Supercritical CO₂ Rankine Cycle Using Low and Medium Temperature Heat Sources

Recovery of industrial waste heat is significant to both energy saving and emission reduction. In the present paper, two types of supercritical CO₂ Rankine cycles with and without internal heat exchanger (IHX) are integrated for analyzing the performance of low and medium temperature industrial heat recovery. Cycles were simulated with Aspen software, by which the influences of the initial temperature, initial pressure and temperature of cooling water were observed. The results indicate that cycle efficiency and net output work increase with growth of initial temperature, yet they fell as temperature of cooling water ramps down for the two types of cycle. For a given initial temperature, the cycle efficiency and net output work have maximum values under various initial pressures. This can be attributed to the power consumption of CO₂ pump, which goes up significantly with increase of initial pressure. The performances of supercritical CO₂ Rankine cycles with and without IHX utilizing four typical industrial heat sources at low and medium temperature were analyzed, which were the heat of non-concentrating solar collector, the exhaust gas heat of a 600MW coal fired plant, the exhaust gas heat of a CFB boiler and the exhaust gas heat of industrial furnace of a cement plant. The optimal cycle efficiencies range from 7% to 12% without IHX and 9% to 15% with IHX, respectively, under the temperature of heat source varying from 130°C to 200°C.