Widespread adoption of renewable energy technologies will arguably benefit from the availability of economically viable distributed thermal power conversion systems. For this reason, considerable efforts have been dedicated in recent years to R&D over mini-organic Rankine cycle (ORC) power plants, thus with a power capacity approximately in the 3–50 kW range. The application of these systems for waste heat recovery from diesel engines of long-haul trucks stands out because of the possibility of achieving economy of production. Many technical challenges need to be solved, as the system must be sufficiently efficient, light, and compact. The design paradigm is therefore completely different from that of conventional stationary ORC power plants of much larger capacity. A high speed turbine is arguably the expander of choice, if high conversion efficiency is targeted, thus high maximum cycle temperature. Given the lack of knowledge on the design of these turbines, which depends on a large number of constraints, a novel optimal design method integrating the preliminary design of the thermodynamic cycle and that of the turbine has been developed. The method is applicable to radial inflow, axial and radial outflow turbines, and to superheated and supercritical cycle configurations. After a limited number of working fluids are selected, the feasible design space is explored by means of thermodynamic cycle design calculations integrated with a simplified turbine design procedure, whereby the isentropic expansion efficiency is prescribed. Starting from the resulting design space, optimal preliminary designs are obtained by combining cycle calculations with a 1D mean-line code, subject to constraints. The application of the procedure is illustrated for a test case: the design of turbines to be tested in a new experimental setup named organic rankine cycle hybrid integrated device (ORCHID) which is being constructed at the Delft University of Technology, Delft, The Netherlands. The first turbine selected for further design and construction employs siloxane MM (hexamethyldisiloxane, C6H18OSi2), supercritical cycle, and the radial inflow configuration. The main preliminary design specifications are power output equal to 11.6 kW, turbine inlet temperature equal to 300 °C, maximum cycle pressure equal to 19.9 bar, expansion ratio equal to 72, rotational speed equal to 90 krpm, inlet diameter equal to 75 mm, minimum blade height equal to 2 mm, degree of reaction equal to 0.44, and estimated total-to-static efficiency equal to 77.3%. Results of the design calculations are affected by considerable uncertainty related to the loss correlations employed for the preliminary turbine design, as they have not been validated yet for this highly unconventional supersonic and transonic mini turbine. Future work will be dedicated to the extension of the method to encompass the preliminary design of heat exchangers and the off-design operation of the system.

A technical equation of state (EoS), according to the perturbed chain statistical associating fluid theory (PC-SAFT), is developed for solvent blends composed of polyethylene glycol dimethylethers (PEGDMEs/glymes), that is, CH 3 O[CH 2 CH 2 O] n CH 3 with n = 3, ..., 9. These solvent blends are employed in industry under the commercial names Selexol or Genosorb, primarily for the physical absorption of H 2 S and CO 2 from acid gases. The molecular parameters for the EoSs of the pure fluids comprising the solvent, notably for the n = 3 and 4 members of the homologous series, are obtained by fitting the PC-SAFT EoS to published vapor pressure and liquid density data. Because of the limited availability of experimental data for the glymes with n ≥ 5, PC-SAFT is used as a predictive tool to determine the molecular parameters for the n = 5, ..., 9 members of the homologous series. To exploit the extrapolative capabilities of PC-SAFT, the n = 1 and 2 members of the homologous series are included in this study. The mixture of glymes is modeled using the van der Waals one-fluid mixing rules with the Lorentz− Berthelot combining rules, whereby the binary interaction parameters k ij among the members of the homologous series are all set to zero. The performance of the mixture EoS is assessed by a comparison of predicted properties with experimental data. The thermodynamic model is also briefly applied to describe the vapor−liquid equilibrium behavior of glymes and their blends with CO 2 . The comparison with available experimental data shows that the resulting model provides a description of the thermodynamic behavior of this system suitable for engineering purposes.

Organic Rankine Cycle (ORC) power systems are receiving increased recognition for the conversion of thermal energy when the source potential and/or its temperature are comparatively low. Mini-ORC units in the power output range of 3–50 kWe are actively studied for applications involving heat recovery from automotive engines and the exploitation of solar energy. Efficient expanders are the enabling components of such systems, and all the related developments are at the early research stage. Notably, no experimental gasdynamic data are available in the open literature concerning the fluids and flow conditions of interest for mini-ORC expanders. Therefore, all the performance estimation and the fluid dynamic design methodologies adopted in the field rely on non-validated tools. In order to bridge this gap, a new experimental facility capable of continuous operation is being designed and built at Delft University of Technology, the Netherlands. The Organic Rankine Cycle Hybrid Integrated Device (ORCHID) is a research facility resembling a state-of-the-art high-temperature ORC system. It is flexible enough to treat different working fluids and operating conditions with the added benefit of two interchangeable Test Sections (TS’s). The first TS is a supersonic nozzle with optical access whose purpose is to perform gas dynamic experiments on dense organic flows in order to validate numerical codes. The second TS is a test-bench for mini-ORC expanders of any configuration up to a power output of 100 kWe. This paper presents the preliminary design of the ORCHID setup, discussing how the required operational flexibility was attained. The envisaged experiments of the two TS’s are also described.

The conventional design of organic Rankine cycle (ORC) power systems starts with the selection of the working fluid and the subsequent optimization of the corresponding thermodynamic cycle. More recently, systematic methods have been proposed integrating the selection of the working fluid into the optimization of the thermodynamic cycle. However, in both cases, the turbine is designed subsequently. This procedure can lead to a suboptimal design, especially in the case of mini- and small-scale ORC systems, since the preselected combination of working fluid and operating conditions may lead to infeasible turbine designs. The resulting iterative design procedure may end in conservative solutions after multiple trial-and-error attempts due to the strong interdependence of the many design variables and constraints involved. In this work, we therefore present a new design and optimization method integrating working fluid selection, thermodynamic cycle design, and preliminary turbine design. To this purpose, our recent 1-stage continuous-molecular targeting (CoMT)-computer-aided molecular design (CAMD) method for the integrated design of the ORC process and working fluid is expanded by a turbine meanline design procedure. Thereby, the search space of the optimization is bounded to regions where the design of the turbine is feasible. The resulting method has been tested for the design of a small-scale high-temperature ORC unit adopting a radial-inflow turbo-expander. The results confirm the potential of the proposed method over the conventional iterative design practice for the design of small-scale ORC turbogenerators.