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14:00   Session 18: Advanced architectures
Chair: Steven Lecompte
20 mins
Jianyong Wang, Jiangfeng Wang, Yiping Dai
Abstract: In this work, an ORC-OFC combined power generation system is proposed to improve the energy conversion efficiency for low grade heat sources. Mathematical models of the system are established to simulate the system under steady-state conditions. Effects of two key thermodynamic parameters including evaporation pressure and flash pressure on the system performance are examined. The analysis indicates that there exists an optimal evaporation pressure and an optimal flash pressure that yield the maximal net power output and system exergy efficiency for the proposed system. Parameter optimizations by genetic algorithm are conducted for ORC, OFC and the proposed system under same heat source and restrictions, and the optimization results of the three systems are compared, showing that the ORC-OFC combined power generation system, with maximal exergy efficiency reaching 16.70%, performs better than the ORC and the OFC.
20 mins
Steven Lecompte, Martijn van den Broek, Michel De Paepe
Abstract: The subcritical ORC (SCORC), sometimes with addition of a recuperator, is the de facto state of the art technology in the current market. However architectural changes and operational modifications have the potential to improve the base system. The ORC architectures investigated in this work are: the transcritical ORC (TCORC), the triangular cycle (TLC) and the partial evaporation ORC (PEORC). Assessing the potential of these cycles is a challenging topic and is brought down to two steps. First, the expected thermodynamic improvement is quantified by optimizing the second law efficiency. Secondly, the influences of technical constraints concerning volumetric expanders are investigated. In the first step, simple regression models are formulated based on an extensive set of boundary conditions. In addition a subset of environmentally friendly working fluids is separately analysed. In the second step, two cases are investigated with the help of a multi-objective optimization technique. The results of this optimization are compared with the first step. As such the effect of each design decision is quantified and analysed, making the results of this work especially interesting for manufacturers of ORC systems.
20 mins
Christoph Kirmse, Aly Taleb, Oyeniyi Oyewunmi, Andrew Haslam, Christos Markides
Abstract: The Up-THERM engine is a novel vapour-phase heat engine with a single moving part (a vertical solid piston) that relies on the phase change of a suitable working fluid to produce sustained thermodynamic oscillations and reciprocating displacement, which can be converted to useful work. In this paper a model of the Up-THERM engine is developed, based on lumped dynamic descriptions of each engine sub-component and by using electrical analogies founded on previously developed thermoacoustic principles [1,2]. This is extended here to include a description of phase change analogous to that used in the model of a similar thermofluidic oscillator known as the Non-Inertive-Feedback Thermofluidic Engine (NIFTE) [3,4] and also to include non-linear descriptions of important sub-components [5]. The predicted efficiency and power output from the Up-THERM model are compared with those of a sub-critical ORC engine, obtained by using a previously developed mathematical model [6]. Both systems are optimized for operation between the same heat sources and sinks, and using the same working fluids; common organic working fluids such as refrigerants and hydrocarbons (and their mixtures) are considered. In some cases, including mixtures, empirical fluid-property data are unavailable; here we employ the SAFT-VR Mie equation of state [6]. Preliminary results indicate that the Up-THERM engine underperforms its ORC counterpart in terms of efficiency and power output. However, owing to its mode of operation and lack of moving parts, the Up-THERM engine does offer a much simpler and more cost-efficient solution than an ORC engine, and is therefore a competitive alternative in terms of cost of electricity or power per unit cost in low-power applications, especially for remote, off-grid settings or those in developing countries where minimising upfront costs is crucial. REFERENCES [1] S. Backhaus, G.W. Swift, “A thermoacoustic-Stirling heat engine: Detailed study”, J Acoust. Soc. Am., v. 107, pp. 3146-3166, 2000. [2] B.J. Huang, M.D. Chuang, “System design of orifice pulse-tube refrigerator using linear flow network analysis”, Cryog., v. 36, pp. 889-902, 1996. [3] C.N. Markides, T.C.B. Smith, “A dynamic model for the efficiency optimization of an oscillatory low grade heat engine”, Energy, v. 36, 6967-6980, 2011. [4] R. Solanki, A. Galindo, C.N. Markides, “The role of heat exchange on the behaviour of an oscillatory two-phase low-grade heat engine”, Appl. Therm. Eng., v. 53, pp. 177-187, 2013. [5] C.N. Markides, A. Osuolale, R. Solanki, G.-B.V. Stan, “Nonlinear heat transfer processes in a two-phase thermofluidic oscillator”, Applied Energy, v. 104, pp. 958-977, 2013. [6] O.A. Oyewunmi, A.I. Taleb, A.J. Haslam and C.N. Markides, “On the use of SAFT-VR Mie for assessing fluorocarbon working-fluid mixtures in organic Rankine cycles for Waste-Heat Recovery”, J. Eng. Gas Turb. Power, revision under review, (2015).
20 mins
Frithjof H. Dubberke, Klaus-Peter Priebe, Jadran Vrabec, Maximilian Rödder, Matthias Neef
Abstract: Employing a zeotropic mixture as a working fluid in ORC allows for an exergetically favourable heat transfer to the evaporator due to the temperature glide. However, during heat discharge via the condenser, the temperature glide becomes a disadvantage. Therefore, a cascaded combination of a two-staged ORC, where the high temperature (HT) cycle is operated with a zeotropic mixture and the low temperature (LT) cycle is operated with a pure fluid in supercritical mode, facilitates both favourable heat uptake from the source as well as heat discharge to the environment [1]. As a test rig for according two-stage cycle innovations, an electrically heated CORC cycle was designed and commissioned at the University of Paderborn. To achieve a high efficiency in each cycle, the design strongly depends on the temperature level of the heat source. The integration of four electrical heating rods as a primary heat source into the HT cycle – each with 50 kW and one of them adjustable – the design enables for the specification of different temperature levels and the LT cycle is supplied with the unused thermal energy of the HT cycle. After successful commissioning of the two-stage CORC, experimental results are used to evaluate cycle and component performance in comparison to the intended design. For this purpose, a detailed cycle simulation is performed using EBSILON®Professional, which can be fed with the operating parameters. The aim is to complement the flexible test rig with a suitable thermodynamic model, which allows for the study of cycle variations, such as fluid changes, hardware design improvements, etc. First results on modeling and experimental validation are presented for a combination of two pure fluids that exemplify heat the integration between the HT and LT cycles. With a validated simulation tool based on energy and mass balances as well as suitable equations of state, the optimization of individual components of the CORC-test rig, such as heat exchangers, pumps, condensators, turbines, as well as working fluids can be carried out rapidly and at low cost. The long-term goal of the present project is to put a two-stage CORC system into practice. REFERENCES [1] B. Liu, P. Rivière, C. Coquelet, R. Gicquel, F. David, “Investigation of a two stage Rankine cycle for electric power plants”, Applied Energy, 100 (2012) 285–294.
20 mins
Hiroshi Kanno, Yusuke Hasegawa, Isao Hayase, Naoki Shikazono
Abstract: ABSTRACT Trilateral cycle is one of heat cycles in which working fluid is pressurized and kept as a single liquid phase during the heating process [1]. The exergy loss can be drastically reduced because of favorable temperature profile matching between the heat source and the working fluid. In the expansion process, working fluid is flashed and becomes liquid-vapor two-phase. This two-phase expander is one of the key components to realize the trilateral cycle system. In the present study, visualization and measurement of two-phase adiabatic expansion in a cylinder for trilateral cycle are carried out. Experimental setup with piston and cylinder which mimics reciprocating expander is constructed and boiling phenomenon is visualized. Working fluids are water and ethanol, and initial temperatures are 100 and 80 ℃ in this study. The piston and cylinder are made of polycarbonate with diameter of Dp = 38, 44 and 55 mm. The piston velocity vp is ranged from 1 to 300 mm/s and inner pressure is measured by the pressure sensor embedded in the piston. Output work is calculated from the P - V diagram. In addition, filter-type sintered metal is fixed on the bottom of the cylinder to enhance boiling. The average pore diameter is ranged from 5 to 75 μm and the effect of initial bubbles on boiling is evaluated. The difference between measured and quasi-static pressures becomes larger and the adiabatic efficiency decreases as piston velocity is increased. When using water, adiabatic efficiency is about 83 % for Dp = 55 mm and vp = 300 mm/s, while the adiabatic efficiency is about 78 % when using ethanol for the same condition. With sintered metal on the bottom of the cylinder, the deterioration of adiabatic efficiency becomes moderate for both working fluids. The adiabatic efficiency for pore diameter 20 μm is about 87 % for water and about 92 % for ethanol when Dp = 55 mm and vp = 300 mm/s. From these results, the initial bubbles captured in the porous metal are effective to improve the efficiency of the expander for trilateral cycle. In an actual reciprocating expander, heat transfer from the working fluid to the setup wall may reduce the adiabatic efficiency while the initial bubbles in the cylinder can improve the boiling. Therefore, a new experiment reproducing the intake and exhaust process is conducted. The feasibility of the two-phase expander is evaluated through this experiment. REFERENCES [1] Smith I.K., Development of the trilateral flash cycle system. 1. Fundamental considerations., Journal of Power and Energy 207 (1993), pp. 179-194.