跨临界-亚临界复叠式有机朗肯循环性能分析

Performance analysis on the transcritical-subcritical cascade organic Rankine cycle system

热源温度高于473.15 K时,复叠式有机朗肯循环(organic Rankine cycle,ORC)可避免高温下工质热分解、膨胀比过大等缺点,相对单级ORC更具优势。跨临界循环相较常规亚临界具有更高的吸热压力及更好的热源匹配性,其与复叠式ORC耦合有望获得更优的热力性能。因此,构建了跨临界-亚临界复叠式ORC(TSORC),以473.15~573.15 K工业烟气为热源,针对5组工质,探究了吸热压力、冷凝压力对系统热力性能的影响,优化系统参数以获得最大净输出功;并与常规亚临界-亚临界复叠式ORC(SSORC)进行对比。结果表明:TSORC相对SSORC可有效增大系统净输出功,最多可提高23.9%;但当热源温度低于"等值热源温度"时,SSORC的输出功反而更大;以R1233zd-R1234ze(E)为工质的TSORC具有最大净输出功。

Cascade organic Rankine cycle(ORC) can avoid thermal decomposition of working fluids and high level of expansion ratio when the heat source is higher than 473.15 K, and thus precedes the single-stage ORC. Transcritical cycle is also superior to subcritical cycle because of higher heat absorption pressure and better temperature match effect with heat source, which can be combined with cascade ORC to improve the system thermodynamic performance. A transcritical-subcritical ORC(TSORC) was proposed in this paper using five working fluids with the heat source ranging from 473.15 to 573.15 K. Effects of pressure in the evaporator and condenser on the system performance were analyzed and these parameters were optimized to maximize the net power output. Thermodynamic performance of TSORC was also compared with sub-subcritical ORC(SSORC). Results show that the TSORC can increase the net power output by 23.9% at most compared with the SSORC. However, the SSORC has a larger net power output over TSORC when heat source temperature is lower than "equal temperature". In addition, the maximum net power output is reported in TSORC using R1233 zd-R1234 ze(E) as working fluids.