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Im 21. Jahrhundert haben einige Länder als Reaktion auf die Nuklearkatastrophe von Fukushima bereits den Ausstieg aus der Kernkraft beschlossen oder zumindest in Erwägung gezogen. Das Ende der Kohleverstromung ist in einigen Ländern bereits umgesetzt oder wie in Deutschland für 2038 zumindest beschlossen, um den globalen Klimawandel zu verlangsamen. Es stellt sich also die Frage, ob die „Dampfmaschine“, das heißt der Rankine-Kreisprozess noch eine Zukunft hat?

Entgegen dem aktuellen Trend, Abwärme im kleinen Leistungsbereich unter 100 kW(el.) mittels einer ORC-Anlage in Strom umzuwandeln, wird im vorliegenden Beitrag das Konzept eines Mikro-Dampfkraftwerks verfolgt. Nachfolgend wird erläutert, warum die Wahl der Turbine auf eine geschwindigkeitsgestufte zweikränzige Curtis-Turbine gefallen ist und das dafür verwendete 1-D-Turbinendesign-Tool wird kurz vorgestellt. Der parametrisierte Ansatz des Turbinenentwurfs in 3-D-CAD/CFD wird erläutert und schließlich werden das CFD-Strömungsfeld und die Leistungskennfelder der entworfenen Turbine diskutiert.

Development of micro ORC systems with 1-15 kW power output for micro-cogeneration and waste heat recovery at the Czech Technical University in Prague, University Centre for Energy Efficient Buildings (CTU UCEEB) has over ten years of history with many successes. These include 6 different ORC units, all with in-house designed rotary vane expanders (RVE) of many versions throughout this development. Among main advantages of the RVE belong relatively simple and robust design at low cost even at very small series of single-unit production and all that with acceptable efficiency. The ORC units operate with hexamethyldisiloxane (MM) working fluid at high pressure ratios and expansion ratios and the isentropic efficiency of RVE has a limit at these conditions around 60%, often however only at values around 50%. While this might be enough on a cost side for commercialization of this technology, in pursuit of higher efficiency solutions, different expander technology needs to be selected. A turbo-expander is a logical choice with prospect of higher efficiency. At the same time, a literature review has found a lack of reported detailed experimental data for micro (5-50 kW) turbo-expanders, possibly hindering global development towards economically feasible solutions. A project named Dexpand, “Optimised expanders for small-scale distributed energy systems” aims at these issues by objectives in designing, optimizing, manufacturing and testing several ORC expanders with MM and isobutane and their subsequent performance mapping and comparison. One major task is a design of a turboexpander for a 120 kWth biomass fired microcogeneration ORC unit currently operated at the CTU UCEEB. An axial impulse single stage turboexpander was selected as a suitable choice, providing a prospect of a decent efficiency at technically manageable rotational speed and size. This paper provides a detail of currently performed design activities, starting from boundary conditions specification, over development and optimization of a 1D model, preliminary 2D CFD calculations and finishing in a state of a robust and detailed 3D CFD model with a real gas model. Note that the working fluid, high molar mass organic vapour, is highly non-ideal in its behaviour and the flow conditions with pressure design ratio around 13 is highly supersonic (nozzle outlet isentropic Mach number exceeds 2). The current results based on 3D CFD indicate a prospect of an isentropic efficiency 71% at mechanical power output of 11 kW. Lastly, ongoing and future work is outlined, which includes aerodynamic optimization based on the developed 3D CFD model and construction design of the entire turbine assembly.

In order to meet the targets set out in the Paris climate agreement of 2015, the energy sector needs to be restructured. In Germany for example the goal is to move out of both nuclear and coal-fired power. In order to fulfil more demanding energy requests alternative sources of energy must be found and made available. One component is the usage of Waste Heat Recovery. This paper deals with the evaluation simulation of an Elektra turbine, which concept is a velocity compounded radial re-entry single wheel turbine. Based on the first turbine design, multiple simulations were carried out in order to maximize the efficiency. With these simulations, optimization opportunities are described and identified. The paper concludes with a comparison of the results achieved by numerical simulation and on the experimental test bench. The main focus for this paper is to analyze the actual flow phenomena to identify possibilities for further improvements. All simulations were calculated with the commercial software FINETM/TURBO 15.1.