The cumulative global capacity of organic Rankine cycle (ORC) power systems for the conversion of renewable and waste thermal energy is undergoing a rapid growth and is estimated to be approx. 2000 MW e considering only installations that went into operation after 1995. The potential for the conversion of the thermal power coming from liquiddominated geothermal reservoirs, waste heat from primary engines or industrial processes, biomass combustion, and concentrated solar radiation into electricity is arguably enormous. ORC technology is possibly the most flexible in terms of capacity and temperature level and is currently often the only applicable technology for the conversion of external thermal energy sources. In addition, ORC power systems are suitable for the cogeneration of heating and/or cooling, another advantage in the framework of distributed power generation. Related research and development is therefore very lively. These considerations motivated the effort documented in this article, aimed at providing consistent information about the evolution, state, and future of this power conversion technology. First, basic theoretical elements on the thermodynamic cycle, working fluid, and design aspects are illustrated, together with an evaluation of the advantages and disadvantages in comparison to competing technologies. An overview of the long history of the development of ORC power systems follows, in order to place the more recent evolution into perspective. Then, a compendium of the many aspects of the state of the art is illustrated: the solutions currently adopted in commercial plants and the main-stream applications, including information about exemplary installations. A classification and terminology for ORC power plants are proposed. An outlook on the many research and development activities is provided, whereby information on new high-impact applications, such as automotive heat recovery is included. Possible directions of future developments are highlighted, ranging from efforts targeting volume-produced stationary and mobile mini-ORC systems with a power output of few kW e , up to large MW e base-load ORC plants.

There is a growing interest in organic Rankine cycle (ORC) turbogenerators because they are suitable as sustainable energy converters. ORC turbogenerators can efficiently utilize external heat sources at low to medium temperature in the small to medium power range. ORC turbines typically operate at very high pressure ratio and expand the organic working fluid in the dense-gas thermodynamic region, thus requiring computational fluid dynamics (CFD) solvers coupled with accurate thermodynamic models for their performance assessment and design. This article presents a comparative numerical study on the simulated flow field generated by a stator nozzle of an existing high-expansion ratio radial ORC turbine with toluene as working fluid. The analysis covers the influence on the simulated flow fields of the real-gas flow solvers: FLUENT, FINFLO, and ZFLOW, of two turbulence models and of two accurate thermodynamic models of the fluid. The results show that FLUENT is by far the most dissipative flow solver, resulting in large differences in all flow quantities and appreciably lower predictions of the isentropic nozzle efficiency. If the combination of the k−ω turbulence model and FINFLO solver is adopted, a shock-induced separation bubble appears in the calculated results. The bubble affects, in particular, the variation in the flow velocity and angle along the stator outlet. The accurate thermodynamic models by Lemmon and Span (2006, “Short Fundamental Equations of State for 20 Industrial Fluids,” J. Chem. Eng. Data, 51(3), pp. 785–850) and Goodwin (1989, “Toluene Thermophysical Properties From 178 to 800 K at Pressures to 1000 Bar,” J. Phys. Chem. Ref. Data, 18(4), pp. 1565–1636) lead to small differences in the flow field, especially if compared with the large deviations that would be present if the flow were simulated based on the ideal gas law. However, the older and less accurate thermodynamic model by Goodwin does differ significantly from the more accurate Lemmon–Span thermodynamic model in its prediction of the specific enthalpy difference, which leads to a considerably different value for the specific work and stator isentropic efficiency. The above differences point to a need for experimental validation of flow solvers in real-gas conditions, if CFD tools are to be applied for performance improvements of high-expansion ratio turbines operating partly in the real-gas regime.

The purpose of the ''International Wet Steam Modeling Project'' is to review the ability of computational methods to predict condensing steam flows. The results of numerous wet-steam methods are compared with each other and with experimental data for several nozzle test cases. The spread of computed results is quite noticeable and the present paper endeavours to explain some of the reasons for this. Generally, however, the results confirm that reasonable agreement with experiment is obtained by using classical homogeneous nucleation theory corrected for non-isothermal effects, combined with Young's droplet growth model. Some calibration of the latter is however required. The equation of state is also shown to have a significant impact on the location of the Wilson point, thus adding to the uncertainty surrounding the condensation theory. With respect to the validation of wet-steam models it is shown that some of the commonly used nozzle test cases have design deficiencies which are particularly apparent in the context of two-and three-dimensional computations. In particular, it is difficult to separate out condensation phenomena from boundary layer effects unless the nozzle geometry is carefully designed to provide near-one-dimensional flow.

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