Dynamics of Melting in PCM Windows via Direct Light Transmission (Energies 2021, 14(3):721)

Source: Heim, Wieprzkowicz et al., Energies 2021, 14(3):721 (open access, full text retrieved). Credibility: High (peer-reviewed, open access, primary measured data; corroborated by 2 research agents).

Key points

• Triple-glazed prototype: two cavities, 4 mm glass panes, 16 mm PCM layer, paraffin RT21HC (melt ~21 °C).
• Direct-light transmittance: 0.06 (solid) → 0.64 (liquid) — a ~10× swing. Case B (PCM inner cavity) tops out at 0.57.
• Melt optics are non-linear: <0.10 through the mushy “period II”, then rapid 0.1→0.64 during non-uniform melting (period III, 30–40 min), with plateaus from interreflections between solid/mushy/liquid sub-layers.
• Refractive index drives opacity: RT21HC liquid nD≈1.434, solid nD≈1.531; glass≈1.52. Solid PCM ≠ glass index → scattering; liquid ≈ glass → clear.
• Full melt: 105–135 min @ 1000 W/m²; 220–250 min @ 500 W/m². Irradiance changes kinetics only, not end transmittance.
• Authors’ caveat: “light transmittance versus temperature cannot be treated as a linear function.”

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