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M.E. Mondejar et al. Renewable and Sustainable Energy Reviews 91 (2018) 126–151 Fig. 14. Examples of different types of expanders for ORC applications: a) Rotor of an ORC axial turbine and core of the generator [113], b) rotor of a radial inflow turbine of a 190 kW ORC [114], c) scroll expander rotor and stator of a 1.5 kW ORC unit [115]. Positive displacement expanders are well-suited for low-tempera- ture and low-capacity power systems (1 − 100 kW), where the expan- sion ratios and the volume flow rates are moderate [7,26,116]. How- ever, their isentropic efficiency is usually lower compared to that of a turbine. On the other hand, the realization of a mini-turbine with a power output in the range of a few or tens of kW is challenging [7]. Thereby, volumetric expanders are often the only alternative at these power capacities. Conversely, turbines become attractive at power outputs greater than 100 kW and/ or when the temperature of the working fluid is between 120 and 350 °C. In these cases, isentropic efficiencies up to 90% are typically attained [7]. Most ORC turbines are axial or radial machines. Axial turbines perform best at high specific speeds Ωs = Ω V̇/Δh3/4, i.e., high volumetric flow rates V̇ and low enthalpy drops Δh. Conversely, radial turbines are mainly used when the volu- metric flow rate is low compared to the enthalpy difference [117,118]. A reason for this is the greater change in tangential momentum that occurs in radial turbines, meaning that one stage of a radial turbine can elaborate the same specific work as two or more axial turbine stages [119]. This results in radial turbines having less mechanical losses for low capacities, owing to the lower number of rotating discs. Radial turbines are also less sensitive to clearance losses compared to the axial counterpart [120]. Moreover, they are more cost-effective than axial expanders, whose cost increases with the number of stages [121]. However, the size of radial turbines increases more rapidly with the volumetric flow than in the case of axial turbines, making the latter more suitable for high capacities with high enthalpy drops. For the ships analyzed in Section 3.3, the design point power of the regenerated ORC module fed by the exhaust gas heat can be up to 700 kW. In this power range, both axial and radial configurations can be adopted. If a direct coupling between the turbine and the electric generator is required, an axial expander is preferable since its optimal rotational speed is typically lower than that of its radial counterpart [119]. Hence, there is no need for a gearbox. The power output of the ORC unit recuperating the heat from the jacket water spans from 50 − 100 kW. Thus, a radial turbine is arguably the most suitable option, especially if the unit is decoupled from the grid by power electronics. In this sense, some ORC systems are equipped with an integrated power module consisting of a radial inflow turbine and an electric generator, in which the high-frequency electric generator can be cooled by the working fluid [122]. Moreover, a screw expander could be also an option if the jacket water is used given the low temperature of this heat source and its capability of handling power capacities up to 1 MW [7,8]. Typically, ORC units considered for recovery of exhaust gas heat would employ an axial flow turbine as the expander due to the large power outputs. In the case of WHR from the jacket cooling water, a radial-flow turbine is also a feasible alternative. 4.3.3. Pumps In an ORC unit, the pumping power may represent up to 10% of the expander power output [8,123], which makes the selection of the pump of significant importance for the optimization of the cycle. The ratio between the pump power consumption and the turbine power output, i.e., back work ratio, has been found to be related with the inverse of the critical temperature of the fluid [8,123], making it lower for working fluids with higher critical temperature. This can explain the greater impact of the pump work on ORC power systems than on SCR plants, where common working fluids have critical temperatures below that of water used in SRC plants. Pumps can be grouped as positive displacement pumps (also called volumetric pumps) and centrifugal pumps. Positive displacement pumps (e.g., diaphragm, rotary-vane or plunger pumps) have effi- ciencies around 40% according to manufacturers’ data [8]. Their vo- lumetric flow rate is proportional to the rotational speed, and almost independent of the pressure ratio, which means that their pressure working range is wide and their performance is barely affected by the pressure ratio [124]. However, the volumetric flow in positive dis- placement pumps is limited by their size. For this reason, volumetric pumps are mainly used in micro-scale and mini-scale ORC systems (< 50 kW). A possible application of these pumps could be ORC sys- tems recuperating the jacket water heat, as in this case the mass flow rate of working fluid is expected to be moderate because the heat available from this source is lower than for exhaust gases. Moreover, the pressure ratio would be lower. In the case of centrifugal pumps, with efficiencies higher than 60% according to manufacturers’ data [8], their volumetric flow rate de- pends not only on their rotational speed, but also on the pressure ratio. Unlike volumetric pumps, the efficiency of centrifugal pumps is greatly influenced by the pressure ratio, which could be of major concern in case with heat sources with high variability. Because centrifugal pumps do not have a volumetric limitation, they are usually adopted for higher power capacities. Therefore the use of centrifugal pumps could be more suitable if the exploited heat source is the exhaust gas heat, as greater volume flow rates of working fluids and higher pressure ratios could be expected. Pumps used for this applica- tion may require a double seal, with the space between the seals con- taining circulated refrigerant oil, in order to reduce the possibility of leakages [124]. The standard method to control the operation of an ORC unit is by varying the pump speed, which allows varying the mass flow rate of working fluid. In this regard, the choice of the pump would have a significant impact on the control of the ORC unit. In volumetric ma- chines in part-load, the mass flow rate is imposed and varies in pro- portion to the rotational speed. Conversely, the head of centrifugal pumps drops monotonically with increasing volume flow rates at con- stant speed operation. Thereby, the mass flow rate in off-design con- ditions depends not only on the pump curve, but also on the part-load characteristics of the heat exchangers and the turbine. The control 140PDF Image | organic Rankine cycle power systems for maritime applications
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