Comparison of micro gas turbine heat recovery systems using ORC and trans-critical CO₂ cycle focusing on off-design performance

In this study, two heat recovery systems, a Rankine cycle (ORC) and a trans-critical CO₂ cycle (tCO₂), were applied to a micro gas turbine(MGT) and their performance was compared. The off-design performance of the two bottoming cycles with the load variation of the MGT was also analyzed. The performance curve, the effectiveness-NTU method, and the Stodola's equation were used to model the off-design operations of all bottoming cycle components, such as a pump, a turbine and heat exchangers. According to the calculations, the power output of the ORC was higher than that of the tCO₂ cycle when the MGT operated at a full load. On the other hand, because the power output variation with the MGT load change is larger in the ORC, the power output of tCO₂ becomes larger when MGT operates at a part load below 75%. Therefore, the tCO₂ is suitable for a heat recovery system utilizing MGT exhaust gas in places where the MGT needs to operate at low loads during many of its operating hours. On the other hand, the ORC is more suitable in applications where the MGT can operate at near full load conditions.