Developments Towards a Liquid Piston Stirling Engine

Abstract

The Stirling engine possesses numerous natural benefits such as functioning from any heat source, quiet operation, and high theoretical efficiency. The limited success of Stirling engines has been partially due to the near-adiabatic operation in the working chambers, necessitating external heat exchangers that add dead space, difficulty sealing low-molecular weight gases at high pressure, and non-ideal piston displacement profiles. As a solution to these difficulties, a liquid piston is proposed that allows the compression and expansion chambers to be designed for a high heat transfer rate. The heat transfer in these chambers can be increased through geometry changes, allowable by the ability of a liquid column to fill an irregular volume. Through a simplistic example, it is demonstrated that a liquid piston can improve the heat transfer rate in the working chambers by 3.5 orders of magnitude over a conventional piston. Creating near-isothermal operation in the working chambers eliminates the need for costly external heat exchangers, while creating a secondary path to transfer heat in and out of the chamber through the liquid. The elimination of the external heat exchangers decreases the dead space in the engine, increasing power and efficiency. The liquid piston also eliminates sealing a gas with sliding seals. Finally, the displacement of the liquid pistons can be carefully controlled in a number of ways to closely match the ideal Stirling cycle.