Potential of Zeotropic Mixtures as Working Fluids in Partial Evaporating Organic Rankine Cycles (PEORC) for Low Temperature Heat Recovery

orc2019 Tracking Number 150

Recent studies on new organic Rankine cycle (ORC) architectures shows partial evaporating ORC (PEORC) architecture to be of improved exergetic efficiency than the simple ORC (SORC)[1]. Previous studies on the use of zeotropic working fluids have shown to improve exergetic efficiency of SORCs owing to their temperature glide characteristics[2]. So far only a few studies have focussed on the use of zeotropes in partial evaporating ORCs operating with finite capacity heat sources, where maximum heat extraction is desirable [3]. Using a zeotrope of high-temperature glide and latent heat in PEORC could lead to increased exergetic efficiency due to the combined effect of improved temperature matching and increased heat extraction. The performance of partial evaporating mixtures should be analyzed from a thermodynamic and economic standpoint. In this study, a zeotropic mixture of R245fa/cyclopentane is analyzed with three cycle architectures: SORC, PEORC and Trilateral cycle (TLC). The effect of vapour fraction and mixture mass fraction on cycle performance is analyzed by optimizing the evaporation temperature for a fixed condenser dew point temperature. PEORC with mixtures achieves maximum heat source utilization at lower vapour fractions. Also, the optimum evaporation temperatures corresponding to maximum power outputs are higher at lower vapour fractions. Thermal efficiency and net power output of mixtures in PEORC is higher than that of pure fluids. Compared to pure fluids, PEORC with mixtures exhibit intermediate volumetric flow ratios, increased heat exchanger UA requirements and expander sizes. For heat source temperatures ranging from 150°C-90°C, the cycles are optimized for maximum power output using Genetic Algorithm. PEORC with R245fa/cyclopentane mixture outperforms SORC with R245fa in terms of net power output by 37-62%, and TLC with R245fa by 2-19%. However, this performance gain results in 2.3-2.8 times higher heat exchanger UA requirements and 1.9-5 times increase in expander volume coefficients than SORC with R245fa. A compound function considering net work output, expander volume coefficient and heat exchanger UA requirements is used to compare various cycles thermo-economically. PEORC with pure R245fa shows the highest thermo-economic performance.