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Energies 2017, 10, 413 19 of 26 the expander with an off-the-shelf electric generator. The size and weight of volumetric expanders increase exponentially for power outputs above 70 kW. This implies an increase in leakages, which in turn lowers the efficiency of the machine. Fiaschi et al. [119] stated that radial turbines and screw expanders represent the only options in the power range of 70–100 kW. The majority of the ORC companies involved in the development of ORC units in the power range of 70–100 kW offer radial turbines. Finally, a few companies developed volumetric expanders (see Table 2). 4.4. Comparative Analysis Volumetric expanders present characteristics that make them a strong candidate in exploiting power in small-scale ORCs and in those applications in which the duty cycle of the heat source is variable. In fact, volumetric machines are able to operate smoothly even when the thermodynamic conditions at the inlet of the machine vary significantly. Another major advantage of volumetric machines is that they operate at low rotational speeds (typically below 3000 rpm) allowing for direct connection with low-speed electric generators. This leads to two advantages. First, low speed electric generators are inexpensive and widely available on the market. Low rotational speeds, which are typical of volumetric machines, allow for the use of common bearings, which increases the longevity of the rotating parts and reduces the frequency of maintenance periods. Second, the low cost of the electric generator helps in reducing the return on investment and in making the technology competitive. The main drawbacks of volumetric expanders derive from the process of lubrication and from leakages, which cause volumetric and fluid dynamic losses. The former decrease volumetric efficiency due to a reduction of the flow rate that contributes to the production of work. Volumetric losses cannot be eliminated, because they occur from the required expansion gap to safeguard moving parts that are subjected to thermal and mechanical stresses. Fluid dynamic losses are classified into concentrated and distributed losses. Concentrated losses are due to the interaction between the fluid and the edges of the moving parts of the machine. Distributed losses are generated by the interaction between the working fluid and the shroud. Turbomachines, as reported in Section 4.2, are generally not suitable for power production below 10 kW, mainly because of their rotational speeds. This imposes a severe challenge when coupling the turbine with the electric generator. Two different options are available to couple the turbo machine to the electric generator. One possibility is to use a gear-box to decouple the rotational speed of the turbine from that of the electric generator. However, this would introduce high mechanical losses [121]. The other option is to adopt a high speed electric generator directly coupled to the expander. The high cost of such machines increases the total cost of the ORC. Figure 10 reports a map of existing high speed electric generators [122]. As can be observed from Figure 10, the options available in the power range of interest are limited to permanent magnet machines. In addition, the balancing of the machine with the turbine implies low tolerances during the manufacturing process. Overall, high speed turbines represent a more efficient alternative to volumetric expanders when the power output is greater than 20 kW. However, the higher cost of turbo expanders (with respect to volumetric machines) renders them cost effective only in applications wherein they considerably outperform volumetric machines; as for example, when the thermodynamic conditions of the heat source are stable over time.PDF Image | Small Scale Organic Rankine Cycle (ORC)
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