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        Kafrit
        November 6th 2019
        News 
        4 min read

        Greenhouse cooling challenges: A new method for evaluating the heat load reduction efficiency of various greenhouse solutions

        Basic farming used open land for agricultural growth. Over time, the areas have shrunk, the population has grown, and there is a need to improve production. One of the common methods is to move to greenhouse crops that provide a more controlled environment for the growing process.

        Greenhouses are built using increasingly sophisticated films. The film allows gaining good control of the ambient temperature, the amount of incoming light, the loss of water from the soil, and the entry of pests, thus streamlining the process and increasing crop yields.

        Sunlight is one of the most influential parameters on the growth. Light is essential to create life without which crops cannot be grown. The various wavelengths are functionally different (figure 1) and some, depending on their dose, have detrimental effects. Therefore, better control and filtering of the desired wavelengths allows us to control this important parameter and streamline the process.

        The electromagnetic spectrum

        The electromagnetic spectrum

        One of the most important controllable parameters is the heat buildup in the growing environment. Reducing heat load is especially important during hot summer days where high temperatures can impair crop yield and quality. Kafrit offers a range of heat load reduction solutions based on two different mechanisms. One is based on filtering the light rays by adding additives that filter the wavelengths that cause heat build-up. Filtering can be selective and focus on the near infrared range, 800-2500 nm. This range of wavelengths causes warming but does not affect the passage of visible light through the film. In addition, filtering can be based on a non-selective mechanism that filters all wavelengths coming from the sun. Selective filtration is obviously preferable because the light waves in the visible range, responsible for the photosynthesis process, can pass while light waves that cause unwanted heating do not penetrate the film. The other mechanism for reducing the heat load is by adding additives that collect/ absorb heat (not filters) and thus reduce the heat rays that penetrate through the film into the greenhouse.

        Further to the extensive work of Kafrit on the subject, a new method has been recently developed to evaluate the effectiveness of the various additives in reducing the heat load. An existing method for the characterization of window filtering was taken as a starting point and changes and adjustments were made to the area of greenhouse films. The computational method makes it possible to characterize a filtration system as a preliminary stage even before field trials are made and thus to go to the field with a limited number of films that are supposed to provide the appropriate response.

        The method is based on a comparison of several measurable and computational parameters of the film that give an indication of its operation (Fig. 2):

        • Light passing through the film in the visible range (Tvisl), between 380 and 800 nm.
        • Solar radiation passing through the film (Tsol), calculation of the radiation in the entire 250-2500nm solar range that has managed to penetrate through the film. The calculation was done by dedicated software written at Kafrit that takes into account the true solar spectrum and the specific filtering capabilities of the film.
        • Selectivity parameter for radiation filtering (Tvis/Tsol> 1). The larger the parameter, the more selective the film, lets visible light pass through but filters light waves causing heat buildup.
        • Solar heat gain coefficient (SHGC), which is a measure of total heat accumulation through the film, both from energy that passed through it in the NIR range and from radiation absorbed in the film. The smaller this parameter, the better the heat filtering capabilities of the film.
        Figure 2: Light passing through a film with filtering of waves in the NIR range

        Figure 2: Light passing through a film with filtering of waves in the NIR range

         

        Based on this method, Kafrit carried out a characterization of 9 films with various additives that operate in a variety of mechanisms, selective filtration, non-selective filtration, and heat absorption in order to examine which additives are particularly effective in preventing heat build-up. The computational results indicated some candidates with good selectivity for filtering the light rays. Then, the films were tested on a laboratory facility specially built for this purpose (Fig. 3). The facility examines the temperature below the film and allows the theoretical results to be verified. Finally, among the various candidates, one film was selected for further field trials. The selected film contained selective filtration by Kafrit’s additive IR 20575 LD. The first experiment was conducted in Italy during 2016 on cucumber and tomato crops under a monolayer film (Fig. 4).

        The experiment was carried out in June and showed a decrease in the average temperature of 5 degrees (the outside temperature was 30 degrees while the temperature inside the greenhouse was 25 degrees). In addition, there was an even greater decrease of 6.8 degrees in the maximum temperature measured this month (the maximum outside temperature was 36.7 degrees while the temperature inside the greenhouse was 29.9 degrees). Field results showed no increase in crop yield but the quality of the resulting vegetables was better. As a result, another round of experiments was conducted in Italy in 2017 using a three-layer film. This experiment is still under examination.

        Figure 3: Kafrit measurement facility for characterizing the heat load reduction of films

        Figure 3: Kafrit measurement facility for characterizing the heat load reduction of films

         

        Figure 4: Growing tomatoes in a greenhouse in Italy for a practical experiment

        Figure 4: Growing tomatoes in a greenhouse in Italy for a practical experiment

         

        In parallel with the method of characterization, a new additive for reducing the heat load, IR00U05 LD, has been developed in Kafrit. The titanium oxide nanoparticle-based additive and the big challenge in development was to maintain a uniform dispersion of the nanoparticles and prevent their accumulation in aggregates. The new additive operates on a filtering mechanism but is less selective relative to IR 20575 LD. In contrast, the additive is easy to use and its cost-benefit ratio is high.

        For further information: Kafrit, Hannah Schwartz, hanna@kafrit.co.il

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