Performance comparison of a novel thermofluidic organic-fluid heat converter and an organic rankine cycle heat engine

The Up-THERM engine is a novel two-phase heat engine with a single moving part–a vertical solid piston–that relies on the phase change of a suitable working fluid to produce a reciprocating displacement and sustained thermodynamic oscillations of pressure and flow rate that can be converted to useful work. A model of the Up-THERM engine is developed via lumped dynamic descriptions of the various engine sub-components and electrical analogies founded on previously developed thermoacoustic principles. These are extended here to include a description of phase change and non-linear descriptions of selected processes. The predicted first and second law efficiencies and the power output of a particular Up- THERM engine design aimed for operation in a specified CHP application with heat source and sink temperatures of 360 ○C and 10 ○C, are compared theoretically to those of equivalent sub-critical, nonregenerative organic Rankine cycle (ORC) engines. Five alkanes (from n-pentane to n-nonane) are being considered as possible working fluids for the aforementioned Up-THERM application, and these are also used for the accompanying ORC thermodynamic analyses. Owing to its mode of operation, lack of moving parts and dynamic seals, the Up-THERM engine promises a simpler and more cost-effective solution than an ORC engine, although the Up-THERM is expected to be less efficient than its ORC counterpart. These expectations are confirmed in the present work, with the Up-THERM engine showing lower efficiencies and power outputs than equivalent ORC engines, but which actually approach ORC performance at low temperatures. Therefore, it is suggested that the Up-THERM can be a competitive alternative in terms of cost per unit power in low-power/temperature applications, especially in remote, off-grid settings, such as in developing countries where minimising upfront costs is crucial