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Mohammed A. Al-Weshahi et al Working fluid selection of low grade heat geothermal Organic Rankine Cycle (ORC) The size of the heat exchangers is playing a role in selecting the optimal fluid for the ORC. Fig. 7 describes the overall heat transfer conductance (UA), the smaller the product the smaller the heat exchanger size to change the refrigerant ORC turbine outlet state phase to saturated liquid phase. Promising fluids such as R134a, R152a, R501 and R717 could be excluded from the shortlisted optimal fluids due to higher UA heat exchange value. Interestingly, R227ea was found with the lowest heat exchanger UA value and, again, R125 with the highest. It is necessary to pinpoint the higher the UA resulting from high absorbed heat from the refrigerants which causes a higher cooling water flow to remove the heat allowing phase changing to the saturated liquid. Although the previous discussed criteria are important the safety and environmental consideration should not be ignored. For example, R600a and R600 showed positive performance and design indicators; however, being under A3 safety categories (higher inflammability) reduced its credit. In addition, R717 showed good thermal efficiency but it was under B2 categories (lower inflammability and higher toxicity) and making the decision to use them, it requires extra precautions. Furthermore, despite HCFC components being of low ODP the environmental preference was always given to the zero ODP fluids. 5. Conclusion The study used validated model of an existing 250 kW ORC energized by hot geothermal source. Maintaining similar design and operating parameters; 25 refrigerants were assessed based on net output power, thermal efficiency, refrigerant pump power consumption, condenser UA, evaporating and condensing pressure, safety concern and environmental concerns. From the results the following conclusions could be inferred: From high net output power, more thermal efficiency and low refrigerant pump power consumption aspects: R141b and R123 were found the best candidates and R125 was the worst. However, refrigerants such as R134a, R236ea, R245ca, R245fa, R717 and R600 were not far from the best working fluids. All of the mixtures were producing lower power output, less than 200kW. Maintaining the acceptable evaporating and condensing pressure lower than 25 bar and higher than 1 bar: promising refrigerants such as R141b, R123, R245ca, R245fa could be rejected for low condensing pressure and R143a and R410A could be discarded due to high evaporating pressure. This study emphasises that at this specific operating condition, low condensing pressure refrigerants could be accepted if the cooling temperature increases. Based on heat exchange UA selection criteria, R134a, R152a, R717 are not preferred due to higher value. It was noted that R227ea achieved the lowest UA. Putting beside previous selection criteria: safety and environmental concerns, the study revealed the R236ea, R236fa and R227ea are the best working fluids for low temperature geothermal application at the specific operating conditions studied. Appendix A: (ASHRAE, 2009) Refrigerant R407A R407B R407C R407D R407E R410A R411A R411B R501 References Alaska Energy geothermal power plant at Chena hot springs, Alaska, Final Report. Aljundi, I. H. (2011), Effect of dry hydrocarbons and critical point temperature on the efficiencies of organic Rankine cycle, Renewable Energy, 36, 1196-1202. Al-Weshahi, M. A., Anderson, A., and Tian, G. (2014), Organic , Applied Energy, DOI: . ASHRAE. (2009), ASHRAE handbook-fundamentals, Atlanta, ASHRAE. Basaran, A., and Ozgener, L. (2013), Investigation of the effect of different refrigerants on performances of binary geothermal power plants, Energy Conversion and Management, 76, 483- 498. Bitzer International, HFC-Refrigerants blend technical information, KT-630-2. BOC GASES, Safety data sheet of R407D, UK, Manchester. Boot, J. L. (1990), Overview of the alternatives to CFCs for domestic refrigerator and freezers, International Journal of Refrigeration, 13, 100-105. Calm, J. M., and Domanski, P. A. (2004), R-22 replacement status, ASHARE Journal, 46, 29-39. Granryd, E. (2001), Hydrocarbons as refrigerants-an overview, International Journal of Refrigeration, 24, 15-24. Herold, K. E., Radermacher, R., and Klein, S. A. (1996), Absorption chillers and heat pumps, 1st edition, Florida, CRC Press. http://webbook.nist.gov/chemistry. Hundy, H. F., Trott, A. R., and Welch, T. C. (2008), Refrigeration and air conditioning, 4th edition, Great Britain, Elsevier Ltd. Kim, H., Shon, Z., Nguyen. H., and Jeon, E. (2011), A review of major chlorofluorocarbons and their halocarbons alternatives in the air, Atoms Environment, 45, 1368-1382. Li, J., Pei, G., Li, Y., Wang, D. et al. (2008), Energetic and exergetic investigation of an organic Rankine cycle at different heat source temperatures, Energy, 38:85-95. Li, T., Zhu, J., and Zhang, W. (2012), Cascade utilization of low temperature geothermal water in oilfield combined power generation, gathering heat tracing and oil recovery, Applied Thermal Engineering, 40, 27-35. Mago, P. J., and Luck, R. (2012), Energetic and exergetic analysis of waste heat recovery from a microturbine using organic rankine cycle. International Journal of Energy Research, Online. Papadopoulos, A. I., Stijepovic, M., Linke, P., and Seferlis, P. (2010), Power generation from low enthalpy geothermal fields by design and selection of efficient working fluids for organic Composition R32/R125/R134a (20%,40%,40%) R32/R125/R134a (10%,70%,20%) R32/R125/R134a (23%,25%,52%) R32/R125/R134a (15%,15%,70%) R32/R125/R134a (25%,15%,60%) R32/R125 (50%,50%) R1270/R22/R152a (1.5%,87.5%,11%) R1270/R22/R152a (3%,94%,3%) R22/R12 (75%,25%) Authority Chena Power. (2007), 400kW Rankine cycle recovering stage heat from MSF desalination distillate water 10.1016/j.apenergy.2014. 02.038 11 | International Journal of Thermal Technologies, Vol.4, No.1 (March 2014)PDF Image | Working fluid selection of low grade heat geothermal Organic Rankine Cycle
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