Abstract
With increasing oil prices and growing interest in cutting green house gases emissions, waste heat recovery techniques appear as a very promising path to enhance internal combustion engine vehicle thermal efficiency. Rankine cycles and thermoelectricity are two possible ways to recover thermal energy. The Rankine cycle shows the highest potential due to its higher cycle efficiency in comparison with the current state-of-the-art thermoelectric materials intrinsic conversion ratio. Research in this area has mainly focused on the optimization of the cycle efficiency and/or output power with respect to the cycle configuration and of the considered working fluids. Several organic fluids have been proposed in the literature. Amongst them, the R245fa seems promising for the operating conditions described in this paper mainly due to its non flammable behaviour and thermodynamic performance. Other criteria, such as expansion machine availability for the selected conditions and fluid, or size of the components for ICE vehicle applications have rarely been taken into account. This paper focuses on the expander selection for both steam and R245fa working fluids. A detailed description and analysis of the most common expansion machines available have been performed. Two main types of expander machines can be distinguished: the turbo (dynamic) and positive displacement (volumetric) types. In the present study the considered expansion machines are: (i) the radial and axial turbine; (ii) the vane expander; (iii) the scroll expander and (iv) the axial piston expander. The advantages and limitations of these machines are considered for both steam and R245fa. The literature review reveals that currently most of the employed positive displacement expanders are obtained by modifying existing compressors. The present analysis shows that higher temperature and pressure differences can be achieved with turbines or multi-stage turbines. The existing turbines are mainly designed for largerscale applications and need to be downsized for small-scale power ranges. Considering the present available expanders, the present study demonstrates that a small-scale turbine expander presents the best compromise between compact design and performance. However, the turbine expander performance is very sensitive to operating conditions. As a result, optimizing the Rankine cycle control is crucial to keep the turbine expander performance high.
Keywords: Waste heat recovery; Rankine cycle; volumetric expanders; turbo expanders; performance.