Windows are an essential element of any building providing light and air. But increasingly rather than simply being visually functional, windows are being viewed as an essential component in an energy efficiency strategy, especially for their heat transmission capability. In cold climates there can be significant heat loss through windows, adding to the heating requirements, while in hot climates there can be significant heat gain, adding to the air conditioning requirements.
While a number of studies on windows have been introduced, one area that needs further investigation is improving the efficiency of existing single-pane windows. To this end the US Department of Energy’s Advanced Research Projects Agency-Energy (ARPA-E) has introduced the Single-Pane Highly Insulating Efficient Lucid Design (SHIELD) programme with US$31 million in funding for 14 projects.
The aim of the programme is to develop innovative window coatings and windowpanes that could cut in half the amount of heat lost through single-pane windows without replacing the full window in commercial and residential buildings.
Many, particularly older buildings have single-pane windows that do not insulate a building or its occupants well. However, complete replacement of single-pane windows with efficient, modern windows is not always feasible due to cost, changes in appearance and other concerns. Retrofitting, rather than replacing single-pane windows, can reduce heat loss and save roughly the amount of electricity needed to power 32 million US homes each year.
“By creating novel materials to retrofit existing single-pane windows, SHIELD technologies can dramatically improve building efficiency and save energy costs for building owners and occupants,” comments ARPA-E Director Dr. Ellen D. Williams.
Innovations for windows
Projects being supported are divided into three categories. The first category seeks transparent, adhesive products that can be applied onto existing windowpanes. These materials must be less than 1/8 inch thick, cost less than $5 per square foot at a manufacturing scale of one million square feet annually, and last for 10 years. The second category includes manufactured windowpanes that can be installed without replacing the window sash that holds the windowpane in place. These windowpanes must be less than ¼ inch thick, cost less than $10 per square foot at manufacturing scale, and last for 20 years. The third category includes innovative technologies that can enable products in the first two categories.
An example in the first category is the project at Oak Ridge National Laboratory to create a highly transparent, multilayer window film comprised of nanoporous silica layers that exhibit superior optical and mechanical properties to improve thermal insulation, a layer of polyvinyl butyral for soundproofing and a layer of low-emissivity film to minimize heat transfer.
Among projects in the second category Arizona State University (ASU) is leading development of a windowpane that incorporates multiple silica and polymer layers that can be aerosol sprayed to reduce costs. Aspen Aerogels, Inc. and partners are fabricating what will be effectively a double-glazed windowpane with a silica aerogel layer to provide thermal insulation and condensation resistance.
The largest project award, to Triton Systems and partners, is for the development of a multilayered low emissivity nanocomposite coating, which can be applied to windowpanes. Nanocomposites are also the basis of the second largest award, to Argonne National Laboratory and partners, in which the aim is to develop a low-cost thin, transparent nanofoam with high thermal insulation and soundproofing properties.