Adjusting the pinch point of heat transfer to improve the thermal performance of the organic Rankine cycle

Abstract

For the thermal performance of the organic Rankine cycle (ORC), there exists a monotonic increase or a maximum value with an optimum point because of the relationship between the heat source temperature and the critical temperature of working fluid. The performance with a monotonic increase is better than that with an optimum value. However, the ORC with a low-grade heat source generally falls into the condition with an optimum value when working fluids have a high critical temperature, for example, R245fa. Early research revealed that this phenomenon is affected by the location of the pinch point of the heat transfer (PPHT) in the heat addition process. Based on this consideration, an approach of adjusting the PPHT and a theoretical cycle: the ORC with adjustable pinch point of heat transfer (APPORC) are proposed, and two new configurations of the ORC with the adjustment of the PPHT are proposed: the organic Rankine cycle with an ejector (EORC) and organic Rankine cycle with a flasher (FORC). The theoretical performance of the APPORC is deduced, and its two innovative reconfiguration cycles are simulated and compared to the basic ORC (BORC). From the theoretical derivation as well as simulation results, the APPORC and its innovative cycles:The EORC and FORC can achieve higher net power output and lower thermal efficiency. The results also show that when the heat source temperature is in a range of 100 C to 160 C, the maximum net power outputs of the EORC and FORC with R245fa as working fluid increase 34.99%-22.57% and 16.96%-11.08%, respectively, compared to the BORC. The exergy destructions of the heat addition process of the EORC and FORC are lower compared to the BORC due to the better temperature matching. For the EORC with zeotropic mixtures R134a/R245fa (0.7/0.3) as working fluid, the increases in maximum net power output of 7.12% and 9.45% are obtained compared to pure fluids R245fa and R134a at a heat source temperature of 120 C. Furthermore, an additional benefit for the EORC with zeotropic mixtures was obtained: The condensation temperature glide can be adjusted to approach the temperature rise of the cooling water.ejector, flasher, optimization, organic Rankine cycle, pinch point of heat transfer (PPHT), temperature matching Abbreviations: APPORC, Organic Rankine cycle with adjustable pinch point; EORC, Organic Rankine cycle with an ejector; FORC, Organic Rankine cycle with a flasher; LGHS, Low-grade heat source; OFC, Organic flash cycle; ORC(BORC), Basic organic Rankine cycle; PPHT, Pinch point of heat transfer.