Effective reduction of building heat loss without insulation materials via the photothermal effect of a chlorophyll thin
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Research Letter
Effective reduction of building heat loss without insulation materials via the photothermal effect of a chlorophyll thin film coated “Green Window” Yuan Zhao, Andrew W. Dunn, and Donglu Shi, The Materials Science and Engineering Program, Department of Mechanical and Materials Engineering, College of Engineering and Applied Science, University of Cincinnati, Cincinnati, OH 45221, USA Address all correspondence to Donglu Shi at [email protected] (Received 28 January 2019; accepted 9 April 2019)
Abstract 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.
Introduction Energy efficiency has to be enhanced by reducing the amount of energy required to provide products and services. In energy saving, one critical issue deals with a large amount of heat loss from public buildings, especially in cold regions. According to a report by the US Department of Energy, building heating, ventilation, and air conditioning (HVAC) accounted for 14.0% of primary energy consumption in the USA. Heat loss through windows in cold weather consumes about 3.9 quads, which is estimated to encompass 28.7% of total HVAC energy consumption.[1] To quantify overall heat flow through windows, the National Fenestration Rating Council (NFRC) has introduced an energy-performance characteristic of windows: the rate of heat loss, or U-factor. The lower U-factor implies better thermal resistance, meaning the heat flow from a warmer room to the colder outside can be better resisted. The current approach in improving window U-factor has been through either spectral selectivity or structural architecture. The former has been mainly achieved, to a limited extent, via lowemissivity (low-E) coating, while the latter by multiple layers of insulation (double-pane or triple-pane).[2,3] Most of the thermally insulated windows in cold climate regions are double-paned, sandwiching transparent insolating materials, such as gases, liquid crystals, vacuum glazing, etc., which creates additional and unnecessary volume and weight, as well as higher cost of the materials.[4,5] Therefore, ARPA-E raised the SHIELD program (Single-Pane Highly Insulating Efficient Lucid Designs), aiming at single-pane windows to replace multi-pane windows, includes double-panes and triple-panes.[1]
U-factor is proportional to the temperature difference, ΔT, between the center of the room and the window interior
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