance comparison between the DPORC and single-pressure ORC (SPORC) is carried out under the same conditions. The results demonstrate that the system net POWs after the four stepwise CTOs are improved by 1.70%, 3.01%, 5.80% and 8.84% over against the initial system net POW, respectively. Reducing the HTA ratios of the HTE and LTE and increasing the total MFR of the working medium can increase the system net POW. Compared with the performance of the SPORC, the system net POW and system net thermal efficiency of the DPORC are increased by 4.91% and 5.51%, respectively. The optimization results can be used to guide the optimal designs of the low-temperature waste heat utilization systems.