Performance analysis of organic rankine cycle with solar- assisted boiler

Abstract

Performance of a 10 kWe basic organic Rankine cycle (ORC) with saturated steam from biofuel-boiler as a heat source was investigated both experiment and simulation. Experimental ORC was selected R245fa as the suitable working fluid due to its high performance (thermal efficiency and cycle exergy efficiency) compared to those with other working fluids. In addition, its fluid properties were environmentally friendly with safe operation. By experimental results, it could be noticed that the electrical output could be increased to the maximum electrical output of 9.5 kW when the saturated steam inlet temperature was increased to 127.2 ℃ with superheating turbine inlet of 26.6 ℃. In addition, the cycle efficiency could be increased from 7.95% to 10.74% while the superheating decreased from 26.6 ℃ to 19 ℃. By simulation, the flow rate of saturated steam-water could be controlled until the steam leaving the ORC evaporator became subcooled liquid, the irreversibility during heat exchange at the evaporator could be reduced. Additionally, solar heat was proposed to preheat the liquid before returning to the boiler then the boiler fuel could be saved. The simulated results showed that the exergy efficiency could be increased from 52.24% to 54.38% when the steam flow rate leaving the ORC evaporator was decreased from 0.0413 kg/s in the form of saturated steam to 0.037 kg/s in the form of subcooled liquid, respectively. With 5-30 units evacuated-tube solar collectors each of 2.37 m² having optical efficiency of 0.57 and overall heat loss coefficient of 0.75 W/m²-K, the optimal solar collector unit and thermal storage capacity were found at 20 units and 1,500 L, respectively, due to the lowest unit cost of electricity (UCE). If rice husk was taken as the heat source of the boiler, the UCEs of the basic ORC without and with 20 units of the solar collector were 7.5 Baht/kWh and 7.48 Baht/kWh, respectively, for the climatic condition of Chiang Mai, Thailand.