In our relentless pursuit of energy-efficient cooling solutions, we often find inspiration in the most unexpected places. In this Behind the Paper story, we explore an innovation that came from an everyday product that many parents are familiar with— baby diapers. In fact, the story of this discovery began when the author Dr. Qiaoqiang Gan was changing his second daughter’s diaper. He noticed a white powder inside, the same water-locking material found in various products like baby diapers and agricultural applications for keeping soil moisture. This substance was intriguing not for its intended use, but rather for its optical property: an exceptional whiteness that indicates high solar reflectance (ability to reflect sunlight). Usually, white materials are excellent candidates for rejecting solar heating.
Driven by curiosity, Dr. Gan collected some of this material from his 2-year-old daughter’s diaper, and placed it outdoors overnight, in the hopes of observing radiative cooling in action. To his astonishment, the material became cooler than the ambient, and radiative cooling indeed took place. Even more surprising was that the white powder spontaneously self-fabricated into a continuous film overnight, and a totally green method for scalable fabrication.
Figure 1 | Fabrication and working principle of sodium polyacrylate (PAAS) photonic film. (a) The transfiguring procedures of the mechanical-robust and free-standing PAAS photonic film from the regular commercial inclusions (PAAS dry powders), generally used as the baby diaper filler, to (b) radiative and evaporative cooling films. (c) The cooling mechanism of the passive radiative and evaporative sodium PAAS photonic film. It absorbs atmospheric moisture during the nighttime and evaporates under direct sunlight. The porous structures backscatter the sunlight, and its infrared molecular vibrations enable high mid-infrared emittance for efficient radiation heat dissipation.
This incredible transformation was facilitated by the moisture in the air at night, which bonded the absorbent powder, sodium polyacrylate (PAAS), into a white film (see Figure 1). This film exhibited not only an excellent resistance to solar absorption but also a remarkable capacity to emit infrared radiation. Moreover, it possessed a unique feature during the day— it could evaporate the water it had absorbed during the night, providing efficient evaporative cooling. Once the water had evaporated into the atmosphere, the remaining white solid film displayed the hardness like a golf ball. What's most impressive is that this entire manufacturing process relied solely on atmospheric moisture without any need for additional chemicals or energy consumption. This represents a green manufacturing process that leaves behind zero carbon emissions. For long-time applications, it reduces energy consumption, featuring an even negative carbon emission.
But how does it all work? Figure 1 illustrates the working principle of the PAAS photonic film. Its porous structure allows it to absorb moisture from its surroundings, particularly in areas with high relative humidity at nighttime. This harvested water then evaporates, carrying heat away from the film when it's exposed to sunlight during the day. The porous structure also scatters sunlight, reducing the solar heating effect. Simultaneously, the film dissipates heat to outer space through the atmospheric window via thermal radiation, achieving hybrid passive radiative cooling. Intriguingly, the PAAS photonic film can regenerate itself by absorbing moisture when ambient humidity increases, forming a perfect daily water cycle that facilitates heat absorption and release.
Now, let's see a practical demonstration of the PAAS photonic film’s radiative cooling and evaporative cooling abilities. We conducted a direct comparison with standard roof shingles under identical outdoor conditions, as depicted in Figure 2. During noontime, when the sun blazed with an average solar intensity of 800 Wm−2, the traditional roof shingle soared to a scorching 40°C above the ambient temperature. In contrast, the PAAS photonic film showed an impressive sub-ambient temperature drop of 5°C, highlighting its superior hybrid cooling performance. This advantage becomes particularly apparent on partly cloudy days when conventional radiative cooling materials struggle to cope with varying conditions. As night falls, the PAAS film absorbs moisture and undergoes a regeneration process. Then, as the day returns, the re-energized PAAS photonic film comes back into action, providing self-sustained and energy-free cooling. This feature underscores not only its environmental sustainability but also its potential to offer a cost-effective and efficient solution to combat rising temperatures and reduce energy consumption for cooling.
Figure 2 | (a) The outdoor cooling experimental setup under direct sunlight. (b) Temperature variations of the sodium polyacrylate (PAAS) photonic film and roof shingle. Insets are photos of the sky in different weather conditions. The red arrow points out the maximum temperature of the shingle. (c) Energy-savings map across the United States with the sodium polyacrylate (PAAS) cooling film. (d) Estimated average annual electricity and predicted average annual CO2 emission reduction per capita and globally.
The potential for energy-saving and carbon emission reduction of the PAAS photonic film is substantial. Using an energy estimation model, we assessed the impact of integrating this film into building rooftops. The results, as shown in Figure 2b in a simulated energy-saving map of over 100 US cities, indicate that up to 104.14 GJ of cooling energy can be conserved for a typical midrise apartment building in a tropical climate zone, equating to an 18.7% reduction in baseline cooling energy consumption. Furthermore, through energy conversion based on the EnergyPlus model simulation, the film's application results in an average saving of 2293.2 kWh of electricity for a mid-rise apartment building. This translates to an 887.9 kg reduction in CO2 emissions and a substantial 14.8 kg reduction per capita. On a global scale, this leads to an estimated annual reduction of 118.4 billion kg in total CO2 emissions, representing roughly a 0.33% reduction from current global emissions.
Katika utafutaji wetu usio na mwisho wa suluhisho la baridi lenye ufanisi wa nishati, mara nyingi hupata msukumo katika maeneo yasiyotarajiwa. Katika hadithi hii ya nyuma ya karatasi, tunachunguza uvumbuzi ambao ulitoka kwa bidhaa ya kila siku ambayo wazazi wengi wanafahamu - vitambaa vya watoto. Kwa kweli, hadithi ya ugunduzi huu ilianza wakati mwandishi Dk Qiaoqiang Gan alikuwa akibadilisha diaper ya binti yake wa pili. "Kutoka kwa mchanganyiko wa dawa za kulevya, ""mchanganyiko wa maji"" uliopatikana katika bidhaa mbalimbali kama vile diapers za watoto na matumizi ya kilimo ya kuweka unyevu wa udongo." "Kama vile ilivyotarajiwa, nyenzo hiyo haikuwa ya kuvutia kwa sababu ya matumizi yake, bali kwa sababu ya uwezo wake wa kuona: rangi nyeupe ya kipekee inayoonyesha uwezo mkubwa wa kutafakari mwangaza wa jua.""" Kwa kawaida, vifaa vyenye rangi nyeupe ni vyenye kufaa sana kukataa joto la jua. Kwa sababu ya udadisi, Dk Gan alikusanya baadhi ya vifaa hivyo kutoka kwenye kitambaa cha binti yake mwenye umri wa miaka miwili, na kuviweka nje usiku kucha, kwa matumaini ya kuona baridi ya mnururisho ikitenda. Kwa mshangao wake, nyenzo hiyo ikawa baridi kuliko mazingira, na kwa kweli baridi ya mnururisho ilitokea. Hata zaidi ya kushangaza ni kwamba poda nyeupe spontaneously self-fabricated katika filamu kuendelea usiku, na njia ya kijani kabisa kwa ajili ya utengenezaji scalable. Picha ya kwanza: Ujenzi wa filamu ya photonic ya polyacrylate ya sodiamu (PAAS) "Kutumia ""Machine ya Urekebishaji wa PEA"" ni njia ya kuunda filamu ya kawaida ya kibiashara ya PAAS (kufuta poda kavu) ambayo hutumiwa kama kujaza diaper ya mtoto, na kuunda filamu ya baridi ya radiative na evaporative." (c) Utaratibu wa baridi ya passively radiative na evaporative sodium PAAS photonic filamu. Huvuta unyevu wa angahewa wakati wa usiku na huyeyuka chini ya mwangaza wa jua. Miundo porous backscatter mwanga wa jua, na vibrations yake infrared molekuli kuwezesha juu ya kati ya infrared emittance kwa ufanisi mionzi joto dissipation. Urefu wa hewa usiku ulifanya kuwa rahisi kwa sababu ya unyevu wa hewa, ambao ulifanya poda ya kuingiza, polyacrylate ya sodiamu (PAAS), kuwa filamu nyeupe. Filamu hiyo haikupinga jua vizuri tu bali pia ilikuwa na uwezo wa kutokeza mnururisho wa infrared. Isitoshe, ilikuwa na uwezo wa pekee wakati wa mchana - iliweza kuharibu maji ambayo ilikuwa imechukua wakati wa usiku, na hivyo kutoa baridi ya kuharibu yenye ufanisi. Baada ya maji hayo kutoweka, sehemu ya juu ya maji hiyo ilionekana kuwa ngumu kama mpira wa gofu. Jambo la kushangaza zaidi ni kwamba mchakato huu wote wa utengenezaji ulitegemea tu unyevu wa angahewa bila haja ya kemikali za ziada au matumizi ya nishati. Hii inawakilisha mchakato wa utengenezaji wa kijani ambao huacha nyuma uzalishaji wa kaboni sifuri. Kwa matumizi ya muda mrefu, hupunguza matumizi ya nishati, na ina hata uzalishaji wa kaboni hasi. Lakini mambo hayo yote hufanyaje kazi? Picha ya 1 inaonyesha kanuni ya kazi ya filamu ya PAAS photonic. Muundo wake wenye umbo la mviringo unamruhusu kuvuta unyevu kutoka kwa mazingira yake, hasa katika maeneo yenye unyevu wa juu wakati wa usiku. Maji hayo huyeyuka na kuondoa joto kutoka kwenye filamu hiyo inapowekwa kwenye jua wakati wa mchana. Muundo huo wenye umbo la mviringo pia hueneza nuru ya jua, na hivyo kupunguza joto linalotokezwa na jua. Wakati huo huo, filamu hiyo huondoa joto kwenye anga ya nje kupitia dirisha la angahewa kupitia mnururisho wa joto, ikifikia baridi ya mnururisho wa mseto. Filamu ya PAAS inaweza kujirekebisha yenyewe kwa kunyonya unyevu wakati unyevu wa mazingira unapoongezeka, na kuunda mzunguko kamili wa maji ya kila siku ambayo inasaidia kunyonya na kutolewa kwa joto. Sasa, hebu tuone maonyesho ya vitendo ya PAAS photonic filamu ya radiative baridi na evaporative baridi uwezo. Kwa mfano, tunaweza kulinganisha shingles ya kawaida na shingles ya kawaida, na kwa mfano, tunaweza kulinganisha shingles ya kawaida na shingles ya kawaida, na shingles ya kawaida, na shingles ya kawaida, na shingles ya kawaida. Wakati wa mchana, wakati jua lilipokuwa liking'aa kwa kiwango cha wastani cha 800 Wm2, shingle ya kawaida ya paa iliongezeka hadi joto la 40 ° C juu ya joto la mazingira. Kwa upande mwingine, filamu ya PAAS ya photonic ilionyesha kushuka kwa joto la chini ya mazingira ya 5 ° C, ikionyesha utendaji wake bora wa baridi ya mseto. Faida hii inakuwa dhahiri hasa juu ya siku sehemu cloudy wakati vifaa vya kawaida radiative baridi mapambano ili kukabiliana na hali tofauti. Wakati wa usiku, filamu ya PAAS huchukua unyevu na hupitia mchakato wa kuzaliana upya. Wakati wa joto, filamu ya PAAS ya photonic inakuja tena, na kutoa baridi ya kujitegemea na isiyo na nishati. Hii sifa inasisitiza si tu uendelevu wake wa mazingira lakini pia uwezo wake wa kutoa suluhisho la gharama nafuu na ufanisi wa kupambana na kuongezeka kwa joto na kupunguza matumizi ya nishati kwa baridi. Picha ya pili: (a) Mfumo wa majaribio ya baridi ya nje chini ya mwangaza wa jua wa moja kwa moja. (b) Mabadiliko ya joto ya sodium polyacrylate (PAAS) photonic filamu na paa shingle. Picha zilizo ndani ni picha za anga katika hali tofauti za hali ya hewa. Mshale mwekundu unaonyesha halijoto ya juu ya shingle. Utafiti wa Uokoaji wa Nishati katika Marekani na Filamu ya Kupumzika ya Polyacrylate ya Sodiamu (d) Takwimu za wastani wa umeme wa kila mwaka na utabiri wa wastani wa kupunguza uzalishaji wa CO2 kwa kila mtu na ulimwenguni. Uwezekano wa kuokoa nishati na kupunguza uzalishaji wa kaboni wa filamu ya photonic ya PAAS ni kubwa. Kutumia mfano wa makadirio ya nishati, tulikadiria athari ya kuunganisha filamu hii kwenye paa za ujenzi. Matokeo, kama inavyoonyeshwa katika Mchoro 2b katika ramani ya kuokoa nishati ya zaidi ya miji 100 ya Marekani, yanaonyesha kwamba hadi 104.14 GJ ya nishati ya baridi inaweza kuhifadhiwa kwa jengo la kawaida la ghorofa ya kati katika eneo la hali ya hewa ya kitropiki, ikilinganishwa na kupunguzwa kwa 18.7% katika matumizi ya nishati ya baridi ya msingi. Kwa kuongezea, kupitia ubadilishaji wa nishati kulingana na mfano wa EnergyPlus, matumizi ya filamu husababisha wastani wa kuokoa umeme wa 229kWh kwa jengo la ghorofa ya kati. Hii inamaanisha kupunguzwa kwa uzalishaji wa CO2 kwa kilo 887.9 na kupunguzwa kwa kilo 14.8 kwa kila mtu. Kwa kiwango cha kimataifa, hii inaongoza kwa kupunguzwa kwa wastani wa kila mwaka kwa kilo 118.4 bilioni katika uzalishaji wa CO2, ikimaanisha kupunguzwa kwa karibu 0.33% kutoka uzalishaji wa sasa wa ulimwengu.