The newest greenhouse film of Hyplast, Kool Lite Plus, reflects the solar heat, which induces a cooler climate in the greenhouse, specifically during the hottest period of the day. The cooling effect is a direct result of using specific pearlescent pigments in the film. These pigments selectively reflect the Near Infrared, the part of the solar spectrum that only provides heat, and is not used by the plants. This patented film can be recognized by its slight nacreous luster.
1. Introduction : Influence of solar radiation to the plant
The electromagnetic spectrum consists of numerous components, going from very high energetic rays (Röntgen rays, Gamma rays and Ultra Violet) to the visible light, then to the Infrared, then radiowaves, etc. which have much lower energy (Fig. 1.).
Solar radiation is a part of the electromagnetic spectrum, and can be subdivided into 3 areas: the UV or Ultra Violet, the visible light, and the Near Infrared.
- The UV radiation has a wavelength between 100 and 380 nm. These high energetic rays are capable of destroying cells and causing damage not only to the plant, but also to human beings and film. Fortunately the most harmful part, with a wavelength under 280 nm (UV-c) is absorbed by the ozone layers. The part that reaches the earth is subdivided into UV-b (280-320 nm) and UV-a (320- 400 nm). This radiation is barely used by the plant for growth. However, these wavelengths influence the formation of colorants and aromas in the plant. Moreover, the UV induces a more compact growth. The majority of the UV-b and UV-a, however, is absorbed by the UV-absorbers, which protect the film against UV-degradation.
- Visible light is the next part of the solar spectrum, with a wavelength between 380 and 780 nm. This part corresponds to the colors violet, blue, green, yellow, orange and red. For the plant, this is the most important part of the spectrum, as it comprises the PAR or Photosynthetic Active Radiation (400 – 700 nm). It is used by the plant for photosynthesis. Light is absorbed by the plant from the PAR-part, primarily the red and blue. The absorbed energy is used by the photosynthesis-pigments (chlorophyll and carotenoids) to convert CO2 and water to carbon hydrates via the process of photosynthesis. These carbon hydrates are used for growth and maintenance of the plant.
The Far Red (700 – 780 nm) is absorbed by the phytochrome, another plant- pigment. It influences the flowering and the cell-elongation of the plant.
￼Fig. 2. Spectral curve of the photosynthetic activity. Absorption spectrum of the extract of photosynthetic active pigments of a leaf (according to Mermier and Baille, 1988).
• Above 780 nm, starts the Near Infrared. This range isn’t specifically absorbed by the plant, but the energy is converted into heat. In addition, these photons have a much longer wavelength than the UV and the visible light, and thus a low energy. Because of its low energy content, this radiation isn’t harmful. In cold circumstances the Near Infrared has a positive influence on the climate in the greenhouse (heating up). On the other hand, it can have a negative influence at noon, i.e. excessive heating of the greenhouse, causing desiccation of the soil and the plant.
Next to the solar radiation, there is still another part of the electromagnetic spectrum that has an important roll for the greenhouse culture: the Longwave Infrared (IR), i.e. the heat irradiated by any object that is cooling down.
The losses of Longwave IR can be very important during cold and clear winter nights. The heat saved during the day is lost by radiation. The tunnel films with high EVA absorb this Longwave IR minimizing the cooling down of the greenhouse and the plants at night.
2. Diffuse light
By the addition of pearlescent pigments the film develops a diffusing character. The incoming light rays are broken on the microscopic pigment particles and diffused over the entire greenhouse. As a direct result, a larger part of the plant gets the right quantity of light. The upper leaves of the plant and possible flowers are protected from an excess of irradiation, and so from burning. In addition, the lower leaves don’t stand in the shadow of the upper leaves anymore. In total, the plant gets more light under a diffuse film than under a transparent film, which can stimulate the production of biomass by 20% or more.
3. High light transmission
The pearlescent pigments used have selectively reflect the Near Infrared (NIR). Thanks to this selective reflection, the light transmission of the film is hardly influenced. The Kool Lite Plus film has a high light transmission in the PAR range (Fig. 5). The difference from a clear standard tunnel film is hardly 5% (VIP measurement) and is equal to that of a diffuse standard film. Moreover, this film has a diffuse character, having more than 50% diffusion. The light transmission data are given in Table 1.
Fig. 5. Transmission spectrum of Kool Lite Plus in comparison with a standard thermic tunnel film in the visible range.
Table 1. Radiometric properties of the Kool Lite Plus film in comparison with a standard clear thermic and a diffuse thermic tunnel film.
The 3 films have a thickness of 200 μm.
4. Selective reflection of Near Infra Red (NIR)
The pearlescent pigments are specifically added to the Kool Lite Plus film to selectively reflect the NIR. As already mentioned, the NIR is the part of the solar spectrum that is not used by the plant and is responsible for the excessive heating of the greenhouse on sunny noons. By using photoselective Kool Lite Plus film, this undesired effect can be restricted.
Because of the reflection of the NIR, the Kool Lite Plus film has a lower transmission in the range 780 – 2500 nm than a standard tunnel film (Fig. 6). The difference in transmission is the most resolute in the range of 780 – 1500 nm, corresponding to the most energetic NIR rays. The photoselective effect of the pearlescent pigments is restricted to the part inducing the most heat of the greenhouse.
Fig. 6. Transmission spectrum of Kool Lite Plus in comparison with a standard clear thermic tunnel film in the UV, visible and the NIR.
5. Cooling effect
• Actual situation :
To keep the temperature in the greenhouse optimum, the growers living in a warm climate are obliged to use screening cloths or to chalk the greenhouse, which also restricts the total amount of light. This limitation is necessary to moderate the temperature but always has a negative influence on the development of the plant by stimulating “stretching” of the light-sensitive plants and inducing other phenomena, i.e. the reduction of the number of flowers per cluster with tomatoes.
• Lower temperature :
Thanks to the selective reflection of the NIR, the Kool Lite Plus film has a lower transmission of these heating rays. The consequence is a cooling effect on the greenhouse climate during the noon hours, which also reduces the stress for the plant. The combination of this property with the high transmission in the PAR range enables a more efficient use of the irradiated solar energy, and in many cases, chalking or screening is unnecessary. The temperature in a greenhouse covered with Kool Lite Plus can be up to 7°C lower than under a thermic diffuse film (Fig. 7). In addition, the temperature will not be influenced in cooler circumstances (Fig. 8).
Evolution of temperature on 4/4/97 in a tunnel covered with Kool Lite Plus in comparison with a standard thermic tunnel film (tomato culture in South-Tunisia).
Evolution of temperature on a cloudy day – 8/4/97 (tomato culture in South-Tunisia).
• Influence on the respiration :
A part of the carbon hydrates produced by the photosynthesis is consumed by respiration. Like photosynthesis, the respiration strongly depends on the temperature. At higher temperatures the loss of carbon hydrates from respiration rises exponentially. Moreover, the synthesis of the carbon hydrates by photosynthesis slows down when the temperature exceeds 35°C. Consequently, there are more assimilates available for plant growth (Fig. 9).
Fig. 9. The effect of the temperature to the assimilation (= photosynthesis) and respiration (according to ‘Kunstlicht in de tuinbouw’, 1992).
The evolution of the assimilation curve is strongly influenced by the light intensity. At higher light intensities the assimilation curve descends at a slower rate than at low light intensities. Consequently, more assimilates are remaining for growth.
Influence to respiration :
In dry climates the relative humidity (RH) outside is very low, which is a stressing factor for the plant. The culture under a film cover enables the grower to create a new climate in the greenhouse, i.e. a higher temperature and relative humidity. However, strong ventilation, which is necessary for temperature control, leads to a sudden decrease of the RH in the greenhouse. The drying out of the climate induces excessive respiration in the plants causing stress and drying out. By the use of Kool Lite Plus, the same temperature and a higher RH than with a standard film can be reached in the greenhouse. The consequence is a better climate for the plant.
6. Thermic effect
To avoid a too strong cooling down of the greenhouse at night, EVA co-polymers with a high percentage of vinyl acetate (as in thermic films) have been chosen as raw material for Kool Lite. EVA, in combination with the mineral substrate of the pearlescent pigment, absorbs the longwave IR-radiation coming from the plants and soil as they cool down. This thermic effect, together with the cooling effect during the day, reduces the difference between the day and night temperatures which is a less stressing environment for the plant.
7. Different versions possible
Kool Lite Plus is available in different versions :
- 7.1. Anti-Fog
By adding tensio-active additives, the water condensating against the film is flowing off. It doesn’t form any drops, but instead, one thin water layer that reflects much less light (Fig. 10). This results in 15% more light transmission and thus higher production. Moreover, the damage to the plant due to falling drops is greatly reduced.
- 7.2. Anti-Dust
The Kool Lite Plus film is also available in an Anti-Dust (abbreviated AD) instead of in an Anti-Fog (AF) version (Fig. 11.). There is much less sticking of the dust to the film and it is washed away when it rains. As a consequence, the light transmission is higher.
Fig. 11. Tunnel covered with an Anti-dust film (at the background) in comparison with an ordinary film (at the front level).
- 7.3. Anti Petal Blackening
In addition to an Anti-Fog or Anti-Dust characteristic, other additives can reduce the appearance of Petal Blackening. (Fig. 12). Petal Blackening is a physiological disease occurring with red roses because of a combination of low night temperatures and high UV irradiation.
Fig. 12 At the left, a rose grown under Anti Petal Blackening Kool Lite Plus,
At the right, the same variety grown under an ordinary film, which shows Petal Blackening.
For more information
RKW Hyplast NV
Tom De Smedt, Export Manager
2320 HOOGSTRATEN | Belgium
T: +32 (0) 3 340 25 53
F: +32 (0) 3 314 23 72
C: +32 (0) 495 41 53 52
Publication date: 4/8/2015
July 2, 2016