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Fig. 2. Daily global radiation S; lines from top to bottom represent S outdoor and inside greenhouses with three different plastic films reference N0, and two levels of near-infrared
radiation-reflecting pigments N1 and N2.
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Fig. 3. Daily photosynthetically active radiation PAR; lines from top to bottom represent PAR outdoor and inside greenhouses with three different plastic films reference N0, and two levels of
near-infrared radiation-reflecting pigments N1 and N2.
The calculated global radiation transmission coefficient by the greenhouses varied through the day. In the early morning, greenhouse transmission coefficients were higher and lower in the late
afternoon; during the day when the sun elevation angle was high from 09:00 to 15:00 the greenhouse transmission coefficients were between these two values. This time-dependent
variability can be attributed to the radiation sensor location. During periods with clouds, the transmission coefficients of the greenhouses were less variable due to the high diffuse radiation
component. The diurnal variation of the greenhouse transmission coefficient has been reported by
Cooman 2002 and
Heuvelink 1996 . The daily average value of the transmission
coefficient is practically more applicable because the variability is smoothed.
4.1.2. Air temperature and air water vapour pressure deficit
Air inside the greenhouses is enclosed by the plastic films and by the insect nets. The plastic film is partly transparent for thermal radiation, but prevents water vapour transmission. Water vapour
is exchanged via the ventilation openings with nets. Diurnal variations in T
Air
and D
Air
are mainly affected by outdoor weather, leaf area index, and ventilation openings.
The daily courses of T
Air
and D
Air
and L are given in Fig. 4
and Fig. 5
. T
Air
and D
Air
were higher inside an empty greenhouse compared to outdoor start of Experiment 1. Additional closing of
the ventilation opening amplified these differences start of Experiments 2 and 3. For example,
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the differences of the average values of the maximum T
Air
initially were around 1 °C and then became 3 °C and the maximum D
Air
of 0.3 kPa became 0.8 kPa.
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Fig. 4. Measured maximum upper lines and average lower lines air temperature and leaf area index for the three greenhouse experiments:
, film type N0; , film type N1;
, film type N2; , outdoor temperature; the diagram shows that the internal climate is
virtually indistinguishable under the three films.
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Fig. 5. Measured maximum upper lines and average lower lines air water vapour pressure deficit and leaf area index for the three greenhouse experiments:
, film type N0; , film type N1;
, film type N2; , outdoor; the diagram shows that the
internal climate is virtually indistinguishable under the three films.
Growing crops inside the greenhouses increased the energy conversion into latent heat and increased the air water vapour content, while decreasing greenhouse T
Air
and D
Air
. This can be seen more clearly in Experiments 2 and 3
Fig. 4 and
Fig. 5 . In later stages of crop growth with
a high leaf area index, the maximum T
Air
was often lower than the maximum T
Out
. The differences between inside and outdoor air water vapour pressure deficit became much higher
when the ventilation walls were partly closed, due to reduced water vapour removal.
In each experimental period, temperature differences between greenhouses due to the differences in cladding optical properties were less than 0.1 °C for the mean values of average temperatures
and less than 0.3 °C for the mean values of maximum temperatures Table 4
. In general, greenhouse climate was close to outdoor climate due to the suitable greenhouse
design, especially with high ventilation openings. The effects of the different greenhouse- covering materials within the applied range are smaller than the effects of the reduction of
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greenhouse ventilation openings. The leaf area index has the highest impact on greenhouse air temperature, implying that a high amount of cooling can be contributed by the crop itself.
4.2. Diurnal course of air temperature and air water vapour pressure deficit
