Modelling and off-design performance optimisation of a trilateral flash cycle system using two-phase twin-screw expanders with variable built-in volume ratio

Giuseppe Bianchi,Matteo Marchionni,Jeremy Miller,Savvas A Tassou

doi:10.1016/j.applthermaleng.2020.115671

Giuseppe Bianchi, Matteo Marchionni + Show 2 more

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https://doi.org/10.1016/j.applthermaleng.2020.115671

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Abstract

This research work presents a numerical chamber model of a two-phase twin-screw expander and its further integration in a one-dimensional model of a Trilateral Flash Cycle (TFC) system for low-grade heat to power conversion applications. The novel feature of the expander is the capability of changing the built-in volume ratio (BIVR) of the machine through a sliding valve in the casing that opens an additional suction port. Lowering the BIVR from 5.06 to 2.63 results in an improvement of the volumetric efficiency from 53% to 77% but also in a reduction of the specific indicated power from 4.77 kJ/kg to 3.56 kJ/kg. Parametric analysis on several degrees of freedom of the full TFC system concluded that expander speed and BIVR are the variables that mostly impact the net power output of the unit. An optimisation study enabled the net power output of the TFC system, at design point, to increase from 81 kW to 103 kW.

Highlights

Low-grade waste heat to power conversion has recently attracted increased attention by the academic and industrial communities
The findings revealed 14–29% higher values of exergetic efficiency in the case of Trilateral Flash Cycle (TFC) depending on the inlet temperatures of heat source and sink
The multi-dimensional look up is carried out as follows: (1) the expander pressure ratio is calculated from the high and low cycle pressures which output from the heat exchangers sub-models; (2) the fluid quality at the expander inlet results from the energy balance at the heater; (3) the expander built-in volume ratio (BIVR) and revolution speed are given as boundary conditions; (4) mass flow rate and isentropic efficiency result from the multi-dimensional interpolation while, beyond the range of independent variables considered, a linear extrapolation method is employed

Summary

Introduction

Low-grade waste heat to power conversion has recently attracted increased attention by the academic and industrial communities. The potential for thermal energy recovery and conversion to electricity from waste heat sources at temperatures below 100 °C has been estimated at 468 TWh on a European scale [1] and 43.2 PWh worldwide [2]. According to the aforementioned studies, the most promising sectors for harvesting this potential are the chemical and petrochemical, the nonmetallic minerals as well as the food and tobacco industries. Nomenclature e specific total internal energy [J kg−1]. Cp specific heat at constant pressure [J kg−1 K−1]. H specific enthalpy [J kg−1] m mass [kg]. M mass flow rate [kg s−1] p pressure [Pa]. R specific internal energy [J kg−1] t time [s] u velocity [m/s] x quality

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