Cooling photovoltaic modules for efficiency gains via spectrally selective back-coating

Photovoltaic (PV) power generation is essential for the global low-carbon transition, yet its efficiency decreases significantly with increasing temperature. Effective thermal regulation is therefore critical to sustain power output, and passive cooling coatings provide a promising solution. This study experimentally investigates the cooling performance of large-area PV modules with back-surface coatings featuring different combinations of solar reflectance and emissivity. Results show that solar reflectance is the primary driver for daytime cooling: compared with black paint group (Group BP) with similarly high mid-infrared emissivity, radiative coupling group (Group RC) reduced the three-day averaged daytime temperature by up to 4.37 °C and suppressed the peak temperature by up to 5.30 °C. Under high-reflectance conditions, mid-infrared emissivity provides a complementary benefit mainly in peak-load mitigation: compared with aluminium foil group (Group AF), Group RC reduced the peak temperature at the hottest position by up to 3.80 °C. These thermal improvements translated into electrical gains: Group RC coating increased daily energy yield by up to 8.7% relative to the untreated module, exceeding Group AF (up to 4.5%), while Group BP caused a performance loss of up to 6.7%. Mechanistically, high reflectance minimizes solar heat absorption by rejecting near-infrared and diffuse radiation incident on the back surface, thereby suppressing net radiative heat gain. Simultaneously, high emissivity facilitates mid-infrared radiative heat dissipation to the immediate surroundings, particularly under peak irradiance, when convective cooling becomes insufficient. The interaction of these optical properties establishes a dual light reflection–thermal radiation mechanism that stabilizes module temperature and improves conversion efficiency. These findings highlight that reflectance governs average temperature reduction, while emissivity ensures peak load dissipation, jointly maximizing electrical yield. This work provides quantitative evidence and physical insight into the thermal–optical coupling mechanisms of radiative coupling coatings, demonstrating their potential as a scalable, zero-energy, and cost-effective solution for enhancing PV performance and durability in high-temperature environments.