Liquid Thermo-Responsive Smart Window Derived from Hydrogel

Abstract

•A liquid-filled smart window with excellent thermochromic performance•Large thermal energy storage capability to shift the electricity load peak•Combining solar regulation and heat storage to cut off building energy consumption•Better soundproof function than double-glazed glass with good scalability Buildings account for 40% of global energy consumption, while windows are the least energy-efficient part of buildings. Thermochromic window responds to temperature automatically to regulate the solar transmission solely. In this work, a smart window that combines good thermochromic performance and large thermal energy storage capability was introduced. At lower temperature, the window is transparent to let in the solar transmission; when heated, the window blocks sunlight automatically to cut off solar gain. The added function of heat storage further reduces the energy consumption and shift the electricity load peak to lower price period. This first thermo-responsive liquid encapsulated window panel offers a think-out-of-box strategy, giving unique advantage of easy fabrication, good uniformity, scalability, and soundproofing. Buildings account for 40% of global energy consumption, while windows are the least energy-efficient part of buildings. Conventional smart windows only regulate solar transmission. For the first time, we developed high thermal energy storage thermo-responsive smart window (HTEST smart window) by trapping the hydrogel-derived liquid within glasses. The excellent thermo-responsive optical property (90% of luminous transmittance and 68.1% solar modulation) together with outstanding specific heat capacity of liquid gives the HTEST smart window excellent energy conservation performance. Simulations suggested that HTEST window can cut off 44.6% heating, ventilation, and air-conditioning (HVAC) energy consumption compared with the normal glass in Singapore. In outdoor demonstrations, the HTEST smart window showed promising energy-saving performance in summer daytime. Compared with conventional energy-saving glasses, which need expensive equipment, the thermo-responsive liquid-trapped structure offers a disruptive strategy of easy fabrication, good uniformity, and scalability, together with soundproof functionality that opens an avenue for energy-saving buildings and greenhouses. Buildings account for 40% of global energy consumption, while windows are the least energy-efficient part of buildings. Conventional smart windows only regulate solar transmission. For the first time, we developed high thermal energy storage thermo-responsive smart window (HTEST smart window) by trapping the hydrogel-derived liquid within glasses. The excellent thermo-responsive optical property (90% of luminous transmittance and 68.1% solar modulation) together with outstanding specific heat capacity of liquid gives the HTEST smart window excellent energy conservation performance. Simulations suggested that HTEST window can cut off 44.6% heating, ventilation, and air-conditioning (HVAC) energy consumption compared with the normal glass in Singapore. In outdoor demonstrations, the HTEST smart window showed promising energy-saving performance in summer daytime. Compared with conventional energy-saving glasses, which need expensive equipment, the thermo-responsive liquid-trapped structure offers a disruptive strategy of easy fabrication, good uniformity, and scalability, together with soundproof functionality that opens an avenue for energy-saving buildings and greenhouses. In 2018, the Intergovernmental Panel on Climate Change (IPCC) reduced the global warming allowance from 2°C to 1.5°C to alarm the great urgency of climate change, which emphasized the significance of energy conservation and carbon emission reduction.1Masson-Delmotte V. Global Warming of 1.5 °C: an IPCC Special Report on the Impacts of Global Warming of 1.5° C Above Pre-Industrial Levels and Related Global Greenhouse Gas Emission Pathways.the Context of Strengthening the Global Response to the Threat of Climate Change, Sustainable Development, and Efforts to Eradicate Poverty. World Meteorological Organization, 2018Google Scholar The building sector consumes approximately 40% of the total energy, while heating, ventilation, and air-conditioning (HVAC) consume half of the energy in buildings.2Ke Y. Zhou C. Zhou Y. Wang S. Chan S.H. Long Y. Emerging thermal-responsive materials and integrated techniques targeting the energy-efficient smart window application.Adv. Funct. Mater. 2018; 28: 1800113Crossref Scopus (173) Google Scholar,3Wang S. Owusu K.A. Mai L. Ke Y. Zhou Y. Hu P. Magdassi S. Long Y. Vanadium dioxide for energy conservation and energy storage applications: synthesis and performance improvement.Appl. Energy. 2018; 211: 200-217Crossref Scopus (76) Google Scholar To address this issue, improving building energy efficiency is critical in energy conservation. Windows are considered as the least energy-efficient part of the building envelope. In hot seasons, most of the window-directed solar energy will be converted into heat and lead to high cooling demand, while in winter, windows are responsible for 30% of energy loss.4Gao Y. Wang S. Kang L. Chen Z. Du J. Liu X. Luo H. Kanehira M. VO2–Sb:SnO2 composite thermochromic smart glass foil.Energy Environ. Sci. 2012; 5: 8234-8237Crossref Scopus (161) Google Scholar, 5Hee W.J. Alghoul M.A. Bakhtyar B. Elayeb O. Shameri M.A. Alrubaih M.S. Sopian K. The role of window glazing on daylighting and energy saving in buildings.Renew. Sustain. Energ. Rev. 2015; 42: 323-343Crossref Scopus (249) Google Scholar, 6Kalnæs S.E. Jelle B.P. Phase change materials and products for building applications: a state-of-the-art review and future research opportunities.Energ. Build. 2015; 94: 150-176Crossref Scopus (312) Google Scholar The most studied energy-saving windows are focused on chromogenic technologies, including electrochromic, photochromic, and thermochromic.7Granqvist C.G. Lansåker P.C. Mlyuka N.R. Niklasson G.A. Avendaño E. Progress in chromogenics: new results for electrochromic and thermochromic materials and devices.Sol. Energy Mater. Sol. Cells. 2009; 93: 2032-2039Crossref Scopus (234) Google Scholar, 8Warwick M.E.A. Binions R. Advances in thermochromic vanadium dioxide films.J. Mater. Chem. A. 2014; 2: 3275-3292Crossref Google Scholar, 9Granqvist C.G. Transparent conductors as solar energy materials: a panoramic review.Sol. Energy Mater. Sol. Cells. 2007; 91: 1529-1598Crossref Scopus (1286) Google Scholar, 10Cui Y.Y. Ke Y. Liu C. Wang N. Chen Z. Zhang L.M. Zhou Y. Wang S. Gao Y.F. Long Y. Thermochromic VO2 for energy-efficient smart windows.Joule. 2018; 2: 1707-1746Abstract Full Text Full Text PDF Scopus (216) Google Scholar, 11Ke Y. Zhang Q. Wang T. Wang S. Li N. Lin G. Liu X. Dai Z. Yan J. Yin J. et al.Cephalopod-inspired versatile design based on plasmonic VO2 nanoparticle for energy-efficient mechano-thermochromic windows.Nano Energy. 2020; 73: 104785Crossref Scopus (26) Google Scholar Examples of chromogenic materials includes hydrogels12Tang L. Wang L. Yang X. Feng Y. Li Y. Feng W. Poly(N-isopropylacrylamide)-based smart hydrogels: design, properties and applications.Prog. Mater. Sci. 2021; 115: 100702Crossref Scopus (92) Google Scholar and liquid crystals.13Gutierrez-Cuevas K.G. Wang L. Zheng Z.G. Bisoyi H.K. Li G. Tan L.S. Vaia R.A. Li Q. Frequency-driven self-organized helical superstructures loaded with mesogen-grafted silica nanoparticles.Angew. Chem. Int. Ed. Engl. 2016; 55: 13090-13094Crossref PubMed Scopus (57) Google Scholar, 14Dong L. Feng Y. Wang L. Feng W. Azobenzene-based solar thermal fuels: design, properties, and applications.Chem. Soc. Rev. 2018; 47: 7339-7368Crossref PubMed Google Scholar, 15Wang L. Bisoyi H.K. Zheng Z. Gutierrez-Cuevas K.G. Singh G. Kumar S. Bunning T.J. Li Q. Stimuli-directed self-organized chiral superstructures for adaptive windows enabled by mesogen-functionalized graphene.Mater. Today. 2017; 20: 230-237Crossref Scopus (141) Google Scholar Among three, thermochromic materials are considered as the most cost-effective, rational stimulus and zero energy input properties.16Liu C. Cao X. Kamyshny A. Law J.Y. Magdassi S. Long Y. VO2/Si-Al gel nanocomposite thermochromic smart foils: largely enhanced luminous transmittance and solar modulation.J. Colloid Interface Sci. 2014; 427: 49-53Crossref PubMed Scopus (65) Google Scholar, 17Seeboth A. Ruhmann R. Mühling O. Thermotropic and thermochromic polymer based materials for adaptive solar control.Materials. 2010; 3: 5143-5168Crossref PubMed Scopus (127) Google Scholar, 18Ke Y. Wang S. Liu G. Li M. White T.J. Long Y. Vanadium dioxide: the multistimuli responsive material and its applications.Small. 2018; 14: e1802025Crossref PubMed Scopus (111) Google Scholar However, it is an inevitable challenge to further enhance its energy-saving capability due to the intrinsic limits of conventional thermochromic materials.19Ke Y. Chen J. Lin G. Wang S. Zhou Y. Yin J. Lee P.S. Long Y. Smart windows: electro-, thermo-, mechano-, photochromics and beyond.Adv. Energy Mater. 2019; 9: 1902066Crossref Scopus (176) Google Scholar All the studied smart windows only regulate light transmission. However, the heating and cooling of the house are much more complicated. High thermal energy storage (TES) materials are widely used in walls,20Li L. Yu H. Liu R. Research on composite-phase change materials (PCMs)-bricks in the west wall of room-scale cubicle: mid-season and summer day cases.Building and Environment. 2017; 123: 494-503Crossref Scopus (38) Google Scholar floors,21Zhou G. He J. Thermal performance of a radiant floor heating system with different heat storage materials and heating pipes.Appl. Energy. 2015; 138: 648-660Crossref Scopus (101) Google Scholar and roofs22Saffari M. Piselli C. de Gracia A. Pisello A.L. Cotana F. Cabeza L.F. Thermal stress reduction in cool roof membranes using phase change materials (PCM).Energ. Build. 2018; 158: 1097-1105Crossref Scopus (46) Google Scholar because they can reduce cooling/heating loads and shift energy load to low price periods.23Zhu N. Li S. Hu P. Wei S. Deng R. Lei F. A review on applications of shape-stabilized phase change materials embedded in building enclosure in recent ten years.Sustain. Cities Soc. 2018; 43: 251-264Crossref Scopus (61) Google Scholar, 24Song M. Niu F. Mao N. Hu Y. Deng S. Review on building energy performance improvement using phase change materials.Energ. Build. 2018; 158: 776-793Crossref Scopus (191) Google Scholar, 25Gil A. Medrano M. Martorell I. Lázaro A. Dolado P. Zalba B. Cabeza L.F. State of the art on high temperature thermal energy storage for power generation. Part 1—concepts, materials and modellization.Renew. Sustain. Energ. Rev. 2010; 14: 31-55Crossref Scopus (1183) Google Scholar, 26Sharma A. Tyagi V.V. Chen C.R. Buddhi D. Review on thermal energy storage with phase change materials and applications.Renew. Sustain. Energ. Rev. 2009; 13: 318-345Crossref Scopus (3552) Google Scholar, 27Kuznik F. David D. Johannes K. Roux J.J. A review on phase change materials integrated in building walls.Renew. Sustain. Energ. Rev. 2011; 15: 379-391Crossref Scopus (707) Google Scholar According to their principle of thermal energy storing, TES materials can be categorized into sensible heat storage material, latent heat material, and chemical heat storage material.28Cabeza L.F. Gutierrez A. Barreneche C. Ushak S. Fernández Á.G. Inés Fernádez A.I. Grágeda M. Lithium in thermal energy storage: a state-of-the-art review.Renew. Sustain. Energ. Rev. 2015; 42: 1106-1112Crossref Scopus (84) Google Scholar,29Gutierrez A. Miró L. Gil A. Rodríguez-Aseguinolaza J. Barreneche C. Calvet N. Py X. Inés Fernández A.I. Grágeda M. Ushak S. Cabeza L.F. Advances in the valorization of waste and by-product materials as thermal energy storage (TES) materials.Renew. Sustain. Energ. Rev. 2016; 59: 763-783Crossref Scopus (71) Google Scholar To ensure a stable and satisfied performance, the TES materials need to have good specific heat capacity. The good specific heat capacity ensures that TES material is able to store large amount of heat, while the relatively high thermal conductivity allows the uniform distribution of the heat stored in the material and improves the heat storage efficiency. Also, TES material needs to meet some physical requirements like high cycle stability, non-corrosiveness, and low system complexity.30Pielichowska K. Pielichowski K. Phase change materials for thermal energy storage.Prog. Mater. Sci. 2014; 65: 67-123Crossref Scopus (1092) Google Scholar The majority building materials such as wood, metal, glass, and concrete generally have low TES less than 100 kJ kg−1 ranging from 10°C–70°C (Figure 1A; Table S1). Some commercially available high TES materials include paraffin, fatty acid, and inorganic salt, a category of phase change materials (PCMs), are not suitable in glass due to the lack of luminous transparency, which is critical for windows. Water, due to its outstanding specific heat capacity (4.2 kJ kg−1 K−1), has a significantly higher TES capability (∼250 kJ kg−1) than the majority of construction materials (Figure 1A; Table S1).54Hasnain S.M. Review on sustainable thermal energy storage technologies, part I: heat storage materials and techniques.Energy Convers. Manag. 1998; 39: 1127-1138Crossref Scopus (911) Google Scholar Hereby, we developed a revolutionary high energy storage thermo-responsive smart window (HTEST smart window), which leverages high solar energy modulation together with high TES capability intrinsic in water-rich thermo-responsive liquid (TRL), which is derived from usual hydrogel.55Zhou Y. Cai Y. Hu X. Long Y. Temperature-responsive hydrogel with ultra-large solar modulation and high luminous transmission for "smart window" applications.J. Mater. Chem. A. 2014; 2: 13550-13555Crossref Google Scholar As showed in Figure 1B, the HTEST window is designed with poly (N-isopropylacrylamide) (PNIPAm) hydrogel particles dispersed in water, which are trapped between two layers of glasses. The conventional thermo-responsive hydrogel is in the gel form and laminated between glasses to regulate the light transmittance solely.56Li X.-H. Liu C. Feng S.-P. Fang N.X. Broadband light management with thermochromic hydrogel microparticles for smart windows.Joule. 2019; 3: 290-302Abstract Full Text Full Text PDF Scopus (97) Google Scholar, 57Zhou Y. Layani M. Wang S. Hu P. Ke Y. Magdassi S. Long Y. Fully printed flexible smart hybrid hydrogels.Adv. Funct. Mater. 2018; 28: 1705365Crossref Scopus (78) Google Scholar, 58Zhou Y. Dong X. Mi Y. Fan F. Xu Q. Zhao H. Wang S. Long Y. Hydrogel smart windows.J. Mater. Chem. A. 2020; 8: 10007-10025Crossref Google Scholar The newly developed TRL experience a similar hydrophilic to hydrophobic transition at the lower critical solution temperature (LCST) as conventional hydrogel; below the LCST, the water molecules are within the PNIPAm macromolecules, which give high transparency, allowing the high solar transmission to heat the room in winter. Once heated above the LCST, the water molecules will be released from the PNIPAm, and the shrinkage particles will cause scattering of the light (Figure 1C). We would like to highlight that different from the in situ synthesis technique used for the production of conventional hydrogel,55Zhou Y. Cai Y. Hu X. Long Y. Temperature-responsive hydrogel with ultra-large solar modulation and high luminous transmission for "smart window" applications.J. Mater. Chem. A. 2014; 2: 13550-13555Crossref Google Scholar,58Zhou Y. Dong X. Mi Y. Fan F. Xu Q. Zhao H. Wang S. Long Y. Hydrogel smart windows.J. Mater. Chem. A. 2020; 8: 10007-10025Crossref Google Scholar the newly developed TRL is synthesized by dispersing PNIPAm particles into water and form the homogeneous solution, which gives it an advantage of free flowing. Although the current thermo-responsive hydrogel has been intensively investigated,59Liu J. An T. Chen Z. Wang Z. Zhou H. Fan T. Zhang D. Antonietti M. Carbon nitride nanosheets as visible light photocatalytic initiators and crosslinkers for hydrogels with thermoresponsive turbidity.J. Mater. Chem. A. 2017; 5: 8933-8938Crossref Google Scholar, 60Owusu-Nkwantabisah S. Gillmor J. Switalski S. Mis M.R. Bennett G. Moody R. Antalek B. Gutierrez R. Slater G. Synergistic thermoresponsive optical properties of a composite self-healing hydrogel.Macromolecules. 2017; 50: 3671-3679Crossref Scopus (43) Google Scholar, 61La T.G. Li X. Kumar A. Fu Y. Yang S. Chung H.J. Highly flexible, Multipixelated thermosensitive smart windows made of tough hydrogels.ACS Appl. Mater. Interfaces. 2017; 9: 33100-33106Crossref PubMed Scopus (62) Google Scholar, 62Zhou Y. Cai Y. Hu X. Long Y. VO2/hydrogel hybrid nanothermochromic material with ultra-high solar modulation and luminous transmission.J. Mater. Chem. A. 2015; 3: 1121-1126Crossref Google Scholar, 63Zhou Y. Wan C. Yang Y. Yang H. Wang S. Dai Z. Ji K. Jiang H. Chen X. Long Y. Highly stretchable, elastic, and ionic conductive hydrogel for artificial soft electronics.Adv. Funct. Mater. 2019; 29: 1806220Crossref Scopus (303) Google Scholar none of the work discusses the thermal capacity concept and none of them are free flow. We adopted the new form of the TRL acting as an energy storage layer with the additional functionality of absorbing and storing the energy. The liquid phase gives a unique advantage of easy fabrication (by simply pouring into the double-glazed glass, Figure 1D; Video S1) as well as the high potential of scaling up and uniformity (Figure 1E), which are difficult and costly in the conventional low-E glass as the expensive setup is a must. eyJraWQiOiI4ZjUxYWNhY2IzYjhiNjNlNzFlYmIzYWFmYTU5NmZmYyIsImFsZyI6IlJTMjU2In0.eyJzdWIiOiI3MTZkNzMxOWUzZTM2OGEyNjQ3OThjNzA0MWEwNTRmZiIsImtpZCI6IjhmNTFhY2FjYjNiOGI2M2U3MWViYjNhYWZhNTk2ZmZjIiwiZXhwIjoxNjQ4MTEyMTA5fQ.eUFgExTYgmQp5Jg01ZqXFQKFZimYpzCUuCH3iBSW-ppNAauKDnyH7Aij2IQrBu9--TGCB2Mo-bd_b_e3twAyXZlKmtkI_PSab9Mqr7xWnuAV8OdqwSJghyA6KUYmRtheEIPYf689vTGDS3l6NOB1WYI0wBiAftKh-dA7MS1ShQCUw0K1IBchV23ehKVA9wLvi68p1Ixfy5qKQda_d_GaWTUDOdcYXiQ3M9rtsGRmIFfLhAmNzTg_8KsAbTwIbeLGbs9_0ProWIHiGd_ZEkwfAj-c8vzSFDdKXy_XTgx6rfnhVjjynv3Ur69YlqNdWqmMEgEGummiILe9zA50R97sng Download .mp4 (1.71 MB) Help with .mp4 files Video S1. Filling Process of the Large-Scale 1-m2 HTEST Window The fabricated HTEST smart window showed a high luminous transmittance (Tlum) of ∼90% and a high solar modulating ability (ΔTsol) of 68.1%. Moreover, the TES of 261 kJ kg−1 is achieved in the temperature range of 10°C∼70°C due to a higher specific heat capacity (Cp) of TRL than water (252 kJ kg−1). The excellent energy-saving performance was proved by the simulation and actual indoor and outdoor demonstration compared with conventional glasses. The HTEST has the added benefits of no constraint of window shape and good potential of soundproofing due to the free-flowing liquid.64Wenmaekers R. Van der Aa B. Pronk A. Couthinho A. van Luxemburg L.C.J. The Sound insulation of water. 34th German Conference on Acoustics, 2008Google Scholar HTEST smart window may revolutionize the window industry with the outstanding energy-saving performance, which has a high potential for commercialization. Figure 2A shows the transmittance spectra of the TRL with thicknesses of 0.1, 1 mm, and 1 cm at 20°C and 60°C, respectively. At room temperature, all the samples show high Tlum as the PNIPAm polymer fibers are thin and elongated to allow the light to pass though (Figure S1A). On the other hand, the transmittance of IR gradually decreases with the increasing of sample thickness. The IR transmittance (TIR) for 0.1 mm sample is 77.0% at room temperature. With thickness increases, TIR decreases to 67.0% and 47.3% for 1 mm and 1 cm sample, respectively. It is worth to mention that the absorption peak at 1,400 and 1,900 nm are due to the water molecules vibration in the TRL. However, when the thickness increased to 1 cm, because of the large thickness, the IR above 1,400 nm is absorbed.21Zhou G. He J. Thermal performance of a radiant floor heating system with different heat storage materials and heating pipes.Appl. Energy. 2015; 138: 648-660Crossref Scopus (101) Google Scholar When the temperature increases, the Tlum for all the samples decreases due to the shrinkage of PNIPAm polymer fibers and formation scattering center in TRL (Figure S1B). However, with the thickness increasing, ΔTsol becomes larger. Figure S2 shows the hysteresis loop and the derivation of transmittance of the TRL against the temperature. It can be observed that the LCST is 32.5°C. Figures 2B and 2C summarize the optical properties for different thickness samples. It can be observed that the transmittance modulation abilities for luminous, IR, and solar wavelength are all increasing with the increase of thickness (Figure 2B). For example, the ΔTlum of 0.1-mm sample is ∼15%, and it increases significantly to ∼90% for the 1-cm sample. Meanwhile, the ΔTsol of 0.1-mm TRL is only 11.3%, and it largely increases to 68.1% for 1-cm TRL. Therefore, the 1-cm sample shows a higher transmittance contrast than the 1- and 0.1-mm samples. Similar to the transmittance modulation ability, the reflectance modulation ability becomes stronger when the sample becomes thicker (Figures 2C and S3). Moreover, the 1-cm sample shows a higher solar reflectance (Rsol, ∼27%) than the other samples (∼23% for 1-mm sample and ∼10% for 0.1-mm sample, respectively), which indicates that the 1-cm sample shows stronger reflection to solar light in the opaque state. The effect of PNIPAm concentration to the thermochromic properties of TRL was further investigated. It can be observed that Tlum of 1-cm TRL decreases from 92.3% to 87.3% when the PNIPAm concentration increases from 0.1% to 20% (Figure S4A); while the maximum value of ΔTsol was observed with 4% PNIPAm TRL (68.1%, Figure S4B). Figure 2D shows the optical photos for different thickness samples at 20°C and 60°C. The optical photos agree with the spectra: at low temperature, all the samples are transparent, the luminous transmittance will not be affected by thickness. On the other hand, when the temperature is above LSCT, no significant transmittance change is observed for the 0.1-mm sample. In contrast, the 1-mm sample becomes translucent, and the 1-cm sample turns opaque, while the flower under the 1-cm sample becomes invisible. Thus, the thermo-responsive optical properties of the TRL were regulated by changing temperature and thickness. Figure 2E shows the curve of Cp with respect to the temperature increase for the TRL and deionized (DI) water. No significant change of Cp is observed from 20°C to 80°C for both TRL and DI water. Moreover, the liquid shows a higher Cp (∼4.35 kJ kg−1 K−1) than that of DI water (∼4.2 kJ kg−1 K−1), while water has significantly higher Cp compared with most of the other materials (Table S1). As the large Cp of water is mainly attributed to the presence of hydrogen bond, the increasing of Cp is due to the introduction of the functional group (amide group and –C=O bond) for the liquid (Figures 2F and S5), which will generate more hydrogen bond and stabilize the water-water hydrogen bond.57Zhou Y. Layani M. Wang S. Hu P. Ke Y. Magdassi S. Long Y. Fully printed flexible smart hybrid hydrogels.Adv. Funct. Mater. 2018; 28: 1705365Crossref Scopus (78) Google Scholar,65Silverstein K.A.T. Haymet A.D.J. Dill K.A. The strength of hydrogen bonds in liquid water and around nonpolar solutes.J. Am. Chem. Soc. 2000; 122: 8037-8041Crossref Scopus (126) Google Scholar,66Tamai Y. Tanaka H. Nakanishi K. Molecular dynamics study of polymer−water interaction in hydrogels. 1. Hydrogen-bond structure.Macromolecules. 1996; 29: 6750-6760Crossref Scopus (209) Google Scholar Therefore, although the latent heat during the phase change at 32°C is negligible (Figure S6), the high Cp of TRL makes it is able to store larger amount of thermal energy than conventional building materials and glass with the same amount of temperature change. As the result, TRL becomes a competitive candidate for energy storage material to regulate the room temperature. Meanwhile, Figure 2G shows the relationship between thermal conductivity and the temperature changes of TRL and DI water. The thermal conductivities are stable at the temperature range from 20°C to 80°C for both liquids. It is worth mentioning that the thermal conductivities of both TRL (0.85W m−1 K−1) and DI water (0.65 W m−1 K−1) are higher than the commonly used TES materials, such as paraffin (0.18–0.19 W m−1 K−1), fatty acids (0.14–0.37 W m−1 K−1), and inorganic salt (Na2HPO4·12H2O, 0.47–0.51 W m−1 K−1).67Fan L. Khodadadi J.M. Thermal conductivity enhancement of phase change materials for thermal energy storage: a review.Renew. Sustain. Energ. Rev. 2011; 15: 24-46Crossref Scopus (685) Google Scholar As a lower thermal conductivity will reduce the energy charging/discharging rate, thereby further reducing the energy storage efficiency of the material, the high thermal conductivity of the TRL makes the temperature distribution of window more uniform and provides the window with higher energy storage efficiency.67Fan L. Khodadadi J.M. Thermal conductivity enhancement of phase change materials for thermal energy storage: a review.Renew. Sustain. Energ. Rev. 2011; 15: 24-46Crossref Scopus (685) Google Scholar, 68Li T.X. Xu J.X. Wu D.L. He F. Wang R.Z. High energy-density and power-density thermal storage prototype with hydrated salt for hot water and space heating.Appl. Energy. 2019; 248: 406-414Crossref Scopus (31) Google Scholar, 69Yuan K. Shi J. Aftab W. Qin M. Usman A. Zhou F. Lv Y. Gao S. Zou R. Engineering the thermal conductivity of functional phase-change materials for heat energy conversion, storage, and utilization.Adv. Funct. Mater. 2020; 30: 1904228Crossref Scopus (88) Google Scholar Moreover, the TRL has a viscosity that comparable to water (TRL: 1.80 cP, water: 1.05 cP); which provides its capability of easy fabrication. From the discussion above, the TRL shows an excellent light regulating ability as well as a good energy storage property. The working principles under different conditions for the HTEST smart window are described in Figure 3A. During the morning and evening in summer, the ambient temperature is not high enough to trigger the phase change of the HTEST window. Therefore, the light (yellow arrows in Figure 3A) will transmit to the room, and the window will keep the transparent state. Meanwhile, the artificial lighting electricity can be saved in the morning due to the sufficient daylighting penetration. Meanwhile, because of the good energy storage property of the TRL, the heat (red arrows in Figure 3A) in the surrounding is difficult to transfer into the room. As a result, the room will be kept at a relatively low temperature. Approaching noon in summer, the outdoor temperature reaches the maximum value of the day, which is above the LCST for the HTEST smart window. The phase change is subsequently activated, and the window becomes translucent/opaque to prevent the sunlight from further heating up the room. Meanwhile, the heat is further stored in the TRL and prevented from entering the room. The heat stored in the liquid is subsequently released, which shifts the peak of the cooling load. On the other hand, the window will keep transparent for the whole day in winter to ensure that the sunlight is able to transmit into the room for heating and lighting purpose. Based on such a working principle, the HTEST smart window is capable of reducing the HVAC energy consumption of buildings through cutting off the energy loss for cooling and increase the thermal comfort of the residence. In order to further investigate the performance of the HTEST smart window, the indoor thermal test was conducted as the proof of concept. The indoor thermal test was designed to explore the solar modulation and TES effects of TRL energy-saving performance. Figure 3B shows the illustration of the experimental setup for the indoor thermal test, which is to test the solar modulation and TES effects on energy saving. Four samples namely normal glass panel (as baseline), 1 cm DI water trapped glass panel, 1 mm and 1 cm TRL trapped glass panel was installed onto four glasshouses (20 cm × 20 cm × 30 cm) to study the temperature change. In order to investigate the energy-saving ability of HTEST smart window more systematically, the PNIPAm particles concentration of 1-mm-thick sample were increased to make it have the similar solar transmittance and optical response (Tsol-1mmTRL = 3.7%) as the 1-cm-thick sample (Tsol-1cmTRL = 1.6%) (Figures 3C and S7), which gives large contrast with the other sets of sample, glass (Tsol-glass = 85%) and 1 cm water (Tsol-1cmwater = 72%) samples. During the test, the temperature of the inner surface of the window (position A), and the air temperature of the geometry center of the box (position B) were recorded, respectively. Figure 3D shows the temperature curve at position A for the four samples. The 1-mm-thick-liquid-trapped window and the normal glass show the highest surface temperature of 90°C and 88°C among the four samples. The temperatures recorded for 1-cm-thick water and 1-cm TRL samples are 46°C and 42°C, respectively. More than 40°C temperature difference was observed on the two sets of samples. As the inner surface temperature of the window is mainly affected by the heat accumulated on the window and the heat transferred through the window, the large difference between the Cp of water and glasses (4.2 versus 0.84 kJ kg−1 K−1) indicated that the heat accumulated through the solar radiation is stored more in the water richer material. Thereby, a largely re