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| Fig. 1: Reversible transparency change of the thermochromic hydrogels. (Image source: W. Wu, after Zhao et al. [2]) |
Windows have been an integral part of human societies for centuries. They protect us from sun burns, ensure our privacy, and, most importantly, help regulate lights and thus temperatures in buildings. As technology improves, there emerged a wide variety of window types, including clear glass windows (the mainstream), Low-E glass windows, and smart windows. Currently, regulation of building temperatures primarily depends on the effectiveness of windows and the HVAC systems. However, most of such HVAC systems require large amounts of energy inputs, making them less cost-effective and energy conservation the key. The clear glass windows, from this perspective, isn't a very ideal option. On the other hand, thermochromic smart windows, including phase change polymer ones and others, could help save energy through reducing sunlight exposures. With such smart windows, HVAC systems will become both more energy effective and economically advantageous.
Thermochromic smart windows, by definition, are a type of passive smart window which respond to the intensity of heat by tinting the window. [1] Fig. 1 shows an example of possible smart windows materials - the "thermochromic hydrogels" ("change transparency reversibly with temperature"). [2] There are several other types of possible smart windows materials including vanadium dioxide ("Metal-insulator phase transition") and "liquid crystal" (molecular orientation adjustment based on temperature changes). [2] In general, smart windows work in relatively simple mechanisms from the perspective of light regulation. Taking smart windows made out of phase-change polymers as an example, they utilize the differences of responses among polymers within the same film to achieve thermo-responsive light modulations. [3]
Specifically, they are able to switch between amorphous and semi-crystalline (or crystalline) states based on their refractive index (RI) differences. [3,4] The hydrophilia differences among polymers, in addition, can cause phase separation and thus enhance the scattering effects. [4] An example of a phase-change polymer film particularly suited for smart windows is TPCC. [4] Through the incorporation of monomers with lower melting point, Liu et al. were able to adjust the co-polymer's phase change temperature into the range of normal daily temperatures, making such co-polymer ideal in daily usages. [4] Similar to what is explained above, the team employs the RI difference mechanism between different polymers. [4] Specifically, when RIs match between polymers, the film stays transparent. [4] The film's stableness, high thermo-responsiveness, and relatively automaticity all exemplify why smart windows made of phase change polymer films (and thus thermochromic ones in general) have the potential to save large amounts of energy. In other words, they have partial ability to automatically regulate light and thus temperature without requiring any external energy sources. Compared to traditional glass windows, in which such thermo-responsive activities are not possible, smart windows raise new possibilities - when teamed up with the HVAC systems - to greatly improve buildings' light and temperature regulation efficiencies.
The question then comes: how much energy can such new smart windows save compared to traditional glass windows when implemented together with the HVAC systems? Indeed, it is challenging to give an exact answer to such a question because of the many existing uncertainties when using such windows. However, the challenge doesn't mean that we can't get an approximation of how much we could potentially save. For instance, Wang et al. have introduced a type of smart windows made with hydrated ionic polymers. [5] The windows demonstrated both a reduction of indoor temperatures by up to 10°C and a most probable energy saving efficiency of 11.4% when being compared to traditional clear windows. [5] With the additional implementation of low-E glass, the energy saving efficiency could reach up to 17.7% in warmer climates. [5]
Such improvements are highly impressive. As a reference, Energy star - a program run by the U.S. Environmental Protection Agency - calculated that offices would consume a site EUI (Energy Use Intensity) of around 52.9 kBTU per square foot. [6] Thus, in a hypothetical scenario which we have a ten-story office building, the building would have consumed around 3.1 GWh of electricity per year (one GWh equals to 3.412 × 109 BTU = 3.6 × 1012 J) when using clear windows and normal HVAC systems. [6] However, if the above smart window (with hydrated ionic polymers) is introduced, an energy amount equal to around 0.35 GWh could be saved each year. In warmer climates, as discussed above, this number can be even higher. Imagining implementing such smart windows to every building within the U.S., the potential savings on energy consumption is almost unlimited.
A subsequent question raises naturally. With the title of the article centering the economics of smart windows, how much money can we actually save by using them? Similarly, there doesn't exist an exact number in this case neither. The monetary savings, if thermochromic smart windows are implemented, depend on several factors that constantly change (e.g., the price of electricity or other energy forms, the maintenance costs, etc.). But, again, we can do some simulations to get a sense of how much we can save up. Back to the hypothetical scenario above, we have an approximate electricity saving of around 0.35 GWh (i.e., 350,000 kWh) per year for a ten-story office building. According to EIA's monthly report, a Kilowatthour of electricity is worth 17.62 U.S. cents in August 2025 for commercial uses. [7] Taking that number, 350,000 kWh of electricity is worth of US $61,670 In other words, switching from traditional clear windows to thermochromic smart windows can save up more than a twentieth million dollars each year. With millions of buildings existing within the U.S., thermochromic smart windows' potential monetary savings are enormous.
Admittedly, thermochromic smart windows are more expensive than traditional ones in terms of prices and maintenance costs. (reference supporting this). Give an estimate of how much needs to be spent - no need to be precise. Therefore, the actual savings through implementing thermochromic smart windows depend largely on how long and for what purposes they will be used for. However, even if it might not as economically feasible as traditional windows at the moment, technology improvements (e.g., ones that lead to decrease in production costs) will likely make them more acceptable in the future. Out of the smart window material options, thermochromic smart windows "are easy to manufacture" and "are relatively low in cost in comparison with their more complex smart window system counterparts", making them ideal alternatives for regular windows in the near future. [1]
Energy sources are not unlimited. With the use of fossil fuels increasing dramatically in the past two centuries, we are depleting our own future. It is thus important for us to start small and save energy. Implementing thermochromic smart windows serves as a great example. With their great potential to effectively respond to heat and thus regulate temperature and light, such smart windows have proved their capacity as promising alternatives to their traditional counterparts.
© Victor Wu. The author warrants that the work is the author's own and that Stanford University provided no input other than typesetting and referencing guidelines. The author grants permission to copy, distribute and display this work in unaltered form, with attribution to the author, for noncommercial purposes only. All other rights, including commercial rights, are reserved to the author.
[1] M. Aburas et al., "Thermochromic Smart Window Technologies For Building Application: A Review," Appl. Energy 255, 113522 (2019).
[2] Y. Zhao et al., "Thermochromic Smart Windows Assisted by Photothermal Nanomaterials," Nanmaterials 12, 3865 (2022).
[3] Y. Xie et al., "A Phase-Changing Polymer Film for Broadband Smart Window Applications," Macromol. Rapid Commun. 41, 2000290 (2020).
[4] Y. Liu et al., "Automatically Modulated Thermoresponsive Film Based on a Phase-Changing Copolymer," Chem. Mater. 33, 7323 (2021).
[5] H. Wang et al., "Hydrated Ionic Polymer For Thermochromic Smart Windows in Buildings," Nat. Commun. 16, 6509 (2025).
[6] "U.S Energy Use Intensity by Property Type," U.S. Environmental Protection Agency, Energy Star, August 2024.
[7] "Electric Power Monthly, October 2025," U.S. Energy Information Adminsitration, October 2025, Table 5.3.