Thermodynamic Tri-Objective Optimisation of a Combined ORC-TFC Cycle for Low-Temperature Power Generation

A Combined ORC-TFC Cycle for Low-Temperature Power Generation

Abstract

This paper presents a new cycle for utilising the energy from low-temperature heat sources that combines the Organic Rankine Cycle (ORC) with the Trilateral Flash Cycle (TFC) via a cascade condenser. The model developed for analysing the cycle uses Excel as a modelling platform with special VBA functions to determine the fluid properties. To verify the functions, they were used to analyse the simple OCR by using R134a, R1234yf, R152a, propane (R290), n-butane (R600), iso-butane (R600a), and ammonia (R717) and the model’sresults are compared with published results obtained by using the EES software. When the performance of the combined cycle was evaluated for a heat-source of 120oC at various values of the cascade-condenser temperature by using R152a as the working fluid, the results showed that a high temperature maximises the cycle’s power and exergetic efficiency but minimises its thermal efficiency. Therefore, a tri-objective optimisation analysis of the cycle was conducted that simultaneously maximises all three parameters. Compared to the temperature that gives the maximum power, the optimised temperature reduced the power by 11.9%, but increased the thermal efficiency by 15.3% and the exergetic efficiency by 5.7%. Compared to the temperature that gives the maxmimum thermal efficiency, the optimised temperature reduced the thermal efficiency by only 5.9%, but increased the power by 68.8% and the exergetic efficiency by 4.3%.