Low-grade heat from industrial exhaust gases can be recovered for electricity generation by various thermodynamic cycles. Previous studies assumed that only sensible heat is available for the heat recovery. However, in fact, the moisture content is commonly high and as the moisture condenses, a significant amount of latent heat is released. To gain further understanding of the effect of the moisture, we carried out comparative analyses among a basic organic Rankine cycle (ORC) and alternative cycles, including organic flash cycle (OFC), Kalina cycle (KCS) and transcritical ORC (T-ORC). Furthermore, the performance of an advanced dual-pressure ORC (DPORC) was evaluated under a wet gas scenario. The results show that by taking into account the moisture condensation heat, the net power output per kg/s of heat source flow increases significantly for all cycles examined although the exergy efficiency, known as an indicator for system irreversibility, tends to drop. Regardless of the level of moisture content in the heat source, ORC and T-ORC always have higher performance than OFC and KCS. In a case study using a moisture content of 0.1 in the heat source, a DPORC can achieve a performance enhancement by 58% compared with a simple ORC.
Keywords kalina cycle (KCS), organic flash cycle (OFC), transcritical ORC (T-ORC), dual-pressure ORC (DPORC), heat recovery