System and component modelling and optimisation for an efficient 10 kWe low-temperature organic Rankine cycle utilising a radial inflow expander

Abstract

Small-scale (10 kWe) organic Rankine cycles for low temperature applications such as heat recovery and solar power present a significant development opportunity but limited prototypes have been developed. This paper aims to address this by describing a system modelling tool which is used to select a working fluid, optimise cycle conditions, and preliminarily size a radial inflow rotor for an experimental test rig. The program is a steady-state sizing and optimisation tool which advances on current models by combining component models and cycle analysis with multi-objective optimisation and turbomachinery design aspects. Sizing and off-design pump and expander models are based on non-dimensional characteristic plots, whilst an additional design program achieves an expander rotor design. A novel objective function couples component and system performance with complexity. Results from an optimisation study indicate that R1234ze is the optimal working fluid for the defined objective function with a predicted net power output of 7.32 kWe, correlating to a cycle efficiency of 7.26%, and evaporator and condenser areas of 1.59 m² and 2.40 m², respectively. However, after considering operating pressures and fluid availability, R245fa has been highlighted as the most suitable fluid for a planned experimental radial expander test rig and a preliminary turbine design is proposed.

Keywords

Organic Rankine cycles radial turbine small-scale expander

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