Harvest Ambient Heat via Constraint-Shaped Phase-Change Cycles: Micro $\Delta T$, Subcooled Liquid, and Liquid-Only Compression

TL;DR

This paper proposes a theoretical phase-change heat engine that harvests ambient thermal energy using micro-temperature differences and liquid-only compression, demonstrating positive net work in an ideal reversible limit.

Contribution

It introduces a novel cycle design leveraging constraint-shaped phase-change processes for ambient energy harvesting at micro-temperature differences.

Findings

Cycle yields positive net work in reversible limit

Energy harvested from ambient micro-temperature difference

Work output per cycle is extremely small

Abstract

Conventional heat engines typically require two distinct thermal reservoirs, with their efficiency strictly bounded by the Carnot limit. We present a theoretical design for a phase-change heat engine that utilizes water as the working fluid undergoing state transitions within geometry-constrained flow paths. The proposed cycle operates under a micro-temperature difference (1--2\,$^\circ$C) and relies on liquid-only compression. The system harvests thermal energy via an \textbf{ambient micro-temperature difference} relative to the environment ($q_{\mathrm{in}} \approx 8.37\,\mathrm{kJ}/\mathrm{kg}$ at 24--26\,$^\circ$C). Expansion work is recovered from the enthalpy drop during flash evaporation. Comprehensive numerical analysis using NIST property data confirms that, in the reversible limit, the cycle yields positive net work while maintaining standard thermodynamic consistency. This study illustrates the theoretical potential for ambient energy harvesting via low-pressure phase change, although the extremely small work output per cycle suggests that hardware realization will require exceptional mechanical precision to overcome parasitic losses.