The physics behind the problem is unforgiving. Overhead conductors, typically aluminum conductor steel-reinforced (ACSR) cables, are limited by the balance between internal resistive Joule heating and passive heat dissipation to the environment. This balance defines the cable’s ampacity, the maximum current it can carry without exceeding its maximum allowable operating temperature, which is usually around 75 degrees Celsius for ACSR conductors. In hot urban climates where air temperatures can surpass 50 degrees Celsius, the thermal gradient between conductor and atmosphere collapses, degrading both convective and radiative cooling precisely when air-conditioning demand peaks. The consequences cascade: every 10-degree increase in conductor temperature raises the electrical resistance of aluminum and its associated Joule losses by roughly 4 percent, while heat accelerates mechanical sag, reduces ground clearance, and promotes annealing that weakens the metal. Recent outage data from China show that heatwaves increase outage frequency by nearly 4 percent and outage duration by about 8 percent, with heatwave-induced outages estimated to cost approximately 3.4 billion US dollars in the analyzed scenario.

Existing mitigation strategies fall short. Dynamic line rating systems squeeze more capacity from existing thermal margins but do nothing to lower conductor temperature itself. High-temperature low-sag conductors tolerate hotter operation but demand extensive and costly retrofitting of transmission infrastructure. Meanwhile, conventional surfaces are optically and thermally suboptimal: aged bare aluminum reflects sunlight only modestly and emits infrared radiation poorly, while common polymeric insulators such as cross-linked polyethylene absorb substantial solar radiation and conduct heat at a sluggish 0.3 watts per meter-kelvin, effectively trapping internally generated heat. The KAUST team recognized that the ideal cable surface must do three things at once: reflect sunlight, radiate heat efficiently in the mid-infrared, and conduct Joule heat from the core to the surface.

That third requirement is what sets this work apart from a decade of radiative cooling research. Most radiative cooling coatings, developed for buildings and textiles, deliberately have low thermal conductivity to block heat from entering. For a cable, which generates its own heat, that strategy is exactly backwards. The researchers inverted another classical concept as well: the critical insulation radius. In pipeline engineering, adding insulation to a cylinder initially increases heat loss by enlarging the surface area, until a critical thickness is reached beyond which insulation suppresses heat transfer. For overhead cables, the team repurposed this counterintuitive principle to maximize heat rejection. Their thermal resistance network model showed that for a coating with conductivity near 1 watt per meter-kelvin on a 10-millimeter-radius cable, total thermal resistance reaches a minimum at a critical thickness of 2.8 millimeters, about 3 percent lower than that of the uncoated conductor. Beyond that thickness, the added conduction path dominates and cooling efficiency declines, so optimal thickness must be tuned to cable radius, wind conditions, and existing insulation.

The material itself is elegantly engineered. Hexagonal boron nitride was chosen because it uniquely combines strong optical scattering, high dielectric contrast, and an intrinsic thermal conductivity exceeding 400 watts per meter-kelvin. A Lorenz-Mie scattering model revealed that solar backscattering is maximized for h-BN particle diameters of 300 to 500 nanometers, coinciding with the peak energy of the solar spectrum, and Monte Carlo ray tracing confirmed the prediction. Reflectance rises with particle loading up to about 10 percent by volume and saturates near 0.91 for coating thicknesses between 0.2 and 2 millimeters. In the final composite, loaded at 40 percent h-BN, the thermal conductivity of the PDMS film jumped from 0.18 to 1.28 watts per meter-kelvin, with cross-sectional electron microscopy revealing a percolated network of platelets that creates efficient phonon transport pathways. Optically, the coating achieves a solar reflectance of 0.92, extending into the ultraviolet for photostability, and a mid-infrared emissivity of 0.95, driven by the vibrational modes of the polymer matrix. Angular-resolved measurements confirmed that reflectance barely depends on incidence angle, essential for a cylindrical cable illuminated by a moving sun.

Durability testing addressed the harsh realities of outdoor service. The pristine coating’s tensile strength of 2.5 megapascals falls short of the 21 megapascals of cross-linked polyethylene, but sandwiching a thin, porous 0.13-millimeter Nylon layer into the film raised tensile strength to roughly 45 megapascals, more than double that of XLPE, while preserving a conductivity of about 1.2 watts per meter-kelvin because the polymer infiltrates the Nylon’s pores. The surface is hydrophobic, with a contact angle of 115.3 degrees, so dust washes off and the coating regains its original white appearance. After 1000 cycles of standardized sandpaper abrasion simulating windblown sand, its optical properties were essentially unchanged. Following 240 hours of accelerated ultraviolet aging, reflectance and emissivity held steady, aided by roughly 95 percent UV reflectance at 340 nanometers. The coating also resisted salt spray and, critically for wildfire-prone regions, did not ignite or deform under direct flame exposure.

Field validation came from outdoor experiments in Thuwal, Saudi Arabia, conducted at KAUST. Over three days of continuous exposure in July 2025, with peak midday solar irradiance near 1100 watts per square meter, the HTCRC-coated cable consistently remained the coolest of three suspended cables, about 5 degrees cooler than bare ACSR and 15 degrees cooler than a commercial insulated cable. To capture the effect of internal heat generation, the team built simulated cables containing resistive wires that mimic Joule heating. Over a seven-day measurement in August 2025, with ambient temperatures reaching 45 degrees Celsius, both the insulated and bare cables exceeded the 75-degree safety threshold, peaking near 80 and 90 degrees respectively, while the coated cable cycled between 50 and 70 degrees, staying about 5 degrees below the limit even at midday. At night, the coated and insulated cables tracked each other closely because both emit infrared strongly, whereas bare aluminum, with an emissivity of only about 0.1, stayed hotter.

Cooler cables also sag less. Using the catenary equation, the standard analytical tool for suspended transmission lines, the team projected sag growth over 10 and 20 years of service and found the coated cable exhibited markedly smaller increases, indicating enhanced long-term dimensional stability and reduced risks of mechanical failure, short circuits, and outages. The thermal margin translates directly into capacity. Applying the CIGRE Technical Brochure 601 heat balance model to an ACSR 450 AlFe 8 conductor, the coated cable achieved a calculated ampacity of 830 amperes, far exceeding the 630 amperes of a black insulated cable and the 710 amperes of a bare silver cable. Under dynamic diurnal conditions with a 40-degree peak ambient temperature and 1000 watts per square meter of irradiance, the coated cable delivered up to 165 amperes more than the insulated cable at solar noon, when conventional conductors hit their thermal bottleneck.

Scaled to real climates, the numbers become strategically significant. A year-long simulation for Riyadh, where summer temperatures frequently exceed 45 degrees Celsius, showed an average daytime ampacity increase of about 10 percent, peaking at 18.3 percent in July. Across 13 representative hot-climate cities worldwide, the projected annual daytime ampacity gain ranges from 6 to 10 percent. Because the coating’s density of about 1.2 grams per cubic centimeter sits within the range of commercial cable insulation materials, retrofitting adds no meaningful weight or tensile load, and the design remains compatible with IEC 60502 insulation standards. The broader implication is a way to break the vicious feedback loop in which rising temperatures degrade transmission capacity exactly when cooling-driven demand peaks. By passively unlocking latent capacity in existing infrastructure, this photonic coating offers cities a scalable, low-cost path to climate resilience at a moment when electrification and the energy appetite of artificial intelligence are pushing grids toward their thermal limits.

Subject of Research: High-thermal-conductivity radiative cooling coatings for overhead power transmission cables

Article Title: Photonic thermal management of overhead cables via high-thermal-conductivity radiative cooling

Article References: Xu, K., Cao, P.-F., El-Demellawi, J. K., Qasem, H., & Gan, Q. (2026). Photonic thermal management of overhead cables via high-thermal-conductivity radiative cooling. Light: Science & Applications, 15(1), Article 385. https://doi.org/10.1038/s41377-026-02485-1

Image Credits: AI Generated

DOI: 10.1038/s41377-026-02485-1

Keywords: radiative cooling, power cables, hexagonal boron nitride, PDMS, ampacity, thermal management, photonic coatings, grid resilience, heatwaves, solar reflectance, mid-infrared emissivity, CIGRE 601