*4.1. Vane Expanders*

The design of the vane expanders is very simple, which translates into low production costs. This machine has a favorable ratio of the output power to its external dimensions. The use of special construction materials makes it possible to eliminate the need for lubrication. Compared to other types of volumetric machines and microturbines, vane expanders are characterized by a lower fluid flow rate and a lower pressure ratio. Moreover, the vane expanders can be hermetically sealed, which is one of the key issues in the cooperation of this type of expander with the ORC system [154]. This type of machine is insensitive to the negative impact of the expansion of the gas-liquid mixture, which is a grea<sup>t</sup> advantage when ORC systems are supplied by heat sources with variable thermal parameters.

Vane compressors and expanders are used in many industries, including mining, refrigeration and pneumatic systems [155]. Vane expanders have an output power from several dozen watts to about a dozen kilowatts. The maximum gas pressure at the inlet to the vane expander is approximately 30 bar. These machines are characterized by rotational speeds from several hundred to 10,000 rpm. Research on vane expanders is carried out, among others at the Wrocław University of Science and Technology. The implemented experimental CHP ORC test stand uses multi-vane expander featuring the maximum power output of 300 W and gas central heating boiler featuring a thermal power of 24 kW serves as a heat source. The view of the test stand and expander is shown in Figure 25.

**Figure 25.** General view of the test stand and expander. Left: 1—plate evaporator, 2—plate condenser, 3—pump, 4—tank 5—multi-vane expander; on the right-general view of the expander [154].

As already mentioned, the design of the vane expanders is simple. Figure 26 shows the components of a vane expander.

**Figure 26.** The individual elements of a vane expander: 1—body, 2—rotor, 3—blades, 4—housing, 5—bearings, 6—rings [154].

Based on the available literature [156–159], it can be stated that vane expanders are devices on a rather smaller scale. The maximum currently available units, when coupled with the ORC system, reach the power of 7.5 kW.

## *4.2. Lobe Expanders*

Rotary lobe expanders are devices that are not currently in mass production. Work on them is still ongoing. Their design is derived from pneumatic motors [160]. The design of such an engine is shown in Figure 27.

**Figure 27.** View on the Armak pneumatic motor [160].

Rotary lobe expanders can be used as heat engines in energy systems powered by different heat sources (e.g., biomass boilers, waste heat recovery boilers, parabolic solar collectors, etc.). The experimental units work in small steam plants. The company that is currently conducting research on this type of expanders is the Polish company Termo2Power. The works are carried out under the research project "PBSE Power Sector Research Program" carried out under the contract with the National Center for Research and Development. Part of the substantive work is carried out by a team from the Faculty of Power and Aeronautical Engineering of the Warsaw University of Technology. The company also conducts tests of its own designs, including multi-stage expanders. These expanders can work with medium pressure from 8 to 40 bar. The inlet medium temperature should not exceed 350 ◦C due to sealing problems. In this type of devices, no labyrinth seals are used, but classic seals that must withstand high temperatures. The capacities of these devices range from single kilowatts up to ca. 150 kW for a single stage expander. It is also practiced to combine expanders into multi-stage systems. Then a system with a power of several hundred kilowatts can be configured. Figure 28 shows the Termo2Power rotary lobe expander and Figure 29 shows this expander connected to the generator and with flexible connectors for supplying and discharging steam.

**Figure 28.** View of the Termo2Power rotary lobe expander [161].

**Figure 29.** View of Termo2Power rotary lobe expander connected to the generator and pipings [161].

The figure below (Figure 30) shows a system with two rotary lobe expanders and interstage superheating. The boiler is visible on the left side. The expanders are coupled to the generators (blue). The power take-off (electric heaters) is visible under the expanders system.

**Figure 30.** View of a two-stage steam rotary lobe expander with inter-stage superheating [161].

In addition to the aforementioned Termo2Power company, Katrix from Australia is another manufacturer that produces lobe expanders. The expander of this company is of a slightly different design from the Termo2Power expander. A description of this expander and analysis of its operation can be found in [162,163]. Its view and principle of operation are shown in the Figure 31.

**Figure 31.** View of a Katrix expander construction [162,163].

Expander made by the Katrix company was under investigation as a part of the electricity generation system. The system based on solar collector to produce hot working fluid can be seen in the Figure 32. The expander connected to the pipings and generator is presented in Figure 33.

**Figure 32.** A solar Rankine microcogeneration system with Katrix expander [162,163].

**Figure 33.** View of a Katrix expander connected to the source of working fluid and generator [162,163].

Roots expanders are the other lobe-type expanders that are currently investigated to be applied in ORCs. Roots expander is a two-shaft, rotary, positive displacement machine, featuring the transverse flow of the working fluid [164–167]. The results of experimental tests of a Roots expander operating in the ORC system are presented in [166]. The tests were carried out using a mixture of R245fa and oil to provide the expander lubrication. The working fluid pressure at the inlet to the machine was varied in the range between 3 and 10.8 bar. The working fluid pressure at the outlet of the machine was ranging between 1.35 and 2.25 bar. The rotational speed of the expander was varying between 1500 and 11,000 rpm. The highest isentropic efficiency of the expander (ca. 50%) was achieved for the rotational speed of ca. 4500 rpm and for small expansion ratio of ca. 1.5. However, for these operating parameters low power output was achieved (ca. 100 W). In order to increase the power output to ca. 3 kW it was necessary to increase the expansion ratio to ca. 3.5.
