"Effect of condensation on light transmission underestimated"

Light transmission is one of the main motivations for choosing a certain type of greenhouse glass. "But the effect of condensation is still often underestimated," says Michiel van Spronsen of Glascom Tuinbouw. The company developed SmartGlass, a type of glass of which the haze adjusts to condensation.

Glascom Tuinbouw is the worldwide representative for SmartGlass. This new type of diffuse glass was designed especially for greenhouses. Van Spronsen: "Growers' wishes were taken into account with the design. This is reflected in its properties."

There was much attention paid to light transmission in developing SmartGlass. "Too often only the perpendicular or hemispherical light transmission of glass is looked at, while light transmission in higher angles on the glass surface is much more relevant in our regions," according to Van Spronsen. "An angle of 0 degrees is perpendicular to the glass, but its much more important to measure the light transmission with angles of 40 to 70 degrees on the greenhouse roof, both in a dry and wet state." The latter is important because of the underestimation of the effect of condensation on the light transmission of glass.

Wageningen UR is currently working on a measurement protocol for determining light transmission of wet materials. The first results of this research are expected soon. With SmartGlass they have already started working with this.

In a dry state SmartGlass’ values are approximately comparable to other diffuse glass products. The major difference occurs when the glass is condensed or wet on one or both sides since SmartGlass has an ‘adaptive haze’: the glass adapts to the circumstances. "If the glass is dry, there is maximum diffusivity, but if the glass is wet, it becomes more or less transparent again," Van Spronsen clarifies. "The diffusivity decreases. Light transmission increases."

Most other types of glass decrease in light transmission when they are condensed. Van Spronsen: "Most glass products are drop-forming (hydrophobic). This could cost up to 6% light transmission.” And it can cost a lot of radiation: "In practice the inside of the roof of, for example, a tomato greenhouse usually has condensation almost the entire day in the period from October to May. Do the math..."

Thanks to its super-smooth surface SmartGlass is hydrophilic. "A water film is formed on the glass that slopes down to the gutter. Unlike droplets this film results in a few percent of light gain." An additional advantage is that no drops fall down from the glass. "And thanks to the super-smooth surface dirt does not stick to the surface and is drained to the gutter by the condensation film. So we dare say that SmartGlass has more or less self-cleaning properties." For those who do want to do some cleaning, SmartGlass can in principle be cleaned with all non-fluorinated cleaners. The use of a roof cleaner is also not a problem. "Due to the uniform diffuse light distribution the application of a temporary coating may not be needed, but – if desired – growers can easily install and remove it without damaging the surface structure of the glass."

SmartGlass is diffuse glass on the basis of toughened float glass, with a choice between 90+ or 91+ (low iron) float glass. Van Spronsen: "Almost all other diffuse glass products currently available are based on figured (rolled) glass. We chose float glass because it has a significantly higher mechanical strength than figured glass."

One of the projects that will use SmartGlass is the Winter light greenhouse. Glascom Tuinbouw is one of the consortium partners in the Winter light project, in which - in collaboration with Wageningen UR and partners - an innovative greenhouse is being developed that should allow the maximum amount of natural light in the greenhouse particularly during the winter months. The Winter light greenhouse will be built at the Innovation and Demonstration Centre of Wageningen UR in Bleiswijk. "The Winter light greenhouse will be fitted with SmartGlass, as this might prove to be the perfect ‘winter glass’. Lots of light transmission in the higher angles on the greenhouse roof (between 40º and 70º) and a higher light transmission if the glass is condensed. But of course SmartGlass also works in the summer months, with maximum diffusivity."