Performance of a Miniature Reciprocating Expander for a Heat Actuated Heat Pump

Abstract

A miniature reciprocating expander is being developed at Oregon State University as part of a larger program to develop a heat actuated heat pump for portable applications. A prototype unit has been demonstrated to produce up to 20 W of shaft power at 2500 rpm using 60 psig, room temperature nitrogen as the input. Assuming adiabatic conditions, the expander appears to operate at up to 80% isentropic efficiency. However, indications are presented that significant heat transfer is occurring and resulting in lower polytropic efficiency. These indications include warmer than expected exhaust temperatures and polytropic expansion exponents below the heat capacity ratio for the fluid. It is anticipated that heat transfer losses will be greatly reduced by using a working fluid with a much lower heat capacity ratio. Nomenclature �s, �p = isentropic and polytropic efficiency, respectively � = shaft rotational position (TDC = 0) Cp = constant pressure specific heat of working fluid D = cylinder diameter hi = specific enthalpy of inlet mass he = specific enthalpy of exhaust