In the bio-optimised greenhouse, the test greenhouse on Ruud van Schie’s site, Wageningen UR Glastuinbouw has researched the effects and possibilities for improvements of the Aircokas concept, in co-operation with Ruud van Schie, Hoogendoorn and Wilk van der Sande. To this end, many measuring instruments and measuring areas have been set up, and the test greenhouse was made independent of the control of the rest of the reference greenhouse.
Explaining the results of the research in the bio-optimised greenhouse is difficult, as a number of changes were made both in the test greenhouse and in the reference greenhouse during research to positively influence the crop. In addition, the horizontal temperature distribution in the test greenhouse during the first half of the crop was rather uneven, which had a strong influence on the plants.
The most important recommendation arising from the research is that the Aircokas principle is positive means in production to prevent extreme circumstances in the area of humidity and temperature. Increased production in combination with energy savings achieved made it possible for the investment to redeem itself.
Below a summary of the most important conclusions from the research.
GREENHOUSE CLIMATE – The greenhouse climate in the bio-optimised greenhouse differed considerably from that in the reference greenhouse in the spring. The average greenhouse temperature for instance from week 10 onwards was almost 2°C higher during the day, and a higher humidity was maintained both during the night and during the day. This led to some 40% less air exchange and, due to this, to an average of almost 200 ppm higher CO2 concentration. There were two problems in the test greenhouse with temperature distribution:
1. The horizontal temperature distribution required a lot of attention. For instance, with a low air output temperature from the LBKs, the emitted air sank too quickly, and was not distributed well across the greenhouse.
2. There were large vertical temperature differences at sunrise. This resulted in fruits becoming wet, as they warmed up less quickly than the greenhouse air.
To improve the distribution of air emitted from the LBKs, air manifolds above the plants were tried out. This only brought insufficient improvements. The best method appeared to be the application of several small support fans, or to increase the emission temperature, which obviously is at the expense of the cooling capacity.
The problems with the vertical temperature distribution were approached from two angles. First, a dew point control was introduced, simulating the fruit temperature and comparing it to the dew point. If these get too close to each other, dehumidification had to be carried out via the LBKs or the vents and a minimum pipe. Since introducing this dew point control, hardly any problems with wet fruit have been observed.
Later, special fans were also tried out, taking the warm air at the top of the greenhouse down to the cold fruit more quickly, so that these warm up earlier and don’t get wet. These fans also save energy, as less dry heating or dehumidification is used.
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ENERGY – You can save a considerable amount of energy in the bio-optimised greenhouse. In the winter, low-value heat in the LBKs is used. You can also manage humidity well with dehumidification., so that more insulating measures, such as a fixed screen, can be used. In summer, heat can be harvested with the LBKs. In addition, savings on the CO2 requirements can be made through misting and a higher fan temperature with a limited air exchange. To which extent energy is saved and production is increased is an economical question, which depends on energy prices and market prices paid for tomatoes.
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CO2 – If the vents can stay completely closed due to cooling, you can save so much on the CO2 consumption that cooling becomes profitable at a cold price between 2 and 4/6 GJ. Following a model calculation, using cooling with opened vents is not profitable with these cold prices though. With open vents, it is better to use misting.
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PLANT SCREENING – Plant screening seemed to stay under reasonable control both in the reference greenhouse and in the test greenhouse in 2006. Only Botrytis attacks cost a lot of attention and production in the spring. The reason for this was found to be the fruits and stems getting wet in the spring, and in the high root pressure. This problem can be prevented by dew point control and vertical air movement. Root pressure can be lowered by using plants without buds, soil cooling and/or a two-stem system. These three measures also have disadvantages though.
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PLANT DEVELOPMENT – The leaves in the test greenhouse stayed smaller and thicker than the ones in the reference greenhouse. It is not known whether this was due to an excess in assimilation or a shortage in calcium in the leaves. This is not seen as a problem however, as the Leaf Area Index (LAI) was mostly higher than 3 in both greenhouses, which is sufficient to catch most of the light.
There was hardly any difference found in the ability for photosynthesis in the leaves. The leaves did react more positively in the test greenhouse, with a lot of light and much CO2 at a higher temperature (± 30°C) than with a lower one (±24°C), while temperature did not have any influence on the photosynthesis in the leaves of the reference greenhouse.
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PRODUCTION – Due to horizontal temperature differences, production per measured area seemed to vary a lot, so that it is difficult to draw firm conclusions about the influence of the bio-optimised greenhouse on production. At a conservative estimate, you can conclude that the production gave a positive impression at the beginning of the crop, but that plants had to suffer much afterwards from the uneven heat distribution from the LBKs. Therefore, the LBKs were not used for cooling during the day from mid-summer onwards, but the misting system was used exclusively instead. As the mist was distributed well throughout the greenhouse, horizontal temperature differences disappeared at the same time.