Design and performance of a miniature free piston expander

Abstract

The availability of free thermal energy provide an opportunity for power generation at miniature length scales (millimeter). Though free piston based expanders at macro length scales (centimeter and above) are found to be suitable for heat energy harvesting, their implementation at miniature scales are plagued by significant parasitic losses, yielding low thermal efficiencies. Through physics-based models we investigate the behavior and performance of a miniature free piston expander that operates on an open cycle. Here, we explore the design space of the free piston expander with an objective to achieve an efficiency of at least 15%. Three observations are reported that contribute to higher efficiency operation: (1) a higher injection pressure;(2) an optimum nondimensionalized duration of injection time of 1.5; and (3) softer springs, lower loads, and heavier pistons. A sample calculation showed that a centimeter-sized expander can generate an output power of 2.24 W at 18% efficiency. We observe that both the performance parameters of the expander, namely efficiency and output power are sensitive to injection pressure of the working fluid compared to the time duration of the working fluid injection. Our study reveals that the miniature free piston expander is promising for low temperature waste-heat harvesting.

The effect of working fluid properties on the performance of a miniature free piston expander for waste heat harvesting

“…To generate a pressure-volume diagram, the model Eqs. [10][11][12][13] are numerically integrated in the order 1→2→3→4→5→1 (Fig. 1b) starting with state 1: ∆V 1 , ∆P 1 , ∆T 1 , ∆V 1 = 0.…”

“…The resulting volume ∆V 3 , pressure ∆P 3 , and temperature ∆T 3 from the injection process (1→2→3) is computed by numerically integrating Eqs. [10][11][12][13] from the initial condition ∆V 1 , ∆P 1 , ∆T 1 , ∆V 1 = 0 for a predetermined time durationt 13 . The volumes, pressures, and temperatures ∆V 4 , ∆P 4 , ∆T 4 and ∆V 1 , ∆P 1 , ∆T 1 from the expansion (3→4) and exhaust (4→5→1) processes are computed by numerically integrating Eqs.…”

“…The volumes, pressures, and temperatures ∆V 4 , ∆P 4 , ∆T 4 and ∆V 1 , ∆P 1 , ∆T 1 from the expansion (3→4) and exhaust (4→5→1) processes are computed by numerically integrating Eqs. [10][11][12][13] from the initial conditions ∆V 3 , ∆P 3 , ∆T 3 , ∆V 3 and ∆V 4 , ∆P 4 , ∆T 4 , ∆V 4 = 0 for timest 34 andt 41 , respectively, such that the piston velocity ∆V 4 = ∆V 1 = 0. It is worth noting that the timest 34 andt 41 are 'a priori ' and are determined during the integration process.…”

“…Like others [8,11], the unique features of the freepiston architecture encouraged prior efforts at developing a FPE for waste heat recovery at the miniature length scales [3,12]. This previous work established many of the working principles [3,12], as well as the technology demonstration of various mechanical components of the FPE [13,14]. Generally, the centimeter-scale FPE under consideration ( Fig.…”

“…As a result, these make an ideal choice for low temperature waste heat harvesting applications. Therefore, this paper builds upon the prior foundation of boiler and FPE design through models and experiments [3,13,12], while concentrating on the effect of working fluid properties (Table 1) on the FPE performance. Unlike other approaches, where the nonlinear spring stiffness of the working fluid is linearized [18], the present study makes no such approximation.…”