Solar-energy harvesting via photothermal thin films for optical thermal insulation

J. Lin, M. Lyu, D. Shi
University of Cincinnati,
United States

Keywords: solar harvesting, photothermal, thermal insulation, energy conversion, optical absorption

Summary:

We have developed a novel concept of Optical Thermal Insulation (OTI) without any intervening medium [1]. Instead of applying a thermal insulator, a transparent photothermal (PT) film can selectively absorb photons near the UV and NIR regions and efficiently convert them to heat, therefore raising the window surface temperature (via free energy). As the inner surface temperature is raised relative to room temperature, the heat transfer at the window inner surface can be effectively reduced via OTI, especially in winter. It must be noted that the PT films are spectral selective with high absorptions near UV and IR, while allowing high visible transmittance, therefore transparent and ideal for façade engineering. Based on this concept, a so-called “Green Window” has been designed for single-pane applications that meet the U-factor specifications of Department of Energy for colder regions of the United Sates [2]. The “Green Window” is coated with thin films of chlorophyll and chlorophyllin retrieved from natural greens. A thin film window coating of naturally occurring chlorophyll or chlorophyllin exhibits strong near UV and NIR absorptions and pronounced photothermal effect, while remaining highly transparent. Upon collecting solar light, considerable heat is created, effectively raising the window surface temperature, leading to a reduced U-factor less than 1.7 W m-2 K-1, even below the values of double-panes. Based on these experimental results, we demonstrate a new concept of “optical thermal insulation” that lifts the dependence on insulating materials making single-pane window highly possible. References: [1] Yuan Zhao, Andrew Dunn, Donglu Shi, MRS Communications: Volume 9, Issue 2, pp. 675-681 (2019) [2] ARPA-E, "Single-Pan Highly Insulating Efficient Lucid Designs (SHIELD) Program Overview," https://arpa-e.energy.gov/sites/default/files/documents/files/SHIELD_ProgramOverview.pdf