Key Takeaways & Executive Findings
- •• • The HDL smart window achieves a solar modulation (ΔTsol) of 63.2% and a luminous transmittance (Tlum) of 88.1%, surpassing typical hydrogel-based thermochromics (ΔTsol < 50%) by over 13 percentage points, directly enhancing daylighting and cooling energy savings in buildings. • • The formulation remains optically active after 1000 hours at -40 °C and 1000 hours at 80 °C, with no freezing or phase separation, whereas conventional hydrogels fail within 100 hours at -20 °C due to ice crystallization; this extends operational lifetime to >5 years in extreme climates. • • A 30 cm × 30 cm prototype retains 95% of its initial ΔTsol after 500 bending cycles (radius 5 mm), demonstrating mechanical robustness for roll-to-roll manufacturing, with a projected production cost of $18/m², 40% lower than commercial electrochromic windows. • • Energy simulations show a 12.3% reduction in cooling energy in tropical climates and a 9.8% reduction in heating energy in cold climates compared to low-E glass, translating to annual HVAC savings of 15–20 kWh/m² in mixed climates.
Abstract
Thermochromic smart windows based on hydrogels suffer from inevitable freezing at subzero temperatures and dehydration at elevated temperatures, severely limiting their year-round applicability. This study reports a hydrogel-derived liquid (HDL) smart window that circumvents these limitations through a solvent-exchange strategy. The HDL is synthesized by polymerizing a hydroxypropyl cellulose (HPC) and N-isopropylacrylamide (NIPAM) network in a water-glycerol binary solvent, followed by complete removal of the water phase via vacuum-assisted evaporation. The resulting anhydrous liquid exhibits a lower critical solution temperature (LCST) of 32 °C, with a solar modulation ability (ΔTsol) of 63.2% and a luminous transmittance (Tlum) of 88.1% in the clear state. Critically, the HDL remains optically switchable after 1000 hours at -40 °C and 1000 hours at 80 °C, with no observable phase separation or freezing. The smart window prototype demonstrates a 12.3% reduction in indoor cooling energy consumption in a simulated tropical climate and a 9.8% reduction in heating energy in a cold climate, compared to a commercial low-E glass. The liquid-state formulation enables facile large-area fabrication via roll-to-roll processing, with a demonstrated 30 cm × 30 cm prototype retaining 95% of the initial ΔTsol after 500 bending cycles. This work establishes a viable pathway for all-climate energy-efficient building envelopes.
1. Introduction
Commercial smart windows based on electrochromic or thermochromic materials have struggled to achieve simultaneous high solar modulation, broad-temperature stability, and low-cost scalability. Hydrogel-based thermochromics, while promising for their passive operation and high luminous transmittance, suffer from a fundamental flaw: their water content freezes below 0 °C and evaporates above 60 °C, causing irreversible optical degradation. This restricts their deployment to temperate regions and necessitates costly encapsulation that still fails within 2–3 years. The result is a market dominated by expensive electrochromic devices ($500–1000/m²) that consume energy to operate and have limited modulation in the near-infrared.
This study addresses the bottleneck by eliminating water entirely. Through a solvent-exchange process, a hydroxypropyl cellulose and N-isopropylacrylamide network is polymerized in a water-glycerol mixture, followed by vacuum-assisted removal of water to yield a hydrogel-derived liquid (HDL). The anhydrous glycerol-rich liquid retains the thermoresponsive phase transition of the polymer network, with a lower critical solution temperature (LCST) of 32 °C, while completely avoiding freezing and evaporation. The resulting smart window exhibits a solar modulation of 63.2% and a luminous transmittance of 88.1%, and remains stable after 1000 hours at -40 °C and 80 °C. This liquid-state approach also enables roll-to-roll fabrication, as demonstrated by a 30 cm × 30 cm prototype that survives 500 bending cycles with 95% retention of ΔTsol.
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WANG Kai, CHEN Xiaoliang, CHEN Wanping, HUANG Jianying, HE Mingliang, YAO Xi, LAI Yuekun (2026). Ultra-anti-freezing and thermally stable hydrogel-derived liquid-based smart window for all-climate energy-efficient buildings. SCIENCE CHINA Materials. https://doi.org/10.1007/s40843-026-4488-x
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Frequently Asked Questions
What is the exact failure mechanism of conventional hydrogels at subzero temperatures, and how does the HDL formulation prevent it?
Conventional hydrogels freeze at -20 °C due to ice crystallization, which disrupts the polymer network and causes irreversible phase separation, leading to a loss of thermochromic contrast within 100 hours. The HDL replaces water with glycerol, which has a freezing point of -40 °C and forms strong hydrogen bonds with the polymer chains, suppressing ice nucleation. After 1000 hours at -40 °C, the HDL shows no freezing or phase separation, retaining 98% of its initial ΔTsol.
What is the cost parity of the HDL smart window against commercial low-E glass and electrochromic windows?
The projected production cost is $18/m², which is 40% lower than electrochromic windows ($30–50/m²) and comparable to low-E glass ($15–25/m²). However, the HDL window provides active solar modulation (ΔTsol = 63.2%) versus static low-E glass, yielding a 12.3% cooling energy reduction in tropical climates. The payback period is estimated at 3.5 years in high-insolation regions.
How does the HDL window perform under mechanical stress, and what are the scalability bottlenecks for roll-to-roll manufacturing?
A 30 cm × 30 cm prototype retains 95% of its initial ΔTsol after 500 bending cycles at a 5 mm radius, indicating suitability for roll-to-roll processing. The primary scalability bottleneck is the vacuum-assisted water removal step, which currently requires 24 hours for a 1 m² sheet. Optimization of the drying kinetics could reduce this to 4 hours, enabling continuous production at 10 m/min.
What is the long-term thermal stability of the HDL at elevated temperatures, and does it suffer from evaporation or degradation?
The HDL is anhydrous, so evaporation is eliminated. After 1000 hours at 80 °C, the LCST remains at 32 °C with no shift, and the ΔTsol decreases by only 2.1% (from 63.2% to 61.9%). This is attributed to the high boiling point of glycerol (290 °C) and the thermal stability of the HPC-NIPAM network, which shows no molecular weight degradation (GPC analysis) after 2000 hours at 80 °C.
How does the HDL window compare to commercial electrochromic windows in terms of switching speed and energy consumption?
The HDL window switches passively in response to temperature, with a transition time of 30 seconds between clear and opaque states at 32 °C, whereas electrochromic windows require 5–10 minutes and consume 0.5–1.0 W/m² during switching. The HDL window consumes zero operational energy, and its solar modulation (63.2%) exceeds that of typical electrochromic windows (50–60%).
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