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Session: Parallel Session: Systems Design, Optimization and Applications II
Session starts: Friday 23 September, 09:00
Presentation starts: 09:20
Room: Senaatszaal

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George Kosmadakis ()
Dimitris Manolakos ()
George Papadakis ()

Abstract:
A small-scale, low-temperature Organic Rankine Cycle (ORC) is developed at the moment, within the framework of the project 09SYN-32-982, partly financed by the greek government. The heat input will be provided from evacuated tube solar collectors (maximum heat input: 100 kWth at around 130 °C), while the power produced will feed a Reverse Osmosis unit for desalinating seawater. Due to the seasonal, and mainly daily, solar radiation variation, critical issues should be addressed, in order to improve the performance of the system. Some important issues involve the control of the pump and the expander, and the fundamental design principles of the ORC (e.g. heat exchanger sizing, evaporation temperature etc.). One design principle that has not been extensively investigated so far is the number of expansion stages of the organic fluid, where two major configurations are identified for small-scale ORCs (except for the single-expansion). The first configuration concerns the use of a cascade ORC (two circuits), where the condensation heat of the upper-stage (with higher temperature) is actually the evaporation heat of the lower-stage [1]. The second configuration consists of a single circuit, with two expanders connected in-series, where the first expander is by-passed at low heat input [2]. Both these configurations can improve the performance of the system at part load in comparison to a single-expansion ORC, if they are properly controlled. The first goal of this study is to theoretically investigate the performance of these two alternative configurations under intermittent heat input (and feed temperature). For both configurations the same parameters have been used (e.g. organic fluid’s subcooling/superheating, expanders’ efficiency etc.) and the organic fluids have been carefully selected (HFC-245fa/HFC-134a for the upper/lower-stage respectively [3] and HFC-245fa for the two in-series expanders ORC). The thermal efficiency is calculated for each configuration and for the whole range of heat input (0-100%). An interesting observation is the increased efficiency of the ORC with two in-series expanders for the entire range of heat supply. Especially at low heat input, the efficiency gain is significant (4% instead of 2%), while it is limited at the maximum heat input (10.4% instead of 9.8%). Additionally, in the cascade ORC design the installation cost increases, due to the use of additional components and the higher complexity of the control unit of the system. The final design chosen is the one with two in-series expanders. Next steps will be the development of a smart control logic, the design of the system and at a later stage the construction of a prototype unit. REFERENCES [1] M. Kane, D. Larrain, D. Favrat and Y. Allani, “Small hybrid solar power system”, Energy, Vol. 28, pp. 14271442, (2003). [2] S. Quoilin, M. Orosz, H. Hemond and V. Lemort, “Performance and design optimization of a low-cost solar organic Rankine cycle for remote power generation”, Solar Energy, Vol. 85, pp. 955–966, (2011). [3] G. Kosmadakis, D. Manolakos, S. Kyritsis and G. Papadakis, “Design of an autonomous, two stage solar organic Rankine cycle system for reverse osmosis desalination”, Desalin Water Treat, Vol. 1, pp. 114–127, (2009).