No more doubts about diffuse glass? Researchers offer method of comparison

Researchers have developed a methodology to compare the performance of growers using clear and diffuse glass. In the future, this method could be used to analyze growers' data in an anonymous and standardized way.

Even though diffuse glass has been widely adopted, there are still some lingering doubts. These have mainly to do with the fact that, according to researchers, glasses not only change the diffusivity (=Hortiscatter) but light transmission too. In practice, these cannot be seen separately in a greenhouse because you always have to deal with the combination of at least 2 factors. That was the reason for doing this research, but also to develop a method to better understand light transmission and diffusivity.

Shedding light 
Five companies from a group of more than 35 potential participants with both a greenhouse with diffuse glass roof and a greenhouse with a clear roof took part in the research. In this way, it was much easier to draw comparisons. These companies grew cucumber, tomato and cherry tomato. In addition, two strawberry growers participated, one with clear glass and the other with clear glass with a double AR coating.

PAR sensor under the glass, and PAR sensor above the crop

8% higher yield in kilograms
The research showed that growers need more figures than just those on light transmission. Although this is a very relevant property of the greenhouse roof, the performance of the entire production system is a better measure than how efficiently the crop can deal with the transmitted or administered light. A greenhouse cover with lower transmission generally results in lower production, as the rule of thumb '1% more light is about 1% more production' still applies to these crops. If the light transmission of a diffuse cover decreases by 10%, there is still a net loss of production, even though the crop uses the transmitted light with a 6% higher efficiency.

Thus, researchers looked into Radiation Use Efficiency (RUE) too, to better understand light transmission and diffusivity. RUE determines how many kilograms are harvested per unit of available solar radiation. The study showed that RUE is on average 8% higher in diffuse greenhouses. In other words: per MJ of global radiation, 8% more kilograms are produced in the diffuse greenhouses compared to the clear greenhouses.

Prior to the study, many companies thought they had to heat more under diffuse glass. However, this study shows that there was no difference in energy consumption between the diffuse and the clear glasshouse, for all participating crops. The energy efficiency in the diffuse greenhouses was on average 12% higher than in the clear greenhouses.

Read more here at Kas als Energiebron

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