Performance analyses of a combined ORC-TFC cycle with various low-GWP refrigerants by using Excel

A combined ORC-TFC cycle with various low-GWP refrigerants by using Excel

Abstract

This paper presents a combined organic cycle for recovering the energy of low-temperature heat sources that utilises the Trilateral Flash Cycle (TFC) in the high-temperature circuit and the Organic Rankine Cycle (ORC) in the lowtemperature circuit with a cascade heat-exchanger in between. Although the combined cycle generally improves the performance of each organic cycle working separately, its actual performance depends on temperature of the heat source and that of the cascade condenser between the two circuits of the cycle as well as the pair of refrigerants used in the two circuits. In this study, the performance of the cycle is initially gauged against that of another combined ORC-TFC cycle with a 300oC heat source using R245fa in the lowtemperature circuit and water in the high-temperature circuit. It is then analysed with a 120oC heat-source by using R152a as a single fluid and R1234yf/R152a as a two-fluid pair and the results are compared to those of the simple ORC using R152a. Finally, the cycle’s performance is analysed with the 120oC heat-source using six working fluids that have zero ozone-layer depletion potential (ODP) and zero or low global-warming potential (GWP), which are R290, R600a, R717, R152a, R1234yf, and R245fa. Comparison of the power produced, thermal efficiency and exergetic efficiency of the cycle at various temperatures in the cascade condenser shows that the best performance with a single fluid is achieved by R245fa, while R1234yf/R152a and R1234yf/R245fa achieved the best performance as pairs. The model developed for analysing the cycle’s performance uses Microsoft Excel with special VBA functions for determining the fluid properties. The VBA functions are first verified by comparing their estimations for the simple ORC and TFC with published data.