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

Ting Peng

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.

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

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\,C) and relies on liquid-only compression. The system harvests thermal energy via an \textbf{ambient micro-temperature difference} relative to the environment ( at 24--26\,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.
Paper Structure (18 sections, 3 equations, 1 figure, 2 tables)

This paper contains 18 sections, 3 equations, 1 figure, 2 tables.

Table of Contents

  1. Cycle implementation: micro-temperature difference and liquid-only compression
  2. Basic scheme and layout
  3. Flow path asymmetry.
  4. Full closed cycle: four states and four processes (Water)
  5. Process 1$\to$2: Pump compresses liquid only.
  6. Process 2$\to$3: Heat absorption.
  7. Process 3$\to$4: Flash expansion.
  8. Net work and efficiency.
  9. Heat and work balance per cycle (per unit mass).
  10. Cycle closure (mass and energy).
  11. Can the cycle's own output sustain the 2 $^\circ$C difference with surplus?
  12. Engineering implementation (mature industrial components)
  13. Mechanism for work extraction (expander) and work consumption (pump).
  14. Key conclusions and application outlook
  15. Continuous operation and scaling to higher power
  16. ...and 3 more sections

Figures (1)

  • Figure 1: Energy flows between environment and cycle components (kJ/kg) for Water.