Article. Energy in fokus - from Kyoto to Copenhagen. - AgroTech

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Article. Energy in fokus - from Kyoto to Copenhagen. - AgroTech

applied conditions make this difficulty

very clear. The U-values for both GFC

variants are higher under applied conditions

compared to laboratory conditions.

Nevertheless, the energy saving potential

represents approximately 40 per cent in

contrast to float glass.

Light transparency

Basically, both materials (microstructured

white glass and an ethylene tetrafluoroethylene

(ETFE) film) display increased

transparency to photosynthetically active

radiation (PAR; approx. 92 per cent), UV-A

radiation (above 80 per cent) and UV-B

radiation (up to 80 per cent) in comparison

to single-layer glass roofing (approx.

90 per cent, 70 per cent and 0 per cent,

respectively). The combination of the two

materials (into a GFC system as described

above) leads to a light transparency of

the photosynthetically active radiation of

almost 90 per cent, and UV-A of approximately

70. These values are comparable

with the ones of floatglas. The UV-B

transmission values for float glass are 0

in comparison to 17 per cent for GFC.

All measurements were conducted under

greenhouse conditions.

Plant product quality

After confirming the improved light conditions

under GFC roofing, it is necessary

to verify the influence of the increased

transparency of PAR and UV-A as well as

UV-B on plant growth and development.

Investigations on plant development were

made under single-layer roofing consisting

of white glass, ETFE and float glass. These

examinations were conducted in commercial

greenhouses as well as in small

research units. The improved light conditions

led to the desired optimization of

plant growth and product quality both in

research greenhouses and also at a commercial

plant grower’s. The example of

red leaf lettuce plants showed the decisive

influence of the UV-B transmission properties

of the roofing material. This led to a

reduction in leaf length and width, which

brought about a more compact growth

form altogether.

With respect to product quality, a considerably

more intensive colouration of the

plants was visibly identifiable. The analysis

of the plants confirmed this impression

based on an increased content of significant

secondary metabolites. For instance,

the anthocyan content of red leaf lettuce

showed a significantly higher value under

UV-B-transmitting roofing material (white

glass and ETFE film).

Outlook

Further investigations are necessary to

evaluate the energy saving potential in

commercial greenhouses considering the

problems of relative air humidity. In this

context, it is important to develop an intelligent

regulation and control algorithm for

operating the air cushion.

For example, there is no air cushion

without ventilation, so the film attaches to

the glass. This state is comparable to single

layer roofing which enables condensation

and the relative air humidity decreases.

But the consequences are reduced energy

savings.


48 ENERGY IN FOCUS

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