The investment of solar energy in life applications has become mandatory to maintain a clean environment and reduce the use of fossil fuels. This work aimed to improve the performance of solar air heater (SAH) by using evacuated tube solar collectors ETSC integrated with nano-enhancer phase change material (NE-PCM). To achieve this purpose, a system consisting of 5 linked collecting panels was designed, fabricated, and experimentally investigated. Each panel included a glass-evacuated tube with two concentric aluminum pipes installed inside. NE-PCM was placed between the inlet and outlet air paths inside the evacuated tube to enhance the heat transfer rate. The performance was investigated with and without NE-PCM at five mass flow rates (0.006, 0.008, 0.01, 0.03, and 0.05 kg/s). Experimental results revealed that the highest temperature was 116, 108, 102, 95, and 93 °C, respectively, for the above mass flow rates without adding NE-PCM. The outlet temperature was decreased by 6–15 °C when using NE-PCM. The SAH efficiency was increased by 29.62% compared to the system without NE-PCM at 0.05 kg/s. The maximum thermal efficiency for the system with NE-PCM was 62.66% at 0.05 kg/s, and the pressure drop was 6.79 kPa under the same conditions. As well known, the hot air is used for a variety of purposes including space heating, food processing, drying of fruit, vegetables, dairy, and solar cooking.

The presented study aligns with global sustainability goals by integrating water harvesting with energy-efficient transactions. Infrastructure and energy in buildings are worth attention to tackle water scarcity and energy consumption management. This research highlights the dual benefits of enhancing water sustainability while optimizing energy use in air conditioning (AC) systems. The methodology includes investigating the potential of harnessing condensate water produced by a network of 113 AC units and a surface area of 3000 m². All data have been collected and evaluated at the faculty of engineering at Horus University in New Damietta, Egypt. The greatest cumulative amount of water produced for one day was 51.24 L in June, 51.56 L in July, 55.65 L in August, and 53.32 L in September. By multiplying the average amount by the number of AC units in the building, the daily water production records 5977.7 L/day, approximately 6 m³. Although this number of air conditioners consumes 236 kW to operate, the operating cost is not considered since water is a byproduct. Therefore, the main cost can be represented by maintenance, renewal, and water treatment when used for drinking. Also, using this water in other applications without treatment, such as irrigation, cleaning, practical experiments, and engineering applications, provides a great return on investment. Graphical Abstract