Effect and comparison of different working fluids on a two-stage organic rankine cycle (ORC) concept

• Efficiency gain in proposed two-stage organic Rankine cycle.
• Working fluid selection influences cycle design.
• Beneficial behavior of proposed ORC compared to state-of-the art cycles.

This paper presents Aspen Plus (V7.3) simulations of a two-stage organic rankine cycle concept with internal heat recovery. The proposed system is compared to state-of-the-art processes with four different working fluids distinguished by the slope of the saturated vapor curve in the corresponding T–s-diagram. The heat source is defined as exhaust gas (490 °C and 1 bar) from an internal combustion engine, which is fired with biogas from a biomass digestion plant. In a first consideration the exhaust gas outlet is constrained to 130 °C to stay above the acid dew point (study 1). In a second study the pinch point of the exhaust gas heat exchanger is set to 10 K. For wet and isentropic fluids the thermodynamic efficiencies of the two-stage cycle exceed the corresponding values of reference processes by up to 2.25%, while the recuperator design benefits dry fluids compared to the two-stage concept.