Numerical Analysis of an Organic Rankine Cycle with Adjustable Working Fluid Composition, a Volumetric Expander and a Recuperator
Numerical Analysis of an Organic Rankine Cycle with Adjustable Working Fluid Composition, a Volumetric Expander and a Recuperator
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DOI：
10.3390/en10040440
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发表时间：
2017-03
期刊：
影响因子：
3.2
通讯作者：
P. Collings;Zhibin Yu
中科院分区：
文献类型：
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作者：
P. Collings;Zhibin Yu
使用环境空气作为冷却剂的传统有机朗肯循环 (ORC) 无法充分利用寒冷月份中较大的温差。然而，改变工作流体成分，使其沸腾温度与环境温度相匹配，已被证明有可能增加全年发电量。先前的研究假设循环压力比能够变化，而涡轮机的等熵效率不会出现重大损失。本文研究了通常使用具有固定膨胀比的正排量膨胀机的小型 ORC 系统是否也可以从这一新概念中受益。首先建立了数值模型，并在此基础上进行了综合分析。结果表明，它可以应用于带有正排量膨胀机的系统，并提高其全年发电量。然而，这种改进小于使用具有可变膨胀比的涡轮膨胀机的系统的改进。此外，这种改进依赖于通过同流换热器的热回收。这是因为具有固定膨胀比的膨胀机在其入口和出口之间具有相对恒定的压力比。这种膨胀机无法利用通过调整冷凝温度以适应冬季较冷的环境条件来增加蒸发器和冷凝器之间的压力比。然而，在安装同流换热器的情况下，膨胀机较高的排气温度可以增加热回收，从而减少蒸发器的热量输入，从而提高热效率和比功率。换热器中传递的热能越高，效率提高就越高。
Conventional Organic Rankine Cycles (ORCs) using ambient air as their coolant cannot fully utilize the greater temperature differential available to them during the colder months. However, changing the working fluid composition so its boiling temperature matches the ambient temperature as it changes has been shown to have potential to increase year-round electricity generation. Previous research has assumed that the cycle pressure ratio is able to vary without a major loss in the isentropic efficiency of the turbine. This paper investigates if small scale ORC systems that normally use positive-displacement expanders with fixed expansion ratios could also benefit from this new concept. A numerical model was firstly established, based on which a comprehensive analysis was then conducted. The results showed that it can be applied to systems with positive-displacement expanders and improve their year-round electricity generation. However, such an improvement is less than that of the systems using turbine expanders with variable expansion ratios. Furthermore, such an improvement relies on heat recovery via the recuperator. This is because expanders with a fixed expansion ratio have a relatively constant pressure ratio between their inlet and outlet. The increase of pressure ratio between the evaporator and condenser by tuning the condensing temperature to match colder ambient condition in winter cannot be utilised by such expanders. However, with the recuperator in place, the higher discharging temperature of the expander could increase the heat recovery and consequently reduce the heat input at the evaporator, increasing the thermal efficiency and the specific power. The higher the amount of heat energy transferred in the recuperator, the higher the efficiency improvement.