CO2跨临界—有机朗肯复合循环的简单计算与特征分析

由于我国地热资源以低品位热能为主,增强型地热系统在发电方面的应用会受限于过低的热效率。理论上,如直接将超临界CO₂与有机工质进行混合,确实有可能提升热动力系统的机械效率。根据该流程设计,对部分参数范围内的CO₂跨临界—异丁烷复合循环的效率进行了估算。计算使用了流量分别为1 kg/s的CO₂和0.25 kg/s的异丁烷流体。计算结果显示,特定状态下的超临界CO₂在膨胀为气态的过程中焦—汤效应显著,流体混合使得异丁烷先蒸发汽化,然后在膨胀中发生冷凝,使得膨胀机内部出现了两相流。冷凝后的异丁烷工质可再次注入压缩机与气体CO₂接触,使得CO₂升温减缓,同时自身获得预热。可借助异丁烷工质降低CO₂的温变范围,改变CO₂膨胀和压缩过程中的多变指数,使其更接近等温过程而非绝热过程,进而提高膨胀和压缩过程的效率。因此,该循环具备显著提高低品位CO₂热动力循环效率的潜力,在未来可用于基于CO₂的地热发电技术。

Simple Calculation and Characteristic Analysis of the Trans-critical CO2-ORC Combined Cycle and the Correspondent Design Strategies

The application of geothermal power in China is limited by its low efficiency since most geothermal reservoirs developed for enhanced geothermal system are low grade heat source,which is not suitable for both water and CO₂ to generate power effectively.Theoretically,a direct blending of supercritical carbon dioxide and organic working fluid may probably improve the mechanical efficiency of thermal power system,specifically in the CO₂ based enhanced geothermal system(EGS).According to the process design,the efficiency of the CO₂ transcritical-isobutane composite cycle is estimated within the parameter range.CO₂ and isobutane flows of 1 kg/s and 0.25 kg/s were used in the calculation.Results showed that:with the decompression of scCO₂ component,the Joule-Thomson effect of CO₂ will come into play,leading to the condensation of organic working fluid,and thus the presence of multi-phased flow in the expander.From the computation,the condensates,if properly be re-utilized in the compression processes,will drastically shorten the range of temperature change of CO₂ and reduce the energy cost for the compressors,while preheating the organic working fluid,paving the way for the fluids blending to complete the cycle.With mass flow rate 4∶1 for CO₂ and isobutaneand a group of initial states of the working fluids carefully selected,increased efficiency can be expected from the design schematics,making the prototype a potentially promising candidate in low grade heat power generation and utilization for CO₂-based EGS in the future.