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Fig. 10 shows a behavior where both the η and the wne at all four temperatures increase. There are several reasons for this: noting firstly, that the input temperature and the discharge pressure of the turbine are fixed; second, we assume that the work produced by this device is that given by equation (2); third, if the Mollier diagram of this fluid is analyzed, it can be seen that, for a constant temperature, when the pressure increases, Δh rises, and Wt along with it, which ensures the increase of both η and the wne of the cycle. It is interesting to note how, for a same inlet pressure to the turbine P1, there is no appreciable increase in η with increasing temperature in the case of the basic cycle. However, for the cycle with IHX, the difference is more representative, both in the same cycle with IHX at different temperatures, as compared to the basic cycle at the same T1, except in the latter case, for temperatures of 60oC and 90oC, in which it is not very noticeable the inclusion of an IHX. This occurs because as was mentioned in the preceding sections, in the case of the basic cycle, the overheating of the fluid does not cause an appreciable increase in η, except for pressures close to the maximum allowed by the T1 studied (i.e. to higher pressure ratios), where this effect starts to be noticeable. As an example for illustrating this, considering an inlet pressure to the turbine of 20 bar, an increase of the inlet temperature to the turbine from 90oC to 150oC, caused a raise in the efficiency of 2.5% for the case of cycle with IHX. For this same pressure and inlet temperature to the turbine of 90oC, the inclusion of the IHX caused an increase in the efficiency of 0.8%, approximately, with respect to the basic cycle. On the other hand, for the same inlet pressure to the turbine P1, there is an appreciable increase in η with increasing temperature in the case of a cycle with IHX due to the recovery of energy. As a consequence, the amount of energy required from the heat source decreases and the overall cycle efficiency increases, as can be seen in the same eq. (1). 5. Conclusions Based on the simulations carried out, the system’s efficiency proposed is a weak function of temperature, because overheating the inlet fluid to the turbine does not cause a significant change in the overall efficiency of the cycle. However, when the pressure ratio in the turbine increases (obviously limited by the temperature of the heat source), much larger values of efficiency are obtained (≈5% more as maximum for the same temperature T1) and also, as the inlet temperature to the turbine rises, the efficiency increases more sharply (≈1% more as maximum for the same pressure ratio P1/P2). Furthermore, adding an internal heat exchanger to the cycle increases significantly the efficiency values obtained (≈3% more as maximum). Moreover, considering the energy analysis carried out, it can be concluded that the ORC with R134a as working fluid is suitable for the production of useful energy using low enthalpy heat, as it is possible to operate in relatively low temperature ranges. In addition, many of the aspects taken into account nowadays in these processes, such as environmental issues, safety and efficient and rational use of energy have been satisfied. Acknowledgements Authors acknowledge all the invaluable comments by Eng. Cecilia Sanz M. from CARTIF. Fredy Vélez. thanks the scholarship awarded by the “Programa Iberoamericano de Ciencia y Tecnología para el Desarrollo”, CYTED, CARTIF Technological Center and University of Valladolid in order to carry out his doctoral thesis, on which this paper is based. References [1] Realpe, A., Diaz-Granados, J.A. and Acevedo, M.T., Electricity generation and wind potential assessment in regions of Colombia. Dyna, vol 171, pp, 116-122, 2012. [2] Vélez, F., Segovia, J., Martín, M.C., Antolín, G., Chejne, F. and Quijano, A., A technical, economical and market review of organic Rankine cycles for the conversion of low-grade heat for power generation. Renewable & Sustainable Energy Reviews, vol. 16, pp. 4175–4189, 2012. [3] Vélez, F., Chejne, F., Antolín, G. and Quijano, A., Theoretical analysis of a transcritical power cycle for power generation from waste energy at low temperature heat source. Energy Conversion and Management, vol. 60, pp. 188–195, 2012. [4] Saleh, B., Koglbauer, G., Wendland, M. and Fischer, J., Working fluids for low temperature organic Rankine cycles. Energy, Vol. 32, pp. 1210– 1221, 2007. [5] Quolin, S., Declaye, S., Tchange, B.F. and Lemort, V., Thermo- Economic optimization of waste heat recovery organic Rankine cycles. Applied Thermal Engineering, vol. 31, pp. 2885-2893, 2011. [6] Tchange, B.F., Papadakis, G., Lambrinos, G. and Frangoudakis, A., Fluid selection for a low-temperature solar organic Rankine cycle. Applied Thermal Engineering, vol. 29, pp. 2468–2476, 2009. [7] Quolin, S., Aumann, R., Grill, A., Schuster, A., Lemort, V. and Spliethoff, H., Dynamic modeling and optimal control strategy of waste heat recovery organic Rankine cycles. Applied Energy, vol. 88, pp. 2183– 2190, 2011. [8] Hung, T.C., Shal, T.Y. and Wang, S.K., A review of organic Rankine cycles (ORC`s) for the recovery of low-grade waste heat. Energy, vol. 22 (7), pp. 661-667, 1997. [9] Chen, H., Goswami, Y. and Stefanakos, E., A review of thermodynamic cycles and working fluids for the conversion of low-grade heat. Renewable & Sustainable Energy Reviews, vol. 14, pp.3059–3067, 2010. [10] Vélez, F., Segovia, J., Martín, M.C., Antolín, G., Chejne, F. and Quijano, A., Comparative study of working fluids for a Rankine cycle operating at low temperature. Fuel Processing Technology, vol. 103, pp.71–77, 2012. [11] U.S. Environmental Protection Agency. Class I Ozone Depleting Substances. [Online]. [date of reference March 11th of 2013] Available at: www.epa.gov/ozone/science/ods/classone.html. [12] Roy, J.P., Mishra, M.K. and Misra, A., Parametric optimization and performance analysis of a waste heat recovery system using organic Rankine cycle. Energy, vol. 35, pp. 5049-5062, 2010. [13] Roy, J.P., Mishra, M.K. and Misra, A., Parametric optimization and performance analysis of a regenerative organic Rankine cycle using low- grade waste heat for power generation. International Journal of Green Energy, vol. 8 (2), pp. 173–196, 2011. Vélez et al / DYNA 81 (185), pp. 153-159. June, 2014. 158PDF Image | Thermodynamic analysis of R134a in an Organic Rankine Cycle for power generation from low temperature sources Analisis termodinamico del R134a en un Ciclo Rankine Organico para la generacionde energía a partir de fuentes de baja temperatura
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