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11:20   System design and optimization IV
Chair: Prof. Paola Bombarda
20 mins
Hiroshi Kanno, Yusuke Hasegawa, Isao Hayase, Naoki Shikazono
Abstract: In the present study, thermodynamic performances of Rankine, trilateral and supercritical cycles with reciprocating expander are assessed. In the trilateral cycle, heat is transferred from the heat source to the single phase working fluid. Thus, the exergy loss of trilateral cycle can be drastically reduced because of favorable temperature profile matching between the heat source and the working fluid. Therefore, trilateral cycle can theoretically achieve highest exergy recovery from the heat source with finite heat capacity. In the expansion process, high pressure saturated liquid is flashed and expanded in the two-phase expander. The quality of working fluid in the expander becomes low, which results in very large expansion ratio of the expander. In this regard, reciprocating expander is suitable for the trilateral cycle. Physical properties of the working fluids are obtained using REFPROP 9.0. Heat sources assumed in this study are hot water and exhaust gas from Diesel engine. The temperature of hot water is 80 ℃ and the temperature of exhaust gas is ranged from 200 to 500 ℃. In the cycle simulation, operating pressure for each cycle with different working fluids is optimized so that to give highest available energy efficiency (availability efficiency, hereafter) for given heat source temperature, pinch point temperature difference and displacement of reciprocating expander (L). In addition, maximum operating pressure (MPa) and rotation velocity of expander (rpm) are evaluated as the criteria to check the feasibility of the cycles. We also investigated the effects of expander’s dead volume and volumetric expansion ratio. When R143a is used as the working fluid and volumetric expansion ratio exceeds 50, the availability efficiency of trilateral cycle becomes about 50 %, which is nearly 56 % larger than that of Rankine cycle for 80 ℃ case. For the 300 ℃ case, water is the optimal working fluid for the trilateral cycle. When volumetric expansion ratio is 400, the availability efficiency of trilateral cycle reaches about 70 %, which is nearly 54 % larger than that of Rankine cycle. However, availability efficiency of trilateral cycle is lower than that of Rankine cycle when volumetric expansion ratio is below 50. To realize the trilateral cycle system, an efficient two-phase expander is the key technology. We are now conducting the visualization experiment and adiabatic efficiency measurement of the two-phase expander. Output work of expander is obtained from p-v diagram. The feasibility of two-phase expander will be evaluated considering the effect of rotation velocity.
20 mins
Hsiao-Wei D. Chiang, Sung Wei Hsu, Chih-Yung Huang
Abstract: Among low-medium heat energy converting to power technologies, transcritical ORC systems have demonstrated to have better thermal efficiency and higher heat recovery rate than subcritical ORC’s. This paper starts with working fluids of R125, R218, R32, R134a, R227ea, R152a, R236fa, R236ea, isobutane, and R245fa, for heating fluid temperature levels of 125C, 150C, 175C, 200C, and 225C. Using the maximum power output of a subcritical cycle as baseline, the effects of different expander inlet temperatures on the transcritical system thermal efficiency, heat recovery rate, and net power output were investigated. Using our performance analysis, transcritical cycle performance behaviors can be studied. As demonstrated, for those working fluids with low critical temperatures, the heat recovery rate is relatively high, and the variation of expander inlet temperature has little effects on the heat recovery. However, for working fluids with high critical temperatures, the heat recovery rate is relatively low, and decreases sharply with elevating expander inlet temperature. It was demonstrated that the temperature difference between the inlet temperature of the heat source stream and the temperature of the working fluid at expander inlet needs to be designed in the range of 30~35C for maximum power output. Consequently, the high critical temperature working fluid would generate higher power output for a heat source with higher temperature level. The results demonstrate that the candidate working fluids for a transcritical cycle can be R227ea, R134a, R236fa, and R245fa for inlet temperatures of the heat source stream of 125C, 150C, 175C, and 200C, respectively. For the 150C inlet temperature of the heat source stream, sensitivity analyses on the isentropic efficiencies of the pump and expander were performed for subcritical and transcritical cycles. The results demonstrate that the pump efficiency has larger effects on the transcritical cycle than the subcritical one, since the transcritical pumping power accounts for a major portion of the power consumption than that of a subcritical cycle. For the increasing expander efficiency effects, both the subcritical and transcritical power outputs are increased similarly. As a result, when the pump and expander isentropic efficiencies are over 80%, the transcritical ORC cycles are demonstrating 20% and more power output than the subcritical cycles.
20 mins
Davide Bonalumi, Marco Astolfi, Roberta Roberto, Matteo Caldera, Matteo Carmelo Romano, Davide Maria Turi, Paolo Silva, Antonio Giuffrida, Costante Invernizzi, Ennio Macchi
Abstract: This work presents innovative cogenerative Organic Rankine Cycles operating with fluids at high temperature (up to 400°C) coupled with a biomass-fired molten salts boiler, that allow to obtain an electrical efficiency over the threshold of 20%. Two fluids have been considered in this study, namely PP9 and TiCl4. The power plants are modeled with Aspen Plus. Both subcritical and supercritical cycles have been investigated. The cycle efficiencies obtained with PP9 is almost of 22%, while with TiCl4 is over 23%.
20 mins
Emeline Georges, Ian Bell, Brandon Woodland
Abstract: Brazed plate heat exchangers present different characteristics that make them suitable for use in ORC systems. The main advantages are the minimal risk of internal leakage, the compact design, efficient heat transfers in both single-phase and two-phase applications, controllable pressure drops and easy maintenance. They are commonly used as the evaporator, condenser or regenerator in Organic Rankine Cycles (ORC). The present paper presents a unique solution technique implemented to solve moving boundaries brazed plate heat exchanger models (Bell, 2011). This solution technique allows one model to solve for the full range of steady state operating conditions possible in the heat exchanger. This includes the most complex case involving phase-change in both fluids. The model is validated with a set of points obtained experimentally for an ORC working with R134a as working fluid. Agreement within 3% error was found when comparing the heat transfer rate at the condenser estimated by the model to the one obtained experimentally.