An Organic Rankine Cycle (ORC) is a closed-loop Rankine cycle that swaps water for an organic fluid with a low boiling point, so it can turn low-grade heat (geothermal brine, engine exhaust, industrial waste streams, 80–260 °C) into electricity that a steam cycle cannot economically use. This 2D view is the same loop as the 3D model, drawn top-down: a racetrack pipe with the pump, evaporator, turbine and condenser at its four sides.
1→2 Pump: w_pump = v_liq · (P_evap − P_cond)
2→3 Evaporator: Q_in = cp_liq·(T_evap − T_cond) + L(T_evap)
3→4 Turbine: w_s,ideal = cp_vap·(T_evap − T_evap·(P_cond/P_evap)^(R/cp_vap))
w_turbine = η_turbine · w_s,ideal
4→1 Condenser: rejects the remaining heat at T_cond
Saturation pressure (Clausius–Clapeyron):
P_sat(T) = P_ref · exp[ −L/R_specific · (1/T − 1/T_ref) ]
Thermal efficiency: η = (w_turbine − w_pump) / Q_in
Carnot limit: η_Carnot = 1 − T_cond / T_evap (both in kelvin)
- Heat-source temp sets the evaporator temperature (with a 12 °C pinch-point loss below the source).
- Working fluid changes the latent heat, specific heats and molar mass used in every equation above — this is why fluid choice is the central ORC design decision.
- Cooling temp sets the condenser temperature; a colder heat sink widens T_evap − T_cond and raises the Carnot limit.
- Turbine efficiency scales the ideal isentropic expansion work down to what a real turbine delivers.
- Mass flow is the working-fluid circulation rate; net power = ṁ · w_net, so it scales plant size without changing efficiency.
- Dots trace fluid parcels around the loop: blue = subcooled liquid, orange = superheated/expanding vapor. The turbine rotor at the top spins faster as net power rises, and the evaporator box glows brighter as the heat-source temperature climbs. Drag to pan, scroll to zoom.
Real-world relevance: ORC units are the standard technology in binary geothermal plants, biomass CHP plants and marine/industrial waste-heat recovery, precisely because they work well below the ~370 °C floor where a water-steam Rankine cycle becomes practical.