Evaluation of greenhouse infrastructure

By Frank Kempkes

The type of covering material of a protected cropping structure can have a huge effect on the indoor climate. Two main types of material are currently in use: glass and plastic. In principle the approach for an experimental setup does not differ between these two basic materials. The most important consideration regarding to the setup of the experiment is the greenhouse plot area. This can depend on location and season of the experiment. Sunlight can be diffuse or direct. If we have a fully clouded sky we can assume the light is diffuse. If we have a clear sky up to 90% of the light can be direct but always at least a small fraction of the light is diffuse. Whether you have a cover with a diffusing effect (as most plastics partially have) or no diffusing effect at all as for ordinary glass, the plot size for the experiment can differ substantially if we consider multispan greenhouses.

Figure 3.7 Shows how transmissivity of light through four contrasting materials depends on its wavelength. Some materials have a complete block of the UV light that can have negative effects on insects. For example, it is known that bumblebees need UV light for their orientation. If the covering material is made of polycarbonate then bumblebees are not able to fl y and pollinate crops. In contrast PMMA transmits almost all UV light which can have negative effect on the life time of used plastics in the greenhouse. Glass is a kind of average between no UV transmittance and complete UV transmittance. For insects the UV A light with a wavelength between 315 and 400 nm is of most importance. Some crops (e.g. red lettuce) need some UV-B to develop their colour.

Figure 3.7 Transmissivity of different materials: Glass, PE fi lm, PMMA and PC. (Figure provided by Wageningen

UR LightLab).

The transmission of light through a material is also dependent on the angle of incidence of the (direct) light (Figure 3.8). But what is the angle of incidence? In Figure 3.9 the maximum elevation of the sun for the longest and shortest day are shown, in the case of the Netherlands, for a typical ‘Venlo’ type greenhouse cover. The direct light of the sun never enters the greenhouse perpendicular through this cover at this location.

Figure 3.8 The relationship between the angle of light incidence (0 o _{means perpendicular and 90}o _{means par-} allel to the material) and the proportion of light transmission from two different glasses from the Asahi glass company (AGC). (Figure provided by Wageningen UR LightLab).

Figure 3.9 The angle of incidence of sunlight falling on a typical 22 o _{greenhouse roof in the Netherlands on the} longest and shortest day of the year. (Figure provided by Wageningen UR LightLab).

If the light is diffuse (sky fully overcast) this is of less importance but remember that most light (in terms of energy sum) is at most locations direct. Even if the cover has diffusing properties these are never perfect. In case of compartments within a bigger greenhouse it is even important to consider what is being grown in the compartments which are not in use for your experiment. Think of different temperature or day length regimes. Also the crop height has to be taken into account. What if your experimental crop is radish and in the south facing compartment is a full grown sweet pepper with a height of 3.5 meters?

These effects as shown in Figure 3.10. In this example the effects of the direct part of the sunlight can only be measured at the left side of the compartment. Also, even in diffuse light the area beside the high crop will have a low light intensity.

Figure 3.10 An example of the effect of crop height in adjacent greenhouse compartments on the direct light

reaching a crop. (Figure provided by Wageningen UR LightLab).

It is clear that the height of the greenhouse will also have an effect on the appropriate plot size. It is best to make a simple drawing of the location and greenhouse(s). Do you have single greenhouses with the whole greenhouse with one covering material or do you have compartments within a bigger greenhouse? In general the roof orientation of all experimental greenhouses/compartments must be the same. Depending on time of the year the roof orientation has an effect on the overall light transmission. In general east-west orientation of the ridge has a higher transmission in winter time at higher latitudes but in summer the north-south orientation shows higher transmissions.

In case of the single greenhouse set up all greenhouses should have the same size, height and ventilation capacity. Do all greenhouses have the same “free” space towards the direction of the sun? Be aware the effect of shade of trees and buildings on part of the greenhouse(s). Make in the drawing of the layout the lowest and highest elevation of the sun during the experiment. Be aware in case you make this layout in mid-summer and the experiment is in winter - the elevation of the sun will be signifi cantly different. Materials in the side walls have, in principle, the same properties as the material in the roof so do not under estimate the effect of the side walls.

It is very diffi cult to combine several covering materials within one greenhouse/compartment. If effects on climate are expected (which is generally the case) each experimental compartment should have its own controls for at least heating and ventilation and maybe even for irrigation if different levels of crop transpiration are expected. If so the humidity can also be affected which can infl uence CO₂ levels and the need for ventilation. What will be the effect on condensation of moisture on the cover of the greenhouse? Each material will have different condensation properties with different effects on light transmission; is the material hydrophobic or hydrophilic?

The light measured inside a greenhouse will be very different depending on the time of year and the time of day - mainly caused by shading effects from construction components. An array of light sensors to measure photosynthetically active radiation (PAR), in µmol/m2_{/s, located high in the greenhouse (above the crop) which}

can be averaged gives a reasonable measure but require regular maintenance - it is important to keep them free from dust and dirt. The sensors need to be levelled horizontally as a few degrees angle can disturb the measurement by more than 5%.