Off-design optimisation of organic Rankine cycle (ORC) engines with piston expanders for medium-scale combined heat and power applications
Off-design optimisation of organic Rankine cycle (ORC) engines with piston expanders for medium-scale combined heat and power applications
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DOI：
10.1016/j.apenergy.2018.12.086
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发表时间：
2019-03
期刊：
影响因子：
11.2
通讯作者：
M. Chatzopoulou;M. Simpson;P. Sapin;C. Markides
中科院分区：
文献类型：
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作者：
M. Chatzopoulou;M. Simpson;P. Sapin;C. Markides
有机朗肯循环(ORC)发动机通常在可变热源条件下运行，因此，最大限度地提高额定和非设计运行时的性能对于该技术的更广泛采用至关重要。在这项工作中，开发了一个设计外优化工具，并用于预测变化的热源条件对ORC运行的影响。与以前假设ORC发动机部件的性能是固定的不同，这里我们明确地考虑这些部件的时变运行特性。当从满载运行的内燃机(ICE)的废气中回收热量时，底部ORC系统首先进行优化，以获得最大功率输出。ORC蒸发器和冷凝器的尺寸采用双管换热器(HEX)模型，膨胀机的尺寸采用活塞膨胀机模型。然后，内燃机在部分负荷下运行，从而改变废气的温度和质量流速。该工具预测换热器中新的非设计换热系数，以及新的最佳膨胀机工作点。结果表明，当不考虑这些部件的非设计工作特性时，ORC发动机的功率输出被低估了17%。特别是，由于降低了系统中的压比和流量，活塞膨胀机的等熵效率在变工况时提高了10-16%，而蒸发器的效率提高了15%，这是因为HEX两端的温差更大，两相蒸发区的换热比例更高。随着内燃机的运行距离其标称点更远，非设计的ORC发动机输出功率的下降幅度小于内燃机。在内燃机60%的部分负荷下(通过电力)，使用R1233zd等流体的优化ORC发动机在其额定容量的77%下运行。生成ORC非设计性能图，用于表征和预测系统性能，ORC系统设计者、制造商和工厂操作员可以使用该图和优化工具来确定实际运行条件下的最佳性能。
Organic Rankine cycle (ORC) engines often operate under variable heat-source conditions, so maximising performance at both nominal and off-design operation is crucial for the wider adoption of this technology. In this work, an off-design optimisation tool is developed and used to predict the impact of varying heat-source conditions on ORC operation. Unlike previous efforts where the performance of ORC engine components is assumed fixed, here we consider explicitly the time-varying operational characteristics of these components. A bottoming ORC system is first optimised for maximum power output when recovering heat from the exhaust gases of an internal-combustion engine (ICE) running at full load. A double-pipe heat exchanger (HEX) model is used for sizing the ORC evaporator and condenser, and a piston-expander model for sizing the expander. The ICE is then run at part-load, thus varying the temperature and mass flow rate of the exhaust gases. The tool predicts the new off-design heat transfer coefficients in the heat exchangers, and the new optimum expander operating points. Results reveal that the ORC engine power output is underestimated by up to 17% when the off-design operational characteristics of these components are not considered. In particular, the piston-expander isentropic efficiency increases at off-design operation by 10–16%, due to the reduced pressure ratio and flow rate in the system, while the evaporator effectiveness improves by up to 15%, due to the higher temperature difference across the HEX and a higher proportion of heat transfer taking place in the two-phase evaporating zone. As the ICE operates further away from its nominal point, the off-design ORC engine power output reduces by a lesser extent than that of the ICE. At an ICE part-load operation of 60% (by electrical power), the optimised ORC engine with fluids such as R1233zd operates at 77% of its nominal capacity. ORC off-design performance maps are generated, for characterising and predicting system performance, which can be used, along with the optimisation tool, by ORC system designers, manufacturers and plant operators to identify optimum performance under real operating conditions.