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Anne Vazquez

Building Exteriors That Clean Themselves?

Titanium Dioxide Being Used To Reduce Maintenance Needs

By Anne Vazquez

In many cases, the aim of operating a sustainable facility intersects with the demands of ensuring quality maintenance. For instance, the pursuit of an environmentally friendly protocol for cleaning must take into account the efficiency of newly adopted products and any new procedures that go along with them. When considering such decisions, a facility manager (fm) will want to be sure the new system operates at the same, if not better, level of effectiveness and efficiency.

Building Exteriors That Clean Themselves! Shown here is an exterior wall on a condominium in Japan. The tiles in the photo at the right were manufactured with titanium dioxide (TiO2), while the tiles in the top photo were not. The two areas have received the same maintenance, and the TiO2 treated section remains free of visible dirt. (Photos: Toto Hydrotect Tile)

Cleaning a building exterior, for instance, can be a time- and labor-intensive task. At its simplest, the process consumes water, detergents, and other cleaning agents. Further, the labor must be scheduled and paid for, and sometimes the process requires closing off a portion of the facility normally accessible to occupants. Additionally, the facilities staff must coordinate and schedule this event every time this is needed. What a boon it would be if the building could clean itself!

While we are not in the realm of magically self-cleaning buildings (yet), there have been developments over the past 10 years, mainly put in practice in Asia and Europe, that hold promise for reducing exterior cleaning demands. Titanium dioxide (TiO2) is a material that can be applied as a thin film to building exteriors to protect against pollutants and debris. The compound can also be incorporated into building materials during the manufacturing process. Concrete, exterior tiles, tensile roof membranes, and windows are among the products currently on the market with TiO2 technology.

The chemical make up of TiO2 makes it a photocatalyst, which enables it to break down dirt that settles on a surface. Simply put, when a photocatalyst is exposed to ultraviolet (UV) light (from the sun, for instance) in the presence of water vapor and oxygen, it creates a charge separation of electrons and electron holes. The electrons disperse on the surface and react with external substances; this causes chemical reactions that form hydroxyl radicals, which then decompose organic compounds (such as dirt and other debris) present on a surface.

The second part of this is that the debris broken down by the photocatalytic reaction can be removed from the surface by rainfall or rinsing. This is because the surface becomes hydrophilic (gains the ability to gather water) after exposure to UV light. In this state, water falling on the surface spreads flat upon it, preventing debris from adhering to the building.

TiO2 also triggers the oxidation of airborne pollutants, including oxides of nitrogen (NOx) and volatile organic compounds (VOCs) and converts them into carbon dioxide (CO2) and water. In essence, it helps to clean the air of pollutants.

While reduced maintenance is desirable for fms, the use of TiO2 for buildings faces several barriers to widespread adoption. For one, using materials with TiO2 comes at a premium. (For instance, the cost of this type of concrete from Italcementi, an Italian manufacturer, is reported to be as high as 30% to 40% more than the traditional product.)

Beyond cost, another point of discussion is the environmental impact of the byproducts of the TiO2 photocatalytic process. Some argue that, while trying to minimize one environmental impact through resource conservation, the presence of TiO2 may add undesirable pollutants to the air. For instance, the chemical reaction involved in photocatalysis releases CO2. The amount is negligible; however, multiplied over thousands of facilities and millions of square feet of surface area, the release of CO2 could work against efforts to reduce greenhouse gas emissions.

An expanded look at the potential of TiO2 for buildings was recently addressed in a study performed by the Lawrence Berkeley National Laboratory (LBNL) in Berkeley, CA. The project investigated the potential of TiO2 to “clean” outdoor air through its ability to oxidize pollutants. Hashem Akbari and Paul Berdahl were the LBNL researchers for the project, and they sought to clarify the potential for reducing air pollution by focusing on measured values of catalytic activity—the rate at which air can be cleaned by a given area of photocatalyst (in this case TiO2). For instance, if one square meter of a catalytic surface can clean 100 cubic meters of air per day, then it has an activity of 100 meters/day.

Commenting on the potential of TiO2 to combat outdoor air pollution, Berdahl said, “Prior to widespread deployment of passive photocatalytic air cleaning technology, large scale meteorological simulations are needed to validate deployment strategies.” (The report also addressed the CO2 concern previously mentioned in this article.)

In exploring sustainability, fms often encounter strategies that remain on the outskirts. Further research and real world installations will eventually reveal whether TiO2 holds promise for mainstream facilities.

Research for this article included information from the LBNL project report, which was prepared for the California Energy Commission, Public Interest Energy Research Program. To download the report, visit www.energy.ca.gov/pier/final_project_reports/index.html (Publication #CEC-500-2007-112).

Do you have any experience with TiO2? Share your thoughts by e-mail to avazquez@groupc.com.

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