# A Review on Two-Phase Volumetric Expanders and Their Applications
###### Abstract  
The importance of volumetric expanders has been increasing in the last decades because several studies confirmed that they lead to improved energy savings, limit the environmental impact, and reduce the energy intensity of industrial and domestic applications. In particular, several applications of the two-phase volumetric expanders, in which the operating fluid consists of liquid and vapor phases, were recently proposed. Nevertheless, the contributions in the scientific literature related to the overview of the state-of-the-art aspects of this technology are rare. For this reason, the present work discussed the potentialities and drawbacks of the available typologies of volumetric expanders that process a two-phase pure working fluid by analyzing a summary of leading studies in this field to go beyond previous efforts in the literature. The analysis revealed that twin-screw machines represent the best candidates, while reciprocating piston devices seemed the least well-adapted because of their reduced tolerance to droplets and high friction losses. Flash evaporation appeared to have the most significant impact on the expander because it affects both inlet and expansion phases, thus, determining the shape of the indicated cycle and the isentropic efficiency. volumetric expanders; two-phase flows; flash expansion; isentropic efficiency; volumetric performance; energy systems +
Footnote †: journal: Applied Sciences  
202212103281032  
expansion in the two-phase region. This general definition allows including all the practical situations in which the vapor quality is so significant as to impact the performance of the machine. As a result, a two-phase expander must withstand high liquid fractions and minimize losses from the suction up to the discharge process. However, some general considerations suggest some advantages of two-phase volumetric machines over the homologous single-phase machines. Indeed, depending on the working fluid, single-phase volumetric devices may require lubricating oil, thus, requiring a dedicated circuit that includes a separator and a pump. Nonetheless, the possible miscibility between the oil and the working fluid may sometimes reduce the lubrication capacity with an increase in friction and wear that worsens the efficiency and reliability of the machine. Another drawback of single-phase volumetric machines is that their adjustment to process two-phase flows requires including other ancillary components in the plant to manage both liquid and vapor phases [1]. Conversely, two-phase volumetric expanders might avoid the need for lubricating oil, since the liquid phase of the working fluid may lubricate the surfaces. Another advantage is that the liquid phase may behave as a sealant, thus, reducing the leakages and increasing the volumetric efficiency. Two-phase volumetric expanders can be applied to generate electrical or mechanical power in the processes for the pressure reduction of a fluid consisting of the liquid and vapor phases. The main applications are the replacement of the pressure reduction valves in several processes (e.g., in industrial steam, flash steam geothermal power plants, and natural gas transport), the replacement of the throttling valves in the vapor compression systems used for refrigeration and air conditioning, the implementation in advanced thermodynamic cycles (e.g., the trilateral flash cycle, Smith cycle, organic double-flash cycle, wet vapor ORC, and combined cooling, heating, and power), in the high-temperature heat pumps, and in the Carnot batteries (i.e., the heat-to-power processes to decarbonize the electrical energy storage sector). [PERSON], in [1], found that two-phase volumetric expanders produce higher mechanical power compared to homologous single-phase machines with the same mass flow rate, inlet pressure, outlet pressure, and fluid quality at the inlet. Nonetheless, two-phase volumetric expanders also show some drawbacks.