Performance Analysis Of The Effect Of The Type Of Heat Exchanger On Organic Rankine Cycle And The Dual-loop Organic Rankine Cycle

Energy crisis and environmental conservation has been becoming the two major challenges faced by the world nowadays and the development and utilization of the low-grade energy has become the most direct and effective way to solve this problem. Among the numerous ways to develop and utilize low-grade energy, Organic Rankine Cycle(ORC) has gained growing attention owing to its many advantages such as simple equipment, range of application. And the research on ORC has become the focus of research on low-grade energy development and utilization at present.In the process of practical engineering application of ORC, the system costs considerably highly on results of lower thermal efficiency leaded by its low temperature of heat source. As a consequence, it is rather import and urgent to improve the performance parameter of the whole system, which becomes the focus of the research at present. This paper selected subcritical Organic Rankine cycle(Sub-ORC) and Dual-loop Organic Rankine Cycle(DORC) system as the study objectives respectively. Through a reasonable simplification and hypothesis, system model is established, and the principles of thermodynamics and heat transfer are used for optimization analysis of performance of the system. This main research contents in this article are as follows:(1) For basic ORC, the performance parameters of the system are selected and studied, and the effect of the critical temperature of the working fluid on above performance parameters are also studied; the operating parameters and system performance parameters are comparison analyzed under optimum operation, and the the selectivity of the working fluid for the heat source temperature is studied; the effect of evaporation pinch temperature difference on the system performance parameters is in research. The results show that: the system thermal efficiency increases with the rise of evaporation temperature; for the high critical temperature of working fluid, the net power output increases with the evaporation temperature, and the net power output get maximum value at the point near critical temperature, and the exergy efficiency is firstly increases and then decreases; for the low critical temperature of working fluid, exergy efficiency has been increasing with the rise of the evaporation temperature, and get maximum exergy efficiency when the temperature is getting close to critical temperature; heat transfer conductance UA decreases with the increase of evaporation temperature, especially obvious for high critical temperature of working fluid, and UA value gradually increases with the increase of the critical temperature of working fluid. The compariion analysis of the ORC with R245 fa, R600 a, R600, n-pentane, n-hexane as working fluid show that: under the heat source temperature 150 oC, taking maximum net power output, condensation pressure, evaporation pressure into consideration, R245 fa is the most appropriate working fluid, and the optimal evaporation temperature is 93℃; for different selected working fluid, the maximum net power output increases with the increase of evaporation temperature; the appropriate working fluid which generally depends on critical temperature is different for different heat source temperature. When the heat source temperature is 150℃, with the increase of evaporator pinch point temperature difference, the net power output,exergy efficiency and the evaporator heat transfer conductance UA will reduce, and heat source outlet temperature will increase.(2) Under the same boundary conditions, the performance parameters of the sub-ORC are studied, and the effect of the type of heat exchanger on the area per unit net power output(APR) which is selected as optimization objective are studied. The results show that: the optimal APR is minimum when system applied the plate heat exchanger, followed by shell-and-tube heat exchanger and plate-fin heat exchanger; when the plate heat exchanger is used in sub-ORC, the optimal APR is between 4.7 and 5.1 m2·k W-1, the other two have great variation; the evaporation pressure of the optimal APR is close for each working fluid R123, R601, R601 a and R600, while changes largly for R245fa; the APR of the plate-fin heat exchanger is relatively large, but it improves the heat exchanger performance by extending the secondary heat transfer surface, especially for the occasion which the convection coefficient of both sides have large difference. In the selected system boundary conditions, the group of the plate heat exchanger and R123 are recommended.(3) The DORC which utilize the middle-temperature low-grade energy is studied hereby. By simplifying assumptions, the system model of thermodynamics and heat transfer is established, and the system performance parameters are analyzed. The results show that: in the process of evaporation pressure rising, for the system with n-pentane/R600, isopentane/R114 as working fluid the thermal efficiency and exergy efficiency firstly increases and then decreases, and the net power output has been reducing, and APR firstly decrease and then increase; while for R141b/R245 fa, the system thermal efficiency and exergy efficiency keep increasing, and the net power output and APR(fluctuate around 4.5 m2·k W-1) are almost unchanged; the working fluid mass flow rate in both cycle keep rising in a small range with the HT evaporation pressure.