Performance enhancement of photovoltaic/thermal systems with heat pump units using alternative refrigerants and nanoparticle TiO2 with linseed oil cooling coating

This study experimentally investigates an integrated photovoltaic/thermal heat pump (PV/T–HP) system enhanced by a biomimicry-inspired, bio-based TiO2 linseed oil nanocoating for hot, high-irradiance operation. A 1.0064 m² monocrystalline PV module (150 W rated) was coupled to a 120 W vapour compression heat pump and tested with three refrigerants (R134a, R600a and R1234yf). Coating thickness was screened in mini-cells (100, 200, and 300 nm), and 100 nm was selected as the optimum based on higher maximum power and lower cell temperature under 1000 W m⁻² illumination. Outdoor measurements were then performed in Kirkuk, Iraq, over four representative seasons (January, March, August and October) using three typical days per condition (n = 3) from 07:00–17:00, comparing baseline PV, PV/T–HP (active cooling) and PV/T–HP + coating (active–passive cooling). Electrical and thermal outputs, PV surface temperature, useful heat gain, and COP were evaluated with uncertainty propagation. Across all seasons and refrigerants, adding the coating to the PV/T–HP increased combined efficiency. The best seasonal cases were 64.93% overall efficiency in winter with R1234yf, and 76.59%, 71.51% and 77.32% in spring, summer and autumn with R600a, respectively (HP-only: 62.97%, 73.44%, 70.39% and 77.32%). Peak COP occurred near solar noon and reached 2.65 (winter, R1234yf), 3.78 (spring, R600a), 4.27 (summer, R600a), and 3.85 (autumn, R600a). The integrated configuration reduced peak PV surface temperature by 17 °C in winter,19 °C in spring, 34 °C in summer, and 24 °C in autumn relative to baseline, explaining the observed electrical gains. These results provide season-specific guidance for refrigerant selection and demonstrate a sustainable pathway that supports SDG 7 and SDG 13.