Passive cooling technology is zero energy consumption and environmentally friendly, and it is possible to reduce dependence on energy-intensive cooling technologies such as compression refrigeration. Generally, in order to achieve efficient cooling, passive systems should reflect as much sunlight as possible to minimize input energy. Recently, an unconventional passive cooling process was demonstrated in a system with a high light absorption rate of approximately 99.3%.

Diagram 1.a, a diagram of the proposed system. The system can collect solar energy to generate steam while achieving passive cooling. The CNT-PAAm cone-shaped aerogel is arranged on the roof of the house and is connected to the room through a heat transfer unit. Due to its high solar energy absorption properties, aerogels can collect solar energy to accelerate the evaporation of water. At the same time, the temperature of the cone-shaped aerogel is much lower than the temperature of the surrounding environment. Therefore, the room can be cooled by a slightly heated cone-shaped aerogel. b, Optical photo of passive cooling and solar steam generation system at the same time.

The main component of the system is an aerogel with a unique cone-shaped structure for heat transfer. Solar thermal energy and environmental thermal energy are used to drive the vaporization of water in the aerogel directional channels, where the water remains in a capillary state. Under a simulated solar irradiation condition, the average evaporation rate of cone-shaped aerogel is as high as 2.23 kg m-2 h-1, and the maximum specific cooling power is 271.56 W m-2. The outdoor prototype shows that on sunny days in summer, the temperature of the cooling chamber can be 13.7°C lower than the temperature of the ambient air. This kind of synchronous solar steam power generation and passive cooling system has huge application potential in the process of heat concentration and environmental cooling.

Figure 6. The outdoor cooling performance of simultaneous passive cooling and solar steam generators. a, the local solar intensity at different times of sunny and cloudy days. b. Measure the temperature of the ambient air, the cooling chamber and the top of the aerogel on a sunny day. c. Measure the temperature of the ambient air, the cooling chamber and the top of the aerogel on a cloudy day. d. Temperature measurement of ambient air, cooling chamber and top of aerogel at night.