*4.4. Piston Expanders*

In a piston expander, the working chamber is formed by the inner surface of the cylinder and the surface of the moving piston. The piston reciprocates in the cylinder between top and bottom dead position and is driven by the crank mechanism. Studies on the possible application of piston expanders in micro steam and ORC systems have proceeded for many years.

The results of research on application of a small piston expander in the ORC system utilizing R134a as a working fluid are reported in [178–180]. The tested expander was manufactured by StarEngine company. The expander features a total displacement of 230 cm<sup>3</sup> and is hermetically coupled in one casing with a generator. The general view of this expander is presented in Figure 37.

**Figure 37.** StarEngine piston expander [178–180].

The experimental research on this expander was carried out for varied heat source and heat sink temperature. Heat source temperature was varied in the range 65–85 ◦C and the temperature of the heat sink was varied in the range 28–27 ◦C. The mass flow rate of the working fluid was varied between 0.05 and 0.14 kg/s, pressure in the evaporator was varied between 11 and 19 bar and pressure in the condenser was varied between 6 and 7 bar. For these experimental parameters, an electric power output of 250–1200 W was achieved while expander rotational speed was ranging between 320 and 1100 rpm. The total efficiency of the expander-generator unit was varying between 38 and 42%. The achieved gross efficiency of ORC system was 4.5% while the achieved net efficiency was 2.2%.

In [181] the results of experimental tests carried out on a prototype oil-free piston expander designed for application in steam distributed generation systems are reported. The electric power output of the tested expander ranges between 740 and 2400 W depending on the parameters of the working medium. During the experiments the thermal parameters of the working fluid at the inlet to the expander were varied in the range 260–340 ◦C and 20–34 bar. The experimentally achieved internal efficiency of the expander was varying between 19 and 40%. This type of steam expander is also promising for application in ORC systems [181]. In [182,183] the results of research on a piston expander featuring a power of 3 kW and applied in the ORC system are presented. The expander is a modified reciprocating compressor with a specially designed control valve.

In [184] the authors presented the results of modeling of the operation of a piston expander designed for application in waste heat recovery system from the passenger car exhaust gases. Modeling results showed the possibility of obtaining an expander power output of 7 kW and an isentropic efficiency between 55 and 70%.

In [185] the results of experimental research on a swash-plate piston expander featuring a displacement of 195 cm3, which was implemented in the ORC system using blowing agen<sup>t</sup> R245fa as a working medium are presented. This type of expander uses a swash-plate to transmit torque from the pistons to the shaft. The cross-section of swash-plate expander is presented in Figure 38.

**Figure 38.** Cross-section of a swash-plate piston expander [185].

The tests of the expander were proceeded for gas inlet pressures ranging between 18 and 30 bar and the rotational speed of the expander ranging between 1000 and 4000 rpm. For these experimental conditions working fluid pressure at the outlet of the expander was ranging between 2.9 and 4.02 bar. The inlet pressure was regulated by changing the mass flow rate of the working fluid by means of a pump. Depending on the operational conditions working fluid was superheated in the range of 4–17 K and working fluid mass flow rate was varied between 29 and 105 g/s. The obtained mechanical power of the expander was ranging between 0.3 and 2 kW while the maximum achieved internal efficiency of the expander was 53%. The mechanical efficiency of the expander was ranging

between 50 and 85%. The results of research on swash-plate expanders were also reported in [186,187].

In addition to the piston expanders discussed above, linear piston expanders are also investigated to be applied in ORC systems. Research results on this type of expansion machines were reported in [188,189]. In a linear piston expander, the linear arrangemen<sup>t</sup> of the cylinders is applied. This design is similar to the boxer arrangemen<sup>t</sup> of cylinders, but does not use a crankshaft. A linear generator is placed between the cylinders. The piston rods of the opposing pistons are connected to each other by a piston rod of a linear generator. The kinetic energy of the reciprocating movement of the piston rod is converted into electricity in a linear generator. The cross-section of linear piston expander is presented in Figure 39. In [190] the test results and design guidelines for a linear piston expander that can be applied in a micro-power ORC systems used for waste heat recovery from automotive engines were presented.

**Figure 39.** Cross-section of a linear piston expander [190].

It has been reported [190] that incomplete expansion (pressure of the working medium in the cylinder after expansion is higher than the pressure of the working medium in the outlet channel from the expander), heat transfer, flow losses during filling and evacuation of the working fluid from cylinder, friction and leakages are the main phenomena limiting the efficiency of linear piston expanders. It has also been shown that appropriate valve control has a significant impact on the linear piston expander operation and pressure losses occurring on the valves. The results of the tests carried out on the prototype of the linear piston expander showed that for gas inlet pressures ranging between 0.13 and 0.21 MPa, the internal efficiency of the expander varies in the range 66.2–93% and decreases with increasing inlet pressure. The highest power output of 22.7 W was achieved for the gas inlet pressure of 0.2 MPa and internal efficiency of 66.2%.

Compared to the other types of volumetric expanders (such as e.g., lobe, screw and Wankel expanders), piston expanders are characterized by a much simpler design; however, they require lubrication, valve timing, and generate vibrations during operation.
