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08:40   Session 8: Scroll expanders
Chair: Eckhard Groll
08:40
20 mins 		System-search EXPERIMENTAL INVESTIGATION OF A SMALL-SCALE TWO STAGE ORGANIC RANKINE CYCLE ENGINE OPERATING AT LOW TEMPERATURE
Erika Ntavou, George Kosmadakis, Dimitris Manolakos, George Papadakis, Dimitris Papantonis
Abstract: A prototype two-stage heat-to-power engine based on the Organic Rankine Cycle (ORC) has been developed for operation at a wide thermal load input range, coming from variable thermal sources, such as evacuated tube solar collectors [1]. The system is used to produce electrical energy through the expansion of a refrigerant (R245fa) in two scroll expanders which are connected in series [2]. The intense fluctuation of the temperature and heat input dictates the use of a two-stage engine, for flexible and efficient operation even at low thermal load, thus these expansion machines can operate within a narrow pressure ratio range, showing high expansion efficiency up to 70%. When operating at high heat input both expanders operate, while for low heat input, the first expander is completely by-passed. The net capacity of the ORC engine is 10 kWe, when supplied with 100 kW of heat at a temperature of 140 °C. This engine has been tested in an appropriate test-rig at the laboratory, using an electrical heater. The power produced by the two hermetic scroll expanders is driven to an electric panel through inverters. The present study examines the ORC engine testing, when the heat transfer fluid is water with temperature of 95 °C. The engine is tested under both single- and two-stage configuration. The measurements have shown that even at such low-temperature the thermal efficiency is adequate (up to 7% for single- and two- stage operation), and that the second expander can operate with increased isentropic efficiency up to 66%, while for the first one this value is much lower, due to its under-expansion (pressure ratio 1.7-2.7 for the first and 3-7 for the second expander). The maximum thermal efficiency is observed for low pump speed, while the highest power production of 3.5 kW was noticed at single-stage operation, insignificantly higher than the 3.3 kW produced at the two-stage operation. Acknowledgement: The present work is conducted within the framework of the project with contract No. 09SYN-32-982, partly funded by the Greek General Secretary of Research and Technology (GSRT). REFERENCES [1] G. Kosmadakis, D. Manolakos, E. Davou and G. Papadakis, “Implementation of a two-stage Organic Rankine Cycle using scroll expanders, operating under variable heat input”, Presented at the 2nd Int. Seminar on ORC Power Systems (ASME-ORC2013), Rotterdam, The Netherlands, 7-8 October 2013. [2] V.M. Nguyen, P.S. Doherty and S.B. Riffat, “Development of a prototype low-temperature Rankine cycle electricity generation system”, Appl. Therm. Eng., Vol. 21, no. 2, pp. 169–181, (2001).
09:00
20 mins 		System-search EXPERIMENTAL STUDY ON ORGANIC RANKINE CYCLE ADOPTING DUAL-EXPANDERS WITH DIFFERENT CAPACITIES IN PARALLEL
Eunkoo Yun, Dokyun Kim, Sang Youl Yoon, Kyung Chun Kim
Abstract: This study aims to experimentally investigate the possibility of an organic Rankine cycle (ORC) adopting dual-expanders with different capacities in parallel. The proposed ORC system was designed to provide competitive advantages over basic ORC system having single expander in case of applications with large waste heat variation. The system could have three operating modes according to the operated expanders; Single mode1, Single mode2, and Dual mode. In Single mode, only one expander (expander1 or expander2) is operated, and in Dual mode both expanders work. Hence, each operating mode has each different heat recovery capacities, and each operating mode has its own design point which can achieve the maximum performance. Therefore, the three heat recovery capacities and the three design points in single ORC system allow more effective heat recovery in applications with large waste heat variations. Two scroll expanders, which have 1 kW and 5 kW class capacities, were utilized in an ORC loop for the experimental test. The ORC test bench consisted of a 100kW heater as the heat source, an ORC loop, and an air-cooled chiller as the heat sink. R245fa was used as working fluid. In order to evaluate the performance characteristics of the system, efficiencies and shaft powers for each operating mode were measured under various evaporative heat transfer conditions. The appropriateness of utilizing proposed ORC system and optimal operation mode which can produce higher power output will be discussed with experimental results.
09:20
20 mins 		System-search THERMO-FLUIDIC AND MECHANICAL LOSSES IN A SCROLL EXPANDER FOR AN R134a ORGANIC RANKINE CYCLE
Karthik G.M., Pardeep Garg, Vinod Srinivasan, Pramod Kumar
Abstract: Unmatched demand and supply of energy keeps motivating the engineering community to efficiently realize even the marginal potential of low temperature heat (~150 °C) which is available in plenty in form of renewable resources such as geothermal or via a low concentration solar field. Further, these sources are distributed in space ranging from kWth to MWth making the scalability a key feature of the technology to be chosen to convert this heat into electricity. Organic Rankine cycle (ORC) is a promising technology which is scalable and can efficiently generate electricity in the above mentioned range. However, the choice of expander becomes crucial at power scales below 100 kWe as the conventional turbine expanders tend to have high rotational speeds (>104 rpm) and suffer from low isentropic efficiencies. Positive displacement device such as a scroll expander is a possibility in the range of 1 to 100 kWe. Existing literature on scroll covers thermal-fluidic losses by using a lumped model to represent the mechanical losses (such as friction between solid components due to relative motion). However, these models do not represent the true mechanical losses of a scroll machine which are dependent on a number of parameters ranging from the basic geometry to thermodynamic interactions, which need to be represented in the model.. The present paper tries to establish the need for optimizing the scroll geometry by simultaneously minimizing both thermo-fluidic and mechanical losses. The methodology is based on a three step approach. First the scroll geometry is generated based on the prescribed operating conditions, next the scroll analyzed using a thermodynamic model. Herein, thermo-fluidic losses due to supply pressure drop, flank and radial leakage are calculated. Finally, the pressure variations are fed to the mechanical losses model wherein the losses at journal bearing, thrust bearing and Oldham coupling are calculated using force and moment analysis. A case study using above approach is performed for an ORC with R134a as a working fluid for various expander inlet temperatures ranging from 100 to 175 °C and a condenser temperature of 45 °C. Condenser pressure of 12 bar results in high densities at the expander exhaust which in turn makes the volumetric flow rates low even at a power scale of 100 kWe. Thus, a 100 kWe R134a scroll is justified despite being traditionally limited to power levels below 10 kWe. Further, for the given operating conditions, scroll involute base circle radius and scroll height are selected as independent variables are optimized for maximum isentropic efficiency. Thermo-fluidic and mechanical losses for the optimized geometric configuration are found to be ~10 and ~15 kW respectively in a 100 kWe scroll, thus establishing the importance of the latter. The procedure described in this paper is a universal design tool applicable to any working fluid or operating conditions for arriving at an optimum scroll geometry for ORC applications.
09:40
20 mins 		System-search A COMPARISON BETWEEN TWO DIFFERENT CFD APPROACHES OF A REAL SCROLL EXPANDER FOR MICRO-ORC APPLICATIONS
Alessio Suman, Carlo Buratto, Nicola Aldi, Michele Pinelli, Pier Ruggero Spina, Mirko Morini
Abstract: In this paper, CFD analyses of real scroll compressor are developed for a two-dimensional real geometry of a scroll compressor obtained by means of a Reverse Engineering (RE) of a commercial scroll compressor to be used as an expander in a microORC system. The analyses are carried out by means of CFD numerical simulations involving two type of approaches: (i) Dynamic Mesh (DM) and (ii) Chimera Strategy (CS). The particularity of these types of transient analyses consists in being able to reproduce the real operation of the machine through a sequence of relative positions between fixed and moving spirals. The results discuss the difference between the two numerical approaches in terms of ability to represent the actual flow features in a positive displacement machine. Analysis of the performance in terms of pressure and mass flow rate profiles, volumetric efficiency and shaft torque are reported.