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Thermodynamic Optimization of an Organic Rankine Cycle for Power Generation from a Low Temperature Geothermal Heat Source Inés Encabo Cáceres Roberto Agromayor Lars O. Nord Department of Energy and Process Engineering Norwegian University of Science and Technology (NTNU) Kolbjørn Hejes v.1B, NO-7491 Trondheim, Norway inese@stud.ntnu.no Abstract The increasing concern on environment problems has led to the development of renewable energy sources, being the geothermal energy one of the most promising ones in terms of power generation. Due to the low heat source temperatures this energy provides, the use of Organic Rankine Cycles is necessary to guarantee a good performance of the system. In this paper, the optimization of an Organic Rankine Cycle has been carried out to determine the most suitable working fluid. Different cycle layouts and configurations for 39 different working fluids were simulated by means of a Gradient Based Optimization Algorithm implemented in MATLAB and linked to REFPROP property library. The heat source was hot water from a geothermal reservoir with an inlet temperature of 120oC and an outlet temperature limit of 75oC. For each working fluid, an optimal configuration was obtained, based on the optimization of the second law efficiency. In addition, a sensitivity analysis for the polytropic efficiencies of the pump and turbine was carried out. Results show that those working fluids with a critical temperature close to the maximum temperature of the cycle give the highest plant efficiencies (being propylene and R1234yf the best ones). Using a recuperator increases the plant efficiency in all cases with exception of wet working fluids. The cycles experiencing the highest sensitivity on the pump performance are those using working fluids with low critical temperatures. Increasing the number of stages of the turbine increases the overall plant efficiency for all working fluids, but some fluids are more sensitive to the turbine efficiency than others. Keywords: Process modelling, process simulation, working fluid selection, parametric optimization, second law efficiency. 1 Introduction Historically, the preferred methods for power generation have been related to Brayton or Rankine power cycles, fueled by natural gas or other fossil fuels (Macchi and Astolfi, 2017). However, during the last years, the increasing concern of the greenhouse effect and climate change has led to an increase of renewable energy, such as wind and solar power. In addition to these listed renewable energies, there is an energy source that shows a promising future due to the advantages it provides when compared to other renewable energies. This developing energy is geothermal energy, and its advantages are related to its availability (Macchi and Astolfi, 2017): it does not depend on the ambient conditions, it is stable, and it offers the possibility of renewable energy base load operation. One of the challenges of geothermal energy is that it does not provide very high temperatures, and this fact has made researches and engineers to focus their studies on how to obtain high thermodynamic efficiencies at low- and medium-temperature heat sources. One of the solutions for this problem are Organic Rankine Cycles (ORCs), which present a simple structure with a high reliability, an easy maintenance and, most importantly, a high potential to produce power from low source temperatures when compared to other power generation cycles (Saleh et al, 2007). The design of the ORC requires a thorough analysis, since there are many parameters affecting its performance, and any change of these parameters will have a major impact on the efficiency of the cycle. With this aim, many different studies have been carried out. (Saleh et al, 2007) analyze the performance of 31 different working fluids, finding that those working fluids with the highest boiling temperatures are giving the highest efficiencies, being cyclopentane the best one for the given conditions and assumptions. (Roy et al, 2011), by means of a parametric ORC optimization, found that R123 is the best working fluid due to its high efficiency and high power production. (Hung et al, 2010) studied the efficiency of an ORC where benzene, ammonia, R11, R12, R1234a and R113, are used as the working fluids, differencing between wet, dry and isentropic working fluids and concluding that isentropic DOI: 10.3384/ecp17138251 Proceedings of the 58th SIMS 251 September 25th - 27th, Reykjavik, IcelandPDF Image | ORC for Power Generation Low Temperature Geothermal Heat Source
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