Feasibility of solar-driven trilateral-like organic Rankine cycle with radial-inflow turboexpander

Abstract

Low-temperature solar collectors coupled with thermal energy storage can enable stable and carbon-free energy production. This work proposes a fully integrated organic Rankine cycle (ORC) with solar field and thermocline direct energy storage. The organic fluid remains liquid inside the solar field and the thermal energy storage, leading to a trilateral-like thermodynamic cycle. As opposed to other trilateral (flash) cycles, the proposed system distinguishes itself by including a turboexpander to deal with two-phase expansion, leading to higher conversion efficiency. In particular, with the same turbine efficiency, the proposed cycle outperforms alternative integrated ORC-solar field configurations by 1.5-3.8 percentage points in thermodynamic cycle efficiency for maximum temperatures between 400−600K. The equivalent electric energy density also increases by 30% to 60%. The problem of the two-phase turbine is tackled by relying on a recently proposed radial-inflow turbine concept. The centripetal stator leverages the retrograde shape of the saturation curve to achieve a complete liquid-to-vapor expansion. As a result, the rotor can handle dry organic vapors without experiencing mechanical damage or additional losses from two-phase interactions. Preliminary turbine designs, obtained through optimization of a validated meanline method, consistently yield isentropic total-to-static efficiencies exceeding 85%, confirming the potential of the proposed system.