Mechanistic Model Of An Oil-Flooded Single-Screw Expander

The power generated from Organic Rankine Cycles (ORCs) is directly related to the performance of the expander used, which in many cases it is adapted from the original compressor. The efficiency of positive displacement expanders depends on several aspects such as internal volume ratio and pressure ratio (under- and over-expansion), leakages, non-adiabatic expansion and heat losses to the ambient and mechanical losses. Thus, lubrication is often required to reduce friction and to seal leakage gaps. In this paper, the effect of oil-flooding on the performance of a single-screw expander is investigated. By varying the oil fraction, it is possible to analyze the combined effects of reducing the leakages and the possibility to achieve a quasi-isothermal expansion. The flooding of the expander is realized by adding an independent oil loop to the conventional ORC as proposed by Woodland et al. (2013). The mechanistic model developed by Ziviani et al. (2014) for a single-screw expander has been extended to include liquid-flooding. The multi-chamber model with the associated governing equations of conservation of mass and energy for the flooded-expansion process is described along with the overall energy balance model of the expander. Insights on the calculation of the working fluid and lubricant oil mixture properties and the solubility effects are provided. The working fluid considered is R245FA and the lubricant oil is ACD100FY. The mechanistic model is implemented in the Python programming language and an example of oil-flooded singlescrew expander model is integrated in the open-source simulation package PDSim (v3.0). REFRENCES Woodland, B. J., Krishna, A., Groll, E. A., Braun, J. E., Horton, W. T., and Garimella, S. V. (2013). Thermodynamic comparison of organic rankine cycles employing liquid-flooded expansion or a solution circuit. Applied Thermal Engineering, 61:859-65. Ziviani, D., Bell, I. H., De Paepe, M., and van den Broek, M. (2014). Comprehensive model of a single screw expander for orc-systems applications. In 22th Int.Compressor Engineering Conf. at Purdue, number 1506