# A Review on Two-Phase Volumetric Expanders and Their Applications
## 3 Two-Phase Volumetric Expanders Applications  
In the literature, several applications of the two-phase volumetric expanders were proposed. In particular, these machines could be used to generate electric or mechanical  
Figure 3: Expansion process on the right and left hand side of the critical point performed by a volumetric machine (reprinted from [6]): thermodynamic diagram (**a**), indicated cycle for the line B (**b**), indicated cycle for the line C (**c**), and indicated cycle for the line D (**d**) with the sharp pressure drop due to the presence of a liquid (in the dashed circle). power in the processes where the required reduction in the fluid pressure during the expansion process takes place mostly in wet vapor phase. Two-phase expanders could be used for the replacement of the pressure reduction valves in industrial steam processes, e.g., food industries. In particular, the steam production takes place in a central boiler at relatively high pressure (typically in the range of 10-20 bar), and then it is transported to various sites in the factory. Here, the steam pressure is locally reduced to a lower value suitable for the processes. Thus, the higher steam density implies a smaller diameter of pipes resulting in considerable savings in the total plant cost. Thus, the opportunity exists to replace the local pressure reduction valves with expanders, and thereby recover power from the expansion process. The extracted shaft power can be used to drive a generator to produce electricity [24]. Similarly, two-phase expanders could be used for the replacement of the pressure reduction valves in flash steam geothermal power plants. Typically, the power is recovered through the circulation of the geothermal fluid in the pressure reduction valve and then a liquid-vapor separator. The separated vapor is passed through a turbine and then it is re-injected back into the ground through an injection well. Nevertheless, in such systems, problems exist with the replacement of the pressure reduction valve. First, the brines, coming from the ground, usually contain significant amounts of silicates and salts, which tend to come out of solution due to the reduction in the pressure and temperature. Second, the relatively low operation pressures and the related large specific volume of steam result in the need for very large and expensive machines [24]. Another application of the two-phase expanders is the replacement of the pressure reduction valves in the transport processes of natural gas [24]. Vapor compression systems are used for refrigeration and air conditioning. The main components are the evaporator, the vapor compressor, the condenser, and the throttling valve. The latter can be replaced by a two-phase expander (Figure 4). This allows the COP increase to be obtained due to the following: (i) less electrical power needs to be supplied to the compressor thanks to the mechanical power provided by the two-phase expander. Indeed, it enables the conversion of the thermodynamic energy of the two-phase fluid, avoiding its dissipation; (ii) higher cooling power supplied to the end-user (having fixed the working fluid mass flow rate and the specific enthalpy at the evaporator outlet) thanks to the lower specific enthalpy at the evaporator inlet. Indeed, the irreversibilities in the two-phase expansion process are lower in the two-phase expander than the throttling valve, with resulting lower specific entropy and vapor phase at the evaporator inlet [24]. Moreover, the transcritical vapor compression refrigeration cycle represents a particular type of vapor compression refrigeration cycle (Figure 5), where the expansion process takes place from the supercritical phase (point 4) to the wet vapor phase (point 5). It is worth noting that the vapor compression refrigeration cycle can be alternatively used as heat pump cycle. In particular, the heat pump cycle provides the end-user (through the condenser) with thermal power at a higher temperature by exploiting the thermal power transferred at a lower temperature with a heat source.