Laboratory-Scale Screening of Boron Nitride-Reinforced Hydrogel Cooling Layers for Passive Thermal Management of Photovoltaic Modules

The rapid expansion of solar photovoltaics has increased interest in passive cooling materials that can reduce module temperature without additional electrical power. This study evaluated boron nitride (BN)-reinforced hydrogel sheets as rear-mounted cooling layers for miniature photovoltaic panels under controlled indoor irradiation. Hydrogels were fabricated from a poly(vinyl alcohol), κ-carrageenan, bovine gelatin, and cellulose nanocrystal matrix with 0, 3, 7, or 10 wt.% hexagonal BN. Each BN loading was tested with and without a flexible graphite foil interlayer. Thermal performance was measured across three independent 3 h irradiation runs, while swelling, three-hour water loss, and time-resolved voltage behavior were also analyzed. The graphite-free 7 wt.% BN sample, BN7G0, produced the strongest thermal response, reaching a mean midpoint temperature of 32.76 ± 0.28 °C and an 8.06 °C reduction relative to the uncooled control. BN10G0 also showed substantial cooling, with a 6.79 °C reduction. The 7 wt.% BN hydrogel had the highest swelling response, 823.81%, and retained most of its mass during irradiation, losing 4.97% over 3 h. However, water loss did not scale directly with cooling, since BN3G0 showed the highest relative water loss but only modest temperature reduction. These results indicate that cooling depended on a balance of water storage, water loss, filler loading, and interfacial configuration rather than evaporation or BN loading alone. Voltage measurements remained stable but did not establish photovoltaic power improvement. Overall, BN7G0 was the strongest formulation within this laboratory-scale screening study and provides a basis for future direct thermal, electrical, and outdoor durability testing.