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4.2 Pump sensitivity analysis Geothermal energy allows to work at part-load conditions when desired. (Landelle et al, 2017) stated efficiency losses of more than a 50% when operating below the nominal power plant conditions, and this, joint to the fact that each working fluid has a single and limited operating range in which it is optimal (Hærvig, 2016) has led to the conclusion that a pump sensitivity analysis should be carried out in this work. In this sensitivity analysis, one of the goals is to determine which working fluid is the optimal and for which range of pump polytropic efficiency, setting the nominal pump efficiency as 70% based on the literature review (Brosukiewicz-Gozdur, 2013). The analysis was then performed for 5 different working fluids. These were: • Toluene: Working fluid operating under subcritical conditions with the lowest power consumption • R161: Working fluid operating under subcritical conditions with the highest power consumption • R1234yf: Working fluid operating under transcritical conditions with the lowest power consumption • CO2: Working fluid operating under transcritical conditions with the highest power consumption • Propylene: Best working fluid for the non- recuperated layout. law efficiency suffers a great variation for all the fluids except for toluene. CO2 shows the greatest variation, because it is the working fluid that requires the highest power consumption in the pump (high operating pressures), and any small change in the pump polytropic efficiency leads to important changes on the net power output and plant efficiency. For pump efficiencies reduced below 30%, the efficiency of the plant turns out to be negative (the power consumption is higher than the power produced in the expander). Even at a 100% pump efficiency, the CO2 would still be the worst working fluid because of other irreversibilities in the cycle. The pump efficiency determines which working fluid is the best one at each operating point. Toluene is found to be the working fluid giving the second worst efficiency for efficiencies higher than a 70%, but, for pump efficiencies lower than 50%, it becomes the best choice. The reason for this is that the BWR for this fluid is low, and this means that any change of the pump polytropic efficiency has a minor impact on the efficiency. Propylene, which was the working fluid giving the best efficiency for the base case is still giving the best results for efficiencies higher than 70%. However, when the pump efficiency drops below this value, it is the second worst working fluid. This gives insight about how important the pump efficiency is for the performance of the cycle. Despite transcritical cycles might appear as the most sensible to pump efficiency changes, some subcritical cycles can be more sensitive. The BWR can be used as a good indicator of the sensitivity of the cycle to the pump efficiency. At the same time, the BWR is a strong function of the critical temperature of the working fluid. Figure 5 also shows that R1234yf and propylene need to move from transcritical to subcritical conditions to reach the optimal performance when the pump efficiency is below 30%, due to the high power consumption that operating at transcritical conditions with a low pump efficiency implies. When comparing the obtained results with the literature, it was found that similar results were obtained in (Brosukiewicz-Gozdur, 2013). Even though this work includes the simulation of working fluids that were not considered by (Brosukiewicz-Gozdur, 2013), the same tendencies were obtained for the same studied working fluids, meaning that propylene is the working fluid with the highest specific power consumption, followed by the propane, while toluene presents the lowest one. 4.3 Turbine stages analysis Since the specific work produced in the turbine is lower in the ORC than for gas and steam turbines, the expansion process can be handled in few stages. Assuming a fixed efficiency for the turbine (independent of factors such as the working fluid) leads to unrealistic results (Astolfi and Macchi, 2015). In this work, 1, 2 and 3 stage axial turbines have been studied in order to determine how this may affect the [%] Figure 5. Effect of the pump polytropic efficiency on the overall plant second law efficiency for different working fluids Efficiencies from 10% to 100% were analyzed for all these fluids. Results for the second law plant efficiency can be found in Figure 5. It can be seen that the second DOI: 10.3384/ecp17138251 Proceedings of the 58th SIMS 259 September 25th - 27th, Reykjavik, Iceland [%]PDF Image | ORC for Power Generation Low Temperature Geothermal Heat Source
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