10:30
Session 1C: Volumetric Expanders (1)
Chair: Michel De Paepe
10:30
20 mins
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Design and modelling of hybrid rSOFC-ORC system
Simone Amicabile, Matteo Testi, Luigi Crema
Abstract: A combined-cogenerative system is here presented and analysed in order to prove the concept and viability of coupling two of the most promising technologies nowadays available on the market of renewable energies and energy storage. A model based on reversible SOFC/SOE system developed within the framework of the EU funded EDEN project (h2eden.eu website, 2015) is coupled with an ORC system to exploit the waste heat coming from the hydrogen conversion process. ORC systems are renowned as one the most promising approaches to recover waste heat owing to its efficiency and
reliability. ORC are able to recover a considerable part of the waste heat energy coming from both the power-to-hydrogen and water-to-hydrogen processes. Indeed, the fuel cell analyzed is able to operate as SOE system producing hydrogen when electric power is available from the grid. The design optimization of the comprehensive layout, however, is nontrivial because there exist many design variables and practical considerations. The model, based on data available from previous experimental test campaigns, is used first to complete the design of the layout and to properly size the system’s components. A simplified approach to implement and study the Steady State (SS) behaviour of the whole system is then
proposed.
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10:50
20 mins
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Extrapolability and limitations of a semi-empirical model for characterizing volumetric expanders
Olivier Dumont, Rémi Dickes, Vincent Lemort
Abstract: Many different modelling approaches can be used to simulate the performance of expansion devices in ORC power systems. A decade ago, researchers proposed a semi-empirical modelling method that can be used to generally characterize volumetric expanders (e.g. scroll, screw, piston or vane machines) [1]. The modelling approach (as depicted in Figure 1) relies on a limited number of physically meaningful equations which decompose the expansion process into six consecutive steps. Besides of under- and over-expansion losses (due to the fixed built-in volumetric ratio of the machine), the model can account for pressure drops at the inlet and outlet ports, internal leakages, mechanical losses and heat losses to the environment. The main interests of the modelling approach are a low CPU time, extrapolability through the use of physical laws and the same formalism for all the volumetric expander technologies (scroll, screw, piston, vane…). The semi-empirical model relies on different parameters that must be properly tuned accordingly to experimental (or manufacturer) data. In practice, however, the reference database used for the model calibration (e.g. the measurements gathered on a test rig) does not necessarily cover the entire range of conditions onto which the model will be evaluated. The capability of the semi-empirical model to behave well in extrapolated conditions has therefore to be assessed. In this work, a detailed analysis of the extrapolation performance of the semi-empirical model is conducted. More specifically, the semi-empirical model behavior is analyzed after being calibrated with different ranges of reference conditions. A study of the minimum reference dataset to ensure a decent modelling accuracy is proposed. Finally, the influence of the parameters guess values and the optimization algorithm on the model calibration is assessed.
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11:10
20 mins
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Experimental investigation on multi-vane expander operating conditions in domestic CHP ORC system
Piotr Kolasiński, Przemysław Błasiak, Józef Rak
Abstract: Small- and micro-power ORC systems designed for domestic use by prosumers should be cheap and reliable. The main part of the ORC system cost is the price of heat exchangers and the expander. Multi-vane expanders are positive displacement volumetric machines which are nowadays considered for application in micro-power domestic ORC systems. The multi-vane expander design is very simple, which translates into low production costs. Compared to the other types of volumetric machines and micro turbines, which are adopted in micro-power domestic ORC systems, multi-vane machines feature a lower gas flow capacity, lower expansion ratios, and an advantageous ratio of the power output to the external dimensions. These machines are insensitive to the negative influence of the gas-liquid mixture expansion. Moreover, the multi-vane expander can be easily hermetically sealed, which is one of the key issues in the ORC system design. Multi-vane expanders feature power outputs of several hundred W to approximately 5 kW. Maximum gas pressure on the inlet to multi-vane expander reaches approximately 10 bar. The issues concerning the application of multi-vane expanders in such systems are innovative and not fully scientifically described. The solution of these problems requires comprehensive study and experimental analysis.
This paper reports the results of experimental investigations carried out on domestic CHP ORC system adopting multi-vane expander under varied operating conditions. The experimental results showed that the expander indicated work varies in the range of 0.96—4.18 kJ/kg while its internal efficiency varies in the range of 17.2—58.3 % depending on the experimental conditions. Moreover, the results of numerical investigations on multi-vane expander operating conditions are presented. The numerical model of the expander was validated using the data obtained from the experiment and analyses were performed in ANSYS CFX software. The results proved that multi-vane expanders are suitable for domestic ORC systems and are a promising alternative to the other types of volumetric expanders used in such systems.
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11:30
20 mins
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Experimental investigation into an ORC-based low-grade energy recovery system equipped with sliding-vane expander using hot oil from an air compressor as thermal source
Stefano Murgia, Gianluca Valenti, Daniele Colletta, Ida Costanzo, Giulio Contaldi
Abstract: Compressed air production is an energy–intensive sector, thus compressor manufacturers are constantly looking for enhancing the efficiency, by acting on several technological aspects. In an air compressor, about 80-90% of the input electric power used is wasted into the environment through the oil circuit, continuously cooled by ambient air blown via a fan. An interesting way to optimize the overall system efficiency is to exploit this waste heat to produce electrical power. Organic Rankine Cycles (ORCs) are a suitable solution for recovering energy from low-grade heat source. In this paper, an experimental analysis of two low-grade ORC-based recovery systems is presented. The thermal source is the hot lubricant of a mid-size air compressor, while the thermal sink is tap water. The first system is tested in a simple cycle configuration while the second in a recuperative one. An extensive experimental campaign is carried out on a test bench composed by sliding-vane expander, pump and plate heat exchangers. The expander differs in terms of geometry and aspect ratio between the two cycles. R236fa is used as working fluid in both the systems. The expander operating conditions are deeply investigated by using piezoelectric pressure transducers to determine the expansion indicated diagram and the expander mechanical efficiency. Experimental results show that the recuperative cycle has a better performance, in terms of cycle efficiency and expander mechanical efficiency, compared with the simple cycle. For this configuration, two off-design conditions are investigated, acting on the pump rotational speed. Finally, an exergy analysis is conducted, in order to evaluate the irreversible losses produced by each component.
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11:50
20 mins
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Two-phase chamber modeling of a twin-screw expander for trilateral flash cycle applications
Giuseppe Bianchi, Stuart Kennedy, Obadah Zaher, Savvas Tassou, Jeremy Miller, Hussam Jouhara
Abstract: Low temperature (<100°C) streams have the largest share of waste heat recovery potential and may represent an attractive opportunity for a sustainable economy. Among the bottoming thermodynamic approaches that have been proposed to convert this waste heat into electricity, the Trilateral Flash Cycle (TFC) proved to be theoretically capable to recover more heat from a low-temperature single-phase heat source than any simple Rankine cycle. However, the commercialization of TFC recovery units has been so far prevented by the lack of an expander technology that should efficiently operate with high mass flow rates of a two-phase flashing flow. A promising candidate to tackle these challenges is the twin-screw technology thanks to its positive displacement nature and the capability to run at high revolution speeds without remarkable efficiency drops.
In the current research work, a twin-screw expander has been modeled in the commercial software GT-SUITE. The modeling activity resulted in a two-phase chamber model based on the coupling of the integral form of the conservation equations and the REFPROP library to calculate the thermophysical properties of the liquid-vapor mixture of the R245fa working fluid at each time step. Using pre-processed geometrical data, the model includes a detailed breakdown of the leakage paths and allows to retrieve key information for a future optimization of the machine such as the indicator diagram and the quality-angle diagram. Parametric analyses were eventually carried out to assess the expander behavior at different operating conditions, namely manometric expansion ratio, revolution speed and inlet quality.
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12:10
20 mins
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Unsteady leakage flow through axial clearance of an ORC scroll expander
Panpan Song, Weilin Zhuge, Yangjun Zhang, Lei Zhang
Abstract: As the scroll-type expander (STE) becomes an attractive expansion device for small-scale organic Rankine systems, performance enhancement of STE could achieve a further improvement of the energy efficiency of ORC system. The gas leakages through the axial and radial clearances have a great effect on the overall performance of STE. The present work proposed a three dimensional numerical technique available for modeling the radial leakage flows through the axial clearances at the tip and root of scroll wrap. The radial leakage flow patterns of both axial clearances are investigated. Asymmetrical distribution of the radial leakage flow through the axial clearances at both sides of working chambers are revealed. Leakage flow difference between the top and bottom axial leakage clearances is comparatively discussed. The effects of radial leakage on the flow fields in the working chambers are also analyzed. The results show the radial leakage flows occur at the axial clearances of both the scroll segments between asymmetrical working chambers and those between symmetrical working chambers. Radial leakage flows through the top and bottom axial clearances are approximately symmetrical about the meshing line. Pressure distribution in the axial clearance passage is uneven along the scroll involute direction, especially in the axial clearance passages between asymmetric chambers. Pressure distortion occurs in the downstream of the axial clearance passage between asymmetric working chambers nearby the mesh point of the scroll wraps. Radial leakage flow leads to the secondary vortex flows and non-uniform pressure distributions in the working chambers.
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