High-efficiency solar thermoelectric conversion enabled by movable charging of molten salts

The study addresses the challenge of intermittency, low energy density, and uneven distribution of solar radiation in solar-thermal-electric systems, which necessitates efficient thermal storage to enable stable electricity generation. Thermoelectric generators (TEGs) benefit from large temperature differences and higher operating temperatures to enhance the figure of merit (ZT) and efficiency. However, latent heat storage materials like molten salts typically suffer from low thermal conductivity, limiting charging rates and overall system performance. Prior work has focused on optical concentration, heat sink design, and cooling optimization, as well as enhancing thermal conductivity of phase change materials—approaches that are less practical for medium-to-high temperature molten salts. This work proposes and demonstrates a solar-thermal-electric system using high-temperature molten salts charged by a magnetically responsive solar-thermal conversion mesh (MSCM) operating in a movable charging mode. The hypothesis is that moving the photothermal absorber with the receding melt front under magnetic attraction will accelerate charging, increase stored thermal energy, sustain larger temperature differentials across the TEG, and thereby improve electrical output and efficiency, including post-illumination operation.

The paper reviews advances in solar-thermal applications including domestic heating, steam generation, desalination, and power plants. It highlights thermoelectric conversion principles and efforts to improve solar-thermal-electric systems via optical concentration, thermal management, and cooling optimization. The need for thermal storage is emphasized to overcome solar intermittency, with latent heat materials offering higher storage capacity and narrow release temperature ranges. Conventional methods to enhance thermal conductivity (carbon materials, metal foams, nanoparticles) are effective mainly at low temperatures and are challenging for molten salts. The authors reference their recent development of magnetically accelerated, movable charging for molten salt phase change materials that doubles charging rates while maintaining storage capacity, motivating its integration with a TEG for improved solar-thermal-electric conversion.

System architecture: A solar-thermal storage subsystem based on molten salts is thermally coupled to a thermoelectric generator (TEG). The hot side consists of an aluminum container (50 mm × 50 mm × 60 mm) filled with 170 g of commercial solar salt (60 wt% NaNO3, 40 wt% KNO3; onset melting ~210 °C; latent heat ~121 J/g). The TEG (TEG1-127-1.0-1.3; 30 mm × 30 mm × 3 mm) is attached to one side of the container using thermal grease; the cold side is connected to a metallic fin (35 mm × 35 mm × 10 mm) cooled by a fan. The container bottom and three sidewalls (excluding the TEG side) are insulated with 1 cm thick glass fiber. Solar-thermal converter (MSCM): A magnetic iron mesh (80-mesh) is coated via dip-coating with a graphite-PDMS composite to form a magnetically responsive solar-thermal conversion mesh. Coating solution: 1 g graphite (~0.5 μm particle size), 1 g PDMS Part A, 0.1 g curing agent Part B, and 20 g n-hexane; mixed at 25 °C for 2 h. Clean iron meshes (ethanol/acetone sonication) are immersed and then cured at 60 °C for 24 h. The resulting MSCM has ~50 μm pores, hydrophobic surface (water contact angle ~154°), broadband high absorptance (>90% UV-Vis, >85% NIR 1.0–2.5 μm), mechanical flexibility, and corrosion resistance in molten salts. Movable charging setup: The MSCM (5 cm diameter) is placed on top of the solid salts; a magnet is positioned beneath the container to pull the MSCM downward as top layers melt, ensuring the mesh follows the receding solid-liquid interface for rapid, volumetric charging. In fixed mode (no magnet), the MSCM remains at the top surface. Operation and measurements: Illumination is provided by a solar simulator at 30 kW/m². Temperatures at hot and cold sides of the TEG are recorded with thermocouples (Agilent 34972A). Output voltage is measured across a load resistor R = 5 Ω to compute output power P = U²/R and total generated electrical energy by integrating power over time. The cold side fan power (0.45 W) is accounted for in efficiency calculation. Efficiency calculation: Overall solar-thermal-electric efficiency η = P / (A qs t' + qc t), where A = 19.6 cm² (MSCM area), qs = 30 kW/m² (solar flux), t' = 60 min (charging duration), qc = 0.45 W (cold-side power consumption), and t = 120 min (generation duration).

• Movable charging yields larger temperature difference across TEG: under 30 kW/m², hot side reached ~200 °C (stabilized after ~20 min) with cold side ~100 °C (ΔT ~100 °C). Fixed mode reached only ~120 °C hot and ~60 °C cold (ΔT ~60 °C).
• Electrical output improvements with movable charging:
  • Maximum output voltage increased to ~3.3 V vs ~2.0 V (reported as 165% of fixed mode).
  • Post-illumination endurance: voltage remained >1.5 V for 15 min vs 8 min (reported increase 187.5%).
  • Output power reached ~0.25 W vs <0.1 W under fixed mode.
  • Total generated electrical energy over 120 min: ~5500 J (movable) vs ~2100 J (fixed).
• Overall solar-thermal-electric conversion efficiency: 2.56% (movable) vs 0.97% (fixed), accounting for 0.45 W cold-side power.
• Demonstration: Electrical power from stored heat illuminated 42 orange LEDs in parallel; continuous power generation persisted after illumination ceased due to latent heat release near constant temperature.
• Materials/optical properties: MSCM exhibited broadband absorptance (>90% UV-Vis; >85% NIR, 0.25–2.5 μm), hydrophobicity (contact angle ~154°), and ~50 μm pore size aiding motion through molten salt.

Movable charging overcomes the low thermal conductivity bottleneck of molten salt latent heat storage by transporting the photothermal absorber into the melt and advancing the solid–liquid interface, enabling rapid, volumetric heat deposition. This enhances the stored thermal energy and sustains a higher temperature difference across the TEG, directly improving voltage, power, and total energy output. The system also benefits from latent heat release at near-constant temperature after illumination ends, supporting continued electricity generation and addressing solar intermittency. The high operating temperatures of molten salts significantly exceed those of low-temperature PCM systems, yielding higher voltages and greater usable electrical energy for practical loads. The authors note potential for further efficiency gains by attaching TEGs to additional container faces and by employing passive cooling strategies (air/water) to eliminate auxiliary fan power, which would increase net efficiency. The approach uses low-cost, reusable materials and straightforward fabrication, suggesting scalability and relevance for combined thermal storage and thermoelectric power in solar-thermal systems.

The work demonstrates a solar-thermal-electric system that integrates a magnetically responsive, movable solar-thermal conversion mesh with high-temperature molten salt storage to accelerate charging and enhance thermoelectric power generation. Compared to fixed charging, the movable mode increased the maximum voltage, extended post-illumination output duration, doubled power, and raised overall efficiency to 2.56%. The system successfully powered over 40 LEDs concurrently, illustrating practical utility. The strategy offers a promising route to improve solar-thermal energy harvesting, storage, and conversion. Future improvements may include deploying TEGs on multiple container faces, adopting passive cooling to eliminate parasitic power consumption, and scaling for broader applications in lighting and heating.

• Only one TEG mounted on a single container side; authors indicate additional TEGs on other sides could raise efficiency.
• Active cooling (fan) was used; its power consumption (0.45 W) was accounted for, but it reduces net efficiency. Passive cooling alternatives are proposed but not demonstrated here.
• Experiments conducted under concentrated solar flux (30 kW/m²) using a solar simulator; outdoor, long-term, and variable-environment performance are not reported.
• System scale is laboratory-scale; durability and long-term compatibility of MSCM in molten salts, and corrosion over extended cycles, are not detailed beyond hydrophobic protection observations.