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The High Road

September 22, 2010

It's easiest and least expensive to correct problems at the source. One simple example is the pencil eraser. Just flip the pencil over, and in as many strokes as it took to make the errant mark, it's obliterated. Likewise, there's toxic waste management, where it's far better to collect your waste into a small container for controlled burial than to release it into the environment, where a tremendous effort is required for remediation. Along the same lines, scientists at the Fraunhofer-Institute for Molecular Biology and Applied Ecology are looking at a novel way of eliminating nitrogen oxide pollution generated by automobiles. The idea is to coat the pavements on which these automobiles drive with titania nanoparticles that photocatalytically convert the nitrogen oxides (NOx) into nitrates.

Titania (titanium dioxide, TiO2) is a rather abundant material that's most commonly used as an opaque white paint pigment. It's only when you want just the titanium metal part of the compound that things get expensive. Titania is a well known photocatalyst that's been proposed for splitting water for hydrogen energy, and for remediation of contaminated water supplies. The idea of putting a photocatalyst into road paving didn't originate at Fraunhofer, although the Germans have scaled things up a bit from previous efforts. As early as 2006,[2] scientists in Hong Kong incorporated titanium dioxide and other catalysts into paving blocks and studied the effects of such variables as the porosity of the blocks, the type of paving material and the percentage of added TiO2. NO removal was better at high block porosity, and using crushed glass in the paving mix improved the photocatalysis by letting more light into the paving surface. Within the parameters of their study, the Hong Kong team found an optimum paving composition using recycled glass, sand, cement and TiO2. Fraunhofer has cited prior experiments in Italy that have shown that the technique works.

F. C. Nüdling Betonelemente, a German paving company, produced titania-coated paving slabs with a range of surface texture, colors, cement composition and titania content to determine an optimum composition. An extended field test by Fraunhofer in variable wind and lighting conditions showed that these paving slabs produced a 20-30 conversion of nitrogen oxides. In calm wind condition, the conversion was as high as 70%. A street in Erfurt, Germany. paved with such slabs, has an average degradation rate of 20 percent for NO2 and 38 percent for NO. These paving stones do not lose their conversion efficiency after two years of service. A group from The Netherlands has developed a mathematical model of photocatalysis in paving materials that should aid further work.[3]

Congestion caused by a road accident, Algarve, Portugal

Hard NOx (Photo by Osvaldo Gago).

Such photocatalytic paving will give roads a dual purpose, but they are useful mostly in cities where there's a high concentration of traffic. What use are the other roadways, aside from transportation? A 2004 study found that the impervious surface area, the area covered by highways, streets, buildings, parking lots and other solid structures, of the continental US was almost as large as the state of Ohio.[5] This area was about 43,000 square miles (112,000 square kilometers). The US interstate highway system alone accounts for 94,000 square kilometers of this total.[4] What if all this area were paved with photovoltaic cells?

As reported in New Scientist magazine,[6] Scott Brusaw of Sagle, Idaho, has produced a prototype of a photovoltaic panel that can act as a road surface. The plan is to produce interlocking panels that are 3.7 meters on a side. That's the standard width of a US interstate highway. With an average insolation of four hours each day, such panels can produce 7.6 kilowatt-hours of energy each day. Of course, there's the problem of storing this energy for non-daylight hours and cloudy days, but the highway systems offer a perfect interconnection infrastructure for routing the power to storage facilities and wherever its needed. There's the further idea that densely populated areas, where more power is needed, have more roads. At this time, each panel would cost about $10,000, which is about four times the cost of the road surface it would replace.

A lot of development is needed for roadway photovoltaics to happen. How can these fragile devices stand the abuse of traffic? Good surface transparency translates to a smooth surface, which is not the type of surface to be driving on during wet weather. Festooning the surface with tiny prisms might solve that problem, but there are so many more; but solving problems is what engineering is all about.


  1. Monika Herrchen, "Paving Slabs That Clean The Air," Fraunhofer-Gesellschaft Research News (August, 2010).
  2. C.S. Poon and E. Cheunga, "NO Removal Efficiency Of Photocatalytic Paving Blocks Prepared With Recycled Materials," Construction and Building Materials, vol. 21, no. 8 (August, 2007), pp.1746-1753.
  3. M.M. Ballari, M. Hunger, G. Husken and H.J.H. Brouwers, "NOx Photocatalytic Degradation Employing Concrete Pavement Containing Titanium Dioxide," Applied Catalysis B: Environmental, vol. 95 (2010), pp. 245-254.
  4. Harvey Leifert, "USA's Built-Up Surfaces Equal Ohio in Area," American Geophysical Union (June 14, 2004).
  5. Total Surface Area Required to Fuel the World With Solar, LandArtGenerator.org (August 13, 2009).
  6. Duncan Graham-Rowe, "Innovation: Sunrise boulevards could bring clean power," New Scientist, August 27, 2010.

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Linked Keywords: Pencil eraser; toxic waste; Fraunhofer-Institute; Molecular Biology; Applied Ecology; nitrogen oxide; titania; nanoparticles; photocatalysis; nitrate; pigment; titanium metal; splitting water; hydrogen energy; Hong Kong; porosity; Italy; Erfurt, Germany; Netherlands; Algarve, Portugal; continental US; US interstate highway system; photovoltaic cells; New Scientist; Sagle, Idaho; prisms.