Abstract
A 195 cm³ swash-plate piston expander was tested in an ORC using R245fa as working fluid. Rotational speeds ranging from 1000 RPM to 4000 RPM and pressure ratios from 7 to 12 were imposed. In total, 65 steady state points were measured. With these measurements, performance maps were generated to point out the influence of rotational speed and levels of pressure on mechanical power and isentropic efficiency. These maps have highlighted the existence of an optimal rotational speed of around 3000 RPM maximizing the mechanical power, while the speed that maximizes the isentropic efficiency lies between 2000 and 2500 RPM. The maximal mechanical power and isentropic efficiency were 2.8 kW and 53%, respectively. Then the measurements were used to analyze the losses. This analysis has shown that, under the expansion and compression limit, the theoretical isentropic efficiency has values comprised between 90% and 70% for pressure ratios of 7 to 12. The filling factor affects the isentropic efficiency for low rotational speeds and low pressure ratios. Indeed, indicated isentropic efficiency is around 60% for 1000 RPM and around 75% for 4000 RPM. These values stay quite constant with the pressure ratio. Finally, a mechanical efficiency comprised between 40% and 90% was observed, which lowers the isentropic efficiency to values comprised between 30% and 53%. Finally, a model based on energy and mass conservation inside a cylinder volume was successfully calibrated and was able to predict mass flow rate, mechanical power and exhaust temperature with good agreement. This model has enabled desegregation of the influence of pressure drops and leakages on the filling factor, then on the isentropic efficiency. This analysis has shown that pressure drops mainly affect the compactness of the expander, and not so much the isentropic efficiency (except for low rotational speeds where pressure drops can lower the isentropic efficiency by 14%). In contrary, leakages have a strong impact. The importance of the different sources of losses varies with the speed. For the optimal speed of 2500 RPM, under-expansion and compression have the strongest impact, followed by mechanical losses, leakages and pressure drops. respectively.
Highlights
The piston expander, or reciprocating expander, is one of the major types of positive displacement expander
The present paper aims first at supplementing the state of the art on piston expanders for ORCs, by describing an exhaustive experimental investigation conducted on a specific design of swash-plate piston expander, characterized by a total capacity of 195 cm3 and fed with R245fa as working fluid
The piston expander was tested under a wide range of operating conditions
Summary
The piston expander, or reciprocating expander, is one of the major types of positive displacement expander. Endo et al (2007) tested a 7-cylinder, 185 cm swash plate piston expander in an ORC integrated into a hybrid vehicle. Galindo et al (2015a,b) tested a 3-cylinder swash-plate piston expander integrated into an ORC using ethanol and coupled with a 2 l gasoline engine. Latz et al (2013) used dimensional analysis and similarity parameters (specific speed and diameter) to select the expansion machine for an ICE WHR application They concluded that piston expanders are best suited for heavy-duty vehicles. Bianchi et al (2019) experimentally investigated a prototype of piston expander made of 3 radial cylinders placed at 120◦, equipped with rotary valves and with a total displacement of 230 cm. The well-known filling factor and isentropic efficiency are defined and disaggregated into several indicators in order to TABLE 1 | Piston expander experimental studies summary
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