Development of a Two-Stage Radial Inflow Turbine for a Mini-ORC

Martin Heylen2^{2}, Vincent Lemort1^{1}, Michel Delanaye2^{2}, Koen Hillewaert3^{3}
^{\star} :
1^{1} Laboratoire de Thermodynamique – Universite de Liège - 17 Bât B49, Allèe de la Découverte – 4000 Liège
2^{2} Mitis SA - 5, rue des Chasseurs Ardennais – 4031 Angleur
3^{3} Design of Turbomachines – Universite de Liège - 9 Bât B52/3, Allèe de la Découverte – 4000 Liège
Mots clés : ORC, Turbine, Energy
Résumé :

Many industrial processes suffer energy loss in the form of wasted thermal energy. In the current context of the energy crisis, it is important to improve the global efficiency of energy systems by converting this wasted thermal energy into useful energy (such as hot water for heating systems) or generating electricity. For this second purpose, Organic Rankine Cycle (ORC) has been developed.

An ORC is a variant of the classical Rankine Cycle (RC) that uses an organic compound instead of water as the working fluid. Compared with the steam-based RC, the ORC can handle low-temperature thermal heat sources.

This paper focuses on the development of a mini-ORC (power generation range: 5-50 kWe) fed by a low-temperature heat source (<150^{\circ}C) where a radial inflow turbine has been chosen for the expansion process. The ORC is developed to allow the coupling with a 10kWe Regenerative Gas Turbine (RGT) developed by Mitis. The RGT’s turbogenerator is composed of a centrifugal compressor and a radial inflow turbine. To ensure rotor stability, it is relying on foil-bearing technology.

In this work, the pump performs the compression of the liquid fluid before being evaporated within the evaporator. The turbogenerator configuration described above has then been converted into a two-stage turboexpander by integrating two radial turbine wheels.

The use of a radial inflow turbine in a mini-ORC brings multiple challenges. Most pressurized working fluids have a very high density at the expander entrance. This results in a very small impeller size, which would lead to a relatively high impact of leakage losses between the blade tip and the shroud. Moreover, the low sound speed of sound of most working fluids leads to lower allowable rotation speeds and therefore lower work capacity of the turbine. Consequently, it is a necessity to select working fluids that have lower densities and speed of sound. One family corresponding to these criteria is the family of hydrocarbons.

To have the possibility to perform several radial inflow turbine designs for a selection of working fluids, a model of the ORC and a 0-D design model of a radial turbine have been developed. The design software is based on the velocity triangles theory and predicts part of the flow physics within the turbine impeller. In comparison with the currently available radial turbine design software, this model does not rely on ideal gas law and properties but integrates a library to correctly account for the complex equations of state for organic fluids.

Finally, 3-D CFD simulations of one of the designed turbines are performed to validate the design strategy and evaluate the off-design performances.

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