Cool the World with Nanoporous Polyethylene Fabric
The paper in Nature Sustainability is here: http://go.nature.com/2og3VuA
As the rapid development of modern society, sustainability is becoming an increasingly important topic. Sustainability requires both exploration of renewable energy sources and reduction of energy consumption. The modern urbanization brings a mushrooming number of skyscrapers, but they consume a huge amount of energy and emit greenhouse gas every second. Only cooling systems of buildings contribute to 15% of electricity consumption and 10% of greenhouse gas emissions globally. Developing new techniques to cut down energy consumption of cooling systems will be significant toward global sustainability.
Controlling the building heat exchange between indoor and outdoor environment, such as improving building heat insulation, is a traditional route to save energy consumption. However, it will be much more efficient if we consider thermal management of every single person. Over 50% of heat dissipation of human body is via mid-infrared light radiation. However, clothes made from traditional textile materials, such as cotton, prevent mid-infrared light from transmitting out. This mid-infrared opacity makes traditional textiles not effective to cool down human body. Therefore, textiles which is transparent to mid-infrared light will have superior cooling effect. People wearing cooler textiles do not need as low set-point of air-conditioner as before. More importantly, every one degree centigrade increase of set-point corresponds to 10% of energy saving for indoor cooling.
In order to realize a mid-infrared transparent textile in practical use, three requirements should be achieved: 1) this textile should be woven/knitted based on fibres with cotton-like softness, to ensure wearing comfort; 2) the fibre material should be intrinsically mid-infrared transparent; 3) the fibre material should be visibly opaque, which is the very basic requirement for clothes.
Polyethylene, one of the most common polymers, is the ideal mid-infrared transparent material. However, normal polyethylene is also transparent to visible light. In our work, we utilized paraffin oil to create nanoscale cavities in polyethylene. At high temperature, paraffin oil is compatible with polyethylene and they can form a homogenous mixture in which paraffin oil occupies nanoscale domains. This mixture was extruded continuously into micro-fibres. Then paraffin oil inside the polyethylene matrix was washed out by solvent leaving behind nanopores. These nanoscale pores can strongly scatter visible light, and thus visible opacity can also be realized. Furthermore, the empty nano-domains can significantly reduce polyethylene’s mechanical hardness, ensuring the nanoporous polyethylene fibre is cotton-like soft.
Pounds of nanoporous polyethylene fibres were efficiently woven/knitted into fabric via industrial weaving/knitting machine. Such fabric has both high mid-infrared transparency and high visible opacity. It is practical to wear nanoporous polyethylene fabric due to its compelling wearability. Moreover, the nanoporous polyethylene fabric can increase set-point of air-conditioner by 2.3 ℃, compared with cotton cloth of similar thickness. The temperature regulation expansion of 2.3 ℃ can help save up to 20.2% of the cooling energy, suggesting that large amount of CO2 emission can be reduced. We expect that the nanoporous polyethylene fabric will not only revolutionize textile for radiative cooling, but also achieve a breakthrough in decreasing energy consumption for global sustainability.