On the optimum axial flow turbine design in Organic Rankine Cycles

Organic Rankine Cycles (ORCs) can effectively recover low grade heat for electricity production from industrial wastes and renewable energy. The general design problem of an ORC system is not trivial due to the choice of several design variables related to the thermodynamic cycle and equipment. Most of the optimization studies in the literature search for the optimum cycle configuration, design parameters and working fluid, disregarding the influence of these choices on expander design and efficiency. Indeed, the latter is often fixed to a constant value implicitly assuming that it will be achieved by a proper expander design in a subsequent design phase. This approach may be weak especially for the working fluids operating in ORCs having a high molecular weight and a low speed of sound. In these applications the high volumetric expansion ratios which may occur even at small temperature differences between turbine inlet and outlet markedly decrease the expander efficiency. Moreover, this efficiency is strongly affected by expander size that may vary from only few kWs up to several MWs. The aim of this study consists in searching for the optimum axial flow turbine design parameters (so called duty parameters) in a wide range of ORC operating conditions. Flow coefficient, loading coefficient and degree of reaction are selected as input values in a mean line design procedure that generates the main turbine geometrical characteristics and evaluates the turbine efficiency according to recent loss correlations. The variation of turbine efficiency with duty parameters is then shown in efficiency charts (like the Smith’s one) to highlight suboptimal design options. This procedure is repeated for a range of ORC duty specifications (mass flow rate and expansion ratio) to detect their influence on turbine efficiency. So, any penalty associated with the selection of non-optimum duty parameters is clearly separated from the efficiency decay deriving from more severe operating conditions (e.g., high expansion ratios and small sizes).