It is well known that additional cooling is essential for human health in hot environments. Widely used cooling technologies such as air conditioners consume large amounts of energy (nearly 15% of global electricity consumption) and are associated with huge greenhouse gas emissions. Radiative cooling is a promising sustainable human body cooling technology as the radiative heat transfer is the primary heat dissipation pathway for the human body, accounting for 40%–60% of the total heat transfer from the human body. More importantly, it dissipates human body heat with no energy input and no carbon species output, making it a promising alternative to existing energy-intensive cooling systems.
In recent years, a variety of high-performance human body radiative cooling textiles have been developed, providing solid evidence of the effectiveness of radiative cooling technology for human body cooling. However, to date, few radiative cooling textiles have been scaled up for actual commercial products. We then pondered the question, what are the main factors that are hindering the large-scale application of radiative cooling technology? To answer this question, we reviewed the previously developed radiative cooling textiles for personal thermal management. It was found that these radiative cooling textiles can be mainly categorized into two groups according to their cooling mechanism, namely transmission-type radiative cooling materials (highly transparent to human body radiation) for indoor applications and emission-type radiative cooling materials (high absorption/emission in human radiation waveband) for sunny outdoor applications.
Obviously, existing radiative cooling textiles can only provide adequate human body cooling effect in limited environments. When the environment changes between indoors and outdoors or changes between sunny and cloudy scenarios, these textiles will exhibit the reduced or even disappearing cooling capacity for human body. Specifically, the existing transmission-type textiles exhibit poor cooling performance when exposed to outdoor heat due to the massive solar thermal load caused by the thickness limitation (a thin thickness to ensure high human radiation transparency, resulting in low solar reflectance). We note that various efforts, including the design of nano/microparticle-based textiles (such as zinc oxide-polyethylene based nanotextiles), have been made to improve the solar reflectance of such materials, it remains a challenge to balance human body compatibility (harmlessness and wearability) with high cooling performance. As for emission-type textiles, they can cool the human body by emitting human heat into the cold outer space through the atmospheric window (8–13 μm). Therefore, there should be a clear heat transfer channel to send radiative heat to outer space through the atmospheric window. When such materials are used indoors or in cloudy outdoor environments and the channel for radiative heat transfer into outer space is blocked, the cooling performance is largely compromised. Based on this, we generated the idea of this work, to develop a radiative cooling textile that can provide effective human body cooling performance under varied weather conditions (including sunny, cloudy, and indoor scenarios), namely an all-weather radiative human body cooling textile.
We speculated that a combination of the main spectral characteristics of the two existing types of radiative cooling textiles (high transmission for human body radiation and high absorption/emission in the atmospheric window waveband) was expected to achieve the above aim. Naturally, we proposed an adaptive (or selective) emission-transmission textile model accordingly, i.e., high human radiation absorption/emission in the atmospheric window band and high human radiation transmission in the non-window band, as well as high reflection in the solar waveband. We then demonstrated its feasibility through theoretical calculations based on blackbody radiation law and steady-state heat transfer theory. Specifically, taking two typical environments, outdoors and indoors, as an example. In sunny outdoor environments under strong sunlight (800 W m−2), the adaptive-type textile enabled a significantly lower skin surface temperature than transmission-type (25.7 °C lower) and emission-type (4.2 °C lower) textiles. For indoor environments, the skin surface temperature with the adaptive-type textile was close to (0.8 °C higher) that with transmission-type textiles but was significantly lower (2.5 °C) than that with emission-type textiles.
As a demonstration, assisted with designs at molecular and nano-scale, we developed a polyoxymethylene (POM) nano-textile with desired adaptive emission-transmission characteristics via a sample and scalable electrospinning method. In addition to having the suitable molecular structure for the desired cooling performance, POM was chosen because it is widely used, inexpensive, commercially available, and has excellent physic-chemical properties (e.g., non-toxicity, high thermal stability, and high mechanical strength), which provide the basis for the scaling up of the environment-adaptive radiative cooling textiles.
To test the actual radiative cooling performance of POM textiles in real hot environments, we collaborated with Prof. Jia Zhu's group at Nanjing University. Multi-scenario radiative cooling thermal measurements (including sunny outdoor, cloudy outdoor and indoor environments) were conducted in Nanjing (a city in southern China) in summer. The results show that the POM textile exhibited a significantly enhanced radiative human body cooling performance that was better than those of typical transmission-type, emission-type, and commercial cotton textiles in sunny outdoor (2.6–8.8 °C cooler), cloudy outdoor (0.7–3.6 °C cooler), and indoor (0.5–1.2 °C cooler) scenarios. In addition to the superior radiative cooling performance, the synthesized POM textile also exhibited many metrics required for wearability, namely, good breathability, high mechanical strength, high waterproofness, high anti-humidity capability, high intense UV resistance, and high outdoor exposure stability.
Finally, COVID-19 has been ravaging the world for the past three years, and medical protective clothing is critical to stopping the spread of the virus. This inspired us to tailor our POM textiles as health hazard protective clothing as a proof of application. As a result, the POM-textile based protective clothing showed significantly better cooling performance than a commercial counterpart in sunny outdoor (5.4 °C cooler), cloudy outdoor (1.3 °C cooler), and indoor scenarios (~1.0 °C cooler). This work provides a new design for both outdoor and indoor human body cooling, contributing to the next generation smart textiles and other applications supporting sustainability.