In recent years, the use of alternative fuels in thermal engine power plants has gained more and more attention, becoming of paramount importance to overcome the use of fuels from fossil sources and to reduce polluting emissions. The present work deals with the analysis of the response to two different gas fuels—i.e., hydrogen and a syngas from agriculture product—of a 30 kW micro gas turbine integrated with a solar field. The solar field included a thermal storage system to partially cover loading requests during night hours, reducing fuel demand. Additionally, a Heat Recovery Unit was included in the plant considered and the whole plant was simulated by Thermoflex^® code. Thermodynamics analysis was performed on hour-to-hour basis, for a given day as well as for 12 months; subsequently, an evaluation of cogeneration efficiency as well as energy saving was made. The results are compared against plant performance achieved with conventional natural gas fueling. After analyzing the performance of the plant through a thermodynamic analysis, the study was complemented with CFD simulations of the combustor, to evaluate the combustion development and pollutant emissions formation, particularly of NO_x, with the two fuels considered using Ansys-Fluent code, and a comparison was made.

The design of organic Rankine cycle (ORC) turbines often requires dealing with transonic flows due to the cycle efficiency requirements and the matching of the temperature profiles with heat sources and sinks, as well as the nature of organic fluids, often featuring high molecular weight. Consequently, the use of convergent–divergent turbine stators has been widely established as a solution in the published literature for use in both axial- and radial-inflow machines. With respect to the latter layout in particular, the available design guidelines are still limited. The present work shows the results of an investigation into a series of ORC radial-inflow convergent–divergent nozzles that differ with respect to the vane count and the designed metal angle of the outlet. These stators were designed by fitting the divergent portion of a sharp-edged minimum-length nozzle, designed by means of the method of characteristics (MoC) adapted to dense gases, into a radial-inflow turbine stator. The geometries were analysed by means of steady-state RANS CFD calculations, and the results were used to assess the influence of the design parameters on the nozzle losses and downstream flow field uniformity, showing that conflicting trends exist between optimum stator efficiency and optimum downstream flow field uniformity.

The use of radial-inflow turbine (RIT) for organic Rankine cycle (ORC) plant sizes below 100 kW is promising, although the application remains challenging. In fact, the single-stage arrangement imposed by economic constraints and hence the large expansion ratio, together with the large molecular weight, which characterizes organic fluids, usually result in highly supersonic flows, so making the use of transonic stators often mandatory. Particularly, the influence of RIT stator design parameters on losses and the level of unsteadiness seen by the subsequent rotor is still scarcely addressed in published literature. The present work investigates the effect of the convergent–divergent stators design parameters and the resulting downstream flow field non-uniformity on the unsteady stator–rotor interaction and loss generation in ORC RIT. To this end, two stator and rotor configurations which differ by the stator design parameters (i.e., discharge metal angle and number of vanes) have been tested by means of three-dimensional (3D) unsteady CFD calculations accounting for real-gas properties. The results show that larger stator–rotor interaction is present for the case featuring higher vane count and lower outlet metal, which also features the largest fluctuations of power output and pressure force on blade, together with a substantially lower average total-to-static efficiency, as a result of a larger stator downstream pitchwise non-uniformity. Ultimately, the results indicate that only aiming for optimal stator efficiency during the stator design phase might lead to highly suboptimal configurations from the whole turbine stage point of view.

In recent years, Organic Rankine Cycle (ORC) technology has received growing interests, thanks to its high flexibility and to the capability to exploit energy sources at temperature levels difficult to be approached with conventional power cycles. These features allow exploiting renewable and renewable-equivalent energy sources, by either improving the energy conversion efficiency of existing plants or using waste heat from industrial process. As far as the expander is concerned, a high potential solution is represented by turbo-expanders, which allow reduction of plant clutter and complexity, so enhancing the potential impact on the diffusion of small power ORC-based plants. The present work concerns the design of a RadialInflow Turbine for a bottoming Organic Rankine Cycle in the tens of kW scale. Design boundary conditions are retrieved by a zero-dimensional model of a solar-assisted micro gas turbine in cogenerating mode. The design process is started by means of an in-house mean-line design code accounting for real gas properties. The code is used to carry out parametric analyses to investigate the design space for several working fluids encompassing different classes, namely refrigerants and siloxanes. The program is used to assess the effect of design variables and working fluid on the turbine performance and turbine design characteristics. Subsequently, the most promising design candidates are selected and three-dimensional first guess stator and rotor geometries are built on these preliminary designs. Stationary and rotating passages are then meshed and analyzed by means of RANS CFD based solution of the stator – rotor interaction.

The successful implementation of an airborne propulsion system based on hydrogen-powered fuel cell technology highly depends on the development of an efficient, lightweight and compact air supply compressor. Meeting these requirements by designing the compressor using conventional single-point preliminary design methods can be challenging, due to the very wide range of corrected mass flow rate and pressure ratio values that the air supply compressor must be able to accommodate. This article presents a multi-point design methodology for the preliminary design of centrifugal compressors of air supply systems. The method is implemented in an in-house code, called TurboSim, and allows to perform single- and multi-objective constrained optimization of vaneless centrifugal compressors. Furthermore, an automatic design point selection method is also available. The accuracy of the compressor lumped-parameter model is validated against experimental data obtained on a high-pressure-ratio single-stage vaneless centrifugal compressor from the literature. Subsequently, the design methodology is applied to optimize the compressor of the air supply system of an actual fuel cell powertrain. The results, compared to those obtained with a more conventional single-point design method, show that the multi-point method provides compressor designs that feature superior performance and that better comply with the specified constraints at the target operating points.