• expander
• Matlab
• modeling
• modellazione
• Wankel
• espansore

In the field of distributed energy generation the ORC (Organic Rankine Cycle) technology represents an attractive solution, as it can be suitable for various applications such as biomass, solar and low-temperature geothermal plants.
Many factors affect the selection of the working fluid and the components, so that the optimal solution may vary significantly depending on the application.
One of the main obstacles to the spread of small (from tens to a hundred of kW) power ORC plants is the lack of an expander technology that combines good performances and low operating costs.
In this context, a volumetric expander derived from a Wankel rotary combustion engine is presented in this paper as a good candidate for this role, because of its high power density, low vibrations and low part count; moreover, the modular architecture of this device makes it suitable for multi-staging in order to increase the overall efficiency.
This paper describes a numerical model of a Wankel rotary expander, wich is based on a thermodynamic analysis of the operative chambers of the device and includes the effects of heat transfer, friction losses and leakage flows; great attention was paid to the dynamic behavior of apex seals as they represent one of the critical points in rotary vane machinery design.
The model predictions showed good accordance with the results of preliminary tests carried out on a Wankel expander prototype designed at the Energy Engineering Department of Pisa University, using air as a working fluid.
An experimental validation of the model was then performed on the same expander feeded with the steam produced by a biomass boiler owned by the Interuniversity Research Centre on Biomass for Energy (CRIBE), the results of which are presented.
The final section of this paper contains a brief description of the data acquisition program developed for the tests.