5th International Seminar on
ORC Power Systems
Athens Greece

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14:00   Session 4A: Volumetric Expanders (1)
Chair: Apostolos Pesyridis
20 mins
Layout and Mechanics Development of a Novel Piston-type Expansion Engine for Waste Heat Recovery
Michael Lang, Christian Bechter, Thomas Amann, Sebastian Schurl, Niko Bretterklieber, Raimund Almbauer
Abstract: Within the wide range of possible waste heat recovery technologies, the Rankine Cycle showed to be widely applicable and established on the market, but mainly for high power systems; if used for smaller applications, either efficiency or costs of the system are often not satisfying. This is mostly caused by the characteristics of the used expansion engine types, mainly being reciprocating piston engines or turbines. Whereas the first show good part load performance and are well suited for small power, they suffer from lubrication problems and life time issues. Turbines on the other hand are preferably used for high power and can do without elaborate lubrication, but suffer under part load conditions and are quite costly. Especially for medium power systems, i.e. waste heat fluxes between 200 and 400 kW, a novel concept for the expansion engine within a Rankine steam cycle was conceived. The aim was to combine the advantages of reciprocating piston engines and of turbines at reasonable costs. The so-called rotational wing-piston expander uses two pivoting shafts, each holding two wing-like pistons, within one housing, that are performing a cyclic movement relative to one another. This way, four working chambers with varying volumes are resulting, each experiencing repetitive compression and expansion. The conversion of the cyclic changing angular velocity to a constant rotation at the output shaft is done via a non-circular gear. This solution offers the possibility of sealing the lubricated gearbox against the steam-flooded section containing the working chambers via rotational seals, being much easier than the sealing within a conventional reciprocating piston engine. This paper treats the design and layout of this novel expansion engine concept, followed by the experimental mechanics development on the test bench. The strategy for creating a robust and effective expansion engine design as well as the most important findings and insights gained during the experimental investigations of the engine are shown.
20 mins
Verification of a 3D-CFD Model of an Oldham Ring Scroll Expander
Saverio Randi, Nicola Casari, Ettore Fadiga, Alessio Suman, Michele Pinelli, Eckhard Groll, Davide Ziviani, Bryce Shaffer
Abstract: Computational Fluid Dynamics (CFD) studies of volumetric machines, such as scroll ones, are becoming more and more widespread due to the interest in these type of devices by Organic Rankine Cycle (ORC) systems manufacturers and because of the increasing computational power, which makes these approaches more affordable than in past years and capable to provide useful information regarding transient phenomena. It is known that the peculiarity of scroll devices is the orbiting motion of one or both spirals. In the first case, to prevent rotation of the moving spiral during machine operation, an Oldham coupling is used. In those applications where this mechanism is placed between the moving and fixed spirals, there could be instants during motion in which one of the inlet or outlet ports, depending if the device is running in compression or expansion mode, is partially overlapped by the Oldham ring. This fact can cause issues such a non-uniform flow profile at the outlet of the port or flow pulsations, leading to vibrations of the machine. In this paper, a CFD analysis of such a scroll expander is showed. The geometry of the machine, provided by the manufacturer, is used to set-up an unsteady, 3D simulation in CONVERGE CFD software. An orthogonal grid is generated and then the scroll surface is mapped on this mesh: in this way, the complex motion of the machine can be simulated, and the gaps can be discretized thanks to an adaptive mesh refinement (AMR) technique, which can guarantee shorter computational time if compared to the Overset method, already presented by the authors in a previous work. Flow field is also solved in both radial and axial gaps, while the motion of both the moving spiral and the Oldham ring is imposed by means of a user-defined function. Working fluid R245fa has been modelled thanks to Redlich-Kwong equation of state. In this way, it has been possible to characterize the operation of the expander for different working conditions, such as rotational speeds and refrigerant inlet temperature and pressures.
20 mins
Development of an Efficient Static Shaft Wankel Expander for Organic Rankine Cycles
Gavin Tozer, Jonri Lomi Ga, Raya Al-Dadah, Saad Mahmoud
Abstract: The Organic Rankine Cycle (ORC) presents one suitable solution to the utilisation of low grade heat from some renewables and waste heat from industrial and power generation processes. However, the efficiency of the ORC will be dependent on the efficiency of the expansion device. The Wankel expander shows promise because the disadvantages of the combustion engine are lost when used as an expansion device. Furthermore, it has the advantages of few moving parts, a high power to weight ratio, low noise and vibrations and simplicity in design and manufacture. However, to achieve a good efficiency, an external valve setup is usually required. This paper presents an alternative form of the Wankel expander in which this is not required and the desirable simplicity is maintained. To analyse this expander computational fluid dynamics was utilised. The simulation results gave a maximum isentropic efficiency of 85% (2bar inlet gauge pressure, 6000RPM, 245W). The case with the maximum power gave 572W (3bar inlet gauge pressure, 9600RPM) but only achieved 73% efficiency. The performance of the expander was then compared to other expanders from literature, where it faired comparably in terms of efficiency and as it has greater simplicity, it could be preferred in many applications.
20 mins
Rotary Vane Expander – Analysis and Prediction of Delayed Chamber Closure
Vaclav Vodicka, Vaclav Novotny, Zbynek Zeleny, Jakub Mascuch
Abstract: Rotary vane expander (RVE) is a perspective solution in the case of requirement for expanders with low power output in range of single kW as they can provide attractive cost to performance ratio, especially in case of small series or single units manufacturing. Specific feature of the RVE are the vanes sliding on the surface of a stator cylinder, thus creating chambers. Permanent contact between the vanes and the stator is essential from the perspective of expander efficiency and lifetime of the vanes. If the vane does not maintain contact with the stator, a phenomenon called vane chatter occurs. In this case, the working chamber is not properly formed, which is associated with excessive leaks of the working fluid from the working chamber. If this occurs during the filling phase, the mass flow of the working fluid through the machine will be greatly increased, while at the same time the overall efficiency of the expander will be significantly reduced. This work provides a mathematical model, which extends previous models describing internal leakage pathways by prediction of the occurrence and extent of the delayed closure of the working chamber in the filling phase. The model is based on combination of 1D quasi-steady flow of working fluid through a set of described leakage pathways combined with a dynamic model of vanes, describing their radial position. The model results are after preliminary assessment in agreement with experimental measurements on an in house developed RVE with MM as working fluid. Vanes of various mass were tested for a given range of rotational speed as the difference in the centrifugal force acting on the vanes has major influence on the proper closure of the working chamber (but also on friction losses). This combined model can serve for future throughout optimization or check of occurrence of delayed chamber closure in RVE design works.
20 mins
Experimental Analysis of a Commercial Twin Screw Compressor Employed as an Expander
Marco Francesconi, Gianluca Pasini, Luca Sani, Marco Antonelli
Abstract: Small scale power plants applicability is often limited by economics constraints. Especially, the need to reduce costs requires a suitable tune of the expander devices, that are often derived from volumetric compressors. For this reason, experimental analysis plays a relevant step in the research of volumetric machine and for their suitability to the market. This work deals about the experimental test of a commercial 3 kWe twin screw compressor which has been used as a saturated steam expander. Inlet pressures between 2 and 8 bar and rotating speeds ranging from 1000 to 5000 rpm were used as test conditions. The isentropic efficiency and the delivered power were considered as key performance indicators.