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1. Utillizing Solar Infrared-Induced Photothermal Heating on Building Windows in Winter

   https://doi.org/10.18086/solar.2020.01.04  

   Zhang, Enhe; Duan, Qiuhua; Zhao, Yuan; Wang, Julian (July 2020, ASES Solar 2020 Proceedings)

   null (Ed.)

   Single-pane windows still account for a large percentage of US building energy consumption. In this paper, we introduced a new solution incorporating the photothermal effect of metallic nanoparticles(Fe3O4@Cu2−xS) into glazing structures to utilize solar infrared and then enhance the window’s thermal performance in winter. Such spectrally selective characteristics of the designed photothermal films were obtained from lab measurements and then integrated into a thermodynamic analytical model. Subsequently, we examined the thermal and optical behaviors of the photothermal single-pane window and compared its overall energy performance with the conventional low-e coated single-pane window, in which typical window properties, dimensions, winter boundary conditions, and solar irradiance were adopted. The numerical analysis results demonstrated that the photothermal window systems could yield 20.4% energy savings relative to the conventional low-e coated windows. This research paves an underlying thermodynamic mechanism for understanding such a nanoscale phenomenon at the architectural scale. From the implementation perspective, the designed photothermal film can be added into the existing single-pane windows for energy-efficient retrofitting purposes. 

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2. Optical thermal insulation via the photothermal effects of Fe ₃ O ₄ and Fe ₃ O ₄ @Cu _2−x S thin films for energy-efficient single-pane windows

   https://doi.org/10.1557/mrc.2020.4  

   Lin, Jou; Zhao, Yuan; Shi, Donglu (March 2020, MRS Communications)

   To address critical energy issues in civic structures, we have developed a novel concept of optical thermal insulation (OTI) without relying on a conventional thermal intervention medium, such as air or argon, as often used in conventional window systems. We have synthesized the photothermal (PT) materials, such as the Fe 3 O 4 and Fe 3 O 4 @Cu 2− x S nanoparticles, that exhibit strong UV and near-infrared (NIR) absorptions but with good visible transparency. Upon coating the inner surface of the window glass with a PT film, under solar irradiation, the inner surface temperature rises due to the PT effect. Subsequently, the temperature difference, Δ T , is reduced between the single pane and room interior. This leads to lower the thermal loss through a window, reflected by the U -factor, resulting in considerable energy saving without double- or triple-glazing. Comparing with the Fe 3 O 4 coatings, Fe 3 O 4 @Cu 2− x S is spectrally characterized with a much stronger NIR absorbance, contributing to an increased PT efficiency under simulated solar irradiation (0.1 W/cm 2 ). PT experiments are carried out via both white light and monochromic NIR irradiations (785 nm). The parameters associated with the thermal performance of the PT films are calculated, including PT conversion efficiency, specific absorption rate (SAR), and U -factor. Based on the concept of OTI, we have reached an optimum U -factor of 1.46 W/m 2 K for a single pane, which is satisfactory to the DOE requirement (<1.7 W/m 2 K). 

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3. Light angle dependence of photothermal properties in oxide and porphyrin thin films for energy-efficient window applications

   https://doi.org/https://doi.org/10.1557/mrc.2020.39  

   1. Lyu M, Lin J (June 2020, MRS communications)

   null (Ed.)

   The photothermal experiments on the incident light angle dependence are carried out using simulated solar light on thin films of both iron oxides (Fe3O4 and Fe3O4@Cu2-xS) and porphyrin compounds (chlorophyll and chlorophyllin). Fe3O4 and Fe3O4@Cu2-xS are synthesized using various solution methods that produce mono-dispersed nanoparticles on the order of 10 nm. Chlorophyll is extracted from fresh spinach and chlorophyllin sodium copper is a commercial product. These photothermal (PT) materials are dispersed in polymethyl methacrylate (PMMA) solutions and deposited on glass substrates via spin coating that result in clear and transparent thin films. The iron-oxide based thin films show distinctive absorption spectra; Fe3O4 exhibits a strong peak near UV and gradually decreases into the visible and NIR regions; the absorption of Fe3O4@Cu2-xS is similar in the UV region but shows a broad absorption in the NIR region. Both chlorophyll and chlorophyllin are characterized with absorption peaks near UV and NIR showing a “U”-shaped spectrum, ideally required for efficient solar harvest and high transparency in energy-efficient single-pane window applications. Upon coating of the transparent PT films on the window inner surfaces, solar irradiation induces the photothermal effect, consequently raising the film temperature. In this fashion, the thermal loss through the window can be significantly lowered by reducing the temperature difference between the window inner surface and the room interior, based on a new concept of so-called “optical thermal insulation” (OTI) without any intervention medium, such as air/argon, as required in the glazing technologies. Single-panes are therefore possible to replace double- or triple panes. As OTI is inevitably affected by seasonal and daily sunlight changes, an incident light angle dependence of the photothermal effect is crucial in both thin film and window designs. It is found that the heating curves reach their maxima at small angles of incidence while the photothermal effect is considerably reduced at large angles. This angle dependence is well explained by light reflection by the thin film surface, however, deviated from what is predicted by the Fresnel’s law, attributable to non-ideal surfaces of the substrates. The angle dependence data provides an important reference for OTI that window exposure to sun is greater at winter solstice while that is considerably reduced in the summer. This conclusion indicates much enhanced solar harvesting and heat conversion via optically insulated windows in the winter season, resulting in much lower U-factors. 

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4. Parametric Energy Simulation Methods for Solar-NIR Selective Glazing Systems

   https://doi.org/10.1088/1742-6596/2069/1/012129  

   Duan, Qiuhua; Zhang, Enhe; Hinkle, Laura; Wang, Julian (November 2021, Journal of Physics: Conference Series)

   Abstract Solar near-infrared (NIR) selective glazing systems have been proposed by incorporating photothermal effects (PTE) of a nanoparticle film into building windows. From an energy efficiency perspective, the nanoscale PTE forms unique inward-flowing heat by heating up the window interior surface temperature under solar near-infrared, significantly improving the window thermal performance. Also, the PTE-driven solar heat gains are dynamic upon solar radiation and weather conditions. However, the PTE on annual building energy use has not been investigated thoroughly, due to the lack of an accurate and appropriate energy simulation method. In this study, we used the EnergyPlus energy management system to develop a parametric energy model and simulation approach in which a solar-temperature-dependent thermal model was embedded into the parametric energy simulation workflow. Applying this method, we examined the solar near-infrared-dependent PTE-induced thermal performances of glazing systems and their effects on annual heating energy use in representative cold climates (i.e., Zones 4, 5, and 6). The results show that the dynamic model considering the PTE demonstrated more heating energy savings, up to 11.64% in cold climates, as opposed to the baseline model that ignored the PTE. This work presents a method to model and simulate the dynamic thermal performance of windows with PTE. 

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5. Effective reduction of building heat loss without insulation materials via the photothermal effect of a chlorophyll thin film coated “Green Window”

   https://doi.org/10.1557/mrc.2019.52  

   Zhao, Yuan; Dunn, Andrew W.; Shi, Donglu (June 2019, MRS Communications)

   One of the critical components of energy savings in buildings is thermal insulation, especially for windows in cold climates. The conventional approach mainly relies on a double-pane design. In this study, a new concept of “Green Window” has been designed for single-pane applications that lower the U-factor. The “Green Window” is structurally and simply composed of a thin film window coating of chlorophyll that exhibits pronounced photothermal effect, while remaining highly transparent. We demonstrate a new concept in “thermal insulation” via optical means instead of solely through thermal insulators or spectral selectivity. This concept lifts the dependence on insulating materials making single-pane window highly possible. 

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