A thermo-economic comparison of the up-therm heat converter and an organic rankine cycle heat engine

Abstract

In this paper we compare a recently proposed two-phase thermofluidic oscillator device termed ‘Up-THERM’ to a basic (sub-critical, non-regenerative) equivalent organic Rankine cycle (ORC) engine. In the Up-THERM heat converter, a constant temperature difference imposed by an external heat source and sink leads to periodic evaporation and condensation of the working fluid, which gives rise to sustained oscillations of pressure and volumetric displacement. These oscillations are converted in a load arrangement into a unidirectional flow, which passes through a hydraulic motor that extracts useful work from the device. A pre-specified Up-THERM design is being considered in a selected application with two n-alkanes, n-hexane and n-heptane, as potential working fluids. One aim of this work is to evaluate the potential of this proposed design. The thermodynamic comparison shows that the ORC engine outperforms the Up-THERM heat converter in terms of power output and thermal efficiency, as expected. An economic comparison, however, reveals that the capital costs of the Up-THERM are lower than those of the ORC engine. Nevertheless, the specific costs (per unit power) favour the ORC engine due to its higher power output. Some aspects of the proposed Up-THERM design are identified for improvement.