Thermodynamic Evaluation of a Combined ORC-TFC Cycle for Low-Temperature Power Generation by using Microsoft Excel

This chapter presents a new power generation cycle for utilising low-grade and renewable energy sources that combines the Organic Rankine Cycle (ORC) with the Trilateral Flash Cycle (TFC). By connecting the two cycles via a cascade condenser and applying the TFC in the high-temperature circuit, the combined cycle enjoys the merits of each cycle while minimising its drawbacks. The thermodynamic performance of the cycle is evaluated by using an Excel-based model that determines the fluid properties with Thermax functions for the refrigerants’ group. The accuracy of these functions is first checked by comparing their estimations with relevant published data for the basic ORC and TFC cycles with a number of relevant fluids. The performance of the combined cycle with hot water at 120oC is compared to those of the simple TFC and ORC by using R152a as the working fluid in all three cycles. It is also compared to those of other two-stage and combined ORC-TFC cycles for a source temperature at 300oC using R234fa in the ORC circuit and water in the TFC circuit. Finally, the cycle is analysed with five fluids of low GWP, which are R1234yf in the ORC and R152a, R600, R600a, or R717 in the TFC. The performance of the best pair found, which is R152a/R1234yf, is compared with that of R152a as the single fluid at the cascade temperature that simultaneously maximises the cycle’s power, thermal efficiency, and exergetic efficiency.