Opportunity
Global warming has caused Earth’s temperature to rise by approximately 0.18°C per decade since 1981, and 2022 was the sixth-warmest year on record. As temperatures increase, cooling has become essential, especially for space cooling, which accounts for nearly 40% of primary building energy usage. Conventional air compression-based air conditioners consume significant energy and contribute heavily to CO₂ emissions, exacerbating environmental pollution and energy shortages, particularly in impoverished regions. Semi-passive and passive cooling methods, such as radiative cooling and thermal sorption, have emerged as energy-saving alternatives, but their effectiveness is often hindered by low solar reflectivity, low emissivity, and absorption of infrared radiation under high humidity. Evaporative cooling, while promising, requires a substantial water supply, which is a challenge in water-scarce areas. Therefore, there is a strong need for an improved passive cooling strategy that operates with minimal energy and water consumption while overcoming the limitations of existing technologies.
Technology
The present invention provides a composite material for passive cooling comprising a porous structure that includes expanded graphite and a surfactant, impregnated with at least one crystalline deliquescent salt selected from chlorides, bromides, sulphates, or nitrates. The expanded graphite acts as a scaffold, and the surfactant (e.g., Triton X-100) renders the graphite super hydrophilic, enhancing water adhesion and mass transfer. The deliquescent salt, such as anhydrous LiCl, absorbs atmospheric moisture during the night and releases it during the day via evaporation, achieving cooling through the latent heat of phase transition. To prevent solution leakage, corrosion, and carryover, the porous structure is encapsulated in a moisture-permeable, liquid-impermeable material like PTFE membrane (pore size ~3 μm). The composite is compressed into shape-stabilized blocks with a density of about 400–600 kg/m³. An apparatus for passive cooling includes a housing with an inlet and outlet, a winding air passage defined by multiple passive cooling units containing the composite sorbent, and a low-power fan to draw air. The fan’s driving power is substantially lower than the cooling power, enabling nearly passive operation. This design allows the composite to continuously absorb and desorb water, providing effective indoor cooling without external energy input for the sorption process.
Advantages
- Passive cooling without additional energy input, relying on natural water absorption and evaporation.
- Enhanced water sorption performance due to super hydrophilic expanded graphite matrix and high-capacity deliquescent salts like LiCl (sorption up to 1.06 g/g at 40% RH, 3.93 g/g at 80% RH).
- Prevention of solution leakage and improved durability through PTFE membrane encapsulation, which also prevents corrosion of metal components.
- Higher cooling efficiency with cooling power up to 630 W/m², significantly outperforming radiative cooling (theoretical max ~160 W/m²).
- Environmental friendliness: no additional water consumption and reduced CO₂ emissions compared to conventional AC.
- Adaptability to diverse climatic conditions, demonstrating effective temperature drops across different temperatures and humidity levels.
- Reduced health risks by avoiding solution carryover and agglomeration associated with hydrous salts.
- Cost-effectiveness by lowering energy consumption and maintenance needs.
Applications
- Indoor passive cooling for residential and commercial buildings.
- Integration into building ventilation channels for sorbent cool roof systems.
- Space cooling in regions with varying climates, including tropical savannas, humid subtropical, and desert climates.
- Energy saving and carbon mitigation in buildings: annual cooling energy reduction of 12.16% to 39% in nine global cities.
- Potential for zero-energy cooling in net-zero energy buildings.
