HVAC exhaust-air-assisted thermal management of rooftop photovoltaics for energy retrofitting toward nearly zero-energy buildings

Retrofitting existing educational buildings toward nearly zero-energy performance is challenging in hot-humid climates, where cooling demand dominates electricity use and high photovoltaic operating temperatures reduce renewable energy yield. This study proposes an integrated retrofit framework for an educational building in Alexandria, Egypt, combining demand-side energy conservation, rooftop photovoltaic generation, and a novel strategy that reuses HVAC exhaust air to cool photovoltaic modules and the underlying roof surface. Hourly dynamic simulations were performed in DesignBuilder to evaluate building energy performance, while ANSYS Fluent was used to assess the thermal response of the exhaust-air-assisted photovoltaic cooling configuration. A 25-year life-cycle economic assessment was conducted, including a two-factor sensitivity analysis of electricity tariff and recoverable FCU value. The retrofit reduced annual electricity consumption from 943.95 to 389.34 MWh, corresponding to a 58.75% reduction. Exhaust-air-assisted cooling reduced PV operating temperature by 4.65–13.37 °C across the investigated climatic conditions and increased annual PV generation from 127.10 to 130.264 MWh/year, corresponding to a 2.49% enhancement. Roof cooling provided an additional 1.20 MWh/year reduction in building demand, resulting in a final demand of 388.14 MWh/year and increasing the PV offset to 33.56%. At the base electricity tariff, the integrated retrofit yielded an estimated first-year electricity-cost saving of approximately USD 27,443/year, with positive economic performance maintained across all investigated sensitivity scenarios. These findings demonstrate the potential of HVAC exhaust-air reuse to enhance PV performance and support nearly zero-energy retrofitting in hot-humid climates.